Quark Mass Difference Research Articles

Quark mass difference effects in hadronic Fermi matrix elements from first principles

It was recently estimated that the strong isospin-symmetry breaking (ISB) corrections to the Fermi matrix element in free neutron decay could be of the order 10−4, one order of magnitude larger than the naïve estimate based on the Behrends-Sirlin-Ademollo-Gatto theorem. To investigate this claim, we derive a general expression of the leading ISB correction to hadronic Fermi matrix elements, which takes the form of a four-point correlation function in lattice gauge theory and is straightforward to compute from first principles. Our formalism paves the way for the first determination of such correction in the neutron sector with fully-controlled theory uncertainties.

The role of U(1) A symmetry breaking in the QCD corrections to the pion mass difference

The charged and neutral pion mass difference can be attributed to both the quantum electrodynamics and QCD contributions. The current quark mass difference (Δm) is the source of the QCD contribution. Here, in a two-flavour non-local Nambu–Jona-Lasinio model, we try to estimate the QCD contribution. Interestingly, we find that the strength of the symmetry-breaking parameter c plays a crucial role in obtaining the pion mass difference while intertwined with the current quark mass difference. To obtain the QCD contribution for the pion mass difference, we scan the parameter space in {Δm, c}, and by comparing this with the existing results, we constrained the parameter space. Further, using a fitted value of c, we determine the allowed range for the Δm in the model. The model estimated Δm ranges enable us to extract the chiral perturbation theory low energy constant, l 7 and verify the dependence of the pion mass difference on Δm. We also find out its dependence on c—it increases with the decreasing value of c, i.e. toward an axial anomaly restored phase.

Impact of charge symmetry breaking on gluon and sea quark distributions in the pion and kaon

In this exploratory study, I present, for the first time, the implications of the charge symmetry breaking (CSB) that arise from the [Formula: see text] and [Formula: see text] quark-mass differences on gluon and sea quark distribution functions of the pion and kaon in the framework of the Nambu–Jona-Lasino (NJL) model, which is a quark-level chiral effective theory of QCD, with the help of the proper-time regularization scheme to simulate color confinement of QCD. From the analysis, one finds that the charge symmetry (CS) gluon distribution for the pion has a good agreement with the prediction results obtained from the recent lattice QCD simulation and JAM global fit QCD analysis at a higher scale of [Formula: see text][Formula: see text]GeV2. The size of the CSB effects on gluon and sea quark distributions for the pion with the realistic ratios of [Formula: see text] at [Formula: see text][Formula: see text]GeV2 are, respectively, estimated by 1.3% and 2.0% at [Formula: see text] in comparison with those for [Formula: see text], while those for the kaon are approximately about 0.3% and 0.5% at [Formula: see text], respectively. A remarkable result is found that the CSB effects on gluon distribution for the kaon are smaller than that for the pion, which has a similar prediction result as that for the CS case.

Charge-symmetry-breaking effects on neutron β decay in nonrelativistic quark models

A formalism for the study of charge-symmetry-breaking (CSB) effects is discussed and used to analyze the effects of charge-symmetry breaking on neutron $\ensuremath{\beta}$ decay. The effect of including CSB reduces the $\ensuremath{\beta}$-decay matrix element by an amount on the order of ${10}^{\ensuremath{-}4}$, a value much larger than the previous estimate. The earlier calculation used the neutron-proton mass difference as the CSB operator instead of the matrix element of the sum of the individual terms between the ground and excited states. The electromagnetic and dynamic effects of the up-down quark mass difference oppose the up-down quark mass difference leading to a small $n\ensuremath{-}p$ mass difference, but add coherently in computing the excitation matrix elements causing large enhancements. The current uncertainty in the value of ${V}_{ud}$ is also on the order of ${10}^{\ensuremath{-}4}$. An improvement of that uncertainty by an order of magnitude would require that charge-symmetry-breaking effects should be included in future analyses.

Amplitudes separation and strong-electromagnetic relative phase in the ψ(2S) decays into baryons

In the framework of a phenomenological model based on an effective Lagrangian, that allows us to exploit all available data, we have obtained the strong, electromagnetic and mixed strong-electromagnetic amplitudes of the $\ensuremath{\psi}(2S)$ decays into baryon-antibaryon pairs. The analysis, repeating the procedure successfully implemented in the case of the leading vector charmonium $J/\ensuremath{\psi}$, revealed a new and quite intriguing phenomenon, that by itself triggered a further quite interesting finding. The phenomenon is the high affinity of the $\ensuremath{\psi}(2S)$ meson with the $\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ pair, at a level that cannot be justified by the model. As a consequence, we excluded the $\mathrm{\ensuremath{\Lambda}}\overline{\mathrm{\ensuremath{\Lambda}}}$ channel from the set of experimental constraints by largely improving the description power of the model, which then led to another interesting result: the QCD regime turned out to be perturbative already at the $\ensuremath{\psi}(2S)$ mass. Finally, the comparison with the results obtained for the $J/\ensuremath{\psi}$ has shown that the main difference with respect to the $\ensuremath{\psi}(2S)$ concerns the SU(3) breaking source due to the quark mass difference. As expected, consequently to its larger mass, the effects of such a difference are less important for the $\ensuremath{\psi}(2S)$ with respect to the $J/\ensuremath{\psi}$ meson.

Dynamics of non-Abelian strings in the theory interpolating from N=2 to N=1 supersymmetric QCD

We study the dynamics of non-Abelian vortex strings supported in ${\mathcal N}=2$ supersymmetric QCD with the U$(N)$ gauge group and $N_f=N$ quark flavors deformed by the mass $\mu$ of the adjoint matter. In the limit of large $\mu$ the bulk four-dimensional theory flows to ${\mathcal N}=1$ supersymmetric QCD. The dynamics of orientational zero modes of the non-Abelian string is described by the world sheet CP$(N-1)$ model. At $\mu=0$ this model has ${\mathcal N}= \left(2,2\right) $ supersymmetry while at large $\mu$ it flows to the non-supersymmetric CP$(N-1)$ model. We solve the world sheet model in the large $N$ approximation and find a rich phase structure with respect to the deformation parameter $\mu$ and quark mass differences. The phases include two strong coupling phases and two Higgs phases. In particular, the Higgs phase at small $\mu$ supports CP$(N-1)$ model kinks representing confined monopoles of the bulk QCD, while in the large-$\mu$ Higgs phase monopoles disappear.

Direct CP Violation from Isospin Symmetry Breaking Effects for the Decay Process B-s→P(V)π0 in PQCD

We investigate the direct CP violation for the decay process of B-s→P(V)π0 (P,V refer to the pseudoscalar meson and vector meson, resp.) via isospin symmetry breaking effects from the π0-η-η′ mixing mechanism in PQCD factorization approach. Isospin symmetry breaking arises from the electroweak interaction and the u-d quark mass difference by the strong interaction, which are known to be tiny. However, we find that isospin symmetry breaking at the leading order shifts the CP violation due to the new strong phases.

Charge symmetry breaking effects in pion and kaon structure

Charge symmetry breaking (CSB) effects associated with the $u$ and $d$ quark mass difference are investigated in the quark distribution functions and spacelike electromagnetic form factors of the pion and kaon. We use a confining version of the Nambu--Jona-Lasinio model, where CSB effects at the infrared scale associated with the model are driven by the dressed $u$ and $d$ quark mass ratio, which because of dynamical chiral symmetry breaking is much closer to unity than the associated current quark mass ratio. The pion and kaon are given as bound states of a dressed quark and a dressed antiquark governed by the Bethe-Salpeter equation, and exhibit the properties of Goldstone bosons, with a pion mass difference given by $m_{\pi^+}^2 - m_{\pi^0}^2 \propto (m_u - m_d)^2$ as demanded by dynamical chiral symmetry breaking. We find significant CSB effects for realistic current quark mass ratios ($m_u/m_d \sim 0.5$) in the quark flavor-sector electromagnetic form factors of both the pion and kaon. For example, the difference between the $u$ and $d$ quark contributions to the $\pi^+$ electromagnetic form factors is about 8\% at a momentum transfer of $Q^2 \simeq 10\,$GeV$^2$, while the analogous effect for the light quark sector form factors in the $K^+$ and $K^0$ is about twice as large. For the Parton distribution functions, we find CSB effects which are considerably smaller than those found in the electromagnetic form factors.

Leading isospin-breaking corrections to pion, kaon, and charmed-meson masses with twisted-mass fermions

We present a lattice computation of the isospin-breaking corrections to pseudoscalar meson masses using the gauge configurations produced by the European Twisted Mass collaboration with $N_f = 2 + 1 + 1$ dynamical quarks at three values of the lattice spacing ($a \simeq 0.062, 0.082$ and $0.089$ fm) with pion masses in the range $M_\pi \simeq 210 - 450$ MeV. The strange and charm quark masses are tuned at their physical values. We adopt the RM123 method based on the combined expansion of the path integral in powers of the $d$- and $u$-quark mass difference ($\widehat{m}_d - \widehat{m}_u$) and of the electromagnetic coupling $\alpha_{em}$. Within the quenched QED approximation, which neglects the effects of the sea-quark charges, and after the extrapolations to the physical pion mass and to the continuum and infinite volume limits, we provide results for the pion, kaon and (for the first time) charmed-meson mass splittings, for the prescription-dependent parameters $\epsilon_{\pi^0}$, $\epsilon_\gamma(\overline{MS}, 2~\mbox{GeV})$, $\epsilon_{K^0}(\overline{MS}, 2~\mbox{GeV})$, related to the violations of the Dashen's theorem, and for the light quark mass difference $(\widehat{m}_d - \widehat{m}_u)(\overline{MS}, 2~\mbox{GeV})$.

η→3π: Study of the Dalitz Plot and Extraction of the Quark Mass Ratio Q.

The η→3π amplitude is sensitive to the quark mass difference m_{u}-m_{d} and offers a unique way to determine the quark mass ratio Q^{2}≡(m_{s}^{2}-m_{ud}^{2})/(m_{d}^{2}-m_{u}^{2}) from experiment. We calculate the amplitude dispersively and fit the KLOE Collaboration data on the charged mode, varying the subtraction constants in the range allowed by chiral perturbation theory. The parameter-free predictions obtained for the neutral Dalitz plot and the neutral-to-charged branching ratio are in excellent agreement with experiment. Our representation of the transition amplitude implies Q=22.0±0.7.

Isospin-violating strong decays of scalar single-heavy tetraquarks

We present a study of the isospin-violating one-pion strong decays of single heavy tetraquarks $X_Q = X(s q \bar q \bar Q)$, $Q=c, b$, and $q=u, d$ with spin-parity $J^P = 0^+$. We assume that the tetraquarks have the configuration of a color diquark and a antidiquark. Three mechanisms of isospin violation can contribute to the decay rate: (1) mixing of the $X(s u \bar u \bar Q)$ and $X(s d \bar d \bar Q)$ tetraquark currents, (2) an explicit $m_d-m_u$ quark mass difference in the quark diagrams describing the corresponding decay transitions and (3) $\pi^0-\eta$ mixing in the final state. Our main results are: (1) It is quite likely that the investigated tetraquark states are isosinglet states with a small admixture of an isotriplet component; (2) The first isospin-breaking mechanism affects the decay rater more significantly than the others; (3) Our calculations contain a size parameter $\Lambda_{X_Q}$, characterizing the distribution of the quarks in the tetraquark state $X_Q$. Absolute decay rates depend very much on the choice of $\Lambda_{X_Q}$, varied from 1 to 2 GeV, reflecting the compactness of the multiquark system.

Diquark mass differences from unquenched lattice QCD**Supported by National Science Foundation of China (11575197, 10835002, 11405178, 11335001), joint funds of NSFC (U1232109), MG and ZL are partially supported by the Youth Innovation Promotion Association of CAS (2015013, 2011013), YC and ZL acknowledge support of NSFC and DFG (CRC110)

We calculate diquark correlation functions in the Landau gauge on the lattice using overlap valence quarks and 2+1-flavor domain wall fermion configurations. Quark masses are extracted from the scalar part of quark propagators in the Landau gauge. The scalar diquark quark mass difference and axial vector scalar diquark mass difference are obtained for diquarks composed of two light quarks and of a strange and a light quark. The light sea quark mass dependence of the results is examined. Two lattice spacings are used to check the discretization effects. The coarse and fine lattices are of sizes 243 × 64 and 323 × 64 with inverse spacings 1/a = 1.75(4) GeV and 2.33(5) GeV, respectively.

Top-quark mass measurements using the ATLAS detector at the LHC

The most recent results of the top-quark mass measurements with the ATLAS detector using data collected from proton-proton collisions at the Large Hadron Collider are presented. Although several decay modes of the top-quark pairs have been used in ATLAS for top-quark mass measurements, only the latest results are presented (single lepton and dilepton channels). The top-quark pole mass from the tt‾ cross-section measurement in the dilepton channel and the top-antitop quark mass difference measurement in the single-lepton channel are also shown. The systematic uncertainties associated to these measurements are discussed in some detail.

Three symmetry breakings in strong and radiative decays of strange heavy mesons

In this paper, we investigate three symmetry breaking effects in strong and radiative decays of strange heavy mesons. We study 1/m_Q corrections within the heavy quark effect theory, as well as SU(3) and SU(2) symmetry breakings induced by light quark mass differences and the \eta-\pi mixing vertex. These effects are studied in a covariant model. The numerical results show that the 1/m_Q corrections of the coupling constants are consistent with \alpha_s \Lambda_{QCD}/m_Q. The SU(3) symmetry violating effect of the strong coupling constant is obviously larger than that of the magnetic coupling constant. The value of the \eta-\pi mixing vertex has some changes because of the renewed data. As compared with the other theoretical calculations and the experimental data, our radiative decay rates are much larger than those of the other theoretical methods, except for \chiPT; however, our branching ratios are close to the experimental data.

Light meson decays from photon-induced reactions with CLAS

Photoproduction experiments with the CEBAF Large Acceptance Spectrometer CLAS at the Thomas Jefferson National Facility produce data sets with competitive statistics of light mesons. With these data sets, measurements of transition form factors for η , ω , and η ′ mesons via conversion decays can be performed using the invariant mass distribution of the final state dileptons. Tests of fundamental symmetries and information on the light quark mass difference can be performed using a Dalitz plot analysis of the meson decay. An overview of preliminary results, from existing CLAS data, and future prospects within the newly upgraded CLAS12 apparatus are given.

Electromagnetic contribution to charge symmetry violation in parton distributions

We report a calculation of the combined effect of photon radiation and quark mass differences on charge symmetry violation (CSV) in the parton distribution functions of the nucleon. Following a recent suggestion of Martin and Ryskin, the initial photon distribution is calculated in terms of coherent radiation from the proton as a whole, while the effect of the quark mass difference is based on a recent lattice QCD simulation. The distributions are then evolved to a scale at which they can be compared with experiment by including both QCD and QED radiation. Overall, at a scale of 5 GeV2, the total CSV effect on the phenomenologically important difference between the d and u-quark distributions is some 20% larger than the value based on quark mass differences alone. In total these sources of CSV account for approximately 40% of the NuTeV anomaly.

Charmed baryon spectroscopy and light flavor symmetry from lattice QCD

We determine the ground state and first excited state masses of singly and doubly charmed spin 1/2 and 3/2 baryons with positive and negative parity. Configurations with $N_f=2+1$ non-perturbatively improved Wilson-clover fermions were employed, with the same quark action also being used for the valence quarks, including the charm. The spectrum is calculated for pion masses in the range $M_\pi \sim 259-460$ MeV at a lattice spacing $a\sim 0.075$ fm. Finite volume effects are studied comparing lattices with two different linear spatial extents ($1.8\,{\rm fm}$ and $2.4\,{\rm fm}$). The physical point is approached from the SU(3) limit keeping the flavour averaged light quark mass fixed. The baryon masses are extrapolated using expansions in the strange-light quark mass difference. Most particles fall into the expected SU(3) multiplets with well constrained extrapolations, the exceptions having a possibly more complex internal structure. Overall agreement is found with experiment for the masses and splittings of the singly charmed baryons. As part of the calculation an analysis of the lower lying charmonium, $D$ and $D_s$ spectra was performed in order to assess discretisation errors. The gross spectra are reproduced, including the $D^*_{s0}$, $D_{s1}$ and $D_1$ mesons, while at this single lattice spacing hyperfine splittings come out $10-20$ MeV too low.

Lattice determination of Sigma-Lambda mixing

Isospin breaking effects in baryon octet (and decuplet) masses are due to a combination of up and down quark mass differences and electromagnetic effects and lead to small mass splittings. Between the Sigma and Lambda this mass splitting is much larger, this being mostly due to their different wavefunctions. However when isospin is broken, there is a mixing between between these states. We describe the formalism necessary to determine the QCD mixing matrix and hence find the mixing angle and mass splitting between the Sigma and Lambda particles due to QCD effects.

SU(3)and isospin breaking effects onB→PPPamplitudes

Several modes of $B$ decays into three pseudoscalar octet mesons $PPP$ are measured. These decays provide useful information for $B$ decays in the standard model (SM). Some powerful tools in analyzing $B$ decays are flavor $SU(3)$ and isospin symmetries. Such analyses are usually hampered by $SU(3)$ breaking effects due to a relatively large strange quark mass which breaks $SU(3)$ symmetry down to isospin symmetry. The isospin symmetry also breaks down when the up and down quark mass difference is nonzero. It is, therefore, interesting to find relations which are not sensitive to $SU(3)$ and isospin breaking effects. We find that the relations among several fully symmetric $B\ensuremath{\rightarrow}PPP$ decay amplitudes are not affected by first-order $SU(3)$ breaking effects due to a nonzero strange quark mass, and also some of them are not affected by first isospin breaking effects. These relations, therefore, hold to good precisions. The measurements for these relations can provide important information about $B$ decays in the SM.

Measurements of top quark properties with the ATLAS detector

A selection of recent top quark measurements performed with the ATLAS detector is presented here, with data taken in 2011 and 2012 at centre-of-mass energies of 7 TeV and 8 TeV, respectively. The analyses cover the determination of the top pole mass by a cross-section measurement, the top/anti-top quark mass difference, the spin correlation and the charge asymmetry. No hints for physics beyond the Standard Model have been found but a great reduction of uncertainties compared to former results was achieved.