A shower containing ∼360 charged and neutral shower particles produced by a magnesium nucleus of 2×10¹⁴e.v. provides a source of mesons whose interactions have been studied over an aggregate track length of about 3 meters in emulsion. It is first shown that in very large showers not only the incident energy can be determined but also the energy of the shower particles. The angular distribution of the shower particles agrees closely and their multiplicity is consistent with Fermi’s theory.

It is shown that the interaction cross-section of charged shower particles is geometric and that at an energy ∈ ∼ 1000µc² they produce on the average 5 to 6 mesons. The mesons are preferentially emitted in the backward direction in the reference system in which the momentum of the incident meson balances that of a nucleon in the target. The excitation produced in the target nucleus is much lower than in nucleon-produced showers of comparable multiplicity.

The charged mesons arenot emitted in pairs by a heavier meson of short lifetime.

Charged shower particles as well as electrons produce electron-positron pairs with a cross-section of the order of that predicted by theory.

The number of neutral mesons decaying intoγ-rays in a time ≤10⁻¹⁴ sec. is nearly equal to the number of charged shower particles.

There exists evidence for the emission of neutral particles, different from neutrons, capable of producing meson showers.

The energy balance of the event suggests that a few shower particles carry an appreciable fraction of the total energy.

A method is described for constructing out of individual sheets of nuclear emulsion a detector of large volume and stopping power which can be used for observing entire chains of successive nuclear interaction and decay processes initiated by high energy particles.

Such a detector which had been exposed for several hours in the stratosphere has proved particularly valuable for detecting K-mesons and Hypersons* and for studying their modes of production and decay.

In an emulsion block detector¹ exposed in the stratosphere three cases of the production ofτ-mesons in nuclear disintegrations have been observed and 5 cases of negativeK-mesons which when brought to rest are captured by nuclei. In one case the production of aτ-meson is associated with the production of a charged hyperon in the same nuclear event. In another case the production of a negativeK-meson is associated with the production of a slow positiveK-meson. The production ratio of slowτ-mesons to slow positiveπ-mesonsτ/π⁺=1·1⁺0·8% and the ratio of slow negativeK-mesons to slow negativeπ-mesonsK⁻/π⁻=0·95±0·5%. The mass of theτ-meson is found to be$$M_\tau = 975 \cdot 9 \pm 2 \cdot 2 m_e $$ and that ofK⁻-mesons$$M_{K^ - } = 927 \pm 75 m_e $$ Proof is given that all three of the decay products of theτ-mesons areπ-mesons. It is suggested that allτ-mesons observed to decay in emulsions are positively charged and that theK⁻-mesons observed by us may be the negative counterpart of this particle. Possible relations between the various phenomenologically distinguished types of chargedK-mesons are discussed.

An unusual example of aτ⁺-meson decay in nuclear emulsions is described. From the measurements made on the momenta and direction of the threeπ-mesons emitted, the event is interpreted as a decay of aτ-meson according to the scheme:$$\tau ^ + \to \underline {\pi ^ + + \pi ^ + + \pi ^ - + \gamma } + Q$$. We have recently observed in a systematic investigation of the origin of slowπ-mesons, an interesting example of aτ-meson brought to rest and decaying in an emulsion block detector flown for about three hours above an altitude of 70,000 ft., at Delhi, India.

The directional intensities of neutrinos of various types produced in the decay of cosmic ray secondaries in the earth’s atmosphere have been estimated. The calculated energy spectra are believed to be uncertain by <20% for neutrino energies up to 100 GeV.

Using these fluxes and making various assumptions about the behaviour of neutrino cross-section with energy, the energy spectra of neutrino-induced muons at large depths underground have been calculated. It is shown that a cross-section increasing only linearly with energy up to ∼ 100 GeV. would account only for about one-fifth of the preliminary counting rate observed in a recent underground experiment. A more rapid increase in cross-section is indicated somewhere between 10 GeV. and 100 GeV. and this is shown to be quantitatively consistent with the existence of a charged intermediate boson of mass 2 GeV. ∼ M_w < 5 GeV. (However see the note added in proof.)

The question of detecting high energy neutrino signals from extra-terrestrial sources is briefly discussed.