PAC34 Kaons

1
L-T Separated Kaon production Cross Sections from
5-11 GeV
P. Bosted, S. Covrig, H. Fenker, R. Ent, D.
Gaskell, T. Horn, M. Jones, J. LeRose, D.
Mack, G.R. Smith, S. Wood, G. Huber, A. Semenov,
Z. Papandreou, W. Boeglin, P. Markowitz, B.
Raue, J. Reinhold, F. Klein, P. Nadel-Turonski,
A. Asaturyan, A. Mkrtchyan, H. Mkrtchyan, V.
Tadevosyan, D. Dutta, M. Kohl, P. Monaghan, L.
Tang, D. Hornidge, A. Sarty, E. Beise, G.
Niculescu, I. Niculescu, K. Aniol, E. Brash, V.
Punjabi, C. Perdrisat, Y. Ilieva, F. Cusanno, F.
Garibaldi, M. Iodice, S. Marrone, P. King, J.
Roche JLab, Regina, FIU, CUA, Yerevan,
Mississippi, Hampton, Mount Allison, Saint
Marys, UMd, JMU, California State, CNU, Norfolk,
WM, South Carolina, INF, Ohio
Hall C Users meeting
30 January 2009
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Meson Reaction Dynamics

• Meson production can be described by the
  t-channel exchange meson pole term in the limit
  of small t and large W
• Pole term is dominated by longitudinally
  polarized photons
• Meson form factor describes the spatial
  distribution of the nucleon

t-channel process

• At sufficiently high Q2, the process should be
  understandable in terms of the handbag diagram
• The non-perturbative (soft) physics is
  represented by the GPDs
• Shown to factorize from QCD perturbative
  processes for longitudinal photons Collins,
  Frankfurt, Strikman, 1997

pointlike
handbag
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Form Factors and GPDs

• Form factors and GPDs are essential to understand
  the structure of nucleons, which make up nucleons
  and mesons (q-q systems)

-

• But measurements of form factors and GPDs have
  certain prerequisites
• Before we can start looking at form factors, we
  must make sure that sL is dominated by the meson
  pole term at low -t
• Before we can learn about GPDs, we must
  demonstrate that factorization applies

p,K
p, K, etc.
f

• A comparison of pion and kaon production data may
  shed further light on the reaction mechanism, and
  intriguing 6 GeV pion results

Hard Scattering
GPD
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Context

• Understanding of the kaon production mechanism is
  important in our study of hadron structure
• GPD studies require evidence of soft-hard
  factorization
• Flavor degrees of freedom provide important
  information for QCD model building and
  understanding of basic coupling constants

• K? and KS have been relatively unexplored
  because of lack of the necessary experimental
  facilities
• There are practically no precision L-T separated
  data for exclusive K production from the proton
  above the resonance region

• Limited knowledge of L/T ratio at higher energies
  limits the interpretability of unseparated cross
  sections in kaon production
• Relative sL and sT contributions are also
  needed for GPD extractions since they rely on the
  dominance of sL

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Transverse Contributions
Hall C 6 GeV data (W1.84 GeV)

• In the resonance region at Q22.0 GeV2 sT is not
  small
• In pion production, sT is also much larger than
  predicted by the VGL/Regge model PRL97192001
  (2006)
• Why is sT so large? Difficult to draw a
  conclusion from current data
• Limited W and Q2 range
• Significant uncertainty due to scaling in xB and
  t

KS
K?
VGL/Regge
KS
K?
High quality sL and sT data for both kaon
and pion would provide important information for
understanding the meson reaction mechanism
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RsL/sT Kaon form factor prerequisite

• Meson form factor extraction requires a good
  reaction model
• Need high quality data to develop these models
• Current knowledge of sL and sT above the
  resonance region is insufficient
• Role of the t-channel kaon exchange in amplitude
  unclear
• Not clear how to understand reaction mechanism
  through current models

VGL/Regge (?2K0.68 GeV2)
Nucleon resonance data scaled to W1.84 GeV
L/T separations above the resonance region
are essential for building reliable models, which
are also needed for form factor extractions
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High Q2 Q-n scaling of sL and sT

• To access physics contained in GPDs, one is
  limited to the kinematic regime where hard-soft
  factorization applies

• A test is the Q2 dependence of the cross section
• sL Q-6 to leading order
• sT Q-8
• As Q2 gets large sL gtgt sT

• The QCD scaling prediction is reasonably
  consistent with recent JLab p sL data, BUT sT
  does not follow the scaling expectation

Kaon production data would allow for a quasi
model-independent comparison that is more robust
than calculations based on QCD factorization and
present GPD models
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Bonus Fp,K - a factorization puzzle?
T. Horn et al., Phys. Rev. Lett. 97 (2006)
192001.
T. Horn et al., arXiv0707.1794 (2007).

• The Q2 dependence of Fp is also consistent with
  hard-soft factorization prediction (Q-2) at
  values Q2gt1 GeV2
• BUT the observed magnitude of Fp is larger than
  the hard QCD prediction
• Could be due to QCD factorization not being
  applicable in this regime
• Or insufficient knowledge about additional soft
  contributions from the meson wave function

A.P. Bakulev et al, Phys. Rev. D70 (2004)
Comparing the observed Q2 dependence of sL,T
and FF magnitude with kaon production would allow
for better understanding of the onset of
factorization
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Motivation Summary

• The charged kaon L/T cross section is of
  significant interest to the study of GPDs and
  form factors at 12 GeV
• Can only learn about GPDs if soft-hard
  factorization applies
• If transverse contributions are large, the
  accessible phase space may be limited
• If sL not dominated by the K pole term at low
  -t, we cannot extract the form factor from the
  data and interpretation of unseparated data
  questionable

• Our theoretical understanding of hard
  exclusive reactions will benefit from L/T
  separated kaon data over a large kinematic range
• Constraints for QCD model building using both
  pion and kaon data
• Understanding of basic coupling constants (S/?
  ratio)
• Quasi model-independent comparison of pion and
  kaon data would allow a better understanding of
  the onset of factorization

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Experiment Overview

• Measure the separated cross sections at varying
  t and xB
• If K pole dominates sL allows for extraction of
  the kaon ff (Wgt2.5 GeV)
• Measure separated cross sections for the
  p(e,eK)?(S) reaction at two fixed values of t
  and xB
• Q2 coverage is a factor of 2-3 larger compared to
  6 GeV at much smaller t
• Facilitates tests of Q2 dependence even if L/T
  ratio less favorable than predicted

Q23.0 GeV2 was optimized to be used for both
t-channel and Q-n scaling tests
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Cross Section Separation

• The virtual photon cross section can be written
  in terms of contributions from transversely and
  longitudinally polarized photons.

• Separate sL, sT, sLT, and sTT by simultaneous fit
  using measured azimuthal angle (fK) and
  knowledge of photon polarization (e)

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Separation in a Multi-Dimensional Phase Space
Low e
High e

• Cuts are placed on the data to equalize the Q2-W
  range measured at the different e-settings

• Multiple SHMS settings (3 left and right of the
  q vector) are used to obtain good f coverage over
  a range of t
• Measuring 0ltflt2p allows to determine L, T, LT and
  TT

SHMS3
SHMS-3
Radial coordinate (-t), Azimuthal coordinate (f)
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Kaon PID

• p/K separation provided by heavy gas Cerenkov
  for pSHMSgt3.4 GeV/c
• For reliable K/p separation above 3 GeV/c an
  aerogel Cerenkov is essential
• Provision has been made in the SHMS detector
  stack for two threshold aerogel detectors

• Four sets of aerogel would provide reliable K/p
  separation over the full momentum range (2.6-7.1
  GeV/c)
• Alternate PID methods (such as RICH) are also
  possible

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Expected Missing Mass Resolution

• Missing mass resolution (30 MeV) is clearly
  sufficient to separate ? and S final states
• Acceptance allows for simultaneous studies of
  both ? and S channels
• Total effect of the ? tail and possible
  collimator punch-through to KS projected to be
  lt1/10 of the size of the tail

ep?eK?(S)
?
SHMSHMS
S
Simulation at Q22.0 GeV2 , W3.0 and high e
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Projections of RsL/sT
VGL/Regge calculation

• Empirical kaon parameterization based on Hall C
  data was used in rate estimates
• Conservative assumptions on the evolution of L/T
  ratio
• Projected ?(L/T)28-60 (10-33 using VGL/Regge)
  for typical kinematics
• PR12-09-011 may indicate larger values of R, with
  associated smaller uncertainties
• Reaching Q28 GeV2 may ultimately be possible

Hall C parameterization
VGL/Regge
Fp param
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Projected Uncertainties for sL and sT
sL
sT

• High quality kaon L/T separation above the
  resonance region

• Projected uncertainties for sL and sT use the L/T
  ratio from Hall C parameterization

PR12-09-011 Precision data for W gt 2.5 GeV
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Projected Uncertainties for the Kaon FF

• If the K pole dominates low -t sL, we would for
  the first time extract FK above the resonance
  region (Wgt2.5 GeV)

• Projected uncertainties for sL use the L/T ratio
  from Hall C parameterization

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Projected Uncertainties for Q-n scaling
p(e,eK)?
xB0.25

• QCD scaling predicts sLQ-6 and sTQ-8
• Projected uncertainties use R from the Hall C
  parameterization

1/Q4
1/Q6
Fit 1/Qn
1/Q8
Is onset of scaling different for kaon than
pion? Kaons and pions together provide quasi
model-independent study
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PR12-09-011 Summary

• L/T separated K cross sections will be essential
  for our understanding of the reaction mechanism
  at 12 GeV
• If transverse contributions are found to be
  large, the accessible phase space for GPD studies
  may be limited
• Basic coupling constants in kaon production (S/?
  ratio)
• If t-channel exchange dominates sL, we can
  perform the first reliable extraction of the kaon
  form factor above the resonance region
• L/T separated K data over a wide kinematic range
  will have a significant impact on our
  understanding of hard exclusive reactions
• Constraints on QCD model building using both pion
  and kaon data
• Quasi model-independent comparison of kaon and
  pion data would allow better understanding of the
  onset of factorization

Request 47 days to provide first precision L/T
separated kaon production data above the
resonance region. Excellent candidate for early
running.
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Backup material
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PR12-09-011 Beam Time
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Systematic Uncertainties
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Overlap with pion data

• No significant improvement in statistics through
  overlap with the approved p(e,ep)n experiments
• Covers only a small region at very high t, which
  is not interesting for studies of form factors or
  GPDs
• Most events are off the focal plane
• exclusive K peak lies at -5.5ltdSHMSlt-2
• Missing Mass tail cut off by SHMS acceptance

• Would require that aerogels installed during pion
  experiments as well

Proposed kaon experiment
Approved pion experiments
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Transverse Contributions in Pion production
VGL sL
VGL sT

• In pion production, magnitude of sT has been
  controversial for a long time
• VGL/Regge model systematically underestimates sT,
  for which it seems to have limited predictive
  power

T. Horn et al., Phys. Rev. Lett. 97, 192001
(2006)
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Bonus Interference Terms
K?(S) as calculated in VGL/Regge model
Q20.4 GeV2

• In the hard scattering limit, these terms are
  expected to scale
• sLT Q-7
• sTT Q-8
• Additional information about the reaction
  mechanism may be obtained for free if one
  performs a full cross section separation

Q23.5 GeV2
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Significance of multiple epsilon points

• Additional epsilon settings require additional
  beam time
• Resulting benefit in systematic uncertainty must
  be weighted against the increased statistical
  uncertainty