[Halld-offline] questions on kinematic variables

[Paul Mattione]

You should be looking at the "time" variable instead of "t0" or "t1".  

t0 and t1 are used to keep the times of the track as it reaches various detectors.  For example, t0 is often the time of the track at the start counter, and t1 at the TOF or BCAL.  These are then used for selecting the RF bunch and performing particle ID.  

The "time" variable is the time of the track at "position."  You should look at this instead.  

For the negative parent IDs, I don't really know what's going on ... I'm not sure, but I think if the particle was produced during interactions in the BCAL and then re-entered the detector it could have a negative PID, but I doubt that explains all of it (e.g. the lambdas).  

 - Paul

On Jan 15, 2014, at 3:13 PM, Kei Moriya wrote:

> 
> 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
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