Neutral Currents in BEBC -
The experiment WA21 (part 4)

Events in BEBC

The image below shows a complex reaction in BEBC. Typical (anti-) neutrino events are however different: They appear out of the nothing. Mainly with very few tracks only; in particular in the H2 liquid, since the reaction length of the π0 disassociating into γs to produce the final electron/positron pairs (visible in the picture as electromagnetic shower) is very rare.

'Rich' neutrino event in BEBC filled with heavy liquid[CERN]
colors removed for clarity

Apart from the specifics of the detector, the interaction of the (anti-) neutrinos determine the event characteristics:

Compared to current collider experiments, this value is quite small and this is one of the reasons why (anti-) neutrino events can be measured by men and don't require sophisticated electronics.

Unlike heavy liquids (serving as isoscalar targets with an equal number of protons and neutrons), the charge composition of the final particles can be well determined in hydrogen, since the reaction takes place on a single proton (with main quark constituents |uud>) and avoiding any so-called nuclear effects.

The main visible measurables in the bubble chamber are:

'Complex' events happen only in case the energy deposited by the (anti-) neutrino is either very large, or certain kinematical conditions are met.

A significant amount of reactions (~ 15 %) can be measured completely in hydrogen:

For CC events the following reaction behavior is well known:

Charged Current Events (CC)

The main reaction of (anti-) neutrinos is due to a charge current process CC. For neutrino interactions a primary W+ is exchanged and for (anti-) neutrino reactions a W-. As a consequence, the struck quark requires to be changed. CC events are flavor changing reactions, making it possible to transform a u-quark into a d-quark and vice versa.

Feynman diagram of a CC (anti-) neutrino proton interaction
(valence quark approximation)

Since the reaction needs to be balanced, in addition the (anti-) neutrino is flavor-changed: A neutrino becomes a µ- and an anti-neutrino turns into a µ+. To keep the electrical charge unchanged in any reaction is one of the main paradigms in physics, while other quantities may not be conserved (exactly).

Historically and from the detector's point of view, CC events are easy to detect and to measure, thus hardly any other interaction of (anti-) neutrinos were considered to exist up to 1970. The existence of neutral currents and the respective heavy intermediate boson (in today words: the Z0) were intensely debated, since it was required by theory to guarantee it's renormalizeability according to Abdus Salam.

Occasionally, CC events are accompanied by more then one muon. This happens naturally, if in the reaction a charm quark is produced (instead of an up quark), decaying into a muon. These muons carry significant lower momentum and posses the opposite charge of the primary muon. Di-muon events with the same charge are believed to happen mainly because of coincidences or failures in the (EMI) muon detection.

Neutral Current Events (NC)

Unlike the dominant CC reactions with a well-measurable muon in the final state, the dynamics of neutral current (NC) reactions are determined by the so-called Weinberg-Angle sin2θw (~0.2) providing essentially the probability the (anti-) neutrino couples with (right- and left-handed components) of the u-or d- (or c-, or s-) quarks. The probability for neutrinos to produce a NC reaction is roughly 1/3, and for anti-neutrinos only about 1/4 compared to CC interactions in this energy range.

According to the theory of Glashow-Salam-Weinberg, the neutral current interaction is suppressed due to the different masses the W+- (about 85 GeV/c2) and Z0 (roughly 95 GeV/c2) intermediate bosons, as expressed by the famous Weinberg-Angle

cos2θw = MW2/MZ2 or

sin2θw = (MZ2 - MW2)/MZ2

While charged currents are attached to the (electric) charge of the quark, neutral currents are sensitive to the weak composition of quarks (and of course in both cases anti-quarks). Now we have to consider the fact that (anti-) neutrinos are (almost) mass-less. This means not only that they travel with (almost) the speed of light (ultrarelativistic), but in addition have a defined helicity state, thus generally are 'left handed' (neutrinos) or 'right handed' (anti-neutrinos).

Feynman diagram of NC neutrino proton interactions
(valence quark approximation) with their chiral weights.

However, the struck quarks (as constituents of matter) in the reactions do carry masses. In our today's understanding the 'rest-mass' of up quarks and down quarks is a few MeV/c2 (strange quarks around 120 MeV/c2 and the heavy charm quarks roughly 1.5 GeV/c2).

Of course, also the quarks have helicity; but due to none-zero masses their particular helicity state depends on the fact how the (anti-) neutrino strucks ('sees') the quark, thus possesses to some extend a kinematical dependency. The helicity of the quarks are typically expressed in terms of their chiral components uL2, uR2, dL2, dR2, as introduced by L.M. Sehgal (University of Aachen).

In comparison to CC reactions, not only

A beautiful historical science story about the discovery of the Neutral Currents can be found in Peter Louis Galison's book "How experiments end".

Hadronic Induced Events (HA)

The production of (unassociated) hadronic induced events (also called N*s) from secondary neutrons (n) and K0L penetrating the bubble chamber is the most severe background with respect to NC events. We have to consider that BEBC' vessel included particular beam windows which allow even charged particles to enter the bubble chamber.

Drawing of a HA event and it's subsequent detection triggering the Veto-IPF [Towers]

Typical reaction chains for HA events are:

  1. CC event (in coils) => µ + n/K0L,
  2. - or -
  3. NC event (in coils) => ν + n/K0L;

either followed by:

The (statistical) discrimination of the background (based on the analysis of Dieter Haidt in 1972 and in collaboration with Paul Musset for the Gargamelle bubble chamber experiment known as B/AS method) is one of most impressive success stories in High Energy Physics. Though critics may say, these observations could have already been made back in 1963, when the 500 l heavy-liquid bubble chamber at CERN was exposed with the first neutrino beams -- and all events without an obvious muon (later called B events) were neglected ...

Though a HA event fakes a NC event, by virtue of the IPF most of the HA events can be successfully classified. However, since neither NC nor HA events carry specific attributes to discriminate, there is still a certain contamination of HA events in the NC events in the NC sample and a loss of NC events due to misidentification.

Exotic events

WA21 had the scope to detect and to analyse 'legacy' events. Experiments like the Beam Dump (WA66) were used to search for exotic reactions. However, the 'filter' to detect new phenomena is the human brain: What you don't expect, you don't see; except for the unfaithful. In modern detectors it is the trigger of the electronics, which biases the experiment, and in addition the Monte Carlo correcting the data.

While the first approach is like running around with pink sun-glasses, the latter is equivalent to expect that you can measure the unknown while extrapolating the known. This might work; or might not, depending on the unknown (you know: the earth is flat).


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