TPC status report Marian Ivanov

TPC reconstruction and the new QA selection criteria Coverity Outlook TPC reconstruction and the new QA selection criteria Coverity TPC calibration and reconstruction New dEdx calculation New space point calibration

TPC calibration strategy Outlook TPC calibration strategy To be presented on Thursday https://twiki.cern.ch/twiki/bin/viewauth/ALICE/PWG1 CalibrationStrategy https://alisoft.cern.ch/AliRoot/trunk/TPC/doc/calib TPC PWG1 QA Automatic extraction of the QA trend graphs on the WEB (currently at the GSI web page ) http://www-alice.gsi.de/TPC/PWG1train/

TPC reconstruction and the new QA selection criteria (0) http://indico.cern.ch/contributionDisplay.py?contribId=6&co nfId=127310 One of the major systematic error in the physics analysis is introduced by selection criteria on the number of clusters. Based on experience with TPC data of 2009 and 2010, it is known that TPC gain and baseline variation influence the distribution of found clusters up to 30% level. This variation is time, energy loss, and running condition dependent and it is only partially described in the MC. However, this variation influences the TPC tracking performance onlyto the level of 1 − nmiss /nf ind . Ongoing work on integration of the new selection criteria into the AliESDtrackCuts and AliPhysiscsSelection

Removing of the remaining defects ongoing Coverity Our records show you currently have 111 defects assigned to you (PWG1 and TPC) Removing of the remaining defects ongoing Main source – obsolete code to be removed After the start of the PbPb pass2

New dEdx calculation (0) Experience from the 2010 data – the dEdx calculation not sufficient robust in respect to the variation of the gas gain Problem: The TPC is working with the gas gain ~ 6600 while in the TDR we expected to run at the gain 20000. Significant fraction (x – momentum,gain, position and PID dependent) of the cluster lost, because did not pass the threshold of electronic. ==> The old dEdx algorithm did not calculates the truncated mean from 0 up to 0.6 but from x-0.6*(1-x) The gain calibration equalize the dEdx for MIP particles. Second order corrections needed to follow the Bethe-Bloch. Such correction time dependent. Solution 1: Use also one pad clusters (July 2010) - the fraction of clusters bellow decreased significantly, but not enogh. Solution 2: Use in the dEdx calculation into account also the clusters bellow threshold. The algorithm to identify “signal below threshold” the same as for the QA variable – number of crossed rows.

New dEdx calculation - PbPb The Altro baseline restoration algorithm was not switched on The validation of the algorithm took 2 weeks. Decided to continue data taking with the same setup. Consequences – the baseline depends on the current event multiplicity (track occupancy). Observed dEdx – linearly decreasing with the occupancy. Solution – Adding the baseline correction (as function of occupancy) as a additive factor to the observed dEdx. Calibration of the baseline shift done in the Pass0.

New space point calibration Transition form the physical + effective” correction to the pure physical correction ( more details during the calibration meeting on Thursday) Improvement of TPC-ITS matching form RMS 0.5 mm down to 0.2 mm Physical correction - TPCExBshape ~ 0.2 -1 cm (magnetic field inhomogenity) Field cage rod shift ~ up to 0.7 cm in critical sector 10-11 ExB twist ~ 1 mm (angle between E and B field) Inter detector alignment ~ +-0.5 mm TPC-ITS (B+ and B-) Sector alignment ~ rms ~ 0.1 mm

Work in progress Make the space point calibration algorithm “user friendly running” on the grid to be repeated after each LHC period Open issue cosmic tracks and high pt V0s (un-binned information) Alignment of the outer detectors - by-product of the TPC calibration (constrains for A side/C side ) Global calibration/alignment? The TPC space point correction classes finalized. To be used also in the MC soon.