arXiv subjects

G. Welke

Publications and source records attributed to G. Welke.

7 recordsLinked to original sources

arXiv1998

${\barΛ}/{\bar p}$ ratios in heavy ion collisions at 11.6 AGeV/c

We attempt to explain the ${\bar Λ}/\bar p$ ratios measured in heavy ion collisions at $11.6~{\rm A\cdot GeV}/c$ beam momentum within a hadronic framework. This ratio is enhanced relative to corresponding ratios in $pp$ collisions, and is large when compared with thermal fits to heavy ion data. Using a detailed cascade calculation, we show that different annihilation cross--sections of $\bar Λ$'s and $\bar p$'s, and the net conversion of $\bar p$'s to $\bar Λ$'s, do not account for the enhancement in central collisions. For larger impact parameters, however, hadronic mechanisms may well suffice to produce the observed enhancement. Uncertainties in elementary cross--sections and formation times are considered.

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arXiv1998

Anti-Lambda/Anti-Proton ratios at the AGS

We attempt to explain the large ${\bar Λ}/\bar p$ ratios measured in heavy ion collisions at $12~{\rm A\cdot GeV}/c$ beam momentum within an hadronic framework. The ratios are large compared to corresponding ratios in $pp$ collisions, and to thermal fits. We show using a simple model and a detailed cascade calculation that different annihilation cross--sections of $\bar Λ$'s and $\bar p$'s, and the net conversion of $\bar p$'s to $\bar Λ$'s, do not account for the enhancement. Uncertainties in elementary cross--sections and formation times are also considered.

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arXiv1996

A theoretical view of practical problems in interferometry

Interferometry is discussed in terms of the representation of the source. In particular, scale-invariant 1-d hydrodynamics is revisited, and extended to the case of unequal transverse masses. It is argued that that kaon emission occurs over a short time interval. Exact results for models of two- and three-dimensional flow are presented, which exhibit altered scaling laws. Such qualitative trends, together with other observables, are vital if one is to draw conclusions about the source.

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arXiv1996

Pion-pair formation and the pion dispersion relation in a hot pion gas

The possibility of pion--pair formation in a hot pion gas, based on the bosonic gap equation, is pointed out and discussed in detail. The critical temperature for condensation of pion pairs (Evans--Rashid transition) is determined as a function of the pion density. As for fermions, this phase transition is signaled by the appearance of a pole in the two--particle propagator. In bose systems there exists a second, lower critical temperature, associated with the appearance of the single--particle condensate. Between the two critical temperatures the pion dispersion relation changes from the usual quasiparticle dispersion to a Bogoliubov--like dispersion relation at low momenta. This generalizes the non-relativistic result for an attractive bose gas by Evans et al. Possible consequences for the inclusive pion spectra measured in heavy--ion collisions at ultra--relativistic energies are discussed.

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arXiv1994

Boson Interferometry after SPS and before RHIC

We re--examine the connection between interferometry and the Wigner representation for source freeze--out, and continuous emission. At the operator level, two equivalent representations of the two--particle spectrum are found, which contradict the standard expression of kinetic theory. The discrepancy is resolved using two toy models. Further, we revisit interferometry in scale--invariant one--dimensional hydrodynamics, and argue that recent experimental results are evidence for a short kaon emission time. Using two exactly calculable models of two-- and three--dimensional flow, it is shown that the saddle point approximation, which is reasonable for one--dimensional flow, is no longer adequate. In these models the scaling law is altered, and we argue that such qualitative trends, together with other observables, are vital if one is to draw conclusions about the unknown source parameters.

hep-ph↗

arXiv1993

Two pion bound states in a hot pion gas

The occurrence of poles in the $2π$ propagator in a hot pion medium is studied. The domain of non--trivial solutions to the corresponding bosonic gap equation is investigated as a function of the chemical potential and temperature of the gas.

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arXiv1993

Nuclear Flow Excitation Function

We consider the dependence of collective flow on the nuclear surface thickness in a Boltzmann--Uehling--Uhlenbeck transport model of heavy ion collisions. Well defined surfaces are introduced by giving test particles a Gaussian density profile of constant width. Zeros of the flow excitation function are as much influenced by the surface thickness as the nuclear equation of state, and the dependence of this effect is understood in terms of a simple potential scattering model. Realistic calculations must also take into account medium effects for the nucleon--nucleon cross section, and impact parameter averaging. We find that balance energy scales with the mass number as $A^{-y}$, where $y$ has a numerical value between 0.35 and 0.5, depending on the assumptions about the in-medium nucleon-nucleon cross section.

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