The mass of the 4\pi^+$

The most precise value for the pion mass was determined from a precision measurement at PSI of the muon momentum in pion decay at rest, \pi^+ \rightarrow \mu^+ + \nu_{\mu}π+→μ++νμ. The result is m_{\pi^+} = 139.570\,21(14)mπ+=139.57021(14)~MeV/c^22. This value is more precise, however, in agreement with the recent compilation of the Particle Data Group for m_{\pi^-}mπ−. The agreement of m_{\pi^+}mπ+ with the recent measurement. This yields a new quantitative measure of CPT invariance in the pion sector: (m_{\pi^+} - m_{\pi^-})/m_{\pi}(\mbox{av}) = (-2.9 \pm 2.0)\cdot 10^{-6}(mπ+−mπ−)/mπ(av)=(−2.9±2.0)⋅10−6, an improvement by two orders of magnitude.

Radiative and Meson Decays of Y(4230) in Flavor SU(3)

The charmonium-like exotic states Y(4230) and the less known Y(4320), produced in e+e− collisions, are sources of positive parity exotic hadrons in association with photons or pseudoscalar mesons. We analyze the radiative and pion decay channels in the compact tetraquark scheme, with a method that proves to work equally well in the most studied D∗→γ/π+D decays. The decay of the vector Y into a pion and a Zc state requires a flip of charge conjugation and isospin that is described appropriately in the formalism used. Rates are found to depend on the fifth power of pion momentum, which would make the final states πZc(4020) strongly suppressed with respect to πZc(3900). The agreement with BES III data would be improved considering the πZc(4020) events to be fed by the tail of the Y(4320) resonance under the Y(4230). These results should renovate the interest in further clarifying the emerging experimental picture in this mass region.

FLUKA Simulations of Pion Decay Gamma-Radiation from Energetic Flare Ions

Gamma-ray continuum at $> 10 $ > 10 MeV photon energy yields information on $\gtrsim 0.2 $ ≳ 0.2 – 0.3 GeV/nucleon ions at the Sun. We use the general-purpose Monte Carlo code FLUktuierenden KAskade (FLUKA) to model the transport of ions injected into thick and thin target sources, the nuclear processes that give rise to pions and other secondaries and the escape of the resulting photons from the atmosphere. We give examples of photon spectra calculated with a range of different assumptions about the primary ion velocity distribution and the source region. We show that FLUKA gives results for pion decay photon emissivity in agreement with previous treatments. Through the directionality of secondary products, as well as Compton scattering and pair production of photons prior to escaping the Sun, the predicted spectrum depends significantly on the viewing angle. Details of the photon spectrum in the $\approx 100$ ≈ 100 MeV range may constrain the angular distribution of primary ions and the depths at which they interact. We display a set of thick-target spectra produced making various assumptions about the incident ion energy and angular distribution and the viewing angle. If ions are very strongly beamed downward, or ion energies do not extend much above 1 GeV/nucleon, the photon spectrum is highly insensitive to details of the ion distribution. Under the simplest assumptions, flares observed near disc centre should not display significant radiation above 1 GeV photon energy. We give an example application to Fermi Large Area Telescope data from the flare of 12 June 2010.

Gamma-ray and X-ray constraints on non-thermal processes in η Carinae

The binary system η Carinae is a unique laboratory that facilitates the study of particle acceleration to high energies under a wide range of conditions, including extremely high densities around periastron. To date, no consensus has emerged as to the origin of the gigaelectronvolt γ-ray emission in this important system. With a re-analysis of the full Fermi-LAT data set for η Carinae, we show that the spectrum is consistent with a pion decay origin. A single population leptonic model connecting X-ray to γ-ray emission can be ruled out. We revisit our physical model from 2015, based on two acceleration zones associated with the termination shocks in the winds of both stars. We conclude that inverse Compton emission from in-situ accelerated electrons dominates the hard X-ray emission detected with NuSTAR at all phases away from periastron and that pion-decay from shock accelerated protons is the source of γ-ray emission. Very close to periastron there is a pronounced dip in hard X-ray emission, concomitant with the repeated disappearance of the thermal X-ray emission, which we interpret as due to the suppression of significant electron acceleration in the system. Within our model, the residual emission seen by NuSTAR at this phase can be accounted for with secondary electrons produced in interactions of accelerated protons, which agrees with the variation in pion-decay γ-ray emission. Future observations with H.E.S.S., CTA, and NuSTAR should confirm or refute this scenario.

The Current-Current Model of the Weak Interaction

This chapter discusses the representation of the weak interaction as a current-current interaction that violates parity and charge conjugation invariance. It describes the experiments that demonstrate that this violation is maximal. The resulting theory is called the V-A theory of the weak interaction. The chapter works out the predictions of the V-A theory for muon and pion decay and high-energy neutrino scattering and shows the comparison to experiment.