6/13/2023 0 Comments Lepton flavorsThe point with error bars shows the SM prediction, while the shaded grey region shows the world experimental results average.Ī possible explanation for this discrepancy is that there is an additional contribution to the decay rate, due to the exchange of a new virtual particle. Experimental results for the two observables probing lepton-flavour universality comparing b → cτ –ν̅ τ and b → cl –ν̅ l decays, l = e, μ. Currently, the SM prediction is roughly three standard deviations away from the global average of results from the LHCb, BaBar and Belle experiments (figure 2). Importantly, since the hadronic inputs that describe the b → c transition do not depend on which lepton flavour is in the final state, the induced uncertainties mostly cancel in the ratios. Two ratios can be measured, since the charm quark is either bound inside a D meson or its excited version, the D*, and the two ratios, R D and R D*, have the very welcome property that they can be precisely predicted in the SM. Rather than materialising as a particle, it leaves its imprint as a very short-range potential that has the property of changing one quark (a b quark) into a different one (a c quark) with the simultaneous emission of a charged lepton and an antineutrino.įlavour universality is probed by measuring the ratio of branching fractions: R D(*) = Br(B → D (* ) τ – ν ̅ τ )/Br(B → D (* )l – ν ̅ l ), where l = e, μ. The W boson, being much heavier than the amount of energy that is released in the decay of the b quark, is virtual. In the SM the b → c τ – ν ̅ τ process is due to a tree-level exchange of a virtual W boson (figure 1, left). Still, the results can be summarised succinctly: all the measured decays agree with SM predictions, with the exception of measurements that probe LFU in two quark-level transitions: b → c τ – ν̅ τ and b → s μ + μ –. Even in the condensed version of the PDG booklet, such listings run to more than 170 pages. A quick look at the Particle Data Group (PDG) booklet, with its long lists of the decays of B mesons, D mesons, kaons and other hadrons, gives an impression of the breadth and depth of the field. Today, flavour physics is a major field of activity. The ν̅ τ in b → cτ –ν̅ τ can also be replaced by a right-handed neutrino, which is not part of the Standard Model. Possible new contributions to this process include exchanges of new colour-singlet states (middle), or new coloured states coupling simultaneously to quarks and leptons (right). The semileptonic decay of the b quark in the Standard Model (left). In other words, the gauge forces, such as the electroweak force, are flavour-universal in the SM, while the exchange of a Higgs particle is not. The Higgs field, on the other hand, distinguishes between fermions of different flavours and endows them with different masses – sometimes strikingly so. It directly follows from the assumption that the SM gauge group, SU(3) × SU(2) × U(1), is one and the same for all three generations of fermions. This “flavour universality” is deeply ingrained in the symmetry structure of the Standard Model (SM) and applies to both the electroweak and strong forces (though the latter is irrelevant for leptons). The three flavours of charged leptons – electron, muon and tau – are the same in many respects. These 12 elementary fermions are grouped into three generations of increasing mass. A similar picture evolved for the leptons: the electron and the muon were joined by the unexpected discovery of the tau lepton at SLAC in 1975 and completed with the three corresponding neutrinos. From the three types known at the time – up, down and strange – the list of quark flavours grew to six. In 1971, at a Baskin-Robbins ice-cream store in Pasadena, California, Murray Gell-Mann and his student Harald Fritzsch came up with the term “flavour” to describe the different types of quarks. If the effect strengthens as more data are gathered, possible explanations range from new gauge forces to leptoquarks. Recent experimental results hint that some electroweak processes are not lepton-flavour independent, contrary to Standard Model expectations.
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