Thursday, October 4, 2018

A New Top Quark Mass Calculation From ATLAS

There is a new paper determining the top quark mass from ATLAS experiment data from the Large Hadron Collider (LHC), although it uses only fairly early data.
The mass of the top quark is measured to be mtop=172.08±0.39(stat)±0.82(syst) GeV. A combination with previous ATLAS mtopmeasurements gives mtop=172.69±0.25(stat)±0.41(syst) GeV.
From here.

From the introduction:
The mass of the top quark mtop is an important parameter of the Standard Model (SM). Precise measurements of mtop provide crucial information for global fits of electroweak parameters [1–3] which help to assess the internal consistency of the SM and probe its extensions. In addition, the value of mtop affects the stability of the SM Higgs potential, which has cosmological implications [4–6]. 
Many measurements of mtop in each tt¯ decay channel were performed by the Tevatron and LHC collaborations. The most precise measurements per experiment in the tt¯ → lepton + jets channel are mtop = 172.85 ± 0.71 (stat) ± 0.84 (syst) GeV by CDF [7], mtop = 174.98 ± 0.58 (stat) ± 0.49 (syst) GeV by D0 [8], mtop = 172.33 ± 0.75 (stat) ± 1.03 (syst) GeV by ATLAS [9] and mtop = 172.35 ± 0.16 (stat) ± 0.48 (syst) GeV by CMS [10]. Combinations are performed, by either the individual experiments, or by several Tevatron and LHC experiments [11]. In these combinations, selections of measurements from all tt¯ decay channels are used. The latest combinations per experiment are mtop = 173.16 ± 0.57 (stat) ± 0.74 (syst) GeV by CDF [12], mtop = 174.95 ± 0.40 (stat) ± 0.64 (syst) GeV by D0 [13], mtop = 172.84 ± 0.34 (stat) ± 0.61 (syst) GeV by ATLAS [14] and mtop = 172.44 ± 0.13 (stat) ± 0.47 (syst) GeV by CMS [10]. 
In this paper, an ATLAS measurement of mtop in the tt¯ → lepton + jets channel is presented. The result is obtained from pp collision data recorded in 2012 at a centre-of-mass energy of √ s = 8 TeV with an integrated luminosity of about 20.2 fb−1 . The analysis exploits the decay tt¯ → W+W−bb¯ → `νqq¯ 0bb¯, which occurs when one W boson decays into a charged lepton (` is e or µ including τ → e, µ decays) and a neutrino (ν), and the other into a pair of quarks. In the analysis presented here, mtop is obtained from the combined sample of events selected in the electron+jets and muon+jets final states. Single-top-quark events with the same reconstructed final states contain information about the top quark mass and are therefore included as signal events
The combined error in the combination is ± 0.48 GeV, which is very low. This excludes masses in excess of 173.65 GeV at the 95% confidence level, which is barely consistent with Particle Data Group indirect measurement of 173.5 GeV.

Both the new calculation and the combined measurement are lighter than previous results from all sources as of a year ago. There was a paper reviewing the most recent mass measurement in both collaborations in January of 2018. At that time, ATLAS was using only Run-1 data.

It is worth observing the range of the combined measurements from the two Tevatron experiments and the two LHC experiments: 172.44 GeV to 174.95 GeV. The spread from the midpoint of that range is ± 1.255 GeV. I think it is safe to say that somebody's error bars are probably underestimated.

It isn't at all clear why such old data is being used in a 2018 publication. As of the most recent information available ATLAS and CMS have each collected almost three times as much data as the 20.2 fb-1 relied upon in this paper.

ATLAS: 57.5 fb-1
CMS: 59.21 fb-1

Also, much of that has been at higher 13 TeV energies than the centre-of-mass energy of √ s = 8 TeV relied upon in this paper, which should also produce better data. Many papers using more recent data have been published concerning top quark physics. For example, this paper from ATLAS and this one in which:  "The data analysed correspond to 79.8 fb−1 of proton--proton collisions at a centre-of-mass energy of s√=13 TeV recorded by the ATLAS experiment at the LHC."

While the percentage error in the top quark mass determination isn't particularly high for QCD, in many circumstances, what matters is the absolute magnitude of the uncertainty, rather than the percentage uncertainty, and in terms of absolute magnitude of the error bars, the uncertainty in the top quark mass dwarfs the uncertainty in all of the other mass measurements in the Standard Model.

A May 3, 2018 paper from CMS also looks at this topic (largely identical in result and data to a January 17, 2018 paper from CMS) using more data and higher energy data:
The mass of the top quark is measured using a sample of tt events containing one isolated muon or electron and at least four jets in the final state, collected by the CMS detector using proton-proton collisions at s= 13 TeV at the CERN LHC. The events are selected from data corresponding to an integrated luminosity of 35.9 fb1. For each event the mass is reconstructed from a kinematic fit of the decay products to a \ttbar hypothesis. Using the ideogram method, the top quark mass is determined simultaneously with an overall jet energy scale factor (JSF), constrained by the mass of the W boson in qq decays. The measurement is calibrated on samples simulated at next-to-leading order matched to a leading-order parton shower. The top quark mass is found to be 172.25±0.08 (stat+JSF)±0.62 (syst) GeV. The dependence of this result on the kinematic properties of the event is studied and compared to predictions of different models of tt production, and no indications of a bias in the measurements are observed.
Other Physics News

A paper in June summed up efforts to more accurately measure the strong force coupling constant. The abstract of the paper notes that:
The latest experimental and theoretical developments in the high-precision determination of the strong coupling αs are briefly reviewed. Six groups of observables: (i) lattice QCD data, (ii) hadronic τ decays, (iii) deep-inelastic e±p data and parton distribution functions (PDF) fits, (iv) event shapes and jet rates in e+e collisions, (v) Z boson hadronic decays, and (vi) top-quark cross sections in pp collisions, are used to extract the current world-average at the Z pole mass, αs(m2Z)=0.1181±0.0011 at next-to-next-to-leading-order (NNLO), or beyond, accuracy. Additional NNLO extractions have recently appeared based on new lattice studies, the R(s) ratio in e+ehadrons, updated PDF fits, energy-energy correlations in e+e collisions, jet cross sections in e±p collisions, and the full set of pptt¯ cross sections at the LHC. Inclusion of these new data into the world-average would slightly increase its value and reduce its uncertainty to αs(m2Z)=0.1183±0.0008. Future αs extraction perspectives with permille uncertainties at future high-luminosity e+e machines -- via W and Z hadronic decays, parton fragmentation functions, and photon F2(x,Q2) structure function in γγ collisions -- are also discussed.

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