Anthropology News, Science News

Particle physicists working with the BaBar detector at Stanford
Linear Accelerator Center have discovered a new particle in the
bottomonium family of "quarkonium" particles. Technically it isn’t a
"new particle" it is a previously unobserved state of particle,
but when we are talking about subatomic particles, their energy states
become a big deal (and their names get very cool). We are in the realms
of the vanishingly small and the discovery of the lowest energy
bottomonium particle may not seem very significant. But in the world of
quantum chromodynamics, this completes the long quest to find
experimental evidence for this elusive meson and may help explain why
there is more matter than anti-matter in the Universe…

Quarkonia are types of mesons containing two quarks: one quark and its
anti-quark (they are therefore "colourless"). They belong to one of two
families: "bottomonium" or "charmonium". As the names suggest,
bottomonium contains a bottom quark and anti-bottom quark; charmonium
contains a charm quark and anti-charm quark. Groups of three quarks
(interacting via the strong force) are baryons (i.e. protons and neutrons) whereas groups of two quarks are mesons.
Mesons are all thought to be made from a quark-antiquark pair and are
therefore of huge importance when studying why there is more matter
than anti-matter in the Universe.

This is where the BaBar detector at the Stanford Linear Accelerator Center (SLAC), CA, comes in. The BaBar international collaboration investigates the
behaviour of particles and anti-particles during the production of the
bottomonium meson (bottom-antibottom quark pairs) in the aim of
explaining why there is an absence of anti-particles in everyday life.

For each particle of matter there exists an
equivalent particle with opposite quantum characteristics, called an
anti-particle. Particle and anti-particle pairs can be created by large
accumulations of energy and, conversely, when a particle meets an
anti-particle they annihilate with intense blasts of energy. At the
time of the big-bang, the large accumulation of energy must have
created an equal amount of particles and anti-particles. But in
everyday life we do not encounter anti-particles. The question,
therefore, is "What has happened to the anti-particles?" – From the BaBar/SLAC collaboration pages.

All matter has a "ground state", or the lowest energy the system is
trying to attain. As particles for instance try to reach this ground
state, they lose energy, often in the form of electromagnetic
radiation. Once reached, the ground state determines the baseline at
which measurements can be made for higher energy states of those
particles. And this is what the BaBar team has done, they have been
able to isolate the lowest possible energy state for the bottomonium
particle (which is far from easy). So what have they named the ground
state of bottomonium? Quite simply: ηb, pronounced "eta-sub-b".

The bottomonium particle was generated during a collision between an
electron and positron. The energy generated by this collision created a
bottom quark and an anti-bottom quark bound together. At this point,
the bottomonium particle was of too high an energy, but it very quickly
decayed, emitting a gamma ray leaving the ηb behind. However, ηb’s are
highly unstable and will quickly decay into other particles, plus they
are very rare and difficult to detect. This particular decay event only
occurs once in every two or three thousand higher energy bottomonium
decays, so many collisions had to be measured and a huge amount of data
had to be gathered by the BaBar detector before a precise measurement
of the ηb ground state could be gained.

"This very significant observation was made possible
by the tremendous luminosity of the PEP-II accelerator and the great
precision of the BaBar detector, which was so well calibrated over the
BaBar experiment’s 8-plus years of operation. These results were highly
sought after for over 30 years and will have an important impact on our
understanding of the strong interactions." – Hassan Jawahery, BaBar Spokesperson, University of Maryland.

If you want to find out more, you can check out the BaBar team’s publication (with the longest list of co-authors I’ve ever seen!) or the SLAC press release.

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