G. Capon

Measurement of the reaction e+e−→ηπ+π− in the center of mass energy interval 1350–2400 MeV

Abstract The e+e−→ηπ+π− reaction has been measured in the center of mass energy interval 1350–2400 MeV by the magnetic detector DM2 at the Orsay storage ring DCI. Under the hypothesis of only one large resonance the cross section is not fit in a satisfactory way. The branching ratio τ−→η−π0ντ=(0.13 ± 0.02)% is deduced via CVC from the above measurement.

Dm2: A Magnetic Detector for the Orsay Storage Ring DCI

DM2 is the magnetic detector built to study e+e- interactions in the c.m. energy range 1.2-3.7 GeV with the Orsay Storage Ring DCI. The magnet is a solenoid of 2 m diameter and 3 m length with a conventional aluminium coil producing a 0.5 T field. Inside the coil, the charged particles detector is made of two proportional chambers with anode and cathode read-outs, 13 layers of drift chambers, one layer of water Cerenkov counters and one layer of scintillation counters. Photon detection is provided outside the 1 rad length coil by stacks of 14 planes of wire tubes with longitudinal read-outs, separated by lead sheets, and by five layers of scintillation counters. Inside the coil, the end caps are covered by shower detectors made of lead sheets and MWPC planes. Muon separation uses the flux return iron or concrete absorbers, and planes of wire tubes with longitudinal read-out. The charged particle detector covers 87% or 4π and the photon detector 82% of 4π. Charged particle identification by time of flight and Cerenkov radiation can discriminate pions from kaons up to 600 MeV/c and kaons from protons up to 1100 MeV/c. The photon detection efficiency is 50% at 50 MeV and near 100% at 100 MeV. It allows a neutral trigger to be operated. The detector is starting operation on the storage ring DCI at Orsay.

Search of glueballs in the Jψ→γφφ decay

In a study of the radiative decay Jψ→γφφ→γK+K−K+K−, from the 8.6 million Jψ produced in the DM2 experiment at DCI, we found a large production of events with φφ mass below the ηc. The branching ratio, for mφφ < 2.9 GeV/c2, is BR(Jψ→γφφ) = (3.1 ± 0.3 ± 0.6) × 10−4. The angular analysis of the events with φφ mass below 2.5 GeV/c2 is consistent with a 2++ assignment. An enhancement near 2.2 GeV/c2 shows a preferred spin-parity JP = 0−.

Pseudoscalar ωω production at threshold in J/ψ→ψωω decay

Abstract The 8.6 million J/ψ collected by the DM2 detector at DCI have been analysed looking for the J/ψ→γωω decay, both ωs decaying into π+π−π0. A clean signal of γωω events is observed with BR(J/ψ→γωω)=(1.41±0.20±0.42)×10 −3. The ωω invariant mass distribution shows a large bump peaked at 1.77 GeV/c2, associated with a pseudoscalar production, as first observed by a previous experiment. No evidence of further structures is found.

Study of the reactione+e−→K+K− in the energy range\(1350 \leqq \sqrt s \leqq 2400 MeV\)

AbstractThee+e−→K+K− cross section has been measured from about 750 events in the energy interval

A measurement of ηc → φφ in the radiative decay of the J/ψ

1350 \leqq \sqrt s \leqq 2400 MeV

First observation of the ηc→ϱ0ϱ0 decay

with the DM2 detector at DCI. TheK± form factor |FF±| cannot be explained by the ρ, ω, ϕ and ρ′(1600). An additional resonant amplitude at 1650 MeV has to be added as suggested by a previous experiment.

First observation of the etac-->varrho0varrho0 decay

Abstract The decays of 8.6 million J ψ produced at DCI in the DM2 detector are analyzed to measure baryon pair production. Angular distributions for J ψ → p p , Λ Λ , Σ 0 Σ 0 are in good agreement with first order QCD predictions. New measurements are given for the branching ratios of these decays.