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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Back up 2018/11/13 EIC user meeting 2014

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

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

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

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

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

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

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

Slow nucleons and centrality categorization in pA Eur. Phys. J. A8, 197 (2000) F.Sikler, hep‐ph/0304065 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

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

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

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