[Eg6_analysis] Meeting tomorrow

[Hovanes Egiyan]

Hi Nathan,

During the last eg6 meeting we briefly discussed what
number to use to indicate our confidence in a certain ID
of the particle based on the dE/dx measurement in RTPC. I think I said
that we could use some probabilities that are defined for each
individual particle type. After giving some more thought on that
I think it should be done differently. It is more complicated than I 
thought,
but it needs to be to be of good use. To summarize in the beginning, I 
think
these number can be useful if someone wants to do something really quick
without developing his own RTPC PID scheme. For detail analysis
people would need to refine RTPC PID.

If we want to specify a probability that a given track was created
by a certain particle based on some measured quantity, for instance
dE/dX, not only we need to know the probability density function for 
that quantity given
the particle ID, but also how often that particular particle is created 
in the
scattering process that we are looking for. That frequency, strictly 
speaking ,
depends on the event topology (and kinematics), since for certain 
selection of particles detected
in CLAS some of the types of the recoil particles will be very unlikely.
For instance, if one measures +2 total charge in CLAS the probability of
having a He4 in RTPC with respect to the probability of having a proton 
(or a deuteron)
becomes smaller compared to the case when the total charge seen in CLAS 
is -1, because
the total charge in CLAS & RTPC & undetected particles needs to be the 
same as
the charge of the original target and electron.  And then if one uses 
some cuts,
like missing mass cuts to select his reaction then these prior 
probabilities can be
completely different.

Therefore, determining the correct probability for particle ID for  
every reaction
by just  one number will be impossible. One can do it for the total sample
of the tracks in the RTPC by somehow figuring out the prior 
probabilities for
the different particle ID regardless of what was detected in CLAS.
One also could pick a particular "important" event topology in CLAS to
define these numbers, for instance four photon topology, or electron and 
a photon.
In the first approximation these would be  reasonable numbers, but then 
every
reaction would need to improve them by "recalibrating" the prior 
probabilities for
different PID for RTPC tracks and reassigning the final PID
probabilities.  One would need to have the dE/dX information to redo
the probability assignment, although the value for the probability
distribution functions (f_{id}(dE/dx)) for the ionization losses
for each PID hypothesis would suffice.  Then the calculation of the
probability that the track was created by a particle with "id" should be:

P(id) = [ a_{id} \times f_{id}( dE/dX ) ]  /  \sum_{ids} ( a_{ids} 
\times f_{ids}( dE/dX ) )

where in the denominator we sum over all IDs we are considering, and 
a_{id} is
the prior probability for sending a particle with such an particle ID 
into the RTPC. These
a_{id}-s can also be kinematics-dependent, but I think this is something 
for each
analysis to decide.

Of course, the person doing his own detail data analysis might argue 
that he'd rather
just look at the dE/dX distribution vs P and use some curves to pick out 
his He4 tracks,
and in  many cases he might be right...

Let me know what you think.

Best,
     Hovanes.


On 08/15/2012 01:34 PM, Stepan Stepanyan wrote:
> Dear all,
>
> We will have eg6 meeting tomorrow, Thursday, August 16 at 10 am in F227.
>
> Preliminary agenda is at
> http://clasweb.jlab.org/rungroups/lowq/wiki/index.php/Meeting_on_August_16_at_10:00am
>
> Calling instruction can be found at -
> http://clasweb.jlab.org/rungroups/lowq/wiki/index.php/Agendas_and_Minutes_of_the_meetings#Agendas
>
>
> Regards, Stepan
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