Exploring the initial stage of high multiplicity proton-proton collisions by determining the initial temperature of the quark-gluon plasma

We have analyzed identified particle transverse momentum spectra in high multiplicity events in p p collisions at LHC energies √ s = 0.9 – 13     TeV published by the CMS Collaboration using the color string percolation model (CSPM). In CSPM color strings are formed after the collision, which decay into new strings through color neutral q − ¯ q pairs production. With the increase in the p p collisions energy number of strings grow and randomly statistically overlap producing higher string tension of the composite strings. The net color in the overlap string area is a vector sum of the randomly oriented strings. The Schwinger color string breaking mechanism produces these color neutral q − ¯ q pairs at time ∼ 1     fm /c, which subsequently hadronize. The initial temperature is extracted both in low and high multiplicity events.The shear viscosity to entropy density ratios η / s are obtained as a function of temperature. For the higher multiplicity events at √ s = 7 and 13 TeV the initial temperature is above the universal hadronization temperature and is consistent with the creation of deconfined matter. The η / s is similar to that in Au + Au collisions at √ s N N = 200     GeV . The small value of η / s above the universal hadronization temperature suggested that the matter is a strongly coupled quark gluon plasma. In these small systems it can be argued that the thermalization is a consequence of the quantum tunneling through the event horizon introduced by the quarks confined in the colliding nucleons and their deceleration due to string formation, in analogy to the Hawking-Unruh radiation which provides a stochastic approach to equilibrium. The disk areas cluster on the nucleon transverse collision area. At the 2 D percolation threshold a macroscopic spanning cluster suddenly occurs at the temperature T i = T h , representing a small connected droplet of q − ¯ q pairs, the quark-gluon plasma (QGP). T h is the universal hadronization temperature ∼ 167.7     MeV . The collision energy dependent buildup of the 2D percolation clusters defines the temperature range 159 ± 9     MeV of the crossover transition between hadrons to the QGP in reasonable agreement with the lattice quantum chromodynamics (LQCD) pseudocritical temperature value of 155 ± 9     MeV . Color string percolation model is the new initial stage paradigm for the study of the high density matter produced in p p and A + A collisions. With CSPM we can directly explore the thermodynamics of the QGP above the universal hadronization temperature.We express our thanks to N. Armesto for fruitful comments. C. P. thanks the grant Maria de Maeztu Unit of excellence MDM-2016-0682 of Spain, the support of Xunta de Galicia under the Projects No. ED431C 2017 and No. FPA 2017-83814 of Ministerio de Ciencia e Innovacion of Spain and FEDERS

Thermal behavior and entanglement in Pb-Pb and p-p collisions

The thermalization of the particles produced in collisions of small objects can be achieved by quantum entanglement of the partons of the initial state as was analyzed recently in proton-proton collisions. We extend such study to Pb-Pb collisions and to different multiplicities of proton-proton collisions. We observe that, in all cases, the effective temperature is approximately proportional to the hard scale of the collision. We show that such a relation between the thermalization temperature and the hard scale can be explained as a consequence of the clustering of the color sources. The fluctuations of the number of parton states decrease with multiplicity in Pb-Pb collisions as long as the width of the transverse-momentum distribution decreases, contrary to the p−p case. We relate these fluctuations to the temperature fluctuations by means of a Langevin equation for the white stochastic noise. We show that the multiplicity parton distribution for events with at least one hard parton collision is a Γ distribution. We use this result to compute the entanglement entropy, showing that the leading term is the logarithm of the number of partons, meaning that the n microstates are equally probable and the entropy is maximal. There is another contribution related to the inverse of the normalized parton number fluctuation, which at very high energy changes the behavior from ln n to ln √nWe are grateful for a grant from the María de Maeztu Unit of Excellence of Spain and the support of Xunta de Galicia under Project No. ED431C2017. This work has been partially carried out under Project No. FPA2017-83814-P of Ministerio de Ciencia, Innovación y Universidades (Spain)S

Production of charged pions, kaons and protons at large transverse momenta in pp and Pb–Pb collisions at √sNN=2.76TeV

Transverse momentum spectra of π±, K±and p(¯ p)up to pT=20GeV/cat mid-rapidity in pp, peripheral (60–80%) and central (0–5%) Pb–Pb collisions at √sNN=2.76TeVhave been measured using the ALICE detector at the Large Hadron Collider. The proton-to-pion and the kaon-to-pion ratios both show adistinct peak at pT≈3GeV/cin central Pb–Pb collisions. Below the peak, pT10GeV/cparticle ratios in ppand Pb–Pb collisions are in agreement and the nuclear modification factors for π±, K±and p(¯ p)indicate that, within the systematic and statistical uncertainties, the suppression is the same. This suggests that the chemical composition of leading particles from jets in the medium is similar to that of vacuum jets.S

Neutral pion production at midrapidity in pp and Pb–Pb collisions at √sNN = 2.76 TeV

Invariant yields of neutral pions at midrapidity in the transverse momentum range 0.6<pT<12GeV/c measured in Pb–Pb collisions at √sNN=2.76 TeV are presented for six centrality classes. The pp reference spectrum was measured in the range 0.4<pT<10GeV/c at the same center-of-mass energy. The nuclear modification factor, RAA, shows a suppression of neutral pions in central Pb–Pb collisions by a factor of up to about 8−10 for 5≲pT≲7GeV/c. The presented measurements are compared with results at lower center-of-mass energies and with theoretical calculations.The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science,World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ and CEA, France; German BMBF and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian OTKA and National Office for Research and Technology (NKTH); Department of Atomic Energy andDepartment of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi - Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); CONACYT, DGAPA, México, ALFA-EC and the EPLANET Program (European Particle Physics Latin American Network) Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), The Netherlands; Research Council of Norway (NFR); Polish Ministry of Science and Higher Education; National Science Centre, Poland; Ministry of National Education/Institute for Atomic Physics and CNCS-UEFISCDI - Romania; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research;Ministry of Education of Slovakia; Department of Science and Technology, South Africa; CIEMAT, EELA,Ministerio de Economía y Competitividad (MINECO) of Spain, Xunta de Galicia (Consellería de Educación), CEADEN,Cubaenergía, Cuba, andIAEA(InternationalAtomicEnergy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio.S

Rapidity and transverse-momentum dependence of the inclusive J/ψ nuclear modification factor in p-Pb collisions at √sNN= 5.02 TeV

We have studied the transverse-momentum (p T) dependence of the inclusive J/ψ production in p-Pb collisions at √sNN= 5.02 TeV, in three center-of-mass rapidity (y cms) regions, down to zero p T. Results in the forward and backward rapidity ranges (2.03 < y cms < 3.53 and −4.46 < y cms < −2.96) are obtained by studying the J/ψ decay to μ + μ −, while the mid-rapidity region (−1.37 < y cms < 0.43) is investigated by measuring the e+e− decay channel. The p T dependence of the J/ψ production cross section and nuclear modification factor are presented for each of the rapidity intervals, as well as the J/ψ mean p T values. Forward and mid-rapidity results show a suppression of the J/ψ yield, with respect to pp collisions, which decreases with increasing p T. At backward rapidity no significant J/ψ suppression is observed. Theoretical models including a combination of cold nuclear matter effects such as shadowing and partonic energy loss, are in fair agreement with the data, except at forward rapidity and low transverse momentum. The implications of the p-Pb results for the evaluation of cold nuclear matter effects on J/ψ production in Pb-Pb collisions are also discussed.The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science, World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia, Conselho Nacional de Desenvolvimento Científico e Tecnol ógico (CNPq), Financiadora de Estudos e Projetos (FINEP), Fundacão de Amparo à Pesquisa do Estado de São Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ and CEA, France; German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian Orszagos Tudomanyos Kutatasi Alappgrammok (OTKA) and National Office for Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi — Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion General de Asuntos del Personal Academico (DGAPA), México; Amerique Latine Formation academique — European Commission (ALFA-EC) and the EPLANET Program (European Particle Physics Latin American Network) Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voorWetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); National Science Centre, Poland; Ministry of National Education/Institute for Atomic Physics and Consiliul National al Cercetarii Stiintifice — Executive Agency for Higher Education Research Development and Innovation Funding (CNCS-UEFISCDI) — Romania; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure shared between Europe and Latin America (EELA), Ministerio de Economía y Competitividad (MINECO) of Spain, Xunta de Galicia (Consellería de Educación), Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio; Ministry of Science, Education and Sports of Croatia and Unity through Knowledge Fund, Croatia. Council of Scientific and Industrial Research (CSIR), New Delhi, India.S

Beauty production in pp collisions at √s=2.76 TeV measured via semi-electronic decays

The ALICE Collaboration at the LHC reports measurement of the inclusive production cross section of electrons from semi-leptonic decays of beauty hadrons with rapidity |y| <0.8and transverse momentum 1 <pT<10GeV/c, in pp collisions at √s=2.76TeV. Electrons not originating from semi-electronic decay of beauty hadrons are suppressed using the impact parameter of the corresponding tracks. The production cross section of beauty decay electrons is compared to the result obtained with an alternative method which uses the distribution of the azimuthal angle between heavy-flavour decay electrons and charged hadrons. Perturbative QCD predictions agree with the measured cross section within the experimental and theoretical uncertainties. The integrated visible cross section, σb→e=3.47 ±0.40(stat)+1.12−1.33(sys) ±0.07(norm)μb, was extrapolated to full phase space using Fixed Order plus Next-to-Leading Log (FONLL) calculations to obtain the total b¯bproduction cross section, σb¯ b=130 ±15.1(stat)+42.1−49.8(sys)+3.4−3.1(extr) ±2.5(norm) ±4.4(BR)μb.S

Production of inclusive ϒ(1S) and ϒ(2S) in p–Pb collisions at √sNN=5.02 TeV

We report on the production of inclusive Υ(1S) and Υ(2S) in p–Pb collisions at √sNN=5.02TeVat the LHC. The measurement is performed with the ALICE detector at backward (−4.46 <ycms<−2.96) and forward (2.03 <ycms<3.53) rapidity down to zero transverse momentum. The production cross sections of the Υ(1S) and Υ(2S) are presented, as well as the nuclear modification factor and the ratio of the forward to backward yields of Υ(1S). A suppression of the inclusive Υ(1S) yield in p–Pb collisions with respect to the yield from pp collisions scaled by the number of binary nucleon–nucleon collisions is observed at forward rapidity but not at backward rapidity. The results are compared to theoretical model calculations including nuclear shadowing or partonic energy loss effectsS

Measurement of charm and beauty production at central rapidity versus charged-particle multiplicity in proton-proton collisions at √s=7 TeV

Prompt D meson and non-prompt J/ψ yields are studied as a function of the multiplicity of charged particles produced in inelastic proton-proton collisions at a centre-of-mass energy of √s=7 TeV. The results are reported as a ratio between yields in a given multiplicity interval normalised to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalised to the average value for inelastic collisions (relative charged-particle multiplicity). D0, D+ and D*+ mesons are measured in five p T intervals from 1 GeV/c to 20 GeV/c and for |y| 1.3 GeV/c and |y| 0. The fraction of non-prompt J/ψ in the inclusive J/ψ yields shows no dependence on the charged-particle multiplicity at central rapidity. Charm and beauty hadron relative yields exhibit a similar increase with increasing charged-particle multiplicity. The measurements are compared to PYTHIA 8, EPOS 3 and percolation calculations.S

Freeze-out radii extracted from three-pion cumulants in pp, p–Pb and Pb–Pb collisions at the LHC

In high-energy collisions, the spatio-temporal size of the particle production region can be measured using the Bose–Einstein correlations of identical bosons at low relative momentum. The source radii are typically extracted using two-pion correlations, and characterize the system at the last stage of interaction, called kinetic freeze-out. In low-multiplicity collisions, unlike in high-multiplicity collisions, two-pion correlations are substantially altered by background correlations, e.g. mini-jets. Such correlations can be suppressed using three-pion cumulant correlations. We present the first measurements of the size of the system at freeze-out extracted from three-pion cumulant correlations in pp, p–Pb and Pb–Pb collisions at the LHC with ALICE. At similar multiplicity, the invariant radii extracted in p–Pb collisions are found to be 5–15% larger than those in pp, while those in Pb–Pb are 35–55% larger than those in p–Pb. Our measurements disfavor models which incorporate substantially stronger collective expansion in p–Pb as compared to pp collisions at similar multiplicity.S