Experiment Proposal - opera - Infn

through the decay topology and its decay modes into an electron, a muon and a single charged hadron
τ − → e − ν τ ν e , τ − → µ − ν τ ν µ , τ − → h − ν τ (nπ 0 )
Despite its distinctive topology the multi-prong channel of the τ has not been considered for the
present estimate of the OPERA sensitivity, due to the less favourable signal to noise ratio.
Measurements of the branching ratio (BR) of the three single-prong decay modes give 17.8%, 17.7%
and 49.5% for the electronic, muonic and hadronic channel, respectively. For the typical τ energies
expected with the CNGS one obtains the decay length distribution shown in Fig. 4, evaluated for the
oscillation parameter values indicated by the Super-Kamiokande results.
Figure 4: τ decay length distribution.
If a τ is produced in a lead plate it will decay either in the same plate (short decays) orfurther
downstream (long decays). For long decays, the τ is detected by measuring the angle between the
charged decay daughter and the parent direction. For example, Fig. 5 shows the distribution of the τ
decay kink angle for the electron channel. For this measurement the directions of the tracks before and
after the kink are reconstructed (in space) by means of the pair of emulsion films sandwiching the lead
plate where the decay vertex occurred (Fig. 3). The τ can also decay in one of the films downstream of the
vertex plate (e.g. in its plastic base). Even then, the kink angle can be reconstructed, albeit with a lower
angular resolution, from the track segments in the emulsion layers on either side of the base. A fraction
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