1. Zvi Citron Low-x Workshop, 31 May 2013 Probing Low-x With 2 Particle Correlations in d+Au Collisions at Zvi Citron בס"ד 1

2. Zvi Citron Low-x Workshop, 31 May 2013 Why d+Au at RHIC? Understanding nucleon structure in nuclei using deuteron+gold (d+Au) collisions –The initial state in Au+Au collisions –Cold nuclear effects –Understand gluon distribution at low x and high parton densities In 2003 (2.74 nb -1 ) and 2008 (80 nb -1 ) d+Au@200 GeV collisions recorded by PHENIX at RHIC Momentum Fraction Times parton density gluons quarks Momentum Fraction, x 2

3. Zvi Citron Low-x Workshop, 31 May 2013 PHENIX Detector at RHIC Muon Arms 1.2 < | η | < 2.4 J/Psi Unidentified charged hadrons Heavy Flavor Central Arms | η | < 0.35 Charged hadrons Neutral pions / η Direct Photon J/Psi Heavy Flavor Muon Piston Calorimeter 3.1 < | η | < 3.9 Neutral pions / η 3

4. Zvi Citron Low-x Workshop, 31 May 2013 4 4 4 PHENIX Muon Piston Calorimeter Small cylindrical holes in Muon Magnet Pistons, Radius 22.5 cm and Depth 43.1 cm d going(forward +y) Au going(backward -y) SOUTH NORTH PbWO 4 Mid-Fwd, x Au ~(0.008-0.040) Fwd-Fwd, x Au ~(0.0008-0.005) Au d 0s0s

5. Zvi Citron Low-x Workshop, 31 May 2013 Forward Hadron Suppression 5 Guzey, Strikman, Vogelsang, PL B603, 173 Forward Single Hadrons p T =2.5 2.5<  <3.5 Forward di-Hadrons Probe:  Narrower x-range  Smaller mean x 2

6. Zvi Citron Low-x Workshop, 31 May 2013 6 Mid-Forward Per-Trigger Correlations |  mid | < 0.35,  fwd = 3.0-3.8 Mid-Forward π 0 π 0 Correlations; Mid-rapidity triggered Central d+Au shows suppression Phys. Rev. Lett. 107, 12301 (2011)

7. Zvi Citron Low-x Workshop, 31 May 2013 7 Fwd-Fwd Per-Trigger Correlations  clus,  0 = 3.0-3.8 Forward rapidity Clusterπ 0 Correlations Use Zero-Yield at Minimum to subtract BG Central d+Au shows significant suppression Possible angular broadening in central d+Au % Phys. Rev. Lett. 107, 12301 (2011) Consistent with CGC based picture

8. Zvi Citron Low-x Workshop, 31 May 2013 8 Pair Nuclear Modification Factor: J dA We define the di-Hadron or “Pair Nuclear Modification factor” J dA Completely analogous to the Hadron Singles “Nuclear Modification factor” R dA One can showwhere For di-hadron studies, I dA has been used most frequently but may be confusing where trigger and associated have different single particle properities.

9. Zvi Citron Low-x Workshop, 31 May 2013 9 Pair Nuclear Modification Factor: J dA Significant Suppression! Increases with Lower p T More central collisions More forward particle selection Single particle R dA Phys. Rev. Lett. 107, 12301 (2011)

10. Zvi Citron Low-x Workshop, 31 May 2013 Estimate Parton Kinematics 10 The advantage of the pair measurement is that it constrains the kinematics. Estimate 2->2 parton kinematics ignoring fragmentation effects, i.e.  1 ~y 3,  2 ~y 4 (not bad…) p T = p T1 /z 1 = p T2 /z 2 : Set z=1 Calculate estimate as if hadrons=partons using bin averages: Plot J dAu vs. x frag variable Expect that x frag underestimates x Au But if ~constant then x frag will be roughly proportional to xdxd x Au y 4, p T y 3, p T xdxd x Au  1, p T1  2, p T2

11. Zvi Citron Low-x Workshop, 31 May 2013 x Au frag Dependence of J dAu 11 Results show systematic dependence of J dAu over large x Au range. No suppression for peripheral d+Au Suppression for central d+Au increases strongly with x Au Interpretation? Indicates very strong shadowing effect. CGC? 60-88% (Peripheral) Trig p T : 1.1-2.0 GeV/c Trig p T : 0.5-7 GeV/c Forward-ForwardMid-Forward 60-88% (Peripheral) Trig p T : 1.1-2.0 GeV/c Trig p T : 0.5-7 GeV/c 0-20% (Central) Note: points for mid-fwd JdA are offset for visual clarity Phys. Rev. Lett. 107, 12301 (2011)

12. Zvi Citron Low-x Workshop, 31 May 2013 d+Au Flow? Ridge? Inspired by observations of flow and ‘ridge’ like phenomena at the LHC in high multiplicity p+p and p+Pb d+Au data was analyzed for a similar signal 12

13. Zvi Citron Low-x Workshop, 31 May 2013 Mid-Mid Hadron Correlations From correlations of charged hadrons at mid-rapidity Pseudorapidity gap(|Δη| ) from 0.48 to 0.7 Subtract ZYAM shaped as constant + Gaussian| Δφ=0 + Gaussian| Δφ=2π Excess in central compared to peripheral can be described with cos2Δ ϕ 13 arXiv:1303.1794

14. Zvi Citron Low-x Workshop, 31 May 2013 Extract Flow Coefficient 14 Assume factorization –2 particle modulation is product of 1 particle anistropies Clear signal Systematic checks to control lingering jet contribution –Vary minimum |Δη| cut –Check same sign (fewer jets) and opposite sign (more jets) pairs Larger than ATLAS result –Be careful about differences between the measurements arXiv:1303.1794

15. Zvi Citron Low-x Workshop, 31 May 2013 Look For Bulk Modulation with Mid-Forward, Mid-Backward 15 Correlation between MPC towers (>3 GeV) and mid rapidity hadrons Rapidity separation reduces jet contamination Long range ‘near side’ structure seen for mid- Au going side correlations in central events Absent in peripheral and mid-d going side Qualitatively consistent with midrapidity results

16. Zvi Citron Low-x Workshop, 31 May 2013 Conclusions Suppression of 2 particle correlations at low x indicates strong shadowing/CGC phenomena Collective flow like properties observed in high multiplicity p+p (7 TeV) and p+Pb (5.02 TeV) extended to d+Au (0.2 TeV!) –Clear need to understand initial state More to come at PHENIX in the next few years –d+Au, 3 He+Au, p+Au, p+Si, p+Cu –MPC-EX upgrade 16

17. Zvi Citron Low-x Workshop, 31 May 2013 MPC-EX upgrade 17 Reconstruct prompt  and   to 80 GeV! Constrain gluon npdf at low x with    spin asymmetry ratios between (polarized) p+p and p+A sensitive to Q a s [Kang and Yuan Phys.Rev., D84:034019, 2011] Coming 2015!

18. Zvi Citron Low-x Workshop, 31 May 2013 Backups 18

19. Zvi Citron Low-x Workshop, 31 May 2013 Open heavy flavor rapidity dependence 19 Clear enhancement in Au-going direction sensitive to high-x in Au (Anti-shadowing regime) Suppression in d-going direction sensitive to low-x (shadowing)

20. Zvi Citron Low-x Workshop, 31 May 2013 2003 Results from Muon Arms 20 d d Au

21. Zvi Citron Low-x Workshop, 31 May 2013 2003 Results from Muon Arms 21 d d Au PRL 96 (2006) 222301 Trigger p T range 0-40% centrality 40-88% centrality I dA 0-40% centrality 40-88% centrality Ratio of d+Au width/pp width

22. Zvi Citron Low-x Workshop, 31 May 2013 EPS09s Mid-Rapidity Perhaps somewhat surprisingly, EPS09s + standard pQCD works well at mid- rapidity, even though other nuclear effects like Cronin are ignored. In any case, agreement is pretty good and Cronin is not too large (~10% effects) 22

23. Zvi Citron Low-x Workshop, 31 May 2013 EPS09s Forward Rapidity Same pQCD calculation for forward inclusive hadrons fails “Problem” with inclusion of Brahms charged pion data in EPS08… New physics has to come into play at forward rapidity? Why? 23

24. Zvi Citron Low-x Workshop, 31 May 2013 LHC mid-y, RHIC fwd-y, same x At LHC mid-rapidity (5 TeV), x T is 25 times lower than at RHIC for the same hadron p T LHC hadron pT = 2 GeV, y = 0, should reach same x as at forward y at RHIC, x ~ 10 -3 Why no suppression? 24

25. Zvi Citron Low-x Workshop, 31 May 2013 LHC mid-y, RHIC fwd-y, same x At LHC mid-rapidity (5 TeV), x T is 25 times lower than at RHIC for the same hadron p T LHC hadron pT = 2 GeV, y = 0, should reach same x as at forward y at RHIC, x ~ 10 -3 Why no suppression? 25

26. Zvi Citron Low-x Workshop, 31 May 2013 Eccentricity Comparison 26

27. Zvi Citron Low-x Workshop, 31 May 2013 Sign Selected Pairs Check 27 More jets – does have larger modulation Fewer jets – modulation remains significant

28. Zvi Citron Low-x Workshop, 31 May 2013 Correlation Functions 28