β-decay

Introduction

The aim of the Na β-decay experiment is to search for signatures of the breaking of time reversal invariance. For this purpose we study a radio-active isotope of sodium, ²¹Na. This isotope lives only 20 seconds before it decays to ²¹ Ne through emission of a positron and a neutrino. This is an example of β-decay. By measuring correlations between the spin and direction of the two emitted particles, we can find the the amount of T-violation in this process. Since the Standard Model predicts that T-violation effects are immeasurably small, a non-zero value will be a clear indication of "new physics". If, on the other hand, the experiment shows no sign of T-violation, it puts constraints on the candidate successors of the Standard Model.

A wide range of experimental techniques

Since ²¹Na is so short lived, it needs to be produced by the AGOR cyclotron and the TRIMP facility. The resulting MeV beam of ²¹Na atoms needs to be slowed down before we can perform our precision experiment. This is done by sending the beam into a so-called magneto-optical trap (MOT), which is in essence a combination of three orthogonal laser beams and two magnetic coils. Next, the atoms are transported to a second MOT. Here the situation can be better controlled to reach the very clean environment needed for the measurements.

First, the nuclear spins of the trapped ions need to be polarized. This can be done by laser techniques. The laser interacts with the valence electrons of Na-21, thereby indirectly manipulating the nuclear spins. Instead of directly detecting the emitted neutrino, which is extremely difficult, we measure the recoil momentum of the daughter Ne-21 ion. To do so, we apply an electric field that accelerates the daughter ion towards a position-sensitive detector. This position, in combination with the time of flight, allows us to determine the initial momentum of the daughter. The emitted positron can also be detected directly. This is done by plastic scintillators and photo multiplier tubes. To cover as many directions as possible, two large detectors will be put around the atom trap.

Having so many detectors producing huge amounts of data, the data acquisition is an important aspect of the experiment. VME computers send the data from the setup over the network to a desktop computer where software, based on CERN programs, deals with the stream of data.