Neutral Currents in BEBC -
The experiment WA21 (part 5)

Extracting the Neutral Current Couplings

Event selection

While it is relative easy to detect CC events in BEBC, the more difficult interaction is the weak neutral current (NC) where the (anti-)neutrino exchanges a Z0 with one of the proton's quarks and leaves the detector with changed momentum, but without a trace.

As discussed, events with this interaction type in the bubble chamber obtain strong competition from background events:

Before the Internal Picket Fence area, the only chance to retrieve a relative clean sample of CC and NC events was to introduce severe cuts on the event samples:

  1. Cuts in the fiducial volume accepting any events, greatly improve the actual geometrical efficiency of the EMI, while reducing the number of events irrespective from their reaction type.
  2. Cuts in the number of charged tracks nch ≥ 3.
  3. Cuts in the total seen hadronic energy EHad,seen > 5 GeV/c2.
  4. Cuts to the seen transverse hadronic momentum of the event, typically pT(Had,seen) > 1.5 GeV/c, discriminating efficiently the N* events and by the same token CC events originating from electron-(anti-)neutrinos interactions.
Definition of the fiducial volume for BEBC [Towers] also visible are the entry and exit gaps in the coils for charged particle beams.

Further, there is the need to correct for events which are falsely labeled in any event category. As a consequence, a Monte Carlos is required to study the effects of the cuts applied, to estimate the real numbers, and to correct for scanning inefficiencies and others. In particular, the subtraction of WBB events depends significant on the corresponding Monte Carlo simulation program, whose results were doubtful to some extend.

None of the methods however, are able to distinguish the following events species:

Obtaining the NC/CC ratio

As a result, the estimation of the correct number of NC and CC events (resulting finally in the ratio of those numbers R) is facilitated by a correction table, which tries faithfully to care about the different event subsamples, potential contaminations, and losses.

By means of the Veto Picket Function, there is the chance to relax in particular the stringent cut on pT,Had,seen, which impacts the statistical precision significantly.

However, any arbitrary cut on the kinematical variables influences the ratio NC/CC since the events populate the phase-space differently (according to the Q2 and yBj distributions).

Uncorrected (including N* events) and corrected values for the ratio R = NC/CC for WA21 events (nch ≥=3; EH > 7.5 GeV/c2) in the neutrino and anti-neutrino beam [Towers]. The prediction of the Monte Carlo assuming sin2ΘW = 0.217 is the shown as solid line.

While comparing the R = NC/CC results from WA21 and other experiments (in particular isoscalar ones, like WA25, CDHS, CHARM ...) one has to keep in mind, that the raw numbers do always depend on the cuts employed.

Neutral Current Couplings and sin2ΘW

In order to determine the weak chiral couplings (model-independently) of the up- and down quark, we need to have four different measurements.

On an isoscalar target (like D2) this can be achieved by simultaneously measuring neutrino and anti-neutrino events selected for proton and neutron reactions (this sounds more easy than it is).

For WA21, we can at least determine the differences or sums of left-handed and right-handed couplings, which have been determined to be:

However, according to the standard model, these values are not independent but rather are derived from the single value of sin2ΘW. As discussed in Simon Towers thesis, we obtain:

Result on the weak neutral couplings for up- and down quarks in comparison to the standard value of sin2ΘW [Towers]

By means of the Standard Model the weak neutral couplings are defined as:

  1. uL = cL = tL = 1/2 - 2/3 sin2ΘW
  2. uR = cR = tR = - 2/3 sin2ΘW
  3. dL = sL = bL = -1/2 + 1/3 sin2ΘW
  4. dR = sR = bR = 1/3 sin2ΘW

Continue with Neutral Current Events