arXiv subjects

George Rupp

Publications and source records attributed to George Rupp.

At least 19 recordsLinked to original sources

arXiv2026

Unquenched Radially Excited $P$-wave Charmonia

The ground-state positive-parity charmonia $\chi_{c0}(1P)$, $\chi_{c1}(1P)$, $h_c(1P)$, and $\chi_{c2}(1P)$ are generally well described in static (``quenched'') quark models, in which dynamical effects of actual or virtual strong decay are neglected. In contrast, the five PDG candidates for $P$-wave charmonia in the energy region 3.85-3.95 GeV, probably including the first radial excitations of the above ones, display a totally different and quite disparate mass pattern. Moreover, two scalar states are listed, viz. $\chi_{c0}(3860)$ and $\chi_{c0}(3915)$, the former one apparently being very broad. Preliminary results will be presented here for the first radial excitations of the lowest $P$-wave $c\bar{c}$ states, obtained with the Resonance-Spectrum Expansion while including in the calculation all OZI-allowed decay channels of the most relevant charm-meson pairs. Employing a generalised scheme of computing coupling constants for decays based on the ${}^{3\!}P_0$ model ensures that no distortion of the spectra will occur due to the different classes of allowed decay channels for the various positive-parity charmonia.

hep-ph↗

arXiv2024

A new 700 GeV scalar in the LHC data?

As an alternative to the metastability of the electroweak vacuum, resulting from perturbative calculations, one can consider a non-perturbative effective potential which, as at the beginning of the Standard Model, is restricted to the pure $\Phi^4$ sector yet consistent with the known analytical and numerical studies. In this approach, where the electroweak vacuum is now the lowest-energy state, besides the resonance of mass $m_h=$ 125 GeV defined by the quadratic shape of the potential at its minimum, the Higgs field should exhibit a second resonance with mass $(M_H)^{\rm Theor}=690\,(30)$ GeV associated with the zero-point energy determining the potential depth. In spite of its large mass, this resonance would couple to longitudinal $W$s with the same typical strength as the low-mass state at 125 GeV and represent a relatively narrow resonance, mainly produced at LHC by gluon-gluon fusion. In this Letter, we review LHC data suggesting a new resonance of mass $(M_H)^{\rm EXP} \sim 682\,(10)$ GeV, with a statistical significance that is far from negligible.

hep-ph↗

arXiv2024

Comment on "Do near-threshold molecular states mix with neighboring $\bar QQ$ states?"

I comment on a paper by Christoph Hanhart and Alexey Nefediev, published in Phys. Rev. D 106, 114003 (2022). The authors discuss the interpretation of mesons close to their lowest decay threshold and present a mechanism for the formation of molecular states. The proposed formalism is then applied to the axial-vector mesons $D_{s1}(2536)$ and $D_{s1}(2460)$, presenting two scenarios for the lighter meson, namely a $D^\star K$ molecule or a compact $c\bar{s}$ state. The authors argue that the latter hypothesis requires a fine-tuning of the mixing angle between the $J^{PC}=1^{++}$ and $J^{PC}=1^{+-}$ $C$-parity eigenstates. In this Comment I show that no such fine-tuning is needed, as demonstrated in an article published in Phys. Rev. D 84, 094020 (2011), where a unitarized quark model was applied to the two $C$-parity eigenstates, coupled to several two-meson channels including $D^\star K$. The coupled-channel dynamics naturally leads to a mixing angle very close to the required one. Moreover, I argue that the $D_1(2420)$ and $D_1(2430)$ axial-vectors, not considered by the authors, as well as a lattice simulation in Phys. Rev. D 90, 034510 (2014), also not mentioned by the authors, do not lend support to a molecular interpretation of the $D_{s1}(2460)$. I conclude with some more general remarks about mesons coupling to $S$-wave thresholds.

hep-ph↗

arXiv2024

Unitary model analysis of $f_0(500)$ pole positions by continuously varying $m_π$: comparison with discrete lattice predictions

Resonance, bound-state, and virtual-state pole positions of the $f_0(500)$ scalar meson are computed as a continuous function of pion mass in the framework of a unitarized and analytic coupled-channel model for scalar mesons, described as dynamical quark-antiquark states. The $f_0(500)$ is modeled with both light and strange $q\bar{q}$ seeds, mixing with each other through the common $S$-wave $ππ$, $K\bar{K}$, and $ηη$ meson-meson decay channels. The few model parameters are fitted to experimental $S$-wave $ππ$ phase shifts up to 1 GeV. In the case of the physical $π^\pm$ mass of 139.57 MeV, resonance poles at $(460-i222)$ MeV and $(978-i37.2)$ MeV are found for the $f_0(500)$ and $f_0(980)$, respectively. Resonance, bound-state, and virtual-state pole trajectories are computed and plotted as a function of pion masses up to 500 MeV, both in the complex-energy and complex-momentum planes. The results are discussed and compared to the most advanced lattice QCD computations employing interpolators that correspond to the $q\bar{q}$ and meson-meson channels in the present model, that is, for a few discrete values of the unphysical pion mass in those lattice calculations.

hep-ph↗

arXiv2024

$\sigma(500)$ resonance pole positions as function of $m_\pi$: analysis with a unitary coupled-channel model

Resonance pole positions of the $f_0(500)$ alias $\sigma(500)$ meson are computed and plotted as a continuous function of pion mass in the framework of a unitary and analytic coupled-channel model for scalar mesons as dynamical $q\bar{q}$ states. The $\sigma$ is described with a light and a strange $q\bar{q}$ seed, mixing with each other mainly through the common $\pi\pi$, $K\bar{K}$, and $\eta\eta$ meson-meson channels. The few model parameters are fitted to experimental $S$-wave $\pi\pi$ phase shifts up to 1 GeV, yielding, in the case of the physical pion mass, resonance poles at $(460-i222)$ MeV for the $\sigma(500)$ and $(978-i37)$ MeV for the $f_0(980)$. Resonance, bound-state, and virtual-state pole trajectories are shown as a function of $m_\pi$ running from 139.57 MeV to 1 GeV. These are compared to recent lattice QCD computations that use interpolating fields corresponding to the model's channels, i.e., for a few discrete $m_\pi $values.

hep-ph↗

arXiv2023

Second resonance of the Higgs field: motivations, experimental signals, unitarity constraints

Perturbative calculations predict that the Standard Model (SM) effective potential should have a new minimum, well beyond the Planck scale, much deeper than the electroweak vacuum. As it is not obvious that gravitational effects can get so strong to stabilize the potential, most authors have accepted the metastability scenario in a cosmological perspective. This perspective is needed to explain why the theory remains trapped into our electroweak vacuum, but requires to control the properties of matter in the extreme conditions of the early universe. Alternatively, one can consider the completely different idea of a non-perturbative effective potential which, as at the beginning of the SM, is restricted to the pure $\Phi^4$ sector yet consistent with the now existing analytical and numerical studies. In this approach, where the electroweak vacuum is the lowest-energy state, besides the resonance of mass $m_h=125$ GeV defined by the quadratic shape of the potential at its minimum, the Higgs field should exhibit a second resonance with mass $690\pm10({\rm stat})\pm20({\rm sys})$ GeV associated with the zero-point energy determining the potential depth. Despite its large mass, this would couple to longitudinal $W$s with the same typical strength as the low-mass state at 125 GeV and represent a relatively narrow resonance of width $\Gamma_H=30\div 38$ GeV, mainly produced at LHC by gluon-gluon fusion. So it is interesting that, in the LHC data, one can find various indications for a new resonance in the expected mass range with a non-negligible statistical significance. As this could become an important new discovery by just adding two missing samples of RUN2 data, we outline further refinements of the theoretical predictions that could be obtained by implementing unitarity constraints, in the presence of fermion and gauge fields, with coupled-channel calculations used for meson spectroscopy.

hep-ph↗

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↗