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TPC dEdx reconstruction in the Hough Transfrom tracking

TPC dEdx reconstruction in the Hough Transfrom tracking. C.Cheshkov, P.Hristov. Outline. Introduction Toy MC Study on several possible dEdx reconstruction approaches Implementation for Hough Transform tracker and some preliminary results Conclusions and To Do. Introduction.

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TPC dEdx reconstruction in the Hough Transfrom tracking

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  1. TPC dEdx reconstruction in the Hough Transfrom tracking C.Cheshkov, P.Hristov HLT Workshop

  2. Outline • Introduction • Toy MC Study on several possible dEdx reconstruction approaches • Implementation for Hough Transform tracker and some preliminary results • Conclusions and To Do HLT Workshop

  3. Introduction • The TPC dE/dx reconstruction in HLT can be useful mainly to separate e and  in the Pt region where we look for J/ • Due to its nature, the Hough Transform tracker need “special” dE/dx reconstruction  so called “on-fly” reconstruction • One has to take great care also about the timing performance of the algorithm HLT Workshop

  4. dEdx Reconstruction Methods • “Truncated mean” • The good old method used in almost 100% of the cases • Caveat: practically impossible to implement in the online Hough tracker  Need to keep ~160 amplitudes for each Hough bin  Huge arrays and therefore extremely slow HLT Workshop

  5. dEdx Reconstruction Methods • “Max. Likelihood Method” • Fitting the Landau distribution by means of max.likelihood gives the “exact” solution for the mean • Mean(A) =  * Log( N /  Exp(-Ai/) ) , where N - #samples and  - width of the Landau distribution • Caveat: Need to know in advance the width  ( ~2) HLT Workshop

  6. dEdx Reconstruction Methods • “Truncated pairs” • The idea is to take 2 ADC amplitudes at once and fill only the smaller • In order to suppress further the divergences coming from the Landau distribution of the signal, compute geometrical mean instead of simple mean: A(i)  log[A(i)] • Not as precise as two previous methods, but no obvious caveats for Hough tracker – small arrays needed, possibly fast computation and no need in additional input params HLT Workshop

  7. dEdx Reconstruction Methods Results from toy MC Likelihood Truncated mean Truncated pair HLT Workshop

  8. Implementation in HT • “Truncated pairs” method has been chosen and implemented recently into the HT • Since we don’t have clustering in HT, identified local maxima in pad and timebin directions are simply taken as dE/dx samples • Using pre-calculated LUTs, the amplitudes are corrected for track-pad crossing angles • Still some work to be done in order to reject further possible overlapping clusters HLT Workshop

  9. Preliminary results Off-line Hough tracker dN/dy=2000, 0.5T HLT Workshop

  10. Conclusions & To Do • It seems that the predictions from the toy MC are qualitively confirmed • However, slightly bigger than expected right tail • Need to check carefully again all the corrections to the amplitudes • Further rejection (or correction for) of possible overlapping clusters • Try the “Max.Likelihood” method at least in the region of >1GeV/c, where the width of the landau distribution should be constant • Optimize the code and check the timings HLT Workshop

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