In the past year, there has
been a lot of discussion about whether the Tevatron
collider at Fermilab can find the Higgs
Hadron Collider can.
There have been all kinds of claims, and even stories of
hints of sightings. In a session today, Brian Winer from
Ohio State University gave a very clear presentation
about just how close the Tevatron is to potentially
finding the Higgs.
Cutting to the chase: Can
the Tevatron find the Higgs? Yes, if Nature is kind.
But first let me set the
What do we know
about the Higgs now?
particle physics predicts that a Higgs boson is most
likely to have a mass of 87 GeV. (Physicists like to
express masses in terms of energies. If you want to
convert to an actual mass number, divide the energy by
the speed of light squared.) The nature of this
prediction is that the Higgs won’t necessarily have that
mass but it is likely to have it somewhere in a range
centered on that number. There is roughly a 2/3 chance
that the Higgs will have mass between 60 GeV and 123
Experiments at the Large
Electron Position collider at CERN showed that the Higgs
does not exist at any mass lower than 114 GeV however,
and the theory predicts that the mass will be less than
160 GeV with a 95% probability. So physicists are really
trying to look in this region of 114 to 160 GeV.
How sensitive do
experiments need to be to find the Higgs?
Experiments have different
sensitivities to the Higgs depending on its mass.
Physicists can estimate how close they are to observing
the Higgs by comparing their measurements to theoretical
At the moment, the
performance of experiments is expressed as a factor of
how much better the measurements need to be to start to
have a chance of being sure that any Higgs-like signal
is really a Higgs boson. It is a game of statistics and
it gets pretty complicated so I’ll just take the
simplest possible path through this, warning that the
full story is much more detailed.
The biggest problem to
start with is that what a collider detector measures is
a set of particles, none of which is specifically the
particle you are looking for. The Higgs decays into
certain sets of particles but other decays look very
similar. So when your detector sees a set of particles,
you need to make extra sure that you are looking at what
you think you are looking at! You can predict how many
of certain types of known particles you should see and
then look for any excess if you are hunting a Higgs or
some other unknown particle.
This “background” of decays
from known particles must all be excluded from analysis
before you can see the Higgs. The big problem is that
the backgrounds are a factor of 100 billion times larger
than the signal from the Higgs! Higgs would make up only
tens of events out of the trillions measured.
Fortunately, detectors are
very good at finding the presence of b and
(they create characteristic jets of
particle from the collision). Higgs will tend to decay
along with other particles so the bs
can be used to identify most of the collisions you are
interested in. In fact, this “b-tagging” can
reduce the background by a factor of one billion.
But as Winer put it today,
“the first factor of a billion is easy, the last factor
of 100 is hard.”
How low can you go?
In trying to get this
factor of 100 down to a factor of 1, a lot depends on
the energy the Higgs has. If the Higgs has an energy at
the bottom end of the range, about 115 GeV, and then you
combine all the data from both the CDF and DZero
experiments, you end up about a factor of 5 short of
where you need to be to have a chance at the Higgs. But
given that the Tevatron is hoping to collect 4-8 times
more data than was used in this analysis, it potentially
comes in range though it would still be a real stretch.
If the Higgs has a mass of
160 GeV, then the combined data already brings us to a
factor of a mere 1.1 times where we need to be. In other
words, a 10% improvement and “Game on!”
Of course, it is not as
simple as collecting 10% more data. That would be too
easy. That level is really the starting point for when
your detectors and dataset have the statistical power to
resolve Higgs hiding in the flood of collisions.
But if the Higgs really
sits around that mass, then the Tevatron has a genuine
chance of finding it soon. Winer said today, “As early
as summer, we can start excluding masses around 160 GeV.”
In other words, the data will be sensitive enough to say
either that there is definitely no Higgs at that mass,
or it will see signals that look like the Higgs and it
will just be a matter of some more data to determine if
they really are Higgs bosons.
Is this the Higgs?
And just to tease you a
little more, this event could even be a Higgs.