arXiv subjects

W. Liu

Publications and source records attributed to W. Liu.

124 records · Page 7Linked to original sources

arXiv2004

Pentaquark baryon production at the Relativistic Heavy Ion Collider

Production of pentaquark $Θ^+$ baryons in central relativistic heavy ion collisions is studied in a kinetic model. Assuming that a quark-gluon plasma is produced in the collisions, we first determine the number of $Θ^+$ produced from the quark-gluon plasma using a parton coalescence model, and then take into consideration its production and absorption in subsequent hadronic matter via the reactions $KN\leftrightarrowΘ$, $KN\leftrightarrowπΘ$, and $\piN\leftrightarrow\bar KΘ$. We find that although the final $Θ^+$ number is affected by hadronic interactions, it remains sensitive to the initial number of $Θ^+$ produced from the quark-gluon plasma, particularly in the case of a small $Θ^+$ width as imposed by the $K^+N$ and $K^+d$ scattering data. Because of small baryon chemical potential in the hot dense matter produced in these collisions, the number of produced anti-$Θ$ is only slightly smaller than that of $Θ^+$.

nucl-th↗

arXiv2003

Photoproduction of pentaquark cascades from nucleons

The cross sections for production of pentaquark $Ξ^+_5$ from the reaction $γp\to K^0K^0Ξ^+_5$ and $Ξ^{--}_5$ from the reaction $γn\to K^+K^+Ξ^{--}_5$ are evaluated in a hadronic model that includes their couplings to both $Σ\bar K$ and $Σ\bar K^*$. With these coupling constants determined from the empirical $πNN(1710)$ and $ρNN(1710)$ coupling constants by assuming that $Ξ^+_5$, $Ξ^{--}_5$, and N(1710) belong to the same antidecuplet of spin 1/2 and positive parity, and using form factors at strong interaction vertices similar to those for pentaquark $Θ^+$ production in photonucleon reactions, we obtain a cross section of about 0.03-0.6 nb for the reaction $γp\to K^0K^0Ξ^+_5$ and about 0.1-0.6 nb for the reaction $γn\to K^+K^+Ξ^{--}_5$ at photon energy $E_γ=4.5$ GeV, depending on the value of the coupling constant $g_{K^*ΣΞ_5}$.

nucl-th↗

arXiv2003

Pentaquark $Θ^+$ production from the reaction $γp\toπ^+K^-Θ^+$

The cross section for $Θ^+$ production from the reaction $γp\toπ^+K^-Θ^+$, which was observed in the CLAS experiment at the Jefferson National Laboratory, is evaluated in a hadronic model that includes couplings of $Θ^+$ to both $KN$ and $K^*N$. With their coupling constants determined from the empirical $πNN(1710)$ and $ρNN(1710)$ coupling constants using the SU(3) symmetry, the cross section for this reaction has been evaluated by taking $Θ^+$ to have spin 1/2 and isospin 0 but either positive or negative parity. We find that the cross section is 10-15 nb if $Θ^+$ has positive parity as predicted by the chiral soliton model. The cross section is reduced by more than a factor of 10 if $Θ^+$ has negative parity as given by lattice QCD studies. For both parities, the differential distribution peaks at small negative four momentum transfer as expected from the dominating $t$-channel kaon-exchange diagram that involves only the coupling of $Θ^+$ to $KN$.

nucl-th↗

arXiv2003

Pentaquark baryon production from photon-neuteron reactions

Extending the hadronic Lagrangians that we recently introduced for studying pentaquark $Θ^+$ baryon production from meson-proton, proton-proton, and photon-proton reactions near threshold to include the anomalous interaction between $γ$ and $K^*K$, we evaluate the cross section for $Θ^+$ production from photon-neutron reactions, in which the $Θ^+$ was first detected in the SPring-8 experiment in Japan and the CLAS experiment at Thomas Jefferson National Laboratory. With empirical coupling constants and form factors, and assuming that the decay width of $Θ^+$ is 20 MeV, the predicted cross section is found to have a peak value of about 280 nb, which is substantially larger than that for $Θ^+$ production from photon-proton reactions.

nucl-th↗

arXiv2003

Cross sections for pentaquark baryon production from protons in reactions induced by hadrons and photons

Using hadronic Lagrangians that include the interaction of pentaquark $Θ^+$ baryon with $K$ and $N$, we evaluate the cross sections for its production from meson-proton, proton-proton, and photon-proton reactions near threshold. With empirical coupling constants and form factors, the predicted cross sections are about 1.5 mb in kaon-proton reactions, 0.1 mb in rho-nucleon reactions, 0.05 mb in pion-nucleon reactions, 20 $μ$b in proton-proton reactions, and 40 nb in photon-proton reactions.

nucl-th↗

arXiv2003

The 21Na(p,gamma)22Mg Reaction and Oxygen-Neon Novae

The 21Na(p,gamma)22Mg reaction is expected to play an important role in the nucleosynthesis of 22Na in Oxygen-Neon novae. The decay of 22Na leads to the emission of a characteristic 1.275 MeV gamma-ray line. This report provides the first direct measurement of the rate of this reaction using a radioactive 21Na beam, and discusses its astrophysical implications. The energy of the important state was measured to be E$_{c.m.}$= 205.7 $\pm$ 0.5 keV with a resonance strength $ωγ= 1.03\pm0.16_{stat}\pm0.14_{sys}$ meV.

astro-ph↗

arXiv2002

Charm meson production from meson-nucleon scattering

Using an effective hadronic Lagrangian with physical hadron masses and coupling constants determined either empirically or from SU(4) flavor symmetry, we study the production cross sections of charm mesons from pion and rho meson interactions with nucleons. With a cutoff parameter of 1 GeV at interaction vertices as usually used in studying the cross sections for $J/ψ$ absorption and charm meson scattering by hadrons, we find that the cross sections for charm meson production have values of a few tenth of mb and are dominated by the s channel nucleon pole diagram. Relevance of these reactions to charm meson production in relativistic heavy ion collisions is discussed.

nucl-th↗

arXiv2002

A Test of CPT and Lorentz Invariance from Muonium Spectroscopy

Upon a suggestion of Kostelecky et al., we have performed a test of CPT and Lorentz invariance from the microwave spectroscopy of muonium. New terms in their standard model extension violating CPT and Lorentz symmetry would appear as anti-correlated frequency shifts, oscillating at the earth's sidereal frequency, in the two ground state hyperfine transitions involving muon spin flip, $ν_{12}$ and $ν_{34}$, which were measured in a strong magnetic field of 1.7 T. No such time dependence was observed in $ν_{12}$ or $ν_{34}$ at the level of 20 Hz, limiting the size of some CPT and Lorentz violating parameters with Planck scale sensitivity, and representing almost an order of magnitude improvement over previous limits for the muon.

hep-ex↗

arXiv2001

Charmonium absorption cross section by nucleon

The $J/ψ$ absorption cross section by nucleon is studied using a gauged SU(4) hadronic Lagrangian but with empirical particle masses, which has been used previously to study the $J/ψ$ absorption cross section by pion and rho meson. Including both two-body and three-body final states, we find that with a cutoff parameter of 1 GeV at interaction vertices involving charm hadrons, the $J/ψ-N$ absorption is at most 5 mb and is consistent with that extracted from $J/ψ$ production from both photo-nuclear and proton-nucleus reactions.

nucl-th↗

arXiv2001

Test of CPT and Lorentz invariance from muonium spectroscopy

Following a suggestion of Kostelecky et al. we have evaluated a test of CPT and Lorentz invariance from the microwave spectroscopy of muonium. Hamiltonian terms beyond the standard model violating CPT and Lorentz invariance would contribute frequency shifts $δν_{12}$ and $δν_{34}$ to $ν_{12}$ and $ν_{34}$, the two transitions involving muon spin flip, which were precisely measured in ground state muonium in a strong magnetic field of 1.7 T. The shifts would be indicated by anti-correlated oscillations in $ν_{12}$ and $ν_{34}$ at the earth's sidereal frequency. No time dependence was found in $ν_{12}$ or $ν_{34}$ at the level of 20 Hz, limiting the size of some CPT and Lorentz violating parameters at the level of $2\times10^{-23}$ GeV, representing Planck scale sensitivity and an order of magnitude improvement in sensitivity over previous limits for the muon.

hep-ex↗

arXiv1997

A Model Approximation Scheme for Planning in Partially Observable Stochastic Domains

Partially observable Markov decision processes (POMDPs) are a natural model for planning problems where effects of actions are nondeterministic and the state of the world is not completely observable. It is difficult to solve POMDPs exactly. This paper proposes a new approximation scheme. The basic idea is to transform a POMDP into another one where additional information is provided by an oracle. The oracle informs the planning agent that the current state of the world is in a certain region. The transformed POMDP is consequently said to be region observable. It is easier to solve than the original POMDP. We propose to solve the transformed POMDP and use its optimal policy to construct an approximate policy for the original POMDP. By controlling the amount of additional information that the oracle provides, it is possible to find a proper tradeoff between computational time and approximation quality. In terms of algorithmic contributions, we study in details how to exploit region observability in solving the transformed POMDP. To facilitate the study, we also propose a new exact algorithm for general POMDPs. The algorithm is conceptually simple and yet is significantly more efficient than all previous exact algorithms.

cs.AI↗

arXiv1993

Effects of spatial size, lattice doubling and source operator on the hadron spectrum with dynamical staggered quarks

We have extended our previous study of the lattice QCD spectrum with 2 flavors of staggered dynamical quarks at $6/g^2=5.6$ and $am_q=0.025$ and 0.01 to larger lattices, with better statistics and with additional sources for the propagators. The additional sources allowed us to estimate the $Δ$ mass and to measure the masses of all mesons whose operators are local in time. These mesons show good evidence for flavor symmetry restoration, except for the masses of the Goldstone and non-Goldstone pions. PCAC is observed in that $m_π^2 \propto m_q$, and $f_π$ is estimated. Use of undoubled lattices removes problems with the pion propagator found in our earlier work. Previously we found a large change in the nucleon mass at a quark mass of $am_q=0.01$ when we increased the spatial size from 12 to 16. No such effect is observed at the larger quark mass, $am_q=0.025$. Two kinds of wall source were used, and we have found difficulties in getting consistent results for the nucleon mass between the two sources.

hep-lat↗

arXiv1993

Simple Hadronic Matrix Elements with Wilson Valence Quarks and Dynamical Staggered Fermions at ${\bf 6/g^2=5.6}$

We have measured some simple matrix elements for pseudoscalar and vector mesons made of Wilson valence quarks and staggered sea quarks at $β=5.6$ at sea quark masses $am_q=0.01$ and 0.025. Our measurements include the decay constants of pseudoscalars (including $f_D$), the wave function at the origin (or decay constant) of vector mesons, and the calculation of quark masses from current algebra. The effects of sea quarks on the simulations are small. We make comparisons to quenched simulations at similar values of the lattice spacing ($1/a \simeq 2$ GeV).

hep-lat↗

arXiv1992

Hadron Spectrum in QCD with Valence Wilson Fermions and Dynamical Staggered Fermions at $6/g^2=5.6

We present an analysis of hadronic spectroscopy for Wilson valence quarks with dynamical staggered fermions at lattice coupling $6/g^2 = β=5.6$ at sea quark mass $am_q=0.01$ and 0.025, and of Wilson valence quarks in quenched approximation at $β=5.85$ and 5.95, both on $16^3 \times 32$ lattices. We make comparisons with our previous results with dynamical staggered fermions at the same parameter values but on $16^4$ lattices doubled in the temporal direction.

hep-lat↗

arXiv1992

QCD Hadron Spectroscopy with Staggered Dynamical Quarks at $β= 5.6$

We present preliminary results from the 1991 HEMCGC simulations with staggered dynamical fermions on a $16^3 \times 32$ lattice at $β= 5.6$ with sea quark masses $am_q = 0.025$ and 0.01. The spectroscopy was done both for staggered valence quarks with mass equal to the sea quark masses and for Wilson valence quarks at six different values for $κ$, 0.1320, 0.1410, 0.1525, 0.1565, 0.1585, and 0.1600. In addition to the measurements performed in our earlier work, we also measured the $Δ$ and other `extended' hadrons for staggered valence quarks and pseudo-scalar decay constants and vector meson matrix elements, the wave function at the origin, for Wilson valence quarks.

hep-lat↗

arXiv1992

Thermodynamics of Lattice QCD with 2 Light Dynamical (Staggered) Quark Flavours on a $16^3\times 8$ Lattice

Lattice QCD with 2 light staggered quark flavours is being simulated on a $16^3\times8$ lattice to study the transition from hadronic matter to a quark gluon plasma. We have completed runs at $m_q=0.0125$ and are extending this to $m_q=0.00625$. We also examine the addition of a non-dynamical "strange" quark. Thermodynamic order parameters are being measured across the transition and further into the plasma phase, as are various screening lengths. No evidence for a first order transition is seen, and we estimate the transition temperature to be $TY_c=143(7) MeV$.

hep-lat↗