(di)-Hadron Production in d+Au Collisions at RHIC

1. (di)-Hadron Production in d+Au Collisions at RHIC Mickey Chiu

2. PHENIX SOUTH MPC NORTH MPC d(forward) Au(backward) • Fwd-Fwd, x~(0.001,0.005) • Mid-Fwd, x~(0.008,0.040) • Mid-Bwd, x~(0.050,0.100) Span rapidity, constrain x regions 2

3. RdAu in 2 forward rapidity Bins Guzey, Strikman, Vogelsang, PL B603, 173 • Large suppression in RdA • That increases with centrality • And increases with larger rapidity • Consistent with previous measurements • However, x covered by single inclusive measurement is over wide range • Includes shadowing, anti-shadowing, (EMC effect) Guzey, Strikman, Vogelsang, PLB603, 173

4. Di-hadron Measurement Peripheral d+Au Correlation Function CORRELATED Npair Underlying event Df “Di-Hadron Nuclear Modification factor” • Notes: • 1. Low pT (but back-to-back peak is selected so possibly clean hard signal, and low pT is desired if one wants to cross over into Qs regime) • Pedestal Determination (Assumed up to twice the width as a systematic). • Di-Hadrons instead of di-jets (but ok if fragmentation unmodified) • Possible indicators of nuclear effects • JdA < 1 • Angular decorrelation of widths

5. p0 (trigger,central)/p0 (associate,forward) p+p d+Au 60-88% NO SIGN OF RIDGE d+Au 0-20% pTt, p0 pTa, p0 mid-fwd

6.  JdA  RGAu Low x, mostly gluons Large Suppression in Central d+Au Eskola , Paukkunen, Salgado, JHP04 (2009)065 EPS09 NLO gluons b=0-100% Q2 = 4 GeV2 xAu High x, mostly quarks Weak effects expected

7. Counting Nucleons in Path d Au Centrality 60-88% 40-60% 20-40% 0-20% bnucleon bnucleon “wee partons” overlap? From Glauber Monte Carlo we can determine the number of nucleons in the path of each nucleon in the deuteron, and correlate that with some measurement in our detector that is correlated to centrality (South BBC, Au-going side).

8. b dependent: Centrality, or b Dependence xfrag ~ 1.6x10-2 xfrag ~ 5x10-3 xfrag ~ 5x10-4 • If we are measuring gluons w/ JdA, then we can perhaps extract impact parameter and x dep of Qs, and possibly extract the value of Qs at RHIC? • Since Ncoll~L~A1/3 ~TA we might be able to understand how gluons recombine with N nucleons? • eg, from above data are we seeing an approx linear dependence on length????

9. Impact Parameter Dependent pdf’s • New impact parameter dependent PDF’s where • N=1 in EPS09 (pdf’s are linearly suppressed with T), N=4 in EPS09s.

10. EPS09s and Pythia Calculation • Using PYTHIA and EPS09s one can extract the JdA expected from nuclear shadowing, and thus extract pdf’s at low x. • EPS09s seems to be a little above the data • Additional suppression of pdf’s in most central collisions

11. 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)

12. 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?

13. LHC mid-y, RHIC fwd-y, same x • At LHC mid-rapidity (5 TeV), xT is 25 times lower than at RHIC for the same hadron pT • LHC hadron pT = 2 GeV, y = 0, should reach same x as at forward y at RHIC, x ~ 10-3 • Why no suppression?

14. fwd-rapidity x x mid-rapidity Wherefore forward rapidity? Au Au Lab frame Nucleus frame bnucleon bnucleon L/ ~ 0.1 fm • Must look at parton rapidity… • Particles at mid-rapidity come from partons of moderate x, while forward particles come from high x • Forward rapidity partons have stronger “coherence” effects due to bigger boost.

15. “pQCD” Approach Kang, Vitev, Xing [arxiv:1112.6021] • Perturbative approach incorporates ISI and FSI for momentum imbalance (multiple scattering broadening), plus energy loss and coherent power corrections

16. CGC Approaches Lappi and Mantsaari, arxiv:1209.2853 Stasto, Xiao, Yuan [arxiv:1109.1817] Hybrid rcBK Approach • Another way the “coherence” effects can manifest itself at forward rapidities is in the Color Glass Condensate • Merger of gluons competing with splitting of gluons, enhanced at large rapidity. • Much work being done and formalism being worked out.

17. Summary • There seem to be some interesting effects in the Au nucleus at x of about 10-3 • Rapidity dependence is very important • Larger “coherence” effects at higher rapidities, since one selects higher rapidity partons • “Coherence” = gluon saturation? Or something else? • Also possibly other explanations (Eloss, eg, rapidity shift) • Single Inclusive vs Di-Hadron • Di-Hadron seems superior • Better control of parton kinematics in di-hadron • Better control of backgrounds • Ability to probe down to lower pT, and therefore Qs • Important: Impact Parameter Dependence starting to be probed • Nuclear thickness dependence crucial • LHC p+A already provides interesting results that one can then test against ideas from what we know already at RHIC

18. Backup Slides

19. MPC Performance Jet1 Jet2 “Trigger” Near North MPC Far Decay photon impact positions for lowand high energy p0s. The decay photons from highenergy p0s merge into a single cluster Sometimes use (EM) clusters, but always corrected to 0 energy Clusters  80% 0 (PYTHIA)

20. RdA Past, di-Hadron Future CNM effects: dynamical shadowing, Energy Loss, Cronin Color Glass Condensate Kharzeev, NPA 748, 727 (2005) (Qiu, Vitev PLB632:507,2006) Kharzeev, Levin, McLerran  Nucl. Phys. A748 (2005) 627 • Di-Hadron Correlations allow one to select out the di-jet from the underlying event • Constrains x range (probe one region at a time) • Probe predicted angular decorrelation of di-jets (width broadening)

21. di-Hadron Signal Peripheral d+Au Correlation Function “ConditionalYield” • Number of di-jet particle pairsper trigger particle after corrections for efficiencies, combinatoric background, and subtracting off pedestal CORRELATED Npair Df “Di-Hadron Nuclear Modification factor” “Sgl-Hadron Nuclear Modification factor” • Possible indicators of nuclear effects • JdA < 1, RdA < 1 • Angular decorrelation of widths • Caveats: • 1. Low pT (but back-to-back peak is selected) • Pedestal Determination (Assumed up to twice the width as a systematic). • Di-Hadrons instead of di-jets (but ok if fragmentation unmodified)