arXiv subjects

E. S. Swanson

Publications and source records attributed to E. S. Swanson.

At least 19 recordsLinked to original sources

arXiv2025

Bottomonium meson spectrum with quenched and unquenched quark models

An open question in hadronic phenomenology concerns the ``unquenching" effects of higher Fock space components on the leading Fock space description of hadrons. We address this by making a comparison of the bottomonium spectrum as computed with the relativized Godfrey-Isgur quark model and an unquenched coupled channel model driven by the ``$^3P_0$" mechanism of hadronic decay. Our results show that both models can describe the spectrum well, indicating that the influence of coupled channel effects can be largely absorbed into the parameters of the quenched quark model. This conclusion is reinforced by a perturbative calculation that shows that the spin-dependence of mass splittings due to mixing with the continuum recapitulates quenched quark model spin-dependent interactions. We also show that softening of the quark-antiquark wavefunction due to continuum mixing improves the description of vector bottomonium decay constants. Together, these results illustrate and substantiate the surprising robustness of simple constituent quark model descriptions of hadrons.

hep-ph↗

arXiv2023

A Constituent Model of Light Hybrid Meson Decays

A model of light hybrid mesons and their strong decays is developed. The model employs a gluonic quasiparticle to describe low energy gluodynamics and uses the QCD Hamiltonian in Coulomb gauge to guide the construction of states and decay amplitudes. We compute the partial widths of the twelve low lying isovector and vector hybrids. Implications of these results on hybrid searches are also made, with the chief conclusions being that direct observation of the vector states will be difficult, that a hybrid $π(1800)$ has distinctive decay characteristics, a narrow $η(1900)$ hybrid should exist, an $η_1(1750)$ should be sought, and that the exotic nature of $J^{PC}= 2^{-+}$ hybrid mesons should be discernible with sufficient data. We argue that the isovector $π_2$ hybrid has been discovered, giving a total of four possible hybrid mesons, $π_1(1600)$, $η_1(1855)$, $π(1800)$, and $π_2(2360)$, which appear to be filling out the lowlying hybrid supermultiplet in the expected fashion.

hep-ph↗

arXiv2023

Light Hybrid Meson Mixing and Phenomenology

A simple constituent model of gluodynamics that is motivated by lattice field theory and the QCD Hamiltonian in Coulomb gauge is applied to descriptions of hybrid meson flavor mixing and vector hybrid configuration mixing. Good agreement with lattice gauge computations is obtained for flavor multiplet masses, while mixing angles are in approximate agreement, given large errors. The configuration mixing results are also in rough agreement with lattice NRQCD calculations. Thus the viability of constituent gluon models of hybrid hadrons and glueballs is supported. The results suggest that a flavor multiplet of vector hybrids should appear with masses of approximately 2100, 2200, and 2300 MeV and that the isovector vector hybrid decay constant is about 20 MeV. Similarly, the $π_1$ exotic hybrid should have isospin partner states near 1760 and 1900 MeV, and it is suggested that the recently seen $η_1$ hybrid signal is the latter state.

hep-ph↗

arXiv2022

Charmonium spectrum in an unquenched quark model

The effects of virtual light quark pairs on the charmonium spectrum are studied. Pair creation is modelled with a ``$^{3}P_{0}$" vertex and intermediate states are summed up to 2S excitations. Quark model parameters are obtained by fitting to 12 well-known charmonium states, allowing for feedback between the decaying particle and the induced mass shifts. Both of these technical steps are new and improve agreement with the experimental spectrum. In general, the masses receive small shifts once model parameters are refit. This is true in almost cases except the $χ_{cJ}(2P)$ multiplet, which experiences upwards mass shifts of order 150 MeV, has the ordering of the multiplet rearranged, and pushes the erstwhile $c\bar{c}$ ${2}^3P_1$ state well above $D^*\bar{D}$ threshold--observations that clarify the nature of the enigmatic $X(3872)$

hep-ph↗

arXiv2022

Production of $P_c$ states in $Λ_b$ decays

We develop a model for the production of the Pc states observed at LHCb in Lambdab - Jpsi p K- decays. With fewer parameters than other approaches, we obtain excellent fits to the Jpsi-p invariant mass spectrum, capturing both the prominent peaks, and broader features over the full range of invariant mass. A distinguishing feature of our model is that whereas Pc(4312), Pc(4380), and Pc(4440) are resonances with Sigmac*-D* constituents, the nature of Pc(4457) is quite different, and can be understood either as a Sigmac-D* threshold cusp, a Lambdac(2595)-D enhancement due to the triangle singularity, or a Lambdac(2595)-D resonance. We propose experimental measurements that can discriminate among these possibilities. Unlike in other models, our production mechanism respects isospin symmetry and the empirical dominance of colour-enhanced processes in weak decays, and additionally gives a natural explanation for the overall shape of the data. Our model is consistent with experimental constraints from photoproduction and Lambdab - Lambdac-D(*)0 K- decays and it does not imply the existence of partner states whose apparent absence in experiments is unexplained in other models.

hep-ph↗

arXiv2022

The LHCb state $P_{ψs}^Λ(4338)$ as a triangle singularity

We present a model for the $J/ψΛ$ spectrum in $B^-\to J/ψΛ\bar{p}$ decays, including the $P_{ψs}^Λ(4338)$ baryon recently observed by the LHCb collaboration. We assume production via triangle diagrams which couple to the final state via non-perturbative interactions which are constrained by heavy-quark and $SU_3$-flavor symmetry. The bulk of the distribution is described by a triangle diagram with a color-favored electroweak vertex, while the sharp $P_{ψs}^Λ(4338)$ enhancement is due to the triangle singularity in another diagram featuring a $1/2^-$ baryon consistent with $Σ_c(2800)$. We predict a comparable $P_{ψs}^Λ(4338)$ signal in $η_c Λ$, and anticipate possible large isospin mixing effects through decays to $J/ψΣ^0$ and $η_c Σ^0$

hep-ph↗

arXiv2022

K-matrix Analysis of $e^+e^-$ Annihilation in the Bottomonium Region

We perform the first global and unitary analysis of $e^+e^-\to b\bar{b}$ cross sections. We analyze exclusive cross sections in the $B\bar{B}$, $B^* \bar{B}(+c.c.)$, $B^*\bar{B}^*$, $B_s^*\bar{B}_s^*$, $Υ(nS)π^+π^-$ and $h_b(nP)π^+π^-$ channels as well as the total inclusive cross section for $b\bar{b}$ production. Pole positions and residues are determined for four vector states, which we associate with the $Υ(4S)$, $Υ(10750)$, $Υ(5S)$ (or $Υ(10860)$), and $Υ(6S)$ (or $Υ(11020)$). We find strong evidence for the new $Υ(10750)$ recently claimed by Belle, although with parameters not well constrained by the data. Results presented here cast doubt on the validity of branching ratios reported earlier using Breit-Wigner parameterizations or ratios of cross sections. We also compare our results with a selection of theoretical calculations for the vector bottomonium spectrum.

hep-ph↗

arXiv2022

Experimental constraints on the properties of $P_c$ states

Using new experimental data on photoproduction and $Λ_b^0$ decays, we derive constraints on the properties of the LHCb $P_c$ states. We conclude that $P_c(4312)$, $P_c(4380)$ and $P_c(4440)$ can be described as $Σ_c\bar D$, $Σ_c^*\bar D$ and $Σ_c\bar D^*$ molecules, but that $P_c(4457)$ does not fit into the same picture. Based on the apparent absence of additional partner states, and the striking disparity between $Λ_c^+\bar D^0$ and $Λ_c^+\bar D^{*0}$ decays, we conclude that $P_c(4440)$ has $3/2^-$ quantum numbers. Using heavy-quark symmetry we predict large experimental signals for $P_c$ states in $η_c p K^-$, $Λ_c^+\bar D^{*0}K^-$, and $Σ_c^{(*)}\bar D K^-$. We also argue that current experimental data on photoproduction is almost at the level of sensitivity required to observe $P_c$ states.

hep-ph↗

arXiv2021

Discriminating among interpretations for the $X(2900)$ states

We make predictions for the production and decays of $X(2900)$ states, and their possible charged partners, in $B^+$ and $B^0$ decays, considering a number of competing models for the states, including triangle diagrams mediated by quark exchange or pion exchange, and resonance scenarios including molecules and tetraquarks. Assuming only isospin symmetry and the dominance of colour-favoured weak decays, we find characteristic differences in the predictions of the different models. Future experimental studies can therefore discriminate among the competing interpretations for the states.

hep-ph↗

arXiv2021

Kinematical Cusp and Resonance Interpretations of the $X(2900)$

We examine whether the LHCb vector $ud\bar{c}\bar{s}$ state $X(2900)$ can be interpreted as a kinematical cusp effect arising from $\bar D^*K^*$ and $\bar D_1 K^*$ interactions. The production amplitude is modelled as a triangle diagram with hadronic final state interactions. A satisfactory fit to the Dalitz plot projection is obtained that leverages the singularities of the production diagram without the need for $\bar{D}K$ resonances. A somewhat better fit is obtained if the final state interactions are strong enough to generate resonances, although the evidence in favour of this scenario is not conclusive.

hep-ph↗

arXiv2019

Molecular Interpretation of the $P_c(4440)$ and $P_c(4457)$ States

A molecular model of the $P_c(4457)$ and $P_c(4440)$ LHCb states is proposed. The model relies on channels coupled by long range pion-exchange dynamics with features that depend crucially on the novel addition of the $Λ_c(2595)\bar D$ channel. A striking prediction of the model is the unusual combination of quantum numbers $J^P(4457) = 1/2^+$ and $J^P(4440) = 3/2^-$. Unlike in other models, a simultaneous description of both states is achieved without introducing additional short-range interactions. The model also gives a natural explanation for the relative widths of the states. We show that the usual molecular scenarios cannot explain the production rate of $P_c$ states in $Λ_b$ decays, and that this can be resolved by including $Λ_c(2595)\bar D$ and related channels. Experimental tests and other states are discussed in the conclusions.

hep-ph↗

arXiv2018

Decays and spectrum of bottom and bottom strange mesons

The strong decay amplitudes and radiative partial widths of orbital and radially excited states of $B$ and $B_s$ mesons are presented. These results are obtained with a nonrelativistic potential quark model, the nonrelativistic reduction of the electromagnetic transition operator, and the "$^3P_0$" model of strong decays. The predictions are compared to experiment where possible and assignments for the recently discovered states, $B_1(5721)$, $B_2^*(5747)$, $B_J(5840)$, $B_J(5970)$, $B_{s1}(5830)$, and $B_{s2}^*(5840)$, are made.

hep-ph↗

arXiv2016

Interpreting the $X(5568)$

A variety of options for interpreting the DØ state, $X(5568)$, are examined. We find that threshold, cusp, molecular, and tetraquark models are all unfavoured. Several experimental tests for unravelling the nature of the signal are suggested.

hep-ph↗

arXiv2016

Issues and Opportunities in Exotic Hadrons

The last few years have been witness to a proliferation of new results concerning heavy exotic hadrons. Experimentally, many new signals have been discovered that could be pointing towards the existence of tetraquarks, pentaquarks, and other exotic configurations of quarks and gluons. Theoretically, advances in lattice field theory techniques place us at the cusp of understanding complex coupled-channel phenomena, modelling grows more sophisticated, and effective field theories are being applied to an ever greater range of situations. It is thus an opportune time to evaluate the status of the field. In the following, a series of high priority experimental and theoretical issues concerning heavy exotic hadrons is presented.

hep-ph↗

arXiv2015

The Physics of the B Factories

This work is on the Physics of the B Factories. Part A of this book contains a brief description of the SLAC and KEK B Factories as well as their detectors, BaBar and Belle, and data taking related issues. Part B discusses tools and methods used by the experiments in order to obtain results. The results themselves can be found in Part C. Please note that version 3 on the archive is the auxiliary version of the Physics of the B Factories book. This uses the notation alpha, beta, gamma for the angles of the Unitarity Triangle. The nominal version uses the notation phi_1, phi_2 and phi_3. Please cite this work as Eur. Phys. J. C74 (2014) 3026.

hep-ex↗

arXiv2015

Cusps and Exotic Charmonia

A simple, causal, and analytic model of final state rescattering is used to describe all available data on the exotic resonances $Z_c(3900)$ and $Z_c(4025)$. The model provides a compelling and accurate representation of experiment with no need for poles in the scattering matrix.

hep-ph↗

arXiv2015

Hybrid Mesons

A review of the theoretical and experimental status of hybrid hadrons is presented. The states $π_1(1400)$, $π_1(1600)$, and $π_1(2015)$ are thoroughly reviewed, along with experimental results from GAMS, VES, Obelix, COMPASS, KEK, CLEO, Crystal Barrel, CLAS, and BNL. Theoretical lattice results on the gluelump spectrum, adiabatic potentials, heavy and light hybrids, and transition matrix elements are discussed. These are compared with bag, string, flux tube, and constituent gluon models. Strong and electromagnetic decay models are described and compared to lattice gauge theory results. We conclude that while good evidence for the existence of a light isovector exotic meson exists, its confirmation as a hybrid meson awaits discovery of its iso-partners. We also conclude that lattice gauge theory rules out a number of hybrid models and provides a reference to judge the success of others.

hep-ph↗

arXiv2014

$Z_b$ and $Z_c$ Exotic States as Coupled Channel Cusps

It is demonstrated that the candidate tetraquark states $Z_b(10610)$, $Z_b(10650)$, $Z_c(3900)$, and $Z_c(4025)$ are coupled channel cusp effects. The model explains in a natural way the masses and quantum numbers of the putative states and the near equality of the widths of the $Z_b$ states. It is argued that the $Z_c(3900)$ and $Z_c(4025)$ should be visible in $\bar B^0 \to J/ψπ^0π^0$ or $B^- \to J/ψπ^- π^0$, but should {\it not} appear in $\bar B^0 \to J/ψπ^+π^-$, in agreement with recent LHCb results. Additional tests for cusp effects are suggested.

hep-ph↗