S. Yokkaichi

Experimental signature of medium modifications for rho and omega mesons in the 12 GeV p+A reactions.

The invariant mass spectra of e+e- pairs produced in 12 GeV proton-induced nuclear reactions are measured at the KEK Proton Synchrotron. On the low-mass side of the meson peak, a significant enhancement over the known hadronic sources has been observed. The mass spectra, including the excess, are well reproduced by a model that takes into account the density dependence of the vector meson mass modification, as theoretically predicted.

Evidence for in-medium modification of the phi meson at normal nuclear density.

Invariant mass spectra of e(+) e(-) pairs have been measured in 12 GeV p + A reactions to detect possible in-medium modification of vector mesons. Copper and carbon targets are used to study the nuclear-size dependence of e(+) e(-) invariant mass distributions. A significant excess on the low-mass side of the phi meson peak is observed in the low betagamma(= beta/square root(1-beta(2))) region of phi mesons (betagamma < 1.25) with copper targets. However, in the high betagamma region (betagamma > 1.25), spectral shapes of phi mesons are well described by the Breit-Wigner shape when experimental effects are considered. Thus, in addition to our earlier publications on rho/omega modification, this study has experimentally verified vector meson mass modification at normal nuclear density.

Observation of rho/omega meson modification in nuclear matter.

We measured the invariant mass spectra of electron-positron pairs in the target rapidity region of 12-GeV p+A reactions. We have observed a significant difference in the mass spectra below the omega meson between p+C and p+Cu interactions. This difference indicates that the spectral shape of mesons is modified at normal nuclear-matter density.

Search for the H dibaryon in (K−, K+) reaction with scintillating fiber active target

Abstract We have studied ( K − , K + ) reactions at P K − = 1.66 GeV/c by using a newly developed active target made of plastic scintillating fibers. The H dibaryon ( S = −2, I ( J π ) = 0(0 + )) has been searched for through the direct process, K − + C → K + + H + X , by the analysis of the K + momentum spectrum with the help of visual data of the scintillating fiber target. No evidence for the H production was observed. Upper limits for the differential cross section for the H production in the forward direction of the K + is 0.04–0.6 μb/sr at the 90% confidence level for the H mass range from 1850 to 2215 MeV/c 2 .

Enhanced ΛΛ production near threshold in the 12C(K−,K+) reaction

Abstract Enhanced production of ΛΛ pairs, above levels predicted by a two-step process model, has been observed near threshold (in the mass range 2.23-2.26 GeV/c2) in the 12C(K−,K+) reaction at 1.66 GeV/c using a scintillating fiber target. The differential cross section for ΛΛ production in the momentum region 0.95≤pK+≤1.3 GeV/c averaged over the range 2.3o≤θK+≤14.7o was found to be 7.6±1.3 ±0.9 μ b/sr, and that for the enhancement was found to be approximately 3 μ b/sr.

Mass production of hydrophobic silica aerogel and readout optics of Cherenkov light

Abstract We have produced 500 l of hydrophobic silica aerogel for the threshold-type Cherenkov detectors to trigger kaons in the momentum region of 0.5– 2.0 GeV /c . The refractive index of the products is 1.034 with piece by piece variation of ±0.004. The transmission length is found to be 25±4 mm at 400 nm . The details of the production scheme and the characteristics of the produced silica aerogel are reported. We also describe our studies of the light collection schemes of the aerogel Cherenkov counters.

Σ+p elastic scattering in the region of 300 ⩽ pΣ ⩽ 600 MeV/c with a scintillating fiber target

Abstract We have performed a hyperon-proton scattering experiment with a scintillating fiber active target. The Σ − p, Λ p and Σ + p scattering have been studied with the same experimental setup. In this paper, we present the differential cross sections of the Σ − p elastic scattering in the momentum region from 400 to 700 MeV /c . This is the first measurement of the Σ − p elastic scattering in the momentum region where the P- and higher waves contributions are important. The present data are in good agreement with the one boson exchange model (Bonn–Julich model A) and the quark cluster model (FSS of Kyoto–Niigata model).

Nuclear-matter modification of decay widths in the phi-->e+e- and phi-->K+K- channels.

The invariant mass spectra of phi-->K+K- are measured in 12 GeV p+A reactions in order to search for the in-medium modification of phi mesons. The observed K+K- spectra are well reproduced by the relativistic Breit-Wigner function with a combinatorial background shape in three betagamma regions between 1.0 and 3.5. The nuclear mass number dependence of the yields of the K+K- decay channel is compared to the simultaneously measured e+e- decay channel for carbon and copper targets. We parameterize the production yields as sigma(A)=sigma0Aalpha and obtain alphaphi-->K+K- -alphaphi-->e+e- to be 0.14+/-0.12. Limits are obtained for the partial decay widths of the phi mesons in nuclear matter.

Development of a time projection chamber using gas electron multipliers (GEM-TPC)

A prototype of time projection chambers using gas electron multipliers (GEM-TPC) has been developed for relativistic heavy ion collision experiments. The performance of the GEM-TPC was investigated using particle beams at KEK with 3 kinds of gases (Ar(90%)-CH/sub 4/(10%), Ar(70%)-C/sub 2/H/sub 6/(30%) and CF/sub 4/). Detection efficiency of 99%, spatial resolution of 80 /spl mu/m in the pad-row direction and double track resolution of 12 mm in the drift direction were achieved. The test results show that the GEM-TPC meets requirements of relativistic heavy ion collision experiments. This paper describes the configuration and performance of the GEM-TPC.

Experimental signature of in-medium mass modification of vector mesons at normal nuclear density

We measured invariant mass spectra of e+e− pairs produced in 12 GeV p + A interaction to investigate in-medium mass modification of vector mesons. We observed an enhancement over the known hadronic sources on the low-mass side of the ω meson mass peak. The obtained ρ/ω ratio indicates that this enhancement mainly results from ρ mesons whose mass was modified in nuclear matter. We performed a toy model calculation based on the prediction using the QCD sum rule. It reproduced well the tendency of our data.