- The absence of a description of gravity compatible with the other forces.
- The standard model does not contain dark matter, which comprises ~20% of out universe, nor does it contain dark energy, which accounts for another 75%.
- The standard model contains many parameters, which must determined from experiment. This in itself is not necessarily a problem, but current belief is that there must be an ab initio way to determine these parameters, without resorting to experiment.
Several methods exist to search for physics beyond the standard model. High energy methods (direct searches) use large accelerators to directly search for these effects while low energy, ultra precise, experiments seek to discover the effects of the “beyond standard model” physics on the low energy observables that can be measured in small scale experiments. Our group focuses on the latter method. The primary tool we use is the ElectroWeak interaction, in particular, we measure the nuclear β decay process of some radioactive isotopes, which can be exactly calculated in the standard model and thus allows us to search for any deviations from the calculation.
In the β decay process a neutron is converted to a proton by the emission of an electron and an anti-neutrino, this is called a β- decay, the opposite process which converts a proton to a neutron is called (naturally) a β+ decay. An example of such a decay is the decay of Neon-19 to Fluorine-19
The energy released in the decay (which comes from the mass difference between the parent and daughter nucleus) is shared between the daughter nucleus, the beta, and the neutrino.
In general, the decay rate for a beta decaying nucleus can be written as.
where aβν, b, c, Aβ, Bν, and D are coefficients which describe, respectively, the correlation between directions of emitted neutrino and beta particle, the energy dependence of the process, and the correlations between the direction of the polarization direction of the emitted beta and neutrino.
In the standard model framework, it is possible to calculate these coefficients to a high degree of accuracy, any deviation from these calculation is a clear signal of beyond standard model physics.
Our primary tool for investigation of these decays are trapped radioactive neutral atoms and ions. The trapped nuclei serve as a source of cold, localized, radioactivity and enable us to make the very precise measurements without any outside interference to the system we are measuring.
Currently we are in the process of designing and building a Magneto-Optical Trap (MOT) setup for studying the beta decay of radioactive Neon atoms. We are also collaborating with groups at the Weizmann Institute and at Lawrence Berkeley National Lab on an effort to perform the same types of measurements using radioactive ions trapped in a special, fully electrostatic, trap.
References:
- N. Severijns et al. Tests of the standard electroweak model in nuclear beta decay. Reviews of Modern Physics (2006)
- J. D. Jackson et al. Possible tests of time reversal invariance in beta decay. Physical Review (1957)
- P. Herczeg. Beta decay beyond the standard model. Progress in Particle and Nuclear Physics (2001)