1. 1 Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

2. Motivation 2 Modeling of minimum bias pile-up is a necessary tool for high p T physics! Physics: improve our understanding of QCD effect. improve, or reject models of particle production which are often available as Monte Carlo event generator. Modeling of minimum bias pile-up is a necessary tool for high p T physics! Physics: improve our understanding of QCD effect. improve, or reject models of particle production which are often available as Monte Carlo event generator. Minimum Bias measurements should be done early on, at low luminosity to remove effect of overlapping proton-proton collisions! NSD (Non Single Diffractive)= DD + ND Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

3. Motivation 3 Pertubative QCD calculations can not be done in the “soft” regime where the transverse momentum transfer between initial and final states is small  Underlying Events (UE) : beam-beam remnants, multiple parton interactions, initial and final state radiation, etc Pertubative QCD calculations can not be done in the “soft” regime where the transverse momentum transfer between initial and final states is small  Underlying Events (UE) : beam-beam remnants, multiple parton interactions, initial and final state radiation, etc Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

4. A Toroidal LHC ApparatuS 4 Inner Detector (|  |<2.5, B=2T): Si Pixels, Si strips, Transition Radiation detector (straws) Precise tracking and vertexing, e/  separation Momentum resolution:  /p T ~ 3.8x10 -4 p T (GeV)  0.015 Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS Minimum Bias Trigger Scintillators

5. Minimum Bias Measurement at ATLAS 5 Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

6. 6 Inner Detector Space Points Minimum Bias Trigger Scintillators Cerenkov Detector (LUCID) Select events with minimal bias using level 1 and level 2 trigger items. Zero degree calorimeter (ZDC) 8.3  ∞ 14 TeV Simulation Dedicated minimum bias trigger Triggering on Minimum Bias Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

7. 7 2.12 < | η | < 2.83 2.83 < | η | < 3.8 Minimum Bias Trigger Scintillators (MBTS) Minimum Bias Trigger Scintillators Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

8. Colliding Bunches! 8 Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

9. Stable Beam Collision Event 9 Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

10. Measurement Overview 10  Select inelastic events with Minimum Bias trigger.  Select primary tracks using impact parameter with respect to a primary vertex.  Require 2 tracks within | η | ≤ 2.5 and p T ≥ 100MeV.  Require 1 track within | η | ≤ 2.5 and p T ≥ 500MeV.  Correct back to the hadron level and measure distributions within | η | 100/500MeV. Primary charged particles: Charged particles with a mean lifetime τ > 0.3 × 10 -10 s directly produced in pp interactions or from subsequent decays of particles with a shorter lifetime 0.9, 7 TeV 2.36 TeV  UE: Leading track p T > 1.0 GeV and all other tracks p T > 0.5 GeV 0.9, 7 TeV Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

11. Offline Selection & Pile-up 11  Less than 0.1% background event after final selection  Pile-up effect:  Remove events with 2 nd vertex with ≥ 4 tracks  Removes 0.2% of events  ~8% pile-up not resolved  <0.03%, except event with highest multiplicity :~1%. Δ Z<3mm, two vertices as merged to one Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS  One MBTS hit out of 32  1 primary vertex  BS constrained Track Selection  |d 0 | vtx ≤ 1.5mm  |Z 0 sin θ | vtx ≤ 1.5mm  Χ 2 prob > 0.01  for p T > 10 GeV 7 TeV: 10M events, 209M tracks 0.9 TeV: 350k events, 4.5M tracks 7 TeV: 10M events, 209M tracks 0.9 TeV: 350k events, 4.5M tracks

12. Low p T Track Validation:100-500 MeV 12 Excellent agreement between data and MC Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

13. 13 Efficiencies Tracking efficiency Vertex efficiencyTrigger efficiency Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

14. 1/N ev dN ev /dN ch 14 At lower p T threshold:  Peak around 10 particles per event  Below and high values not well described by current MC At high p T threshold:  AMBT1 describes full spectrum(~10%)  Other tunes have different shapes in intermediate regions 0.9, 7 TeV 2.36 TeV Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

15. 1/N ev dN ch /d η 15  Very little shape variation between the MC models  Difference mostly in normalization Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

16. 16  Measurements span 12 orders of magnitude  Large disagreements at lowest p T  At intermediate p T much better agreement with AMBT1 tune Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

17. 17  Shape at high n ch well-modeled  AMBT1 reproduces the spectrum the best  Low n ch shape is sensitive to ND, SD, DD fractions Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

18. Systematic Uncertainties p T 18 The different p T bins are normalized such that the integral | η | <1.0 is the same. Data 7 TeV MC ND 7 TeV Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

19. Underlying Event Studies at ATLAS 19 Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

20. 20 Underlying Event sensitivity  Charged tracks in the minimum bias events are used for the study  Align leading p T track at ϕ =0  Define three equal regions in | Δ ϕ |  Transverse region is most sensitive to UE, perpendicular to the hardest scattering axis  Measure track-based observables in all regions  Charged particle multiplicity vs p T lead  Scalar p T sum vs p T lead  vs p T lead  ϕ distribution of track density Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

21. 21 Charged Particle multiplicity More tracks are present in UE than MC Tune DW provides good description of other regions Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

22. 22 Scalar p T sum of charged particles Plateau level 10- 15% higher than MC predictions As expected, Towards region is higher than away regions. Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

23. 23 of charged particles Plateau level slightly higher than MC As expected, toward region higher than away region Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

24. 24 ϕ distribution of charged densities Emergence of jet structure as p T requirement of leading tracking increased Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

25. Summary 25  Charged particle momentum down to 100 MeV is being measured.  Measurements are inclusive:  No correction for diffractive events (~20%)  More work on tuning and diffractive models needed to fully describe the data  UE measurement provides valuable input to models:  Transverse region is more active and energetic than expected  Measured lies above the MC expectations.  Formation of jets structure is different from MC predictions. 7 TeV: dn/d η (@ η = 0) = 5.635 ± 0.002(stat.) ± 0.149(syst.) 0.9 TeV: dn/d η (@ η = 0) = 3.486 ± 0.008(stat.) ± 0.077(syst.) ATLAS-COM-PHYS-2010-416 ATLAS-COM-PHYS-2010-390 ATLAS-COM-PHYS-2010-416 ATLAS-COM-PHYS-2010-390 Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

26. 26 Back up slides Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

27. N ch Unfolding 27  Primary particle spectrum is redistributed due to tracking inefficiency (20-30%)  n sel ≥2 spectrum unfolded to hadron level using migration matrix from MC  Iterative Bayesian unfolding  Correct for events that migrated to n sel =0,1 that are not measured Systematic from tracking efficiency uncertainty Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

28. MC Closure Test 28 Closure test works within 1%  Similar method as for Nch  Larger off-diagonal elements  Cannot be more clever than the MC  Limitation is that tails need to be well modeled by the MC  Systematic from input p T spectrum in matrix  Set to flat in MC for 1 st iteration  Converges after 7 (vs 3 for default) iterations Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

29. MC Closure Test ….. 29 Closure test works within 1% Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

30. 30 Offline Selection Track Cuts  Quality  Inside-out + low p T tracking algorithms  B-layer hit if expected  ≥ 1 pixel hit  ≥ 2:4:6 SCT hits  For: p T ≥100:200:300 MeV  |d 0 | vtx ≤ 1.5mm  |Z 0 sin θ | vtx ≤ 1.5mm  Χ 2 prob > 0.01  for p T > 10 GeV  Phase-space  | η | ≤ 2.5  p T ≥ 100 MeV Event Cuts  GRL selection  First 7 runs @ 7 TeV  L ≈ 168*1.13 ≈ 190 μ b -1  All 2009 data @ 900 GeV  As in paper  Exclude LB during lumi scans  Pass L1_MBTS_1 trigger  1 primary vertex  BS constrained  requires ≥2 tracks  Pile-up veto  No second vertex with ≥4 tracks  ≥ 2 good tracks 7 TeV: 10M events, 209M tracks 0.9 TeV: 350k events, 4.5M tracks 7 TeV: 10M events, 209M tracks 0.9 TeV: 350k events, 4.5M tracks Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

31. 31  Motivation  Detector status and data taking  Minimum Bias observables measured at ATLAS  Underlying observables measured at ATLAS  Summary and Conclusions Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS Outline

32. Badly Measured tracks 32 Non-Gaussian tails in resolution very hard to model in MC Even in MC: significant fraction of observed high p T tracks from low p T particles. Fake reduced from 30% to 6% in 30 to 50 GeV but higher p T tracks (> 50 GeV) is not trustable => not included in the analysis. Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS Before χ 2 cut

33. Material Interaction Map in MC 33 Pixel services (PP0) Pixel services (PP0) 1m Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

34. Beam Background & Pile-up 34 Beam background before selection Beam background after selection  Less than 0.1% event after final selection  Pile-up effect:  Remove events with 2 nd vertex with ≥ 4 tracks  Removes 0.2% of events  ~8% pile-up not resolved  <0.03%, except event with highest multiplicity :~1%. Δ Z<3mm, two vertices as merged to one Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

35. Non-Primaries 35  At low p T  Non-primaries from photon conversions to electrons dominate at high |d 0 |  Other types of secondaries dominate at low |d 0 |  Need to apply b-layer hit cut in order to get enough sensitivity in tails to fit both contributions  Cross-check all results with fit to z 0 distribution Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS

36. Correction procedure 36  The correction for event level distributions (N Ch ) is given by  The correction for track level distributions (P T, η ) is given by:  Iterative Bayesian unfolding method applied to both number of particles (n ch ) and p T.  Correct for events with ≥ 2 particles but ≤ 2 tracks:  vs n ch : bin by bin correction for average p T then n ch migration. f sec (p T ): Fraction of secondaries, f okr (p T, η ): Fraction of track for which the corresponding primary particles are outside the kinematic range, originate from resolution effects Underlying Event studies & Charged particle multiplicities in inelastic pp events with the ATLAS