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

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

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) R dAu in 2 forward rapidity Bins Guzey, Strikman, Vogelsang, PLB603, 173

Di-hadron Measurement  Peripheral d+Au Correlation Function CORRELATED N pair “Di-Hadron Nuclear Modification factor” Possible indicators of nuclear effects J dA < 1 Angular decorrelation of widths Notes: 1. Low p T (but back-to-back peak is selected so possibly clean hard signal, and low pT is desired if one wants to cross over into Q s regime) 2.Pedestal Determination (Assumed up to twice the width as a systematic). 3.Di-Hadrons instead of di-jets (but ok if fragmentation unmodified) Underlying event

 0 (trigger,central)/  0 (associate,forward) p+p d+Au 0-20% d+Au 60-88% p T t,  0 p T a,  0 mid-fwd NO SIGN OF RIDGE

Large Suppression in Central d+Au b=0-100% Q 2 = 4 GeV 2 x Au EPS09 NLO gluons Eskola, Paukkunen, Salgado, JHP04 (2009)065 Low x, mostly gluons  J dA  R G Au High x, mostly quarks Weak effects expected

Counting Nucleons in Path dAu b nucleon 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). “wee partons” overlap? Centrality 60-88% 40-60% 20-40% 0-20%

Centrality, or b Dependence If we are measuring gluons w/ J dA, then we can perhaps extract impact parameter and x dep of Qs, and possibly extract the value of Qs at RHIC? Since  N coll  ~L~A 1/3 ~T A 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???? xfrag ~ 1.6x10 -2 xfrag ~ 5x10 -3 xfrag ~ 5x10 -4 b dependent:

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.

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

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)

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?

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 ~ Why no suppression?

Wherefore forward rapidity? Au b nucleon Au b nucleon 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. Lab frameNucleus frame L/  ~ 0.1 fm x mid-rapidity x fwd-rapidity

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

CGC Approaches Lappi and Mantsaari, arxiv: Stasto, Xiao, Yuan [arxiv: ] 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. Hybrid rcBK Approach

Summary There seem to be some interesting effects in the Au nucleus at x of about 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 (E loss, 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 p T, and therefore Q s 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

Backup Slides

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

JdA Centrality Dependence Fit using EPS09 parametric function: Evaluate J dA in 3 bins at  x frag  = 5x10 -4, 5x10 -3, 1.6x10 -2

CNM effects: dynamical shadowing, Energy Loss, Cronin R dA Past, di-Hadron Future Kharzeev, NPA 748, 727 (2005) 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) Kharzeev, Levin, McLerran Nucl. Phys. A748 (2005) 627 Color Glass Condensate (Qiu, Vitev PLB632:507,2006)

di-Hadron Signal  Peripheral d+Au Correlation Function CORRELATED N pair “Di-Hadron Nuclear Modification factor” Possible indicators of nuclear effects J dA < 1, R dA < 1 Angular decorrelation of widths “Sgl-Hadron Nuclear Modification factor” “Conditional Yield” Number of di-jet particle pairs per trigger particle after corrections for efficiencies, combinatoric background, and subtracting off pedestal Caveats: 1. Low p T (but back-to-back peak is selected) 2.Pedestal Determination (Assumed up to twice the width as a systematic). 3.Di-Hadrons instead of di-jets (but ok if fragmentation unmodified)