Forward near forward azimuthal correlations in p+p and d+Au collisions

1. Forward near forward azimuthal correlations in p+p and d+Au collisions Xuan Li (Shandong Uni & BNL) Apr. 13th, 2012 Xuan Li

2. Outline • Introduction • Motivation • Forward near-forward correlations at STAR • Systematic studies • West ZDC neutron tag • Summary & Outlook Xuan Li

3. The soft gluon x is related to associated particle in correlations y = rapidity, λ = 0.3 • At fixed low Q2 (>Λ2), the gluon density increases rapidly as x decreases. The state transfers from dilute parton gas to Color Glass Condensate (CGC). PYTHIA simulation Iancu and Venugopalan, hep-ph/0303204 arXiv:hep-ex/0502040 Eur.Phys.J.C43:427-435,2005 xmin = xg Xuan Li

4. STAR Detectors Proton (Deuteron) Proton (Gold) • The schematics of STAR in RHIC run8. Forward Meson Spectrometer (FMS) with 2.5<η<4.0 East Beam Beam Counter (BBC) Endcap electro-magnetic calorimeter (EEMC) with 1.08<η<2.0 Barrel electro-magetic calorimeter (BEMC) with -1<η<1 Xuan Li

5. Rapidity dependence of azimuthal correlations • At fixed low Q2 (>Λ2), the gluon density increases rapidly as x decreases. FMS-FMS Correlation FMS-EEMC Correlation FMS-BEMC Correlation How sharp is the transition? Iancu and Venugopalan, hep-ph/0303204 ln(2GeV/c)2 • Nearly continuous EM system (spans -1<η<4) at STAR provides acceptance for azimuthal correlations at different pseudo-rapidity. Xuan Li

6. The event reconstruction in the EEMC • The event is reconstructed based on the energy deposition in the EEMC. E (GeV) One event of the energy deposition in the EEMC with FMS π0 trigger (pt>2.0GeV/c) in p+p collision at √s = 200GeV. • The π0 usually is the leading particle inside a jet measured in the EM calorimeter. • The initial gluon state is independent of the final fragmentation process. Jet-like clusters can be surrogates of fragment partons. Xuan Li

7. FMS (π0)-EEMC (jet-like cluster) correlations (Low pt) PT(FMS) > 2.0 GeV/c ; 1.0 GeV/c < PT(EEMC) < PT(FMS) y FMS π0 Δφ z y EEMC jet-like cluster • σpp = 0.7978 ± 0.012, σdAu= 0.8935± 0.0157 • bpp= 0.0008± 0.00001, bdAu= 0.0097± 0.00006 • To suppress underlying event contribution, we use 600MeV tower threshold for the EEMC and 0.4GeV/c2 as the lower mass limit for the reconstructed jet-like cluster. • σdAu-σpp = 0.096 ± 0.020. Significant broadening from p+p to d+Au collisions. x Xuan Li

8. FMS (π0)-EEMC (jet-like cluster) correlations (High pt) PT(FMS) > 2.5 GeV/c ; 1.5 GeV/c < PT(EEMC) < PT(FMS) y FMS π0 Δφ z y EEMC jet-like cluster • σpp = 0.7154 ± 0.014, σdAu= 0.8449± 0.0179 • bpp= 0.0007± 0.00002, bdAu= 0.0094± 0.00007 • To suppress underlying event contribution, we use 600MeV tower threshold for the EEMC and 0.4GeV/c2 as the lower mass limit for the reconstructed jet-like cluster. • σdAu-σpp = 0.1295 ± 0.0229. Significant broadening from p+p to d+Au collisions. x Xuan Li

9. Systematic uncertainties on the width differences • (I) Cone radius R • (II) EEMC tower energy threshold • (III) Pseudo-rapidity cuts for the EEMC jet-like cluster • (IV) Mass cut lower limit for the EEMC jet-like cluster Xuan Li

10. Take item III for example • We change the standard 1.1<η<1.9 to 1.3<η<1.7, the corrected coincidence probability is, • See the fitted parameters in next slide. Xuan Li

11. Take item III for example • Fitted parameters with 1.3<η<1.7 cuts. • The width differences with low pt selections are ∆(σdAu − σpp) = 0.1308 ± 0.0296 and the results with high pt selections are ∆(σdAu − σpp ) = 0.1428 ± 0.0333. Xuan Li

12. Systematic uncertainties • Consider the four sources on slide 9 • Based on the systematic studies shown above, the correlation width differences between p+p collisions and d+Au collisions for the coincidence probability of FMS π0 and EEMC jet-like cluster are with high pt cuts and with low pt cuts. Xuan Li

13. Theory predictions on the pedestal • From leading twist to double parton scattering. M. Strikman, W.Vogelsang Phys.Rev.D 83,034029, 2011 arXiv: hep-ph/1009.6123 • The contribution from (b) can be studied by comparing the pedestal (uncorrelated part) of the correlations in d+Au and p+Au collisions. • A deuteron beam facing neutron tag is used in d+Au collisions as a p+Au approach. Xuan Li

14. What has been done in FMS-FMS correlations • FMS-FMS π0-π0 correlations. dAu pp pAu ArXiv:1109.0649 • Multi-parton interactions appear to contribute to the pedestal in d+Au collisions but not p+Au collisions. • Other basic aspects of the azimuthal correlations appear to be unchanged between d+Au and p+Au collisions. Xuan Li

15. FMS-EEMC correlations in p+Au approach • The coincidence probability of azimuthal correlation. • PtFMS>2.0GeV/c and 1.0GeV/c<ptEEMC<2.0GeV/c (MEEMC>0.2GeV/c2) • The p+Au approach only impacts on the pedestal, the other qualities like the width and the integral of the correlation peak are analogous like in d+Au collisions. • The ratio defined as the correlation integral over the pedestal is comparable between this study and forward-forward analysis. Xuan Li

16. Summary • Forward studies at RHIC provide opportunities to explore the initial state of the proton and the nuclei. • The behavior of gluons is one fundamental question to understand the characteristics of the matter. • The rapidity dependences of the correlations present a smooth transition process from dilute parton gas to dense CGC state. Xuan Li

17. Outlook of nucleus gluon saturation study The final state π0s or jet-like clusters are complex objects that can include not only color interactions from initial states but also from final states. • A Electron Ion Collider (EIC)? • Go to lower x than fixed • target experiment. • DIS process is much cleaner • than the hadron-hadron • interaction. Xuan Li

18. Backup Xuan Li