Last month, Admir Greljo (University of Mainz) and Abdollah Mohammad (Kansas State University, US) during the Collider Cross Talk, gave a comprehensive overview of the theoretical and experimental aspects in leptoquarks searches.

Leptoquarks are hypothetical particles that can turn quarks into leptons and vice versa and they can be either scalar (spin-zero) or vector (spin-one) particles. Moreover, they participate both in QCD and electroweak interactions in addition to the direct quark-lepton coupling as they have both a color and an electroweak charge.

Recently there has been some renewed interest in leptoquarks. The reason is that these particles seem well equipped to address some of the hottest topic in the search for new physics that lie beyond the Standard Model. Moreover, recent hints of lepton universality violation in semileptonic B-meson decays strengthened the interest in leptoquarks.

As Admir Greljo explains: “Leptoquarks are quite common in models beyond the Standard Model. Such particles typically arise as composite resonances of a hypothetical new strong dynamics at the TeV scale. They help us to address the electroweak scale stabilization problem (the smallness of the Higgs boson mass) in a natural way”. He adds: “Another paradigm predicting leptoquarks is a model of quark-lepton unification strongly motivated by the charge quantization as well as hinted gauge coupling unification. Thirdly, supersymmetry with the R-parity violation is a motivated theoretical framework predicting leptoquarks.”

In fact there is a wide range of possible quantum numbers for leptoquarks which however can be restricted both by theoretical assumptions and results from current experimental searches. Greljo notes: “The leptoquark zoo contains only a handful of distinct particles which, however, exhibit a very rich phenomenology (see for example here). If they exist, they could leave a footprint in precisely measured low-energy observables such as flavour transitions and electroweak tests, but also lead to a spectacular signature in the ATLAS and CMS detectors.“

In fact, direct limits come from their production cross sections at colliders, while indirect limits are calculated from the bounds on the leptoquark-induced four-fermion interactions, which are observed in low-energy experiments. LEP, Tevatron and LHC experiments search for pair production of the leptoquark states.

At the LHC, there are two main leptoquark production mechanisms at play. Firstly, leptoquarks would be copiously produced in pairs via strong interactions followed by the prompt decay to leptons and jets. “This is indeed a conventional assumption in most experimental searches” says Greljo. After creation, a leptoquark would split almost immediately into a quark and a lepton and could be identified by looking for their decay products. Quarks, since they can’t exist isolated, quickly create many quark-antiquark pairs and form a ``jet'' of particles that can be identified by the large energy deposition in the calorimeter. The lepton can be an electron, muon, tau or a neutrino. An electron is identified by the presence of an isolated track in a tracking chamber and energy deposition in the electromagnetic portion of a calorimeter. Neutrinos are identified by ``missing'' energy since they escape the detector, carrying energy away.

Another important mechanism at the LHC, is the production of a single leptoquark in association with a lepton due to the direct quark-lepton coupling. Greljo says: “The later process is an important complementary perspective which is not yet fully exploited by the experimental collaborations. Finally, it should be noted that leptoquarks can lead to non-resonant effects in the high energy tails of the dilepton invariant mass”.

Particle collisions that look like this are used to search for leptoquarks. This figure is specifically for (electron + up/down quark) type collisions. (Image credits: Fermilab Today). 

There have been extensive searches for leptoquarks both by ATLAS and CMS experiments. These searches include all three generation leptoquarks using both 8 TeV and 13 TeV data. Nonetheless, so far there is no smoking gun for the existence of the leptoquarks.

The most recent results from CMS experiment on the pair production of the third generation scalar leptoquarks in the events with two taus and two b-jets, exclude leptoquarks with masses below 850 GeV at 95% confidence level using 12.9 fb-1 of 13 TeV data. The scalar sum of the transverse momenta of the two tau leptons (which one decays to a muon or an electron and the other decays to hadrons), two jets and missing transverse energy, denoted by ST, is used as the final observable.

To obtain the limit on the product of the cross section and branching ratio of leptoquarks to a lepton and a quark, the ST distribution of all standard model backgrounds plus signal hypothesis is compared to that of data. Data shows agreement with the background-only hypothesis which excludes the presence of a signal. In the following plot (left) the ST distribution is shown for the semileptonic decay of the tau into a muon. Similar plot exists for the electron channel as well. The 95% confidence level has obtained by combining both channels. The limit depends on the branching fraction of the third-generation leptoquark to a tau lepton and b quark, and is usually denoted by β. The observed and expected exclusion limit in terms of the β is depicted in the right plot.

Similar analysis has been performed on the third generation leptoquarks where both tau leptons decay hadronically using 2.3 fb^-1 of 13 TeV data. For the case of β =1, the observed exclusion limit is set to about 740 GeV. Exploring the LQ in the first and second generations using 2.6 fb^-1, also reveals no indication of the signal and the limits are set on the product of cross section and branching ratio of the LQ which is equivalent to 1130 and 1165 GeV, respectively.

CMS has also explored leptoquarks in other final states (i.e. top plus taus) and through different production mechanism such as single produced leptoquark. ATLAS has also conducted several searches for both pair-produced and singly-produced leptoquarks in different generations, all leading to set a limit in the lack of presence of a signal on top of the standard model background. However, since the last public results, LHC has provided much more integrated luminosity and both CMS and ATLAS experiments are analysing the entire 2016 and possibly 2017 data to shed light on the existence of leptoquarks with the largest dataset we had ever.

Searches for leptoquarks are also motivated by the observed anomalies in the B meson decays. Greljo explains: “Flavour experiments (LHCb, Belle, and BaBar) have recently puzzled the high energy physics community with strong (yet inconclusive) hints on lepton universality violation in decays of B-mesons.  Theorists suggested a consistent picture of new physics explaining these effects while being simultaneously in agreement with previous data at low and high energies (see for example here). In these models, leptoquarks are an essential ingredient. In fact, leptoquarks are expected in the ballpark for direct searches at the LHC. It is therefore of utmost importance for ATLAS and CMS to invest more resources in the LQ searches in years to come.”

This “Cross Collider” talk showed that the physics of Leptoquarks is a very rich and mature subject and at the same time, a rapidly evolving field on both experimental and theoretical fronts. It is only through the collective effort of the whole community that we will be able to make progress in the quest for new physics.

Note: The image in the cover of this article was taken from the University of the Witwatersrand website.