Transcript Physics programs (1)

Investigation of Exotic Baryon States in
Photoproduction Reactions with CLAS
Spokespersons:
K. Hicks (Ohio U.) and S. Stepanyan (Jlab)
and the CLAS Collaboration
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Scientific motivation
Previous experiments
Q+ spectator reaction analysis
Q+ exclusive reaction analysis
Beam request
Summary
Jefferson Lab PAC 24, 6/16/2003
Goals of the proposal:
• Measure photoproduction reactions on the
deuteron with CLAS in the energy region
where the strangeness S=+1 pentaquark
(known as the Q+) has been seen.
• Get statistics to conclusively determine
whether or not the Q+ exists, and if so
measure the decay angular distribution.
Why is searching for the Q+ important?
• QCD does not prohibit q4q states, but early
searches have failed to produce evidence for
pentaquarks. Now there is a definite theoretical
prediction of mass, width, spin and isospin of a
S=+1 state (structure uudds).
• If found, the Q+ would be the first hard evidence
of a new class of particle: the pentaquark.
• One of the central activities at Jefferson Lab is to
understand N* resonances. Do pentaquarks
contribute to the resonance spectrum?
Brief review of the model
• The Q+ was predicted by calculations in the chiral
soliton model.
– These predictions motivated experimenters to look in a
particular mass range for this S=+1 isosinglet state.
– Our experimental results are not dependent on this
theoretical interpretation.
– At present, this model provides guidance for
experimental measurements.
• Other theoretical work is in progress.
The standard baryon decuplet representation
Here, hypercharge Y
versus isospin I3 is
plotted, where
I3 = Q – Y/2
and
Y=B+S
for baryon number B
and strangeness S.
The Anti-decuplet predicted by Diakonov et al.
Width < 15 MeV
The Q+ is an isospin singlet with an anti-strange quark!
The mass prediction is based on the N*(1710).
Previous Experiments
• The LEPS collaboration at SPring-8 has
announced evidence for the Q+(1540).
 gn  K-Q+  K-K+(n)
(4.6 sigma)
• The DIANA collaboration at ITEP has also
announced evidence for the Q+(1539).
K+n  Q+ K0p
(4.4 sigma)
• The CLAS collaboration (two different channels).
 gdK-pQ+  K+K-p(n)
(5.0 sigma)
 gp  K*0Q+  p+K-K+(n) (4.3 sigma)
Spectator Reactions (used by LEPS)
K─
g
g n ( p)  Q+ K - ( p)
+
+
Q K n
Q+
n
p
+
L* (1520)  K - p
gN  f(1020) N K+K- N
n
p
K+
g p (n)  L (1520) K (n)
*
K+
g
p
n
K─
L*
p
n
LEPS results (includes vetoing tagged protons)
• The solid line is the final sample (veto protons).
• The dashed lines show the “background” (proton required).
• The statistical significance is ~4.6 s (+/- 1.0 due to BG error).
ITEP results (submitted for publication)
Exclusive Reaction (CLAS)
The K- has a significant probability to re-interact with the proton.
This rescattering effect can also be seen in other reaction channels.
Neutron identified via missing mass
“loose” timing cuts
“tight” timing cuts
Essential Background Cuts
CLAS g2 results for the Q+
“loose” timing cuts
Dashed: background from L(1520) cut
“tight” timing cuts
No signal is seen in the other flavor channel
pK+ mass spectrum
Same cuts as before
This suggests that the Q+ peak is an isosinglet.
Q+ Evidence from a proton target
gp  p+ K- K+ (n). CLAS analysis by V. Koubarovski
Decay angular distribution
This is for the L(1520), a spin 3/2 particle, using 653 events.
A similar study for the Q+ would require about 20 times our current statistics!
g2 decay angular distribution
Signal (peak)
Background (side)
For J=1/2, we expect a flat angular distribution.
Beam time request
• Estimates from the number of Q+ events and the
g2 running conditions along with:
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Trigger only on two-track events
Raise the beam current
Increase the target thickness
 factor of >20 in statistics in 30 days.
• Why a factor of 20 (or more)?
 Decay angular distribution: intrinsic spin.
 Determine isospin assignment.
 Excitation function: production mechanism.
 Systematic studies: uncertainties and backgrounds
Summary
• Recent experiments suggest the existence of the Q+.
• We need sufficient statistics to conclusively prove
whether or not the Q+ really exists.
• If it does exist, we need sufficient statistics to
measure the decay angular distribution to get its spin,
and also learn about the energy dependence of its
production.
Could K+N phase shifts see the Q+??
W=1540
Particle Data Group Listing of Baryons
• 4-star resonances are well-established
• 3-star resonances are very likely, but need further study
N*(1710) P11 decay widths
• PDG(2002): mass 1680-1740, width 50-250
– 10% Np, 40-90% Npp, 5-25% LK, 6% Nh
• Manley (1992): mass 1717, width 478 (226)
– 9% Np, 49% Dp, 37% LK “effective”
• Manley (2003): mass 1718, width 368 (85)
– 8% Np, 10% Dp, 18% LK, 73% Nh !!
• Dytman (2002): mass 1675, width 154 (70)
– 16% Np, 32% Dp, 11% LK, 1% Nh ??
The only real agreement is that this N* couples weakly to Np.
Prediction from the chiral soliton
model for the N*(1710)
• Magnetic moments for the octet and
decuplet can be calculated in this model
– Chiral limit: get same m’s as NR quark model!
• Use symmetries: extend to antidecuplet
– Get transition mNN* for octetantidecuplet
• “photoproduction of the baryon antidecuplet
…is strongly suppressed on the proton
target. It occurs mostly on the neutron”
GRAAL h photoproduction
W=1.7 GeV
The effect of removing the N*(1710) from LK production
The Q+ search group at CLAS
Particle ID, ntuples
Luminita Todor
Eugene Pasyuk
Monte Carlo
Dave Tedeschi
Data Analysis
Stepan Stepanyan
Valeri Koubarovski
Ken Hicks
Dan Carman
Reinhard Schumacher
Elton Smith
Bernhard Mecking
Volker Burkert
Monte Carlo simulations
Time difference for
Kp
and
+
Kp