Pentaquark Searches at JLab

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Pentaquark Searches at JLab
Patrizia Rossi Laboratori Nazionali di Frascati -
INFN for the CLAS Collaboration
QCD 05 4-8th July 2005 Montpellier (France)
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Goal of the Experimental Pentaquarks Program _at_
JLab
The goal of the experimental pentaquark program
at Jefferson Lab is to have a comprehensive
program to search for pentaquarks with high
statistics and high resolution photoproduction
experiments.
New experiments seeking evidence of pentaquarks
with the CLAS detector in Hall B were approved in
2003-2004 with the goal of confirming previous
results and explore new kinematics with at least
a factor 10 increase in statistics
g10 deuteron Eg 1.0-3.5 GeV
data taking completed in 2004 g11 proton
Eg 1.6-3.8 GeV data taking
completed in 2004 eg3 deuteron Eg
4.0-5.4 GeV data taking completed in
2005 Super-g proton Eg 3.8 5.7
GeV planned for 2006
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Hall B Cebaf Large Acceptance Spectrometer
Tagger

• Broad angular coverage
• (8 ? 140 in LAB frame)
• Charged particle momentum
• resolution 0.5 forward dir

• Tagged photon beam with
• energy resolution dk/k 0.1

• Eg (20 - 95) Ee

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G11 Spettroscopy of Exotic baryon with
CLASSearch for Ground and Excited States
Proposed measurement and Primary Goals Search
for ?(1540) and possible excited states in ?-p
interaction above threshold (E?1.6 3.8 GeV)

• collect more than 10 times the statistics of
  previous
• measurements
• establish the mass of any observed peak with 2
  MeV accuracy
• determine total and differential cross section
• Status of the experiment
• New experiment approved by JLab PAC25 in January
  2004.
• Run in May-July 2004, with a total of 7109
  triggers recorded (Luminosity 70 pb-1)
• Data calibration and processing completed in
  January 2005this reaction

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G11 Channel Identification

• strangeness is tagged detecting the K
• the K0 is detected via its KS component decaying
  
• into ?????? ?
• final state is identified using the missing mass
  technique
• hyperons which give the same final state are
  removed (?(1520), ?, ?-)
• using the full statistics (70 pb-1) 165000
  events selected

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G11 nK Mass Spectrum

• the nK mass spectrum is smooth

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G11 Evaluation of an Upper Limit on the Q
X-Section
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G11 Upper Limit on the g p ? Q K0 Cross
Section
N(95 CL) derived using Feldman and Cousins
approach
Upper limit (95 CL) ????p ?????K0 lt 1-4 nb
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G11 Comparison with SAPHIR results
SAPHIR
Counts
Counts

• Kinematics
• Energy limited to
• 2.6 GeV
• no hyperon rejection
• Selection of forward
• angles of the K0 in
• the ?-p center of mass

K0
cos?CM(K0) gt 0.5
?
M(nK) (GeV)
M(nK0) (GeV)
g11_at_CLAS
Counts
Counts
?(1520)
?(1540) ?
M(nK0) (GeV)
M(nK) (GeV)
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G11 Comparison with SAPHIR results
SAPHIR
Counts
Counts
K0
cos?CM(K0) gt 0.5
?
M(nK) (GeV)
M(nK0) (GeV)
g11_at_CLAS
Counts
Counts
?(1520)
?(1540) ?
M(nK0) (GeV)
M(nK) (GeV)
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G10 Search for T on Deuterium
Proposed measurement and Primary Goals Search
for ?(1540) in ?-d interaction in the energy
range E? 0.8 3.59 GeV

• Collected data with two values of the CLAS torus
  magnetic field
• For each configuration collected data with 10
  times the statistics of the published result
• Determine total and differential cross section
• Status of the experiment
• Run in May-July 2004, with a total luminosity of
  50 pb-1
• Indipendent analysis of several reactions by
  different groups reaction

?d?nKK-p T?nK ?d?pK0 K-(p)
T?pK0 K0???- ?d??Kn T?nK
?? p?- ?d??K0p T?pK0 ?? p?-
K0???-
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G10 Comparison with Published Data (g2a)
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G10 Comparison with Published Data

• Published results on Q from analysis of g2a data
  cannot be reproduced in the analysis of high
  statistics g10 data
• The statistical significance in the published
  data is an unlucky coincidence of the statistical
  fluctuation and the underestimate of the
  background in the mass region of 1.54 GeV/c2.

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G10 Upper Limit on X-section for gd?QpK-,
Ppgt0.35 GeV/c
No peak is found under minimal kinematical cuts

• Fit with the sum of 3rd degree polynomial and a
  Gaussian function with fixed width. Gaussian
  s5.5 MeV/c2, mean running from 1.48 to 1.72
  GeV/c2 .
  
  Number of signal events number of events
  fluctuating into Gaussian peak
• Acceptance calculation 4 body phase space event
  generator, modified to match kinematics of
  detected particles with data

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G10 Upper Limit on X-section for gd?QpK-,
Ppgt0.35 GeV/c
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G10 Upper limit on the elementary cross section
gn?QK-
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G10 Upper limit on the elementary cross section
gn?QK-

• With Fermi momentum being the only source of an
  energetic spectator proton, the cross section
  upper limit is 20 nb _at_ M(nK)1.529 GeV
• If we assume a more sophisticated model for an
  energetic spectator and take the ?(1520)
  production as a guide, the cross section upper
  limit is 4-5 nb _at_ M(nK)1.529 GeV(model
  dependent).

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Conclusion
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Conclusion (contd)
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Conclusion (contd)

• A clear peak is seen in the pK and pK-
• invariant mass distribution in dAU coll.
• _at_ 200 GeV and AuAu coll. _at_ 62.4 GeV.
• No signal is seen in pp coll. _at_ 200 GeV.
• The peak mass is 1530 MeV, width 15 MeV