Strange nuclei
22 August 2001
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Physicists at the Brookhaven National Laboratory in New York have this week announced the production of a "significant number" of nuclei containing two strange quarks. The protons and neutrons of normal matter are made of two types of quark - called up and down - but particles containing the heavier strange quarks are made in the high energy collisions of cosmic rays
in the atmosphere. In the experiment at Brookhaven, a beam of negative
kaons (particles containing a strange quark) struck a beryllium target
and on very rare occasions produced a "doubly-strange nucleus"
consisting of a proton, a neutron, and two lambda particles - particles like neutrons but with a down quark replaced by a strange quark.
This week's image shows a colour rendition of the first observation
of a "hypernucleus" containing one lambda particle, which was discovered late in 1952 by two Polish physicists, Marian Danysz and Jerzy Pniewski*. The image shows the tracks of particles in nuclear emulsion - special photographic emulsion for detecting particles. A cosmic ray coming in from the top right (marked p on the original black and white version)
has collided with a nucleus in the emulsion to create the big star of
tracks. One of the fragments from the collision disintegrates lower
down the image (at B on the original) to produce three new tracks. The
faintest of these, travelling towards the lower left (and marked 3 on
the original), is probably due to a pion. The total energy released in
the disintegration is consistent with the decay of a lambda particle in
the original nuclear fragment. The discovery has been celebrated in
Poland both on a stamp and a postmark. (The horizontal bar indicates a
distance of 0.05 mm.)
*Credit: Danysz and Pniewski, Philosophical Magazine 44 348 (1953)
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