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["Fisyak, Yuri V" <fisyak AT bnl.gov>]

Hi Cameron,

1. Yes, relative errors on dE/dx  have stored on both MuDst and PicoDst. StPicoTrack:: dEdxError()

In TFG version (TFG23b)  using this error you can recalculate nSigma  via StPicoTrack::dEdxPull  with modified StdEdxModel in  StdEdxPull::Eval

…

#if defined (__TFG__VERSION__)

//_________________

Float_t StPicoTrack::dEdxPull(Float_t mass, UChar_t fit, Int_t charge) const {

  Float_t z = -999.;

  Float_t momentum  = gMom().Mag();

  Float_t betagamma = momentum * TMath::Abs(charge) / mass;

  Float_t dedx_measured, dedx_resolution = -1;

  if (! fit) { // I70

    dedx_measured = 1e-6*dEdx();

    dedx_resolution = dEdxError();

  }

  else if ( fit == 1) {     // Ifit

    dedx_measured = 1e-6*dEdx();

    dedx_resolution = dEdxError();

  }

  else {     // dNdx

    dedx_measured = dNdx();

    dedx_resolution = dNdxError();

  }

  if (dedx_resolution <= 0) return z;

  z = StdEdxPull::Eval(dedx_measured,dedx_resolution,betagamma,fit,charge);

  return z;

}

…
       

2. The question is for what particle you are trying to estimate the systematical errors of PiD selection and what do you call as systematical error.

1. For dE/dx calibation we are using MiP i.e. for pions it should work. For proton and heavier particle we have systematics in dE/dx model especially for low pT
2. There are two types of systematic errors when you use PiD cuts:

                                                              i.      You loose selected particle (P) with |nSigma_P| > cut value, e assume that nSigma_P has gaussian distribution with μ = 0 and σ = 1. This assumption is not always true. We have some systematics in dE/dx measurement which should be accounted.

                                                            ii.      You have admixute from unselected particles (U)  which depends on total number of U particle in your smample, and the particle dE/dx position_U and sigma_U. nsigma_P distribution for U particle is not gaussian    

 

                                                      Yuri Fisyak

STAR                                           Phone: +1 631 344 3913
Brookhaven National Laboratory  Fax:    +1 631 344 4206
510A/1-161
http://www.star.bnl.gov/~fisyak       E-mail: fisyak AT bnl.gov

 

 

 

From: Cameron Racz <cracz001 AT ucr.edu>
Date: Friday, March 17, 2023 at 11:46 AM
To: "Fisyak, Yuri V" <fisyak AT bnl.gov>
Cc: cebra <cebra AT physics.ucdavis.edu>, videbaek via Star-tpc-l <star-tpc-l AT lists.bnl.gov>
Subject: Re: [Star-tpc-l] TPC dE/dx resolution in FXT

 

Hi Yuri,

 

Thank you this is very helpful! I think I understand that this factor of sigma used to calculate ’nsigma’ is not a constant and varies for each track. 

 

I do see that the dE/dx uncertainties are in the MuDsts from the conclusions slide, are they in the PicoDsts right now?

 

What I’m essentially trying to do is use what we know about the dE/dx resolution to figure out what the ‘resolution’ would be for the nsigma values in my flow analysis so that I can calculate accurate systematic uncertainties coming from the dE/dx measurements. For example if I know that dE/dx is good within +- 7% then I could make variations on nsigma of +- X% to calculate the systematic uncertainties. If this is something that someone knows that would be great for me to find out.

 

Thanks!

 

Cameron Racz

Ph.D. Candidate 

Heavy-ion Physics Group

University of California, Riverside

 

 

 

 

On Mar 17, 2023, at 9:49 AM, Fisyak, Yuri V <fisyak AT bnl.gov> wrote:

 

Hi Cameron, please have a look on slides 3-4 of

https://drupal.star.bnl.gov/STAR/system/files/nSigma.STAR_.Lehigh.072018.v4.pdf

with explanation what does mean sigma of dE/dx

                                                              Yuri Fisyak

STAR                                           Phone: +1 631 344 3913
Brookhaven National Laboratory  Fax:    +1 631 344 4206
510A/1-161
http://www.star.bnl.gov/~fisyak       E-mail: fisyak AT bnl.gov

 

 

 

From: Star-tpc-l <star-tpc-l-bounces AT lists.bnl.gov> on behalf of Cameron Racz via Star-tpc-l <star-tpc-l AT lists.bnl.gov>
Reply-To: Cameron Racz <cracz001 AT ucr.edu>
Date: Friday, March 17, 2023 at 10:14 AM
To: cebra <cebra AT physics.ucdavis.edu>
Cc: videbaek via Star-tpc-l <star-tpc-l AT lists.bnl.gov>
Subject: Re: [Star-tpc-l] TPC dE/dx resolution in FXT

 

Hi Daniel,

 

Thank you for the info! That makes sense and does help. If you do find that study though that would also be great to see. 

 

While I’m here I did have one other question about the nsigma value we get from the TPC. When I found the definition here (https://arxiv.org/abs/nucl-ex/0505026 section 3.1) it is of course the logarithm of the dE/dx measurement divided by the expected value, but it also showed a factor of 1/sigma_X out in front where sigma_X “is the dE/dx resolution of the TPC”. I just wanted to verify that this factor is the same resolution we’re talking about here (8%, 7%, etc) because I can’t find any details on this factor aside from this statement. So, for example, if the resolution is 8% then sigma_X = 0.92?

 

Thanks,

 

Cameron Racz

Ph.D. Candidate 

Heavy-ion Physics Group

University of California, Riverside

 

 

 

 

On Mar 16, 2023, at 11:42 PM, Daniel Cebra <cebra AT physics.ucdavis.edu> wrote:

 

Hi Cameron,

   The dE/dx resolution in FXT mode is actually better than in collider mode. Empirically dE/dx resolution goes as L^-0.37, where L is the tracking length. Since tracks pass through the TPC at long angles, there is a longer tracking length then in collider mode. 

  For midrapidity tracks for the 3.0 GeV system, there is 200 cm of track length, whereas in collider mode there is 140 cm for track length. Using the empirical track length dependence, we expect that the dE/dx resolution in FXT mode should be 78% of that in collider mode (i.e. ~7%).  Note that the dE/dx resolution is eta (or rapidity dependent - for target rapidity tracks, the resolution would again be 8%; for rapidity 1.5 (which hits the top far corner the dE/dx resolution should be ~6%).

 

  OK... that is the theoretical improvement in dE/dx resolution in FXT mode.  I will need to do some digging to verify that experimentally we see that expected improvement in resolution. I know that I looked into this a while ago, and my recollection was that we did see the expected improvement in dE/dx resolution, but I will have to find that study to confirm these expectations.

 

  For reference, the paper to quote for empirical dE/dx resolution in tracking detectors is:

Allison and Cobb, Ann. Rev. Nucl. Part. Sci. 30, 253 (1980)

 

Best, Daniel

 

On Thu, Mar 16, 2023 at 8:26 PM Van Buren, Gene via Star-tpc-l <star-tpc-l AT lists.bnl.gov> wrote:

Forwarding Cameron's query to the TPC list (not my area of expertise).

 

-Gene

Begin forwarded message:

 

Hi Gene, 

 

I have a quick question about the TPC and I thought you might be the one who would know. I’ve been looking into the dE/dx resolution of the TPC in order to try to estimate systematic uncertainties related to nsigma. I’ve found that it was established that the TPC dE/dx resolution is ~8% for pions, kaons, and protons, but is there any information on what the resolution would be in fixed target mode? Would it be safe to assume 8% in fixed target mode as well? I’m working on 2018 sqrt(s_NN) = 3.0 GeV specifically if that matters.

 

Thank you!

 

Cameron Racz

Ph.D. Candidate 

Heavy-ion Physics Group

University of California, Riverside

 

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