|This plot shows the mjet1 versus mjet2 distribution for the data taken in this experiment.|
At CDF, protons of energy 1 TeV, or 1 trillion electronvolts, collided with antiprotons of equal energy.
In many of these events, we observe a phenomenon called a jet. A jet is a spray of particles all moving in the same direction and typically originating from a practically massless subatomic particle, which is why it is also expected to have a low mass. It is fascinating that in some cases these jets have masses that are a substantial fraction of their energy.
Scientists have studied events in which a very large fraction — at least 40 percent — of the collision energy is transformed into just two such jets. Based on the internal structure of these jets, we have found that they appear mostly to come from very energetic quarks.
There are six different flavors of quarks, with five of the six having masses that are small compared to the masses of the jets we see in these two-jet (or “dijet”) events.
If these jets originate from the lighter quarks, then we would expect to see a high occurrence of jets with low masses. The above figure plots the masses of one jet against the other, and indeed we see that most of the events in our sample have two jets where each has a mass between 40 and 60 GeV/c2, or between 50 and 70 proton masses. This amount of mass is consistent with predictions of quantum chromodynamics, the theory describing the strong interaction.
But what if some of these massive two-jet events were really coming from the production of the super-massive top quark, which has a mass of 173.34 ± 0.76 GeV/c2? We then would expect to see a cluster of events in which both jets had masses between about 140 and 200 GeV/c2. Although there are roughly 30 such events in our data, as seen in the figure, it is only slightly more than we might have expected from the very occasional production of two very massive jets from the lighter quarks.
We can use these data to set an upper limit on the rate of top quarks being produced at these very high energies at about 40 femtobarns, or no more often than about one collision in every trillion. Our current understanding of the strong interactions is that the expected rate of top quark production corresponding to two-jet events is about 5 femtobarns.