ARTICLES NATURE PHYSICS DOI: 10.1038/NPHYS4208 ET (GeV) 02468 1 0 1 2 1 4 1 6 1 8 Photon PID efficiency 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 ATLAS Data, 480 μb−1 MC Data, 480 μb−1 MC Ee T − p T trk2 (GeV) 02 468 1 0 1 2 1 4 1 6 1 8 Photon reconstruction efficiency 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 ATLAS Pb + Pb √sNN = 5.02 TeV Pb + Pb √sNN = 5.02 TeV ba Figure 2 | Photon identification and reconstruction efficiencies. a, Photon PID efficiency as a function of photon ET extracted from FSR event candidates. b, Photon reconstruction efficiency as a function of photon ET (approximated with Ee T� ptrk2 T ) extracted from
! eCe� events with a hard-bremsstrahlung photon. Data (filled markers) are compared with MC simulations (open markers). The statistical uncertainties arising from the finite size of the data and simulation samples are indicated by vertical bars. sum of cluster transverse energies (E cl1 T C E cl2 T ). The e ciency grows from about 70% at ( E cl1 TCE cl2 T )D 6 GeV to 100% at ( E cl1 T C E cl2 T )> 9 GeV . The e ciency is parameterized using an error function fit, which is then used to reweight the simulation. Due to the extremely low noise, very high hit reconstruction e ciency and low conversion probability of signal photons in the pixel detector (around 10%), the uncertainty due to the requirement for minimal activity in the ITD is negligible. The MBTS veto e ciency was studied using
!C� events (D e,) passing a supporting trigger and it is estimated to be (98 2)%. Photons are reconstructed from EM clusters in the calorimeter and tracking information provided by the ITD, which allows the identification of photon conversions. Selection requirements are applied to remove EM clusters with a large amount of energy from poorly functioning calorimeter cells, and a timing requirement is made to reject out-of-time candidates. An energy calibration specifically optimized for photons 38 is applied to the candidates to account for upstream energy loss and both lateral and longitudinal shower leakage. A dedicated correction 39 is applied for photons in MC samples to correct for potential mismodelling of quantities that describe the properties (shapes') of the associated EM showers.
The photon particle identification (PID) in this analysis is based
on three shower-shape variables: the lateral width of the shower
in the middle layer of the EM calorimeter, the ratio of the energy
dierence associated with the largest and second largest energy
deposits to the sum of these energies in the first layer, and the
fraction of energy reconstructed in the first layer relative to the total
energy of the cluster. Only photons with ET> 3 GeV andjj< 2.37,
excluding the calorimeter transition region 1.37 <jj< 1.52, are
considered. The pseudorapidity requirement ensures that the pho-
ton candidates pass through regions of the EM calorimeter where
the first layer is segmented into narrow strips, allowing for good
separation between genuine prompt photons and photons coming
from the decay of neutral hadrons. A constant photon PID e ciency
of 95% as a function of with respect to reconstructed photon can-
didates is maintained. This is optimized using multivariate analysis
techniques40, such that EM energy clusters induced by cosmic-ray
muons are rejected with 95% e ciency.
Preselected events are required to have exactly two photons
satisfying the above selection criteria, with a diphoton invariant
mass greater than 6 GeV . T o reduce the dielectron background,
a veto on the presence of any charged-particle tracks (with
pT> 100 MeV ,jj< 2.5 and at least one hit in the pixel detector)
is imposed. This requirement further reduces the fake-photon
background from the dielectron final state by a factor of 25,
according to simulation. It has almost no impact on
!
signal events, since the probability of photon conversion in the pixel detector is relatively small and converted photons are suppressed at low ET (36 GeV) by the photon selection requirements. According to MC studies, the photon selection requirements remove about 10% of low- ET photons. T o reduce other fake-photon backgrounds (for example, cosmic-ray muons), the transverse momentum of the diphoton system ( p
T ) is required to be below 2 GeV . T o reduce background from CEP gg!
reactions, an additional requirement on diphoton acoplanarity, Aco D 1�1
=< 0.01, is imposed. This requirement is optimized to retain a high signal e ciency and reduce the CEP background significantly, since the transverse momentum transferred by the photon exchange is usually much smaller than that due to the colour-singlet-state gluons 41. Performance and validation of photon reconstruction Since the analysis requires the presence of low-energy photons, which are not typically used in ATLAS analyses, detailed studies of photon reconstruction and calibration are performed. High-pT
!C� production with a final-state radiation
(FSR) photon is used for the measurement of the photon PID
e ciency. Events with a photon and two tracks corresponding to
oppositely charged particles withpT> 1 GeV are required to pass the
same trigger as in the diphoton selection or the supporting trigger.
The1R between a photon candidate and a track is required to be
greater than 0.2 to avoid leakage of the electron clusters from the
! eCe� process to the photon cluster. The FSR event candidates are identified using a p tt T < 1 GeV requirement, where p tt T is the transverse momentum of the three-body system consisting of two charged-particle tracks and a photon. The FSR photons are then used to extract the photon PID e ciency, which is defined as the probability for a reconstructed photon to satisfy the identification criteria. Figure 2a shows the photon PID e ciencies in data and simulation as a function of reconstructed photon ET. Within their statistical precision the two results agree. The photon reconstruction e ciency is extracted from data using
! eCe� events where one of the electrons emits a hard-bremsstrahlung photon due to interaction with the material of the detector. Events with exactly one identified electron, two reconstructed charged-particle tracks and exactly one photon are studied. The electron ET is required to be above 5 GeV and the pT 854 © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. NATURE PHYSICS | VOL 13 | SEPTEMBER 2017 | www.nature.com/naturephysics NATURE PHYSICS DOI: 10.1038/NPHYS4208 ARTICLES γγ acoplanarity 0.00 0.01 0.02 0.03 0.04 0.05 0.06 Events/0.005 0 2 4 6 8 10 12 14 Signal selection no Aco requirement Data, 480 μb−1 γγ → γγ MC γγ → e+e− MC CEP γγ MC mγγ (GeV) 0 5 10 15 20 25 30 Events/3 GeV 0 2 4 6 8 10 12 Signal selection with Aco < 0.01 ATLAS Pb + Pb √sNN = 5.02 TeV Data, 480 μb−1 γγ → γγ MC γγ → e+e− MC CEP γγ MC ATLAS Pb + Pb √sNN = 5.02 TeV b a Figure 3 | Kinematic distributions for γ γ →γ γ event candidates. a, Diphoton acoplanarity before applying the Aco < 0.01 requirement. b, Diphoton invariant mass after applying the Aco < 0.01 requirement. Data (points) are compared to MC predictions (histograms). The statistical uncertainties on the data are shown as vertical bars. of the track that is unmatched with the electron (trk2) is required to be below 2 GeV. The additional hard-bremsstrahlung photon is expected to have E γ T ≈(Ee T −ptrk2 T ). The ptrk2 T < 2 GeV requirement ensures a sufficient 1R separation between the expected photon and the second electron, extrapolated to the first layer of the EM calorimeter. The data sample contains 247 γ γ →e+e−events that are used to extract the photon reconstruction efficiency, which is presented in Fig. 2b. Good agreement between data and γ γ →e+e− MC simulation is observed and the photon reconstruction efficiency is measured with a 5–10% relative uncertainty at low ET (3–6 GeV). In addition, a cross-check is performed on Z →µ+µ−γ events identified in pp collision data from 2015 corresponding to an inte- grated luminosity of 1.6 fb−1. The results support (in a similar way to ref. 42) the choice to use the three shower-shape variables in this photon PID selection in an independent sample of low-ET photons. The photon cluster energy resolution is extracted from data using γ γ →e+e−events. The electrons from the γ γ →e+e− reaction (see Supplementary Information) are well balanced in their transverse momenta, with very small standard deviation, σpe+ T −pe− T < 30 MeV, much smaller than the expected EM calorimeter energy resolution. Therefore, by measuring (Ecl1 T −Ecl2 T ) distributions in γ γ →e+e−events, one can extract the cluster energy resolution, σEcl T . For electrons with ET < 10 GeV, the σEcl T /Ecl T is observed to be approximately 8% both in data and simulation. An uncertainty of δσEγ T /σEγ T = 15% is assigned to the simulated photon energy resolution and takes into account differences between σEcl T in data and σEγ T in simulation. Similarly, the EM cluster energy scale can be studied using the (Ecl1 T +Ecl2 T ) distribution. It is observed that the simulation provides a good description of this distribution, within the relative uncertainty of 5% that is assigned to the EM cluster energy-scale modelling. Background estimation Due to its relatively high rate, the exclusive production of electron pairs (γ γ →e+e−) can be a source of fake diphoton events. The contribution from the dielectron background is estimated using γ γ →e+e−MC simulation (which gives 1.3 events) and is verified using the following data-driven technique. Two control regions are defined that are expected to be dominated by γ γ →e+e− backgrounds. The first control region is defined by requiring events with exactly one reconstructed charged-particle track and two identified photons that satisfy the same preselection criteria as for the signal definition. The second control region is defined similarly to the first one, except exactly two tracks are required (Ntrk = 2). Good agreement is observed between data and MC simulation in both control regions, but the precision is limited by the number of events in data. A conservative uncertainty of 25% is therefore assigned to the γ γ →e+e−background estimation, which reflects the statistical uncertainty of data in the Ntrk = 1 control region. The contribution from a related QED process, γ γ →e+e−γ γ , is evaluated using the MadGraph5_aMC@NLO MC generator43 and is found to be negligible. The Aco < 0.01 requirement significantly reduces the CEP gg →γ γ background. However, the MC prediction for this process has a large theoretical uncertainty; hence, an additional data-driven normalization is performed in the region Aco>b, where b is a value greater than 0.01 which can be varied. Three values of b (0.01, 0.02, 0.03) are used, where the central value b=0.02 is chosen to derive the nominal background prediction and the values b = 0.01 and b = 0.03 to define the systematic uncertainty. The normalization is performed using the condition: f norm,b gg→γ γ = (Ndata (Aco > b) −Nsig (Aco >b) −Nγ γ →e+e−(Aco>b))/Ngg→γ γ (Aco >b), for each value of b, where Ndata is the number of observed events, Nsig is the expected number of signal events, Nγ γ →e+e−is the expected background from γ γ →e+e−events and Ngg→γ γ is the MC estimate of the expected background from CEP gg →γ γ events. The normalization factor is found to be f norm gg→γ γ = 0.5 ± 0.3 and the background due to CEP gg →γ γ is estimated to be f norm gg→γ γ ×Ngg→γ γ (Aco < 0.01) = 0.9 ± 0.5 events. To verify the CEP gg →γ γ background estimation method, energy deposits in the ZDC are studied for events before the Aco selection. It is expected that the outgoing ions in CEP events predominantly dissociate, which results in the emission of neutrons detectable in the ZDC20. Good agreement between the normalized CEP gg →γ γ MC expectation and the observed events with a ZDC signal corresponding to at least 1 neutron is observed in the full Aco range (see Supplementary Information for details). Low-pT dijet events can produce multiple π0 mesons, which could potentially mimic diphoton events. The event selection requirements are efficient in rejecting such events, and based on studies performed with a supporting trigger, the background from hadronic processes is estimated to be 0.3 ± 0.3 events. MC studies show that the background from γ γ →q¯q processes is negligible. Exclusive neutral two-meson production can be a potential source of background for LbyL events, mainly due to their back-to- back topology being similar to that of the CEP gg →γ γ process. The cross-section for this process is calculated to be below 10% of the CEP gg →γ γ cross-section44,45 and it is therefore considered to NATURE PHYSICS | VOL 13 | SEPTEMBER 2017 | www.nature.com/naturephysics © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. 855