Measurement of the mass difference between top and antitop quarks
CMS Collaboration, S Chatrchyan, V Khachatryan, A M Sirunyan, E Aguiló, C Amsler, V Chiochia, S De Visscher, C Favaro, M Ivova Rikova, B Millan Mejias, P Otiougova
A measurement of the mass difference between the top and the antitop quark (Δm t = m t − ) is performed using events with a muon or an electron and at least four jets in the final state. The analysis is based on data collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.96±0.11 fb−1, and yields the value of Δm t = −0.44±0.46 (stat.) ±0.27 (syst.) GeV. This result is consistent with equality of particle and antiparticle masses required by CPT invariance, and
... rovides a significantly improved precision relative to existing measurements. Abstract A measurement of the mass difference between the top and the antitop quark (∆m t = m t − m t ) is performed using events with a muon or an electron and at least four jets in the final state. The analysis is based on data collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 4.96 ± 0.11 fb −1 , and yields the value of ∆m t = −0.44 ± 0.46 (stat.) ± 0.27 (syst.) GeV. This result is consistent with equality of particle and antiparticle masses required by CPT invariance, and provides a significantly improved precision relative to existing measurements. The standard model of particle physics is a local gauge-invariant quantum field theory in which symmetries play a fundamental role that includes the dependence of system properties under specific transformations such as charge conjugation (C), parity or space reflection (P) and time reversal (T). These individual symmetries and the combined CP symmetry are known to be violated in weak interactions, but the CPT combination appears to be conserved in nature  . A major consequence of CPT conservation is that the mass of any particle must equal that of its antiparticle. We focus on a measurement of the mass difference between the top and antitop quark. Since quarks carry color charge and hadronize into colorless particles before decaying, they cannot be observed as free quarks. The lone exception is the top quark, which due to its short lifetime decays before hadronization. The mass difference between the top quark and its antiquark was measured previously by the D0 and CDF experiments, and showed no significant deviation from zero    . This letter reports a measurement of the difference between the mass of the top quark (t) and of its antiparticle (t), with significantly reduced uncertainties, using tt events produced in protonproton collisions at √ s = 7 TeV, recorded with the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC)  . We select events where one W boson, either from the top or antitop quark, decays into qq (t → bW + → bqq , or its charge conjugate), and the other W decays leptonically (t → bW + → b + ν , or its charge conjugate), where the lepton is a muon or an electron. The data are split into − and + samples that contain, respectively, three-jet decays of the associated top or antitop quarks. For each event category, the Ideogram likelihood method  is used to measure the mass of the top quark (m t ) or antitop quark (m t ), and the difference between the masses in the two categories of lepton charge is taken as the mass difference ∆m t ≡ m t − m t . The Ideogram method was used previously [7, 8] to measure the mass of the top quark. The procedure incorporates a kinematic fit of the events to a tt hypothesis that is modified specifically for this analysis to consider only the top or antitop quark that decays to three jets. The CMS detector The central feature of the CMS apparatus is a superconducting solenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. The field volume houses the silicon-pixel and siliconstrip trackers, a crystal electromagnetic calorimeter (ECAL) and a brass/scintillator hadron calorimeter. The inner tracker reconstructs charged-particle trajectories within the pseudorapidity range |η| < 2.5, where the pseudorapidity is defined in terms of the polar angle θ relative to the counterclockwise-rotating proton beam as η ≡ − ln (tan θ/2). The tracker provides an impact parameter resolution of ≈ 15 µm and a transverse momentum (p T ) resolution of ≈ 1.5% for 100 GeV particles. The energy resolution is < 3% for the electron energies in this analysis. Muons are measured for |η| < 2.4 using gaseous detection planes based on three technologies: drift tubes, cathode-strip and resistive-plate chambers. Matching outer muon trajectories to tracks measured in the silicon tracker provides a transverse momentum resolution of 1 − 6% for the p T values in this analysis. In addition to the barrel and endcap detectors, CMS has extensive forward calorimetry. A more detailed description of the CMS detector can be found in Ref. . 4 Event reconstruction and selection 3 Data and simulation This analysis is based on a data sample corresponding to an integrated luminosity of 4.96 ± 0.11 fb −1 collected in pp collisions at a center-of-mass energy of 7 TeV and recorded with the CMS detector. Events are selected through a trigger requiring an isolated electron or muon with p T > 25 or 17 GeV, respectively, accompanied by at least three jets of p T > 30 GeV in each event. The acquired data are compared to a set of simulated pp collisions at √ s = 7 TeV. Most signal and background events are generated with the matrix-element generator MADGRAPH 4.4.12 , interfaced to PYTHIA 6.4.22  for the parton showering, where tt events are generated accompanied by up to three extra partons. The MLM algorithm  is used for matching the matrix-element partons to their parton showers. Singly produced top-quark events are generated with the POWHEG event generator  and generic multijet events with PYTHIA. The simulation of multijet events is used just to normalize a multijet-enriched control sample of data needed in the analysis (described below). The simulation also includes effects of pileup in pp collisions, which refers to additional pp interactions that can occur during the same bunch crossing or in those immediately preceding or following the primary generated process. The simulated event samples are normalized to the theoretical cross section for each process, as calculated with FEWZ  for W and Z production, with PYTHIA for multijet production, and MCFM  for all other contributing processes. The generated events are then passed through the full CMS detector simulation based on GEANT4 , and eventually reconstructed using the same algorithms as used for data.