arXiv subjects
Publications and source records attributed to Peter Lichard.
Using the quantum field theory, we derive a Breit-Wigner-type formula for the $e^+ e^-$ annihilation into a vector meson and its antiparticle, and relate the formula parameters to observable quantities. The formula's soundness is checked by fitting the $e^+ e^- \to D^{\ast+}D^{\ast-}$ data published by the BESIII Collaboration in 2022.
Using precise data from the BESIII collaboration, we obtained compelling evidence of a subthreshold pole in the $e^+ e^-\rightarrow D_s^+ D_s^-$ process with mass of (3896$^{+13}_{-22}$)~MeV. The latter suggests the identification of this subthreshold pole with the $G$(3900) state, which behaves as a resonance in the $e^+ e^-\rightarrow D\bar D$ processes.
We show that the $\psi(2\mathrm S)$ subthreshold pole influences the cross section of the electron-positron annihilation into the $\mathrm D^+\mathrm D^-$ and $\mathrm D^0\bar\mathrm D^0$ final states. We perform a fit to the merged BES \cite{bes2008} and BESIII \cite{besiii2024} data, providing the cross section for $e^+e^-$ annihilation into those final states. The statistical significance of the $\psi(2\mathrm S)$ as a subthreshold pole is 8.2$\sigma$. A fit assuming it and seven resonances excels with the fit quality characterized by $\chi^2/\mathrm{NDF}=240.4/284$, which means a $p$-value of 97\%.
We present the arguments that the X(1750) has the isospin $I=0$, and as such, it belongs to the $\phi$ family. We base our argumentation on the X(1750) appearing in the $e^+e^-\rightarrow K^+ K^-$ process as a narrow resonance and not appearing in the $e^+e^-\rightarrow\pi^+\pi^-$ process at all.
Fitting the recent $e^+e^-\raϕ\,η$ data by the Belle collaboration with a theoretical formula reveals, besides the dominant $ϕ(1680)$ resonance, two narrow resonances: expected $ϕ(2170)$ resonance and an unexpected resonance with the mass of about 1851 MeV. Close proximity to the $X(1835)$ resonance suggests that the new resonance may be interpreted as the $p \bar p$ baryonium in an excited state. Follow-up analysis found the same resonance also in $e^+e^-\raω\,η$ data by CMD-3 experiment.
We suggest that the decay modes of kaons with a pion and a pionium ($π^+π^-$ atom) in the final state can constitute a not yet considered background to the very rare decay $K\toπν\barν$. In fact, a part of pioniums may escape the decay region before decaying into two $π^0$s (or to $π^0π^0γ$ in the case of excited pionium). To illustrate the importance of this background, we show that it may even explain, under some assumptions, the unexpected $K_L$ decay events that appeared in the KOTO experiment.
We suggest a few particle physics processes in which excited 2p pionium $\mathrm{A}^\prime_{2π}$ may be observed. They include the $e^+e^-\toπ^+π^-$ annihilation, the $V^0\toπ^0\ell^+\ell^-$ and $K^\pm\toπ^\pm\ell^+\ell^-$ ($\ell=e,μ$) decays, and the photoproduction of two neutral pions from nucleons. We analyze available experimental data and find that they, in some cases, indicate the presence of 2p pionium, but do not provide definite proof.
We show that the behavior of the excitation curves of some $e^+e^-$ annihilation processes close to the nucleon-antinucleon threshold can be explained either by the $ρ(1900)$ resonance itself or by its interference with other resonances. Besides the six-pion annihilation and the $e^+e^- \to K^+K^-π^+π^-$ and $e^+e^- \to Φπ^0$ processes, we also analyze the final states $ηπ^+ π^-$, $K^+ K^- π^+ π^- π^0$, and $K^+ K^-π^0 π^0$, the behavior of which around 1.6 GeV has not yet attracted attention. Analysis of the data on the $e^+e^- \to p \bar p$ and $e^+e^- \to n \bar n$ reactions clearly shows that the $ρ(1900)$ resides above the $n \bar n$ threshold.
The role of the a_1(1420) resonance in the three-pion decays of the tau lepton is investigated using a phenomenological model. For all data before 2008, roughly equal fit quality is achieved when the basic a_1(1260) resonance is supplemented with either a_1(1640) or a_1(1420). However, two recent and more precise data sets require resonances with masses that are not very far from that of a_1(1420). This suggests that the axial-vector resonance that accompanies the a_1(1260) in the three-pion decays of the tauon is a_1(1420), not a_1(1640), as believed up to now. More data are needed to demonstrate this definitely.
Encouraged by a recent observation of the B_c^+ -> B_s^0 + pi^+ decay by the LHCb collaboration we present the meson dominance predictions for other weak decays of the B_c^+ into B_s^0 or B^0 in the form of branching ratios to the observed decay.
It is shown that the pi^0 transition form factor F(Q_1^2,Q_2^2) differs substantially from its one-real-photon limit F(Q_1^2,0) even for rather small values of Q_2^2 (approx 0.1 GeV^2), which cannot be excluded in experiments with one "untagged" electron. It indicates that the comparison of data with theoretical calculations, which usually assume Q_2^2=0, may be untrustworthy. Our phenomenological model of the pi^0 transition form factor is based on the vector-meson-dominance hypothesis and all its parameters are fixed by using the experimental data on the decays of vector mesons. The model soundness is checked in the two-real-photon limit, where it provides a good parameter-free description of the pi^0 -> 2 gamma decay rate, and in the pi^0 Dalitz decay. The dependence of F(Q_1^2,Q_2^2) on Q_1^2 at several fixed values of Q_2^2 is presented and the comparison with existing data performed.
The e+e- --> pi+pi-pi0pi0 reaction cross section as a function of the incident energy is calculated using a model that is an extension of our recently published model of the e+e- annihilation into four charged pions. The latter considered the intermediate states with the pi, rho, and a_1 mesons and fixed the mixing angle of the a_1-rho-pi Lagrangian and other parameters by fitting the cross section data. Here we supplement the original intermediate states with those containing omega(782) and h_1(1170), but keep unchanged the values of those parameters that enter both charged and mixed channel calculations. The inclusion of omega is vital for obtaining a good fit to the cross section data, while the intermediate states with h_1 further improve it. Finally, we merge our models of the e+e- --> pi+pi-pi0pi0 and e+e- --> pi+pi-pi+pi- reactions and obtain a simultaneous good fit.
The NA60 experiment has measured low-mass muon pair production in In-In collisions at 158 A GeV with unprecedented precision. We show that this data is reproduced very well by a dynamical model with parameters scaled from fits to measurements of hadronic transverse mass spectra and Hanbury-Brown and Twiss correlations in Pb-Pb and Pb-Au collisions at the same energy. The data is consistent with in-medium properties of $ρ$ and $ω$-mesons at finite temperature and density as deduced from empirical forward-scattering amplitudes. Inclusion of the vacuum decay of the $ρ$-meson after freeze-out is necessary for an understanding of the mass and transverse momentum spectrum of dimuons with $M \apprle 0.9 {\rm GeV}/c^2$.
The excitation curve of e+e- annihilation into four charged pions in the rho(770) region is calculated using three existing models with rho mesons and pions in intermediate states supplemented by Feynman diagrams with the a_1(1260)pi intermediate states. A two-term phenomenological Lagrangian of the a_1-rho-pi interaction is used. The mixing angle is determined by fitting the e+e- --> pi+pi-pi+pi- cross section data of the Novosibirsk CMD-2 collaboration and also its combination with the low-energy part of the BaBar collaboration data. It is shown that the inclusion of the a_1-pi intermediate states succeeds in obtaining a good agreement with the data on both cross section and the rho^0 --> pi+pi-pi+pi- decay width. When moving to energies above 1 GeV, the rho(1450) and rho(1700) resonances are taken into account to get excellent agreement with the BaBar data over the full energy range up to 4.5 GeV.
In order to reveal possible mass shifts of the vector mesons in a dense strongly interacting medium presumably created in high energy heavy ion collisions it is necessary to know their free masses reliably. The rho(770) mass quoted in the last two editions of the Review of Particle Physics is significantly larger than the values quoted in previous editions. The new value is mostly influenced by the results of recent experiments CMD-2 and SND at the VEPP-2M e+e- collider at Novosibirsk. We show that the values of the mass and width of the rho(770) meson measured in the e+e- -> pi+pi- annihilation depend crucially on the parametrization used for the pion form factor. We propose a parametrization of the rho(770) contribution to the pion form factor based on the running mass calculated from a single-subtracted dispersion relation and compare it with the parametrization based on the formula of Gounaris and Sakurai used recently by the CMD-2 collaboration. We show that our parametrization gives equally good or better fits when applied to the data of the CMD-2, SND, and KLOE collaborations, but yields much smaller values of the rho(770) mass, consistent with the photoproduction and hadronic reactions results. Our fit to the KLOE data becomes exceptionally excellent (confidence level of 99.88%) if an energy shift of about 2 MeV in the rho-omega region is allowed.
The rate of the beta decay of neutral kaons is calculated within the meson dominance approach taking into account the relation between the KKρand ππρcoupling constants which follows from the vector meson dominance in electromagnetic interactions and isospin symmetry. The decay rate transforms into the following branching fraction summed over the charge states indicated: B(K0L -> K+- e-+ nu)=(2.53+-0.10)x10^{-9}. The error is dominated by our estimate of isospin violating effects. Experimental aspects of such a measurement are discussed.
We investigate the mechanism of K+ -> pi+ l+ l- (l= e, mu) decays in which a virtual photon is emitted either from the incoming K+ or the outgoing pi+. We point out some inconsistencies with and between two previous calculations, discuss the possible experimental inputs, and estimate the branching fractions. This mechanism alone fails to explain the existing experimental data by more than one order-of-magnitude. But it may show itself by its interference with the leading long-range mechanism dominated by the a_1^+ and rho^0 mesons.
We investigate the relations between the K_{l3} and tau --> K pi nu_tau decays using the meson dominance approach. First, the experimental branching fractions (BF) for K^-_{e3} and K^0_{e3} are used to fix two normalization constants (isospin invariance is not assumed). Then, the BF of tau^- --> K^*(892)^- nu_tau is calculated in agreement with experiment. We further argue that the nonzero value of the slope parameter lambda_0 of the K^-_{mu3} and K^0_{mu3} form factors f_0(t) implies the existence of the tau^- --> K_0^*(1430)^- nu_tau decay. We calculate its BF, together with BF's of the K^-_{mu3}, K^0_{mu3}, tau- --> K^- pi^0 nu_tau, and tau- --> antiK^0 pi^0 nu_tau decays, as a function of the lambda_0 parameter. At some value of lambda_0, different for charged and neutral kaons, calculated BF's seem to match existing data and a prediction is obtained for the tau^- --> K pi nu decays going through the K_0^*(1430)^- resonance.