Annual report+Search for Θ via π p → K −X reaction at E522 experiment−K. Miwa (Kyoto Univ.) for KEK-PS E522 collaboration

Counts/3MeV

We have performed the E522 experiment at the K2 beamline of KEK 12GeV Proton
Synchrotron in November 2002 and February 2004. The main objective of this experiment was to search for H-dibaryon resonance with (K − , K + ) reaction. At KEK-PS E224
experiment, the enhancement was observed at threshold region of the double-Λ system
which are decay products from S=-2 system[1]. In this present experiment we aimed tosearch this enhancement with ten times larger statistics[2].
Besides this reaction, we optionally took (π − , K − ) data, because the Θ+ search via
mesonic reaction was important and the K2 beamline was unique beamline which could
provide a high-momentum π beam. In this report, we mainly mention about this topics.
We used π − beam extracted at 1.87 and 1.92 GeV/c. As a target, we used a scintillation
fiber (SCIFI) target which consisted of ((CH)n )and a bulk target of polyethylene which
consisted of ((CH2 )n ). We irradiated 2.9×109 and 3.0×109 π − beam to the SCIFI andthe polyethylene targets respectively at beam momentum 1.87GeV/c. At at 1.92GeV/c,7.4×109 π − beam were irradiated to only polyethylene target.
The experimental set up consists of two parts; one part is a beamline spectrometer toanalyze momentum of each incident beam particle with the resolution of ∆P/P =0.5%,
and the other part is forward spectrometer to detect scattered particles.
Figure 1 shows the missing mass spectrumof (π − , K − ) reaction at 1.92GeV/c. It seems
Preliminarythat there is a bump around 1.53GeV/c2 . How500
Pπ=1.87GeV/cever there is a possibility that the structure is400
only a statistical fluctuation. We fitted this300
histogram with the background of cubic function and gaussian peak. The peak position was
200+1.912C data
1530.6 +2.2
MeV/c2 (prelimi−1.9 (stat.)−1.3 (syst.)
1002nary), the width was σ=4.1+3.0−1.5 MeV/c (prelim0inary) which was consistent with the expected
1.3
1.35
1.4
1.45
1.5
1.55
1.6
1.65
1.7
Mass (GeV/c )resolution of 5.6MeV/c2 within error. The statistical significance is not so large. Therefore we
Figure 1: Missing mass spectrum of (π − , K − )derive the upper limit of the production crossreaction 1.92GeV/c. The hatched histograms
sectionand obtained the upper limit of the difare the carbon target data which is normalized−−
by the target and beam count, and these his- ferential cross section via (π , K ) reaction to bedσ
tograms represent the contribution of carbon dΩ = 3.2µb/sr (preliminary) at 90% confidencenuclei in SCIFI and polyethylene target.
level. Assuming that K − is produced isotropically at center of mass system, 10.8% of K − isaccepted by the spectrometer. Then, if the K − is produced with s-wave, the upper limitof the total cross section is obtained to be σ = 4.1µb (preliminary) at 90% confidence
level. These values are much smaller cross section than other hadron resonance and isalso smaller than some theoretical calculations.
2References[1] J.K. Ahn et al., Phys. Lett. B 444, (1998) 267.
[2] C.J. Yoon et al., in: Proc. of the YITP workshop “Multi-quark Hadrons; for, five andmore ?”, http://www.slac.stanford.edu/econf/C0402171/files/manuscripts/Yoon.pdf