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
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.
Production cross section times branching ratio
σxBR(H→WW(*)) excluded with the current DØ
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
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
(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
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
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)