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Register a new userCollective String Interactions in AdS/QCD and High-Multiplicity pA Collisions
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QCD strings originate from high-energy scattering in the form of Reggeons and Pomerons, and have been studied in some detail in lattice numerical simulations. Production of multiple strings, with their subsequent breaking, is now a mainstream model of high energy $pp$ and $pA$ collisions. Recent LHC experiments revealed that high multiplicity end of such collisions show interesting collective effects. This ignited an interest in the interaction of QCD strings and multi-string dynamics. Holographic models, collectively known as AdS/QCD, developed in the last decade, describe both hadronic spectroscopy and basic thermodynamics, but so far no studies of the QCD strings have been done in this context. The subject of this paper is to do this. First, we study in more detail the scalar sector of hadronic spectroscopy, identifying glueballs and scalar mesons, and calculate the degree of their mixing. The QCD strings, holographic images of the fundamental strings, thus have a gluonic core and a sigma cloud. The latter generates $sigma$ exchanges and collectivization of the strings, affecting, at a certain density, the chiral condensate and even the minimum of the effective string potential, responsible for the very existence of the QCD strings. Finally, we run dynamical simulations of the multi-string systems, in the spaghetti setting approximating central $pA$ collisions, and specify conditions for their collectivization into a black hole, or the dual QGP fireball.
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We study the early stages of central pA and peripheral AA collisions. Several observables indicate that at a sufficiently large number of participant nucleons the system undergoes a transition into a new explosive regime. By defining a string-string interaction through the sigma meson exchange and performing molecular dynamics simulation, we argue that one should expect a strong collective implosion of the multi-string spaghetti state, creating significant compression of the system in the transverse plane. Another consequence is the collectivization of the sigma clouds of all strings into a chirally symmetric fireball. We find that these effects happen provided the number of strings $N_s > 30$ or so, as only such a number can compensate a small sigma-string coupling. These findings should help us to understand the subsequent explosive behavior, observed for the particle multiplicities roughly corresponding to this number of strings.
Holographic AdS/QCD models of the Pomeron unite a string-based description of hadronic reactions of the pre-QCD era with the perturbative BFKL approach. The specific version we will use due to Stoffers and Zahed, is based on a semiclassical quantization of a tube (closed string exchange or open string virtual pair production) in its Euclidean formulation using the scalar Polyakov action. This model has a number of phenomenologically successful results. The periodicity of a coordinate around the tube allows the introduction of a Matsubara time and therefore an effective temperature Teff on the string. We observe that at the LHC energies and for sufficiently small impact parameter, Teff approaches and even exceeds the Hagedorn temperature of the QCD strings. Based on studies of the stringy thermodynamics of pure gauge theories we suggest that there should exist two new regimes of the Pomeron: the near-critical and the post-critical ones. In the former one, string excitations create a high entropy string ball, with high energy and entropy but small pressure/free energy. If heavy enough this ball becomes a (dual) black hole (BH). As the intrinsic temperature of the string exceeds the Hagedorn temperature, the ball becomes a post-critical explosive QGP ball. The hydrodynamical explosion resulting from this scenario was predicted by us to have radial flow exceeding that ever seen even in heavy ion collisions, which was recently confirmed by CMS and ALICE at LHC. We also discuss the elastic scattering profile, finding some hints for new phases in it, as well as two-particle correlations.
Quarkonium production mechanism in high multiplicity small collision systems has recently been pursued in the color-glass-condensate (CGC) effective theory combined with non-relativistic QCD (NRQCD) factorization, allowing to study initial state interactions. Quarkonium polarization, potentially measured in future experiments, would help elucidate the quarkonium production mechanism at high multiplicities. In this paper, we provide predictions on $J/psi$ polarization parameters in high multiplicity proton-proton ($pp$) and proton-nucleus ($pA$) collisions within the CGC+NRQCD framework. Theoretical predictions are given for $J/psi$ rapidity $2.5< y_{J/psi}<4$, charged-particle multiplicity pseudorapidity $|eta_{ch} | <1$ and energies $sqrt{S}=13mathrm{~TeV}$ for $pp$, $sqrt{S}=8.16mathrm{~TeV}$ for $pA$ collisions. Considering two leptonic frame choices (Collins - Soper and helicity) we find a weak polarization of $J/psi$ that additionally decreases with growing event activities. No significant system size dependence between $pp$ and $pA$ collisions is obtained - this could be a new constraint to initial state interactions in small collision systems.
We study the physics with finite nuclear density in the framework of AdS/QCD with holographic baryon field included. Based on a mean field type approach, we introduce the nucleon density as a bi-fermion condensate of the lowest mode of the baryon field and calculate the density dependence of the chiral condensate and the nucleon mass. We observe that the chiral condensate as well as the mass of nucleon decrease with increasing nuclear density. We also consider the mass splitting of charged vector mesons in iso-spin asymmetric nuclear matter.
Gluons at small x in high-energy nuclei overlap in the longitudinal direction, so the nucleus acts as a single source of gluons, like higher Fock components in a single nucleon, which contribute to inelastic collisions with a high multiplicity of produced hadrons. This similarity helps to make a link between nuclear effects in pA and high-multiplicity pp collisions. Such a relation is well confirmed by data for the J/Psi production rate in high-multiplicity pp events measured recently in the ALICE experiment. Broadening of J/Psi transverse momentum is predicted for high-multiplicity pp collisions.
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