arXiv:1201.3293v2 [hep-ex] 7 May 2012
Search for excited leptons in proton–proton collisions
at
√
s
= 7 TeV with the ATLAS detector
The ATLAS Collaboration
The ATLAS detector is used to search for excited leptons in the electromagnetic radiative decay channel ℓ∗ →ℓγ. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 2.05 fb−1. No evidence for excited leptons is found, and limits are set on the compositeness scale Λ as a function of the excited lepton mass mℓ∗. In the special case where Λ = mℓ∗, excited electron and muon masses below 1.87 TeV and 1.75 TeV are excluded at 95% C.L., respectively.
PACS numbers: 12.60.Rc, 13.85.-t
I. INTRODUCTION
The Standard Model (SM) of particle physics is an extremely successful effective theory which has been ex-tensively tested over the past forty years. However, a number of fundamental questions are left unanswered. In particular, the SM does not provide an explanation for the source of the mass hierarchy and the generational structure of quarks and leptons. Compositeness mod-els address these questions by proposing that quarks and leptons are composed of hypothetical constituents named preons [1]. In these models, quarks and leptons are the lowest-energy bound states of these hypothetical parti-cles. New interactions among quarks and leptons should then be visible at the scale of the constituents’ binding energies, and give rise to excited states. At the Large Hadron Collider (LHC), excited lepton ℓ∗production via
four-fermion contact interactions can be described by the effective Lagrangian [2] Lcontact= g2 ∗ 2Λ2j µj µ, where g2
∗is the coupling constant, Λ is the compositeness
scale, and jµ is the fermion current
jµ= ηLfLγµfL+ηL′f ∗ LγµfL∗+η ′′ Lf ∗ LγµfL+h.c.+(L → R).
For simplicity and consistency with recent searches, the following prescription is used: g2
∗ = 4π, ηL= η′L= η ′′ L=
1, and ηR = ηR′ = η ′′
R = 0 such that chiral symmetry
is conserved [3][4]. The above ansatz ignores underly-ing preon dynamics and is valid as long as the mass of the excited leptons is below the scale Λ. In the well-studied case of the homodoublet-type ℓ∗
[2, 5, 6], the rel-evant gauge-mediated Lagrangian describing transitions between excited and ground-state leptons is
LGM= 1 2Λℓ ∗ Rσµν gfτ a 2 W a µν+ g ′ f′Y 2Bµν ℓL+ h.c.,
where ℓLis the lepton field, Wµν and Bµνare the SU (2)L
and U (1)Y field strength tensors, g and g′are the
respec-tive electroweak couplings, and f and f′are
phenomeno-logical constants chosen to be equal to 1. The LGMterm
allows the decay of excited leptons via the electromag-netic radiative mode ℓ∗±
→ ℓ±γ, a very clean signature
which is exploited in this search. For a fixed value of Λ, the branching ratio B(ℓ∗±
→ ℓ±
γ) decreases rapidly with increasing ℓ∗
mass. For Λ = 2 TeV, B(ℓ∗ ±
→ ℓ±
γ) is 30% for mℓ∗ = 0.2 TeV and decreases exponentially to about 2.3% for mℓ∗ = 2 TeV.
Previous searches at LEP [7], HERA [8], and the Teva-tron [9] have found no evidence for such excited leptons. For the case where Λ = mℓ∗, the CMS experiment has excluded masses below 1.07 TeV for e∗
and 1.09 TeV for µ∗
at the 95% credibility level (C.L.) [10].
II. ANALYSIS STRATEGY
This article reports on searches for excited electrons and muons in the ℓ∗
→ ℓγ channel based on 2.05 fb−1
of 7 TeV pp collision data recorded in 2011 with the AT-LAS detector [11]. The benchmark signal model consid-ered is based upon theoretical calculations from Ref. [2]. In this model, excited leptons may be produced singly via qq → ℓ∗
ℓ or in pairs via qq → ℓ∗ℓ∗, due to contact
interactions. As the cross section for pair production is much less than for single production, the search for ex-cited leptons is based on the search for events with ℓℓγ in the final state: three very energetic particles, isolated, and well separated from one another.
For both the e∗
and µ∗
searches, the dominant back-ground arises from Drell-Yan (DY) processes accompa-nied by either a prompt photon from initial or final state radiation (Z + γ) or by a jet misidentified as a photon (Z +jets). The dominant irreducible Z +γ background re-sults in the same final state as the signal, whereas Z +jets background can be highly suppressed by imposing strin-gent requirements on the quality of the reconstructed photon candidate. Small contributions from t¯t and di-boson (W W , W Z and ZZ) production are also present in both channels. W + jets events, as well as semileptonic decays of heavy flavor hadrons, and multijet events can be heavily suppressed by requiring the leptons and pho-tons to be isolated and thus have a negligible contribution
to the total background.
The signature for excited leptons can present itself as a peak in the invariant mass of the ℓ+γ system because the width of the ℓ∗
is predicted to be narrower than the detec-tor mass resolution for excited lepton masses mℓ∗ .0.5Λ. This peak could be easily resolved from the Z + γ back-ground. However, it is difficult to identify which of the two leading leptons in the event comes from the ℓ∗
de-cay. To avoid this ambiguity, one can search for an ex-cess in the ℓℓγ invariant mass (mℓℓγ) spectrum. This
ap-proach is effective for the whole mℓ∗−Λ parameter space probed as one can search for an excess of events with mℓℓγ > 350 GeV, which defines a nearly background-free
signal region. Optimization studies demonstrate that the observable mℓℓγ provides better signal sensitivity than
mℓγ, particularly for lower ℓ∗masses. The analysis
strat-egy therefore relies on mℓℓγ for the statistical
interpreta-tion of the results.
III. ATLAS DETECTOR
ATLAS is a multi-purpose detector with a forward-backward symmetric cylindrical geometry and nearly 4π coverage in solid angle. It consists of an inner track-ing detector immersed in a 2 T solenoidal field, electro-magnetic and hadronic calorimeters, and a muon spec-trometer. Charged particle tracks and vertices are re-constructed in silicon-based pixel and microstrip track-ing detectors that cover |η| < 2.5 and transition radia-tion detectors extending to |η| < 2.0 [12]. A hermetic calorimetry system, which covers |η| < 4.9, surrounds the superconducting solenoid. The liquid-argon electro-magnetic calorimeter, which plays an important role in electron and photon identification and measurement, is finely segmented. It has a readout granularity varying by layer and cells as small as 0.025 × 0.025 in η × φ, and extends to |η| < 2.5 to provide excellent energy and position resolution. Hadron calorimetry is provided by an iron-scintillator tile calorimeter in the central rapid-ity range |η| < 1.7 and a liquid-argon calorimeter with copper and tungsten as absorber material in the rapidity range 1.5 < |η| < 4.9. Outside the calorimeter, there is a muon spectrometer which is designed to identify muons and measure their momenta with high precision. The muon spectrometer comprises three toroidal air-core magnet systems: one for the barrel and one per end-cap, each composed of eight coils. Three layers of drift tube chambers and/or cathode strip chambers provide precision (η) coordinates for momentum measurement in the region |η| < 2.7. A muon trigger system consist-ing of resistive plate chambers in the barrel and thin-gap chambers for |η| > 1 provides triggering capability up to |η| = 2.4 and measurements of the φ coordinate.
IV. SIMULATED SAMPLES
The excited lepton signal samples are generated based on calculations from Ref. [2] at leading order (LO) with CompHEP 4.5.1 [13] interfaced with Pythia 6.421 to handle parton showers and hadronization [14, 15], us-ing MRST2007 LO* [16] parton distribution functions (PDFs). Only single production of excited leptons is simulated, with the ℓ∗ decaying exclusively via the
elec-tromagnetic channel. The Z + γ sample is generated with Sherpa 1.2.3 [17] using CTEQ6.6 [18] PDFs, re-quiring the dilepton mass to be above 40 GeV. To avoid phase-space regions where matrix elements diverge, the angular separation between the photon and leptons is required to be R(ℓ, γ) = p(∆η)2+ (∆φ)2 > 0.5 and
the transverse momentum (pT) of the photon is required
to be pγT > 10 GeV. To ensure adequate statistics at
large mℓℓγ, an additional Z + γ sample is generated with
pγT > 40 GeV, and is equivalent to ∼ 300 fb−1 of data.
The Z + jets background is generated with Alpgen 2.13 [19], while the t¯t background is produced with MC@NLO 3.41 [20]. In both cases, Jimmy 4.31 [21] is used to de-scribe multiple parton interactions and Herwig 6.510 [22] is used to simulate the remaining underlying event and parton showers and hadronization. CTEQ6.6 PDFs are used for both backgrounds. To remove overlaps be-tween the Z + jets and the Z + γ samples, Z + jets events with prompt energetic photons are rejected if the photon-lepton separation is such that R(ℓ, γ) > 0.5. The diboson processes are generated with Herwig using MRST2007 LO* PDFs. For all samples, final-state photon radia-tion is handled via photos [23]. The generated samples are then processed through a detailed detector simula-tion [24] based on Geant4 [25] to propagate the particles and account for the detector response. A large sample of MC minimum bias events is then mixed with the sig-nal and background MC events to simulate pile-up from additional pp collisions. Simulations are normalized on an event-by-event basis such that the distribution of the number of interactions per event agrees with the spec-trum observed in data.
Although Sherpa includes higher-order QCD contri-butions beyond the Z + γ Born amplitude, such as the real emission of partons in the initial state, it omits vir-tual corrections. For this reason, the Z + γ cross sec-tion is calculated at next-to-leading order (σNLO) using
MCFM [26] with MSTW 2008 NLO PDFs [27]. The the-oretical precision of the σNLO estimate is ∼ 6%, and the
ratio σNLO/σSHERPA is used to determine a correction
factor as a function of mℓℓγ. The Z + jets cross section
is initially normalized to predictions calculated at next-to-next-to-leading order (NNLO) in perturbative QCD as determined by the FEWZ [28] program using MSTW 2008 NNLO PDFs. Since the misidentification of jets as photons is not well modeled, the Z +jets prediction is ad-justed at the analysis level using data-driven techniques described below. Cross sections for diboson processes are known at NLO with an uncertainty of 5%, while the
t¯t cross section is predicted at approximate-NNLO, with better than 10% uncertainty [29, 30].
V. DATA AND PRESELECTION
The data, which correspond to a total integrated lumi-nosity of 2.05 fb−1, were collected in 2011 during stable
beam periods of 7 TeV pp collisions. For the e∗ search,
events are required to pass the lowest unprescaled sin-gle electron trigger available. For the first half of the data this corresponds to a pe
T threshold of 20 GeV, and
a pe
T threshold of 22 GeV for the later runs. For the
µ∗ search, a single muon trigger with matching tracks in
the muon spectrometer and inner detector with combined pµT> 22 GeV is used to select events. In addition, events
with a muon with pµT> 40 GeV in the muon
spectrome-ter are also kept. Collision candidates are then identified by requiring a primary vertex with a z position along the beam line of |z| < 200 mm and at least three associated charged particle tracks with pT> 0.4 GeV.
The lepton selection consists of the same requirements used in the ATLAS search for new heavy resonances de-caying to dileptons [31]. Electron candidates are formed from clusters of cells in the electromagnetic calorimeter associated with a charged particle track in the inner de-tector. For the e∗ search, two electron candidates with
pe
T > 25 GeV and |η| < 2.47 are required. Electrons
within the transition region 1.37 < |η| < 1.52 between the barrel and the endcap calorimeters are rejected. The mediumelectron identification criteria [32] on the trans-verse shower shape, the longitudinal leakage into the hadronic calorimeter, and the association with an inner detector track are applied to the cluster. The electron’s reconstructed energy is obtained from the calorimeter measurement and its direction from the associated track. A hit in the first active pixel layer is required to sup-press background from photon conversions. To further suppress background from jets, the leading electron is re-quired to be isolated by demanding that the sum of the transverse energies in the cells around the electron direc-tion in a cone of radius R < 0.2 be less than 7 GeV. The core of the electron energy deposition is excluded and the sum is corrected for transverse shower leakage and pile-up from additional pp collisions to make the isolation vari-able essentially independent of pe
T [33]. In cases where
more than two electrons are found to satisfy the above requirements, the pair with the largest invariant mass is chosen. To minimize the impact of possible charge misidentification, the electrons are not required to have opposite electric charges.
Muon tracks are reconstructed independently in both the inner detector and muon spectrometer, and their momenta are determined from a combined fit to these two measurements. For the µ∗
search, two muons with pµT > 25 GeV are required. To optimize the
momen-tum resolution, each muon candidate is required to have a minimum number of hits in the inner detector and to
have at least three hits in each of the inner, middle, and outer layers of the muon spectrometer. This requirement results in a muon fiducial acceptance of |η| < 2.5. Muons with hits in the barrel-endcap overlap regions of the muon spectrometer are discarded because of large residual mis-alignments. The effects of misalignments and intrinsic position resolution are otherwise included in the simu-lation. The pµT resolution at 1 TeV ranges from 13% to 20%. To suppress background from cosmic rays, the muon tracks are required to have transverse and longitu-dinal impact parameters |d0| < 0.2 mm and |z0| < 1 mm
with respect to the primary vertex. To reduce back-ground from heavy flavor hadrons, each muon is re-quired to be isolated such that ΣpT(R < 0.3)/pµT< 0.05,
where only inner detector tracks with pT> 1 GeV enter
the sum. Muons are required to have opposite electric charges. In cases where more than two muons are found to satisfy the above requirements, the pair of muons with the largest invariant mass is considered.
The dielectron and dimuon distributions are inspected for consistency with background predictions to ensure that the resolution and efficiency corrections were ad-justed properly in the simulation. Excellent agreement is found around the mass of the Z, in terms of both the peak position and width of the dilepton invariant mass distributions. For the mass range 70 < mℓℓ< 110 GeV,
the number of events observed in data agrees to within 1% of the background predictions for both the electron and muon channels. Furthermore, the tails of the pe
Tand
pµT distributions in the simulation are found to closely match the data.
The presence of at least one photon candidate with pγT> 20 GeV and pseudorapidity |η| < 2.37 is then neces-sary for the events to be kept. Photons within the transi-tion region between the barrel and the endcap calorime-ters are excluded. Photon candidates are formed from clusters of cells in the electromagnetic calorimeter. They include unconverted photons, with no associated track, and photons that converted to electron-positron pairs, as-sociated to one or two tracks. All photon candidates are required to satisfy the tight photon definition [34]. This selection includes constraints on the energy leakage into the hadronic calorimeter as well as stringent requirements on the energy distribution in the first sampling layer of the electromagnetic calorimeter, and on the shower width in the second sampling layer. The tight photon definition is designed to increase the purity of the photon selection sample by rejecting most of the jet background, including jets with a leading neutral hadron (usually a π0) that de-cays to a pair of collimated photons. To further reduce background from misidentified jets, photon candidates are required to be isolated by demanding that the sum of the transverse energies of the cells within a cone R < 0.4 of the photon be less than 10 GeV. As for the electron isolation, the core of the photon energy deposition is ex-cluded and the sum is corrected for transverse shower leakage and pile-up. Because no background predictions are simulated for R(ℓ, γ) < 0.5, photons are required to
be well separated from the leptons with R(ℓ, γ) > 0.7. This requirement has a negligible impact on signal ef-ficiency. Finally, if more than one photon in an event satisfies the above requirements, the one with the largest pTis used in the search.
For the above selection criteria, the total signal ac-ceptance times efficiency (A × ǫ) is ∼ 56% in the e∗
channel for masses me∗ > 600 GeV. This value includes the acceptance of all selection cuts and the reconstruc-tion efficiencies, and reflects the lepton and photon an-gular distributions. In comparison, A × ǫ is ∼ 32% for mµ∗ > 600 GeV. The lower acceptance in the µ
∗
channel is due to the stringent selection on the muon spectrome-ter hits used to maximize the pµTresolution, in particular the limited geometrical coverage of the muon spectrom-eter with three layers of precision chambers.
VI. BACKGROUND DETERMINATION
All background predictions are evaluated with simu-lated samples. These include the dominant and irre-ducible Z + γ background, as well as Z + jets events where a jet is misidentified as a photon. The rate of jet misidentification is overestimated in the simulation so the Z + jets predictions are adjusted to data as described be-low. Small contributions from t¯t and diboson production are also present at low mℓℓγ. Background from multijet
events and semileptonic decays of heavy flavor hadrons are heavily suppressed by the isolation requirements and are negligible in the signal region.
The Z + jets estimates are adjusted to data in a con-trol region defined by mℓℓγ < 300 GeV. This region
rep-resents less than 1% of the signal parameter-space for mℓ∗ ≥ 200 GeV. The nominal strategy consists of count-ing the number of events in data in this control region and comparing it to the MC background predictions. The excess of background events found in the simulation is at-tributed to the mis-modeling of the rate of jets misidenti-fied as photons, and the number of Z+jets events is scaled down accordingly. As a result, the number of events in the control region is the same in the MC simulations as in data as shown in Table I. The Z + jets estimates are validated using various data-driven methods, notably by using misidentification rates evaluated in jet-enriched samples, and applying these rates to Z + jets data sam-ples using an approach similar to the one described in Ref. [34]. The main reason for the overestimation of the jet misidentification rate in the simulation is due to the mis-modeling of the jet shower shapes. A Z + jets en-riched sample was used to correct the shower shapes of jets in the simulations, such that the efficiency for jets to pass the tight photon requirement in the MC simula-tion is comparable to the rate measured in data. This correction depends strongly on the generator used (e.g. Pythiavs Alpgen) and results in a 15% uncertainty in the Z + jets background estimate.
The largest difference between the nominal Z + jets
background determination and the alternative estimates is assigned as a systematic uncertainty and dominates the total error in the Z + jets estimates presented in Ta-ble I. The corresponding scaling factors applied to the Z + jets simulation are 0.51 ± 0.14 and 0.61 ± 0.21 for the e∗
and µ∗
channels, respectively, i.e. within uncertainties of one another. Furthermore, the ratio of the number of Z + jets events outside the control region to the number of events inside is found to be the same in the MC sim-ulations as in the data-driven techniques: 0.06 for both the e∗
and µ∗
channels. This finding indicates that the jet misidentification rate as a function of the jet pT is
modeled properly.
TABLE I: Data yields and background expectations inside (mℓℓγ < 300 GeV) and outside the mℓℓγ control region after adjusting the Z + jets background. The uncertainties shown are purely statistical, except for the Z + jets background for which the total uncertainty is dominated by systematic un-certainties.
Region [GeV] Z + γ Z + jets diboson t¯t data meeγ< 300 306 ± 8 138 ± 38 8.3 ± 0.8 2.4 ± 0.5 455 meeγ> 300 25 ± 2 8.1 ± 1.6 0.8 ± 0.2 0.5 ± 0.2 29 mµµγ< 300 255 ± 8 89 ± 31 4.9 ± 0.6 0.9 ± 0.3 350 mµµγ> 300 14 ± 1 5.4 ± 1.4 0.9 ± 0.3 0.1 ± 0.1 19
Comparisons between data and the resulting back-ground expectations for the pℓ
T, p γ
T, mℓγ and mℓℓγ
distri-butions are shown in Figs. 1 to 4. No significant discrep-ancies are observed between data and the simulations. In particular, the background prediction for the photon pT
shape matches the data for both the e∗ and µ∗ searches,
which suggests that the tuning of the jet misidentification rate for the Z + jets background is adequate.
VII. SIGNAL REGION OPTIMIZATION
The signal search region is optimized as a function of mℓ∗ using simulated events by determining the lower bound on mℓℓγ that maximizes the significance defined
as
SL=
r
2 lnh(1 + S/B)S+Be−Si,
where S and B are the number of signal and background events, respectively. The optimum threshold value is found to be mℓℓγ = mℓ∗+ 150 GeV. Additionally, to im-prove the sensitivity particularly at low mℓ∗, background contributions from DY processes are suppressed further by requiring events to satisfy mℓℓ> 110 GeV. The signal
efficiency for these two additional requirements is > 99% for mℓ∗ ≥ 200 GeV.
Because few events survive the complete set of re-quirements, the shape of the Z + γ and Z + jets back-grounds are individually fitted using an exponential func-tion exp(P0+ P1× mℓℓγ) over the mass range 250 GeV <
[GeV] T electron p 50 100 150 200 250 Events / 20 GeV -1 10 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[GeV] T muon p 50 100 150 200 250 Events / 20 GeV -1 10 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 1: Lepton pTdistributions for the e∗(top) and µ∗ (bot-tom) channels. The expected background uncertainties shown correspond to the sum in quadrature of the statistical uncer-tainties as well as the uncertainty in the Z +jets normalization measured in the control region.
mℓℓγ < 950 GeV. The sum of these two fits is then used
to obtain the total background prediction for mℓℓγ >
350 GeV. The resulting background estimates and data yields are shown in Table II for the e∗ and µ∗ searches,
as well as in Figs. 5 and 6.
VIII. SYSTEMATIC UNCERTAINTIES
In this section, the dominant systematic uncertainties in the Z + γ and Z + jets background predictions are first described, followed by a description of the experimental systematic uncertainties that affect both the background and signal yields, and by a discussion of the theoretical uncertainties which affect both the e∗
and µ∗
.
The dominant systematic uncertainty in the irre-ducible Z + γ background comes from the fit of its back-ground shape and normalization due to the limited num-ber of events with mℓℓ> 110 GeV. This uncertainty
[GeV] T photon p 20 40 60 80 100 120 140 160 180 200 220 Events / 15 GeV -1 10 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[GeV] T photon p 20 40 60 80 100 120 140 160 180 200 220 Events / 15 GeV -1 10 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 2: Photon pTdistributions for the e∗(top) and µ∗ (bot-tom) channels. The expected background uncertainties shown correspond to the sum in quadrature of the statistical uncer-tainties as well as the uncertainty in the Z +jets normalization measured in the control region.
creases with mℓ∗ from about 20% at 200 GeV to 100% for mℓ∗ > 800 GeV. The second largest uncertainty in the Z + γ background is of theoretical nature and arises from the NLO computations. This uncertainty is obtained by varying the renormalization and factorization scales by factors of two around their nominal values and combin-ing with uncertainties ariscombin-ing from the PDFs and values of the strong coupling constant αs. For mℓ∗ = 200 GeV (mℓ∗ > 800 GeV), the resulting theoretical uncertainty in the number of Z + γ background events in our signal region is 7% (10%) for both channels.
The uncertainty in the Z + jets normalization is deter-mined to be 38% (35%) for the e∗ (µ∗) channel, which
covers the range of values obtained by the different es-timates as well as their uncertainties in the mℓℓγ <
300 GeV control region. Uncertainties in the Z + jets prediction from the shape of the fitted distribution are added in quadrature to the normalization uncertainty.
[GeV] γ e m 50 100 150 200 250 300 350 400 pairs / 25 GeV γ e 1 10 2 10 3 10 Data 2011γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[GeV] γ µ m 50 100 150 200 250 300 350 400 pairs / 25 GeV γµ 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 3: Distributions of the invariant mass of the ℓγ systems for the e∗ (top) and µ∗ (bottom) channels. Combinations with both the leading and subleading leptons are shown. The expected background uncertainties shown correspond to the sum in quadrature of the statistical uncertainties as well as the uncertainty in the Z + jets normalization measured in the control region. For both channels, one event lies outside the mass range shown.
signal and background yields include the uncertainty from the luminosity measurement of 3.7% [35], and un-certainties in particle reconstruction and identification as described below.
A 3% systematic uncertainty is assigned to the pho-ton efficiency. This value is obtained by comparing the signal efficiency with and without photon shower shape corrections (2%), by studying the impact of material mis-modeling in the inner detector (1%) and the by deter-mining the reconstruction efficiency for various pile-up conditions (1%) [36].
The electron trigger and reconstruction efficiency is evaluated in data and in MC simulations in several η × φ bins to high precision. Correction factors are applied to the simulations accordingly and have negligible uncer-tainties. A 1% systematic uncertainty in the electron
[GeV] γ ee m 100 150 200 250 300 350 400 450 500 Events / 40 GeV 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[GeV] γ µ µ m 100 150 200 250 300 350 400 450 500 Events / 40 GeV 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 4: Distributions of the invariant mass for the ℓℓγ sys-tem for the e∗(top) and µ∗(bottom) channels. The expected background uncertainties shown correspond to the sum in quadrature of the statistical uncertainties as well as the uncer-tainty in the Z + jets normalization measured in the control region. For both channels, one event lies outside the mass range shown.
efficiency at high pT is assigned. This uncertainty is
es-timated by studying the electron efficiency as a function of the calorimeter isolation criteria.
The calorimeter energy resolution is dominated at high pT by a constant term which is 1.1% in the barrel and
1.8% in the endcaps. The simulation is adjusted to re-produce this resolution at high energy, and the uncer-tainty in this correction has a negligible effect on pe
Tand
pγT. The calorimeter energy scale is corrected by study-ing J/ψ → ee and Z → ee events. Calibration constants are obtained for several η regions and deviate at most by 1.5% of unity, and have small uncertainties. Thus, un-certainties on the calorimeter energy scale and resolution result in negligible uncertainties in the background and signal yields.
The combined uncertainty in yields arising from the trigger and reconstruction efficiency for muons is
[GeV] γ e m 200 300 400 500 600 700 800 900 pairs / 50 GeV γ e -3 10 -2 10 -1 10 1 10 2 10 3 10 4 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ) = (0.5, 7.5) TeV Λ , e* (m ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[GeV] γ µ m 200 300 400 500 600 700 800 900 pairs / 50 GeV γµ -3 10 -2 10 -1 10 1 10 2 10 3 10 4 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ) = (0.5, 7.5) TeV Λ , * µ (m ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 5: Distributions of the invariant mass of the ℓγ sys-tems for the e∗(top) and µ∗(bottom) channels after requiring mℓℓ> 110 GeV. Combinations with both the leading and sub-leading leptons are shown. The expected background uncer-tainties shown correspond to the sum in quadrature of the sta-tistical uncertainties as well as the uncertainty in the Z + jets normalization measured in the control region. Note that the last bin contains the sum of all entries with mℓγ> 950 GeV.
mated to increase linearly as a function of pµT to about 1.5% at 1 TeV. This uncertainty is dominated by a con-servative estimate of the impact of large energy loss from muon bremsstrahlung in the calorimeter which can affect reconstruction in the muon spectrometer. The uncer-tainty from the resolution due to residual misalignments in the muon spectrometer propagates to a change in the number of events passing the mµµγ cut, and affects the
sensitivity of the search. The muon momentum scale is calibrated with a statistical precision of 0.1% using the Z → µµ mass peak. Thus, uncertainties on the muon momentum scale and resolution result in negligible un-certainties in the background and signal yields.
An additional 1% systematic uncertainty is assigned to the e∗
and µ∗
signal efficiencies to account for the fact that the dependence on Λ is neglected in this
[GeV] γ ee m 200 400 600 800 1000 1200 1400 Events / 100 GeV -2 10 -1 10 1 10 2 10 3 10 Data 2011 γ Z + Z + jets Bkg. uncertainty ) = (0.5, 7.5) TeV Λ , e* (m ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[GeV] γ µ µ m 200 400 600 800 1000 1200 1400 Events / 100 GeV -2 10 -1 10 1 10 2 10 3 10 Data 2011Z + γ Z + jets Bkg. uncertainty ) = (0.5, 7.5) TeV Λ , * µ (m ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 6: Distributions of the invariant mass for the ℓℓγ sys-tem for the e∗(top) and µ∗(bottom) searches after requiring mℓℓ > 110 GeV. The Z + jets and Z + γ backgrounds were fitted, and the total uncertainties from the fit as well as the uncertainty in the Z +jets normalization measured in the con-trol region are displayed as the shaded area. Note that the last bin contains the sum of all events with mℓℓγ> 1450 GeV.
ysis. This uncertainty is obtained by studying the signal A × ǫ for various excited lepton masses and composite-ness scales. Theoretical uncertainties from renormaliza-tion and factorizarenormaliza-tion scales and PDFs have negligible impact on the signal efficiency and are not included in the results presented below.
IX. RESULTS
A summary of the data yields and background expec-tations as a function of a lower bound on mℓℓγ is shown
in Table II for the e∗ and µ∗ searches. The
uncertain-ties displayed correspond to the sum in quadrature of the statistical and systematic uncertainties. The significance of a potential excited lepton signal is estimated by a p-value, the probability of observing an outcome at least
as signal-like as the one observed in data, assuming that a signal is absent. The lowest p-values obtained are 3% in the e∗ channel (for m
eeγ > 950 GeV), and 17% in the
µ∗ channel (for m
µµγ > 850 GeV), which indicates that
the data are consistent with the background hypothesis. Given the absence of a signal, an upper limit on the ℓ∗
cross section times branching ratio σB is determined at the 95% C.L. using a Bayesian approach [37] with a flat, positive prior on σB. Systematic uncertainties are incor-porated in the limit calculation as nuisance parameters. The limits are translated into bounds on the composite-ness scale as a function of the mass of the excited leptons by comparing them with theoretical predictions of σB for various values of Λ.
The expected exclusion limits are determined using simulated pseudo-experiments (PE) containing only SM processes, by evaluating the 95% C.L. upper limits for each PE for each fixed value of mℓ∗. The median of the distribution of limits represents the expected limit. The ensemble of limits is used to find the 1σ and 2σ envelopes of the expected limits as a function of mℓ∗.
Figure 7 shows the 95% C.L. expected and observed limits on σB(ℓ∗
→ ℓγ) for the e∗
and µ∗
searches. For mℓ∗ > 0.9 TeV, the observed and expected limits on σB are 2.3 fb and 4.5 fb for the e∗and µ∗, respectively. The
green and yellow bands show the expected 1σ and 2σ con-tours of the expected limits. When the expected number of background events is zero, there is an effective quan-tization of the expected limits obtained from the PE, and no downward fluctuation of the background is pos-sible. These effects explain the behavior of the 1σ and 2σ contours of the expected limits for large ℓ∗
masses. Theoretical predictions of σB for three different values of Λ are also displayed in Fig. 7, as well as the theoreti-cal uncertainties from renormalization and factorization scales and PDFs for Λ = 2 TeV. These uncertainties are shown for illustrative purposes only and are not included in determining mass limits. The mass limits obtained for various Λ values are used to produce exclusion limits on the mℓ∗− Λ plane as shown in Fig. 8. In the special case where Λ = mℓ∗, masses below 1.87 TeV and 1.75 TeV are excluded for excited electrons and muons, respectively.
TABLE II: Data yields and background expectation as a function of a lower bound on mℓℓγ= mℓ∗+ 150 GeV. The uncertainties represent the sum in quadrature of the statistical and systematic uncertainties. The probability for the background only hypothesis (p-value) is also provided.
mℓℓγ region e∗search µ∗search
[TeV] Z + γ total bkg data p-value Z + γ total bkg data p-value
> 0.35 10.1 ± 1.9 11.5 ± 2.2 8 0.92 5.2 ± 1.4 6.0 ± 1.6 6 0.40 > 0.45 4.6 ± 1.0 5.1 ± 1.2 2 0.83 3.1 ± 0.8 3.4 ± 0.9 3 0.42 > 0.55 2.1 ± 0.7 2.3 ± 0.8 1 0.80 1.8 ± 0.6 2.0 ± 0.7 1 0.72 > 0.65 0.98 ± 0.47 1.02 ± 0.49 1 0.32 1.09 ± 0.49 1.14 ± 0.51 1 0.72 > 0.75 0.45 ± 0.29 0.46 ± 0.30 1 0.16 0.65 ± 0.39 0.67 ± 0.39 1 0.28 > 0.85 0.20 ± 0.16 0.21 ± 0.17 1 0.11 0.39 ± 0.29 0.39 ± 0.29 1 0.17 > 0.95 0.09 ± 0.09 0.10 ± 0.09 1 0.03 0.23 ± 0.21 0.23 ± 0.21 0 0.78 > 1.05 0.05 ± 0.05 0.05 ± 0.05 0 0.81 0.14 ± 0.14 0.14 ± 0.14 0 0.92 X. CONCLUSIONS
The results of a search for excited electrons and muons with the ATLAS detector are reported, using a sample of √
s = 7 TeV pp collisions corresponding to an integrated luminosity of 2.05 fb−1. The observed invariant mass
spectra are consistent with SM background expectations. Limits are set on the cross section times branching ratio σB(ℓ∗
→ ℓγ) at 95% C.L. For mℓ∗ > 0.9 TeV, the ob-served upper limits on σB are 2.3 fb and 4.5 fb for the e∗
and µ∗
channels, respectively. The limits are translated into bounds on the compositeness scale Λ as a function of the mass of the excited leptons. In the special case where Λ = mℓ∗, masses below 1.87 TeV and 1.75 TeV are excluded for e∗
and µ∗
, respectively. These limits are the most stringent bounds to date on excited leptons for the parameter-space region with mℓ∗ ≥ 200 GeV.
XI. ACKNOWLEDGEMENTS
We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently. We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CON-ICYT, Chile; CAS, MOST and NSFC, China; COL-CIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; ARTEMIS, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNAS, Geor-gia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS,
[TeV] e* m 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) [fb] γ e → B(e* σ 10 2 10 Observed limit Expected limit σ 1 ± Expected σ 2 ± Expected = 2.0 TeV Λ = 5.0 TeV Λ = 8.0 TeV Λ ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[TeV] * µ m 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 ) [fb] γµ → * µ B( σ 10 2 10 Observed limit Expected limit σ 1 ± Expected σ 2 ± Expected = 2.0 TeV Λ = 5.0 TeV Λ = 8.0 TeV Λ ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 7: Cross section × branching ratio limits at 95% C.L. as a function of e∗ and of µ∗ mass. Theoretical predictions for excited leptons produced for three different compositeness scales are shown, as well as the theoretical uncertainties from renormalization and factorization scales and PDFs for Λ = 2 TeV. For mℓ∗ > 0.9 TeV, the observed limit on σB is 2.3 fb (4.5 fb) for e∗(µ∗). [TeV] e* m 0.5 1 1.5 2 2.5 3 [ T e V ] Λ 1 2 3 4 5 6 7 8 9 10 11 Observed limit Expected limit σ 1 ± Expected Λ > e* m -1 CMS 36 pb -1 D0 1 fb ATLAS = 7 TeV s -1 L dt = 2.05 fb
∫
[TeV] * µ m 0.5 1 1.5 2 2.5 3 [ T e V ] Λ 1 2 3 4 5 6 7 8 9 10 11 Observed limit Expected limit σ 1 ± Expected Λ > * µ m -1 CMS 36 pb -1 CDF 370 pb ATLAS = 7 TeV s -1 L dt = 2.05 fb∫
FIG. 8: Exclusion limits in the mℓ∗−Λ parameter space for e∗ and µ∗. Regions to the left of the experimental limits are excluded at 95% C.L. No limits are set for the hashed region as the approximations made in the effective contact interaction model do not hold for mℓ∗ > Λ. The best limits from the Tevatron experiments as well as from the CMS experiment based on 36 pb−1are also shown.
Japan; CNRST, Morocco; FOM and NWO, Netherlands; RCN, Norway; MNiSW, Poland; GRICES and FCT, Portugal; MERYS (MECTS), Romania; MES of Rus-sia and ROSATOM, RusRus-sian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Can-tons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Lever-hulme Trust, United Kingdom; DOE and NSF, United
States of America.
The crucial computing support from all WLCG part-ners is acknowledged gratefully, in particular from CERN and the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Tai-wan), RAL (UK) and BNL (USA) and in the Tier-2 fa-cilities worldwide.
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L. Courneyea168, G. Cowan75, C. Cowden27, B.E. Cox81, K. Cranmer107, F. Crescioli121a,121b, M. Cristinziani20,
G. Crosetti36a,36b, R. Crupi71a,71b, S. Cr´ep´e-Renaudin55, C.-M. Cuciuc25a, C. Cuenca Almenar174,
T. Cuhadar Donszelmann138, M. Curatolo47, C.J. Curtis17, C. Cuthbert149, P. Cwetanski60, H. Czirr140, P. Czodrowski43, Z. Czyczula174, S. D’Auria53, M. D’Onofrio72, A. D’Orazio131a,131b, P.V.M. Da Silva23a,
C. Da Via81, W. Dabrowski37, T. Dai86, C. Dallapiccola83, M. Dam35, M. Dameri50a,50b, D.S. Damiani136,
H.O. Danielsson29, D. Dannheim98, V. Dao49, G. Darbo50a, G.L. Darlea25b, W. Davey20, T. Davidek125,
N. Davidson85, R. Davidson70, E. Davies117,c, M. Davies92, A.R. Davison76, Y. Davygora58a, E. Dawe141,
I. Dawson138, J.W. Dawson5,∗, R.K. Daya-Ishmukhametova22, K. De7, R. de Asmundis101a, S. De Castro19a,19b,
P.E. De Castro Faria Salgado24, S. De Cecco77, J. de Graat97, N. De Groot103, P. de Jong104, C. De La Taille114,
H. De la Torre79, B. De Lotto163a,163c, L. de Mora70, L. De Nooij104, D. De Pedis131a, A. De Salvo131a,
U. De Sanctis163a,163c, A. De Santo148, J.B. De Vivie De Regie114, S. Dean76, W.J. Dearnaley70, R. Debbe24,
C. Debenedetti45, D.V. Dedovich64, J. Degenhardt119, M. Dehchar117, C. Del Papa163a,163c, J. Del Peso79,
T. Del Prete121a,121b, T. Delemontex55, M. Deliyergiyev73, A. Dell’Acqua29, L. Dell’Asta21, M. Della Pietra101a,i,
D. della Volpe101a,101b, M. Delmastro4, N. Delruelle29, P.A. Delsart55, C. Deluca147, S. Demers174, M. Demichev64, B. Demirkoz11,k, J. Deng162, S.P. Denisov127, D. Derendarz38, J.E. Derkaoui134d, F. Derue77, P. Dervan72,
K. Desch20, E. Devetak147, P.O. Deviveiros104, A. Dewhurst128, B. DeWilde147, S. Dhaliwal157, R. Dhullipudi24 ,l,
A. Di Ciaccio132a,132b, L. Di Ciaccio4, A. Di Girolamo29, B. Di Girolamo29, S. Di Luise133a,133b, A. Di Mattia171,
B. Di Micco29, R. Di Nardo47, A. Di Simone132a,132b, R. Di Sipio19a,19b, M.A. Diaz31a, F. Diblen18c, E.B. Diehl86,
J. Dietrich41, T.A. Dietzsch58a, S. Diglio85, K. Dindar Yagci39, J. Dingfelder20, C. Dionisi131a,131b, P. Dita25a,
S. Dita25a, F. Dittus29, F. Djama82, T. Djobava51b, M.A.B. do Vale23c, A. Do Valle Wemans123a, T.K.O. Doan4,
M. Dobbs84, R. Dobinson29,∗, D. Dobos29, E. Dobson29,m, J. Dodd34, C. Doglioni49, T. Doherty53, Y. Doi65,∗,
J. Dolejsi125, I. Dolenc73, Z. Dolezal125, B.A. Dolgoshein95,∗, T. Dohmae154, M. Donadelli23d, M. Donega119,
J. Donini33, J. Dopke29, A. Doria101a, A. Dos Anjos171, M. Dosil11, A. Dotti121a,121b, M.T. Dova69, J.D. Dowell17,
A.D. Doxiadis104, A.T. Doyle53, Z. Drasal125, J. Drees173, N. Dressnandt119, H. Drevermann29, C. Driouichi35,
M. Dris9, J. Dubbert98, S. Dube14, E. Duchovni170, G. Duckeck97, A. Dudarev29, F. Dudziak63, M. D¨uhrssen29, I.P. Duerdoth81, L. Duflot114, M-A. Dufour84, M. Dunford29, H. Duran Yildiz3a, R. Duxfield138, M. Dwuznik37,
F. Dydak29, M. D¨uren52, W.L. Ebenstein44, J. Ebke97, S. Eckweiler80, K. Edmonds80, C.A. Edwards75,
N.C. Edwards53, W. Ehrenfeld41, T. Ehrich98, T. Eifert142, G. Eigen13, K. Einsweiler14, E. Eisenhandler74,
T. Ekelof165, M. El Kacimi134c, M. Ellert165, S. Elles4, F. Ellinghaus80, K. Ellis74, N. Ellis29, J. Elmsheuser97,
M. Elsing29, D. Emeliyanov128, R. Engelmann147, A. Engl97, B. Epp61, A. Eppig86, J. Erdmann54, A. Ereditato16,
D. Eriksson145a, J. Ernst1, M. Ernst24, J. Ernwein135, D. Errede164, S. Errede164, E. Ertel80, M. Escalier114,
C. Escobar122, X. Espinal Curull11, B. Esposito47, F. Etienne82, A.I. Etienvre135, E. Etzion152, D. Evangelakou54,
H. Evans60, L. Fabbri19a,19b, C. Fabre29, R.M. Fakhrutdinov127, S. Falciano131a, Y. Fang171, M. Fanti88a,88b,
A. Farbin7, A. Farilla133a, J. Farley147, T. Farooque157, S.M. Farrington117, P. Farthouat29, P. Fassnacht29,
P. Federic143a, O.L. Fedin120, W. Fedorko87, M. Fehling-Kaschek48, L. Feligioni82, D. Fellmann5, C. Feng32d,
E.J. Feng30, A.B. Fenyuk127, J. Ferencei143b, J. Ferland92, W. Fernando108, S. Ferrag53, J. Ferrando53, V. Ferrara41,
A. Ferrari165, P. Ferrari104, R. Ferrari118a, D.E. Ferreira de Lima53, A. Ferrer166, M.L. Ferrer47, D. Ferrere49,
C. Ferretti86, A. Ferretto Parodi50a,50b, M. Fiascaris30, F. Fiedler80, A. Filipˇciˇc73, A. Filippas9, F. Filthaut103,
M. Fincke-Keeler168, M.C.N. Fiolhais123a,h, L. Fiorini166, A. Firan39, G. Fischer41, P. Fischer20, M.J. Fisher108,
M. Flechl48, I. Fleck140, J. Fleckner80, P. Fleischmann172, S. Fleischmann173, T. Flick173, A. Floderus78,
L.R. Flores Castillo171, M.J. Flowerdew98, M. Fokitis9, T. Fonseca Martin16, D.A. Forbush137, A. Formica135,
A. Forti81, D. Fortin158a, J.M. Foster81, D. Fournier114, A. Foussat29, A.J. Fowler44, K. Fowler136, H. Fox70,
P. Francavilla11, S. Franchino118a,118b, D. Francis29, T. Frank170, M. Franklin57, S. Franz29, M. Fraternali118a,118b,
S. Fratina119, S.T. French27, F. Friedrich43, R. Froeschl29, D. Froidevaux29, J.A. Frost27, C. Fukunaga155,
E. Fullana Torregrosa29, J. Fuster166, C. Gabaldon29, O. Gabizon170, T. Gadfort24, S. Gadomski49,
G. Gagliardi50a,50b, P. Gagnon60, C. Galea97, E.J. Gallas117, V. Gallo16, B.J. Gallop128, P. Gallus124, K.K. Gan108,
Y.S. Gao142,e, V.A. Gapienko127, A. Gaponenko14, F. Garberson174, M. Garcia-Sciveres14, C. Garc´ıa166, J.E. Garc´ıa
Navarro166, R.W. Gardner30, N. Garelli29, H. Garitaonandia104, V. Garonne29, J. Garvey17, C. Gatti47,
G. Gaudio118a, B. Gaur140, L. Gauthier135, I.L. Gavrilenko93, C. Gay167, G. Gaycken20, J-C. Gayde29, E.N. Gazis9, P. Ge32d, C.N.P. Gee128, D.A.A. Geerts104, Ch. Geich-Gimbel20, K. Gellerstedt145a,145b, C. Gemme50a,
A. Gemmell53, M.H. Genest55, S. Gentile131a,131b, M. George54, S. George75, P. Gerlach173, A. Gershon152,
C. Geweniger58a, H. Ghazlane134b, N. Ghodbane33, B. Giacobbe19a, S. Giagu131a,131b, V. Giakoumopoulou8,
V. Giangiobbe11, F. Gianotti29, B. Gibbard24, A. Gibson157, S.M. Gibson29, L.M. Gilbert117, V. Gilewsky90,
D. Gillberg28, A.R. Gillman128, D.M. Gingrich2,d, J. Ginzburg152, N. Giokaris8, M.P. Giordani163c,
R. Giordano101a,101b, F.M. Giorgi15, P. Giovannini98, P.F. Giraud135, D. Giugni88a, M. Giunta92, P. Giusti19a,
B.K. Gjelsten116, L.K. Gladilin96, C. Glasman79, J. Glatzer48, A. Glazov41, K.W. Glitza173, G.L. Glonti64,
J.R. Goddard74, J. Godfrey141, J. Godlewski29, M. Goebel41, T. G¨opfert43, C. Goeringer80, C. G¨ossling42,
T. G¨ottfert98, S. Goldfarb86, T. Golling174, A. Gomes123a,b, L.S. Gomez Fajardo41, R. Gon¸calo75,
J. Goncalves Pinto Firmino Da Costa41, L. Gonella20, A. Gonidec29, S. Gonzalez171, S. Gonz´alez de la Hoz166,
G. Gonzalez Parra11, M.L. Gonzalez Silva26, S. Gonzalez-Sevilla49, J.J. Goodson147, L. Goossens29,
P.A. Gorbounov94, H.A. Gordon24, I. Gorelov102, G. Gorfine173, B. Gorini29, E. Gorini71a,71b, A. Goriˇsek73,
E. Gornicki38, S.A. Gorokhov127, V.N. Goryachev127, B. Gosdzik41, M. Gosselink104, M.I. Gostkin64,
I. Gough Eschrich162, M. Gouighri134a, D. Goujdami134c, M.P. Goulette49, A.G. Goussiou137, C. Goy4,
S. Gozpinar22, I. Grabowska-Bold37, P. Grafstr¨om29, K-J. Grahn41, F. Grancagnolo71a, S. Grancagnolo15,
V. Grassi147, V. Gratchev120, N. Grau34, H.M. Gray29, J.A. Gray147, E. Graziani133a, O.G. Grebenyuk120,
T. Greenshaw72, Z.D. Greenwood24,l, K. Gregersen35, I.M. Gregor41, P. Grenier142, J. Griffiths137,
N. Grigalashvili64, A.A. Grillo136, S. Grinstein11, Y.V. Grishkevich96, J.-F. Grivaz114, M. Groh98, E. Gross170,
J. Grosse-Knetter54, J. Groth-Jensen170, K. Grybel140, V.J. Guarino5, D. Guest174, C. Guicheney33,
A. Guida71a,71b, S. Guindon54, H. Guler84,n, J. Gunther124, B. Guo157, J. Guo34, A. Gupta30, Y. Gusakov64,
V.N. Gushchin127, P. Gutierrez110, N. Guttman152, O. Gutzwiller171, C. Guyot135, C. Gwenlan117, C.B. Gwilliam72,
A. Haas142, S. Haas29, C. Haber14, H.K. Hadavand39, D.R. Hadley17, P. Haefner98, F. Hahn29, S. Haider29, Z. Hajduk38, H. Hakobyan175, D. Hall117, J. Haller54, K. Hamacher173, P. Hamal112, M. Hamer54,
A. Hamilton144b,o, S. Hamilton160, H. Han32a, L. Han32b, K. Hanagaki115, K. Hanawa159, M. Hance14, C. Handel80,
P. Hanke58a, J.R. Hansen35, J.B. Hansen35, J.D. Hansen35, P.H. Hansen35, P. Hansson142, K. Hara159, G.A. Hare136,
T. Harenberg173, S. Harkusha89, D. Harper86, R.D. Harrington45, O.M. Harris137, K. Harrison17, J. Hartert48,
F. Hartjes104, T. Haruyama65, A. Harvey56, S. Hasegawa100, Y. Hasegawa139, S. Hassani135, M. Hatch29, D. Hauff98,
S. Haug16, M. Hauschild29, R. Hauser87, M. Havranek20, B.M. Hawes117, C.M. Hawkes17, R.J. Hawkings29,
A.D. Hawkins78, D. Hawkins162, T. Hayakawa66, T. Hayashi159, D. Hayden75, H.S. Hayward72, S.J. Haywood128,
E. Hazen21, M. He32d, S.J. Head17, V. Hedberg78, L. Heelan7, S. Heim87, B. Heinemann14, S. Heisterkamp35,
L. Helary4, C. Heller97, M. Heller29, S. Hellman145a,145b, D. Hellmich20, C. Helsens11, R.C.W. Henderson70,
M. Henke58a, A. Henrichs54, A.M. Henriques Correia29, S. Henrot-Versille114, F. Henry-Couannier82, C. Hensel54,
T. Henß173, C.M. Hernandez7, Y. Hern´andez Jim´enez166, R. Herrberg15, A.D. Hershenhorn151, G. Herten48, R. Hertenberger97, L. Hervas29, G.G. Hesketh76, N.P. Hessey104, E. Hig´on-Rodriguez166, D. Hill5,∗, J.C. Hill27,
N. Hill5, K.H. Hiller41, S. Hillert20, S.J. Hillier17, I. Hinchliffe14, E. Hines119, M. Hirose115, F. Hirsch42,
D. Hirschbuehl173, J. Hobbs147, N. Hod152, M.C. Hodgkinson138, P. Hodgson138, A. Hoecker29, M.R. Hoeferkamp102,
J. Hoffman39, D. Hoffmann82, M. Hohlfeld80, M. Holder140, S.O. Holmgren145a, T. Holy126, J.L. Holzbauer87,
Y. Homma66, T.M. Hong119, L. Hooft van Huysduynen107, T. Horazdovsky126, C. Horn142, S. Horner48,
J-Y. Hostachy55, S. Hou150, M.A. Houlden72, A. Hoummada134a, J. Howarth81, D.F. Howell117, I. Hristova15,
J. Hrivnac114, I. Hruska124, T. Hryn’ova4, P.J. Hsu80, S.-C. Hsu14, G.S. Huang110, Z. Hubacek126, F. Hubaut82,
F. Huegging20, A. Huettmann41, T.B. Huffman117, E.W. Hughes34, G. Hughes70, R.E. Hughes-Jones81,
M. Huhtinen29, P. Hurst57, M. Hurwitz14, U. Husemann41, N. Huseynov64,p, J. Huston87, J. Huth57, G. Iacobucci49,
