String fragmentation in supercooled confinement and implications for dark matter

A strongly-coupled sector can feature a supercooled confinement transition in the early universe. We point out that, when fundamental quanta of the strong sector are swept into expanding bubbles of the confined phase, the distance between them is large compared to the confinement scale. We suggest a modelling of the subsequent dynamics and find that the flux linking the fundamental quanta deforms and stretches towards the wall, producing an enhanced number of composite states upon string fragmentation. The composite states are highly boosted in the plasma frame, which leads to additional particle production through the subsequent deep inelastic scattering. We study the consequences for the abundance and energetics of particles in the universe and for bubble-wall Lorentz factors. This opens several new avenues of investigation, which we begin to explore here, showing that the composite dark matter relic density is affected by many orders of magnitude.

String fragmentation with a time-dependent tension

Motivated by recent theoretical arguments that expanding strings can be regarded as having a temperature that is inversely proportional to the proper time, $$\tau $$ τ , we investigate the consequences of adding a term $$\propto 1/\tau $$ ∝ 1 / τ to the string tension in the Lund string-hadronization model. The lattice value for the tension, $$\kappa _0 \sim 0.18\,{\mathrm {GeV}}^2\sim 0.9\,{\mathrm {GeV}}/{\mathrm {fm}}$$ κ 0 ∼ 0.18 GeV 2 ∼ 0.9 GeV / fm , is then interpreted as the late-time/equilibrium limit. A generic prediction of this type of model is that early string breaks should be associated with higher strangeness (and baryon) fractions and higher fragmentation $$\langle p_\perp \rangle $$ ⟨ p ⊥ ⟩ values. It should be possible to use archival ee data sets to provide model-independent constraints on this type of scenario, and we propose a few simple key measurements to do so.

Some properties of the pT regions observed at the LHC energies

The inclusive spectrum of the charged particles, [Formula: see text]0- and [Formula: see text]-mesons produced in the pp collisions at LHC energies were analyzed by fitting them with exponential functions. It was found the spectra were composed of several p[Formula: see text] regions, which could be characterized by the length of the regions [Formula: see text] and two free fitting parameters [Formula: see text] and [Formula: see text]. The study of the [Formula: see text] dependences of the parameters [Formula: see text] and [Formula: see text] and of the energy dependencies of the [Formula: see text], [Formula: see text] and [Formula: see text] showed that the regions can be classified into two groups depending on the values of the [Formula: see text], [Formula: see text] and [Formula: see text]. The values of the [Formula: see text] and [Formula: see text] for the first group don’t depend on colliding energy and the type of the particles (though the values of [Formula: see text] increase linearly with energy) whereas the characteristics in the second group of regions show strong dependencies. It was found that the ratio of the length for the [Formula: see text]-mesons to one for the [Formula: see text]0-mesons is approximately equal to the ratio of their mass: [Formula: see text]. Assuming that the values of the [Formula: see text] are directly proportional to the string tension the result could be considered as evidence in favor of parton string fragmentation dynamics. The increase in the lengths for the [Formula: see text]-mesons’ regions is accompanied by an increase of the values for the parameter [Formula: see text]. It can mean that the [Formula: see text]-mesons were produced at smaller values of [Formula: see text] compared with that for [Formula: see text]0-mesons. The results show that for the first group of regions the lengths of the regions are [Formula: see text]3–5 times greater than the lengths of neighboring, lower p[Formula: see text] regions. For the second group of regions the lengths of the regions are [Formula: see text]1–2 times greater than the lengths of neighboring lower p[Formula: see text] region. In the framework of the string fragmentation and hadronization dynamics, this could mean that the particles in the group [Formula: see text] of regions are produced through previous-generation strings decays into [Formula: see text]3–5 strings while those in group [Formula: see text] originate from previous-generation strings decays into [Formula: see text]2 strings.

Pseudorapidity dependence of multiplicity and transverse momentum fluctuations at the SPS energies

A search for the critical behavior of strongly interacting matter was done by studying the event-by-event fluctuations of multiplicity and transverse momentum of charged hadrons produced in inelastic p+p collisions and central Be+Be and Ar+Sc collisions at the NA61/SHINE experiment. Results for energy dependence of the scaled variance of the multiplicity distribution and for two families of strongly intensive measures of multiplicity and transverse momentum fluctuations Δ[PT, N] and Σ[PT, N] were presented. The study was performed in rapidity-integrated way and for different pseudorapidity regions, which corresponds to changing the rapidity averaged baryo-chemical potential and the value of temperature at the freeze-out stage. The strongly intensive measure Σ[NF, NB], evaluated in rapidity separated windows, was used in the analysis of short- and long-range multiplicity correlations and is considered to be sensitive for the initial conditions of particle production such as string fragmentation and fusion.

Enhancement of strange baryons in high-multiplicity proton–proton and proton–nucleus collisions

We investigate the enhancement of yields of strange and multi-strange baryons in proton–proton (p+p), proton–lead (p+Pb), and lead–lead (Pb+Pb) collisions at Large Hadron Collider (LHC) energies from a dynamical core–corona initialization model. We first generate partons just after the collisions by using event generators. These partons dynamically generate the quark gluon plasma (QGP) fluids through the source terms in the hydrodynamic equations. According to the core–corona picture, this process tends to happen where the density of generated partons is high and their transverse momentum is low. Some partons do not fully participate in this process when they are in dilute regions or their transverse momentum is high, and subsequently fragment into hadrons through string fragmentation. In this framework, the final hadrons come from either chemically equilibrated fluids as in the conventional hydrodynamic models or string fragmentation. We calculate the ratio of strange baryons to charged pions as a function of multiplicity and find that it monotonically increases up to $dN_{\mathrm{ch}}/d\eta \sim 100$ and then saturates above. This suggests that the QGP fluids are partly created and that their fraction increases with multiplicity in p+p and p+Pb collisions at LHC energies.

Nuclear Stopping in Central Au+Au Collisions at RHIC Energies

Nuclear stopping in central Au+Au collisions at relativistic heavy-ion collider (RHIC) energies is studied in the framework of a cascade mode and the modified ultrarelativistic quantum molecular dynamics (UrQMD) transport model. In the modified mode, the mean field potentials of both formed and “preformed” hadrons (from string fragmentation) are considered. It is found that the nuclear stopping is increasingly influenced by the mean-field potentials in the projectile and target regions with the increase of the reaction energy. In the central region, the calculations of the cascade model considering the modifying factor can describe the experimental data of the PHOBOS collaboration.