The reasons for carrying out experiments with high energy neutrinos are fairly obvious. Until two years ago, our knowledge of weak interactions came only from the study of processes like nuclear
B
decay,
n
decay,
y
decay,
u
capture, etc., involving energy transfers of the order of 100MeV or less. A fairly comprehensive theory of
B
decay, capable of accounting for the great bulk of the experimental data, was built up over the years which, however, left open a number of questions which could only be answered by looking at weak interactions at much higher energies, in the multi-GeV region. In principle, these high energy weak processes might be studied experimentally using a variety of beams of energetic particles (such as protons, muons, electrons). In practice, however, a beam of neutrinos, which are unique in that they can undergo
only
the weak interaction, offer tremendous advantages over other types of particle, since only then are the events to be studied not swamped by an enormous background due to the effects of strong or electromagnetic interactions. The possibility of carrying out experiments with high energy neutrino beams from accelerators was discussed several years ago by many people, notably by Schwartz, Pontecorvo and Lee & Yang. The cross-sections for the interaction of neutrinos with matter was known to be extremely low, of the order of 10
-38
cm
2
/nucleon for a neutrino energy of 1 GeV. If the neutrino beams were to be produced by the decay in flight of pions or kaons produced in conventional proton synchrotrons, the fluxes were such that one might reasonably expect to observe about one interaction per day per ton of detector, and it seemed doubtful if this event rate would be sufficient for quantitative experiments. In fact, the first successful, large-scale neutrino experiment, carried out in 1962 by a group at Brookhaven from Columbia University (Danby
et al.
1962) yielded a striking and definite result from only a handful of events; namely, that there are
two
types of neutrino, v
e
and v
fl
, associated with the electron (e) and the muon (y) respectively. More recently (during 1963), major technical advances, particularly at C. E. R. N. have increased the available neutrino fluxes by between one and two orders of magnitude. These developments have been, first, a considerable increase in proton beam intensity, from the region of a few times 10
11
protons/pulse to nearly 10
12
protons/pulse; secondly, the successful extraction of nearly the entire proton beam from the synchrotron, so that one is able to take advantage of the higher intensities of pions and kaons emitted in the forward direction from an external target, instead of taking the beam off at a considerable angle (5° to 10°) from an internal target; thirdly, the development of magnetic focusing devices (‘horns’) which collimate the pion beams from the target into a narrow forward cone. The corresponding neutrino event rates are then of the order of one per ton per hour, rather than one per ton per day, and, with the certainty of further order-of-magnitude increases in proton beam currents (for example at the Argonne ZGS accelerator), the possibility of quantitative neutrino experiments looks very good indeed. Certainly, the prospects are much brighter than anyone thought a few years ago.
LEvEL: Low-Energy Neutrino Experiment at the LHC
