Nuclear modification of
transverse momentum spectra of
identified particles in Pb-Pb collisions at
√sNN= 2.76 TeV with ALICE
Simone SchuchmannGoethe-University of Frankfurt
International School of Nuclear Physics Probing the Extremes of Matter with Heavy Ions 2012, Erice
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 2
Outline
● Motivation
● L and K0s
– Reconstruction
– Nuclear modification factors: RCP and RAA
– Comparison to unidentified
charged particles and p(p), p+;-,K+;- RAA
● Conclusions
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 3
Outline
Sub-detectors of interest in this talk: ALICE TPC and ITS
● Motivation
● L and K0s
– Reconstruction
– Nuclear modification factors: RCP and RAA
– Comparison to unidentified
charged particles and p(p), p+;-,K+;- RAA
● Conclusions
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 4
Motivation I
● Why looking at high pT?– Diagnostic potential to probe the created medium via :
● the measurement of yields and particle ratios● comparison between AA and pp collisions
→ Parton energy loss and modified fragmentation pattern as a measure of the medium influence
● Charged particle RAA:– Strong suppression in Pb-Pb collisions
compared to pp around pT = 6-8 GeV/c– Rise towards high pT
● → Identified particles?● → Baryons versus mesons?
DEg > DEq > DEQ ? arXiv: 1208.2711v1
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 5
Motivation II
● Baryon to meson enhancement for light flavoured particles at low to intermediate pT
● No centrality dependence at high pT
● What is the effect on RCP and RAA?
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 6
Motivation III
● Baryon to meson enhancement likely to be a bulk effect (M.Veldhoen, arXiv:1207.7195)
● Particle production in jets seems not to be affected by medium (A. Ortiz Velasquez,QM12)
● What is the effect on RCP and RAA?
PYTHIA pp
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 7
Motivation III
● Baryon to meson enhancement likely to be a bulk effect (M.Veldhoen, arXiv:1207.7195)
● Particle production in jets seems not to be affected by medium (A. Ortiz Velasquez,QM12)
● What is the effect on RCP and RAA? Wiedemann et al. arXiv: 0707.3494v1
contradicts? →
= pp
PYTHIA pp
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 8
Motivation IV
● Note that
p(AA) / p(AA) > p(pp) / p(pp)
RAA(p) > RAA(p)
● Is this the case?
Wiedemann et al. arXiv: 0707.3494v1
= pp
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 9
Measurement of identified particles
Measurement of identified particles at high pT:
● L and K0s: secondary vertex analysis of V0 decays
– Reconstruction via weak decay products:
K0s p+ p- L p- p
= Invariant mass analysis
→ Identification via topological track reconstruction
→ no PID necessary
● p(p), p+;-,K+;-: identification via relative energy loss dE/dx in the TPC
L
p -
p
primary vertex
secondary vertex
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 10
K0s and L Analysis
Fit of the background
Function: polynomial of 1st or 2nd order Function: Gaussian + polynomial of 1st order
Fit of the peak for mass and resolution extraction
22/05/2011
22/05/2011
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 11
RCP
: comparison K0s – charged
● RCP of charged kaons match the K0
s RCPat low pT
● RCP similar for charged particles and K0
s
● Strong suppression of K0s at
high pT
RCP=dN central /dpTdN periph/dpT
⋅⟨N coll ⟩ periph⟨N coll ⟩central
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 12
RCP
: K0s and L
● Low pT: L enhanced over K0s
(baryon to meson enhance-ment) up to pT = 8 GeV/c
● High pT :
– RCP for L and K0s
compatible– Similar to RCP of
charged particles
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 13
RAA
: peripheral L and K0s
RAA=dN AA/dpTdN pp/dpT
⋅ 1⟨N coll ⟩AA
● Low pT:
– L enhanced over suppressed K0
s
● High pT :
– L and K0s are
suppressed similarly
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 14
RAA
: central vs peripheral
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 15
RAA
: comparison to charged
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 16
Identified charged particles: p,p,K
● Separation of particles at relativistic rise not perfect → need for statistical PID● → Extract particle fractions via
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 17
RAA: p,p,K versus L and K0s
● Low pT: RAA of protons exceeds the pions, kaons → qualitative similar to L, K0s
● High pT: RAA ~ 0.12 at pT = 6 GeV/c. Universal behaviour at higher pT within systematics
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 18
RAA: p,p,K versus L and K0s
● … same picture for peripheral collisions
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 19
RAA: flavour independence at high-pT (?)
● No significant difference between baryons and mesons at high pT● → no indication for different gluon and quark energy loss, or modified
fragmentation in medium
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 20
RAA: flavour independence at high-pT (?)
● No significant difference between baryons and mesons at high pT● → no indication for different gluon and quark energy loss, or modified
fragmentation in medium● … also no significant difference for charmed
hadrons observed● Is this understood: DEg > DEq > DEQ ?
17.09.2012 Simone Schuchmann, Goethe-Universität Frankfurt 21
Summary
● L, K0s, p(p), p+;-, K+;- RCP, RAA measured in Pb-Pb and pp at √sNN= 2.76 TeV up
to pT = 16 - 20 GeV/c.
● Strong suppression observed at high pT (pT > 8 GeV/c):
– p(p),p+;-,K+;-, L and K0s compatible
● At lower pT (pT < 5 GeV/c):
– p and p, L and K0s different in RAA ( RCP):
→ Baryon to meson enhancement● No indication for parton type dependence of energy loss at high pT observed in
ALICE so far