Digging deeper in the data

Failure to find quick discoveries in CMS data has led researchers to dig a little deeper, hoping to find the big breakthrough.

Before the LHC began operations, scientists were making many confident predictions that new physical phenomena would be everywhere, just waiting for the intrepid physicist to find it. And I’m not talking about the discovery of the Higgs boson, which, no matter how amazing an accomplishment, was predicted half a century ago. I’m talking about a discovery that reveals something new about the nature of the universe.

We now know that the energy level at which the LHC ran from 2010–2012 wasn’t enough for the hoped-for proliferation of discoveries, but that doesn’t mean a discovery isn’t to be had. Rather it means that any discovery will require some cleverness to uncover. Further, to ensure that a discovery isn’t missed, it means that in any attempt to find something new, scientists should search broadly and be open-minded to new possibilities.

Today’s CMS result satisfies both of those criteria. Physicists looked at events in which six jets were present and, following an algorithm, split the jets into two groups of three. This analysis had two main thrusts. The first was simply to look for objects decaying into three jets, without any preconceived ideas. This was a nice and general search to see if anything unexpected turned up.

The second thrust is a little more intricate, as it was a search for an unusual form of supersymmetry. Models that incorporate supersymmetry predict that in addition to the familiar particles of the Standard Model, there is an entire cast of supersymmetric particles with properties that parallel the properties of the familiar particles.

In supersymmetric models, there is an important quantity called R parity. This is just a number that identifies a particle as being a supersymmetric particle (with R parity = -1) or a Standard Model particle (with R parity = +1). In most models, R parity is “conserved,” which means that whenever a supersymmetric particle is created and subsequently decays, one of its decay products must also have an R parity of -1. Eventually the decay chain leads to the lightest supersymmetric particle (which, incidentally, is a candidate for dark matter).

However, there is a class of supersymmetric theories in which R parity is not conserved. In these theories, the decay products of the supersymmetric particle can only be the familiar particles of the Standard Model. One such possibility occurs when a supersymmetric gluon, called a gluino, decays. If R parity is not conserved, the decay products of a gluino could be three jets.

CMS scientists saw no evidence that there was an excess of three-jet production. This was a general observation that ruled out an undiscovered particle that decayed into three jets. This same measurement could be interpreted more precisely for models of gluino production in which R parity was violated. This study resulted in stringent limits being set on this kind of gluino production. Thus for both general investigation and the one on gluinos, the measurement was a very successful addition to CMS’s long list of important accomplishments.

Don Lincoln

These US CMS scientists contributed to this analysis.
After seven years, Joel Butler has stepped down as US CMS program operations manager. Patricia McBride has taken over for Joel. The US CMS community is grateful for Joel’s many years of service and wishes the best of luck to Patricia in her challenging new role.