Research Highlight

Electron's super-heavy cousins evade detection

doi:10.1038/nindia.2019.72 Published online 6 June 2019

Particle physicists at the Indian Institute of Science Education and Research (IISER) in Pune report that the most extensive hunt yet for 'vector-like leptons' – a new kind of fundamental particles predicted by some theorists – has not met any success.

The discovery of these particles – heavier cousins of electron – would have thrown new light on the most fundamental building blocks of the universe, Sourabh Dube and graduate student Anshul Kapoor report in a preprint paper submitted to Physical Review-D 1. The duo has been working in collaboration with scientists at the Compact Muon Solenoid (CMS), a general-purpose detector at the Large Hadron Collider (LHC) of CERN in Geneva.

Leptons are a class of particles that form an important part of the 'Standard Model' of physics, a theory that explains how fundamental particles interact. Negatively charged electrons are the best examples of leptons, to which two other particles (muons and "tau" leptons) that can only be produced in high energy collisions, also belong.

A depiction of how the production of vector-like leptons might look like in the CMS experiment at the LHC. This image shows 4 muons (red trajectories) passing through the muon detectors of the CMS experiment.

© CMS e-Lab / I2U2

Leptons can be left-handed or right-handed (like the left hand appearing to be the right hand in a mirror). But physicists know that left and right-handed leptons do not behave similarly (their mirror world differs from the real world) and thus "violate parity". However, many particle physics theories predict the existence of two new leptons, whose left- and right-handed components will not behave differently – which means they will not violate parity.

"If we find these 'vector-like' leptons, it will fundamentally alter our views about how parity is violated in nature and would have a significant effect on our understanding of the most fundamental building blocks of the universe," the authors report.

Dube and Kapoor searched for these particles by analyzing proton-proton collision data collected by the CMS experiment in 2016 and 2017. But they found no evidence of vector-like leptons in the mass range of 120-790 GeV, the report says. "If at all nature has a vector-like lepton pair, it must be super heavy," the researchers conclude.

"In the future, we may find it, or maybe it does not exist at all," they say.


1. CMS Collaboration. Search for vector-like leptons in multilepton final states in proton-proton collisions at √s = 13 TeV. (2019) e-Print: