Dear Claus,
everybody has Tachions so don't worry about showing
negative mass**2. If you are looking for rare particles you
could plot mass**2(TOF) versus mass**2(RICH). However these
variables would
not be independent since they both use the momentum. However you
could plot the velocity as determined from TOF versus the
velocity from the RICH ring radius. This should be a nice band at
45 degrees with some noise caused by misidentified tracks.
Yours Michael
Quoting "Claus O. E. Jorgensen" <ekman@nbi.dk>:
>
> Hi all,
>
> Thanks to Peter I realized that it's maybe not so clever to calculate
> the
> mass from the RICH rings (some may even say that it's a bit stupid).
> One
> should rather calculate the mass^2 (like for the time-of-flight walls).
>
> The RICH mass is calculated from the momentum and the radius of the
> reconstructed rings (or the cherenkov angle theta). The mass is
> calculated from
>
> m = p sqrt( (n*cos(theta))^2 - 1 )
>
> The problem is just that (n*cos(theta))^2 - 1) can be negative, if the
> measured theta/radius is bigger than the theoretical maximum
> theta/radius. Fortunately the resolution of the RICH is so good that
> this
> does not happen very often - we loose approx 3% of the calculated masses
>
> in the high field runs (1/2 field), and probably much less in the lower
> field runs.
>
> The right way to do it, is to use mass^2 all the way (as for the
> time-of-flight walls):
>
> m^2 = p^2 * (n*cos(theta))^2 - 1)
>
> If you use the RichPid objects the radius should be ok, but if you make
> cuts in mass I guess you will loose the few cases where
> (n*cos(theta))^2)>1.
>
> I would like to make the changes in brat soon - that is introduce mass2
> instead of mass in the BrRichPid class. This will of course cause some
> trouble for those that use the mass from the BrRichPid objects. If
> there are no strong objections I will commit the changes one of these
> days.
>
> Comments and suggestions are welcome!
>
> Claus
>
> +-------------------------------------------------------------+
> | Claus Jørgensen |
> | Cand. Scient. Phone : (+45) 33 32 49 49 |
> | Cell : (+45) 27 28 49 49 |
> | Niels Bohr Institute, Ta-2, Office : (+45) 35 32 53 07 |
> | Blegdamsvej 17, DK-2100, E-mail : ekman@nbi.dk |
> | University of Copenhagen Home : www.nbi.dk/~ekman/ |
> +-------------------------------------------------------------+
>
>
Michael Murray, Cyclotron TAMU, 979 845 1411 x 273, Fax 1899
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