Search for the Higgs Boson at DØ in the Trilepton Channel James Kraus University of Mississippi 1 Outline The Standard Model and the Higgs Boson Fermilab Tevatron and the DØ experiment Higgs Production and Higgs Searches at DØ – Motivation for the trilepton Higgs search Search for the Higgs in the trilepton + Missing Transverse Energy (MET) final state Discussion of DØ and Tevatron combinations – Comparison with LHC results 2 Standard Model Describes 3 of the 4 Fundamental Forces – Strong Force • Force carrier is gluon • Binds quarks into protons, neutrons • Holds protons & neutrons in nucleus – Electromagnetic Force • Force carrier is photon • Acts on charged particles • Responsible for electricity, chemical bonds 3 Standard Model Describes 3 of the 4 Fundamental Forces – Strong Force – Electromagnetic Force – Weak Force νe ± 0 137 137 • Force carriers W , Z Cs Ba • Responsible for some radioactive 55 56 decays, nuclear fission e- – Gravity • Planetary motion, falling apples 4 Standard Model Describes 3 of the 4 Fundamental Forces – Strong Force – Electromagnetic Force – Weak Force νe ± 0 137 137 • Force carriers W , Z Cs Ba • Responsible for some radioactive 55 56 decays, nuclear fission e- – Gravity • Planetary motion, falling apples Gravity is not included in 5 Standard Model Standard Model The Standard Model arranges 6 quark flavors and 6 lepton flavors into 3 generations of matter Quarks experience The generations strong, weak, and are arranged by electromagnetic forces increasing mass, but are otherwise Leptons do not similar experience the strong force. Neutrinos only interact via the weak 6 force Electroweak Theory Salam, Glashow, & Weinberg develop Electroweak theory in 1967 (Nobel prize in 1979) – Describes EM and Weak forces with a single theory – Predicted existence of massive, neutral Z boson Electroweak symmetry must be broken to give us the observed electromagnetic and weak forces With electroweak symmetry unbroken, particles are massless... 7 A Massive Problem u d s c b W Boson Z Boson 80 GeV 91 GeV (Photon & Gluon = Massless) Top Quark e e 173 GeV 1 GeV ≈ mass of the proton 8 A Massive Solution 2010 Sakurai Prize ... for "elucidation of the properties of spontaneous symmetry breaking in four-dimensional relativistic gauge theory and of the mechanism for the consistent generation of vector boson masses." Englert Brout Higgs Guralnik Hagen Kibble PRL 13, 321-323 PRL 13 508-509 PRL 13, 585-587 9 (1964) (1964) (1964) The Higgs Boson in the Standard Model Higgs mechanism responsible for electroweak symmetry breaking Interactions with Higgs field gives particles their masses – No prediction of the strength of this interaction The Higgs is the last undiscovered particle predicted by the Standard Model – Standard Model does not predict the mass of the Higgs 10 Fermilab Fermilab is a national lab located in Batavia, IL, outside Chicago − Is home to US-based high energy physics experiments, including Tevatron To Chicago 11 The Fermilab Tevatron DØ Run II of Fermilab Tevatron March 2001 to Sept. 2011 − p-p collisions with energy of 1.96 TeV = 1960 mp One collision every 396 ns − >1.7 million collision/min Technology − Superconducting magnets Size − CDF 4 mile circumference 12 Fermilab Site Home to a healthy herd of bison 13 Fermilab at Night 14 The DØ Experiment η θ A multipurpose Pseudorapidity = -ln(tan( /2)) particle detector Innermost detectors are the trackers, followed by calorimetry and muon chambers Using information from all subdetectors, we can reconstruct the pp collision 15 Electrons, Photons Lighter particles (e, γ) are stopped in the first few layers of the the calorimeter Hadrons make it farther into the detector Only charge particles leave tracks Track matching allow us to distinguish e from γ 16 Hadronic Jets When a high momentum quark or gluon is produced in a collision, results in a Jet of hadronic particles into the detector 17 Muons Muons can pass through our entire detector – µ are massive enough not to be stopped like e or γ in EM calorimeter – Don't feel strong force, so hard interactions are rare Muons are charged, so they leave tracks Place trackers outside the calorimeter to detect passage, and match to a 18 track in inner tracker Missing Transverse Energy Neutrinos do not interact with our detector MET muons – Infer their presence through missing transverse energy jet – Transverse = ⊥ to beamline To conserve momentum, vector sum pT = 0 – If non-zero, then either • Energy Mis-measurement electron • Missed one or more particles (usually neutrinos) 19 Higgs Production at the Tevatron The Higgs at Tevatron produced by gluon fusion (gg→H), associated production (qq→W/Z+H), and vector boson fusion (qq→qqH) 20 Higgs Decay Modes The Higgs decays primarily to bottom quarks at low mass, WW at higher mass The searches for the Higgs at the Tevatron focus on H→bb at low mass, H→WW at high mass. Also use H → ττ, H → γγ due to low backgrounds 21 Higgs Backgrounds Or, You've been looking for over 10 years, why haven't you found it already? The rate of Higgs production is small compared to backgrounds − Bottom decays swamped by QCD background except for associated ZH/WH production − WW cleaner, but still large backgrounds 22 Trilepton Higgs Search New search channel at DØ – not in July 2011 combination H0 ZH and WH production followed by H → WW decay For WH → WWW → lνlνlν, where l = e, µ – Ratio of eee:eeµ:µµe:µµµ = 1:3:3:1 For ZH → ZWW → lllνX, ratio is 1:2:2:1 Currently, only considering the eeµ and µµe final states, with others to be added in future 23 Higgs Decay Modes The expected Higgs signal from VH→VWW is between one tenth and one hundreth that from H→WW However, eliminates almost all W, Z, Wγ, and WW backgrounds So there is a gain in the signal over background in the three lepton channels compared to the 2 lepton channels 24 Trilepton Backgrounds Principle backgrounds for the trilepton search – WZ → lνll – ZZ → llll Also have backgrounds with 2 real leptons and a Jet or photon fake – Z + Jets – Z + γ ν ν – tt→l bl b 25 Z + Jets q Z/γ* Z→l+l- + Jets cross section is the largest of all the backgrounds Fakes lepton However, it is suppressed by low q g J→l fake rates – Z + Jet fake rate is estimated by passing simulated Z+Jet events through our MC detector model Z + Jet events have no real missing energy – Variables that are sensitive to the missing energy separate this background from our signal 26 Z + γ Z + γ significant background for µµe final state Only difference between electrons and photons in our detector is a track 27 Z + γ Z + γ significant background for µµe final state Only difference between electrons and photons in e + - our detector is a track γ e Sometimes the photon undergoes interaction with the tracker, leading to γ→ee – If one electron gets most of the photon energy, will be reconstructed as an electron 28 Estimating the Z + γ Background Correctly estimating the γ→e fake rate in MC difficult We use a data-driven method to estimate this background – Calculate the γ→e fake ratio from a photon sample in data – Reconstruct µµγ events, where the photon is not matched to a track – Apply the fake ratio to the data events to get the background estimate 29 Top Background Like Z + Jets, tt requires Jet to fake lepton – tt events have real missing energy – b-quark Jets have a higher fake rate than light quark Jets – The tt background is the smallest SM background, but tends to be more signal-like, so it is significant Fakes lepton 30 WZ and ZZ Backgrounds WZ → lνll and ZZ → llll are estimated from MC – Currently, there is no dedicated 4l Higgs search at DØ, so these events are included in our search – Of all backgrounds, WZ most closely mimics signal 31 Trilepton Selection Cuts We have requirements on both eeµ and µµe channels – 3 good leptons must be found – M(ee) or M(µµ) > 15 GeV – One lepton with pT > 15 GeV, all with pT > 10 GeV – All leptons from same pp collision – Δ R = √ Δ η 2 + Δ ϕ 2 > 0.3 between any two leptons 32 Reducing Backgrounds in µµe Channel The µµe channel has larger backgrounds than eeµ – From Z backgrounds – Jet → e fake rate higher than Jet→µ fake rate, γ fake e but not µ Have additional cuts on µµe to reduce backgrounds – Remove final state radiation (FSR) – Photon is radiated off a muon after Z decay, so µµγ will have Z mass – Also, no real missing energy, so can keep most signal in the Z mass range by cutting only low MET events 33 Reducing Backgrounds in µµe Channel The µµe channel has larger backgrounds than eeµ – From Z backgrounds – Jet → e fake rate higher than Jet→µ fake rate, γ fake e but not µ Have additional cuts on µµe to reduce backgrounds 34 Reducing Backgrounds in µµe To reduce background more, turn to variables related to MET Special MET – If ∆φ between MET and nearest object < 90°. specialMET = MET×sin∆φ, otherwise = MET MET Significance – Based on the uncertainty on the energy measured in the 35 detector Reducing Backgrounds in µµe We require either – Special MET > 15 GeV or – MET Significance > 3 Removes most Z + Jets and Zγ, leaves most signal 36 Event Yields After all cuts, have 175 trilepton events Less than 3 expected signal events S/B ~ 0.15 Errors reflect uncertainty due to sample size. With all statistical and systematic errors, expected 37 and observed agree within 1σ Signal vs.