Leptoquarks are hypothetical particles that would interact withquarks andleptons. Leptoquarks are color-tripletbosons that carry bothlepton andbaryon numbers. Their otherquantum numbers, likespin, (fractional)electric charge andweak isospin vary among models. Leptoquarks are encountered in various extensions of theStandard Model, such astechnicolor theories, theories of quark–lepton unification (e.g.,Pati–Salam model), orGUTs based onSU(5),SO(10),E6, etc. Leptoquarks are currently searched for in experimentsATLAS andCMS at theLarge Hadron Collider inCERN.
In March 2021, there were some reports to hint at the possible existence of leptoquarks as an unexpected difference in howbottom quarks decay to create electrons or muons. The measurement has been made at a statistical significance of 3.1σ, which is well below the 5σ level that is usually considered a discovery.[1]
Leptoquarks, if they exist, must be heavier than any of the currently knownelementary particles, otherwise they would have already been discovered. Current experimental lower limits on leptoquark mass (depending on their type) are around1 TeV/c2 (i.e., about 1000 times theproton mass).By definition, leptoquarksdecay directly into aquark and alepton or an antilepton. Like most of otherelementary particles, they live for a very short time and are not present in ordinary matter.However, they might be produced in high energy particle collisions such as in particlecolliders or fromcosmic rays hitting the Earth's atmosphere.
Like quarks, leptoquarks must carrycolor and therefore must also interact withgluons. Thisstrong interaction of theirs is important for their production inhadroncolliders (such as theTevatron orLHC).
Several kinds of leptoquarks, depending on theirelectric charge, can be considered:
If a leptoquark with a given charge exists, itsantiparticle with an opposite charge and which would decay intoconjugated states to those listed above, must exist as well.
A leptoquark with given electric charge may, in general, interact with any combination of a lepton and quark with given electric charges (this yields up to3 × 3 = 9 distinct interactions of a single type of a leptoquark). However, experimental searches usually assume that only one of those "channels" is possible. Especially, aQ = 2⁄3 charged leptoquark that decays into apositron and adown quark is called a "firstgeneration leptoquark", a leptoquark that decays intostrange quark andantimuon is a "second-generation leptoquark" etc. Nevertheless, most theories do not bring much of a theoretical motivation to believe that leptoquarks have only a single interaction and that thegeneration of the quark and lepton involved is the same.[2]
Existence of pure leptoquarks would not spoil thebaryon number conservation. However, some theories allow (or require) the leptoquark to also have a diquark interaction vertex. For example, aQ = 2⁄3 charged leptoquark might also decay into two down-type antiquarks. Existence of such a leptoquark-diquark would causeprotons to decay. The current limits on proton lifetime are strong probes of existence of these leptoquark-diquarks. These fields emerge ingrand unification theories; for example, in theGeorgi–Glashow SU(5) model, they are calledX and Y bosons.
In 1997, an excess of events at the HERA accelerator created a stir in theparticle physics community, because one possible explanation of the excess was the involvement of leptoquarks.[3] However, later studies performed both at HERA and at theTevatron with larger samples of data ruled out this possibility for masses of the leptoquark up to around275–325 GeV/c2.[4] Second generation leptoquarks were also looked for and not found.[5]
Current best limits on leptoquarks are set byLHC, which has been searching for the first, second, and third generation of leptoquarks and some mixed-generation leptoquarks[6]and have raised the lower mass limit to about1 TeV/c2.[7] For leptoquarks coupling to a neutrino and a quark 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.[8]
For leptoquarks coupling to electrons and up or down quarks, experiments of atomic parity violation and parity-violating electron scattering set the best limits.
TheLHeC project to add an electron ring to collide bunches with the existingLHC proton ring is proposed as a project to look for higher-generation leptoquarks.[9]
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