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VBF H-> in CMS at LHC. Jessica Leonard University of Wisconsin - Madison Preliminary Examination. Outline. Motivation for Higgs The Higgs -> tau tau signal The CMS detector Monte Carlo Event Selection Simulation Results Future plans. Standard model.
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VBF H-> in CMS at LHC Jessica Leonard University of Wisconsin - Madison Preliminary Examination
Outline • Motivation for Higgs • The Higgs -> tau tau signal • The CMS detector • Monte Carlo • Event Selection • Simulation Results • Future plans
Standard model • One particle we haven’t seen yet: Higgs! • Gives mass to W, Z
Higgs Physics • More info on Why We Need the Higgs?? Talk about: Higgs required to give mass to W and Z, also couples with most other particles -- coupling strength determines masses of those particles
General Higgs Production • Gluon-gluon fusion high rate, but high QCD background • Vector boson fusion lower rate, but lower background
Higgs decays • Bb~ most prominent signal below ~100 GeV, tau is second • Tau jets easier to identify than b jets
VBF to di- • H-> • Relatively high rate for low-mass Higgs • Distinct signal • VBF • Relatively high rate • Identification of Higgs production via tagged jets • qqH->: Good potential for discovery!
LHC Startup Stage 1 Initial commissioning 43x43156x156, 3x1010/bunch L=3x1028 - 2x1031 Starts in 2007 Shutdown • Year one (+) operation • Lower intensity/luminosity: • Event pileup • Electron cloud effects • Phase 1 collimators • Equipment restrictions • Partial Beam Dump • 75 ns. bunch spacing (pileup) • Relaxed squeeze Stage 2 75 ns operation 936x936, 3-4x1010/bunch L=1032 - 4x1032 Stage 3 25 ns operation 2808x2808,3-5x1010/bunch L=7x1032 - 2x1033 Long Shutdown Phase 2 collimation Full Beam Dump Scrubbed Full Squeeze Stage 4 25 ns operation Push to nominal per bunch L=1034
Experiments at the LHC • pp s = 14 TeV Ldesign = 1034cm-2 s-1 • Heavy ions (e.g. Pb-Pb ats ~ 1000 TeV) ATLAS and CMS : pp, general purpose 27 Km ring 1232 dipoles B=8.3 T (NbTi at 1.9 K) First Collisions 2007 Physics in 2008
CMS detector (temp slide) • Components with slides already: • ECAL, HCAL, tracker, trigger, muon • Should other components have slides? • magnet, preshower, . . .
CMS Detector CALORIMETERS HCAL ECAL Plastic scintillator/brass sandwich 76k scintillating PbWO4 crystals IRON YOKE MUON ENDCAPS Cathode StripChambers (CSC) Resistive PlateChambers (RPC) TRACKER PixelsSilicon Microstrips 210 m2 of silicon sensors 9.6M channels Superconducting Coil,4 Tesla MUON BARREL Resistive Plate Drift Tube Chambers (RPC) Chambers (DT)
Tracker Tracker coverage extends to ||<2.5, with maximum analyzing power in ||<1.6 Silicon pixel detectors used closest to the interaction region Silicon strip detector used in barrel and endcaps
ECAL • >80,000 PbWO4 crystals • high density • small Moliere radius (2.19 cm) • radiation resistant • Precise measurements of electron/photon energy and position • Each crystal 22mm x 22mm • x = 0.0175 x 0.0175 barrel, increases to 0.05 x 0.05 in endcap • Covers || < 3 • Resolution:
sample • 80,000 PbWO4 crystals • high density • small Moliere radius (2.19 cm) • radiation resistant • Precise measurements of electron/photon energy and position • Each crystal 22mm x 22mm • x = 0.0175 x 0.0175 barrel, increases to 0.05 x 0.05 in endcap • Covers || < 3 • Resolution:
HCAL • HCAL sampling calorimeter (barrel, endcap) • 50 mm copper plates and 4 mm scintillator tiles • Measures energies and positions of central jets • Covers || < 3 • Energy resolution: • HF extends coverage to || = 5 • Steel plates and 300 m quartz fibers - withstand high radiation • Measures energies and positions of forward jets • Resolution:
Muon System • Muon chambers identify muons and provide position information for track matching. • Drift tube chambers max area 4m x 2.5m cover barrel to ||=1.3 • Cathode strip chambers in endcaps use wires and strips to measure r and , respectively. Coverage ||=0.9 to 2.4. • Resistive plate chambers capture avalanche charge on metal strips. Coverage ||<2.1
Produced Observed Jets and Hadronization • Colored partons produced in hard scatter → “Parton level” • Colorless hadrons form through fragmentation → “Hadron level” • Collimated “spray” of real particles → Jets • Particle showers observed as energy deposits in detectors → “Detector level”
Jet algorithm • Info on how we find • Jets and • Tau jets (identification requirements)
Calorimeter Trig. 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 towerspass Fine Grain & H/E • One group of 5 EM ET < Thr. • Jet or t ET • 12x12 trig. tower ET sliding in 4x4 steps w/central 4x4 ET > others • t: isolated narrow energy deposits • Energy spread outside t veto pattern sets veto • Jet tif all 9 4x4 region t vetoes off
Jet Finding: Cone Algorithm • Maximize total ET of hadrons in cone of fixed size • Procedure: • Construct seeds (starting positions for cone) • Move cone around until ET in cone is maximized • Determine the merging of overlapping cones • Issues: • Overlapping cones • Seed , Energy threshold • Infrared unsafe • σ divergesas seed threshold → 0 R
Tau Triggering Require a “narrow” jet in the calorimetry. Require confirmation from the tracking, and isolation around the narrow jet.
Monte Carlos • How do we know all our algorithms actually work? • Simulate the entire event, run it through the actual reconstruction. We know what the “right” answer is, so we can tell how well our reconstruction algorithms work.
Parton Level QCD Cross section Hadron Level Model Fragmentation Model Detector Level Detector simulationbased on GEANT Monte Carlos (MCs) Parton Level Hadron Level Detector Simulation Factorization: Long range interactions below certain scale absorbed into proton’s structure
Event Simulation • PYTHIA used to simulate events at parton-level and hadron-level. • FIND OUT MORE ABOUT DET-LEVEL SIM!
Lund String Fragmentation • Used by MCs (or just “PYTHIA”) to describe hadronization and jet formation. • Color “string" stretched between q and q moving apart • Confinement with linearly increasing potential (1GeV/fm) • String breaks to form 2 color singlet strings, and so on., until only on mass-shell hadrons remain.
decays in detector • Higgs decays isotropically, so signature in general is in central detector (as opposed to forward) • -> W* + , then • W* -> lepton + l OR • W* -> u + dbar e.g., more hadronization possible (single- and triple-prong events) • What do these look like in the detector? • lepton + l : electron (ECAL energy + track) or muon (muon chamber energy + track) + missing energy • hadrons : hadronic jet (HCAL energy + odd number of tracks), energy deposit must be small and contiguous --> tagged as “ jet”
Generate events • ~50,000 H-> events generated; no constraints on decays. Higgs mass set to 130 GeV.
H-> final states and triggers • Note: Here “jet” means energy deposit consistent with • ->jj • L1: single or double (93, 66 GeV) ??? • HLT: double ??? • ->j • L1: single • HLT: single , + jet • ->ej • L1: single isolated e, e + jet • HLT: single isolated e, e + jet
H->->l++single-prong event offline selection • e and candidates identified • Additional electron requirements: • E/p > 0.9 • Tracker isolation • Hottest HCAL tower Et < 2 GeV • Highest-pt lepton candidate with pt > 15 GeV chosen • Lepton track identifies the other tracks of interest: within z = 0.2 cm at vertex
H->->l++single-prong event offline selection (cont.) • candidates identified; jet formed around each and passed through t-tagging requirements • Require -jet charge opposite lepton charge • Hottest HCAL tower Et > 2 GeV if coincides with electron candidate • -jet Et > 30 GeV
H->->l++single-prong event offline selection (cont.) • Jets are the 2 highest-Et jets with Et > 40 GeV, not including e and candidate • Jets must be within || < 4.5, as well as having different signs in h • Require hj1j2 > 4.5, fj1j2 < 2.2, invariant mass Mj1j2 > 1 TeV • Require transverse mass of lepton-MisEt system < 40 GeV
H->->2 1-prong • Backgrounds: ttbar, Drell-Yan Z/*, W+jet, Wt, QCD multi-jet
