# Presentation: 196444

## STAR TPC dE/dx resolution

- Yuri Fisyak
- STAR TPC  review, BNL,
- October 6-7, 2006

## Outline

- Requirements on dE/dx resolution
- Calibration
- Procedure
- Bichsel Model
- Parameters
- Resolution
- Problems anticipated with luminosity increase and detector aging

## Requirement on dE/dx resolution

**In STAR CDR **(p. 4C-33)** for relative resolution of dE/dx truncated mean (****Δ****)**** **** it was used Walenta, Allison, Cobb formula **
** ****Δ **** = ****σ****(dE/dx) / (dE/dx) =  0.47 N****-0.46**** (Ph)****-0.32****,**
**where N – no. of dE/dx samples, **
**P – pressure (= 1atm),**
**h sample length (cm).**
**For STAR TPC**
**Inner sector N = 12, h = 1.15 cm : ****Δ****Inner**** = 14.3%,**
**Outer sector N = 32, h = 1.95 cm : ****Δ****Outer**** =  7.7%,**
**Total : ****Δ**** = 1/√(1/ ****Δ****2****Inner**** + 1/****Δ****2****Outer**** )          = ****6.8%****                                      for 90° tracks with total track length in TPC 76.2 cm.**
**The simulation (H.Bichsel, see below) gives resolution 7.3%.**
**Requirement for dE/dx resolution achievable with our calibration procedure has been set as ****7.5% ****for tracks with length in TPC equal to 76.2 cm.**

## dE/dx Resolution and  ±1σ particle separation

- 2σ e-π
- separation
- up to
- p = 6.3 GeV/c

## Calibration procedure

**Calibration sample is **
**good clusters : used in track fit, with no overlaps ; **
**good global tracks  with ****Track length in TPC > 20[or 40] cm;**
**tracks with 0.4 < p < 0.5 GeV/c (~MIP for pions: ****βγ**** ****= p/m = 4), ****i.e. **
**calibration point is MIP for ****π****,**** and **
**βγ**** dependence**** is completely defined by ****Bichsel model****.**
**Create histograms of **
**Z = log {(dE/dx) measured / (dE/dx) predicted for ****π****},**
**where prediction is taken from ****Bichsel model.**
**Fit this histogram by 5 Gaussians with respect to position (****μ****) and ****σ**** of ****π****, with **** fixed relative positions of K- ****π****, P-****π****, e-****π****, d-****π****. The position ****(****μ****) ****of ****π**** ****defines the calibration correction**.
**Define calibration parameters which provide condition** **μ = 0**

- Sector 23
- row 8

- Sector 23
- row 38

## Bichsel model

**H.Bichsel model now has official reference:         NIM A 562 (2006) 154-197**
**This is a Photo Absorption Model, which accounts for atom excitation levels. In our case it is done for P10: 90% Ar + 10 %CH****4:**
**Bichsel prediction for dE/dx dependence on ****βγ**** is quite different from Bethe Bloch model used in GEANT3**
**Bichsel prediction for dE/dx has dependence on dx. **

## Calibration parameters

- Parameters are grouped in 3 categories:
**fixed or out of offline control**,
**calibrated offline, mainly depend on gas state**, **and**** **
**used to account specific conditions for the given run.**
**The list of parameters includes:**
**“****tpcGain” correction,**
**ADC/Clustering nonlinearity correction,**
**Cuts of sectors borders, and positions near TPC membrane and gating grid. ****Dependence of the Gain on distance from Chamber edges****,**
**Gas gain correction for sector/row****,**
**Correction for Electron Attachment due to O****2****,****,**
**Dependence of the Gain on Gas Density due to Pressure****,**
**Dependence of the Gain on Gas Density due to Temperature****,**
**Global track multiplicity dependence****,**
**Dependence of the Gain on total charge accumulated so far****,**
**Dependence of the Gain on space charge near the wire****,**
**dX correction****,**
**Dependence of the Gain on interception angle****,**
**Variation truncated mean for track dE/dx and its relative error  versus its length in TPC .**

## tpcGain

- (“Misleading”) name “tpcGain” is assigned to correction for electronics amplification.
- This correction is obtained by running “pulser” and provides equalization of electronics amplification over channels.
- This correction is applied on the level of on-line clustering. Offline we don’t have any handle on this correction.
- This correction is recalculated a few times during the run. This correction has to be updated after each power shutdown (including shutdown of the STAR magnet).

## ADC/Clustering nonlinearity correction

- The nonlinearity  is coming from:
- Saturations in ADC
- ASIC thresholds in readout electronics. We lose some  time sequences.
- This gives nonlinearity in cluster charge.
- Correction has been obtained on level of clusters from simulation.

- Pulser

- Outer

- Inner

- Simulation versus Reconstruction

- Pulser

## Gas gain correction for sector/row

- sector

- row

## Sector / row correction

**Variation of gas gain for sector row is  ~6% before calibration and **
**~1% after**

## Correction for Electron Attachment due to O2

**The correction: **
**~e****+AdO****,**
**	****where**
**            ****d – drift distance (cm),**
**            ****O – concentration of Oxygen (ppm)**
**This correction is fixed with ALEPH **
**       ****A = 1.75e-6 (1/cm/ppm O****2****)**

## Dependence of the Gain on Gas Density due to Pressure

- log(P[bar])

- Before correction

- After Correction

## Parameters accounting condition for specific run

**Total charge in TPC**
**Global track multiplicity dependence,**
**Dependence of the Gain on total charge accumulated on a wire so far,**
**Dependence of the Gain for a given cluster  due to previous clusters (undershoot).**
**We observe the above dependencies (up to 0.3****σ**** for AuAu central events) ****but till now we don’t have a reasonable model to describe them. This is main issue which should be addressed with increasing luminosity.**
**Effect of residual distortions (?)**
**dX correction,**
**Dependence of the Gain on interception angle,**
**...**
**We observed extra dependencies which are not described by Bichel model. For now it is attributed to residual distortions.**

## Variation truncated mean for track dE/dx and its relative error  versus its length in TPC

-     Due to dE/dx dependence on dx there is a dependence of truncated mean <dE/dx> for tracks on their length in TPC at the level ~2%

- Before correction

- After correction

## Resolution just after applying tpcGain correction before full calibration

- 9.6% @ 76 cm

## Run IV dE/dx resolution

## Run V dE/dx resolution

## Run VI dE/dx resolution

## Problems anticipated with luminosity increase and detector aging

- Calibration procedure described above has been done on level ~1%.
- We have to control distortion in TPC to provide this level (dx)
- We have to understand effect of charge accumulated in an event on measurement cluster charge (dE).
- With detector aging probably we need to introduce more parameters in offline calibration (from Slow Control, ...)
- DAQ 1000 will require to revisit whole dE/dx calibration procedure