Searching from top to bottom for lepton unity

22 March 2023 | By

The fundamental particles of the Universe are part of a multi-generational family, with several branches (leptons, quarks and bosons). The similarity between the generations of different branches is an outstanding puzzle of the Standard Model – one that could be solved by the existence of leptoquarks. These hypothetical particles would allow leptons and quarks to transform into each other, giving a unified picture of the fundamental particles that form matter.

Leptoquarks would be spin 0 (scalar) or spin 1 (vector) particles, and would interact via the strong force due to their colour charge. Vector leptoquarks would be a new force-carrying particle, with enhanced interactions with gluons (Yang-Mills couplings) and possibly with other new heavy particles. At the LHC, vector leptoquarks would be dominantly produced in pairs if their mass is about 1 TeV, with their production rate depending largely on the mass. However, as it costs more energy to produce two particles, researchers are also looking for signs of singly-produced leptoquarks. This production rate would additionally depend on the leptoquark’s interaction (coupling) strength to quarks and leptons.

The ATLAS Collaboration has studied the full LHC Run-2 dataset (139 fb-1) looking for singly-produced and pair-produced leptoquarks interacting with third-generation quarks and leptons. Their new results focused on leptoquark interactions with either a bottom quark and a tau lepton, or a top quark and a light lepton (i.e electron or muon). Researchers used dedicated collision-event categorisations to precisely estimate and discriminate multiple background processes, including the associated production of top-quark pairs with ‘jets’ of particles or with W/Z boson processes. They found no significant excess above the Standard-Model prediction in either search.

The ATLAS Collaboration has released an impressive set of searches for leptoquarks interacting with third-generation leptons or quarks – placing valuable constraints on leptoquark parameters.

The bottom line

When heavy particles are produced in LHC collisions, they lead to final state particles with large momenta transverse to the beam direction. For the search for singly-produced leptoquarks, researchers compared the data distribution of the scalar sum of transverse momenta of two tau leptons and the most energetic jet of particles originating from a bottom quark (b-jet) to that of the Standard-Model background prediction. Leptoquark events would contain tau-leptons and b-jets of higher transverse momenta than those produced in typical Standard Model processes.

Researchers were thus able to extract constraints on the leptoquark mass and coupling parameters for the combined single and pair-produced leptoquark signals. For lower coupling strengths (up to 1), ATLAS physicists report a lower limit on the vector leptoquark mass of 1.58 TeV, assuming that the leptoquark has Yang-Mills couplings. At higher couplings (up to 2.5), single production plays a key role and extends the lower limit on leptoquark mass to about 2 TeV. Combined measurements from low-energy experiments have shown that B-meson decays into tau leptons are about three standard deviations larger than those into muons compared to Standard Model predictions. This could suggest that leptoquarks have a higher chance of interacting with third-generation leptons. The ATLAS results exclude some of the possible range of leptoquark coupling and mass values needed to explain these B-anomalies (see Figure 1).

Figure 1: Exclusion limits in the mass vs coupling plane for singly-produced scalar leptoquark (green) and combined singly- and doubly-produced scalar leptoquark signals (blue). Regions to the left of lines are excluded. The grey area shows the preferred region where the chosen leptoquark model can explain observed B-anomalies. (Image: ATLAS Collaboration/CERN)
Figure 2: Comparison between data and background predictions in the distributions of parameterised neural network (PNN) score in events with two hadronically decaying tau-leptons and one or two b-jets. Signal prediction in red is shown 5-fold for 1.4 TeV scalar leptoquark mass. (Image: ATLAS Collaboration/CERN)

ATLAS physicists also performed a dedicated search for pair-produced leptoquarks decaying into a b-quark and tau-lepton. To better separate signal from background, they used a neural network based on several useful event quantities (e.g. the transverse momenta of the tau-leptons) and dependent on the leptoquark mass (see Figure 2). This improved the leptoquark mass bounds and set a lower limit on the vector leptoquark mass of 1.75 TeV, regardless of the coupling strength.

Figure 3: Comparison between data and background predictions (solid line after the likelihood fit, dashed line before likelihood fit) in the effective mass distributions in events with at least three leptons, two of which are muons, at least two jets, one of which is b-jet. Signal prediction is shown for 1.6 TeV scalar leptoquark mass. (Image: ATLAS Collaboration/CERN)

Tackling the top

In another new result, researchers looked for leptoquark pairs decaying exclusively to a top quark and an electron or a muon. As multiple particles originate from these decay chains, leaving a unique signature in the ATLAS detector, researchers were able to cleanly select for collision events that matched the leptoquark signature (see Figure 3). They set lower bounds on the scalar and vector leptoquark masses. For example, a scalar leptoquark with a mass below 1.64 TeV was excluded for the assumption of its exclusive decays into top quark and muon, and similar constraints were obtained for the scalar leptoquark coupling to a top quark and an electron. These are by far the most stringent limits on these decay channels.

ATLAS physicists have established an impressive set of Run-2 searches for leptoquarks interacting with third-generation leptons or quarks, and placed valuable constraints on leptoquark parameters. Further theoretical and experimental developments will improve the reach of these searches, continuing the hunt for the unification of leptons and quarks.

About the event display: Visualisation of an interesting event candidate in the search of leptoquark pairs where each leptoquark is assumed to decay into a top-quark and an electron. This event contains a muon (red), two electrons (blue), five jets (cones) including a b-jet (blue cone), and the large missing transverse momentum (dashed). The scalar sum of the transverse momenta of muon, electron, jets and missing transverse momenta is 2.1 TeV. (Image: ATLAS Collaboration/CERN)

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