Authors: Scientific American Editors
The Higgs Boson
Searching for the God Particle
From the Editors of Scientific American
Cover image: CERN
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THE HIGGS BOSON
Searching for the God Particle
From the Editors of Scientific American
Table of Contents
Introduction: A Good Beginning
by Jesse Emspak
In 1846 astronomers found a new planet, Neptune, because the mathematics of Newton's laws, when applied to the orbit of Uranus, said some massive body had to be there. They eventually found it, using the best telescopes available to peer into the sky.
The search for the Higgs boson, which current theories say gives matter its mass, is being conducted in a similar way. One consequence of the prevailing theory of physics, called the Standard Model, is that there has to be some field that gives particles their particular masses. With that there has to be a corresponding particle, made by creating waves in the field. Peter Higgs, a physicist at the University of Edinburgh, outlined the idea in the 1960s. (Higgs wasn't the first to propose the idea; but he did work out a relativistic model for it.)
To create the ripples in the Higgs field, one needs to smash particles together at high energies. That would take the construction of particle accelerators able to reach energies measured in trillions of electronvolts – enough to push a proton to speeds a few meters per second slower than light. Just as it would take the invention of the telescope to seek Neptune, it wasn't until the Large Hadron Collider was built that anyone could see evidence of the Higgs.
The Higgs demonstrates the power of a good theory. The mathematics said something had to be there. If it wasn't, then something was very wrong with the Standard Model, or there was some subtlety that physicists had missed. So the search for answers continued, and finally in July 2012, a Higgs-like particle was found – and further, near the energies scientists expected to find it.
This book shows a bit of that scientific process. We start with an introduction to the physics involved. The second part outlines the reasons why particle physicists thought the Higgs – or something like it – had to exist.
The next section deals with the search. Much of it involved billion-dollar projects, and for a while the cancellation of the Superconducting Supercollider seemed to dash the hopes that there would be a collider powerful enough to find the Higgs. But the Large Hadron Collider stepped into the breach.
The Higgs search is important to understanding the world around us. Even more than finding a new planet, it helps us answer questions that seem intuitive: why does anything have mass at all? What do we mean by "mass?" Once those might have been purely metaphysical or philosophical debates.
It doesn't solve any problems. But being able to study the Higgs boson, like all the best science, puts us in a better position to ask the right questions. It's a good beginning.
Elementary Particles and Forces
By Chris Quigg
The notion that a fundamental simplicity lies below the observed diversity of the universe has carried physics far. Historically the list of particles and forces considered to be elementary has changed continually as closer scrutiny of matter and its interactions revealed microcosms within microcosms: atoms within molecules, nuclei and electrons within atoms, and successively deeper levels of structure within the nucleus. Over the past decade, however, experimental results and the convergence of theoretical ideas have brought new coherence to the subject of particle physics, raising hopes that an enduring understanding of the laws of nature is within reach.
Higher accelerator energies have made it possible to collide particles with greater violence, revealing the subatomic realm in correspondingly finer detail; the limit of experimental resolution now stands at about 10
centimeter, about a thousandth the diameter of a proton. A decade ago physics recognized hundreds of apparently elementary particles; at today's resolution that diversity has been shown to represent combinations of a much smaller number of fundamental entities. Meanwhile the forces through which these constituents interact have begun to display underlying similarities. A deep connection between two of the forces, electromagnetism and the weak force that is familiar in nuclear decay, has been established, and prospects are good for a description of fundamental forces that also encompasses the strong force that binds atomic nuclei.
FUNDAMENTAL SCHEME OF NATURE, according to current theory, embraces 12 elementary particles
and four forces
. All the particles listed are thought to be structureless and indivisible; among their properties are an identical amount of spin, given by convention as 1/2, and differing values of electric charge, color charge and mass, given as energy in millions of electron volts (MeV) divided by the square of the speed of light (c). Only the pairs of leptons and quarks at the top of each column are found in ordinary matter; the other particles are observed briefly in the aftermath of high-energy collisions. The four forces thought to govern matter vary in range and strength; although the strong force is the most powerful, it acts only over a distance of less than 10
centimeter, the diameter of a proton. All the forces are conveyed by force particles, whose masses are given in billions of electron volts (GeV) divided by the square of the speed of light. Because of its weakness, gravity has not been studied experimentally by particle physicists.
Illustration by Andrew Christie
Of the particles that now appear to be structureless and indivisible, and therefore fundamental, those that are not affected by the strong force are known as leptons. Six distinct types, fancifully called flavors, of lepton have been identified. Three of the leptons, the electron, the muon and the tau, carry an identical electric charge of –1 ; they differ, however, in mass.
The electron is the lightest and the tau the heaviest of the three. The other three, the neutrinos, are, as their name suggests, electrically neutral. Two of them, the electron neutrino and the muon neutrino, have been shown to be nearly massless. In spite of their varied masses all six leptons carry precisely the same amount of spin angular momentum. They are designated spin –1/2 because each particle can spin in one of two directions. A lepton is said to be right-handed if the curled fingers of a right hand indicate its rotation when the thumb points in its direction of travel and left-handed when the fingers and thumb of the left hand indicate its spin and direction.
For each lepton there is a corresponding antilepton, a variety of antiparticle.
Antiparticles have the same mass and spin as their respective particles but carry opposite values for other properties, such as electric charge. The antileptons, for example, include the antielectron, or positron, the antimuon and the antitau, all of which are positively charged, and three electrically neutral antineutrinos.
In their interactions the leptons seem to observe boundaries that define three families, each composed of a charged lepton and its neutrino. The families are distinguished mathematically by lepton numbers; for example, the electron and the electron neutrino are assigned electron number 1, muon number 0 and tau number 0. Antileptons are assigned lepton numbers of the opposite sign. Although some of the leptons decay into other leptons, the total lepton number of the decay products is equal to that of the original particle; consequently the family lines are preserved.
The muon, for example, is unstable.
It decays after a mean lifetime of 2.2 microseconds into an electron, an electron antineutrino and a muon neutrino through a process mediated by the weak force. Total lepton number is unaltered in the transformation. The muon number of the muon neutrino is 1, the electron number of the electron is 1 and that of the electron antineutrino is –1. The electron numbers cancel, leaving the initial muon number of 1 unchanged. Lepton number is also conserved in the decay of the tau, which endures for a mean lifetime of 3 X 10
The electron, however, is absolutely stable. Electric charge must be conserved in all interactions, and there is no less massive charged particle into which an electron could decay. The decay of neutrinos has not been observed.
Because neutrinos are the less massive members of their respective families, their decay would necessarily cross family lines.
Where are leptons observed? The electron is familiar as the carrier of electric charge in metals and semiconductors.
Electron antineutrinos are emitted in the beta decay of neutrons into protons. Nuclear reactors, which produce large numbers of unstable free neutrons, are abundant sources of antineutrinos. The remaining species of lepton are produced mainly in high-energy collisions of subnuclear particles, which occur naturally as cosmic rays interact with the atmosphere and under controlled conditions in particle accelerators. Only the tau neutrino has not been observed directly, but the indirect evidence for its existence is convincing.
EVIDENCE OF QUARKS, two narrow jets of particles emerge from the collision and mutual annihilation of an electron and an antielectron, or positron. The annihilation releases energy, which gives rise to matter. The detected particles have a variety of masses and spings; some are neutral
and some electrically charged
. If the particles arose directly from the annihilation, they would be expected to follow widely divergent paths. The focused character of the jets suggests instead that each jet developed from a single precursor: a quark or an antiquark. They are the immediate products of the photon of electromagnetic energy released in the collision, which is diagrammed on the left using arrows to represent the relative motion of the particles. The event shown was recorded in the JADE detector of the PETRA accelerator at the Deutsches Elektronen-Synchotron (DESY) in Hamburg. The paths of the particles were reconstructed by computer from ionization tracks and from the pattern of energy
deposited as the particles struck the inner layer of the 2.4-meter-long cylindrical detector.
Illustration by Andrew Christie
Subnuclear particles that experience the strong force make up the second great class of particles studied in the laboratory. These are the hadrons;
among them are the protons, the neutrons and the mesons. A host of other less familiar hadrons exist only ephemerally as the products of high-energy collisions, from which extremely massive and very unstable particles can materialize. Hundreds of species of hadron have been catalogued, varying in mass, spin, charge and other properties.
Hadrons are not elementary particles, however, since they have internal structure. In 1964 Murray Gell-Mann of the California Institute of Technology and George Zweig, then working at CERN, the European laboratory for particle physics in Geneva, independently attempted to account for the bewildering variety of hadrons by suggesting they are composite particles, each a different combination of a small number of fundamental constituents.
Gell-Mann called them quarks. Studies at the Stanford Linear Accelerator Center (SLAC) in the late 1960's in which high-energy electrons were fired at protons and neutrons bolstered the hypothesis. The distribution in energy and angle of the scattered electrons indicated that some were colliding with pointlike, electrically charged objects within the protons and neutrons.
Particle physics now attributes all known hadron species to combinations of these fundamental entities. Five kinds, also termed flavors, of quark have been identified-the up
quarks-and a sixth flavor, the top
quark, is believed to exist. Like the leptons, quarks have half a unit of spin and can therefore exist in leftand right-handed states. They also carry electric charge equal to a precise fraction of an electron's charge: the
quarks have a charge of
–1/3, and the
and the conjectured
quark have a charge of +2/3. The corresponding antiquarks have electric charges of the same magnitude but opposite sign.
Such fractional charges are never observed in hadrons, because quarks form combinations in which the sum of their charges is integral. Mesons, for example, consist of a quark and an antiquark, whose charges add up to –1, 0 or + 1. Protons and neutrons consist respectively of two
quarks and a
quark, for a total charge of + 1 , and of a
quark and two
quarks, for a total charge of 0.
Like leptons, the quarks experience weak interactions that change one species, or flavor, into another. For example, in the beta decay of a neutron into a proton one of the neutron's
quarks metamorphoses into a
quark, emitting an electron and an antineutrino in the process. Similar transformations of
quarks have been observed. The pattern of decays suggests two family groupings, one of them thought to contain the
quarks and the second the
quarks. In apparent contrast to the behavior of leptons, some quark decays do cross family lines, however; transformations of
quarks and of
quarks into d quarks have been observed. It is the similarity of the two known quark families to the families of leptons that first suggested the existence of a
quark, to serve as the partner of the
quark in a third family.
In contrast to the leptons, free quarks have never been observed. Yet circumstantial evidence for their existence has mounted steadily.
One indication of the soundness of the quark model is its success in predicting the outcome of high-energy collisions of an electron and a
Because they represent matter and antimatter, the two particles annihilate each other, releasing energy in the form of a
The quark model predicts that the energy of the photon can materialize into a quark and an antiquark. Because the colliding electron-positron pair had a net momentum of 0, the quark-antiquark pair must diverge in opposite directions at equal velocities so that their net momentum is also 0.
The quarks themselves go unobserved because their energy is converted into additional quarks and antiquarks, which materialize and combine with the original pair, giving rise to two jets of hadrons
(most of them pions, a
species of meson). Such jets are indeed observed, and their focused nature confirms that the hadrons did not arise directly from the collision but from single, indivisible particles whose trajectories the jets preserve.
The case for the reality of quarks is also supported by the variety of energy levels, or masses, at which certain species of hadron, notably the psi and the upsilon particles, can be observed in accelerator experiments.
Such energy spectra appear analogous to atomic spectra:
they seem to represent the quantum states of a
bound system of two smaller components.
Each of its quantum states would represent a
different degree of excitation and a
different combination of the components'
spins and orbital motion.
To most physicists the conclusion that such particles are made up of quarks is irresistible.
The psi particle is held to consist of a
c quark and its antiquark, and the upsilon particle is believed to comprise a
b quark and its antiquark.