No Evidence of a New Generation of Quarks (Yet)


Physicists working on the Compact Muon Solenoid experiment at the Large Hadron Collider at CERN, Switzerland have yet to find evidence of a fourth-generation heavy quark, the so-called “vector quark” or T quark. But they’re still looking and they have come up with some pretty ingenious ways to search.

There are several UC Davis physicists on the 3,000-plus CMS science team, including professors Robin Erbacher, John Conway, Maxwell Chertok, Michael Mulhearn and Mani Tripathi. Postdoctoral researcher Justin Pilot and graduate student Reyer Band also worked on the T quark search.

Quarks are fundamental particles that combine in different ways to produce larger sub-atomic particles including neutrons and protons. Detecting quarks involves smashing other particles together at extremely high energies then analyzing the spray of particles, or jet, to see what came out.

The Large Hadron Collider is currently the world’s most powerful particle-smashing instrument. In 2012, the machine produced the first evidence for the long-predicted, but never before seen, Higgs boson.

Following discovery of the Higgs boson, scientists at the LHC turned their attention to other particles, including a possible fourth generation of quarks. Third-generation quarks were discovered in the 1970s, but new types of heavy quarks (which can only be seen at very high energies) would help explain the properties of the Higgs boson.

Using special relativity to understand quark decay

Theorists predict that T quarks could decay in three ways: to a bottom quark and a W boson, to a top quark and a Z boson, or to a top quark and a Higgs boson. Assuming that T quarks exist, of course.

Top, bottom, W and Z quarks and Higgs bosons also decay in specific ways, into characteristic jets of particles. The problem is, from our point of view these particles are moving at nearly the speed of light and the jets look pretty much the same.

A new approach uses Einstein’s Theory of Special Relativity to change the frame of reference so that the jets appear as they would if you, the observer, were moving at the same speed.

The researchers used machine learning to train an algorithm on 126 possible jet patterns, adjusted by special relativity. Then they used the algorithm to scan through a year’s worth of data collected from the collider.

They did not find evidence of T quarks in data from 2016 and ruled out T quarks up to a certain mass. But they are refining their methods and continuing the search through data from 2017 and 2018.

Professors emeritus Winston Ko, Richard Lander and David Pellett have also been part of the CMS team, together with research scientists Richard Breedon, Timothy Cox and John Smith.

Andy Fell, UC Davis News and Media Relations, for the Egghead Blog