The muon g − 2 anomaly confronts new physics in e± and μ± final states scattering
Luc Darmé, Giovanni Grilli di Cortona, Enrico Nardi
Abstract
A bstract The 4.2 σ discrepancy between the standard model prediction for the muon anomalous magnetic moment a μ and the experimental result is accompanied by other anomalies. A crucial input for the prediction is the hadronic vacuum polarization $$ {a}_{\mu}^{\mathrm{HVP}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>a</mml:mi> <mml:mi>μ</mml:mi> <mml:mi>HVP</mml:mi> </mml:msubsup> </mml:math> inferred from σ had = σ ( e + e − → hadrons) data. However, the two most accurate determinations of σ had from KLOE and BaBar disagree by almost 3 σ . Additionally, the combined data-driven result disagrees with the most precise lattice determination of $$ {a}_{\mu}^{\mathrm{HVP}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>a</mml:mi> <mml:mi>μ</mml:mi> <mml:mi>HVP</mml:mi> </mml:msubsup> </mml:math> by 2 . 1 σ . We show that all these discrepancies could be accounted for by a new boson produced resonantly around the KLOE centre of mass energy and decaying promptly yielding e + e − and μ + μ − pairs in the final states. This gives rise to three different effects: (i) the additional e + e − events will affect the KLOE luminosity determination based on measurements of the Bhabha cross section, and in turn the inferred value of σ had ; (ii) the additional μ + μ − events will affect the determination of σ had via the (luminosity independent) measurement of the ratio of π + π − γ versus μ + μ − γ events; (iii) loops involving the new boson would contribute directly to the prediction for a μ . We discuss in detail this possibility, and we present a simple model that can reconcile the KLOE and BaBar results for σ had , the data-driven and the lattice determinations of $$ {a}_{\mu}^{\mathrm{HVP}} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>a</mml:mi> <mml:mi>μ</mml:mi> <mml:mi>HVP</mml:mi> </mml:msubsup> </mml:math> , the predicted and measured values of a μ , while complying with all phenomenological constraints.