1. Rapidity dependence of azimuthal correlations for pp and dAu Xuan Li (Shandong Uni. &BNL) For the STAR Collaboration WWND 2011 (Winter Park)

2. Outline Motivation – Background – STAR forward-mid rapidity, forward-forward rapidity correlations. New results in pseudo-rapidity interval between 1 and 2. – Cluster finder introduction. – π 0 candidates searching. – Preliminary correlation results. – Jet-like event correlations. Conclusions & Outlook Xuan Li2

3. Motivation The nucleon gluon density at low x. Xuan Li3 The nucleon gluon density is known in the 0.0001<x<0.3. Can’t increase indefinitely. Saturation? Fixed Target Experiments Rapid rise of the gluon density at low-x evident from  F 2 (x)/  lnQ 2 at fixed x (Prytz relation) Rizvi E. Talk presented at the “International Euro Physics Conference on High Energy Physics”, July 2003 arxiv:hep-ph/0201195 Proton gluon density lepton Nucleonγ* quark gluon

4. Gluon saturation is expected at low x Parton gas approach saturation through evolution. Xuan Li4 Parton saturation is expected at low x and low Q 2. At a given x, nuclei (mass number A) gluon density ≈ A 1/3 × nucleon gluon density, leading to the expectation Q s 2 ≈A 1/3 x β. [hep-ph/0304189] Current fixed target data provides 0.02<x<0.3 range for nuclear gluon density. How sharp is the transition?

5. How to access low x gluon Forward inclusive production. Xuan Li5 Large rapidity inclusive  production   ~4  probes asymmetric partonic collisions. Mostly high-x valence quark + low-x gluon. pp data is in agreement with perturbative QCD. Suppression of forward inclusive particle in dAu data is better described in Color Glass Condensate (CGC) predictions. Phys. Rev. Lett. 97.152302

6. How to access low x gluon Inclusive π 0 to correlated π 0 -π 0. Xuan Li6 Phys. Lett. B603 (2004) 173 From inclusive π 0 to π 0 -π 0 Forward π 0 -forward π 0 are more sensitive to low x gluon than inclusive production. Inclusive production measures integral of broad x range. If we measure π 0 -π 0, we can probe limited x range for gluon.

7. How to measure the sensitivity Effects on the azimuthal correlations from different parton distribution for 2->2. scattering. Xuan Li7 The away side peak height is correlated with the parton density distribution. arXiv:hep-ex/1005.2378 Near side peak Away side peak

8. Back to back correlations pQCD 2  2 process =back-to-back di-jet (Works well for p+p) With high gluon density, 2  1 (or 2  many) process = Mono- jet ? Xuan Li8 CGC predicts suppression of back-to-back correlation. Kharzeev, Levin, McLerran Nucl. Phys. A748 (2005) 627

9. Color Glass Condensate (CGC) prediction Test the phase boundary at fixed Q 2. Xuan Li9 Fix P T, look through different x region Fix P t for π 0 at forward rapidity, and vary the rapidity of the associated π 0. Vary the P t to study the boundary. K. Golec-Biernat, M. Wustoff, Phys. Rev. D 59, 014017

10. STAR Detectors The full view of STAR Xuan Li10 Forward Meson Spectrometer (FMS) Endcap electro-magnetic calorimeter (EEMC) Barrel electro-magetic calorimeter (BEMC) Proton (Gold) Proton (Deuteron) East Beam Beam Counter (BBC)

11. STAR Detectors The detectors of STAR used for correlations. Xuan Li11 Front view of north half of FMS FMS measuring range 2.3<η<4. Δ ϕ =0.058, Δη=0.1 for large cells. BEMC measuring range -1<η<1. Tower range Δ ϕ =0.05, Δη=0.05. EEMC measuring range 1<η<2 Tower range Δ ϕ =0.1, Δη=0.057-0.099 Nearly hermetic electro-magnetic calorimeters cover -1<η<4.

12. Forward-mid rapidity correlation Xuan Li12 FMS-BEMC(TPC) correlation Triggering on the forward rapidity π 0, the rapidity of the associated π 0 is correlated with the x bj of the soft parton involved in the partonic scattering. Probe nuclei gluon density at 0.008 < x < 0.07. PYTHIA simulation Arxiv:hep-ex/0502040

13. Forward-mid rapidity correlation FMS-BEMC(TPC) azimuthal correlation. Back to back azimuthal correlation peak looks similar in pp and dAu data. There is no dramatic broadening from pp to dAu. Forward-mid rapidity correlations are not near the saturation region. Xuan Li13 arXiv: hep-ex/0907.3473 P T (FMS)>2.5GeV/c 1.5GeV/c<P T (BEMC/TPC)<P T (FMS)

14. Forward-forward rapidity correlation Xuan Li14 FMS-FMS correlation Look at forward-forward correlation to access lowest x region. Probe gluon density at 0.0009 < x < 0.005. PYTHIA simulation

15. Forward-forward rapidity correlation FMS-FMS azimuthal correlation. Similarity of near side peak in pp and dAu data. There is significant broadening from pp to dAu in forward-forward rapidity azimuthal correlations in the away side peak. Xuan Li15 PP datadAu data arXiv:hep-ex/1005.2378

16. Forward-forward rapidity correlation Centrality cut on the dAu data. Xuan Li16 dAu centrality averaged dAu peripheral dAu central J.L. Albacete, C. Marquet arXiv:1005.4065 PP data (1)The suppression of the height of the away side peak in the central dAu collisions suggests forward-forward correlations at low x are consistent with gluon saturation in nuclei. (2) The degree of broadening is consistent with multi-parton scattering in central dAu collisions associated with saturation.

17. Forward-near forward rapidity correlation Xuan Li17 FMS-EEMC correlation Probe gluon density at intermediate region which is 0.003<x<0.02. PYTHIA simulation

18. FMS π 0 triggered event Within the FMS high tower triggered data, selecting events where FMS di-photon invariant mass is less than 0.2GeV/c 2 and P t is larger than 2.5 GeV/c. Xuan Li18 For example, the invariant mass of photon pair in the FMS. pp data

19. Introduction to cluster finder BEMC (EEMC) geometry EEMC η range [1.08,2.0], BEMC η range [-1,1]. Use minimum bias cluster finder instead of constrained cluster finder to reproduce FMS-BEMC correlations. Then apply the same method to approach FMS-EEMC correlations. Xuan Li19 X Y ϕ η EEMC Z X(Y) ϕ η BEMC

20. Event display Run 8 pp FMS triggered data. Cluster is energy threshold bounded group of towers. The threshold for the BEMC tower is 70MeV, and for EEMC is 100MeV. Sorting the tower energy, then add the towers near the high tower to construct cluster. Xuan Li20 EEMCBEMC cluster

21. BEMC invariant mass with di-cluster Xuan Li21 Take one cluster as photon candidate, then pair two clusters with energy ratio cuts. In addition to π 0 selection cuts, we apply P t cuts for example 1.5GeV/c<P t <2.5GeV/c. There are π 0 candidates in pp and dAu data. π 0 candidates

22. π 0 -π 0 azimuthal correlation with new cluster finder (FMS-BEMC) FMS photon pair P t > 2.5GeV/c and mass<0.2 GeV/c 2. BEMC di-cluster 1.5GeV/c < P t < 2.5GeV/c and mass<0.2GeV/c 2. Xuan Li22 FMS triggered pp dataFMS triggered dAu data Width 0.709± 0.019(stat.) Width 0.754 ± 0.024(stat.) The FMS-BEMC results with this cluster finder are consistent with preliminary results shown in arXiv: hep-ex/0907.3473. arXiv: hep-ex/0907.3473

23. New Rapidity (1<η<2) EEMC di-cluster invariant mass Take one cluster as photon candidate, then pair two clusters with energy ratio cuts. In addition to π 0 selection cuts, we apply P t cuts for example 1.5GeV/c<P t <2.5GeV/c. Xuan Li23 There are π 0 candidates in pp and dAu data. π 0 candidates

24. π 0 -π 0 azimuthal correlation (FMS-EEMC) FMS photon pair P t > 2.5GeV/c and mass<0.2 GeV/c 2. EEMC di-cluster 1.5GeV/c < P t < 2.5GeV/c and mass<0.2GeV/c 2. Xuan Li24 FMS triggered pp data FMS triggered dAu data Width 0.897 ± 0.060(stat.) Width 0.967 ± 0.120(stat.) The π 0 in EEMC require 2 clusters.

25. π 0 -π 0 azimuthal correlation (FMS-EEMC) with lower pt cuts FMS photon pair P t > 2.0GeV/c and mass<0.2 GeV/c 2. EEMC di-cluster 1.0GeV/c < P t < 2.0GeV/c and mass<0.2GeV/c 2. Xuan Li25 FMS triggered pp data(10%)FMS triggered dAu data(10%) Width 0.833 ± 0.048(stat.) Width 1.032 ± 0.179(stat.)

26. Initial look at Jet-like events (super-cluster) in BEMC How to get jet-like event – Use single cluster as seed, then find a cone with R ( )<0.5. – Require mass of ‘jet’ >0.2GeV/c 2 to emphasize ‘jetty events’. – Distance between jet center and seed less than 3cm to reduce bias effects. The mass of the jet-like event with 1.5GeV/c<P t <2.5GeV/c. Xuan Li26 pp dAu

27. π 0 +jet-like azimuthal correlation (FMS-BEMC) FMS photon pair P t > 2.5GeV/c. BEMC jet-like candidate 1.5GeV/c < P t < 2.5GeV/c. Xuan Li27 FMS triggered pp dataFMS triggered dAu data No significant broadening in FMS-BEMC π 0 +jet-like correlations. Width 0.761 ± 0.061(stat.)Width 0.832 ± 0.008(stat.)

28. Initial look at Jet-like events (super-cluster) in EEMC How to get jet-like event – Use single cluster as seed, then find a cone with R ( )<0.5. – Require mass of ‘jet’ >0.2GeV/c 2 to emphasize ‘jetty events’. – Distance between jet center and seed less than 3cm to reduce bias effects. The mass of the jet-like event with 1.5GeV/c<P t <2.5GeV/c. Xuan Li28 ppdAu

29. π 0 +jet-like azimuthal correlation (FMS-EEMC) FMS photon pair P t > 2.5GeV/c. EEMC jet-like candidate 1.5GeV/c < P t < 2.5GeV/c. Xuan Li29 Significant broadening in FMS-EEMC π 0 +jet-like correlations. FMS triggered pp dataFMS triggered dAu data Width 0.751 ± 0.042(stat.) Width 0.903 ± 0.014(stat.)

30. Conclusions There are π 0 and jet-like candidates in the EEMC tower clusters. There are hints of broadening in the away side peak for FMS-EEMC π 0 +jet-like azimuthal correlation. The transition from parton gas to CGC saturation state is not sharp. Xuan Li30 Outlook To add ESMD information for π 0 reconstruction. Use self consistent jet finder for jet-like events. Get corrected normalized azimuthal correlation.

31. Introduction to EEMC shower maximum detector The SMD helps distinguish photon from charged hadrons. EM shower Pre-shower1 Pre-shower2 Tower Post-shower Shower Maximum Detector ( SMD ) The electro-magnetic shower passing through EEMC Front view of SMD U/V plane

32. Event display of π 0 in the EEMC EEMC has shower maximum detector (ESMD). Xuan Li32 Photon 1 EEMC tower E (GeV)ESMD strips E (MeV) Photon 2 SMD will help define the photon position and the energy sharing between two photons.

33. Backup Xuan Li33

34. Current measured nuclei gluon density at low x. Xuan Li34 Current fixed target data provide 0.02<x<0.3 range for nuclei gluon density [Phys. Rev. C70 (2004)044905].

35. The nuclei gluon density prediction Transverse density of nuclei definition. At a given x, nuclei (mass number A) gluon density ≈ A 1/3 × nucleon gluon density, leading to the expectation Q s 2 ≈A 1/3 x β. Current fixed target data provides 0.02<x<0.3 range for nuclear gluon density. Xuan Li35 Transverse density of light nuclei The distribution of nucleons in the nucleus, The transverse distribution is defined as [hep-ph/0304189]

36. Partonic scattering for forward π 0 production Xuan Li36 <z><z> <xq><xq> <xg><xg> NLO pQCD Jaeger,Stratmann,Vogelsang,Kretzer

37. FMS coverage Xuan Li37

38. STAR Detectors In η, ϕ space Xuan Li38 Nearly hermetic electro-magnetic calorimeter cover -1<η<4.

39. Forward-forward rapidity correlation Centrality determination in dAu. Multiplicity in dAu measured by the east beam beam counter (BBC) at STAR reflects the centrality. Xuan Li39 PP data Peripheral Central dAu data arXiv:hep-ex/1005.2378 The impact parameter is related with the charge sum in the east BBC by a model. East BBC charge sum Average impact parameter (fm) 0 - 5006.8 ± 1.7 2000 - 40002.7 ± 1.3

40. Centrality cuts in dAu Xuan Li40

41. Motivation Xuan Li41 FMS-FMS correlation FMS-EEMC correlation FMS-BEMC(TPC) correlation Triggering on the forward rapidity π 0, the rapidity of the associated π 0 is correlated with the soft parton involved in the partonic scattering. Provide direct sensitivity to nuclei gluon density at 0.001< x < 0.02.

42. Energy threshold studies For example, BEMC tower energy deposited (GeV) in pp data. Xuan Li42 With energy threshold 35MeV With energy threshold 70MeV Energy threshold is selected to suppress noise.

43. Event display Run 8 dAu fms triggered data. Xuan Li43 EEMCBEMC

44. BEMC dAu number of raw tower hits P 6/27/201544 MB dataMB simulation

45. Cluster width definition BEMC Unfold the barrel, and put in the R ϕ, Z plane. Xuan Li45 Z X(Y) R ϕ Z RϕRϕ Large width Small width

46. Cluster width definition EEMC In xy plane. Xuan Li46 X Y Large width Small width X Y

47. Simulated π 0 decay kinematics Projection on the EEMC, with the π 0 P t in [1.25GeV/c, 2.5GeV/c] and Zγγ<0.7 cuts. Xuan Li47 Dγγ VS η of π 0 Most of the π 0 events are in EEMC single clusters. For FMS π 0 events, Cuts on the single cluster is (1)1.25GeV/c<P t <2.5GeV/c (2)Zγγ < 0.7

48. dAu FMS triggered data FMS di-photon invariant mass. Xuan Li48 With FMS photon pair which has mass less than 0.2GeV/c 2 and P t larger than 2.5 GeV/c.

49. π 0 jet-like azimuthal correlation (FMS-BEMC) FMS photon pair P t > 2.5GeV/c. BEMC jet-like candidate 1.5GeV/c < P t < 2.5GeV/c and mass<0.3GeV/c 2. Xuan Li49 FMS triggered pp data (60%)FMS triggered dAu data(30%) Width 0.722 ± 0.048Width 0.728 ± 0.028 No significant broadening in FMS-BEMC π 0 jet-like correlations.

50. BEMC efficiency studies Xuan Li50 Ermes Braidot thesis Fig 5.18 π 0 efficiency in the BEMC