Science & Technology Almanac

By Lynn Lauerman | Go to book overview

DISCIPLINE BY DISCIPLINE:


Physics

PHYSICS: NEWS

B MESONS

B Mesons and Antimatter

Particle physics teams at the Stanford Linear Accelerator Center (SLAC) in California and the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, have been trying to figure out why matter, not antimatter, is the basis of the universe. According to decades of scientific research, it is because antimatter is not quite the opposite of matter, as the name suggests. In 2001, the SLAC and KEK teams showed this assertion to be true using short-lived subatomic particles called B mesons and anti-B mesons.

Particle physics theory states that antimatter operates under the same physical laws as matter; antimatter is merely opposite in charge and spin parity Since the 1960s, however, physicists have known that this is not necessarily the case: Antimatter can exhibit an asymmetry called a charge-parity (CP) violation. When antimatter displays this violation, antiparticles decay faster than expected. For example, in the early universe, this faster decay rate destroyed some antimatter before it could combine with matter and annihilate both particles. Thus, when all the antimatter was annihilated, there was matter left over to make up the known universe.

Physicists represent matter/antimatter asymmetry in B/anti-B meson experiments as a dimensionless number with possible values from −1 to 1, with 0 representing no CP violation; a positive number would indicate a CP violation in favor of matter, and a negative number, a violation in favor of antimatter. The standard model of particle physics predicts a value of 0.72. Special machines at SLAC and KEK created billions of B mesons with reactions that also create a few thousand anti-B mesons, and the matter/antimatter pairs were studied to calculate a CP value. SLACs value was reported to be 0.34±0.20, and KEK’s value was 0.58±0.33. Although these figures did not agree totally with the standard model, they did show conclusively that matter naturally dominates over antimatter.


BOSE-EINSTEIN CONDENSATE

Helium Bose-Einstein Condensate

Since 1995, scientists have been studying a state of matter called a Bose—Einstein (B-E) condensate, which is an ultracold atomic cloud with unique properties. After first creating a B-E condensate with rubidium, researchers also made them with sodium, lithium, and hydrogen. In March 2001, scientists at two separate French labs, the Institut d’Optique at Orsay and the École Normale Supérieure, each reported that they had created a B-E condensate with helium. The new condensate will help scientists delve further into the atomic physics world of particle structure.

An individual atom can behave as both a particle and a wave; this characteristic is a scientific phenomenon known as wave-particle duality. Each atom has a quantum state with certain characteristics and a specific waveform. In a B-E condensate, these properties are exploited. When the condensate cooled to very low temperatures, the atoms’ kinetic energies are lowered enough to allow the individual quantum states to overlap, thereby creating a superatom with a single quantum state.

Until now, Bose—Einstein condensates have contained atoms only in their lowest energy state, or ground state. In the helium B-E condensate, however, the helium atoms are in a metastable state (a state in which each atom carries an internal energy of 20 electron volts). Researchers devised a new way to detect the particles in the condensate by taking advantage of the helium atoms’ high energies. The researchers allowed atoms from the condensate to drop onto a platelike detector. There, the release of each atom’s stored energy knocked an electron loose from the detector and created a measurable signal. Thus, counting the individual atoms in the cloud may permit experiments in which physicists can closely compare the numbers of

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Science & Technology Almanac
Table of contents

Table of contents

  • Title Page i
  • Contents iii
  • Preface v
  • Chemistry 41
  • Computers & Internet 63
  • Earth Sciences 87
  • Environment 107
  • Health & Medicine 133
  • Life Sciences 175
  • Mathematics 203
  • Meteorology & the Weather 223
  • Physics 247
  • Technology 259
  • Legislation & Regulation 293
  • Media Watch 297
  • Prizes 311
  • Biographies 337
  • Appendix One 435
  • Appendix Two 439
  • Glossary 468
  • Bibliography 485
  • Index 489
  • About the Editors 514
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