Quark Charge Research Articles

Subsets of configurations and canonical partition functions

We explain the physical nature of the subset solution to the sign problem in chiral random matrix theory: The subset sum is shown to project out the canonical determinant with zero quark charge from a given configuration. As the grand canonical chiral random matrix partition function is independent of the chemical potential, the zero quark charge sector provides the full result.

W helicity, top quark spin and charge

An overview of the recent measurements of top quark charge, spin correlations, and top quark and W boson polarisation in tevents is presented. The measurements were performed at the Fermilab Tevatron collider in p ¯ p collision data at p s = 1:96 TeV by the CDF and D0 experiments and in pp collision data at p s = 7 TeV by the ATLAS and CMS experiments

Measurements of the top-quark properties at CMS

We present different measurements of the top quark properties in production and decay. The measurements are performed using the LHC proton-proton ( pp ) collisions at 7 TeV and 8 TeV center-of-mass energies collected by the CMS detector during 2011 and 2012 runs, respectively. The study of production properties consists of charge asymmetry, spin correlation and the vector boson associated production of . The top quark properties in decay are studied via the measurement of the b-quark content, R = ℬ( t → Wb )/ℬ( t → Wq ), the top quark charge, the FCNC process of t → Zq and the W-helicity fractions. The results of the last measurement are used to set limits on the tWb anomalous couplings. The W-helicity fractions are measured in the electroweakly produced (single-) top quarks, too.

Top properties intt̅events at CMS (includes mass)

Selected results from the following topics are presented: Measurements of several top quark properties are obtained from the CMS data collected in 2011 at a center-of-mass energy of 7 TeV. The results include measurements of the top quark mass, the W helicity in top decays, the top quark charge, and of the $t\bar{t}$ spin correlation and the search for anomalous couplings.

Top quark pair properties: spin correlation, charge asymmetry and complex final states at LHC in ATLAS

In proton-proton collisions at the LHC, pairs of top and anti-top quarks are expected to be mostly produced through gluon fusion, in contrast to production at the Tevatron, where quark annihilation dominates. Making use of the large number of top quark pairs, we present measurements of the spin correlation between top and anti-top quarks as well as of the top quark charge asymmetry, which constitute important tests of QCD and are sensitive to new physics. We also discuss top production in association of photons and Z bosons.

Quark charge balance function and hadronization effects in relativistic heavy ion collisions

We calculate the charge balance function of the bulk quark system before hadronization and those for the directly produced and the final hadron system in high energy heavy ion collisions. We use the covariance coefficient to describe the strength of the correlation between the momentum of the quark and that of the anti-quark if they are produced in a pair and fix the parameter by comparing the results for hadrons with the available data. We study the hadronization effects and decay contributions by comparing the results for hadrons with those for the bulk quark system. Our results show that while hadronization via quark combination mechanism slightly increases the width of the charge balance functions, it preserves the main features of these functions such as the longitudinal boost invariance and scaling properties in rapidity space. The influence from resonance decays on the width of the balance function is more significant but it does not destroy its boost invariance and scaling properties in rapidity space either. The balance functions in azimuthal direction are also presented.

Constraints on universal extra dimensions fromW′searches

We obtain constraints on three universal extra-dimensional models utilizing limits from the CMS Collaboration on ${W}^{\ensuremath{'}}$ production and decay into a single-top-quark final state. We find a weak constraint on the minimal universal extra-dimensional model due to small Kaluza-Klein number violating terms. In contrast, the ${W}^{\ensuremath{'}}$ search puts a strong limit on the size of the Dirac mass term of the quarks in split universal extra-dimension models. In nonminimal universal extra-dimension models, the ${W}^{\ensuremath{'}}$ search constrains the splitting between the boundary-localized kinetic terms of the gauge bosons and the quarks. Each of these bounds can be translated into constraints on the mass splitting between the Kaluza-Klein excitations of the $SU(2)$ charged quarks and the Klauza-Klein excitations of the $W$ boson.

Associated production of charged Higgs bosons and top quarks with POWHEG

The associated production of charged Higgs bosons and top quarks at hadron colliders is an important discovery channel to establish the existence of a non-minimal Higgs sector. Here, we present details of a next-to-leading order (NLO) calculation of this process using the Catani–Seymour dipole formalism and describe its implementation in POWHEG, which allows to match NLO calculations to parton showers. Numerical predictions are presented using the PYTHIA parton shower and are compared to those obtained previously at fixed order, to a leading order calculation matched to the PYTHIA parton shower, and to a different NLO calculation matched to the HERWIG parton shower with MC@NLO. We also present numerical predictions and theoretical uncertainties for various Two Higgs Doublet Models at the Tevatron and LHC.

Top quark and leptonic charge asymmetries for the Tevatron and LHC

We compute, for $t \bar t$ production at the LHC and at the Tevatron, several charge asymmetries to NLO QCD, including also the electromagnetic and weak-interaction corrections. We calculate these asymmetries both inclusively and with additional kinematic cuts and compare our results, where possible, with recent experimental results and with SM predictions. The $t \bar t$ asymmetries induce also corresponding asymmetries for the charged leptons from semileptonic top-quark decay. Although these asymmetries are, in the SM, smaller than the corresponding ones for top quarks, they are expected to be measurable quite precisely. In fact, measurement of a lepton asymmetry in $\ell$ + jets events was reported by the D$\emptyset$ and CDF experiments. We analyze and compute to NLO in the gauge couplings leptonic charge asymmetries for dileptonic and semileptonic $\ttbar$ events, with and without acceptance cuts, at the Tevatron and the LHC.

Top quark forward-backward asymmetry and charge asymmetry in the left-right twin Higgs model

In order to explain the Tevatron anomaly of the top quark forward-backward asymmetry ${A}_{\mathrm{FB}}^{t}$ in the left-right twin Higgs model, we choose to give up the lightest neutral particle of $\stackrel{^}{h}$ field as a stable dark matter candidate. Then a new Yukawa interaction for $\stackrel{^}{h}$ is allowed, which can be free from the constraint of same-sign top pair production and contribute sizably to ${A}_{\mathrm{FB}}^{t}$. Considering the constraints from the production rates of the top pair ($t\overline{t}$), the top decay rates, and $t\overline{t}$ invariant mass distribution, we find that this model with such new Yukawa interaction can explain ${A}_{\mathrm{FB}}^{t}$ measured at the Tevatron while satisfying the charge asymmetry ${A}_{C}^{t}$ measured at the LHC. Moreover, this model predicts a strong correlation between ${A}_{C}^{t}$ at the LHC and ${A}_{\mathrm{FB}}^{t}$ at the Tevatron, i.e., ${A}_{C}^{t}$ increases as ${A}_{\mathrm{FB}}^{t}$ increases.

Implications of the effective axial-vector coupling of the gluon on top-quark charge asymmetry at the LHC

We study different top quark charge asymmetries and the variation of $t\bar t$ total cross section induced by the effective axial-vector coupling of gluon in the LHC experiments. We show that rapidity cut-dependent asymmetries are more sensitive to the new physics than the independent ones. We also study the dependence of the asymmetries and variations of total $t\bar t$ cross sections on the invariant mass of $t\bar t$ system and show that it would be necessary to measure those quantities as functions of $m_{tt}$ at the LHC. In the context of considered new physics scenario, 7 TeV LHC has enough sensitivity either to confirm the Tevatron top asymmetry anomaly or to rule it out. In the latter case the LHC is able to put stringent constraint on the new physics scale $\Lambda$ in this framework.

Search for same-sign top-quark production and fourth-generation down-type quarks in pp collisions at $ \sqrt {s} = {7}\;{\text{TeV}} $ with the ATLAS detector

Abstract A search is presented for same-sign top-quark production and down-type heavy quarks of charge −1/3 in events with two isolated leptons (e or μ) that have the same electric charge, at least two jets and large missing transverse momentum. The data are selected from pp collisions at $ \sqrt {s} = {7}\;{\text{TeV}} $ recorded by the ATLAS detector and correspond to an integrated luminosity of 1.04 fb−1. The observed data are consistent with expectations from Standard Model processes. Upper limits are set at 95 % confidence level on the cross section of new sources of same-sign top-quark pair production of 1.4-2.0 pb depending on the assumed mediator mass. Upper limits are also set on the pair-production cross-section for new heavy down-type quarks; a lower limit of 450 GeV is set at 95 % confidence level on the mass of heavy down-type quarks under the assumption that they decay 100 % of the time to W t.

Heavy quarks in a magnetic field

Abstract The motion of a heavy charged quark in a magnetic field is analyzed in the vacuum of strongly coupled CFT. The motion of the quark is dissipative. It moves in spiral until it eventually comes to rest. The world-sheet geometry is locally AdS2 but has a time dependent horizon. The string profile in the static gauge extends from the boundary till a point where an embedding singularity exists. Connections with other circular string motions are established.

RESTORATION OF PARITY SYMMETRY THROUGH PRESYMMETRY

Presymmetry, the hidden symmetry underlying the charge and generational patterns of quarks and leptons, is utilized for repairing the left–right asymmetry of the standard model with Dirac neutrinos. It is shown that the restoration of parity is consequent with an indispensable left–right symmetric residual presymmetry. Thus, presymmetry substantiates left–right symmetry and the experimental search for the latter is the test of the former, with the nature of neutrinos as a crucial feature that can distinguish the left–right symmetry alone and its combination with presymmetry. This phenomenological implication is in accordance with the fact that Majorana neutrinos are usually demanded in the first case, but forbidden in the second.

Top quark anomalous couplings at the International Linear Collider

We present a study of the experimental determination of the forward-backward asymmetry in the process ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}t\overline{t}$ and in the subsequent $t\ensuremath{\rightarrow}Wb$ decay, studied in the context of the International Linear Collider. This process probes the elementary couplings of the top quark to the photon, and the $Z$ and the $W$ bosons at a level of precision that is difficult to achieve at hadron colliders. Measurement of the forward-backward asymmetry requires excellent $b$ quark identification and determination of the quark charge. The study reported here is performed in the most challenging all-hadronic channel ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}b\overline{b}q\overline{q}q\overline{q}$. It includes realistic details of the experimental environment, a full Monte Carlo simulation of the detector, based on the Silicon Detector concept, and realistic event reconstruction. The forward-backward asymmetries are determined to a precision of approximately 1% for each of the two choices of beam polarization. We analyze the implications for the determination of the $t\overline{t}Z$ and $Wt\overline{b}$ couplings.

Cooper pairsʼ magnetic moment in MCFL color superconductivity

We investigate the effect of the alignment of the magnetic moments of Cooper pairs of charged quarks that form at high density in three-flavor quark matter. The high-density phase of this matter in the presence of a magnetic field is known to be the Magnetic Color–Flavor-Locked (MCFL) phase of color superconductivity. We derive the Fierz identities of the theory and show how the explicit breaking of the rotational symmetry by the uniform magnetic field opens new channels of interactions and allows the formation of a new diquark condensate. The new order parameter is a spin-1 condensate proportional to the component in the field direction of the average magnetic moment of the pairs of charged quarks. The magnitude of the spin-1 condensate becomes comparable to the larger of the two scalar gaps in the region of large fields. The existence of the spin-1 condensate is unavoidable, as in the presence of a magnetic field there is no solution of the gap equations with nonzero scalar gaps and zero magnetic moment condensate. This is consistent with the fact that the extra condensate does not break any symmetry that has not already been broken by the known MCFL gaps. The spin-1 condensate enhances the condensation energy of pairs formed by charged quarks and the magnetization of the system. We discuss the possible consequences of the new order parameter on the issue of the chromomagnetic instability that appears in color superconductivity at moderate density.

Erratum: Maxwell-Chern-Simons hydrodynamics for the chiral magnetic effect [Phys. Rev. C81, 062201(R) (2010)

The rate of vacuum changing topological solutions of the gluon field, sphalerons, is estimated to be large at the typical temperatures of heavy-ion collisions, particularly at the Relativistic Heavy Ion Collider. Such windings in the gluon field are expected to produce parity-odd bubbles, which cause separation of positively and negatively charged quarks along the axis of the external magnetic field. This chiral magnetic effect can be mimicked by Chern-Simons modified electromagnetism. Here we present a model of relativistic hydrodynamics including the effects of axial anomalies via the Chern-Simons term.

QCD phase diagram using PNJL model with eight-quark interactions

We present the phase diagram and the fluctuations of different conserved charges like quark number, charge and strangeness at vanishing chemical potential for the 2+1 flavor Polyakov Loop extended Nambu--Jona-Lasinio model with eight-quark interaction terms using three-momentum cutoff regularisation. The main effect of the higher order interaction term is to shift the critical end point to the lower value of the chemical potential and higher value of the temperature. The fluctuations show good qualitative agreement with the lattice data.

Searches for Physics Beyond the Standard Model

Jefferson Lab has now demonstrated ablility to test the fundamental symmetries of nature, and thereby probe for new physics beyond the Standard Model. Here we review the tremendous advances in precision parity-violation measurements with CEBAF that enable searches for new physics. This has been demonstrated with a determination of the weak charge of the proton, which is found to be in agreement with the prediction of the standard electroweak theory, and at a precision that rules out relevant new physics to the TeV scale. We also review the planned future experiments which aim to further test the electroweak theory at Jefferson Lab, including a further improvement on the proton weak charge, an ultra-precise Møller measurement, and a probe of the axial quark charges in PVDIS.

Higher extended technicolor representations and fermion generations

In usual extended technicolor (ETC) theories based on the group ${\rm{SU}(N_{ETC}})_{ETC}$, the quarks of charge 2/3 and -1/3 and the charged leptons of all generations arise from ETC fermion multiplets transforming according to the fundamental representation. Here we investigate a different idea for the origin of SM fermion generations, in which quarks and charged leptons of different generations arise from ETC fermions transforming according to different representations of ${\rm{SU}(N_{ETC}})_{ETC}$. Although this mechanism would have the potential, {\it a priori}, to allow a reduction in the value of $N_{ETC}$ relative to conventional ETC models, we show that, at least in simple models, it is excluded by the fact that the technicolor sector is not asymptotically free or by the appearance of fermions with exotic quantum numbers which are not observed.