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Standard Model Higgs Boson Searches at the D0 Experiment

We present the latest searches for the Standard Model Higgs boson at a centre-of-mass energy of sqrt(s) = 1.96 TeV with the D0 detector at the Fermilab Tevatron collider.


The Higgs Boson
To explain the origin of mass for the W and Z Bosons in the Standard Model (SM) the Higgs mechanism predicts a breaking of electroweak symmetry. A consequence of this symmetry breaking is a heavy scalar boson whose mass is not predicted by the SM.
Direct searches for the Higgs Boson were performed at the LEP experiments in the process e + e − → ZH with a centre of mass energy of 206.6 GeV. A direct mass limit at m H => 114.4 GeV is set at the 95% confidence level [3](all limits in this paper will be shown at 95% CL). This limit is slightly below the maximum available kinematic limit due to a small excess observed in the LEP data.
Indirect limits have been placed on the Higgs boson at the LEP, SLD and Tevatron experiments from electroweak precision measurements [1]. The SM fit best value for the mass of the Higgs boson is m H = 76 +36 −24 GeV [2]. The upper limit achieved from this fit is m H < 144 GeV. When the direct mass limit is taken into account this limit is increased to m H < 182 GeV (Figure 1).

Higgs searches at DØ
The fit for the Higgs Mass shown in Figure 1 shows that a light Higgs Boson is favoured which is accessible to the Tevatron experiments. The Tevatron accelerator collides pp with  a centre of mass energy = 1.96 TeV at an instantaneous luminosity up to around 3×10 32 cm −2 s −1 . At the time of writing, the DØ experiment has collected nearly 5 fb −1 of data and results shown here include between 1 -3 fb −1 of data analysed. The dominant production mechanism for a SM Higgs boson at the Tevatron is via gluon-gluon fusion. An order of magnitude lower is associated production with a W/Z boson. At high masses the main search channel is gluon-gluon fusion with the Higgs boson decaying to two W bosons which subsequently decay leptonically. However, at low Higgs masses (m H < 135 GeV) the Higgs decays into bb pairs which are extremely difficult to resolve against the large multijet background at a hadron collider. As such associated production is searched for where events are tagged by the leptonic decay of the W/Z boson.
One of the most sensitive channels at low mass consists of a final state of two b jets from the Higgs boson and a charged lepton ℓ and a neutrino from the W. All three leptonic decays of the W are analysed at DØ , however, the most sensitive are the decays to electrons and muons. They will be focused on here using 1.7 fb −1 of data. Events are selected with one or two b tagged jets with transverse momentum p T > 20 GeV, an isolated electron/muon with p T > 15 GeV and missing transverse energy E miss T > 20 GeV. The main backgrounds after selection are W +jets and tt production. To improve separation between the signal and the irreducible background a Neural Network (NN) is trained taking a number of kinematic and topological variables as input. The output of this NN (Figure 2) are used to set limits on Higgs production. The analysis sets a limit on σ W H = 10.9xσ SM at m H = 115 GeV [4] (where σ SM is the cross section predicted for this process by the Standard Model).
At high mass H → W W is searched for in the leptonic final state ℓνℓν. This analysis uses approximately 3 fb −1 of data and searches in three independent final states (ee, eµ, µµ). To select these events two high-p T leptons are required and E miss T > 25 GeV to account for the neutrinos. The main feature of this final state to differentiate from di-boson production is the small angle between the leptons arising from spin-correlations between them due to the spin-0 nature of the Higgs boson. This is in contrast to the Many other channels have been studied in the search for the SM Higgs boson with some more sensitive than others. However, to obtain the best limit on Higgs production at DØ a combined limit is set. Systematic uncertainties are taken into account using both Modified Frequentist and Bayesian approaches to ensure the result to does not depend upon the details of the statistical method applied. The results across the full mass range for the DØ combination are shown in Figure 3. To further improve the limits on a SM Higgs boson a combination is performed with the CDF experiment (currently only for high mass). The most interesting feature of this combination is that the observed limit crosses one at m H = 170 GeV (Figure 3), resulting in an exclusion of that Higgs boson mass at the 95% CL [4].