Just as physicists were getting used to the idea of all particle physics measurements agreeing with each other and with the all-embracing Standard Model, a major experiment at Fermilab has announced a surprising neutrino measurement.
The NuTeV collaboration at Fermilab’s Tevatron compares the different types of neutrino interaction and finds a vital parameter to be 0.2277, not 0.2227. At the level of precision being explored in particle physics, this is a major upset and needs confirmation. However, the NuTeV experiment has now terminated, and the Fermilab Tevatron has ceased operations for fixed target studies, such as those using neutrino beams.
Over a period of 15 months in 1996 and 1997, NuTeV shone beams of neutrinos and their antiparticles at a 700 tonne target. The energy of the neutrinos was 125 GeV, and that of the antineutrinos was 115 GeV.
Neutrinos do not interact readily with matter – at NuTeV, only one in a billion neutrinos registered a hit inside the target. Together, some 2 million neutrino and antineutrino hits were patiently collected. Neutrinos, which are electrically neutral, almost massless, particles, can interact with other matter through the weak nuclear interaction in one of two ways. In the classic form, related to nuclear beta decay, the neutrino changes a nuclear proton into a neutron (or vice versa) and an electrically charged muon (or electron) is emitted. This type of reaction shuffles round the charges of the participating particles and is therefore known as a “charged current”.
In 1973, neutrinos were also discovered to be capable of interacting with matter without permuting electric charge – a “neutral current”. This discovery was vital evidence in favour of the then new “electroweak” theory, which unifies weak interactions and electromagnetism. By looking to see whether neutrino interactions were accompanied by a muon, NuTeV could distinguish between charged and neutral current interactions – only the former produced an outgoing muon.
By comparing the ratio of neutral and charged current production by neutrinos and antineutrinos, the experiment finds a value for the vital mixing parameter (Weinberg angle), which dictates the weights of the electromagnetic and weak effects in the combined electroweak theory and also relates the masses of the Z boson (the particle that mediates the neutral current) and the W boson (the charged-current carrier).
The history of the neutrino has been full of surprises
Beginning with high-energy neutrino studies at CERN in the late 1970s, and continuing with precision measurements of Z and W properties in proton-antiproton colliders at CERN and Fermilab and at electron-positron colliders at CERN and SLAC in the 1990s, physicists had built up a precision picture of weak interaction parameters. However, the latest NuTeV result does not fit in with this.
The history of the neutrino has been full of surprises. The prediction of such a bizarre particle was itself a surprise, and the fact that neutrinos could be observed was another. More came when physicists discovered that neutrinos come in different forms, which could even transform into each other. Is the latest NuTeV result a blip or another neutrino surprise? Only time will tell.
Muon magnetism OK
A heroic reappraisal of complicated calculations means that an intriguing physics anomaly has gone away, but in an unexpected way. Last year a precision measurement of the muon’s magnetism at Brookhaven reported a slight disagreement with the expected value. In some quarters, this was heralded as possible evidence for new physics. After carefully re-examining the underlying calculations, experts found that there was, in fact, a mathematical error in the predicted value. The Brookhaven team’s measured value has not changed, but is no longer a puzzle. The muon’s magnetism looks to be in order.
