Showing papers on "Meson published in 2019"

The $XYZ$ states: experimental and theoretical status and perspectives

Abstract: The quark model was formulated in 1964 to classify mesons as bound states made of a quark-antiquark pair, and baryons as bound states made of three quarks. For a long time all known mesons and baryons could be classified within this scheme. Quantum Chromodynamics (QCD), however, in principle also allows the existence of more complex structures, generically called exotic hadrons or simply exotics. These include four-quark hadrons (tetraquarks and hadronic molecules), five-quark hadrons (pentaquarks) and states with active gluonic degrees of freedom (hybrids), and even states of pure glue (glueballs). Exotic hadrons have been systematically searched for in numerous experiments for many years. Remarkably, in the past fifteen years, many new hadrons that do not exhibit the expected properties of ordinary (not exotic) hadrons have been discovered in the quarkonium spectrum. These hadrons are collectively known as $XYZ$ states. Some of them, like the charged states, are undoubtedly exotic. Parallel to the experimental progress, the last decades have also witnessed an enormous theoretical effort to reach a theoretical understanding of the $XYZ$ states. Theoretical approaches include not only phenomenological extensions of the quark model to exotics, but also modern non-relativistic effective field theories and lattice QCD calculations. The present work aims at reviewing the rapid progress in the field of exotic $XYZ$ hadrons over the past few years both in experiments and theory. It concludes with a summary on future prospects and challenges.

B →P and B →V form factors from B-meson light-cone sum rules beyond leading twist

Abstract: We provide results for the full set of form factors describing semileptonic B-meson transitions to pseudoscalar mesons π, K, $$ \overline{D} $$ and vector mesons ρ, K∗, $$ {\overline{D}}^{\ast } $$ . Our results are obtained within the framework of QCD Light-Cone Sum Rules with B-meson distribution amplitudes. We recalculate and confirm the results for the leading-twist two-particle contributions in the literature. Furthermore, we calculate and provide new expressions for the two-particle contributions up to twist four. Following new developments for the three-particle distribution amplitudes, we calculate and provide new results for the complete set of three-particle contributions up to twist four. The form factors are computed numerically at several phase space points using up-to-date input parameters, including correlations across phase space points and form factors. We use a model ansatz for all contributing B-meson distribution amplitudes that is self-consistent up to twist-four accuracy. We find that the higher-twist two-particle contributions have a substantial impact on the results, and dominate over the three-particle contributions. Our numerical results, including correlations, are provided as machine-readable files in the supplementary material. We discuss the qualitative phenomenological impact of our results on the present b anomalies.

Isospin breaking decays as a diagnosis of the hadronic molecular structure of the P c (4457 )

Abstract: The LHCb Collaboration announced the observation of three narrow structures consistent with hidden-charm pentaquark states. They are candidates of hadronic molecules formed of a pair of a charmed baryon and an anticharmed meson. Among them, the Pc(4457) mass is consistent with earlier predictions of a ΣcD¯* molecule with I=1/2. We point out that if such a picture were true, one would have B(Pc(4457)→J/ψΔ+)/B(Pc(4457)→J/ψp) at the level ranging from a few percent to about 30%. Such a large isospin breaking decay ratio is two to three orders of magnitude larger than that for normal hadron resonances. It is a unique feature of the ΣcD¯* molecular model, and can be checked by LHCb.

Study of doubly heavy tetraquarks in Lattice QCD

Abstract: We present the results of a lattice calculation of tetraquark states with quark contents ${q}_{1}{q}_{2}\overline{Q}\overline{Q},{q}_{1},{q}_{2}\ensuremath{\subset}u,d,s,c$ and $Q\ensuremath{\equiv}b$, $c$ in both spin-0 ($J=0$) and spin-1 ($J=1$) sectors. This calculation is performed on three dynamical ${N}_{f}=2+1+1$ highly improved staggered quark ensembles at lattice spacings of about 0.12, 0.09, and 0.06 fm. We use the overlap action for light to charm quarks, while a nonrelativistic action with nonperturbatively improved coefficients with terms up to $\mathcal{O}({\ensuremath{\alpha}}_{s}{v}^{4})$ is employed for the bottom quark. While considering charm or bottom quarks as heavy, we calculate the energy levels of various four-quark configurations with light quark masses ranging from the physical strange quark mass to that of the corresponding physical pion mass. This enables us to explore the quark mass dependence of the extracted four-quark energy levels over a wide range of quark masses. The results of the spin-1 states show the presence of ground state energy levels which are below their respective thresholds for all the light flavor combinations. Further, we identify a trend that the energy splittings, defined as the energy difference between the ground state energy levels and their respective thresholds, increase with decreasing the light quark masses and are maximum at the physical point for all the spin-1 states. The rate of increase is, however, dependent on the light quark configuration of the particular spin-1 state. We also present a study of hadron mass relations involving tetraquarks, baryons, and mesons arising in the limit of infinitely heavy quarks and find that these relations are more compatible with the heavy quark limit in the bottom sector but deviate substantially in the charm sector. The ground state spectra of the spin-0 tetraquark states with various flavor combinations are seen to lie above their respective thresholds.

Toward the determination of heavy-quark transport coefficients in quark-gluon plasma

Abstract: Several transport models have been employed in recent years to analyze heavy-flavor meson spectra in high-energy heavy-ion collisions Heavy-quark transport coefficients extracted from these models with their default parameters vary, however, by up to a factor of 5 at high momenta To investigate the origin of this large theoretical uncertainty, a systematic comparison of heavy-quark transport coefficients is carried out between various transport models Within a common scheme devised for the nuclear modification factor of charm quarks in a brick medium of a quark-gluon plasma, the systematic uncertainty of the extracted drag coefficient among these models is shown to be reduced to a factor of 2, which can be viewed as the smallest intrinsic systematical error band achievable at present time This indicates the importance of a realistic hydrodynamic evolution constrained by bulk hadron spectra and of heavy-quark hadronization for understanding the final heavy-flavor hadron spectra and extracting heavy-quark drag coefficient The transverse transport coefficient is less constrained due to the influence of the underlying mechanism for heavy-quark medium interaction Additional constraints on transport models such as energy loss fluctuation and transverse-momentum broadening can further reduce theoretical uncertainties in the extracted transport coefficients

Hadronic interaction model sibyll 2.3c and inclusive lepton fluxes

Abstract: Muons and neutrinos from cosmic ray interactions in the atmosphere originate from decays of mesons in air-showers. Sibyll-2.3c aims to give a precise description of hadronic interactions in the relevant phase space for conventional and prompt leptons in light of new accelerator data, including that from the LHC. Sibyll is designed primarily as an event generator for use in simulation of extensive air showers. Because it has been tuned for forward physics as well as the central region, it can also be used to calculate inclusive fluxes. The purpose of this paper is to describe the use of Sibyll-2.3c for calculation of fluxes of atmospheric leptons.

Threshold cusps and triangle singularities in hadronic reactions

Abstract: The spectrum of hadrons is the manifestation of color confinement of quantum chromodynamics. Hadronic resonances correspond to poles of the S-matrix. Since 2003, lots of new hadron resonant structures were discovered in the mass regions from light mesons to hadrons containing a pair of a heavy quark and an antiquark. Many of them are candidates of exotic hadrons, and they are usually observed as peaks in invariant mass distributions. However, the S-matrix also has kinematical singularities due to the on-shellness of intermediate particles for a process, such as two-body thresholds and triangle singularities, and they can produce peaks as well. On the one hand, such singularities may be misidentified as resonances; on the other hand, they can be used as tools for precision measurements. In this paper, we review the threshold cusps and various triangle singularities in hadronic reactions, paying attention to their manifestations in phenomena related to exotic hadron candidates.

Light-meson leptonic decay rates in lattice QCD+QED

Abstract: The leading electromagnetic and strong isospin-breaking corrections to the decay rates of light pseudoscalar mesons are computed for the first time in a lattice QCD simulation. Combining with experimentally measured decay rates, the authors obtain the Cabibbo-Kobayashi-Maskawa matrix element $|{V}_{u\phantom{\rule{0}{0ex}}s}|=0.22538(46)$, the most precise value, so far.

Meson screening masses in (2 +1 )-flavor QCD

Abstract: We present lattice QCD results for mesonic screening masses in the temperature range $140\text{ }\text{ }\mathrm{MeV}\ensuremath{\lesssim}T\ensuremath{\lesssim}2500\text{ }\mathrm{MeV}$. Our calculations were carried out using ($2+1$) flavors of the highly improved staggered quark action, with a physical value for the strange quark mass and two values of the light quark mass corresponding to pion masses of 160 and 140 MeV. Continuum-extrapolated results were obtained using calculations with a variety of lattice spacings corresponding to temporal lattice extents ${N}_{\ensuremath{\tau}}=6--16$. We discuss the implications of these results for the effective restoration of various symmetries in the high temperature phase of QCD, as well as the approach toward the perturbative limit.

Determination of the pole position of the lightest hybrid meson candidate

Abstract: Mapping states with explicit gluonic degrees of freedom in the light sector is a challenge, and has led to controversies in the past. In particular, the experiments have reported two different hybrid candidates with spin-exotic signature, pi¬_1(1400) and pi_1 (1600), which couple separately to eta pi and eta'pi. This picture is not compatible with recent Lattice QCD estimates for hybrid states, nor with most phenomenological models. We consider the recent partial wave analysis of the eta^(')pi system by the COMPASS Collaboration. We fit the extracted intensities and phases with a coupled-channel amplitude that enforces the unitarity and analyticity of the S matrix. We provide a robust extraction of a single exotic pi_1 resonant pole, with mass and width 1564 +/- 24 +/- 86 and 492 +/- 54 +/- 102 MeV, which couples to both eta^(')pi channels. We find no evidence for a second exotic state. We also provide the resonance parameters of the a_2 (1320) and a_2^' (1700).

Baryogenesis and dark matter from B mesons

Abstract: We present a new mechanism of baryogenesis and dark matter production in which both the dark matter relic abundance and the baryon asymmetry arise from neutral $B$ meson oscillations and subsequent decays. This setup is testable at hadron colliders and $B$ factories. In the early universe, decays of a long lived particle produce $B$ mesons and antimesons out of thermal equilibrium. These mesons/antimesons then undergo $CP$ violating oscillations before quickly decaying into visible and dark sector particles. Dark matter will be charged under the baryon number so that the visible sector baryon asymmetry is produced without violating the total baryon number of the Universe. The produced baryon asymmetry will be directly related to the leptonic charge asymmetry in neutral $B$ decays: an experimental observable. Dark matter is stabilized by an unbroken discrete symmetry, and proton decay is simply evaded by kinematics. We will illustrate this mechanism with a model that is unconstrained by dinucleon decay, does not require a high reheat temperature, and would have unique experimental signals---a positive leptonic asymmetry in $B$ meson decays, a new decay of $B$ mesons into a baryon and missing energy, and a new decay of $b$-flavored baryons into mesons and missing energy. These three observables are testable at current and upcoming collider experiments, allowing for a distinct probe of this mechanism.

First Measurement of Charm Production in its Fixed-Target Configuration at the LHC.

Abstract: The first measurement of heavy-flavor production by the LHCb experiment in its fixed-target mode is presented. The production of J/ψ and D0 mesons is studied with beams of protons of different energies colliding with gaseous targets of helium and argon with nucleon-nucleon center-of-mass energies of sNN=86.6 and 110.4 GeV, respectively. The J/ψ and D0 production cross sections in pHe collisions in the rapidity range [2, 4.6] are found to be σJ/ψ=652±33(stat)±42(syst) nb/nucleon and σD0=80.8±2.4(stat)±6.3(syst) μb/nucleon, where the first uncertainty is statistical and the second is systematic. No evidence for a substantial intrinsic charm content of the nucleon is observed in the large Bjorken-x region.

Meson condensation.

Abstract: We give a pedagogical review of the properties of the various meson condensation phases triggered by a large isospin or strangeness imbalance. We argue that these phases are extremely interesting and powerful playground for exploring the properties of hadronic matter. The reason is that they are realized in a regime in which various theoretical methods overlap with increasingly precise numerical lattice QCD simulations, providing insight on the properties of color confinement and of chiral symmetry breaking.

R -parity violation and light neutralinos at CODEX-b, FASER, and MATHUSLA

Abstract: The $LQ\overline{D}$ operator in $R$-parity-violating supersymmetry can lead to meson decays to light neutralinos and neutralino decays to lighter mesons, with a long lifetime. Since the high-luminosity LHC is expected to accumulate as much as $3/\mathrm{ab}$ of data, several detectors proposed to be built at the LHC may probe unexplored regions in the parameter space, for long-lived neutralinos. We estimate the sensitivity of the recently proposed detectors, CODEX-b, FASER, and MATHUSLA, for detecting such light neutralinos singly produced from $D$- and $B$-meson decays in a list of benchmark scenarios, and discuss the advantages and disadvantages of the proposed detectors in this context. We also present our results in a model-independent fashion, which can be applied to any long-lived particle with mass in the GeV regime.

Λ and Λ ¯ spin interaction with meson fields generated by the baryon current in high energy nuclear collisions

Abstract: We propose a dynamical mechanism which provides an interaction between the spins of hyperons and antihyperons and the vorticity of the baryon current in noncentral high energy nuclear collisions. The interaction is mediated by massive vector and scalar bosons, which is well known to describe the nuclear spin-orbit force. It follows from the Foldy-Wouthuysen transformation and leads to a strong-interaction Zeeman effect. The interaction may explain the difference in polarizations of $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ hyperons as measured by the STAR Collaboration at the BNL Relativistic Heavy Ion Collider. The signs and magnitudes of the meson-baryon couplings are closely connected to the binding energies of hypernuclei and to the abundance of hyperons in neutron stars.

Parton Distribution Functions from a Light Front Hamiltonian and QCD Evolution for Light Mesons.

Abstract: We obtain the pion and the kaon parton distribution functions from the eigenstates of a light front effective Hamiltonian in the constituent quark-antiquark representation suitable for low-momentum scale applications. By taking these scales as the only free parameters, the valence quark distribution functions of the pion, after QCD evolution, are consistent with the data from the FNAL-E615 experiment. The ratio of the up quark distribution of the kaon to that of the pion also agrees with the CERN-NA3 experiment. Supplemented by known parton distribution functions for the nucleons, we further obtain the cross section consistent with experimental data for the ${\ensuremath{\pi}}^{\ensuremath{-}}\text{nucleus}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}X$ Drell-Yan process.

Mesons with Beauty and Charm: New Horizons in Spectroscopy

Abstract: Using a modified, retuned, Cornell potential that includes the running and large distance freezing of the strong coupling constant, the authors compute the spectroscopy, production mechanisms, and decays of excited ${B}_{c}$ meson.

Pions in magnetic field at finite temperature

Abstract: Pions in an external magnetic field are investigated in the frame of a Pauli-Villars regularized Nambu--Jona-Lasinio model The meson propagators in terms of quark bubbles in Ritus and Schwinger schemes are analytically derived, and pion masses are numerically calculated in the Ritus scheme For neutral and charged pions at finite temperature, there exist mass jumps at the Mott transition points due to the discrete energy levels of the two constituent quarks in the magnetic field

η ' and η mesons with connection to anomalous glue

Abstract: Properties of hadrons are emergent from the more fundamental quark and gluon degrees of freedom of quantum chromodynamics (QCD). In the hadronic zoo, \ensuremath{\eta} and \ensuremath{\eta}\ensuremath{'} mesons are unique laboratories of gluon dynamics: measurements of \ensuremath{\eta} and \ensuremath{\eta}\ensuremath{'} production are sensitive to behavior of QCD symmetries at finite density and temperature. This review surveys \ensuremath{\eta} and \ensuremath{\eta}\ensuremath{'} meson physics as a probe of QCD dynamics emphasizing recent advances in experiment and theory.

Masses of Light and Heavy Mesons and Baryons: A Unified Picture

Abstract: We compute masses of positive-parity spin-$1/2$ and -$3/2$ baryons composed of $u$, $d$, $s$, $c$ and $b$ quarks in a quark-diquark picture. The mathematical foundation for this analysis is implemented through a symmetry-preserving Schwinger-Dyson equations treatment of a vector-vector contact interaction, which preserves key features of quantum chromodynamics, such as confinement, chiral symmetry breaking and low-energy Goldberger-Treiman relations. This study requires a computation of diquark correlations containing these quarks which in turn are readily inferred from solving the Bethe-Salpeter equations of the corresponding mesons. Therefore, it serves as a unified formalism for a multitude of mesons and baryons. It builds on our previous works on the study of masses, decay constants and form factors of quarkonia and light mesons, employing the same model. We use two sets of parameters, one which remains exactly the same for both the light and heavy sector hadrons, and another where the coupling strength is allowed to evolve according to the available mass scales of quarks. Our results are in very good agreement with the existing experimental data as well as predictions of other theoretical approaches whenever comparison is possible.

Phenomenology of GeV-scale scalar portal

Abstract: We review and revise the phenomenology of the scalar portal — a new scalar particle with the mass in GeV range that mixes with the Higgs boson. In particular, we consider production channels B → SK1(1270) and $$ B\to S{K}_0^{\ast }(700) $$ and show that their contribution is significant. We extend the previous analysis by comparing the production of scalars from decays of mesons, of the Higgs bosons and direct production via proton bremsstrahlung, deep inelastic scattering and coherent scattering on nuclei. Relative efficiency of the production channels depends on the energy of the beam and we consider the energies of DUNE, SHiP and LHC-based experiments. We present our results in the form directly suitable for calculations of experimental sensitivities.

Neutral meson properties in hot and magnetized quark matter: A new magnetic field independent regularization scheme applied to an NJL-type model

Abstract: A magnetic field independent regularization scheme (zMFIR) based on the Hurwitz-Riemann zeta function is introduced. The new technique is applied to the regularization of the mean-field thermodynamic potential and mass gap equation within the SU(2) Nambu-Jona-Lasinio model in a hot and magnetized medium. The equivalence of the new and the standard MFIR scheme is demonstrated. The neutral meson pole mass is calculated in a hot and magnetized medium and the advantages of using the new regularization scheme are shown.

Towards a precise determination of the scattering amplitudes of the charmed and light-flavor pseudoscalar mesons

Abstract: We study the scattering of the light-flavor pseudoscalar mesons ( $$\pi , K, \eta $$ ) off the ground-state charmed mesons ( $$D,D_s$$ ) within chiral effective field theory. The recent lattice simulation results on various scattering lengths and the finite-volume spectra both in the moving and center-of-mass frames, most of which are obtained at unphysical meson masses, are used to constrain the free parameters in our theory. Explicit formulas to include the S- and P-wave mixing to determine the finite-volume energy levels are provided. After a successful reproduction of the lattice data, we perform a chiral extrapolation to predict the quantities with physical meson masses, including phase shifts, inelasticities, resonance pole positions and the corresponding residues from the scattering of the light pseudoscalar and charmed mesons.

The energy-momentum tensor of spin-1 hadrons: formalism

Abstract: We provide the complete decomposition of the local gauge-invariant energy-momentum tensor for spin-1 hadrons, including non-conserved terms for the individual parton flavors and antisymmetric contributions originating from intrinsic spin. We state sum rules for the gravitational form factors appearing in this decomposition and provide relations for the mass decomposition, work balance, total and orbital angular momentum, mass radius, and inertia tensor. Generalizing earlier work, we derive relations between the total and orbital angular momentum and the Mellin moments of twist-2 and 3 generalized parton distributions, accessible in hard exclusive processes with spin-1 targets. Throughout the work, we comment on the unique features in these relations originating from the spin-1 nature of the hadron, being absent in the lower spin cases.

Exclusive semileptonic decays of D and Ds mesons in the covariant confining quark model

Abstract: Recently, the BESIII collaboration has reported numerous measurements of various D(s) meson semileptonic decays with significantly improved precision. Together with similar studies carried out at BABAR, Belle, and CLEO, new windows to a better understanding of weak and strong interactions in the charm sector have been opened. In light of new experimental data, we review the theoretical description and predictions for the semileptonic decays of D(s) to a pseudoscalar or a vector meson. This review is essentially an extended discussion of our recently published results obtained in the framework of the covariant confining quark model.