FOR RELEASE: Monday, May 1st
Neil Russell (906 227-1058, nrussell@nmu.edu)
Northern Michigan
University, Marquette MI, USA
Robert Bluhm (207 872-3250, rtbluhm@colby.edu)
Colby College,
Waterville ME, USA
V. Alan Kostelecky (812 855-6923, kostelec@indiana.edu)
Indiana
University, Bloomington IN, USA
Neil Russell can be reached during the Long Beach conference at the Renaissance Hotel.
Popular Version of Paper Q21.004
Monday, May 1, 14:36 p.m.
APS April 2000 Meeting, Long Beach,
CA
In 1905 Albert Einstein discovered that space and time are not independent of each other as commonly thought, but instead are intertwined to form a four-dimensional spacetime. Although space and time measurements become relative in Einstein's theory, the laws of physics remain the same for everyone. This equivalence is linked to a symmetry, called Lorentz symmetry, that relates physical quantities in different spacetime coordinate systems. Lorentz symmetry has become a cornerstone of modern particle physics and its description of interactions of elementary particles, including electrons, neutrinos, and quarks.
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| Figure 1: The antihydrogen atom is the antimatter counterpart of the hydrogen atom. Its nucleus is the antiproton, the antiparticle of the hydrogen atom's proton nucleus. An antiproton has the same mass as a proton, but opposite charge. Orbiting around this nucleus is a positron, which can be regarded basically as a positively charged electron. When produced and trapped for study, antihydrogen atoms will need to be carefully isolated from ordinary matter to prevent them releasing energy through annihilation. For example, if the hydrogen atoms in a single drop of water annihilated an equal number of anithydrogen atoms, the energy released could keep a light bulb burning continuously for months. |
About fifty years after Einstein's discovery it was shown that Lorentz symmetry in the context of particle physics implies another important symmetry, this time between matter and antimatter. This result, known as the CPT theorem, predicts that matter and antimatter have certain identical properties. Both the CPT theorem and Lorentz symmetry have been verified experimentally to exceptional precision.
The CPT theorem gets its name from three transformations relating the properties of matter and antimatter. These are charge conjugation (C), parity inversion (P) and time reversal (T). In simple terms, C is a change in the sign of electrical charge, P is essentially a mirror reflection in space, and T is a reversal of the direction of time. The CPT theorem basically states that if all three transformations are applied to a physical system, the equations describing the transformed system are unchanged. This implies a close relationship between the properties of matter and antimatter. In particular, particles and antiparticles such as protons and antiprotons should have identical masses, and antiatoms made up of antiparticle constituents should have the same characteristic light spectra as their matter counterparts.
The presently accepted theory in particle physics, called the standard model, has exact CPT and Lorentz symmetry. Although no experiments contradict the standard model, most physicists believe that the standard model will someday be replaced by a more fundamental theory. A primary reason for this is that the standard model cannot adequately describe gravity, one of the most important forces in nature. Although gravity is well understood on the large scale of planets, stars, and galaxies, understanding the way gravity works at the small scale of elementary particles remains a challenging puzzle.
An effort centered at Indiana University to challenge and test the robustness of CPT and Lorentz symmetry has been underway for over ten years. The idea developed by the Indiana group is that violations of CPT and Lorentz symmetry could occur spontaneously. In general, spontaneous breaking occurs when a symmetry obeyed by the force laws governing a system is broken by the system itself. For example, in a permanent magnet the forces between the atoms in the magnet are independent of spatial direction (rotational symmetry), but the north and south poles of the magnet itself are aligned along a particular direction (spontaneously broken rotational symmetry). If CPT and Lorentz symmetry undergo a similar process, then observable violations of these symmetries may exist in nature even though the fundamental laws of physics have CPT and Lorentz symmetry.
Based on this idea, the Indiana group has developed a theory that extends the standard model of particle physics to include possible effects from spontaneous breaking of CPT and Lorentz symmetry. This standard-model extension shows that miniscule but meaningful differences could exist between the properties of matter and antimatter.
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| Figure 2: The black lines show the energy-level splitting in the ground state of a hydrogen atom in a magnetic field. If an antihydrogen atom is placed in a similar field, the current belief is that identical splittings occur. According to the Indiana theory, this splitting would differ for the two species because of the violation of Lorentz and CPT symmetry. As shown here, the transition between the c and d levels is larger for antihydrogen than for hydrogen. This difference would vary over the period of a day as the magnetic field direction rotated with the earth. The scale of the diagram is substantially exaggerated. In fact, it seems unlikely that any experiment will directly resolve the difference between the red and blue lines for many years, although experiments constraining the variation of the blue lines alone have already been performed. |
The paper we are presenting at this meeting uses the standard-model extension to investigate theoretically the spectra of hydrogen and antihydrogen. We show that the spectrum of the hydrogen atom could differ from that of its antimatter counterpart, the antihydrogen atom, as a result of the violation of CPT and Lorentz symmetry. Measurement of such a difference would provide direct experimental evidence of matter-antimatter asymmetry in nature.
Two experiments with antihydrogen at the European Laboratory for Particle Physics (CERN), named ATRAP and ATHENA, will be able to test for CPT and Lorentz violation by studying antihydrogen spectroscopy. These difficult experiments will take many years to complete because unlike ordinary hydrogen, which is available in copious quantities throughout the universe, antihydrogen is hard to obtain. To date only about 20 atoms of anithydrogen have been produced, first at CERN in 1996 (New York Times, Jan. 5, 1996) and soon after at Fermilab near Chicago. The planned experiments at CERN will rely on a recently commissioned machine, the Antiproton Decelerator, to produce numerous antiprotons suitable for creating antihydrogen.
Our paper also discusses experimental searches for CPT and Lorentz violation in hydrogen alone. In the context of the standard-model extension, the precise colors of lines in the spectrum of hydrogen (the transition frequencies) depend on the orientation of the atom. This means that as the Earth rotates there can be daily (sidereal) time variations in the hydrogen spectrum.
A recent experiment by the Walsworth group at the Harvard-Smithsonian Center has performed precision measurements with hydrogen masers that place tight constraints on a combination of effects from CPT and Lorentz violation predicted by the standard-model extension. The results of this experiment will be presented (Q21.007) in the same session as our paper, on Monday afternoon.
Different investigations conducted by our group find that various other experiments could be sensitive to CPT- and Lorentz-violating effects. For example, in an earlier work we investigated effects due to CPT and Lorentz violation on the behavior of the electron and its antimatter twin, the positron. Nobel laureate Hans Dehmelt and coworkers at the University of Washington in Seattle recently published results of searches for these effects using Penning traps as the confining mechanism for the particles. Their work sharply constrained certain parameters in the standard-model extension (Physical Review Letters Vol. 83, 13 September 1999, page 2116, and 6 December 1999, page 4694).
The standard-model extension also implies potential signals in experiments with spin-polarized solids, muons, neutral mesons, light, and various atomic systems. More information, including a list of publications and animations illustrating some of the effects, can be found at the Indiana website about CPT and Lorentz violation.
http://physics.indiana.edu/~kostelec/faq.html
http://physics.indiana.edu/~kostelec/mov.html
http://www.indiana.edu/~cpt98/