Getting to the Heart of Matter: New Ways to Understand the Mass We See and Don't See in the World Around Us Extraordinary progress has been made over the past decade in understanding masses of "elementary" particles, the building blocks that make up all matter we know about in the universe. That progress was accompanied by great surprises: supposedly mass-less neutrinos have been shown to possess mass. Most of the mass in our galaxy and elsewhere in the universe appears to be in the form of "dark matter", particles not yet detected in the laboratory. In the "Standard Model" of particle physics, mass derives from interactions of quarks and leptons -- the constituents of all known matter -- with a new kind of particle or field, called the "Higgs particle". The Higgs particle also remains elusive. New scientific tools -- including the Large Hadron Collider which will be coming online next year -- may be able to shed light on the nature of dark matter and the Higgs particle. If history is any guide, the new tools will also create their own surprises. The talk will cover some of the recent highlights in the historic quest to understand the basic forces and building blocks of all matter and energy and it will preview the new tools and how they plan to carry on this quest. 1 Einstein (1905, ff.) E hc m = E = mm= c2 λ and, the World is made of atoms! Last year, we celebrated the centenary of Einstein’s annus mirabilis: 5 remarkable papers that changed physics. We will use 3 equations of Einstein in our examination of mass In the universe. The first, and to most physicists, the most radical of that year, proposed that light— radiation—was not continuous, but consisted of particles we now call photons which carry a definite quantum of energy depending on their wavelength. This is the work for which he received the Nobel Prize. His formula for representing this idea is the centerpiece of this slide. The formula on the left came from his final paper of 1905 in an addendum to his “relativity” paper; in slightly different form, it is the most famous formula in science. In the present form, however, it achieves its “Einsteinian” significance: “The mass of a body is a measure of its energy content;” He went on to say: “If theory agrees with the facts, then radiation carries inertia between emitting and absorbing bodies.” (Comment: energy from the stars; accelerators.) The third formula is Einstein’s somewhat later postulate, the equivalence principle, stating that the two Newtonian concepts of mass, inertial and gravitational, are equivalent. Taken as icons, these three formulas represent modern physics, the joining of relativity and quantum theory to describe our world of atoms and nuclei in all their diversity and Einstein’s theory of gravity, “General Relativity.” His other two papers that year essentially closed any remaining debate on the atomic picture of the structure of matter. 2 Goals of particle physics to find out: What is the world made of? How does it work? Answer: it depends on where you look. Physics recognizes 4 basic forces in nature: gravity, electromagnetism, a strong force responsible for binding nuclei of atoms and a weak force involved in certain kinds of radioactive decay. At the largest scales we know about, gravity is the dominant force, shaping the universe around us. At our scale, gravity is important, but we begin to sense the importance of the other forces and by the time we examine physics at the scale of atoms and smaller, gravity is completely negligible and the other forces dominate. 20th century physics revealed the nature of these forces at the atomic scale and smaller, the particles that make up all the matter around us, and the theoretical tools needed to understand their interactions down to distances ~1/1000 the size of atomic nuclei, our present horizon of understand of the very small. At much smaller distances—the “Planck scale”—it is expected that gravity will return to be the dominant force. 3 E hc m = E = c2 λ Progress in understanding the structure of atoms and their nuclei during the 2nd half of the 20th century was achieved by remarkable advances in particle accelerators and detectors. The photo above shows Fermilab, outside Chicago, which has operated the world’s highest energy accelerator—the Tevatron—over the past 20 years (along with other, lower-energy machines). In the Tevatron, counter-rotating beams of protons and their antiparticle, antiprotons, collide in the center of large detectors, such as “CDF” shown, which analyze particles produced in the collisions. Physicists use detectors to “see” trails of ionization left by charged particles produced in the collisions as they pass through matter. In these experiments, direct use is made of Einstein’s relationship between mass and energy. New massive particles are created from the energy achieved by the accelerator. The study of the new particles and their interactions reveals details of the underlying physics down to scales of distance related to energy by Einstein’s “photon” relationship. The 50 “golden-years” of accelerator-driven particle physics can be broken into two major periods: 1947-1974 experimental and theoretical discoveries leading to the “Standard Model”, post-1974 testing the SM and searching for physics “beyond the Standard Model” 4 Standard Model ca. Nov. 1974 — today nt t n c n m t u b e m s e d Leptons Quarks + forces unified through hidden Mass of quarks and charged leptons symmetries, all described by acquired through interactions with a relativistic quantum field theory new field filling all space: The Higgs The Standard Model represents the culmination of the 20th Century quest to understand the basic building blocks of all matter and the forces that operate between building blocks. It posits matter as being made from two classes of objects: quarks and leptons. Quarks participate in the strong nuclear force and are the distinguishing constituents of the nuclei of atoms. Leptons do not respond to the strong force, but interact with the other known forces, electromagnetism, weak and gravity, in the same way as do quarks. The Standard Model is based on symmetry principles requiring the quarks and leptons to have exactly zero mass. It accommodates the evident fact that particles carry mass by introducing a new field which fills all space, the “Higgs field”. Quarks and the charged leptons acquire their masses by they way they “attach” to the Higgs field. The precise date I’ve given to the acceptance of the Standard Model corresponds to the start of a revolutionary set of experiments which began on this day in 1974 at the Stanford Linear Accelerator Center. That was a Saturday; by the following Monday the world of particle physics had entered its “November Revolution” which, over the next few weeks and months, joined in a remarkable consensus on the validity of the Standard Model. To date, there are no experimental conflicts with predictions of the SM, but as we shall see, there certainly are important questions not understood within the Standard Model. Review what was so special about the Nov. revolution and the next generation of discoveries to where we stand today. 5 Weighing Nothing E hc m = E = c2 λ How mass is described in the Standard Model: The SM uses elegant symmetry arguments to describe and unify the strong, electromagnetic and weak forces. The symmetries invoked rely on having all particles—quarks and leptons—exactly massless. The symmetry is broken in our world by the introduction of a new field filling all space, the Higgs field. Interactions between the quarks and charged leptons and the Higgs field given them their apparent mass. Nuclei of atoms gain additional mass from the energy content associated with binding quarks in nuclei. The diversity of masses between, say, the electron and the top quark is large, about 1 : 1/3 million The three neutrinos sit outside this picture. They don’t stick to the Higgs field in the way quarks and charged leptons do. They don’t bind together to acquire internal energy and, hence, mass. They were expected to retain their masslessness in the real world. It was known since the early days of nuclear physics that neutrinos must have tiny masses at most, much less than the electron, for example. Starting nearly 40 years ago, there started to appear data indicating something was wrong with neutrinos that travel long distances, first from the sun, then in the upper reaches of the earth’s atmosphere and now in accelerator experiments. 2002 NP, SNO & MINOS @ UT Unavoidable conclusion: neutrinos have definite mass, small but not zero. 6 Weighing Nearly Everything mm= For decades, astronomers have sought to measure the total mass “out there” in our universe. In both Newton’s theory of gravity and Einstein’s, an object in orbit around other things must move with a speed that depends on the total mass inside its orbit. Too slow and it falls in; too fast and it sails off. So, by measuring the speed of stars orbiting on the fringes of galaxies, one can determine the total mass of all the stuff in the galaxy! (Naturally, some simple equations from Einstein tell how to relate shifts in the apparent color of stars to their speed around their galaxy—just like Doppler radar!) Historically big putdown: what we actually see in galaxies—all the stars and other exotic things—represents only a few percent of the mass of the galaxy, itself! Another several percent is probably in non-luminous ordinary matter, like planets and dust clouds and the like.