E. A. Pozharova

Search for a light dark photon in muonium decay

About 0.85 × 105 events involving stopped μ+ and muonium decay were observed in a nuclear photoemulsion with the aim of searches for a light dark photon (DPh) in the decay process DPh → e+e−. With a probability of about 10−5, no event of the decay μ+ → е+νμνе accompanied by an electron–positron pair was observed. In the interval of 1.1 MeV ≤ mDPh ≤ 60 MeV, the mixing parameter ε2 was estimated at about 10−5 to 10−4.

Interaction of 32-GeV positrons with photoemulsion nuclei

An analysis of evaporated particles originating from the interaction of 32-GeV positrons with photoemulsion nuclei was performed. The yields of protons, deuterons, tritons, and alpha particles were measured, and the results were compared with evaporation-theory predictions. The energy spectra of protons, deuterons, and tritons agree with a Maxwellian distribution. The excitation energy of photoemulsion nuclei complies with the theoretical estimate for the emission of these particles from the equilibrium state.

Observation of Electrons with Energies up to 180 keV Accompanying μ+ Decay

It is shown that during the stopping and decaying of positively charged muon (μ+) in a nuclear emulsion, there arises with a probability of about 10−2 electron with energies between 10 and 180 keV. The energy spectrum and angular distribution of these electrons are measured. A hypothesis on the possible mechanism behind the appearance of electrons corresponding to μ+ decay is put forth.

Evaporated particles in the annihilation of K − mesons in a nuclear photoemulsion

The emission of light nuclei (p, d, t) in the annihilation of K− mesons stopped in a nuclear photoemulsion is measured. The deuteron-to-proton yield ratio is found to be d/p = 0.32 ± 0.05. The observed excess of evaporated protons is due to intranuclear-nucleon correlations.

Observation of the production of

About 37 × 103 events of alpha-particle interaction with photoemulsion nuclei were scanned. Among them, 339 events involving the production of a 8Li fragment and the decay 8Li →8 Be → 2α were found. Over the region extending up to 60 MeV, the energy spectrum of 8Li is consistent with the mechanism of fragment evaporation from an excited nucleus. At 8Li energies in excess of 70 MeV, the existence of a 8Li fragment in target nuclei before their interaction with a projectile is possibly observed with a probability of about 10−5.

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The azimuthal asymmetry is measured for the emission of Z ≥ 2 particles from the interaction of 22Ne, 24Mg, 32S, 56Fe, 197Au, and 207Pb nuclei with photoemulsion nuclei that is induced by semicentral collisions characterized by projectile energies in the range Epr = 1.88–200 GeV per nucleon and by impact-parameter (b) values in the range 0.12 ≤ b/bmax ≤ 0.70. The results of these measurements are compared with the results of similar measurements for protons. It is found that, at a low energy of Epr ≈ 2 GeV per nucleon, the ratio of the azimuthal-anisotropy coefficients v2 for Z ≥ 2 particles and protons is 6 ± 2, but that, for energies in the region Epr ≥ 4 GeV per nucleon, the coefficients in question agree with each other. This may suggest that, at low energies, Z ≥ 2 particles are predominantly formed at an early stage of the development of a collective flow. For Epr ≥ 4 GeV per nucleon, these particles are presumably formed at the stage of nuclear-matter expansion. Other possible explanations for the results of the observation of an elliptic flux of Z ≥ 2 particles are discussed.

Li fragments in alpha-particle interaction with photoemulsion nuclei

The mean number ⌨Nb〉 of particles evaporated in the interaction of 22Ne, 32S, and 56Fe nuclei with photoemulsion nuclei was measured as a function of the number of alpha particles emitted within the fragmentation cone. It is found that ⌨Nb〉 decreases with increasing number of the alpha particles and increases with increasing number of projectile nucleons involved in the interaction with a target nucleus and that ⌨ Nb〉 is a linear function of the excitation energy Eex of the target-nucleus residue. The maximum experimental value of the mean number of evaporated particles is ⌨Nbmax〉 ≅ 12–13, which corresponds to Eexc ≅ 540 ± 60 MeV.

Measurement of the elliptic flux of Z {>=} 2 charged particles in collisions of relativistic nuclei with photoemulsion nuclei

Angular correlations between secondary particles emitted in the interaction between 56Fe nuclei of momentum 2.5 GeV/c per nucleon and photoemulsion nuclei were measured. These secondaries are partitioned into four groups: s, g, b, and f. For particles from each pair of the above groups, the average values of the collinearity factor and of the asymmetry parameter, as well as the parameter η of the function α(1+η cos ε) approximating the distribution of azimuthal-angle differences ε, were calculated.

Determination of nuclear excitation energies from the number of evaporated particles

The azimuthal anisotropy of the emission of fragments and relativistic particles in collisions between 56Fe nuclei of momentum 2.5 GeV/c per nucleon and photoemulsion nuclei is measured. For semicentral collisions at impact-parameter values in the range 0.12 ≤ b/bmax ≤ 0.70, charged fragments and relativistic particles are predominantly emitted in the direction orthogonal to the nuclear-reaction plane. The azimuthal-asymmetry parameter P2 for fragments whose charge numbers are Z = 1, 2 and Z ≥ 3 takes values of, respectively, −0.192±0.057, −0.28±0.07, and −0.39±0.12. Evaporated b particles have an isotropic azimuthal distribution.