Isolated electrons and muons in events with missing transverse momentum at HERA 

In the case of neutrinos, the clue lies in the momentum which a neutrino carries away from an electron-proton collision or "event". Since the incoming electron and proton approach each other "head-on", basic mechanics tells us that in the aftermath of the event an outgoing particle heading transverse to the direction of the initial electron or proton must be balanced by another particle heading the other way. This is a familar scenario on a billiard table - if the cue ball heading down the centre of the table strikes a target ball sitting on the centre spot, then if one ball richochets to the right the other will certainly go left. Thus if physicists add up the momenta of all the particles they observe in an event and are left with a gap, there's good evidence that a neutrino escaped that way.

Neutrinos are commonly produced at HERA, in so-called charged current interactions, where the incoming electron is converted into a neutrino. In these events a quark is struck out of the proton and rapidly develops into a collimated spray of particles ("a jet"). The characteristic signature of these events is the combination of such a jet and a neutrino (inferred from the imbalance in summed momenta). However, in identifying this type of event, a small subset of events have been found where there is also a high energy electron or muon observed, well separated from the jet. The Standard Model, the state-of-the-art theory of modern particle physics, says that events such as these are usually the result of a heavy W boson being fleetingly produced, before decaying into an electron or muon, and a neutrino.

Examining the data that H1 collected during the period 1994-2000, eighteen events are identified which contain an isolated electron or muon and missing transverse momentum. The Standard Model predicts 12.4 ± 1.7 events of this kind, the majority coming from the decays of W bosons. Whilst these numbers are broadly in agreement with each other, the comparison becomes more interesting when the selection is restricted to those events which have a particularly powerful jet. When this jet punches sideways (transversely) into the detector with a momentum of more than 25 GeV (approximately the momentum the electrons or positrons have when fully accelerated by HERA), ten events are found compared to a Standard Model expectation of 2.9 ± 0.5. The chance of a statistical fluctuation producing this number of observed events is around 0.15%.

One of the selected events (containing an isolated electron) is illustrated below in an "event display". This shows the tracks of charged particles and energy depositions in the H1 detector, both in a radial view (top right) and in a sideways projection (left). In this event the decay electron can be seen in the lower-left section of the radial view. The jet emerging from the proton can be seen lower-right. The empty upper half of the radial view is the evidence for the neutrino. It is events such as this, with a particularly powerful jet, that are of greatest interest to the physicists studying these processes. Since the jet is so powerful it is thought that it develops from a quark recoiling against a heavy particle. The fact that more events of this kind have been seen than are predicted could indicate that a mechanism which is not currently part of the Standard Model is enhancing the frequency of W bosons being produced. It might even be the case that a different heavy particle is being momentarily produced and decaying into products similar to those expected from W boson decay. Only more data and more careful detective work will reveal the truth.