Fermilab CMS research associates Abhijith Gandrakota, Nicholas Smith and Oz Amram on the ski lift at La Thuile.
Every spring, particle physics researchers gather at the Planibel Hotel in northern Italy for a series of conferences where they share the most exciting recent analyses. Between the wine and visits to the ski slopes, scientists give talks that describe their discoveries. This year, four of Fermilab’s CMS postdoctoral researchers were among those invited to share their work and the work of their colleagues.
The tradition of meeting in the Alps each spring is an old one, and there is more than one conference each year. The first Rencontres de Moriond conference was held in 1966, while the La Thuile conference began in 1987. Attendance at the two conferences is by invitation only, reflecting the selectivity of the topics discussed.
This year, the La Thuile conference took place in early March. At it, Fermilab research associate Lesya Horyn presented selected electroweak measurements from both ATLAS and CMS. These results included the simultaneous production of W bosons, photons and jets, which explore rare types of interactions called anomalous quartic gauge couplings, or aQGC. The analysis set very stringent limits on the process.
Horyn also presented an analysis measuring for the first time the simultaneous production of a Z boson and two photons as a function of the combined mass of the three objects. She also presented a study of the joint polarization of W and Z bosons. Again, this was the first time this result has been shared with the scientific community.
In contrast, the Moriond conference took place at the end of March. Here, three Fermilab research associates gave presentations.
The first was by Oz Amram, who presented, for both the ATLAS and CMS collaborations, studies of W and Z production in association with quarks. Because W and Z production is relatively well understood, these analyses are powerful probes of QCD, the model of strong interactions. Also, because these processes can be both calculated and measured with excellent precision, they are an ideal laboratory in which to search for unknown physics.
Research associate Abhijith Gandrakota presented searches for new particles. Generically, new particles are called resonances. The name originates from the same phenomenon as when you blow on the top of a bottle. Blow too fast or too slow and nothing happens; but, blow at the right speed, and the bottle makes a sound.
For particles, it’s the same thing. If you smash beam particles together at random speeds, they just scatter. However, if you get the energy just right, lots of particles can be made. Because of Einstein’s equation E = mc2, energy and mass are equivalent. Thus, the right collision energy results in particles with a specific mass being created.
Both ATLAS and CMS are conducting dozens of searches for new particles, and Gandrakota could only present a select subset. He presented searches for low-mass particles decaying into pairs of leptons, and a search for what is called a b* (b-star) quark, which is a new quark heavier than a top quark. Other analyses include looking for what is called a Z’ (Z-prime), which is a hypothetical light cousin of the well-known Z particle. He also presented a search for what are called “leptoquarks,” which are hypothetical particles that are like a quark and a lepton blended together. While none of these searches resulted in discoveries, they were successful at ruling out some mass ranges, which means future searches will have fewer places to look.
Finally, Fermilab research associate Nick Smith presented a series of what are called “indirect measurements.” One set of measurements capitalized on the LHC’s unique capability to make Higgs bosons. While the Higgs boson was discovered over a decade ago, it remains an open question as to whether the particle discovered in 2012 is the same one that Peter Higgs and others predicted over half a century ago. To establish this, researchers need to explore how the potential energy changes as a function of the strength of the Higgs field. One way to do this is to search for events in which two Higgs bosons are created. In addition, events in which two Higgs bosons are created can shed light on exactly how they interact with their decay products.
Another class of measurements presented by Smith involve the simultaneous production of four fermions (electrons, muons, taus and neutrinos). In events in which top quarks are produced, the standard model sets stringent limits on which combinations are possible. Looking for events in which these rules are not honored is a great way to look for new physics.
Yet another analysis that Smith presented explores what are called “flavor-changing neutral currents,” which is just a fancy way of saying decays of quarks that don’t change the electric charge of the quark in the decay. This is just decays of top quarks into charm or up quarks, or decays of bottom quarks into strange or down quarks. These types of decays are forbidden by the Standard Model, which again makes them a very sensitive laboratory in which to search for new physics.
None of the analyses presented hinted at new physics, but they all highlighted just how valuable the high-energy and high-statistics LHC datasets are. As a sign of just how important these particular results were regarded by the events’ organizers, they all received prominent mention in the closing talk of the conferences.
Of course, ATLAS and CMS are just getting started. Over the next two decades, each experiment expects to increase their datasets at least tenfold and probably much more than that. And these young Fermilab physicists will be leading the field forward into the future.
Don Lincoln is a senior scientist in the CMS department of the Particle Physics Directorate.
CMS department communications are coordinated by Fermilab distinguished scientist Pushpa Bhat.