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Eef van Beveren

Publications and source records attributed to Eef van Beveren.

At least 19 recordsLinked to original sources
arXiv2023

RSE production amplitude and possible evidence of a (pseudo)scalar boson at about 57 GeV

Threshold enhancements predicted by the Resonance-Spectrum-Expansion (RSE) production amplitude and observed by the BaBaR Collaboration in open-bottom production above the $B\bar{B}$ threshold, as well as by several collaborations in open-charm production above the $D\bar{D}$ threshold, can also be seen in diphoton amplitudes at energies above 100 GeV. One such threshold effect is visible in tau-tau and muon-muon data of the L3 Collaboration at LEP, and in the diphoton and four-lepton data of the ATLAS and CMS Collaborations at LHC, as it is enhanced by the nearby presence of the Higgs resonance. This supports the assumption of pair production at about 115 GeV. An accumulation of single-photon and dimuon data around 28 GeV observed by the L3 and the CMS Collaborations, respectively, lend further credit to the hypothesis of the existence of a (pseudo)scalar boson with a mass of about 57 GeV.

hep-ph↗
arXiv2023

Comment on "Scrutinizing pion-pion scattering in light of recent lattice phase shifts"

In a recent paper by Xiu-Li Gao, Zhi-Hui Guo, Zhiguang Xiao, and Zhi-Yong Zhou, Phys. Rev. D 105, 094002 (2022), here referred to as I, $S$-wave $ππ$ scattering phase shifts obtained in a lattice-QCD calculation are analyzed using dispersive $S$-matrix methods. We question the reliability of the conclusion from this analysis that, for a pion mass of 391 MeV, the lattice phases favor the presence of both a $σ$-meson bound state and a nearby virtual state. Our main criticism concerns the neglect of the $S$-wave $K\bar{K}$ channel, which was considered alongside additional $s\bar{s}$ interpolating fields in the lattice computation used by the authors of I and also in typical coupled-channel models. As an illustration, some results from such a recent model are presented as well. Concluding remarks concern possible improvements of the analysis in I as well as further model tests.

hep-ph↗
arXiv2021

Espectroscopia Mesónica Moderna: o Papel Fundamental da Unitariedade

The importance of implementing unitarity constraints in meson spectroscopy is very briefly outlined for Portuguese students of engineering sciences and therefore non-experts in the field. After explaining the profound differences between meson spectroscopy and atomic spectroscopy, attention is paid to the shortcomings of standard Breit-Wigner parametrisations in the case of broad and/or overlapping resonances. Finally, the manifestly unitary Resonance-Spectrum-Expansion model, which lies at the heart of a recent invited review paper by the present authors, is graphically presented, together with a simple yet typical application to the long-controversial $K_0^\star(700)$ resonance.

physics.pop-ph↗
arXiv2021

Modern meson spectroscopy: the fundamental role of unitarity

The importance of $S$-matrix unitarity in realistic meson spectroscopy is reviewed, both its historical development and more recent applications. First the effects of imposing $S$-matrix unitarity on meson resonances is demonstrated in both the elastic and the inelastic case. Then, the static quark model is revisited and its theoretical as well as phenomenological shortcomings are highlighted. A detailed account is presented of the mesons in the tables of the Particle Data Group that cannot be explained at all or only poorly in models describing mesons as pure quark-antiquark bound states. Next the earliest unitarised and coupled-channel models are revisited, followed by several examples of puzzling meson resonances and their understanding in a modern unitarised framework. Also, recent and fully unquenched lattice descriptions of such mesons are summarised. Finally, attention is paid to production processes, which require an unconventional yet related unitary approach. Proposals for further improvement are discussed.

hep-ph↗
arXiv2020

Understanding the $f_0(980)$ and $a_0(980)$ masses as well as their widths

The low and approximately equal masses of the scalar mesons $f_0(980)$ and $a_0(980)$, as well as their relatively small decay widths, are impossible to understand in terms of standard $P$-wave quark-antiquark states. Here, these mesons are studied in a unitarised quark-meson model, together with the other light isoscalar scalar $f_0(500)$, as members of a complete scalar nonet below about 1 GeV. They are shown to be dynamical states generated by a combination of quark-confinement and strong-decay interactions, resulting in a large breaking of $SU(3)_{\rm flavour}$ symmetry. This is illustrated with several pole trajectories in the complex-energy plane as a function of the model's decay coupling constant. Also, experimental evidence is presented of a still much lighter scalar boson called $E(38)$, which may correspond to a novel kind of mesons predicted by V. N. Gribov, as an observable manifestation of a condensate of light quarks.

hep-ph↗
arXiv2020

$Z_0(57)$ and $E(38)$: possible surprises in the Standard Model

With the reported observation of the Higgs boson at the LHC, the Standard Model of particle physics seems to be complete now as for its particle content. However, several experimental data at low and intermediate energies indicate that there may be two surprises. First we propose a tentative new boson $Z_0(57)$, with a mass of about 57 GeV, on the basis of small enhancements we observe in several experiments, using recent data obtained at the LHC as well as much older ones from LEP. If confirmed, we interpret this new particle as a pseudoscalar or scalar partner of a composite $Z$ vector boson. Secondly, we advocate the existence of a very light spinless boson $E(38)$, probably a scalar, with a mass of 38 MeV and decaying into two photons. Theoretical arguments and experimental signals supporting such a novel light boson will be presented, including a recent direct experimental confirmation at the Joint Institute for Nuclear Research in Dubna.

hep-ph↗
arXiv2019

Dramatic implications of unitarity for meson spectroscopy

An unambiguous definition of meson resonance masses requires a description of the associated phase shifts in terms of a manifestly unitary $S$-matrix and its complex poles. However, the commonly used Breit-Wigner (BW) parametrisations can lead to appreciable deviations. We demonstrate this for a simple elastic resonance, viz. $ρ(770)$, whose pole and BW masses turn out to differ by almost 5 MeV. In the case of the very broad $f_0(500)$ and $K_0^\star(700)$ scalar mesons, the discrepancies are shown to become much larger, while also putting question marks at the listed PDG BW masses and widths. Furthermore, some results are reviewed of a manifestly unitary model for meson spectroscopy, which highlight the potentially huge deviations from static model predictions. Finally, a related unitary model for production amplitudes is shown to explain several meson enhancements as non-resonant threshold effects, with profound implications for spectroscopy.

hep-ph↗
arXiv2018

Dimuon enhancement at 28 GeV and tentative (pseudo)scalar partner of the Z boson at 57.5 GeV

The CMS Collaboration at the LHC recently reported an accumulation of data around 28 GeV in the invariant-mass distribution of muon pairs in association with a b quark jet and at least a second jet. This is analysed here in the light of the possible existence of a (pseudo)scalar boson with mass of about 57.5 GeV. We find that part of the data may originate in the radiative decay of Z bosons into pairs consisting of the lighter boson and a photon, giving rise to dimuon decay products that either stem from the photon or from the (pseudo)scalar boson.

hep-ph↗
arXiv2018

Heavy quarkonia: the beauty and the beasts

New enhancements in the charmonium and bottomonium spectra observed since 2003 are very briefly reviewed. Special attention is paid to $χ_{c1}(3872)$ (formerly $X(3872)$) owing to its remarkable proximity to the $\bar{D}^{\star0}\!D^0$ threshold, which allows modelling as a quasibound axial-vector $c\bar{c}$ state with a large $\bar{D}^{\star0}\!D^0$ admixture. In contrast, the interpretation of many other charmonium-like and bottomonium-like states is still very controversial and some may not even correspond to genuine resonances. Accordingly, several entries in the PDG tables have been wildly changing over the years. Three representative states are reviewed here as non-resonant enhancements due to threshold effects, viz. $ψ(4260)$, $ψ(4660)$, and $Υ(10580)$.

hep-ph↗
arXiv2018

Scalar mesons: fifty years of challenging the quark model

Half a century of work on the light scalar mesons $f_0(500)$, $f_0(980)$, $K_0^\star(700)$, and $a_0(980)$ is briefly reviewed. After summarising all light scalar candidates in the Review of Particle Physics since 1963, a selection of different theoretical and phenomenological descriptions is presented, including pure meson-meson models, a tetraquark construction, unitarised quark-meson models, unitarised effective chiral approaches, and a very recent lattice-QCD simulation.

hep-ph↗
arXiv2017

Unquenching and unitarising mesons in quark models and on the lattice

Mesons with masses below their lowest OZI-allowed strong-decay thresholds have very small widths. Thus, it is usually believed that they can be safely treated as pure quark-antiquark bound states in spectroscopy models. However, unitarised and coupled-channel models from decades ago already indicated that this may not be the case, owing to significant virtual meson-loop contributions. Recent unquenched lattice calculations that include two-meson interpolators besides the usual $q\bar{q}$ ones confirm the latter conclusion, in particular for the enigmatic narrow $D_{s0}^\star(2317)$, $D_{s1}(2460)$, and $X(3872)$ states. Here, we briefly review some predictions of some old and new quark models that go beyond the static description of mesons, also in comparison with up-to-date lattice results.

hep-ph↗
arXiv2017

General unquenching properties of two-meson scattering and production amplitudes

Besides the unitarity and symmetry requirements for a multi-resonance scattering amplitude, several other natural conditions can easily exclude unrealistic proposals. In particular, the behaviour of singularities under the variation of model parameters yields important information. We discuss how resonance poles should move in the complex-energy plane when coupling constants and masses are varied, how resonances above threshold can turn into bound states below threshold and how the light-quark spectrum can be turned into the spectrum of heavy quarks, with one and the same analytic expression for the scattering amplitude. Moreover, it is shown that perturbative approximations usually do not satisfy those natural conditions.

hep-ph↗
arXiv2017

Why static bound-state calculations of tetraquarks should be met with scepticism

Recent experimental signals have led to a revival of tetraquarks, the hypothetical $q^2\bar{q}^2$ hadronic states proposed by Jaffe in 1976 to explain the light scalar mesons. Mesonic structures with exotic quantum numbers have indeed been observed recently, though a controversy persists whether these are true resonances and not merely kinematical threshold enhancements, or otherwise states not of a true $q^2\bar{q}^2$ nature. Moreover, puzzling non-exotic mesons are also often claimed to have a tetraquark configuration. However, the corresponding model calculations are practically always carried out in pure and static bound-state approaches, ignoring completely the coupling to asymptotic two-meson states and unitarity, especially the dynamical effects thereof. In this short paper we argue that such static predictions of real tetraquark masses are highly unreliable and provide little evidence of the very existence of such states.

hep-ph↗
arXiv2016

Summary of the 2015 LHCb workshop on multi-body decays of D and B mesons

This document contains a summary of the LHCb workshop on multi-body decays of D and B mesons, held at CBPF, Rio de Janeiro, in July 2015. The workshop was focused on issues related to amplitude analysis of three- and four-body hadronic decays. In addition to selected LHCb results, contributions from guest theorists are included.

hep-ex↗
arXiv2016

Unquenching the meson spectrum: a model study of excited $ρ$ resonances

Quark models taking into account the dynamical effects of hadronic decay often produce very different predictions for mass shifts in the hadron spectrum. The consequences for meson spectroscopy can be dramatic and completely obscure the underlying confining force. Recent unquenched lattice calculations of mesonic resonances that also include meson-meson interpolators provide a touchstone for such models, despite the present limitations in applicability. On the experimental side, the $ρ(770)$ meson and its several observed radial recurrences are a fertile testing ground for both quark models and lattice computations. Here we apply a unitarised quark model that has been successful in the description of many enigmatic mesons to these vector $ρ$ resonances and the corresponding $P$-wave $ππ$ phase shifts. This work is in progress, with encouraging preliminary results.

hep-ph↗
arXiv2015

Unquenched quark-model calculation of excited $ρ$ resonances and P-wave $ππ$ phase shifts

The $ρ(770)$ vector resonance, its radial recurrences, and the corresponding P-wave $ππ$ phase shifts are investigated in an unquenched quark model with all classes of relevant decay channels included, viz. pseudoscalar-pseudoscalar, vector-pseudoscalar, vector-vector, vector-scalar, axialvector-pseudoscalar, and axialvector-vector, totalling 26 channels. Two of the few model parameters are fixed at previously used values, whereas the other three are adjusted to the $ρ(770)$ resonance and the lower P-wave $ππ$ phases. Preliminary results indicate the model's capacity to reproduce these phases as well as the $ρ$ mass and width. However, at higher energies the phase shifts tend to rise too sharply. A possible remedy is an extension of the model so as to handle resonances in the final states for most of the included decay channels. Work in progress.

hep-ph↗
arXiv2015

No serious meson spectroscopy without scattering

The principal purpose of meson spectroscopy is to understand the confining force, which is generally assumed to be based on low-energy QCD. This is usually done in the context of quark models that ignore the dynamical effects of quark-pair creation and decay. Very recent lattice calculations confirm much earlier model results showing that neglecting such effects, in the so-called quenched approximation, may give rise to discrepancies of hundreds of MeV, and so distort the meson spectra resulting from quark confinement only. Models attempting to mimic unquenching through a redefinition of the constituent quark mass or screening of the confining potential at larger interquark separations are clearly incapable of accounting for the highly non-perturbative and non-linear effects on mesonic bound-state and resonance poles, as demonstrated with several published examples.

hep-ph↗