Experiment Proposal - opera - Infn

Figure 154: Average PH max around one strip.
thick lead absorbers are used. The beam telescope includes the BPCs and scintillator counters C1-C3.
An additional larger counter C µ , installed downstream behind a wall of 1.6 m iron and 2 m concrete,
was used to tag beam muons. The setup was exposed to 15 GeV/c mixed beam of negative pions and
muons, shifted diagonally by 2.5 cm away from the brick wall centre and defocussed to illuminate the
area of ∼ 5 × 5 cm 2 (defined by the trigger counter C1).
We analysed two types of events: muon events, simulating CC neutrino interactions with a single
leading track emerging from the brick (e.g. quasi-elastic events), and pion interaction events simulating
deep-inelastic muon-less neutrino events. Muons are selected by requiring a hit in the C µ counter and
less than 6.5 mip for the total PH in the first three strip planes. Events with no hit in C µ and more
than 6.5 mip in the first three planes are identified as pions, mostly interacting in the first brick wall.
In each scintillator strip plane, the centres of gravity of the pulse-heights are calculated. Only strips
with signals above 1 p.e. are retained. The pulse-heights are expressed in mip units to equalise all strips.
The vertex position in one projection is determined as the mean value of the centres in two planes with
the same orientation. The actual beam impact point in each event is accurately measured by the BPCs.
The difference ∆ (vertex) between the true and the reconstructed vertex positions is plotted in Fig. 73.
For muon events with predominantly one strip hit per plane the RMS vertex resolution is 0.62 cm.
The ∆ (vertex) distribution for muons can be approximated by two Gaussians with σ of 0.46 cm and
1.8 cm, the broader one containing less than 1% of events.
For pion interactions the hit multiplicity is much higher (up to all 16 strips per plane) and the RMS
spread in ∆ (vertex) increases to 1.8 cm. The pion ∆ (vertex) distribution, when approximated by
two Gaussians, shows a narrow peak containing about two thirds of the events and having a σ of about
0.8 cm and a broad one with a σ of 2.2 cm.
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