Revealing the Hidden Nature of Space and Time Charting the Course for Elementary Particle Physics (2006) / Chapter Skim
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2. Key Questions in Particle Physics
2. Key Questions in Particle Physics
Pages 33-55
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From page 33... ...
· What do the properties of particles reveal about the nature and origin of matter and the properties of space and time? · What are dark matter and dark energy, and how has quantum mechanics influenced the structure of the universe?
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From page 34... ...
This process culminated a half century later with the emergence of the Standard Model. The Standard Model, in a remarkably concise way, describes and explains many of the phenom ena that underlie particle physics and captures with astonishing precision an in credible range of observational data.
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Why are the force-carrying particles so different, with the photon being detectable by our eyes while the W and Z particles can be observed only with the most sophisticated equipment? Settling this question, which would explain why the weak interactions are weak, is a major goal of particle physics for the coming decade.
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From page 36... ...
The Higgs particle, which is the particle associated with the Higgs field, has not yet been seen. One major goal of upcoming accelerator experiments is to discover whether a simple Higgs particle causes the breaking of the symmetry between the weak interactions and electromagnetism, as in the Standard Model, or whether there is some more complicated mechanism.
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Model is mathematically inconsistent if the Higgs particle -- or whatever replaces it -- is too much heavier than the W and Z Thus, combined with experimental measurements, the Higgs particle should weigh no more than around 300 GeV.
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Of the superpartners predicted by supersymmetry, the lightest neutral superpartner particle, a neutralino, is thought to be an excellent candidate to account for some or all of the dark matter in the universe. Theoretical arguments strongly suggest that some of the new supersymmetric particles will be produced at the LHC.
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From page 39... ...
WHAT DO THE PROPERTIES OF PARTICLES REVEAL ABOUT THE NATURE AND ORIGIN OF MATTER AND THE PROPERTIES OF SPACE AND TIME? Though particle physics focuses on the fundamental particles of the universe, it involves far more than just developing a taxonomy of esoteric phenomena studied in accelerator laboratories.
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A new source of naturally occurring particles was discovered in 1912: Earth is constantly bombarded with cosmic rays from space. Besides giving physicists a fascinating new window from which to explore the universe, cosmic rays made possible fundamental discoveries about nature, mainly because cosmic rays have higher energies than do the particles emitted by radioactive atoms.
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From page 41... ...
At the same time, quantum mechanics shows that three generations is the minimum number that can accommodate a mechanism known as CP violation, which allows matter and antimat ter to behave slightly differently and which may have been critical in the formation and evolu tion of the universe. FIGURE 2-3-1 The particles of the Standard Model.
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Since all known neutrino types are very light, this tells us that there is no fourth generation of particles that follows the same pattern as the first three with a very light neutrino.
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By 1962, using high-energy neutrino beams created at accelerators, the second-generation neutrino was discovered; an experiment at Brookhaven National Laboratory demonstrated that the neutrinos created along with muons in meson decays are distinct from the first-generation neutrinos created in decays of radioactive atoms. The discovery of the second generation was completed when evidence for the charm quark was found at particle accelerators, beginning with the discovery of the J/ particle (which consists of a charm quark and an anticharm quark)
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Observing neutrino effects is hard, but an even bigger challenge for particle physics has been to detect and measure the masses of the neutrinos. Those masses still have not been precisely determined, yet they are suspected to be very impor tant clues about particle unification.
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From that small remnant of the cosmos's origin, according to Sakhavov, stars, planets, and people ultimately formed. For this to work, Sakharov showed, two very subtle particle physics effects would be needed.
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, the matter and antimatter would all have disappeared, leaving only pho tons and dark matter. The result would have been a very dull universe.
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Extremely precise measurements of parameters provide extremely sensitive tests of particle theory: Thus, extra precision has important dividends. There are several avenues to achieving greater precision.
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If experiments at the LHC do not discover a Higgs particle within the expected range, the mechanism that produces particle masses must be more complicated than the hypothesis incorporating a single Higgs boson. As the examples in the previous paragraphs demonstrate, precise measure ments in particle physics do not always require the highest possible energies to probe for new physics effects (see Box 2-5)
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From page 49... ...
The most recent measurement of the muon magnetic moment, reported at Brookhaven in 2004, has a precision of about one part in ten billion, which also places this measure among the most precise in nature. Comparisons between the experimentally measured magnetic moments of the electron and muon and the precise predictions of the Standard Model place important restrictions on the allowed masses of the new particles predicted in some extensions of the Standard Model.
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If dark matter is a cloud of elemen tary particles, it may be detectable in sensitive particle detectors placed deep un derground for shielding from ordinary cosmic rays. Calculations show that a cloud of Terascale particles would have just about the right properties to agree with what is known about dark matter.
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The overwhelming scientific interest in dark matter and dark energy is driven by the fact that these seemingly exotic substances were discovered because of their very real effects on the structure and evolution of the universe. Another challenging idea about cosmology is the idea of the inflationary universe, which is closely linked to particle physics.
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From page 52... ...
. Second, by studying how the number of spots and energy concentration vary with the spot size, cosmologists can derive a precise measure of the composition of the universe, providing the best evidence that the universe contains 4 percent ordinary matter, 20 percent dark matter, and more than 75 percent dark energy.
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From page 53... ...
ROLES OF ACCELERATOR- AND NON-ACCELERATOR-BASED EXPERIMENTS This recent history of particle physics underscores the interplay between experiments involving accelerators and those that do not involve accelerators. For instance, nonaccelerator experiments have helped drive the scientific frontiers of particle physics and have brought the field into closer contact with nuclear physics, cosmology, and astrophysics.
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This discovery validated and solidified the Dirac theory of antiparticles. Even though the positron, found in cosmic rays, was already known to exist, physicists were not sure about every particle having an antiparticle.
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There is no question that accelerators have been essential in particle physics and there is a clear role for them in uncovering the secrets of the Terascale. Indeed, much of the drama surrounding the Terascale comes from the expectation that accelerators will finally expose and then directly investigate the cracks in the Standard Model.
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