Hadronic resonance production in Pb+Pb collisions

from the ALICE experimentAnders Knospe

on behalf of the ALICE CollaborationThe University of Texas at Austin

29 May 2012

2 Outline• Introduction• f Yield• K*0 Mass and Width• Hadron-Resonance Correlations• Summary

3 Introduction• Temperature and Lifetime of Fireball

– Resonance formation through recombination– Rescattering prevents resonance reconstruction– Statistical Model, UrQMD, fit to resonance/non-resonance ratios

• Chiral Symmetry Restoration– Resonances can be produced with jets in QGP– Produced off-shell or interact with medium mass shift– Interactions with medium width broadening (reduced lifetime)

• For T=250 MeV: f lifetime may change by up to factor of 10*

G. Torrieri and J. Rafelski, Phys. Lett. B 509 239 (2001)

Statistical Model

* L. Holt and K. Haglin, J. Phys. G31, S245 (2005)

4 Hadron-Resonance Correlations• To probe QGP: compare resonances that passed

through medium with those that did not– Hadron-resonance correlations

jet, le

ading

hadro

n

Resonance in near side: no medium interaction

Resonance in away side:Low pT (below ~2 GeV/c): dominated by interactions in hadronic mediumHigh pT: dominated by interactionswith early hadronic or partonic medium

Resonance transverse to jet:Thermal production in hadronic medium

Method proposed by:C. Markert, R. Bellwied, I. Vitev, Phys. Lett. B 669 92-97 (2008)

f Yield

6 Event & Track Selection• Pb+Pb collisions at

√sNN=2.76 TeV– 9.5 M events– |vz| < 10 cm

• fK-K+ (B.R.=0.489)• PID with TPC dE/dx:

|nsK|<2

TPC: PID and Tracking

ITS: Tracking

Centrality estimate: from measurement of multiplicity in V0 detector (scintillator arrays on each side of collision vertex)

7 Signal Extraction• Invariant mass

– |ypair|<0.5• Combinatorial Background

– Event Mixing (default)– Like-Sign

• Residual Background Fit: polynomial• Yield: integrate histogram, subtract

residual background• Peak Fit: res. back. + BW peak

– Find yield, mass, widthBACKGROUND SUBTRACTION

Background Fit Region(exact boundaries varied)

f Analysis

8 Uncorrected f Yields• Statistics up to pT~10 GeV/c• Coming for Quark Matter:

– Corrected Spectra– dN/dy– Ratios: f/p, f/K

Uncorrected f Yields

9 Particle Ratios• f/p:

– NA49 (17.5 GeV) and STAR (200 GeV): A+A higher than p+p

– PHENIX 200 GeV Au+Au• f/p = 0.015 ± 0.003• lower than STAR Au+Au• Consistent with STAR and

ALICE p+p– Ratio for p+p appears to saturate

at LHC energies: Is this also true for A+A?

• f/K: A single trend for both p+p and A+A over a wide energy range– Will Pb+Pb agree?– Au+Au 200 GeV:

• PHENIX f/K = 0.0911 ± 0.021• STAR higher than PHENIX

Lower energy: seePRC 71 064902 (2005) andPRC 79 064903 (2009)

?

?

K*0 Mass and Width

11 Signal Extraction• 3.4 M events• K*0p±K+ (BR=0.67)• PID with TPC dE/dx:

|nsp|<2, |nsK|<2• Calculate invariant mass of pK

pairs– rapidity cut: |ypair|<0.5

• Combinatorial Background:– Event Mixing– Like-Sign Pairs

• Fit signal: BW peak + linear background

• Uncorrected spectra:– Statistics up to pT~10 GeV/c– Corrected spectra for QM

2012

Uncorrected K*0

12 K*0 Mass and Width• No centrality dependence• Mass:

– High pT: Pb+Pb consistent with p+p and HIJING– Low pT: Pb+Pb consistent with p+p (deviation from PDG not due to medium)

– Low pT: Pb+Pb not consistent with HIJING• detector effects in simulation under investigation

• Width: consistent with simulation; no width change beyond 10 MeV/c2

K*0 Mass K*0 Width

13

Hadron-Resonance Correlations

14 Hadron-Resonance Correlations

Want to select for resonances produced early in collision and reduce contribution from hadronic phase: look for high-pT resonances in away side

C. Markert, R. Bellwied, I. Vitev, Phys. Lett. B 669 92-97 (2008)

15 Angular Correlations• Angular Correlation of trigger hadron with a f meson

– pT(h)>3 GeV/c– pT(f)>1.5 GeV/c

Pb+Pb

p+p

16 Mass and Width vs. Df

mass/expected value width/expected value

p+pp+p

Pb+Pb Pb+Pb

• f mass and width as a function of angle (Dφ) w.r.t. leading hadron

• pT(h)>3 GeV/c• pT(f)>1.5 GeV/c• Measured values divided

by expected value• No clear difference in

behavior between p+p and Pb+Pb

• In Pb+Pb: no mass shift or width broadening observed in away side

• However: f signal may be dominated by non-jet f for this pT range

12/9/2011

12/9/2011

12/9/2011 12/9/2011

17 Summary• K*0 and f signals observed in Pb+Pb collisions at 2.76 TeV

– Corrected f and K*0 spectra and ratios to non-resonances will be presented at Quark Matter 2012

• K*0 mass: Pb+Pb consistent with p+p and with HIJING at high pT

– any mass shift is not due to a medium effect• K*0 width: Pb+Pb consistent with HIJING• h-f correlations: away side looks like near side

– No mass shift or width broadening in away side– Will things be different for higher pT(f) or higher pT(trigger)?

• We do not observe the signatures of chiral symmetry restoration.