Quark Charge Research Articles

Sea quark contributions to the electric polarizability of hadrons

We present a lattice QCD calculation of the polarizability of the neutron and other neutral hadrons that includes the effects of the background field on the sea quarks. This is done by perturbatively reweighting the charges of the sea quarks to couple them to the background field. The main challenge in such a calculation is stochastic estimation of the weight factors, and we discuss the difficulties in this estimation. Here we use an extremely aggressive dilution scheme to reduce the stochastic noise to a manageable level. The pion mass in our calculation is 306 MeV and the lattice size is 3 fm. For neutron, we find that $\alpha_E = 2.70(55) * 10^{-4}fm^3$, which is the most precise lattice QCD determination of the polarizability to date that includes sea effects.

Measurement of the electric charge of the top quark intt¯events

We present a measurement of the electric charge of top quarks using $t\bar{t}$ events produced in $p\bar{p}$ collisions at the Tevatron. The analysis is based on fully reconstructed $t\bar{t}$ pairs in lepton+jets final states. Using data corresponding to 5.3 $\rm fb^{-1}$ of integrated luminosity, we exclude the hypothesis that the top quark has a charge of $Q=-4/3\,e$ at a significance greater than 5 standard deviations. We also place an upper limit of 0.46 on the fraction of such quarks that can be present in an admixture with the standard model top quarks ($Q=+2/3\,e$) at a 95\% confidence level.

Deconfinement transition in two-flavor lattice QCD with dynamical overlap fermions in an external magnetic field

We study the influence of an external magnetic field on the deconfinement transition in two-flavour lattice QCD with physical quark charges. We use dynamical overlap fermions without any approximation such as fixed topology and perform simulations on a $16^3 \times 6$ lattice and at a pion mass around $500MeV$. The pion mass (as well as the lattice spacing) was determined in independent runs on $12^3 \times 24$ lattices. We consider two temperatures, one of which is close to the deconfinement transition and one which is above. Within our limited statistics the dependence of the Polyakov loop and chiral condensate on the magnetic field supports the "inverse magnetic catalysis" scenario in which the transition temperature decreases as the field strength grows for temperature not to far above the critical temperature.

Three sterile neutrinos inE6

Candidates for unification of the electroweak and strong interactions include the grand unified groups SU(5), SO(10), and the exceptional group ${\mathrm{E}}_{6}$. The 27-dimensional fundamental representation of ${\mathrm{E}}_{6}$ contains exotic fermions, including weak isosinglet quarks of charge $\ensuremath{-}1/3$, vectorlike weak isodoublet leptons, and neutral leptons which are singlets under both left-handed and right-handed SU(2). These last are candidates for light ``sterile'' neutrinos, hinted at by some recent short-baseline neutrino experiments.In order to accommodate three families of quarks and charged leptons, an ${\mathrm{E}}_{6}$ model must contain three 27-plets, each of which contains a sterile neutrino candidate $n$. The mixing pattern within a 27-plet is described, and experimental consequences are discussed.

Quark scalar, axial, and pseudoscalar charges in the Schwinger-Dyson formalism

We calculate the scalar, axial, and pseudoscalar charges of the quark in the Schwinger-Dyson formalism of Landau gauge QCD. It is found that the dressed quark scalar density of the valence quark is significantly enhanced against the bare quark contribution, and the result explains qualitatively the phenomenologically known value of the pion-nucleon sigma term and also that given by lattice QCD. Moreover, we show that the Richardson's interquark potential suppresses the quark scalar density in the Higashjima-Miransky approximation. This fact suggests that the quark scalar density is an observable that is sensitive to quark confinement. For the quark axial charge, we find that it is suppressed due to the gluon dynamics. The result of the quenched analysis agrees qualitatively with the experimental data of the isovector axial coupling constant $g_A$. We show that the suppression of the quenched axial charge is due to a mechanism similar to that of the quark tensor charge. In the Schwinger-Dyson equation with the leading unquenching quark-loop contribution the quark axial charge is more suppressed, due to the anomaly effect. The quark pseudoscalar density is found to be large, and is divergent as the bare quark becomes massless. This result is in agreement with the phenomenological current algebraic analysis, and explains well the dominance of the pion-pole contribution.

LHC accessible second Higgs boson in the left-right model

A second Higgs doublet arises naturally as a parity partner of the standard model (SM) Higgs, once the SM is extended to its left-right symmetric version (LRSM) to understand the origin of parity violation in weak interactions, as well as to accommodate small neutrino masses via the seesaw mechanism. The flavor-changing neutral Higgs (FCNH) effects in the minimal version of this model (LRSM), however, push the second Higgs mass to more than 15 TeV, making it inaccessible at the LHC. Furthermore, since the second Higgs mass is directly linked to the ${W}_{R}$ mass, discovery of a ``low'' mass ${W}_{R}$ (${M}_{{W}_{R}}\ensuremath{\le}5--6\text{ }\text{ }\mathrm{TeV}$) at the LHC would require values for some Higgs self-couplings larger than 1. In this paper we present an extension of LRSM by adding a vectorlike $SU(2{)}_{R}$ quark doublet which weakens the FCNH constraints, allowing the second Higgs mass to be near or below the TeV range and a third neutral Higgs below 3 TeV for a ${W}_{R}$ mass below 5 TeV. It is then possible to search for these heavier Higgs bosons at the LHC without conflicting with FCNH constraints. A right-handed ${W}_{R}$ mass in the few TeV range is quite natural in this class of models without having to resort to large scalar coupling parameters. The CKM mixings are intimately linked to the vectorlike quark mixings with the known quarks, which is the main reason why the constraints on the second Higgs mass are relaxed. We present a detailed theoretical and phenomenological analysis of this extended left-right model and point out some tests as well as its potential for discovery of a second Higgs at the LHC. Two additional features of the model are a $5/3$-charged quark and a fermionic top partner with masses in the TeV range.

Single top-quark productioncross-section at the LHC in ATLAS

Measurements of the single top-quark production cross-sections with the AT- LAS detector in ppcollisions at the Large Hadron Collider (LHC) are presented. Results of measurements of single top-quark production in the t- and Wt-channel, as well as the determination of the CKM matrix element |Vtb| are shown. Further, the separate mea- surement of the top- and antitop-quark t-channel production cross-section and the ratio of the two are presented, and several searches for exotic processes in single top-quark production are discussed. The latter include results from searches for flavour changing neutral currents, extra heavy charged gauge bosons (W � ), excited quarks, and searches for anomalous CP-violating couplings in the Wtb vertex.

Early Results from the QweakExperiment

A subset of results from the recently completed Jefferson Lab Qweak experiment are reported. This experiment, sensitive to physics beyond the Standard Model, exploits the small parity-violating asymmetry in elastic ep scattering to provide the first determination of the protons weak charge Qweak(p). The experiment employed a 180 uA longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons corresponding to Q2 of 0.025 GeV2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of Qweak(p) to 4.2 percent (combined statistical and systematic error), which implies a measure of sin2(thetaw) at the level of 0.3 percent, and to help constrain the vector weak quark charges C1u and C1d. The experimental method is described, with particular focus on the challenges associated with the worlds highest power LH2 target. The new constraints on C1u and C1d provided by the subset of the experiments data analyzed to date will also be shown, together with the extracted weak charge of the neutron.

Magnetic moments of bottom baryons: Effective mass and screened charge

We calculate the magnetic moments of low lying heavy flavor bottom baryons using effective quark mass and shielded quark charge scheme. We obtain the magnetic moments of both $J^P = 1/2^+$ and $J^P = 3/2^+$ baryon states. We compare our predictions with other theoretical approaches.

Measurement of the top quark charge in pp collisions at $ \sqrt{s}=7 $ TeV with the ATLAS detector

AbstractA measurement of the top quark electric charge is carried out in the ATLAS experiment at the Large Hadron Collider using 2.05 fb−1of data at a centre-of-mass energy of 7 TeV. In units of the elementary electric charge, the top quark charge is determined to be 0.64 ± 0.02 (stat.) ± 0.08 (syst.) from the charges of the top quark decay products in single lepton$ t\overline{t} $candidate events. This excludes models that propose a heavy quark of electric charge −4/3, instead of the Standard Model top quark, with a significance of more than 8σ.

Quark tensor charge and electric dipole moment within the Schwinger-Dyson formalism

We calculate the tensor charge of the quark in the QCD-like theory in the Landau gauge using the Schwinger--Dyson formalism. It is found that the dressed tensor charge of the quark is significantly suppressed against the bare quark contribution, and the result agrees qualitatively with the analyses in the collinear factorization approach and lattice QCD. We also analyze the quark confinement effect with the phenomenological strong coupling given by Richardson and find that this contribution is small. We show that the suppression of the quark tensor charge is due to the superposition of the spin flip of the quark arising from the successive emission of gluons that dress the tensor vertex. We also consider the relation between the quark and the nucleon electric dipole moments by combining with the simple constituent quark model.

TOP AND HIGGS PHYSICS AT THE HADRON COLLIDERS

This review summarizes the recent results for top quark and Higgs boson measurements from experiments at Tevatron, a proton–antiproton collider at a center-of-mass energy of [Formula: see text], and the Large Hadron Collider, a proton–proton collider at a center-of-mass energy of [Formula: see text]. These results include the discovery of a Higgs-like boson and measurement of its various properties, and measurements in the top quark sector, e.g. top quark mass, spin, charge asymmetry and production of single top quark.

Baryon inhomogeneities in a charged quark gluon plasma

We study the generation of baryon inhomogeneities in regions of the quark gluon plasma which have a charge imbalance. We find that the overdensity in the baryon lumps for positively charged particles is different from the overdensity due to the negatively charged particles. Since quarks are charged particles, the probability of forming neutrons or protons in the lumps would thus be changed. The probability of forming hadrons having quarks of the same charges would be enhanced. This might have interesting consequences for the inhomogeneous nucleosynthesis calculations.

Exclusion of exotic top-like quarks with−4/3electric charge using jet-charge tagging in single-leptontt¯events at CDF

We report on a measurement of the top-quark electric charge in tt¯ events in which one W boson originating from the top-quark pair decays into leptons and the other into hadrons. The event sample was collected by the CDF II detector in s=1.96 TeV proton-antiproton collisions and corresponds to 5.6 fb−1. We find the data to be consistent with the standard model and exclude the existence of an exotic quark with −4/3 electric charge and mass of the conventional top quark at the 99% confidence level.4 MoreReceived 16 April 2013DOI:https://doi.org/10.1103/PhysRevD.88.032003© 2013 American Physical Society

Top Quark Charge Asymmetry Measurement with ATLAS at the LHC

In the Standard Model top-antitop pair production is foreseen to be charge asymmetric in quark-antiquark annihilation at NLO. Although the dominant tt̄ production mechanism in pp collisions at the LHC is gluon-gluon fusion, QCD predicts a small excess of top quarks produced at higher absolute rapidities than antitops. The measurement of the top quark charge asymmetry is therefore an important test of QCD and is also sensitive to new physics. We present measurements of this asymmetry using data corresponding to an integrated luminosity of 1.04 fb−1 and 4.70 fb−1 of pp collisions at = 7 TeV collected by the ATLAS detector in the single lepton and dilepton channels respectively.

Top Quark Asymmetry Results at the LHC

In 2011, an integrated luminosity of more than 5 fb−1 at 7 TeV has been delivered by the Large Hadron Collider (LHC). The measurements of the top-antitop quark charge asymmetry in top quark pair production have been performed at the ATLAS and CMS experiments. The measured charge asymmetry values from both experiments are within the uncertainties in agreement with the SM theory predictions. An overview of the latest results of these measurements by the time of TOP2012 conference will be presented.

Top B physics at the LHC.

In top-pair events where at least one of the tops decays semileptonically, the identification of the lepton charge allows us to tag not only the top quark charge but also that of the subsequent b quark. In cases where the b also decays semileptonically, the charge of the two leptons can be used to probe CP violation in heavy flavor mixing and decays. This strategy to measure CP violation is independent of those adopted so far in experiments, and can already constrain non standard model sources of CP violation with current and near future LHC data. To demonstrate the potential of this method we construct two CP asymmetries based on same-sign and opposite-sign leptons and estimate their sensitivities. This proposal opens a new window for doing precision measurements of CP violation in b and c quark physics via high p(T) processes at ATLAS and CMS.

A scaler-based data acquisition system for measuring parity-violating asymmetry in deep inelastic scattering

An experiment that measured the parity-violating asymmetries in deep inelastic scattering was completed at the Thomas Jefferson National Accelerator Facility in experimental Hall A. From these asymmetries, a combination of the quark weak axial charge could be extracted with a factor of five improvement in precision over world data. To achieve this, asymmetries at the 10−4 level needed to be measured at event rates up to 600 kHz and the high pion background typical to deep inelastic scattering experiments needed to be rejected efficiently. A specialized data acquisition (DAQ) system with intrinsic particle identification (PID) was successfully developed and used: the pion contamination in the electron samples was controlled at the order of 2×10−4 or below with an electron efficiency of higher than 91% during most of the production period of the experiment, the systematic uncertainty in the measured asymmetry due to DAQ deadtime was below 0.5%, and the statistical quality of the asymmetry measurement agreed with the Gaussian distribution to over five orders of magnitudes. The DAQ system is presented here with an emphasis on its design scheme, the achieved PID performance, deadtime effect and the capability of measuring small asymmetries.

Bound-state field-theory approach to proton-structure effects in muonic hydrogen

A bound-state field theory approach to muonic hydrogen is set up using a variant of the Furry representation in which the lowest-order Hamiltonian describes a muon in the presence of a point Coulomb field, but the origin of the binding field is taken to be three charged quarks in the proton which are modeled as Dirac particles that move freely within a spherical well. Bound-state field theory techniques are used to evaluate one- and two-photon effects. Particular attention is paid to two-photon exchange diagrams, which include the effect of proton polarizability. In addition the modification of the electromagnetic self energy of the proton by the electric field of the muon is examined. Finally, the model is used to carry out a calculation of the static electric polarizability of the proton.

Transverse-Momentum Resummation for Top-Quark Pairs at Hadron Colliders

We develop a framework for a systematic resummation of the transverse momentum distribution of top-quark pairs produced at hadron colliders based on effective field theory. Compared to Drell-Yan and Higgs production, a novel soft function matrix is required to account for the soft gluon emissions from the final states. We calculate this soft function at the next-to-leading order, and perform the resummation at the next-to-next-to-leading logarithmic accuracy. We compare our results with parton shower programs and with the experimental data at the Tevatron and the LHC. We also discuss the implications for the top quark charge asymmetry.