sPHENIX tracking discussion

["Lajoie, John G [PHYSA]" <lajoie AT iastate.edu>]

Hi Dennis, Jin, Spencer:

 

You guys are awesome – I send out a question in one airport and I have answers when I land in another!

 

The specific question I am trying to respond to concerns the energy-energy correlations plots we made and showed at DIS. Specifically, can you really get down to 10^-2 in separation between particles in sPHENIX?  I think the answer is “certainly”, and in principle we can get down to a few times 10^-3, but the acceptance quickly becomes a complicated function of relative momentum and position in the detector.

 

Of course, all of the action in these correlations is at larger angles (QCD angular ordering), at the smallest angles you just reconstruct a trend that represents free-streaming hadrons.

 

Once we get our new postdocs on-board, I think studying this in detail will be a very good first-project.

 

Thanks again for the quick responses!

 

John

 

From: Perepelitsa, Dennis <dvp AT bnl.gov>
Sent: Saturday, July 30, 2022 5:56 PM
To: Klein, Spencer <SRKlein AT LBL.GOV>
Cc: Huang, Jin <jhuang AT bnl.gov>; Lajoie, John G [PHYSA] <lajoie AT iastate.edu>; sphenix-tracking-l AT lists.bnl.gov
Subject: Re: [Sphenix-tracking-l] check my numbers....

 

Hi John and all,

 

In case it is useful, I attach here a plot of the charged-particle multiplicity distribution in jets of different cone sizes. That may help give a sense of the “typical” inter-particle separation in jets. (The solid histograms are for charged particles, while the dashed ones are for all.)

 

We made this plot in response to a review question in 2019, which I believe was about an issue similar to yours - although now I cannot remember the exact details…

 

Dennis

 

Dennis V. Perepelitsa

Associate Professor, Physics Department

University of Colorado Boulder

on sabbatical at Brookhaven National Laboratory

 

On Jul 30, 2022, at 3:50 PM, Spencer Klein via sPHENIX-tracking-l <sphenix-tracking-l AT lists.bnl.gov> wrote:

 

Hi John:

Here is my 2 cents worth. 

This seems like a rather difficult thing to determine from a back-of-the-envelope calculation.  It is very rare to have two tracks with the same momentum (or even close enough that their radii-of-curvature are indistinguishable.   So, the chance of overlap is mostly independent in each layer, and, in each layer, the chance of overlap will depend on the local track density i. e. higher in jets.  This seems hard to work out without a Monte Carlo. 

Spencer

On 7/30/22 11:22, Huang, Jin via sPHENIX-tracking-l wrote:

Dear John and tracking team

 

Interesting question. Just to share few thoughts, and hope to be useful.

 

1. For azimuthal angular separation, it is 3mrad for the TPC outer pad row, which is worse but not too far from MVTX pixels. And TPC separate tracks in 3D (charge/p, eta, phi), as you also pointed out
2. We probably need 2-3 pixel/pad separation to cleanly separate two track’s clusters due to charge sharing among pixels/pads in both MVTX and TPC.
3. In the end, the limiting factor may be the tracking pattern recognition and fake rejection algorithms. And they may need a special tune for fine separations.

 

Therefore, a more defendable answer could come from a quick plot from the current MDC2 HIJING simulation, to plot track-pair reconstruction efficiency as function of the 3D separation in (charge/p, eta, phi) space, normalized by the product of two stand-alone single track reconstruction efficiency in the same centrality and momentum bin.

 

Cheers

 

Jin

 

 

______________________________

 

Jin HUANG

 

Physicist, Ph.D.

Brookhaven National Laboratory

Physics Department, Bldg 510 C

Upton, NY 11973-5000

 

Office: 631-344-5898

Cell:   757-604-9946

______________________________

 

From: sPHENIX-tracking-l <sphenix-tracking-l-bounces AT lists.bnl.gov> On Behalf Of Lajoie, John G [PHYSA] via sPHENIX-tracking-l
Sent: Saturday, July 30, 2022 1:00 PM
To: sphenix-tracking-l AT lists.bnl.gov
Subject: [Sphenix-tracking-l] check my numbers....

 

Dear sPHENIX Tracking folks: 

 

Just to make sure I’m not going off the rails here, check my numbers for me….

 

I’m interested in the *minimum* separation between two tracks that sPHENIX can resolve, as part of this effort to use N-particle correlations to look at jet substructure.  I can get a start on estimating this by looking at the parameters of the MVTX.  If I assume that the tracks are not allowed to share *any* MVTX hits, and the MVTX has a 28um pixel, then the best I can do is:  

 

~28um/24mm = 1.2 e-3

 

Assuming they have to be separated at the first MVTX layer. If they can share hits in the MVTX, but I require the last hit to be unique, I get

 

~28um/39mm = 0.7e-3

 

This is a very back-of-the-envelope estimate, and basically assumes to equal momenta particles.  If they are not equal mpomenta then they will separate based on momentum in both the MVTX, and certainly in the TPC, which would allow you to go to even smaller angles.  

 

Of course, this depends a bit on how the tracking is handled.   Are multiple TPC tracks allowed to be matched to MVTX tracks?

 

Sorry if these questions are somewhat naïve, just trying to think through this….

 

John

 

 

John Lajoie

he, him, his

Harmon-Ye Professor of Physics

Iowa State University

 

(515) 294-6952

lajoie AT iastate.edu

 

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