Signal of the pion string at high-energy collisions

Abstract

We study the possible signals of a pion string associated with the QCD chiral phase transition in LHC Pb - Pb collision at energy $ \sqrt{s}=5.5$ TeV. In terms of the Kibble-Zurek mechanism we discuss the production and evolution of the pion string. The pion string is not topologically stable, it decays into neutral pions and sigma mesons which in turn decay into pions. Our results show that all the neutral pions from the pion string are distributed at the low momentum and the ratio of neutral to charged pions from the pion string violates the isospin symmetry. For the momentum spectra of the total pions, the signal from the sigma particle decay which is from the pion string will be affected by the large decay width of the sigma significantly.Comment: 8 pages, 3 figures, one reference added, title changed, version accepted for publication in Phys. Rev.

Influence of in-mediumNNcross sections, symmetry potential, and impact parameter on isospin observables

“…choose to simulate nuclear collisions with the code ImQMD05 developed at the Chinese Institute of Atomic Energy (CIAE) [42][43][44]. Details of this code are described in section II.…”

“…In this section, we will describe in more detail the theoretical formalisms used in the ImQMD05 code [41][42][43][44]. Within this code, the nucleonic mean fields acting on nucleon wavepackets are derived from a potential energy density functional where the potential energy U includes the full Skyrme potential energy with the spin-orbit term omitted:…”

“…The in-medium isospin dependent nucleon-nucleon (NN) differential cross section in free space is adopted from Ref. [46], and isospin dependent Pauli blocking effects are the same as in [42][43][44]. Asymptotic clusters are constructed by means of the minimum tree spanning method widely used in QMD calculations, in which particles with relative momentum smaller than P 0 and relative distance smaller than R 0 coalesce into one cluster.…”

Effect of isospin-dependent cluster recognition on the observables in heavy ion collisions

“…At any time of the reaction process, fragments can be recognized by a minimum spanning tree (MST) algorithm [7,17]. In this algorithm, nucleons which have a neighbor within a distance of coordinate and momentum of |r i − r j | ≤ R 0 and |p i − p j | ≤ P 0 belong to a fragment.…”

“…Typical values of R 0 and P 0 used in the QMD approaches are about 3.5 fm and 250 MeV /c [7,17]. This approach has been quite successful in explaining certain fragmentation observables, such as charge distributions of the emitted particles, intermediate mass fragments multiplicities [7,[17][18][19], yield ratios of free neutron to proton (n/p), and the double n/p ratios in heavy ion collisions [15]. On the other hand, the MST method fails to describe other details in the production of nucleons and light charged particles [7,17,20].…”