From ADC counts to the Higgs boson: photons for physics measurements with the ATLAS experiment at the LHC Run 1

HAL Id: tel-01312862

https://tel.archives-ouvertes.fr/tel-01312862

Submitted on 9 May 2016

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

From ADC counts to the Higgs boson: photons for physics measurements with the ATLAS experiment at

the LHC Run 1

Marco Delmastro

To cite this version:

Marco Delmastro. From ADC counts to the Higgs boson: photons for physics measurements with

the ATLAS experiment at the LHC Run 1. High Energy Physics - Experiment [hep-ex]. Universite

Grenoble Alpes, 2016. �tel-01312862�

LAPP-H- 2016 - 01

Université Grenoble Alpes Mémoire présenté par

Marco Delmastro

pour obtenir le diplôme de

Habilitation à Diriger des Recherches

Spécialité: Physique des Particules

From ADC counts to the Higgs boson:

photons for physics measurements with the ATLAS experiment

at the LHC Run 1

Soutenu le 23 / 3 / 2016 devant le Jury composé de : Pr. Matteo C acciari (LPTHE, Paris) Rapporteur Dr. Lydia I conomidou -F ayard (LAL, Orsay) Rapporteur Dr. Giovanni L amanna (LAPP, Annecy-le-Vieux) President du Jury

Dr. Yves S irois (LLR, Palaiseau) Rapporteur

Dr. Isabelle W ingerter -S eez (LAPP, Annecy-le-Vieux) Examinateur

LAPP-H- 2016 - 01

Université Grenoble Alpes Mémoire présenté par

Marco Delmastro

pour obtenir le diplôme de

Habilitation à Diriger des Recherches

Spécialité: Physique des Particules

From ADC counts to the Higgs boson:

photons for physics measurements with the ATLAS experiment

at the LHC Run 1

Soutenu le 23 / 3 / 2016 devant le Jury composé de : Pr. Matteo C acciari (LPTHE, Paris) Rapporteur Dr. Lydia I conomidou -F ayard (LAL, Orsay) Rapporteur Dr. Giovanni L amanna (LAPP, Annecy-le-Vieux) President du Jury

Dr. Yves S irois (LLR, Palaiseau) Rapporteur

Dr. Isabelle W ingerter -S eez (LAPP, Annecy-le-Vieux) Examinateur

a Irene e Giulia

Acknowledgments

No man is an island, Entire of itself,

Every man is a piece of the continent, A part of the main.

— J ohn D onne ( 1624 )

Experimental particle physics is a team game. No result discussed in this manuscript would have been possible without the hard and dedicated work of many people, whom I am deeply grateful to for having shared with me the joys and pains of cutting-edge research. The achievement I count as mine are only as good as the groups I had the chance and the proviledge to contribute to.

My greatest thank goes to Isabelle Wingerter-Seez, my mentor and friend, which has always been there all these years to share ideas, projects and concerns; and from whom I learned, among all things, the importance – and the burden – of always choosing what is right over what is convenient.

I am also deeply indebted to Guillaume Unal, from whom I learned over the years, and I still learn every day, everything I know about calorimetry, photons, the Higgs boson, and much more.

The ATLAS Liquid Argon community has warmly welcomed me at the beginning of my career, and generously hosted all my first attempts to contribute to the ATLAS experiment. I am in par- ticular grateful to Martin Aleksa, Luis Hervas, Walter Lampl, Lydia Fayard, Hong Ma and Pavol Strizenec for their support and friendship over the years. I spent many exciting years working within the ATLAS EGamma group, where I had the chance to work with many clever and gener- ous people, many of whom I am honored to count among my friends. My thanks go in particular to Laurent Serin, Daniel Froidevaux, Fabrice Hubaut, Andrea Bocci, Kerstin Tackmann, Marcos Jimenez and Maarten Boonekamp with whom I spent many days and nights, doing that ground- work that is the base of experimental particle physics, and actually is particle physics. I would also like to thank Kevin Einsweiler and Tancredi Carli, with whom I had the opportunity to work with in different occasions, wearing various hats. From both of them I learned rigor and scientific honesty, and the possibility of being an all-rounded physicist even in today high-energy-physics landscape.

Among all the people I had the opportunity to share my path with studying physics with photons in ATLAS, a few had become very good friends, and today I feel close to them in many aspects of my life beyond physics. I warmly thank Leonardo Carminati and Giovanni Marchiori for their friendship, and their willingness to always share hard work, ideas and coffees.

I would like to thank the LAPP laboratory in Annecy, and in particular the former and current directors, Yannis Karyotakis and Giovanni Lamanna, for welcoming me. I am grateful to the LAPP ATLAS Group, for willingly accepting another Italian in the crew. I thank in particular Lucia Di Ciaccio, for leading me since the beginning through the jungle of the French system, and for supporting me at all steps of my career. A special thank also goes to Nicolas Berger, Remi Lafaye and Jessica Leveque. My gratitude also goes to my HDR Referees, Lydia Fayard, Yves Sirois and Matteo Cacciari, for finding the time to read and digest this manuscript.

Finally, last by not least, my deepest thanks to the women of my life, Irene and Giulia, for

their patience and your willingness to stand my grumpy days, my long nights, my rumblings and

tiredness, and for offering me all the time I sacrificed from family to physics. No result in this

manuscript would have been possible without your help and support.

The White Rabbit put on his spectacles.

“Where shall I begin, please your Majesty?” he asked.

“Begin at the beginning,” the King said gravely,

“and go on till you come to the end: then stop.”

— L ewis C arrol , A lice ’ s A dventures in W onderland

Contents

Foreword 9

Notes to the reader . . . . 10

1 From ADC counts to the Higgs boson and beyond 11

1 . 1 Exploring physics above the electroweak symmetry breaking scale with photons . . . 11 1 . 2 The experimental tools . . . . 11 1 . 3 Photons in ATLAS: from cell energies to physics objects . . . . 22 1 . 4 Physics measurements with photons in the final state with the ATLAS detector at the

LHC . . . . 32

2 Reconstruction of the LAr calorimeter cell energies 35

2 . 1 Overview of the ATLAS LAr signal reconstruction . . . . 36 2 . 2 Optimal Filtering Coefficients optimization for the 2012 data taking . . . . 37 2 . 3 Final remarks and thoughts for the future . . . . 47 3 Electron and photon energy calibration with the ATLAS detector 49 3 . 1 A brief history of the ATLAS electron and photon energy calibration . . . . 49 3 . 2 Improved electron and photon energy calibration using LHC Run 1 data . . . . 51 3 . 3 Final remarks and thoughts for the future . . . . 87 4 Data-driven measurements of the photon identification efficiency in ATLAS 89 4 . 1 Data-driven corrections to the photon shower shapes . . . . 90 4 . 2 Measurements of the photon identification efficiency using 4 . 9 fb

⁻¹

of pp collision

data at √

s = 7 TeV collected in 2011 . . . . 93 4 . 3 Measurements of the photon identification efficiency using 20 . 3 fb

⁻¹

of pp collision

data at √

s = 8 TeV collected in 2012 . . . . 113 4 . 4 Final remarks and thoughts for the future . . . . 114 5 Measurements of prompt photon and di-photon production in ATLAS 119 5 . 1 Motivations . . . . 119 5 . 2 Measurement of the inclusive isolated prompt photon cross section with √ 880 nb

⁻¹

at

s = 7 TeV . . . . 120 5 . 3 Measurement of the inclusive isolated prompt photon cross section with √ 35 pb

⁻¹

at

s = 7 TeV and comparison with various PDF sets . . . . 136 5 . 4 Measurement of the isolated di-photon cross-section with 37 pb

⁻¹

at √

s = 7 TeV . . . 137 5 . 5 Measurement of the isolated di-photon cross-section with 4 . 9 fb

⁻¹

at √

s = 7 TeV . . . 155 5 . 6 Final remarks and thoughts for the future . . . . 166

6 Measurement of the Higgs boson mass 169

6 . 1 From the Higgs boson discovery to the measurement of its properties with the H → γγ decay . . . . 169 6 . 2 Measurement of the Higgs boson mass from the H → ^{γγ and H} → ^ZZ

^∗

→ ⁴ ` channels

in pp collisions at center-of-mass energies of 7 and 8 TeV . . . . 169

6 . 3 Final remarks and thoughts for the future . . . . 190

7 Search for scalar resonances decaying into photon pairs 193 7 . 1 Search for scalar diphoton resonances in the mass range 65 – 600 GeV with √

s = 8 TeV data . . . . 193 7 . 2 Final remarks and thoughts for the future . . . . 201 8 Planning the road ahead: new physics searches, Higgs boson properties, ATLAS detector

upgrades and future collider studies 203

8 . 1 Motivations . . . . 203 8 . 2 Search for new physics at LHC Run 2 . . . . 205 8 . 3 Measurement of the properties of the H ( 125 ) boson at LHC Run 2 and beyond . . . . 207 8 . 4 Precision measurement of SM processes . . . . 209 8 . 5 ATLAS detector upgrades . . . . 211 8 . 6 Physics studies for post-LHC colliders . . . . 212

8

Foreword

The discovery at the Run 1 of the Large Hadron Collider (LHC) [ 1 ]) of a new particle with properties compatible with those of a Higgs boson by the ATLAS [ 2 ] and CMS [ 3 ] experiments [ 4 , 5 ] represents a milestone of particle physics. The discovery of a Higgs boson [ 6 – 11 ] perfectly completes the framework of the Standard Model (SM) of particle physics [ 12 – 17 ], crowning with success more than 50 years of experimental and theoretical efforts. Such a discovery comes from a long road, and would not have been possible without the dedicated efforts of thousands of physicists, engineers and technicians, spending years to develop, prototype, build and operate the LHC, the experiments and the computing grid, and then processing and analyzing the data.

I have been a member of the ATLAS Collaboration since 2000 , when the detector was far from being realized and installed, and the data taking era would have still taken years to come. Already at that time, the crucial interplay between the excellent performance of the detector and the details of the physics process under study was evident, and much effort was devoted by the collaboration to prepare well in advance an experiment that would have permit a solid scientific success. The discovery of the Higgs boson by ATLAS in 2012 , is in my opinion, an excellent representation of this interplay: in many of the search channels that proved fundamental to the discovery, only the optimal reconstruction and identification of the objects found in the corresponding final states, based on the excellent performance of the sub-detectors on which these tasks were based, allowed to reach the necessary level of significance required to discover the new particle.

Since the beginning of my career in ATLAS, I have been working on physics analyses with photons in the final state, both focused on their production in QCD process or on the search of the Higgs boson in the decay channel in photon pairs H → γγ. In this context, the interplay between the detector response understanding, the optimization of the performance physics objects, and the measurements of the physics process under study is particularly evident. Photons, either coming from the Higgs boson decays or produced by QCD processes, are reconstructed in ATLAS from the energy deposited in the electromagnetic calorimeter cells by the shower they induce. Background events to these processes, mainly composed by photons coming from decays of neutral hadrons in jets, are suppressed by leveraging the different properties of the electromagnetic showers, re- constructed thanks to the fine granularity of the ATLAS electromagnetic calorimeter. The mass of the Higgs boson is measured with the best precision in ATLAS in the γγ decay mode, exploiting the excellent calibration of the photon energy response. In all cases, the ultimate physics goals are obtained by virtue of a deep understanding of the behavior of the electromagnetic calorimeter, from its basic signal reconstruction to the clustering properties and object reconstruction, to their final energy calibration.

In this manuscript, prepared to obtain the “Habilitation à Diriger des Recherches”, I will review my activities in ATLAS ranging from the signal reconstruction and calibration in the Liquid Ar- gon calorimeters, to the performance of reconstructing, identifying and calibrating photons, to the physics studies exploiting photons in the final state, including the measurement of QCD prompt photon cross-sections, the search and discovery of the Higgs boson and the measurement of its properties in the γγ decay channel, and the search for physics beyond the SM in final states involv- ing photons. This selection of my research results is presented with the intent to demonstrate the crucial interplay between detector, performance and physics measurements. In this respect, it is my hope that the coherence of all these activities and of my professional path will clearly emerge.

This manuscript is structured as follows. In Chapter 1 I will frame the physics measurements with prompt photons in the final state performed by ATLAS at the LHC using pp collisions, de- scribe the experimental setup, and explain how photons are reconstructed, identified and calibrated by ATLAS.

In Chapter 2 I will briefly review the signal reconstruction and cell energy calibration in the AT-

LAS LAr electromagnetic calorimeter. I will report the different LAr signal reconstruction strategies adopted for the pp data taking at √

s = 7 TeV in 2011 and at √

s = 8 TeV in 2012 , discussing their impact on the electron and photon energy scale. This chapter is meant to sample my activities as co-coordinator of the ATLAS Liquid Argon Electronic Calibration working group, a role that I covered between 2005 and 2010 , and as co-convener of the ATLAS Liquid Argon Software and Data Preparation working group, between 2011 and 2012 .

In Chapter 4 I will review how the approach to understand the efficiency of the photon identifi- cation in ATLAS evolved since the beginning of the LHC data, going from approximate to refined data-driven measurements. This chapter will sample my activities as coordinator of the ATLAS e/γ Photon Identification working group, a role that I covered between 2011 and 2012 , and as co-convener of the ATLAS e/γ Combined Performance group, between 2012 and 2014 .

In Chapter 3 I will discuss the development of the improved calibration scheme used by ATLAS for the final measurement of the Higgs boson mass using all the data collected at √

s = 7 TeV and 8 TeV. This chapter is meant to sample my long-standing and still-ongoing commitment to the calibration of the response of electrons and photons in ATLAS, and my activities as co-convener of the ATLAS e/γ Combined Performance group, a role that I covered between 2012 and 2014 .

In Chapter 5 I will describe two of the prompt photon measurements that I mostly contributed to, the measurements of the including photon cross section and of the diphoton cross section in pp collisions at √

s = 7 TeV. This chapter is meant to sample results from one of my major research lines since the beginning of LHC startup in 2009 . In this context, between 2010 and 2011 I was convener of the ATLAS Standard Model Direct Photons working group.

In Chapter 6 I will briefly review the analysis leading to the discovery of the Higgs boson and the role of the H → ^γγ channel. I will then report the measurement of the Higgs boson mass combining the H → ^{γγ an H} → ^ZZ

^∗

→ ⁴ ` channels, culminating in 2014 a long physics program I was active in since before the beginning of LHC data taking.

In Chapter 7 I will report the results of a search for additional scalar resonances decaying in photon pairs using data at √

s = 8 TeV, that was the in-house effort of the ATLAS LAPP Photon Group in 2014 . The same search, extended to the data provided by the LHC at √

s = 13 TeVin 2015 , represents today my main research activity.

Finally, in Chapter 8 , I will try to lay a plan for the road ahead. Photons at ATLAS have proved to be excellent tools for discovery at the LHC Run 1 , and are already playing a similar role at the LHC Run 2 that started in 2015 . In this context, I will outline how all activity areas I was active in since the beginning of my carreer (detector prototyping, development and operation; performance optimization; precision measurement of SM processes; Higgs physics; search for physics beyond the SM) will continue to have an interplay in my future activities, and will represent the foundation of a rich and consistent research program in the years to come.

Notes to the reader

Some sections of this manuscript are reissues of published works, for which I have been a main contributor and editor. They are listed for the reader convenience in Table 1 , along with the corre- sponding publication reference.

The reader more interested in the experimental aspects of the ATLAS measurements with pho- tons will concentrate on Chapters 2 , 3 and 4 of the manuscript, while she will find the bulk of the physics results in Chapters 5 , 6 and 7 .

Section Ref.

2 . 2 [ 18 ] 3 . 2 [ 19 ] 4 . 2 [ 20 ] 5 . 2 [ 21 ] 5 . 3 [ 22 , 23 ]

Section Ref.

5 . 4 [ 24 ] 5 . 5 [ 25 ] 6 . 2 [ 26 ] 7 . 1 [ 27 ]

Table 1 : Correspondence between Sections and specific publications.

10

Chapter 1

From ADC counts to the Higgs boson and beyond

1.1 Exploring physics above the electroweak symmetry breaking scale with photons

To date, the Standard Model (SM) of particle physics [ 12 – 17 ] provides the most successful de- scription of the interactions of elementary particles, by coherently combining the SU ( 2 )

L

⊗ ^U ( 1 )

Y

model of electroweak (EW) interactions with the SU ( 3 )

C

color gauge theory of the strong interac- tions (Quantum Chromodynamics, QCD). All basic interactions in the SM are described by gauge theories, where the form of the couplings of the bosons that mediate these interactions are deter- mined by the underlying gauge symmetries. Because of this structure, the SM Lagrangian cannot explicitly accommodate mass terms for the weak vector bosons W

^±

and Z without spoiling its gauge invariance and renormalizability properties. On the other hand, Spontaneous EW Sym- metry Breaking (EWSB) can be obtained by introducing in SM Lagrangian a doublet of complex scalars fields, with a non-zero vacuum expectation value of its neutral component [ 6 – 11 ]. As a con- sequence of this extension, three of the four degrees of freedom introduced with the doublet confer masses to the weak force carriers W

^±

and Z, while the photon remains massless. The remaining degree of freedom corresponds to a new fundamental scalar boson, electrically neutral, commonly referred to as the Higgs boson. This mechanism can also explain the mass of the quarks and lep- tons, through Yukawa interactions with the scalar field and its conjugate. The couplings of the Higgs boson to the other particles in the SM are predicted to be proportional to the particle masses.

The mass of the new boson itself is not predicted by the theory, but must be below 1 TeV to ensure that the longitudinal W boson scattering amplitude does not violate unitarity for √

s & ^{1 TeV.}

The Large Hadron Collider (LHC [ 1 ]) has been designed and built to allow experimental access to the scale of the EWSB, approximatively ranging between the values of the masses of the weak bosons ( ∼ 100 GeV) and 1 TeV, by providing proton-proton collisions at multi-TeV center-of-mass (CM) energy. The ATLAS (A Toroidal LHC ApparatuS) [ 2 ], a general-purpose detector operating at the LHC, aims to explore physics above the EWSB scale. Its ambitious physics program encom- passes both the search for the Higgs boson, and the quest of direct signs of physics beyond the SM. At the same same time, ATLAS measures with high precision the properties of SM processes at the unprecedented energies provided by the LHC collisions, both to validate the validity of the theory in this regime, and to unveil potential sign of deviations from the SM expectations. In all these contexts direct photons, either coming from the decay of a new particle or produced in the hard scattering, are excellent tools for discoveries and high-precision measurements.

1.2 The experimental tools

1.2.1 The Large Hadron Collider

The LHC is a two-ring-superconducting-hadron accelerator and collider built and operated in the surroundings of Geneva, Switzerland, by the European Organization for Nuclear Research (CERN).

It was conceived to provide proton-proton (pp) collisions at a nominal CM energy √

s = 14 TeV, as

$&3/ GBR GBR _-)$ GBR

UIFHVJEF

-*/"$

(SBO4BTTP

/PSUI"SFB

-*/"$

*POT

&BTU"SFB

5* 5*

55 55

$5'

55 55

55

Fo

"-*$&

"5-"4

-)$C

$.4

$/(4

OFVUSJOPT

OFVUSPOT

Q Q

414

*40-%&

#0045&3

"%

-&*3 O5P'

-)$

14

Figure 1 . 1 : Illustration of the LHC accelerator complex. The interaction points hosting the four main LHC experiments are indicated with yellow circles.

well as collisions between heavy nuclei (Pb–Pb at √

s

NN

= 2.76 TeV and p–Pb at √

s

NN

= 5.02 TeV, to four main experiments. The project has a long history [ 28 ] dating back to 1984 [ 29 ]; the machine development was approved by the CERN Council in December 1994 , while the construction of a single-stage 14 TeV collider was finally endorsed in December 1996 .

The LHC is installed in the existing 26 . 7 km underground tunnel, comprising eight straight sectors and eight arcs laying between 45 m and 170 m below the surface, and was constructed between 1984 and 1989 for the CERN electron-positron collider LEP [ 30 ]. LEP operated at CERN between 1989 and 2000 , when it was closed to free the tunnel for the LHC assembly, thereafter started in 2002 .

The LHC hosts two general-purpose experiments, ATLAS [ 2 ] and CMS [ 3 ], both aiming at a peak luminosity of L = 10

³⁴

cm

²

s

⁻¹

for pp operation, and two dedicated experiments focusing either on precision measurements of the flavor and CP sector of the Standard Model and the search for new physics in rare B meson decays (LHCb [ 31 ]), or on the study of strongly interacting matter and the quark-gluon plasma produced in nucleus-nucleus collisions (ALICE [ 32 ]). Two smaller experiments are also located in the LHC tunnel, aiming to the measurement of the total pp cross section (TOTEM [ 33 ]) or the very-forward production of neutral particles (LHCf [ 34 ]).

The high beam intensity required for a luminosity of L = 10

³⁴

cm

²

s

⁻¹

excludes the use of anti-proton beams, as used for example in the Tevatron [ 35 ], and hence precludes the use of a com- mon vacuum and magnet system for both circulating beams. Proton bunches in the LHC circulate therefore in opposite directions in two separate rings, under separate magnetic fields to drive the counter-rotating beams. Because of the small size of the tunnel, the LHC uses twin-bore dipole magnets, consisting of two sets of coils and beam channels within the same mechanical structure and cryostat. The opposite magnetic fields, reaching a maximum intensity of 8 . 3 T, are simultane- ously provided by 1232 , 15 m-long, superconducting NbTi dipoles, kept at cryogenic temperatures ( 1 . 9 K) by superfluid liquid helium. The two circulating beams only share common sections of beam pipe at the insertion regions, where the experimental detectors are located. Protons are injected into the LHC rings at an energy of 450 GeV from the injector chain Linac 2 – Proton Syn- chrotron Booster (PSB) – Proton Synchrotron (PS) – Super Proton Synchrotron (SPS) (Fig. 1 . 1 ), and then accelerated to the LHC nominal beam energy. In the nominal configuration, the LHC proton beams are composed each by 2808 bunches, with a nominal bunch spacing of 25 ns, corresponding to a 40 MHz bunch-crossing frequency.

The first LHC proton beams circulated on September 10 , 2008 . Nine days later the operations were interrupted by an accident caused by the overpressure of gaseous helium, produced by the

12

Month in 2010 Month in 2011 Month in 2012

Jan Apr Jul Oct Jan Apr Jul Oct Jan Apr Jul Oct

]-1 s-2 cm33 Peak Luminosity [10

0 2 4 6 8

10 s = 7 TeV s = 7 TeV s = 8 TeV

ATLAS

Online Luminosity

Figure 1 . 2 : Peak instantaneous luminosity delivered by the LHC to ATLAS per day versus time during the pp runs of 2010 , 2011 and 2012 . The ATLAS online luminosity measurement is used.

Month in Year

Jan Apr Jul Oct

]

-1

Delivered Luminosity [fb

0 5 10 15 20 25 30 35

= 7 TeV s 2010 pp

= 7 TeV s 2011 pp

= 8 TeV s 2012 pp

ATLAS Online Luminosity

Figure 1 . 3 : Cumulative luminosity versus day delivered to ATLAS during stable beams and for pp collisions. This is shown for 2010 (green), 2011 (red) and 2012 (blue) running.

heating generated by a faulty electrical connection between two magnets [ 36 ]. Repairs required one year of work, and it was decided to limit the beam energy with respect to the nominal 7 TeV per beam, until further consolidation work on the accelerator would be done. Operations resumed at the end of 2009 .

In March 2010 collisions of 3 . 5 TeV proton beams were successfully established, and then deliv- ered to the experiments throughout the rest of 2010 and 2011 . A total of about 5 . 6 fb

⁻¹

of integrated luminosity at √

s = 7 TeV was delivered to ATLAS and CMS, thanks to an instantaneous peak lu- minosity ramping from an initial 10

²⁷

cm

⁻²

s

⁻¹

to 3.65 × ¹⁰

³³

^cm

⁻²

^s

⁻¹

^{. In 2012} the beam energy was increased to 4 TeV, mainly motivated by the expected increase in sensitivity to the Higgs bo- son production [ 37 , 38 ]. The instantaneous peak luminosity, constantly above 10

³³

cm

⁻²

s

⁻¹

, rose to 7.7 × ¹⁰

³³

^cm

⁻²

^s

⁻¹

, close to its design value, allowing the machine to deliver a total of about 23 . 3 fb

⁻¹

of pp collisions at √

s = 8 TeV to both ATLAS and CMS. Figure 1 . 2 and 1 . 3 respectively show the evolution of the instantaneous and integrated luminosities delivered by the LHC to AT- LAS [ 39 , 40 ]. A summary of the nominal parameters of the LHC proton beams is presented in Table 1 . 1 , with the actual values corresponding to the 2010 , 2011 and 2012 data-taking periods. The three-dimensional ellipsoidal distribution of the pp collision points, as determined in ATLAS from the reconstructed event vertices, had typical transverse sizes of 22 µm in 2011 and 15 µm in 2012 , and typical longitudinal width of 60 mm in 2011 and 50 mm in 2012 [ 41 ].

At the end of February 2013 the LHC operations have been stopped, and the accelerator has

entered a two-years shot-down period dedicated to maintenance and upgrade activities. These

operations were in particular meant to address the issues associated to the 2008 incident, including

the extensive investigation and potential repair of all electrical connections between the magnets,

parameter nominal 2010 2011 2012

beam energy [TeV] 7 3 . 5 3 . 5 4

number of colliding bunches per beam 2808 368 1380 1380

protons per bunch 1.15 × ¹⁰

¹¹

^1.2 × ¹⁰

¹¹

^1.5 × ¹⁰

¹¹

^1.6 × ¹⁰

¹¹

time between collisions [ns] 25 150 50 50

peak instantaneous luminosity ( L ⁾ [ cm

⁻²

s

⁻¹

] 1.0 × ¹⁰

³⁴

^2.1 × ¹⁰

³²

^3.7 × ¹⁰

³³

^7.7 × ¹⁰

³³

integrated luminosity per year ( R

L dt) [fb

⁻¹

] 80 0 . 048 5 . 6 23 . 3

Table 1 . 1 : Nominal values of the main parameters of the LHC proton beams, and their values for three data-taking periods.

and the installation of security systems capable of mitigating the damage in case of a similar overpressure event [ 42 ]. The LHC was scheduled to resume activities in 2015 with a pp run at a center-of-mass energy close to the design value, and has began its Run 2 operations delivering over the 2015 period about 3 . 2 fb

⁻¹

at √

s13 TeV.

Fig. 1 . 4 show the values of the cross-sections of several processes in pp and p p ¯ collision as a function of the CM energy [ 43 ]. The total cross-section is orders of magnitude larger than those of the processes targeted by the physics programs of the LHC experiments, thus imposing a very strict triggering strategy to the experiments. The inelastic cross sections at √

s = 14 TeV is σ

^inel_pp

∼ 80mb: considering the nominal value of LHC luminosity, bunch spacing and protons per bunch, this corresponds to an average µ = 25 inelastic events in each bunch-crossing overlapping with the interesting event. This phenomenon is referred to as “in-time pileup”, and has consequences on the optimization strategies when reconstructing, calibrating and selecting objects in the final state. Similarly, the remnants of the inelastic collisions happened in previous bunch-crossings (“out-of-time pileup”) have consequences on the treatment of the detector calibration, when this is dominated by a response time longer then the LHC bunch spacing time, as in the case of the ATLAS Liquid Argon calorimeters (see Sec. 1 . 2 . 3 ).

During the 2011 run, at a peak luminosity of 3.7 × ¹⁰

³³

^cm

⁻²

^s

⁻¹

^{and a 50} ns bunch separation, the number of inelastic cross section per bunch crossing was on average µ = 9 . 1 , and reached values as high as 20 . In 2012 , with the same bunch spacing and a peak luminosity of 7.7 × ¹⁰

³³

^cm

⁻²

^s

⁻¹

^, it had an average value of µ = 20 . 7 , reaching values as high as 40 (Fig. 1 . 5 ). These scenarios lead to difference choices in the treatment of the signal emerging from the ATLAS electromagnetic calorimeters (see Chap. 2 ) and entangled consequences on the calibration of the energy response of the ATLAS detector to photons (see Chap. 3 ).

1.2.2 The ATLAS experiment

The ATLAS experiment [ 2 ] is a general-purpose particle physics detector with a forward-backward symmetric cylindrical geometry and near 4 π coverage in solid angle. It is composed by a series of sub-detectors, illustrated in the sketch of Fig. 1 . 6 . ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the center of the LHC ring, and the y-axis points upward. Cylindrical coordinates ( r, φ ) are used in the transverse plane, φ being the az- imuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ as η = − ^{ln tan} ( θ/2 ) . Angular distances are measured in R ≡ ^p ( ∆η )

²

+ ( ∆φ )

²

. In the following, a brief description of all detector components will be given, with a particular focus on the inner tracking system and on the electromagnetic calorimeter system, given their crucial role in the recon- struction, identification and response calibration of photons. In particular, the LAr electromagnetic calorimeters will be described in more details in Section 1 . 2 . 3 .

The ATLAS inner detector (ID, Fig. 1 . 7 ) consists of three subsystems: at small radial distance r from the beam axis (50.5 < r < 150 mm), pixel silicon detectors are arranged in three cylindrical layers in the barrel and in three disks in each end-cap; at intermediate radii (299 < r < 560 mm), double layers of single-sided silicon microstrip detectors are used, organized in four cylindrical layers in the barrel and nine disks in each end-cap; at larger radii (563 < r < 1066 mm), a straw tracker with transition radiation detection capabilities divided into one barrel section (with 73 layers of straws parallel to the beam line) and two end-caps (with 160 layers each of straws radial to the beam line) is used. These three systems are immersed in a 2 T axial magnetic field provided by a superconducting solenoid. The inner detector has full coverage in φ. The silicon pixel and

14

0.1 1 10 10^-7

10^-6 10^-5 10^-4 10^-3 10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴ 10⁵ 10⁶ 10⁷ 10⁸ 10⁹

10^-7 10^-6 10^-5 10^-4 10^-3 10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴ 10⁵ 10⁶ 10⁷ 10⁸ 10⁹

σ σσ σ_ZZ σ σσ σ_WW

σ σ σ σ_WH σ σ σ σ_VBF MH=125 GeV

WJS2012

σ σ σ

σjet(E_T^jet > 100 GeV) σ

σσ

σ_jet(E_T^jet > √√√√s/20)

σ σ σ σggH

LHC Tevatron

e v e n ts / s e c f o r L = 1 0

33

c m

-2

s

-1 σ

σ σ σ_b σ σ σ σtot

proton - (anti)proton cross sections

σ σ σ σ_W σ σ σ σZ

σσσ σt

σ σ σ σ (((( n b ))))

√

√√

√ s (TeV)

{

Figure 1 . 4 : Cross sections of various physics processes in pp or p p ¯ collisions as a function of the center of mass energy √ s. The scale on the right-hand side show the corresponding event rate per second at a luminosity L ∼ ¹⁰

³³

^cm

⁻²

^s

⁻¹

^.

Mean Number of Interactions per Crossing

0 5 10 15 20 25 30 35 40 45

/0.1]

-1

Recorded Luminosity [pb

0 20 40 60 80 100 120 140 160

180 ATLAS Online Luminosity

> = 20.7 µ , <

Ldt = 21.7 fb-1

∫

= 8 TeV, s

> = 9.1 µ , <

Ldt = 5.2 fb-1

∫

= 7 TeV, s

Figure 1 . 5 : Luminosity-weighted distribution of the mean number of interactions per crossing for

the 2011 and 2012 data. Details on the calculation are found in Ref. [ 39 ].

Figure 1 . 6 : Schematic view of the ATLAS detector, indicating The components of the different subdetectors are.

microstrip subsystems cover the pseudorapidity range | ^η | < 2.5, while the transition radiation tracker (TRT) acceptance is limited to the range | ^η | < 2.0. The inner detector allows an accurate reconstruction of tracks from the primary proton-proton collision region, and also identifies tracks from secondary vertices, permitting the efficient reconstruction of photon conversions in the inner detector up to a radius of ≈ ^{80 cm.}

The electromagnetic (EM, Fig. 1 . 8 ) calorimeter is a lead/LAr sampling calorimeter with an accordion geometry, and it is described in more details in Section 1 . 2 . 3 .

The hadronic calorimeter (Fig. 1 . 8 ), surrounding the EM calorimeter, consists of an iron/scintillator tile calorimeter in the range | ^η | < ¹ . 7 , with depth around 7 . 4 interaction lengths (TileCal), and of two copper/LAr calorimeters spanning 1 . 5 < | ^η | < ³ . 2 , with depth around 9 interaction lengths (LAr Hadronic EndCaps, HEC). The acceptance is extended by two copper/LAr and tungsten/LAr forward calorimeters (LAr FCal) up to | ^η | = ⁴ . 9 .

The muon spectrometer (Fig. 1 . 9 ), located beyond the calorimeters, consists of three large air- core superconducting toroid systems with precision tracking chambers providing accurate muon tracking for | ^η | < ² . 7 and fast detectors for triggering for | ^η | < ² . 4 .

The ATLAS trigger system used during the LHC Run 1 operations is organized in three levels:

the hardware–based first level trigger (L 1 ) and the software–based High Level Trigger (HLT), com- prised of the Level- 2 (L 2 ) and the Event Filter (EF). The reconstruction in the HLT is seeded by the L 1 result and exploits the full granularity of the ATLAS sub-detectors. Only data contained in the regions of the detector identified by the L 1 , each region corresponding to ∼ ² % of the detector, are processed. At L 2 fast reconstruction algorithms are deployed whereas at EF level simplified offline reconstruction algorithms are used.

1.2.3 The ATLAS Liquid Argon electromagnetic calorimeter

The ATLAS electromagnetic (EM, Fig. 1 . 10 ) calorimeter [ 44 ] is a lead/LAr sampling calorimeter with an accordion geometry (Fig. 1 . 11 ). It plays a crucial role during the operation of the LHC, since physics channels involving electrons and photons in the final state form a crucial part of the ATLAS physics program. Achieving the required precision and discovery reach has placed strin- gent requirements on the performance of the calorimeter [ 45 ]. The uniformity of the calorimeter

16

Figure 1 . 7 : Schematic views of the ATLAS inner tracking system (left: full; right: detail of barrel region, with distances with respect to the beam pipe).

Figure 1 . 8 : Schematic views of the ATLAS calorimeter systems.

Figure 1 . 9 : Schematic views of the ATLAS muon spectrometer systems.

Figure 1 . 10 : Schematic views of the ATLAS Liquid Argon calorimeters.

18

47 cm

readout electrode absorber

P lead glue kapton outer copper layer

outer copper layer inner copper layer

stainless steel

HV HV

liquid argon gap liquid argon gap (~2 mm)

Figure 1 . 11 : Accordion structure of the LAr EM Barrel (EMB) calorimeter. The top figure is a view of a small sector of the EMB calorimeter in a plane transverse to the LHC beams. Honeycomb spacers, in the LAr gap, position the electrodes between the lead absorber plates.

response over a large acceptance is particularly important for the overall resolution. This drives several design choices for the calorimeter: lead-liquid argon calorimetry provides a good energy resolution and homogeneity even in the presence of strong radiation; the accordion geometry avoids readout cracks between calorimeter modules, thus also providing good uniformity.

The LAr EM calorimeter is divided into a barrel section (EMB), covering the pseudorapidity region | ^η | < ¹ . 475 ,

¹

and two endcap sections (EMEC), covering 1 . 375 < | ^η | < ³ . 2 . The barrel and endcap sections are divided into 16 and 8 modules in φ, respectively. The transition region between the EMB and the EMEC, 1.37 < | ^η | < 1.52, has a large amount of material in front of the first active calorimeter layer ranging from 5 to almost 10 radiation lengths (X

₀

). Fig. 1 . 12 shows the most accurate measurement of the material upstream the EMC, as constrained by data-driven studies [ 19 ]. The determination of the material budget in front of the EM calorimetry system, and its impact on the precise calibration of the electron and photon energies, is discussed in details in Chapter 3 .

A high voltage system (HV, Fig. 1 . 11 ) generates an electric field of about 1 kV/mm, which allows ionisation electrons to drift in the LAr gap. The absorbers are made of steel-coated lead and serve as ground electrodes, with prepreg layers that glue the steel coats on the lead sheets compensating for the varying lead thickness (Figure 1 . 11 ). The read-out electrodes consist of three copper layers separated by Kapton layers. The two outer layers carry the high-voltage while the inner one is used as signal layer (Figure 1 . 11 ). In the EMB, the HV is constant along η, while in the EMEC, where the gap varies continuously with radius, it is adjusted in steps along η. The HV supply granularity is typically in sectors of ∆η × ∆φ = 0.2 × ^0.2.

A triangular current pulse is produced when charged particles ionize the liquid argon in the gaps, with a short rise time smaller than 1 ns (Fig. 1 . 13 ). In the barrel, the high voltage is 2 kV and the typical size of the drift gap on each side of the electrode is about 2 mm, yielding a total drift time of about 450 ns. The amplitude of the ionization current pulse collected in a calorimeter cell is proportional to the energy deposited by the electromagnetic shower in that portion of the calorimeter, and its reconstruction is the starting step toward the measurement of the energy deposited by the initial particle in the detector. The triangular ionisation current is collected by the readout electrodes, and brought to the Front End Board readout electronics (FEB),

1The EMB is split into two half-barrel modules which cover the positive and negativeηregions.

η|

|

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0X/X

0 0.5 1 1.5 2 2.5 3

Services TRT SCT Pixel Beam-pipe ATLAS Simulation

η|

|

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0X/X

0 2 4 6 8 10 12 14 16

Up to calorimeter Up to presampler ATLAS Simulation

Figure 1 . 12 : Amount of material traversed by a particle, X/X

₀

, as a function of | ^η | , in the improved simulation, up to the ID boundaries (left), and up to the PS and the EM calorimeter (right). The contributions of the different detector elements, including the services and thermal enclosures are shown separately by filled color areas.

where the signal is first amplified by a current-sensitive pre-amplifier. In order to accommodate a large dynamic range, and to optimise the total noise due to electronics and inelastic pp collisions coming from previous bunch crossings (out-of-time pile-up), the signal is shaped by a bipolar filter (Fig. 1 . 13 ) and simultaneously amplified with three linear gains called low (LG), medium (MG) and high (HG). For each channel, these three amplified signals are sampled at a 40 MHz clock frequency and stored on a switched capacitor array, awaiting the level- 1 trigger decision;

upon receipt, the sample corresponding to the maximum amplitude of the physical pulse stored in MG is first digitised by a 12 -bit analog-to-digital converter (ADC). Based on this sample, a hardware gain selection is used to choose the most suited gain. The samples of the chosen gain ( 5 in normal operation during LHC Run 1 ) are digitised and routed via optical fibres to the read- out drivers, where the pulse amplitudes and the corresponding cell energies are reconstructed from them by mean of the Optimal Filtering (OF) technique [ 46 , 47 ]. The details of this reconstruction and calibration procedure, and the implications on the response calibration of electrons and photons, are discussed in Chapter 2 .

Both the barrel and endcap calorimeters are longitudinally segmented into three shower-depth layers for | ^η | < 2.5 (Fig. 1 . 14 ). The first one (L 1 , Strips), in the ranges | ^η | < ¹ . 4 and 1 . 5 < | ^η | < ² . 4 , has a thickness of about 4 . 4 X

₀

and is segmented into high-granularity strips in the η direction, typically 0 . 003 × ^{0 . 1 in} ∆η × ∆φ in EMB, sufficient to provide an event-by-event discrimination between single photon showers and two overlapping showers coming from the decay of neutral hadrons in jets. The second layer (L 2 , Middle), which collects most of the energy deposited in the calorimeter by photon and electron showers, has a thickness of about 17 X

₀

and a granularity of 0 . 025 × ^{0 . 025 in} ∆η × ∆φ. A third layer (L 3 , Back), which has a granularity of 0 . 05 × ^{0 . 025} in ∆η × ∆φ and a depth of about 2 X

₀

, is used to correct leakage beyond the EM calorimeter for high-energy showers. Each calorimeter cell is built out of several LAr gaps connected in parallel:

for L 2 and L 3 , there are 4 ( 3 ) double-gaps in parallel in the barrel (endcap) respectively; there are four times as many gaps per cell in L 1 , given the coarser granularity of the readout in the azimuthal direction [ 2 ]. In front of the accordion calorimeter, a thin presampler layer (PS), covering the pseudorapidity interval | ^η | < ¹ . 8 , is used to correct for energy loss upstream of the calorimeter.

The PS consists of an active LAr layer with a thickness of 1 . 1 cm ( 0 . 5 cm) in the barrel (endcap) and has a granularity of ∆η × ∆φ = 0.025 × ^0.1.

The EM calorimeter globally counts 173 , 312 readout channels, the majority of the about 184 , 000 channels of the whole LAr system including the HEC and FCAL hadronic calorimeters. This manuscript being focused on physics with photons in the final state, I will discuss in details the reconstruction and calibration of the EMC cells, even if many of the procedures discussed in Chap- ter 2 where developed and optimized for all LAr calorimeters, including the hadronic subdetectors.

The construction of the EM calorimeter took place between 1999 and 2006 [ 48 , 49 ], and its instal- lation in the ATLAS cavern was completed at the end of 2006 . I joined the ATLAS Collaboration and the ATLAS LAr Group in 2000 , and I am active in the LAr community since then. Before LHC start-up, the main challenge was to commission the associated electronics and to automate all of the calibration steps for all channels [ 50 ]. Many of these steps were established on smaller scale

20

Figure 1 . 13 : Form of the ionization signal (a) and the shaped ionization signal (b). The dots indicate an ideal position of samples separated by 25 ns.

∆ϕ= 0.0245

∆ η = 0.025 37.5mm

/8 = 4.69mmm

∆ η = 0.0031

∆ϕ=0.0245x4 36.8mmx

Trigger Tower

∆ϕ= 0.0982

∆ η = 0.1

16X0

4.3X₀

2X0

1500 mm

470 m m

η ϕ

η =0

Stri p cel l s i n L ay er 1

Square cel l s i n L ay er 2 1.7X0

Cells in Layer 3

∆ϕ×∆η = 0.0245× 0.05

Cells in PS

∆η×∆ϕ= 0.025 × 0.1

Trigger Tower

=147.3mm4

Figure 1 . 14 : Scheme of the segmentation and cell granularity of the LAr EM calorimeter, at η == 0.

The represented segmentation is approximatively constant in the barrel, and reproduced in the

endcaps with varying cell sizes. The PS extends up to η = 1.8.

during the testbeam campaigns held between 2000 and 2004 [ 51 – 62 ], and needed to be adapted to the full detector scale, to the ATLAS common software framework, and to the expected LHC cycle. Cosmic muon data have been taken regularly for commissioning purposes since 2006 : in particular, at the end of the summer and during autumn of 2008 , stable cosmic muon runs were taken with the detector fully operational and using various trigger menus [ 63 – 66 ]. During the LHC data taking, the focus shifted toward the continuous calibration and monitoring of the electronic and energy response [ 67 , 68 ], the assessment of the data quality [ 69 ], and the refined optimization of the calibration procedure in front of the LHC running conditions [ 18 , 70 ]. A selection of my contributions to these endeavors is discussed in Chapter 2 , with particular attention paid to their connection and impact on the electron and photon energy calibration in the 8 TeV data collected by ATLAS in 2012 .

1.3 Photons in ATLAS: from cell energies to physics objects

1.3.1 Photon reconstruction and calibration

Photons are reconstructed in ATLAS in the region | ^η | < 2.47 starting from energy deposits (clusters) in the LAr EM calorimeter. To reconstruct the EM clusters, the EM calorimeter is divided into a grid of N

η

× ^N

φ

towers of size ∆η × ^{∆φ = 0 . 025} × ^{0 . 025} . Inside each of these elements, the energy of all cells in all longitudinal layers is summed into the tower energy. These clusters are seeded by towers with total transverse energy above 2 . 5 GeVand searched for by a sliding-window algorithm [ 71 ], with a window size of 3 × ^{5 towers.}

Conversion vertices with two associated ID tracks (“double-track conversions”) are reconstructed by performing a constrained vertex fit using the parameters of the two participating tracks under the condition that the photon is a massless particle [ 45 ]. Conversion vertices with only one track assigned to them (“single-track conversions”) are mostly located at larger radial positions inside the tracker, and correspond to the case when one of the two produced electron tracks failed to be reconstructed either because it has p

_T

< 500 MeV, or when the two tracks are very close to each other and cannot be adequately separated. In this case, since a vertex fit cannot be performed, the conversion vertex is placed at the location of the first measurement of the participating track.

Clusters matched to a well-reconstructed ID track, originating from a vertex found in the beam interaction region, are classified as electrons [ 72 ]. If the track matched to the cluster is instead consistent with a photon conversion and a conversion vertex is reconstructed, the corresponding candidates are considered as converted photons: they are classified as single-track or double-track conversions depending on the number of assigned electron-tracks. Clusters without matching tracks are classified as unconverted photons [ 73 ]. A final algorithm allows the recovery of photon candidates initially classified as electron ones, for instance because they were wrongly matched to fake electron tracks, either with low transverse momentum (below 2 GeV) or large imbalance between their energy E measured in the EM calorimeter and their momentum p measured in the inner tracker (E/p > 10).

Following this classification, the candidate initial cluster is rebuilt to account for its origin. Clus- ters associated to electron candidates are rebuilt using an area of calorimeter cells corresponding to 3 × ^{7 and 5} × ⁵ ∆η × ∆φ = 0 . 025 × ^{0 . 025} cells in the EMB and EMEC respectively. Clusters associated to converted photons are instead enlarged to a 3 × ⁷ cluster size in the EMB, to account to the larger shower size in the φ direction, due to the bending of the trajectories of the electron and positron originated in the conversion. 3 × ⁵ clusters are used for unconverted photons in the EMB, due to their smaller lateral size. A 5 × ⁵ cluster size is used in the EMEC for converted and unconverted photons. These lateral cluster sizes were optimized to take into account the different overall energy distributions in the barrel and endcap calorimeters while at the same time minimising the impact of pile-up and electronic noise on the total energy.

The tracking, vertexing and track-cluster matching requirements used to reconstruct photon candidates are optimised differently for the √

s = 7 and 8 TeV data, in order to cope with the different level of pile-up. For the 7 TeV data taking the track selection criteria are presented in Ref. [ 74 ] while the vertex-to-cluster matching ones are detailed in Ref. [ 73 ]. The criteria used for the 8 TeV data taking have been tightened to significantly reduce the number of fake conversion vertices, while keeping a constant efficiency for high-p

_T

conversions, and to reduce the number of fake tracks, in particular those which have hits only in the transition-radiation detector. Moreover,

22

to improve the track parameter resolution, tracks with hits in the silicon detectors are refitted with a gaussian-sum-filter technique [ 75 ].

The calibrated cluster energy is determined by applying correction factors computed by a cal- ibration scheme based on the full detector simulation, which is described in details in Chapter 3 . The relative photon energy resolution after the cluster calibration can be parameterized as:

σ E = √ ^a

E ⊕ _E ^b ⊕ ^{c, ( 1 . 1 )}

where a, b and c are η-dependent parameters. The sampling term a, associated to the sam- pling structure of the LAr calorimeter, mostly contributes at low energy; its design value is ( 9-

− ¹⁰ ) %/ p

E [ GeV ] at low | ^η | , and is expected to worsen as the amount of material in front of the calorimeter increases at larger | ^η | . The noise term b is about 350 × ^{cosh η} MeV for a 3 × ^{7 cluster} in the EMB, and for a mean number of interactions per bunch crossing µ = 20; it is dominated by the pile-up noise at high η. At higher energies the relative energy resolution tends asymptotically to the constant term c, which has a design value of 0 . 7 %.

1.3.2 Photon identification

The baseline photon identification algorithms in ATLAS rely on rectangular cuts

²

using calorimetric variables, described in details in Ref. [ 73 ], sketched in Fig. 1 . 15 and listed in Table 1 . 2 . These dis- criminating variables (DV’s) can be grouped in three main categories: hadronic leakage, variables using the second longitudinal compartment (middle layer) of the EMC and variables using the first longitudinal compartment (strip layer) of the EMC. Figures 1 . 16 and 1 . 17 shows examples of the normalized distributions of the calorimetric discriminating variables in the region 0 < | ^η | < 0.6 for E

_T

> 20 GeV for true and fake photons before any selection, for candidates reconstructed as unconverted and converted photon respectively. All quantities for true photons are obtained from photon candidates reconstructed in a sample of γ-jet events and matched to true prompt photons, while all figures for fake photons are obtained from photon candidates reconstructed in a sample of QCD jj events, and not matched to true photons originating from parton bremsstralung.

The selection cuts on the DV’s have been optimized in different pseudorapidity regions, aiming for the largest background rejection for a fixed signal identification efficiency of around 85 %, by using a full simulation of both signal and background processes in the nominal description of the ATLAS detector. Two levels of identification are defined: aloose selection, based on the information from the hadronic leakage and the EMC Middle layer DV’s R

_η

and w

_η2

; a stricter tight selection, including tighter cuts on the same DV’s used by the loose selection, an additional cut on one middle layer quantity and especially cuts on quantities computed from the energy deposit in the strip layer, which with its fine granularity provides good γ − ^π

⁰

separation. Figure 1 . 18 shows event displays of electromagnetic shower associated to photon candidates, respectively passing and failing the tight selections. The double structure of the energy deposit in the strip layer, typical of two superimposing showers from the photons coming from the decay of a neutral hadron, is clearly seen in Fig. 1 . 18 (b).

For both the √

s = 7 TeV data collected in 2011 and the √

s = 8 TeV collected in 2012 , the loose selection applies the same cuts to both unconverted and converted photons, while the tight selection cuts have been separately optimized for unconverted and converted photons [ 73 ].

Figure 1 . 19 shows as an example the total expected selection efficiencies for loose and tight selections optimized for the 7 TeV data-taking, for true prompt photons in a sample of γ-jet events in the region 0 < | ^η | < 1.37 and 1.52 < | ^η | < 2.37 having true E

T

> 20 GeV, as a function of the pseudorapidity (Figures 1 . 19 (a) and 1 . 19 (c)) and transverse energy (Figures 1 . 19 (b) and 1 . 19 (d)).

Since the very beginning of the ATLAS data taking, non negligible discrepancies have been observed between the DV distributions recorded in data and the ones predicted by the MC [ 77 , 78 ].

A partial responsible for such discrepancy was identified in 2011 in the description of the LAr absorber in the MC description of the EMC. The initial G eant4 (G 4 , [ 79 ]) description used a single mixed material averaging the properties of the absorber lead, glue and steel layers to improve the simulation time of the electromagnetic shower development in the EMC ( 2010 simulation, G 4 . 9 . 2 ).

2Specifically for the search of the SM Higgs boson in theH →^γγusing the√

s=7TeV collected by ATLAS in2011, a dedicated selection based on neural-network algorithm aiming to improve the signal-to-background ratio was developed [76].

Not reviewed, for internal cir culation only

2 JAMES SAXON, BRIG WILLIAMS

Variables and Position

Energy Ratios

Shower Shapes

Widths

Strips 2nd Had.

Ratios f

1

, f

side

R

⌘

*, R R

Had.

* Widths w

s,3

, w

s,tot

w

⌘,2

* - Shapes E , E

ratio * Used in PhotonLoose.

- -

η φ

Width in a 3×5 (Δη×Δφ) region of cells in the second layer.

ws3 = w1 uses ±1 strips (three total);

wstot is defined similarly, but uses 20×2 strips.

R ⌘ = E _{3 7} ^S2 E _{7 7} ^S2

f

1

= E

S1

E

Tot.

f

_side

= E

^S1_{7 1}

E

_{3 1}^S1

E

_{3 1}^S1

φ η

Strips

Hadronic Second Layer

Figure 1. Sketch of the ten isEM variables used in this optimization.

1.1. The ‘isEM’ Variables. The input variables used for these neural nets are the same as those used for the ‘PhotonTight’ cuts-based optimization. There are six variables in the first layer of the liquid argon calorimeter (the strips), three variables from the second layer, and a final variable (R

Had.

) that characterizes the leakage into the hadronic calorimeter. A sketch of these variables is given in Fig. 1.

Among the strip variables:

§ f

1

“ E

^S1

{ E

Tot.

measures the fraction of the total shower energy deposited in the strips. The signal and background overlap largely, and the Tight cuts use this only as a ‘safety’ variable, cutting those events with less than 0.5% deposited in the strips. The NNs, however, are able to take advantage of the di↵erences in shapes.

§ f

side

“ `

E

_7ˆ1^S1

´ E

_3ˆ1^S1

˘

{ E

_3ˆ1^S1

quantifies the fractional enegy deposited outside of the three center- most strips. Signal-like showers deposit their energy preferentially at lower f

side

.

§ w

s,3

“ w

1

“ b

∞

Eip∞i´imaxq²

Ei

measures the ‘width’ in the strips, using only the neighbouring strips (i “ ˘ 1). Using 20 strips in ⌘ instead of just three, we get w

s,tot.

. Signal showers tend to be narrower than the background.

Two highly-correlated variables characterize the ‘two-peak-ness’ of the shower. These help to reject the primary, ⇡

⁰

Ñ background:

§ E “ E

_max,2^S1

´ E

^S1_min

is defined for a shower with tow maxima, as the height of the second maximum above the minimum between them (see Fig. 1.

§ E

ratio

“

^EE^max,1_max,1^{S1S1 ´}`^EE^S1max,2S1_max,2

characterizes the di↵erence in the magnitudes of two peaks in a single shower.

Three more variables come from the second layer of the calorimeter:

§ R

⌘

“ E

₃^S2_ˆ7

{ E

^S2_7ˆ7

characterizes the width in the ⌘ direction. Signal tends to have R

⌘

values signfi- cantly closer to 1 than the background. R

⌘

is one of the most powerful discriminants, and it is one of three variables used in the Loose cuts.

§ R

'

“ E

₃^S2_ˆ3

{ E

₃^S2_ˆ7

characterizes the width in the ' direction. Like R

⌘

, signal photons tend to have R

'

values closer to 1 than the background.

Figure 1 . 15 : Sketch of the calorimetric discriminating variables used for photon identification by ATLAS, from Ref. [ 76 ].

When a detailed description of the absorbers was introduced ( 2011 simulation, G 4 . 9 . 4 ), the discrep- ancy between the DV distributions in data and MC was partially mitigated, while not completely removed (see Fig. 1 . 20 for examples). More recently, the impact of different G 4 physics lists, imple- menting alternative models for the electron multiple scattering and bremsstralung and the photon conversion, was also investigated [ 80 ], unfortunately not leading to any sizable improvement in the DV data/MC agreement. It was therefore clear that, because of the residual discrepancy in DV distributions in data and MC, the central value of electron and photon identification efficiency one could compute from the MC simulation of the signal processes in ATLAS would suffer of large systematic errors, and data-driven corrections to the MC and direct measurement of the selection efficiency in data were needed. An overview of the results of both efforts is presented in Chapter 4 .

1.3.3 Photon isolation

In order to further separate prompt photons from their main background, consisting in photons from the decay of neutral mesons in jets, photon candidates are required to be isolated from nearby hadronic activity. The level of this surrounding hadronic activity is either measured by the local response in the calorimeters (calorimetric isolation), of by that in the ID (track isolation). Calori- metric isolation has been widely used in measurements of the production cross sections of isolated photon [ 22 , 24 , 25 , 78 , 82 – 84 ], while track isolation has been first introduced as a complement to the calorimetric isolation in the context of the measurement of the Higgs boson properties in the H → ^γγ decay (see as an example [ 85 ]).

Calorimetric isolation

The “cone” calorimetric isolation transverse energy is defined for photons (and electrons) as the scalar sums of the transverse energy in all cells in the EM and hadronic calorimeters within a cone of radius R around the photon candidate axis. A rectangular core of cells (5 × ^{7 in η} × ^φ) nearest to the photon is excluded from the sum, aiming to remove the photon energy from the sum (Fig. 1 . 21 (a)). The core of the photon (electron) shower is excluded, and the isolation transverse energy is corrected for the leakage of the photon shower into the isolation cone. The contributions

24