[Halld-cal] pi0 reconstruction update

[Shepherd, Matthew]

Hi all,

I was thinking about the resolution dependence
on energy and whether an effect should be visible
in going from 1 GeV to 2 GeV...  This reminded me
of an important issue we learned when studying
beam test data.  (I'm sure many of you with calorimetry
experience already know this.)

I suspect the dominant mechanism for improving
delta(E) / E as a function of energy is not increased
photo-statistics but instead a reduction in fluctuations
of energy lost due to sparsification.  It is also a statistical
effect, but of a little different nature.

The readout threshold is set by pulse height and
corresponds to around 50 MeV per block.  
So if a 500 MeV photon has a few blocks nearby 
with close to but less than 50 MeV of energy, it is easy 
to see fluctuations in the energy the level of tens of 
percent.  

[This is also another reason for gain balancing using
hardware HV settings and not software.  Low gain
channels have high energy thresholds and data for
them is simply lost if their HV is not turned up.]

Matt

---------------------------------------------------------------------
Matthew Shepherd, Associate Professor
Department of Physics, Indiana University, Swain West 265
727 East Third Street, Bloomington, IN 47405

Office Phone:  +1 812 856 5808

> On Nov 25, 2014, at 9:39 PM, Shepherd, Matthew <mashephe at indiana.edu> wrote:
> 
> 
> Hi all,
> 
> Attached is an updated version of the slides
> from today's meeting.  I'm increasingly convinced
> that the peak I showed is indeed pie and not turkey.
> I might have two flavors of pie also:  one with a little
> bit of strangeness stirred in.
> 
> The definitive proof will be in a plot of E_cluster / p_track.
> There are some scale factors that have to be adjusted 
> depending on where in the analysis chain one gets the 
> E_cluster value.  (I guess it is a semi-blind test for the 
> person who is producing the plot.)
> 
> I also expect E/p for electrons to have some mild momentum 
> dependence.  As p increases the ratio should also 
> increase because of the fact that the DAQ sparsification 
> thresholds are fractionally less significant for larger 
> energies.  (I think I see this same effect as a slight 
> dependence of the pi0 mass on the photon energy cut.)  
> The effect should be reduced with increased PMT HV 
> or lower threshold.
> 
> Matt
> 
> 
> 
> 
> 
> ---------------------------------------------------------------------
> Matthew Shepherd, Associate Professor
> Department of Physics, Indiana University, Swain West 265
> 727 East Third Street, Bloomington, IN 47405
> 
> Office Phone:  +1 812 856 5808
> 
> <update on turkey or pie.pdf>_______________________________________________
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