# Corona effect in AA collisions at LHC

## TL;DR

This paper investigates the corona effect in AA collisions at LHC, proposing a finite formation time for produced media and explaining R_{AA} and v_2 behaviors with a parton energy loss model, contrasting with earlier flat R_{AA} predictions.

## Contribution

It introduces a modified hypothesis accounting for a finite formation time and parton energy loss, explaining the rising R_{AA} and azimuthal anisotropy at LHC energies.

## Key findings

- R_{AA} increases with p_t at LHC, contrary to previous flat predictions.
- A fixed energy loss of about 7 GeV explains R_{AA} dependencies.
- Predicted v_2 at high p_t aligns with observed data.

## Abstract

Following our earlier finding based on RHIC data on the dominant jet production from nucleus corona region, we reconsider this effect in nucleus-nucleus collisions at LHC energy. Our hypothesis was based on experimental data, which raised the idea of a finite formation time for the produced medium. At RHIC energy and in low density corona region this time reaches about 2 fm/c. All observed high p_t particles are produced in the corona region and have a chance to escape during this 2 fm/c. After that, the formed high density matter absorbs all jets. Following this hypothesis, the nuclear modification factor R_{AA} should be independent on particle momentum and be flat versus p_t. At the same time, we describe the finite azimuthal anisotropy of high p_t particles, v_2. A separate prediction held that, at LHC energy, the formation time in the corona region should be two times smaller, about 1 fm/c. New data at LHC show that R_{AA} is not flat and is rising with p_t. We add to our original hypothesis an assumption that a fast parton traversing the produced medium loses the fixed portion of its energy. A shift of about 7 GeV (I would call it as a parton binding energy) from the original power law p^{-6} production cross section in pp explains well all the observed R_{AA} dependencies. The shift of about 7 GeV is also valid at RHIC energy, where the cross section follows a power law with about p^{-8} and this shift explains a very slow rise of R_{AA} seen for neutral pions with p_t above 15 GeV/c. We also show that the observed at LHC dependence of v_2 at high p_t and our previous predictions agree.