[Halld-offline] problem with spread of values for particle gun

[Kei Moriya]

Dear offliners,

I wanted to let you know that there is a little bug
when generating particles with a spread in
momentum and angle with the particle gun.

When generating events with the particle gun,
I specify the central value of p, theta, and phi,
along with the full width of each value.
What I have noticed is that if I specify for
example
c   particle  momentum  theta  phi  delta_momentum delta_theta delta_phi
KINE   114      5.0       30.   0.      0.5              30        360.

which means generate protons(offset=100+PID=14) with
p = 5.0 +/- 5
theta = 30 +/- 15
phi = 0 +/- 180.

What I have noticed is that the decimal point for
the momentum values are essential, since without
the decimal point, the spread becomes 0!
I assume this is a simple bug of taking the value
as an int instead of a float.

The plot attachd show the difference when
a "." is added to delta_momentum so that the
top plot (with decimal point) has a full width
of 5 (2.5<p<7.5), while the bottom (without decimal
point) has no width.

I assume the place to correct this is within
gukine.F in sim-recon/sim-recon/src/programs/Simulations/HDGeant/
but I'll let the experts fix this so I don't
break anything.

One thing that comes as a consequence of this is that
the plots I showed last week had theta exactly
at 0.25,0.75,1.25, ... degrees, and NO spread.
I'm not sure how many other people are using the
particle gun, but you may want to check that you
are actually getting a spread in your values.

	Kei

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