Comments on neutron production. Mark Strikman. Relativistic heavy ion collisions.
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Comments on neutron production
Relativistic heavy ion collisions
Unique feature of RHIC / LHC - determination of the reaction plane. Becomes interesting to take another look at the process of formation of the spectator system in the heavy ion collision (discussed often in the framework of the abrasion - ablation model).
What happens when a piece of the nucleus is chopped off? How the residual system decays?
With our configuration generator + Glauber model we can perform more microscopic treatment.
Lead - Lead collision event at b=6 fm
using event generator of Alvioli and MS which generates correct SRC between nucleons in the colliding nuclei)
● spectator nucleons which were near wounded nucleons
Estimate of the total excitation energy - generalization of the Koltun sum rule for the excitation of the nucleus in the (e,e’p) reactions.
Excitation energy of spectator system
s=spectator nucleons; w=wounded nucleons;
is dominated by np SRCs. <Vpp>: <Vnn>: <Vpn>=1:1:9
Correct nuclear configurations in MC are critical for calculation of
Pb-Pb for plab=159 GeV/c
Dashed curve - Effects of SRC emission and surface evaporation are subtracted & elastic NN scattering included
Average kin. energy/ nucleon
drops with increase of b
Asymmetry of nucleon emission v1;
Correlation of asymmetries from two nuclei
For small b the emission spectrum is close to momentum distribution in the nucleus (near the surface)
Differences with pA
At LHC nucleon makes a whole of radius 1.6 fm
Number of wounded nucleons, N, in Glauber for small b is N~ 15
Difference with AA - more important secondary interactions
say pions with energies < few GeV- known to contribute a factor of 2 for Ep=400 GeV, bigger chance for knocked out nucleons to be absorbed/loose energy.
Glauber: For central impact parameters
What happens when one nucleon is removed from arbitrary point in the nucleus
Nneutrons ~ 60 !!???
expect large fluctuations in particular due to difference between Gribov-Glauber and Glauber approximations. Dispersion from cross section fluctuations is ~ 0.2 leading to fluctuations of
Nneutrons ~ 40 -- 80
Plus cascade fluctuations
Fast nucleon production
The average number of neutrons emitted in incoherent J /ψ production in Au + Au UPCs at RHIC and Υ production in Pb + Pb UPCs at the LHC as a function of the recoil nucleon momentum, pN = √ |t|.
Large impact parameters:
angular asymmetry especially
for fast (En/(EA/A) > 1.2
At -t > 0.07 GeV2 separation of scattering
off nucleus and proton is pretty easy in UPC
Tverskoi, Zhalov, MS