experimental overview on exclusive processes

Experimental overview on Experimental overview on exclusive processesexclusive processes

Delia Hasch

1st Duality Workshop June 06-08, 2005; LNF-INFN, Frascati (Italy)

Q2

t

Q2>>, t<<

hard exclusive processeshard exclusive processes

• high luminosity ~1/Q4, 1/Q6

• high Q2 hard regime

• high resolution exclusivity

Quantum number of final state selects different GPDs:Vector mesons ( H EH E

Pseudoscalar mesons (: H E H E

DVCS () depends on H, E, H, EH, E, H, E~~ ~~

~~ ~~

prerequisities perspectives

DVCS asymm | VECTOR MESONS asymm | PS MESONS asymm

Compass: 100-190 GeV

Jlab: HallA,Bup to 6 GeV e

H1, ZEUS distinct signature in 4 det.

HERMES, COMPASS, JLab:

missing mass/energy technique:

CLAS: MX ~ 30 MeV

HERMES: MX ~ 400 MeV



Hall A: p , n Hall B: p, d

HERMES: p , d unpol. H, D, 4He, 14N, 20Ne, Kr, Xe

H1, ZEUS, HERMES:27.5 GeV e, e

H1, ZEUS: p (820 GeV)

Compass: d , p

prerequisitesprerequisites

CLAS

HERMES

Deeply virtual Compton ScatteringDeeply virtual Compton Scattering

γpepe '' Bethe-HeitlerHallA,B HERMES H1/ZEUS

BHDVCS



NLO analysis of H1, ZEUS cross sections

H H E ~

DVCS cross section DVCS cross section first measurement oft –slope:d/dt = d/dt|t=0∙exp(-b t)

b 6.02±0.350.39 GeV2

|e)(),,( b|tq xqtxH |e)(),,( b|tg xgxtxH

[A.Freund, M. McDermott EPJC23(2002)]

absolute normalisation!

comparison to NLO QCD:

band width given by b measurement



Hg Hq

DVCS asymmetries DVCS asymmetries )τττ(τ |τ||τ|dσ BH

*DVCSDVCS

*BH

2DVCS

2BH

DVCS-BH interference leads to non-zero azimuthal asymmetry

BH

II

d

dd

)(d)(d

)(d)(d)(A

LU ~ sin∙Im{F1H + (F1+F2)H +kF2E}~

UL ~ sin∙Im{F1H + (F1+F2)(H + …}~

UT ~ sin∙Im{k(F2H - F1E) + … }

separating GPDs through polarisation: LU

beam target

= xB/(2-xB ),k = t/4M2 kinematically suppressed

H, H, E

H, H

H, E

~

~



H H E ~

beam spin asymmetrybeam spin asymmetryγpepe ''

V

[PRL87,2001] [PRL87,2001]

PRELIMINARY

(not for circulation)

0.15 < xB< 0.41.5 < Q2 < 4.5 GeV2

-t < 0.5 GeV2

- VanderhaeghenVanderhaeghen --



H H E ~

CLAS

BSA on nuclear targetsBSA on nuclear targets

… study dependence of coherent production on nuclei

heavier targets : 4He, 14N, 20Ne, Kr, Xe



H H E ~

target spin asymmetrytarget spin asymmetryγpepe ''

V

PRELIMINARY




H H ~

CLAS

target spin asymmetrytarget spin asymmetry

- VanderhaeghenVanderhaeghen --

transverse TSA: sensitivity to GPD E (Jq)



H H E ~

beam charge asymmetrybeam charge asymmetry(only @ HERA)

γpepe ''/- γpede ''/-



H H E ~

beam charge asymmetrybeam charge asymmetry(only @ HERA)

GPD calculations: different parametrisations for H

- VanderhaeghenVanderhaeghen (1999/02)(1999/02) ––

HERA: 2004/05 e beam (5x)

AC: high sensitivity to GPD models (tiny ep sample ~10pb-1)

… H1/ZEUS ?



H H E ~

DVCS DVCS DVMS DVMS

ρ0 2u+d, 9g/4

ω 2u-d, 3g/4

s, g

ρ+ u-d



H H E ~

H E

DVCS theoretically cleanest process to study GPDs

- azimuthal asymmetries from interference term relate directly to amplitude

hard exclusive meson production: flavour separation

H E ~ ~

! factorisation theorem for L only !

vs W in bins of Qvs W in bins of Q22 J/

• ~ Wwith • no dependence on Q2

general transition to hard behaviour at high Q2+M2



H E

LL separation from angular separation from angular distributiondistribution

if SCHC holds (VM retains * helicity):

0r0r 311

0411 Im

0400

0400

T

L

r1

r

δ)(ε

1

σ

σR

0

2002 < 10%

[Vanderhaegen et.al. (1999)]

--- 2-gluon exchange--- quark exchange

CLAS: analysis of L for 0 from 6 GeV data (higher W !) going on …



H E

00 transverse TSA transverse TSAinterference of H and E:

S ~ STsin S E∙H

- - Goeke, Polyakov & VanderhaeghenGoeke, Polyakov & Vanderhaeghen (2001)(2001) - -

EE related to JJqq TSA sensitive to J Jqq

0pepe ''

theorySS

SSS A

)(N)(N

)(N)(N

S

1)(A

TUT

same x-dependence behaviour asGPD calculations



H E

cross sectioncross sectionComparison with GPD model for L: [Vanderhaegen, Guichon, Guidal (1999)]

• Q2 dependence: in general agreement with the theoretical expectation• LO calculations too small

power corrections (k┴ ,soft overlap) overestimate data

Q2 dependence:

T suppressed by 1/Q2



H E ~ ~

cross section ratioscross section ratios

production: no-

pole

H dominates

[Mankiewicz,Piller & Radyushkin (1999)]

x

~measure at HERMES Jlab:

PRELIMINARY




H E ~ ~

++ transverse TSA transverse TSAinterference of H and E:

S ~ STsin S E∙H

~ ~

~~

gives access to relative size of H and E ~ ~

soon from HERMES Compass, Jlab …

TA

RG

ET

SP

IN A

SY

MM

ET

RY

L p → + n

- - Frankfurt, Pobylitsa, Polyakov & StrikmanFrankfurt, Pobylitsa, Polyakov & Strikman (1999)(1999) - -

- - Belitsky & MBelitsky & Müüllerller (2001)(2001) - -- - Frankfurt, Polyakov, Strikman & VanderhaeghenFrankfurt, Polyakov, Strikman & Vanderhaeghen (2000)(2000) - -



H E ~ ~

dedicated experimentsdedicated experiments

DVCS with recoil: 2006/07

background “free” DVCS:

• semi-inclusive bg: 5% <<1%

• associated bg: 10% 1%



JLab/Hall A JLab/CLAS

Inner Calorimetrer and supraconducting magnet within CLAS torus

e

e’p

γ

Dedicated, high statistics, DVCS experiments

→ Detection of 3 particles e, p and γ in final state

→ Firmly establish scaling laws (up to Q2 ~ 5 GeV2), if observed, or deviations thereof understood, first significant measurement of GPDs.

→ Large kinematical coverage in xB and t leads to femto-tomography of the nucleon

→ Opens the way for an ambitious program with JLab@12GeV (CLAS12 and other)

JLab dedicated DVCS experiments in 2004 - 2005JLab dedicated DVCS experiments in 2004 - 2005

M. Garcon

conclusionsconclusions



• first exciting results on DVCS and DVMS available

polarisation provides new observables sensitive

to different (combinations) GPDS

• dedicated experiments for DVCS measurements going

on at JLab or starting soon at HERMES

new facilities (high luminosity/energy/resolution) needed

to map GPDs in an global analysis of all hard exclusive

processes

H E ~ ~

H E H H E ~

backup slidesbackup slidest – dependence of BSA for photon and pion production:

PRELIMINARY


factorisation theorem factorisation theorem predictionprediction

for fixed xB and t

asymptotically

6Q

16Q

1LL

σ ),(QFactor Kin.σ red2

L x

4Q

1

2Q

12Q

1spins

2red |M|σ

fit: 1/Qp

p=1.9±0.5p=1.7±0.6p=1.5±1.0

experimental overview on exclusive processes

Documents

dvcs dvms h e dvcs

e beam

h vanderhaeghen

particles e

ppole h

high q2 m2 h e sl separation

p p n frankfurt

hermes jlab