T.C. Jude

1
Strangeness Photoproduction At Threshold Energies

• Hyperon photoproduction
• Crystal Ball detector at the MAMI-C facility
• A new method of K meson detection
• Cross section measurements
• Future plans

T.C. Jude D.I. Glazier, D.P. Watts The
University of Edinburgh
2
Hyperon Photoproduction

• Important for investigations of nucleon
  resonances. Predicted nucleon resonances which
  have not been observed experimentally may couple
  to strange decay channels 1
• A crucial test of QCD based chiral perturbation
  theories in the strange quark sector

1 S Capstick and W Roberts, Phys. Rev. D58,
074011 (1998)
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
3
Cross-Section Measurements

• No detailed measurements of g p g K L at
  threshold

• Significant discrepancies exist between
  cross-sections measured at JLab 2 and ELSA 3
• Fits to these data sets reveal large differences
  in the roles of nucleon resonances when
  describing the photoproduction process 4

The large angular coverage and intense photon
beam over the range 0.9 to 1.4 GeV make the
Crystal Ball ideal for strangeness measurements
at threshold
2 R. Bradford et al., Phys Rev. C 73, 035202
(2006) 3 K.H. Glander et al., Eur Phys. J. A
19, 251 (2004) 4 T. Mart arXiv0803.0601v1
nucl-th (2008)
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
4
Experimental Apparatus

• The Crystal Ball detector - 672 NaI crystals
  covering 93 of 4p steradians
• Edinburgh PID - 24 Plastic Scintillators parallel
  to the beam
• TAPS - Segmented BaF2 detector. Used as a forward
  wall for the Crystal Ball

T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
5
K meson detection in segmented calorimeters

• Identify the K decay within the crystals of the
  Crystal Ball and TAPS
• Eliminates the need of expensive magnetic
  spectrometers

K mnm ( 63) Mean lifetime of K
12 ns pp0 ( 21)
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
6
(a) Energy of the Second Cluster
(a) Time difference between first and second
clusters
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
7
2D cut on K loci in the DE-E analysis
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
8
Detection of S0 decay

• Identify photons in the Crystal Ball and TAPS
• Apply angular and energy cuts
• Boost photon into S0 rest frame peak of energy
  on S0 - L mass difference

Simulation
Real Data
From the K events identified (in simulation)
50 of KS0 events correctly identified by
detection of the decay photon 15 of KL events
misidentified (Still in progress)
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
9
Fitting to K Missing Mass Plots

• Fit To K missing mass plots to measure g(p,K)L
  and g(p,K)S0 contributions
• Further cuts on K cluster size (lt 3) and K
  punch through energy (lt 340 MeV reconstructed
  from polar angle)

For determining K yield for particular energy
and polar angle ranges, integrate over the Lambda
peak (blue)
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
10
g(p,K)L Cross Section Measurements

• July 2007 data, 36500 K events that pass cuts
• 350 MeV Crystal Ball energy sum, Multiplicity 2

0.6
Detection Efficiency 20 million g(p,K)L events
with phase space generation and A2 GEANT4
simulation. Approximately 10
Next Slide Black points this data Blue Points
JLab data (2006)2 Red Points ELSA data
(2004)3
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
11
Preliminary
Preliminary
Preliminary
Preliminary
Black points this data, Blue Points JLab data,
Red Points SAPHIR data
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
12

• Cross sections for forward angles (K centre of
  mass polar angle lt 760)
• Approaching region of cross section discrepancy
  with previous data

Preliminary

• Future beam times greater proportion of K in
  forward directions (change of electronic
  triggers)
• Reduce statistical error by 2

T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
13
Future Plans

• More data resolve g(p,K)L cross sections at an
  unprecedented resolution
• Increased beam energy measure the L(1405)
  structure
• Circular polarised beam measure polarisation
  observables, CX and CZ at threshold
• Deuterium target study the reaction
  g(n(ps),K)S-

T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
14
Future Plans

• Strangeness photoproduction with transversely
  polarised target 6

• Measure the beam-target polarisation observables
  E and G (longitudinal pol. target), and H and F
  (transversely pol. target)
• Sensitive to missing nucleon resonances for
  example D13(1900)

E
G
6 A. Thomas, Helicity Dependence of Meson
Photoproduction on the Proton, 13th Crystal Ball
Meeting, Mainz, Germany, March 2009
T. C. Jude Strangeness Photoproduction At
Threshold Energies IOP
Conference, Birmingham, April 2009
15
Strangeness Photoproduction At Threshold Energies
T.C. Jude D.I. Glazier, D.P. Watts The
University of Edinburgh