New Lepton Universality Data To Be Announced Tuesday, 18 October 2021

[ohwilleke]

TL;DR Summary

The LHCb experiment will announce its new experimental results regarding lepton universality in B meson decays, a leading anomaly from the Standard Model at the LHC in two new decay chains on Tuesday, October 18, 2021.

One of the Standard Model's rules is that charged leptons (i.e. the electron, muon and tau lepton) are identical to each other in their properties except for their masses (and that their anti-particles are identical to them except for a charge-parity flip).​

But, in two kinds of rare semi-leptonic decays of B mesons (which are composite particles made up of bound quark-antiquark valance quarks with a b quark as one of the quarks), decays with electrons/positrons are much more common than decays with muons/antimuons. The difference in the number of first and second generation charged leptons produced in these decays is greater than their mass differences alone can explain in the Standard Model.​

This anomaly is not observed in many other kinds of decays that can produce more than one kind of charged lepton without violating mass-energy conservation.​

Theoretical explanations have been tricky. It is easy to imagine a process that produces lepton universality violations. But, lepton universality is experimentally confirmed in a wide range of particle and meson decays believed to be mediated by W boson interactions and decays. It isn't obvious why these interactions involving B meson decays which are also assumed in the Standard Model to be mediated by W boson interactions and decays would be different.

The most popular published new physics explanations favor vector (i.e. spin-1) "leptoquarks". But, another possibility is that there might be some background process or missing decay channel, for example, that is not being accounted for properly within the Standard Model, or for example, a flaw in the event cutoffs use to measure it that is including something in addition to the decays it is seeking to measure or excluding some of the relevant decays.

Are these two results experimental flukes or errors? Or are they proof of new physics to be determined? New data from two different kinds of semi-leptonic B meson decays from LHCb tomorrow will help us get to the bottom of this question.

New tests of lepton universality using rare B decays with 𝐾0𝑆 mesons in the final state at LHCb

by Harry Victor Cliff (University of Cambridge (GB))​

Tuesday Oct 19, 2021, 11:00 AM → 12:00 PM Europe/Zurich
500/1-001 - Main Auditorium (CERN)
Description
In recent years several intriguing anomalies have been reported in the rare decays of B mesons. Most prominently, in March 2021 a measurement by LHCb of the ratio known as 𝑅𝐾 showed 3.1 standard deviation significance for the violation of lepton universality in 𝐵+→𝐾+ℓ+ℓ− decays, where ℓ is either a muon or an electron. Previous measurements of the ratio 𝑅𝐾∗ using 𝐵0→𝐾∗0ℓ+ℓ− also hinted at the violation of lepton universality, albeit at a lower statistical significance.
In this seminar we present the first tests at LHCb of lepton universality using these decays’ isospin partners - 𝐵0→𝐾0𝑆ℓ+ℓ− and 𝐵+→𝐾∗+ℓ+ℓ− .
These tests have lower statistical sensitivity than 𝑅𝐾 and 𝑅𝐾∗ due to the presence of a 𝐾0S in the final state. The measurements are performed with data recorded by the LHCb experiment in 2011-2012 and 2016-2018, corresponding to an integrated luminosity of 9 fb−1.

Signups to the videoconference are available here.

The fact that they are doing a videoconference rather than just making a routine arXiv preprint posting or posting to the LHCb website suggests that they have probably seen new evidence of lepton non-universality previously seen in two other channels, although we'll have to wait to be sure and to see how significant the result appears to be.

Select background reading:

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).

A new pre-print is entitled "Test of lepton family universality and search for lepton and baryon number violation at Belle"

 

[ohwilleke]

The preprint says no BSM physics were detected in these decays (contrary to my expectations):

Tests of lepton universality in B0→K0Sl+l− and B+→K∗+ℓ+ℓ− decays where l is either an electron or a muon are presented. The differential branching fractions of B0→K0Sl+l− and B+→K∗+ℓ+ℓ− decays are measured in intervals of the dilepton invariant mass squared. The measurements are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of 9fb−1. The results are consistent with the Standard Model and previous tests of lepton universality in related decay modes. The first observation of B0→K0Sl+l− and B+→K∗+ℓ+ℓ− decays is reported.

LHCb collaboration, "Tests of lepton universality using B0→K0Sl+l− and B+→K∗+ℓ+ℓ− decays" arXiv:2110.09501 (October 18, 2021).

This tends to support an interpretation of the prior lepton universality violation indications as a fluke or some sort of unrecognized systemic error because these are isospin partners of the cases where apparent violations were seen. The body text of the Letter explains this while also noting that the reduced precision of this measurement is also a factor:

Forces in the SM couple to the charged leptons with equal strength, which is referred to as lepton universality. Therefore, these ratios are predicted to be very close to unity, with small corrections due to the muon-electron mass difference. Furthermore, these ratios benefit from precise cancellation of the hadronic uncertainties that affect predictions of the branching fractions and angular observables. Significant deviation from unity in such ratios would therefore constitute unambiguous evidence of BSM physics.

The ratio RK∗0 , measured by the LHCb collaboration using the data collected in the q 2 regions 0.045 < q2 < 1.1 GeV2 /c4 and 1.1 < q2 < 6.0 GeV2 /c4 , is in tension with the SM predictions at 2.2–2.4 and 2.4–2.5 standard deviations (σ), respectively. A measurement of RK+ performed in the region 1.1 < q2 < 6.0 GeV2 /c4 deviates from the SM by 3.1 standard deviations. The analogous ratio measured using Λ 0 b → pK−` +` − decays, RpK, is consistent with the SM within one standard deviation. All four measurements show a deficit of b→ sµ+µ − decays with respect to b→ se+e − decays.

In addition, angular observables and branching fractions of b→ sµ+µ − decays have been measured, with several in tension with the SM. However, the extent to which they may be affected by residual quantum chromodynamics contributions remains uncertain. Intriguingly, it is possible to account for all these anomalies simultaneously through the modification of the b→ s coupling in a model-independent way. Such a modification can be generated by the presence of a heavy neutral boson or a leptoquark, as well as in models with supersymmetry, extra dimensions, and extended Higgs sectors.

The B0→ K0 S ` +` − and B+→ K∗+` +` − decays are the isospin partners of B+→ K+` +` − and B0→ K∗0 ` +` − decays and are expected to be affected by the same NP contributions. Testing lepton universality by measuring the ratios RK0 S and RK∗+ can therefore provide important additional evidence for or against NP. However, while these decays have similar branching fractions to their isospin partners, O(10−6 ) to O(10−7 ), 1Charge conjugate processes are implied throughout. 1 they suffer from a reduced experimental efficiency at LHCb due to the presence of a long-lived K0 S or π 0 meson in the final state.