# Presentation: 265852

## Status of dE/dx Calibration

- Yuri Fisyak
- October 17, 2002

## Outlook

- Why ?
- What ?
- Where ?
- Conclusions

## Why ?

- Last calibration was done in March, 2002:
- σ(dE/dx)/(dE/dx) = 8.2% for 76 cm track
- STAR CDR (p. 4C-33)
- σ(dE/dx)/(dE/dx) = 0.47 N-0.46(Ph)-0.32, P = 1atm
- σInner   = 14.3%, h = 1.15 cm, N = 12;
- σOuter  =   7.7%, h = 1.95 cm, N = 32;
- σ        =  6.8% for 76 cm track in TPC
- H.Bichsel simulation:
- σ        =  7.3 % for 11 * 1.2 cm + 31 * 2.0 cm
- I am not happy that ~1% is missing

## Why (cont.) ?

- Comparison new and old tcl
- H.Bichsel’s calculations reproduce data only qualitatively. Is this due to calibration procedure ?
- H.Bichsel claims that we have non linearity in dE measurement. Can we check this ?
- Can dE/dx calibration be done in one pass? Can we move it into fast online ?

## What does calibration include?

**Applying pad correction obtained from pulser data (Fabrice did this and I will not talk about)**
**For good clusters (used in fit, no overlaps) **
**For good global tracks (No. fit points ****≥ 30, Track length in TPC > 40 cm)**
**Z =log[(dE/dx)****measured****/(dE/dx)****predicted for ****π****]**

**Fit Z-distribution with Gauss(****μ****,****σ****)**** + pol3 in +/-3****σ**** range. It is supposed that we have ~80% ****π**** and ****μ**** corresponds to ****π****.**
**μ**** should not depend on****	**
**Time, ****Pressure**
**Sector, row**
**Drift distance**
**...****	**

## What does prediction mean ?

**0.45 GeV/c**

- Hans Bichsel made presentation about his model during the last collaboration meeting. He is preparing STAR TN on this subject.
- PAI model (Photo Absorption model for Ionization energy loss, V.Grishin V.Ermilova,S.Kotelnikov, NIM  A309(1991) 476) gives the same predictions but PAI simulation program (provided by P.Nevski) has problem at low β (it was not tuned for this range). For this reason I will use Bichsel’s calculations.

## Resolution before calibration

- 9.6% @ 76 cm

## What does March calibration mean?

- March calibration included:
- The same procedure as for Year 1 data
- As prediction it was used Sirrf
- Calibration was done for all tracks (no restriction  on momentum)
- Time dependence : overall gain correction factor each few hours (1-4)
- Sector and pad row correction
- Drift distance correction
- Result : σ = 9.6 % → σ = 8.2 %

## Red – 40% truncation

**Red**** – 40% truncation**
**Green**** – 30% truncation**
**Black  - fit**

- 8.2% @ 76 cm

## 9.7%

**9.7%**

**9.9% ****=>9.8% after setting convolution flag**

**New tcl a little bit worse than old one**

## New calibration

- Calibration is based on tracks with 0.4 < p < 0.5 GeV/c (~MIP for pions: βγ = p/m = 4).
- Calibration has been done for new tcl only.
- As prediction it was used Bichsel’s calculation with dx dependence (see next slide) i.e.
-  **Z =log[(dE/dx)****measured****/(dE/dx)****predicted for ****π****(****βγ****,****dx)]**
- This calibration gives σ = 8.8% (instead of 8.2% obtained in March because it was done only for 0.4<p<0.5GeV/c but resolution is obtained for all momenta).

## Slide 12

## Calibration done for p in [0.4,0.5] GeV/c with new tcl

- σ = 8.8% @76 cm

## Check of ADC nonlinearity

- For uniquely identified tracks:
- σ < 15% and
- v = log[(dEdx)/(dE/dx)J], where J = [e,π,K,p,d]
- |v| < 3σ for only J, and
- |(dE/dx)J - (dE/dx)k| >5σ, for J≠K
- Plot predicted dE versus    measured dE

**Inner**

**Outer**

**It is seen clean nonlinearity besides saturation (with offset ****~****1keV). **
**It is not clear what is origin for this nonlinearity:**
**ADC, ****it has to be checked with pulser data**
**tcl ? due to threshold effects? **

## Time dependence

**Gain variation versus time.     ~5%**

**Pressure variation versus time**

**Oxygen concentration variation versus time**

## Time dependence (cont.)

- Drift distance correction:
**e****+AdO**
- where A = 1.75e-6 (1/cm/ppmO2),
- 		provided by  A.Lebedev (ALEPH data)
-             d – drift distance (cm),
-             O – concentration of Oxygen (ppm)

## Pressure

**Both Drift distance and Pressure correction allow to remove** **time dependence.**
**TPC Gain Monitor cannot be used (at least now) because its results depend on magnetic field.**

## < 1 %

**< 1 %**

## After pad level correction from pulser calibration it still exists gain variation versus sector and row  

## Dependence of gain correction versus drift distance (after accounting absorption) for Inner rows has opposite behavior with respect to expected. ADC nonlinearity is overcorrected !? 

## 7.4% @ 76 cm

**7.4% @ 76 cm**

## For both + and -

**For both + and -**

## For positive only

**For positive only**

## Negative only

**Negative only**

## Bichsel shapes 

- Inner

- fit by φ(μ+(1+σ)z), where  φ(z) is Bichsel shape and z = log(dE/dEmost probable);

## Outer

- Both inner and outer rows are reasonably well described by Bichsel shape.

## Conclusions

- In sense of dE/dx old and new tcl give comparable results.
- It is observed significant nonlinearity in dE with respect to H.Bichsel calculation.
- Accounting this nonlinearity allows to improve resolution by ~1% (8.2%=>7.4%) which is matched with our expectation.
- To understand the nonlinearity is necessary to get pulser data  with different signal amplitudes.
- If the linearity will be understood than it will be possible to move dE/dx calibration in fast online.
- With new calibration the Bichsel shape of dE/dx distribution describes data pretty well. Thus we can claim that model calculations gives quantitative agreement with data.