tauSPECT

Introduction

The storage experiment τSPECT deals with the determination of the free neutron lifetime. Unlike former storage experiments the neutrons are not confined in material walls but with magnetic fields. After various storage times the remaining neutrons are counted. The number of neutrons compared to the corresponding storage time shows an exponential decay which is used to determine the neutron lifetime.

β-decay

Every physical system wants to minimise its energy. Neutrons and protons are quite similar in their constituents (both consist of three valence quarks - neutron: udd and proton: uud) so that the slightly heavier neutron with mn = 939 MeV is able to decay into the lighter proton with mp = 938 MeV under radiation of a W- boson. The latter converts into an electron and an electron antineutrino.

Beam- vs. storage experiment

The free neutron lifetime can in general be determined by two different types of experiment:

Beam experiments use a cold neutron beam and detect the decay protons from a predefined volume. The ratio of this number to the total number of neutrons in the beam depends exponentially on the neutron lifetime. The special difficulty with this type of experiment is the necessity of two different types of detector (one for protons and one for neutrons) which typically have different efficiencies. They need to be understood precisely for a comparison of measured protons and neutrons.

In storage experiments ultracold neutrons are confined in a volume and after a variable storage time the surviving neutrons are counted. The result is an exponential decay in the number of counted neutrons for different storage times, which ideally depends only on the neutron lifetime if no other loss mechanisms occur.

Both types of experiment show results with a precision of less than one second. Nevertheless the measured lifetime is around 888 s with the beam method and around 878 s with the storage method. This discrepancy (also known as neutron lifetime puzzle) is currently not (yet) understood.