for the ALICE Collaboration RAA in ALICE Michael Linus Knichel for the ALICE Collaboration High-pT Probes of High-Density QCD at the LHC, École Polytechnique, Palaiseau May 31st, 2011 Michael L. Knichel HPHD2011 31.05.2011 1

Content ALICE detector (subsystems used for the RAA analysis) Tracking, high pT capabilities pp measurements – reference for RAA Pb-Pb spectra RAA for charged particles and Centrality dependence Comparison to CMS/RHIC Comparison to models RAA vs. Reaction plane angle ϕ Particle Identification at high pT via dE/dx in the TPC Charged pion spectra and RAA RAA of Λ and K0s Summary Michael L. Knichel HPHD2011 31.05.2011 2

Motivation • Heavy Ion Physics ↔ Study of nuclear matter at extreme conditions • RHIC : “The production of charged hadrons at high pT is suppressed in central Au-Au collision compared to the superposition of nucleon-nucleon collisions” • Dominant production mechanism for high pT charged hadrons – Fragmentation of high pT partons from hard scatterings at an early stage of nuclear collisions • Suppression is probably due to energy loss of partons, traversing a hot, dense QCD medium → the Quark-Gluon-Plasma (QGP) • Understanding the suppression goes towards understanding the medium properties of the QGP → What happens at LHC energies ? Michael L. Knichel HPHD2011 31.05.2011 3

Introduction ALICE Collaboration, Phys. Lett. B696 (2011) Supression of high pT particles in Pb-Pb collisions at LHC was one of the first ALICE results Showing strongest suppression in central collisions at pT around 7 GeV RAA increases for pT above 7 GeV/c σINELPP = 64 mb Nuclear overlap functionTAA from Glauber model Michael L. Knichel HPHD2011 31.05.2011 4

ALICE detector Central Barrel Detectors: Inner Tracking System (ITS) 6 layers of Silicon detectors: Pixel: 2 layers (tracking, vertex, trigger) Each 2 layers of Drift and Strip (tracking, PID via dE/dx) Forward/Trigger detector: V0 scintillator hodoscopes: Trigger, centrality determination ZDC scintillating fibre calorimeter (+-116 m from IP): centrality, timing Michael L. Knichel HPHD2011 31.05.2011 5

ALICE detector EMCal Pt resolution: In the 0.5 T magnetic field a track with 100 GeV/c is almost straight The deviation from a straight line is only 5 mm over the track length of 2.5 m Very high space point precision needed, achieved with many measured points PHOS Central Barrel Detectors: Time Projection Chamber (TPC): tracking, PID Time of Flight detector (TOF): PID Transition radiation detector (TRD): e/π separation, possibility to improve pT resolution at high pT Michael L. Knichel HPHD2011 31.05.2011 6

Event and Track selection Use of combined tracking Information from TPC and ITS (Silicon detectors) Reconstructed primary vertex within 10 cm of nominal position Centrality quality cuts (for Pb-Pb) Select long tracks in the TPC, two hits in the ITS, one in the Pixels Tracks must point to the primary vertex (7σ) Tracks must pass some additional quality cuts All Events taken with Minimum Bias Trigger Total Numbers: ~15M Events in 2.76 ATeV Pb-Pb (2010) ~65M Events in 2.76 TeV pp (2011) (after offline trigger and event selection) Michael L. Knichel HPHD2011 31.05.2011 7

Transverse momentum resolution Analysis with combined tracking information from ITS and TPC pT resolution looks very similiar in pp and Pb-Pb σ(pT)/pT ~ 20% at 100 GeV/c Resolution estimated from covariance matrix of the Kalman Filter algorithm used for tracking Veryfied with invariant mass analysis of K0s -> π+ π - decays up to 12 GeV/c Systematic uncertaintiy on the resolution ±20% (estimated from K0 decay, relative uncertainty constant with pT) Factor 2 improvement expected soon (better alignment/calibration) Further improvement with TRD Michael L. Knichel HPHD2011 31.05.2011 8

Proton-Proton charged particle spectra Corrections from Simulations Differences between data and MC: main source of systematics Pb-Pb very similiar to 7 TeV, 900 GeV 2.76 TeV data sample analyzed was taken without SDD (Drift) detector Lower efficiency, larger systematics for the 2.76 TeV data Small differences pos/neg Contamination of secondary tracks form weak decays of K0s and Λ derived from data Track cuts remove most secondaries Remaining second. <1% above 4 GeV Michael L. Knichel HPHD2011 31.05.2011 9

Proton-Proton charged particle spectra pT spectra at √s = 0.9, 2.76, 7 TeV 7 TeV: up to 100 GeV/c 2.76 TeV: up to 35 GeV/c For pT > 50 GeV/c: systematics from pt resolution dominant For 2.76 TeV uncertainties ~10% + 3% (normalization) Michael L. Knichel HPHD2011 31.05.2011 10

Proton-Proton 2.76 TeV Sufficient statistics only up to ~35 GeV/c in pT Phythia8 describes the data very well above 1GeV/c As reference for RAA we are interested in an extended pT range of up to 100 GeV/c Michael L. Knichel HPHD2011 31.05.2011 11

A Reference for Pb-Pb For RAA we use the measured yields bin-by-bin up to 5 GeV/c, above 5 GeV/c we fit a modified Hagedorn function to the data For 5-35 GeV we use this fit as a reference to avoid fluctuations in the reference Above 35 GeV we extrapolate using the fit Systematic uncertatinties includes using different function (powelaw) and fit ranges Good agreement with Pythia8 and alternative references CMS: arXiv:1104.3547v1 Michael L. Knichel HPHD2011 31.05.2011 12

Centrality VZERO Amplitude used as centrality estimator, SPD for systematics outliers in the multiplicity correlations, of SPD vs V0, TPC vs V0, ZDC vs V0 σ = 64 mb A. Dainese Michael L. Knichel HPHD2011 31.05.2011 13

Spectra in Pb-Pb Text Charged particle spectra measured up to 100 GeV/c versus collision centrality Visible change of the pT shape from central to peripheral collisions Michael L. Knichel HPHD2011 31.05.2011 14

RAA compared to published new analysis: new pp reference based on 2.76 TeeV pp data all Pb-Pb statistics in publication: pp reference (interpolation between AliCE data 0.9 - 7 TeV using power law) 20% of Pb-Pb statistics Michael L. Knichel HPHD2011 31.05.2011 15

RAA: Comparison to RHIC Similar shape Stronger suppresion at LHC Indicates denser medium rise above 7 GeV measured with ALICE Michael L. Knichel HPHD2011 31.05.2011 16

RAA vs. Centrality Charged particles: • RAA shown in pT range (0.15 – 100 GeV/c) • Different suppression pattern depending on collision centrality • RAA has minimum at pT = 6-7 GeV/c in all Centralities • Hint of leveling off above 50 GeV/c Michael L. Knichel HPHD2011 31.05.2011 17

RAA in bins of pT low pT: approximate scaling with multiplicity density, matching also RHIC results Michael L. Knichel HPHD2011 31.05.2011 18

RAA in bins of pT high pT: weak suppression, no significant centrality dependence low pT: approximate scaling with multiplicity density, matching also RHIC results Michael L. Knichel HPHD2011 31.05.2011 19

RAA: Comparison to CMS QM2011 talk by Andre Yoon Agreement up to 50 GeV/c within the uncertainties Different systematic uncertainties above 50 GeV/c Some tension at highest pt, but agreement within the (presently large) systematic uncertainties Comparison of Pb-Pb spectra CMS/ALICE ongoing Michael L. Knichel HPHD2011 31.05.2011 20

RAA: Comparison to models Bass et al, PRC79, 024901 pronounced pT dependence of RAA at LHC sensitivity to details of the energy loss distribution Michael L. Knichel HPHD2011 31.05.2011 21

RAA vs. Reaction plane angle Can be build from RAA and v2 Different path length in the medium: in plane vs. out of plane Michael L. Knichel HPHD2011 31.05.2011 22

Particle Identification via dE/dx almost constant separation of pions to K, p above p = 3 GeV/c Track-by-track PID possible in the region, where the bands are clearly separated (low momentum) At high momentum, the differences of energy loss in the relativistc rise can be used for statistical PID Michael L. Knichel HPHD2011 31.05.2011 23

Particle Identification in the relativistic rise from TPC dE/dx Michael L. Knichel HPHD2011 31.05.2011 24

Charged pion fraction Meaure the ratio pions/charged as a function of pt (both in pp and Pb-Pb) Michael L. Knichel HPHD2011 31.05.2011 25

Charged pion spectra Multiply by the charged particle spectra to get pion spectra for pp and Pb-Pb Michael L. Knichel HPHD2011 31.05.2011 26

Charged pion RAA Charged pion RAA up to 20 GeV/c in pT agrees with charged particle RAA - in peripheral events - for pT > 6 GeV/c is smaller than charged particle RAA for pT < 6 GeV/c Michael L. Knichel HPHD2011 31.05.2011 27

Measurement of Λ and K0s Recontruction via weak decays K0S  π+ π - Λ  π - p Invariant mass analysis Extraction of yields Michael L. Knichel HPHD2011 31.05.2011 28

RCP of Λ and K0s K0s - RAA very similar to that of charged particles: strong suppression of K0s at high pT Λ - RAA significantly larger than charged at intermediate pT: enhanced hyperon production counteracting suppression for pT > 8 GeV/c, Λ and K0s - RAA similar to charged particle RAA: strong high-pT suppression also of Λ Michael L. Knichel HPHD2011 31.05.2011 29

RCP of Λ and K0s K0s - RAA very similar to that of charged particles: strong suppression of K0s at high pT Λ - RAA significantly larger than charged at intermediate pT: enhanced hyperon production counteracting suppression above 8 GeV/c, Λ and K0s - RAA are similar to charged: strong high-pT suppression also of Λ Λ enhancement reaches out to higher pT than at RHIC magnitude of RCP compatible Michael L. Knichel HPHD2011 31.05.2011 30

RAA of Λ and K0s high-pT: suppression of Λ and K0s similar to charged particles intermediate pT: moderate enhancement in peripheral collisions  central RAA similar to RCP Michael L. Knichel HPHD2011 31.05.2011 31

RAA of Λ and K0s strong nuclear enhancement of Λ observed at RHIC not seen at the LHC Michael L. Knichel HPHD2011 31.05.2011 32

All ALICE RAA Michael L. Knichel HPHD2011 31.05.2011 33

Summary charged particle pT spectra in Pb-Pb at √sNN = 2.76 TeV measured with ALICE at the LHC new reference pT spectrum derived from pp collisions at √s = 2.76 TeV strong suppression charged particle production observed at pT < 50 GeV/c comparison to RHIC data suggests that suppression scales with the charged particle density at pT > 50 GeV/c, no strong centrality dependence of charged particle production is observed results on identified particles will allow to disentangle the interplay between quark and gluon energy loss, and recombination mechanisms at intermediate pT Michael L. Knichel HPHD2011 31.05.2011 34