Although we can claim that the overall results at the LHC for all the decay channels, the signal strengths, and the spin and parity obtained as of July 2013 are compelling in their compatibility with the standard-model Higgs boson, there are still problematic uncertainties in some decay channels and differences between the ATLAS and CMS results. As we know, details matter in physics. In 1916, the experimental difference between Einstein’s theory of general relativity and Newtonian gravity was the tiny effect of 43 arc seconds per century in the perihelion advance of the planet Mercury. This is only about one percent of the total perihelion advance observed. However, such a tiny difference resulted in a huge change in our understanding of what gravity is. Another example of the importance of small details is Kepler’s discovery that he could not resolve a tiny effect in the astronomical data of Danish astronomer Tycho Brahe by using circular orbits for the planets. This forced him into having to accept that the orbits of the planets are elliptical and it heralded the downfall of Ptolemaic astronomy and the acceptance of the Copernican heliocentric paradigm.
If the data to come out of the upgraded LHC in the coming years continue to strengthen the case for the new boson being the long-sought standard-model Higgs boson, then this will represent an enormous achievement in the long history of particle physics, as experiment validates theory. It will also represent one of the greatest technological feats in history, and should guarantee continued funding for the LHC and its successors.
Yet the discovery will not signal the end of the struggle to decipher the particle code of nature. As we know, the standard-model Higgs boson brings with it difficult problems to be solved, such as the Higgs mass hierarchy problem and the gauge hierarchy problem. These are in addition to the long-standing cosmological constant problem and the puzzle of how to combine Einstein’s general relativity theory with quantum mechanics. After the celebratory party welcoming the standard-model Higgs boson officially into the company of elementary particles, physicists must get back to work and solve these problems that come in on the wings of the Higgs boson. We hope that they will be solved by the time-honored method of working out theoretical ideas and then verifying them by experiment, rather than resorting to pseudo-science resolutions such as the multiverse. The new boson, whether it is the standard-model Higgs boson or a different kind of Higgs boson, will offer new challenges for future generations of physicists working toward a deeper understanding of the origins and structure of matter.
ACKNOWLEDGMENTS
I thank my wife, Patricia, for her enduring and superb help in editing the book. Her tireless efforts in preparing the manuscript and her ongoing support made the book become a reality. I thank my agent Chris Bucci at Anne McDermid & Associates, Toronto, for his support and encouragement and for finding the right publisher. I am indebted to my colleagues Martin Green, Viktor Toth, and Vincenzo Branchina for reading the manuscript at an early stage and making many valuable suggestions for improving the book. I thank Bob Holdom and Alvaro de Rujula for helpful discussions. I am grateful to my editor Jeremy Lewis at Oxford University Press for being interested in the topic of particle physics and for publishing the book; thanks also to Erik Hane, Cat Ohala, and Bharathy Surya Prakash, who have been helpful in the final stages of the book preparation. I thank the John Templeton Foundation for their generous financial support for much of the research that forms the basis of this book, and for the writing of the book itself. I also thank my home institute, the Perimeter Institute for Theoretical Physics, for their support, and for providing opportunities for the exchange of ideas, which is a major way that theoretical physics progresses.
GLOSSARY
abelian and nonabelian groups named after the 19th-century Norwegian mathematician Niels Henrik Abel, abelian groups satisfy the commutative rule (the results of a mathematical operation do not depend on the order of the elements [for example, (a × b) = (b × a)], whereas nonabelian groups do not satisfy this rule; their elements do not commute
accelerator a machine that propels charged particles in beams at high speeds using electromagnetic fields; also known as a collider
action the integral over time of the Lagrangian function, but for an action pertaining to fields, it may be integrated over spatial variables as well; it has the physical dimensions of angular momentum
anode in a cell, battery, or apparatus with an electrical current, the anode is the positively charged electrode, which attracts negatively charged ions, most often electrons
anthropic principle the idea that our existence in the universe imposes constraints on its properties; an extreme version claims that we owe our existence to this principle
antimatter particles with opposite electric charge from their matter counterparts; predicted by Paul Dirac in 1928; the first antimatter particles, discovered by Carl Anderson in 1932, were positrons, or positively charged electrons
ATLAS and CMS detectors two of four huge detectors at the large hadron collider (LHC) that collect and measure the debris from the collisions of proton beams; the acronyms stand for “a toroidal LHC apparatus” and “compact muon solenoid”
bare mass the mass of a particle in the absence of interactions with other particles and their fields
baryon a subatomic particle composed of three quarks, such as the proton and neutron
beta decay a type of radioactive decay in which a nucleus of an atom emits an electron or a positron, and the nucleus receives its optimal ratio of protons and neutrons; beta decay is mediated by the weak force
Big Bang theory the theory that the universe began with a violent explosion of space-time, and that matter and energy originated from an infinitely small and dense point
black body an idealized physical system that absorbs all the electromagnetic radiation that hits it, and emits radiation at all frequencies with 100 percent efficiency
boson a particle with integer spin, such as photons, mesons, and gravitons, that carries the forces between fermions
bubble chamber a chamber containing a superheated liquid, such as liquid hydrogen, through which electrically charged particles move, producing bubbles that allow the particles to be tracked and detected; the bubble chamber was invented by Donald Glaser in 1952
cathode a negatively charged electrode through which a current flows out of an electrical device; see anode
cloud chamber a sealed container filled with supersaturated water or alcohol vapor that ionizes when a charged particle interacts with it; an ion acts as a condensation nucleus around which a mist forms, allowing the track of the particle to be detected
collider a kind of accelerator in which directed beams of particles may collide against a stationary target or as two beams colliding head-on; colliders allow beams of particles to accelerate to very high kinetic energies when impacting other particles
color charge a charge carried by quarks and gluons that explains how quarks are confined inside hadrons, such as the proton and neutron; by combining their three colors (red, green, and blue), the resulting hadrons show white as their color charge; color charge also explains how three spin-½ quarks can be in the same quantum state inside a proton without violating Pauli’s exclusion principle; quarks exchange gluons and create a strong color force field that binds the quarks together, becoming stronger as the quarks get farther apart
confinement the phenomenon of colored, charged quarks not being directly observed because they do not occur as free particles; quarks are confined in hadrons (three quarks) and in mesons (one quark and one antiquark)
Cooper pairs (of electrons) two electrons bound together at low temperature, forming the basis of the 1957 Bardeen, Cooper, and Schrieffer (BCS) theory of low-temperature superconductivity; the three shared the 1972 Nobel Prize
cosmic microwave background (CMB) the first significant evidence for the Big Bang theory; initially found in 1964 and studied further by NASA teams in 1989 and the early 2000s, the cosmic microwave background is a smooth signature of microwaves everywhere in the sky, re
presenting the “afterglow” of the Big Bang; infrared light produced about 400,000 years after the Big Bang had red-shifted through the stretching of spacetime during 14 billion years of expansion to the microwave part of the electromagnetic spectrum, revealing a great deal of information about the early universe
cosmic rays very high-energy particles originating outside the solar system and penetrating the earth’s atmosphere; they are composed mainly of protons and atomic nuclei that produce showers of secondary particles; recent data from the Fermi space telescope show that cosmic rays originate primarily from the supernovae of massive stars
cosmic void a vast, empty space in the universe between matter filaments that contains very few or no galaxies
cosmological constant a mathematical term that Einstein inserted into his gravity field equations in 1917 to keep the universe static and eternal; although he later regretted this and called it his “biggest blunder,” cosmologists today still use the cosmological constant, and some equate it with the mysterious dark energy
Coulomb force the electrostatic inverse-square interaction between electrically charged particles, discovered in 1785 by the French physicist Charles Augustin de Coulomb
coupling constant the strength of an interaction between particles or fields; electric charge and Newton’s gravitational constant are coupling constants
coupling strength the strength of the force exerted in an interaction between particles
cross-section in a collider, the area of a material with nuclei that acts as a target for a beam of particles hitting it
dark energy a mysterious form of energy that has been associated with negative pressure vacuum energy and Einstein’s cosmological constant; it is hypothesized to explain the data on the accelerating expansion of the universe; according to the standard model of cosmology, the dark energy, which is spread uniformly throughout the universe, makes up about 70 percent of the total mass and energy content of the universe
dark matter invisible, not-yet-detected unknown particles of matter, representing about 25 percent of the total matter-energy in the universe according to the standard model; its presence is necessary if Newton’s and Einstein’s gravity theories are to fit data from galaxies, clusters of galaxies, and cosmology that show much stronger gravity than is predicted by the theories; together, dark matter and dark energy mean that 96 percent of the matter and energy in the universe is invisible
degree of freedom an independent parameter in a physical system that describes that system’s configuration
Dirac equation a relativistic wave equation formulated by Paul Dirac in 1928 that is consistent with both quantum mechanics and the theory of special relativity; it describes the quantum physics of elementary spin-½ particles such as electrons, and can be generalized to apply to the curved spacetime of Einstein’s general relativity; the wave equation also predicted the existence of antimatter, such as the positron
drift chamber (wire chamber) a proportional counter with a wire under high voltage running through a metal conductor enclosure that is filled with a gas (such as an argon–methane mix), so that an ionizing particle will ionize surrounding atoms; the ions and electrons are accelerated by an electric field acting on the wire, producing an electric current proportional to the energy of the particle that is to be detected, which allows an experimentalist to count particles and determine their energy
electromagnetism one of the four fundamental forces of nature, the others being gravitation, the weak interaction (radioactivity), and the strong interaction; the electromagnetic field interacts through the photon, which is its force carrier, with the electric charge of elementary particles such as the electron
electroweak theory the unified description of the electromagnetic and weak interactions of particles; although they appear to be different forces at low energies, they merge into one above the unification energy 246 GeV
elementary particle a particle that does not have any substructure such as smaller particles; considered one of the basic building blocks of matter in the universe
energy cutoff a maximum value of energy, momentum, or length in particle physics, usually chosen to make infinite quantities in calculations become finite
ether (or aether) the medium through which it was believed for centuries that energy and matter moved, something more than a vacuum and less than air; its origins were in the Greek concept of “quintessence,” but during the late 19th century, the Michelson–Morley experiment disproved the existence of the ether
femtobarn a unit of cross-section area where a barn is equal to 10−28 m2, a millibarn (mb) is equal to 10−31 m2, and a femtobarn (fb) is equal to 10−43 m2
fermion a particle with half-integer spin like protons and electrons, which make up matter
field a physical term describing the forces between massive bodies in gravity and electric charges in electromagnetism; Michael Faraday discovered the concept of field when studying magnetic conductors
fine-tuning the unnatural cancelation of two or more large numbers involving an absurd number of decimal places, when one is attempting to explain a physical phenomenon; this signals that a true understanding of the physical phenomenon has not been achieved
gauge boson in the standard model of particle physics, a carrier of a force, such as the photon, which is the carrier of the electromagnetic interaction; the W and the Z bosons, which are the carriers of the weak interaction; and the gluon, which is the carrier of the strong interaction
gauge invariance in electromagnetism, the property that a class of scalar and vector potentials, related by gauge transformations, retains the same electric and magnetic fields; Maxwell’s field equations are such that the electric field and the magnetic field can be expressed in terms of the derivative of a scalar field (scalar potential) and a vector field (vector potential); gauge invariance has been extended to more general theories such as nonabelian Yang–Mills fields and gravitational fields
Geiger counter a particle detector that measures ionizing radiation, such as the emission of alpha particles, beta particles, or gamma rays by atomic nuclei
general relativity Einstein’s revolutionary gravity theory, created in 1916 from a mathematical generalization of his theory of special relativity; it changed our concept of gravity from Newton’s universal force to the warping of the geometry of spacetime in the presence of matter and energy
GeV a unit of energy equal to one billion electron volts, or a gigaelectron volt
gluon the exchange particle or gauge boson for the strong force that binds quarks to make hadrons; gluons are analogous to photons, which carry the electromagnetic force between two electrically charged particles
graviton the hypothetical smallest packet of gravitational energy, comparable to the photon for electromagnetic energy; the graviton has not yet been seen experimentally
gravity as first expressed by Isaac Newton, a force by which physical bodies attract each other proportional to their masses, and inversely proportional to the square of the distance between them; Einstein’s general relativity theory described gravitation as the curvature of spacetime by matter; gravity is the weakest of the known four fundamental forces
group (in mathematics) in abstract algebra, a set that obeys a binary operation that satisfies certain axioms; for example, the property of addition of integers makes a group; the branch of mathematics that studies groups is called group theory
hadron a particle composed of quarks bound together by the strong force; there are two kinds: baryons, such as protons and neutrons, made of three quarks, and mesons, such as pions, made from one quark and one antiquark; in Greek, hadrós means “stout” or “thick”
Higgs field the field associated with the Higgs boson, which is theorized to impart mass to the known elementary particles of the standard model
Higgs particle or boson theorized to be an elementary particle by Peter Higgs in 1964, its probable discovery was announced on July 4, 2012, and confirmed more confidently on March
14, 2013; this particle plays a pivotal role in the standard model
horizon problem identified in the standard Big Bang model of cosmology during the late 1960s, it occurs because widely separated regions of the early universe had the same temperature and other physical properties, yet were unable to communicate with each other because of the large distances between them; the finite measured speed of light prevents the regions from being connected causally; inflation theory and the variable speed of light theory have been proposed to resolve the horizon problem
identity in mathematics, an equality relation A = B, such that A and B contain numbers or variables; A = B is an identity when it is true for all values of the functions making up A and B; in algebra, the identity element e when combined with any element x of S gives the same x: ex = xe = x for all x in S
inflation a theory proposed during the early 1980s by Alan Guth and others to resolve the flatness, horizon, and homogeneity problems in the standard Big Bang model; the very early universe is pictured as expanding exponentially fast in a fraction of a second
inverse femtobarn (fb−1) a measurement of particle collision events per femtobarn of target cross-section, which serves as the conventional unit for time-integrated luminosity
isotopic spin or isospin proposed by Werner Heisenberg in 1932 to explain the properties of the newly discovered neutron, it is a quantum number related to the strong interaction such that particles with different charges that are affected equally by the strong force can be treated as different states of the same particle, with isospin values determined by the number of charge states; physically, isospin is a dimensionless quantity—in other words, it does not have the units of angular momentum or spin, but its name comes from the fact that the mathematics of isospin are similar to those describing spin
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