Strange particles and the weak interaction In the 1950s, with development of
particle accelerators and studies of
cosmic rays,
inelastic scattering experiments on protons (and other atomic nuclei) with energies about hundreds of
MeVs became affordable. They created some short-lived
resonance "particles", but also
hyperons and
K-mesons with unusually long lifetime. The cause of the latter was found in a new quasi-
conserved quantity, named
strangeness, that is conserved in all circumstances except for the
weak interaction. The strangeness of heavy particles and the
μ-lepton were first two signs of what is now known as the
second generation of fundamental particles. The weak interaction revealed soon yet another mystery. In 1957
Chien-Shiung Wu proved that it does not
conserve parity. In other words, the mirror symmetry was disproved as a fundamental
symmetry law. Throughout the 1950s and 1960s, improvements in particle accelerators and
particle detectors led to a bewildering variety of particles found in high-energy experiments. The term
elementary particle came to refer to dozens of particles, most of them
unstable. It prompted Wolfgang Pauli's remark: "Had I foreseen this, I would have gone into botany". The entire collection was nicknamed the "
particle zoo". It became evident that some smaller constituents, yet invisible, form
mesons and
baryons that counted most of then-known particles.
Deeper constituents of matter The interaction of these particles by
scattering and
decay provided a key to new fundamental quantum theories.
Murray Gell-Mann and
Yuval Ne'eman brought some order to mesons and baryons, the most numerous classes of particles, by classifying them according to certain qualities. It began with what Gell-Mann referred to as the "
Eightfold Way", but proceeding into several different "octets" and "decuplets" which could predict new particles, most famously the , which was detected at
Brookhaven National Laboratory in 1964, and which gave rise to the
quark model of hadron composition. While the
quark model at first seemed inadequate to describe
strong nuclear forces, allowing the temporary rise of competing theories such as the
S-matrix theory, the establishment of
quantum chromodynamics in the 1970s finalized a set of fundamental and exchange particles (). It postulated the fundamental
strong interaction, experienced by quarks and mediated by
gluons. These particles were proposed as a building material for hadrons (see
hadronization). This theory is unusual because individual (free) quarks cannot be observed (see
color confinement), unlike the situation with composite atoms where electrons and nuclei can be isolated by transferring
ionization energy to the atom. Then, the old, broad
denotation of the term
elementary particle was deprecated and a replacement term
subatomic particle covered all the "zoo", with its hyponym "
hadron" referring to composite particles directly explained by the quark model. The designation of an "elementary" (or "fundamental") particle was reserved for
leptons, quarks, their
antiparticles, and
quanta of fundamental interactions (see below) only.
Quarks, leptons, and four fundamental forces Because the quantum field theory (see
above) postulates no difference between particles and
interactions, classification of elementary particles allowed also to classify interactions and
fields. Now a large number of particles and (non-fundamental) interactions is explained as combinations of a (relatively) small number of fundamental substances, thought to be
fundamental interactions (incarnated in fundamental
bosons), quarks (including antiparticles), and
leptons (including antiparticles). As the theory distinguished
several fundamental interactions, it became possible to see which elementary particles participate in which interaction. Namely: • All particles participate in gravitation. • All charged elementary particles participate in electromagnetic interaction. • As a consequence, neutron participates in it with its
magnetic dipole in spite of zero electric charge. This is because it is composed of
charged quarks whose charges
sum to zero. • All fermions participate in the weak interaction. • Quarks participate in the strong interaction, along gluons (its own quanta), but not leptons nor any fundamental bosons other than gluons. The next step was a reduction in number of fundamental interactions, envisaged by early 20th century physicists as the "
united field theory". The first successful modern
unified theory was the
electroweak theory, developed by
Abdus Salam,
Steven Weinberg and, subsequently,
Sheldon Glashow. This development culminated in the completion of the theory called the Standard Model in the 1970s, that included also the strong interaction, thus covering three fundamental forces. After the discovery, made at
CERN, of the existence of
neutral weak currents, mediated by the
Z boson foreseen in the standard model, the physicists Salam, Glashow and Weinberg received the 1979
Nobel Prize in Physics for their electroweak theory. The discovery of the
weak gauge bosons (quanta of the
weak interaction) through the 1980s, and the verification of their properties through the 1990s is considered to be an age of consolidation in particle physics. While accelerators have confirmed most aspects of the Standard Model by detecting expected particle interactions at various collision energies, no theory reconciling general relativity with the Standard Model has yet been found, although
supersymmetry and
string theory were believed by many theorists to be a promising avenue forward. The
Large Hadron Collider, however, which began operating in 2008, has failed to find any evidence whatsoever that is supportive of supersymmetry and string theory, and appears unlikely to do so, meaning "the current situation in fundamental theory is one of a serious lack of any new ideas at all." This state of affairs should not be viewed as a crisis in physics, but rather, as
David Gross has said, "the kind of acceptable scientific confusion that discovery eventually transcends."
Gravitation, the fourth fundamental interaction, is not yet integrated into particle physics in a consistent way.
Higgs boson s and two electrons, visible as lines. As of 2011, the
Higgs boson, the quantum of a field that is thought to provide particles with
rest masses, remained the only particle of the Standard Model to be verified. On July 4, 2012, physicists working at CERN's
Large Hadron Collider announced that they had discovered a new subatomic particle greatly resembling the Higgs boson, a potential key to an understanding of why elementary particles have masses and indeed to the existence of diversity and life in the universe.
Rolf-Dieter Heuer, the director general of CERN, said that it was too soon to know for sure whether it is an entirely new massive particle – one of the heaviest subatomic particles yet – or, indeed, the elusive particle predicted by the
Standard Model, the theory that has ruled physics for the last half-century. It is unknown if this particle is an impostor, a single particle or even the first of many particles yet to be discovered. The latter possibilities are particularly exciting to physicists since they could point the way to new deeper ideas,
beyond the Standard Model, about the nature of reality. For now, some physicists are calling it a "Higgslike" particle.
Joe Incandela, of the
University of California, Santa Barbara, said, "It's something that may, in the end, be one of the biggest observations of any new phenomena in our field in the last 30 or 40 years, going way back to the discovery of quarks, for example." The groups operating the large detectors in the collider said that the likelihood that their signal was a result of a chance fluctuation was less than one chance in 3.5 million, so-called "five sigma," which is the gold standard in physics for a discovery.
Michael Turner, a cosmologist at the University of Chicago and the chairman of the physics center board, said Confirmation of the Higgs boson or something very much like it would constitute a rendezvous with destiny for a generation of physicists who have believed the boson existed for half a century without ever seeing it. Further, it affirms a grand view of a universe ruled by simple and elegant and symmetrical laws, but in which everything interesting in it being a result of flaws or breaks in that symmetry. According to the Standard Model, the Higgs boson is the only visible and particular manifestation of an
invisible force field that permeates space and imbues elementary particles that would otherwise be massless with mass. Without this Higgs field, or something like it, physicists say all the elementary forms of matter would zoom around at the speed of light; there would be neither
atoms nor life. The Higgs boson achieved a notoriety rare for abstract physics. To the eternal dismay of his colleagues, Leon Lederman, the former director of
Fermilab, called it the "God particle" in his book of the same name, later quipping that he had wanted to call it "the goddamn particle". Professor Incandela also stated, Dr.
Peter Higgs was one of six physicists, working in three independent groups, who in 1964 invented the notion of the cosmic molasses, or Higgs field. The others were
Tom Kibble of
Imperial College, London;
Carl Hagen of the
University of Rochester;
Gerald Guralnik of
Brown University; and
François Englert and
Robert Brout, both of
Université Libre de Bruxelles. One implication of their theory was that this Higgs field would produce its own quantum particle if hit hard enough by the right amount of energy. The particle would be fragile and fall apart within a millionth of a second in a dozen different ways depending upon its own mass. Unfortunately, the theory did not predict the particle mass making it difficult to find. The particle eluded researchers at a succession of particle accelerators. Further experiments continued and in March 2013 it was tentatively confirmed that the newly discovered particle was a Higgs Boson. Although they have never been seen, Higgs-like fields play an important role in theories of the universe and in string theory. Under certain conditions, according to the strange accounting of Einsteinian physics, they can become suffused with energy that exerts an antigravitational force. Such fields have been proposed as the source of an enormous burst of expansion, known as inflation, early in the universe and, possibly, as the secret of the dark energy that now seems to be speeding up the expansion of the universe. ==Notes==