Angular Structure of the In-Medium QCD Cascade

Area of Science:

• High-energy particle physics
• Quantum Chromodynamics (QCD)
• Medium-induced phenomena

Background:

• Understanding particle interactions within dense media is crucial for interpreting high-energy collision data.
• Previous studies have explored aspects of parton showers in QCD, but the specific dynamics of angular broadening in extended media require further investigation.

Purpose of the Study:

• To investigate the angular broadening of medium-induced QCD cascades.
• To derive and solve the equation governing the evolution of average transverse momentum squared for gluons within a QCD cascade.
• To identify and characterize different regimes of momentum broadening based on medium size and gluon properties.

Main Methods:

• Derivation of an evolution equation for the average transverse momentum squared of gluons.
• Analytical solution of the derived equation as a function of medium length.
• Identification of distinct physical regimes based on the analytical solutions.

Main Results:

• Two distinct regimes of transverse momentum broadening were identified.
• In a smaller medium, transverse momentum increases with medium size, consistent with standard momentum broadening.
• In a larger medium with soft gluons, transverse momentum saturates, independent of medium size, due to multiple branching.

Conclusions:

• The study reveals a novel saturation regime in QCD cascades within large media.
• The observed momentum broadening behavior is qualitatively consistent with recent LHC data on dijet asymmetry.
• This work provides a theoretical framework for understanding particle interactions in dense QCD environments.