Ze+e- + Jets with CMS at the LHC Christos Lazaridis University of Wisconsin-Madison Preliminary Examination December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Outline The Standard Model Large Hadron Collider Compact Muon Solenoid Z + Jets Previous results Monte Carlo studies Summary/Future Plans December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination The Standard Model Quarks u and d quark make nucleons in atom Leptons electrons complete the atoms Force Carriers γ: electromagnetism W/Z : weak interaction gluons: strong interaction Higgs boson gives mass to particles Not discovered yet Particle Interactions: In the SM, the elementary particles that make up matter are a set of fermions that interact primarily through the exchange of vector bosons. The elementary fermions include the leptons and the quarks whose bound states form particles (mesons and baryons) via strong interactions mediated by gluons. December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Z + Jets Verification of the Standard Model at high Q2 A QCD regime never probed before! Events at scale where pQCD expected to be valid: Z mass scale Due to high cross section, useful as a high-statistics detector calibration tool Z + Jets are irreducible backgrounds for interesting SM processes (e.g. top production) for new physics searches (e.g. Higgs) The colored partons produced through the hard scatter form colorless hadrons through fragmentation --> collimated spray or real particles (Jets) December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination New physics in Z+Jets Resonances in the ZZ spectrum Higgs Z’ : Final state jets : Final state e+/e- December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Large Hadron Collider p-p collider 14 TeV total collision energy 27 km circumference protons travel around circumference in 90 μsec Bunches cross every 25 ns 8 μm bunch transverse radius December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Magnets Superconducting NbTi magnets Operating @1.9K 1232 dipoles bend proton beam around ring, B = 8T Quadrupoles focus beam in transverse plane Arrows show direction of magnetic field December 18, 2007 Christos Lazaridis Preliminary Examination

Compact Muon Solenoid Design field: 4 Tesla diameter: 6 m length: 12.5 m December 18, 2007 Christos Lazaridis Preliminary Examination

Compact Muon Solenoid Built ← Surface assembly hall Building of CMS is completed underground ↓ Solenoid Endcap Discs: Designed, assembled & installed by Wisconsin December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Tracker Tracker coverage extends to || < 2.5 Silicon pixel detectors closest to interaction region Silicon strip detectors in barrel and endcaps cover an area of 210m2 Resolution: Strong magnetic field Very good tracker granularity Pixels: Lower Occupancy (4 &7 cm – 7 & 11 cm – cell size 150 x 150 μmx) SiStrips: Faster readout, easier to track with Resolution gradually degrading to (60Pt +- 0.5)% as eta 2.5 210 m^2 silicon 66M channels SiPixels, 2.8M for SiStrip Pixels provide 2-3 hits/track, precision ~10μm in transverse and 15μm in z. SiStrips provide up to 14 hits per track 10 – 60 μm in transverse Some strips are double sided to provide similar accuracy in longitudinal measurements Material thickness 0.35X0 central 1.4X0 at barrel/endcap transition drops to .8X0 at |eta| = 2.5 Radiation ength It is both the mean distance over which a high-energy electron loses all but 1/e of its energy by bremsstrahlung, and 7/9 of the mean free path for pair production by a high-energy photon. It is also the appropriate scale length for describing high-energy electromagnetic cascades. December 18, 2007 Christos Lazaridis Preliminary Examination

Electromagnetic Calorimeter Measures e/ energy and position to ||<3 ~76,000 lead tungstate crystals High density (8.2 gr/cm3) Short radiation length (9 mm) Small Moliere radius (22 mm) Resolution: 36 supermodules / 1700 crystals each Lead tungstate density 23 cm long 26 X0 Small moliere radius matches front face • The electronic noise and pileup energy • The shower containment and photostatictics • The constant term (detector non-uniformity, calibration uncertainty) The scintillation light from the crystals must be captured by a photodetector, amplified and digitized. December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Hadronic Calorimeter HCAL samples showers to measure their energy/position Barrel/Endcap region (|| < 3) Brass/scintillator layers Resolution: Forward region (3 < || < 5) Steel plates/quartz fibers Resolution: Stochastic term/constant term December 18, 2007 Christos Lazaridis Preliminary Examination

Detecting electrons and jets Leave a track Deposit energy in the ECAL Jets Leave tracks Deposit energy in the ECAL/HCAL A collimated spray of high energy hadrons December 18, 2007 Christos Lazaridis Preliminary Examination

Proton-proton collisions at LHC Proton-proton 2838 bunches/beam Crossing Rate 40 MHz Protons/bunch 1011 Bunch RT 8 μm Luminosity 1034 cm-2s-1 Startup: 1030 – 1032 cm-2s-1 Interaction Rate: f : revolution frequency n : number of bunches/beam in the storage ring. N : number of particles/bunch A : cross section of the beam December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination CMS Trigger 40 MHz crossing frequency Interaction rate ~1 GHz Selects interesting events Level 1 Trigger: Hardwired processors High Level Trigger: Farm of processors L1 Trigger Requirements: Output: 100 kHz (50 kHz for initial running) Latency: 3 μsec Data collection, decision, propagation HLT designed to output 100 Hz Trigger Rejection ~4x105 December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Level 1 Trigger Information from Calorimeters and Muon detectors Electron/photon triggers Jet/MET triggers Muon triggers Highly complex Trigger primitives: ~5000 electronics boards Regional/Global: 45 crates, 630 boards Flexibility Most algorithms implemented in reprogrammable FPGAs *** 7 microsec to readout tracker Readout + processing: < 1 microsec Signal collection & distribution: ≈ 2 microsec *** The 18 crates of the RCT: Receive input from the CMS calorimeters Combined with the GCT outputs to the GT the 4 highest isolated/non-isolated electrons, jets, τ-jets, missing ET and total ET per event December 18, 2007 Christos Lazaridis Preliminary Examination

Calorimeter Trigger Towers   φ η CRACK! Eta: preferred over polar angle θ  particle production constant as a function of eta December 18, 2007 Christos Lazaridis Preliminary Examination

Electron Trigger Algorithms Electron (Hit Tower + Max) 2-tower ET + Hit tower H/E Hit tower 2x5-crystal strips >90% ET in 5x5 (Fine Grain)‏ Isolated Electron (3x3 Tower)‏ Quiet neighbors: all towers pass Fine Grain & H/E One group of 5 EM ET < Threshold December 18, 2007 Christos Lazaridis Preliminary Examination

RCT Trigger Supervisor An online framework to configure, test, operate and monitor the CMS Trigger Each subsystem is represented by a “cell” Cells communicate via XML- formatted commands My contribution so far: A control panel for the RCT that permits Specific Crate/Card selection Command execution Crate Selection Card Selection Command Selection December 18, 2007 Christos Lazaridis Preliminary Examination

Z+Jets Characteristics Total Z production cross section: 56 nb Z → e+e- cross section: 1.5 nb Total cross section x Branching Ratio ( ~ 3%) 105 events/100pb-1 at LHC startup luminosity (1032 cm-2s-1) Looking for 2 electrons N jets, N = 0...5 Within detector coverage Reconstructing Z Invariant Mass Z PT December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination CDF Z+4 Jet events CDF has found two Z+4 Jet events in 1.7 fb-1 of p-pbar collisions at 1.96 TeV CDF Public Note 8827 December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination CDF Z→e+e- peak Requirements: Two electrons with ET > 25 GeV At least one central electron: |ηe|<1 Second electron central or forward |ηe|< 1 or 1.2 < |ηe|< 2.8 Z mass window: 66 < Mee < 116 GeV/c2 ΔR (e, jet) > 0.7 Jets found using a cone algorithm with ΔR = 0.7 pT,jet > 30 GeV/c and |yjet| < 2.1 Mee Invariant Mass Events with at least one jet CDF Public Note 8827 December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Z+Jets Cross Section MCFM: Monte Carlo for FeMtobarn processes Z + Jets Inclusive Cross Section CMS will have large data samples at high jet pT. Will probe proton PDF and investigate scale choices Data/Theory Ratio MCFM: designed to calculate cross-sections for various femtobarn-level processes at hadron-hadron colliders Shaded bands show systematic uncertainties Dashed lines: PDF uncertainties Dash-dot lines: scale uncertainties CDF Public Note 8827 December 18, 2007 Christos Lazaridis Preliminary Examination

Monte Carlo Studies Dataset used: Z+N jets, N = 0…5 Generated with: ALPGEN 2.12 PYTHIA 6.409 Detector simulation with GEANT4 8.2.p01 Detector electronics simulation and event reconstruction and physics objects provided by CMS framework Matrix element generator: multi-parton processes in hadronic collisions General purpose MC generator: for hadronization and showering Simulation of the passage of particles through matter December 18, 2007 Christos Lazaridis Preliminary Examination

Selecting Ze+e- events # Jets σ (pb) Ze+e- 1.5x103 70K 1 3.2x102 15K 2 9.9x101 5K 3 6.9x101 3.5K 4 1.8x101 0.9K 5 9.3x100 0.4K Events to be produced with 100pb-1 of data Cross sections as computed by Alpgen December 18, 2007 Christos Lazaridis Preliminary Examination

Signal Identification Strategy We will use the following cuts: Starting with: 105 Z→e+e- events (with 100 pb-1) Requirement Reason 2 Electrons ET > 15 GeV, |η| < 2.4 Above trigger threshold: Efficient for W and Z events N Jets; N = 0...5, ET > 15 GeV, |η| < 2.4 Jet ET higher than QCD 80 GeV < Mee < 100 GeV Mass of the e+e- pair close to Z mass December 18, 2007 Christos Lazaridis Preliminary Examination

Offline Electron Reconstruction Create “super-clusters” from clusters of energy deposits using Level-1 E/M calorimeter information In area specified by Level-1 trigger ET > threshold Match super-clusters to hits in pixel detector Electrons leave a hit (track) Photons do not Combine with full tracking information Track seeded with pixel hit Final cuts made to isolate electrons ET γ e- Tracker Strips • ET/pT cut Pixels pT December 18, 2007 Christos Lazaridis Preliminary Examination

Trigger on Electrons for Z + 0-5 jets Automatic L1 trigger for electrons above 63 GeV Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets x103 max value of 63 GeV (7 bits) # electrons/100 pb-1 Isolated Electron Et December 18, 2007 Christos Lazaridis Preliminary Examination

Highest Et Reconstructed Electrons Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets x103 x103 # electrons/100 pb-1 # electrons/100 pb-1 electron Electrons with: ET > 15 GeV -2.4 < η < 2.4 electron December 18, 2007 Christos Lazaridis Preliminary Examination

Highest Et Electrons η/φ Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets x103 η1st Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets x103 # electrons/100 pb-1 CRACK! φ1st # electrons/100 pb-1 Electron η Electron φ December 18, 2007 Christos Lazaridis Preliminary Examination

ΔR Matching to Generated Electrons Comparing the ΔR separation of the electrons generated and reconstructed # electrons No match for ΔR > 0.03 December 18, 2007 Christos Lazaridis Preliminary Examination

Electron Finding Efficiency Events where both electrons are fully reconstructed Entries: Overflows: 141530 135 Matched MCe 185 All MCe December 18, 2007 Christos Lazaridis Preliminary Examination

Z Invariant Mass Reconstructed Z Invariant Mass x103 With 100 pb-1 of data we should see ~100 Z + 5 Jets events! # evts/100 pb-1 2 isolated electrons with pt > 25 GeV -2.4 < η < 2.4 Z+0 Jets fit: Range 80-100 GeV Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets Z+4 Jets Z+5 Jets Mee December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Reconstructed pt(Z) x103 Measure differential cross section of Z production Compare with MC predictions # evts/100 pb-1 Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets December 18, 2007 Christos Lazaridis Preliminary Examination

Offline Jet Reconstruction Iterative cone Draw a cone ΔR = 0.5 around a seed with Et > threshold The computed direction seeds a new cone Iterate until the cone position is stable Stable cone ≡ a jet; towers are removed from the list of input objects no jet merging R December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Highest Et Jets Iterative Cone Rcone = 0.5 Pt > 15 GeV -2.4 < η < 2.4 Some jets (e.g. in Z+0 jets events) are underlying soft QCD radiation Second Jet Highest Jet x102 x102 # jets/100 pb-1 # jets/100 pb-1 Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets It's Z+0 partons according to Alpgen, but eventually we get a soft jet December 18, 2007 Christos Lazaridis Preliminary Examination

Signal Identification Starting with: 105 Z→e+e- events (with 100 pb-1) Requirement Reason Events 2 Electrons ET > 15 GeV, |η| < 2.4 Above trigger threshold: Efficient for W and Z events 0.6 x 105 events (60 % of initial) N Jets; N = 0...5, ET > 15 GeV, |η| < 2.4 Jet ET higher than QCD 4.8 x 104 events (48 % of initial) 80 GeV < Mee < 100 GeV Mass of the e+e- pair close to Z mass 4 x 104 events (40 % of initial) December 18, 2007 Christos Lazaridis Preliminary Examination

Preliminary Examination Summary/Future Plans Z + Jets is a high cross section channel 105 events/100pb-1 (LHC startup luminosity) Measure Z + Jets total and differential cross sections Can be used for detector calibration Clear Z→e+e- peak Plans: Work on the RCT, key component in electrons triggering Study/Improve reconstruction of electrons & jets Take data Tune Monte Carlo generators and PDFs Discover new physics! December 18, 2007 Christos Lazaridis Preliminary Examination