Charged Mesons Research Articles

Kaon to Pion Ratio in SU(3) PNJL Model

The article is devoted to investigation of the sharp peak in the $${{K}^{ + }}/{{\pi }^{ + }}$$ ratio, known as a ”horn”. We used the SU(3) PNJL model to show how the splitting of kaon mass can appear in dense matter and how it can affect the kaon to pion ratio. In the model we calculated the $$K/\pi $$ ratio of both positive and negative charged mesons, choosing temperature and baryonic chemical potential along the chiral phase transition line to show how matter properties can affect to the mesons ratio.

Measurements of CP asymmetries in charmless four-body {varLambda } ^0_{b} and {varXi } ^0_{b} decays

A search for C!P violation in charmless four-body decays of {varLambda } ^0_{b} and {varXi } ^0_{b} baryons with a proton and three charged mesons in the final state is performed. To cancel out production and detection charge-asymmetry effects, the search is carried out by measuring the difference between the C!P asymmetries in a charmless decay and in a decay with an intermediate charmed baryon with the same particles in the final state. The data sample used was recorded in 2011 and 2012 with the LHCb detector and corresponds to an integrated luminosity of 3 text { fb} ^{-1} . A total of 18 C!P asymmetries are considered, either accounting for the full phase space of the decays or exploring specific regions of the decay kinematics. No significant C!P-violation effect is observed in any of the measurements.

Photoproduction and electroproduction of Λ(1405) via γ(*)p→K*+π0Σ0

We study the photo- and electro-productions of the vector kaon off the proton, i.e., $\gamma^{(*)}p\to K^{*+}\pi^0\Sigma^0$, and investigate the line shape of the $\pi^0\Sigma^0$ invariant mass in an effective Lagrangian approach with the inclusion of a $K^*N\Lambda^*$ interaction. Relevant electromagnetic form factors for the neutral hyperons and charged strange mesons are constructed by considering experimental and theoretical information. We find that the $\Lambda^*$ peak is clearly observed for the photo- and electro-productions with the finite $K^*N\Lambda^*$ interaction, whereas the clear peak signals survive only for the electro-production, when we ignore the interaction. These different behaviors can be understood by different $Q^2$ dependences in the $K^*$ electromagnetic and $K^*\to\gamma K$ transition form factors. We suggest a photon-polarization asymmetry $\Sigma$ to extract information of the $K^*N\Lambda^*$ interaction. It turns out that $\Sigma$ near the $\Lambda^*$ peak region becomes negative with a finite $K^*N\Lambda^*$ interaction while positive without it for $Q^2 = 0$, due to the different naturalities of $K$ and $K^*$ exchanges. For $Q^2\ne 0$, we observe more obvious signals in the peak region due to the additional contribution of the longitudinal virtual photon for $\Lambda^*$.

Charged-lepton-flavor violation in |\u0394S| = 1 hyperon decays

Studies of lepton-flavor violation in strangeness-changing (|ΔS| = 1) transitions have a long tradition in the kaon sector where they provide some of the strongest limits on physics beyond the standard model. Recent hints of violation of lepton-flavor universality in B-meson decays have revived interest in lepton-flavor violation as the two phenomena appear simultaneously in many extensions of the standard model. At the same time, the LHCb experiment has produced new results for the hyperon process Σ+ → pμ+μ− and may be in a position to study other rare hyperon decay modes. With this in mind, we investigate |ΔS| = 1 hyperon decays into different-flavor lepton pairs e±μ∓ in a model-independent manner and contrast the coverage of parameter space that can be achieved with what is known from kaon modes. We include a comparison with selected two-body leptonic decays of charged mesons, with K → πν overline{v} modes, with μ → e conversion, and with lepton-flavor violating decays of other neutral mesons, all of which constrain the same parameter space in a complementary way.

Generalized parton distributions from charged current meson production

In this paper we prove that the simultaneous study of both $\rho$- and $\pi$-meson production by charged currents in Bjorken kinematics allows for a very clean extraction of the leading twist Generalized Parton Distributions of the target, with inherent control of the contribution of higher-twist corrections. Also, it might provide target-independent constraints on the distribution amplitudes of the produced mesons. We expect that such processes might be studied either in neutrino-induced or in electron-induced processes. According to our numerical estimates, the cross-sections of these processes are within the reach of JLab and EIC experiments.

Light flavour hadron and inclusive charged particle pT-spectra measured with the ALICE experiment at CERN

Light flavor hadron and inclusive charged particle spectra provide an interesting probe of the particle production mechanisms and the transport properties of QCD matter. Charged particle and neutral meson inclusive pT-differential cross sections in pp, p-Pb, Pb-Pb and Xe-Xe collisions have been measured by ALICE. Charged particles are studied in different multiplicity (for pp and p-Pb where available) and centrality classes (for A-A). The comparison of spectra at similar〈dNch/dη〉, the nuclear modification factors, the mean pTand particle ratios will also be presented considering those are observables important to get information on the properties of the medium formed in the collisions. Comparisons with model calculations will also be discussed.

Secret interactions of neutrinos with light gauge boson at the DUNE near detector

Secret interactions of neutrinos with light new gauge bosons, $Z^\prime$, can lead to a rich phenomenology in supernova explosion as well as in the early Universe. This interaction can also lead to new decay modes for charged mesons, $\pi^+ (K^+) \to e^+ \nu Z'$, and subsequently to $Z'\to \nu \bar{\nu}$. After demonstrating that such an interaction can be accommodated within viable electroweak symmetric models, we study how the near detector (ND) of DUNE can probe this scenario. We also discuss how the DUNE ND can make it possible to reconstruct the flavor structure of the $Z^\prime$ coupling to neutrinos.

Basis light front quantization for the charged light mesons with color singlet Nambu–Jona-Lasinio interactions

We apply the basis light front quantization (BLFQ) approach to describe the valence structures of the charged light meson ground states. Specifically, the light front wavefunctions of $\pi^\pm$, $\rho^\pm$, $K^\pm$, and $K^{*\pm}$ are obtained as the eigenvectors of the light front effective Hamiltonians with confinement potentials supplemented by the color singlet Nambu--Jona-Lasinio (NJL) interactions. We adjust our model such that the spectrum of these ground states and the charge radii of the pseudoscalar states agree with experimental results. We present the elastic form factors and parton distribution amplitudes (PDAs) as illustrations of the internal structures of the pseudoscalar pions and kaons in terms of valence quarks.

Energy and system dependence of nuclear modification factors of inclusive charged particles and identified light hadrons measured in p–Pb, Xe–Xe and Pb–Pb collisions with ALICE

We report recent ALICE results on primary charged particle and neutral meson production in pp, p–Pb, Pb–Pb and Xe–Xe collisions at LHC energies. In this article, measurements of the nuclear modification factors RAA of primary charged particles and of light neutral mesons in Pb–Pb, in Xe–Xe and in p–Pb collisions in a wide pT range and different centrality classes are discussed. We compare the nuclear modification factors obtained for different collision systems as a function of transverse momentum, collision centrality as well as charged particle multiplicity (dNch/dη). We also present comparison of experimental results to model calculations.

Lorentz-violating nonminimal coupling contributions in mesonic hydrogen atoms

We have studied a Lorentz-violating scalar electrodynamics in which the gauge and scalar fields interact via a nonminimal covariant derivative containing CPT-odd and CPT-even backgrounds. By considering the complex scalar field describes a charged meson we have analyzed the contributions to the mesonic hydrogen energy. By using the experimental data for the 1S strong correction shift and for the pure QED transitions 4P→3P, the best upper-bound for the CPT-odd coupling is <10−3GeV−1 and for the CPT-even one is <10−2GeV−2. Besides, both the nonminimal couplings generate radiative higher-order derivative corrections that are contained in the nonminimal photon sector of the standard model extension. We also have studied their respective contributions to the bound-state energies of the hydrogen-like atoms and determined similar upper-bounds for the Lorentz-violating coefficients.

Testing of almost all the hadronic interaction models by comparing calculated muon energy spectrum with data

Uncertainties of the model energy spectra of the most energetic secondary charged mesons are discussed. Computer simulations of the partial energy spectra of the atmospheric vertical muons induced by primary cosmic particles with various fixed energies in terms of hadronic interactions models had been carried out with the help of the CORSIKA package. These partial spectra have been convolved with the contemporary spectra of the primary cosmic particles in the energy range 0.1-10 000 TeV. Results of simulations are compared with the contemporary data of the atmospheric vertical muon flux. Comparison shows that all models underestimate the production of secondary charged π±-mesons (and K±-mesons) by a factor of ~ 1.4 ÷ 2 at the highest energies. This underestimation induces a more rapid development of extensive air showers in the atmosphere and results in uncertainties in estimates of energy and composition of the primary cosmic particles.

New results on energy and momentum conservation in meson production for A+A collisions at SPS energies

We review our recent findings on electromagnetic (EM) effects in Ar+Sc and Pb+Pb collisions at CERN SPS energies, and discuss them in the context of our earlier results on the EM splitting of π+/π- meson directed flow. A specific space-time picture emerges, on the basis of which we construct a simple model of the collision. Starting from local energy and momentum conservation, we nicely describe the centrality dependence of π meson rapidity distributions in Pb+Pb collisions at $\rad{}{s_{NN} } = 17.3\,{\rm{GeV}}$. We discuss the resulting implications on the role of energy and momentum conservation for the dynamics of meson production in A+A collisions. We present our conclusions on the link between the initial stages of the nuclear collision and the final state observables connected to strong and electromagnetic phenomena in charged meson emission. We also discuss the possibility to use the latter EM phenomena to obtain new insight into the spectator space-time evolution and excitation energy.

Deeply virtual meson production on neutrons

In this paper we analyze in detail how the measurements of exclusive electroproduction of mesons on neutrons would complement the studies of generalized parton distributions (GPDs) of the proton, providing independent experimental observables. Some of these processes on neutrons have very distinctive features, and thus we expect that measurements on liquid deuterium would allow to clearly distinguish them from similar processes on protons, giving a very clean probe of the GPD. In the case of charged meson production, all produced hadrons are charged, and for this reason we expect that the kinematics of this process could be easily reconstructed. We estimate the cross-sections in the kinematics of the Jefferson Laboratory experiments using current phenomenological GPD models.

Light-by-light scattering in the presence of magnetic fields

The low-energy light-by-light cross section as determined by the nonlinear Euler-Heisenberg QED Lagrangian is evaluated in the presence of constant magnetic fields in the center-of-mass system of the colliding photons. This cross section has a complicated dependence on directions and polarizations. The overall magnitude decreases as the magnetic field is increased from zero, but this trend is reversed for ultrastrong magnetic fields $B\gtrsim B_c$, where the cross section eventually grows quadratically with the magnetic field strength perpendicular to the collision axis. This effect is due to interactions involving the lowest Landau level of virtual Dirac particles; it is absent in scalar QED. An even more dramatic effect is found for virtual charged vector mesons where the one-loop cross section diverges at the critical field strength due to an instability of the lowest Landau level and the possibility of the formation of a superconducting vacuum state. We also discuss (the absence of) implications for the recent observation of light-by-light scattering in heavy-ion collisions.

N* Photoexcitation Results from Bonn

To investigate the excitation spectrum of the nucleons, photoproduction of mesons off polarized or unpolarized nucleons poses an ideal tool. The ELSA accelerator in Bonn provides electrons with energies up to 3.2 GeV, which can produce photon beams by bremsstrahlung off different radiator targets. Currently, there are two different experiments taking data: the BGO-OD and the CBELSA/TAPS experiment. Both are able to extract polarization observables for different final states, including charged and uncharged mesons. This contribution reports on the selection of different final states of both experiments and discusses the extracted observables. In addition, a truncated partial wave analysis is shown, which gives first insight into the sensitivity of the observables.

Neutral and charged scalar mesons, pseudoscalar mesons, and diquarks in magnetic fields

We investigate both (pseudo)scalar mesons and diquarks in the presence of external magnetic field in the framework of the two-flavored Nambu--Jona-Lasinio (NJL) model, where mesons and diquarks are constructed by infinite sum of quark-loop chains by using random phase approximation. The polarization function of the quark-loop is calculated to the leading order of $1/N_c$ expansion by taking the quark propagator in the Landau level representation. We systematically investigate the masses behaviors of scalar $\sigma$ meson, neutral and charged pions as well as the scalar diquarks, with respect to the magnetic field strength at finite temperature and chemical potential. It is shown that the numerical results of both neutral and charged pions are consistent with the lattice QCD simulations. The mass of the charge neutral pion keeps almost a constant under the magnetic field, which is preserved by the remnant symmetry of QCD$\times$QED in the vacuum. The mass of the charge neutral scalar $\sigma$ is around two times quark mass and increases with the magnetic field due to the magnetic catalysis effect, which is an typical example showing that the polarized internal quark structure cannot be neglected when we consider the meson properties under magnetic field. For the charged particles, the one quark-antiquark loop contribution to the charged $\pi^{\pm}$ increases essentially with the increase of magnetic fields due to the magnetic catalysis of the polarized quarks. However, the one quark-quark loop contribution to the scalar diquark mass is negative comparing with the point-particle result and the loop effect is small.

Collective flows of pions in Au+Au collisions at energies 1.0 and 1.5 GeV/nucleon

Based on the newly updated version of the ultrarelativistic quantum molecular dynamics (UrQMD) model, the pion potentials obtained from the in-medium dispersion relation of the $\Delta$-hole model and from the modified phenomenological approach are further introduced. Both the rapidity $y_0$ and transverse-velocity $u_{t0}$ dependence of directed $v_1$ and elliptic $v_2$ flows of ${\pi}^{+}$ and ${\pi}^{-}$ charged mesons produced from Au+Au collisions at two beam energies of 1.0 GeV/nucleon and 1.5 GeV/nucleon and within a large centrality region of $0<b_0<0.55$ are scanned. Calculations with pion potentials as well as without considering the pion potential are compared to the newly experimental data released by the FOPI collaboration at GSI. It is found that the directed flow is more sensitive to the pion potential than the elliptic one, and the attractive pion potential from the phen.B mode of the phenomenological approach is too strong to describe the flow data and can be safely ruled out. The relatively weak pion potential from the $\Delta$-hole model can supply a good description for the FOPI data of both flows as functions of both centrality and rapidity. A two-peak/valley structure occurs in the transverse-velocity dependent directed flow but the elliptic flow drops monotonously with increasing $u_{t0}$. Finally, both $v_1$ and $v_2$ flows with large $u_{t0}$ from semi-central heavy ion collisions can be taken as sensitive probes for the pion potential.

Meson properties in magnetized quark matter

We study neutral and charged meson properties in magnetic field. Taking bosolization method in a two-flavor Nambu--Jona-Lasinio model, we derive effective meson Lagrangian density with minimal coupling to the magnetic field, by employing derivative expansion for both the meson fields and Schwinger phases. We extract from the effective Lagrangian density the meson curvature, pole and screening masses. As the only Goldstone mode, the neutral pion controls the thermodynamics of the system and propagates the lang range quark interaction. The magnetic field breaks down the space symmetry, and the quark interaction region changes from a sphere in vacuum to a ellipsoid in magnetic field.

Measurement of branching fractions of charmless four-body \u039bb0 and \u039eb0 decays

A search for charmless four-body decays of Λb0 and Ξb0 baryons with a proton and three charged mesons (either kaons or pions) in the final state is performed. The data sample used was recorded in 2011 and 2012 with the LHCb experiment and corresponds to an integrated luminosity of 3 fb−1. Six decay modes are observed, among which Λb0 → pK−π+π−, Λb0 → pK−K+K−, Ξb0 → pK−π+π− and Ξb0 → pK−π+K− are established for the first time. Their branching fractions (including the ratio of hadronisation fractions in the case of the Ξb0 baryon) are determined relative to the Λb0 → Λc+π− decay.

Search for CP violation in the D+→π+π0 decay at Belle

We search for $CP$ violation in the charged charm meson decay $D^{+}\to\pi^{+}\pi^{0}$, based on a data sample corresponding to an integrated luminosity of $\rm 921~fb^{-1}$ collected by the Belle experiment at the KEKB $e^{+}e^{-}$ asymmetric-energy collider. The measured $CP$ violating asymmetry is $[+2.31\pm1.24({\rm stat})\pm0.23({\rm syst})]\%$, which is consistent with the standard model prediction and has a significantly improved precision compared to previous results.