Gordon D. Cates, Jr.University of Virginia

P-05-009 Collaboration Meeting - March 14, 2005

E-05-009:High resolution study of the

resonance in the nK+ system

• Implications of current experimental information.• Update on experimental design and sensitivity.

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.

The case for P-05-009

• 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-05-009 would be the first experiment to measure its width directly.

• P-05-009 can conservatively and cleanly confirm or kill the Spring-8 result at the 10 sigma level.

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 + K0 p) (silver plated) = 0.4 +/- 0.1 (stat) +/- 0.1(syst) b. ≈ 8 MeV

If it’s real, it’s narrow!!

1 MeV≤ ≤ 8 MeV

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-05-009; E = 4 GeV).

• Guidal, Polyakov, and Vanderhaeghen:

8 nb ≤ ≤ 64 nb (Spring-8; E = 2 GeV)

1.6 nb ≤ ≤ 13 nb (P-05-009; E = 4 GeV).

At worst we would see a 7 sigma effect

The cross-section implications of Spring-8Reconstructing

p K+ (1520) K+ K pReconstructing

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 Takashi’s comments: 25 nb < + < 250 nb

A quick preview:The sensitivity of P-05-009

• 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)

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-05-009: 13 events would represent 1 sigma over background.

– 1 sigma above background implies .3 nb < 2GeV < .8 nb

PR05-009 will have 4 - 8 times the sensitivity of CLAS

What prevents us from discounting the + if CLAS sees a negative result?

• Window of opportunity is at the edge of CLAS’s sensitivity due to large instrumental resolution.

• CLAS’s 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 CLAS’s acceptance.

• Numerous medium-energy experiments still see an effect.

There is a pressing need for a high-resolution experiment.

PAC27 report

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 Takashi’s lower estimate)

9 - 16 sigma detection