?'-photoproduction with CLAS

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?'-photoproduction with CLAS
M. Dugger, J. Ball, P. Collins, E. Pasyuk, B.
Ritchie and the CLAS collaboration
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• Motivation
• Experimental conditions
• Analysis technique
• Background subtraction
• Acceptance
• Normalization and uncertainties
• Comparison with models
• Summary

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Motivation

• The motivation is usual for CLAS collaboration
  Understanding of baryon spectrum.
• ?', as well as ? provide isospin filter, they
  can only originate from I1/2 states.
• Prior to 1998, only 18 ?' photoproduction events
  had been measured (11 events from ABBHHM buble
  chamber and 7 events from AHHM streamer chamber)
• In 1998 SAPHIR published results extracted from
  an additional 250 ?' exclusive events
• In CLAS experiment we detected over 2x105 ?'
  photoproduction events and used them to extract
  differential cross sections

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Experimental conditions

• G1c run (fall 1999)
• Beam energy 2.4 and 3.1 GeV
• 18 cm LH2 target
• Trigger At least one charged particle
  (any TOF counter)x(asyncSTxMOR)

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Event selection

• Detect proton and use missing mass technique
• GPID for particle ID
• Photon energy correction and momentum corrections
  obtained by Mike Williams from kinematical fit.
• Standard energy loss correction
• Bin in photon energy and cos ?cm

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Background subtraction

• Accidental background
• Physics background multipion production.
• Use MC to get a shape for each component.
• Fit to the experimental spectrum with weight
  factors in each kinematical bin.
• Subtract from experimental spectrum. What is left
  is the yield of ?'.

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Acceptance/efficiency

• Empirical efficiency using reaction ?p?p??-
• A good match to the phase space for the ?'
  photoproduction.
• Uncertainty of the efficiency is about 1-2

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Photon flux and absolute normalization

• Calculated photon flux using GFLUX code.
• Corrected for stolen hits in tagger
• Corrected for trigger inefficiency. Used g2a data
  taken with charged/neutral trigger.
• To verify that the normalization is reasonable we
  extracted ?0 photoproduction cross sections from
  the same data set using the same procedures and
  tools.

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CLAS/CB-ELSA/SAID
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CLAS/CB-ELSA/SAID
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SAID/CLAS

• For E? from 0.675 to 1.525 GeV our entire set of
  ?0 differential cross sections comprised of 19
  energy bins each with 12 bins in cos ?cm (228
  points in total) was easily fit with a single
  overall constant factor a1.02 (?21.3)
• If we perform independent fit for each energy
  bin, the the distribution of the norm factors has
  a Gaussian shape with the same mean value 1.02
  and standard deviation of 0.038
• This overall agreement throughout the energy
  range implies that the absolute normalization
  technique is correct and also indicates that
  detector acceptance is well understood.

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Cross sections CLAS-SAPHIR
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Cross sections
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Models

• Consistent analysis of the reactions ?p?p?' and
  pp?pp?' within a relativistic meson exchange
  model K. Nakayama and H. Haberzettl, PRC 69,
  065212 (2004)
• Our implementation of the model by A. Sibirtsev
  et al. nucl-th/0303044

• Both models include s-, t-, and u-channel
  contributions
• Both include S11(1535) and P11(1710) which are
  seen in ?p channel
• Both include ?/? mesons in t-channel
• NH includes in addition two S11 and two P11
  resonances with relatively small couplings
  S11(1626), S11(2082), P11(2094), P11(2474)
• NH model favors inclusion of spin 3/2 resonances
  P13(1940), D13(1730) and D13(2090)

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Solid line NH model Dashed line our
calculations using recipe of Sibirtsev et al.
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Contributions from different mechanisms

• S11(1535) describes overall initial rise an fall
  of the total cross section below 1.7 GeV
• P11(1710) helps to get gradually increasing
  forward-angle peaking
• J3/2 resonaces are useful for getting the right
  shape near 900 for the energies 1.7-1.9 GeV
• t-channel exchange is responsible for rapid rais
  of forward cross section at highest energies
• u-channel contributes to the rise at backward
  angles at higher energies

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Singlet axial charge
Since the ?' is the only flavor singlet of the
fundamental pseudoscalar meson nonet, the study
of the reaction can also help yield information
on the role of glue states in excitation of
nucleon. The flavor singlet axial charge of
nucleon gA0 is related to the h'-nucleon-nucleon
and gluon-nucleon-nucleon coupling constants
through the flavor-singlet Goldberger-Treiman
relation
When first measured J. Ashman et al., Nucl.
Phys. B 328,1 (1989) the singlet axial charge
was found to have a value of gA00.200.35 At
that time, the importance of the second term was
unappreciated, and this low value was surprising
since g?'NN is concidered to be correlated with
a fraction of the nucleon spin carried by its
constituent quarks. Neglecting gluonic portion
was one of the cause of the so-called spin
crisis The value of gA0 found from NH model fit
shown here is 1.49 while our calculation yield
1.46. These numbers are consistent with
theoretical estimate of T. Feldmann J. Mod.
Phys. A 15, 159 (2000) which is 1.4
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Cross sections and asymmetries
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Summary

• The differential cross sections measured with
  CLAS are the first high quality data for ?'
  photoproduction
• The data indicate for the first time contribution
  from S11(1535) and P11(1710) nucleon resonances
  to the ?'N channel, the two resonances previously
  found to be couple to ?N channel
• These data are useful in guiding future
  experimental and theoretical investigations.
• Measurement of polarization is extremely
  important.
• G8b data set can be analyzed to extract beam
  asymmetry.