Symmetries run very deep in physics, particularly so in the Standard Model. The fact that physics is the same (i.e. symmetric) in one place or another leads to the conservation of momentum, for example. But for the universe to exist as we see it today, some of these symmetries need to be broken. The violation of charge-parity symmetry, for example, explains some of the differences between matter and antimatter. Are there unexplored symmetries that could be broken?

So far, two unbroken symmetries are electric charge and lepton flavour. Electrons, muons and taus are three flavours of particles called charged leptons. According to the Standard Model, the only difference between these leptons should be their mass, with each flavour equally likely to interact with a force particle. This is known as lepton flavour universality. Both of these symmetries are separately respected, but could a combination of charge and lepton-flavour symmetry break in a similar way to charge-parity?

ATLAS physicists are investigating this question by studying the differences between positively- and negatively-charged electrons (e^±) and muons (μ^±). According to the Standard Model, there should be an equal number of LHC collision events with e⁺μ^– and e^–μ⁺; the ratio (ρ) of the rates of these events should be close to 1. But venturing into the wild-west of beyond-the-Standard Model (BSM) physics, the combined charge and lepton flavour symmetry might not need to be respected in these interactions. This raises the tantalising possibility that, if the measured ratio were to be larger than 1, it could be evidence for new physics!

The ATLAS Collaboration has just released a new paper on the measurement of ρ – the first of a kind from an LHC experiment. The measurement is an exceptionally powerful probe of new physics processes, as it relies solely on ATLAS data, without the use of simulated data to estimate background processes.

Achieving this result was far from straightforward, as there are many experimental biases that affect ρ. For example, the LHC collides positively-charged protons, giving rise to more events with W⁺ than W^– bosons, which in turn give rise to more positive leptons than negative leptons. This, in combination with hadronic jets being incorrectly reconstructed as an electron more frequently than as a muon, leads to a bias for e^–μ⁺ (with a "fake" e^–) over e⁺μ^– (with a "fake" µ^–). Another bias comes from the fact that ATLAS is made of matter rather than antimatter! As particles pass through the layers of sub-detectors, only electrons (e^–) can be knocked out of atoms, again favouring e^–μ⁺ over e⁺μ^– in events with a muon. In the end, all of the experimental biases together push ρ to be less than 1.

Because of this, ATLAS physicists chose only to investigate BSM theories that could make ρ significantly larger than 1. Two concrete examples studied in the paper are R-parity-violating supersymmetry and scalar leptoquarks. However, the search was also designed to be generic and sensitive to any new process that could create more e⁺μ^– than e^–μ⁺ pairs.

In order to take the most accurate measurement possible, physicists deployed several strategies to overcome their experimental biases. For instance, they corrected for small local differences in reconstruction efficiencies of μ⁺ and μ^– and included the “fake lepton” background described above.

The resulting ρ measurement is shown in Figure 1. These data show that, after correcting for biases, ρ is about 1. This means there is no evidence for new physics that strongly prefers e⁺μ^– over e^–μ⁺, and no evidence of any strong, unaccounted-for biases in the way that the ATLAS detector records data. Instead, ATLAS physicists were able to set limits on BSM physics models. An example for the R-parity-violating supersymmetry model is shown in Figure 2, where supersymmetric muons are excluded up to masses of about 640 GeV.

The innovative approach of this new result helps pave the way for future searches that are less reliant on simulated data which, while incredibly useful for many ATLAS searches, can be limited in its accuracy and statistical precision. Though the combined charge and lepton-flavour symmetry remains unbroken, ATLAS physicists will continue to try new and innovative ways to test the Standard Model.

Learn more

• A search for an unexpected asymmetry in the production of e⁺μ^– and e^–μ⁺ pairs in proton–proton collisions recorded by the ATLAS detector at 13 TeV (arXiv: 2112.08090, see figures)
• See also the full lists of ATLAS Conference Notes and ATLAS Physics Papers.