Jets in Heavy Ion Collisions at the LHC. Andreas Morsch CERN. Outline. What are the new opportunities but also experimental challenges of jet physics on Heavy Ion Collisions ? How can jets be reconstructed in the high multiplicity heavy ion events ?
Jets in Heavy Ion Collisions at the LHC
Simplistically:Jet(E)→Jet(E-DE)+ soft gluons (DE)
Baier, Dokshitzer, Mueller, Peigne, Schiff (1996); Zakharov (1997); Wiedemann (2000); Gyulassy, Levai, Vitev (2000); Wang ...
Coherent sum over scatterings with
free path length l and mean qT transfer m
Medium characterized by
Expect large effects !
Needs large range of E to measure DE(E)
Decrease of leading particle pT
Increased mult. of low-pT
Particles from radiation.
Increase of pT rel. to jet-axis
Energy outside jet cone
Dijet energy imbalance
p+p @ s = 200 GeV STAR Au+Au @ sNN = 200 GeV
In central Au-Au collisions standard jet reconstruction algorithms fail due to
the large energy from the underlying event (125 GeV in R< 0.7)
and the relatively low accessible jet energies (< 20 GeV).
Use leading particles very successfully as a probe.
Phys. Rev. Lett. 91, 072304 (2003).
Suppression of inclusive hadron yield
Disappearance of away-side correlations
Eskola et al., hep-ph/0406319
RAA~0.2-0.3 for broad range of
Surface emission bias limits
100 GeV Jet
Mean value shifts to
Salgado, Wiedemann, Phys. Rev. Lett. 93: 042301 (2004)
Sensitive to out-of-cone radiation.
J. Casalderrey-Solana and XNW, arXiv:0705.1352 [hep-ph].
A. Accardi et al., hep-ph/0310274
CERN TH Yellow Report
pT < 2 GeV
* For dN/dy = 5000.
Jets reconstructed from charged particles:
Energy contained in sub-cone R
Need reduced cone sizes and transverse momentum cut !
Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1).
R and pT cut have to be optimized according to the background conditions.
E ~ R2
Background reduced by 0.42 = 0.16 but 88% of signal preserved.
Loop1: Background estimation from cells outside jet cones
Loop2: UA1 cone algorithm to find centroid
using cells after background subtraction
Standard ATLAS solution -cone algorithm (R = 0.4) - is intensively studied with different samples
Jet position resolution
Jet energy resolution
Jet finding & energy measurement work for ET > 40 GeV (15 GeV in pp)
Jet reconstruction using charged particles measured by TPC + ITS
And neutral energy from EMCAL.
Attention: ALICE quotes fluctuations relative to ideal jet with R = 1.0
2 GeV 1GeV
2 GeV 1GeV
Background estimated for
Pb-Pb using HIJING
Ideal: No background
Background fluctuates down
Background fluctuates up
Bias towards higher Bg
Statistical error for Ejet = 100 GeV, 104 events
Systematics of Background Subtraction
Robust signal but underestimation of jet energy
biases x to lower values.