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We argue that the anomalous magnetic moment of the electron (a e ) can be used to probe new physics. We show that the present bound on new-physics contributions to a e is 8 × 10 −13 , but the sensitivity can be improved by about an order of magnitude with new measurements of a e and more refined determinations of α in atomic-physics experiments. Tests on new-physics effects in a e can play a crucial role in the interpretation of the observed discrepancy in the anomalous magnetic moment of the muon (a µ ). In a large class of models, new contributions to magnetic moments scale with the square of lepton masses and thus the anomaly in a µ suggests a new-physics effect in a e of (0.7±0.2)×10 −13 . We also present examples of new-physics theories in which this scaling is violated and larger effects in a e are expected. In such models the value of a e is correlated with specific predictions for processes with violation of lepton number or lepton universality, and with the electric dipole moment of the electron.

We propose a new experiment to measure the running of the electromagnetic coupling constant in the spacelike region by scattering high-energy muons on atomic electrons of a low-Z target through the elastic process μ e → μ e. The differential cross section of this process, measured as a function of the squared momentum transfer t = q 2 < 0, provides direct sensitivity to the leading-order hadronic contribution to the muon anomaly a HLO μ . By using a muon beam of 150 GeV, with an average rate of ∼1.3 ×10 7 muon/s, currently available at the CERN North Area, a statistical uncertainty of ∼0.3% can be achieved on a HLO μ after two years of data taking. The direct measurement of a HLO μ via μe scattering will provide an independent determination, competitive with the time-like dispersive approach, and consolidate the theoretical prediction for the muon g-2 in the Standard Model. It will allow therefore a firmer interpretation of the measurements of the future muon g-2 experiments at Fermilab and J-PARC. a

Abstract:We update the constraints on two-Higgs-doublet models (2HDMs) focusing on the parameter space relevant to explain the present muon g − 2 anomaly, ∆a µ , in four different types of models, type I, II, "lepton specific" (or X) and "flipped" (or Y). We show that the strong constraints provided by the electroweak precision data on the mass of the pseudoscalar Higgs, whose contribution may account for ∆a µ , are evaded in regions where the charged scalar is degenerate with the heavy neutral one and the mixing angles α and β satisfy the Standard Model limit β − α ≈ π/2. We combine theoretical constraints from vacuum stability and perturbativity with direct and indirect bounds arising from collider and B physics. Possible future constraints from the electron g − 2 are also considered. If the 126 GeV resonance discovered at the LHC is interpreted as the light CP-even Higgs boson of the 2HDM, we find that only models of type X can satisfy all the considered theoretical and experimental constraints.

After a brief review of the muon g−2 status, we analyze the possibility that the present discrepancy between experiment and the Standard Model (SM) prediction may be due to hypothetical errors in the determination of the hadronic leading-order contribution to the latter. In particular, we show how an increase of the hadro-production cross section in low-energy e + e − collisions could bridge the muon g−2 discrepancy, leading however to a decrease on the electroweak upper bound on MH , the SM Higgs boson mass. That bound is currently MH < ∼ 150 GeV (95%CL) based on the preliminary top quark mass Mt = 172.6(1.4) GeV and the recent determination ∆α (5) had (MZ) = 0.02768(22), while the direct-search lower bound is MH > 114.4 GeV (95%CL). By means of a detailed analysis we conclude that this solution of the muon g−2 discrepancy is unlikely in view of current experimental error estimates. However, if this turns out to be the solution, the 95%CL upper bound on MH is reduced to about 130 GeV which, in conjunction with the experimental lower bound, leaves a narrow window for the mass of this fundamental particle. A. IntroductionThe measurement of the anomalous magnetic moment of the muon a µ by the E821 experiment at Brookhaven, with a remarkable relative precision of 0.5 parts per million [1], is challenging the Standard Model (SM) of particle physics. Indeed, as each sector of the SM contributes in a significant way to the theoretical prediction of a µ = (g − 2)/2 (g is the muon's gyromagnetic factor), this measurement allows us to test the entire SM and provides a powerful tool to scrutinize viable "new physics" appendages to this theory [2].The SM prediction of the muon g−2 is conveniently split into QED, electroweak (EW) and hadronic (leadingand higher-order) contributions: [7], is the O(α 3 ) contribution of diagrams containing hadronic vacuum polarization insertions [10]. The second term, also of O(α 3 ), is the hadronic light-by-light contribution; as it cannot be determined from data, its evaluation relies on specific models. Recent determinations of this term vary between 80(40) × 10 −11 [11] and 136(25) × 10 −11 [12]. The most recent one, 110(40) × 10 −11 [13], lies between them. If we add this result to the leading-order hadronic contribution, for example the value of Ref. [7] (which also provides a recent calculation of the hadronic contribution to the effective fine-structure constant, later required for our analysis), and the rest of the SM contributions, we obtain a The term a HLO µ can alternatively be computed incorporating hadronic τ -decay data, related to those of hadroproduction in e + e − collisions via isospin symmetry [14,15]. Unfortunately there is a large difference between the e + e − -and τ -based determinations of a HLO µ , even if isospin violation corrections are taken into account [16]. The τ -based value is significantly higher, leading to a small (∼ 1σ) ∆a µ difference. As the e + e − data are more directly related to the a HLO µ calculation than the τ ones, the latest analyses do no...

Investigation at a φ-factory can shed light on several debated issues in particle physics. We discuss: i) recent theoretical development and experimental progress in kaon physics relevant for the Standard Model tests in the flavor sector, ii) the sensitivity we can reach in probing CPT and Quantum Mechanics from time evolution of entangled kaon states, iii) the interest for improving on the present measurements of non-leptonic and radiative decays of kaons and η/η′ mesons, iv) the contribution to understand the nature of light scalar mesons, and v) the opportunity to search for narrow di-lepton resonances suggested by recent models proposing a hidden dark-matter sector. We also report on the e + e − physics in the continuum with the measurements of (multi)hadronic cross sections and the study of γγ processes.

We present the achievements of the last years of the experimental and theoretical groups working on hadronic cross section measurements at the low-energy e + e − colliders in Beijing, Frascati, Ithaca, Novosibirsk, Stanford and Tsukuba and on τ decays. We sketch the prospects in these fields for the years to come. We emphasise the status and the precision of the Monte Carlo generators used to analyse the hadronic cross section measurements obtained as well with energy scans as with radiative return, to determine luminosities and τ decays. The radiative corrections fully or approximately implemented in the various codes and the contribution of the vacuum polarisation are discussed.

This article reviews and updates the Standard Model prediction of the τ lepton g−2.Updated QED and electroweak contributions are presented, together with new values of the leading-order hadronic term, based on the recent low energy e + e − data from BaBar, CMD-2, KLOE and SND, and of the hadronic light-by-light contribution. The total prediction is confronted to the available experimental bounds on the τ lepton anomaly, and prospects for its future measurements are briefly discussed.