Experimental Issues in Low-x Physics and Transverse Single Spin Asymmetries EIC Meeting at Stony Brook L.C. Bland Brookhaven National Laboratory 7 December 2007

2 Questions to be Addressed A naïve experimental view Are phenomena observed in inclusive/semi-inclusive deep inelastic electron scattering universal, meaning related to phenomena in hard scattering particle production in hadronic interactions? What is the justification to use particle production in hadronic interactions as a tool to probe for universality? Why are transverse single spin asymmetries interesting? And, are there universal aspects between semi-inclusive deep inelastic scattering and hadronic interactions? Why is the gluon density, especially at low Bjorken x, interesting? And, how can universal aspects of the gluon density be explored in hadronic interactions? What is on the horizon for addressing these questions?

3 Kinematics large- p T physics in p+p collisions p beam -p beam large p T  or jet or  or … Largest p T reached by detecting produced particles at ~ 90  (midrapidity,  ~0)

4 Kinematics large- x F (with sufficient p T ) physics in p+p collisions p beam -p beam large p L  or jet or  or … Large p L (produced particle at large  ) is required to reach large Feynman- x, x F = p L / p beam = 2 p L /  s

5 RHIC Polarized Collider BRAHMS & PP2PP STAR PHENIX AGS LINAC BOOSTER Pol. H - Source Spin Rotators (longitudinal polarization) Siberian Snakes 200 MeV Polarimeter RHIC pC Polarimeters Absolute Polarimeter (H  jet) AGS pC Polarimeter Strong AGS Snake Helical Partial Siberian Snake PHOBOS Spin Rotators (longitudinal polarization) Siberian Snakes 2006: 1 MHz collision rate; Polarization=0.6

gluon quark pion or jet quark RHIC Spin/Low-x Probes Polarized proton collisions / d+Au collisions Describe p+p particle production at RHIC energies (  s  62 GeV) using perturbative QCD at Next to Leading Order, relying on universal parton distribution functions and fragmentation functions

7 √s=23.3GeV√s=52.8GeV Do we understand forward   production in p + p? At  s < 200 GeV, not really… 2 NLO collinear calculations with different scale: p T and p T /2 Bourrely and Soffer [Eur. Phys. J C36 (2004) 371], data references therein to ISR and fixed target results  data /  pQCD appears to be function of , √s in addition to p T Collinear NLO pQCD underpredicts the data at  s < 60 GeV xFxF     Ed 3  dp 3 [  b/GeV 3 ] Data-pQCD difference at p T =1.5GeV xFxF       Ed 3  dp 3 [  b/GeV 3 ]

8 Does pQCD describe particle production at RHIC? Compare cross sections measured for p+p   +X at  s=200 GeV to next-to-leading order pQCD calculations S.S. Adler et al. (PHENIX), PRL 91 (2003) J. Adams et al. (STAR), PRL 92 (2004) ; and PRL 97 (2006) Cross sections agree with NLO pQCD down to p T ~2 GeV/c over a wide range, 0 <   3.8, of pseudorapidity (  = -ln tan  /2) at  s = 200 GeV.

9 STAR-Forward Cross Sections Similar to ISR analysis J. Singh, et al Nucl. Phys. B140 (1978) 189. Expect QCD scaling of form:  Require  s dependence (e.g., measure   cross sections at  s = 500 GeV) to disentangle p T and x T dependence

10 <z><z> <xq><xq> <xg><xg> Large rapidity  production   <4  probes asymmetric partonic collisions Mostly high-x valence quark + low-x gluon 0.3 < x q < < x g < 0.1 nearly constant and high 0.7 ~ 0.8 Large-x quark polarization is known to be large from DIS Directly couple to gluons  probe of low x gluons NLO pQCD Jaeger,Stratmann,Vogelsang,Kretzer Forward   production in a hadron collider pdpd p Au  qq gg ENEN xqpxqp xgpxgp   ENEN (collinear approx.)

11 Q: What is the justification to use particle production in hadronic interactions as a tool to probe for universality? A: Particle production cross sections agree with next-to- leading order pQCD calculations at central and forward rapidities at  s = 200 GeV. To do:Measure at  s = 500 GeV to establish x F, x T and p T scaling

12 Transverse Single-Spin Asymmetries (A N ) Probing for orbital motion within transversely polarized protons

13 Expectations from Theory What would we see from this gedanken experiment? F  0 as m q  0 in vector gauge theories, so A N ~ m q /  s or, A N ~ for  s = 200 GeV Kane, Pumplin and Repko PRL 41 (1978) 1689

14  s=20 GeV, p T = GeV/c   0 – E704, PLB261 (1991) 201.   +/- - E704, PLB264 (1991) 462. QCD theory expects very small (A N <10 -3 ) transverse SSA for particles produced by hard scattering. A Brief and Incomplete History… The FermiLab E-704 experiment found strikingly large transverse single- spin effects in p  +p fixed-target collisions with 200 GeV polarized proton beam (  s = 20 GeV).

15 Two of the Explanations for Large Transverse SSA Spin-correlated k T Require experimental separation of Collins and Sivers contributions Collins/Hepplemann mechanism requires transverse quark polarization and spin-dependent fragmentation Sivers mechanism requires spin-correlated transverse momentum in the proton (orbital motion) and color-charge interaction. SSA is present for jet or  final state initial state

16 Transverse Single-Spin Asymmetries World-wide experimental and theoretical efforts Transverse single-spin asymmetries are observed in semi-inclusive deep inelastic scattering with transversely polarized proton targets  HERMES ( e  ); COMPASS (  ); and planned at JLab Transverse single spin asymmetries are observed in hadron-pair production in e + e  collisions (BELLE) Intense theory activity underway

17 FPD  =2  =  1 FPD++ East-side West-side Inclusive  0 in forward region:  4<  <  3 (FPD), 2.5<  <4 (FPD++) RUN6 configuration x z y

18 Overview of transverse spin runs at STAR with forward calorimetry: 2001→2006 Run2Run3Run5Run6 detector EEMC and FPD prototypes 6 matrices of FPD full FPD (8 matrices) East FPD West FPD++ ~15~30~45~ ±3.3/±4.0±3.7/± /3.3 sampled FOM (P 2 L) in Run 6 is ~50 times larger than from all the previous STAR runs, and ~ 725 times larger than for Run 2

19 FPD++ Physics for Run6 Run-5 FPD We staged a large version of the FPD to prove our ability to detect jet-like events, direct photons, etc. with the STAR FMS The center annulus of the run-6 FPD++ is similar to arrays used to measure forward   SSA. The FPD++ annulus is surrounded by additional calorimetry to increase the acceptance for jet-like events and direct  events.

20   Identification and Spin Dependence Large rapidity measurements require careful calibration Left/right symmetric detectors cancels many sources of systematic errors Spin effect is visible in the raw spin-sorted yields

21 π 0 A N at √s=200 GeV – x F -dependence A N at positive x F grows with increasing x F A N at negative x F is consistent with zero Small errors of the data points allow quantitative comparison with theory predictions STAR Preliminary results: hep-ex/ Final results presently under review by STAR

22 Evidence for Universality? Uses phenomenological fit to  Cahn effect to get kT dependence;  Sivers moments from HERMES ;  generalized parton model

23 A N (p T ) in x F -bins Combined data from three runs at =3.3, 3.7 and 4.0 In each x F bin, does not significantly changes with p T Measured A N is not a smooth decreasing function of p T as predicted by multiple theoretical models (hep-ex/ ) D’Alesio & Murgia PRD 70 (2004) Kouvaris, Qiu, Vogelsang, Yuan PRD 74 (2006) STAR

24 Q: Why are transverse single spin asymmetries interesting? A: Leading-twist, collinear pQCD expects them to be zero. Experimentally, they are non-zero and large, at  s=200 GeV, where cross sections are described by pQCD. They are also large at lower  s, where cross sections are not described by pQCD. Transverse single spin asymmetries may provide a window onto orbital motion of partons within the proton.

25 Q.Are there universal aspects for transverse single spin asymmetries between semi-inclusive deep inelastic scattering and hadronic interactions? A.Assuming existing p  +p  +X data has contributions only from the Sivers effect (related to twist-3 collinear approach), then maybe, since the x F dependence can be described. But, the fixed- x F, p T dependence is not described by theory. Also, experimental separation of Sivers/Collins contributions are not possible in p  +p  +X  must go to particle correlations, or to produced particles (e.g., real/virtual  ) for which fragmentation (Collins) contributions are not allowed

26 Low-x

27 Deep Inelastic Scattering from Nuclear Targets Kinematic Coverage Restricted to Fixed Target Experiments (no EIC, yet) From Hirai, Kumono, Nagai PRC 70 (2004) , and references therein Growth of gluon distribution at low- x within the proton cannot continue forever Gluon density in nucleus only known to x ~0.02 since g(2 x )~  F 2 ( x, Q 2 )/  ln( Q 2 )

28 Gluon Saturation and the Color Glass Condensate  = ln(1/ x ) Iancu, Venugopalan hep-ph/ Does the low- x gluon density saturate, and is this a high- energy phase of matter? Would a Color Glass Condensate be universal for both nuclear DIS and hadronic probes of nuclei at high energy?

29 Hints of Gluon Saturation from Large-Rapidity Particle Production in d+Au Collisions at RHIC? d+Au   +X cross sections at  s NN = 200 GeV and  =4.0 [STAR, Phys.Rev.Lett. 97 (2006) ] NLO pQCD calculations using gluon shadowing [Guzey, Strikman and Vogelsang PLB 603 (2004) 173] CGC model calculation [Dumitru, Hayashigaki, Jalilian-Marian, Nucl.Phys. A770 (2006) 57] Large-rapidity d+Au cross sections are suppressed Data are best described by CGC model calculation Many other possible explanations of suppression

30 Can Particle Correlations Quantify a CGC? 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 back-to-back particle correlations in d+Au should be suppressed relative to p+p p+p: Di-jet

31 are suppressed at small and consistent with CGC picture are similar in d+Au and p+p at larger ( ) as expected by HIJING 25<E  <35GeV Fixed  as E & p T grows An initial glimpse: correlations in d+Au π 0 : | | = 4.0 h ± : | η | 0.5 GeV/c ~ 1.0 GeV/c ~ 1.3 GeV/c As expected by HIJING PRL 97, (2006)

32 Q: Why is the gluon density, especially at low Bjorken x, interesting? A: At sufficiently low-x gluon splitting and gluon recombination should balance each other, resulting in gluon saturation. Present-day capabilities in DIS are not conclusive whether the gluon density saturates. An EIC will address this question. Even with an EIC, establishing the universal aspects of a saturated low-x gluon density is important to establish its reality. This may be possible at RHIC and should be pervasive at the LHC, if the CGC is real.

33 The Future (starts now…)

34 FMS construction completed installation and commissioning during Run 7 (NOW) FMS ½ Wall Pb. Glass FMS Wall FMS for Run 7 NOW!! Near full EM coverage -1<  <4 Pairs of Forward Pions same side correlations (Fragmentation – Collins) Event by event “x” measurement from two jets. Opposite side correlated pions (dijets) – Sivers effect –d-Au (Gluon saturation in Nuclei) Other future objectives –Forward Lepton pairs –Charm PHYSICS OBJECTIVES Outlook Forward Meson Spectrometer Installation completed 2007 d-Au gold nuclei 0.001< x <0.1 1.A d-Au measurement of the parton model gluon density distributions x g(x) in gold nuclei for 0.001< x <0.1. For 0.01<x<.1, this measurement tests the universality of the gluon distribution. macroscopic gluon fields. (again d-Au) 2.Characterization of correlated pion cross sections as a function of Q 2 (p T 2 ) to search for the onset of gluon saturation effects associated with macroscopic gluon fields. (again d-Au) transversely polarized protons resolve the origin of the large transverse spin asymmetries forward   production. (polarized pp) 3.Measurements with transversely polarized protons that are expected to resolve the origin of the large transverse spin asymmetries in reactions for forward   production. (polarized pp) Au Au FMS Commissioning April 2007 Summed Energy (ADC cnts) Summed Energy (ADC cnts) Cell multiplicity Cell multiplicity

Forward Meson Spectrometer Les Bland 1, Ermes Braidot 2, Hank Crawford 3, Anatoli Derevschikov 4, Jim Drachenberg 5, Jack Engelage 3, Len Eun 6, Carl Gagliardi 5, Andrew Gordon 1, Steve Heppelmann 6, Eleanor Judd 3, Dmitri Morozov 4, Larissa Nogach 4, Akio Ogawa 1, Hiromi Okada 1, Chris Perkins 3, Nikola Poljac 6, Alexander Vasiliev 4 1Brookhaven National Laboratory 2Utrecht University 3University of California (Berkeley) / Space Sciences Institute 4IHEP, Protvino 5Texas A&M University 6Penn State University 7Zagreb University STAR

36 Frankfurt, Guzey and Strikman, J. Phys. G27 (2001) R23 [hep-ph/ ]. Pythia Simulation constrain x value of gluon probed by high- x quark by detection of second hadron serving as jet surrogate. span broad pseudorapidity range (-1<  <+4) for second hadron  span broad range of x gluon provide sensitivity to higher p T for forward    reduce 2  3 (inelastic) parton process contributions thereby reducing uncorrelated background in  correlation. Universality check low-x

37 Transverse Spin and FMS Acceptance of FMS and projected RHIC performance will enable… further reach for inclusive   and heavy mesons spin-dependent near-side correlations (     )  separation of Sivers and Collins effects spin-dependent away-side correlations (   -jet)  isolation of Sivers effect embark on spin-dependent inclusive  and  +jet Note that entire delivered luminosity is useful for measurements based on run-6 operations. Projections for run-8, assuming split of longitudinal:transverse=2:1 at STAR

38 Transverse Spin Direct  Theory expects repulsive color charge interactions to result in an opposite sign to spin-correlated momentum imbalance for  +jet. Magnitude of effect requires >10 5 events to see significant effect. Comparison of run-6 data to simulation provides indication that prompt  can be extracted. The large acceptance and dynamic range of the FMS is needed to veto daughter  from   ,… decays. Expect >0.5M  events into small fiducial volume in 30 pb -1 sample with E  >25 GeV. Bacchetta et al., Phys. Rev. Lett. 99, (2007)

39 The Future is later, too

40 Sivers in SIDIS vs Drell Yan Important test at RHIC of the fundamental QCD prediction of the non-universality of the Sivers effect! requires very high luminosity (~ 250pb -1 ) Transverse-Spin Drell-Yan Physics at RHIC L. Bland, S.J. Brodsky, G. Bunce, M. Liu, M. Grosse-Perdekamp, A. Ogawa, W. Vogelsang, F. Yuan

41 Simple QED example: DIS: attractive Drell-Yan: repulsive Same in QCD: As a result: Non-universality of Sivers Asymmetries: Unique Prediction of Gauge Theory !

x Sivers Amplitude 0 Experiment SIDIS vs Drell Yan: Sivers| DIS = − Sivers| DY *** Test QCD Prediction of Non-Universality *** HERMES Sivers Results Markus Diefenthaler DIS Workshop Műnchen, April RHIC Drell Yan Projections

43 Rapidity and Collision Energy Large rapidity acceptance required to probe valence quark Sivers function

44 Backups

45 Benchmarking Simulations p+p  J/  +X  l + l  +X,  s=200 GeV e  e  |  |<0.35     1.2<|  |<2.2 J/  is a critical benchmark that must be understood before Drell-Yan PHENIX, hep-ex/

46 Dilepton Backgrounds J/  ’’ cc bb  Drell-Yan Isolation needed to discriminate open heavy flavor from DY

47 A N (p T ) at x F > 0.4 Run3+Run5 data (hep-ex/ ): Run6 data (hep-ex/ ): more precise measurements consistent with the previous runs in the overlapping p T region complicated dependence on p T, but not in agreement with theoretical predictions Online calibration of CNI polarimeter Hint of A N decrease with increasing p T at p T ~1-2 GeV/c residual x F -dependence? => A N mapping in (x F,p T ) plane is required

48 RHIC Spin Goals Understanding the Origin of Proton Spin Transverse Spin PRD 70 (2004) Spin Sum Rules Longitudinal Spin Understanding the origin of proton spin helps to understand its structure

49 Summary Firmly established that large transverse single spin asymmetries are observed at  s = 200 GeV, where generally cross sections agree with pQCD calculations. Large transverse single spin asymmetries are observed only at large x F ; midrapidity asymmetries are small. Large x F spin asymmetries show the same pattern for 20   s  200 GeV First observation of p T dependence at fixed-x F, enabled by the run-6 luminosity/performance  Some aspects of the theory are still not understood Intense theory activity is underway to understand these spin effects. Most theorists agree the Sivers mechanism is responsible for the dynamics  evidence for partonic orbital angular momentum?

50 Acceptance of FPDSTAR Inclusive  0 Strong x F - p T correlation because of limited acceptance FPD xFxF p T GeV/c

51 Study of the p T dependence needs large acceptance Acceptance of FPD and FPD++STAR Inclusive  0 FPD++ FPD xFxF p T GeV/c