The top quark is the heaviest elementary particle in the Standard Model. For this reason, a precise determination of its mass mt is part of the LHC physics program. The most accurate determinations of mt rely on the kinematic

reconstruction of the top decay products and on the use of Monte Carlo event generators. In this contribution I discuss the impact of different theoretical descriptions for the process of top-pair production at the LHC on the extraction of mt

Ferrario Ravasio, S.

arXiv

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DOI 10.1393/ncc/i2019-19168-8Colloquia: IFAE 2018IL NUOVO CIMENTO 42 C (2019) 168Accurate predictions for top-quark pair production at the LHCS. Ferrario Ravasio(1)(2)(1) INFN, Sezione di Milano Bicocca - Milano, Italy(2) Università degli Studi di Milano-Bicocca - Milano, Italyreceived 31 January 2019Summary. — The top quark is the heaviest elementary particle in the StandardModel. For this reason, a precise determination of its mass mt is part of the LHCphysics program. The most accurate determinations of mt rely on the kinematicreconstruction of the top decay products and on the use of Monte Carlo event gen-erators. In this contribution I discuss the impact of different theoretical descriptionsfor the process of top-pair production at the LHC on the extraction of mt.1. – IntroductionThe phenomenology of the top-quark is driven by the large value of its mass mt. It isthe only quark that decays instead of hadronizing, giving us the opportunity to study theproperties of a bare quark. Due to radiative corrections, the value of mt has an impacton many parameters of the Standard Model (SM) like the Higgs self-couplings and thebosons masses.The most accurate determinations of mt are the so-called “direct measurements”.Monte Carlo (MC) generators are used to simulate templates of kinematic distributionssensitive to the top-quark mass. These templates are produced varying the input massmt of the generator, in order to extract the parametric dependence of the distributionson mt. The mt value that fits the data the best is the extracted top-quark mass. Thismethod has, however, an important drawback: the extracted mt value strongly dependson the accuracy of the MC generator employed. This is a strong motivation for thetheoretical community to improve the accuracy of the MC generators.In this contribution I compare the predictions of two next-to-leading-order (NLO)event generators for tt̄ production implemented in the POWHEG framework [1-4],namely the hvq [5] and the bb̄4 generators [6] for some observables that can be usedto determine the top-quark mass. These NLO generators must be matched with a gen-eral purpose Shower Monte Carlo (SMC) program, like Pythia8 [7] or Herwig7 [8, 9]. Ialso show differences between distributions obtained with the two SMCs. The resultspresented here have first been published in ref. [10].Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 S. FERRARIO RAVASIO2. – Monte Carlo generators and simulated samplesNLO (QCD) events for tt̄ production in pp collisions at a center-of-mass energy√s =8 GeV have been produced using:• The hvq [5] generator, which describes the process of tt̄ production at NLO. Thetop decay is included at LO, using a re-weighting procedure that allows to partiallytake into account off-shell and spin-correlation effects between the t and the t̄ decayproducts• The bb̄4 generator [6], that implements the process pp → bb̄e+νeμ−ν̄μ at NLO.Thus, as well as for top production, it describes its decay at NLO including off-shelland spin-correlations effects and the interference between radiation in productionand in decay. Furthermore, all the processes which yield the same final state andtheir quantum interference are correctly included.Selection cuts to suppress the Wt topology, which is present only in bb̄4, are adopted [10].In order to implement the bb̄4 generator, a resonance-aware formalism has been devel-oped, which is coded in the new POWHEG BOX RES framework [6]. By default, it generatesmultiple emissions, one from the production process and one from each decayed reso-nance. Pythia8 [7] (or Herwig7 [8,9]) completes the NLO events, by adding subsequentemissions in the soft and collinear approximation (provided by the parton shower, PS)and translating the partonic final state into a hadronic one. In order not to spoil theNLO accuracy of the result, the PS must veto emissions that have a transverse momen-tum larger than the POWHEG radiation. This is done per default both by Pythia8 andHerwig7 if the emissions come from the production process. We extended this veto pro-cedure also to the case of radiation from decayed top quarks. Since Pythia8 implementsa transverse momentum ordered PS, it is enough to require that the first emission hasa transverse momentum smaller than the one of the corresponding POWHEG emission.Herwig7 is an angular-ordered PS, thus all the emissions must be inspected.3. – Comparison between the NLO generators showered with Pythia8We choose simple observables O that can be directly related to the top-quark mass:(1) O = Oc + B(mt − mct),where mct is our reference mass mct = 172.5 GeV and Oc and B are parameters thatcan be fitted using a MC generator. Neglecting the differences among the B coefficientsbetween the generators, the differences in the extracted mass can be written as(2) Δmt = −ΔOcB.The first observable we examine is the peak of the invariant mass of the reconstructedtop quark (W and b-jet system), i.e., mmaxWbj . In this case Δmt ≈ −ΔmmaxWbj. Even if weapply a Gaussian smearing of 15 GeV to mimic experimental resolution effects, we findremarkable agreement between the peak position extracted using bb̄4 and hvq.It is also intriguing to have a look at purely leptonic observables, as proposed by theauthors of ref. [11]. There is an overall agreement between bb̄4 and hvq predictions, apartACCURATE PREDICTIONS FOR TOP-QUARK PAIR PRODUCTION AT THE LHC 3from observables sensitive to spin correlations, that are only approximatively describedat LO by hvq. Two examples are given by 〈m(e+μ−)〉, which yields to Δmt ≈ 1.5 GeV,and 〈p⊥(e+μ−)〉, which leads to Δmt ≈ −2 GeV.4. – Comparison between Herwig7 and Pythia8 predictionsQuite large differences arise when we shower the events with Herwig7 instead ofPythia8. If we look at the reconstructed-top mass peak, the Herwig7 prediction is1 GeV smaller than the Pythia8 if bb̄4 is employed, 0.5 GeV in the case of hvq. A worseagreement is found when leptonic observables are considered: the mt value extractedwith Herwig7 is roughly 2.5 GeV larger than the Pythia8 one for both NLO generators.This is not completely unexpected, given the different nature of the Herwig7 PS withrespect to the transverse-momentum shower implemented by Pythia8.5. – ConclusionsWhen using Pythia8, the differences between hvq and bb̄4 are large enough to use thenewest generator, but not large enough to completely overturn the current measurementsthat are based on hvq. When Herwig7 is used, we do not have a nice and consistentpicture, however we do believe that the option of dismissing Herwig7 is not soundlymotivated. The difference between the two PSs may be due to higher-order effects, thatmust be taken into account when estimating theoretical uncertainties. When a realisticanalysis is performed, the parameters of a MC are tuned to reproduce the data fairly.This could improve the agreement between Herwig7 and Pythia8.∗ ∗ ∗The author thanks T. Ježo, P. Nason, L. Cieri and M. Rocco for suggestions on thepaper.REFERENCES[1] Nason P., JHEP, 11 (2004) 040.[2] Frixione S., Nason P. and Oleari C., JHEP, 11 (2007) 070.[3] Alioli S., Nason P., Oleari C. and Re E., JHEP, 06 (2010) 043.[4] Ježo T. and Nason P., JHEP, 12 (2015) 065.[5] Frixione S., Nason P. and Ridolfi G., JHEP, 09 (2007) 126.[6] Ježo T., Lindert J. M., Nason P., Oleari C. and Pozzorini S., Eur. Phys. J. C, 76(2016) 691.[7] Sjöstrand T. et al., Comput. Phys. Commun., 191 (2015) 159.[8] Bahr M. et al., Eur. Phys. J. C, 58 (2008) 639.[9] Bellm J. et al., Eur. Phys. J. C, 76 (2016) 196.[10] Ferrario Ravasio S., Ježo T., Nason P. and Oleari C., Eur. Phys. J. C, 78 (2018)458.[11] Frixione S. and Mitov A., JHEP, 09 (2014) 012.

Accurate predictions for top-quark pair production at the LHC

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