Exact baryon, strangeness, and charge conservation in hadronic gas models

Abstract

Relativistic heavy ion collisions are studied assuming that particles can be described by a hadron gas in thermal and chemical equilibrium. The exact conservation of baryon number, strangeness and charge are explicitly taken into account. For heavy ions the effect arising from the neutron surplus becomes important and leads to a substantial increase in e.g. the π − /π + ratio. A method is developed which is very well suited for the study of small systems.

Multiplicity fluctuations in a hadron gas with exact conservation laws

“…[5]. More recently, it has been pointed out [6,7] that in the canonical ensemble (CE) with exact conservation of charges, scaled second moment (scaled variance) of the multiplicity distribution of any particle does not converge to the corresponding GC value even in the thermodynamic limit, unlike the mean [8,9]. This was fairly understood among experts in statistical mechanics [10], but probably it has been shown explicitely for the canonical relativistic gas for the first time in refs.…”

Chemical factors in canonical statistical models for relativistic heavy ion collisions

“…Strangeness enhancement in Au-Au system does not definitely look like being of canonical origin, whereas Si-Au and Si-Al follow roughly the curve at the normal nuclear density Whereas the kaon enhancement with increasing centrality seems not to be compatible with a purely canonical effect, inelastic pp and central p-A and A-A systems considered in ref. [8] show a different behaviour with increasing volume. Therein, it was found that K/π ratios clearly follow the canonical curves.…”

“…The integral can be turned into a sum over many indices of modified Bessel functions I n (x), yet its evaluation is very time consuming and becomes impractical with baryon number larger than ∼ 10 [8]. Therefore, we have chosen another approach, which has been applied in elementary collisions [7,17].…”

“…Also, the * Electronic address: antti.keranen@oulu.fi † Electronic address: becattini@fi.infn.it strangeness enhancement with increasing system size in p-A reactions can be explained by a canonical effect [8].…”

“…The main difficulty of canonical calculations is related to the involved large values of baryon number and electric charge, which make the numerical computation of canonical partition functions quite hard. We present here an efficient numerical method to carry out such calculations which enabled us to implement exact charge conservation up to baryon number O(100) whereas with old methods it was only possible to reach B ∼ 20 [8] with very long computation times.…”