Neutrinos, CP Violation Highlight DPF 2000

B meson decay candidate in the BELLE detector.

What are the parameters describing neutrino oscillation? Does the observed violation of CP symmetry agree with the predictions of the standard model? These were among the questions for which preliminary answers were suggested at the August meeting of the Division of Particles and Fields on the campus of Ohio State University.

In a plenary talk on Thursday morning, August 10, Kate Scholberg of Boston University highlighted the recent announcement of the direct observation, by the DONUT detector at Fermilab, of the tau neutrino, the last of the leptons predicted by the standard model, thereby formally completing the structure for which so much evidence has accumulated over the last couple of decades.

She went on to review the data, not all mutually consistent, determining the parameters of neutrino oscillation. These data come from three basic sources: the deficit of electron neutrinos from the sun, the deficit of muon neutrinos produced in the atmosphere, and the direct observation of muon neutrino to electron neutrino oscillations in terrestrial experiments.

The latest results on the solar and atmospheric neutrinos come from SuperKamiokande, a giant water Cerenkov detector in Japan. In addition, the direct observation of neutrino oscillation by the LSND experiment at Los Alamos, has been looked for but not yet seen by the KARMEN experiment at Rutherford-Appleton Laboratories in England. Given the current sensitivity, however, these two accelerator experiments are not inconsistent.

If all the neutrino experiments are taken at face value, Scholberg said, they present a puzzle: one can deduce from them three different mass-squared differences for the various neutrino families, but if there are only three families, there can be only two independent such differences, and they do not agree with the data. One looks to several experiments in the next few years to help clarify the situation: the Sudbury Neutrino Observatory in Canada (funded as well by the US and the UK) together with other solar neutrino experiments, the BooNE experiment at Fermilab which will test the LSND results, and a series of long baseline experiments, in Japan, the US and Europe, which send beams of neutrinos from an accelerator to a neutrino detector located many hundreds of miles away.

Two experiments to measure CP violation parameters reported results at DPF2000. The BELLE experiment, at the KEK facility in Japan, was described in a talk by Jorge Rodriguez of the University of Hawaii. The experiment studies the decay of B mesons, which are composed of one bottom quark and one ordinary anti-quark, and their antiparticles the B-bar mesons. If CP (which is a symmetry related to the exchange of a particle with its antiparticle) is violated as inferred from the standard model, then, under certain circumstances, there should be a difference in the time dependence of the decay of B and B-bar mesons into the same final state. Because these are rare events the statistics in these early data are relatively poor, and so far BELLE's results are consistent either with the standard-model prediction or with zero. According to Rodriguez, a significant improvement in statistical accuracy is expected within the next couple of years.

The second experiment is Babar, located at the Stanford Linear Accelerator Center, a detector built by a collaboration of about 550 physicists from nine countries. The results were reported at DPF2000 by Yury Kolomensky of Caltech. Like the KEKB accelerator, the PEP-II accelerator at SLAC features asymmetric colliding beams, which means that the B's and B-bars are moving down the beam pipe after they are produced, allowing experimenters to deduce the time-dependence of the decays from the positions of the decay vertices. Like BELLE, Babar needs more statistics to differentiate between the standard model and zero, and they have plans to increase their data sample tenfold within the next two years. As Kolomensky says, these precision measurements will test the standard model predictions and provide new insights into the nature of CP violation.