TPC Hit Errors (2008)

NOTE: this was for 2008

 These are the TPC hit errors as parameterized by Victor Perevoztchikov in order to normalize pulls (and thus chi squares) of ITTF tracks versus z, dip angle, and crossing angles:

error_xy = sqrt([0]+[1]*((200.-y)/100.)/(cos(x)*cos(x))+[2]*tan(x)*tan(x))
error_z  = sqrt([0]+[1]*((200.-y)/100.)*(1.+tan(x)*tan(x))+[2]*tan(x)*tan(x))

where y in the equations is the z coordinate in the TPC, and x in the equations is the crossing angle for error_xy, and the dip angle for error_z. The parameters are:

Inner TPC:

error_xy->SetParameters(0.0004,0.0011513,0.01763);
error_z ->SetParameters(0.00093415,0.0051781,0.014985);

Outer TPC:

error_xy->SetParameters(0.0012036,0.0011156,0.063435);
error_z ->SetParameters(0.0026171,0.0045678,0.052361);

Plotting these as a function of z and crossing/dip angles gives:

Inner TPC:

Outer TPC:

EPS versions of these images are attached.

 

TPC Hit Errors (Run 9)

 As was TPC Hit Errors (2008), hit errors were found for Run 9 by Victor Perevoztchikov.

I created a macro which can be used by anyone to generate these plots, obtainable by CVS checkout from offline/users/genevb/hitErrors.C. Attached to this page are text files used as input for pp500 and pp200 (Victor ran on data with timestamps of 20090326.082803 and 20090425.093105 respectively), and eps versions of these plots.

 

pp500:

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pp200:

 

-Gene Van Buren

TPC Pt and DCA resolution

[Update on 2010-06-21: global track momentum resolution has now also been studied using Cosmic rays study, version 2, with some as yet unexplained good performance at very high p_T.]

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Jim Thomas has written code to model transverse momentum (p_T) and pointing resolution near the primary vertex (DCA) of tracks using various detectors. The code has been tuned to match TPC performance under low luminosity conditions, but assumes (for simplicity) TPC hit errors of 0.06 cm in rφ and 0.15 cm in z (which can be compared with the TPC Hit Errors (2008)), throwing tracks at η = 0.5 and including hits in "nearly all" 45 padrows (padrows 1 and 13 are dropped).

Shown below are the pt resolutions for various options:

• Black filled triangles: embedded antiprotons at half field
• Black open triangles: embedded pions at half field
• Cyan: model prediction for global (anti)proton tracks at half field
• Green: model prediction for global pion tracks at half field
• Blue: model prediction for global pion tracks at full field
• Red: model prediction for primary pion tracks at full field (primary vertex known to 1 mm)
• Magenta: model prediction for primary pion tracks at full field (primary vertex known to 0.3 mm)

 

Note that the embedding data was shown in Figure 10 of the TPC NIM paper [1]. At low p_T, the resolution is dominated by multiple coulomb scattering (MCS) effects. At high p_T, the momentum resolution approaches a C * p_T² dependence (note that C can be considered the inverse transverse momentum resolution, as δ(1/p_T) = δ(p_T) / p_T² for small δ(p_T)). For the above curves, C (in units of inverse momentum [c/GeV]) is approximately:

• Cyan and Green: 0.018
• Blue: 0.009
• Red: 0.005
• Magenta: 0.003

Here is the same model run with only using every other padrow of the TPC:

 For the above "less hits" curves, C is approximately 50% higher in all cases:

• Cyan and Green: 0.027
• Blue: 0.013
• Red: 0.008
• Magenta: 0.0045

Additional studies of the momentum resolution come from Yuri Fisyak for Monte Carlo simulations (similar to a thorough embedding study done using TPT in 2002 by Jen Klay) in the following plots for globals and primaries in AuAu200 and pp500 [with pileup] collisions. The comparison is best made with the blue [AuAu globals] and magenta [AuAu primaries] lines above. The data matches reasonably well with the "less hits" model curves, with the exception of an additional offset of about 0.5% for global tracks. The pileup in the pp500 simulation probably causes its results for primaries to have even further degraded resolutions than the red [pp primaries] lines above.

 

AuAu200 [Run 10 FF setup]:

 

pp500 [Run 9 RFF setup, with pileup]:

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Shown below are the DCA resolutions from the same model described earlier, using tracks with "nearly all" hits, for 

• Cyan: model prediction for global (anti)proton tracks at half field
• Green: model prediction for global pion tracks at half field
• Blue: model prediction for global pion tracks at full field

 

[note: I do not know what the dashed lines are]

As can be seen, the field makes only a small impact on the DCA resolution. When using "good" (high quality) tracks for calibrations, I regularly find a mean DCA resolution for global tracks in full field of just under 3 mm.

EPS versions of these images are attached.

 

References:

1. M. Anderson et al., Nucl. Instr. and Meth. A499 (2003) 659-678