Editing LHCb experiment (section)

=== Lepton flavour universality{{anchor|Lepton_flavour_universality_anchor}} ===
{{See also|Lepton#Universality}}

In the Standard Model, couplings of charged [[lepton]]s (electron, muon and tau lepton) to the gauge bosons are expected to be identical, with the only difference emerging from the lepton masses. This postulate is referred to as "lepton flavour universality". As a consequence, in decays of b hadrons, electrons and muons should be produced at similar rates, and the small difference due to the lepton masses is precisely calculable.

LHCb has found deviations from this predictions by comparing the rate of the decay <math>\mathrm{B}^+ \to \mathrm{K}^+ \mu^+ \mu^-</math> to that of <math>\mathrm{B}^+ \to \mathrm{K}^+  \mathrm{e}^+ \mathrm{e}^-</math>,<ref>{{Cite web|title=How universal is (lepton) universality?|url=https://home.cern/news/news/accelerators/how-universal-lepton-universality|access-date=2021-03-21|website=CERN|language=en}}</ref> and in similar processes.<ref>{{Cite web|title=LHCb explores the beauty of lepton universality|url=https://home.cern/news/news/physics/lhcb-explores-beauty-lepton-universality|access-date=2021-03-21|website=CERN|language=en}}</ref><ref>{{Cite web|date=2021-10-19|title=LHCb tests lepton universality in new channels|url=https://cerncourier.com/a/lhcb-tests-lepton-universality-in-new-channels/|access-date=2021-10-27|website=CERN Courier|language=en-GB}}</ref> However, as the decays in question are very rare, a larger dataset needs to be analysed in order to make definitive conclusions.

In March 2021, LHCb announced that the anomaly in lepton universality crossed the "3 [[Standard deviation|sigma]]" [[statistical significance]] threshold, which translates to a [[p-value]] of 0.1%.<ref>{{Cite web|title=Intriguing new result from the LHCb experiment at CERN|url=https://home.cern/news/news/physics/intriguing-new-result-lhcb-experiment-cern|access-date=2021-03-23|website=CERN|language=en}}</ref> The measured value of <math>R_{\rm K} = \frac{\mathcal{B}(\mathrm{B}^+ \to \mathrm{K}^+ \mu^+\mu^-)}{\mathcal{B}(\mathrm{B}^+ \to \mathrm{K}^+ \mathrm{e}^+\mathrm{e}^-)}</math>, where symbol <math>\mathcal{B}</math> denotes probability of a given decay to happen, was found to be <math>0.846^{+0.044}_{-0.041}</math> while the Standard Model predicts it to be very close to unity.<ref name="LHCb 2022">{{Cite journal |last1=LHCb collaboration |last2=Aaij |first2=R. |last3=Beteta |first3=C. Abellán |last4=Ackernley |first4=T. |last5=Adeva |first5=B. |last6=Adinolfi |first6=M. |last7=Afsharnia |first7=H. |last8=Aidala |first8=C. A. |last9=Aiola |first9=S. |last10=Ajaltouni |first10=Z. |last11=Akar |first11=S. |date=22 March 2022 |title=Test of lepton universality in beauty-quark decays |url=https://www.nature.com/articles/s41567-021-01478-8 |journal=Nature Physics |language=en |volume=18 |issue=3 |pages=277–282 |arxiv=2103.11769 |doi=10.1038/s41567-021-01478-8 |bibcode=2022NatPh..18..277L |s2cid=232307581 |issn=1745-2473}}</ref> In December 2022 improved measurements discarded this anomaly.<ref name="LHCb 2023B">{{Cite journal |last=LHCb collaboration |date=2023 |title=Test of Lepton Universality in ''b'' → ''s'' ℓ<sup>+</sup> ℓ<sup>−</sup> decays |journal=Physical Review Letters |volume=131 |issue=5 |page=051803 |doi=10.1103/PhysRevLett.131.051803 |pmid=37595222 |arxiv=2212.09152 |s2cid=254854814 }}</ref><ref name="LHCB 2023A">{{Cite journal |last=LHCb collaboration |date=2023 |title=Measurement of lepton universality parameters in ''B''<sup>+</sup> → ''K''<sup>+</sup> ℓ<sup>+</sup> ℓ<sup>−</sup> and ''B''<sup>0</sup>→''K''<sup>∗0</sup>ℓ<sup>+</sup>ℓ<sup>−</sup> decays |journal=Physical Review D |volume=108 |issue=3 |page=032002 |doi=10.1103/PhysRevD.108.032002 |arxiv=2212.09153 |s2cid=254853936 }}</ref><ref>{{Cite web |title=Improved lepton universality measurements show agreement with the Standard Model |url=https://lhcb-outreach.web.cern.ch/2022/12/20/improved-lepton-universality-measurements-show-agreement-with-the-standard-model/ |access-date=2023-01-08 |language=en-US}}</ref>

In August 2023 joined searches in leptonic decays <math>b\rightarrow s\ell^+\ell^-</math> by the LHCb and semileptonic decays <math>b\rightarrow s\ell\nu</math> by Belle II (with <math>\ell=e,\mu</math>) set new limits for universality violations.
<ref  name="LHCb 2023B" /><ref name="LHCB 2023A" /><ref>{{Cite journal |last1=Belle II Collaboration |last2=Aggarwal |first2=L. |last3=Ahmed |first3=H. |last4=Aihara |first4=H. |last5=Akopov |first5=N. |last6=Aloisio |first6=A. |last7=Anh Ky |first7=N. |last8=Asner |first8=D. M. |last9=Atmacan |first9=H. |last10=Aushev |first10=T. |last11=Aushev |first11=V. |last12=Bae |first12=H. |last13=Bahinipati |first13=S. |last14=Bambade |first14=P. |last15=Banerjee |first15=Sw. |date=2023-08-02 |title=Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic $B$-Meson Decays at Belle II |url=https://link.aps.org/doi/10.1103/PhysRevLett.131.051804 |journal=Physical Review Letters |volume=131 |issue=5 |pages=051804 |doi=10.1103/PhysRevLett.131.051804|pmid=37595249 |arxiv=2301.08266 |bibcode=2023PhRvL.131e1804A |s2cid=256080428 }}</ref><ref>{{Cite journal |last=Wright |first=Katherine |date=2023-08-02 |title=Standard Model Stays Strong for Leptons |url=https://physics.aps.org/articles/v16/s91 |journal=Physics |language=en |volume=16 |issue=5 |pages=s91 |doi=10.1103/PhysRevLett.131.051804|pmid=37595249 |arxiv=2301.08266 |bibcode=2023PhRvL.131e1804A |s2cid=256080428 }}</ref>