[Halld-pid-upgrade] Fwd: Re: thoughts after PID session [from Mike]

[Matthew Shepherd]

On Oct 8, 2012, at 5:58 PM, Michael Williams wrote:

> One final question: Do we have any decent predictions for the size of the hybrid rates?  Paul is looking at S/B but we don't really care about S, we care about S*f where f is the fraction of the signal that is from a hybrid.   If the bkgd is of comparable size to the hybrid contribution, then it could be a problem (regardless of how big the non-hybrid signal is).   

Yes, it can be a problem, but it is more complicated than just a PID problem.

Definitions:

S:  number of events with some topology of stable hadrons
f: fraction of these events that come from exotic resonance decay
B: number of events that are not of the signal topology

The PID system is going to help us with S/B.  We hope that nature helps us with f so that S*f is bigger than B.  The idea that S*f is big is one of the arguments for building a photoproduction experiment -- we have to test it.  I don't think we have a good idea of how big f is.

There are some measurements for S.  See:

http://argus.phys.uregina.ca/cgi-bin/private/DocDB/ShowDocument?docid=809

which is an old compilation of cross sections.

I would tend to focus on S/B in the PID studies, since that is what the PID system will do.  If we want to go a step further we can use PID system to maximize S/B (or some other FOM.. maybe S^2/(S+B)) and then use the size of B to infer some sensitivity limit on f or the hybrid production cross section.  

It may be more clear in a presentation to say a system gives us sensitivity down to a certain value for S*f, than picking an f, which has a big uncertainty, and quoting S*f/B.  We can ask:  which system gives us the most sensitivity to exotics?  All assumptions that go into such a study would be based on some data or a Pythia model.  

Matt