ATLAS Finds No Evidence Of Lepton Universality Violations In W-Boson Decays

In the Standard Model of Particle Physics, electrons, muons and tau leptons have identical properties except rest mass. But there is mixed experimental evidence from W boson mediated decays of B mesons (i.e. two quark composite particles containing bottom quarks) to suggest that different charge leptons decay differently (much like quarks of different generations do pursuant to the CKM matrix). 

This study is the first and only experimental data point from the Large Hadron Collider addressing the question from the perspective of the ratio of tau leptons to muons. It does so in a "cleaner" experimental design with less room for unexpected systemic errors or theoretical issues than the B meson decay experiments that have shown signs of lepton universality violations. 

The results support the Standard Model rule known as "lepton universality" with results within less than one standard deviation of the Standard Model expectation. There are multiple models for what could cause lepton universality violations in B meson decays, however, and it can't rule out all of them definitively.

The Standard Model of particle physics encapsulates our current best understanding of physics at the smallest scales. A fundamental axiom of this theory is the universality of the couplings of the different generations of leptons to the electroweak gauge bosons. 

The measurement of the ratio of the rate of decay of W bosons to τ-leptons and muons, R(τ/μ)=B(W→τντ)/B(W→μνμ), constitutes an important test of this axiom. A measurement of this quantity with a novel technique using di-leptonic tt¯ events is presented based on 139 fb^−1 of data recorded with the ATLAS detector in proton--proton collisions at s√=13 TeV. Muons originating from W bosons and those originating from an intermediate τ-lepton are distinguished using the lifetime of the τ-lepton, through the muon transverse impact parameter, and differences in the muon transverse momentum spectra. 

The value of R(τ/μ) is found to be 0.992±0.013[±0.007(stat)±0.011(syst)] and is in agreement with the hypothesis of universal lepton couplings as postulated in the Standard Model. This is the most precise measurement of this ratio, and the only such measurement from the Large Hadron Collider, to date.

ATLAS Collaboration "Test of the universality of τ and μ lepton couplings in W-boson decays from tt¯ events with the ATLAS detector" arXiv (July 28, 2020).

The body text of the paper provides the following background to this experiment:

It is a fundamental axiom and remarkable feature of the Standard Model (SM) that the couplings of the electroweak gauge bosons (W, Z) to charged leptons, g(l) (l = e, µ, τ), are independent of the mass of the leptons. This fundamental assumption is referred to as lepton-flavour universality and is tested in this paper by measuring the ratio of the fraction of on-shell W boson decays, branching ratios (B), to τ-leptons and muons, R(τ/µ) = B(W → τντ)/B(W → µνµ). The measurement exploits the large number of top and anti-top quark pair (tt¯) events produced in proton-proton (pp) collisions at the Large Hadron Collider (LHC). Given the large B(t → W q), close to 100%, this gives a very large sample of W boson pairs. These are used in a tag and probe technique to obtain a large sample of clean and unbiased W boson decays to muons and τ-leptons. The τ-leptons are identified through their decay to muons. The displacement of the τ decay vertex and the different muon transverse momentum (pT) spectra are used to distinguish between muons from the W → τντ → µνµντ ντ and W → µνµ processes, to extract R(τ/µ). This is achieved by utilising the precise reconstruction of muon tracks obtainable by the ATLAS experiment. 

Previously, R(τ/µ) has been measured by the four experiments at the Large Electron–Positron Collider (LEP), yielding a combined value of 1.070 ± 0.026. This deviates from the SM expectation of unity[1] by 2.7σ, motivating a precise measurement of this ratio at the LHC. Other experimental measurements of the ratio B(W → τν(τ))/B(W → lν(l)), where l is either an electron or a muon, have not yet reached the precision of the LEP results. 

The equivalent ratio for the two light generations, B(W → µνµ)/B(W → eνe), has been accurately measured by the LEP, LHCb and ATLAS experiments, and is found to be consistent with the SM prediction at the 1% level. Additionally, while most low-energy experiments show good agreement, to very high precision, with the hypothesis of universality of lepton couplings, recent results from LHCb, Belle and BaBar show some tension with the SM, further motivating this analysis. 

This measurement relies on precise knowledge of the branching ratio of τ-leptons decaying to muons to extrapolate to the full W → τντ branching ratio. The value of (17.39 ± 0.04)% measured by the LEP experiments is used in the analysis. The relative uncertainty of 0.23% is included in the measured value of R(τ/µ) and is a subdominant component of the overall uncertainty.

[1] The phase space effects due to the masses of the decay products on this ratio are very small (∼ 5 × 10−4 ) and hence can be neglected [2]. 

Previous coverage of the question of lepton universality violations at this blog:

* Lepton Flavor Violations In B Meson Decays Still Seen In Experimental Evidence (updated March 27, 2019)

* A Review Of Lepton Universality In B Meson Decays (September 18, 2018)

* B Quark Decays Still Anomalous (June 25, 2018)

* Maybe b quark decays aren't weird after all (January 18, 2018)

* Some B Meson Decay Anomalies Disappear In Run-2 Data (May 10, 2017)