[Halld-offline] questions on kinematic variables

[Kei Moriya]

Dear offliners,

I had a few questions regarding some of the kinematic variables
that are stored for events generated using genr8. The example
reaction I use is
gamma + p -> K+ K+ Xi-(1820),
Xi-(1820) -> Lambda K-
so that the primary particles are K+, K+, K-, Lambda.
The Lambda is decayed within hdgeant.

1. parentid=-1
For the primary generated particles it seems that they are always 0
and for secondary particles they are usually positive numbers.
However, as you can see in
http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/parentid.pdf
where I have plotted the parentid's by particle types,
protons, pi-'s, neutrons sometimes have negative parentid's.
Does anybody know which part of which program sets these up,
and why they are sometimes negative?

2. parentid for Lambda
If you look at the top right panel in the same plot, you will
see that there are 5264 Lambda's with parentid -1 (out of 300k events).
If I look at the number of Lambda's generated for each event, there
are 5252 events with 2 Lambda's, and 6 events with 3 Lambda's.
Does anybody know why multiple Lambda's will be generated and what
the generation mechanism is? The "mech" variable within DKinematicData
is always 0 for events generated through genr8 and it would
be nice if this were set up. I've noticed that this problem of
having multiple Lambda's show up in other channels where Lambda's
are created.

You can compare
- |p| vs theta
- vertex y vs x
- vertex rho (radius in xy-plane) vs z
for the primary and parentid=-1 Lambda's with the following
plots:

http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/genpvstheta_Lambda.pdf
http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/genpvstheta_negativeparentid_Lambda.pdf

http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/genvy_vx_Lambda.pdf
http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/genvy_vx_negativeparentid_Lambda.pdf

http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/genvrho_vz_Lambda.pdf
http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/genvrho_vz_negativeparentid_Lambda.pdf

It does seem that the momentum of the parentid=-1 Lambda's
have less momentum than the primary ones, and many are created
in dense material regions of the detector outside of the target.
The primary vertices have been generated over a cyllindrical
region of rho<3 cm, 50<z<80 cm.

3. t1
Does anybody know how the variable t1 should be set? For events
generated within genr8, t1 always seems to be 0.

4. t0
For the variable t0, primary particles are always 0, and
for non-primaries they seem to be recorded in seconds,
not ns. Does anybody know where this is being set?

The following is for the pi- from Lambda decay:
http://dustbunny.physics.indiana.edu/~kmoriya/2014-01-15-Xi1820/t0_pim.pdf
The values have been multiplied by 10^9, and you can see that
the values are on the order of a few ns, and coincides
with the Lambda flight time. The gray histogram is for all
pi-'s, and the blue is for pi-'s decaying from a Lambda.

---

If there are any additional plots you would like to see
or further studies, please let me know.

Thanks,
	Kei