Carl Gagliardi – d+Au Collisions at STAR – DIS’06 1 d+Au Collisions at STAR Outline d+Au collisions and saturation physics at RHIC Recent STAR results STAR plans for the future Carl A. Gagliardi Texas A&M University for the Collaboration STAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 2 E-M Calorimeter Time of Flight Projection Chamber STAR detector

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 3 Mid-rapidity p+p at RHIC and NLO pQCD PRL 91, PHENIX π 0 Calculations by W. Vogelsang At 200 GeV, pQCD does a very good job describing mid-rapidity yields STAR (h + +h - )/2 BRAHMS (h + +h - )/2

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 4 Mid-rapidity d+Au PRL 91, Pedestal&flow subtracted Inclusive yields and back-to-back di-hadron correlations are very similar in p+p and d+Au collisions In contrast, Au+Au collisions are very different from p+p and d+Au – but that’s not the subject of this talk STAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 5 Mid-rapidity vs. forward rapidity Mid Rapidity Forward Rapidity CTEQ6M Gluon density can’t grow forever. Saturation may set in at forward rapidity when gluons start to overlap. Can be explored by comparing p(d)+A to p+p.

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 6 Forward particle production in d+Au collisions BRAHMS, PRL 93, Sizable suppression in charged hadron production in d+Au collisions relative to p+p collisions at forward rapidity BRAHMS

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 7  related to rapidity of produced hadrons. As y grows Expectations for a color glass condensate D. Kharzeev, hep-ph/ Iancu and Venugopalan, hep-ph/ Are the BRAHMS data evidence for gluon saturation at RHIC energies?

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 8 Recent saturation model calculation Very good description of the p T dependence of the BRAHMS d+Au → h − + X cross section at η = 3.2 (Dumitru, Hayashigaki, and Jalilian-Marian, NP A765, 464)

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 9 Is saturation really the explanation? Difficult to explain BRAHMS results with standard shadowing, but in NLO pQCD calculations ~ 0.02 is not that small (Guzey, Strikman, and Vogelsang, PL B603, 173) In contrast, <~ in CGC calculations (Dumitru, Hayashigaki, and Jalilian-Marian, NP A765, 464 ) Basic difference: pQCD: 2  2 CGC: 2  1

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 10 Do we understand forward π 0 production in p + p? Bourrely and Soffer, EPJ C36, 371: NLO pQCD calculations underpredict the data at low  s from ISR Ratio appears to be a function of angle and √s, in addition to p T √s=23.3GeV√s=52.8GeV xFxF xFxF           Ed 3  dp 3 [  b/GeV 3 ] NLO calculations with different scales: p T and p T /2 Data-pQCD differences at p T =1.5GeV

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 11 The error bars are statistical plus point-to-point systematic Consistent with NLO pQCD calculations at 3.3 < η < 4.0 Data at low p T trend from KKP fragmentation functions toward Kretzer. PHENIX observed similar behavior at mid-rapidity. p+p    +X at 200 GeV nucl-ex/ STAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 12 d+Au    +X at 200 GeV p T dependence of d+Au π 0 cross section at = 4.0 is best described by a LO CGC calculation. (Dumitru, Hayashigaki, and Jalilian-Marian, NPA 765, 464) nucl-ex/ STAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 13  dependence of R dAu nucl-ex/ STAR Observe significant rapidity dependence. pQCD calculations significantly over predict R dAu.

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 14 Any difference between p+p and d+Au? Kharzeev, Levin, McLerran gives physics picture (NPA748, 627) d+Au: Mono-jet? P T is balanced by many gluons Dilute parton system (deuteron) Dense gluon field (Au) Color glass condensate predicts that the back-to-back correlation from p+p should be suppressed p+p: Di-jet

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 15 Back-to-back correlations with the color glass (Kharzeev, Levin, and McLerran, NP A748, 627) The evolution between the jets makes the correlations disappear.

Carl Gagliardi – d+Au Collisions at STAR – DIS’ <E  <35GeV 35<E  <45GeV HIJING predicts similar correlations in d+Au as PYTHIA predicts for p+p. Only significant difference is combinatorial background level. Forward + mid-rapidity di-hadron correlations

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 17 STAR are suppressed at small and consistent with CGC picture are similar in d+Au and p+p at larger and as expected by HIJING 25<E  <35GeV Fixed  as E & p T grows Forward + mid-rapidity correlations in d+Au nucl-ex/ π 0 : | | = 4.0 h ± : | η | 0.5 GeV/c ~ 1.0 GeV/c ~ 1.3 GeV/c

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 18 STAR Forward Meson Spectrometer FMS increases areal coverage of forward EMC from 0.2 m 2 to 4 m 2 Addition of FMS to STAR provides nearly continuous EMC from -1< η <+4 Available for Run 7

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 19 p+p and d+Au    +   +X correlations with forward   hep-ex/ p+p in PYTHIAd+Au in HIJING Conventional shadowing will change yield, but not angular correlation. Saturation will change yield and modify the angular correlation. Sensitive down to x g ~ in pQCD scenario; few x in CGC scenario.

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 20 Conclusions Forward rapidity inclusive π 0 production at RHIC is well described by pQCD calculations d+Au results at RHIC provide hints that saturation effects are becoming important Future STAR measurements will elucidate the dynamics underlying forward inclusive particle suppression at RHIC RHIC may be the ideal accelerator to explore the onset of saturation

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 21 STAR The STAR Collaboration U.S. Labs: Argonne, Lawrence Berkeley, and Brookhaven National Labs U.S. Universities: UC Berkeley, UC Davis, UCLA, Caltech, Carnegie Mellon, Creighton, Indiana, Kent State, MIT, MSU, CCNY, Ohio State, Penn State, Purdue, Rice, Texas A&M, UT Austin, Washington, Wayne State, Valparaiso, Yale Brazil: Universidade de Sao Paolo China: IHEP - Beijing, IPP - Wuhan, USTC, Tsinghua, SINAP, IMP Lanzhou Croatia: Zagreb University Czech Republic: Nuclear Physics Institute England: University of Birmingham France: Institut de Recherches Subatomiques Strasbourg, SUBATECH - Nantes Germany: Max Planck Institute – Munich University of Frankfurt India: Bhubaneswar, Jammu, IIT-Mumbai, Panjab, Rajasthan, VECC Netherlands: NIKHEF/Utrecht Poland: Warsaw University of Technology Russia: MEPHI – Moscow, LPP/LHE JINR – Dubna, IHEP – Protvino South Korea: Pusan National University Switzerland: University of Bern STAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 22

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 23 Pseudo-rapidity yield asymmetry vs p T Back/front asymmetry in 200 GeV d+Au consistent with general expectations of saturation or coalescence; doesn’t match pQCD prediction. Au direction / d direction PRC 70, STAR

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 24 Nuclear Gluon Density World data on nuclear DIS constrains nuclear modifications to gluon density only for x gluon > 0.02 e.g., see M. Hirai, S. Kumano, T.-H. Nagai, Phys. Rev. C70 (2004) and data references therein

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 25 One calculation within the saturation picture R dAu R CP Saturation model calculation with additional valence quark contribution (Kharzeev, Kovchegov, and Tuchin, PL B599, 23)

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 26 x values in saturation calculations In CGC calculations, the BRAHMS kinematics corresponds to <~ (Dumitru, Hayashigaki, and Jalilian-Marian, NP A765, 464 )

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 27 Many recent descriptions of low-x suppression Saturation (color glass condensate) ● Jalilian-Marian, NPA 748 (2005) 664. ● Kharzeev, Kovchegov, and Tuchin, PLB 599 (2004) 23; PRD 68 (2003) ● Armesto, Salgado, and Wiedemann, PRL 94 (2005) ● Dumitru, Hayashigaki, and Jalilian- Marian, NPA 765 (2006) 464 Multiple scattering ● Qiu and Vitev, PRL 93 (2004) ; hep-ph/ Shadowing ● R. Vogt, PRC 70 (2004) ● Guzey, Strikman, and Vogelsang, PLB 603 (2004) 173. Parton recombination ● Hwa, Yang, and Fries, PRC 71 (2005) Factorization breaking ● Kopeliovich et al., PRC 72 (2005) ● Nikolaev and Schaefer, PRD 71 (2005) Others? A short list (probably incomplete) ●...

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 28 NLO pQCD S. Kretzer Forward π 0 production at a hadron collider Large rapidity π production ( η ~4) probes asymmetric partonic collisions Mostly high-x quark + low-x gluon 0.3 < x q < < x g < 0.1 nearly constant and high ~ A probe of low-x gluons N N  qq gg ENEN xqpxqp xgpxgp   ENEN (collinear approx.)

Carl Gagliardi – d+Au Collisions at STAR – DIS’06 29 Constraining the x-values probed in hadronic scattering Guzey, Strikman, and Vogelsang, Phys. Lett. B 603, 173 Measure two particles in the final state to constrain the x-values probed Log 10 (x Gluon )  Gluon TPC Barrel EMC FTPC FPD For 2  2 processes FPD: |  |  4.0 TPC and Barrel EMC: |  | < 1.0 Endcap EMC: 1.0 <  < 2.0 FTPC: 2.8 <  < 3.8 Collinear partons: ● x + = p T /  s (e +  1 + e +  2 ) ● x  = p T /  s (e  1 + e  2 ) Log 10 (x Gluon )