In addition to the previous version (see text below) I applied a cut on the
cluster charge (about 1.5 MIP) and I kept the ,ultiplicity cut, but looser.

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This parameters have been obtained from 2005 data. 
The definitions of eta2, eta3 and eta4 have been modified according to the
discussion with Prof. Landi.
In particular:
- the baricenter with 2,3 or 4 strips is calculated even if not all the strip
are over threshold
- the correction function for eta4 is defined in the range -0.5:0.5 
and not -1:1 as it was before.

For the moment the multiplicity selection is kept.
The next improved parameters sets should have:
- no multiplicity cut (eventually replaced with a cut on the cluster charge)
- defined as a function of the effective angle (corrected for the magnetic
field)
- implemented the systematic shift due to charge distribution asimmetry