Mon·Jun 10, 2019Monday·June 10, 2019

1. 01*

   Multiplicity dependence of light (anti-)nuclei production in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

   ALICE Collaboration

   The measurement of the deuteron and anti-deuteron production in the rapidity range $-1<y<0$ as a function of transverse momentum and event multiplicity in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV is presented. (Anti-)deuterons are identified via their specific energy loss $\mathrm{d}E/\mathrm{d}\mathrm{x}$ and via their time-of-flight. Their production in p-Pb collisions is compared to pp and Pb-Pb collisions and is discussed within the context of thermal and coalescence models. The ratio of integrated yields of deuterons to protons (d/p) shows a significant increase as a function of the charged-particle multiplicity of the event starting from values similar to those observed in pp collisions at low multiplicities and approaching those observed in Pb-Pb collisions at high multiplicities. The mean transverse momenta are extracted from the deuteron spectra and the values are similar to those obtained for p and $\Lambda$ particles. Thus, deuteron spectra do not follow mass ordering. This behaviour is in contrast to the trend observed for non-composite particles in p-Pb collisions. In addition, the production of the rare ${}^3\mathrm{He}$ and ${}^3\overline{\mathrm{He}}$ nuclei has been studied. The spectrum corresponding to all non-single diffractive p-Pb collisions is obtained in the rapidity window $-1<y<0$ and the $p_{\mathrm{T}}$-integrated yield d$N$/d$y$ is extracted. It is found that the yields of protons, deuterons, and ${}^3\mathrm{He}$, normalised by the spin degeneracy factor, follow an exponential decrease with mass number.

   nucl-exhep-exPLB(2020)·83 citations
2. 02*

   $p_T$-Dependent Particle Number Fluctuations From Principal Component Analyses in Hydrodynamic Simulations of Heavy-Ion Collisions

   Fernando G. Gardim🇧🇷 · Frédérique Grassi🇧🇷 · Pedro Ishida🇧🇷 · Matthew Luzum🇧🇷 · Jean-Yves Ollitrault🇫🇷

   We carry out a principal component analysis of fluctuations in a hydrodynamic simulation of heavy-ion collisions, and compare with experimental data from the CMS collaboration. The leading and subleading principal components of elliptic and triangular flow reproduce the trends seen in data. By contrast, the principal components of multiplicity fluctuations show an interesting difference in their $p_T$ dependence for simulations compared to experimental data. Specifically, the leading component increases with $p_T$ in hydrodynamics, while it is constant in experiment. In order to understand how the leading and subleading modes arise, we construct a toy model where the principal components have a simple analytic form. We show how the PCA components depend on fluctuations of the average transverse momentum and of the total multiplicity, as well as correlations between the two, and we verify that hydrodynamic simulations agree with the predictions of the toy model. The difference in the momentum trend is likely due to the fact that hydrodynamic models typically have transverse momentum fluctuations that are larger than seen experimentally.

   ↳ nucl-thhep-phnucl-exPRC(2019)·30 citations
3. 03*

   Canonical statistical model analysis of p-p, p-Pb, and Pb-Pb collisions at the LHC

   Volodymyr Vovchenko🇩🇪 · Benjamin Dönigus🇩🇪 · Horst Stoecker🇩🇪

   The system-size dependence of hadrochemistry at vanishing baryon density is considered within the canonical statistical model (CSM) with local exact conservation of three conserved charges, allowing for a possibility of strangeness undersaturation, i.e. ${\gamma }_S\le 1$. Exact baryon number conservation is found to be even more important than that of strangeness in the canonical suppression picture at the LHC, in contrast to intermediate and low collision energies. The model is applied to p-p, p-Pb, and Pb-Pb data of the ALICE collaboration. A chemical equilibrium CSM with a fixed $T_{\mathrm{ch}}=155$ MeV describes the trends seen in most yield ratios. However, this vanilla version of CSM predicts an enhancement of the $\varphi /\pi$ ratio at smaller multiplicities, in stark contrast to the suppression seen in the data. The data are described with a 15% relative accuracy level whence a multiplicity dependence of both the temperature and the strangeness saturation parameter ${\gamma }_S\le 1$ is accepted. Both the canonical suppression and the strangeness undersaturation effects are small at $d{N}_{\mathrm{ch}}/d\eta \gtrsim 100$, but they do improve substantially the description of hadron yields in p-p collisions, in particular the $\Omega$ yields. A possibility to constrain the rapidity correlation volume using net-proton fluctuation measurements is pointed out.

   ↳ hep-phnucl-exnucl-thPRC(2019)·140 citations
4. 04*

   Pion-induced production of hidden-charm pentaquarks $P_c(4312),P_c(4440)$, and $P_c(4457)$

   Xiao-Yun Wang🇨🇳 · Jun He🇨🇳 · Xu-Rong Chen🇨🇳 · Quanjin Wang🇨🇳 · Xinmei Zhu🇨🇳

   The production of the hidden-charm pentaquarks $P_c$ via pion-induced reaction on a proton target is investigated within an effective Lagrangian approach. Three experimentally observed states, $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$, are considered in the calculation, and the Reggeized $t$-channel meson exchange is considered as main background for the reaction ${\pi }^{-}p\to J/\psi n$. The numerical results show that the experimental data of the total cross section of the reaction ${\pi }^{-}p\to J/\psi n$ at $W\simeq 5$ GeV can be well explained by contribution of the Reggeized $t$ channel with reasonable cutoff. If the branching ratios $Br[P_c\to J/\psi N]\simeq 3\%$ and $Br[P_c\to \pi N]\simeq 0.05\%$ are taken, the average value of the cross section from the $P_c(4312)$ contribution is about 1.2 nb/100 MeV, which is consistent with existing rude data at near-threshold energies. The results indicate that the branching ratios of the $P_c$ states to the $J/\psi N$ and $\pi N$ should be small. The shape of differential cross sections shows that the Reggeized $t$-channel provides a sharp increase at extreme forward angles, while the differential cross sections from the $P_c$ states contributions are relatively flat. High-precision experimental measurements on the reaction ${\pi }^{-}p\to J/\psi n$ at near-threshold energies are suggested to confirm the LHCb hidden-charm pentaquarks as genuine states, and such experiments are also helpful to understand the origin of these resonance structures.

   ↳ hep-phhep-exnucl-exPLB(2019)·39 citations
5. 05*

   A Baseline Study of the Event-shape and Multiplicity Dependence of Chemical Freeze-out Parameters in Proton-Proton Collisions at $\sqrt{s}$ = 13 TeV Using PYTHIA8

   Rutuparna Rath🇮🇳 · Arvind Khuntia🇵🇱 · Sushanta Tripathy🇮🇳 · Raghunath Sahoo🇮🇳

   The event-shape and multiplicity dependence of the chemical freeze-out temperature ($T_{\text{ch}}$), freeze-out radius ($R$), and strangeness saturation factor (${\gamma }_s$) are obtained by studying the particle yields from the PYTHIA8 Monte Carlo event generator in proton-proton (pp) collisions at the centre-of-mass $\sqrt{s}$ = 13 TeV. Spherocity is one of the transverse event-shape techniques to distinguish jetty and isotropic events in high-energy collisions and helps in looking into various observables in a more differential manner. In this study, spherocity~classes are divided into three categories, namely (i) spherocity integrated, (ii) isotropic, and~(iii) jetty. The~chemical freeze-out parameters are extracted using a statistical thermal model as a function of the spherocity class and charged particle multiplicity in the canonical, strangeness canonical, and grand canonical ensembles. A clear observation of the multiplicity and spherocity class dependence of $T_{\text{ch}}$, $R$, and ${\gamma }_s$ is observed. A final state multiplicity, $N_{\mathrm{ch}}\ge$ 30 in the forward multiplicity acceptance of the ALICE detector appears to be a thermodynamic limit, where the freeze-out parameters become almost independent of the ensembles. This~study plays an important role in understanding the particle production mechanism in high-multiplicity pp collisions at the Large Hadron Collider (LHC) energies in view of a finite hadronic phase lifetime in small systems.

   ↳ hep-phhep-exnucl-exnucl-thPhysics(2020)·7 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.