The search for new physics in top quark decays at the LHC is reviewed in this paper. Results from ATLAS [1] and CMS [2] experiments on top quark decays within the Standard Model are presented together with the measurements of the W boson polarizations and the study of the structure of the Wtb vertex. As a natural step forward, the experimental status on measurements sensitive to top quark couplings to gauge bosons (γ, Z, W and H) is reviewed as well as possible top quark decays Beyond the Standard Model (MSSM and FCNC).(undefined)info:eu-repo/semantics/publishedVersio

Onofre, A.

arXiv

The search for new physics in top quark decays at the LHC is reviewed in this paper. Results from ATLAS and CMS experiments on top quark decays within the Standard Model are presented together with the measurements of the W boson polarizations and the study of the structure of the Wtb vertex. As a natural step forward, the experimental status on measurements sensitive to top quark couplings to gauge bosons (\gamma, Z, W and H) is reviewed as well as possible top quark decays Beyond the Standard Model (MSSM and FCNC).The search for new physics in top quark decays at the LHC is reviewed in this paper. Results from ATLAS and CMS experiments on top quark decays within the Standard Model are presented together with the measurements of the W boson polarizations and the study of the structure of the Wtb vertex. As a natural step forward, the experimental status on measurements sensitive to top quark couplings to gauge bosons (\gamma, Z, W and H) is reviewed as well as possible top quark decays Beyond the Standard Model (MSSM and FCNC)

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Searches for new physics in top decays at the LHC

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arXiv:1212.6898v1  [hep-ex]  31 Dec 2012Searches for new physics in top decays at the LHCAntonio Onofre , On behalf of the ATLAS and CMS CollaborationsDepartamento de F´ısica,Universidade do MinhoCampus de Gualtar,4710 - 057 Braga, Portugal.Proceedings of CKM 2012, the 7th International Workshop on the CKM Unitarity Triangle, Univer-sity of Cincinnati, USA, 28 September - 2 October 20121 AbstractThe search for new physics in top quark decays at the LHC is reviewed in this paper. Results fromATLAS [1] and CMS [2] experiments on top quark decays within the Standard Model are presentedtogether with the measurements of the W boson polarizations and the study of the structure of theWtb vertex. As a natural step forward, the experimental status on measurements sensitive to topquark couplings to gauge bosons (γ, Z, W and H) is reviewed as well as possible top quark decaysBeyond the Standard Model (MSSM and FCNC).2 IntroductionThe top quark is the heaviest known quark (mt = 173.18 ± 0.94 GeV [3]) ever discovered. Witha charge of +2/3|e| (e is the electron charge) and spin 1/2, the top quark completes the 3 familystructure of the Standard Model (SM). One of the stricking features of this quark is preciselythe value of its mass, so distinctive from the other fermions, which may suggest it plays a morefundamental role in the eletroweak symmetry breaking mechanism. In the SM, top quarks areexpected to decay mostly through t → bW , being the branching ratios Br(t → sW ) ≤ 0.18% andBr(t → dW ) ≤ 0.02% diminut when compared with the dominant decay channel [4]. The lifetime,at least one order of magnitude smaller than the typical hadronization time scale O(10−23) s, wasmeasured to be τt = (3.29+0.90−0.67) × 10−25 s [5]. This implies top quarks decay before hadronizationmay take place and spoil the spin information propagated to the final state decay products. Onceagain, top quarks show remarkable distintive features when compared to the other quarks. At theLHC, top quarks are mainly produced in pairs through gluon fusion or quark anti-quark annihilationwith a predicted cross section of 164.6+11.4−15.7 pb (approx. NNLO) at 7 TeV [6]. There is howevera significant number of top quarks which are produced singly through the t-channel (gq′ → qtb),associated production (bg → Wt) and s-channel (qq′ → tb) single top production. The productioncross sections are, respectively, 64.6+2.7−2.0 pb [7] (NNLO+soft gluon corrections), 15.7 ± 1.1 pb [8](approx. NNLO) and 4.6± 0.2 pb [9] (NNLL resummation). In this paper the experimental statusof top quark measurements sensitive to gauge bosons couplings (W, γ, Z and H) and decays beyondthe SM are reviewed. As most analysis described in the paper rely on the lepton+jets and dileptonfinal states, or variations from these, it is appropriate to shortly mention them.In the lepton+jets channel, one isolated lepton (e or µ) is required in the event selection as wellas at least 4 jets for both ATLAS [10, 11] and CMS [12, 13]. While for muons, the transverse mo-mentum (pT) selection is similar for both experiments, selected electrons at ATLAS are harder (with1transverse energy, ET, above 25 GeV) than in CMS (ET > 20 GeV). Softer jets, with pT > 25 GeV,are also selected by ATLAS compared to CMS (pT > 30 GeV). The use of a transverse missingenergy (EmissT ) cut together with the requirement that at least one jet had to be tagged as a b-jetwas performed by ATLAS while CMS required at least two b-tagged jets with no EmissT condition.In the dilepton channel, events with two isolated leptons (e or µ) and two jets, from which atleast one is b-tagged, are selected by both CMS [14, 13] and ATLAS [15]. While at least 2 oppositecharge leptons are requested by CMS, exactly 1 pair is imposed by ATLAS. In order to reducepotential contributions from the Z+jets background, events are required to have the invariant massof the two opposite charge leptons (mℓ+ℓ−) outside a mass window of at least 10 GeV around thecentral value of the Z mass. Both CMS and ATLAS impose EmissT cuts to events and ATLAS, forthe e±µ∓, requires the scalar sum of jets and leptons transverse energies (HT) to be above 130 GeV.3 The Wtb vertex structureIn the SM, the Wtb vertex has a (V −A) structure where, V and A, are the vector and axial-vectorcontributions to the vertex. One way of probing the structure of the vertex, is to study top quarkdecays to W -bosons and b-quarks (t → bW ). The W bosons produced in these decays can havelongitudinal, left-handed or right-handed polarizations with fractions F0, FL and FR, respectivelyequal to 0.687 ± 0.005, 0.311 ± 0.005 and 0.0017 ± 0.0001 at NNLO [16], in the SM. The helicityfractions were extracted from a direct fit to the top quark decay products angular distribution,cos θ, using templates from simulation. Alternatively, angular asymmetries (A+, A− and AFB),built from cos θ, were also used by ATLAS [17]. The angle θ is measured between the momen-tum direction of the charged lepton from the decay of the W boson, and the reversed momentumdirection of the b-quark from the decay of the top quark, both boosted into the W boson restframe. CMS [18] measured the helicity fractions to be F0 = 0.567±0.074(stat)±0.047(syst), FL =0.393±0.045(stat)±0.029(syst) and FR = 0.040±0.035(stat)±0.044(syst). The correspondent mea-surements from ATLAS [19] are, F0 = 0.67±0.03(stat)±0.06(syst), FL = 0.32±0.02(stat)±0.03(syst)and FR = 0.01±0.01(stat)±0.04(syst). All these measurements are consistent with SM expectations.The measured fractions were used to probe the existence of anomalous Wtb couplings. Exclusionlimits on the real components of the anomalous couplings gL and gR were set at 68% and 95% CL(Figure 1). The region of gR around 0.8 in the ATLAS plot is disfavored by the current experimentalmeasurements on the single top quark production cross sections at Tevatron and LHC. Once thelimits depend on Vtb, it is worth mentionning the current experimental status of these measurements(Table 1) which assume the SM and negligeable decays to t→ sW and t→ dW , when compared tothe dominant channel. The current Vtb measurements precision is at the level of 5%-10%.4 Top quark couplings to gauge bosonsFollowing the discussion on the Wtb vertex and Vtb, it is natural to ask what is the current LHCstatus on the measurements of the couplings of the top quarks to other bosons (γ,Z,W and H).Several dedicated analyses were developed by both ATLAS and CMS, relying to a great extenton the lepton+jets and dilepton channels discussed previously, with the exception that activityfrom the gauge bosons is expected to be present as well in the events. These are particularlychallenging channels which should be improved as both CMS and ATLAS collect more data overtime. ATLAS [20] looked into ttγ production and, by fitting the data with templates for promptphotons (well isolated) and hadron fakes, measured the cross section to σ(ttγ) = 2.0± 0.5 (stat)±0.7 (syst)±0.08 (lumi.) pb, for events with photons with pT >8 GeV for which the expected SM crosssection is 2.1±0.4 pb (at 7 TeV). For the ttZ channel ATLAS [21] observes 1 candidate after event2Figure 1: CMS(left) and ATLAS(right) allowed regions, at 68% and 95% CL, on the anomalouscoupling plane (gL,gR), for VR=0 and VL=1.selection against a background of 0.28+1.57−0.14. A 95%CL limit of 0.71 pb was set on the ttZ production(which compares with a SM prediction of 0.14 pb). CMS [22] searched, in two independent channels(three and two same charge leptons), for ttV (V =W,Z). While the ttV combined cross section wasmeasured to be σ(ttV ) = 0.51+0.15−0.13(stat)+0.05−0.04(syst) pb at 7 TeV, the ttZ cross section (obtainedfrom the trilepton channel) was σ(ttZ) = 0.30+0.14−0.11(stat)+0.04−0.02(syst) pb (Figure 2).Both ATLAS [23] and CMS [24] searched for ttH production using data collected at 7 TeV witha total integrated luminosity of 5 fb−1. While ATLAS studied the single lepton channel, where theHiggs boson is assumed to decay to a pair of b-quarks, CMS looked into both the single and dileptonchannels. As no evidence for signal was found, combined upper limits, at 95% CL, were set oncross section times branching ratio by ATLAS and CMS to σttH× Br(H → bb) ≤13.1 pb (10.5 pbexpected), σttH×Br(H → bb) ≤3.8 pb (4.6 pb expected), respectively.5 Top quark decays beyond the SMIn the pursuit of new physics in the top quark sector, the decays play a fundamental role. Not onlythey may reveal violation of symmetries naturaly observed in the SU(3)×SU(2)×U(1) gauge theoryof strong and electroweak interactions (which do not require introducing global separate conservationATLAS Measurement (SM)t-channel (@ 7TeV) |Vtb| = 1.13+0.14−0.13(arXiv:1205.3130) |Vtb| ≥0.75 @ 95% CL (|Vtb in [0,1])]t-channel (@ 8TeV,5.8 fb−1) |Vtb| = 1.04+0.10−0.11(ATLAS-CONF-2012-132) |Vtb| ≥0.80 @ 95% CL (|Vtb in [0,1])]Wt-prod. (PLB 716 (2012) 142) |Vtb| = 1.03+0.16−0.19CMS Measurement (SM)t-channel (@ 7TeV) |Vtb|=1.02±0.046(exp)±0.017(th.)(arXiv:1209.4533) |Vtb| ≥0.92 @ 95% CL (|Vtb in [0,1])]t-channel (@ 8TeV,5. fb−1) |Vtb|=0.96±0.08(exp)±0.02(th.)(CMS PAS TOP-12-011) |Vtb| ≥0.81 @ 95% CL (|Vtb in [0,1])]Wt-prod. (@ 7TeV) |Vtb| = 1.01+0.16−0.13(exp.)+0.03−0.04(th.)(arXiv:1209.3489) |Vtb| ≥0.79 @ 95% CL (|Vtb in [0,1])]Table 1: Current status of Vtb measurements from both ATLAS and CMS.3 [GeV]cone20Tp0 2 4 6 8 10 12 14 16 18 20 / binγ# 01020304050 e+jetsdataγtt bkgtnon-tγtbkg telectron fakeshadron fakes PreliminaryATLAS-1 L dt = 1.04 fb∫Figure 2: (left) ATLAS template fit to ttγ events in the electron channel; (middle) ATLAS expectedand observed numbers of events for the ttZ signal region; (right) CMS measurements of the ttVproduction cross section.laws), as typically happens for baryon or lepton number conservation, but also the presence of newPhysics Beyond the SM, as predicted for instance by the Minimal Supersymmetric Model (MSSM),could manifest itself. It could even happen that decays with very low SM branching ratios, likeFlavor Changing Neutral Currents (FCNC), non visible at tree level and very much suppressed atone loop due to the GIM mechanism in the SM (Br < 10−12), could be experimentally detected. Inaddition to the theorectical motivation to search for these decays, the stronger motivation is perhapsmore of experimental nature: to look for signs of new physics where the SM expects essentially nocontribution.Using 5 fb−1 of data collected at 7 TeV, CMS [25] looked for the decay of top quarks that violatebaryon number conservation (t → UDL+). U,D and L play the role of an up quark, down quarkand charged lepton, respectively. The top quark decay is expected to produce one isolated electronor muon, two jets, and no neutrino in the final state. As no significant excess of events was found,an upper limit at 95% CL was set on the branching ratio of the baryon number violating top quarkdecay to be 0.0067.Both CMS [26] and ATLAS [27] looked for MSSM decays of top quarks to charged Higgs (t →H+b) when mH± < mt. If tanβ is particularly suited, it may happen the dominant decay modeof the charged Higgs is through the decay H+ → cs. As no sign of new physics was found, best95% CL upper limits were set on the branching fractions Br(t → H+b) in the range 2-4% for Higgsboson masses between 80 and 160 GeV and assuming Br(H+ → τ+ντ ) = 1.In the SM, FCNC decays (t→ qX,X = γ, Z, g,H) are forbidden at tree level and much smallerthan the dominant decay channel. ATLAS [28] and CMS [29] searched for signs of t → qZ and, asno new physics was found, observed limits at 95% CL were set on Br(t → qZ) ≤ 0.73% (ATLAS)and Br(t→ qZ) ≤ 0.27% (CMS).6 ConclusionsTop quark studies are well under way at the LHC. Although no new physics was seen yet in thetop quark physics, the performance of the LHC and the ATLAS and CMS experiments has beenremarkable. The LHC is entering a new era of precision studies for the top quark physics.The work of A.Onofre was supported by Fundac¸a˜o para a Cieˆncia e Tecnologia under projectCERN/FP/123619/2011.4References[1] ATLAS Collaboration, JINST 3, S08003 (2008).[2] CMS Collaboration, JINST 3 (2008) S08004.[3] T. Aaltonen et al. [CDF and D0 Collaborations], Phys. Rev. D 86, 092003 (2012)[arXiv:1207.1069 [hep-ex]].[4] J. Beringer et al. (Particle Data Group), Phys. Rev. D 86, 010001 (2012)[5] V. M. Abazov et al. [D0 Collaboration], Phys. Rev. D 85, 091104 (2012) [arXiv:1201.4156[hep-ex]].[6] M. Aliev, H. Lacker, U. Langenfeld, S. Moch, P. Uwer and M. Wiedermann, Comput. Phys.Commun. 182, 1034 (2011) [arXiv:1007.1327 [hep-ph]].[7] N. Kidonakis, Phys. Rev. D 83, 091503 (2011) [arXiv:1103.2792 [hep-ph]].[8] N. Kidonakis, Phys. Rev. D 82, 054018 (2010) [arXiv:1005.4451 [hep-ph]].[9] N. Kidonakis, Phys. Rev. D 81, 054028 (2010) [arXiv:1001.5034 [hep-ph]].[10] ATLAS Collaboration, Phys. Lett. B 711, 244 (2012) [arXiv:1201.1889 [hep-ex]].[11] ATLAS Collaboration, arXiv:1207.5644 [hep-ex].[12] CMS Collaboration, Phys. Rev. D 84, 092004 (2011) [arXiv:1108.3773 [hep-ex]].[13] CMS Collaboration, arXiv:1211.2220 [hep-ex].[14] CMS Collaboration, JHEP 1211, 067 (2012) [arXiv:1208.2671 [hep-ex]].[15] ATLAS Collaboration, JHEP 1205, 059 (2012) [arXiv:1202.4892 [hep-ex]].[16] A. Czarnecki, et al., Phys. Rev. D 81, 111503 (2010)[17] J. A. Aguilar-Saavedra, et al., Eur. Phys. J. C 50, 519-533 (2007)[18] CMS Collaboration, CMS-PAS-TOP-11-020, February (2012).[19] ATLAS Collaboration, JHEP 1206, 088 (2012) [arXiv:1205.2484 [hep-ex]].[20] ATLAS Collaboration, ATLAS-CONF-2011-153.[21] ATLAS Collaboration, ATLAS-CONF-2012-126.[22] CMS Collaboration, CMS-PAS-TOP-12-014.[23] ATLAS Collaboration, ATLAS-CONF-2012-135.[24] CMS Collaboration, CMS-PAS-HIG-12-025.[25] CMS Collaboration, CMS-PAS-B2G-12-002.[26] CMS Collaboration, JHEP 1207, 143 (2012) [arXiv:1205.5736].[27] ATLAS Collaboration, JHEP 1206, 039 (2012) [arXiv:1204.2760].[28] ATLAS Collaboration, JHEP 1209, 139 (2012) [arXiv:1206.0257].[29] CMS Collaboration, arXiv:1208.0957.5

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Searches for new physics in top decays at the LHC

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