1. Characterizing eA collision geometry with forward neutrons at an EIC
Liang Zheng @BNL & CCNU
On behalf of the BNL EIC science task force
2018/11/13
EIC user meeting 2014

2. Outline Motivations Collision geometry definition in DIS
Simulation framework
Constraint on the eA collision geometry
Applications in the measurements
2018/11/13
EIC user meeting 2014

3. Motivation
Semi-inclusive DIS (SIDIS) in eA program at EIC studies various nuclear effects dependent on the collision geometry
Initial state
Saturation
Final state
Cold nuclear medium energy loss
Performed usually by varying nuclear mass A
Additional handle to the geometry for the same nuclear type
2018/11/13
EIC user meeting 2014

4. Collision geometry definition in DIS
Anatomy of an eA collision
Deep inelastic scattering off a nucleon: primary interaction
Intra-nuclear cascade process: secondary interactions
Nuclear remnant breaks up depending on the excitation energy: evaporation
2018/11/13
EIC user meeting 2014

5. Collision geometry definition in DIS
b : impact parameter
d : projected traveling distance in medium
R : distance from involved nucleon to the center of nucleus
Nevap: number of particles (neutrons) from evaporation d
Nsec
Nevap
In medium traveling distance d can be depicted by Nevap (measurable)
2018/11/13
EIC user meeting 2014

6. eA collision geometry monitor
Why neutrons
Dominant products from evaporation
Purified by bending magnets
How to measure
Zero degree calorimeter (ZDC)
Cover most forward rapidity
η > 0
η < 0 A e-
Forward neutrons identified as a promising collision geometry monitor
Total
Evap
Secd
Prim
Total
Evap
Secd
Prim
ZDC
10%
2018/11/13
EIC user meeting 2014

7. Simulation framework
Event final state simulated with DPMJET eA generator tuned with E665 data
PHOJET -> primary interaction
HADRIN -> secondary interaction
FLUKA -> evaporation
ZDC response -> energy smearing
Phys. Rev. Lett. 74, 5198 (1995)
Coherent scattering effect in DPMJET
Shadowing in total cross section Glauber-Gribov formalism
Lessons from FNAL-E665
Size dependence
Very low neutron multiplicity
Input for parameter tuning
Set up a framework to simulate the generation of forward neutron and detector response
2018/11/13
EIC user meeting 2014

8. Measuring forward neutrons
ZDC parameter:
Acceptance: θ<4 mrad
Energy resolution:
Neutron energy resolution
2018/11/13
EIC user meeting 2014

9. Selecting traveling distance by forward neutrons
En range
<d>±RMS
<b>±RMS
66-100%
[0,237]
5.89±2.81
4.70±1.82
33-66%
[237,743]
7.51±2.77
4.40±1.73
0-33%
[743,4329]
9.70±2.67
3.78±1.61
Traveling distance well constrained in current scheme
2018/11/13
EIC user meeting 2014

10. Selecting impact parameter by forward neutrons
For initial state, impact parameter b is most relevant.
d>1.5R0
d<0.25R0
Impact parameter direction
From d to centrality
Peripheral: not so well controlled
Central: guaranteed by selecting the largest traveling distance d
Virtual photon direction
To find the most central collisions, one must go with the maximized d.
2018/11/13
EIC user meeting 2014

11. Selecting impact parameter with additional measurements in forward rapidity
Strong neutron emission
Large traveling distance
Nch forward
4<η<6
Forward neutron cut +
Double cut or
Even larger d?
Forward neutron cut +
Np in forward
2018/11/13
EIC user meeting 2014

12. Selecting impact parameter with additional measurements in forward rapidity
<b>±RMS
10% N
3.40±1.51
10% N+P
3.16±1.43
10% N+FwCh
3.15±1.39
5% N
3.24±1.46
5% N+P
2.98±1.39
5% N+FwCh
3.03±1.35
Double cut (require both at the same time):
Nn (ZDC) and Nch (4<η<6)
Nn (ZDC) and Np (ZDC)
Forward neutron cut is sufficient
2018/11/13
EIC user meeting 2014

13. Traveling distance dependent measurements: energy loss
Observable:
Nucl. Phys. B 780 (2007) 1
Formation length:
See talks by T. Ullrich & W. Brooks
2018/11/13
EIC user meeting 2014

14. Traveling distance dependent measurements: energy loss
<d>±RMS
66-100%
5.89±2.81
33-66%
7.51±2.77
0-33%
9.70±2.67
d > Lc
Formed inside the nucleus
d < Lc
Formed outside the nucleus
An extra dimension to analyze space-time feature of hadronization
Charge pion, Hermes kinematics and acceptance cuts
2018/11/13
EIC user meeting 2014

15. Impact parameter dependent measurements: dihadron correlations
See talks by Y. Kovchegov & T. Ullrich
“peripheral”
“central”
Is the b dependent effect observable in dihadron correlations?
2018/11/13
EIC user meeting 2014

16. Impact parameter dependent measurements: dihadron correlations
10 GeV x100 GeV
Q2 = 1, y = 0.7
z1=z2 = 0.3, pT1>2, 1< pT2<pT1
Glauber density
Minibias: c(b)eff=0.58
0-10%: c(b) eff=0.7
90-100%: c(b) eff=0.53
Central: larger Qs, stronger suppression
Peripheral: smaller Qs, less suppression
Observable effect in most central events for dihadron correlations
2018/11/13
EIC user meeting 2014

17. Summary Geometry definition of nuclear DIS Experimental observable
Where the interaction happens: traveling distance, impact parameter
Experimental observable
Forward neutrons correlated with geometry through the secondary collisions in the nucleus
ZDC can be used to measure the forward neutrons
Underlying geometry especially traveling distance can be well controlled
Utilizing forward neutron measurement can enhance our experimental control on the collision geometry in eA at EIC
2018/11/13
EIC user meeting 2014

18. Back up
2018/11/13
EIC user meeting 2014

19. Collision geometry definition in DIS
What is a good collision geometry definition
Delivers sensible physics message
Depicted by observable experimental measurement
Collision geometry in AA
Collision geometry in eA
Final state multiplicity
Npart, Ncoll, b
Hot medium effect
Initial gluon density ?
Cold medium energy loss
2018/11/13
EIC user meeting 2014

20. Collision geometry in lepton-nucleus
d : in medium traveling distance.
R : distance from involved nucleon to the center of nucleus.
b : impact parameter.
Nn: number of neutrons in forward region
2018/11/13
EIC user meeting 2014

21. Model description
We are using DPMJET3 package for the e+A event simulation
Nuclear geometry sampled according to Glauber-Gribov model.
Collision geometry
Elementary interaction treated by PHOJET.
Cascade process handled by HADRIN
Primary interaction
Target remnant evaporation and break up included by FLUKA.
Intranuclear cascade
Nuclear remnant evaporation
2018/11/13
EIC user meeting 2014

22. Slow nucleons and centrality categorization
pA /AA sensitive to number of binary collisions
eA sensitive to the number of intranuclear cascade collisions
2018/11/13
EIC user meeting 2014

23. Detector Requirement Charged particles bended away from beam direction
Neutrons shoot in the zero degree directly
2018/11/13
EIC user meeting 2014

24. Detector Requirement Fit with input from secondary number distributionMC
Black:fitted
Red:MC
2018/11/13
EIC user meeting 2014

25. Coherence in the shadow
Evap Nn distribution
Total
Npar=1
Npar=2
Npar=3
Npar=4
Npar=5
eAu 10x100 GeV
Npar=1 88.4%
10x100
100x1000
2018/11/13
EIC user meeting 2014

26. Slow nucleons and centrality categorization in pA
Eur. Phys. J. A8, 197 (2000)
F.Sikler, hep‐ph/
Slow particles
Initial reaction produces fast forward particles
Secondary interactions triggered by the forward particles knock free a number of nucleons
The nuclear system thermalize and nucleons evaporate
2018/11/13
EIC user meeting 2014

27. Slow nucleons and centrality categorization in pA
From fixed target to collider:
Large lorentz boost to the target.
1. Black tracks boosted to the most forward region
2. Gray tracks spread to a large rapidity range
Clean measurement to the black tracks using ZDC
2018/11/13
EIC user meeting 2014

28. Simulation tool
We are going to use DPMJET package for the event simulation
PHOJET
HADRIN
FLUKA
Phys. Rev. Lett. 74, 5198 (1995)
2018/11/13
EIC user meeting 2014

29. Dihadron correlations in central/peripheral (glauber density)
Minbias: c(b)eff=0.58
0-10%: c(b) eff=0.7
90-100%: c(b) eff=0.53
Peripheral
Minibias
Central
2018/11/13
EIC user meeting 2014