Fermilab CMS scientists seek most massive particles yet at the LHC

Fermilab Associate Scientist and Wilson Fellow Jennifer Ngadiuba (left) and Fermilab Research Associate Irene Zoi (right) study this data in collaboration with other CMS scientists. Photo: Irene Dutta

Searching for new fundamental particles that may be produced at particle colliders is exciting — but it is more exciting to actually discover one! Those of us on the CDF and DZero experiments in 1995 at Fermilab’s Tevatron will remember the excitement of discovering the top quark, at 173 GeV, the most massive elementary particle known so far.

The Higgs boson is lighter, at 125 GeV, but was much harder to find because it interacts only weakly with other particles. Discovered in 2012, it needed the higher energy and higher intensity beams at CERN’s Large Hadron Collider, or LHC. The top quark and the Higgs boson were expected to be the last of the so-called Standard Model of particles and forces. But, despite the success of the Standard Model, most theorists are convinced there are more particles to discover. These particles from new physics “Beyond the Standard Model” could be either too massive or too weakly interacting to have shown up in our experiments so far.

Many Fermilab scientists are using the Compact Muon Solenoid, one of the two huge particle detectors at the LHC, for this research. The CMS collaboration has just submitted a new paper, describing the search for new particles 10 times more massive than the top quark, even more than 10 uranium nuclei.

Several theoretical ideas allow for the possibility of such massive, but extremely short-lived, particles in a class called “bosons.” Bosons are defined as having spin, or intrinsic angular momentum, of zero, one or two units. (Particles of matter, such as electrons and quarks, have ½ unit of spin.) Bosons are force-carrier particles, such as photons (electromagnetic force), gluons (strong force) and massive vector bosons W and Z (weak force). The most massive is the Higgs boson H, discovered in 2012 by the ATLAS and CMS collaborations, special because it has no spin and no electric charge, just mass, like an excited piece of vacuum.

The heavy bosons W and Z are extremely unstable and decay before flying about 10-15 m, the diameter of a proton. The average lifetime of the H is about 500 times that of the W — still very short. All these bosons decay in several different ways, one being a pair of quarks that manifest themselves as two narrow sprays, or “jets,” of light particles. The CMS collaboration’s new search focused on the possibility that a much more massive particle X, or particles since there could be several, which would be very unstable, can decay to pairs of these bosons: WW, WZ, ZZ, WH or ZH. Each of these can decay in turn into two quarks, resulting in two narrow jets.

An illustration showing (left) a slow W, Z or H-boson decaying to a pair of quarks, each resulting in a spray of particles, or jet, in a cone. Right: If the boson has high momentum, or high “boost,” the two jets merge into one single, wide jet with sub-structure corresponding to the two original quarks. Image: CMS collaboration

When each boson has high momentum, as expected for a very massive X, the two jets merge into one “fat jet” with sub-structure. The two quarks produced in each of the boson’s decays are emitted close to each other, giving rise to a single, wide jet. When studied in detail, one can see the substructures of these wide jets, which reflect the two original quarks. This peculiar signature, illustrated in the figure, can be used to discriminate against the background events, where the wide jets do not show any substructures.

CMS scientists have been using powerful algorithms, based on state-of-the-art machine learning techniques, to identify the wide jets resulting from boson decays. The new analysis developed further the categorization of events by focusing on each combination of boson pairs, increasing the sensitivity of the search for a massive particle decaying into a pair of bosons.

A computer reconstruction, called an “event display,” of particles emerging from a proton – proton collision in CMS. The colliding proton beams circulate in the (gray) vacuum pipe. Tracks emerging from the collision are shown as lines, the most energetic being nearly straight. The particles deposit their energy in the surrounding “calorimeter” as shown by the blue boxes. The green “towers” show the detailed reconstruction of the particle jets. Image: CMS collaboration

This sophisticated analysis of the data, together with predictions of the distributions expected with no new X particle, did not reveal compelling evidence for an X with mass as high, in some categories, as 4,500 GeV. The data does show intriguing excesses at masses of 2.1 and 2.9 TeV, with a maximum local significance of 3.6 standard deviations from SM expectations. But the global significance is only 2.3 standard deviations, suggesting the excesses may just be upward “statistical fluctuations.”

Now the LHC is running again, providing more data, which will tell us how excited to be about these excesses and whether they will turn into discoveries.

For further information, read a physics briefing by the collaboration.

Mike Albrow is a scientist emeritus in the CMS department at Fermilab.

CMS communications are coordinated by Fermilab distinguished scientist Pushpa Bhat.