Decay Of Charged Pions Research Articles

Propagation of angular momentum in charged pion decay and related processes

There are confusions about angular momentum propagation in scattering or decay processes involving the transition between particle systems that appear to transform differently under Lorentz transformations. This paper provides an analysis of the transformation properties of the states and interactions for a few typical processes within the standard model of particle physics, and performs explicit calculations showing how angular momentum transfers in these processes. We shall show explicitly (a) how a state with zero angular momentum evolves via interactions mediated by a single vector boson of spin one, and (b) that angular momentum conservation is completely consistent with the calculation in quantum field theory. A discussion is also given about the phenomenological consequences of the theoretical results obtained in this study.

Generator for Large Fluxes of Kaons and Pions Using Laser-Induced Nuclear Processes in Ultra-Dense Hydrogen H(0)

Laser-induced nuclear reactions in ultra-dense hydrogen H(0) produce mesons with both relatively low kinetic energy and with high kinetic energy. The kaons have up to 100 MeV of kinetic energy, thus a velocity of 0.55 c. Each laser pulse of >0.1 J of energy and length of 5 ns produces 1013 mesons. The operation of the meson generator is here demonstrated by measuring all decay times for mesons in the ns range after induction by a pulsed laser. These decay times are the unique fingerprints of the mesons, and they also produce the kinetic energy of the mesons created from their time-dilated decay. The charged pion decay time at rest from this generator is measured to be 25.92 ± 0.04 ns (standard fit error), in reasonable agreement with the tabulated results of 26.033 ns. A similar accuracy is found for the other mesons as for the charged kaons, with 96 MeV of kinetic energy, at 14.81 ± 0.05 ns. The same general behaviour is found with both deuterium and normal hydrogen forming the ultra-dense phase H(0) on the laser target. This meson generator gives intense meson showers useful for many types of particle physics experiments at a small fraction of the cost of using particle accelerators. A particle accelerator would need an energy of at least 1021 eV to produce a similar shower of 1013 mesons. Thus, the described generator is among the most intense meson sources that exist. Other important applications include nuclear energy generation and particle (pion) radiation for cancer treatment.

Quiescent luminosities of transiently accreting neutron stars with neutrino heating due to charged pion decay

We study the quiescent luminosities of accreting neutron stars by a new mechanism as neutrino heating for additional deep crustal heating, where the neutrino heating is produced by charged pions decay from the nuclear collisions on the surface of neutron star during its active accretion. For low mass neutron star($\lesssim1.4~M_{\odot}$), as the neutrino heating is little($\lesssim1$ MeV per accreted nucleon) or there would be no neutrino heating, the quiescent luminsoity will be not affected or slightly affected. While for massive neutron star ($\gtrsim2~M_{\odot}$), the quiescent luminosity will be enhanced more obviously with neutrino heating in the range 2-6 MeV per accreted nucleon. The observations on cold neutron stars such as 1H 19605+00, SAX J1808.4-3658 can be explained with neutrino heating if a fast cooling and heavy elements surface are considered. The observations on a hot neutron star such as RX J0812.4-3114 can be explained with neutrino heating if the direct Urca process is forbidden for a massive star with light elements surface, which is different from the previous work that the hot observations should be explained with small mass neutron star and the effect of superfluidity.

Pion electronic decay and lepton universality

In common with a number of simple processes involving elementary particles, charged pion decays are profoundly shaped by applicable Standard Model (SM) symmetries and properties. Given the highly precise SM theoretical description, pion decays are used as selective probes of SM parameters, and of possible SM extensions. The PEN experiment at PSI is studying the \pi^+ \to e^+\nu_e(\gamma)π+→e+νe(γ), or \pi_{e2(\gamma)}πe2(γ) decay. The primary goal is to reach the relative precision of 5 \times 10^{-4}5×10−4 in R_{e/\mu}^\piRe/μπ, the branching ratio for \pi_{e2(\gamma)}πe2(γ) decay. We review the PEN research program, its present status, and prospects.

Making the Universe at 20 MeV

We present a testable mechanism of low-scale baryogenesis and dark matter production in which neither the baryon nor lepton number are violated. Charged $D$ mesons are produced out of equilibrium at tens of MeV temperatures. The $D$ mesons quickly undergo $CP$-violating decays to charged pions, which then decay into dark-sector leptons without violating the lepton number. To transfer this lepton asymmetry to the baryon asymmetry, the dark leptons scatter on additional dark-sector states charged under the lepton and baryon number. Amusingly, this transfer proceeds without electroweak sphalerons, which are no longer active at such low scales. We present two example models which can achieve this transfer while remaining consistent with current limits. The required amount of $CP$ violation in charged $D$ meson decays, while currently allowed, will be probed by colliders. Additionally, the relevant decays of charged pions to dark-sector leptons have been constrained by the PIENU and Paul Scherrer Institute experiments and will be further explored in upcoming experiments.

Radio-to-Gamma-Ray Synchrotron and Neutrino Emission from Proton–Proton Interactions in Active Galactic Nuclei

We explore possible physical origin of correlation between radio wave and very-high-energy neutrino emission in active galactic nuclei (AGN), suggested by recently reported evidence for correlation between neutrino arrival directions and positions of brightest radio-loud AGN. We show that such correlation is expected if both synchrotron emitting electrons and neutrinos originate from decays of charged pions produced in proton-proton interactions in parsec-scale relativistic jet propagating through circum-nuclear medium of the AGN.

Weak decays of magnetized charged pions in the symmetric gauge

We consider the decay $\pi^- \to l \, \bar{\nu}_l$ ($l=e^-,\,\mu^-$) in the presence of an arbitrary large uniform magnetic field, using the symmetric gauge. The consequences of the axial symmetry of the problem and the issue of angular momentum conservation are discussed in detail. In particular, we analyze the projection of both the canonical and the mechanical total angular momenta along the direction of the magnetic field. It is found that while the former is conserved in the symmetric gauge, the latter is not conserved in both the symmetric and Landau gauges. We derive an expression for the integrated $\pi^- \to l \, \bar{\nu}_l$ width that coincides exactly with the one we previously found using the Landau gauge, providing an explicit test of the gauge independence of that result. Such an expression implies that for nonzero magnetic fields the decay width does not vanish in the limit in which the outgoing charged leptons can be considered as massless, i.e. it does not exhibit the helicity suppression found in the case of no external field.

Neutrino Emissions from Tidal Disruption Remnants

Abstract We study high-energy neutrino emissions from tidal disruption remnants (TDRs) around supermassive black holes. The neutrinos are produced by the decay of charged pions originating in ultrarelativistic protons that are accelerated there. In the standard theory of tidal disruption events (TDEs), there are four distinct phases from the debris circularization of stellar debris to super- and sub-Eddington to radiatively inefficient accretion flows (RIAFs). In addition, we consider the magnetically arrested disk (MAD) state in both the super-Eddington accretion and RIAF phases. We find that there are three promising cases to produce neutrino emissions: the super-Eddington accretion phase of the MAD state and the RIAF phases of both the non-MAD and MAD states. In the super-Eddington MAD state, the enhanced magnetic field makes it possible to accelerate the protons to with the other given appropriate parameters. The neutrino energy is then at the peak of the energy spectrum. For M bh ≳ 107.7 M ⊙, the neutrino light curve is proportional to , while it follows the standard decay rate for . In both cases, the large luminosity and characteristic light curves diagnose the super-Eddington MAD state in TDEs. In the RIAF phase of the non-MAD state, we find and , and its light curve is proportional to . This indicates that one can identify whether the existing RIAFs are the TDE origin or not. TDRs are potentially a population of hidden neutrino sources invisible in gamma-rays.

Neutral and charged pion properties under strong magnetic fields in the NJL model

In the framework of the Nambu--Jona-Lasino (NJL) model, we study the effect of an intense external uniform magnetic field on neutral and charged pion masses and decay form factors. In particular, the treatment of charged pions is carried out on the basis of the Ritus eigenfunction approach to magnetized relativistic systems. Our analysis shows that in the presence of the magnetic field three and four nonvanishing pion-to-vacuum hadronic form factors can be obtained for the case of the neutral and charged pions, respectively. As expected, it is seen that for nonzero magnetic field the $\pi^0$ meson can still be treated as a pseudo Nambu-Goldstone boson, and consequently the corresponding form factors are shown to satisfy various chiral relations. For definite parametrizations of the model, numerical results for $\pi^0$ and $\pi^\pm$ masses and decay constants are obtained and compared with previous calculations given in the literature.

Weak Decay of Magnetized Pions.

The leptonic decay of charged pions is investigated in the presence of background magnetic fields. In this situation, Lorentz symmetry is broken, and new fundamental decay constants need to be introduced, associated with the decay via the vector part of the electroweak current. We calculate the magnetic field dependence of both the usual and a new decay constant nonperturbatively on the lattice. We employ both Wilson and staggered quarks and extrapolate the results to the continuum limit. With this nonperturbative input, we calculate the tree level electroweak amplitude for the full decay rate in strong magnetic fields. We find that the muonic decay of the charged pion is enhanced drastically by the magnetic field. We comment on possible astrophysical implications.

Can Winds Driven by Active Galactic Nuclei Account for the Extragalactic Gamma-Ray and Neutrino Backgrounds?

Abstract Various observations are revealing the widespread occurrence of fast and powerful winds in active galactic nuclei (AGNs) that are distinct from relativistic jets, likely launched from accretion disks and interacting strongly with the gas of their host galaxies. During the interaction, strong shocks are expected to form that can accelerate nonthermal particles to high energies. Such winds have been suggested to be responsible for a large fraction of the observed extragalactic gamma-ray background (EGB) and the diffuse neutrino background, via the decay of neutral and charged pions generated in inelastic pp collisions between protons accelerated by the forward shock and the ambient gas. However, previous studies did not properly account for processes such as adiabatic losses that may reduce the gamma-ray and neutrino fluxes significantly. We evaluate the production of gamma rays and neutrinos by AGN-driven winds in detail by modeling their hydrodynamic and thermal evolution, including the effects of their two-temperature structure. We find that they can only account for less than ∼30% of the EGB flux, as otherwise the model would violate the independent upper limit derived from the diffuse isotropic gamma-ray background. If the neutrino spectral index is steep with Γ ≳ 2.2, a severe tension with the isotropic gamma-ray background would arise as long as the winds contribute more than 20% of the IceCube neutrino flux in the 10–100 TeV range. At energies ≳ 100 TeV, we find that the IceCube neutrino flux may still be accountable by AGN-driven winds if the spectral index is as small as Γ ∼ 2.0–2.1.

Hadronic model for the non-thermal radiation from the binary system AR Scorpii

Abstract AR Scorpii is a close binary system containing a rotation powered white dwarf and a low-mass M type companion star. This system shows non-thermal emission extending up to the X-ray energy range. We consider hybrid (lepto-hadronic) and pure hadronic models for the high energy non-thermal processes in this binary system. Relativistic electrons and hadrons are assumed to be accelerated in a strongly magnetised, turbulent region formed in collision of a rotating white dwarf magnetosphere and a magnetosphere/dense atmosphere of the M-dwarf star. We propose that the non-thermal X-ray emission is produced either by the primary electrons or the secondary e± pairs from decay of charged pions created in collisions of hadrons with the companion star atmosphere. We show that the accompanying γ-ray emission from decay of neutral pions, which are produced by these same protons, is expected to be on the detectability level of the present and/or the future satellite and Cherenkov telescopes. The γ-ray observations of the binary system AR Sco should allow us to constrain the efficiency of hadron and electron acceleration and also the details of the radiation processes.

Phenomenological implications of very special relativity

We discuss several phenomenological implications of Very Special Relativity (VSR). It is assumed that there is a small violation of Lorentz invariance and the true symmetry group of nature is a subgroup called SIM(2). This symmetry group postulates the existence of a fundamental or preferred direction in space-time. We study its implications by using an effective action which violates Lorentz invariance but respects VSR. We find that the problem of finding the masses of fundamental fermions is in general intractable in the presence of VSR term. The problem can be solved only in special cases which we pursue in this paper. We next determine the signal of VSR in torsion pendulum experiment as well as clock comparison experiment. We find that VSR predicts a signal which is different from other Lorentz violating theories and hence a dedicated data analysis is needed in order to impose reliable limits. Assuming that signal is absent in data we determine the limits that can be imposed on the VSR parameters. We also study the implications of VSR in particle decay experiments taking the charged pion and kaon decay as an example. The effective interaction between the charged pion and the final state leptons is related to the fundamental VSR mass terms through a loop calculation. We also predict a shift in the angular dependence of the decay products due to VSR. In particular we find that these no longer display azimuthal symmetry with respect to the momentum of the pion. Furthermore the azimuthal and polar angle distributions show time dependence with a period of a sidereal day. This time dependence provides us with a novel method to test VSR in future experiments.

Metastable pions in dense media

We study the leptonic decay of charged pions in a compact star environment. Considering leptons as a degenerated Fermi system, pions are tightly constrained to decay into these particles because their Fermi levels are occupied. Thus, pion decay is only possible through thermal fluctuations. Under these circumstances, pion lifetime is larger and hence can be considered to reach a metastable state. We explore restrictions under which such a metastability is possible. We also study conditions under which pions and leptons already in chemical equilibrium can reach simultaneously the thermal equilibrium, and obtain the neutrino emissivity from metastable pions. Scenarios that favor this metastable state are protoneutron stars.

Search for massive neutrinos in π + → e + ν e ${\pi }^{+}{\rightarrow }e^{+}{\nu }_{e}$ decay

The PIENU experiment aims to measure the branching ratio of the charged pion decay with precision of < 0.1 %. This measurement is much sensitive to search for massive neutrinos coupled to electrons in ${\pi }^{+}{\rightarrow }e^{+}{\nu }_{e}$ decay. The initial analysis was completed and the upper limit on the neutrino mixing parameter |U e i |2 in the neutrino mass range of 0 to 55 MeV/c 2 was improved by a factor of 1.5, and the sensitivity for the mass range of 68 to 129 MeV/c 2 was improve by a factor of up to 4.

AN ALL-SKY SEARCH FOR THREE FLAVORS OF NEUTRINOS FROM GAMMA-RAY BURSTS WITH THE ICECUBE NEUTRINO OBSERVATORY

ABSTRACT We present the results and methodology of a search for neutrinos produced in the decay of charged pions created in interactions between protons and gamma-rays during the prompt emission of 807 gamma-ray bursts (GRBs) over the entire sky. This three-year search is the first in IceCube for shower-like Cherenkov light patterns from electron, muon, and tau neutrinos correlated with GRBs. We detect five low-significance events correlated with five GRBs. These events are consistent with the background expectation from atmospheric muons and neutrinos. The results of this search in combination with those of IceCube’s four years of searches for track-like Cherenkov light patterns from muon neutrinos correlated with Northern-Hemisphere GRBs produce limits that tightly constrain current models of neutrino and ultra high energy cosmic ray production in GRB fireballs.

Galactic sources of high energy neutrinos: Expectation from gamma-ray data

The recent results from ground based $\gamma$-ray detectors (HESS, MAGIC, VERITAS) provide a population of TeV galactic $\gamma$-ray sources which are potential sources of High Energy (HE) neutrinos. Since the $\gamma$-rays and $\nu$ -s are produced from decays of neutral and charged pions, the flux of TeV $\gamma$-rays can be used to estimate the upper limit of $\nu$ flux and vice versa; the detectability of $\nu$ flux implies a minimum flux of the accompanying $\gamma$-rays (assuming the internal and the external absorption of $\gamma$-rays is negligible). Using this minimum flux, it is possible to find the sources which can be detected with cubic-kilometer telescopes. I will discuss the possibility to detect HE neutrinos from powerful galactic accelerators, such as Supernova Remnants (SNRs) and Pulsar Wind Nebulae (PWNe) and show that likely only RX J1713.7-3946 , RX J0852.0-4622 and Vela X can be detected by current generation of instruments (IceCube and Km3Net). It will be shown also, that galactic binary systems could be promising sources of HE $\nu$ -s. In particular, $\nu$-s and $\gamma$-rays from Cygnus X-3 will be discussed during recent gamma-ray activity, showing that in the future such kind of activities could produce detectable flux of HE $\nu$-s

High-energy neutrino emission from the core of low luminosity AGNs triggered by magnetic reconnection acceleration

The detection of astrophysical very high energy (VHE) neutrinos in the range of TeV-PeV energies by the IceCube observatory has opened a new season in high energy astrophysics. Energies ~PeV imply that the neutrinos are originated from sources where cosmic rays (CRs) can be accelerated up to ~ 10^{17}eV. Recently, we have shown that the observed TeV gamma-rays from radio-galaxies may have a hadronic origin in their nuclear region and in such a case this could lead to neutrino production. In this paper we show that relativistic protons accelerated by magnetic reconnection in the core region of these sources may produce VHE neutrinos via the decay of charged pions produced by photo-meson process. We have also calculated the diffuse flux of VHE neutrinos and found that it can be associated to the IceCube data.

NEUTRINO CONSTRAINTS TO THE DIFFUSE GAMMA-RAY EMISSION FROM ACCRETION SHOCKS

Accretion of gas during the large scale structure formation has been thought to give rise to shocks that can accelerate cosmic rays. This process then results in an isotropic extragalactic gamma-ray emission contributing to the extragalactic gamma-ray background observed by the Fermi-LAT. Unfortunately this emission has been difficult to constrain and thus presents an uncertain foreground to any attempts to extract potential dark matter signal. Recently, IceCube has detected high-energy isotropic neutrino flux which could be of an extragalactic origin. In general, neutrinos can be linked to gamma rays since cosmic-ray interactions produce neutral and charged pions where neutral pions decay into gamma rays, while charged pions decay to give neutrinos. By assuming that isotropic high-energy IceCube neutrinos are entirely produced by cosmic rays accelerated in accretion shocks during the process of structure formation, we obtain the strongest constraint to the gamma-ray emission from large scale structure formation (strong) shocks and find that they can make at best ~20% of the extragalactic gamma-ray background, corresponding to neutrino flux with spectral index 2, or ~10% for spectral index 2.46. Since typical objects where cosmic rays are accelerated in accretion shocks are galaxy clusters, observed high-energy neutrino fluxes can then be used to determine the gamma-ray emission of a dominant cluster type and constrain acceleration efficiency, and thus probe the process of large scale structure formation.

ON THE ROLE AND ORIGIN OF NONTHERMAL ELECTRONS IN HOT ACCRETION FLOWS

We study the X-ray spectra of tenuous, two-temperature accretion flows using a model involving an exact, Monte Carlo computation of the global Comptonization effect as well as general relativistic description of both the flow structure and radiative processes. In our previous work we found that in flows surrounding supermassive black holes, thermal synchrotron radiation is not capable of providing a sufficient seed photons flux to explain the X-ray spectral indices as well as the cut-off energies measured in several best-studied AGNs. In this work we complete the model by including seed photons provided by nonthermal synchrotron radiation and we find that it allows to reconcile the hot flow model with the AGN data. We take into account two possible sources of nonthermal electrons. First, we consider e+- produced by charged-pions decay, which should be always present in the innermost part of a two-temperature flow due to proton-proton interactions. We find that for a weak heating of thermal electrons (small delta) the synchrotron emission of pion-decay e+- is much stronger than the thermal synchrotron emission in the considered range of bolometric luminosities, L Ledd. The small-delta model including hadronic effects in general agrees with the AGN data, except for the case of a slowly rotating black hole and a thermal distribution of protons. For large-delta, the pion-decay e+- have a negligible effect and then in this model we consider nonthermal electrons produced by direct acceleration. We find an approximate agreement with the AGN data for the fraction of the heating power of electrons which is used for the nonthermal acceleration eta~0.1.