Speaker
Description
Following our earlier finding based on RHIC data about the dominant jet production from nucleus corona region, we reconsider this effect in nucleus-nucleus collisions at LHC energy. Our hypothesis was based on the 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. In the center of interaction region it's about 0.7 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 can describe at RHIC 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 from the original power law $p^{-6}$ production cross section in $pp$ explains well all the observed $R_{AA}$ dependencies at all centrality. 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. It is very attractive to call this value of 7 GeV as a parton binding energy.
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