There may be particles that can convert quarks into leptons and vice versa: the leptoquarks. What is a leptoquark?
Leptoquark: Quarks and Leptons
All stable, known (baryonic) matter consists of quarks and leptons. To be precise: the lightest versions of this, the “first generation”. These are the electron and the up and down quark. The second generation are muon and charm / strange quarks. The third, heaviest generation consists of the tauon and top / bottom quarks. The second and third generations have very short lives and, as far as we know, only occur in particle accelerators and cosmic rays. Protons consist of two up quarks and one down quark; neutrons consist of one up quark and two down quarks. Below is a handy overview of all particles in the Standard Model. Electron, muon and tauneutrino are virtually massless.
Quarks have a curious charge. Up quarks +2/3, down quarks -1/3. Remarkable, because electrons do have a whole charge: -1. Would there be more fundamental particles than electrons? This question inspired the inventors of the leptoquark model. But what exactly are leptoquarks?
To be or not to be: do leptoquarks exist?
Physicists are always looking for simplicity. In theory, the Standard Model (SM) can be a lot simpler. For example, if the quarks and leptons could be converted into each other. This with an extremely heavy messenger particle. This particle must then be able to change the baryon number and lepton number: the leptoquark. One thing we know: this hypothetical particle must be a lot heavier than all previously discovered particles. The mass must be at least a TeV / c ^ 2. In other words, be at least a thousand times as heavy as a proton. This is above the maximum power of the CERN particle accelerator. Otherwise we would have discovered this particle by now.
When a leptoquark falls apart, it happens in a lepton and a quark. The theories with leptoquarks collected dust in the drawer so far. But now things are different, because the measurements are no longer correct with the SM. The tension therefore rises.
Unexplained b-meson anomalies caused by the leptoquark?
Measurements of the decay process of b-mesons deviate from the predictions of the Standard Model. These deviations are quite substantial, but have not yet been established reliably enough (with six sigma). That is why the researchers at CERN are still cautious. This is also the only place where the measured values deviate from the prediction of the Standard Model. No wonder the tension is mounting. Are we now on the trail of new physics?
No leptoquarks found, but lower limit of mass determined
The Large Hadron Collider has yielded an enormous amount of data in recent years, to which numerous analyzes are now being applied. It Compact Muon Solenoid (CMS) team looked for third-generation leptoquarks in a data sample of proton-proton collisions. This sample comes from the collision data of the Large Hadron Collider (LHC) with an energy of 13 TeV. These were recorded by the CMS experiment between 2016 and 2018. 
Specifically, the team looked for pairs of leptoquarks that turn into a top-down quark and a tauon or tau neutrino, as well as 'loose' leptoquarks that are produced along with a tau neutrino and transform into a top quark and a tauon. The CMS researchers found no indication that such leptoquarks were produced in the collisions. 
However, they were able to set lower limits for their mass. If third generation leptoquarks exist, they are at least 0.98–1.73 TeV / c ^ 2 in mass.  This in turn depends on their spin and the strength of their interaction with a quark and a lepton. These limits are some of the strictest yet for third generation leptoquarks. Thanks to these boundaries, part of the leptoquark mass range that could explain the B meson anomalies can be excluded. Including all the theories that require the existence of these lighter leptoquarks.
Good news: we will continue to exist for the time being
That's good news, at least if this high mass also applies to first-generation leptoquarks. Because the heavier the leptoquark, the smaller the chance that it will spontaneously form from the vacuum and disintegrate a proton. So protons will remain stable for quadrillion years. And since we consist largely of protons, that's quite a comforting thought ...
1. CMS sets new bounds on the mass of leptoquarks, CERN, 2020
2. The flavor of new physics, CERN Courier, 2019
3. Search for singly and pair-produced leptoquarks coupling to third-generation fermions in proton-proton collisions at s√ = 13 TeV, CERN (submitted to Physical Review Letters B)