Search for the Higgs Boson in H→WW(*) decays at DØ

One of the most fundamental questions addressed by particle physicists today is the origin of mass. Theory holds that particles acquire mass by interacting with a field which permeates space. Within this theory an unstable massive particle, called the Higgs boson, is associated with this field. The theory predicts all the different parameters of the Higgs boson depending on its mass which is the only unknown quantity of the Higgs particle. If the Higgs boson exists, it can be produced in particle collisions at the Tevatron.
Higgs to WW production rate excluded with the current DØ data (and LEP experiments), along with the expectations from an alternative model
Figure 1: Production cross section times branching ratio
σxBR(H→WW(*)) excluded with the current DØ data (blue)
and LEP experiments (yellow), along with the
expectations from the Standard Model (red) and
an alternative model (dark red).

If the mass of the Higgs particle is in the range of 135 to 200 GeV, it will predominantly decay to W boson pairs. W bosons cannot be observed directly with the DØ detector, because after their production they decay immediately into other particles. In 2/3 of the cases the decay products are two quarks that appear as jets of particles in our detector. In 30% of the time the W bosons decay into a lepton (l) and its corresponding neutrino (ν). The lepton can either be an electron, muon or tau. While electrons and muons can be detected directly in the detector, the tau is unstable and can only be observed in its subsequent decay into an electron or muon. The tau may also decay in quarks, but these events are not considered since it is difficult to distinguish these hadronic tau decays from jets.

Other particles produced in proton anti-proton collisions can lead to similar final states as the Higgs decay into W boson pairs. In most of the collisions the final state consists of quarks and since the direct production of quarks happens many million times more often, it is impossible to distinguish the hadronic W decays from the Higgs decay. In addition the DØ detector is optimized to detect events involving electrons and/or muons. Thus, final states including either two electrons, two muons or one electron and one muon, are the best to reliably detect the W bosons from the Higgs decay.

Furthermore there are still other processes that can mimic the decay of the Higgs into W bosons. For example two electrons or muons can also be produced via a photon or Z boson which is called the Drell Yan process. However the main background is the pair production of W bosons , which consists exactly of two W bosons as the Higgs decays itself. But our situation is not hopeless since we can exploit different features of Higgs decay into W boson pairs to suppress other backgrounds. One important fact is that the Standard Model Higgs boson has spin zero, whereas the W bosons have spin 1. In order to conserve angular momentum, the spins of the W bosons from H→WW(*) decays must be anti-correlated. Thus, the two leptons from the H→WW(*) decays have a smaller opening angle of their momentum vectors compared to leptons from most of the other backgrounds.

DØ has used Tevatron Run II data taken from April 2002 until August 2004 to search for the Higgs in the H→WW(*) decay mode. To take into account the signal kinematic characteristics that change with the Higgs boson mass, a Higgs boson mass dependent selection has been developed. Applying different selection criteria we observe 20-30 events at the end depending on the Higgs boson mass selection. In this sample we expect about 0.7 events to be from a Higgs boson of 160 GeV mass. All other remaining events are consistent with the expectation from background processes that mimic a H→WW(*) decay.

So far, no evidence for the elusive Higgs particle has been found. But, we can calculate upper limits on the production rate of H→WW(*) decays, since otherwise the Higgs particle would have been observed in our detector. This limit means, that the Higgs events do not occur with a rate larger than this limit with very high probability (95% confidence level CL). The plot shows the DØ limits on the production rate of the Higgs as a function of mass (blue) and compares that to the limits set at LEP (yellow box) and the predictions of the Standard model (red) and one alternative model (dark red) that predicts four families of fermions instead of three.

In the next years we will record 20-40 times more data that will allow us to highly improve our current results in the Higgs search. If we will be able to observe the Higgs boson in the mass range from 140 to 180 GeV depends on how much data we will be getting, but we will exclude this mass region, if the Higgs boson does not exist.

An article on this analysis has been submitted to Physical Review Letters in August 2005.

For more information on this analysis please contact Johannes Elmsheuser (Ludwig-Maximilians-Universität, München, Germany) or Marc Hohlfeld (LAL, Orsay, France)

by JE and MH