Fermilab LPC researchers find unexpected events with CMS

For decades, researchers at Fermilab have been world leaders — precisely measuring known physics and scouring the data, looking for hints that our current theories are incomplete and that we need to revisit and revise the Standard Model. These hints might even require that it be replaced by a newer, more accurate model. This heritage continued at the recent Rencontres de Moriond conference, held annually in the Italian town of La Thuile, where new results could indicate that discoveries are right around the corner.

One physics measurement, led by physicists at the Fermilab LHC Physics Center, caused quite a buzz in the particle physics community. It studied the simultaneous creation of four high-energy “jets,” which are created when quarks and gluons are knocked out of a collision between two protons. The recent result is hard to reconcile with accepted theory and suggests that undiscovered phenomena could exist.

CMS event in which four jets were produced. The Standard Model predicts that events like these should be very rare, implying that possibly new physical phenomena is at work. Photo: CMS collaboration

Senior scientist Rob Harris has long been recognized as a world leader in the study of high-energy jets produced at a hadron collider. He and Rutgers University postdoctoral researcher Marc Osherson have been digging through CMS data, looking for these “four jet” events. Osherson is part of a group led by Professor Eva Halkiadakis, a former LPC Fellow.

Because the Standard Model has been so thoroughly explored at lower-energy facilities like the Fermilab Tevatron and the CERN LEP accelerator, many researchers believe that the best path forward is to create the highest-energy collisions current technology can provide, and this is only possible at the CERN Large Hadron Collider. Furthermore, collisions mediated by the strong nuclear force are the most common, and it is through the collision and creation of quarks and gluons that researchers will be able to study the highest-energy particle production. It is for this reason that researchers are keen to investigate the highest-energy jets possible.

Earlier measurements studied events in which two protons collided and produced two high-energy jets. The idea is that the energy from the proton collisions would create some sort of new state of matter or involve new physics that would turn into quarks and/or gluons. However, the data from these earlier studies agreed quite well with the Standard Model, suggesting no need for changes to currently accepted theory.

So, researchers considered another scenario, one in which four jets were simultaneously produced. These jets were grouped in pairs, and each pair came from an intermediate particle. Because physicists do not have a well-accepted theory to guide them, they used the placeholder “X” to denote the intermediate particle.

Essentially, they were looking for the process where two protons (p) collided and created two X particles. The X particles each then decayed into two jets (j) (i.e., pp → XX → (jj)(jj)) in what is called “non-resonant” production. Researchers also considered another hypothesis, where there was yet another intermediate particle that they called a Y, and the Y then decayed into the two X particles (i.e., pp→Y→XX→(jj)(jj)), which is called “resonant” production. Non-resonant production is similar to how top quarks were discovered at the Tevatron, and resonant production is similar to how the Higgs boson was discovered at the LHC.

The fascinating thing is that researchers found two events that were consistent with a particle with a mass of 8 TeV decaying into two objects, each with a mass of 2 TeV. Events with these properties are very unlikely in the Standard Model and point to the possibility that new physics exists.

The researchers used specific theoretical models to evaluate the significance of this observation. They found that for resonant production, the observation had a local significance of 3.9 and a global significance of 1.6 for an object Y with a mass of 8.6 TeV and a mass of 2.15 TeV for X. A local significance is the significance of specifically what you observed, while a global significance considers all possibilities. A significance above 3.0 is considered to be evidence of the existence of something.

For non-resonant prediction, the Y particle isn’t produced. Instead, this analysis had a global significance of 3.6 and a local significance of 2.5 for an X particle with a mass of 0.95 TeV.

These significances are tantalizingly close to the significance of 3 necessary for claiming evidence of a discovery, but still far away from the significance of 5 required to claim that something has been observed. So, it’s too early to get very excited.

However, with the imminent restart of the LHC, scientists are delighted at the prospect of collecting additional data to see if this observation is the first signs of new physics or just a statistical fluke. Even more exciting is that the LHC will resume operations at a collision energy of 13.6 TeV, which is higher than the previous operating energy of 13 TeV. This small increase in collision energy will double the production rate of events with the properties similar to those already observed.

Measurements involving quarks are not the only place where tantalizing measurements are observed. Experiments studying leptons have also found exciting hints of new physics. In the spring, the g-2 experiment unveiled a deviation from SM expectation with a significance of 4.2, which indicates the possibility of undiscovered particles that interact with electrons and muons.

And the LHCb experiment followed up in the studies of b quark decays and found that the decays into electrons and muons weren’t the same. This disagrees with predictions from the Standard Model, which means this is another possible hint of undiscovered physics.

The next several years are going to be very exciting.

Don Lincoln is a senior scientist in the CMS department.