Searching for Higgs particles, in
a nearly supersymmetric world...
One of the most
fundamental properties of an elementary particle (i.e. particle that has no
substructure) is its mass. Different particles have different masses,
from the very light electron, to the nearly million times more massive top
quark. The modern theory of particle physics called the Standard Model (SM)
cannot explain the origin of mass and why various particles have the masses that
they do. Instead, it postulates the existance of a new particle, the Higgs
boson, and the corresponding Higgs field that fills the Universe (!).
Particles become heavy because they are coupled to (interact with) the Higgs
field. It also follows from this model that one can create Higgs particles for
example in experiments at accelerators. Since the Higgs particle itself is
predicted to be heavy, a great amount of energy must be concentrated into a
small volume of space to have any hope of creating one. The only machine capable
of doing this today is the Tevatron proton-antiproton collider
at Fermilab, and the D0 experiment is one of the two detectors that is
specifically designed to watch for traces of Higgs particles.
But why are the Higgs particles
heavy? Where do they get their masses from? No one really knows... but
it's worse than that! Within the SM framework the Higgs particle mass is not
even finite. However, an extension of the model, called
supersymmetry, helps to control the
Higgs mass. The 'symmetry' of supersymmetry is
that any particle with half-integer spin (like 1/2 or 3/2) has a partner,
identical in all respects, but with integer spin (like 0,1,or 2). It is
called 'super' because it solves many problems in
particle physics! We know that the theory of supersymmetry is not completely
correct since there is not an integer-spin electron observed in experiments, for
instance. However, if the theory was almost correct, there might be an
integer-spin electron, but it's just a bit too heavy for us to see. This
'near-supersymmetry' would be good enough to solve the mystery of the Higgs particle masses.
If we do live in a nearly supersymmetric world, the Higgs particles would be a bit
different. First of all, there would be at least 5 kinds of them! This is
required for the theory to be 'well behaved' so that calculated masses are
finite. Theory also predicts that some of the Higgs particles might be made at
the Tevatron in a very special way. Instead of being
produced alone, they would often be accompanied by two bottom quarks.
Furthermore, Higgs particles themselves would also tend to decay into a pair of
bottom quarks. The result would then be an excess, a signal, of events with many
bottom quarks, where a pair of these bottom quarks would have energies that add
up to the Higgs mass.
The
number of multi-bottom-quark events we would expect to see with the D0 detector
can be accurately predicted using computer simulations of the physical processes
we know about, and models of the detector geometry, materials, and electronics.
The predictions are compared to what is observed from about 2 years of the
experimental data. The result is shown in Figure 1, where the solid line is the
prediction, assuming no Higgs particles, and the points with error bars are the
data. The signal of Higgs events, if it were to exist, would have a size and
shape like the dotted line. Since no excess in data is seen, we conclude that
either
a) Higgs
particles don't really exist b)
they exist but are too heavy for the Tevatron to create in large numbers c) the world is not
nearly-supersymmetric d) the
parameters of supersymmetry do not lead to a large number of Higgs particles to
be created
Figure 1: A comparison of the data to
predictions
The parameter of the supersymmetric theories which most directly controls
the number of Higgs events we would expect to see is called tanB (pronounced 'tan', like a sun-tan, and the Greek letter: 'beta'). So we can set limits on the possible
value of tanB, for each assumed Higgs mass, as shown in Figure 2. The two
different blue lines correspond to two slightly different models of
supersymmetry. The other shaded regions have been ruled out by other experiments
in the past, like the Large Electron Positron collider
at CERN. Since supersymmetry has not been discovered, we certainly don't know
the values of its parameters!
Figure 2: Limits on tanB
versus the Higgs mass
As we continue to take data with the D0 detector, we will become more and
more sensitive to lower tanB values and heavier Higgs boson masses. Some
theorists believe that the Higgs particles may be just outside our current
reach. If we are lucky, therefore, we could discover Higgs particles within the
next few years, marking a major milestone in the history of particle physics,
and our understanding of the world around us!
An article on this analysis has
been submitted to Physical Review Letters in April 2005. Please e-mail for an
additional information.
Searching for Higgs particles, in
a nearly supersymmetric world...
