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Photoproduction of the S (1385) resonance at LEPS

Photoproduction of the S (1385) resonance at LEPS. K. Hicks & D. Keller, Ohio U. LEPS Collaboration Meeting May 1, 2008. Motivation - 1. Very few data exist for the S (1385). It is an important part of the decuplet group. It is difficult to disentangle from the L (1405).

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Photoproduction of the S (1385) resonance at LEPS

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  1. Photoproduction of the S(1385) resonance at LEPS K. Hicks & D. Keller, Ohio U. LEPS Collaboration Meeting May 1, 2008

  2. Motivation - 1 • Very few data exist for the S(1385). • It is an important part of the decuplet group. • It is difficult to disentangle from the L(1405). • In order to study the shape of the L(1405), which is of controversial composition, it is useful to know more about the S(1385). • Some suggest the L(1405) is molecular. • Few question the 3-quark nature of S(1385).

  3. Motivation - 2 • A recent theoretical model by Oh, Ko and Nakayama is available to interpret data. • Reference: arXiv: 0712.4285. • Suggests that new resonances (predicted by Capstick & Roberts) help to fit the CLAS data. • The contribution from t-channel K* exchange is predicted to be negligible. (Is it surprising?) • The LEPS detector, at forward angles, is complementary to the large-angle CLAS data.

  4. Comparison to Data Note: CLAS data are preliminary (L. Guo et al.); the older data (pre-1970) are shown in red. Resonance strength is small—most strength is from t-channel.

  5. SPring-8 and LEPS The front end of the LEPS detector, showing the cryogenic target (left) followed by tracking chambers with the dipole magnet on the right. The 8 GeV stored electron beam facility in Japan, located about 100 km west of Osaka. It rings the top of the mountain.

  6. Data and Analysis • The desired reaction is: g n  K+p- X. • The target was liquid deuterium (LD2). • Both K+ and p- are detected at forward angles. • Use standard “clean-up” cuts to get clean PID • Use missing mass technique to isolate: • S- p- n (here, “X”=n). • S*-  p-L (here, “X”=L). • There is very little background: • gp  K+p- X+ (requires X to be positive). • gp  K+S*0  K+p0L (small, can be subtracted).

  7. MM(K+) v. MM(K+p-) Hydrogen target Deuterium target

  8. Cross Sections: ratio method • The LD2 data had normalization problems. • The beam flux is unreliable for part of the run. • The gn  K+S- cross sections are known: • H. Kohri et al., PRL 97, 082003 (2006). • LEPS acceptance calculations are reliable. • Have agreement with world data for LH2 data. • Let R = (NS*-/AccS*-) / (NS-/AccS-). • Then dsS*- = R (dsS-).

  9. Experimental and MC Ratios All plots are a function of the photon beam energy, from 1.5-2.4 GeV. In each case, the angular bin is, in order, cos(q) = 0.6-0.7, 0.7-0.8, 0.8-0.9, 0.9-1.0.

  10. Cross Section Compared to Theory LEPS: PRELIMINARY! Calculation by Oh, Ko, Nakayama Cross sections appear to be fairly flat in both Eg and qK.

  11. f-dependence: Data and MC Monte Carlo LD2 Data K+S- K+S- K+S*- K+S*- f (radians) f (radians)

  12. Polarization Dependence K+S- Solid = Pol 1 + Pol 2 Dashed = Pol 1 only Dotted = Pol 2 only K+S*- f (radians)

  13. Beam Asymmetry Ave. over all Eg and all q. K+S*- K+S- Kohri’s S- measured ave. Asym = +0.7 For the S*-, the Asym changes sign!

  14. Theory: Asym for S*0 From: Oh, Ko, Nakayama

  15. Summary • New cross sections for gn  K+S*- were presented for the first time. • There is very little background. • In general, the cross sections do not rise as rapidly at forward angles as predicted. • The OKY model may need to be adjusted • Beam asymmetry (for a single bin) is in good agreement for S-, new data for S*-.

  16. Questions • Is there a correction for beam flux in the two polarization states? • There seems to be a zero-offset in the Asym. • What is the dependence on f-acceptance for a given range of cos(q)? • The average over all q could skew the Asym.

  17. Sample fits to S*- peak

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