Neutral Meson Research Articles

Branching Fractions and CP Violations in Decays of Charged and Neutral Beauty Mesons to Pion and Kaon Mesons

Branching Fractions and CP Violations in Decays of Charged and Neutral Beauty Mesons to Pion and Kaon Mesons

Effects of isospin violation in the e+e−→ B(*) overline{B} (*) cross sections

Model estimates are obtained for the influence of Coulomb effects on the ratio of the cross sections for the production of charged and neutral B overline{B} and B* overline{B} * pairs in e+e− annihilation. It is shown that the difference between the masses of charged and neutral mesons obtained under the assumption that this ratio is constant on a scale of the order of the beam energy spread can differ from the true one by δM ∼ 0.03 MeV at the energy of Υ(5S) and by δM ∼ 0.4 MeV at the energy of Υ(4S). Thus, the errors given in the PDG for the difference between the masses of charged and neutral B mesons, based on the results obtained at the energy of Υ(4S), are strongly underestimated. Similar measurements at the energy of Υ(5S) will have an order of magnitude smaller systematic shift for the mass difference. This circumstance should be taken into account when planning future experiments at the B factory in KEK.

Search for new physics with rare charm decays at LHCb

The LHCb experiment is playing a crucial role in the study of rare and forbidden decays of charm hadrons, which are unique probes for hints of physics beyond the Standard Model. These proceedings present results of the search for semileptonic three-body decays of the form $D_{(s)}^+$ \rightarrow $h^\pm$ \ellp $\ell^{(')\mp}$, where $h^\pm$ is a charged pion or kaon and $\ell^{(')\mp}$ is an electron or a muon. Furthermore, studies of the four-body neutral meson charm decays $D^0 \rightarrow \pi^+ \pi^- \mu^+ \mu^-$ and $D^0 \rightarrow K^+ K^- \mu^+ \mu^-$ are discussed. Prospects for leptonic and semileptonic rare charm decays at LHCb in the future are presented in the end.

RGE effects on the LFV scale from meson decays

We consider the lepton-flavor violating (LFV) lepton-quark dimension-6 operators and analyze their contributions to the LFV leptonic decays of vector, pseudoscalar, and scalar neutral mesons Mrightarrow ell _1 ell _2 as well as to mu (tau ) rightarrow ell ee, ell gamma gamma decays. These operators contribute to the purely leptonic processes via quark loop. On the basis of quark-hadron duality, we relate these loops to the appropriate meson-exchange contributions. In this way, we extract lower bounds on the individual scales of the studied LFV operators from the experimental and phenomenological limits on the leptonic decays of mesons and leptons. As a byproduct, we shall obtain new limits on the LFV leptonic decays of flavored mesons from the experimental bounds on the three-body lepton decays. We study the effects of QED and QCD radiative corrections to the LFV lepton-quark operators in question. We derive for them the one-loop matrix of the RGE evolution and examine its effect on the previously derived tree-level limits on these operators. We show that the QED corrections are particularly relevant due to operator mixing. Specifically, for some of them the limits on their individual LFV scales improve by up to 3 orders of magnitude.

Overview of Direct Photon and Neutral Meson Measurements with ALICE at the LHC

Overview of Direct Photon and Neutral Meson Measurements with ALICE at the LHC

Dyson-Schwinger equations and the muon g-2

We present a brief introduction to the Dyson-Schwinger equations (DSEs) approach to hadron and high-energy physics. In particular, how this formalism is applied to calculate the electromagnetic form factors $\gamma^* \gamma^* \to \textbf{P}^0$ and $\gamma^* \textbf{P}^\pm \to \textbf{P}^\pm$ (with $\textbf{P}^\pm$ and $\textbf{P}^0$ charged and neutral ground-state pseudoscalar mesons, respectively) is discussed. Subsequently, the corresponding contributions of those form factors to the muon anomalous magnetic moment ($g-2$) are estimated. We look forward to promoting the DSE approach to address theoretical aspects of the muon $g-2$, highlighting some calculations that could be carried out in the future.

On the radius of spatial analyticity for the Klein-Gordon-Schrödinger system

In this paper, we study the persistence of spatial analyticity for the solutions to the Klein-Gordon-Schrödinger system, which describes a physical system of a nucleon field interacting with a neutral meson field, with analytic initial data. Unlike the case of a single nonlinear dispersive equation, not much is known about nonlinear dispersive systems as it is harder to show the spatial analyticity of coupled equations simultaneously. The only results known so far are rather recent ones for the Dirac-Klein-Gordon system which governs the physical system when the nucleon is described by Dirac spinor fields in the case of relativistic fields. In contrast, we aim here to study the Klein-Gordon-Schrödinger system that works in the non-relativistic regime. It is shown that the radius of spatial analyticity of the solutions at later times obeys an algebraic lower bound as time goes to infinity.

Properties of neutral mesons in a hot and magnetized quark matter: Size-dependent effects

In this work we investigate the combined finite-size and thermomagnetic effects on the properties of neutral mesons in a hot medium, in the context of the Nambu--Jona-Lasinio model. In particular, by using the mean-field approximation and the Schwinger proper time method in a toroidal topology with periodic and antiperiodic conditions, we evaluate the chiral phase transition, the critical coupling, the constituent quark mass and meson observables like the ${\ensuremath{\pi}}^{0}$ and $\ensuremath{\sigma}$ meson masses and pion decay constant under the change of the size, temperature and strength of external magnetic field. The results indicate that the observables are strongly affected by the conjoint effects of relevant variables and also by the periodicity of the boundary conditions chosen, and the net result will depend on the balance of these competing phenomena.

Chiral condensates and screening masses of neutral pseudoscalar mesons in thermomagnetic QCD medium

We point out that chiral condensates at nonzero temperature and magnetic fields are in strict connection to the space-time integral of corresponding two-point neutral meson correlation functions in the pseudoscalar channel via the Ward-Takahashi identity. Screening masses of neutral pseudoscalar mesons, which are defined as the exponential decay of the corresponding spatial correlation functions in the long distance, thus are intrinsically connected to (inverse) magnetic catalysis of chiral condensates. To study this we performed lattice simulations of ($2+1$)-flavor QCD on ${32}^{3}\ifmmode\times\else\texttimes\fi{}{N}_{t}$ lattices with pion mass ${M}_{\ensuremath{\pi}}\ensuremath{\simeq}220\text{ }\text{ }\mathrm{MeV}$ in a fixed scale approach having temperature $T\ensuremath{\in}[17,281]\text{ }\text{ }\mathrm{MeV}$ and magnetic field strength $eB\ensuremath{\in}[0,2.5]\text{ }\text{ }{\mathrm{GeV}}^{2}$. We find that screening lengths, i.e., inverses of screening masses of ${\ensuremath{\pi}}^{0}$, ${K}^{0}$ and ${\ensuremath{\eta}}_{s\overline{s}}^{0}$, turn out to have the similar complex $eB$ and $T$ dependences of the corresponding chiral condensates. Although the transition temperature is found to always decrease as $eB$ grows, we show that the suppression due to magnetic fields becomes less significant for hadron screening length and chiral condensates with heavier quarks involved, and ceases to occur for ${\ensuremath{\eta}}_{s\overline{s}}^{0}$ and strange quark chiral condensate. The complex $eB$ and $T$ dependences of both screening masses and chiral condensates, reflecting the crossover nature of the QCD transition, are attributed to the competition between sea and valence quark effects. These findings could be useful to guide low-energy models and effective theories of QCD.

Interpretation of Neutral Charm Mesons near Threshold as Unparticles.

The existence of the X(3872) resonance extremely close to the D^{*0}D[over ¯]^{0} threshold implies that neutral charm mesons have an approximate nonrelativistic conformal symmetry. Systems consisting of these mesons with small kinetic energies produced in a short-distance reaction are unparticles in the sense that they can be created by operators with definite scaling dimensions in a nonrelativistic conformal field theory. There is a scaling region in which their energy distribution has power-law behavior with an exponent determined by the scaling dimension of the operator. The unparticle associated with two neutral charm mesons produces a peak in the recoil momentum spectrum of K^{±} in inclusive decays of B^{±} that has been observed. The scaling dimensions of the unparticles associated with three neutral charm mesons are calculated. They can be determined experimentally by measuring the invariant mass distributions for XD^{0} or XD^{*0} in inclusive prompt production at the Large Hadron Collider.

Measurement of Parameters of Neutral Mesons Produced in Meson–Nucleus Interactions in Hyperon-M Experiment

Measurement of Parameters of Neutral Mesons Produced in Meson–Nucleus Interactions in Hyperon-M Experiment

Light pseudoscalar meson masses under strong magnetic fields within the SU(3) Nambu–Jona-Lasinio model

We calculate the pole masses of pseudoscalar mesons in a strongly magnetized medium within the framework of the SU(3) Nambu-Jona--Lasinio model, using a magnetic field-independent regularization scheme. We employ both a constant and a magnetic field-dependent coupling $G(B)$, the latter being fitted to reproduce lattice QCD results for the pseudocritical chiral transition temperature. Numerical results for the pole masses are obtained for definite parametrizations of the model. For neutral mesons, the use of $G(B)$ provides closer agreement with lattice QCD results, which reveal a decrease of the mass with the external field. On the contrary, charged mesons masses are enhanced by $B$, showing no sign of the non-monotonous behavior found in recent lattice QCD simulations.

Neutral and charged mesons in magnetic fields

Neutral and charged mesons in magnetic fields

TeV-scale vector leptoquark from Pati-Salam unification with vectorlike families

In this paper, we show an explicit way to realize a TeV-scale vector leptoquark from the Pati-Salam (PS) unification with extra vectorlike families. The leptoquark mass is constrained to be heavier than PeV-scale by the measurement of a flavor violating kaon decay, $K_L \to \mu e$, in conventional models. This strong constraint can be avoided by introducing vectorlike families consistently with the quark and lepton masses and CKM and PMNS matrices. The other flavor violating processes are also suppressed. In this model, the vector leptoquark can be sufficiently light to explain the recent $b\to s\mu\mu$ anomaly, while the $b\to c\tau\nu$ anomaly is difficult to be explained due to the strong constraints from the $Z^\prime$ boson and vectorlike quark searches at the LHC. When the $b\to s\mu\mu$ anomaly is explained, we show that $\mathcal{O}({0.2})$ % tuning is required in the fermion matrix, the future experiments in $\mu\to e\gamma$ and $\mu$-$e$ conversions will cover the most available parameter space, and sizable neutral meson mixings, induced by the extra Higgs doublets, are unavoidable.

Prosthetic socket rotation stability - A mock limb study on different suspension systems

Decays of mesons produced in cosmic ray induced air showers in Earth’s atmosphere can lead to a flux of light exotic particles which can be detected in underground experiments. We evaluate the energy spectra of the light neutral mesons π0, η, ρ0, ω, ϕ and J∕ψ produced in interactions of cosmic ray protons and helium nuclei with air using QCD inspired event generators. Summing up the mesons produced in the individual hadronic interactions of air showers, we obtain the resulting fluxes of undecayed mesons. As an application, we re-consider the case of millicharged particles created in the electromagnetic decay channels of neutral mesons.

The charm of 331

We perform a detailed analysis of flavour changing neutral current processes in the charm sector in the context of 331 models. As pointed out recently, in the case of Z′ contributions in these models there are no new free parameters beyond those already present in the Bd,s and K meson systems analyzed in the past. As a result, definite ranges for new Physics (NP) effects in various charm observables could be obtained. While generally NP effects turn out to be small, in a number of observables they are much larger than the tiny effects predicted within the Standard Model. In particular we find that the branching ratio of the mode D0→ μ+μ−, despite remaining tiny, can be enhanced by 6 orders of magnitude with respect to the SM. We work out correlations between this mode and rare Bd,s and K decays. We also discuss neutral charm meson oscillations and CP violation in the charm system. In particular, we point out that 331 models provide new weak phases that are a necessary condition to have non-vanishing CP asymmetries. In the case of ∆ACP, the difference between the CP asymmetries in D0→ K+K− and D0→ π+π−, we find that agreement with experiment can be obtained provided that two conditions are verified: the phases in the ranges predicted in 331 models and large hadronic matrix elements.

Observation of the Mass Difference between Neutral Charm-Meson Eigenstates.

A measurement of mixing and CP violation in neutral charm mesons is performed using data reconstructed in proton-proton collisions collected by the LHCb experiment from 2016 to 2018, corresponding to an integrated luminosity of 5.4 fb^{-1}. A total of 30.6 million D^{0}→K_{S}^{0}π^{+}π^{-} decays are analyzed using a method optimized for the measurement of the mass difference between neutral charm-meson eigenstates. Allowing for CP violation in mixing and in the interference between mixing and decay, the mass and decay-width differences are measured to be x_{CP}=[3.97±0.46(stat)±0.29(syst)]×10^{-3} and y_{CP}=[4.59±1.20(stat)±0.85(syst)]×10^{-3}, respectively. The CP-violating parameters are measured as Δx=[-0.27±0.18(stat)±0.01(syst)]×10^{-3} and Δy=[0.20±0.36(stat)±0.13(syst)]×10^{-3}. This is the first observation of a nonzero mass difference in the D^{0} meson system, with a significance exceeding seven standard deviations. The data are consistent with CP symmetry and improve existing constraints on the associated parameters.

1/Nc approximation and universality of vector mesons

In this work, we argue that the observed differences in the value of the vector coupling constant extracted from the decays $\ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\pi}$ (${g}_{\ensuremath{\rho}}=6.0$), $\ensuremath{\rho}\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}$ (${g}_{\ensuremath{\rho}}=5.0$) and $\ensuremath{\omega}\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}$ (${g}_{\ensuremath{\rho}}=5.7$), where $l=e$, $\ensuremath{\mu}$, are an indication of the important role played by the $1/{N}_{c}$ corrections in the description of these processes. We show that an emission of a photon by charged meson loops in the ${\ensuremath{\rho}}^{0},\ensuremath{\omega},\ensuremath{\phi}\ensuremath{\rightarrow}\ensuremath{\gamma}$ transitions is a key process that allows to describe above vector meson decays into two leptons with a single value ${g}_{\ensuremath{\rho}}=6.0$. Our result supports the idea of universality of neutral vector mesons and clarifies the role of accounting of $1/{N}_{c}$ corrections to its fulfilment.

How to Detect the Lightest Glueball

We suggest a procedure for detecting the lightest glueball in a head-on collision of photons whose center-of-mass energy is in the range 1.3–2 GeV. We use recent evidence for scattering of light by light in Large Hadron Collider experiments as a phenomenological basis for our suggestion. With this evidence, the cross section of the lightest glueball creation in γγ collisions is estimated to be ~60 nb. The predominant mode of the lightest glueball decay, predicted from the gauge/gravity duality, proves to be the decay into two neutral vector mesons ρ0ρ0. Because the ρ0 decays into π+π–, a drastic increase of the π+π–π+π– yield is expected as the center-of-mass energy approaches the value of the lightest glueball mass. This fact will be the unique signature of the lightest glueball detection.

On the group generated by C, P and T: I 2 = T 2 = P 2 = ITP = −1, with applications to pseudo-scalar mesons

We study faithful representations of the discrete Lorentz symmetry operations of parity P and time reversal T , which involve complex phases when acting on fermions. If the phase of P is a rational multiple of π then P 2n = 1 for some positive integer n and it is shown that, when this is the case, P and T generate a discrete group, a dicyclic group (also known as a generalised quaternion group) which are generalisations of the dihedral groups familiar from crystallography. Charge conjugation C introduces another complex phase and, again assuming rational multiples of π for complex phases, TC generates a cyclic group of order 2m for some positive integer m. There is thus a doubly infinite series of possible finite groups labelled by n and m. Demanding that C commutes with P and T forces n = m = 2 and the group generated by P and T is uniquely determined to be the quaternion group. Neutral pseudo-scalar mesons can be simultaneous C and P eigenstates, T commutes with P and C when acting on fermion bi-linears so neutral pseudo-scalar mesons can also be T eigenstates. The T -parity should therefore be experimentally observable and the CPT theorem dictates that T = CP.