arXiv subjects

T. Barnes

Publications and source records attributed to T. Barnes.

85 records · Page 5Linked to original sources

arXiv1995

Hybrid and Conventional Mesons in the Flux Tube Model: Numerical Studies and their Phenomenological Implications

We present results from analytical and numerical studies of a flux tube model of hybrid mesons. Our numerical results use a Hamiltonian Monte Carlo algorithm and so improve on previous analytical treatments, which assumed small flux tube oscillations and an adiabatic separation of quark and flux tube motion. We find that the small oscillation approximation is inappropriate for typical hadrons and that the hybrid mass is underestimated by the adiabatic approximation. For physical parameters in the ``one-bead" flux tube model we estimate the lightest hybrid masses (${}_ΛL = {}_1 P$ states) to be 1.8-1.9~GeV for $u\bar u$ hybrids, 2.1-2.2~GeV for $s\bar s$ and 4.1-4.2~GeV for $c\bar c$. We also determine masses of conventional $q\bar q$ mesons with $L=0$ to $L=3$ in this model, and confirm good agreement with experimental $J$-averaged multiplet masses. Mass estimates are also given for hybrids with higher orbital and flux-tube excitations. The gap from the lightest hybrid level (${}_1P$) to the first hybrid orbital excitation (${}_1D$) is predicted to be $\approx 0.4$~GeV for light quarks $(q=u,d)$ and $\approx 0.3$~GeV for $q=c$. Both ${}_1P$ and ${}_1D$ hybrid multiplets contain the exotics $1^{-+}$ and $2^{+-}$; in addition the ${}_1P$ has a $0^{+-}$ and the ${}_1D$ contains a $3^{-+}$. Hybrid mesons with doubly-excited flux tubes are also considered. The implications of our results for spectroscopy are discussed, with emphasis on charmonium hybrids, which may be accessible at facilities such as BEPC, KEK, a Tau-Charm Factory, and in $ψ$ production at hadron colliders.

hep-ph↗

arXiv1994

CEBAF at Higher Energies

This report summarizes topics in hadron spectroscopy and production which could be addressed at CEBAF with an energy upgrade to $E_γ=8$ GeV and beyond. The topics discussed include conventional meson and baryon spectroscopy, spectroscopy of exotica (especially molecules and hybrids), CP and CPT tests using $ϕ$ mesons, and new detector and accelerator options.

hep-ph↗

arXiv1994

Hadronic Molecules and Low-Energy Hadron-Hadron Scattering Amplitudes

Recently there has been considerable interest in the subject of molecules, which are weakly bound states of hadron pairs. The question of the existence of molecules is closely related to the more general problem of the determination of low energy hadron-hadron scattering amplitudes, which is widely believed to require nonperturbative methods. In this contribution we report on quark model calculations using a simple perturbative scattering mechanism, one gluon exchange followed by constituent interchange. We refer to the associated diagrams as ``quark Born diagrams". For the cases chosen to isolate this mechanism, I=2 $ππ$, I=3/2 K$π$ and KN the results are usually in good agreement with experimental S-wave scattering amplitudes given standard potential model parameters, and for NN we find perturbative results very similar to the nonperturbative hard cores of Oka and Yazaki. We also discuss our findings for other less familiar channels; these include predictions of vector-vector bound states, one of which may be the $θ(1710)$.

hep-ph↗

arXiv1994

The Status of Molecules

This report summarizes the experimental and theoretical status of hadronic molecules, which are weakly-bound states of two or more hadrons. We begin with a brief history of the subject and discuss a few good candidates, and then abstract some signatures for molecules which may be of interest in the classification of possible molecule states. Next we argue that a more general understanding of $2\to 2$ hadron-hadron scattering amplitudes will be crucial for molecule searches, and discuss some of our recent work in this area. We conclude with a discussion of a few more recent molecule candidates (notably the $f_0(1710)$) which are not well established as molecules but satisfy some of the expected signatures.

hep-ph↗

arXiv1994

Inelastic Neutron Scattering from the Spin Ladder Compound (VO)2P2O7

We present results from an inelastic neutron scattering experiment on the candidate Heisenberg spin ladder vanadyl pyrophosphate, (VO)2P2O7. We find evidence for a spin-wave excitation gap of $E_{gap} = 3.7\pm 0.2$ meV, at a band minimum near $Q=0.8 A^{-1}$. This is consistent with expectations for triplet spin waves in (VO)2P2O7 in the spin-ladder model, and is to our knowledge the first confirmation in nature of a Heisenberg antiferromagnetic spin ladder.

cond-mat↗

arXiv1994

The susceptibility and excitation spectrum of (VO)$_2$P$_2$O$_7$ in ladder and dimer chain models

We present numerical results for the magnetic susceptibility of a Heisenberg antiferromagnetic spin ladder, as a function of temperature and the spin-spin interaction strengths $J_\perp$ and $J_{||}$. These are contrasted with new bulk limit results for the dimer chain. A fit to the experimental susceptibility of the candidate spin-ladder compound vanadyl pyrophosphate, (VO)$_2$P$_2$O$_7$, gives the parameters $J_\perp = 7.82$ meV and $J_{||} = 7.76$ meV. With these values we predict a singlet-triplet energy gap of $E_{gap} = 3.9$ meV, and give a numerical estimate of the ladder triplet dispersion relation $ω(k)$. In contrast, a fit to the dimer chain model leads to $J_1=11.11$ meV and $J_2=8.02$ meV, which predicts a gap of $E_{gap} = 4.9$ meV.

cond-mat↗

arXiv1994

$I=0,1$ $ππ$ and $I=1/2$ $Kπ$ Scattering using Quark Born Diagrams

We extend the quark Born diagram formalism for hadron-hadron scattering to processes with valence $q\bar q$ annihilation, specifically $I=0$ and $I=1$ $ππ$ and $I=1/2$ $Kπ$ elastic scattering. This involves the $s$-channel hybrid annihilation process $q^2\bar q^2 \to q\bar q g \to q^2\bar q^2$ and conventional $s$-channel $q\bar q$ resonances, in addition to the $t$-channel gluon exchange treated previously using this formalism. The strength of the $t$-channel gluon amplitude is fixed by previous studies of $I=2$ $ππ$ and $I=3/2$ $Kπ$ scattering. The $s$-channel resonances $ρ(770)$, $K^*(892)$, $f_0(1400)$ and $K_0^*(1430)$ are incorporated as relativized Breit-Wigner amplitudes, with masses and energy-dependent widths fitted to experimental phase shifts. The strength of the $s$-channel gluon ``hybrid" annihilation diagrams is problematical since the perturbative massless-gluon energy denominator must be modified to account for the effect of confinement on the energy of the virtual hybrid state. Our naive expectation is that near threshold the hybrid diagrams are comparable in magnitude but opposite in sign to the contribution predicted using massless perturbative gluons. Fitting the strength of this amplitude to $I=0$ $ππ$ and $I=1/2$ $Kπ$ S-wave data gives a result consistent with this expectation. We find good agreement with experimental phase shifts from threshold to 0.9 GeV in $ππ$ and 1.6 GeV in $Kπ$ using this approach. We conclude that the most important contribution to low energy S-wave scattering in these channels arises from the nonresonant quark Born diagrams, but that the low-energy wings of the broad $s$-channel resonances $f_0(1400)$ and $K^*_0(1430)$ also give important contributions near threshold. The nonresonant contributions are found to be much smaller in $L\;

hep-ph↗

arXiv1993

Signatures for Hybrids

In this review talk I summarize theoretical expectations for properties of hybrid mesons and baryons, and discuss the prospects for identifying these states experimentally. This is an expanded version of an Invited Contribution to the Conference on Exclusive Reactions at High Momentum Transfers, Marciana Marina, Elba, Italy, 24-26 June 1993.

hep-ph↗

arXiv1993

Charmonium Physics at a Tau-Charm Factory

This talk summarizes the status of the charmonium system, with particular emphasis on outstanding problems in QCD spectroscopy which a tau-charm factory can address.

hep-ph↗

arXiv1993

Hadronic Molecules and Scattering Amplitudes from the Nonrelativistic Quark Model

This report summarizes recent calculations of low-energy hadron-hadron scattering amplitudes in the nonrelativistic quark potential model, which assume that the scattering mechanism is a single interaction (usually OGE) followed by constituent interchange. We refer to the scattering diagrams as ``quark Born diagrams". For the cases chosen to isolate this mechanism, I=2 $ππ$, I=3/2 K$π$, KN and NN, the results are usually in good agreement with experimental S-wave scattering amplitudes given standard potential-model parameters. These calculations also lead to predictions of vector-vector bound states, one of which may be the $θ(1710)$. This assignment can be tested by searches for K$\bar{\rm K}ππ$ and $ϕπ^0γ$ decay modes of the $θ(1710)$.

hep-ph↗

arXiv1993

NN Core Interactions and Differential Cross Sections from One Gluon Exchange

We derive nonstrange baryon-baryon scattering amplitudes in the nonrelativistic quark model using the ``quark Born diagram" formalism. This approach describes the scattering as a single interaction, here the one-gluon-exchange (OGE) spin-spin term followed by constituent interchange, with external nonrelativistic baryon wavefunctions attached to the scattering diagrams to incorporate higher-twist wavefunction effects. The short-range repulsive core in the NN interaction has previously been attributed to this spin-spin interaction in the literature; we find that these perturbative constituent-interchange diagrams do indeed predict repulsive interactions in all I,S channels of the nucleon-nucleon system, and we compare our results for the equivalent short-range potentials to the core potentials found by other authors using nonperturbative methods. We also apply our perturbative techniques to the N$Δ$ and $ΔΔ$ systems: Some $ΔΔ$ channels are found to have attractive core potentials and may accommodate ``molecular" bound states near threshold. Finally we use our Born formalism to calculate the NN differential cross section, which we compare with experimental results for unpolarised proton-proton elastic scattering. We find that several familiar features of the experimental differential cross section are reproduced by our Born-order result.

nucl-th↗

arXiv1992

Kaon-Nucleon Scattering Amplitudes and Z$^*$-Enhancements from Quark Born Diagrams

We derive closed form kaon-nucleon scattering amplitudes using the ``quark Born diagram" formalism, which describes the scattering as a single interaction (here the OGE spin-spin term) followed by quark line rearrangement. The low energy I=0 and I=1 S-wave KN phase shifts are in reasonably good agreement with experiment given conventional quark model parameters. For $k_{lab}> 0.7$ Gev however the I=1 elastic phase shift is larger than predicted by Gaussian wavefunctions, and we suggest possible reasons for this discrepancy. Equivalent low energy KN potentials for S-wave scattering are also derived. Finally we consider OGE forces in the related channels K$Δ$, K$^*$N and K$^*Δ$, and determine which have attractive interactions and might therefore exhibit strong threshold enhancements or ``Z$^*$-molecule" meson-baryon bound states. We find that the minimum-spin, minimum-isospin channels and two additional K$^*Δ$ channels are most conducive to the formation of bound states. Related interesting topics for future experimental and theoretical studies of KN interactions are also discussed.

nucl-th↗

arXiv1992

I=3/2 $K π$ Scattering in the Nonrelativisitic Quark Potential Model

We study $I=3/2$ elastic $Kπ$ scattering to Born order using nonrelativistic quark wavefunctions in a constituent-exchange model. This channel is ideal for the study of nonresonant meson-meson scattering amplitudes since s-channel resonances do not contribute significantly. Standard quark model parameters yield good agreement with the measured S- and P-wave phase shifts and with PCAC calculations of the scattering length. The P-wave phase shift is especially interesting because it is nonzero solely due to $SU(3)_f$ symmetry breaking effects, and is found to be in good agreement with experiment given conventional values for the strange and nonstrange constituent quark masses.

hep-ph↗