Neutral Currents in BEBC -
The experiment WA21 (part 6)

Neutral Current Events

The Situation

My diploma thesis (worked out at the Physics Institute at the University Bonn) introduced a new method to identify hadronic-induced events: N*s accompanied by a thrugoing muon -- and proved it to be reliable. Together with a careful analysis of the Internal Picket Fence IPF's data (evaluated by Maurice Ritter and Simon Towers from Oxford University) it seemed to be possible to reduce any background for NC events such it is feasible to start analyzing neutral current reactions: Yes we can!

With this attitude, I moved as PHD student to the Max-Planck-Institute for Physics in Munich to the only existing and strong German member of the famous 'Aachen-Bonn-Munich-London-Oxford' WA21 collaboration.

After some analysis on the current situation, it became obvious that the decision quality of the PICKET program

  1. was by far not good enough concerning disentangling the different event species and left a significant amount of Don't Know Events,
  2. didn't help to understand (and utilize in a none-trivial way) the information regarding the unassociated hits in the IPF,
  3. was restricted to the assumption that any track leaving BEBC shall be muon, which is of course not true.

After the first promising results, I concluded that the individual systematical approaches (in particular muon and neutral star detection) did not lead to a qualified description of NC events. Rather, even after carefully cleaning the input data in 1985 at CERN (by means of an enhanced QUACK = Quality And Check and SYNTHESIS program), it became obvious, that a systemic method is necessary to finally identify NC events with the required quality. Thus, I decided to develop a new data-reconstruction program providing an unbiased and complete description of the initial data.

Reconstruction of a NC event as provided by the SEVEN program. The dotted lines refer to potential γs hitting the IPF and providing an electro-magnetic cascade in the BEBC vessel triggering the IPF.
Reconstruction of a HA event by means of SEVEN.
The unassociated IPF activity can be recognized originating from the primary interaction outside BEBC.

I worked on this new data-reconstruction program called SEVEN (including roughly about 20K LoC) between 1986 and 1988 in FORTRAN 4. SEVEN is essentially a program to provide the filtered input for any analysis on tape; irrespective of the reaction type/target. The physics community referrers to this as Data Summary Tape (DST).

We remember and keep the tradition alife if we label (now 'triggered' by the PICKET program):

  1. A events consist of events with detected muons (= CC),
  2. B events have no clear signature of muons (= potential NC),
  3. C events are identified N* events; either by means of the Veto Picket Function or by means of associated thrugoing muon (earlier called AS events);
  4. D events (Don't know) can neither be attached conclusively to any hypothesis above.

Extracting the final NC event sample

Even after utilizing all detector capabilities, it is not possible to disentangle the event species completely, as already discussed. Main contaminations are CC events with muons below 2 GeV/c momentum and all sorts of WBB background events, while N* events could almost completely removed from the raw sample.

NC candidate event samples according to the estimated CMS energy W. The thin lines denote the contribution from the different sets as assumed by the LUND Monte Carlo, while the thick line is their superposition.
The dots with error bars refer to the raw data as obtained by SEVEN.

Different from the CC contamination (which was estimated and statistically substracted) from the data, I always used a linear combination (thanks to Wolfgang Wittek) of events from both the neutrino and anti-neutrino beam since in both beams we expect NC events from the 'wrong' neutrino type (with different contibutions). Thus, the sample of corrected NC events were unfolded from the raw samples of events.

The actual analysis routines taking care of this approach were comparable in size to the SEVEN program and I completed those in the following months, including the required unsmearing corrections.

Measurables in NC Interactions

Regarding any NC analysis, it became obvious, that the only measurable physical quantities are:

Unlike CC events, it is simply not possible (except of the rare cases of a 0-C fit) to determine the incident (anti-) neutrino energy and to calculate from here the kinematical values. Essentially, one is left with some crude and partial knowledge of the hadronic finale state.

However, with some care, I analysed the Neutral Current events closely to the CC events. My analysis completes the goals of the experiment WA21 with respect to the initial items b) and d).

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