First identified particle studies at the CERN-LHC with the ALICE experiment Panos Christakoglou a,b for the ALICE Collaboration XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 1 a NIKHEF b Utrecht University

Outline  The ALICE experiment at LHC o Experimental setup o PID design performance  Commissioning of the PID detectors o Results from the ITS o Results from the TPC o Results from the TOF  First physics measurements o Identified particle spectra o Baryon number transport  Conclusion XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 2

Experimental setup XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 3

PID in ALICE  PID for hadrons in the central barrel (|η| < 0.9) o ITS: Excellent (3σ) separation from ~50 MeV/c for π o TPC: π from 200 MeV/c, Κ from 300 MeV/c o TOF: K up to 2.5 GeV/c, p up to 3.5 GeV/c o HMPID: intermediate to high momenta  Photons with the dedicated spectrometer (PHOS)  Electrons in the central barrel: o ITS + TPC for low momentum o TRD from P ~ 1GeV/c (beauty measurements feasible  Muons reconstructed in the forward region (muon arm: -4 < η < -2.4) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 4 PID – momentum reach

ITS commissioning XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 5  Design goals o Optimal resolution for primary vertex and track impact parameter  Minimize distance of innermost layer from beam axis ( ≈ 3.9 cm) and material budget o Maximum occupancy (central PbPb) < few % o dE/dx information in the 4 outermost layers for particle ID in 1/β 2 region  Installed in  Successfully commissioned during the cosmics runs in 2008 and LayerDet. Type Radius (cm) Length (cm) Resolution (  m)PbPb dN/dy=6000 rr ZPart./cm 2 Occupancy (%) 1SPD SPD SDD SDD SSD SSD

ITS alignment XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 6  First order alignment with cosmics using the SPD trigger o Coincidence between top outer SPD layer and bottom outer SPD layer (rate: 0.18 Hz)  Preliminary results show: o Space point resolution for the SPD of ~14 μm in rφ (matching between upper and lower part of a track). o Residual misalignment spread of the SSD modules on the ladders of ~5μm  Outlook: o Inclusion of the SDD o Alignment with magnetic field with beams (p t dependence). ALICE collaboration 2010 JINST 5 P03003

ITS calibration and PID (SDD + SSD) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 7  Charge distribution measured on all the SDD modules o ~95% of the detector included in the readout during data taking  Charge distribution measured on all the SSD modules o 90% of the detector active in 2009 o <1.5% bad strips o S/N ~40 dE/dx resolution ~ 13%

 Main TPC characteristics o Cylindrical in shape, with an active volume that has an inner radius of about 85 cm, an outer radius of about 250 cm, and an overall length along the beam direction of 500 cm (largest TPC ever built) o Cylindrical field cage filled with ~90 m 3 of Ne/CO 2 /N 2 gas mixture (90:10:5) o Drift time of ~90 μs o Many (>90) 3D points (+dE/dx) per track  Since August 2009 more than 750 million events (cosmics, krypton, and laser) recorded, with and without B- field. o First round of calibrations (dE/dx, momentum, alignment, gain) completed before collisions. TPC commissioning XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 8

XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 9 J. Alme et al. (ALICE TPC), arXiv: (subm. to NIM) TPC particle identification dE/dx resolution ~ 5% September 2009 December 2009

TOF XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 10  18 sectors, 2π covering in |η| < 0.9  Granularity: 2.5x3.5 cm2 at ∼ 3.7 m from the primary vertex;  Resolution at present ∼ 130 ps (calibration only from cosmic rays)

Analysis details XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 11 Event Selection Online TriggerYes Offline TriggerYes |V z |<1 0 |V x |<1 |V y |<1 Track Selection – Primary like cuts dca cm Track Selection – Quality cuts Number of TPC clusters80 Number of TPC clusters (dE/dx)80 x 2 /N TPCclusters 3.5 Number of ITS clusters2 Hit on SPD1 || SPD2Yes Analysis of the p+p events collected during the first LHC run in December Accepted events: 250K

Hadron spectra – π, Κ, p XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 12  Subtract measured signal (dE/dx or tof) from the expected one for each p t bin  Yields extracted from a fit with a convolution of three Gaussians  Compare results with a simple 3σ cut ITS TPC TOF

Hadron spectra - π, Κ, p (cont) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 13  ITS points to be added: new reconstruction pass based on the latest calibration constants (SDD)  Excellent agreement between TPC and TOF in the overlap regions: max. difference of 6%

K ± from Kinks XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 14 π- kinks M. Spyropoulou-Stassinaki, P. Ganoti, A. Belogianni, F. Roukoutakis  Preliminary results in good agreement with the K ± spectra obtained from the dE/dx and TOF PID

Baryon number transport – Motivation XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 15  Who is the carrier of the baryon number? o Valence quarks: G.C. Rossi and G. Veneziano, Nucl. Phys B123 (1977) 507. o Gluonic field: B.Z. Kopeliovich and B. Zakharov, Z. Phys. C43 (1989) 241.  Different approaches based on theory: o QGSM considers the baryon as a bound quark-diquark state. BN transport implies breaking the diquark pair. o Gluonic mechanism aka String Junction (SJ). BN transport implies the stopping of the SJ. u u d u u d Conventional approach - QGSM Within QGSM one expects an asymmetry ~0 at LHC energies  No BN transported at mid-rapidity from the fragmentation region du u SJ String Junction BN transport even at large rapidity gaps (large energies).  Veneziano: Probability exponentially suppressed (a J : SJ intercept – model dependent)  Kopeliovich: Probability constant with rapidity LHC’s large rapidity gap between the incoming protons (y p = ±9.6) will give us the opportunity to distinguish between the different models.

Baryon number transport – Experimental results XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 16 BRAHMS: Phys. Lett. B607 (2005) 42 p+p - = 200GeV p+p Au+Au 20% central PYTHIA HIJING-B STAR: Phys.Rev.Lett.86 (2001) 4778 Lessons from RHIC A. Falkiewicz: DIS2008, London Lessons from HERA  Proton excess at mid- rapidity can be attributed to the BN transport from the beam at RHIC energies  No sign of any rapidity or p t dependence  PYTHIA systematically overestimates the proton ratio  HIJING-B describes the experimental data  Initially reported an asymmetry of ~8% for Δy ~ 7. o Results never published due to large systematic uncertainties caused by the high beam-gas rate.  Results reported at the DIS2008 are compatible with an asymmetry for Λs ~0

Baryon number transport – Corrections (I) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 17  Apply corrections that don’t cancel out in the ratio: o Absorption o Cross section in transport codes o Secondaries (mainly protons) Absorption  Extract corrections based on MC sample  Rely on the proper description of the material in the simulation  Rely on the interaction cross-section used in the transport code (GEANT3)

Baryon number transport – Corrections (II) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 18 Bendiscioli and Kharzeev, Riv.Nuovo Cim.17N6, (1994) Detailed study published in 2008: ALICE-INT v1  Scaling factor for the absorption corrections defined as ε GEANT /ε FLUKA  ~2.5% decrease of the absorption correction

Baryon number transport – Corrections (III) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 19  Fit the dca distributions of primary antiprotons which are background free.  Use the same function and fit the primary protons scaled by the ratio (free parameter).  Fit the dca distributions of secondaries (protons coming from the interaction of particles with the material).  Use the same fitting functions to fit the dca distributions from real data.

Baryon number transport – Results (I) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 20 Fully corrected ratio: stat. error – lines, syst. error – shaded are around data points  Preliminary results show no sign of any rapidity or p t dependence.  Experimental points are compared with different model predictions that include variation of BN transport mechanisms: o HIJING-B clearly underestimates the y dependence, whereas it predicts a decrease of the ratio with increasing p t not seen in p+p collisions. o QGSM’s (only y information) version with a SJ ≠ 0 is systematically below the data points o The different PYTHIA tunes describe the data well.

Baryon number transport – Results (II) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 21  Mid-rapidity ratio for ALICE extracted by fitting the y dependence plot at |y| < 0.5.  Preliminary results show still a small excess of baryons over antibaryons (asymmetry of ~1.9%) at √s = 900 GeV/c.

ALICE is up and running!!! XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 22 THANK YOU FOR YOUR ATTENTION

BACKUP XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 23

Strangeness (K s 0 ) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 24

Strangeness (Λ) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 25

Cascades (Ξ) XXVIII Workshop on Recent Advances in Particle Physics and Cosmology,Thessaloniki 26