Result

The top (CDF) and bottom (DZero) images show the expected and observed 95 percent confidence level upper limits on the production rate of a Higgs boson as a multiple of the Standard Model prediction, assuming standard model decay branching ratios. The solid, horizontal line shows the prediction for the Higgs boson according to the Standard Model. We determine our measurement by how our data relates to this solid line. The figures have two squiggly lines: one dotted and one solid….

This analysis searched for another generation of quarks even heavier than the top quark discovered at Fermilab in 1995. There are many fascinating measurements that have been made at particle colliders over the years, but perhaps my favorite was done at the LEP accelerator at CERN. After varying the beam energy, physicists made a plot showing the production of Z bosons in comparison to theoretical curves for the existence of two, three and four neutrinos. The data essentially demonstrated that…

The production of two Z bosons in a single collision is among the rarest phenomena expected to occur at the Tevatron. It is about 20,000 times rarer than finding a single W boson. Exploiting the entire Run II DZero data set netted just 10 events of this kind. The weak nuclear force is the weakest interaction that physicists at the Tevatron have successfully studied. Like all subatomic forces, we can understand it as being caused by the exchange of force…

The plot shows the rapidity difference between the reconstructed top and anti-top quarks in data, and the predicted signal and background simulations. The observed forward backward asymmetry in the data is Afb = 14 ± 5 percent. The corresponding reconstructed forward-backward asymmetry, after correcting for experimental effects, is Afb = 42 ± 16 percent, which can be compared to the Standard Model prediction of Afb = 6 ± 1 percent Symmetries have long been an important part of characterizing physical…

Collision events in the figure above are arranged according to degree of momentum imbalance: balanced events pile up on the left while unbalanced events are put on the right. One of the supersymmetric models that CMS ruled out is shown in yellow: if this model were true, the physicists would have observed twice as many unbalanced events (See figure 2 in the paper for details). Editor’s note: This is the first CMS Result written by Jim Pivarski, a CMS collaborator….

In the telephone game a message is garbled by changes in how it is repeated. Today’s article describes how we defeat a similar subatomic process using the world’s largest collection of top quarks. When I was young, a bunch of us would play the telephone game. In this game, a person would whisper a message to someone. That person would whisper it to someone else and, after a series of whispers, we’d compare what was the last person heard to…

In the insert figure, the measured top quark mass is shown at the bottom of the curve. In the larger image, the blue points show the data from the most recent measurement and the red lines show how well the data fit with a prediction of the signal and background combined. The black line shows the predicted background only. The top quark is the heaviest currently known elementary particle, with a mass nearly equal to that of a gold atom….

Particle collisions that look like this are used to search for leptoquarks. This figure is specifically for (electron + up/down quark) type collisions. Leptoquarks are theoretical particles. While there are theories that suggest these particles are real, we live in a world full of leptons (the electron is the most familiar example) and quarks (which are generally found in the nucleus of an atom.) We don’t understand why there are two types of particles (quarks and leptons) and we have…

Sand fairies redux

The top figure shows the normal production and decay processes of top quarks. In the bottom figure, the magenta section shows a FCNC process. This type of event is forbidden in the Standard Model and, if it were observed, would require rewriting of the books. In an earlier article, I compared the search for something that the Standard Model deems impossible to searching for tiny fairies in the sand on a beach. I also promised that this wouldn’t be the…

Some of the Feynman diagrams that contribute to the decay of a D0 (a meson composed of a charm quark and up anti-quark) to a pair of muons. The loops (square or triangle) in the diagrams are an indication that the processes they represent are suppressed. This decay has no processes without loops in the Standard Model. But new physics can have processes without loops and these can dominate over the Standard Model contribution, so this decay becomes an interesting…