Central Questions in Nucleon Structure

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1 Central Questions in Nucleon Structure Werner Vogelsang BNL Nuclear Theory QCD and Hadron Physics Town Meeting, 01/13/2007

2 Exploring the nucleon: Of fundamental importance in science Know what we are made of! Test our ability to use QCD: Asymptotic Freedom, Factorization Explore and Understand QCD: Lattice, Models Nucleon as tool for discovery: RHIC Heavy Ions, LHC Tevatron High-E T jets NuTeV anomaly,

3 We can probe the quark-gluon structure of the Nucleon in short-distance processes: 1/Q Diverse probes: DIS, DVCS, Drell-Yan, pp jetx,

4 Questions to ask: What are the momentum distributions of quarks, anti-quarks, and gluons? p = x P What flavor symmetries hold-- or how are they broken? vs. vs.? Isospin-symmetry between p and n? How are quarks and gluons distributed spatially?

5 How do partons carry the proton spin-1/2? (Spins & orbital angular momenta) What difference does vs. make? What novel features arise? How are quarks and gluons correlated?

6 These are central questions of the field. The challenge is: Map out the Nucleon Its complete spin, flavor, gluon landscape We have a pretty good picture of some aspects We are learning about others We are still in the dark in many cases We ll have a good chance to get all the answers with present and next-generation facilities!

7 Momentum distributions of quarks and gluons

8 An important part of our picture of the nucleon: Gluons rule at small-x! We know a lot, but

9 but some aspects little understood, for example: sea quarks and gluon at high-x, valence at very-high-x Botje CTEQ SU(6) pqcd sc.diquark Testing ground for models of Proton wave function Measure at Jlab-12 GeV sea: DY Not an academic problem: LHC Measure at HERA, EIC

10 EIC A. Bruell, R. Ent One observable among many: df 2 /dlog(q 2 ), ep jet+jet+x, charm,

11 Helicity structure of the Nucleon g 1L = g 1L =

12 A major motivation : Explore the proton spin! q+q spin contribution Gluon spin contribution Orbital ang. momenta Quotable properties of the proton

13 EMC, SMC, E , HERMES q and q spins carry of proton spin Hirai, Kumano, Saito x How sure are we? And, what are the details?

14 Rests on a number of things: small-x extrapolation of structure function at small-x, typically Q 2 small as well. Higher twists? To really nail it down, need measurements at lower x. And: at current x, but higher Q 2 EIC use of SU(3) symmetry: Bjorken

15 if all true, current picture is: is it correct? would like to know more:

16 Important applications for models : + Many models predict Thomas,Signal,Cao; Holtmann,Speth,Fässler; Diakonov,Polyakov,Weiss; Glück,Reya; Schäfer,Fries; Kumano; Wakamatsu; Various avenues for addressing these questions

17 At RHIC: PV u unpol. W+charm at RHIC-II

18 In lepton scattering : SIDIS HERMES Major topic at Jlab-12 GeV

19 EIC: J. Seele also, at EIC: e W - u ν d Q 2 >225 GeV 2 L=2/fb get sin 2 (Θ W )?

20 Bjorken s sum rule high-order perturbation theory target-mass corrections Twist-4 matrix elements Precision QCD. Currently tested at ~10%. Can it be tested at ~1 or 2%?

21 The gluon spin distribution Δg Not much information until recently: Bag model Chen,Ji axial anomaly Altarelli et al.

22 NOW: PHENIX, STAR Δg/g HERMES, COMPASS x

23 Challenge will be to really extract Δg and its integral: Need Global analysis Eventually, for integral will likely need additional information, in particular from smaller x ~ RHIC 200 GeV midrap. data

24 RHIC at 500 GeV, and with jet+jet, gamma+jet at forward kinematics An Electron-Ion Collider!

25 A. Bruell, R. Ent

26 A. Bruell, R. Ent at small x

27 What s the structure of a Transversely polarized Nucleon?

28 Transversity: h 1 = Ralston,Soper; Jaffe,Ji; in helicity basis: + _ + _ Helicity-flip!

29 the physics involved: odd chirality helicity-flip, χsb no mixing with gluons tensor charge difference to helicity probes relativistic / dynamical effects

30 Opportunities for measurement? not in inclusive DIS, but: π + _ + _ Collins effect azimuthal asym. this effect actually appears to be there : HERMES

31 First glimpse of Transversity! (Anselmino et al.) information on Collins fragm. fct. has become available from BELLE in (up to sign)

32 The future : SIDIS at COMPASS, Jlab-12 GeV Collins-type asymmetries at RHIC Drell-Yan: q(x 1 ) µ + µ - q(x 2 ) RHIC / RHIC-II GSI, J-PARC azimuthal asymmetries in SIDIS at EIC

33 Transverse spin offers further new insights into Nucleon structure. correlation Sivers

34 Where would this show up? SMC, HERMES, COMPASS, CLAS Seen!

35 In pp scattering: involved (in disguised form) in large left-right asymmetries π L R E704 STAR (BRAHMS, too)

36 What s the physics of the Sivers functions? _ Probes overlap of proton wave fcts. with J z = ±1/2 involves orbital angular momentum T-invariance of QCD: they involve a rescattering in the color field of the remnant Brodsky, Hwang, Schmidt; Collins; Belitsky, Ji, Yuan; Boer, Mulders, Pijlman Attractive!

37 intuitive (model-based) picture : Burkardt

38 profound physics implication: process-dependence of Sivers functions DIS: attractive DY: repulsive hugely important in QCD -- tests much of what we know about description of hard processes

39 Many avenues for important measurements: Drell-Yan RHIC / RHIC-II GSI, J-PARC correlations in pp jet+jet X at RHIC (now data!) detailed studies of azimuthal asymmetries in SIDIS at EIC at high Q 2

40 What s the spatial structure of the Nucleon?

41 Over the last decade, theory has understood that parton distributions and form factors are special cases of a much more powerful representation of nucleon structure: Generalized Parton Distributions Müller,Robaschik; Ji; Radyushkin GPD

42 What we dream of: Tomographic images of the nucleon: Burkardt; Ji At EIC: spatial distribution of sea and glue

43 Quantify orbital motion of partons in nucleon Ji + L + glue GPDs have potential to take our picture of the nucleon to a new level.

44 HE setup: e +/- (10 GeV) + p (250 GeV) L = cm -2 s pb -1 /day LE setup: e +/- ( 5 GeV) + p ( 50 GeV) L = cm -2 s pb -1 /day Sandacz Precision of DVCS unpolarized cross sections for one out of 6 Q 2 intervals (8 < Q 2 < 15 GeV 2 ) data <Q 2 > = 10.4 GeV 2 Projected results W [GeV] also: gluon imaging with exclusive J/Ψ Frankfurt,Strikman,Weiss

45 The challenge is: Map out the Nucleon Its complete spin, flavor, gluon landscape Sea q, Δq, δq Gluons g, Δg Valence q, Δq, δq Form factors GPDs: Spatial distrib. Orbital ang. mom. We ll have a good chance to get all the answers with present and next-generation facilities!

46 a little further into the future: prompt photons

47 N. Saito

48

49 RHIC offers great possibilities to probe Δg : Jäger,Schäfer, Stratmann,WV Jäger,Stratmann,WV; Signer et al. Gordon,WV; Contogouris et al.; Gordon, Coriano Stratmann, Bojak Weber; Gehrmann; Kamal NLO corrections known in all cases.