Recent results from PHENIX on the evolution of hot QCD

Volume: 70 Host publication title: 1st International Conference on New Frontiers in PhysicsPeer reviewe

Charged-particle multiplicity fluctuations in Pb-Pb collisions at root s(NN)=2.76 TeV

Measurements of event-by-event fluctuations of charged-particle multiplicities in Pb-Pb collisions at root s(NN) = 2.76 TeV using the ALICE detector at the CERN Large Hadron Collider (LHC) are presented in the pseudorapidity range |eta| < 0.8 and transverse momentum 0.2 < p(T) < 2.0 GeV/c. The amplitude of the fluctuations is expressed in terms of the variance normalized by the mean of themultiplicity distribution. The. and p(T) dependences of the fluctuations and their evolution with respect to collision centrality are investigated. The multiplicity fluctuations tend to decrease from peripheral to central collisions. The results are compared to those obtained from HIJING and AMPT Monte Carlo event generators as well as to experimental data at lower collision energies. Additionally, the measured multiplicity fluctuations are discussed in the context of the isothermal compressibility of the high-density strongly-interacting system formed in central Pb-Pb collisions.Peer reviewe

Measurement of the production cross section of prompt Xi(0)(c) baryons at midrapidity in pp collisions at root s=5.02 TeV

The transverse momentum (p(T)) differential cross section of the charm-strange baryon Xi(0)(c) is measured at midrapidity (vertical bar y vertical bar < 0.5) via its semileptonic decay into e(+)Xi(-)nu(e) in pp collisions at root s = 5.02TeV with the ALICE detector at the LHC. The ratio of the p(T)-differential Xi(0)(c)-baryon and D-0-meson production cross sections is also reported. The measurements are compared with simulations with different tunes of the PYTHIA 8 event generator, with predictions from a statistical hadronisation model (SHM) with a largely augmented set of charm-baryon states beyond the current lists of the Particle Data Group, and with models including hadronisation via quark coalescence. The pT-integrated cross section of prompt Xi(0)(c)-baryon production at midrapidity is also reported, which is used to calculate the baryon-to-meson ratio Xi(0)(c)/D-0 = 0.20 +/- 0.04 (stat.)(-0.07)(+0.08) (syst.). These results provide an additional indication of a modification of the charm fragmentation from e(+) e(-) and e-p collisions to pp collisions.Peer reviewe

Direct observation of the dead-cone effect in quantum chromodynamics

A Publisher Correction to this article was published on 07 July 2022: https://doi.org/10.1038/s41586-022-05026-zIn particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD)(1). These partons subsequently emit further partons in a process that can be described as a parton shower(2), which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m(Q) and energy E, within a cone of angular size m(Q)/E around the emitter(3). Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques(4,5) to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.Peer reviewe

Measurement of K*(892)(+/-) production in inelastic pp collisions at the LHC

The first results on K*(892)(+/-) resonance production in inelastic pp collisions at LHC energies of root s = 5.02, 8, and 13 TeV are presented. The K*(892)(+/-) (;)has been reconstructed via its hadronic decay channel K*(892)(+/-) -> K-S(0) + pi(+/-) with the ALICE detector. Measurements of transverse momentum distributions, p(T)-integrated yields, and mean transverse momenta for charged K*(892) are found to be consistent with previous ALICE measurements for neutral K*(892) within uncertainties. For p(T) > 1 GeV/c the K*(892)(+/-) transverse momentum spectra become harder with increasing centre-of-mass energy from 5.02 to 13 TeV, similar to what previously observed for charged kaons and pions. For p(T) < 1 GeV/c the K*(892)(+/-) yield does not evolve significantly and the abundance of K*(892)(+/-) relative to K is rather independent of the collision energy. The transverse momentum spectra, measured for K*(892)(+/-) at midrapidity in the interval 0 < p(T) < 15 GeV/c, are not well described by predictions of different versions of PYTHIA 6, PYTHIA 8 and EPOS-LHC event generators. These generators reproduce the measured p(T)-integrated K*(+/-)/K ratios and describe well the momentum dependence for p(T) < 2 GeV/c. (c) 2022 European Organization for Nuclear Research. Published by Elsevier B.V.Peer reviewe

Multiharmonic Correlations of Different Flow Amplitudes in Pb-Pb Collisions at root s(NN)=2.76 TeV

The event-by-event correlations between three flow amplitudes are measured for the first time in Pb-Pb collisions, using higher-order symmetric cumulants. We find that different three-harmonic correlations develop during the collective evolution of the medium when compared to correlations that exist in the initial state. These new results cannot be interpreted in terms of previous lower-order flow measurements since contributions from two-harmonic correlations are explicitly removed in the new observables. A comparison to Monte Carlo simulations provides new and independent constraints for the initial conditions and system properties of nuclear matter created in heavy-ion collisions.Peer reviewe

Measurement of beauty and charm production in pp collisions at s√ = 5.02 TeV via non-prompt and prompt D mesons

The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper the production of (anti-)deuterons is studied as a function of the charged particle multiplicity in inelastic pp collisions at root s = 13 TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity (dN(ch)/d eta similar to 26) as measured in p-Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p-Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and statistical hadronisation models (SHM).Peer reviewe

Charm-quark fragmentation fractions and production cross section at midrapidity in pp collisions at the LHC

Peer reviewe

First measurements of N-subjettiness in central Pb-Pb collisions at root s(NN)=2.76 TeV

The study of (anti-)deuteron production in pp collisions has proven to be a powerful tool to investigate the formation mechanism of loosely bound states in high-energy hadronic collisions. In this paper the production of (anti-)deuterons is studied as a function of the charged particle multiplicity in inelastic pp collisions at root s = 13 TeV using the ALICE experiment. Thanks to the large number of accumulated minimum bias events, it has been possible to measure (anti-)deuteron production in pp collisions up to the same charged particle multiplicity (dN(ch)/d eta similar to 26) as measured in p-Pb collisions at similar centre-of-mass energies. Within the uncertainties, the deuteron yield in pp collisions resembles the one in p-Pb interactions, suggesting a common formation mechanism behind the production of light nuclei in hadronic interactions. In this context the measurements are compared with the expectations of coalescence and statistical hadronisation models (SHM).Peer reviewe