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Using evaporated neutron number distribution as a saturation signature tagger

EIC taskforce meeting

2014/4/17

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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.

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Nn range <Nn> <d>±RMS

75-100% [0,3] 2.10 7.09±2.69

50-75% [4,8] 6.35 7.92±2.50

25-50% [9,13] 11.42 9.34±2.50

0-25% [14,38] 18.42 11.17±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

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75-100%50-75%25-50%0-25%

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Neutron number distribution as a tagger for the saturation physics

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Nn ?

<Nn>

iterations

Fix geo config, impact b

Sample interaction collect Nn

1. Probe interacts coherently with all nucleons2. 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

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

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Averaged Non Averaged

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

RMS shown as the error bar in every bin

RMS shown as the error bar in every bin

<Nn>

<Nn>

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eAu 10 GeV x 100 GeV

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Black: 10<Q2<20Red: 1<Q2<2

eAu 10 GeV x 100 GeV

Kinematics dependence of neutron number distribution

Shape of neutron number distribution does not depend on the kinematics

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Red:Saturated

eAu 10x100 Averaged eAu 10x100 Non Averaged

1<Q2<2 1<Q2<2

Significant difference between the sat/nosat break up neutron distribution

Red: from a 50-50 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.

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Solid: NonAveragedDashed: Averaged

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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

+ +

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Primary interaction Intranuclear cascade Nuclear remnant evaporation

Stages of neutron production

All finalCascadeEvap

ZDC cut

Evaporated neutrons fully accepted, contaminations under control.

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% in ZDC

Primary 0.2

Cascade 14.64

Evap 85.16

+ +

eAu 10 GeV x 100 GeV

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Cascade neutron and geometry

Intranuclear cascade

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

Longer traveling distance

More chance for secondary collisions

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1. 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:

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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.

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Back up

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eAu 10 GeV x 100 GeV

75-100%50-75%25-50%0-25%

Counts

Counts

A handle to the eA collision geometry

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Sources of neutron production

eAu EvapeAu NonEvapenep

Black: Evap+CascadeRed:Primary

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Number of neutrons in eta Number of neutrons in E

Number of neutrons in pTeAu 10 GeVx100 GeV0.01<y<0.951<Q2<20 GeV2

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)

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Number of neutrons in eta Number of neutrons in E

Number of neutrons in pTeCa 10 GeVx100 GeV0.01<y<0.951<Q2<20 GeV2

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)

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The two bump structures in Nn

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CaCuXeAuPb

CaCuXeAuPb

R = 1.12*A1/3+0.545*4.605

A An R

Ca 40 20 6.34

Cu 64 35 6.99

Xe 131 77 8.12

Au 197 118 9.03

Pb 207 125 9.14

Red: <Nn>Black:Nn

RMS

n

Ca

Cu

Xe

AuPb

A depencence of neutron number distribution

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