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Comments on neutron production. Mark Strikman. Relativistic heavy ion collisions.

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Comments on neutron production

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Comments on neutron production

Mark Strikman

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)



● protons


● 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


Qualitative expectations:

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)

Spectator emission


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

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