1. Cosners House Forum on LHC startup April 07 1 Minimum bias studies etc…. Craig Buttar University of Glasgow

2. Cosners House Forum on LHC startup April 07 2 Outline LHC start up Minimum bias events –Why study them? –What are they? Triggering minimum bias –Minimum bias trigger scintilators (MBTS) –Random trigger and track trigger Tracking –Reconstructing low p T tracks –Vertex reconstruction –Using pixel tracklets Underlying event –What is it and why measure it –Measurement with ATLAS Summary

3. Cosners House Forum on LHC startup April 07 3 Events/xing <<1 LHC Pilot run: 2007 S. Myers At L=10 29, can get ~10M MB events in 1 day with 30% eff

4. Cosners House Forum on LHC startup April 07 4 Possible Beam Parameter Evolution at 7TeV/beam S. Myers

5. Cosners House Forum on LHC startup April 07 5 Why measure min bias? Not exactly what the LHC was built for! But….. Physics: measure dN/d | =0 –Compare to NSD data from SppS and Tevatron MB samples for pile-up studies –Calorimeter –Physics analyses –Benchmark for sLHC 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

6. Cosners House Forum on LHC startup April 07 6 What are minimum bias events? pp collisions at s = 14TeV PYTHIA6.323PHOJET1.12 σ tot σ elas σ single dif (ABXB) σ single dif (ABAX) σ double dif σ non-dif 101.5 mb 22.2 mb 7.2 mb 10.3 mb 54.7 mb 119.1 mb 34.5 mb 5.5 mb 4.1 mb 69.5 mb inelastic events Trigger bias: Typically select non-single diffractive (NSD) events

7. Cosners House Forum on LHC startup April 07 7 What are minimum bias events? Minimum bias are inelastic collisions of two protons –Includes very rare high-pt scatters and very common low-pt scatters Minimum bias is an experimental definition –Defined by experimental trigger and analysis Relation between experiment and physics is: f i are acceptances for different physics processes determined by the trigger

8. Cosners House Forum on LHC startup April 07 8 Event characteristics –Non-single diffractive~non-diffractive inelastic –Soft tracks p T peak ~250MeV –Approx flat distribution in to | |~3 and in –N ch ~30; | |<2.5 Trigger rates – ~70mb (NSD!) –R~700kHz @ L=10 31 cm -2 s -1 M. Leyton

9. Cosners House Forum on LHC startup April 07 9 Triggering minimum bias Use forward scintillators (MBTS) Random trigger –inefficient where there are <<1 events/xing –Validate at L2/EF to reduce inefficiency Other triggers: jet, lepton triggers –High pt thresholds bias

10. Cosners House Forum on LHC startup April 07 10 MBTS Trigger scintillation counters mounted on end of LAr calorimeter covering same radii as ID η=2.0 η=3.8 interaction point Beam-pipe +η pannel MBTS UA5 To compare to UA5 and CDF data need to understand composition of the sample trigger bias Currently generate inelastic and diffractive parts using PYTHIA –Need to investigate other simulations- PHOJET

11. Cosners House Forum on LHC startup April 07 11 Noise spectrum fits well to a Gaussian with σ=2.52pC Beyond 9pC (almost 4σ), non- Gaussian behavior is possible Accidental rate must be suppressed to ~Hz, limited by EF output-rate of 100Hz suppression required of 10 -6 -10 -7 at 900GeV and 14TeV Noise and Accidental Suppression – Lab tests Eric Feng

12. Cosners House Forum on LHC startup April 07 12 Trigger Efficiencies and Bias Shown below for each coincidence logic: - Required counter threshold to suppress accidental rate to 1Hz - Corresponding trigger efficiency for each type of event Coincidence LogicCounter Threshold [pC] Trigger Efficiency 900 GeV14 TeV Non-DiffSingle/ Double Diff. Non-DiffSingle/ Double Diff. 1+110.7598%35%99%42% 2+2 7.7597%27%98%36% 3+3 6.2594%20%97%30% 4+4 5.2590%15%95%24% 5+5 4.5085%10%92%19% 6+6 4.0079%7%89%14% Eric Feng

13. Cosners House Forum on LHC startup April 07 13 Track Trigger L2 track trigger to validate random trigger when <<1 events/xing LVL 1 LVL 2 Event-Filter 1 kHz eventrate 100 Hz eventrate 13 MHz eventrate reduction by a factor 10 storage pass-through random trigger R. Kwee

14. Cosners House Forum on LHC startup April 07 14 Selecting MinBias Events At L2 can use –no. of spacepoints from pixel and SCT little bias –no. of reconstructed tracks, needed to reject beamgas, halos pt-track > ~1GeV biased Rejection of empty events, compare to cosmic runs =3 =0 noise spacepoints from ¼ sct-barrel from cosmics datasimulation of noise in whole SCT R. Kwee

15. Cosners House Forum on LHC startup April 07 15 Rejecting beam-gas To reject beam gas need track reconstruction SCT-cut at 20 sp pixel-cut at 900 sp minbias99.286.3 noise00.4 beamgas77.331.6 Diffractive (SD+DD) 50.215.0 R. Kwee Beamgas Diffractive

16. Cosners House Forum on LHC startup April 07 16 CMS MB trigger: Trigger Towers Use Forward HCAL –Towers: N vs P Strongly correlated Trigger should cut on nTowers hit on N side and P side –e.g. N>10 and P>10 ORCA Simulation

17. Cosners House Forum on LHC startup April 07 17 Tracking in MB events Acceptance limited in rapidity and pt Rapidity coverage –Tracking covers | |<2.5 p T problem –Need to extrapolate by ~x2 Need to understand low pt charge track reconstruction 1000 events dN ch /d dN ch /dp T Black = Generated (Pythia6.2) Blue = TrkTrack: iPatRec Red = TrkTrack: xKalman Reconstruct tracks with: 1) pT>500MeV 1) pT>500MeV 2) |d 0 | < 1mm 2) |d 0 | < 1mm 3) # B-layer hits >= 1 3) # B-layer hits >= 1 4) # precision hits >= 8 4) # precision hits >= 8 p T (MeV)

18. Cosners House Forum on LHC startup April 07 18 Where is the Momentum Limit ? 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 150MeV 50MeV 400MeV A.Salzburger

19. Cosners House Forum on LHC startup April 07 19 Track reconstruction efficiencies MC/Track cuts: η < 2.5, z0 < 150 mm, d0 < 1 mm Efficiency: fraction of MC tracks with > 50% matched hits Pt=200MeV Tracking efficiency >20-50% for pt>200MeV M.Leyton p T (MeV)

20. Cosners House Forum on LHC startup April 07 20 Dan Clemens Track Reconstruction efficiencies by track insertion efficiency by track insertion reconstruction with New Tracking Plot by M.Leyton for comparison : truth matched Insert a track into a minimum bias event by overlapping a simulated single track event. Reconstruct merged event and look for inserted single track to determine efficiency.

21. Cosners House Forum on LHC startup April 07 21 Track multiplicity X Vertex Residuasl Minbias Vertex Reconstruction I. Dawson, K. Prokofiev 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 σ NSD. –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

22. Cosners House Forum on LHC startup April 07 22 Tracklet Method Primary vertex (standard tracking) Hits in B-layer Hits in Layer 1 η Φ Each hit assigned η,Φ relative to primary vertex Calculate Δη,ΔΦ for each pair Tracklet defined (now) as Δη<0.01,ΔΦ<0.15 Peter Steinberg, BNL Exploring methods with sensitivity at lower pT with even fewer pixel-hits

23. Cosners House Forum on LHC startup April 07 23 Single Event Φ η Hits in B-layer and L1. Black circles are Truth-primary charged hadrons. Clustered pairs of hits visible near truth hadrons. Peter Steinberg, BNL

24. Cosners House Forum on LHC startup April 07 24 Momentum Reconstruction p T = 0.011/ΔΦ Poor persons tracking code. spatial resolution kicks in above ~2 GeV Peter Steinberg, BNL

25. Cosners House Forum on LHC startup April 07 25 Charged primary hadrons Reconstructed Peter Steinberg, BNL at low pT (down to 100 MeV) the recontructed and truth pT match well, at higher pT reconstruction gets distorted Momentum reconstruction

26. Cosners House Forum on LHC startup April 07 26 Inclusive dN ch /dη Current parameters good for η~0, large backgrounds at |η|~2 Peter Steinberg, BNL Charged primary hadrons Reconstructed Focus on |η|<1, look at multiplicity distributions linear response,good resolution

27. Cosners House Forum on LHC startup April 07 27 CMS Tracking Similar to ATLAS pixel tracklets

28. Cosners House Forum on LHC startup April 07 28 What is the underlying event High P T scatter Beam remnants ISR Hard and soft physics ISR beam remnants central particle production colour flow/string drawing

29. Cosners House Forum on LHC startup April 07 29 VBF Signal (H WW l l ) forward tagging jets correlated isolated leptons low hadronic activity in central region central Higgs production Tagging jet H W W Z/W Important discovery channel For Higgs in mass range 120-200GeV

30. Cosners House Forum on LHC startup April 07 30 ModelParameter Simple MSTP(82)=1 PARP(81)=1.9 Complex MSTP(82)=4 PARP(82)=1.9 Tuned MSTP(82)=4 PARP(82)=1.8 PARP(84)=0.5 ModelCJV (eff)LEPACC (eff)All Vetoes (eff) Simple0.943 ± 0.0030.610 ±0.0010.084 ±0.001 Complex0.885 ± 0.0030.575 ±0.0010.076 ±0.001 Tuned0.915 ± 0.0030.589 ±0.0010.080 ±0.001 Uncertainty at the level of ~6% on CJV Pt>20GeV and ~3% on lepton Giving a total uncertaintly in the range ~8%

31. Cosners House Forum on LHC startup April 07 31 Average multiplicity of charged particles in the underlying event associated to a leading jet with P t ljet (GeV). Average p T sum (GeV) of charged particles in the underlying event associated to a leading jet with P t ljet (GeV). Describing the region transverse to the leading jet

32. Cosners House Forum on LHC startup April 07 32 Min-Max analysis Leading Jet Back-to-Back TransMAX and transMIN sensitive to radiation and soft UE respectively Back-to-back sample suppresses radiation difference between tranMAX region and transMIN in leading jet and b-2-b jet sample

33. Cosners House Forum on LHC startup April 07 33 E T jet #1 = 1.63TeV E T jet #2 = 1.16TeV Measuring the underlying event at ATLAS

34. Cosners House Forum on LHC startup April 07 34 First physics studies with the underlying event ~ 15 days of data taking in 2007 enough to cover up to p T (leading jet) ~ 40 GeV ~ 15 days of data taking in 2007 enough to cover up to p T (leading jet) ~ 40 GeV ATLAS - s=900 GeV ATLAS - s=14 TeV s=14TeV s=900GeV Reconstructed tracks Multiplicity of charged particles with p T > 0.5 GeV and | |< 1 in region transverse to leading jet

35. Cosners House Forum on LHC startup April 07 35 Selecting the underlying event: | η track | < 2.5, p T track > 1.0 GeV/c N jets > 1, | η jet | < 2.5, E T jet >10 GeV, J1J2J3 J4 J5J6 charged particles (generator level) J1J2J3 J4 J5 J6 Measuring the underlying event in TeV jets

36. Cosners House Forum on LHC startup April 07 36 Ratio / Leading jet E T (GeV) Selecting the underlying event: |η track | < 2.5, p T track > 1.0 GeV/c N jets > 1, |η jet | < 2.5, E T jet >10 GeV,

37. Cosners House Forum on LHC startup April 07 37 summary Minimum bias and underlying event are uncertain at LHC energies –Energy extrapolation for SppS and CDF not known –dN/d | =0 measurement can be done in early running Triggering –Both ATLAS and CMS are developing minimum bias triggers –Need to understand trigger bias to compare to UA5,CDF…. Tracking –Low pt tracking is challenging but progress by both ATLAS+CMS Underlying event can be measured with early data –Need to ensure overlap between jet and mb samples Waiting for data

38. Cosners House Forum on LHC startup April 07 38 Extra slides

39. Cosners House Forum on LHC startup April 07 39 We are taking REAL DATA ! SR1 cosmics test stand –Final TRT barrel –Final SCT barrel –1 sector cabled for both Day zero start-up looks promising!

40. Cosners House Forum on LHC startup April 07 40 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 First Results

41. Cosners House Forum on LHC startup April 07 41 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

42. Cosners House Forum on LHC startup April 07 42 Non-diffractive+SD+DD Non-diffractive only Diffractive events Coincidences in MBTS PYTHIA charged-particle level

43. Cosners House Forum on LHC startup April 07 43 MBTS response Non-diffractive inelastic events (sample 005001) Non-diffractive inelastic events: energy is deposited on both sides (typically) Single and double diffractive inelastic events (sample 005002) Single diffractive events: energy is deposited on either side Double diffractive events A.Moraes

44. Cosners House Forum on LHC startup April 07 44 How many events? For dN/d require ~10k For UE need ~20M MB events to get some with leading jets P T ~30GeV Need to overlap with jet triggers –Prescaled P T >25GeV ~40M events gives ~10 hits/strip in barrel SCT CMS quote ~18M to intercalibrate their calorimeter with MB

45. Cosners House Forum on LHC startup April 07 45 Performance goals

46. Cosners House Forum on LHC startup April 07 46 Machine considerations

47. Cosners House Forum on LHC startup April 07 47 LHC operational cycle

48. Cosners House Forum on LHC startup April 07 48

49. Cosners House Forum on LHC startup April 07 49 Physics inefficiencies What fraction of primary particles reach the various detector layers, versus pT? Charged particle component of MB events dominated by +-, with ~10% K +-. First look at - 1)Pixel system should provide 3 space points down to ~100MeV/c. 2)Down to ~150MeV/c, particles can traverse SCT magnetic field, but reduced efficiency below 300MeV/c due to inelastic interactions in ID material. 2

50. Cosners House Forum on LHC startup April 07 50 Physics inefficiencies What about the charged kaons? 1)Much larger inefficiencies evident, due to increased material interactions and decays. 2)For pT=300MeV/c, ~10% of K do not reach final Pixel layer. For pT=100MeV/c, K do not even make it into the Pixel system! 3)In the SCT system, 15% of 500MeV/c K do not reach last SCT layer. Below 300MeV/c no spacepoints in outer SCT. 3