least four jets, the W+jets background still contributes the number of events competitive to the number of signal events after event selection.
However, using the information of b-tagging, e.g. the requirement ofat least one b-tagged jet, it is more effectively eliminated, since the crosssectionof the W+bb+jets with at least 2 additional partons is much smaller, about 3 pb. It is found that c-jet can be misidentified as b-tagged jet. Therefore the W+cc+jets and W+c+jets backgrounds are also considered as contamination to b-tagged sample. The W+cc+jets and W+c+jets events have been included in the W+jets MC samples in Appendix A in Table A.3. The crosssectionof W+cc+jets and W+c+jets is 1.8 nb with inclusive number of partons and it is reduced to 90 pb with at least 2 additional partons. The content and crosssectionof W+cc+jets and W+c+jets are detailed in the note . In Figure 4.2 the leading order Feynman diagrams of W+c and W+cc processes are shown. The W+bb process has the similar diagram, while the corresponding W+b is highly suppressed in the theory of SM. Inclusive W+jets, including contributions from W+jets with heavy flavors, is the dominant background to the signal. Therefore, it is measured using data driven methods, which is detailed in Chapter 5.
A search is performed for top-quark pairs (t¯ t) produced together with a photon (γ) with transverse energy greater than 20 GeV using a sample of t¯ t candidate events in final states with jets, missing transverse momentum, and one isolated electron or muon. The dataset used corresponds to an integrated luminosity of 4.59 fb −1 of proton–proton collisions at a center-of-mass energy of7TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. In total 140 and 222 t¯ tγ candidate events are observed in the electron and muon channels, to be compared to the expectation of 79 ± 26 and 120 ± 39 non-t¯ tγ background events respectively. The productionof t¯ tγ events is observed with a significance of 5.3 standard deviations away from the null hypothesis. The t¯ tγ productioncrosssection times the branching ratio (BR) of the single-lepton decay channel is measured in a fiducial kinematic region within the ATLAS acceptance. The measured value is σ fid
eration; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, 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.
incorporating three large superconducting toroid magnets (each with eight coils). The in- ner detector (ID) is immersed in a 2 T axial magnetic field and provides charged-particle tracking in the pseudorapidity 2 range |η| < 2.5. The high-granularity silicon pixel detector covers the vertex region and typically provides three measurements per track, the first hit being normally in the innermost layer. It is followed by the silicon microstrip tracker, which provides typically eight measurements (four space-points) per track. These silicon detectors are complemented by the transition radiation tracker, which covers a region up to |η| = 2.0. The transition radiation tracker also provides electron identification information based on the fraction of hits above a high energy-deposit threshold corresponding to transition radi- ation. The calorimeter system covers the pseudorapidity range |η| < 4.9. Within the region |η| < 3.2, electromagnetic calorimetry is provided by barrel and endcap high-granularity lead/liquid-argon (LAr) calorimeters. An additional thin LAr presampler covers |η| < 1.8 to correct for energy loss in material upstream of the calorimeters. Hadronic calorimetry is provided by a steel/scintillating-tile calorimeter, segmented radially into three barrel struc- tures within |η| < 1.7, and two copper/LAr hadronic endcap calorimeters, that cover the region 1.5 < |η| < 3.2. The solid angle coverage is completed in the region of 3.1 < |η| < 4.9 with forward copper/LAr and tungsten/LAr calorimeter modules optimized for electromag- netic and hadronic measurements respectively. The muon spectrometer (MS) comprises separate trigger and high-precision tracking chambers measuring the deflection of muons in a magnetic field generated by superconducting air-core toroids. The precision chamber system covers the region |η| < 2.7 with three layers of monitored drift tubes, complemented by cathode strip chambers in the forward region, where the background is highest. The muon trigger system covers the range |η| < 2.4 with resistive plate chambers in the barrel, and thin gap chambers in the endcap regions. A three-level trigger system is used to select interesting events. The Level-1 trigger is implemented in hardware and uses a subset of detector information to reduce the event rate to a design value ofat most 75 kHz. This is followed by two software-based trigger levels which together reduce the event rate to about 400 Hz.
The gluon component of the fake τ leptons is charge symmet- ric; therefore it is expected to have the same shape in SS events as in OS events and should contribute the same number of fake
τ leptons in each sample. The contribution of fake τ leptons from gluons can be removed by subtracting the distribution of any quantity for SS events from the corresponding distribution for OS events. The multi-jet background also cancels, as can be seen in Table 1. The resulting distributions are labeled OS- SS. Similarly, since each sample is expected to have an almost equal contribution from b-jets and b-jets, the small b-jet com- ponent should also be removed by OS-SS (asymmetric single b production is negligible compared to bb production). The only jet types remaining in the OS-SS distributions are light-quark
We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Aus- tralia; BMWF, 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 and ERC, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNAS, Georgia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; RCN, Norway; MNiSW, Poland; GRICES and FCT, Portugal; MERYS (MECTS), Romania; MES of Russia and ROSATOM, Russian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, 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.
We acknowledge the support of ANPCyT, Ar- gentina; YerPhI, Armenia; ARC, Australia; BMWF, 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 and ERC, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNAS, Georgia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Mo- rocco; FOM and NWO, Netherlands; RCN, Norway; MNiSW, Poland; GRICES and FCT, Portugal; MERYS (MECTS), Romania; MES of Russia and ROSATOM, Russian Federation; JINR; MSTD, Serbia; MSSR, Slo- vakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, Spain; SRC and Wallenberg Foun- dation, 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.
A.M. Nairz 29 , Y. Nakahama 29 , K. Nakamura 155 , I. Nakano 110 , G. Nanava 20 , A. Napier 161 , M. Nash 77,c , N.R. Nation 21 , T. Nattermann 20 , T. Naumann 41 , G. Navarro 162 , H.A. Neal 87 , E. Nebot 80 , P.Yu. Nechaeva 94 , A. Negri 119a,119b , G. Negri 29 , S. Nektarijevic 49 , S. Nelson 143 , T.K. Nelson 143 , S. Nemecek 125 , P. Nemethy 108 , A.A. Nepomuceno 23a , M. Nessi 29,t , S.Y. Nesterov 121 , M.S. Neubauer 165 , A. Neusiedl 81 , R.M. Neves 108 , P. Nevski 24 , P.R. Newman 17 , V. Nguyen Thi Hong 136 , R.B. Nickerson 118 , R. Nicolaidou 136 , L. Nicolas 139 , B. Nicquevert 29 , F. Niedercorn 115 , J. Nielsen 137 , T. Niinikoski 29 , N. Nikiforou 34 , A. Nikiforov 15 , V. Nikolaenko 128 , K. Nikolaev 65 , I. Nikolic-Audit 78 , K. Nikolics 49 , K. Nikolopoulos 24 , H. Nilsen 48 , P. Nilsson 7 , Y. Ninomiya 155 , A. Nisati 132a , T. Nishiyama 67 , R. Nisius 99 , L. Nodulman 5 , M. Nomachi 116 , I. Nomidis 154 , M. Nordberg 29 , B. Nordkvist 146a,146b , P.R. Norton 129 , J. Novakova 126 , M. Nozaki 66 , M. Noˇziˇcka 41 , L. Nozka 113 , I.M. Nugent 159a , A.-E. Nuncio-Quiroz 20 , G. Nunes Hanninger 86 ,
M. Livan 123a,123b , A. Lleres 58 , J. Llorente Merino 35a , S.L. Lloyd 79 , F. Lo Sterzo 153 , E.M. Lobodzinska 45 , P. Loch 7 , F.K. Loebinger 87 , K.M. Loew 25 , A. Loginov 179,∗ , T. Lohse 17 , K. Lohwasser 45 ,
M. Lokajicek 129 , B.A. Long 24 , J.D. Long 169 , R.E. Long 75 , L. Longo 76a,76b , K.A. Looper 113 ,
J.A. Lopez 34b , D. Lopez Mateos 59 , B. Lopez Paredes 141 , I. Lopez Paz 13 , A. Lopez Solis 83 , J. Lorenz 102 , N. Lorenzo Martinez 64 , M. Losada 21 , P.J. Lösel 102 , X. Lou 35a , A. Lounis 119 , J. Love 6 , P.A. Love 75 , H. Lu 62a , N. Lu 92 , H.J. Lubatti 140 , C. Luci 134a,134b , A. Lucotte 58 , C. Luedtke 51 , F. Luehring 64 , W. Lukas 65 , L. Luminari 134a , O. Lundberg 148a,148b , B. Lund-Jensen 149 , P.M. Luzi 83 , D. Lynn 27 , R. Lysak 129 , E. Lytken 84 , V. Lyubushkin 68 , H. Ma 27 , L.L. Ma 36b , Y. Ma 36b , G. Maccarrone 50 , A. Macchiolo 103 , C.M. Macdonald 141 , B. Maˇcek 78 , J. Machado Miguens 124,128b , D. Madaffari 88 , R. Madar 37 , H.J. Maddocks 168 , W.F. Mader 47 , A. Madsen 45 , J. Maeda 70 , S. Maeland 15 , T. Maeno 27 , A. Maevskiy 101 , E. Magradze 57 , J. Mahlstedt 109 , C. Maiani 119 , C. Maidantchik 26a , A.A. Maier 103 , T. Maier 102 , A. Maio 128a,128b,128d , S. Majewski 118 , Y. Makida 69 , N. Makovec 119 , B. Malaescu 83 , Pa. Malecki 42 , V.P. Maleev 125 , F. Malek 58 , U. Mallik 66 , D. Malon 6 , C. Malone 30 , S. Maltezos 10 , S. Malyukov 32 , J. Mamuzic 170 , G. Mancini 50 , L. Mandelli 94a , I. Mandi´c 78 , J. Maneira 128a,128b , L. Manhaes de Andrade Filho 26b , J. Manjarres Ramos 163b , A. Mann 102 , A. Manousos 32 ,
``νν + X are selected by requiring two reconstructed oppositely-charged leptons and a large transverse mo- mentum imbalance due to the neutrinos, which escape detection. There are four main backgrounds, all of com- parable size: (i) W +jetsproduction with a jet misidenti- fied as a lepton; (ii) Drell-Yan production, which includes Z/γ ∗ → `` where the observed momentum imbalance is due to mismeasurements and Z/γ ∗ → τ τ → `` + 4ν; (iii) topproduction (t¯ t and W t), which also produces two W bosons, but is not considered signal and is suppressed by vetoing candidates with jets; (iv) other diboson pro- cesses, which include W Z production decaying to ```ν where one charged lepton is lost, ZZ with one Z decay- ing to charged leptons and one Z decaying to neutrinos, and W γ with the photon misidentified as an electron.
This Letter presents measurements of the W ± Z productioncrosssectionin pp collisions at a centre-of- mass energy of √ s = 13 TeV. The data sample analysed was collected in 2015 by the ATLAS experiment at the LHC, and corresponds to an integrated luminosity of 3.2 fb −1 . The W and Z bosons are reconstruc- ted using their decay modes into electrons or muons. The inclusive productioncrosssection is measured in a fiducial phase space and extrapolated to the total phase space. This Letter also reports the ratio of the cross sections at 13 TeV and 8 TeV [ 4 ], as well as the ratio of the W + Z /W − Z cross sections, which is sensitive to the parton distribution functions (PDF). Finally, the productioncrosssection is also measured as a function of the jet multiplicity. This distribution provides an important test of perturbative quantum chromodynamics (QCD) for diboson production processes. The W ± Z diboson process is particularly well suited for this measurement, since the WW final state has a very large background from top-quark produc- tion when associated jets are present, and the ZZ final state has substantially fewer events. The reported measurements are compared with the SM cross-section predictions at the next-to-leading order (NLO) in QCD [ 5 , 6 ] and the total crosssection is also compared to a very recent calculation at next-to-next-to- leading order (NNLO) in QCD [ 7 ].
FIG. 12: Differential cross sections multiplied by the di-muon
branching fraction, d 2 σ/dp
T dy × Br(Υ → µ + µ − ), for Υ (1S)
production extrapolated to the full phase space for (top) cen- tral and (bottom) forward rapidities. Points with error bars indicate results of the measurements with total (statistical and systematic) uncertainties. The maximal envelope of vari- ation of the result due to spin-alignment uncertainty is indi- cated by the solid band. Also shown are predictions of direct production with the NNLO* Color Singlet Mechanism (CSM) and inclusive predictions from the Color Evaporation Model (CEM). These theory predictions are shown as a ratio to the data in the lower panes for CEM (middle) and CSM (bottom), along with detail of the variations of the cross-section mea- surement under the four anisotropic spin-alignment scenarios as a ratio to the nominal data.
Measurements of differential cross-sections intop quark pair events require full kinematic reconstruction of the t ¯t system. The reconstruction is performed using a likelihood fit of the measured objects to a theoretical leading-order representa- tion of the t ¯t decay. The same reconstruction method as in Ref.  is used. The likelihood is the product of three fac- tors. The first factor is the product of Breit-Wigner distribu- tions for the productionof W bosons and top quarks, given the four-momenta of the true t ¯t decay products. The sec- ond factor is the product of transfer functions representing the probabilities for the given true energies of the t ¯t decay products to be observed as the energies of reconstructed jets, leptons and as missing transverse energy. The third factor is the probability to b-tag a certain jet, given the parton it is associated with. The pole masses of the W bosons and the top quarks in the Breit-Wigner distributions are set to 80.4 GeV and 172.5 GeV, respectively.
S. Cheatham 167a,167c , A. Chegwidden 93 , S. Chekanov 6 , S.V. Chekulaev 163a , G.A. Chelkov 68,k ,
M.A. Chelstowska 92 , C. Chen 67 , H. Chen 27 , K. Chen 150 , S. Chen 35b , S. Chen 157 , X. Chen 35c , Y. Chen 70 , H.C. Cheng 92 , H.J Cheng 35a , Y. Cheng 33 , A. Cheplakov 68 , E. Cheremushkina 132 ,
R. Cherkaoui El Moursli 137e , V. Chernyatin 27,∗ , E. Cheu 7 , L. Chevalier 138 , V. Chiarella 50 , G. Chiarelli 126a,126b , G. Chiodini 76a , A.S. Chisholm 32 , A. Chitan 28b , M.V. Chizhov 68 , K. Choi 64 , A.R. Chomont 37 , S. Chouridou 9 , B.K.B. Chow 102 , V. Christodoulou 81 , D. Chromek-Burckhart 32 , J. Chudoba 129 , A.J. Chuinard 90 , J.J. Chwastowski 42 , L. Chytka 117 , G. Ciapetti 134a,134b , A.K. Ciftci 4a , D. Cinca 46 , V. Cindro 78 , I.A. Cioara 23 , C. Ciocca 22a,22b , A. Ciocio 16 , F. Cirotto 106a,106b , Z.H. Citron 175 , M. Citterio 94a , M. Ciubancan 28b , A. Clark 52 , B.L. Clark 59 , M.R. Clark 38 , P.J. Clark 49 , R.N. Clarke 16 , C. Clement 148a,148b , Y. Coadou 88 , M. Cobal 167a,167c , A. Coccaro 52 , J. Cochran 67 , L. Colasurdo 108 , B. Cole 38 , A.P. Colijn 109 , J. Collot 58 , T. Colombo 166 , G. Compostella 103 , P. Conde Muiño 128a,128b , E. Coniavitis 51 , S.H. Connell 147b , I.A. Connelly 80 , V. Consorti 51 , S. Constantinescu 28b , G. Conti 32 , F. Conventi 106a,l , M. Cooke 16 , B.D. Cooper 81 , A.M. Cooper-Sarkar 122 , F. Cormier 171 , K.J.R. Cormier 161 , T. Cornelissen 178 , M. Corradi 134a,134b , F. Corriveau 90,m , A. Cortes-Gonzalez 32 , G. Cortiana 103 ,
due to final states with non-prompt muons, such as those from semileptonic b- or c-hadron decays. The multijet background normalisation and shape are estimated from data using the “Matrix Method” (MM) technique.
The MM exploits differences in the properties used for lepton identification between prompt, isolated leptons from W and Z boson decays (referred to as “real leptons”) and those where the leptons are either non-isolated or result from the misidentification of photons or jets (referred to as “fake leptons”). For this purpose, two samples are defined after imposing the event selection described in Sect. 4 , differing only in the lepton identification criteria: a “tight” sample and a “loose” sample, the former being a subset of the latter. The tight selection employs the final lepton identification criteria used in the analysis. For the loose selection, the lepton isolation requirements are omitted for both the muon and electron channels, and the quality requirements are also loosened for the electron channel. The method assumes that the number of selected events in each sample (N loose and N tight ) can be expressed as a linear combination of the numbers
2 ATLAS detector
ATLAS is a multipurpose detector [ 14 ] that provides nearly full solid angle 1 coverage around the inter- action point. This analysis exploits all major components of the detector. Charged-particle trajectories with pseudorapidity |η| < 2.5 are reconstructed in the inner detector, which comprises a silicon pixel de- tector, a silicon microstrip detector and a transition radiation tracker (TRT). The innermost pixel layer, the insertable B-layer [ 15 ], was added before the start of the 13 TeV LHC operation, at a radius of 33 mm around a new, thinner beam pipe. The inner detector is embedded in a 2 T axial magnetic field, allowing precise measurementof charged-particle momenta. Sampling calorimeters with several di fferent designs span the pseudorapidity range up to |η| = 4.9. High-granularity liquid argon (LAr) electromagnetic (EM) calorimeters are used up to |η| = 3.2. Hadronic calorimeters based on scintillator-tile active material cover |η| < 1.7 while LAr technology is used for hadronic calorimetry in the region 1.5 < |η| < 4.9. The calorimeters are surrounded by a muon spectrometer within a magnetic field provided by air-core toroid magnets with a bending integral of about 2.5 Tm in the barrel and up to 6 Tm in the end-caps. Three layers of precision drift tubes and cathode-strip chambers provide an accurate measurementof the muon track curvature in the region |η| < 2.7. Resistive-plate and thin-gap chambers provide muon triggering capability up to |η| = 2.4.
(Received 30 June 2014; published 11 December 2014)
This article presents measurements of the t-channel single top-quark (t) and top-antiquark (¯t) total productioncross sections σðtqÞ and σð¯tqÞ, their ratio R t ¼ σðtqÞ=σð¯tqÞ, and a measurementof the inclusive productioncrosssection σðtq þ ¯tqÞ in proton-proton collisions at p ﬃﬃﬃ s ¼ 7TeVat the LHC. Differential cross sections for the tq and ¯tq processes are measured as a function of the transverse momentum and the absolute value of the rapidity of t and ¯t, respectively. The analyzed data set was recorded with the ATLAS detector and corresponds to an integrated luminosity of 4.59 fb −1 . Selected events contain one charged lepton, large missing transverse momentum, and two or three jets. The cross sections are measured by performing a binned maximum-likelihood fit to the output distributions of neural networks. The resulting measurements are σðtqÞ ¼ 46 1ðstatÞ 6ðsystÞ pb, σð¯tqÞ ¼ 23 1ðstatÞ 3ðsystÞ pb, R t ¼ 2.04 0.13ðstatÞ 0.12ðsystÞ, and σðtq þ ¯tqÞ ¼ 68 2ðstatÞ 8ðsystÞ pb, consistent with the Standard Model expectation. The uncertainty on the measured cross sections is dominated by systematic uncertainties, while the uncertainty on R t is mainly statistical. Using the ratio of σðtq þ ¯tqÞ to its theoretical prediction, and assuming that the top-quark-related CKM matrix elements obey the relation jV tb j ≫ jV ts j; jV td j, we determine jV tb j ¼ 1.02 0.07.
The cross sections as a function of the azimuthal angle and of the η–φ distance between the jets are shown in Figs. 5 and 6 , respectively. In these figures, the region at high angular separation is where the flavour-creation process is expected to dominate. This is visible in the peaks at ∆φ ∼ π and ∆R ∼ 3. The NLO predictions are above the data in Fig. 5 for low ∆φ values, where the b¯b pair is more likely produced together with at least one other jet. They reproduce well the shape of the data distribution for ∆φ & 1, but underestimate the crosssection by a factor two in the same region. Good agreement between data and simulation with LO generators is seen. The di fferential crosssection as a function of ∆R, shown in Fig. 6 , is well reproduced by Powheg. The ratio of MC@NLO predictions to the data is ∼ 0.5 for ∆R . 2, and is above the data in the intermediate ∆R region. The LO predictions do not show strong deviations from data apart from an excess for ∆R values below ∼ 0.7.
7.4 Non-perturbative corrections
The N jetti , BlackHat+Sherpa, and MCFM results do not include non-perturbative effects from hadronisation
and the underlying event. These corrections are computed for each bin with Sherpa 2.2.1 [ 37 ] combining matrix element calculations with up to two parton emissions at LO in pQCD. The calculation uses the NNPDF 3.0 PDF set and dynamic renormalisation and factorisation scales determined by the CKKW scale-setting procedure. The corrections are typically around 2–3% and are applied to the predictions for all measured distributions. Statistical uncertainties in these corrections and the systematic uncertainty, defined by the envelope of variations of the starting scale of the parton shower, the recoil scheme, the mode of shower evolution and the number of emitted partons from the matrix element, are included in the respective theory uncertainties. For the W + /W − predictions, no non-perturbative corrections are required as these effects cancel out in the ratio. The impact of QED radiation, which is considered as part of the dressed-electron definition in the measured cross sections, on the parton-level theoretical predictions is investigated using Sherpa 2.2.1 with the same set-up as the NLO Sherpa predictions described above and found to be very small. No correction for this effect is applied.