EIC Collaboration Meeting, Hampton University,

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Exclusive Meson Production with EIC
Tanja Horn (JLab) Antje Bruell (JLab) Garth Huber
(University of Regina) Christian Weiss (JLab)

• EIC Collaboration Meeting, Hampton University,
• 19-23 May 2008

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Outline

• Exclusive processes physics motivation
• Cross section parameterization
• Monte Carlo simulations input for detector
  design
• L/T separations and the pion form factor

3
Exclusive Processes Physics motivation

• Study of high-Q2 exclusive processes essential
  part of physics program for ep collider
• Reaction mechanism QCD factorization
• Information about GPDs, meson wave functions
  (baryon/meson structure)

• Experimental challenge
• Small cross sections, s(mesonN) 1/Q8
• Detection of the recoil nucleon
• Differential measurements in x, Q2, t
• cf. GPD White Paper for NSAC Long-Range Plan,
  presented at Rutgers Town Meeting Jan-07

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Exclusive Processes Collider Energies
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Exclusive Processes EIC Potential and Simulations
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1H(e,ep)n at EIC Cross Section Parameterization
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MC Simulations

• Rate predictions including simulations of the
  detector restrictions
• Input for detector design
• Momentum and angular distributions for various
  particles
• Case studies
• H(e,ep)n
• H(e,ep)p
• H(e,eK) ?

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Exclusive MC Generator

• Exclusive EIC Monte Carlo
• Based on HERMES GMC
• New event generator using standard cernlib
  functions
• Includes cross section model by Ch. Weiss model
  for p production
• Can be easily extended to other channels, e.g.
  p, K? etc.

• MC agrees with fixed target data from Jlab

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1H(e,ep)n Momentum and Angular Distributions
neutrons
electrons
p

• Kinematically, electrons and pions are separated

Q2gt1 GeV2

• The neutron is the highest energy particle and is
  emitted in the direction of the proton beam

p
n
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1H(e,ep)n Scattered Electron

• Most electrons scatter at angles lt25
• BUT access to the high Q2 region of interest for
  GPD studies requires larger electron angles

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1H(e,ep)n Scattered Neutron

• Low t neutrons are emitted at very small angles
  with respect to the beam line, outside the main
  detector acceptance
• A separate detector placed tangent to the proton
  beam line away from the intersection region is
  required

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1H(e,ep)n Scattered Pion
Q2 (GeV2)
P (GeV)
Pion Lab Angle (deg)
Pion Lab Angle (deg)

• The pion cross section is peaked in the direction
  of the proton
• At larger Q2 pion angles and momenta are smaller
• within the capability of the detector (pp and Q2
  are uncorrelated)
• provide good missing mass resolution

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Event Topologies

• The most straightforward way to assure
  exclusivity of the 1H(e,ep)n reaction is by
  detecting the recoil neutron
• The neutron acceptance is limited to lt0.27 by a
  magnet aperture close to the interaction point

• Alternatively, the neutron can be reconstructed
  from missing momentum
• Missing mass resolution has to be good enough to
  exclude additional pions

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Rates and coverage in different Event Topologies
Assume 100 days, Luminosity10E34
Detect the neutron
Missing mass reconstruction
10ltQ2lt15
10ltQ2lt15
15ltQ2lt20
15ltQ2lt20
35ltQ2lt40
35ltQ2lt40
G ds/dt (ub/GeV2)
G ds/dt (ub/GeV2)
0.02ltxlt0.05
0.05ltxlt0.1
0.01ltxlt0.02
0.05ltxlt0.1
-t (GeV2)
-t (GeV2)

• Neutron acceptance limits the t-coverage
• The missing mass method gives full t-coverage for
  xlt0.2

Assume dp/p1 (pplt5 GeV)
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• At higher energies, the missing mass resolution
  deteriorates, so need to detect the neutron
• At lower energies, the missing mass
  reconstruction works well, but neutron detection
  is more difficult
• With Ee5 GeV and Ep 50 GeV can ensure
  exclusivity over the full region in (x,-t, Q2)
  using a combination of the two methods
• Overlap region between the two methods allows for
  cross checks

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Systematic uncertainty on the rate estimate
Assume 100 days, Luminosity10E34
10ltQ2lt15
15ltQ2lt20
35ltQ2lt40

• Data rates obtained using two different
  approaches are in reasonable agreement
• Ch. Weiss Regge model
• T. Horn p empirical parameterization

0.02ltxlt0.05
0.05ltxlt0.1
0.01ltxlt0.02
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Statistical uncertainty in the measurement
Assume 100 days
Luminosity 1031
G ds/dt (ub/GeV2)

• High luminosity is essential to achieve the
  experimental goals

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1H(e,ep)p Momentum and Angular Distributions
Photon from p decay
electrons
p
protons

• Similar to p, but additional complication due to
  photons from p decay

Q2gt1 GeV2

• p decay photon opening angle places a constraint
  on the calorimetry

p
2? opening angle
tlt1GeV2
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1H(e,ep)p p Decay Photons
1 ? 35mm / 2m

• Opening angle is small and requires fine
  calorimeter granularity
• JLab/BigCal 38x38mm, H1 forward calorimeter
  35x35mm
• High energy photons at large angles can be
  detected
• At high momentum, charged particles are difficult
  to measure

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1H(e,eK)? Momentum and Angle Distributions
Assume 100 days, Luminosity10E34

• Kinematics overall similar to the pion case
• Some p- from ? decay might be detected in an
  outbending toroidal field

?
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Rate estimate for K?
15ltQ2lt20
10ltQ2lt15
35ltQ2lt40

• Using an empirical fit to kaon electroproduction
  data from DESY and JLab

0.01ltxlt0.02
0.02ltxlt0.05
0.05ltxlt0.1
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1H(e,ep)n L/T Separation Experiments

• Pion Form Factor, Fp(Q2)
• Excellent opportunity for studying the QCD
  transition from effective degrees of freedom to
  quarks and gluons.
• i.e. from the strong QCD regime to the hard QCD
  regime.
• 2. Longitudinal Photon, Transverse Nucleon
  Single-Spin Asymmetry, A-p
• Especially sensitive to spin-flip GPD which
  can only be probed via hard exclusive
  pseudoscalar meson production.
• 3. QCD and GPD scaling tests
• Scan vs Q2 at fixed xB to test Hard QCD scaling
  predictions
• sL1/Q6, sT1/Q8
• Scan sL vs xB at fixed Q2 to distinguish pole and
  axial contributions in GPD framework.

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Determination of F? via Pion Electroproduction
At low Q2lt0.3 GeV2, the ? form factor can be
measured exactly using high energy ? scattering
from atomic electrons. ? F? determined by the
pion charge radius 0.6570.012 fm.

• To access higher Q2, one must employ the
  p(e,e?)n reaction.
• the t-channel process dominates ?L at
• small tlt0.02 GeV2.

In the actual analysis, a model incorporating the
? production mechanism and the spectator
nucleon is used to extract F? from ?L.
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L/T separations in exclusive p production

• Cross Section Extraction
• Determine sT e sL for high and low e
• Isolate sL, by varying photon polarization, e

Ee3 GeV Ep5 GeV
e0.64
e0.40
Ee5 GeV Ep2 GeV

• L/T separations require sufficiently large ?e to
  avoid magnification of the systematic uncertainty
  in the separation

• Requires special low energies for at least one e
  point and cannot be done with the standard EIC

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Different accelerator mode

• The ability to use 5-15 GeV protons will allow
  many high priority L/T-separation experiments
  which are otherwise not possible.
• The proton accelerator needs a mode where the
  injector is not run to its full energy.
• This beam is injected into the main proton
  accelerator, which is used as a storage ring.
• The costs to implement this low energy mode will
  be reduced if this flexibility is included at the
  planning stage.
• Achieving the high luminosity required for this
  experiment may not be possible

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Recoil Polarization Technique

• In parallel kinematics can relate sL/sT to recoil
  polarization observables

• From R and the simultaneous measurement of s0 one
  can obtain sL

• Requires only one epsilon setting
• Polarized proton beam
• Additional model assumptions needed in general if
  the reaction is not elastic

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Kinematic Reach (Pion Form Factor)

• Assumptions
• High e 5(e-) on 50(p).
• Low e proton energies as noted.
• ?e0.22.
• Scattered electron detection over 4p.
• Recoil neutrons detected at ?lt0.35o with high
  efficiency.
• Statistical unc ?sL/sL5
• Systematic unc 6/?e.
• Approximately one year at L1034.

Preliminary
Excellent potential to study the QCD transition
nearly over the whole range from the strong QCD
regime to the hard QCD regime.
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Projected uncertainties for Q-n scaling
EIC Ee5 GeV, Ep50 GeV
Preliminary

• Transition region 5-15 GeV2 well mapped out even
  with narrow fixed x and t
• careful with detector requirements

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Outlook

• Extend studies to vector mesons
• Resolution studies
• Test additional requirements from e.g. p and K?
• At high energies, calorimeter granularity needs
  to be better than 35x35mm
• Requirements on magnets, e.g. toroidal fields for
  KL

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Summary

• High Q2 studies of exclusive processes are an
  essential part of the physics program for an ep
  collider
• For beam energy 5 on 50 two methods are available
  to ensure exclusivity over the full range in
  (x,-t,Q2)
• At high energies, need a separate detector
  tangent to proton direction to detect the
  exclusive final state limited acceptance
• At low energies, missing mass reconstruction
  works well
• Overlap in certain kinematic regions allows for
  cross checks between the two methods
• High luminosity (10E34) is essential for these
  studies

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Other
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1H(e,ep)n Momentum and Angular Distributions

• Kinematically, electrons and pions are separated

p
neutrons
electrons
Q2gt1 GeV2

• The neutron is the highest energy particle and is
  emitted in the direction of the proton beam

p
n
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1H(e,ep)p p Decay Photons
6 on 15
Opening Angle (deg)
Opening Angle (deg)
3 on 30
5 on 50
10 on 250
p Lab Angle (deg)

• Separating the p decay photons is getting more
  difficult as the energy increases, but recall
  that pion momenta are low at high Q2

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Systematic uncertainty on the p rate estimate
10ltQ2lt15
15ltQ2lt20

• Data rates obtained using two different
  approaches are in reasonable agreement
• Ch. Weiss sT from Regge model
• T. Horn sT from p empirical parameterization

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Missing Mass Resolution
Assume dp/p0.5
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Longitudinal Photon, Transverse Nucleon
Single-Spin Asymmetry, A-p
where ds is the exclusive p(e,ep)n cross
section using longitudinal photons ß is the angle
between the proton polarization vector and the
reaction plane.

• Measure A-p to access the spin-flip GPD
• Requires a transversely polarized proton beam,
  and an L/T-separation.
• The asymmetry vanishes in parallel kinematics, so
  the p must be detected at ?pqgt0, -t up to 0.2Q2.

A.V.Belitsky, hep-ph/0307256

• A-p vs xB
• LO
• Q24
• Q210

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QCD Scaling Tests

• To access physics contained in GPDs, one is
  limited to the kinematic regime where hard-soft
  factorization applies
• No single criterion for the applicability, but
  tests of necessary conditions can provide
  evidence that the Q2 scaling regime (partonic
  picture) has been reached

• One of the most stringent tests of factorization
  is the Q2 dependence of the p electroproduction
  cross section
• sL scales to leading order as Q-6
• sT scales as Q-8
• As Q2 becomes large sL gtgt sT

Factorization
Q2 ?

• Factorization theorems for meson
  electroproduction have been proven rigorously
  only for longitudinal photons Collins,
  Frankfurt, Strikman, 1997

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Low e data from Jlab12?
JLAB Ee12
EIC Ee5 GeV, Ep50 GeV
e0.99
e0.3-0.7

• L/T separations at EIC will benefit from Jlab12
  measurements