Using evaporated neutron number distribution as a saturation signature tagger EIC taskforce meeting 2014/4/171

2 A little bit recap 1.We found the correlation between number of forward neutron production and the traveling distance after collision in the nuclear. 2.This correlation can be utilized to characterize eA collision geometry. 3.By binning in produced forward neutron number, underlying traveling distance can be largely constrained.

N n range ±RMS %[0,3] ± %[4,8] ± %[9,13] ± %[14,38] ±2.49 Counts Neutron number handle constrains the collision geometries Collisition geometry variable d has been effectively constrained by the neutron number handle from nuclei break up 2014/4/ % 50-75% 25-50% 0-25%

Neutron number distribution as a tagger for the saturation physics 2014/4/174

N n ? iterations Fix geo config, impact b Sample interaction collect N n 1. Probe interacts coherently with all nucleons 2. No collision geometry sensitivity in z direction! Saturated: Averaged: How does the nuclei break up in the saturated case? Assumed to be the same as averaged configurations 2014/4/175 All the following simulations based on evaporated neutrons from DPMJET + FLUKA for eAu collisions

AveragedNon Averaged The averaged (saturated) vs non averaged (non saturated) RMS shown as the error bar in every bin 2014/4/176 eAu 10 GeV x 100 GeV

Black: 10<Q 2 <20 Red: 1<Q 2 <2 eAu 10 GeV x 100 GeV Kinematics dependence of neutron number distribution Shape of neutron number distribution does not depend on the kinematics 2014/4/177

Red:Saturated eAu 10x100 Averaged eAu 10x100 Non Averaged 1<Q 2 <2 Significant difference between the sat/nosat break up neutron distribution Red: from a mixture of Averaged/Non Averaged distributions Saturated case effectively cast into a mixture of the averaged and non averaged distribution. Difference from the nonsaturated distribution can be reckoned as the saturation signature. 2014/4/178 Solid: NonAveraged Dashed: Averaged

2014/4/179 Primary interaction Intranuclear cascade Nuclear remnant evaporation Pick 1 nucleon from initial geometry: e+p/n -> X+n All ep/en underlying processes are possible. Secondary interactions with the rest of the nucleon before flying outside h + N -> h ( * ) + N ( * ) h = pi/K/p/n, N=p/n Need only mass, charge, excitation energy, no memory for prior history Event generation process ++

Primary interaction Intranuclear cascade Nuclear remnant evaporation Stages of neutron production All final Cascade Evap ZDC cut Evaporated neutrons fully accepted, contaminations under control. 2014/4/1710 % in ZDC Primary0.2 Cascade14.64 Evap eAu 10 GeV x 100 GeV

Cascade neutron and geometry Intranuclear cascade 2014/4/1711 A correlation pattern observed in the intranuclear cascade neutron number and collision geometry. Longer traveling distance More chance for secondary collisions

2014/4/ Measure neutron number distribution with ZDC in a wide kinematics range. 2.In the nonsaturated regime, this measurement can be used as a handle for underlying collision geometry. 3.In the saturated regime, we can compare the neutron number distribution with that from the nonsaturated region to find if saturation exists. Strategies to make the neutron number distribution:

Summary Neutron number distribution from nucleus break up is sensitive to the underlying collision geometry. Possible applications in determining impact parameter for measurements like dihadron correlations and hadron attenuation. In addition, we propose to utilize this measurement as a saturation tagger. Assuming the saturated forward neutron distribution can be simulated by averaged iterations, saturation phenomena can be significantly discriminated by scanning through the kinematics regime. ZDC can be used to measure this neutron distribution efficiently with the systematics under control. 2014/4/1713

Back up 2014/4/1714

eAu 10 GeV x 100 GeV % 50-75% 25-50% 0-25% Counts A handle to the eA collision geometry 2014/4/1715

Sources of neutron production eAu Evap eAu NonEvap en ep Black: Evap+Cascade Red:Primary 2014/4/1716

Number of neutrons in etaNumber of neutrons in E Number of neutrons in p T eAu 10 GeVx100 GeV 0.01<y<0.95 1<Q 2 <20 GeV 2 FS (KS=1/-1) Evap (KS=-1) Cascade (KS=1) E>80 (KS=1) FS (KS=1/-1) Evap (KS=-1) Cascade (KS=1) FS (KS=1/-1) Evap (KS=-1) Cascade (KS=1) NoSec (KS=1) Two different mechanisms: 1.Cascade neutrons (wide energy spectrum) 2.Target remnant evaporation neutrons(narrow energy spectrum, mostly accepted by ZDC) 2014/4/1717

Number of neutrons in etaNumber of neutrons in E Number of neutrons in p T eCa 10 GeVx100 GeV 0.01<y<0.95 1<Q 2 <20 GeV 2 FS (KS=1/-1) Evap (KS=-1) Cascade (KS=1) E>80 (KS=1) FS (KS=1/-1) Evap (KS=-1) Cascade (KS=1) FS (KS=1/-1) Evap (KS=-1) Cascade (KS=1) NoSec (KS=1) Two different mechanisms: 1.Cascade neutrons (wide energy spectrum) 2.Target remnant evaporation neutrons(narrow energy spectrum, mostly accepted by ZDC) 2014/4/1718

The two bump structures in N n 2014/4/1719

Ca Cu Xe Au Pb Ca Cu Xe Au Pb R = 1.12*A 1/ *4.605 AAnAn R Ca Cu Xe Au Pb Red: Black:N n RMS n Ca Cu Xe Au Pb A depencence of neutron number distribution 2014/4/1720