If you’re a Chicago Bears fan, it doesn’t take much imagination to be able to identify some of the traits that an ideal quarterback would have. You can probably even come up with a numerical criterion or two for that ideal quarterback. Passing yardage and completion percentage come to mind, although even high numbers for those are less than ideal if your quarterback throws a lot of interceptions (nope, not naming any names here). So, what you really want is to look at several metrics all at once to determine your quarterback selection criteria, or, better yet, to combine those metrics into one number. Scouts, analysts and fans accomplish the latter using Quarterback Rating: an equation that combines all of the aforementioned metrics and then some to produce a single number whose value gives you a better measure of how good a quarterback is than any single measured statistic can.
Particle physicists similarly have “measured statistics” in the form of particle energies and masses measured in detectors. The challenge is in using these measured quantities to differentiate between events potentially resulting from new physics and events resulting from known Standard Model processes. One theory that describes such new physics, supersymmetry, would address many open questions in particle physics, yet it remains elusive. To help identify any potential supersymmetric event, physicists form their own versions of quarterback ratings: variables that combine several measured quantities into one that helps identify those events.
In a recent a recent paper submitted to the Journal of High Energy Physics, physicists performed a search for supersymmetric events in the most recent data set of 13-TeV collisions collected by the CMS detector. These events consist of jets and energy imbalance, or “missing energy,” that could be caused by supersymmetric particles escaping undetected. In order to help identify events that may result from supersymmetry, the CMS search uses a variable “MT2,” a modified version of a variable called “transverse mass” (MT), which was first devised to measure the W boson mass in the early 1980s. The value of MT2 is calculated for each event and combines the measured jet energies and observed energy imbalance to produce a quantity that peaks higher for supersymmetric events than for most Standard Model processes.
The search yielded only events that were consistent with known Standard Model processes. The increase in the LHC’s energy between Run 1 and Run 2 did allow CMS physicists to place significantly higher limits on the possible masses of supersymmetric particles than with prior searches. The hunt for supersymmetry doesn’t end there though. Just as football scouts have advanced metrics other than quarterback rating to assess potential star players, physicists have “advanced metrics” other than MT2 to apply to CMS data. Expect more such searches as Run 2 of the LHC continues.