diHadron Production in dAu Collisions at RHIC Mickey

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

PHENIX d(forward) Au(backward) 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) Span rapidity, constrain x regions 2

Rd. Au in 2 forward rapidity Bins Guzey, Strikman, Vogelsang, PL B 603, 173 • Large suppression in Rd. A • 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, antishadowing, (EMC effect) Guzey, Strikman, Vogelsang, PLB 603, 173

Di-hadron Measurement Peripheral d+Au Correlation Function CORRELATED Npair Underlying event Df “Di-Hadron Nuclear Modification factor” Notes: Low p. T (but back-to-backofpeak is selected so possibly clean hard signal, and • 1. Possible indicators nuclear effects low p. T is desired if one wants to cross over into Qs regime) • Jd. A < 1 Determination (Assumed up to twice the width as a systematic). 2. Pedestal Angularinstead decorrelation ofok widths 3. • Di-Hadrons of di-jets (but if fragmentation unmodified)

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

Large Suppression in Central d+Au Eskola , Paukkunen, Salgado, JHP 04 (2009)065 EPS 09 NLO gluons b=0 -100% Q 2 = 4 Ge. V 2 x. Au High x, mostly quarks Weak effects expected Low x, mostly gluons Jd. A RGAu

Counting Nucleons in Path d Au bnucleon Centrality 60 -88% 40 -60% 20 -40% 0 -20% 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).

Centrality, or b Dependence xfrag ~ 1. 6 x 10 -2 xfrag ~ 5 x 10 -3 xfrag ~ 5 x 10 -4 b dependent: • If we are measuring gluons w/ Jd. A, then we can perhaps extract impact parameter and x dep of Qs, and possibly extract the value of Qs at RHIC? • Since Ncoll ~L~A 1/3 ~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? ?

Impact Parameter Dependent pdf’s • New impact parameter dependent PDF’s where • N=1 in EPS 09 (pdf’s are linearly suppressed with T), N=4 in EPS 09 s.

EPS 09 s and Pythia Calculation • Using PYTHIA and EPS 09 s one can extract the Jd. A expected from nuclear shadowing, and thus extract pdf’s at low x. • EPS 09 s seems to be a little above the data • Additional suppression of pdf’s in most central collisions

EPS 09 s Mid-Rapidity • Perhaps somewhat surprisingly, EPS 09 s + standard p. QCD works well at midrapidity, even though other nuclear effects like Cronin are ignored. • In any case, agreement is pretty good and Cronin is not too large (~10% effects)

EPS 09 s Forward Rapidity • Same p. QCD calculation forward inclusive hadrons fails • “Problem” with inclusion of Brahms charged pion data in EPS 08… • New physics has to come into play at forward rapidity? Why?

LHC mid-y, RHIC fwd-y, same x • At LHC mid-rapidity (5 Te. V), x. T is 25 times lower than at RHIC for the same hadron p. T • LHC hadron p. T = 2 Ge. V, y = 0, should reach same x as at forward y at RHIC, x ~ 10 -3 • Why no suppression?

Wherefore forward rapidity? Au Lab frame x x bnucleon Au Nucleus frame fwd-rapidity mid-rapidity 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.

“p. QCD” 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

CGC Approaches Stasto, Xiao, Yuan [arxiv: 1109. 1817] Lappi and Mantsaari, arxiv: 1209. 2853 Hybrid rc. BK 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.

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 p. T, 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

Backup Slides

MPC Performance North MPC “Trigger” Near Far Jet 1 Jet 2 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)

Rd. A Past, di-Hadron Future Color Glass Condensate Kharzeev, NPA 748, 727 (2005) CNM effects: dynamical shadowing, Energy Loss, Cronin (Qiu, Vitev PLB 632: 507, 2006) Kharzeev, Levin, Mc. Lerran Nucl. Phys. A 748 (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)

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