Test of lepton universality in beauty-quark decays

Abstract

The standard model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown that a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton–proton collision data collected with the LHCb detector at CERN’s Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the standard model, such as a new fundamental interaction between quarks and leptons.

Beyond the Standard Model with Lepton Flavor Universality Violation

“…Among the different decays involving b → s + − transitions, the ratios R(K ( * ) ) = Br(B → K ( * ) µ + µ − )/Br(B → K ( * ) e + e − ) are particularly prominent. They are measured by LHCb [6,7] (and Belle [8,9]) and their theory predictions are very clean (within the SM) since the dependence on the form factors drops out to an excellent approximation. Moreover, there is a long list of semi-leptonic b → s + − observables that significantly deviate from the SM predictions, like the optimized observable P µ 5 [10,11] 1 , but also total branching ratios (sensitive to form factors) like Br(B → K * µ + µ − ) [14], Br(B → Kµ + µ − ) [15] and Br(B s → φµ + µ − ) [16].…”

“…However, taking into account the information on the various q 2 bins of the muon to electron ratio, one can see that NP is only preferred by ≈ 3 σ [182] meaning that the excess of 4 σ cannot be fully explained. 6 ∆A F B : A good fit to data requires a non-zero Wilson coefficient of the tensor operators. Importantly, among the set of renormalizable models, only two scalar LQ can generate this operator at tree-level and only the SU (2) L singlet gives a good fit to data [183].…”

“…See Ref [154]. for a recent overview on NP explanations of aµ 6. Note that even though here an effect in electrons is required, while b → s + − points towards NP mainly related to muons, this is not a priori a contradiction.…”

Accelerate to the next level

“…1 ). Using proton-proton collision data recorded in 2011, 2012 and 2015-2018 at centre-of-mass energies of 7, 8 and 13 TeV, respectively, the LHCb Collaboration now presents a prime example of how accelerators advance our fundamental understanding of particle physics in this issue 2 .…”

Angular distributions for $Λ_b\to Λ^*_J (pK^-)J/ψ$ Decays

“…Thus the B → K + − and B → K * + − have received great attentions in the past decades and have provide very stringent constraints on NP models beyond the SM [7][8][9][10][11][12][13][14][15][16]. Meanwhile in these decays, the so-called flavor anomalies are also found [17][18][19][20][21][22][23][24][25]. For instance, LHCb has presented its latest measurement of the ratio of branching fractions [24,25]: R K ≡ B(B → Kµ + µ − ) B(B → Ke + e − ) = 0.846 +0.042+0.013 −0.039−0.012 , 1.1GeV 2 < q 2 < 6GeV 2 , R K * ≡ B(B → K * µ + µ − ) B(B → K * e + e − ) = 0.69 +0.11 −0.07 ± 005, 1.1GeV 2 < q 2 < 6GeV 2 , (…”