In order to search for the weak neutral current in the nucleon-nucleoninteraction, we investigated P-odd asymmetries in the reactions ⁶Li(n,α)t and ¹⁰B(n,α)⁷Li* →⁷ Li+γ, using a multi-section ionization chamber and large scintillation crystals, respectively. These detectors, an integral method to detect the events, and the most intensive neutron beam in the world, PF1B, allowed us to measure the asymmetries with a record sensitivity of about 4·10^-8.

The existence of neutral weak currents is well established in the Standard Model of particle physics. Such currents were found earlier in leptonic and semi-leptonic processes but have not yet been observed in the weak nucleon-nucleon interaction. The problem here is that the tiny effects caused by the weak interaction are overlaid by the strong interaction. Therefore, the only way to detect these effects is to measure the P-odd asymmetries since the parity violation is specific to the weak interaction.

The importance to measure the weak neutral constant for the nucleonnucleon interaction is evident. Its value should significantly depend on the vacuum average value of the energy density of the quark condensate in a nucleus, which is of fundamental importance in nuclear physics and in quantum chromodynamics. The main methodical difficulty is to measure the P-odd asymmetry coefficients with values as low as ~10^-8 in the corresponding nuclear reactions. In the present work, we measured the P-odd asymmetry coefficients with a sensitivity that is approaching to 10^-8.

P-odd effects in neutron-induced reactions have been measured earlier in fission, in neutron optics, and in (n,γ) and (n,p) reactions with heavy and medium heavy nuclei. Such effects could be detected because the high number of nucleons in such nuclei might cause their enhancement by 2- 3 orders of magnitude. However, since these enhancement factors can not be calculated precisely enough, the measured P-odd effects do not allow one to determine the coupling constants of the underlying weak interaction itself. Reactions like n+p→d+γ with a very small number of interacting nucleons provide the most direct way to measure these coupling constants, as any uncertain enhancement factors are absent there. Since the asymmetry in the γ-ray emission is expected at Aγ 2·10^-8, an accuracy of better than 5·10^-9 is required. Such an accuracy has not been achieved so far. However, this requirement could be relaxed by using reactions with light nuclei like ⁶Li, ⁷Li, or ¹⁰B. Such nuclei are well described in the framework of cluster and multicluster models where they are considered to consist of a few well-known clusters, e.g. ⁶Li α+n+p α+d or ⁷Li α +2n+p α + t. Then a reaction of a neutron with a light nucleon can be considered as a reaction between a few nucleons or nuclei in the field of one or a few a-particles, mediated by π⁰ -mesons (the neutral current) and π^+/- -mesons (the charged current). The P-odd asymmetry coefficients can be calculated in terms of the constants of π-nucleon coupling [1,2] and the measurement of such Podd coefficients would allow one to calculate the weak neutral constant of the π-meson exchange. We consider measurements of the P-odd asymmetry in the reactions ⁶Li(n,α)t (in the triton channel) and ¹⁰B(n,α)⁷Li* → ⁷Li+γ (in the γ-quantum channel) as the optimal and most perspective ones [3]. Estimated effects in these reactions are significantly larger and the experiments are much easier due to huge reaction crosssections of 1000-4000 barn, compared to those in the very few-nucleon nuclei. Besides, these experiments are simpli- fied since the particle to be observed can be clearly distinguished from other reaction products. In the ⁶Li experiment, the triton absorption length is about 5 times longer than that of the α-particle, which permits to absorb the a-particles before they enter the gas ionisation chambers [4] used for the triton detection. In the ¹⁰B experiment, only the γ-quantum can enter the detector (two large NaI scintillators). The expected asymmetry coefficients for the triton emission in ⁶Li(n,α)t and the γ-quantum emission in ¹⁰B(n,α)⁷Li* → ⁷Li+γ are α_t1.55·10^-7 and αγ 4.6·10^-8, respectively [2]. For the calculation of these values, the minimal theoretical estimation for the weak neutral constant ƒ_π=2·10^-7 was used. The values α_t and α_γ were measured recently at the PF1B neutron beam at the Institute Laue-Langevin, and the constant ƒ_π was estimated in both cases. We obtained α_t =(-8.1±3.9)·10^-8 and αa_γ=(-2.3±3.8)·10^-8, corresponding to ƒ_π=(0.1±0.7)·10^-7 and ƒ_π=(-0.7±0.96)·10^-7, respectively [5]. The estimated constant of the weak neutral current appeared to be lower than the minimal theoretical estimation ƒ_π=2·10^-7.

The achieved record sensitivity improves the preceding level. However, a further increase in the sensitivity is necessary in order to obtain statistically significant non-zero effects for the P-odd asymmetry coefficients. Even if the weak neutral current would not exist in the nucleon-nucleon interaction, some non-zero values of the P-odd asymmetry coefficients would be measured at the sensitivity level corresponding to the well-established weak charged currents. Recent improvements in the experimental equipment and in the neutron beam should allow us to reach a sensitivity of 10^-8 for the asymmetry coefficients, which should be sufficient to measure for the first time the nonzero effects due to the weak neutral or at least due to the weak charged currents.

This work was supported by a grant RFBR, Moscow.