sPHENIX tracking discussion

["Yasuyuki Akiba" <akiba AT rcf.rhic.bnl.gov>]

Dear Tony

 

> He presented some best case (i.e. overly optimistic) simulations

> of charge detected in a 2 mm drift region in the DC showing that

> about 1/3 of the time the high momentum track is missed.
This is completely incorrect.

 

What Tom showed is that when a 200 MeV track, which makes 6mm long electron cloud in the 2mm sampling region due to the large inclination angle, overlap with a high pT track, the signal peak from the high pT track is lost at 30% probability in his definition of “lost”. His definition of “lost” is that the pulse height of the 200 MeV background hit exceeds that of the high pT track.

 

I should point out that number of 200 MeV track is small. And the long (6mm) cloud is due to the fact that 200 MeV track is almost at the limit of PT acceptance. The track with pT<180MeV/c doesn’t reach to the DC (R>80cm). So what Tom has done is to take an extreme case that a low pT track with a long background tail coverlap with high pT track. Even this rare case, he showed that 70% of the time, one can find the signal peak of the high pT track. So basically what he showed confirmed what I have been saying. Pulse height information helps two track separation.

 

I should also point out that his criterial of “lost” is not a right one. For my purose, “found” should be that I can se leading edge produced by the high pT track. In this definition, the “lost” probability should becomes smaller value.

 

 

Tom didn’t consider the probability of such overlap happens. The average <PT> of charged particles in Au+Au collisions is >0.5 GeV/c. The inclination angle of a 0.5 GeV/c track is about 0.4 radian, and the length of electron cloud produces by such a track is about 1mm. The average distance of the track in the DC is about 1cm. So the occupancy is 10% if the two track separation is 1mm. It is 15% of the two track separation is 1.5mm, which is the official number of PHENIX DC. It is written in a NIM paper that PHENIX DC can separate two track at 1.3mm at 50% probability. Ed O’Brien calculated the occupancy to be 20% with dN/dy=750 and 1.5mm separation in his notes, and Tom agreed with this 20% number. So even if I use Ed’s number 80% of the time the hit of the track of interest is clean from overlap.

 

As I wrote in one of my previous message, and also in my presentation, what I need is to have a few such clean hits free from overlap with background track. In the prosed DC, there are 6 anode layers. If the occupancy is 10%, 98% of track of interest has at least 4 such clean hits. Even if the occupancy is 20%, 98% of track has at least 3 clean hits. These 4 or 3 hits are all I need to confirm the track and to measure its position.

 

I can also increase the number of layers. The total number of read-out channel of the proposed DC is only 3K channel (3K of 8bits FADC). I can easily double the layer with a very modest cost increase. This makes the number of layer to be 12. I think this will give a very robust tracking even if the occupancy is 20% or even at 30%. If we have 12 layers, the probability that a track as at least 7 clean hits is 98% for 20% occupancy. Even with 30% occupancy a track should have 5 clean hits at 99% probability.

 

Sincerely yours,

              Y. Akiba

From: sphenix-tracking-l-bounces AT lists.bnl.gov [mailto:sphenix-tracking-l-bounces AT lists.bnl.gov] On Behalf Of Frawley, Anthony
Sent: Friday, July 15, 2016 8:30 PM
To: sphenix-tracking-l AT lists.bnl.gov
Subject: [Sphenix-tracking-l] Minutes of July 15, 2016 tracking meeting

 

Hi All,

Here are my notes from this morning's sPHENIX tracking meeting.

Cheers
Tony

Krista Smith - Effect of intermediate tracker mass on resolution
------------------------------------------------------------------------------------
Krista showed results of simulations with 3 maps layers + an intermediate tracker layer + TPC.
The intermediate tracker thickness and radius were varied to see the effect on Upsilon mass resolution.

Yasuyuki asked if the simulation included included the internal radiation tail. No it did not. Yasuyuki said he would ask Gaku to provide the code for adding this to the vector meson generator.

Mike McCumber - MAPS and software updates
----------------------------------------------------------------
Mike said the LANL proponents will get together next week to map out the path ahead now that the LDRD has been funded.
He went over the software projects that need to be completed to improve the realism and performance of the TPC tracking code.

After the TPC work, Mike will return to his effort to implement event pileup in the simulations. He estimated 2 weeks for completion of the pileup modifications. Carlos and Mike are talking about how to collaborate on this.

Tom Hemmick - My last words on the DC idea
-------------------------------------------------------------
Tom responded to the suggestion that very high occupancy in the proposed DC outer tracker does not matter because high momentum tracks can be separated by waveform analysis. He presented some best case (i.e. overly optimistic) simulations of charge detected in a 2 mm drift region in the DC showing that about 1/3 of the time the high momentum track is missed.

Tom considers that the DC is not viable as an outer tracker for sPHENIX, and he advocated pursuing an interpolating pad chamber as an alternative to the TPC.

Yasuyuki Akiba - EMC-Si Tracker - DC
-------------------------------------------------
Yasuyuki talked qualitatively about the idea of using an inner tracker, the EMC and a DC outer tracker as the sPHENIX tracker. In this proposal the DC is not considered to be a standalone tracker - its purpose is to confirm tracks found by the inner tracker and the EMC, and improve the momentum resolution.

slide 10: Tom commented that this is not the electric field configuration we use - it cannot hold the gain. When built, this voltage configuration was found to be untenable, and we had to find a configuration that worked.

Working on the tools to do a full simulation. Will need a new tracking code, which Gaku is working on.

Gunther commented that it is only 7 weeks until the tracking review. A couple of people commented that it would be a serious mistake to go into the tracking review with any tracker proposal that is not fully understood before the review. Ultimately, the collaboration will have to decide what is presented at the review.