The Search For Vh U2192 Vww Standard Model Higgs Production In The Trilepton Signature With 5 9fb 1 Of Data From P Bar P Collisions At U221a S
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The Search for VH {u2192} VWW Standard Model Higgs Production in the Trilepton Signature with 5.9fb-1 of Data from P$\bar{p}$ Collisions at {u221A}s
We present here the search for Standard Model V H → VWW → lll + ET (missing energy due to neutrinos) production, where V is a W or Z weak vector boson, which uses up to 5.9 fb-1 of integrated luminosity. This analysis has recently added to the CDF high-mass Higgs group three new signal topologies characterized by a tri-lepton signature, which are chosen to isolate the V H → VWW associated production signals in the three-lepton signature. As such, we define three new regions for a WH analysis, a ZH 1-jet analysis, and a ZH ≥ 2-jet analysis with which we expect to contribute an additional 5.8% (for mH = 165 GeV) acceptance to the current H → WW dilepton analysis. The ZH trilepton regions are defined by events passing a Z-boson selection: events having at least one lepton pairing (among three possible pairings) with opposite sign, same flavor, and a dilepton invariant mass within [76.0, 106.0] GeV–a ± 15 GeV window around the Z-boson mass. TheWH trilepton region is then defined as the set of trilepton events that are complement to those chosen by the Z-boson selection. These three new event topologies make a substantial contribution to the H → WW group result. As a measure of the sensitivity of this search, we compute the median expected limit on the at 95% confidence level (?C.L.?) on the production cross section (effectively the rate of production) for a StandardModel Higgs boson and report the result as a ratio to the theoretical production cross section. An observed limit ratio of one or less at a given mass would rule out the production of a Standard Model Higgs boson at that mass with 95% confidence. At mH = 165 GeV, the WH analysis expected limits reach 7.2 times the standard model cross section; the ZH 1-jet analysis is set at 29 times the expected standard model cross section; the ZH ≥ 2-jet analysis is set at 9.9 times the expected standard model cross section; and the combined trilepton analysis is set at 4.9 times the expected standard model cross section. We announce that the combination of this trilepton V H → VWW Higgs boson search and the previous CDF dilepton H → WW search achieves an expected median limit of 1.00 at 165 GeV/c2. The expected median limit of 1.00 indicates we anticipate a 50% probability of ruling out the existence of a Standard Model Higgs boson with a mass of 165 GeV/c2. This is the first time a single hadron collider experiment has achieved sensitivity to the production of a Standard Model Higgs boson. We do not see evidence for a significant signal of Higgs bosons in the data and place observed limits on the production of a Standard Model Higgs boson of 165 GeV/c2 at 1.08 times Standard Model production cross section. Matthew.
Search for the Standard Model Higgs Boson in ZH {u2192} V$\bar{v}$b$\bar{b}$ Channel in P$\bar{p}$ Collisions at {u221A}s
A search for the standard model Higgs boson is performed in 5.2 fb-1 of p$ar{p}$ collisions at p √s = 1.96 TeV, collected with the DØ detector at the Fermilab Tevatron. The final state considered is a pair of b jets with large missing transverse energy, as expected from p$ar{p}$→ ZH → v$ar{v}$b$ar{b}$ production. The search is also sensitive to the WH → lvb$ar{b}$ channel, where the charged lepton is not identified. Boosted decision trees are used to discriminate signal from background. Good agreement is observed between data and expected backgrounds, and, for a Higgs-boson mass of 115 GeV, a limit is set at 95% C.L. on the cross section multiplied by branching fraction of (p$ar{p}$ → (Z/W)H) × (H → b$ar{b}$) that is a factor 4.57 expected and 3.73 observed larger than the value expected from the standard model.
Measurement of ?(p$\bar{p}${u2192}Z) Br(Z{u2192}?+?-) and Search for Higgs Bosons Decaying to ?+?- at {u221A}s
The resonant production of tau-lepton pairs is as interesting for the study of Standard Model (SM) physics as the production of lighter leptons pairs. For new phenomena, such as Higgs boson production or in case new particles beyond the SM would arise, the detection of (resonant) pairs of tau leptons becomes much more interesting. This is due to the fact that tau leptons are much heavier than the other leptons, which increases the chance that these new phenomena would be observed first in this channel. Unfortunately their clean detection is far more difficult than that of muons or electrons. The cross section times branching ratio ?. Br for the process p$ar{p}$ → Z → ?+?- was measured at √s = 1.96 GeV using 1.0 fb-1 of data collected by the D0 experiment. This measurement was performed in the channel in which one of the tau leptons decays to a muon and neutrinos, while the other decays either hadronically or to an electron and neutrinos. A set of 1511 events, of which about 20% estimated background, passed all selection criteria. The trigger and muon reconstruction efficiencies, as well as the efficiency for track reconstruction were obtained from data using the 'tag and probe' method on Z → ?+?- events. The multijet background was estimated from the sample of events which passed all selection criteria but in which the muon and the tau candidate had the same charge. The W → ?v + jets background was modeled by Monte Carlo simulations, but normalized to data. All the other backgrounds, as well as the efficiency for Z → ?+?- events were estimated using simulated events normalized to the theoretical calculations of cross sections at next-to-leading order or next-to-next-to-leading order. The energy of the tau candidates was corrected for the estimated response of the charged pions in the calorimeter, which is of the order 50-80%. Since the charged pion response in data was not well reproduced by the default simulation of hadronic interactions (Geisha), a different simulation (gCALOR) was used to obtain an estimated charged pion response consistent with the one measured in data. This tau energy correction method makes use of the superior resolution of the track momentum measurement compared to the resolution of the tau candidate energy as measured by the calorimeter, which leads to a better data--simulation agreement and a decrease of 10% in the resolution of the visible mass peak. The result of this measurement is ?(p$ar{p}$ → Z) • Br(Z → ?+?-) = 240 ± 8(stat) ± 12(syst) ± 15(lumi) pb, in good agreement with the theoretical predictions of 241.6$+3.6top{3.2}$ pb [79] or 251.9$+5.1top{-12}$ pb [93-95], as well as with other measurements performed by the D0 and CDF experiments in all channels in which the Z boson decays leptonically [96-100]. This is the most precise Z boson cross section measurement to date performed in the tau lepton channel at hadron colliders. The analysis demonstrates the ability of the D0 experiment to identify tau leptons decaying hadronically with good efficiency and high purity, a challenging task in p{bar p} collisions where the number of jets resembling tau leptons is very high. This achievement forms a solid basis for other analyses using hadronic tau lepton decays, such as the search for the Higgs boson decaying into tau-lepton pairs, which was performed for the last part of this thesis.