The Top Quark: worth its weight in gold
Scientists built the Standard Model of particle physics as a description
of the basic nature of the Universe. The top quark is the sixth, and
according to the Standard Model, the last to be found. It is one of the
fundamental particles of the Standard Model, a
"particle zoo" of 6 quarks, 6 leptons, and 4 force-carrying bosons like
the photon (the force carrier of light). This Table shows all we currently
Fig 1. Elementary particles of the Standard Model
The missing piece
Until a decade ago, almost all particles predicted by this model were
found experimentally except for one piece missing, the top quark. The
Standard Model requires that quarks (and also leptons) come in pairs.
Therefore after the discovery of the b quark in 1977 there was a quark
without a partner. All matter as we know it is formed with from the first
generation of quarks and leptons (see figure above). Although top quarks
are not part of the Universe as we know it today, scientists believe
that they existed naturally at the beginning of our Universe, along with
other currently non-existent particles.
After decades of very difficult searches, the top quark was discovered in
1995 by the CDF and DØ experiments at Fermilab. The Fermi National Accelerator Laboratory (Fermilab), located in Batavia, Il, creates the
conditions necessary for nature to produce top quarks. During the
discovery of the top quark, the Fermilab accelerator, called the Tevatron,
collided 900 GeV protons and 900 GeV anti-protons (1 GeV is the kinetic
energy that a 1 billion volt battery could give to 1 proton), allowing the
creation of top quarks. Anti-protons are like protons but have
opposite charge. Now, this accelerator has been updated to collide
protons with higher energy, that is 980 GeV protons and 980 GeV
anti-protons. More energy means more top quarks !
CDF and DØ are very large collaborations of more than 500 people.
These experiments are complementary of one another, providing cross checks
of their results. Top quarks very rarely get produced in proton collisions
with anti-protons. Therefore, these experiments consist of very
complicated instruments called detectors which use thousands of data
readout channels (like pixels on a digital camera) to identify top quarks
from other more common processes.
... But predictions help
The Standard Model gives scientists guidance on how the top quark will be
produced and how it will decay. From the theory, we expect top quarks to
be produced predominantly in pairs. The top quark existence is very short.
It decays into a lighter mass b quark and a W boson. The existence of the
top quark is inferred by the observation of its decay products. Each W
boson also decays shortly after being produced, therefore signatures of
top pair production depend on the decay of the two W bosons in the event.
Most of the times, the W decays into 2 quarks (2/3 of the time) which
after a process called "hadronization" appear as a collimated spray or jet
of particles in the detector. The rest of the times, the W can decay to
one of the three flavors of charged lepton and neutrino.
The lepton, neutrino and b quark from the decay of the top quark (and
subsequent decay of the W) have some particular kinematic properties. The
lepton for example tends to be very energetic compared to leptons coming
from other decays. These differences can be used to select which events
are top quarks. To study top quarks scientists select candidate events that are
similar to what the Standard Model would predict if a top and anti-top
quark were produced in the collision. Despite care, this carefully selected sample
still contains a large fraction of other types of physics events called
background. The major background is when W bosons and jets are created without
coming from the top quark together with other light particles. Many techniques
are applied to suppress the background further until we consider that we have a
cleaner sample of top events. Then we ask two experimental questions:
What percent of the time are top quarks produced when we
collide 900 GeV protons into 900 GeV antiprotons? What is the mass of the top
These two questions are correlated. The first one refers to the concept of a cross section. A cross section is a description of how often a process will occur under specified conditions. Experimentally the procedure consists of counting the number of times top quarks are seen in our detector and dividing by the total number of collisions observed. The unit measurement of cross sections is the barn (b), defined as 10^-28 m^2, which is about the size of a large atomic nucleus. The bigger the area, the more likely one is to get a collision which will produce the desired outcome. The fact that top quark are very rare, translates into a very small cross section and is therefore measured in picobarns (pb = 10^-12 b), this is a million million times smaller than a large nucleus! One of the main reason why there is a small probability of producing top quarks is because they are very heavy. The energy of the Tevatron accelerator is close to the energy threshold necessary to produce a pair of top quarks.
The importance of this cross section measurement is that it gives us a tool to test of the correctness of the Standard Model, so we can compare our experimental result to theoretical calculations. Since the top quark mass is not predicted by the Standard Model (actually the SM doesn't predict any particle mass), the theory allows prediction of the cross section in terms of the top quark's mass. The graph to the right plots the top quark pair production cross section versus its mass (with a cross corresponding to the uncertainties.)
The Dě and CDF results for the cross section and mass are shown as a red
dot and lightblue square with lines corresponding to the uncertainties of
our measurement. These are the results from data collected previously (Run
I of the Tevatron). The figure also shows some lines for various
theoretical predictions. We can see that at this time the experimental
results are in good agreement with theoretical predictions, within our
Fig 4. Fermion masses.
In fact, the only Standard Model particle it may be lighter than
is the Higgs boson. The Higgs boson was input into the Standard Model to
account for why the photon does not have mass, but the Z and W bosons do.
The Higgs also gives mass to the quarks and leptons, giving more mass to
ones that it interacts with the most. The problem is that while the Higgs
theoretically does the job it is supposed to do, it has not yet been
discovered to actually exist. To help experimenters look for the Higgs
boson, theorists have used the Standard Model to predict a relationship
between the mass of the top quark and the W boson, which allows one to
predict the mass of the Higgs boson. This relationship is extremely
sensitive to the top mass, and since the top mass has big errors (as in
the plot above), there is still much uncertainty in the predicted Higgs
mass. If experimenters knew the predicted mass of the Higgs boson, they
would know its cross-section and how it would decay, allowing their
searches for the Higgs to be more precise. Therefore, if the top quark
mass measurement is improved, the Higgs mass can be more precisely found,
and we will be one step closer to answering the question of how particles
There are other properties of the top quark that need to be tested.
Does it behave as the Standard model predicts? Its large mass causes
scientists to speculate about the role of the top quark in the Standard
Model. Being so much more massive than all the other quarks (see figure
above), the top quark might play a special part in particle physics. It
can also test the Standard Model at energies that have not been studied
before, where we may find that our interpretation of the building blocks
of nature is not correct or not precise enough. And physicists are always
looking for new physics beyond the Standard Model.
University of Rochester, DØ Experiment