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Using evaporated neutron number distribution as a saturation signature tagger

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### Using evaporated neutron number distribution as a saturation signature tagger

EIC taskforce meeting

We found the correlation between number of forward neutron production and the traveling distance after collision in the nuclear.

This correlation can be utilized to characterize eA collision geometry.

By binning in produced forward neutron number, underlying traveling distance can be largely constrained.

Neutron number handle constrains the collision geometries

Counts

75-100%

50-75%

25-50%

0-25%

Collisition geometry variable d has been effectively constrained by the neutron number handle from nuclei break up

How does the nuclei break up in the saturated case?

Nn ?

Saturated:

1. Probe interacts coherentlywith all nucleons

2. No collision geometry sensitivity in z direction!

Assumed to be the same as averaged configurations

Fix geo config, impact b

Averaged:

iterations

Sample interaction collect Nn

All the following simulations based on evaporated neutrons from DPMJET + FLUKA for eAu collisions

<Nn>

The averaged (saturated) vs non averaged (non saturated)

eAu 10 GeV x 100 GeV

Averaged

Non Averaged

<Nn>

<Nn>

RMS shown as the error bar in every bin

RMS shown as the error bar in every bin

Kinematics dependence of neutron number distribution

eAu 10 GeV x 100 GeV

Shape of neutron number distribution does not depend on the kinematics

Black: 10<Q2<20

Red: 1<Q2<2

Significant difference between the sat/nosat break up neutron distribution

eAu 10x100 Non Averaged

eAu 10x100 Averaged

1<Q2<2

1<Q2<2

Red:Saturated

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.

Red: from a 50-50 mixture of Averaged/Non Averaged distributions

Solid: NonAveraged

Dashed: Averaged

+

+

Nuclear remnant evaporation

Intranuclear cascade

Primary interaction

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

Pick 1 nucleon from initial geometry:

e+p/n -> X+n

All ep/en underlying processes are possible.

+

+

Nuclear remnant evaporation

Intranuclear cascade

Primary interaction

Evaporated neutrons fully accepted, contaminations under control.

eAu 10 GeV x 100 GeV

All final

Cascade

Evap

ZDC cut

A correlation pattern observed in the intranuclear cascade neutron number and collision geometry.

Intranuclear cascade

Longer traveling distance

More chance for secondary collisions

Strategies to make the neutron number distribution:

Measure neutron number distribution with ZDC in a wide kinematics range.

In the nonsaturated regime, this measurement can be used as a handle for underlying collision geometry.

In the saturated regime, we can compare the neutron number distribution with that from the nonsaturated region to find if saturation exists.

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.

0.01<y<0.95

1<Q2<20 GeV2

Number of neutrons in pT

FS (KS=1/-1)

Evap (KS=-1)

Cascade (KS=1)

NoSec (KS=1)

Two different mechanisms:

Cascade neutrons (wide energy spectrum)

Target remnant evaporation neutrons(narrow energy spectrum, mostly accepted by ZDC)

Number of neutrons in eta

Number of neutrons in E

FS (KS=1/-1)

Evap (KS=-1)

Cascade (KS=1)

E>80 (KS=1)

FS (KS=1/-1)

Evap (KS=-1)

Cascade (KS=1)

0.01<y<0.95

1<Q2<20 GeV2

Number of neutrons in pT

FS (KS=1/-1)

Evap (KS=-1)

Cascade (KS=1)

NoSec (KS=1)

Two different mechanisms:

Cascade neutrons (wide energy spectrum)

Target remnant evaporation neutrons(narrow energy spectrum, mostly accepted by ZDC)

Number of neutrons in eta

Number of neutrons in E

FS (KS=1/-1)

Evap (KS=-1)

Cascade (KS=1)

E>80 (KS=1)

FS (KS=1/-1)

Evap (KS=-1)

Cascade (KS=1)

A depencence of neutron number distribution

R = 1.12*A1/3+0.545*4.605

Red: <Nn>

Black:NnRMS

Pb

Au

Xe

Cu

Ca

n

Ca

Cu

Xe

Au

Pb

Ca

Cu

Xe

Au

Pb

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