CMS has observed the Ξ baryon by looking for events containing both lambda baryons and pi mesons. If these two particles are observed and came from a single parent particle, we expect to see a narrow peak in our data. This measurement used data taken in December, including the data taken during the period when the LHC ran at record-breaking energy, 20 percent higher than the Tevatron.
Some things are far stranger than others, and this week I’m reporting on the strangest thing CMS has seen so far. “Strange” in this context doesn’t mean “weird” but rather describes something about the quarks inside the newly rediscovered particle.
There are many particles that are produced at the LHC. One particular type is called the baryon. Baryons are a class of particles that contain exactly three quarks. The most familiar baryons are the proton and neutron, which are made of up and down quarks. Indeed, the up and down quarks are all that is necessary to make up our universe. However, we know that four other kinds of quarks exist: strange, charm, bottom and top. Finding particles with these additional quarks is an important way-station in the CMS collaboration’s journey to fully understanding our detector.
One interesting particle is the Ξ (the Greek letter xi) baryon. Discovered in 1964, this type of particle is also called the cascade particle because of its distinctive decay pattern. All baryons contain three quarks, but in order to be a Ξ baryon, two of those quarks must be strange quarks. Scientists observe these particles by their decay into a π (pi) meson and a λ (lambda) baryon. The lambda baryon contains a single strange quark and CMS collaborators observed the lambda baryon in December.
CMS physicists searched in their data, looking for collisions in which lambda baryons and pi mesons were created. They then asked the question “If these two particles were the decay product of a single particle, what would be the mass of the parent particle?” They then plotted the mass of the potential parent. If the two particles didn’t have a single parent, all masses would occur with nearly equal frequency. However, if they had a single parent, we expect to see a narrow peak on a wide background. As seen in the figure above, these scientists clearly observed the Ξ baryon with the mass we expect from earlier measurements.
With this success, CMS continues its rediscovery of the Standard Model.
By Don Lincoln
This story first appeared in Fermilab Today on March 12, 2010.
