![]() ![]() This principle is known as conservation of lepton number. When particles interact, generally the number of leptons of the same type (electrons and electron neutrinos, muons and muon neutrinos, tau leptons and tau neutrinos) remains the same. The masses of the leptons also obey a simple relation, known as the Koide formula, but at present this relationship cannot be explained. The charged leptons have two possible spin states, while only one helicity is observed for the neutrinos (all the neutrinos are left-handed, and all the antineutrinos are right-handed). All known charged leptons have a single unit of negative or positive electric charge (depending on whether they are particles or antiparticles) and all of the neutrinos and antineutrinos have zero electric charge. All six of these particles have corresponding antiparticles (such as the positron or the electron antineutrino). The other is a nearly massless neutral particle called a neutrino (such as the electron neutrino). One is a massive charged particle that bears the same name as its flavor (like the electron). Each flavor is represented by a pair of particles called a weak doublet. As the detector signature of each resonant decay is similar to that of its corresponding non-resonant decay, systematic uncertainties that would otherwise dominate the calculation of these efficiencies are suppressed.There are three known flavors of lepton: the electron, the muon, and the tau. The efficiency of the non-resonant B + → K + e + e − decay therefore needs to be known only relative to that of the resonant B + → J/ ψ( → e + e −) K + decay, rather than relative to the B + → K + μ + μ − decay. In this equation, each branching fraction can be replaced by the corresponding event yield divided by the appropriate overall detection efficiency ( Methods), as all other factors needed to determine each branching fraction individually cancel out. 2, 65), the R K ratio is determined via the double ratio of branching fractions Since the J/ ψ → ℓ + ℓ − branching fractions are known to respect lepton universality to within 0.4% (refs. To help overcome the challenge of modelling precisely the different electron and muon reconstruction efficiencies, the branching fractions of B + → K + ℓ + ℓ − decays are measured relative to those of B + → J/ ψ K + decays 64. In the remainder of this paper, the notation B + → K + ℓ + ℓ − is used to denote only decays with 1.1 < q 2 < 6.0 GeV 2 c −4, which are referred to as non-resonant, whereas B + → J/ ψ( → ℓ + ℓ −) K + decays are denoted resonant. The B + hadron contains a beauty antiquark, \(\overline\) resonances, such as the ϕ(1020) meson. Measurable quantities can be predicted precisely in the decays of a charged beauty hadron, B +, into a charged kaon, K +, and two charged leptons, ℓ + ℓ −. One method to search for new physics is to compare measurements of the properties of hadron decays, where hadrons are bound states of quarks, with their SM predictions. Particle physicists have therefore been searching for ‘new physics’, that is, new particles and interactions that can explain the SM’s shortcomings. The SM is unable to explain cosmological observations of the dominance of matter over antimatter, the apparent dark matter content of the Universe, or the patterns seen in the interaction strengths of the particles. However, it is clear that the model is incomplete. The standard model (SM) of particle physics provides precise predictions for the properties and interactions of fundamental particles, which have been confirmed by numerous experiments since the inception of the model in the 1960s. ![]()
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