Physics Letters B 785 (2018) 226–231
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Influence of anisotropic / creation on the
−
multiplicity in
subthreshold proton–nucleus collisions
Miklós Zétényi
a,b,∗
, György Wolf
a
a
Wigner Research Centre for Physics, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary
b
ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany
a r t i c l e i n f o a b s t r a c t
Article history:
Received
28 March 2018
Received
in revised form 7 July 2018
Accepted
7 August 2018
Available
online 30 August 2018
Editor:
V. Metag
Keywords:
Proton–nucleus
collision
Subthreshold
particle production
Strangeness
production
baryon production
Transport
models
We present an analysis of
−
baryon production in subthreshold proton–nucleus (p + A) collisions in
the framework of a BUU type transport model. We propose a new mechanism for production in the
collision of a secondary or hyperon and a nucleon from the target nucleus. We find that the
−
multiplicity in p + A collisions is sensitive to the angular distribution of hyperon production in the
primary N + N collision. Using reasonable assumptions on the unknown elementary cross sections we
are able to reproduce the
−
multiplicity and the
−
/( +
0
) ratio obtained in the HADES experiment
in p+Nb collisions at
√
s
NN
= 3.2 GeV energy.
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP
3
.
1. Introduction
A sensitive tool to investigate the properties of the dense
medium formed in nuclear collisions is the study of particle pro-
duction
near and below the kinematical threshold valid in an ele-
mentary
N +N collision. For the subthreshold production of a par-
ticle
species, the energy has to be provided by the medium, e.g. via
Fermi motion, in-medium modification of masses, or multiple col-
lisions.
The threshold for strangeness production is relatively high
because strangeness conservation in strong interactions requires
the simultaneous creation of two (anti)strange particles. Therefore
the study of subthreshold strangeness production can provide in-
formation
about the high density medium reached in heavy-ion
collisions at higher energies.
In
hadron–nucleus (p + A, π + A) collisions the properties of
the medium at normal nuclear matter density can be studied.
These reactions are also an important intermediate step between
hadronic and heavy-ion collisions, since they are cleaner, and fewer
particle production channels are possible as the collision of two
secondaries is unlikely.
*
Corresponding author.
E-mail
addresses: zetenyi.miklos@wigner.mta.hu (M. Zétényi),
wolf.gyorgy@wigner.mta.hu (G. Wolf).
The HADES collaboration has measured the yield of the doubly
strange
−
baryon in p+Nb collisions at the subthreshold energy
of
√
s
NN
= 3.2GeV [1], and in Ar+KCl collisions at the deeply sub-
threshold
energy of
√
s
NN
= 2.61 GeV [2]. They also performed
a statistical model analysis of the obtained
−
yields along with
their results for π, η, , kaon, ω and φ multiplicities in the same
reactions [3]. They found that the THERMUS statistical model gives
a good description of all particle yields except for that of the
−
baryon, which the model underestimates by a factor 15 in the case
of the reaction Ar+KCl and by a factor 8 in the case of the reaction
p+Nb.
The Ar+KCl reaction was also analyzed in terms of a different
statistical model in which kaon multiplicity was taken into account
on an event-by-event basis [4]. The probability to find n kaons in
the final state follows a Poisson distribution. The estimate of the
multiplicity is based on the observation that the doubly strange
baryon is produced simultaneously with two kaons, therefore
only events with at least two kaons contribute to production.
The value of the //K
+
multiplicity ratio in the Ar+KCl reaction
obtained in this model is smaller by a factor of 8 than the value
obtained in by HADES and by a factor of 3 smaller than the exper-
imental
lower bound.
production in heavy-ion collisions at the energies of the
SIS18 synchrotron at GSI has been studied in terms of relativis-
tic
transport models, which are able to follow the non-equilibrium
https://doi.org/10.1016/j.physletb.2018.08.054
0370-2693/
© 2018 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by
SCOAP
3
.