Charged particle spectra and nuclear modification factor in lead-lead collisions at √ s NN = 2 . 76 TeV with the ATLAS detector at the LHC Petr Balek for the ATLAS collaboration 16 August 2012 IPNP, Charles University in Prague Petr Balek 16 August 2012 1 / 19
introduction why to study charged particle spectra? ◮ to understand properties of hot dense matter created in HI collisions ◮ to understand the mechanism of energy loss of partons study ratio of central (C) and peripheral (P) spectra: N coll ( C ) · N events , C d 2 N C / d η d p T 1 R CP = 1 N coll ( P ) · N events , P d 2 N P / d η d p T as a peripheral bin we take 60–80% previously measured R CP and spectra up to 30 GeV high luminosity collected by ATLAS in 2011 allow us to extend previous measurements Petr Balek 16 August 2012 2 / 19
ATLAS detector – Inner detector p T > 0 . 5 GeV | η | < 2 . 5 full φ coverage Petr Balek 16 August 2012 3 / 19
determination of centrality centrality based on energy deposited in Forward Calorimeter – 3 . 1 < | η | < 4 . 9 model – based on Glauber calculation convoluted with p+p data Petr Balek 16 August 2012 4 / 19
event selection used data 2010 and 2011: minimum bias 2010 – 7 µ b − 1 , 50.7M events ◮ MBTS or ZDC minimum bias 2011 – 7 µ b − 1 , 50.7M events ◮ total energy > 50 GeV ◮ or signal from ZDC+track hard probes 2011 – 0.14 nb − 1 , 998M sampled events ◮ unprescaled jet trigger; anti-k T , E T > 20 GeV Petr Balek 16 August 2012 5 / 19
track selection each track is required to have: at least 2 Pixel hits, one in the innermost layer at least 7 SCT hits no Pixel or SCT holes on track 0.7 on track on track ATLAS Preliminary 2010 Data 0-5% Pb+Pb 9.5 2010 MC 0-5% 0.6 4.5 s = 2.76 TeV 2010 Data 60-80% NN 〉 -1 L = 7 µ b 〉 〉 2010 MC 60-80% Pixel hits SCT Hits int 9.0 SCT Holes 0.5 4.0 8.5 0.4 〈 〈 〈 8.0 0.3 3.5 7.5 0.2 3.0 7.0 0.1 0.0 -2 -1 0 1 2 -2 -1 0 1 2 -2 -1 0 1 2 η η η Petr Balek 16 August 2012 6 / 19
track selection -1 Z axis: dN / (dp d(d )) [GeV mm -1 ] track T 0 -1 2 3 4 5 6 7 8 9 10 10 10 10 10 10 10 10 10 1 10 Preliminary a) b) c) d) ATLAS [mm] 4 Pb+Pb s = 2.76 TeV 3 NN -1 L = 15nb int 0 d 2 1 0 -1 -2 -3 60-80% 0-5% 0-5% 0-5% -4 2010-11MB 2010-11MB 2011HP 2010-11MB 2 2 2 2 1 10 10 1 10 10 1 10 10 1 10 10 p [GeV] p [GeV] p [GeV] p [GeV] T T T T Pixel measure d 0 and z 0 sin θ precisely due to high occupancy, SCT measure p T of some tracks incorrectly this caused populating area of high p T and high d 0 significance cuts on d 0 and z 0 sin θ solve this problem Petr Balek 16 August 2012 7 / 19
raw spectra reconstruction ATLAS Preliminary MB 3 10 standard Pb+Pb s = 2.76 TeV NN HP -1 2 Data 2010-11 L = 0.15 nb 10 int Centrality 60-80% Centrality 0-5% 10 10 10 1 1 1 1.3 1.3 standard 2 2 HP jet-matched/HP 1.2 1.2 1.2 HP jet-matched/MB jets 1.1 1.1 HP jet-matched/MB (10-80%) 1.0 1 1 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.6 0.6 0.6 2 2 1 10 10 1 10 10 p [GeV] p [GeV] T T tracking cuts able to deal with signal/background up to 1/100 need matching to jets from ∼ 60 GeV for spectra and R CP , muon and electrons (and positrons) are subtracted – these are µ ± and e ± from W decay Petr Balek 16 August 2012 8 / 19
fakes and secondaries correction low- p T tracks are corrected for fakes and secondary particle based on PYTHIA events embedded into HIJING Fraction 1.00 η η η | |<1.00 1.50<| |<1.75 2.25<| |<2.50 0.95 30-40% 10-20% 0.90 ATLAS Preliminary 5-10% Pb+Pb s =2.76TeV NN 0-5% simulation 2 4 2 4 2 4 p [GeV] p [GeV] p [GeV] T T T Petr Balek 16 August 2012 9 / 19
efficiency correction efficiency estimated with PYTHIA events embedded into HIJING (2010) or data (2011) evaluate efficiencies in 7 η bins and 9 centrality bins used bin-by-bin unfolding to correct spectra for momentum resolution we use iterative approach - once fully corrected spectra are obtained, MC spectrum is reweighed to follow data Preliminary ATLAS 0.9 Efficiency 0.9 0.9 0.9 Pb+Pb s = 2.76 TeV NN 0.8 0.8 0.8 0.8 simulation 0.7 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 60-80% 30-40% 0.4 0.4 0.4 0.4 10-20% 0-5% 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 | |<1. 1.5<| |<1.75 2.25<| |<2.5 η η η 0.1 0.1 0.1 0.1 2 2 2 2 2 2 1 10 10 1 10 10 1 10 10 p [GeV] p [GeV] p [GeV] T T T Petr Balek 16 August 2012 10 / 19
systematic uncertainties Source Spectra R CP Tracking cuts 4% 3% PV pointing cuts 1-4% 1-3% Truth association 1% 2% Efficiency 3-20% 5% Feed down 0-3% – Material budget 2-5% – Momentum scale 0-6% – Trigger efficiency 2% 1% Electroweak decays 1% 1% N coll ratios – 3.8-11.7% in uncertainty of efficiency is hidden all unfolding uncertainties Petr Balek 16 August 2012 11 / 19
spectra use minimum bias events for p T < 30 GeV and hard probes events for p T > 30 GeV ] 2 10 η η η -2 | |<1.0 1.0<| |<2.0 2.0<| |<2.5 ) [GeV -2 T 10 dp η /(d ch -6 10 ) dN ATLAS Preliminary T (0-5)% p Pb+Pb s =2.76TeV (30-40)% π NN -10 10 1/(2 Data 2010 + 2011 (50-60)% (60-80)% -1 L = 0.15nb int 2 2 2 1 10 10 1 10 10 1 10 10 p [GeV] p [GeV] p [GeV] T T T Petr Balek 16 August 2012 12 / 19
R CP η η η | |<1.0 1.0<| |<2.0 2.0<| |<2.5 CP 1 R 0.2 ATLAS Preliminary (50-60)% / (60-80)% Pb+Pb s =2.76TeV (30-40)% / (60-80)% NN Data 2010 + 2011 (0-5)% / (60-80)% -1 L = 0.15nb int 2 2 2 1 10 10 1 10 10 1 10 10 p [GeV] p [GeV] p [GeV] T T T R CP has no η dependency Petr Balek 16 August 2012 13 / 19
R CP CP 1 R η | |<2.5 ATLAS Preliminary 0.2 (50-60)% / (60-80)% Pb+Pb s =2.76TeV NN (30-40)% / (60-80)% Data 2010 + 2011 (0-5)% / (60-80)% -1 L = 0.15nb int 2 1 10 10 p [GeV] T Petr Balek 16 August 2012 14 / 19
R CP 2 ATLAS CP Pb+Pb s = 2.76 TeV NN ∫ R 1.5 anti- k R = 0.2 µ -1 L dt = 7 b t 1 0.5 50 - 60 % 0 1 0.5 30 - 40 % CP 1 0 R 1 0.5 10 - 20 % 0 η 1 | |<2.5 ATLAS Preliminary 0.2 (50-60)% / (60-80)% 0.5 Pb+Pb s =2.76TeV NN 0 - 10 % (30-40)% / (60-80)% Data 2010 + 2011 -1 (0-5)% / (60-80)% 0 L = 0.15nb int 40 60 80 100 120 140 160 180 200 2 1 10 10 p [GeV] p [GeV] T T compatible with jet measurement Petr Balek 16 August 2012 15 / 19
R CP 1 CMS Collaboration, Eur. Phys. J. C72 (2012) 1945 0.8 CP 1 1 0.6 R 0.4 0.2 0.8 CP R 0.6 1.2 1 η | |<2.5 0.4 0.8 ATLAS Preliminary 0.2 0.6 (50-60)% / (60-80)% Pb+Pb s =2.76TeV NN 0.2 (30-40)% / (60-80)% 0.4 Data 2010 + 2011 -1 (0-5)% / (60-80)% 0-5% / 50-90% L = 0.15nb 0.2 int 0 2 1 2 3 4 5 67 10 0 20 30 100 1 10 10 p [GeV] p (GeV/c) T T CMS has different peripheral bin ATLAS used lower threshold for jet trigger → better statistics at high p T Petr Balek 16 August 2012 16 / 19
conclusion measured spectra and R CP within | η | < 2 . 5 and 0.5 GeV < p T < 150 GeV minimum seen at around 7 GeV; above show rise up to high p T very good statistics up to ∼ 70 GeV; higher p T limited by statistics of peripheral bin compatible with level of suppression measured in jets Petr Balek 16 August 2012 17 / 19
backup Petr Balek 16 August 2012 18 / 19
raw spectra ] -1 2 10 ATLAS Preliminary MB reconstruction [GeV Pb+Pb s = 2.76 TeV HP reconstruction NN MB standard Data 2010-11 HP standard 1 -1 L = 0.15 nb T HP standard, in jets int /dp µ ± HP standard, Centrality 0-5% track Centrality 60-80% dN -4 10 events 1/N -8 10 2 2 1 10 10 1 10 10 p [GeV] p [GeV] T T good agreement between MB and HP at high p T Petr Balek 16 August 2012 19 / 19
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