Hard exclusive production at HERMES

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Hard exclusive production at HERMES
Cynthia Hadjidakis
2nd workshop on the QCD structure of the
Nucleon Rome, 12-16 June, 2006

• Generalized Parton Distributions
• Compton scattering (DVCS)
• Exclusive mesons production
• Summary and perspectives

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Hard exclusive production of photons and mesons
Q2
4 Generalized Parton Distributions (GPDs) H
H conserve nucleon helicity E
E flip nucleon helicity
Q2gtgt, tltlt

-2 x

xx
x-x
Pseudoscalar mesons (p, h)
Vector mesons (r, w, f)
DVCS (g) depends on 4 GPDs
t
t
30(DIS)
1
( H(x,x,t0) E(x,x,t0) ) x dx Jquark
1/2 DS D Lz
-1
?0 2ud, 9g/4
? 2u-d, 3g/4
f s, g
? u-d
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HERMES kinematics coverage
GPDs formalism Q2gtgt, tltlt
HERMES ltQ2gt2.4 (1-10) GeV2, -t lt 0.5 GeV2

• collider experiments
• H1, ZEUS
• 10-4ltxBlt0.021 gluons in the proton
• fixed target experiments
• COMPASS, HERMES
• ? 0.006/0.02ltxBlt0.3 gluons/valence and sea
  quarks
• CLAS
• ? 0.15ltxBlt0.6 valence quarks

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HERMES spectrometer
Tracking system dP/P 2 , dq lt 1
mrad (charged) Particle Identification RICH,
TRD, preshower, calorimeter Photon calorimeter
dP/P 5 for high energy photon
no recoil detection e p ? e g (p) only e and
g detected Exclusive reaction signed via the
missing mass technique MX ( e p e g
) Exclusive reaction selected with a cut on MX
Background contamination estimated with
non-exclusive MC
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Deep Virtual Compton Scattering e p ? e p g
for HERMES kinematics DVCS ltlt Bethe-Heitler
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DVCS asymmetries
IDs
? Different charge e e- (only at HERA!)
H

H

H, H
DsUT sinf ImH - E
H, E
Suppressed by kinematical factor
x xB/(2-xB ),k -t/4M2
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Beam spin and charge asymmetry
Beam Spin Asymmetry
Beam Charge Asymmetry
PRL87,2001
symmetrization f ? f (cancel sin f terms from
polarized beam)
hep-ex/0605108, subm. to PRL
e/- p ? e/- p g (MXlt1.7 GeV) - P1 P2 cos f
P3 cos 2f P4 cos 3f
L140 pb-1
L10 pb-1
P1 -0.010.02 P2 0.060.03 P3 0.020.03
P4 0.030.03
lt-tgt 0.12 GeV2,ltxBgt 0.1, ltQ2gt 2.5 GeV2
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Beam charge asymmetry t-dependence
DVCS


? H, H, E, E
GPD calculation different parameterization for
H
Vanderhaegen et.al. (1999)

• H double distribution q(x) with skewing
  effect
• D-term or not
• t dependence
• Regge-inspired t-dependence
• factorized t-dependence (ebt)

? AC sensitive to GPD-models
tiny e-p sample (L10 pb-1) HERA 2004-2005 e-
beam (x10)
P1 -0.010.02 P2 0.060.03 P3 0.020.03
P4 0.030.03
lt-tgt 0.12 GeV2,ltxBgt 0.1, ltQ2gt 2.5 GeV2
symmetrization f ? f (cancel sin f terms from
polarized beam)
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Longitudinal target spin asymmetry
DVCS


? H, H, E, E
Lp 50 pb-1 Ld 170 pb-1
sin f in agreement with GPD models
unexpected large sin 2f (NLO contributions) from
qGq correlations twist-3 GPDs?
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Transverse target
spin asymmetry
DVCS


? H, H, E, E

AUT sin(f-fS) cos(f) ImH - E cos(f-fS)
sin(f) ImH
GPD calculation Goeke et.al. (2001) ,
Ellinghaus et.al. (2005) H double
distribution Regge-inspired t-dep. D-term E
double distribution sensitive to Jq Ju
(Jd0) factorized t-dep. (dipole form factor)
L 64 pb-1
? First (model dependent) constraints on Ju and
Jd ! talk by Zhenyu Ye
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DVCS on nuclear target
DVCS


? H, H, E, E
GPDs modification in nuclear matter spatial
distribution of energy, angular momentum and
shear forces inside the nuclei

• coherent nuclear DVCS (-tlt0.05 GeV2) different
  from proton DVCS
• incoherent nuclear DVCS similar to proton DVCS
  (small BH cross section on neutron at small t)

• proton and deuteron data consistent
• highest t-bin may be affected by associated
  production (30)

2H (720 pb-1), 4He (30 pb-1), 14N (50 pb-1), Ne
(86 pb-1), Kr (135 pb-1), Xe (80 pb-1)
study of properties of quarks and gluons inside
nuclei
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Beam spin asymmetry on nuclear target
DVCS


? H, H, E, E
L30 pb-1
L86 pb-1

• ? clear sin f amplitude in the exclusive region
  for Ne and Kr
• ? soon Anucleus/Aproton (He, N, Ne, Kr, Xe )
• t-dependence (separation of coherent and
  incoherent part)
• A-dependence for coherent production

Guzey et al. (2003), Liuti et al. (2005)
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Factorization theorem for meson production
Q2
Q2gtgt, tltlt
hard scale
t
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Vector Mesons
VECTOR MESONS
? H, E
cross sections

• ? dx H(x,x,t) E(x,x,t) 2

E kinematically suppressed at low t H double
distribution q(x)/G(x) with skewing
effect factorized t-dependence (ebt with slope
from data)
transverse target spin asymmetry
AUT Im( H .E )
E double distribution sensitive to Jq
factorized t-dependence (dipole form
factor) higher order corrections cancel scaling
region reached at lower Q2
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e p ? e r0 (p) exclusive r0 selection
VECTOR MESONS
r0 ?p p- hh- detected
Missing energy DE (M2X-M2p)/2Mp (MX e p
e h h- )
0.6 lt M2hlt 1.0 GeV
-t-ttminlt0.4 GeV2
DE lt 0.6 GeV
Monte Carlo simulation of non-exclusive (DIS)
background
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extraction of sL r0 ? pp- angular distributions
VECTOR MESONS
g-p CMS
23 SDMEs (15 unpolarised, 8 polarised) extracted
in 3-D F, f, cos q
r rest frame
f
p
e
p
g
e
L250 pb-1
r
p
F
q
p-
if SCHC holds (VM retains g helicity)
? violation of SCHC
? at Q2 2 GeV2, sLsT
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r0 longitudinal cross sections
VECTOR MESONS
? H, E
EPJC17,2000
L 106 pb-1
Frankfurt et.al. (1996)

• more data to come r, f, w, r

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r0 transverse target spin asymmetry
VECTOR MESONS
? H, E
Vinnikov et.al. (2005)
L64 pb-1
xB
x
GPD model calculations (quarksgluons GPDs) E
related to Jq ? TSA sensitive to Jq 2 times more
data with 2005 sL/sT separation
? talk by Armine Rostomyan
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Pion pairs production e p (d)? e p (d) p p-
PION PAIRS
? H, E
Legendre moment
ltP1gt sensitive to the interference between
different pp- isospin states
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Legendre Moment Mpp dependence
PION PAIRS
? H, E
PLB599,2004
L250 pb-1
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Pseudoscalar Mesons
PS MESONS
cross sections

• ? dx H(x,x,t) E(x,x,t) 2

E kinematically suppressed at low t H double
distribution Dq(x) with skewing
effect factorized t-dependence

target spin asymmetry


AUT Im( H .E )
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p cross section measurement
PS MESONS
L/T separation not possible
sT suppressed by 1/Q2
? at large Q2, sL dominates
L250 pb-1
supported by REGGE model
Laget (2005)
Q2 dependence is in general agreement with the
theoretical expectation Corrections to LO (k- and
soft overlap) calculations overestimate the data
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Transverse target spin asymmetry for exclusive p
PS MESONS


gL p ? p n
Frankfurt et al. (1999)


Belitsky et al. (2001)
L 145 pb-1
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Future analysis recoil detector
clean reaction identification improve statistical
precision (Lp 750 pb-1, Ld 200 pb-1)
? talk by Ralf Kaiser
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CONCLUSION


? H E
? H E
? H, H, E, E
GPDs probed by hard exclusive photon and meson
production
H DVCS BSA, BCA excl. r0 sL excl. pions pairs
E TTSA DVCS, excl. r0
Corrections to leading order are needed to
describe the cross sections Leading order
calculations describe asymmetries
Jan. 06 polarized target removed, recoil
detector installed and under commissioning ?
HERMES dedicated to exclusive processes!

Asymmetries powerful tool to constrain GPD models


Reaction Observable GPDs

ep?epg BCA, BSA, L(T)TSA H (2ud) ep?ep?0
sL H (2ud) TTSA
H.E ep?epf
sL H (s) ep?ep? sL
H (2u-d) ep?eppp-
Legendre Moment H
ep?epp stot E
(u-d) TTSA
H.E ep?epp0 stot H
(2ud)



Hpp0 2/3 Hu/p 1/3 Hd/p Hpp Hu/p -
Hd/p

Polarisation provides observable sensitive to
different combinations of GPDS


dedicated experiments for exclusive measurements
starting soon at HERMES
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HERMES at DESY
e-beam e/e-, Ee27.5 GeV, PB 55 spin
rotators _at_ HERMES for longitudinal beam
polarization
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Longitudinal target spin asymmetrysin 2f


? H, H, E, E
unexpected large sin 2f from qGq correlations
twist-3 GPDs? upper limits for qGq correlations
twist-3 GPDs D. Mueller
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Model dependent constraint on Ju and Jd


? H, H, E, E
30
Exclusive p production e p ? e p(n)
Missing Mass2 (p-g-p)2 e p ? e p-n use of
p- yield to subtract the non exclusive background
e p ? e p X