Study of Z  e + e - + Jets with CMS at the LHC Christos Lazaridis University of Wisconsin-Madison Preliminary Examination

November 27, 2007Christos Lazaridis Preliminary Examination 2Outline The Standard Model Large Hadron Collider Compact Muon Solenoid Z + Jets –Previous results –Monte Carlo studies Summary/Future Plans

November 27, 2007Christos Lazaridis Preliminary Examination 3 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:

November 27, 2007Christos Lazaridis Preliminary Examination 4 Z + Jets Verification of the Standard Model at high Q 2 – 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)

November 27, 2007Christos Lazaridis Preliminary Examination New physics in Z+Jets

November 27, 2007Christos Lazaridis Preliminary Examination 6 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

November 27, 2007Christos Lazaridis Preliminary Examination 7Magnets Superconducting NbTi magnets – – 1232 dipoles bend proton beam around ring, B = 8T – Quadrupoles focus beam in transverse plane Arrows show direction of magnetic field

November 27, 2007Christos Lazaridis Preliminary Examination 8 Compact Muon Solenoid Design Mass: T Length: 21.5 m Diameter: 15.0 m Solenoid field: 4 Tesla diameter: 6 m length: 12.5 m

November 27, 2007Christos Lazaridis Preliminary Examination 9 Compact Muon Solenoid Built ← Surface assembly hall Building of CMS is completed underground ↓ Endcap Discs: Designed, assembled & installed by Wisconsin Solenoid

November 27, 2007Christos Lazaridis Preliminary Examination 10 Tracker coverage extends to |  | < 2.5 Silicon pixel detectors closest to interaction region Silicon strip detectors in barrel and endcaps – – cover an area of 210m 2 Resolution: δ p T /p T ≈ (15p T (TeV)  0.5)% – – Strong magnetic field – – Very good tracker granularity Tracker

November 27, 2007Christos Lazaridis Preliminary Examination Electromagnetic Calorimeter Measures e/  energy and position to |  | < 3 ~76,000 lead tungstate crystals Short radiation length Small Moliere radius Resolution:

November 27, 2007Christos Lazaridis Preliminary Examination 12 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 fibersResolution:

November 27, 2007Christos Lazaridis Preliminary Examination 13 Detecting electrons and jets Electrons – Leave a track – Deposit energy in the ECAL Jets – Leave tracks – Deposit energy in the ECAL/HCAL A collimated spray of high energy hadrons

November 27, 2007Christos Lazaridis Preliminary Examination 14 Proton-proton collisions at LHC Startup: – cm -2 s -1 Luminosity L = Particle Flux/Time Interaction Rate:

November 27, 2007Christos Lazaridis Preliminary Examination 15 CMS Trigger 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 ~4x MHz crossing frequency Interaction rate ~1 GHz

November 27, 2007Christos Lazaridis Preliminary Examination 16 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 *** 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 E T and total E T per event ***

November 27, 2007Christos Lazaridis Preliminary Examination 17    φ η Calorimeter Trigger Towers CRACK!

November 27, 2007Christos Lazaridis Preliminary Examination 18 Electron Trigger Algorithms

November 27, 2007Christos Lazaridis Preliminary Examination 19 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 Command Selection Card Selection Crate Selection

November 27, 2007Christos Lazaridis Preliminary Examination 20 Total Z production cross section: 56 nb – –Z → e + e - cross section: 1.5 nb Total cross section x Branching Ratio ( ~ 3%) – –10 5 events/100pb -1 at LHC startup luminosity (10 32 cm -2 s -1 ) Looking for – –2 electrons – –N jets, N = – –Within detector coverage Reconstructing – –Z Invariant Mass – –Z P T Z+Jets Characteristics

November 27, 2007Christos Lazaridis Preliminary Examination 21 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

November 27, 2007Christos Lazaridis Preliminary Examination 22 CDF Z → e + e - peak Requirements: – Two electrons with E T > 25 GeV – At least one central electron: | η e |<1 – Second electron central or forward | η e |< 1 or 1.2 < | η e |< 2.8 – Z mass window: 66 < M ee < 116 GeV/c 2 – ΔR (e, jet) > 0.7 Jets found using the MidPoint algorithm with R = 0.7 – p T,jet > 30 GeV/c and |y jet | < 2.1 M ee Invariant Mass Events with at least one jet CDF Public Note 8827

November 27, 2007Christos Lazaridis Preliminary Examination 23 Z+Jets Cross Section MCFM: Monte Carlo for FeMtobarn processes Data/Theory Ratio Z + Jets Inclusive Cross Section CDF Public Note 8827 Shaded bands show systematic uncertainties Dashed lines: PDF uncertainties Dash-dot lines: scale uncertainties CMS will have large data samples at high jet p T. Will probe proton PDF and investigate scale choices

November 27, 2007Christos Lazaridis Preliminary Examination 24 Monte Carlo Studies Dataset used: – Z+N jets, N = 0…5 – Generated with: ALPGEN 2.12 PYTHIA 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

November 27, 2007Christos Lazaridis Preliminary Examination Selecting Z  e + e - events Events to be produced with 100pb -1 of data Cross sections as computed by Alpgen # JetsCS (pb) Ze+e-Ze+e- 01.5x K 13.2x K 29.9x10 1 5K 36.9x x x

November 27, 2007Christos Lazaridis Preliminary Examination Signal Identification Starting with: 10 5 Z→e + e - events (with 100 pb -1 ) CutReason 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 < M T,ee < 100 Mass of the e + e - pair close to Z mass We will use the following cuts:

November 27, 2007Christos Lazaridis Preliminary Examination 27 Offline Electron Reconstruction Pixels Tracker Strips ETET pTpT E T /p T cut Create “super-clusters” from clusters of energy deposits using Level-1 E/M calorimeter information – In area specified by Level-1 trigger – E T > 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

November 27, 2007Christos Lazaridis Preliminary Examination 28 Trigger on Electrons for Z jets Automatic L1 trigger for electrons above 63 GeV Isolated Electron E t max value of 63 GeV (7 bits)  # electrons/100 pb -1 Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets x10 3

November 27, 2007Christos Lazaridis Preliminary Examination 29 Highest E t Reconstructed Electrons Electrons with: E T > 15 GeV E T > 15 GeV -2.4 < η < < η < 2.4 Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets # electrons/100 pb -1 # electrons/10 pb -1 electron x10 3

November 27, 2007Christos Lazaridis Preliminary Examination 30 Highest E t Electrons η / φ η 1st φ 1st Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets # electrons/100 pb -1 Electron φ Electron η # electrons/100 pb -1 Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets CRACK! x10 3

November 27, 2007Christos Lazaridis Preliminary Examination 31 ΔR Matching to Generated Electrons No match for ΔR > # electrons Comparing the ΔR separation of the electrons generated and reconstructed

November 27, 2007Christos Lazaridis Preliminary Examination 32 Electron Finding Efficiency Entries: Overflows: Matched MC e 185 All MC e Events where both electrons are fully reconstructed

November 27, 2007Christos Lazaridis Preliminary Examination 33 Z Invariant Mass With 100 pb -1 of data we should see ~100 Z + 5 Jets events! 2 isolated electrons with p t > 25 GeV 2 isolated electrons with p t > 25 GeV -2.4 < η < < η < 2.4 Z+0 Jets fit: Range GeV Range GeV Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets Z+4 Jets Z+5 Jets # evts/100 pb -1 M ee x10 3

November 27, 2007Christos Lazaridis Preliminary Examination 34 Reconstructed p t (Z) Measure differential cross section of Z production Measure differential cross section of Z production Compare with MC predictions Compare with MC predictions Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets # evts/100 pb -1 x10 3

November 27, 2007Christos Lazaridis Preliminary Examination 35 Offline Jet Reconstruction Iterative cone – Draw a cone ΔR = 0.5 around a seed with E t > 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

November 27, 2007Christos Lazaridis Preliminary Examination 36 Highest E t Jets Some jets (e.g. in Z+0 jets events) are underlying soft QCD radiation Second Jet Second Jet Iterative Cone Iterative Cone - R cone = 0.5 P t > 15 GeV P t > 15 GeV -2.4 < η < < η < 2.4 Highest Jet Highest Jet Z+0 Jets Z+1 Jets Z+2 Jets Z+3 Jets (x10 2 ) # jets/10 pb -1

November 27, 2007Christos Lazaridis Preliminary Examination Signal Identification Starting with: 10 5 Z→e + e - events (with 100 pb -1 ) CutReasonEvents 2 Electrons Et > 15 GeV, |η| < 2.4 Above trigger threshold: Efficient for W and Z events 0.6 x 10 5 events (60 % of initial) N Jets; N = 0...5, Et > 15 GeV, |η| < 2.4 Jet ET higher than QCD 4.8 x 10 4 events (48 % of initial) 80 < M T,ee < 100 Mass of the e + e - pair close to Z mass 4 x 10 4 events (40 % of initial)

November 27, 2007Christos Lazaridis Preliminary Examination 38 Summary/Future Plans Z + Jets is a high cross section channel – 10 5 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!