Science in action

In 2011, CDF published a paper that showed an unexpected bump in the plot showing the mass of a hypothetical particle that decayed into two jets. This bump was seen only in events with a W boson and two or more jets. CMS conducted a similar search and the data (points) does not seem to agree with a prediction (yellow) based on the CDF data.

Every so often, the blogosphere and Twitterverse hear about a physics measurement and go a little crazy. This happened last year when the CDF collaboration reported a study of events in which a W boson and at least two jets were observed. If they assumed the two jets came from a single particle and plotted the mass distribution of that hypothetical particle, they observed more events with a mass of about 150 GeV than was predicted by the Standard Model. They observed the excess only in events with a W boson and two or more jets. If this observation was confirmed by other experiments, it could have heralded a significant discovery.

The DZero experiment subsequently studied their data and found no such excess. This could mean that there was no discovery to be made or it could mean that DZero missed seeing something truly interesting. In a case like this, it’s good to have a third experiment weigh in to break the tie.

CMS studied their data, sifting out about 100,000 events in which a W boson and at least two jets were created. When a plot was made of the mass of a hypothetical particle that decayed into two jets, no excess was found and placed CMS firmly in the DZero camp.

One should be cautious in interpreting this new measurement, as the Tevatron beams collided protons and antiprotons, while the LHC collides pairs of protons. Still, the evidence disfavors reports of the CDF excess. In addition to this experimental result, several new theoretical models were created to account for the observed CDF bump and these models were definitively ruled out by CMS data.

—Don Lincoln

These analyzers contributed to this analysis.
In order for scientists to analyze CMS data, it must first be validated. Validation means verifying that all equipment is operating properly and that the subsequent reconstruction of the data is done successfully. This ensures that the recorded data has no known flaws. After validation, we can be certain that the bits and bytes that were recorded by the detector were successfully converted into physical objects (jets, muons, electrons, etc.) that subsequent analyzers can use to make measurements. Validation of CMS data is an enormous endeavor, involving physicists from all over the world. The people pictured here are some of the U.S. scientists who play a key role in this complex and crucial task.