RPCs - where do we stand?

1. RPCs - where do we stand? HARP Collaboration meeting, 7 July 2003 Jörg Wotschack

2. The team • Oxford: • G. Barr, Ch. Pattison, S. Robins, A. De Santo (now at Royal Holloway College) • IHEP Protvino: • V. Koreshev • CERN: • (M. Bogomilov), J. Wotschack J.Wotschack

3. Data analysis ... so far based on • 2001 ntuples: Large angle data (Ta 3 GeV) • 150‘000 RPC hits in barrel (few hundert/pad) • Only ~50% of hits correlated with ‚good‘ tracks • 2002 ntuples: various smaller data sets • Partly incomplete information in ntuples • Statistics insufficient • Not useful for systematic analysis • Calibration scan data (4 spare RPCs) J.Wotschack

4. Results - calibration Main results are based on calibration/scan data • Time-slewing correction - unique for all RPC pads (ADS, VK) • Systematic studies of response as function of beam impact point (VK) • Little dependance on position along pad (z) • Significant dependence on hit distance to PA (charge dependent) • Alignment in z and phi wrt TPC (ChP) • TDC calibration (Ch. Wiebusch): ps/TDC count J.Wotschack

5. Corr=-4.7+5.97 104/Q-2.65 107/Q2+1.66 1010/Q3 Ttime-slewing correction Corr=a+b/Q+c/Q2+d/Q3 Q=Qmeas-Qped+Qoffset Qoffset =394.4 Qmeas- Qped

6. 3 TDC counts TDC counts Variation of response across pad with distance to preamplifier; Slope = (TPA-Tend)/L [TDC counts/mm] Variation of response along pad (z)

7. Results - particle identification 3 GeV Ta data (2001) - thin target T0‘s not yet well determined J.Wotschack

8. Missing ... • Good T0 calibration of pads • Required precision: ~100 ps (3 TDC counts) • Need sufficient statistics (>1000 hits/pad) and reliably reconstructed tracks • Three methods are being used to solve the problem J.Wotschack

9. Method I • Use charged tracks with known momentum and particle id; measure the tracklength and the time and compare to nominal time of flight • Best method in principle • Relies on good knowledge of p, L, and particle type • Protons: 10% momentum error leads to an error of 20 TDC counts for a 1m long proton track for pp=0.5 GeV/c • Pions: dp/p=10% for 200 MeV/c corresp. to ~100 ps • Separation of e/π not possible in this step J.Wotschack

10. Method II (overlapping pads) • Tracks through overlaps b/w pads • Same track through pads A and B • Gives relative response b/w all pads in one pad ring; i.e., can adjust all pads in ring to same scale • Independent of momentum measurement and particle id • Requires still absolute t0 determination for the eigth pad rings in barrel; same difficulties as in Method I but statistically much better • Of limited use in forward RPCs J.Wotschack

11. T1-T2 (ns) Tracks through overlapping pads Ideal tool to test T0 determination Time difference in ns for identical tracks measured in two overlapping pads after t0 determination with neutrals. Example of a good case Gives (convoluted) time resolution of the two pads involved. s=280 ps

12. Method III (neutrals) • Use photons converting in material in front of (or in) RPCs • Signature: no track pointing to RPC pad but good signal (in time and charge) in pad • Advantages: • independent of momentum measurement • Straight tracks • Known beta (relativistic particles) • Disadvantages: • no tracklength info, averages over pad; but mean values are well known J.Wotschack

13. Selection of photons • Select beam protons and ITC trigger • Require at least one good track coming from target, confirmed by RPC hit, i.e., there was an interaction in time • Scan over all RPC pad hits • Require that there is no track pointing to this pad or close by • Exclude small charges (qdc < 100 counts); small charge signals are expected from Compton scattering of ~MeV photons. Mainly (back)scattering photons not coming from the target J.Wotschack

14. Signal • Typical time spectrum of hits in RPC pad w/o associated track Tail from not tagged charged particles Low energy backscattered photons J.Wotschack

15. Example for problem case • Known problem areas: • TPC sector boundaries • Dead areas in TPC Other difficulties: • Cross-talk induced effects • Difficult pattern recognition • Overlays J.Wotschack

16. Tracks or no tracks Phi0 from TPC RPC hits/chamber All fitted tracks Dead channels (1/8 pads in chb) Extrapolation to RPCs >9 hits/track >12 hits/track 2 chambers/bin J.Wotschack

17. Strategy • Proceed in parallel with all three methods • Expect (in the end) to find the same calibration constants from all three methods • Produce ‚private‘ LA ntuples with thin and low z material targets (Be or Al, pbeam = 8.9 GeV) • Minimize π0 conversion in target (large rad. length) => enhanced number of π0 conversions in TPC f.c. • Minimize re-interaction in target (large inter. length) => cleaner sample of tracks pointing to IP • Keep all TPC clusters (also those not connected to a track) • When barrel is done try methods on forward RPCs J.Wotschack

18. Conclusions • Internal RPC calibration parameters largely understood • Missing: T0 determination of RPC pads, i.e., absolute time scale for each individual RPC channel • Three methods are proposed (charged, overlap, neutral) to cross-check results • Needs large statistics data sets with complete TPC info (clusters) and well reconstructed tracks • Using neutrals requires selection of clean sample of no-track hits in RPCs and therefore all TPC hits in ntuples • ‚Private‘ ntuple production about to start ... J.Wotschack