γdy(
NLOσd
2 2.2 2.4 2.6 2.8 3 3.2 3.4
Figure 1.11: k-factorσtNLO¯tγ /σtLO¯tγ, with µR = 2mtand µF =√ ˆ
s, as a function of photon transverse momentum (left) and of the photon rapidity (right) for pp collisions at √
s= 7 TeV [54]. The uncertainty, from scale variations, is overlaid as a band on thek-factor as function of the transverse momentum.
1.5 Summary
The top quark, because of its large mass, decays before producing bound states, therefore it allows for a unique possibility to probe directly the quark-photon vertex. The measurement of the t¯tγ production cross section paves the way for a direct probe of thetγ vertex. A direct measurement of the top quark’s electric charge would allow to increase the confidence in the exclusion of ICQ models. Furthermore, (small) deviations from the SM prediction of the top-EW couplings can provide hints of new phenomena appearing at a higher scale of that accessible by the experiment.
The comparison of experimental data with the NLO theoretical prediction of thet¯tγ cross section is the starting point of this programme.
CHAPTER 2
The experimental setup and its performance
This chapter is meant as a quick description of the experimental setup used in this measurement for the reader who is not familiar with the detector components and its nomenclature.
Section 2.1 describes the accelerator complex and the beam parameters of the Large Hadron Collider at CERN. Section 2.2 briefly explains the different detection techniques used in the ATLAS experiment. Section 2.6 is devoted to the review of the data acquisition and processing. Section 2.7 reviews the detector operations during the run 1 period (2010-2013) and the detector performance is overviewed in Sec. 2.8. In the last two sections a more detailed emphasis is given to data quality monitoring and performance of the silicon micro strip detector, as the author was directly involved in these activities [60].
2.1 The accelerator complex
The ATLAS experiment is located at an interaction point (IP) of the Large Hadron Collider (LHC) [61]. The LHC itself is a two-ring-superconducting-proton (ion) accelerator situated at the Euro-pean Centre for Nuclear Research (CERN) in Geneva, Switzerland. The LHC is part of a wide accelerator complex hosted by CERN.
2.1.1 Pre-accelerators
Before protons are accelerated to a centre-of-mass energy (√
s) of the order of 7 TeV by the LHC ring, a range of pre-accelerators is used to bring the proton energy in steps close to the TeV threshold. Figure 2.1 illustrates the complex accelerator system and the LHC experiments situated at CERN. Hydrogen gas is ionised and accelerated by linear accelerators, such as the Linac1 and the Linac2. The ions passing through Radiofrequency (RF) conductor cavities are accelerated to about 50 MeV. A small circular accelerator system, the Proton Synchrotron Booster (PSB), accelerates protons up to 1.4 GeV which are then injected into the Proton Synchrotron (PS).
The PS subsequently injects protons to the Super Proton Synchrotron (SPS) where they reach gradually an energy of450GeV. Clockwise and anticlockwise injector systems feed those protons into the LHC .
24
Figure 2.1: Overview of the accelerator complex at CERN [62].
2.1.2 The Large Hadron Collider
The LHC consists of 1232 liquid helium cryogenic dipole magnets filling two thirds of a circle of approximatively 27 km circumference, the rest comprising beam focusing quadrupole magnets, and accelerating cavities. The ring is divided in eight straight sections and in eight arced sections.
A cross section view of a dipole element is shown in Fig. 2.2. The underground tunnel excavated for the Large Electron Positron (LEP) now hosts the LHC . Lying on molasse and limestone rock beads for, respectively, 90% and 10% of its length, the tunnel is situated between 100 m and 45 m underground with a 1.4% inclination gradient pointing towards Geneva. Two transfer tunnels link the LHC to the remainder of the CERN accelerator complex. Technical aspects are detailed elsewhere [63]. For a bunched gaussian-distributed beam containingNb particles per bunch, interacting at a frequencyfrev, and accelerated at speeds ofγr, the instantaneous luminosity (LLumi) can be defined as [61]:
LLumi= Nb2nbfrevγrF
4πεnβ? (2.1)
where the normalised transverse emmitance(εn)and the beta function at the point of collision (β?) characterise the geometrical properties of the beam. The luminosity is corrected by a geometrical factorF which depends on the angle formed by the opposite direction of the two colliding beams (θc), on the longitudinal (transverse) gaussian width of the colliding beams at the IP σz (σ?).
Experimental Setup 26
2008 JINST 3 S08001
Figure 3.3: Cross-section of cryodipole (lengths in mm).
an important operation for the geometry and the alignment of the magnet, which is critical for the performance of the magnets in view of the large beam energy and small bore of the beam pipe.
The core of the cryodipole is the “dipole cold mass”, which contains all the components cooled by superfluid helium. Referring to figure3.3, the dipole cold mass is the part inside the shrinking cylinder/He II vessel. The dipole cold mass provides two apertures for the cold bore tubes (i.e. the tubes where the proton beams will circulate) and is operated at 1.9 K in superfluid helium. It has an overall length of about 16.5 m (ancillaries included), a diameter of 570 mm (at room temperature), and a mass of about 27.5 t. The cold mass is curved in the horizontal plane with an apical angle of 5.1 mrad, corresponding to a radius of curvature of about 2’812 m at 293 K, so as to closely match the trajectory of the particles. The main parameters of the dipole magnets are given in table3.4.
The successful operation of LHC requires that the main dipole magnets have practically iden-tical characteristics. The relative variations of the integrated field and the field shape imperfections must not exceed⇠10 4, and their reproducibility must be better than 10 4after magnet testing and during magnet operation. The reproducibility of the integrated field strength requires close control of coil diameter and length, of the stacking factor of the laminated magnetic yokes, and possibly fine-tuning of the length ratio between the magnetic and non-magnetic parts of the yoke. The struc-tural stability of the cold mass assembly is achieved by using very rigid collars, and by opposing the electromagnetic forces acting at the interfaces between the collared coils and the magnetic yoke with the forces set up by the shrinking cylinder. A pre-stress between coils and retaining structure
– 23 –
Figure 2.2: Cross section view of a dipole element of the LHC [61].
The total amount of data recorded by the experiments, situated in the IP s of the ring, depends upon the choice of those parameters. Design and typical operating values for those parameters are shown on Tab. 2.1.
Year Design
Parameter 2011 2012
Beam energy 3.5 TeV 4 TeV 7 TeV
β? 1.0 m 0.6 m 0.55 m
1/frev 50 ns 50 ns 25 ns
nb 1380 1374 2808
< Nb> 1.45×1011 protons 1.65×1011 protons 1.10×1011 protons
Intial εn 2.5 mm mrad 2.5 mm mrad 3.75 mm mrad
LmaxLumi 3.7×1033cm−2s−1 7.7×1033cm−2s−1 1.0×1034cm−2s−1
Stored beam energy 110 MJ 140 MJ 362 MJ
Table 2.1: Overview of proton proton beam parameters of the LHC during the machine operations of 2011 and 2012. Paramters are compared with respect to their design values [61, 64, 65].