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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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)

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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

Ttime-slewing correction Corr=a+b/Q+c/Q 2 +d/Q 3 Q=Q meas -Q ped +Q offset Q offset =394.4 Corr= /Q /Q /Q 3 Q meas - Q ped

Variation of response along pad (z) Variation of response across pad with distance to preamplifier; Slope = (T PA -T end )/L [ TDC counts/mm] TDC counts 3 TDC counts

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 7 Results - particle identification 3 GeV Ta data (2001) - thin target T0‘s not yet well determined

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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 p p =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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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

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. T1-T2 (ns) Tracks through overlapping pads  =280 ps Ideal tool to test T0 determination

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 14 Signal Typical time spectrum of hits in RPC pad w/o associated track Tail from not tagged charged particles Low energy backscattered photons

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 16 Tracks or no tracks RPC hits/chamber Phi0 from TPC All fitted tracks Extrapolation to RPCs >9 hits/track >12 hits/track Dead channels (1/8 pads in chb) 2 chambers/bin

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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, p beam = 8.9 GeV) Minimize π 0 conversion in target (large rad. length) => enhanced number of π 0 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

HARP Coll. Meeting, 7/7/2003J.WotschackRPC status / 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...