First experimental results are presented on event-by-event net-proton fluctuation measurements in Pb–Pb collisions at sNN=2.76 TeV, recorded by the ALICE detector at the CERN LHC. The ALICE detector ...is well suited for such studies due to its excellent particle identification capabilities and large acceptance, which is crucial for fluctuation analysis. The studies are focussed on second order cumulants, but the analysis technique used is more general and will be applied, in the near future, also to higher order cumulants.
Ultra-peripheral Pb-Pb collisions, in which the two nuclei pass close to each other, but at an impact parameter greater than the sum of their radii, provide information about the initial state of ...nuclei. In particular, heavy vector meson production, where the particle mass sets a hard scale, proceeds in such collisions by photon-gluon interactions, and gives access to nuclear PDFs. The ALICE collaboration has published measurements of J/ψ and ψ(2S) photoproduction in ultra-peripheral collisions in LHC Run 1 at forward (J/ψ) and mid-rapidity, and has obtained a substantially larger data set in 2015 from LHC Run 2, allowing much more detailed studies of the production mechanism to be performed. In particular, the increased energy and more detailed measurements in the forward region in Run 2 give access to significantly lower values of Bjorken-x than in previous studies. In this talk, the latest available results from Run 2 will be given.
Two- and multi-particle azimuthal correlations have proven to be an excellent tool to probe the properties of the strongly interacting matter created in heavy-ion collisions. Recently, the results ...obtained for multi-particle cumulants have been interpreted as evidence for collectivity in the small collision systems (pp and p–Pb) providing new insights into the systems' fluctuating initial conditions. In this article, first ALICE results on two- and multi-particle cumulants as a function of charged hadron multiplicity produced at midrapidity (|η| < 1.0) in pp collisions at s=13 TeV are reported. The results are compared with measurements in p–Pb collisions at sNN=5.02 TeV and in Pb–Pb collisions at sNN=2.76 TeV. A new method for the 4-particle cumulant with an η gap denoted as cn{4,|Δη|} will be presented, which is capable of further suppressing non-flow effects in multi-particle cumulants. The results allow for further understanding of the origin of multi-particle correlations in pp collisions.
The availability at the LHC of the largest collision energy in pp collisions allows a significant advance in the measurement of J/ψ production as function of event multiplicity. The interesting ...relative increase observed with data at the LHC at s=7 TeV and at RHIC at s=0.2 TeV is studied now at unprecedented multiplicities at s=13 TeV. The measurement at mid-rapidity in the dielectron channel with ALICE is presented and discussed in comparison to predictions from available theoretical models and to data at lower energies.
The aim of this work is to explore possible medium modifications to the substructure of inclusive charged jets in Pb-Pb relative to proton-proton collisions by measuring a set of jet shapes. The set ...of shapes includes the radial moment, g, and the momentum dispersion pTD. They provide complementary information on the fragmentation and can help to discriminate between two different scenarios: intra-jet broadening or collimation as a result of jet quenching. The shapes are measured in Pb–Pb collisions at sNN=2.76 TeV with a constituent cutoff of 0.15 GeV/c and jet resolution R=0.2. New techniques for background subtraction are applied and a two-dimensional unfolding is performed to correct the shapes to particle level. The corrected jet shapes for jet pT 40≤pT,jet≤60 GeV/c are presented and discussed. The observed jet shape modifications suggest that the in-medium fragmentation is harder and more collimated than vacuum fragmentation as obtained by a PYTHIA calculation. The PYTHIA calculation is validated with proton-proton data at 7 TeV.
We present the ALICE results on the nuclear modification factor and elliptic flow of electrons and muons from open heavy-flavour hadron decays at mid-rapidity and forward rapidity in Pb–Pb collisions ...at sNN=2.76 and 5.02 TeV for different centrality intervals. The results are compared to model calculations that include interactions of heavy quarks with the medium.
The study of identified particle production as a function of transverse momentum (pT) and event multiplicity in proton-proton (pp) collisions at different center-of-mass energies (s) is a key tool ...for understanding similarities and differences between small and large collisions systems. We report on the production of π±, K±, KS0, p(p‾), Λ(Λ‾), Ξ± and Ω± measured in pp collisions in a wide range of center-of-mass energies with ALICE 1. The multiplicity dependence of identified particle yields is presented for s=7 and 13 TeV and discussed in the context of the results obtained in protonlead (p-Pb) and lead-lead (Pb-Pb) collisions, unveiling remarkable and intriguing similarities. The production rates of strange hadrons are observed to increase more than those of non-strange particles, showing an enhancement pattern with multiplicity which does not depend on the collision energy. Even if the multiplicity dependence of spectral shapes can be qualitatively described by commonly-used Monte Carlo (MC) event generators, the evolution of integrated yield ratios is poorly described by these models.
ALICE measured the nuclear modification factor (RAA) and elliptic flow (v2) of D mesons (D0, D+, D⁎+ and Ds+) in semi-central Pb–Pb collisions at sNN=5.02TeV. The increased amount of data in ...semi-central Pb–Pb events obtained with the LHC Run 2 provides access to more precise measurements for the RAA and the non-strange D-meson v2 with respect to Run 1 results, as well as to the first measurement of the Ds-meson v2 at LHC energies. The first application of the Event Shape Engineering technique in the analysis of the v2 coefficient of D0 and D+ mesons is also illustrated.
We present the charged-particle pseudorapidity density in pp, p–Pb, and Pb–Pb collisions at sNN=5.02 TeV over a broad pseudorapidity range. The distributions are determined using the same ...experimental apparatus and methodologies, thereby minimizing systematic uncertainties, and providing clear and model-independent observations on the system-size dependence of the particle production at relativistic energies. An increase of particle production in Pb–Pb collisions near mid-rapidity, relative to pp collisions, is observed. The relative linearity in p–Pb collisions of the ratio to the smaller system indicate a coherent particle production throughout the longitudinal extend of the collision zone.
ALICE at the LHC provides unique capabilities to study charmonium production at low transverse momenta (pT). At central rapidity, (|y|<0.8), ALICE can reconstruct J/ψ via their decay into two ...electrons down to zero pT. Results at mid-rapidity on the inclusive J/ψ nuclear modification factor RAA as a function of centrality in Pb-Pb collisions at sNN=5.02 TeV and comparisons to lower energies and models are shown. In addition, the yield of the J/ψ produced with very low pT in the most peripheral collisions and the J/ψ elliptic flow (v2) are also presented.
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