Medium Modification of Jet Shapes and Jet Multiplicities

TL;DR

It is found that the kt broadening of jet multiplicity distributions provides a very sensitive probe of the properties of dense QCD matter, whereas the sensitivity of jet energy distributions is much weaker.

Abstract

Medium-induced parton energy loss is widely considered to underly the suppression of high-pt leading hadron spectra in 200 GeV/A Au+Au collisions at RHIC. Its description implies a characteristic kt-broadening of the subleading hadronic fragments associated to the hard parton. However, this latter effect is more difficult to measure and remained elusive so far. Here, we discuss how it affects genuine jet observables which are accessible at LHC and possibly at RHIC. We find that the kt-broadening of jet multiplicity distributions provides a very sensitive probe of the properties of dense QCD matter, whereas the sensitivity of jet energy distributions is much weaker. In particular, the sensitive kinematic range of jet multiplicity distributions is almost unaffected by the high multiplicity background.

Figures (3)

• Figure 1: The gluon energy distribution (\ref{['eq1']}) as a function of the rescaled gluon energy $\omega/\omega_c$ and the rescaled gluon transverse momentum $\kappa$.
• Figure 2: LHS: The jet shape (\ref{['eq5']}) for a 50 GeV and 100 GeV quark-lead jet which fragments in the vacuum (dashed curve) or in a dense QCD medium (solid curve) characterized by $\omega_c = 62$ GeV and $\omega_c\, L = 2000$. RHS: the corresponding average medium-induced energy loss for $E_t = 100$ GeV outside a jet cone $R$ radiated away by gluons of energy larger than $E_{\rm cut}$. Shaded regions indicate theoretical uncertainties discussed in the text.
• Figure 3: Comparison of the vacuum and medium-induced part of the gluon multiplicity distribution (\ref{['eq8']}) inside a cone size $R=\Theta_c$, measured as a function of $k_t$ with respect to the jet axis. Removing gluons with energy smaller than $E_{\rm cut}$ from the distribution (dashed and dotted lines) does not affect the high-$k_t$ tails.