[G14_run] Questions about information of the initial photon

[Paul Mattione]

I haven't checked the calibrations in a while, but it may be that a lot of the photons are being lost because they're hitting tagger counters that have misaligned timing (I was seeing this problem with earlier runs).  If the tagger timing is jumping around frequently we may eventually need to perform the calibrations on a run-by-run basis, but it may be too early to worry about doing that now.  

We don't measure the flight time.  We measure when the charged particle hits the TOF paddles, and can derive an event start time from the time of the hits in the start counter.  There is a very real possibility that a TOF mass corresponding to an electron is actually a pion, it depends on the momentum of the particle.  Our timing resolution with the TOF is often not good enough to separate electrons from pions above a few hundred MeV, that's why we have Cherenkov counters for electron runs (are they off for g14?).  Look at track beta vs momentum to see how good the separation is.  

For PID, all particles that fall within your 3-sigma pion PID cuts are either electrons, muons, or pions (or kaons at high momentum).  Just assume they're all pions.  Events with electrons and kaons will probably just be background underneath your signal peaks, and the muons are probably from pion decay, so there's no harm in keeping them because they're likely signal events anyway.  

 - Paul

On Mar 24, 2012, at 10:22 AM, Dao Ho wrote:

> Dear all,
> 
> Previously  I did some quick analysis of the K0 Lambda channel for g14. It
> turn out that I had a very low statistics result, so I naively assumed
> something was wrong with my particle selection conditions. Now, I check to
> see how much statistics I have rejected with each of my selection
> conditions. So far, the most important player in my selection conditions
> is the information about the initial photon (from the beam).
> 
> I choose only events with None Zero photons in the RF bucket.
> I choose only events with the tagger ID (tagrid) being the same for all
> charges in those events. Consequently, I reject events with zero tagrID
> (found no photon matching).
> 
> 
> To my surprise, with "good" non-helicity events (at least 1 charged track
> events) about half of the time I see ZERO photon in the RF bucket. In
> other words, my selection removes 50% statistics because the events are
> recorded with zero photon in the RF bucket. So my question is how come
> this number too high? Can we do anything about it?
> 
> My second finding is about events with zero tagrID (found no matching
> photons). This condition also removes about 10% of the "good" non-helicity
> events. Any comment?
> 
> 
> 
> Here I would like to ask about the PID when cooking. My understanding is
> that we measure the TOF (flight time), the fight distance, and the
> curvature of a charge track. We use these to determine the beta and the
> momentum of the particle. From these quantities we can derive the mass of
> the particle (the so-call TOF mass). Will there be any chance that a
> particle with a TOF mass of electron (<5MeV) is really the pion?
> My concern is if our measure TOF is smaller than the real TOF by some
> nanoseconds, we would get a bigger beta, and a smaller mass. Is my concern
> a legitimate concern?
> 
> This question is because in my selection scheme of +,+, -,- tracks. Half
> of the plus tracks has the mass of the positron, and the same thing
> happens with the negative tracks. Naturally, I suspect that there are
> pairs of e+/- created by photon, so I look for events with both e+/-. The
> number of events (4 tracks, 2+, 2-) with the electron/positron pair is
> about 10%. As a result, I find out there are a lot of events with only ONE
> positron/electron track.
> By the way, I consider the charged particle is electron if it TOF mass is
> less than 5MeV.
> 
> Please let me know if you have suggestions or comments.
> Sincerely,
> Dao Ho
> 
> 
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