After a spring that turned out to be more hectic than I expected, I am finally getting back to working on a physics analysis with the CDF detector. I am trying to measure the mass and lifetime of a particle called the B_c meson – a bound state of a bottom quark and a charm quark. There is nothing surprising (yet?) about the B_c – it should and does exist, but we need to measure its properties experimentally to confirm or refute the predictions. Hints of the B_c were first reported by experiments at the LEP electron-positron collider, but its existence was solidly established by CDF several years ago. Currently, the Tevatron is the only accelerator that can produce the B_c, so both CDF and D0 are studying it.
As with most unstable particles, we only measure the trajectories and energies (or momenta) of the particle’s decay products, not the particle itself. The B_c can decay into many different states, but I am using the B_c → J/ψ π decay channel; the J/ψ is a charm-anticharm quark meson that itself decays into a pair of muons, while the π (pion) is a well-known up-down quark meson (the pion decays, too, but usually not before we measure its momentum). We can use conservation of energy and momentum to reconstruct the mass of the B_c. Seeing how far the J/ψ π origin point (vertex) is from the collision point of the incoming proton and antiproton beams indicates how far the B_c traveled before decaying – for the B_c, most decay before traveling even a few hundred microns, corresponding to a lifetime of less than 1 ps = 1 trillionth of a second.
The J/ψ is a particularly useful particle in experimental high-energy physics – its decay to two muons is often used as a “trigger” to help record the process of interest. We start with a sample of events with a J/ψ and then search for events containing a π that also originates from the same point as the J/ψ muons. Most events passing our selection criteria are background that is difficult to eliminate. For any given event, we don’t know if it contains a true B_c → J/ψ π (our signal) or something that just looks like one. We need to look at events where there is no signal to understand how the background affects the mass and lifetime measurements. We then combine the data from the selected events (signal + background) to make the measurements.
In the end, there will likely be only a couple hundred signal events scattered among the hundreds of thousands of background. It’s not easy to extract those measurements, especially the lifetime. Convincing myself and my colleagues that we understand the background will be challenging. I sure wish I had more time to spend on the analysis – it’s hard to make steady progress when working on it only a morning here or an afternoon there. In a moment like this, I envy students who can spend nearly all their time on their physics analyses.
