STAR HardProbes PWG

[Helen Caines <helen.caines AT yale.edu>]

Hi Gabe,

  Thanks for putting in writing what you were telling us last Thursday, having the chance to chew over the explanation has helped me understand your logic. I have a couple more questions below. You can feel free to wait until Thursday to answer them, but I wanted to write them down while I was thinking about it.

Helen

 Compare rate at which jets are found per event to the same rate in data
Q: Do you have any comparison plot?

 

This was Helen’s question in the meeting yesterday, and it is a good question. I plan on presenting again next week with a few more plots to explain our logic for the choice of the 3 track cut.

For now I will state our general thinking again:

Pseudo-embedding and Mixed Event methods aim to capture two different sources of correlated background. Pseudo-embedding looks for additional background signal that gets pulled along due to upward fluctuation around a real jet signal. Mixed Event looks for completely combinatorial objects that appear as jets on their own. While these are two distinct sources, there was originally some overlap in the resulting corrections. The 3 track minimum for combinatorial jets aims to eliminate this double-counting. The two cases this eliminates are 1 and 2 track jets.

1 track jets (tracks above 9 Gev/c) are real jets according to our definition. Thus, they must be excluded from a study aiming to identify fake jets.

This I now get, indeed 1 track with 9 GeV (you jet cut-off I assume) can be taken as having to have come from a hard scattering. Its a functional definition that I suspect most/all can agree with.  I think you also stated that there’s a high pT cut off for tracks you use in the mixed events. Do I remember correctly and what cut-ff is that?

2 track jets are accounted for in the pseudo-embedding correction. If a 1 track jet is found in p+p and embedded into Au+Au background, it has no correlation with any other track in the event, so if it forms a 2 track jet that reaches our 9GeV threshold, this is identical to a combinatorial 2 track fake jet. Thus we must exclude these from one of the corrections to avoid double-counting.

Doesn’t this  assertion depend on how low in pT you define a hard scattering i.e. what pT you inject PYHTIA events down to,  and what’s the lowest pT track you put into the mixed events? Do you inject PYTHIA “jets” down to a single track with pT  = 4.5 GeV? And mix event with only 4.5 GeV and below?

3 Track jets are where the corrections become distinct. If a 2 track jet is found in p+p and embedded into Au+Au background, it retains association between the two original tracks, forming a jet cone on its own. If it picks up a third track from Au+Au background, this is exactly what we are looking for with pseudo-embedding. It is distinct from combinatorial jets due to the presence of a “real” jet.

The same holds true for all >3 track jets. This is the regime where collections of low p_T tracks can form fake, fully combinatorial jets.

Agreed