QCD Spin Physics: Some Recent Highlights
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QCD Spin Physics:Some Recent Highlights
Werner VogelsangTübingen University
IWHSS12, Lisboa, 04/16/2012
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Know what we are made of !
Understand the strong force:“QCD”
Use protons as tool for discovery (e.g. LHC )
Exploring the nucleon: a fundamental quest
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25 years since the “proton spin crisis”:
We have come a long way …
• what role do gluons play for the proton spin ?
• and strangeness ?
• what orbital angular momenta do partons carry ?
• what’s the 3D image of the nucleon in terms of quarks and gluons spacially ( GPDs) ? in momentum (TMDs) ?
A far-developed theory framework to address this
Plus, first-rate experimental facilities
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Today’s talk :
Not a comprehensive review – a collection of recent results with emphasis on relevance for COMPASS
Nucleon helicity PDFs
• status, latest results
• theory efforts
Transverse-spin phenomena
Role of fragmentation functions
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Nucleon Helicity Parton Distributions
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• NLO (MS) “global analysis” de Florian, Sassot, Stratmann, WV
Probes of nucleon helicity structure:
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Status ~ 2009 :
bestconstrained
driven by SIDIS
indicationsfor
strangenesspuzzle?
SIDIS
DIS +SU(3)
small in measured regime !
de Florian, Sassot, Stratmann, WV
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Recent developments: new input from experiments
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• recent COMPASS semi-inclusive data:
not covered by HERMES
not available from HERMES
DSSV+(Stratmann,Sassot)
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• implications for ?
• tendency toward negative low-x also from SIDIS ?
Stratmann @ DIS2011
• heavily relies on kaon fragmentation Leader,Sidorov,Stamenov
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• still have a lot to learn about strangeness:
HERMES
ATLAS
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New devlopments on :
gives gluon with
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DSSV
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Lepton scattering:
(yet to be included in global analysis)
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NLO: Jäger, Stratmann, WV
COMPASS cross section for
thesis C. Hoeppner / A. Morreale
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Fixed-target regime:Large logarithmic terms
all-order resummation relevant
only soft/collinear gluons allowed
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Fixed order
NNLO
NLOResu
mm
atio
n
LL NLL
······
LO
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All-order resummation: Laenen, Oderda, Sterman; Catani et al.;Kidonakis, Sterman; Bonciani et al.;de Florian, WV;Almeida, Sterman, WV
Mellin moment in
soft-gluon effects exponentiate :
Leading logarithms:
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hBeyond leading logs:
soft & coll. gluons (LL)
large-angle soft gluons (NLL)
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de Florian, Pfeuffer, Schäfer, WV (prel.)
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News on fragmentation functions
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Fragmentation functions Dqh(z):
• encode hadronization in hard-scattering reactions
• universality / factorization
• crucial for pQCD description of hadron-production data:
• global analyses with uncertainty estimates:de Florian, Stratmann, Sassot (DSS) e+e-, ep, pp
Albino, Kniehl, Kramer (AKK) e+e-, pp
Hirai, Kumano, Nagai. Sudoh (HKNS) e+e-
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direct only
corrected mistake in NLO calculation !
thesis C. Hoeppner
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(M. Stratmann at INT workshop 02/2012)
LEP data mostly fix singlet !
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BELLE (M. Leitgab at DIS 2012)
• marks new era of precision fragmentation functions. Evolution e+e- vs. RHIC(pp)
• beware of large-z effects / power corrections !
DSS (LO)
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courtesy de Florian,Pinto,Sassot,Stratmann (w/o HERMES data)
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courtesy de Florian,Pinto,Sassot,Stratmann (w/o HERMES data)
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Transverse-spin phenomena
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E704
STAR,BRAHMS,PHENIX
HERMES, COMPASS, JLab
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• single large scale p
• single-inclusive processes, e.g.
• power-suppressed ~1/p in QCD
• collinear factorization
• probe qqg correlations, e.g.
• two-scale processes: small & measured q << Q
• SIDIS at HERMES,COMPASS
• TMD factorization for simplest observables, e.g. Sivers
• crucial role of gauge links / non-universality
• spin-orbit correlations, “lensing”, 3D imaging
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TMDs and twist-3 functions are closely related:
• at operator level:
Boer,Mulders,Pijlman,Ji, Qiu,WV,Yuan; Koike,WV,YuanZhou,Yuan,LiangBacchetta,Boer,Diehl,Mulders
• it means we can confront SIDIS and RHIC data
Kang,Qiu,Yuan,WV a sign puzzle
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from
from pp
Kang,Qiu,Yuan,WV
more mundane : can one get away with nodes in x / kT?
What to conclude ?
inconsistency in QCD formalism for single-spin ?
• Collins-type effect dominant in ?
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Kang,ProkudinJoint fit to SIDIS and pp data:
SIDIS pp (STAR)
Works (reasonably) well for SIDIS and STAR, but failsFor BRAHMS!
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Kang,ProkudinHas ramifications for DY spin asymmetry:
Strengthens case for study of DY “sign change” !
AnDY, COMPASS, E906,W bosons at RHIC
w/o node
node
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q
d/dq
QCD
q ~ Q coll.fact.
well-known feature: emergence of Sudakov logarithms
QCD evolution of TMDs Collins,Aybat,Rogers,QiuKang,Xiao,YuanAybat,Prokudin,RogersAnselmino,Boglione,Melis
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• can be resummed to all orders in strong coupling
Collins,Soper,Sterman;…Koike, Nagashima,WVKang,Xiao,Yuan
(e.g. Drell-Yan, simplified)
Sudakov exponent
• can be formulated to give evolution of TMDs in terms of
Mert Aybat,Rogers,Collins,QiuKang,Xiao,Yuan
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• for Sivers function obtain (~ Sudakov)
where
non-pert.piece
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• initial phenomenology very encouraging
Anselmino,Boglione,Melis
Aybat,Prokudin,Rogers
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Takes this field to new level !
However, a number of caveats:
• various simplifying assumptions
• results will depend on large-b prescription: b* only one possible choice choice of parameters such as g2, bmax
• matching to large-kT tail (“Y-term” / TF)
• so far “just” leading log
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Enjoy IWHSS12 !
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…well described by NLO at RHIC …but data much higher than NLO at fixed-target energies !
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• spin sum rule / orbital angular momentum: a long-standing debate
Recently: WakamatsuChen et al.Leader
News
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Leader,Sidorov,Stamenov
Forte et al. (NNPDF)
• other “contemporary” analyses of polarized (SI)DIS data:
Sissakian, Shevchenko, Ivanov
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There are many observables that are sensitive to OAM. Question is connection to spin sum rule
NewsSivers <-> OAM in a quantitative way ?
Bacchetta,Radici
• combines: Ji’s expression for in terms of GPDs connection between moment of and GPD (Burkardt’s lensing idea) joint fit of SIDIS Sivers asymmetries and magnetic moments
• OAM from Wigner distributions ? Lorce,Pasquini
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A far-developed theory framework to address this:
(F. Yuan at DIS 2012)
plus, first-rate experimental facilities
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• connection to hyperon β-decays, SU(3)
• strangeness?
(axial charges)
Bjorken; Ellis, Jaffe;Sehgal;Karliner, Lipkin;Ratcliffe;…
?