Current Quark Research Articles

Lepton flavor universality violation without new sources of quark flavor violation

We show that new physics models without new flavor violating interactions can explain the recent anomalies in the $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ transitions. The $b\ensuremath{\rightarrow}s{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ arises from a ${Z}^{\ensuremath{'}}$ penguin which automatically predicts the $V\ensuremath{-}A$ structure for the quark currents in the effective operators. This framework can either be realized in a renormalizable $U(1{)}^{\ensuremath{'}}$ setup or be due to new strongly interacting dynamics. The dimuon resonance searches at the LHC are becoming sensitive to this scenario since the ${Z}^{\ensuremath{'}}$ is relatively light, and could well be discovered in future searches by ATLAS and CMS.

Top Quark Mass and Properties Measurements with the ATLAS Detector

The top quark is the heaviest elementary particle and unique among the known quarks since it decays before forming hadronic bound states. This makes measurements of the top quark itself particularly interesting as one can access directly the properties of a bare quark. The latest measurements of these properties with the ATLAS detector are reported using data from 8 TeV and 13 TeV proton-proton collisions at the Large Hadron Collider at CERN. The latest measurements of the top quark mass using template methods, as well as others aiming to measure the mass in a well-defined scheme, are presented. In addition, measurements for the top quark decay width, top quark spin observables and [Formula: see text]-boson helicity in events with top quark pairs ([Formula: see text]) are presented and compared to the Standard Model predictions. The cross-section measurements of top quark pair production in association with a photon, [Formula: see text]- or [Formula: see text]-boson are also reported and compared to the most accurate theoretical calculations. These measurements probe the top quark electroweak couplings. Limits on the rate of flavour-changing neutral current processes in the production or decay of the top quark are also shown.

Top-quark properties and mass measurements with the ATLAS detector

The top quark is unique among the known quarks in that it decays before it has an opportunity to form hadronic bound states. This makes measurements of its properties particularly interesting as one can access directly the properties of a bare quark. The latest measurements of these properties with the ATLAS detector at the LHC [1] are presented. Measurements of top-quark spin observables in top-antitop events, each sensitive to a different coefficient of the spin density matrix, are presented and compared to the Standard Model predictions. The helicity of the W boson from the top decays and the production angles of the top quark are further discussed. Limits on the rate of flavour changing neutral currents in the production or decay of the top quark are reported. The production of top-quark pairs in association with W and Z bosons is also presented. The measurement probes the coupling between the top quark and the Z boson. The cross-section measurement of photons produced in association with top-quark pairs is also discussed. These process are all compared to the best available theoretical calculations. The latest ATLAS measurements of the top-quark mass in lepton+jets, dilepton, and all-hadronic final states are also reported. In addition, measurements aiming to measure the mass in a well-defined scheme are presented.

Quasi-distribution amplitudes for pion and kaon via the nonlocal chiral-quark model

We investigate the pseudoscalar (PS) meson ([Formula: see text] and [Formula: see text]) quasi-distribution amplitude (QDA), which is supposed to be an asymptotic analog to the meson distribution amplitude (DA) [Formula: see text] in the limit of the large longitudinal PS-meson momentum, i.e. [Formula: see text], in the non-perturbative (NP) region. For this purpose, we employ the nonlocal chiral-quark model (NLChQM) in the light-front (LF) formalism with a minimal Fock-state for the mesons [Formula: see text][Formula: see text][Formula: see text] at the low-energy scale parameter of the model [Formula: see text][Formula: see text][Formula: see text][Formula: see text]1 GeV. As a trial, we extract the transverse-momentum distribution amplitude (TMDA) from the light-front wave function (LFWF) within the model, and convert it to QDA with help of the virtuality-distribution amplitude (VDA). By doing that, we derive an analytical expression for the NP QDA with the current-quark mass correction up to [Formula: see text]. Numerically, we confirm that the obtained TMDA reproduces the experimental data for the photon-pion transition form factor [Formula: see text] at the low-[Formula: see text] qualitatively well. We also observe that the obtained QDA approaches to DA as [Formula: see text] increases, showing the symmetric and asymmetric curves with respect to [Formula: see text] for the pion and kaon, respectively, due to the current-quark mass difference [Formula: see text]. Assigning [Formula: see text], the moments [Formula: see text] are computed, using the pion and kaon QDAs, and there appear only a few percent deviations in the moments for [Formula: see text] in comparison to the values calculated directly from DAs. It turns out that the higher moments are more sensitive to the change of [Formula: see text], whereas the lower ones depend less on it.

Constituent quark masses in Poincaré-invariant quantum mechanics

The masses of the quarks in the Poincaré-invariant quantum mechanics are the constituent masses. Even in this framework it is possible to obtain an estimate of the constituent quark masses from the Ward identity for the axial current ant the current quark masses.

Top quark measurements at ATLAS

The top quark is the heaviest known fundamental particle. As it is the only quark that decays before it hadronizes, it allows us to probe the properties of bare quarks at the Large Hadron Collider. Highlights of a few recent precision measurements by the ATLAS Collaboration of the top quark using 13 TeV and 8 TeV collision data will be presented: top-quark pair and single top production cross sections including differential distributions will be presented alongside measurements of top-quark properties, including results using boosted top quarks, probe our understanding of top-quark production in the TeV regime. Measurements of the top-quark mass and searches for rare top quark decays are also presented.

Centre vortex removal restores chiral symmetry

The influence of centre vortices on dynamical chiral symmetry breaking is investigated through the light hadron spectrum on the lattice. Recent studies of the quark propagator and other quantities have provided evidence that centre vortices are the fundamental objects underpinning dynamical chiral symmetry breaking in gauge theory. For the first time, we use the chiral overlap fermion action to study the low-lying hadron spectrum on lattice ensembles consisting of Monte Carlo, vortex-removed, and vortex-projected gauge fields. We find that gauge field configurations consisting solely of smoothed centre vortices are capable of reproducing all the salient features of the hadron spectrum, including dynamical chiral symmetry breaking. The hadron spectrum on vortex-removed fields shows clear signals of chiral symmetry restoration at light values of the bare quark mass, while at heavy masses the spectrum is consistent with a theory of weakly interacting constituent quarks.

Tachyonic instability of the scalar mode prior to the QCD critical point based on the functional renormalization-group method in the two-flavor case

We establish and elucidate the physical meaning of the appearance of an acausal mode in the sigma mesonic channel, found in the previous work by the present authors, when the system approaches the $\mathrm{Z}_{2}$ critical point. The functional renormalization group method is applied to the two--flavor quark--meson model with varying current quark mass $m_q$ even away from the physical value at which the pion mass is reproduced. We first determine the whole phase structure in the three-dimensional space $(T, \mu, m_q)$ consisting of temperature $T$, quark chemical potential $\mu$ and $m_q$, with the tricritical point, $\mathrm{O}(4)$ and $\mathrm{Z}_{2}$ critical lines being located; they altogether make a wing-like shape quite reminiscent of those known in the condensed matters with a tricritical point. We then calculate the spectral functions $\rho_{\sigma, \pi}(\omega, p)$ in the scalar and pseudoscalar channel around the critical points. We find that the sigma mesonic mode becomes tachyonic with a superluminal velocity at finite momenta before the system reaches the $\mathrm{Z}_{2}$ point from the lower density, even for $m_q$ smaller than the physical value. One of the possible implications of the appearance of such a tachyonic mode at finite momenta is that the assumed equilibrium state with a uniform chiral condensate is unstable toward a state with an inhomogeneous $\sigma$ condensate. No such an anomalous behavior is found in the pseudoscalar channel. We find that the $\sigma$-to-$2\sigma$ coupling due to finite $m_q$ play an essential role for the drastic modification of the spectral function.

Unexpected pattern of transitions to radial excitations by heavy quark current

Motivated by the claimed possibility of a large contribution of the first radial excitation of the $D^{(\ast )}$ to the $B$ semileptonic decay into charmed mesons, also invoked to solve the "$1/2$ vs. $3/2$ semileptonic puzzle", we discuss the transitions to heavy-light radial excitations by a heavy $b \to c$ quark current. We first consider a HQET sum rule, which provides a bound on the slopes of Isgur-Wise functions which we then calculate in the Bakamjian-Thomas framework which both guaranties covariance in the heavy quark limit and satisfies a set of HQET sum rules. We observe a remarkable property that for a large variety of wave functions the transition to the first radial excitation is very small while the transition matrix element to the second radial excitation is large and dominant in saturating the HQET sum rule. This is opposite to what is found in non-relativistic models, where the transition to the first radial excitation dominates the sum rule. The relative magnitude of the transition to the second excitation appears to be weakly dependent on the dynamical scale (radius of the bound states), and the same holds true for the slope of the elastic transition. These features could be tested in the heavy mass limit of lattice QCD. This pattern is shown to be related to the general structure of the Bakamjian-Thomas model, it is independent of the spin structure of the approach and derives mainly from the Lorentz transformation of the spatial wave function, a feature often disregarded in quark models.

Wilson coefficients of dimension 6 gluon operators with spin indices in the heavy quark current correlation functions

We calculate the Wilson coefficients of dimension 6 gluon operators with spin indices in the two point correlation function of the heavy scalar, pseudoscalar, and axialvector currents. Our result completes the list of all Wilson coefficients of gluon operators up to dimension 6 for the correlation functions between heavy quark currents without derivatives. We then use the result to investigate the stability of the QCD sum rule results for the ηc, J/Ψ, χc0, and χc1 mesons near Tc. While the inclusion of the dimension 6 operators increases the stability of the sum rules for all currents, all of them break down slightly above Tc with that for the J/ψ persisting to relatively highest temperature.

Generalized Nambu-Goldstone pion in dense matter: A schematic Nambu–Jona-Lasinio model

Chiral symmetry is always broken in cold, dense matter, by chiral condensation at low densities and by diquark condensation at high density. We construct here, within a schematic Nambu-Jona-Lasinio (NJL) model, the corresponding generalized Nambu-Goldstone pion, $\pi_G$. As we show, the $\pi_G$ mode naturally emerges as a linear combination of the $\langle \bar{q}q\rangle$ vacuum pion $\pi$ and the $\langle qq \rangle$ diquark condensate pion $\tilde{\pi}$, with $q$ the quark field, and continuously evolves with increasing density from being $\pi$-like in the vacuum to $\tilde{\pi}$-like in the high density diquark pairing phase. We calculate the density-dependent mass, decay constant, and coupling to quarks of the $\pi_G$, and derive a generalized Gell-Mann--Oakes--Renner (GMOR) relation in the presence of a finite bare quark mass $m_q$. We briefly discuss the implications of the results to possible Bose condensation of $\pi_G$ in more realistic models.

Narrow exotic tetraquark mesons in large- Nc QCD

Discussing four-point Green functions of bilinear quark currents in large-$N_c$ QCD, we formulate rigorous criteria for selecting diagrams appropriate for the analysis of potential tetraquark poles. We find that both flavor-exotic and cryptoexotic (i.e., flavor-nonexotic) tetraquarks, if such poles exist, have a width of order $O(1/N_c^2)$, so they are parametrically narrower compared to the ordinary $\bar qq$ mesons, which have a width of order $O(1/N_c)$. Moreover, for flavor-exotic states, the consistency of the large-$N_c$ behavior of "direct" and "recombination" Green functions requires two narrow flavor-exotic states, each coupling dominantly to one specific meson-meson channel.

Polyakov loop effects on the phase diagram in strong-coupling lattice QCD

We investigate the Polyakov loop effects on the QCD phase diagram by using the strong-coupling (1/g^2) expansion of the lattice QCD (SC-LQCD) with one species of unrooted staggered quark, including O}(1/g^4) effects. We take account of the effects of Polyakov loop fluctuations in Weiss mean-field approximation (MFA), and compare the results with those in the Haar-measure MFA (no fluctuation from the mean-field). The Polyakov loops strongly suppress the chiral transition temperature in the second-order/crossover region at small chemical potential, while they give a minor modification of the first-order phase boundary at larger chemical potential. The Polyakov loops also account for a drastic increase of the interaction measure near the chiral phase transition. The chiral and Polyakov loop susceptibilities have their peaks close to each other in the second-order/crossover region. In particular in Weiss MFA, there is no indication of the separated deconfinement transition boundary from the chiral phase boundary at any chemical potential. We discuss the interplay between the chiral and deconfinement dynamics via the bare quark mass dependence of susceptibilities.

NJL model with the modified quark-dependent coupling strength G

In this paper, the coupling strength G of the Nambu–Jona-Lasinio (NJL) model is modified by incorporating quark’s feedback into the gluon propagator. The modified two-flavor NJL model with the quark-dependent coupling strength is explored. The quark condensate in this framework has a conspicuous agreement with the lattice quantum chromodynamics (QCD) results at finite temperature. Then, it is compared with the original NJL model in both zero (chiral limit) and nonzero current quark mass. The QCD phase diagram and susceptibilities are investigated in the temperature–chemical potential [Formula: see text] plane. Therefore, the pseudo-critical temperature [Formula: see text] and the critical end point (CEP) are worked out and compared with original NJL model or lattice QCD results. In addition, the pion mass and decay constant are studied at finite temperature.

QCD topological susceptibility from the nonlocal chiral quark model

We investigate the QCD topological susceptibility $\chi_t$ by using the nonlocal chiral quark model (NL$\chi$QM). This model is based on the liquid instanton QCD-vacuum configuration in which $\mathrm{SU}(3)$ flavor symmetry is explicitly broken by the current quark mass $(m_{u,d},m_s)\approx(5,135)$ MeV. To compute $\chi_t$, the local topological charge density operator $Q_t(x)$ is derived from the effective partition function of NL$\chi$QM. We take into account the contributions from the leading-order (LO) ones $\sim\mathcal{O}(N_c)$ in the $1/N_c$ expansion. We also verify that the analytical expression of $\chi_t$ in NL$\chi$QM satisfy the Witten-Veneziano (WV) and the Leutwyler-Smilga (LS) formulae. Once the average instanton size and inter-instanton distance are fixed with $\bar{\rho}=1/3$ fm and $\bar{R}=1$ fm, respectively, all the associated model parameters are all determined self-consistently within the model, including the $\eta$ and $\eta'$ weak decay constants. We obtain the results such as $F_{\eta}=96.77$ MeV and $F_{\eta'}=102.53$ MeV for instance. Numerically we observe that $\chi_{t}=(165.57\,\mathrm{MeV})^4$ in our full calculation. This value is comparable with its empirical one $\chi_t=(175\pm5\,\mathrm{MeV})^4$. We also find that our $\chi^\mathrm{WV}_t=\chi^\mathrm{QL}_t=(194.30,\mathrm{MeV})^4$ in the quenched limit and $\chi^\mathrm{LS}_t=(162.54\,\mathrm{MeV})^4$ in the chiral limit. Consequently, we conclude that $\chi_{t}<\chi^\mathrm{QL}_t$. Our result also implies that the $(10\sim20)\,\%$ decrease with the dynamical quark contributions.

Transverse momentum dependent quark and gluon distributions of light nuclei

We investigate the unpolarized transverse momentum dependent (TMD) structure of light nuclei in the modified chiral quark exchange model (QEM), for the first time. To this end, we calculate the TMD quark and gluon distributions inside the bound state nucleons of the light nuclei based on the modified chiral quark model in which the TMD bare quark distributions of the bounded nucleons are needed. In order to compute these bare distributions, we first obtain the bare quark momentum densities using the QEM and then calculate the TMD bare distributions applying a theoretical method in which the light-cone variables are used. Finally, considering the nucleon structure of helium, tritium and deuteron nuclei, we obtain their TMD quark and gluon densities at low Q2 scale. It is shown that our results have appropriate properties that are expected for the TMD distribution functions.

Masses of the lowest spin-0 and spin-1 meson nonets: Explicit symmetry breaking effects

We extend a known multi-quark three-flavor Lagrangian of the Nambu-Jona-Lasinio type, which includes a set of effective interactions proportional to the current quark masses, to include the multi-quark interactions of vector and axial-vector types. It is shown that the mass spectrum of the four low-lying meson nonets are in agreement with current phenomenological expectations. The role of the new interactions is analyzed in detail.

Analytic properties of the quark propagator from an effective infrared interaction model

In this paper, I investigate the analytic properties of the quark propagator Dyson-Schwinger equation (DSE) in the Landau gauge. In the quark self-energy, the combined gluon propagator and quark-gluon vertex is modeled by an effective interaction (the so-called Maris-Tandy interaction), where the ultraviolet term is neglected. This renders the loop integrand of the quark self-energy analytic on the cut plane $\ensuremath{-}\ensuremath{\pi}&lt;arg(x)&lt;\ensuremath{\pi}$ of the square of the external momentum. Exploiting the simplicity of the truncation, I study solutions of the quark propagator in the domain $x\ensuremath{\in}[\ensuremath{-}5.1,0]\phantom{\rule{0.16em}{0ex}}{\text{GeV}}^{2}\ifmmode\times\else\texttimes\fi{}i[0,10.2]\phantom{\rule{0.16em}{0ex}}{\text{GeV}}^{2}$. Because of a complex conjugation symmetry, this region fully covers the parabolic integration domain for Bethe-Salpeter equations (BSEs) for bound state masses of up to 4.5 GeV. Employing a novel numerical technique that is based on highly parallel computation on graphics processing units (GPUs), I extract more than 6500 poles in this region, which arise as the bare quark mass is varied over a wide range of closely spaced values. The poles are grouped in 23 individual trajectories that capture the movement of the poles in the complex region as the bare mass is varied. The raw data of the pole locations and residues is provided as Supplemental Material, which can be used to parametrize solutions of the complex quark propagator for a wide range of bare mass values and for large bound-state masses. This study is a first step towards an extension of previous work on the analytic continuation of perturbative one-loop integrals, with the long-term goal of establishing a framework that allows for the numerical extraction of the analytic properties of the quark propagator with a truncation that extends beyond the rainbow by making adequate adjustments in the contour of the radial integration of the quark self-energy.

Setting the scale for the CLS 2+1 flavor ensembles

We present measurements of a combination of the decay constants of the light pseudoscalar mesons and the gradient flow scale $t_0$, which allow to set the scale of the lattices generated by CLS with $2 + 1$ flavors of non-perturbatively improved Wilson fermions. Mistunings of the quark masses are corrected for by measuring the derivatives of observables with respect to the bare quark masses.

Minkowski space pion model inspired by lattice QCD running quark mass

Abstract The pion structure in Minkowski space is described in terms of an analytic model of the Bethe–Salpeter amplitude combined with Euclidean Lattice QCD results. The model is physically motivated to take into account the running quark mass, which is fitted to Lattice QCD data. The pion pseudoscalar vertex is associated to the quark mass function, as dictated by dynamical chiral symmetry breaking requirements in the limit of vanishing current quark mass. The quark propagator is analyzed in terms of a spectral representation, and it shows a violation of the positivity constraints. The integral representation of the pion Bethe–Salpeter amplitude is also built. The pion space-like electromagnetic form factor is calculated with a quark electromagnetic current, which satisfies the Ward–Takahashi identity to ensure current conservation. The results for the form factor and weak decay constant are found to be consistent with the experimental data.