Photoproduction of the (1385) resonance at LEPS

K. Hicks & D. Keller, Ohio U.

LEPS Collaboration Meeting

May 1, 2008

Motivation - 1

• Very few data exist for the (1385).– It is an important part of the decuplet group.– It is difficult to disentangle from the (1405).

• In order to study the shape of the (1405), which is of controversial composition, it is useful to know more about the (1385).– Some suggest the (1405) is molecular.– Few question the 3-quark nature of (1385).

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.

Comparison to Data

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

SPring-8 and LEPS

The 8 GeV stored electron beam facilityin Japan, located about 100 km west of Osaka. It rings the top of the mountain.

The front end of the LEPS detector, showing the cryogenic target (left) followed by tracking chambers with the dipole magnet on the right.

Data and Analysis

• The desired reaction is: n K+ - X.– The target was liquid deuterium (LD2).– Both K+ and - are detected at forward angles.– Use standard “clean-up” cuts to get clean PID– Use missing mass technique to isolate:

• - - n (here, “X”=n).• *- - (here, “X”=).

• There is very little background:– p K+ - X+ (requires X to be positive).– p K+ *0 K+ 0 (small, can be subtracted).

MM(K+) v. MM(K+-)

Hydrogen target Deuterium target

Cross Sections: ratio method

• The LD2 data had normalization problems.– The beam flux is unreliable for part of the run.

• The n K+- 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 = (N*-/Acc*-) / (N-/Acc-).

– Then d*- = R (d-).

Experimental and MC Ratios

In each case, the angular bin is, in order, cos() = 0.6-0.7, 0.7-0.8, 0.8-0.9, 0.9-1.0.

All plots are a function of the photon beam energy, from 1.5-2.4 GeV.

Cross Section Compared to Theory

LEPS: PRELIMINARY!

Calculation by Oh, Ko, Nakayama

Cross sections appear to be fairly flat in both E and K.

-dependence: Data and MC

-

-

-

-

(radians) (radians)

LD2 Data Monte Carlo

Polarization Dependence

K+-

K+-

(radians)

Solid = Pol 1 + Pol 2Dashed = Pol 1 onlyDotted = Pol 2 only

Beam AsymmetryK+- K+-

Kohri’s - measured ave. Asym = +0.7 For the *-, the Asym changes sign!

Ave. over all E and all .

Theory: Asym for *0

From: Oh, Ko, Nakayama

Summary

• New cross sections for n K+*- 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 -, new data for *-.

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 -acceptance for a given range of cos()?– The average over all could skew the Asym.

Sample fits to *- peak