EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)
Submitted to: Physical Review Letters CERN-PH-EP-2015-028
17th March 2015
Search for a Heavy Neutral Particle Decaying to eµ, eτ, or µτ in pp Collisions at √
s = 8 TeV with the ATLAS Detector
The ATLAS Collaboration
Abstract
This Letter presents a search for a heavy neutral particle decaying into an opposite-sign different-flavor dilepton pair, e±µ∓, e±τ∓, or µ±τ∓ using 20.3 fb−1 of pp collision data at
√s = 8 TeV collected by the ATLAS detector at the LHC. The numbers of observed can- didate events are compatible with the Standard Model expectations. Limits are set on the cross section of new phenomena in two scenarios: the production of ˜ντin R-parity-violating supersymmetric models and the production of a lepton-flavor-violating Z0vector boson.
c
2015 CERN for the benefit of the ATLAS Collaboration.
Reproduction of this article or parts of it is allowed as specified in the CC-BY-3.0 license.
arXiv:1503.04430v1 [hep-ex] 15 Mar 2015
Lepton-flavor-violation (LFV) in the charged sector, if observed at present sensitivities, would be a clear signature of new physics. Such signatures occur in several new physics scenarios, including R-parity- violating (RPV) [1] supersymmetry (SUSY) [2–10] and models with an additional heavy neutral gauge boson Z0[11] allowing for LFV couplings.
In RPV SUSY, the lagrangian terms allowing LFV can be expressed as 12λi jkLiLj¯ek + λ0i jkLiQjd¯k [1], where L and Q are the S U(2) doublet superfields of leptons and quarks; e and d are the S U(2) singlet superfields of leptons and down-like quarks; λ and λ0 are Yukawa couplings; and the indices i, j and kdenote fermion generations. A τ sneutrino (˜ντ) may be produced in pp collisions by d ¯dannihilation and subsequently decay to eµ, eτ, or µτ. Although only ˜ντis considered here in order to compare with previous searches performed at the Tevatron, the results of our analysis apply to any sneutrino flavor.
The Sequential Standard Model (SSM), where the Z0boson is often assumed to have the same quark and lepton couplings as the Standard Model (SM) Z boson, can be extended to include LFV couplings for the Z0. The Z0 → eµ, eτ, or µτ couplings (Q12, Q13, or Q23) [12] are typically expressed as fractions of the SSM Z0 →`+`−(`= e, µ, τ) coupling.
The CDF [13], D0 [14], and ATLAS [15] collaborations have searched for a ˜ντin LFV final states and placed limits for various ˜ντ mass hypotheses. Both the CDF [16] and ATLAS [17] collaborations have placed limits on Q12as a function of the Z0mass.
This Letter describes a search for a neutral heavy particle (˜ντor Z0) decaying into e±µ∓(eµ), e±τ∓had (eτ), or µ±τ∓had(µτ) using pp collision data collected at √
s= 8 TeV, where τhadis a τ lepton that decays into hadrons. The ATLAS detector is described in detail elsewhere [18]. Events are selected with a three-level trigger system that requires one or two leptons (e or µ) with high transverse momentum (pT). The dataset has a total integrated luminosity of 20.3 ± 0.6 fb−1, where the uncertainty is derived following the same methodology as that detailed in Ref. [19].
Electrons are required to have pT > 25 GeV, |η| < 1.37 or 1.52 < |η| < 2.47 [20], and satisfy the
“tight” selection in Ref. [21], which was modified in 2012 to reduce the impact of additional inelastic ppinteractions, termed pileup. Muon candidates must have pT > 25 GeV, |η| < 2.4 and be reconstructed in both the inner tracker detector and the muon spectrometer. The muon momenta measured by the inner detector and muon spectrometer must match within five standard deviations of their combined uncertainty.
Good quality reconstruction and pT resolution at high momentum are ensured by requiring a minimum number of associated hits on the inner detector track [22] and in each of the three muon spectrometer stations.
Candidate events must contain at least one primary interaction vertex reconstructed with more than three associated tracks with pT > 400 MeV. If there is more than one such vertex, the one with the highest sum of p2T of associated tracks is chosen. The longitudinal impact parameter is required to be smaller than 2 mm for candidate electrons and smaller than 1 mm for candidate muons. It is further required that the transverse impact parameter is less than six times its resolution for candidate electrons, and that the transverse impact parameter is smaller than 0.2 mm for candidate muons. A calorimeter isolation criterion E∆R<0.2T /ET < 0.06 and a tracker isolation criterion p∆R<0.4T /pT < 0.06 are applied for both the electrons and muons, where E∆R<0.2T is the transverse energy deposited in the calorimeter within a cone of size∆R = p(∆η)2+ (∆φ)2 = 0.2 around the lepton, and p∆R<0.4T is the sum of the pT of tracks with pT > 1 GeV within a cone size of 0.4 around the lepton. ETand pTare the lepton transverse energy and momentum, respectively.
Hadronic decays of τ leptons are characterized by one or three charged tracks associated to a narrow energy cluster in the calorimeters [23]. This search uses τhad candidates with only one charged track due to reduced identification and reconstruction efficiency for high-pT τ decays with three charged tracks.
Boosted-decision-tree multivariate discriminators, based on tracking and calorimeter information, are used to reject jets and electrons misidentified as τhad. The τhad candidates must have |η| < 2.47 and ET > 25 GeV. Candidates with |η| < 0.03 are removed to exclude a region with increased misidentification of electrons due to reduced coverage by the inner detector and calorimeters.
Jets are reconstructed from clusters of energy in the calorimeter using the anti-kt algorithm [24] with radius parameter R= 0.4. Jet energies are calibrated using Monte Carlo (MC) simulation and a combina- tion of several in situ calibrations [25]. The calculation of the missing transverse momentum vector ~pTmiss (with magnitude ETmiss) is based on the vector sum of the calibrated pT of reconstructed jets, electrons, and muons, as well as calorimeter energy clusters not associated with reconstructed objects [26].
Candidate signal events are required to have exactly two leptons, of opposite charge and of different flavor, satisfying the above lepton selection criteria. The two leptons are required to be back-to-back in the azimuthal plane with |∆φ``0| > 2.7, where ∆φ``0 is the φ difference between the two leptons. Due to the presence of the undetected neutrino in the τ decay, the ETof the τhad candidate is required to be less than the ET(pT) of the electron (muon) for the eτhad(µτhad) channel.
A collinear neutrino approximation is used to reconstruct the dilepton invariant mass (m``0) in the eτhad and µτhad channels. In the hadronic decay of a τ lepton from a heavy resonance, the neutrino and the resultant jet are nearly collinear. The four-vector of the neutrino is reconstructed from the ~pTmissand η of the τhadjet. Four-vectors of the electron or muon, τhad candidate and neutrino are then used to calculate m``0. For eτhad and µτhad signal events, the above technique significantly improves the mass resolution and search sensitivity.
Events with m``0< 200 GeV form a validation region to verify the background modeling, and events with m``0> 200 GeV are used as the search region.
The SM processes that produce `±`0∓final states can be divided into two categories: processes that pro- duce two prompt leptons such as Z/γ∗→ττ, t¯t, single-top Wt channel, diboson production, and processes where one or more photons or jets are misidentified as leptons, predominantly W/Z+ γ, W/Z+jets, and multijet events. The decay of a τ to an electron or a muon is considered as prompt production. For the eτhad(µτhad) channel, additional background can originate from the Z/γ∗→ ee (µµ) process if one lepton is misidentified as a τhad candidate. The contributions of these processes are even larger with respect to the Z/γ∗ → ττ background, since the final states e or µ are usually harder than those from leptonic τ decay.
Contributions from processes in the prompt two-lepton category, as well as photon-related and Z/γ∗ → ee(µµ) backgrounds, are estimated using MC simulation [27]. The detector response model is based on the geant4 program [28]. Lepton reconstruction and identification efficiencies, energy scales, and resolutions in the MC simulation are corrected to the corresponding values measured in the data. Pileup is included to match distributions observed in the data. Top quark production is generated with mc@nlo v4.06 [29] for t¯t and single-top, the Drell–Yan process (Z/γ∗→``) is generated with alpgen v2.14 [30], and diboson processes are generated with herwig v6.520.2 [31]. Samples of Wγ and Zγ events are generated with sherpa v1.04 [32]. These generated samples are normalized to the most accurate available cross-section calculations. For the dominant backgrounds, the Drell–Yan processes are corrected to next- to-next-to-leading order (NNLO) [33], and t¯t is corrected to NNLO, including soft-gluon resummation to next-to-next-to-leading-logarithm order [34].
Since it is difficult to model misidentification of jets as leptons, particularly at high pT, the W+jets and multijet backgrounds are determined from control regions in the data. The W+jets background is determ- ined in a control region selected with the same criteria as used for the signal selection except requiring ETmiss > 30 GeV (to enhance the W contribution) and requiring that the electron or muon pT be less than 150 GeV (to eliminate potential signal). Simulation studies indicate that there is negligible multijet back- ground in this control region. For the eτhadand µτhadchannels, the number of events in the control region is corrected for the other SM background sources using MC samples. For the eµ channel, the number of W+jets events in the control region is too small to yield a statistically meaningful measurement. Instead, the control region is enlarged by removing the isolation criterion on one lepton, and the W+jets contribu- tion is estimated using the lepton E∆R<0.2T /ETdistribution to fit the data with the MC predictions for other SM processes (dominant at low values of the isolation variable) and W+jets (dominant at large values).
For the W+jets background in all three channels, the extrapolation factor from the control region to the signal region and the shape of the m``0distribution are taken from the W+jets MC sample.
The contribution from multijet production is estimated from a control region with the same selection as the signal region except that the leptons are required to have the same electric charge, under the assumption that the probability of misidentifying a jet as a lepton is independent of the charge. The number of events in this same-sign control region is corrected for contributions from backgrounds with prompt leptons and from W+jets backgrounds using the procedure described above. The assumption of charge independence of the jet misidentification rates is tested in a multijet-enriched region with two nonisolated leptons.
After subtraction of other SM backgrounds, the ratio of opposite-sign to same-sign events is found to be 1.07 ± 0.06 (stat.) ± 0.02 (syst.).
The background estimates are verified in validation regions defined with the same selection criteria as for the signal regions but with 1.0 < |∆φ``0| < 2.7. In these validation regions, simulation studies show the backgrounds have compositions similar to the signal regions, and the predictions agree with the data within 20% over the entire m``0 range. An uncertainty of 20% is hence placed on the total background estimate that is used in the final results.
MC signal events are generated with herwig v6.520.2 for ˜ντ and pythia v8.165 [35] for Z0. Samples are produced with ˜ντ and Z0 masses ranging from 0.5 to 3 TeV. Signal cross sections are calculated to next-to-leading order for ˜ντ, and NNLO for Z0. The theoretical uncertainties are taken from an envelope of cross-section predictions using different parton distribution function (PDF) sets and factorization and renormalization scales [36,37].
The selection efficiency, including τ decay branching ratio if τ is involved, for m˜ντ = 2 TeV are 42%, 14%, and 10% in the eµ, eτ and µτ channels, respectively. The corresponding numbers for a Z0 boson with mZ0= 2 TeV are 37%, 11%, and 9%. The systematic uncertainties on the signal efficiency vary from 3% to 6% depending on the resonance mass and decay mode. The primary contributions are due to the number of MC events, and the uncertainties related to the muon and τ pTscales.
The observed and expected event yields in both the validation and search m``0 regions for all three final states are in good agreement, as summarized in Table1. The m``0distributions (Fig.1) show no significant excess above the SM expectation in any of the three modes. The dominant contributions to the uncertainty bands in Fig.1are due to the number of MC events, the MC cross-section uncertainties, the EmissT scale and resolution, and the uncertainty in the shape of the m``0distribution for W+jets.
Upper limits are placed on the production cross section times branching ratio [σ(pp → ˜ντ/Z0)×BR(˜ντ/Z0→
``0)]. For each ˜ντor Z0mass, m, the search region is defined to be m ± 3σ``0, where σ``0 is the standard deviation of the simulated signal m``0distribution. The relative width of the signal m``0distribution ranges
Table 1: Estimated SM backgrounds and observed event yields for the validation (m``0 < 200 GeV) and search (m``0 > 200 GeV) regions. Both the statistical and systematic uncertainties are included. Due to correlations, the total uncertainties are not exactly the sum in quadrature of the components.
m``0 < 200 GeV m``0 > 200 GeV
Process Neµ Neτhad Nµτhad Neµ Neτhad Nµτhad
Z/γ∗→ττ 6000±400 11000± 900 11200± 700 28± 12 72± 21 99± 33
Z/γ∗→ ee — 6100±1100 — — 430± 70 —
Z/γ∗→µµ — — 19500±1300 — — 410± 80
t¯t 4220±290 690± 60 580± 50 1640±120 700± 60 550± 40
Diboson 1440± 80 321± 29 258± 17 474± 30 197± 17 141± 11
Single top quark 470± 40 87± 11 60± 7 202± 17 90± 10 73± 8
W+jets 54± 18 17000±4000 14000±4000 8± 4 3600±700 2800±600
Multijet 227± 32 4800±1000 700± 800 58± 12 340±210 100±190
Total 12400±600 40400±2900 46000±4000 2400±130 5400±500 4200±400
Data 12954 41304 48304 2474 5336 4184
from 3% to 17% for different mass points, channels and models. To reduce the statistical error, if the up- per side of the search region is greater than 1 TeV, all events above 1 TeV are used. To further reduce the effect of fluctuations in the high-mass region due to low MC event counts, the number of background events in each mass window is estimated using a double exponential fit to the total background m``0 dis- tribution. The fit uncertainty is taken into account in the limit-setting procedure, including a contribution from varying the fit function range.
Events / 20 GeV
−1
10 1 10 102
103
104
105 Data
(1 TeV) ν∼τ
τ τ
→ Z Others
Z' (0.75 TeV) Jet fake
t t Total SM = 8 TeV, 20.3 fb-1
s ATLAS
µ e
Dilepton Mass [GeV]
0 200 400 600 800 1000 1200
Data/SM
1 2
Events / 20 GeV
−1
10 1 10 102
103
104
105 Data
(1 TeV) ν∼τ
→ ll Z Others
Z' (0.75 TeV) Jet fake
t t Total SM = 8 TeV, 20.3 fb-1
s ATLAS
τ e
Dilepton Mass [GeV]
0 200 400 600 800 1000 1200
Data/SM
1 2
Events / 20 GeV
−1
10 1 10 102
103
104
105 Data
(1 TeV) ν∼τ
→ ll Z Others
Z' (0.75 TeV) Jet fake
t t Total SM = 8 TeV, 20.3 fb-1
s ATLAS
τ µ
Dilepton Mass [GeV]
0 200 400 600 800 1000 1200
Data/SM
1 2
Figure 1: Observed and predicted eµ, eτhad, µτhadinvariant mass distributions. The contributions of the different processes are also shown: “Others” includes diboson and single-top while “Jet fake” refers to W+jets and multijet.
All overflows are included in the rightmost bin. Signal simulations are shown for m˜ντ= 1 TeV and mZ0 = 0.75 TeV.
The couplings λ0311 = 0.11 and λi3k = 0.07 (Q``0 = 1) are used for the RPV (Z0) model. The uncertainty bands include both the statistical and systematic uncertainties.
A frequentist technique [38] is used to set the expected and observed upper limits as a function of m˜ντ and mZ0. The likelihood of observing the number of events in data as a function of the expected number of signal and background events is constructed from a Poisson distribution for each m``0bin. Systematic un- certainties are taken into account with Gaussian-distributed nuisance parameters. A 95% confidence level (CL) limit is then determined. The expected exclusion limits are determined, using simulated pseudoex-
periments containing only background processes, as the median of the 95% CL limit distributions for each set of pseudoexperiments at each value of m˜ντ or mZ0, including systematic uncertainties. The ensemble of limits is also used to assess the 1σ and 2σ uncertainty envelopes of the expected limits.
Figure2 shows the observed and expected cross section times branching ratio limits as a function of m˜ντ(mZ0), together with the ±1σ and ±2σ uncertainty bands. For a ˜ντmass of 1 TeV, the observed limits on the production cross section times branching ratio are 0.5 fb, 2.7 fb, and 9.1 fb for the eµ, eτ and µτ channels, respectively. The corresponding limits for a Z0boson mass of 1 TeV are 1.0 fb, 4.0 fb and 9.9 fb for the eµ, eτ and µτ channels, respectively.
Theoretical predictions of cross section times branching ratio [33,39] are also shown, assuming λ0311 = 0.11 and λi3k = 0.07 for the ˜ντ and Qi j = 1 for the Z0, consistent with benchmark couplings used in previous searches.
For these benchmark couplings, the lower limits on the ˜ντmass are 2.0 TeV, 1.7 TeV, and 1.7 TeV for the eµ, eτ and µτ channels, respectively. The corresponding lower limits on the Z0mass are 2.5 TeV, 2.2 TeV and 2.2 TeV for the eµ, eτ and µτ channels, respectively. The observed lower mass limits are a factor of three to four higher than the best limits from the Tevatron [13,14] and also more stringent than the previous limits from ATLAS [15] for the same couplings.
In summary, a search has been performed for a heavy particle decaying to eµ, eτhad, or µτhad final states using 20.3 fb−1of pp collision data at √
s= 8 TeV recorded by the ATLAS detector at the LHC. The data are found to be consistent with SM predictions. Limits are placed on the cross section times branching ratio for an RPV SUSY ˜ντand a LFV Z0boson. These results considerably extend previous constraints from the Tevatron and LHC experiments.
[GeV]
ν∼τ
m
500 1000 1500 2000 2500 3000
)[fb]τµ/τ/eµ BR(e×σ 1
10 102
103
104
= 8 TeV, 20.3fb-1
s ATLAS
µ
→ e ν∼τ
[GeV]
τ(Z')
mν∼
500 1000 1500 2000 2500 3000
)[fb]τµ/τ/eµ BR(e×σ
−1
10 1 10 102
103
104
µ
→ e
Z' mν∼τ [GeV]
500 1000 1500 2000 2500 3000
)[fb]τ BR(e×σ
1 10 102 103 104
Theory 95% CL Observed limit 95% CL Expected limit
σ
±1 95% CL Expected limit
σ
±2 95% CL Expected limit
τ
→ e ν∼τ
[GeV]
τ(Z')
mν∼
500 1000 1500 2000 2500 3000
)[fb]τ BR(e×σ
−1 10
1 10 102 103 104
τ
→ e
Z' mν∼τ [GeV]
500 1000 1500 2000 2500 3000
)[fb]τµ BR(×σ
1 10 102 103 104
τ µ
τ→ ν∼
[GeV]
τ(Z')
mν∼
500 1000 1500 2000 2500 3000
)[fb]τµ BR(×σ
−1 10
1 10 102 103 104
τ µ
→ Z'
Figure 2: The 95% CL limits on cross section times branching ratio as a function of ˜ντmass (top plots) and Z0mass (bottom plots) for eµ (left), eτ (middle), and µτ (right). Theory curves are for the arbitrary choice of couplings λ0311 = 0.11 and λi3k = 0.07 for ˜ντ and Q``0 = 1 for Z0. The gray band around the theory curve represents the theoretical uncertainty from the PDFs and factorization and renormalization scales.
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; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark;
EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Geor- gia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT and NSRF, Greece; RGC, Hong Kong SAR, China; ISF, MINERVA, GIF, I-CORE and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW and NCN, Poland; GRICES and FCT, Portugal; MNE/IFA, Romania; MES of Russia and ROSATOM, Rus- sian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MIZŠ, Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Cantons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF, United States of America.
The crucial computing support from all WLCG partners 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 (Taiwan), RAL (UK) and BNL (USA) and in the Tier-2 facilities worldwide.
References
[1] R. Barbier et al., Phys. Rep. 420, 1 (2005), [arXiv:hep−ph/0406039].
[2] H. Miyazawa, Prog. Theor. Phys. 36 (6), 1266 (1966).
[3] P. Ramond, Phys. Rev. D 3, 2415 (1971).
[4] Y. A. Golfand and E. P. Likhtman, JETP Lett. 13, 323 (1971), [Pisma Zh. Eksp. Teor. Fiz.
13:452,1971].
[5] A. Neveu and J. H. Schwarz, Nucl. Phys. B 31, 86 (1971).
[6] A. Neveu and J. H. Schwarz, Phys. Rev. D 4, 1109 (1971).
[7] J. Gervais and B. Sakita, Nucl. Phys. B 34, 632 (1971).
[8] D. V. Volkov and V. P. Akulov, Phys. Lett. B 46, 109 (1973).
[9] J. Wess and B. Zumino, Phys. Lett. B 49, 52 (1974).
[10] J. Wess and B. Zumino, Nucl. Phys. B 70, 39 (1974).
[11] P. Langacker, Rev. Mod. Phys. 81, 1199 (2009), [arXiv:0801.1345].
[12] B. Murakami, Phys. Rev. D 65, 055003 (2002), [arXiv:hep−ph/0110095].
[13] T. Aaltonen et al. (CDF Collaboration), Phys. Rev. Lett. 105, 191801 (2010), [arXiv: 1004.3042]; A.
Abulencia et al. (CDF Collaboration), Phys. Rev. Lett. 96, 211802 (2006), [arXiv:hep−ex/0603006].
[14] V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 105, 191802 (2010), [arXiv:1007.4835];
V. M. Abazov et al. (D0 Collaboration), Phys. Rev. Lett. 100, 241803 (2008), [arXiv:0711.3207].
[15] ATLAS Collaboration, Phys. Lett. B 723, 15 (2013), [arXiv:1212.1272].
[16] T. Aaltonen et al. (CDF Collaboration), Phys. Rev. Lett. 105, 191801 (2010), [arXiv:1004.3042].
[17] ATLAS Collaboration, Eur. Phys. J. C 71, 1809 (2011), [arXiv:1109.3089].
[18] ATLAS Collaboration, JINST 3, S08003 (2008), [arXiv:0901.0512].
[19] ATLAS Collaboration, Eur. Phys. J. C 73, 2518 (2013), [arXiv:1302.4393].
[20] ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the center of the detector and the z-axis along the beam direction. The x-axis points toward 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 azimuthal angle around the beam direction. Pseudorapidity is defined in terms of the polar angle θ as η= − ln tan(θ/2). Transverse projections are defined relative to the beam axis.
[21] ATLAS Collaboration, Eur. Phys. J. C 74, 2941 (2014), [arXiv:1404.2240].
[22] ATLAS Collaboration, Eur. Phys. J. C 74, 3130 (2014), [arXiv:1407.3935].
[23] ATLAS Collaboration, ATLAS-CONF-2013-064, https://cds.cern.ch/record/1562839 .
[24] M. Cacciari, G. P. Salam, and G. Soyez, J. High Energy Phys. 04 (2008) 063, [arXiv:0802.1189].
[25] ATLAS Collaboration, Eur. Phys. J. C 73, 2304 (2013), [arXiv:1112.6426].
[26] ATLAS Collaboration, Eur. Phys. J. C 72, 1844 (2012), [arXiv:1108.5602].
[27] ATLAS Collaboration, Eur. Phys. J. C 70, 823 (2010), [arXiv:1005.4568].
[28] S. Agostinelli et al., Nucl. Instrum. Methods Phys. Res., Sect. A 506, 250 (2003).
[29] S. Frixione and B.R.Webber, J. High Energy Phys. 06 (2002) 029, [arXiv:hep−ph/0204244]; S. Frix- ione, E. Laenen and P. Motylinski, J. High Energy Phys. 03 (2006) 092, [arXiv:hep−ph/0512250];
S. Frixione et al., J. High Energy Phys. 07 (2008) 029, [arXiv:0805.3067].
[30] M.L.Mangano, M.Moretti, F.Piccinini, R.Pittau, A.Polosa, J. High Energy Phys. 07 (2003) 001, [arXiv:hep−ph/0206293].
[31] G. Corcella et al., J. High Energy Phys. 01 (2001) 010, [arXiv:hep−ph/0011363].
[32] T. Gleisberg et al., J. High Energy Phys. 02 (2009) 007, [arXiv:0811.4622]; T. Gleisberg and S. Hoeche, J. High Energy Phys. 12 (2008) 039, [arXiv:0808.3674]; S. Schumann and F. Krauss, J. High Energy Phys. 03 (2008) 038, [arXiv:0709.1027]; S. Hoeche, F. Krauss, S. Schumann and F. Siegert, J. High Energy Phys. 05 (2009) 053, [arXiv:0903.1219].
[33] Y. Li and F. Petriello, Phys. Rev. D 86, 094034 (2012), [arXiv:1208.5967].
[34] M. Czakon, P. Fiedler, and A. Mitov, Comput. Phys. Commun.185, 2930 (2014), [arXiv:1112.5675];
M. Czakon and A. Mitov, Phys. Rev. Lett. 110, 252004 (2013), [arXiv:1303.6254.
[35] T. Sjöstrand, S. Mrenna, and P. Skands, J. High Energy Phys. 05 (2006) 026, [arXiv:hep- ph/0603175]; T. Sjöstrand, S. Mrenna, and P. Skands, Comput. Phys. Comm. 178, 852 (2008), [arXiv:0710.3820].
[36] M. Kramer et al.,[arXiv:1206.2892].
[37] ATLAS Collaboration, Phys. Rev. D 90, 052005 (2014), [arXiv:1405.4123].
[38] G. Cowan et al., Eur. Phys. J. C 71, 1554 (2011), [arXiv:1007.1727].
[39] S. Wang et al., Chin. Phys. Lett. 25, 58 (2008), [arXiv:0706.3079].
The ATLAS Collaboration
G. Aad85, B. Abbott113, J. Abdallah152, O. Abdinov11, R. Aben107, M. Abolins90, O.S. AbouZeid159, H. Abramowicz154, H. Abreu153, R. Abreu30, Y. Abulaiti147a,147b, B.S. Acharya165a,165b,a,
L. Adamczyk38a, D.L. Adams25, J. Adelman108, S. Adomeit100, T. Adye131, A.A. Affolder74,
T. Agatonovic-Jovin13, J.A. Aguilar-Saavedra126a,126f, M. Agustoni17, S.P. Ahlen22, F. Ahmadov65,b, G. Aielli134a,134b, H. Akerstedt147a,147b, T.P.A. Åkesson81, G. Akimoto156, A.V. Akimov96,
G.L. Alberghi20a,20b, J. Albert170, S. Albrand55, M.J. Alconada Verzini71, M. Aleksa30,
I.N. Aleksandrov65, C. Alexa26a, G. Alexander154, T. Alexopoulos10, M. Alhroob113, G. Alimonti91a, L. Alio85, J. Alison31, S.P. Alkire35, B.M.M. Allbrooke18, P.P. Allport74, A. Aloisio104a,104b,
A. Alonso36, F. Alonso71, C. Alpigiani76, A. Altheimer35, B. Alvarez Gonzalez90, M.G. Alviggi104a,104b, K. Amako66, Y. Amaral Coutinho24a, C. Amelung23, D. Amidei89, S.P. Amor Dos Santos126a,126c, A. Amorim126a,126b, S. Amoroso48, N. Amram154, G. Amundsen23, C. Anastopoulos140, L.S. Ancu49, N. Andari30, T. Andeen35, C.F. Anders58b, G. Anders30, K.J. Anderson31, A. Andreazza91a,91b, V. Andrei58a, S. Angelidakis9, I. Angelozzi107, P. Anger44, A. Angerami35, F. Anghinolfi30, A.V. Anisenkov109,c, N. Anjos12, A. Annovi124a,124b, M. Antonelli47, A. Antonov98, J. Antos145b, F. Anulli133a, M. Aoki66, L. Aperio Bella18, G. Arabidze90, Y. Arai66, J.P. Araque126a, A.T.H. Arce45, F.A. Arduh71, J-F. Arguin95, S. Argyropoulos42, M. Arik19a, A.J. Armbruster30, O. Arnaez30, V. Arnal82, H. Arnold48, M. Arratia28, O. Arslan21, A. Artamonov97, G. Artoni23, S. Asai156, N. Asbah42,
A. Ashkenazi154, B. Åsman147a,147b, L. Asquith150, K. Assamagan25, R. Astalos145a, M. Atkinson166, N.B. Atlay142, B. Auerbach6, K. Augsten128, M. Aurousseau146b, G. Avolio30, B. Axen15,
M.K. Ayoub117, G. Azuelos95,d, M.A. Baak30, A.E. Baas58a, C. Bacci135a,135b, H. Bachacou137, K. Bachas155, M. Backes30, M. Backhaus30, E. Badescu26a, P. Bagiacchi133a,133b, P. Bagnaia133a,133b, Y. Bai33a, T. Bain35, J.T. Baines131, O.K. Baker177, P. Balek129, T. Balestri149, F. Balli84, E. Banas39, Sw. Banerjee174, A.A.E. Bannoura176, H.S. Bansil18, L. Barak30, S.P. Baranov96, E.L. Barberio88, D. Barberis50a,50b, M. Barbero85, T. Barillari101, M. Barisonzi165a,165b, T. Barklow144, N. Barlow28, S.L. Barnes84, B.M. Barnett131, R.M. Barnett15, Z. Barnovska5, A. Baroncelli135a, G. Barone49, A.J. Barr120, F. Barreiro82, J. Barreiro Guimarães da Costa57, R. Bartoldus144, A.E. Barton72,
P. Bartos145a, A. Bassalat117, A. Basye166, R.L. Bates53, S.J. Batista159, J.R. Batley28, M. Battaglia138, M. Bauce133a,133b, F. Bauer137, H.S. Bawa144,e, J.B. Beacham111, M.D. Beattie72, T. Beau80,
P.H. Beauchemin162, R. Beccherle124a,124b, P. Bechtle21, H.P. Beck17, f, K. Becker120, M. Becker83, S. Becker100, M. Beckingham171, C. Becot117, A.J. Beddall19c, A. Beddall19c, V.A. Bednyakov65, C.P. Bee149, L.J. Beemster107, T.A. Beermann176, M. Begel25, J.K. Behr120, C. Belanger-Champagne87, W.H. Bell49, G. Bella154, L. Bellagamba20a, A. Bellerive29, M. Bellomo86, K. Belotskiy98,
O. Beltramello30, O. Benary154, D. Benchekroun136a, M. Bender100, K. Bendtz147a,147b, N. Benekos10, Y. Benhammou154, E. Benhar Noccioli49, J.A. Benitez Garcia160b, D.P. Benjamin45, J.R. Bensinger23, S. Bentvelsen107, L. Beresford120, M. Beretta47, D. Berge107, E. Bergeaas Kuutmann167, N. Berger5, F. Berghaus170, J. Beringer15, C. Bernard22, N.R. Bernard86, C. Bernius110, F.U. Bernlochner21, T. Berry77, P. Berta129, C. Bertella83, G. Bertoli147a,147b, F. Bertolucci124a,124b, C. Bertsche113, D. Bertsche113, M.I. Besana91a, G.J. Besjes106, O. Bessidskaia Bylund147a,147b, M. Bessner42, N. Besson137, C. Betancourt48, S. Bethke101, A.J. Bevan76, W. Bhimji46, R.M. Bianchi125,
L. Bianchini23, M. Bianco30, O. Biebel100, S.P. Bieniek78, M. Biglietti135a, J. Bilbao De Mendizabal49, H. Bilokon47, M. Bindi54, S. Binet117, A. Bingul19c, C. Bini133a,133b, C.W. Black151, J.E. Black144, K.M. Black22, D. Blackburn139, R.E. Blair6, J.-B. Blanchard137, J.E. Blanco77, T. Blazek145a, I. Bloch42, C. Blocker23, W. Blum83,∗, U. Blumenschein54, G.J. Bobbink107, V.S. Bobrovnikov109,c,
S.S. Bocchetta81, A. Bocci45, C. Bock100, M. Boehler48, J.A. Bogaerts30, A.G. Bogdanchikov109,
C. Bohm147a, V. Boisvert77, T. Bold38a, V. Boldea26a, A.S. Boldyrev99, M. Bomben80, M. Bona76, M. Boonekamp137, A. Borisov130, G. Borissov72, S. Borroni42, J. Bortfeldt100, V. Bortolotto60a,60b,60c, K. Bos107, D. Boscherini20a, M. Bosman12, J. Boudreau125, J. Bouffard2, E.V. Bouhova-Thacker72, D. Boumediene34, C. Bourdarios117, N. Bousson114, S. Boutouil136d, A. Boveia30, J. Boyd30,
I.R. Boyko65, I. Bozic13, J. Bracinik18, A. Brandt8, G. Brandt15, O. Brandt58a, U. Bratzler157, B. Brau86, J.E. Brau116, H.M. Braun176,∗, S.F. Brazzale165a,165c, K. Brendlinger122, A.J. Brennan88, L. Brenner107, R. Brenner167, S. Bressler173, K. Bristow146c, T.M. Bristow46, D. Britton53, D. Britzger42,
F.M. Brochu28, I. Brock21, R. Brock90, J. Bronner101, G. Brooijmans35, T. Brooks77, W.K. Brooks32b, J. Brosamer15, E. Brost116, J. Brown55, P.A. Bruckman de Renstrom39, D. Bruncko145b, R. Bruneliere48, A. Bruni20a, G. Bruni20a, M. Bruschi20a, L. Bryngemark81, T. Buanes14, Q. Buat143, P. Buchholz142, A.G. Buckley53, S.I. Buda26a, I.A. Budagov65, F. Buehrer48, L. Bugge119, M.K. Bugge119, O. Bulekov98, H. Burckhart30, S. Burdin74, B. Burghgrave108, S. Burke131, I. Burmeister43, E. Busato34, D. Büscher48, V. Büscher83, P. Bussey53, C.P. Buszello167, J.M. Butler22, A.I. Butt3, C.M. Buttar53, J.M. Butterworth78, P. Butti107, W. Buttinger25, A. Buzatu53, R. Buzykaev109,c, S. Cabrera Urbán168, D. Caforio128,
O. Cakir4a, P. Calafiura15, A. Calandri137, G. Calderini80, P. Calfayan100, L.P. Caloba24a, D. Calvet34, S. Calvet34, R. Camacho Toro49, S. Camarda42, D. Cameron119, L.M. Caminada15,
R. Caminal Armadans12, S. Campana30, M. Campanelli78, A. Campoverde149, V. Canale104a,104b, A. Canepa160a, M. Cano Bret76, J. Cantero82, R. Cantrill126a, T. Cao40, M.D.M. Capeans Garrido30, I. Caprini26a, M. Caprini26a, M. Capua37a,37b, R. Caputo83, R. Cardarelli134a, T. Carli30, G. Carlino104a, L. Carminati91a,91b, S. Caron106, E. Carquin32a, G.D. Carrillo-Montoya8, J.R. Carter28,
J. Carvalho126a,126c, D. Casadei78, M.P. Casado12, M. Casolino12, E. Castaneda-Miranda146b,
A. Castelli107, V. Castillo Gimenez168, N.F. Castro126a,g, P. Catastini57, A. Catinaccio30, J.R. Catmore119, A. Cattai30, J. Caudron83, V. Cavaliere166, D. Cavalli91a, M. Cavalli-Sforza12, V. Cavasinni124a,124b, F. Ceradini135a,135b, B.C. Cerio45, K. Cerny129, A.S. Cerqueira24b, A. Cerri150, L. Cerrito76, F. Cerutti15, M. Cerv30, A. Cervelli17, S.A. Cetin19b, A. Chafaq136a, D. Chakraborty108, I. Chalupkova129,
P. Chang166, B. Chapleau87, J.D. Chapman28, D.G. Charlton18, C.C. Chau159, C.A. Chavez Barajas150, S. Cheatham153, A. Chegwidden90, S. Chekanov6, S.V. Chekulaev160a, G.A. Chelkov65,h,
M.A. Chelstowska89, C. Chen64, H. Chen25, K. Chen149, L. Chen33d,i, S. Chen33c, X. Chen33f, Y. Chen67, H.C. Cheng89, Y. Cheng31, A. Cheplakov65, E. Cheremushkina130,
R. Cherkaoui El Moursli136e, V. Chernyatin25,∗, E. Cheu7, L. Chevalier137, V. Chiarella47, J.T. Childers6, G. Chiodini73a, A.S. Chisholm18, R.T. Chislett78, A. Chitan26a, M.V. Chizhov65, K. Choi61,
S. Chouridou9, B.K.B. Chow100, V. Christodoulou78, D. Chromek-Burckhart30, M.L. Chu152, J. Chudoba127, A.J. Chuinard87, J.J. Chwastowski39, L. Chytka115, G. Ciapetti133a,133b, A.K. Ciftci4a, D. Cinca53, V. Cindro75, I.A. Cioara21, A. Ciocio15, Z.H. Citron173, M. Ciubancan26a, A. Clark49, B.L. Clark57, P.J. Clark46, R.N. Clarke15, W. Cleland125, C. Clement147a,147b, Y. Coadou85, M. Cobal165a,165c, A. Coccaro139, J. Cochran64, L. Coffey23, J.G. Cogan144, B. Cole35, S. Cole108, A.P. Colijn107, J. Collot55, T. Colombo58c, G. Compostella101, P. Conde Muiño126a,126b, E. Coniavitis48, S.H. Connell146b, I.A. Connelly77, S.M. Consonni91a,91b, V. Consorti48, S. Constantinescu26a,
C. Conta121a,121b, G. Conti30, F. Conventi104a, j, M. Cooke15, B.D. Cooper78, A.M. Cooper-Sarkar120, K. Copic15, T. Cornelissen176, M. Corradi20a, F. Corriveau87,k, A. Corso-Radu164, A. Cortes-Gonzalez12, G. Cortiana101, G. Costa91a, M.J. Costa168, D. Costanzo140, D. Côté8, G. Cottin28, G. Cowan77,
B.E. Cox84, K. Cranmer110, G. Cree29, S. Crépé-Renaudin55, F. Crescioli80, W.A. Cribbs147a,147b, M. Crispin Ortuzar120, M. Cristinziani21, V. Croft106, G. Crosetti37a,37b, T. Cuhadar Donszelmann140, J. Cummings177, M. Curatolo47, C. Cuthbert151, H. Czirr142, P. Czodrowski3, S. D’Auria53,
M. D’Onofrio74, M.J. Da Cunha Sargedas De Sousa126a,126b, C. Da Via84, W. Dabrowski38a, A. Dafinca120, T. Dai89, O. Dale14, F. Dallaire95, C. Dallapiccola86, M. Dam36, J.R. Dandoy31, A.C. Daniells18, M. Danninger169, M. Dano Hoffmann137, V. Dao48, G. Darbo50a, S. Darmora8,
J. Dassoulas3, A. Dattagupta61, W. Davey21, C. David170, T. Davidek129, E. Davies120,l, M. Davies154, P. Davison78, Y. Davygora58a, E. Dawe88, I. Dawson140, R.K. Daya-Ishmukhametova86, K. De8,
R. de Asmundis104a, S. De Castro20a,20b, S. De Cecco80, N. De Groot106, P. de Jong107, H. De la Torre82, F. De Lorenzi64, L. De Nooij107, D. De Pedis133a, A. De Salvo133a, U. De Sanctis150, A. De Santo150, J.B. De Vivie De Regie117, W.J. Dearnaley72, R. Debbe25, C. Debenedetti138, D.V. Dedovich65, I. Deigaard107, J. Del Peso82, T. Del Prete124a,124b, D. Delgove117, F. Deliot137, C.M. Delitzsch49, M. Deliyergiyev75, A. Dell’Acqua30, L. Dell’Asta22, M. Dell’Orso124a,124b, M. Della Pietra104a, j, D. della Volpe49, M. Delmastro5, P.A. Delsart55, C. Deluca107, D.A. DeMarco159, S. Demers177, M. Demichev65, A. Demilly80, S.P. Denisov130, D. Derendarz39, J.E. Derkaoui136d, F. Derue80, P. Dervan74, K. Desch21, C. Deterre42, P.O. Deviveiros30, A. Dewhurst131, S. Dhaliwal107,
A. Di Ciaccio134a,134b, L. Di Ciaccio5, A. Di Domenico133a,133b, C. Di Donato104a,104b, A. Di Girolamo30, B. Di Girolamo30, A. Di Mattia153, B. Di Micco135a,135b, R. Di Nardo47, A. Di Simone48, R. Di Sipio159, D. Di Valentino29, C. Diaconu85, M. Diamond159, F.A. Dias46, M.A. Diaz32a, E.B. Diehl89, J. Dietrich16, S. Diglio85, A. Dimitrievska13, J. Dingfelder21, F. Dittus30, F. Djama85, T. Djobava51b, J.I. Djuvsland58a, M.A.B. do Vale24c, D. Dobos30, M. Dobre26a, C. Doglioni49, T. Dohmae156, J. Dolejsi129, Z. Dolezal129, B.A. Dolgoshein98,∗, M. Donadelli24d, S. Donati124a,124b, P. Dondero121a,121b, J. Donini34, J. Dopke131, A. Doria104a, M.T. Dova71, A.T. Doyle53, E. Drechsler54, M. Dris10, E. Dubreuil34, E. Duchovni173, G. Duckeck100, O.A. Ducu26a,85, D. Duda176, A. Dudarev30, L. Duflot117, L. Duguid77, M. Dührssen30, M. Dunford58a, H. Duran Yildiz4a, M. Düren52, A. Durglishvili51b, D. Duschinger44, M. Dwuznik38a, M. Dyndal38a, C. Eckardt42, K.M. Ecker101, W. Edson2, N.C. Edwards46, W. Ehrenfeld21, T. Eifert30, G. Eigen14, K. Einsweiler15, T. Ekelof167, M. El Kacimi136c, M. Ellert167, S. Elles5, F. Ellinghaus83, A.A. Elliot170, N. Ellis30, J. Elmsheuser100, M. Elsing30, D. Emeliyanov131, Y. Enari156, O.C. Endner83, M. Endo118, R. Engelmann149, J. Erdmann43, A. Ereditato17, D. Eriksson147a, G. Ernis176, J. Ernst2, M. Ernst25, S. Errede166, E. Ertel83, M. Escalier117, H. Esch43, C. Escobar125, B. Esposito47, A.I. Etienvre137, E. Etzion154, H. Evans61, A. Ezhilov123, L. Fabbri20a,20b, G. Facini31,
R.M. Fakhrutdinov130, S. Falciano133a, R.J. Falla78, J. Faltova129, Y. Fang33a, M. Fanti91a,91b, A. Farbin8, A. Farilla135a, T. Farooque12, S. Farrell15, S.M. Farrington171, P. Farthouat30, F. Fassi136e, P. Fassnacht30, D. Fassouliotis9, A. Favareto50a,50b, L. Fayard117, P. Federic145a, O.L. Fedin123,m, W. Fedorko169,
S. Feigl30, L. Feligioni85, C. Feng33d, E.J. Feng6, H. Feng89, A.B. Fenyuk130, P. Fernandez Martinez168, S. Fernandez Perez30, S. Ferrag53, J. Ferrando53, A. Ferrari167, P. Ferrari107, R. Ferrari121a,
D.E. Ferreira de Lima53, A. Ferrer168, D. Ferrere49, C. Ferretti89, A. Ferretto Parodi50a,50b, M. Fiascaris31, F. Fiedler83, A. Filipˇciˇc75, M. Filipuzzi42, F. Filthaut106, M. Fincke-Keeler170, K.D. Finelli151, M.C.N. Fiolhais126a,126c, L. Fiorini168, A. Firan40, A. Fischer2, C. Fischer12, J. Fischer176, W.C. Fisher90, E.A. Fitzgerald23, M. Flechl48, I. Fleck142, P. Fleischmann89,
S. Fleischmann176, G.T. Fletcher140, G. Fletcher76, T. Flick176, A. Floderus81, L.R. Flores Castillo60a, M.J. Flowerdew101, A. Formica137, A. Forti84, D. Fournier117, H. Fox72, S. Fracchia12, P. Francavilla80, M. Franchini20a,20b, D. Francis30, L. Franconi119, M. Franklin57, M. Fraternali121a,121b, D. Freeborn78, S.T. French28, F. Friedrich44, D. Froidevaux30, J.A. Frost120, C. Fukunaga157, E. Fullana Torregrosa83, B.G. Fulsom144, J. Fuster168, C. Gabaldon55, O. Gabizon176, A. Gabrielli20a,20b, A. Gabrielli133a,133b, S. Gadatsch107, S. Gadomski49, G. Gagliardi50a,50b, P. Gagnon61, C. Galea106, B. Galhardo126a,126c, E.J. Gallas120, B.J. Gallop131, P. Gallus128, G. Galster36, K.K. Gan111, J. Gao33b,85, Y. Gao46, Y.S. Gao144,e, F.M. Garay Walls46, F. Garberson177, C. García168, J.E. García Navarro168,
M. Garcia-Sciveres15, R.W. Gardner31, N. Garelli144, V. Garonne119, C. Gatti47, A. Gaudiello50a,50b, G. Gaudio121a, B. Gaur142, L. Gauthier95, P. Gauzzi133a,133b, I.L. Gavrilenko96, C. Gay169, G. Gaycken21, E.N. Gazis10, P. Ge33d, Z. Gecse169, C.N.P. Gee131, D.A.A. Geerts107, Ch. Geich-Gimbel21,
M.P. Geisler58a, C. Gemme50a, M.H. Genest55, S. Gentile133a,133b, M. George54, S. George77, D. Gerbaudo164, A. Gershon154, H. Ghazlane136b, N. Ghodbane34, B. Giacobbe20a, S. Giagu133a,133b,
