Dec 21, 2020
Physicists from the CMS (Compact Muon Solenoid) Collaboration at CERN’s Large Hadron Collider (LHC) report the results of a new search for leptoquarks produced singly and in pairs in proton-proton collisions.
A collision event recorded by CMS at the start of the data-taking run of 2018. CMS sifts through such collisions up to 40 million times per second looking for signs of leptoquarks. Image credit: Thomas McCauley / Tai Sakuma / CMS / CERN.
At the most fundamental level, matter is made up of two types of particles: leptons, such as the electron, and quarks, which combine to form protons, neutrons and other composite particles.
Under the Standard Model, both leptons and quarks fall into three generations of increasing mass. Otherwise, the two kinds of particles are distinct.
But some theories that extend the Standard Model predict the existence of new particles called leptoquarks that would unify quarks and leptons by interacting with both.
Searches for these hypothetical particles have been an important part of the LHC’s research program since the beginning.
In a paper to be published in the journal Physics Letters B, the CMS Collaboration reports the results of its latest search for leptoquarks that would interact with third-generation quarks and leptons.
Such third-generation leptoquarks are a possible explanation for an array of tensions with the Standard Model, which have been seen in certain transformations of particles called B mesons but have yet to be confirmed. There is therefore an additional reason for hunting down these particles.
The CMS physicists looked for third-generation leptoquarks in a data sample of proton-proton collisions that were produced by LHC at an energy of 13 TeV and were recorded by the CMS experiment between 2016 and 2018.
Specifically, they looked for pairs of leptoquarks that transform into a top or bottom quark and a tau lepton or tau neutrino, as well as for single leptoquarks that are produced together with a tau neutrino and transform into a top quark and a tau lepton.
The researchers didn’t find any indication that such leptoquarks were produced in the collisions.
However, they were able to set lower bounds on their mass: they found that such leptoquarks would need to be at least 0.98-1.73 TeV in mass, depending on their intrinsic spin and the strength of their interaction with a quark and a lepton.
“These results are the most stringent limits to date on the presence of leptoquarks that couple preferentially to the third generation of fermions,” the scientists said.
“They also probe the parameter space preferred by the B-physics anomalies in several models, excluding relevant portions.”
“As theories predict leptoquark masses as high as many tens of TeV, the pursuit of this promising solution for the unification of quarks and leptons must continue.”
CMS Collaboration. 2020. Search for singly and pair-produced leptoquarks coupling to third-generation fermions in proton-proton collisions at s√= 13 TeV. Physics Letters B, in press; arXiv: 2012.04178
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