CPT '07: à la recherche de la violation de la symétrie de Lorentz

La réunion 2007 sur la symétrie de Lorentz et la CPT a mis en évidence l'intense activité de la recherche en physique visant à mettre à l'épreuve la symétrie de Lorentz ainsi que d'autres propriétés fondamentales de la nature. Les efforts entrepris pour trouver une trace expérimentale de la violation de Lorentz et de la violation de CPT sont motivés en partie par la recherche d'une théorie permettant de concilier mécanique quantique et gravitation. D'autres apports sont venus de développements théoriques connus sous le nom d'extension du modèle standard. Lors de la réunion, il a été question de divers travaux d'expérimentation, allant d'expériences effectuées auprès d'accélérateurs de particules jusqu'à l'étude d'effets gravitationnels au moyen de la télémétrie laser lunaire.

Lorentz symmetry and the closely related CPT symmetry, which combines charge conjugation (C), parity reversal (P) and time reversal (T), are well tested properties of nature. Nevertheless, efforts to find experimental evidence of Lorentz and CPT violation have increased in number, motivated in part by the quest for a theory to unite quantum mechanics and gravity. Further impetus has come from the introduction of a framework for Lorentz and CPT violation known as the Standard Model Extension (SME) which encompasses the panorama of physical theories. Since its development by Alan Kostelecký and co-workers at Indiana University in the 1990s, the SME has been used widely to guide experimental efforts and allow comparisons of results from different experiments.

This field is the topic of a series of meetings that have run triennially since 1998 at the Indiana University physics department, bringing together researchers to share results and ideas. In 2007, the fourth meeting on Lorentz and CPT Symmetry (CPT '07) was held on 8–11 August, with contributed and invited talks, and a poster session during the conference reception.

The meeting opened with a welcome from Bennett Bertenthal, dean of the university's College of Arts and Sciences. Ron Walsworth of Harvard University and the Harvard Smithsonian Center for Astrophysics gave the first scientific talk, in which he reflected on the progress in experimental studies of Lorentz violation since 1997, when the SME coefficients first appeared in their current form. He also discussed his group's current work to upgrade its noble-gas maser, with the aim of improving the sensitivity to a variety of SME coefficients.

Accelerator-based tests

The SME has opened a rich variety of possibilities for Lorentz violation in the context of neutrino oscillations. Recent work has shown that some, or perhaps even all, of the oscillation effects seen in existing data may be attributable to Lorentz violation. Talks included a presentation by Rex Tayloe of the MiniBooNe collaboration at Fermilab, who provided an overview of the recent data and considered their relation to earlier results from the Liquid Scintillator Neutrino Detector experiment at Los Alamos. He also discussed the three-parameter "tandem" model based on SME coefficients.

The physics of neutral-meson oscillations provides an abundance of theoretical possibilities for Lorentz and CPT violation that can be tested in current and planned experiments. Antonio Di Domenico of the KLOE collaboration showed the first results of a search for CPT-violating effects using K mesons at the DAΦNE collider in Frascati. New results also came from David Stoker of University of California, Irvine, who presented the first constraints on all four coefficients for CPT violation in the Bd system, based on results from the BaBar experiment at SLAC.

Possible signals of Lorentz violation in the muon system hinge on variations in the anomaly frequency, which could be detected by performing instantaneous frequency comparisons and sidereal-variation searches. The results described by Lee Roberts from Boston University and the Muon (g-2) Collaboration at Brookhaven represent the highest-sensitivity test of Lorentz and CPT violation for leptons, and improve previous results with muonium, electrons and positrons by about an order of magnitude.

Lorentz and CPT violation may be detectable using antihydrogen spectroscopy. Such tests would involve looking for sidereal changes in the spectra, or looking for direct differences between the spectra of antihydrogen and conventional hydrogen. Theoretical considerations have shown that the hyperfine transitions are of particular interest in these tests. Ryugo Hayano, spokesperson for the ASACUSA collaboration at CERN, provided details of his group's progress towards tests of this type (see "ASACUSA moves towards new antihydrogen experiments"). Niels Madsen of Swansea gave an overview of the status of the ALPHA experiment at CERN, which has the potential to test Lorentz and CPT symmetry in a variety of ways using trapped antihydrogen (CERN Courier July/August 2007 p13).

Gravitational and astrophysical effects

There have been extensive studies of signals of Lorentz violation in the gravitational sector during the past few years, and the results include the identification of 20 coefficients for Lorentz violation in the pure-gravity sector of the minimal SME. The meeting featured presentations of the first ever measurements of SME coefficients in the gravitational sector by two experimental groups. Holger Müller of Stanford University announced measurements of seven such coefficients for Lorentz violation, based on work with Mach–Zehnder atom interferometry. Other first results were unveiled by James Battat of Harvard University, placing limits on six gravitational-sector SME coefficients from the analysis of more than three decades of archival lunar-laser-ranging data from the McDonald Observatory in Texas and the Observatoire de la Côte d'Azur in France. The Apache Point Observatory in New Mexico should achieve further improvements in lunar-laser ranging, down to a sensitivity level of 1 mm, as Tom Murphy of the University of California at San Diego explained.

The fundamental nature of Lorentz symmetry means that there are subtle conceptual issues to be addressed. Roman Jackiw of MIT considered one example, and showed that symmetry breaking may be a mask for co-ordinate choice in a diffeomorphism invariant theory. Robert Bluhm of Colby College in Maine provided a comprehensive discussion of the Nambu–Goldstone and massive fluctuation modes about the vacuum in gravitational theories. Other theoretical topics included approaches to deriving the Dirac equation in theories that violate Lorentz symmetry, presented by Claus Lämmerzahl of Bremen University, and Chern–Simons electromagnetism, which Ralf Lehnert of MIT described.

Satellites offer unique opportunities to probe Lorentz symmetry in a low-gravity environment. Tim Sumner of Imperial College gave an overview of approaches to space-based experiments with high-sensitivity instruments, and looked at ESA's upcoming plans. James Overduin of Stanford University likewise reviewed the ongoing analysis of data from the Gravity Probe B satellite.

Cosmological and astrophysical sources also provide a number of intriguing possibilities for testing the fundamental laws and symmetries of nature. Matt Mewes of Marquette University presented recent work using the cosmic microwave background to place limits on Lorentz-violation coefficients in the renormalizable and non-renormalizable sectors of the SME. The Pioneer anomaly – apparent deviations in the paths of spacecraft in the outer solar system, such as the Pioneer 10 and 11 – provides a possibility for new physics, as Michael Nieto of Los Alamos described, giving perspectives on the underlying physics that may be responsible for the observations. Synchrotron radiation and inverse Compton scattering from high-energy astrophysical sources may also show sensitivity to a variety of SME coefficients, as Brett Altschul of the University of South Carolina explained.

Atomic-physics tests

There have been tests of the electromagnetic sector of the SME for several years using low-energy experiments that include optical and microwave cavity oscillators, torsion pendulums, atomic clocks, and interferometric techniques. Experimental innovations have led to steadily improving resolutions and the ability to access better the geometrical components of the SME coefficient space.

Achim Peters of the Humboldt University in Berlin announced improvements by a factor of 30 on certain photon-sector SME coefficients using a cryogenic precision optical resonator on a rotating turntable. Another test in the photon sector has been performed by Michael Tobar of the University of Western Australia, who has used a Mach–Zehnder interferometer to improve the sensitivity to one particular coefficient by six orders of magnitude. The team at Princeton University has recently developed innovations for a second-generation comagnetometer. Sylvia Smullin and group leader Mike Romalis described this work, and also presented the results from the experiment's first-generation predecessor.

The Eöt-Wash torsion pendulum group at the University of Washington in Seattle has made major contributions to the search for Lorentz violation, including several of the tightest constraints on SME coefficients in the electron sector, which they recently generated using a spin-polarized torsion pendulum with a macroscopic intrinsic spin. Group member Blayne Heckel described preliminary results achieving yet greater sensitivity to a number of electron coefficients using a further refined version of the apparatus.

In all, the 2007 meeting on CPT and Lorentz symmetry highlighted the intense efforts of the physics community in testing Lorentz symmetry and other fundamental properties of nature. Should the minuscule traces of Lorentz violations be found, it would be a paradigm-changing event, leading to profound alterations to our current theories describing the forces of nature.