Exclusive p+ production at HERMES
Cynthia Hadjidakis
on behalf of the HERMES collaboration
Baryons 2004, Palaiseau, France
October 25-29, 2004
 Generalized Parton Distributions
 Exclusive p+ production at HERMES
 Cross section measurements
 Ongoing and future analysis
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Factorization theorem for meson production
g *L
M
Q2>>, t<<
- Collins, Frankfurt & Strikman (1997) -
F
~~
H,E,H,E
N
4 Generalized Parton Distributions (GPDs)
~
H
H
conserve nucleon helicity
~
E
E
flip nucleon helicity
Vector mesons
N’ (r, w, f)
Pseudoscalar
mesons (p, h)
t
~ ~
p+ production: H E
~
At low t and large x, E dominated by the pion pole
~
~
Constrain pole E and non pole H would help the Fp extraction
Factorization for longitudinal photons only
sT suppressed by 1/Q2 → at large Q2, sL dominates
ds L
dt
Cynthia Hadjidakis
asymptotically
for fixed x and t
1
Q6
Baryons 2004
Palaiseau, 25th October 2004
HERMES Spectrometer
e+
27.5 GeV
Tracking system: dP/P = 2 %, dq < 0.6 mrad
Particle Identification: RICH, TRD, preshower, calorimeter
lepton identification 99%
p identification
98% for 2<pp<15 GeV
e+ p → e+ p+n
Cynthia Hadjidakis
e+ and p+ detected, no recoil detection
Exclusive reaction signed via the missing mass technique
missing mass = (e+p-e’-p+)
Baryons 2004
Palaiseau, 25th October 2004
Exclusivity for e p → e p+(n)
Missing Mass resolution does not allow to separate exclusive events from background
e p → e p-n : use of p- yield to subtract the non exclusive background
e p → e p+ X
e p → e p+ n
p+ enhancement
Exclusive peak clearly centered at the nucleon mass
Mean and width in agreement with exclusive MC
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Cross section determination
+
σ γ pπ n ( x, Q 2 ) 
*
L
x Q2
G(<x>,<Q2>)
N excl
p+
L x Q 2 Γ( x ,  Q 2 ) ( x, Q 2 )
integrated luminosity
range in x and Q2
virtual photon flux factor
Nexcl number of p+ background subtracted
1996-2000: hydrogen target (unpolarized and polarized)
14.2 M DIS events (L=283 pb-1) – 3.5k exclusive p+
(x,Q2): detection probability using 2 exclusive MC (different GPD
parameterization)
- Mankiewicz, Piller & Radyushkin (1999) - Vanderhaeghen, Guichon & Guidal (1999) -
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Exclusive distributions: Monte Carlo comparison
- Vanderhaeghen, Guichon & Guidal (1999) 1
tuned t-dependence
● Data
 Monte Carlo
(arbitrary norm.)
Q2 (GeV2)
xB
-t (GeV2)
○ Mankiewicz et al.
● Vanderhaeghen et al.
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Cross section: Q2 dependence for different x ranges
Comparison with GPD based model:
-Vanderhaeghen, Guichon & Guidal (1999) -
stot = sT + e sL
L/T separation not possible
… but:
sT suppressed by 1/Q2
Hermes kinematics:
0.80  e  0.96
→ at large Q2, sL dominates
Q2 dependence is in general agreement with the theoretical expectation
Power corrections (k┴ and soft overlap) calculations overestimate the data
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
- Vanderhaeghen et al. (1999) -
p+ production:
~
pseudoscalar contribution E
~
pseudovector contribution H
~ ~
Leading Order (H+E)
LO + power corrections (k┴
and soft overlap)
Q2=2.4 GeV2
t=tmin
Q2=2.4 GeV2
t=-0.3 GeV2
Hermes kinematics
- Vanderhaeghen et al. (1999) -
- Huang, Wu & Wu (2004) CEA, Cornell, Jlab
Pion Electromagnetic
Form Factor
LO + power corrections
Leading Order
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Cross section: Q2 dependence for different x ranges
Comparison with REGGE model:
- Laget (2004) -
hadronic degrees of freedom
p+
g*
p
n
exchanged trajectories p, r
the longitudinal part
dominates the total cross
section
REGGE calculations overestimate the data
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Cross section: Q2 dependence for different x ranges
Factorization theorem prediction:
sL
asymptotically
for fixed x and t
1
Q6
σ L  Kin. Factor (Q 2 , x) σ red
1
Q4
σ red   | M |2
spins
1
Q2
fit: 1/Qp
p=1.9±0.5
p=1.7±0.6
p=1.5±1.0
Q2 dependence in agreement with theoretical expectation
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Ongoing analysis with a transversally polarized target
- Frankfurt, Pobylitsa, Polyakov & Strikman (1999) - Frankfurt, Polyakov, Strikman & Vanderhaeghen (2000) - Belitsky & Müller (2001) g*L p → p+ n
TARGET SPIN ASYMMETRY
Transverse target spin asymmetry
~
~
interference between E and H
~ ~
sS: |ST| sin (f-fS) E H
~ ~
TSA linear dependence E.H
~ ~
/ cross section quadratic combination (E+H)2
TSA higher order corrections cancel: scaling region reached at lower Q2
2002-2005: run with a transverse polarized target
6 M DIS expected (4 M on disk) - Nexcl ~ 1.5 k
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Future analysis for pseudoscalar mesons
Recoil detector
mid 2005
Detection of the
recoiling proton
p0 production: no p-pole
e p→e p+ n / e p→e p0 p
Mankiewicz,Piller & Radyushkin (1999)
axial anomaly
and SU(3) breaking effects
h/p0 on proton and deuteron
Eides,Frankfurt & Strikman (1999)
h/p0
deuteron
Pseudoscalar
meson ratios
proton
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Conclusion
 GPDs can be probed by hard exclusive meson production
 Measurement of the total cross section for hard exclusive p+ at
HERMES
 Comparison with GPD and REGGE based model
 Q2 dependence for different x ranges in agreement with
factorization theorem expectation
 Transverse target spin asymmetry for p+: ongoing analysis
 summer 2005: start run with recoil detector
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Exclusivity for e p → e p+(n)
Ee=27.5 GeV
Ee=12 GeV
mean=0.880.05
sigma=0.290.07
mean=0.870.05
sigma=0.680.04
Missing Mass 2 (GeV2)
Missing Mass 2 (GeV2)
For different beam energy, same exclusive peak at the nucleon mass
L/T separation not possible
stot = sT + e sL
Hermes kinematics: e  0.80
sT suppressed by 1/Q2 → at large Q2, sL dominates
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
GPDs: Q2 and x dependence
dsL/dt (nb/GeV2)
- Vanderhaeghen, Guichon & Guidal (1999) -
t=tmin
Q2=2.4 GeV2
kperp+overlap
~ ~
LO (H+E)
~
H
~
E
LO
overlap
kperp
x
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Binning choice
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Power corrections
-Vanderhaeghen, Guichon & Guidal (1999) -
intrinsic transverse momentum of the active quark (kperp)
(Q2>>) → 0
soft overlap contribution → no gluon exchange
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004
Piller – VGG
- Mankiewicz, Piller & Radyushkin (1999) - Vanderhaeghen, Guichon & Guidal (1999) -
arbitrary scale
xB=0.1
Q2=2.4 GeV2
VGG
~ t
Tuned VGG (E*e )
Piller parametrisation
t=tmin
xB=0.1
Hermes kinematics
Q2
t=tmin
Q2= 2.4 GeV2
xB
Cynthia Hadjidakis
g*p  p+n
-t
different dependence
for VGG and Piller
t=-0.4 GeV2
Q2= 2.4 GeV2
xB
Baryons 2004
Palaiseau, 25th October 2004
p+ production
g *L
p+
Fas
~
H pseudovector contribution
~
E pseudoscalar contribution (pion pole)
~~
H,E
p
n
- Vanderhaeghen, Guichon & Guidal (1999) -
At low t, dominance of the pion pole:
~
E related to Fp
~
~
Constrain pole E and non pole H
would help the p FF extraction
Cynthia Hadjidakis
Baryons 2004
Palaiseau, 25th October 2004