Evidence for top quark production in nucleus-nucleus collisions
EEvidence for top quark production in nucleus-nucleuscollisions
Georgios Konstantinos Krintiras a , ∗ a The University of Kansascern.ch/gkrintir
E-mail: [email protected]
Using 1 . ± . − of lead-lead ( A = σ tt ) is extracted from likelihood fits to a multivariate discrim-inator using lepton kinematic variables in dilepton final states and two methods. One methodrelies on the leptonic information alone, and the second one exploits, in addition, the presenceof bottom quarks. The measured σ tt is 2 . + . − . and 2 . + . − . µ b in the two cases, respectively,consistent with predictions from perturbative quantum chromodynamics. We demonstrate, for thefirst time, that top quark decay products (leptonically decaying W bosons and bottom quarks) canbe identified, irrespective of any possible final-state interactions with the quark-gluon plasma. HardProbes20201–6 June 2020Austin, Texas ∗ Speaker (on behalf of the CMS Collaboration)Supported by the Nuclear Physics program DE-SC0019389 of the U.S. Department of Energy © Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ nu c l - e x ] S e p vidence for top quark production in nucleus-nucleus collisions Georgios Konstantinos KrintirasDroplets of quark-gluon plasma (QGP), a state of strongly interacting quantum chromodynam-ics (QCD) matter, are produced in heavy nuclei high-energy collisions. A detailed study of the top(t) quark decay products, i.e., bottom (b) quarks and W bosons [1], provides novel insights intothe mechanisms of QGP-induced parton energy loss. On one hand, b quarks are ideally suited toserve as a “standard candle” of the amount of energy suppression for b quark jets [2] emergingalmost simultaneously with the heavy ion collisions [3]. On the other hand, hadronically decayingW bosons are not immediately resolved by the medium, and can probe the QGP density evolutionat different space-time scales [4, 5]. The length of the time delay can be constrained by selecting arange of reconstructed top quark transverse momentum ( p T ).At the LHC, top quark cross sections are dominated by pair production (tt ) via gluon-gluon(gg) fusion, i.e., the partonic reaction gg → tt + X , and are computable with high accuracy inperturbative quantum chromodynamics (QCD) [6–8]. The top quark is thus a theoretically preciseprobe of the initial state, and in particular the gluon parton distribution function (PDF) [9]. Topquark pair production is characterized by final states [1] comprising the decay products of the twoW bosons and two b jets.Although the feasibility of top quark studies with nuclear collisions was demonstrated re-cently [10], they remained inaccessible in nucleus-nucleus collisions because of the amount ofdata accumulated so far at the CERN LHC. At the end of 2018, LHC provided its four majorexperiments with lead-lead (PbPb) collisions at a nucleon-nucleon (NN) center-of-mass energy √ s NN = .
02 TeV and an unprecedented collision rate. The amount of data recorded by the CMSexperiment [11] corresponds to an integrated luminosity of about 1 . ± . − [12], making thefirst-ever measurement of top quark production in nucleus-nucleus collisions possible [13].
1. Methods
We first identify tt signal events based on the leptonic W ± → (cid:96) ± ν decays, with (cid:96) beingeither an electron (e) or muon ( µ ). The resulting dilepton final states (e ± µ ∓ , µ + µ − , and e + e − )involve two energetic, oppositely charged (OS) leptons that are isolated from nearby hadronicactivity, momentum imbalance from the undetected neutrinos ( ν ), and two b jets. Two methodsare employed: (i) making use of the final-state dilepton kinematic properties alone (“2 (cid:96) OS ”), and(ii) imposing extra requirements on the number of “b-tagged jets” (“2 (cid:96) OS + b-tags”). Since thefeasibility to reconstruct b jets in the QGP may be impacted by the sizeable suppression evidencedin data, the 2 (cid:96) OS + b-tagsmethod relies on a data-based estimate for the b jet identification (“tagging”)performance, measured in terms of the b jet identification efficiency ε b . Finally, we extract the ttcross section ( σ tt ) from a combined maximum-likelihood fit to a multivariate discriminator usinglepton kinematic variables, independently for the 2 (cid:96) OS and 2 (cid:96) OS + b-tags methods.The NN → tt + X process (N = p , n) is simulated with the MadGraph5_amc@nlo(v2.4.2) [14] program at next-to-leading order (NLO) with the EPPS16 NLO nuclear PDF [15].Drell–Yan production of quark-antiquark annihilation into lepton-antilepton pairs through Z bosonor virtual-photon exchange (referred to as “Z / γ ∗ ”) and W boson production with additional jets,are generated with the same program at NLO, corrected with scaling factors derived from data.Subdominant contributions from single top quark plus W boson events (tW) and WW, WZ, and ZZproduction (collectively referred to as “VV”) are simulated with the powheg [16] NLO generator.2 vidence for top quark production in nucleus-nucleus collisions Georgios Konstantinos KrintirasGradient boosted decision trees (BDTs) are set up to maximally discriminate genuine leptonswith high p T between the tt signal and the Z / γ ∗ background. The BDTs exploit kinematic propertiesof the leading- and subleading- p T leptons (referred to as “ (cid:96) ” and “ (cid:96) ”, respectively). The maximum-likelihood fits are performed on binned BDT distributions, simultaneously in the e ± µ ∓ , µ + µ − , ande + e − final states, and accounting for all sources of uncertainty, i.e., statistical and systematic, andtheir correlations. The simultaneous analysis of these events allows the measurement of the tt signalstrength µ , defined as the ratio of the observed σ tt to the expectation from theory. The best-fit valueof µ and its uncertainty ∆ µ (corresponding to a 68% confidence level) are extracted separately withthe 2 (cid:96) OS and 2 (cid:96) OS + b-tags methods. For the latter, we correlate the number of tt signal events inthe b-tagged jet categories based on multinomial probabilities, using ε b and a parameter ( δ QGP )accounting for medium-induced suppression of ε b . We allow ε b being different for the two b jets,i.e., ε b → ε ∗ b = ( − δ QGP ) × ε b , motivated by a path-length dependence of the parton energy loss.
2. Results
The “postfit predicted” (i.e., the tt signal and various sources of background are adjustedaccording to the fit procedure of Section 1) and observed BDT distributions are shown in Fig. 1 inthe tt -enriched e ± µ ∓ final state with the 2 (cid:96) OS ( µ = . + . − . , left) and 2 (cid:96) OS + b-tags ( µ = . + . − . ,right) methods. We found the classifier to separate well the tt signal from the Z/ γ ∗ background in the µ + µ − and e + e − final states, enhancing our confidence about its applicability to the e ± µ ∓ final state.The inclusive σ tt is then obtained multiplying the best fit µ value by the theoretical expectation.Accounting for the acceptance corrections, we measure σ tt to be 2 . + . − . and 2 . + . − . µ b in thecombined e ± µ ∓ , µ + µ − , and e + e − final states, with a relative total uncertainty of 32 and 34% in the2 (cid:96) OS and 2 (cid:96) OS + b-tags methods, respectively. E v en t s Data ttVV tWNonprompt * g Z/Total unc
CMS = 5.02 TeV) NN s ( -1 m e D a t a / P r ed E v en t s Data ttVV tWNonprompt * g Z/Total unc
CMS = 5.02 TeV) NN s ( -1 m e [ , / [ [ / , / [[ / , ] [ , / [ [ / , / [[ / , ] [ , ] BDT00.511.52 D a t a / P r ed Figure 1:
Postfit predicted (histograms) and observed (points) binned BDT (left, middle) and sphericity(right) distributions in the e ± µ ∓ final state with the 2 (cid:96) OS (left, right) and 2 (cid:96) OS + b-tags (middle) methods. Thecomparison between the tt signal and the background-subtracted data is shown for the postfit distributions asinset panel (right). The vertical bars on the points represent the statistical uncertainty in data (68% Clopper-Pearson intervals). The hatched regions show the postfit uncertainty in the sum of tt signal and background.The lower panels display the ratio of the observed data to the predictions, including the tt signal, with barsand bands, respectively, representing the statistical and total uncertainties in the prediction [13]. The compatibility of the data with the background-only hypothesis is evaluated using a profile-likelihood ratio as a test statistic, including all sources of systematic uncertainty. The probability3 vidence for top quark production in nucleus-nucleus collisions
Georgios Konstantinos Krintirasfor the background to mimic an excess of events larger than that observed in data is quantified using p values, expressed in terms of Gaussian tail probabilities and given in units of standard deviation( σ ). The background-only hypothesis is excluded with observed (expected) statistical significanceof 3 . .
8) and 4 . . σ with the 2 (cid:96) OS and 2 (cid:96) OS + b-tags methods, respectively.Figure 2 presents the measured σ tt , including the measurement at √ s = .
02 TeV [9] inproton-proton (pp) collisions, and compared to predictions from perturbative QCD [7]. - b] m [ s CMS
NNLO+NNLL TOP++NNPDF30 NNLONNLO+NNLL TOP++CT14 NNLO = 5.02 TeV)s, ( -1 pp, 27.4 pb ) (scaled by A b-tag +jets/l+N OS JHEP 03 (2018) 115
NNLO+NNLL TOP++CT14 NLOEPPS16 NLOCT14 NNLO x = 5.02 TeV) NN s, ( -1 PbPb, 1.7 nb OS b-tag +N OS syst ¯ Exp unc: stat, stat scale ¯ Th unc: PDF, PDF
Figure 2:
The inclusive tt cross section measured in the combined e ± µ ∓ , µ + µ − , and e + e − final states inPbPb collisions (divided by the mass number squared, A ), compared to predictions [7], and pp results at √ s NN = .
02 TeV [9]. The total experimental error bars (theoretical error bands) include statistical andsystematic (PDF and QCD scale) uncertainties added in quadrature [13].
3. Summary
In summary, the top pair production cross section ( σ tt ) is measured for the first time in nucleus-nucleus collisions, using lead-lead collision at √ s NN = .
02 TeV with an integrated luminosity of1 . ± . − . The measurement makes use of at least one pair of oppositely charged electrons ormuons, and, separately, jets originating from the bottom quarks. The extracted σ tt is 2 . + . − . and2 . + . − . µ b, respectively, consistent with the expectations from scaled proton-proton data as wellas perturbative quantum chromodynamics calculations. The observed statistical significance of thett signal against the background-only hypothesis is 3 . . References [1] Particle Data Group, M. Tanabashi et al., “Review of particle physics”,
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