The Top Quark, Part 2

November 14, 2010
  • Particle Physics
  • Physics

The top quark is the heaviest of the quarks. It weights about 172 GeV (Giga Electron Volts), which is more than 40 times the mass of the second heaviest quark (the bottom quark). No one knows exactly why it has such a high mass. But in particle physics, there are very few coincidences, and many believe that the awkward distribution of masses of the known quarks must come from some yet-unknown physical process. Possibly it's related to something called ElectroWeak Symmetry Breaking, which is the process in which the Higgs Boson plays a major role.

Because the top is so heavy, it takes a lot of energy to make one in a collider (contrary to what one may expect, heavy things are hard to discover. In general, light particles are easy to find because they are made so readily). So, how does one "make" a top quark? There are many ways for a top to be made at the LHC. One example is leads to the creation of a top quark and the top quark's anti-particle, known simply as an anti-top. (Anti particles are often written as the particle with a bar over it. So, the anti-top is written as a t with a bar over it. Because of this, the processes that lead to the creation of a top and anti-top quark together are collectively known as "tee tee-bar.")

When two protons are accelerated to high energies and made to fly into each other, as they are at the LHC, there is some chance that two quarks within the protons will collide and cause a gluon to fly out of the quark collision with a very high energy (recall that gluons are the particles that hold quarks together within protons). If this gluon has enough energy, it can decay (turn into) a top quark and an anti top quark. Because of conservation of energy, the gluon must be ejected from the protons with more energy than twice the mass of the top quark, or about 344 GeV (remember, mass and energy are the same thing. That, among many other things, is what Einstein taught us. So, the gluon must have energy equal to the mass of the top plus the mass of an anti-top, both of which have the same mass of 172 GeV).

Because Top quarks are so heavy, they don't exist for very long. Almost instantly after it comes into existence (after only 51025 seconds), a Top quark will decay into a Bottom quark and a W boson. Similarly, the W boson quickly can decay into more quarks or into leptons (electrons or muons). Thus, in the end we are left with a bunch of quarks and leptons flying out into the detector. From those "daughter" particles, physicists must search for evidence that the parent Top quarks were even there in the first place.

The image (a) above is the process I just described, and the image (b) is a similar process where two gluons collide and eject another gluon, which then turns into Top Quarks.

So, why is this interesting? The final state of top quark production as described above (leptons and lots of quarks) is very similar to final states of new physics, including Higgs physics. So, if you are searching for the Higgs, you first have to know what Top events look like at the LHC. If the theory of the Higgs predicts an excess of events of a certain type, and Top Quark production can contribute to that excess as well, one must have precise knowledge of how much Top background is present in order to state with confidence that the rest of the excess comes from Higgs. So, understanding the Top is a very necessary precursor to discovering a wide range of new physics at the LHC.