1. Xiaoxuan Chu 1 Brookhaven National Lab & Central China Normal University For the BNL EIC science task force Studying quasi-real photon structure at Electron-Ion-Collider

2. Outline Xiaoxuan Chu 2  Background and motivation  Photon structure at EIC  Di-jet & Di-hadron method  Test if simulation results match HERA data  Separation of direct/resolved process  Reconstruct x γ  Preliminarily analysis of particles inside di-jet  Summary

3. Background Xiaoxuan Chu 3  In ep collision: the exchanged physical photon can be approximated as a superposition of the bare photon state and the hadronic photon state.  The internal structure of photons is a manifestation of quantum fluctuations. Photon split into parton content

4. Xiaoxuan Chu 4  Photon Parton Distribution Functions (PDFs).  Density of partons  With large uncertainties x γ is defined as the momentum fraction of the parton from photon. Motivation Unpolarized photon structure: arXiv:9504004, arXiv:9710018, Eur. Phys. J. C 10, 363{372 (1999), DESY 97-164 Polarized photon structure: (critical input for ILC  option) no data theory: Z. Phys. C 74, 641—650 (1997) and arXiv:971125

5. Xiaoxuan Chu 5 Motivation Q 2 =Q 2 eff Question: does the kink occur at the same Q 2 for eA and for polarised ep Photon structure as fct. of Q 2

6. Motivation Incredible precision for all polarized and unpolarized ep & eA structure functions can determine the kink with high precision but there is more Xiaoxuan Chu 6

7. 7  Di-hadron method  Two hadrons with highest p T.  Di-jet method  Two jets with highest p T. “resolved process” category  hadronic photon.  x γ is small than 1.  Di-jet/di-hadron production. Similar with pp collision  point-like photon(no substructure).  x γ is close to 1. LODIS: NO di-jet produced PGF: Di-jet produced “direct process” category Di-jet/di-hadron method

8. 27GeV×820GeV 0.2<y<0.83 E jet1 T, E jet2 T >7.5 GeV E jet1 T +E jet2 T >20GeV |E jet1 T - E jet2 T |/(E jet1 T +E jet2 T )< 0.25 |Δη jets |<1 0<η jet1 +η jet2 <4 Xiaoxuan Chu 8  Reconstructing x γ provides a good way to separate direct/resolved contribution.  Parton densities in the photon can be extracted by measuring di-jet cross section.  Our simulation can match the existing data perfectly. Photon PDF test (dijet method) Eur.Phys.J. 1998C1:97-107

9. Xiaoxuan Chu 9 Correlation between reconstructed x rec γ and the true x used in the simulation indicates the di-jet observation is an excellent tool. Photon PDF test (dijet method)

10. Photon structure at EIC ParameterSet Ee20 GeV Ep250 GeV Q2Q2 10 -9 － 1 x 10 -9 － 0.99 Proton PDF setCTEQ5 N evt (million)25 σ (microbarn)54.7 L int (pb -1 )0.457 Data Generation: Xiaoxuan Chu 10 CTEQ5 shows the best description of cross section at low Q 2

11. Photon structure at EIC Di-jet produced: Resolved photon: 10<process<69 Direct photon: 130<process<137 Separate the two types! Xiaoxuan Chu 11 Resolved 11 qq – q qbar 12 q qbar – q qbar 13 q qbar – gg 28 gq(qg) – gq(qg) 53 gg – q qbar 68 gg - gg 99 γ q – q 131 γ T q –qg 132 γ L q –qg 135 γ T g –q qbar 136 γ L g –q qbar Direct Soft VMD 91 - 95

12. Di-hadron: Collection: 1.Two highest p T 2.π/K/p 3.Back to back 4.p trig T >2GeV, p asso T >1GeV Di-jet: Collection: 1.Two highest p T 2.Each particle p T >250MeV in jet 3.Back to back 4.p trig T >5GeV, p asso T >4.5GeV Xiaoxuan Chu 12 Photon structure study at EIC

13. η LAB Separation For both methods: - At positive η LAB, especailly η LAB >2, the cross section is dominated by resolved process. Xiaoxuan Chu 13 η distribution of associate hadron/jet shows the same results Di-hadron methodDi-jet method

14. Reconstructing x γ measurement Xiaoxuan Chu 14 Di-hadron methodDi-jet method  Both di-hadron and di-jet methods can help us separate resolved/direct process.  Di-jet method provides a better way to reconstruct x γ.

15. Di-jet method shows better separation of resolved and direct photon. Xiaoxuan Chu 15 x γ separation Di-hadron method Di-jet method If we choose different x rec γ cut, how well can we separate resolved/direct processes:  small x γ : mainly resolved contribution  large x γ : mainly direct contribution. Di-jet Di-hadron

16. Dijet cross section Xiaoxuan Chu 16  The simulation shows the capability to measure the cross section for di-jet production, with high accuracy in a wide kinematic range at EIC and extract the photon PDFs from a global fit. Probing Q 2 range as low as 10 -4

17. Xiaoxuan Chu 17 Preliminarily analysis of particles inside di-jet beamparton tgtparton remnant final partons  Choose the particle with highest p T in trigger jet(associate jet).  leading particle  Trace back the parent particle of leading till it is a parton.  final parton  Trace back the parent parton of the final parton.  initial parton leading particle fragmentation first parton parent initial partons

18. Xiaoxuan Chu 18 Preliminarily analysis of particles inside di-jet Pseudo-Rapidity distribution of trigger jet from photon/proton side:  Lepton beam polarisation  Redo everything for polarised photons  polarized photon PDFs  No data at all  critical for  option of ILC

19. Summary  In resolved process, photon has hadronic structure.  Di-jet produced in resolved and direct process can be separated at EIC.  Photon PDFs can be extracted in resolved process by reconstructing x γ.  x rec γ is exactly correlated with true x γ.  Di-jet cross section can be measured at EIC.  Photon PDFs can be extracted by analyzing initial and final parton related to the di-jet.  Jets from photon side and proton side are different. Xiaoxuan Chu 19

20. Back up Photon PDF set DO-G LO LAC-G/GAL-G LO GS-G LO GS-G-96 LO GRV-G/GRS-G LO ACFGP/AFG-G NLO WHIT-G LO SAS-G(v1/v2) LO Xiaoxuan Chu 20

21. Why jets come from remnant photon: Remnant photon Q 2 Outgoing electron ηOutgoing electron energy Remnant photon energy Back up Xiaoxuan Chu 21

22. Xiaoxuan Chu 22 Back up Final parton of trigger jet: Hard scattering Photon remnant Proton remnant