Leptoquarks strike back

Leptoquarks are hypothetical scalar particles that carry both color and electroweak charges. Nothing like that exists in the Standard Model, where the only scalar is the Higgs who is a color singlet. In the particle community, leptoquarks enjoy the similar status as Nickelback in music: everybody's heard of them, but no one likes them.  It is not completely clear why... maybe they are confused with leprechauns, maybe  because they sometimes lead to proton decay, or maybe because they rarely arise in cherished models of new physics.  However,  recently there has been some renewed interest in leptoquarks.  The reason is that these particles seem well equipped to address the hottest topic of this year - the B meson anomalies.

There are at least 3 distinct B-meson anomalies that are currently intriguing:

1.  A few sigma (2 to 4, depending who you ask) deviation in differential distribution of B → K*μμ decays, 
2.  2.6 sigma violation of  lepton flavor universality in  B → Kμμ vs B → Kee decays, 
3.  3.5 sigma violation of lepton flavor universality, but this time in  B → Dτν vs B → Dμν decays. 

Now, leptoquarks with masses in the TeV ballpark can explain either of these anomalies.  How? In analogy to the Higgs, leptoquarks may interact with the Standard Model fermions via Yukawa couplings. Which interactions  are possible is determined by  its color and electroweak charges. For example, this paper proposed a leptoquark transforming as (3,2,1/6) under the Standard Model gauge symmetry (color SU(3) triplet like quarks, weak SU(2) doublet like Higgs,  hypercharge 1/6).  Such particle can have the following Yukawa couplings with b- and s-quarks and muons:

 If both  λb and λs  are non-zero then a tree-level leptoquark exchange can mediate the b-quark decay  b → s μ μ.  This contribution  adds up to the Standard Model amplitudes  mediated by loops of W bosons, and thus affects the B-meson observables. It turns out that the first two anomalies listed above can be fit if the leptoquark mass is in the 1-50 TeV range, depending on the magnitude of λb and λs.

Also the 3rd anomaly above can be easily  explained by leptoquarks. One example from this paper is a leptoquark transforming as (3,1,-1/3) and coupling to matter as

This particle contributes to  b → c τ ν, adding up to the tree-level W boson contribution, and is capable of explaining the apparent excess of semi-leptonic B meson decays into D mesons and tau leptons observed by the BaBar, Belle, and LHCb experiments. The difference to the previous case is that this leptoquark has to be less massive, closer to the TeV scale, because it has to compete with the tree-level contribution in the Standard Model.

There are more kinds of leptoquarks with different charges that allow for Yukawa couplings to matter. Some of them could also explain the 3 sigma discrepancy of the experimentally measured muon anomalous magnetic moment with the Standard Model prediction. Actually, a recent paper says that the (3,1,-1/3) leptoquark discussed above can explain all B-meson and muon g-2 anomalies simultaneously, through a combination of tree-level and loop effects.  In any case, this is something to look out for in this and next year's data.  If a leptoquark is indeed the culprit for the B → Dτν excess, it should be within reach of the 13 TeV run (for the 1st two anomalies it may well be too heavy to produce at the LHC).   The current reach for leptoquarks is up to 1 TeV mass (strongly depending on model details),  see e.g. the recent ATLAS and CMS analyses. So far these searches have provoked little public interest, but that may change soon...