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LHCb results: Paper of the Month-Special Edition

LHCb results: Paper of the Month-Special Edition "Test of lepton universality in beauty-quark decays"

                                                     Paper of the Month-Special Edition by Mario Fernández Navarro (SOTON ESR)


The last 23rd of March, the LHCb collaboration presented an updated measurement of the ratio RK, which is defined as the number of times a B+ meson decays to a charged kaon and a muon-antimuon pair (K+μ+μ) divided by the number of times a B+ meson decays to a charged kaon and a positron-electron pair (K+e+e). These decays involve the transition of a beauty antiquark into a strange antiquark , a process that is highly suppressed within the Standard Model (SM), taking place only at the loop level (see Fig. 1). Furthermore, according to the SM, both the decay channel to muons and the decay channel to electrons must be equivalent, preserving a fundamental principle of the SM known as “lepton universality” (the mediator bosons of the SM couple to the three generations of charged leptons with the same strength).
Therefore, RK is predicted to be 1, up to corrections of order 1% due to the different charged lepton masses.

     Figure 1: SM contribution to RK. Figure taken from [1].


However, the updated measurement obtained by the LHCb collaboration is [1], which deviates from the SM prediction with a statistical significance of 3.1 standard deviations . This result indicates evidence of the breaking of lepton universality in beauty-quark decays, and is one of the stronger hints for new physics beyond the SM coming from LHC measurements. This updated measurement of RK, shown as a black point with error bars in Fig. 2, is consistent with previous results and supersedes them, being the most precise measurement to date. However, in order to claim a discovery, the statistical significance must reach the 5σ level. Therefore, we must be cautious as there is still the possibility that this anomaly is due to fluctuations of the data.


 Figure 2: Comparison between RK measurements. Figure taken from [2].


There has been a strong theoretical effort in the last years aiming to explain the RK anomaly, and this effort will increase due to the new measurement. Up to now, some of the most promising candidates to explain the RK anomaly are tree-level exchange of a hypothetical, electrically neutral and flavourful Z' boson (Fig. 3a), along with the contribution of a hypothetical leptoquark (LQ) coupling with different strengths to the different types of charged leptons (Fig. 3b).



(a) Z' exchange diagram contributing to RK.                                                  (b) Leptoquark diagram contributing to RK. Figure taken from [1].
                                                                                                                      Figure 3      


Finally, the LHCb collaboration has also reported an improved measurement of the very rare decay. The measured branching fraction   [2] is in excellent agreement with the SM prediction of (3.66 ± 0.14)×10−9, with a statistical significance in the 10σ level. All candidate models of physics beyond the SM will have to demonstrate their compatibility with this important result, including models which propose explanations of hints for lepton universality anomalies, like RK.

[1] R. Aaij et al. [LHCb], “Test of lepton universality in beauty-quark decays”, [arXiv:2103.11769 [hep-ex]].


[2] Forthcoming LHCb paper, results available in