Neutron Lifetime Measurement with Pulsed Cold Neutron Beam inside Superfluid Liquid Helium*

To shed light to the free neutron lifetime problem, we propose a novel approach to measure the neutron beta decay lifetime using a pulsed cold neutron beam travelling through superfluid helium. The measurement will have different sources of systematic errors compared to the earlier measurements. When neutron wavelength is larger than16.5 Å the scattering on superfluid ⁴He is absent due to the requirement of the conservation of total energy and momentum of the neutron and the quasiparticle excitations.  When a neutron beta decays and/or a residual ³He captures a neutron in superfluid He, hundreds of 16 eV UV photons will be generated and detected. The number of the beta decays, ΔN_β, and ³He neutron captures, ΔN_3He, can be counted through the detection of the UV pulses. The neutron lifetime τ_β is then calculated from the ratio of ΔN_β / ΔN_3He =τ_βn_3He σ(v)v, where n_3He is the ³He concentration in the superfluid and σ(v) is the neutron-³He capture cross section for neutron with speed of v.  By precisely measuring ΔN_β, ΔN_3He, n_3He and σ(v)v,  we will be able to determine the neutron lifetime. We will discuss the measurement method, experimental setup, and the source of systematic errors.