Search for the Higgs boson

The Standard Model of particle physics is a theory which explain the
constitution of all matter which makes up our own bodies and the World
around us. It describes all matter as being made of infinitesimal
matter particles. It also explains the various known forces between
particles, except gravity. The matter particles interact, atract,
repel, and fuse with each other, by exchanging or emitting a different
kind of particle, the force carriers or intermediate bosons.

The Standard Model was tested in particle colliders, for example, with
increasing energy and accuracy over the years. The theory was, so far,
much more successful than its creators ever imagined, and all its
predictions were successfully verified. Except one. The missing piece
is the Higgs boson, which plays a central role in the mathematical
edifice of the Standard Model.

According to the theory, there is a field which permeates all space,
called the Higgs field. It is the particles' interaction with this
field that gives them mass. The Higgs boson is a direct consequence of
the mechanism that gives mass to particles. So if we find signs of the
Higgs boson, this tells us that the mechanism imagined by Prof. Peter
Higgs and others is most likely true.

I propose to carry out a research programme to search for signs of the
Higgs boson using data from the ATLAS experiment, one of the
experiments the Large Hadron Collider (LHC) at CERN. This is a 27km
circunference particle accelerator built underground in the outskirts
of Geneva. Inside it, protons are accelerated to very near the speed
of light. The protons are then made to collide in the centre of one of
four giant experiments. The LHC successfully started its operation on
the 10th of September 2008. After an accident involving some of its
magnets, it stopped for repairs and will restart operations in Spring
2009.

The experimental data that we currently have provides us with clues of
how and where to search for it. These indicate that the Higgs boson
has a mass not much above that of 122 protons. I intend to investigate
collisions where the Higgs boson is produced together with two heavy
"top" quarks. Since both the Higgs and the top quarks are unstable and
decay almost immediately, their presence needs to be inferred from
examining the complex results of proton collisions.

The production of these events is also a very rare process, which
means examining a huge amount of proton collisions. This is why the
LHC creates 40 million collisions per second. Since only a small
fraction of these collisions can be recorded, the experiments use a
"trigger" system to decide, in real time, which collisions are
potentially interesting. Only those are kept for later analysis. My
first goal will be to ensure that the ATLAS trigger system will select
most Higgs events while rejecting as much as possible of the
non-interesting events.

In a second step, I plan to help identify and analyse events
containing the top quark. This quark, discovered only in 1996, has the
highest mass of all known fundamental particles. This makes it a very
interesting object to examine. In addition, observing it in a new
experiment would constitute an important milestone and good
preparation for searching for the Higgs boson.

Only then will it be possible to search for the Higgs boson in the
proposed channel. And several techniques of data analysis will need to
be tested and optimised. After all this, I may find signs of the Higgs
boson in the mode that I intend to investigate. If the signs are
there, then it will be important to measure the properties of the new
particle, to verify if this really is the particle predicted by the
Standard Model.

But we know that the Standard Model is not the full story. We
would expect a more complete theory to also explain gravity, for
example. So while finding the Higgs boson would be a great discovery,
finding a new particle that doesn't fit in the theory could be even
greater!..