Vector Mesons and DVCS at Jefferson Lab
Valery Kubarovsky Jefferson Lab Newport News, VA April 12, 2001
XIX Interna onal Workshop on Deep-Inelas c Sca ering and Related Subjects Outlook
• Introduc on • DVCS with unpolarized target • DVCS with polarized targets • Vector meson electroproduc on • JLAB 12 upgrade • Conclusion Descrip on of hadron structure in terms of GPDs
Nucleon form factors Structure functions GPDs transverse charge & quark longitudinal correlated quark momentum current densities momentum (polarized distributions (polarized and and unpolarized) unpolarized) in transverse distribu ons space q q γ∗ γ
(x + ξ)P (x ξ)P DVCS and DVMP − GP D • Factoriza on theorem (1 + ξ)P (1 ξ)P • Access to fundamental degrees of − freedom
DVCS: • the clearest way to access the GPDs • Only γT photons par cipate in DVCS ~ ~ • Interference with BH process H,E DVMP:
• Factoriza on proven only for σL 2 H, E σT/σL~1/Q • Meson distribu on amplitude • Gluon exchange required • Vector and pseudoscalar meson produc on allows to separate flavor and separate the helicity-dependent GPDs form helicity independent. Accessing GPDs through polariza on
σ+ - σ- Δσ - A = σ+ + σ = 2σ ξ ~~ xB/(2-xB) k = t/4M2 Polarized beam, unpolarized proton target: ~ ( ,t) ΔσLU ~ sinφ {F1H + ξ(F1+F2)H +kF2E }dφ H ξ
Kinematically suppressed
Unpolarized beam, longitudinal proton target: ~ ~ ΔσUL ~ sinφ {F1H+ξ(F1+F2)(H +ξ/(1+ξ)E ) -.. }dφ H ( ξ ,t ), H(ξ,t)
Kinematically suppressed
Unpolarized beam, transverse proton target:
ΔσUT ~ cosφ {k(F2H – F1E ) + ….. }dφ H(ξ,t), E(ξ,t) Kinematically suppressed H(ξ,t), E(ξ,t)… are CFF JLab Site: The 6 GeV Electron Accelerator
Hall-B CLAS Hall-A
Hall-C
3 independent beams with energies up to 6 GeV Dynamic range in beam current: 106 Electron polarization: 85% CEBAF Large Acceptance Spectrometer CLAS
424 crystals, 18 RL, CLAS Lead Tungstate Electromagne c Calorimeter Poin ng geometry, APD readout σ+ - σ- Δσ - DVCS Beam Spin Asymmetry ALU ALU = σ+ + σ = 2σ
• VGG parameterization CLAS data 2
reproduces –t > 0.5GeV ) 2 0.2 # sin! # behavior, and overshoots 0.1 1+" cos! 0.3 (GeV 2 0 0.2 asymmetry at small t. Q BSA -0.1 0.1 (integrated) 0 -0.2 3 • The latter could indicate that 0 90 180 270 360 VGG misses some important ! (deg) e1-dvcs contributions to the DVCS Hall-A cross section. CLAS (previous) VGG model • Regge model (J-M Laget) is in fair VGG + twist3 agreement in some kinema c bins with 2 Laget model our results.
• The Regge mode seems to be working at 0.5 1 1.5 low Q2 while the GDP approach gets -t (GeV2) be er at larger Q2. This is expected
1 F.-X. Girod et al., PRL 100 (2008) 162002 0.1 0.2 0.3 0.4 xB Extrac on of Compton Form Factors from CLAS DVCS data
• ALU and AUL CLAS results only • Im H(t) Im H~ (t) are extracted M. Guidal, Phys.Le .B689:156-162,2010 • Im H~ (t) fla er than Im H(t)
The fact that H is "fla er" in t than H, hints that the axial charge of the nucleon ~ is more concentrated than the electromagne c charge. This is related to the fact that the axial form factor is also fla er than the EM form factors. We see that via different formalism (GDPS vs FFs) and reac on (DVCS vs elas c), one reaches the same conclusions. "(ep #ep$) DVCS x-sections from e1dvcs F.X. Girod
!
F.X. Girod Hyon-Suk Jo Alex Kubarovski
Four dimensional grid 2 CLAS PRELIMINARY Q , xB, t, φ
Radiative corrections and π0 contamination accounted ! ! DVCS target spin asymmetry ! (ep ! ep") eg1-dvcs - completed data taking at 2009
E. Seder Polarizations: Beam: ~80% NH3 proton ~70% Beam energy ~5.7 GeV
Longitudinal Polarized target
Longitudinal target SSA will be extracted in bins in Q2, x and t 11 DVCS double spin asymmetry eg1-dvcs - completed data taking at 2009
(N ++ + N !! )! (N +! + N !+ ) A = LL ++ !! +! !+ S. Pisano fPbeamPt arget (N + N )! (N + N )
Fitting function:
2 2 ALL = ! + "cos# +$cos # +%sin #
N+/-: number of DVCS events with a positive (negative) target/beam polarization
Pbeam/T: beam/target polarization f: diluition factor Hall A
• Proton DVCS, helicity dependent and independent cross sec ons were measured at Q2=(1.5, 1.9, 2.3) GeV2 -t=(0.17, 0.23, 0.28, 0.33) GeV2
xB=0.36 • Neutron DVCS, helicity dependent cross sec on on deuterium. Sensi ve to E(!,t) Q2=1.9 GeV2
xB=0.36 • Completed data taking at 2010, which included measurements of DVCS on proton and deuterium at two different energies with the aim to separate Re [DVCS*BH] and |DVCS|2 terms. Imaginary Part of the Interference Term
VGG model
• VGG model agrees in slope with the data but lies 30% above • Q2 independent in all t bins • Provide support for the factoriza on at Q2>2 GeV2 Constraint on Jd and Ju
Helicity-dependent Jlab Hall-A neutron and HERMES transversity polarized proton data constrain in a model dependent way on the total up and down quark contribu ons to the proton spin.
1 1 1 J = !" + L = x[H (x,!,t = 0)+ E (x,!,t = 0)]dx q 2 q q 2 $ #1 q q Exclusive Meson Produc on + + + 0 ep ! en" + ep ! en" , " ! # # 0 0 + $ ep ! ep" 0, " 0 ! ## ep ! ep" , " ! # # ep ! ep$, $ ! ## ep ! ep%, % ! # +# $# 0 ep ! ep&, & ! K +K $
CLAS6: lots of data. New proposal being prepared CLAS12: Exp. # E12-06-108 for PAC 38
K. Lukashin et al., Phys.Rev.C63:065205,2001 (φ, 4.2 GeV) C. Hadjidakis et al., Phys.Le .B605:256-264,2005 (ρ0,4.2 G eV) L. Morand et al., Eur.Phys.J.A24:445-458,2005 (ω, 5.75GeV) J. Santoro et al., Phys.Rev.C78:025210,2008 (φ, 5.75GeV) S. Morrow et al., Eur.Phys.J.A39:5-31,2009 (ρ0, 5.75GeV) A. Fradi, Orsay Univ. PhD thesis (ρ+ ,5.75 GeV)
Vector Mesons Quark and Gluon GPDs New d" (# * p $en% + ) & 'tebt dt
!
CLAS data. The first measurement of the ρ+ exclusive electroproduc on ρ+ t-slope parameter
d" (# * p $en% + ) & 'tebt 2) dt b(xB,Q
Slope parameter is ! decreasing with xB. This indicates that the size of the interac on region decreases as xB1
xB )
2 9
Vector mesons t-slope parameter ï CLAS (5.754 GeV) FermiLab (1977) CLAS (5.754 GeV) CLAS (5.754 GeV) CORNELL E665 CORNELL CLAS (5.754 GeV) 1 8 CORNELL 2 2 2 b (GeV HERMES H1 H1 W = Q ( !1)+ m NMC ZEUS ZEUS N 7 xB 6
5
4 New
3 2 0 + 1 l t q l
0 W 10 10 10 10 10 10 10 10 W (GeV) W (GeV) W (GeV) W (GeV) )
2 9 ï
CLAS (5.754 GeV) CLAS (5.754 GeV) CLAS (5.754 GeV) 8 CLAS (5.754 GeV) DESY 0 DESY DESY b (GeV + SLAC l SLAC t SLAC q Daresbury l 7 6 New
5
4
3
2 1 Q2 0 0123456 0123456 0123456 0123456 Q2 (GeV2 ) Q 2 (GeV2) Q 2 (GeV2) Q 2 (GeV2)
• b increases with W : the size of the nucleon increases as one probes the high W values (i.e. the sea quarks). Sea quarks tend to extend to the periphery of the nucleon. * + σL, σΤ separa on ! L p ! n" S-channel helicity conserva on
2 2 σ 2 2 1.00 1 10ï1 10ï1 10ï2 2 2 Q =1.25 10ï2 Q =1.75 10ï3 10ï3 0 0.5 1 1.5 2 2.5 3 0 0.5 1 1.5 2 2.5 3 W(GeV) W(GeV) 2.00 10 1 1 Q2=2.75 10ï1 10ï1 Q2=2.25 10ï2 10ï2 0 0.5 1 1.5 2 2.5 3 2 2.5 3 3.5 4 W(GeV) W(GeV) 3.00 1 1 2 Q =3.25 ï1 10 10ï1 Q2=3.75 10ï2 10ï2 10ï3 2 2.5 3 3.5 4 4.5 0 0.5 1 1.5 2 2.5 3 W(GeV) W(GeV) 4.00 1 + mL(l ) [[email protected] GeV] 2 + 10ï1 Q =4.25 mL(l ) [VGG] ( +) [GK] ï2 mL l 10 W 2 2.5 3 3.5 4 4.5 5 W(GeV) GPD fails to describe data by more than order of magnitude * 0 " L p # p$ Fails to describe data W<5 GeV Describes well for W>5 GeV Regge model VGG model ! σL VGG with D-term W GK model •Popular GK and VGG models can not provide the right W-dependence of the cross-sec on •This does not mean that we can’t access GPD in vector meson electroproduc on •For example, model with the addi on of q- qbar exchange (M.Guidal) together with standard VGG model successfully describes data !"#$%&'()"**#%"(+) * φ and ρ0 * 0 " L p # p$ Goloskokov, Kroll " L p # p$ φ ρ0 CLAS ZEUS 102 Cornell ZEUS E665 HERMES 2 p) [nb] p) Cornell p) [nb] p) $ # 10 H1 ! 1 ! p-> CLAS p-> H1 * * 10 " " ( ( L L ! ! GPD GK GPD GK 100 101 4 6 81020 40 60 100 4681020 40 60 100 W[GeV] W[GeV] (a) (b) •φ mesons - gluon GPD are dominant 0 FIGURE 5. The longitudinal•ρ 0 and cross ω - sea quarks and/or gluons dominant. section for (a) exclusive $,(b)exclusive # production (adapted from Ref. [25]). The curve/error GPD approach describes well data for W>5 band show the GPD–based model calculationGeV of Ref. [25]. transition GPDs, which can be related to the usual flavor–diagonal GPDs in the proton using SU (3) flavor symmetry (see Ref. [29] for other interesting “ratio observables”). Pseudoscalar meson production (%,&,K)inthesmall–sizeregimeprobesthepolar- ized GPDs in the nucleon. A particular feature of %+ (and to some extent also K+) production is the existence of a “pole term” in the GPD, in which the QCD operator mea- suring the quark density is connected to the nucleon by t–channel %+(K+) exchange; it gives a contribution to the amplitude proportional to the pion form factor, which is in fact the basis for measuring the latter in electroproductionexperiments[30].Betterin- sight into the relation between the “pole” and the “non–pole”componentoftheGPDand their relative importance is necessary not only for improving the extraction of the pion form factor, but also for isolating the non-pole component related to the nucleon helicity structure. It could come e.g. from model–independent comparisons of %+ (which has a pole) and %0 (no pole) electroproduction data (see Ref. [29] for strange channels). Ex- perimental studies of exclusive pseudoscalar meson production are challenging because the L/T virtual photon cross sections have to be separated by comparing data taken at different beam energies (Rosenbluth method). There are intriguing suggestions that !T in exclusive pion production above the resonance region could be described as the limit of semi-inclusive production via the fragmentation mechanism [31]; if confirmed, this could greatly aid the analysis of such processes. GPDs IN SMALL–x PHYSICS AND pp SCATTERING The notion of the transverse spatial distribution of partonsconveyedbytheGPDshas many important applications in small–x physics and high–energy pp collisions with hard processes. Measurements of the t–dependence of exclusive J/' photo/electroproduction at HERA (cf. Fig. 4b) and FNAL have provided a rather detailed picture of the transverse spatial distribution of gluons with 10−4 < x < 10−2 [22]. In particular, these experiments have shown that the nucleon’s gluonic transverse size at Q2 ∼ few GeV2 is substantially smaller than its size in soft hadronic interactions at high energies, and increases less Generalized parton distributions: Status and perspectivesFebruary12,20098 JLab 12 GeV Upgrade CLAS12 • High luminosity • Large acceptance • Wide kinematic coverage • High precision The JLab 12 GeV project offers an unprecedented fron er of intensity and precision for the study of deep exclusive sca ering. Kinema c Bins at Jlab 12 GeV CLAS12 Hall-A Le : CLAS12 kinema cs for DVCS and DVMP on unpolarized H2 and longitudinally Polarized targets. The colors and density are propor onal to the rela ve count rates. Right: Hall A kinema cs for DVCS and π0 electroproduc on. Beam me is adjusted for roughly equal counts in all bins. Jlab Upgrade Program Deeply Virtual Exclusive Meson Electroproduc on ep ! ep! 0 !!!!!!!ep ! ep" ep ! ep#!!!!!!!!ep ! ep$0 ep ! ep% ep ! en$+ Deep Virtual Compton Sca ering ep ! ep! !!!!!!!!en ! en! • Kinema cs: Q2 from 3 – 10 GeV2 -t from .5 to 10 Gev2 W from 2-4 GeV Summary 2015 • Jlab DVCS experiments provide important data, crucial for the extrac on of GPDs in a wide kinema cal region • DVCS with polarized and unpolarized targets provides precise informa on on H and H ~ • The most extensive set of π0, η, ρ+,ρ0,ω, and φ electroproduc on to date has been obtained with the CLAS spectrometer. • Jlab 12 GeV program of DVCS, pseudoscalar and vector meson electroproduc on will provide unique informa on about the: - transi on between so long-range phenomena and hard short range - quark momentum and spin distribu ons of the nucleons. - quark and gluon GPDs DVCS 104 0 nb/GeV 1 10-1 -1 10 10-2 -2 10 0 50 100 150 200 250 300 350 q degrees DVMP DVCS 104 0 nb/GeV 1 10-1 -1 10 10-2 -2 10 0 50 100 150 200 250 300 350 q degrees DVMP DVCS kinema cs ! (ep ! ep") 2 Q -t ) ) 1 2 2 4.5 0.9 (GeV 2 -t (GeV Q 4 0.8 0.7 3.5 0.6 3 Kinema cal coverage 0.5 2 2.5 0.4 (xB,Q ) and (xB,-t) 2 0.3 0.2 1.5 0.1 1 0 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 xB xB xB xB 8000 16000 Exclusive cuts 7000 14000 Missing Energy 0 ! 6000 12000 And π subtrac on 5000 10000 / 9 MeV / 0.007 4000 8000 3000 6000 events events N N 2000 4000 1000 2000 0 0 0 0.5 1 1.5 2 -0.5 0 0.5 1 1.5 "! Y (!) EX (GeV) Helicity dependent (top) and independent DVCS Cross sec ons Twist-2 Complete fit Q2=2.3 GeV2 BH Twist-2 Twist-3 The helicity independent cross sec ons show the significant contribu on from the sum of the interference and DVCS terms as compared to the pure BH cross sec on