Distorting the Hump-backed Plateau of Jets with Dense QCD Matter

Abstract

The hump-backed plateau of the single inclusive distribution of hadrons inside a jet provides a standard test of the interplay between probabilistic parton splitting and quantum coherence in QCD. The medium-induced modification of this QCD radiation physics is expected to give access to the properties of the dense medium produced in relativistic heavy ion collisions. Here, we introduce a formulation of medium-induced parton energy loss, which treats all leading and subleading parton branchings equally, and which - for showering in the vacuum - accounts for the observed distribution of soft jet fragments. We show that the strong suppression of single inclusive hadron spectra measured in Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC) implies a characteristic distortion of the hump-backed plateau; we determine, as a function of jet energy, to what extent the soft jet fragments can be measured above some momentum cut. Our study further indicates that the approximate pT-independence of the measured nuclear modification factor does not exclude a significant Q^2-dependence of parton energy loss.

Figures (3)

• Figure 1: The single inclusive hadron distribution as a function of $\xi = \ln \left[E_{\rm jet}/p\right]$. Data taken from $e^+e^-$ collision experiments TASSO Braunschweig:1990yd and OPAL Abbiendi:2002mj, $E_{\rm jet} = \sqrt{s}/2$. Lines through data obtained from the MLLA result (\ref{['eq3']}). Dashed and dash-dotted curves labeled "in medium" are calculated with a medium-modification $f_{\rm med} = 0.8$ of the LO splitting functions.
• Figure 2: The $p_T$-dependence of the nuclear modification factor $R_{AA}$, calculated for a medium-enhanced parton splitting with $f_{\rm med}$. Data taken from Adler:2003ii.
• Figure 3: The change of the total hadronic multiplicity inside jets of different energy $E_{\rm jet}$, as a function of the soft background cut $p_T^{\rm cut}$ above which this multiplicity is measured. Medium-effects are modeled by $f_{\rm med} = 0.8$.