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Gordon D. Cates, Jr. University of Virginia P Collaboration Meeting - March 14, 2005 E : High resolution study of the resonance in the nK + system Implications of current experimental information. Update on experimental design and sensitivity.

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From the November 2004 Science Either we should stop giving people talks on this subject, or we should resolve it one way or the other.

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The case for P The question of the pentaquark remains high profile and unresolved. CLAS, the most sensitive of current experiments, can confirm it, but not kill it. If the + is confirmed, P would be the first experiment to measure its width directly. P can conservatively and cleanly confirm or kill the Spring-8 result at the 10 sigma level.

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Constraints on the width of the + from the relationship to cross section Elastic K + p, K + d scattering (gold plated), and K + Xe reaction: –Nussinov, < 6 MeV. –Arndt, Strakovsky, and Workman, < a few MeV –Cahn and Trilling looked at the K + Xe data from Diana, = 0.9 +/- 0.3 MeV. COSY data (p p + K 0 p) (silver plated) – = 0.4 +/- 0.1 (stat) +/- 0.1(syst) b. – 8 MeV If its real, its narrow!! 1 MeV 8 MeV

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What are the implications of 1 MeV 8 MeV for photoproduction? Liu and Ko: 15 nb 120 nb (Spring-8; E = 2 GeV). 7.5 nb 60 nb (P ; E = 4 GeV). Guidal, Polyakov, and Vanderhaeghen : 8 nb 64 nb (Spring-8; E = 2 GeV) 1.6 nb 13 nb (P ; E = 4 GeV). At worst we would see a 7 sigma effect

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The cross-section implications of Spring-8 Reconstructing p K + (1520) K + K p Reconstructing n K + K + K n w/proton w/out proton plastic hydrogen I have a few comments on your estimation of the Theta+ cross section. The number of Theta+ events is about 17 and not 36. You should consider the branching ration of L*->NK, which is 45%. On the other hand, the Theta+ decays into NK 100% So, if we assume the same acceptance for the L* and Theta+, the cross-section of the Theta+ would be about 1/4th of the L* cross-section. To get more realistic number, we need to know angular and energy dependences of the L* and Theta+ production. But I think these dependences cannot change the above estimation by order of magnitude. Best regards, Takashi Implications of Takashis comments: 25 nb < + < 250 nb

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A quick preview: The sensitivity of P If were 25 nb at Spring-8 –Liu and Ko: around 29 sigma (s=940, bg=140) –Vanderhaeghen: around 16 sigma (s=375, bg=200) If were 10 nb at Spring-8 –Liu and Ko: around 16 sigma (s=380, bg=150) –Vanderhaeghen: around 8.5 sigma (s=150, bg=170)

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Least optimistic case for 25 nb at Spring-8 Here a 3 MeV width and 4.5 nb cross section (at E = 4 GeV) is assumed, consistent with the Vanderhaeghen calculation. This would imply roughly a 23 nb cross section for Spring-8.

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Implication of a negative CLAS result For CLAS: if 43 events represented 50 nb (low statistics run ) –1 sigma above background implies 2GeV = 3 nb (statistics x 10) For P : 13 events would represent 1 sigma over background. –1 sigma above background implies.3 nb < 2GeV <.8 nb PR will have times the sensitivity of CLAS

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What prevents us from discounting the + if CLAS sees a negative result? Window of opportunity is at the edge of CLASs sensitivity due to large instrumental resolution. CLASs limitation on forward acceptance requires rescattering for expected production mechanism. –If + proceeds via a t-channel K + exchange, the spectator proton would not make it into CLASs acceptance. Numerous medium-energy experiments still see an effect. There is a pressing need for a high-resolution experiment.

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PAC27 report

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Summary We request 550 hours of beam time. Instrumental resolution will be 2.1 MeV FWHM, the best of any proposed experiment. We will detect 17 events/day assuming 4.5 nb production cross section at our photon energies ( 4 GeV), or 340 events over 20 days. We can be ready in 1 year. Conditional approval would allow us to begin work on long-term items. Implications of a 10 nb cross section at Spring-8 (well below Takashis lower estimate) sigma detection

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