Pion-photon transition form factor and new physics in theτsector

Abstract

Recent measurement of the à form factor of the neutral pion in the high Q 2 region disagrees with a priori predictions of QCD-based calculations. We comment on existing explanations, and analyze a possibility that this discrepancy is not due to poorly understood QCD effects, but is a result of some new physics beyond the standard model (SM). We show that such physics would necessarily involve a new neutral light state with mass close to the mass of 0 , and with stronger than 0 couplings to heavier SM flavors such as c, , and b. It is found that only the coupling to the lepton can survive the existing constraints and lead to the observed rise of the pion form factor relative to Q À2 at high Q 2 . We perform numerical fits to data and determine the allowed range of masses and couplings for such new particles. This range of masses and couplings could also reduce or eliminate the tension between the e þ e À and decay determinations of the hadronic vacuum polarization. Dedicated experimental analysis of pair production in association with such new states should provide a conclusive test of the new physics hypothesis as an explanation of the pion form factor rise. We also comment on the calculations of the pion form factor in the chiral quark model, and point out a possible dynamical origin of the quark mass scale inferred from the form factor measurement.

Dark forces coupled to nonconserved currents

“…• ττ X production at B and tau/charm factories: A unique capability of B-factories is the possibility to study τ + τ − pairs in a well-controlled environment. If X has an axial coupling to the τ , then the e + e − → τ + τ − X process gives (m τ /m X ) 2 enhanced X L emission, in analogy to the e + e − → τ + τ − S signatures discussed in [108,109]. Sensitivity estimates show that BaBar and Belle searches should be able to probe [109], for leptonic X decays.…”

Muon anomalous magnetic moment through the leptonic Higgs portal

“…The search for exotic particles in association with taus is a relatively unexplored subject, with only one specific case analyzed to date [45,46]. The production cross section for (31) can be calculated analytically.…”

Search for newπ0-like particles produced in association with aτ-lepton pair

“…It is fixed in the fit and scanned between 110 and 160 MeV=c 2 , covering the expected range of impostor mass values [8]. The step size is 0.5 MeV=c 2 , corresponding to less than half the estimated mass resolution.…”

“…A recent suggestion [8] proposes that the observed lack of asymptotic behavior might be due to the production of new particles or states, tentatively named "pion impostors" and generically denoted ϕ [9]. Two classes of models are considered.…”

“…These models predict large coupling strengths between the new objects and the τ lepton, comparable to the strength of the strong force, leading to an observable increase of F π 0 ðQ 2 Þ through virtual loops with τ leptons. The couplings of the new particles to heavy quarks and other Standard Model (SM) particles are constrained by experimental data to be an order of magnitude or more smaller [8].…”