Decays Of Light Mesons Research Articles

Charged pions asymmetry due to interference from the decay of light neutral axial mesons

Abstract Effective three meson couplings, based in flavor U(3) quark-antiquark
 interactions,
 are considered to describe two
light neutral axial meson
Strong decays, 
 $A \sim f_1(1285)$ and $f_{1S} (1420)$,
and the charged rho meson decay
in the following channels:
$A \to \rho^\mp (770)+ \pi^\pm$,
 $A\to a_0^\mp (980) +\pi^\pm$ and
$\rho^\pm \to \pi^\pm \pi^0$.
By considering neutral meson mixings, such as $f_1(1285) - a_1^0(1260)$, $f_{1S}(1420) - a_1^0(1260)$ and $\eta-\pi^0$ for the channels investigated, the leading three-meson interactions may lead to decay amplitudes that undergo interference and a resulting ratio of decay rate into charged pions, $\Gamma_{\pi^+}/\Gamma_{\pi^-}$, slightly smaller than one. For that it will be assumed that the produced charged vector mesons or charged scalar mesons undergo absorption, decay into channels without charged pions, or any other inelastic process that may suppress their decays into charged pions.

Light meson decays at BESIII

Due to the high production rate of light mesons in J/ψ radiative and hadronic decays, the largest sample of J/ψ events accumulated at the BESIII detector offers a unique laboratory for investigating light meson decays. In recent years the BESIII experiment has made significant progresses in the study of η′ decays, including the search for the η′→γγη and η′→π0π0π0π0 decays, the measurement of the branching fraction of and the search for a CP-violating asymmetry in the η′→π+π−e+e− decay, and the observation of the η′→π+π−μ+μ− decay. This paper gives brief introductions to these works.

Comprehensive study of light axial vector mesons with the presence of triangle singularity

We present a systematic study of the productions and decays of light axial vector mesons with $J^{PC}=1^{+\pm}$ in charmonium decays. In the quark model scenario, the two axial vector nonets are connected with each other by the Gell-Mann-Okubo mass relation through the mixing between two $K_1$ states (i.e. $K_1(1270)$ and $K_1(1400)$) with configurations of $^3P_1$ and $^1P_1$. The mixing angles between $f_1$ and $f_1'$ (i.e. $f_1(1285)$ and $f_1(1420)$), and between $h_1$ and $h_1'$ (i.e. $h_1(1170)$ and $h_1(1415)$) can be reliably constrained. We then introduce the intermediate $K^*\bar{K}+c.c.$ meson loop transitions in the description of the productions and decays of these axial vector mesons. The presence of the nearby $S$-wave $K^*\bar{K}+c.c.$ to which these axial vector mesons have strong coupling strengths, turns out to be crucial for understanding many puzzling questions related to their productions and decays. This is because that the $S$-wave $K^*\bar{K}+c.c.$ rescatterings by the kaon exchange satisfy the triangle singularity (TS) condition in some of these cases and the TS mechanism can introduce special interference effects in the exclusive decays of these light axial vector mesons.

Light meson decays at BESIII

Due to the high production rate of light mesons in J/ψ decays, the sample of 1.3 × 109J/ψ events accumulated at BESIII offers a unique laboratory for investigating light meson decays, especially for η and η′ decays. In this paper, several recent progresses on η′ decays at BESIII are introduced, including the first direct measurement of the absolute branching fractions of five dominant η′ decay modes, the search for η′ → γγη decay and the search for η′ → π0π0π0π0 decay.

Light meson physics at BESIII.

Studies of light meson decays are important tools to perform precision tests of the effective field theories, determine transition form factors and test fundamental symmetries. With very high statistics data samples, the Beijing Spectrometer III (BESIII) experiment provides a unique laboratory for light meson studies and is contributing significantly to a variety of these investigations. A brief review of recent progress in light meson decay studied at the BESIII experiment, including detailed studies of common decay dynamics, searches for rare/forbidden decays and new particles, is presented. Finally, together with descriptions of different experimental techniques, prospects for future studies of light mesons are discussed in some detail.

Signals of the QCD axion with mass of 17 MeV/c2 : Nuclear transitions and light meson decays

The QCD axion remains experimentally viable in the mass range of O(10 MeV) if (i) it couples predominantly to the first generation of SM fermions; (ii) it decays to $e^+ e^-$ with a short lifetime $\tau_a\lesssim 10^{-13}\,$s; and (iii) it has suppressed isovector couplings, i.e., if it is piophobic. Remarkably, these are precisely the properties required to explain recently observed anomalies in nuclear de-excitations, to wit: the $e^+e^-$ emission spectra of isoscalar magnetic transitions of $^{8\!}$Be and $^{4\!}$He nuclei showed a "bump-like" feature peaked at $m_{e^+e^-}\sim 17$ MeV. In this article, we argue that on-shell emission of the QCD axion (with the aforementioned properties) provides an extremely well-motivated, compatible explanation for the observed excesses in these nuclear de-excitations. The absence of anomalous features in other measured transitions is also naturally explained: piophobic axion emission is strongly suppressed in isovector magnetic transitions, and forbidden in electric transitions. This QCD axion hypothesis is further corroborated by an independent observation: a $2-3\,\sigma$ deviation in the measurement of $\Gamma(\pi^0\to e^+e^-)$ from the Standard Model theoretical expectation. This article also includes detailed estimations of various axionic signatures in rare light meson decays, which take into account contributions from low-lying QCD resonance exchange, and, in the case of rare Kaon decays, the possible effective implementations of $\Delta S=1$ octet enhancement in chiral perturbation theory. These inherent uncertainties of the effective description of the strong interactions at low energies result in large variations in the predictions for hadronic signals of the QCD axion; in spite of this, the estimated ranges for rare meson decay rates obtained here can be probed in the near future in $\eta/\eta^\prime$ and Kaon factories.

Strong decays of strange quarkonia in a corrected P03 model

Extensively applied to both light and heavy meson decay and standing as one of the most successful strong decay models is the $^{3}P_{0}$ model, in which $q\bar{q}$ pair production is the dominant mechanism. In this paper we evaluate strong decay amplitudes and partial widths of strange $S$ and $D$ state mesons, namely $ \phi(1020)$, $\phi (1680) $, $ \phi (2050) $, $\phi_1 (1850) $, $\phi_2 (1850) $ and $ \phi_3 (1850) $, in the bound-state corrected $^{3}P_{0}$ decay model (C$^{3}P_{0}$). The C$^{3}P_{0}$ model is obtained in the context of the Fock-Tani formalism, which is a mapping technique.

{hbox {BHLS}}_2, a new breaking of the HLS model and its phenomenology

Previous studies have shown that the Hidden Local Symmetry (HLS) Model, supplied with appropriate symmetry breaking mechanisms, provides an Effective Lagrangian (BHLS) able to encompass a large number of processes within a unified framework. This allowed one to design a global fit procedure which provides a fair simultaneous description of the e^+ e^- annihilation into six final states (pi ^+pi ^-, pi ^0gamma , eta gamma , pi ^+pi ^-pi ^0, K^+K^-, K_L K_S), the dipion spectrum in the tau decay and some more light meson decay partial widths. In this paper, additional breaking schemes are defined which improve the BHLS working and extend its scope so as to absorb spacelike processes within a new framework ({hbox {BHLS}}_2). The phenomenology previously explored with BHLS is fully revisited in the {hbox {BHLS}}_2 context with special emphasis on the phi mass region using all available data samples. It is shown that {hbox {BHLS}}_2 addresses perfectly the close spacelike region covered by NA7 and Fermilab data; it is also shown that the recent lattice QCD (LQCD) information on the pion form factor are accurately predicted by the {hbox {BHLS}}_2 fit functions derived from fits to only annihilation data. The contribution to the muon anomalous magnetic moment a_mu ^{mathrm{th}} of these annihilation channels over the range of validity of {hbox {BHLS}}_2 (up to simeq 1.05 GeV) is updated within the new {hbox {BHLS}}_2 framework and shown to strongly reduce the former BHLS systematics. The uncertainty on a_mu ^{mathrm{th}}(sqrt{s}< 1.05 , hbox {GeV}) is much improved compared to standard approaches relying on direct integration methods of measured spectra. Using the {hbox {BHLS}}_2 results, the leading-order HVP contribution to the muon anomalous moment is a_mu ^{mathrm{HVP-LO}}= 686.65 pm 3.01 +(+1.16,-0.75)_{mathrm{syst}} in units of 10^{-10}. Using a conservative estimate for the light-by-light contribution, our evaluation for the muon anomalous magnetic moment is a_mu ^{mathrm{th}}=left[ 11,659,175.96 pm 4.17 +(+1.16,-0.75)_{mathrm{syst}}right] times 10^{-10}. The relationship between the dispersive and LQCD approaches to the rho ^0–gamma mixing is also discussed which may amount to a shift of delta a_mu [pi pi ]_{rho gamma }=+(3.10pm 0.31) times 10^{-10} at LO+NLO, presently treated as additional systematics. Taking also this shift into account, the difference a_mu ^{mathrm{th}}-a_mu ^{mathrm{BNL}} exhibits a significance not smaller than 3.8 sigma .

Decays of light mesons in the relativistic quark model

In the framework of the relativistic quark model based on the point form of the Poincaré-invariant quantum mechanics, the parameters were fixed using the integral representations of the lepton decay constants of pseudoscalar and vector mesons containing u-, d- and s-quarks. As a result of numerical calculations using the oscillator wave function, the basic parameters of the model are obtained using the pseudoscalar density constant and current quark masses. The analysis showed that the obtained calculation results in the framework of the model and the experimental data on the lepton decays of hadrons agree well with each other. As a result, the calculation method is generalized to the case of hadronic transitions with γ-quantum emission and a subsequent calculation of the integral representations of radiative decay constants of pseudoscalar and vector mesons. The obtained values of the anomalous magnetic moments are compared with the baryon data. As a test of the model, the authors studied the behavior of the form-factors of radiative decays of vector mesons with a subsequent comparison to the modern experimental data in the q &lt; 0.5 GeV range where the resonance effects are insignificant. As a result, self-consistent descriptions of lepton and radiative transitions were obtained within the framework of the model proposed by the authors.

Meson decay studies from MAMI A2

Decays of the light mesons π0, η, ω, and η’ provide a unique laboratory to test fundamental aspects of hadron physics. Precision studies of such diverse topics as the light quark mass ratio, π−π scattering lengths, and searches for physics beyond the Standard Model are possible. Additionally, Dalitz decays of light mesons provide a way of measuring the electromagnetic meson transition formfactors in the time-like region. The A2 tagged photon facility at the Mainz Microtron provides a high yield of light mesons produced by photo-induced reactions on protons, which makes the experiment ideal for high precision measurements of meson decays. Presented here are the contributions made by the A2 collaboration to such studies.

Recent Results on Meson Decays from A2

Light meson decays are used to investigate topics related to fundamental aspects of particle physics. Precision measurements of meson Dalitz decays give input to theoretical evaluations of the Hadronic Light-by-Light contribution (HLbL) to the anomalous magnetic moment of the muon. The A2 collaboration, using the Crystal Ball/TAPS setup at MAMI, has recently published several high precision results on transition form factors which are related to HLbL. Pseudoscalar η’ decays allow for studies of topics like ππ scattering lengths, effective field theories and fundamental symmetries. In 2014 the collaboration had a dedicated experimental campaign with one of its main goals to measure the dynamics of η’→ π0π0η with high precision. A brief overview of the experimental setup, physics motivations, analyses and results are given.

Light meson decays at BESIII

The BESIII experiment has accumulated the world’s largest data samples of 1.3 billion J/ψ events and 0.45 billion ψ(3686) events, which provide a unique opportunity to investigate light meson decays. The η and η′ decays are sensitive tools for investigations of π − π and η − π interactions, symmetry breaking, and serve as a test of chiral perturbation theory. In recent years considerable results on η and η′ decays were achieved at the BESIII experiment. In this proceeding, we present the significant progresses on amplitude analyses of Dalitz decays η' → 3π,η' → ηππ, and radiative decay η' → γπ+π−, η' → γγπ0. Additionally, the $a_0^0\left( {980} \right) - {f_0}\left( {980} \right)$ mixing is an important probe to the nature of those two lightest scalar mesons. The first observation of $a_0^0\left( {980} \right) - {f_0}\left( {980} \right)$ mixing is also presented, which will help to improve the understanding of the nature of $a_0^0\left( {980} \right)$ and f0(980).

Ambiguities in model-independent partial-wave analysis

Partial wave analysis is an important tool for analyzing large data sets in hadronic decays of light and heavy mesons. It commonly relies on the isobar model, which assumes multihadron final states originate from successive two-body decays of well-known undisturbed intermediate states. Recently, analyses of heavy-meson decays and diffractively produced states have attempted to overcome the strong model dependences of the isobar model. These analyses have overlooked that model-independent, or freed-isobar, partial-wave analysis can break the orthogonality of partial waves and introduce mathematical ambiguities into results. We show how these ambiguities arise and present general techniques for identifying their presence and for correcting for them. We demonstrate these techniques with specific examples in both heavy-meson decay and pion--proton scattering.

ForwArd Search ExpeRiment at the LHC

New physics has traditionally been expected in the high-$p_T$ region at high-energy collider experiments. If new particles are light and weakly-coupled, however, this focus may be completely misguided: light particles are typically highly concentrated within a few mrad of the beam line, allowing sensitive searches with small detectors, and even extremely weakly-coupled particles may be produced in large numbers there. We propose a new experiment, ForwArd Search ExpeRiment, or FASER, which would be placed downstream of the ATLAS or CMS interaction point (IP) in the very forward region and operated concurrently there. Two representative on-axis locations are studied: a far location, $400~\text{m}$ from the IP and just off the beam tunnel, and a near location, just $150~\text{m}$ from the IP and right behind the TAN neutral particle absorber. For each location, we examine leading neutrino- and beam-induced backgrounds. As a concrete example of light, weakly-coupled particles, we consider dark photons produced through light meson decay and proton bremsstrahlung. We find that even a relatively small and inexpensive cylindrical detector, with a radius of $\sim 10~\text{cm}$ and length of $5-10~\text{m}$, depending on the location, can discover dark photons in a large and unprobed region of parameter space with dark photon mass $m_{A'} \sim 10~\text{MeV} - 1~\text{GeV}$ and kinetic mixing parameter $\epsilon \sim 10^{-7} - 10^{-3}$. FASER will clearly also be sensitive to many other forms of new physics. We conclude with a discussion of topics for further study that will be essential for understanding FASER's feasibility, optimizing its design, and realizing its discovery potential.

KLOE Results on Hadron Physics and Perspectives for KLOE-2

Recent results obtained with KLOE data on light meson decays, together with perspectives for the current KLOE-2 run, are reviewed.

A Numerical Study of Renormalization Group Transformations on Multiscale Lattices

The RBC and UKQCD Collaborations have shown that light hadron masses and meson decay constants measured on 2+1 flavor Mobius DWF ensembles generated with the Iwasaki gauge action and a dislocation suppressing determinant ratio (DSDR) term show few percent O(a2) scaling violations for ensembles with a-1 = 1 GeV. We call this combination the ID+MDWF action and this scaling implies that, to a good approximation, these ensembles lie on a renormalization group trajectory, where the form of the action is unchanged and only the bare parameters need to be tuned to stay on the trajectory. Here we investigate whether a single-step APE-like blocking kernel can reproduce this trajectory and test its accuracy via measurements of the light hadron spectrum and non-perturbative renormalization. As we report, we find close matching to the renormalization group trajectory from this simple blocking kernel.

Recent Results on Meson Decays from A2

Light meson decays are used to investigate topics related to fundamental aspects of particle physics. Precision measurements of meson Dalitz decays give input to theoretical evaluations of the Hadronic Light-by-Light contribution (HLbL) to the anomalous magnetic moment of the muon. The pseudoscalar η' decays allow for studies of topics like ππ scattering lengths, effective field theories and fundamental symmetries. The A2 collaboration, using the Crystal Ball/TAPS setup at MAMI, has recently published several high precision results on transition form factors which are related to HLbL. The value obtained for the slope parameter of the π0 electromagnetic (e/m) transition form factor (TFF) is απ = 0.030 ± 0.010tot. The slope parameters for the e/m TFFs of ηγ and ωπ0 are [see formula in PDF] and [see formula in PDF], respectively. In 2014 the collaboration had a dedicated experimental campaign for the production of η' mesons. One of the main goals was to measure the dynamics of η' → ηπ0π0 with high precision. An overview of the physics cases, the experimental setup and the results are given.

Event generators for decays at BESIII**Supported by National Natural Science Foundation of China (NSFC) (11205117, 11575133, 11675184), the China Scholarship Council (201506275156), the Wuhan University PhD Short-time Mobility Program 2016, the Joint Funds of the NSFC and Henan Province (U1504112)

The light unflavoured meson decays are valuable for testing non-perturbative quantum chromodynamics and exploring new physics beyond the Standard Model. This paper describes a series of event generators, including , , , , , and , which have been developed for investigating decay dynamics. For most of these generators, their usability has been validated in BESIII analyses for determining the detection efficiency, and background studies. The consistency between data and Monte Carlo shows that these generators work well in the BESIII simulation, and will also be useful for ongoing BESIII analyses and other experiments for studying physics.

Light Meson Decay with Gaussian Confinement in a JKJ Decay Model

A form to describe hadron strong decays is in terms of quark and gluon degrees of freedom in microscopic decay models. Initially we assume that strong decays are driven by the same inter-quark Hamiltonian which determines the spectrum, and that it incorporates gaussian confinement. An [Formula: see text] decay matrix element of the JKJ Hamiltonian involves a pair-production current matrix elements times a scattering matrix element. Diagrammatically this corresponds to an interaction between an initial line and produced pair. In this work we apply the model to the light meson sector and calculate the decay rate, comparing with the experimental values.

Light Meson Decays at BESIII

At present the world’s largest sample of 1.3 billion J/ψ events was accumulated at the BESIII detector, which offers a unique place to study light meson decays. The recent results on the light meson decays are reviewed in this talk. An emphasis is put on the significant progresses on the study of η/η′ decays, including Dalitz plot analysis of η/η′ → πππ , observation of new decay modes (η′ → π + π − π +(0) π −(0) , η′ → ρ ± π ∓ , η′ → γe + e − and η′ → e + e − ω ), study of η′ → γπ + π − and search for the rare decay of η′ → Kπ . In addition, a prospect on the Dalitz plot analysis of ω → π + π − π 0 is presented.