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Minimum bias studies

This study outlines the importance of minimum bias events in particle physics research and discusses triggering and tracking techniques used in the ATLAS experiment. It also highlights the potential applications and future directions of minimum bias studies.

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Minimum bias studies

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  1. Minimum bias studies Craig Buttar UK ATLAS Physics Durham 06

  2. Outline • Minimum bias events • Why study them? • What are they? • Triggering • How many events do we want? • Random trigger • Minimum bias trigger scintilators (MBTS) • Tracking • Reconstructing low pT tracks • Using pixel hits • Vertex reconstruction • HLT • Minimum bias group • Summary, future work/open issues UK ATLAS Physics Durham 06

  3. Why measure min bias? Not exactly what the LHC was built for! But….. • Physics: measure dN/dh|h=0 • Compare to NSD data from SppS and Tevatron • MB samples for pile-up studies • Calorimeter • Physics analyses • Overlap with underlying events • analyses eg VBF, Jets… • Demonstrate that ATLAS is operational • Inter-calibration of detector elements • Uniform events • Alignment • Baseline for heavy ions UK ATLAS Physics Durham 06

  4. Soft pp collisions inelastic events Trigger bias: Typically select non-single diffractive (NSD) events UK ATLAS Physics Durham 06 A. Moraes et al. – SN-ATLAS-2006-057

  5. Event characteristics • Event characteristics • Non-single diffractive~non-diffractive inelastic • Soft tracks pTpeak~250MeV • Approx flat distribution in h to |h|~3 and in f • Nch~30; |h|<2.5 • Trigger rates • s~70mb (NSD!) • R~700kHz @ L=1031cm-2s-1 M. Leyton UK ATLAS Physics Durham 06

  6. How many events? • For dN/dh require ~10k • For UE need ~20M MB events to get some with leading jets PT~30GeV • Need to overlap with jet triggers • Prescaled PT>25GeV • ~40M events gives ~10 hits/strip in barrel SCT • CMS quote ~18M to intercalibrate their calorimeter with MB UK ATLAS Physics Durham 06

  7. Random trigger • 75ns @ L=1031cm-2s-1 gives <nx>~0.05 • Probability of 1 event/xing =0.05 • Assume 1 month of running with eff=50% • For 20M events to tape from HLT • 1kHz L1 random trigger • Process in EF to remove empty events • eg track reconstruction • 20Hz output from HLT • Assumes 100% efficiency in identifying MB and rejecting backgrounds • How to identify MB in EF  Later • Probably need x2 safety factors at L1 and EF • There is a built in safety factor – empty events are small • For lower luminosity ÷10 need to increase L1 random rate by x10 to maintain 20Hz HLT rate UK ATLAS Physics Durham 06

  8. η=2.0 interaction point η=3.8 +η pannel Beam-pipe MBTS MBTS • Trigger scintillation counters mounted on end of LAr calorimeter covering same radii as ID • Cover 2<|h|<4 • Can be used for first data BUT! • Not rad-hard • Uses 1/8th of tilecal readout  Lifetime unknown • At L1 S/N is ‘modest’ • Now in simulation can be tuned to measurement in the summer • Can do better at L2 with precision readout • Trigger logic not defined • Likely to be coincidence between hits in forward and backward hemisphere UK ATLAS Physics Durham 06

  9. Coincidences in MBTS PYTHIA charged-particle level Non-diffractive+SD+DD Diffractive events Non-diffractive only UK ATLAS Physics Durham 06

  10. Single and double diffractive inelastic events (sample 005002) Single diffractive events: energy is deposited on either side Double diffractive events MBTS response Non-diffractive inelastic events (sample 005001) Non-diffractive inelastic events: energy is deposited on both sides (typically) UK ATLAS Physics Durham 06 A.Moraes

  11. 1000 events dNch/d h Black = Generated (Pythia6.2) Blue = TrkTrack: iPatRec Red = TrkTrack: xKalman dNch/dpT Reconstruct tracks with: 1) pT>500MeV 2) |d0| < 1mm 3) # B-layer hits >= 1 4) # precision hits >= 8 pT (MeV) Tracking in MB events • Acceptance limited in rapidity and pt • Rapidity coverage • Tracking covers |h|<2.5 • pT problem • Need to extrapolate by ~x2 • Need to understand low pt charge track reconstruction UK ATLAS Physics Durham 06

  12. I. Dawson, K. Prokofiev Athena 11.0.41 # charged rec. tracks with associated PV particle # charged particles produced in PV volume εprimary = Track reconstruction efficiencies in minbias events xKalman • Started looking at primary particle reconstruction efficiencies in minbias events: • usually want to measure primary particle distributions such as dNch/dη. • Primary charged particles selected as those originating within a volume around interaction point • Hit based truth matching using TrackTruthCollection (map between reconstructed and truth). • NB need to develop method independent of truth! • Try to understand inefficiences and limits in reconstructing tracks in this low-pT regime: • Physics or algorithm, hardware? h< 0.5 (CSC11.005001.pythia_minbias.digit.RDO.v11000401) UK ATLAS Physics Durham 06

  13. Where is the Momentum Limit ? 50MeV • Tracker is in principle sensitive to soft tracks • Pt = 400 MeV - tracks reach end of TRT • Pt = 150 MeV - tracks reach last SCT layer • Pt = 50 MeV - tracks reach all Pixel layers  Do not need to run with low field • Event graphics using Fatras simulation • Tools are there to tune for such tracks A.Salzburger 400MeV 150MeV M.Elsing Talk in SM meeting UK ATLAS Physics Durham 06

  14. A.Poppleton, I.Dawson ~450 particles h< 0.5 Progress so far Reconstructed with Athena 12.0.0 Low-pT track reconstruction • A.Poppleton extending iPatRec to reconstruct tracks at lower pT: • Not clear what limit is? • Need to investigate inefficiences and fake rates in this new pT regime – lots of testing (CSC11.005001.pythia_minbias.digit.RDO.v11000401) • Problem getting good efficiencies below pT ~ 300MeV – under investigation. UK ATLAS Physics Durham 06

  15. I. Dawson, K. Prokofiev X Vertex Residuasl Track multiplicity Minbias Vertex Reconstruction • How well can we reconstruct primary vertex position of a minbias event? • A good vertex reconstruction efficiency in multi-pp bunch crossings may allow measurement of σNSD. • Knowledge of primary vertex position can improve signal to noise, eg, H →γγ (UE to Higgs similar to MB) • Use VxBilloirTools package to fit vertex to minbias tracks. • Shown here are residuals (difference between reconstructed and truth) Typically ~250μm precision UK ATLAS Physics Durham 06

  16. Tracking Strategy for soft Particles • Step 1: Primary track reconstruction (= current NewTracking) • Track candidates in Pixels+SCT - ambiguity solution - TRT extension similar in xKalman and iPatRec • Step 2: Secondary track reconstruction (release 13) • TRT leftover segments - extension to SCT and Pixels - global ambiguity • Step 3 (New): Soft particle reconstruction after primary vertexing • Select soft track candidates pointing directly to established primary vertex - this would mostly reuse reconstruction tools from the common tracking framework - dedicated search tuned specifically for very soft tracks (maybe even for pixel only tracks) - direct control of soft tracking, no interference with default reconstruction - Michael Leyton interested in this… M.Elsing Talk in SM meeting UK ATLAS Physics Durham 06

  17. Pixel tracklets To avoid pt-cut-off use pixel layers 0 and 1 DR hits in layer 1 DR hits relative to tracks Minimise for layer 1 hits relative to layer 0 How to get efficiencies? UK ATLAS Physics Durham 06 P.Steinberg - preliminary

  18. MB identification at HLT • How to identify minimum bias in random trigger using HLT? • After offline studies, implement track reconstruction in EF • Min number of well reconstructed ID tracks • This is (probably) ok for NSD sample but is biased • Need to look at bias introduced ptmin~0.5GeV • Lower pT cut-off possible but time consuming? • Look at pixel hits • Reduce pT-bias • Faster algorithm • Reduce MBTS rate • At L2 and/or EF use high gain to improve S/N • Endup with two complementary MB triggers UK ATLAS Physics Durham 06

  19. Minimum bias activities • Minimum bias activities can be found at: • Min bias Twiki page: https://uimon.cern.ch/twiki/bin/view/Atlas/MinbiasWorkingGroup • ~Weekly meetings • Triggering • MBTS • Random+HLT • Tracking • Low-pT track • Tracklet based • Simulations • minbias p+p • Beam-gas, beam-halo • Measurements • Pt-spectra and dN/dh • Calorimeter Volunteers welcome! UK ATLAS Physics Durham 06

  20. Work to do • MBTS • Measure low gain S/N for L1 • optimise logic • Invesitgate refinement at L2/EF using high gain readout • Random • How to operate a low luminosity -- can event size be minimised? • Best way to identify non-empty events in EF • start looking at tracking • Offline tracking • Study how to lower pT-threshold • Investigate bias of triggers • Compare to different physics models • How to reject empty events, generate minimally biased stream • Study use of calorimeter and LUCID • Looking at dN/dh for p0 • Extend rapidity range of triggering UK ATLAS Physics Durham 06

  21. Extra slides UK ATLAS Physics Durham 06

  22. Day zero start-up looks promising! We are taking REAL DATA ! • SR1 cosmics test stand • Final TRT barrel • Final SCT barrel • 1 sector cabled for both UK ATLAS Physics Durham 06

  23. First Results • Offline reconstruction worked with little problems within hours • Debug cabling… • First results are very positive • See residuals and pulls • No alignment or detailed calibration yet ! • Reflects excellent mechanical precision… • More for Ringberg Workshop in 2 weeks • I am definitely very optimistic for first data taking having seen this ! • Detector is on a good way ! • Pixels are recovering from cooling problem delay • Finish 3 layer system in time UK ATLAS Physics Durham 06

  24. 11 M.Leyton ~3.3 ~3.0 ~2.4 A.Moraes Track pT > 0.5 GeV Track reconstruction efficiencies in minbias events • Involved in InDet software validation • Using InDetRecStatisitics package • Looking at pion reconstruction efficiencies in Athena 12.0.0 • Investigating inefficiencies in minbias events at AOD level. • Using cone method to match tracks to particles. UK ATLAS Physics Durham 06

  25. Tracking: Startup-Initial Alignment • Very first alignment will be based on: • Mechanical precision • Detailed survey data • Cosmics data (SR1/Pit) • Minimum bias events and inclusive bb • Studies indicate good efficiencies after initial alignment • Example taken from T.Golling • Precision will need Zs and resonances to fix energy scales, constrain twists, etc… M.Elsing Talk in SM meeting UK ATLAS Physics Durham 06

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