Dear Borge,
Julia's algorithm does not require a
fixed radius. Rather one would adapt the radius
based on the knowledge of the momentum. For
each track of momentum P their will be 3 possible
ring with the pion one being largest, then the
kaons and finally the smallest for the proton
if it is above threshold.
Yours Michael
Quoting Borge Svane Nielsen <borge@alf.nbi.dk>:
> 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
> |
> +------------------------------------------------------------------------+
>
>
Michael Murray, Cyclotron TAMU, 979 845 1411 x 273, Fax 1899
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