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Measurement of the Ratio of Inclusive Cross-Sections

Measurement of the Ratio of Inclusive Cross-Sections. at. 25 February 2005. D issertation presented by Y ı ld ı rım D. Mutaf Suny, Stony Brook. Introduction Introduction to Higgs searches Higgs expectations from Fermilab Motivation for studies on Z+b production

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Measurement of the Ratio of Inclusive Cross-Sections

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  1. Measurement of the Ratio of Inclusive Cross-Sections at 25 February 2005 Dissertation presented byYıldırım D. Mutaf Suny, Stony Brook

  2. Introduction Introduction to Higgs searches Higgs expectations from Fermilab Motivation forstudies on Z+b production Experimental Apparatus Introduction to Fermilab RunII Program and DØ detector Sub-detectors and particle identification methods b-tagging primer Measurement of σ(Z+b) / σ(Z+j) Methodology & jet properties (with MC comparisons) Extraction of the Z+b/Z+j ratio Systematic uncertainties and final result Improvements for Z+bb Studies Muon Isolation Likelihood B-tagging Optimizations Summary Outline Ph.D. Dissertation, 25 Feb. 2005

  3. Introduction

  4. Tevatron Run II program is committed to both SM and non-SM Higgs searches Production cross-sections are very low and analyses need large integrated luminosity SM Higgs searches are pursued in W/Z associated production for low mass Higgs due to cleaner signatures bb decay mode (low mass) Gluon fusion becomes relevant for high mass Higgs because of WW decay mode (high mass) Higgs Searches at Tevatron Ph.D. Dissertation, 25 Feb. 2005

  5. Higgs reach of Tevatron was first estimated rigorously in 1999 Many assumptions are made in this study with simplistic simulations A similar study is performed recently in 2003 Full DØ and CDF detector simulations are used with more realistic reconstruction Instead of parameterizations, used realistic detector performance and resolutions b-tagging efficiencies, di-jet mass resolution etc. Higgs Expectations from Tevatron • The conclusion is that more realistic Higgs searches can exceed earlier expectations • There is even more room to improve • Especially with more sophisticated analyses methods… Ph.D. Dissertation, 25 Feb. 2005

  6. Motivation for Z+b • Run II Luminosity is not yet enough for a Higgs analysis • However, understanding the background processes for the “tiny” Higgs signal is of paramount importance • At DØ, we recently performed a study of the b-jets produced with Z’s • And measured the cross-section ratio for the production of Z+b-jet to Z+inclusive jet processes • This measurement • Adds to our understanding of the major Higgs backgrounds • Also provides a validation of the b-tagging methods at DØ • First step towards the measurement of the Z+bb cross-section • And also provides an indirect probe of the b-quark density inside the proton Ph.D. Dissertation, 25 Feb. 2005

  7. Experimental Apparatus

  8. TeVatron collider complex is upgraded to pursue the challenges of Higgs searches as well as other interesting topics Shorter bunch crossing Need faster readout/electronics Larger CM energy (~10%) Increased production cross section Higher luminosity Need better triggering Need radiation hard detectors TeVatron Upgrade Program Ph.D. Dissertation, 25 Feb. 2005

  9. DØ performance Easy to see the evolution from commissioning to high efficiency data taking TeVatron is delivering steady and high luminosities Available data as of last week is ~ 550 pb-1 Run II Performance Ph.D. Dissertation, 25 Feb. 2005

  10. DØ Run II Experiment • Upgrade of the very successful Run I experiment (1992-96) • top discovery, largest W collection, QCD measurements … • Versatile detector ready for challenging physics tasks • Higgs searches • Direct searches, understanding backgrounds • Precision measurements of MW and Mt • As well as other top & W properties (single top?) • Searches for Physics beyond SM • SUSY, Large Extra Dimensions, Technicolor… Ph.D. Dissertation, 25 Feb. 2005

  11. Calorimetry New electronics/trigger Preshower detectors Central & Forward Muon Detection New forward detectors New electronics & scintillators for trigger Tracking System Entirely new tracker with several components Tracking, vertexing, btagging and triggering DAQ / Trigger Entirely new systems DØ Run II Upgrade Ph.D. Dissertation, 25 Feb. 2005

  12. Calorimeter • Same detector as Run I but improved electronics and trigger system • Liquid Argon with interspersedUranium absorbers • Accommodate 396 ns crossing • Uniform, hermetic calorimeter with fine segmentation and large coverage • |η|<4.2 with 0.1x0.1 ηφ towers • EM, FH and CH layers • Single particle energy resolutions • e: E/E ≈ 15%/√E • e.g. 3.3% @ 20 GeV • : E/E ≈ 45%/√E • e.g. 10.0% @ 20 GeV Ph.D. Dissertation, 25 Feb. 2005

  13. Muon Detectors • Completely new forward muon system • Also added central scintillators • Provides muon detection up to |η|<2.0 • Cosmic ray rejection • Triggering with fast scintillators • Momentum measurement with the solid iron Toroid magnet Ph.D. Dissertation, 25 Feb. 2005

  14. DØ Run II Tracking System • 2 T solenoid magnetic field • Super-conducting • New Silicon Microstrip Tracker (SMT) • Necessary for improved dca resolution • Tracking / vertexing / b-tagging • New Central Fiber Tracker (CFT) • 8 double scintillating fiber layers • Axial & stereo coverage up to |η|<2.4 (all layers) • Incorporated into Level 1 trigger system • Preshower Systems (Forward/Central) • Electron / Photon discrimination • Improve reconstruction via cluster matching • central track and/or calorimeter towers Ph.D. Dissertation, 25 Feb. 2005

  15. ~ 800,000 read-out channels ~ 10 cm ~ 1.2 m Silicon Microstrip Detector (SMT) • Hybrid system (barrel & disk) • Essential piece of detector for track & vertex reconstruction • > 95 % hit efficiency • r-φ hit resolution ~ 10 μm • z hit resolution ~ 40 μm • Enabling b-tagging up to |η|<2.4 Ph.D. Dissertation, 25 Feb. 2005

  16. 8 layers of scintillator fibers 1.8 meter or 2.6 meter long Axial & stereo (3o angle) 77k read-out channels Input to Level-1 track triggers Including CFT Central Fiber Tracker (CFT) Ph.D. Dissertation, 25 Feb. 2005

  17. 3D reconstruction of tracks from the hits in SMT & CFT DØ has several alternative tracking reconstruction methods Tracking in simulation and real detector shows difference 2D (r-φ) 3D (r-φ-z) Hits in transverse plane From Hits to Tracks Ph.D. Dissertation, 25 Feb. 2005

  18. b-tagging methods generally rely on the following properties of B hadrons Semi-leptonic decays Presence of soft muon in the jet Long life-time of B hadrons Reconstruction of the displaced vertex where b decays Order of 1 mm decay length Vertex decay length resolution Track impact parameter distance from the primary vertex where the hard scatter occurs Track IP resolution p b Identification Primer SV IP PV p Ph.D. Dissertation, 25 Feb. 2005

  19. Secondary Vertex Reconstruction Cluster tracks and form 2 track “seed” vertices Add tracks to the seed according to final vertex χ2 Select vertices based on quality Collinearity and χ2of the vertex fit Define three types of vertices (loose, medium, tight) based on Momentum & IP of the vertex tracks Decay length & its significance (wrt PV) Tagging b’s Tag a calorimeter jet if a secondary vertex is matched to the vertex Secondary Vertex Method TIGHT b-tag efficiency Ph.D. Dissertation, 25 Feb. 2005

  20. This method uses the signed impact parameter significance of the tracks (IP/σIP) in the jets Negative IP tracks are mostly associated with PV and due to PV uncertainties Positive IP tracks are from the decays of long lived particles Construct a likelihood for the tracks the probability of originating from Primary Vertex Jet Track PV IP Track Jet IP PV Impact Parameter Method - I Ph.D. Dissertation, 25 Feb. 2005

  21. The probability due to tracks from PV is flat by definition Large & positive IP tracks peak around “zero-probability” Combine the probabilities of the tracks in jets to get a jet probability Cut on jet probability for b-tagging TIGHT b-tag efficiency jet η jet pT Impact Parameter Method - II Ph.D. Dissertation, 25 Feb. 2005

  22. Measurement of σ(Z+b) / σ(Z+j)

  23. Introduction • As we pointed out before, b-quark production in association with Z boson has paramount importance for Higgs searches • Z+b-jet production has recently been investigated at DØ • Besides studying general b-jet production and properties, we performed a measurement of the following ratio: • Being a cross-section ratio, this measurement is in general insensitive to detector inefficiencies and systematic uncertainties • Except the factors that affect b-jets and light jets differently (eg. b-tagging) Ph.D. Dissertation, 25 Feb. 2005

  24. This analysis is performed in two different channels* Dimuon Analysis (Z→μμ) Dielectron Analysis (Z→ee) * Actual dissertation research consists only the dimuon channel Apart from the reconstruction of Z in these channels, the hadronic (jet) parts of the two analyses are identical Both analyses are based on data collected Aug’02-Sep’03 correspond to ~180 pb-1 luminosity however, the actual luminosity figure is irrelevant to this measurement Methodology A schematic view of the Z+jet signature shown for dimuon decay of Z Ph.D. Dissertation, 25 Feb. 2005

  25. Pμ PTrel Pjet Dimuon Analysis (Z→μμ) • Muon Reconstruction at DØ • Uses hits in the muon drift detectors and scintillators • Muon detector track segments are reconstructed from the hit information • Three muon criteria are used consistent with the quality of muon hits • Tight, Medium & Loose • Scintillator timing cuts are used to reject the cosmic background • Optional central track matching • Muon & Z(μμ) Selection (specific to this analysis) • Event must have triggered any MUON trigger (single muon, dimuon etc…) • 2 Loose muons with both muons matched to reconstructed central tracks • isolated i.e. pTrel with respect to closest jet must be larger than 10 GeV • |ημ| < 2.0 and pTμ> 15 GeV • 65.0 < Mμ1μ2< 115.0 GeV Ph.D. Dissertation, 25 Feb. 2005

  26. Dimuon Analysis (Z→μμ) Z→μμ candidate with an additional jet underlying event • Shown above is the invariant mass of all selected dimuon candidates • In this dataset, we found about 11500 inclusive Z(μμ) candidate events within the [65-115] GeV mass range Ph.D. Dissertation, 25 Feb. 2005

  27. Dielectron Analysis (Z→ee) • Electron Reconstruction at DØ • Clusters of energy (simple cone algorithm) in EM layers of the calorimeter • Energy deposited in EM layers should be > 90 % of the total energy inside cone • Shower shape consistent with electrons (not π0/γ) • χ2 from H-Matrix using discriminating variables like shower width, total energy, fraction at EM etc… • Optional central track matching • Electron & Z(ee) Selection (specific to this analysis) • Event must have triggered one of the two 2EM triggers • 2 EM clusters • isolated i.e. must be less than 20% • |ηe| < 2.5 and pTe > 15 GeV • At least one of the electrons must have a reconstructed central track • 80.0 < Me1e2< 100.0 GeV Ph.D. Dissertation, 25 Feb. 2005

  28. Shown above is the invariant mass of all selected dielectron candidates In this dataset, we found about 15600 inclusive Z(ee) candidate events within the [80-100] GeV mass range Dielectron Analysis (Z→ee) Z→ee candidate with 2 jets (1 b-tagged) Ph.D. Dissertation, 25 Feb. 2005

  29. Estimation of Background • Major backgrounds to Z(ee/μμ)+jet signature • Drell/Yan continuum for dilepton production • Multi-jet background (mostly important for tagged sample) • Dimuon Analysis • Background is evaluated for the two contributions • Multi-jet background is estimated from the isolation criteria • Dielectron Analysis • Background is estimated as a convolution of these two processes • Fitting of the side-band mass distributions Ph.D. Dissertation, 25 Feb. 2005

  30. Jet reconstruction and selection are identical in both the dimuon and dielectron analysis Jet reconstruction at DØ Clustering of the calorimeter towers starting with high energy seeds (jet Emin = 8 GeV) ΔR=0.5 standard cone algorithm jets are used in this analysis Jets are passed through several quality criteria Remove energy depositions in hadronic layers due to electron, photons etc. Remove hot calorimeter towers… Jets are applied Energy Scale (JES) corrections to account for Calorimeter response and baseline subtraction (noise, pile-up etc.) Out-of-cone showering, muonic decay energy compensation ... Jets - I Ph.D. Dissertation, 25 Feb. 2005

  31. Jets - II • The kinematic cuts for jets in this analysis • |ηjet| < 2.5 and pTjet > 20 GeV • For b-tagging, we require the following for calorimeter jets • In order to be “taggable”, the jets must be matched to clusters of tracks made of at least 2 tracks (more track cuts are introduced) • In this analysis, we use the Secondary Vertex (SV) tagger • Use TIGHT reconstructed SV’s with decay length significance > 7 • Match taggable jets to SV’s if ΔRSV-JET<0.5 Ph.D. Dissertation, 25 Feb. 2005

  32. Jets - III • The number of inclusive Z+jet events in both channels are shown above… • Shown on right is the jet multiplicities for dimuon channel alone • Points are data with statistical error bars • Error boxes represent the uncertainty due to JES Ph.D. Dissertation, 25 Feb. 2005

  33. Taggable jet kinematics distributions observed in data are compared to expectations from Z+j ALPGEN MC Differences of jet reconstruction in MC & Data are accounted for… Expectations are overall in good agreement with jets observed in data Jets in Z+j samples are a composition of different flavors Heavy jets : b/c Light jets : u/d/s/g Need to decompose the flavors to extract Z+b component Taggable Jet Kinematics Ph.D. Dissertation, 25 Feb. 2005

  34. b-tagged Jets • After b-tagging, we are left with 49 Z+b-tagged jet events • Combined dielectron (27) & dimuon (22) • Not all of these events are “truly” b-quark events due to fake contamination from the b-tagging method • εb ≈ 33.13 % , εc ≈ 8.42 % , εl ≈ 0.24 % (none zero) (μμ) εb(pT,η) εl(pT,η) Ph.D. Dissertation, 25 Feb. 2005

  35. Secondary Vertices (only from μμ) X 2 = 42.89 without b-content X 2= 30.89 with added b-content X 2 = 34.59 without b-content X 2= 1.39 with added b-content Ph.D. Dissertation, 25 Feb. 2005

  36. Cross-Check of SV b-tagging • Decay length distribution comparison with expectation confirms that most these vertices are mostly from the decay of heavy hadrons • However, we performed a cross-check of the analysis by using different b-tagging methods • Soft Muon Tagging – Z(ee) • Rather independent cross-check of SV tagger (no reliance on track IP) • Measure tag efficiencies from MC and correct for muon and track reconstruction efficiencies in data • Mistag measured from jets in data • Estimate that 10.3 ± 1.3 events should be tagged • Consistent with 12 actually tagged • Impact Parameter Tagging – Z(μμ) • Measure correlation between IP and SV b-tagging algorithms in dijet data events for actual b-jets • Use MC for charm and light jets • Out of 22 SV tagged events, expect 14.3 ± 3.4 events tagged by IP • Consistent with 14 tagged events Ph.D. Dissertation, 25 Feb. 2005

  37. Z+jet Zb Zc Zq Bckg. Taggability Z+taggable-jet Zb Zc Zq Bckg. Analysis b-tagging Z+bjet Zb Zc Zq B Extraction of Jet Flavors & Ratio • # Z + taggable-jet events is 1658 (ee) & 1406 (μμ) • # Z + b-jet events is 27 (ee) & 22 (μμ) • These numbers are not pure (composition is a mixture of b/c/l) Ph.D. Dissertation, 25 Feb. 2005

  38. Extraction of Jet Flavors & Ratio • Basically solve for the following equation to get the correct mixture of jet flavors… • Two equations, three unknowns (i.e. NB, NC, NL) • Introduce another equation by fixing NC/NB to most recent NLO calculations* * J. Campbell, K. Ellis, F. Maltoni, S. Willenbrock, Phys. Rev. D69 (2004) 074021 Ph.D. Dissertation, 25 Feb. 2005

  39. Measuring Jet Efficiencies - I • Taggability efficiency is measured in data (light-jet) to be 78.6 % • For heavy flavor jet taggability, we scale this with a correction factor obtained from MC  80.7 % • Jet reconstruction efficiencies are assumed to affect heavy and light jets in similar ways • Assign a 2 % systematic uncertainty to this assumption Ph.D. Dissertation, 25 Feb. 2005

  40. Measuring Jet Efficiencies - II • The efficiencies are corrected for the inclusiveness of the events i.e. jet multiplicities • If you require one or more jets, the efficiency of such a selection will basically be different for events having 1 jet, 2 jets, 3 jets … • Only apply this for light jets since all of the jets can be considered to be of the same flavor • However for heavy jets, since we don’t know the exact flavor of each jet in the event, we don’t make the correction in this way • Consult theory and take the ratio of Z+QQ in inclusive Z+Q events and correct the heavy-jet efficiencies for this factor (fbb = 10.8 %, fcc = 6.7 %) Ph.D. Dissertation, 25 Feb. 2005

  41. Extraction of Jet Flavors & Ratio • The ratio of cross-sections is defined as: • The number of events, Ni are added statistically from both channels • Our result is comparable to the NLO prediction of 0.018 ± 0.004* μμ ee * J. Campbell, K. Ellis, F. Maltoni, S. Willenbrock, Phys. Rev. D69 (2004) 074021 Ph.D. Dissertation, 25 Feb. 2005

  42. Systematic Uncertainties • Measurement of Efficiencies • b/c-tag efficiency 2.4 % • The errors come from the measurement of the b-tagging efficiencies in data, errors related to fitting results etc • Mistag efficiency 5.8 % • Due to the different results obtained from different selection of jets in data • Taggability efficiency 0.5 % • The uncertainty factor from the fluctuations observed for the heavy-jet taggability correction factor obtained from MC • Jet Reconstruction efficiency 2.1 % • The uncertainty associated with the assumption that the jet reconstruction efficiencies are similar for light and heavy jets whereas observed as different in MC Ph.D. Dissertation, 25 Feb. 2005

  43. Systematic Uncertainties (more) • Other major uncertainties • Background Estimation 8.6 % • Uncertainty due to estimation of the multi-jet background in Z+jet both before and after b-tagging (fit errors, assumptions etc.) • Z+Q or Z+(QQ) 5.4 % • Uncertainty due to merging of two b-quarks from gluon splitting into a single jet cone • Measure the increases in tag efficiencies for such cases from MC • Obtain the relative fractions of Z+Q vs Z+(QQ) final states from theory Ph.D. Dissertation, 25 Feb. 2005

  44. Systematic Uncertainties (more) • Other major uncertainties • Jet Energy Scale 6.5 % • Uncertainty due to the correction of jet energies for detector resolution etc. • Theoretical Input 2.7 % • Uncertainty due to fixing the NC to NB ratio to NLO calculations • ZQQ Correction Factor Uncertainty 1.6 % • Uncertainty due to correcting the heavy jet efficiencies for QQ contributions Ph.D. Dissertation, 25 Feb. 2005

  45. Final Result • The overall effect of all the systematic uncertainties is ~ 12 % • The final result with the full systematic uncertainty • Again, theoretical prediction for this ratio is 0.018 ± 0.004 • Our measurement is consistent with the theoretical NLO predictions Ph.D. Dissertation, 25 Feb. 2005

  46. Discussion • This result is the first Z+b measurement at hadron colliders and carry big importance for future studies • We confirmed and demonstrated the b-tagging capabilities at DØ • Z+b production (both jet kinematics and the cross-section ratio) is consistent with the theoretical predictions • Increasing confidence for Higgs background studies • Study of Z+b production also a good starting step for similar signature Higgs and other background studies • The ratio we measured has the largest contribution from the b-quark in the proton sea • It’s an indirect experimental constraint on the b-quark PDF which is crucial for • Single top production • hb(b) higgstrahlung process Ph.D. Dissertation, 25 Feb. 2005

  47. Improvements for Z+bb Studies

  48. Introduction • DØ (TeVatron) is committed to Higgs searches • Even though the challenges and time pressure are hard to beat • The initial measurements related to Higgs is produced for the similar signature backgrounds • Like Z+bb, W+bb … • Current data-set is about 3 times larger than the data used for the ratio analysis • Yet, we still would have 3 to 4 Z+bb events with similar analysis strategy • These low yields require more sophisticated analysis methods Ph.D. Dissertation, 25 Feb. 2005

  49. Introduction II • At DØ, we’ve worked out a preliminary strategy to increase the yield for Z+bb • Employing simple multivariate techniques, the sensitivity to Z+bb signal is increased by a large amount • We studied the following optimizations: • Muon Isolation • Instrumental for multijet vs Z discrimination • B-tagging operating point optimization • Finding the optimum combination of Signal (b-jet) and Background (mistag) • Optimization for two b-jet system • Further b-tagging optimization specific to two b-jet system Ph.D. Dissertation, 25 Feb. 2005

  50. Muon Isolation • For muon isolation, several variables are investigated and their performances are compared in “signal” and “background” events • Momentum scaling of the variables enhances their discrimination power • Combine the most powerful variables in a single isolation discriminant • Using the background distribution of the isolation discriminant, we create an isolation likelihood Ph.D. Dissertation, 25 Feb. 2005

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