High energy nuclear collisions at collider facilities probe the phase boundary between ordinary hadronic matter (protons, neutrons, etc.) and quark matter (soup of quarks and gluons). We study the collective expansion generated as matter passes through this phase boundary. Such effects as thermophoresis, diffusion and viscosity are modeled as possible generators of radial flow. We find that quark diffusion is responsible for a significant flow signal; much stronger than is observed in experiments. We therefore suggest that the absence of strong flow in the experiments might signal the absence of the mixed phase of hadronic plus quark matter.
Ms. Peters, pictured below, gave an oral presentation in the
2003 Student Research Colloquium.
The free-space lifetime of the phi meson is 45 fm/c (1 fm/c is roughly ten to the minus 23 seconds). The fireball generated in high energy nuclear collisions is expected to persist for a time somewhat less than this; by maybe a factor of 2 or 3. The phi mesons therefore decays outside the fireball. This is unfortunate since decays inside would signal interesting nuclear (medium) effects. We model the lifetime of the phi meson at finite temperature, where the spectral properties are quite different from the vacuum. We find that at finite temperature the lifetime of the phi is significantly reduced. Indications are such that at temperatures above 150 MeV (fraction of a trillion Kelvin) the phi lifetime drops below the fireball's lifetime. The phi therefore might decay inside the fireball and carry signals of medium-modified dynamics.
Mr. Kakuk is pictured below during his poster presentation at the
2003 Student Research Colloquium.
Student Research (2001-2002)
- Effects of Broad Hadron Distribution on Low Mass Dilepton Signals (Ms. Judith Peters, advised by K. Haglin)
- Charmonium Dissociation in Hadronic Matter (Mr. Theron Blount, advised by K. Haglin)
- Modifications of the Lifetime of the Omega Particle (Mr. Michael Kakuk, advised by K. Haglin)
In high-energy nucleus-nucleus central collisions, a sizable fraction of the beam energy is converted into hadronic matter produced in the vicinity of the center of mass of the colliding system. The rho meson is a particularly important constituent of this matter due to its direct dilepton decay channel. Dileptons are ideal probes for studying the in-medium properties of hadrons since they probe the entire volume of the system and carry information on the thermodynamical state of the medium at the point of production. Therefore, the invariant mass distribution of dileptons is crucial to understanding the in-medium spectral function of vector mesons. This model seeks to improve upon existing models by describing production of dileptons with the width of the rho mesons and the participating pions broadened to included in-medium decay and collision where appropriate. The impact of radial flow on the production of dileptons will also be addressed.
Ms. Peters, pictured below, presented results as a poster in the
2002 Student Research Colloquium.
High energy nuclear collisions create a short-lived, heated and compressed system of subatomic particles called mesons. One such meson, the $J/\psi$, is a rare and fairly massive object consisting of a charm quark plus its antiquark. When $J/\psi$ collides with lighter particles, it can suffer breakup and lose its identity in favor of other, less rate and lighter particles. Modeling the rate at which these conversions take place is critical in understanding the overall production of $J/\psi$. Nuclear collision events which have a decreased production of $J/\psi$ are expected to involve, at least for some brief moments, the presence of a plasma of deconfined quarks and gluons. The plasma represents a new phase of matter in the sense that it has never before been identified in the laboratory. We predict the likelihood of $J/\psi$ survival using relativistic kinetic theory, complete with dynamical system details and with energy, momentum and other appropriate conservation laws. The survival probability calculation can be compared with experimental yields. Breakup scattering rates, system cooling rates, and particle distributions are among the parameters of our model that we vary in attempts to better understand the experiments.
Mr. Blount is pictured below standing between two attendees and is
explaining his poster at the
2002 Student Research Colloquium.
Current experiments in heavy ion colliders involve gold and lead beams generating a ``fireball'' of strongly interacting matter at temperatures approaching a trillion Kelvin. Due to the fireball's brief existence, it cannot be examined directly; instead, it can be studied by sifting through the particle bebris. Individual particle properties, such as the lifetimes of the particles involved, affect the final state in identifiable ways. It is expected that the extreme conditions within the fireball alter particle lifetimes as compared to free space. This research involves exploring the in-medium lifetime of the omega meson. We find that the omega decays, on average, sooner in the medium than it would in free space. This impacts the experiments by opening up the possibility that some omega particles decay inside the fireball, changing its apparent mass distribution dramatically.
Mr. Kakuk is pictured below during his oral presentation at the
2002 Student Research Colloquium.
