Leptoquarks are hypothetical particles that carry information between quarks and leptons of a given generation that allow quarks and leptons to interact. They are color-triplet bosons that carry both lepton and baryon numbers. They are encountered in various extensions of the Standard Model, such as technicolor theories or GUTs based on Pati–Salam model, SU(5) or E6, etc. Their quantum numbers like spin, (fractional) electric charge and weak isospin vary among theories.
Leptoquarks, predicted to be nearly as heavy as an atom of lead, could only be created at high energies, and would decay rapidly. A third generation leptoquark, for example, might decay into a bottom quark and a tau lepton. Some theorists propose that the 'leptoquark' observed by HERA and DESY could be a new force that bonds positrons and quarks or be examples of preons found at high energies. Leptoquarks could explain the reason for the three generations of matter. Furthermore, leptoquarks could explain why the same number of quarks and leptons exist and many other similarities between the quark and the lepton sectors. At high energies, when leptons that do not feel the strong force and quarks that cannot be separately observed because of the strong force become one, it could form a more fundamental particle and describe a higher symmetry. There would be three kinds of leptoquarks made of the leptons and quarks of each generation.
In 1997, an excess of events at the HERA accelerator created a stir in the particle physics community, because one possible explanation of the excess was the involvement of leptoquarks. However, later studies performed both at HERA and at the Tevatron with larger samples of data ruled out this possibility for masses of the leptoquark up to around 275–325 GeV. Second generation leptoquarks were also looked for and not found. More recent studies, performed at the LHC, have raised the excluded range to about 1 TeV. For leptoquarks to be proven to exist, the missing energy in particle collisions attributed to neutrinos would have to be excessively energetic. It is likely that the creation of leptoquarks would mimic the creation of massive quarks.
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