New Notations for Elementary Particles

In recent years a large number of new elementary particles have been discovered (see¹). The newly discovered particles were given various notations, and these were chosen, as a rule, at random, without any system. This led to the fact that in some cases one and the same particle was denoted by different symbols (for example, $\tau^0$, $V_2^0$, $V_4^0$) and, conversely, it happened that one and the same notation had different meanings for different investigators. In connection with the situation that has arisen, it became necessary to order the classification and choice of notations for particles already discovered and newly being discovered.

At the beginning of this year, a joint letter by a group of physicists engaged in the study of new particles was published in a number of journals², proposing that the following classification and corresponding notation for elementary particles be introduced:

1. Particles are divided, according to their mass, into three groups, denoted by capital Latin letters:

a) light mesons ($L$-mesons), which include $\pi$-mesons, $\mu$-mesons, and all still lighter mesons, if any should be discovered;

b) heavy mesons ($K$-mesons), which include all particles heavier than the $\pi$-meson but lighter than the proton;

c) particles whose mass is greater than the mass of the neutron but less than the mass of the deuteron, called hyperons ($Y$-particles).

II. Individual kinds of particles are denoted by Greek letters (as distinct from the notation for groups of particles). The only exceptions are the generally accepted notations for neutrons ($n$) and protons ($p$). Particles belonging to $L$- and $K$-mesons are denoted by lowercase Greek letters, and hyperons by uppercase letters.

The group of $K$-mesons includes the following particles known at the present time:

1. Tau-mesons ($\tau$), whose existence has been established with sufficient certainty, decay according to the scheme $\tau \to 3\pi$.

2. Kappa-mesons ($\varkappa$), whose existence is highly probable, decay into a $\mu$-meson and two neutral particles ($\nu$) of unknown nature ($\varkappa \to \mu + 2\nu$).

3. Chi-mesons ($\chi$) decay according to the scheme $\chi \to \pi + \nu$, the nature of the $\nu$-particle being unknown.

4. Theta-mesons ($\theta^0$) are those particles which were formerly denoted as $\vartheta^0$, $V^0_2$, $V^0_4$ and which decay according to the scheme $\theta^0 \to \pi^\pm + (\pi^\mp \text{ or } \mu^\mp)$. If it turns out that decay schemes of $\theta^0$-mesons occur with different energy releases ($Q$), then these $\theta^0$-mesons should be denoted by corresponding indices ($\theta^0_1$, $\theta^0_2$, etc.).

The group of hyperons ($Y$-particles) includes:

1) $\Lambda^0$-particles, formerly denoted as $V^0_1$-particles and characterized by the decay scheme $\Lambda^0 \to p + \pi^-$. If $\Lambda^0$-particles decaying according to the same scheme, but with different energy releases ($Q$), are found, they will be denoted by corresponding indices ($\Lambda^0_1$, $\Lambda^0_2$, etc.);

2) $\Lambda^+$-particles, corresponding to $\Lambda^0$-particles and decaying according to one of two possible schemes, $\Lambda^+ \to n + \pi^+$ or $\Lambda^+ \to p + \pi^0$.

III. The phenomenological classification, based on the experimental characteristics of the decay process, is retained in the same form in which it existed until now.

1. The phenomenon of decay in flight of a $K$-meson or $Y$-particle is denoted as $V$-decay, with $V^0$-decay for the case of decay of a neutral particle, and $V^\pm$-decay for the case of a charged particle.

2. The phenomenon of decay of a stopped $K$-meson or $Y$-particle is denoted as $S$-decay.

The introduction of order into the classification and notation of new particles will not only eliminate the possibility of confusion and misunderstandings, but, one may think, will help clarify whether there are real connections between particles that at first glance are different. In particular, there is an opinion3 that the particles presently combined into the group of $K$-mesons are essentially particles of one type, but decay according to different schemes. Although this question requires further experimental investigations, from this point of view such a combining of particles into separate groups should be recognized as reasonable.

M. F.

CITED LITERATURE

1. A. I. Alikhanov, UFN 50, No. 4, 481 (1953).
2. E. Amaldi, C. D. Anderson, P. M. S. Blackett, W. B. Fretter, L. Leprince-Ringuet, B. Peters, C. F. Powell, G. D. Rochester, B. Rossi, R. W. Thompson, Nature 173, No. 4394, 123 (1954); Nuovo Cim. 11, No. 2, 213 (1954); Naturwissenschaften 41, No. 3, 56 (1954).
3. C. F. Powell, Nature 172, No. 4376, 477 (1953).