sPHENIX tracking discussion

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

Dear Ed,

 

I see you have concern that the DC I proposed have difficulty to reconstruct tracks since it need to handle order of magnitude higher particle density than PHEINX DC and the occupany is high.

 

First, I have a mistaken in my slides a the general meeting. The maximum drift length of the proposed DC is the same as that of PHEIX DC, i.e. 2 cm. This still makes the number of track per anode reduced by a factor of two.

 

In any case, the main factor to determine reconstruction of tracks is not the average number of hit per anode, but the two particle separation. Here you calculate that the effective occupancy will be 20% assuming that dN/deta=750 and two track separation of 1.5mm. I assume that two track separation of 1mm can be achievable. Also, using the measured dN/deta = 620 (PPG019), I calculate the effective occupancy to be 10%. I agree that to achieve two track separation of 1 mm is not a trivial task. But since PHENIX DC can achieve two track separation of 1.5mm (a NIM paper of PHENIX DC wrote that the separation is 1.3mm at 50% efficiency) with TMC chip, I think 1mm separation with FADC readout is achievable. This is certainly an important R and D item.

 

One thing I want to point out is that the proposed DC is not intended to work as a stand-alone tracking device. The track reconstruction will be done by the entire tracking system consisting of “Pixel”, 4 layers of Si strips, EMCal and DC. A track is first reconstructed by EMC-Pixel-beam center. These three “pixel” points (3D points) define a circle for a track road. Then the track is confirmed by 4 layers of strips in R-phi plane. After that, DC hits are used to confirm the track again and to measure the position of the track near the EMC. With the precise position determined by the DC near EMCAL, and the precise direction vector information determined by “pixel”-strips, we can determine the momentum of the track at high precision. Gaku’s simulation has already demonstrated that this combination will have upsilon mass resolution better than 100 MeV with full sPHENIX GEANT4 simulation.

 

In this scheme, DC is only used

(1) to confirm a track that is already reconstructed, and

(2) to refine its momentum. It is not expected that DC can reconstruct tracks as a stand-alone tracker.

In fact, DC alone cannot reconstruct a track since DC doesn’t have any UV wire. So it cannot determine the z-coordinate of the track. But the z-coordinate of the track is provided by EMCAL, as a PAD chamber provide the z-coordinate in the PHENIX DC.

 

I agree with you that occupancy of 10% is high for a stand-alone tracker. However, I think the 10% occupancy is fine for a DC whose function is solely to confirm a track that is already reconstructed. If occupancy is 10%, some of the 6 DC hits will be masked by another background track. But some others are isolated. As long as there are sufficient number of those isolated hits remain, the DC can deliver its intended functionality. The probability of having 4 “isolated” hit out of 6 wires under 10% occupancy is more than 98%. We can then use only those 4 clean hits to confirm the track and to measure its position. This is why I think the proposed DC will work in central AuAu collisions.

 

If the occupancy is higher, say, 20%, this probably of having 4 isolated hits is reduced to 90% .But the probability of 3 “isolated hit” is more than 98%. So we can use these 3 isolated hits. We can also increase the number of wires. We can, say, increase the number of anode layers to 10 instead of 6. In this case, the probability to have more than 5 isolated hit out of 10 is more than 99% even if the occupancy is 20%. The number of read-out in this case is only about 5K channels so the cost of the DC is still modest.

 

We will make a realistic simulation with full HIJING event to evaluate the effect two track separation. This will answer the question how many layers we needs as a function of track separation. We can also evaluate how many strip layers are needed.

 

Please note that the proposed system is a MINIMUM system to make the cost as low as possible. If simulation shows that the efficiency or fake rate is not satisfactory, we can always increase the number or layers of strips and / or DC with a modest cost.

 

The present sPHENIX tracker program is not designed to work well for a projective detector like DC. We need a reconstruction program that is designed to work with the proposed tracking system. Gaku is working on it. Once the reconstruction code is working, we can quantitatively evaluate how many layers we really need with full GEANT4 simulation of HIJING events.

 

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 EdwardOBrien
Sent: Friday, July 1, 2016 8:13 PM
To: sphenix-tracking-l AT lists.bnl.gov; sphenix-tpc-l AT lists.bnl.gov; John Haggerty <haggerty AT bnl.gov>; mills, James A <mills AT bnl.gov>; dlynch AT bnl.gov; Sourikova, Irina V <irina AT bnl.gov>; Ernst, Robert E <ernst AT bnl.gov>; eobrien AT bnl.gov
Subject: [Sphenix-tracking-l] Comparison of PHENIX DC and proposed sPHENIX DC

 

    Dear all,
     I have compared the design and performance parameters
    of the PHENIX Drift Chamber and the proposed sPHENIX Drift Chamber
    as described at the sPHENIX general meeting on June 24.
    Tom Hemmick and I have gone over these number and agree
    with the conclusion of this summary document.
    I'd be happy to answer any questions. Thanks.

    Ed