Minimum-Bias and Early QCD Physics in ALICE

A Large Ion Collider Experiment (ALICE) is the dedicated heavy-ion experiment at the Large Hadron Collider (LHC). In addition to its heavy-ion physics program, it also has a rich proton-proton physics program benefiting from a detector with a low momentum cut-off (pT about 50 MeV/c) and a small material budget (about 11% of a radiation length until the outer wall of the main tracking detector, the Time-Projection Chamber). ALICE has excellent means of particle identification (PID) with methods ranging from specific energy loss and time of flight to transition and Cherenkov radiation. The good primary and secondary vertex resolution allows for measurements of strangeness and heavy flavor with low backgrounds. ALICE has taken proton-proton collision data at 0.9, 2.36, and 7 TeV. In this article results of the first minimum-bias and soft-QCD measurements are presented. Inclusive pseudorapidity, multiplicity, and transverse momentum distributions are discussed as well as distributions of identified particles including strange particles. Further, results on two-pion Bose-Einstein correlations and the antiproton-to-proton ratio in collisions at the LHC are shown.Comment: Proceedings of the Hadron Collider Physics Symposium 201

Multi-strange baryon production in Pb-Pb and pp collisions at sNN\sqrt{s_{NN}} = 2.76 TeV with the ALICE experiment at the LHC

The production of $\Xi^{-}$ and $\Omega^{-}$ baryons and their anti-particles in Pb-Pb and pp collisions at $\sqrt{s_{NN}}$ = 2.76 TeV has been measured by the ALICE collaboration. The transverse momentum spectra at mid-rapidity (|y| < 0.5) in pp and Pb-Pb collisions for five centrality intervals have been compared with model predictions. Hyperon yields and spectra in Pb-Pb collisions, normalized to the corresponding measurements in pp at the same centre-of-mass energy, allow the study of the strangeness enhancement and the nuclear modification factor as a function of the transverse momentum ($p_{T}$) and collision centrality.Comment: 4 pages, 3 figures. Proceedings of the Strangeness in Quark Matter Conference (SQM 2013), 22nd - 27th July 2013, published by the Open Access Journal of Physics: Conference Series (JPCS), in the IOP conference serie

KS0{\rm K}_{\rm S}^{0} and Λ\Lambda Production in Charged Particle Jets in p--Pb Collisions at sNN=5.02\sqrt{s_{\rm NN}}=5.02 TeV with ALICE

We study the production of ${\rm K}_{\rm S}^{0}$ mesons and $\Lambda$ baryons in jets in p--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV with ALICE at the LHC. The $p_{\rm T}$-differential density of the particles produced in jets is compared to the inclusive distributions and the $\Lambda/{\rm K}_{\rm S}^{0}$ ratio is reported in bins of multiplicity of the collisions. The hard scatterings are selected on an event-by-event basis using the anti-$k_{\rm T}$ clustering algorithm with resolution parameter $R=0.2,~0.3$ and $0.4$, reconstructed from charged particles with a minimum $p_{\rm T,jet}$ of $10$ (or $20$) GeV/$c$.Comment: 4 pages, 2 figures, Quark Matter 2014 proceeding, submitted to Nucl. Phys.

Measurement of identified charged hadron spectra with the ALICE experiment at the LHC

The ALICE experiment features multiple particle identification systems. The measurement of the identified charged hadron $p_{t}$ spectra in proton-proton collisions at $\sqrt{s}=900$ GeV will be discussed. In the central rapidity region ($|\eta|<0.9$) particle identification and tracking are performed using the Inner Tracking System (ITS), which is the closest detector to the beam axis, the Time Projection Chamber (TPC) and a dedicated time-of-flight system (TOF). Particles are mainly identified using the energy loss signal in the ITS and TPC. In addition, the information from TOF is used to identify hadrons at higher momenta. Finally, the kink topology of the weak decay of charged kaons provides an alternative method to extract the transverse momentum spectra of charged kaons. This combination allows to track and identify charged hadrons in the transverse momentum ($p_{t}$) range from 100 MeV/c up to 2.5 GeV/$c$. Mesons containing strange quarks (\kos, $\phi$) and both singly and doubly strange baryons (\lam, \lambar, and \xip + \xim) are identified by their decay topology inside the TPC detector. Results obtained with the various identification tools above described and a comparison with theoretical models and previously published data will be presented.Comment: 11 pages, 14 figures, contribution to conference proceedings of the 27th Winter Workshop on Nuclear Dynamic

Transverse momentum spectra of hadrons identified with the ALICE Inner Tracking System

The Inner Tracking System is the ALICE detector closest to the beam axis. It is composed of six layers of silicon detectors: two innermost layers of Silicon Pixel Detectors (SPD), two intermediate layers of Silicon Drift Detectors (SDD) and two outermost layers of Silicon Strip Detectors (SSD). The ITS can be used as a standalone tracker in order to recover tracks that are not reconstructed by the Time Projection Chamber (TPC) and to reconstruct low momentum particles with $p_{t}$ down to 100 MeV/c. Particle identification in the ITS is performed by measuring the energy loss signal in the SDD and SSD layers. The ITS allows to extend the charged particle identification capability in the ALICE central rapidity region at low $p_{t}$: it is possible to separate $\pi/K$ in the range 100 MeV/c $< p_{t} <$ 500 MeV/c and $K/p$ in the range 200 MeV/c $< p_{t} <$ 800 MeV/c. The identification of hadron in the ITS will be discussed in detail, different methods used to extract the $p_{t}$ spectra of $\pi, K$ and $p$ will also be described.Comment: 2 pages, 2 figures, submitted as contribution to PLHC2011 conference proceeding

Transverse momentum distribution of charged particles and identified hadrons in p-Pb collisions at the LHC with ALICE

Hadron production has been measured at mid-rapidity by the ALICE experiment at the LHC in proton-lead (p-Pb) collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The transverse momentum ($p_{\rm T}$) distribution of primary charged particles and of identified light-flavoured hadrons ($\pi^{\pm}$, K$^{\pm}$, K$^{0}_{\rm S}$, p, $\bar{\rm p}$, $\Lambda$, $\bar{\Lambda}$) are presented in this report. Charged-particle tracks are reconstructed in the central barrel over a wide momentum range. Furthermore they can be identified by exploiting specific energy loss (d$E$/d$x$), time-of-flight and topological particle-identification techniques. Particle-production yields, spectral shapes and particle ratios are measured in several multiplicity classes and are compared with results obtained in Pb-Pb collisions at the LHC. The measurement of charged-particle transverse momentum spectra and nuclear modification factor R$_{\rm pPb}$ indicates that the strong suppression of high-$p_{\rm T}$ hadrons observed in Pb-Pb collisions is not due to initial-state effects, but it is rather a fingerprint of jet quenching in hot QCD matter. The systematic study of the hadronic spectral shapes as a function of the particle mass and of particle ratios as a function of charged-particle density provides insights into collective phenomena, as observed in Pb-Pb collisions. Similar features, that could be present in high-multiplicity p-Pb collisions, will also be discussed.Comment: 7 pages, 5 figures, presented at the The European Physical Society Conference on High Energy Physics - EPS-HEP2013, 18-24 July 2013, Stockholm, Swede

Energy dependence of the saturation scale and the charged multiplicity in pp and AA collisions

A natural framework to understand the energy dependence of bulk observables from lower energy experiments to the LHC is provided by the Color Glass Condensate, which leads to a "geometrical scaling" in terms of an energy dependent saturation scale Q_s. The measured charged multiplicity, however, seems to grow faster (~\sqrt{s}^0.3) in nucleus-nucleus collisions than it does for protons (~\sqrt{s}^0.2), violating the expectation from geometric scaling. We argue that this difference between pp and AA collisions can be understood from the effect of DGLAP evolution on the value of the saturation scale, and is consistent with gluon saturation observations at HERA.Comment: RevTeX, 8 pages, 4 figures. V2: modified discussion of fragmentation, published in EPJ