Hi all, Concerning ring finding algoritms, I think one should be careful assuming fixed radius in our case. We have pions, kaons and protons, many of them close to the Cherenkov threshold, i.e. not saturated rings. To use Julia's method would mean first to determine the momentum and then assume in sequence that the particle is a pion/kaon/proton, do the ring finding and fitting to a circle and then at the end produce probabilities for each of the hypotheses. This is indeed a viable method, but did does introduce systematic effects. Adding knowledge about the centre of the circle is just one more parameter in the game. This kind of method is reported as one option in the paper I quoted. The other type of method relies a little more on knowing the centre, but is probably less biased when it comes to pre-knowledge about the radius. It is basically Claus' concept: start with a centre, histogram radii from the centre and determine a best radius - possibly followed by a refitting of the hits in the found ring to a (slightly) shifted centre and radius. In either case, the worst problem is to suppress background hits not belonging to the ring. It pays to have a good understanding of the background sources: white noise, other reconstructable tracks, not reconstructable tracks, etc. The background is typically not flat, because the geometry in the RICH is limited, i.e. you are always close to the edge of the detector. Another thing relating to Claus' mail. Gain matching is of limited value. All you really need to know is if the signal is above threshold to be counted as a photon. Trying to count photons in a given channel from the measured pulse height is difficult to impossible due to the signal spread of a single photon. I am not sure, Claus, if your dR signals were weighted with ADC count. I think they shouldn't be. Cheers, Borge +------------------------------------------------------------------------+ | Børge Svane Nielsen E-mail: borge@nbi.dk | | Technical Coordinator http://www.nbi.dk/TECHI | | Niels Bohr Institute Phone: (+45) 3532 5433 | | University of Copenhagen Fax: (+45) 3532 5465 | | Blegdamsvej 17, DK-2100 Copenhagen O, Denmark | +------------------------------------------------------------------------+
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