Neutral Currents in BEBC -
The experiment WA21 (part 2)

The Experiment WA21

The experiment WA2 was guided by Gerald Myatt from Oxford University and spokesman for the BEBC User Group BUG was the already deceased Douglas Morrison (D.R.O.M).

In WA21, the neutrino's target was simply Hydrogen (H2), the cleanest environment one can think about. However, by the very same token, a target with reduced interaction capability (low statistics) and considerable losses due to undetected neutral particles, in particular γs from π0 decays.

Neutrino flux monitoring was realized by systems supervised by Horst Wachsmuth, who also passed-by recently. Electronics, the External Muon Identifier EMI, and in particular the Internal Picket Fence IPF were subject of Henrik Foeth.

Occasionally, some veterans showed up and were interested in new results, in particular Donald Cundy and Donald Perkins (who passed by in November 2022) with whom I shared partly an office, and who suspiciously starred at my Star War's X-Wing Fighter, I set up on my desk as a good-by present from my Wohngemeinschaft.

Other (quasi-permanent) staff-members were Josef Mittendorfer and Peter Schmid (both from Austria), who was busy this time with the Russian Scientist Andrei Sacharow and on Andy Parker's and Mandy Cooper-Sarkar's (both from Oxford's RAL) office door stuck an irritating drawing of Lady Di and Prince Charles ...  

Famous WA21 CC Charm Event: Roll 204, Frame 995 [CERN]

Shortly after WA21 came into operation, some very nice events were detected, which gave rise to high expectations:

Abstract of WA21

This experiment is a high statistics exposure of BEBC filled with hydrogen to both neutrino- and antineutrino beams. The principal physics aims are:

  1. The study of the production of charmed mesons and baryons using fully constrained events.
  2. The study of neutral current interactions on the free proton.
  3. Measurement of the cross-sections for production of exclusive final state N* and D-meson resonances.
  4. Studies of hadronic final states in charged and neutral current reactions.
  5. Measurement of inclusive charged current cross-sections and structure functions.

The Beam Line

(Anti-) Neutrino Beams

The CERN's Neutrino Beams resulted from protons spitted out of the SPS at energies of 450 GeV/c2 (400 GeV/c2 for anti-protons respectively) in bunches including about 1013 protons (significant less for anti-protons) over a pulse period of 25 msec (the beam spill) and led to a target in the West Area. This happens in case the particles are at their maximum momentum and at high flux - which requires a few seconds to rebuild the beam, since the beam line is considerable emptied after the ejection of the protons, anti-protons respectively.

West Area Neutrino Beam Line [CERN]
During operation, the 290 m long beam line is evacuated, thus the produced particles can decay without interactions.

In order to supervise the ejection, a particular Beam Current Transformer (BCT) was used to tell the SPS team online the efficiency hitting the target initially and perhaps to refine the (anti-) proton beam position.

SPS Beam Current Transformer [CERN]

Typically, the beam hits a target of relative few nucleons (Beryllium) per atom or becomes dumped. In both cases, a lot of short-lived light charged secondaries are produced, mainly π+/- and strange particles K+/-, both called mesons. Those particle species decay into leptons and neutrinos, and in particular into muon-neutrinos (or muon-anti-neutrinos) accompanied by a muon or an anti-muon to be measured and eventually being stopped before reaching the detector.

SPS neutrino beam target [CERN]

Depending on the target and the later focusing mechanisms, we have to consider three different types of neutrino beams:

A significant improvement in producing neutrino beams was invented already a decade ago by Simon Van der Meer's magnetic horn operating at currents of about 150 Ampére to 'guide' and focus the primary produced charged mesons.

Inner part of the van-der-Meer Magnetic Horn [CERN]

Wide Band Background and Neutrino Flux Measurements

Even under ideal conditions, any (anti-) neutrino beam is populated with different species of neutrinos: electron and muon (and perhaps also) tau neutrinos and their anti-partners. However, due to the primary charged particles produced at that energy, namely pions (90 %) and kaons (9 %) with high rates, and their corresponding decay the muon(-anti)-neutrinos dominate the (anti-) neutrino beam.

Of course, the final (anti-)neutrino flux can not directly be measured in the beam line, however, due to the two-body decay the muon flux provides a relative exact knowledge of the primary produced particles. Therefore, the WA beam line included particular silicon muon detectors used as Neutrino Flux Monitors (NFM) to calibrate the descending neutrino detectors for later analysis. The NFM detectors could be moved during the run perpendicular to the (neutrino) beam line to allow an evaluation of the radial distribution of the muons and hence the spread of the neutrino beam.

Neutrino Flux Monitors (NFM) in the beam line [CERN]

Both, charged pions and kaons decay primarily into a charged muon and the corresponding (anti-)neutrino. At the end of the evacuated decay tunnel approximately 5 % of the pions and 20 % of the kaons suffered from that fate. Due to the different masses of the primary particles, neutrinos from kaon decays carry more momentum then those of pion decays and can be clearly distinguished in the observed (anti-) neutrino spectrum. As a result, (anti-) neutrinos from kaons dominate the high-energy region of the (anti-) neutrino beams.

In order to stop the remaining pions and kaons after the decay tunnel, an additional shielding is provided with 185 m of iron and 175 m of earth. The last part of the beam line is the magnetic muon shield consisting of a 10 m long torodial magnet which tries to focus the residual muons towards the beam axis, mainly to avoid rescattering but in addition enhancing absorption of those, in order to reduce the muon background.

In the WBB the final (anti-) neutrinos carrying an average energy of 35 GeV/c2 / 25 GeV/c2, depending of course on the cuts applied.

In order to estimate the amount of 'wanted' and 'unwanted' particles and to provide an estimate of the total (anti-) neutrino flux, a Monte-Carlo calculation was employed which was fed by the data received from the BCT and NFM (telling the charge of the measured muons, which accompany the anti/neutrinos) and some theoretical models about particle generation and particle decay (and their so-called branching-ratios).

Finally, the 'seen' (anti-) neutrino flux depends on the positions of the detector along the beam line. In CERN's West Area hall the first detector was BEBC (at a distance of 820 m from the SPS beam target), followed by Jack Steinberger's (and Konrad Kleinknecht's) CDHS (WA1; at 890 m) and finally Klaus Winter's CHARM (WA18; at 910 m).

Monte-Carlo simulation of SPS'sneutrino (solid) and anti-neutrino beam (dashed) and the neutrino background in the anti-neutrino beam (dash-dotted); the superimposed diagram shows the measured spectrum of neutrinos from (energy corrected, weighted and provisionally scaled) CC events in BEBC recorded in the WBB neutrino beam.
Like above; but now with the superimposed data from (corrections as above) CC events in BEBC recorded in the WBB anti-neutrino beam.

From the above figures one can see that the primary component of the WBB (anti-) neutrino beam (SPS energy 450 GeV/c, run 1983/5) is consistent with the Monte-Carlo simulation (calculated at 400 GeV/c), while the background is typically underestimated by an order of magnitude!

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