# Hard exclusive processes -

date post

26-Jan-2016Category

## Documents

view

27download

0

Embed Size (px)

description

### Transcript of Hard exclusive processes -

Hard exclusive processes -Andrzej SandaczSotan Institute for Nuclear Studies, Warsaw DVCS at fixed target experiments and HERA Transverse target spin asymmetry for 0 production Vector Meson production at HERA EIC Collaboration Meeting, Stony Brook University, December 8, 2007 experimental results and simulations for EIC DVCS at EIC Exclusive + and 0 production

- Generalized Parton DistributionsGPDsx+xx-xP1P2hardsoftg*gFactorisation:Q2 large, -t
GPDs properties, link to DIS and form factorsJis sum rule total angular momentum carried by quark flavour q(helicity and orbital part)

GPD= a 3-dimensional picture of the partonic nucleon structure or spatial parton distribution in the transverse plane H(x, =0, t) H(x,, rx,y ) probability interpretation Burkardt

Holy Grails or the main goals x Px yrz E

The imaginary part of amplitude T probes GPD at x = xThe real part of amplitude T depends on the integral of GPD over xObservables and their relationship to GPDsDGLAPDGLAP

ERBL T

DVCS Asymmetries measured up to nowDsUT ~ sin(f-s)cosIm{k(H - E) + } polarization observables: kinematically suppressedH HH, E~ different charges: e+ e- (only @HERA!):Hx = xB/(2-xB ),k = t/4M2

CLAS: DVCS - BSAIntegrated over tAccurate data in a large kinematical domain

Difference of polarized cross sectionsUnpolarized cross sectionsQ2 = 2.3 GeV2 xB = 0.36Results from JLAB Hall A E00-100Twist-2Twist-3PRL97, 262002 (2006)extracted twist-3 contribution small

No Q2 dependence: strong indication for scaling and handbag dominanceGPD !!!dominant term at small |t|s1int ~

Towards E and Ju, JdHermes DVCS-TTSA:Hall A nDVCS-BSA:x=0.36 and Q2=1.9GeV2 Neutron obtained combining deuteron and proton

F1 small and u & d cancel in

Present status of the MODEL-DEPENDENT Ju-Jd extraction

Unpolarised DVCS cross sections from HERA DVCS at small xB (< 0.01) mostly sensitive to Hg, HseaQ and W dependence: NLO predictionsbands reflect experimental error on slope b: 5.26 < b < 6.40 GeV-2

t dependence of DVCS cross sectionNew H1 results on slopes - DIS07DIS06

Lessons from DVCS at H1/ZEUS

Hard exclusive meson productionnecessary to extract longitudinal contribution to observables (L , ) allows separation and wrt quark flavoursFlavour sensitivity of DVMP on the proton factorisation proven only for Lconserve flip nucleon helicity T suppressed of by 1/Q2 wave function of meson (DA ) additional information/complication quarks and gluons enter at the same order of S

Dipole models for exclusive VM production at small x at very small x huge NLO corrections, large ln(1/x) terms (BFKL type logs) pQCD models to describe colour dipole-nucleon cross sections and meson WF Martin-Ryskin-Teubner (MRT) Phys.Rev. D62 (2000) 014022 Farshaw-Sandapen-Shaw (FSS) Phys.Rev. D69 (2004) 094013 Kowalski-Motyka-Watt (KMW) Phys.Rev. D74 (2006) 074016 Dosch-Fereira (DF) hep-ph/0610311 (2006) Frankfurt-Koepf-Strikman (FKS) Phys.Rev. D57 (1998) 512sensitivity to different gluon density distibutionssensitivity to 0 wave function at small x sensitivity mostly to gluonsdipole transv. size W-dep. t-dep. large weak steep small strong shallow

Recent ZEUS results on exclusive 0 production W-dependence W steeper energy dependence with increasing Q2 96-00 data: 120 pb-1 significant increase of precision + extended Q2 range arXiv:0708.1478[hep-ex]

W-dependence for hard exclusive processes at small xJ/ steep energy dependence for all vector mesons in presence of hard scaleQ2 and/or M2 universality of energy dependence at small x? at Q2+M2 10 GeV2 still significant difference between and J/

d / dt - 0ZEUS example (1 out of 6) Fit shallower t-dependence with increasing Q2

t-dependence for hard exclusive processes at small xrecent data suggestive of possible 15% difference between and J/

Selected results on R = L/T for 0 productionthe same W- and t-dependence for L and T i.e. contribution of large qqbar fluctuations of transverse * suppressedthe same size of the longitudinal and transverse * involved in hard 0 productionL T bL bT

Comparison to dipole modelsextensive comparison of the models to recent ZEUS 0 data inbelow just selected examples considered models describe qualitatively all features of the data reasonably well recent ZEUS data are a challenge; none of the models gives at the momentsatisfactory quantitative description of all features of the dataarXiv:0708.1478[hep-ex]

Comparison to GPD model Goloskokov-KrollHand-bag model; GPDs from DD using CTEQ6 power corrections due to kt of quarks includedboth contributions of *L and *T calculated H1 ZEUSZEUS (recent)W=75 GeVW=90 GeVW=90 GeV/10complete calculationleading twist only (in collinear approx.) leading twist prediction above full calculation, even at Q2 = 100 GeV2 10% sea quark contribution, including interference with gluons, non-negligible 25% at Q2 = 4 GeV2 contribution of T decreases with Q2, but does not vanish even at Q2 = 100 GeV2 20%arXiv:0708,3569[hep-ph]

Transverse target spin asymmetry for exclusive 0 production So far GPD E poorly constrained by data; mostly by Pauli form factorsGive access to GPD E related to the orbital angular momentum of quarksJis sum ruleqqThe asymmetry defined as to disentangle contributions from L and T the distribution of 0 decay polar angle needed in additionDiehl and Sapeta Eur. Phys. J.C 41, 515 (2005)

Method for L/T separation used by HERMESAngular distribution W(cos , , s) and Unbinned Maximum Likelihood fit a prerequisite for the method: determination of acceptance correction as a function of cos , and s A. Rostomyan and J. Dreschler arXiv:0707.2486

0 transverse target spin asymmetry from HERMES Transversely polarised proton target, PT 75% 2002-2005 data, 171.6 pb-1 for the first time 0 TTSA extracted separately for *L and *T

0 transverse target spin asymmetry from HERMES in a model dependent analysis data favours positive Ju in agreement with DVCS results from HERMES

0 transverse target spin asymmetry from COMPASS Transversely polarised deuteron target (6LiD), PT 50% 2002-2004 data in bins of = S: obtained using Double Ratio Method raw asymmetry from the fit to DR()dilution factor f 0.38u (d) are for upstream (downstream) cell of polarised targetarrows indicate transverse polarisation of corresponding cells

0 transverse target spin asymmetry from COMPASS asymmetry for deuteron target consistent with zero ongoing work on: longitudinal/transverse separation separation of incoherent/coherent in 2007 data taken with transversely polarised proton target (NH3)new

Exclusive + production from HERMES at leading twist L sensitive to GPDs H and E ~~ L/T separation not possible at HERMES, T expected to be supressed as 1/Q2 e p e n +

Beam spin asymmetry in exclusive 0 production from CLASpreliminary at leading twist L sensitive to GPD H no t-channel pion-pole (in contrast to exclusive + production) any non-zero BSA would indicate L-T interference, i.e. contribution not described in terms of GPDs ~e p e p 0

Cross sections for exclusive 0 production from JLAB HALL A DVCS Collab. from P. Bertin, Baryon-07t-slope close to 0, maybe even small negative E00-110h = 1 is the beam helicity

dashed prediction for L from VGG model using GPDs Vanderhaeghen, Guichon, Guidal

Summary for existing measurements New precise data on cross sections result in significantly more stringent constraints on the models for GPDs To describe present data on DVMP, both at large and small x, including power corrections (or higher order pQCD terms) is essential First experimental efforts to constrain GPD E and quark orbital momentum Results on DVCS promissing; good agreement with NLO for HERA dataindication of scaling and handbag dominance in valence region

Precision of DVCS unpolarized cross sections at eRHIC (1) eRHIC measurements of cross section will provide significant constraintsFor one out of 6 W intervals (30 < W < 45 GeV)Lint = 530 pb-1(2 weeks)Q2 [GeV2]eRHIC HE setup = 37 GeV (*p p) [nb]HE setup: e+/- (10 GeV) + p (250 GeV) L = 4.4 1032 cm-2s-1 38 pb-1/day

EIC measurements of cross section will provide significant constraintsFor one out of 6 Q2 intervals (8 < Q2 < 15 GeV2)W [GeV] = 10.4 GeV2 (*p p) [nb] also significantly extend the range towards small WPrecision of DVCS unpolarized cross sections at eRHIC (2)LE setup: e+/- ( 5 GeV) + p ( 50 GeV) L = 1.5 1032 cm-2s-1 13 pb-1/dayeRHIC Lint = 530 pb-1Lint = 180 pb-1HE setup: e+/- (10 GeV) + p (250 GeV) L = 4.4 1032 cm-2s-1 38 pb-1/day

Towards 3D mapping of parton structure of the nucleon at EIC = 0.125 GeV-2(assumed for illustration)simultaneous data in several (6) Q2 binsSufficient luminosity to do triple-differential measurements in x, Q2, t at EIC!Lint = 4.2 fb-1

Lepton charge asymmetry at eRHICBMK use improved charge asymmetries ACcos and ACsinmodel of Belitsky, Mueller, Kirchner (2002) for GPDs at small xBparameters of sea-quark sector fixed using H1 DVCS data (PL B517 (2001))except magnetic moment sea (-3 < sea < 2), which enters Jis sum rule for Jq sea = 2sea = 2sea = -3sea = -3 measurements of asymmetries at EIC sensitive tool to validate models of GPDs HE setup, Lint = 530 pb-1 divided equally between e+ and e-

Summary for DVCS at eRHIC Wide kinematical range, overlap with HERA and COMPASS 1.5 10-4 < xB < 0.2 - sensitivity to quarks and gluons 1 < Q2 < 50 GeV2 - sensitivity to QCD evolution DVCS cross sections - significant improvement of precision wrt HERA Intereference with BH - pioneering measurements for a collider powerfull tool to study DVCS amplitudes full exploratory potential, if e+ and e- available as well as longitudinaly and transversely polarized protons Sufficient luminosity to do tripple-differential measurements in xB, Q2, t

Backup slide

A simulation of DVCS at eRHICHE setup: e+/- (10 GeV) + p (250 GeV) L = 4.4 1032 cm-2s-1 38 pb-1/dayacceptance of Central Detector (improved ZDR) 2 < lab < 178diam. of the pipe - 20 cm, space for Central Detector: +/- 280 cm from IPevent generator: FFS (1998) parameterization with R=0.5, = 0.4 and b = 6.2 GeV-2DVCS + BH + INT cross sectiondue to acceptance and to reasonably balance DVCS vs. BH following kinematical range chosenacceptance simulated by kinematical cutskinematical smearing: parameterization of resolutions of H1 (SPACAL, LArCal) + ZEUS (, ) + expected for LHC (e , e )LE setup: e+/- ( 5 GeV) + p ( 50 GeV) L = 1.5 1032 cm-2s-1 13 pb-1/day2.5 < W < 28 GeVHE setupLE setup