Lorentz invariance goes under the spotlight

28 September 2010

Experimental advances and new results at the CPT ’10 meeting.


Experimental tests of relativity and theoretical developments in relativity violation have flourished over the past few years. This interest has been strengthened by recent results in particle physics that appear to deviate from the predictions of the Standard Model. Two examples are the evidence for anomalous CP violation in the Bs system and the indications that antineutrinos might not have the same properties as neutrinos. These and many other frontier topics were discussed at CPT ’10, the fifth Meeting on CPT and Lorentz Symmetry, which was held in Bloomington, Indiana, on 28 June – 2 July. Speakers from four continents presented dozens of new limits on coefficients for Lorentz violation in the Standard-Model Extension (SME). In the opening scientific talk, James Bjorken of SLAC not only delivered an analysis of vacuum structures associated with emergent QED and torsion but also took the opportunity to put the meeting in perspective by looking back at the development of the SME over the past 15 years, as well as referring to the opening presentation at the CPT ’01 meeting by Nobel Laureate Yoichiro Nambu.

From antimatter to Antarctica

The CERN antimatter collaborations ALPHA, ATRAP, ASACUSA and AEgIS all provided updates on recent progress. Makoto Fujiwara of TRIUMF described how the ALPHA group developed a technique for evaporative cooling of trapped antiprotons down to temperatures of 9 K. The group is designing the apparatus to enable future hyperfine spectroscopy in antihydrogen. The ATRAP collaboration has made a number of advances using trapping techniques. Spokesperson Gerald Gabrielse of Harvard discussed results gained from ATRAP and other projects with single, trapped particles, including an improved measurement of the electron’s magnetic moment and a method for cooling single, trapped protons. Masaki Hori of the Max Planck Institute of Quantum Optics discussed techniques developed for performing spectroscopy on a beam of antiprotonic helium by the ASACUSA collaboration. The group has developed a titanium sapphire laser that should reduce spectral-line widths in future experiments.

Antihydrogen will provide new opportunities to study the interaction of neutral antimatter with the gravitational field. Marco Giammarchi of INFN/Milan described how the AEgIS collaboration aims to measure the local gravitational acceleration of antihydrogen to about 1% by detecting the fall of an antihydrogen beam travelling at some 500 m/s over a distance of about 1 m. Alan Kostelecký of Indiana and Jay Tasson of Whitman College have recently completed a study of Lorentz violation involving gravitational couplings to matter and antimatter. They use toy models to demonstrate that Lorentz-violating effects could appear in antihydrogen spectroscopy without observable effects in hydrogen and could cause gravimetric properties of antihydrogen and hydrogen to differ. Gravitational experiments can also place limits on the a-type coefficients in the SME. These are unmeasurable with a single particle species in the Minkowski space–time context and in principle could be large without having been detected to date. Experiments with the potential for interesting results include ones involving free-fall gravimeters, as well as weak equivalence principle tests with free fall and with satellites. In this context, Paul Worden of Stanford described the latest developments from the Gravity Probe B and STEP satellite programmes.

Atom interferometers have the potential to explore untested regions in the matter-gravity sector of the SME coefficient space. The caesium interferometer built by Holger Müller’s group at the University of California, Berkeley, is currently the highest-resolution atomic gravimeter. It has generated improved limits on half a dozen pure-gravity SME coefficients during 2009, which Müller described in an overview of the current results and interests within his group.

Atom-based co-magnetometers built by groups at Princeton University, the Harvard-Smithsonian Center for Astrophysics and the University of Mainz have contributed a number of sharp bounds on SME coefficients in the fermion sector. The three groups presented the status of their programmes at CPT ’10. Mike Romalis’ group at Princeton has commissioned a new apparatus, CPT-II, which is mounted on a turntable and is more compact than the one presented at the last meeting in the series, CPT ’07. At this year’s meeting, Romalis presented results from the device, a K-He co-magnetometer, which was run for 143 days between July 2009 and April 2010, allowing sidereal signals to be separated from diurnal ones. These new data represent the highest energy-resolution to date of any spin-anisotropy experiment. A future improvement of two orders of magnitude is feasible by using neon in the magnetometer, and systematic effects due to the Earth’s rotation could be evaded by running the experiment at the Amundsen-Scott South Pole Station in Antarctica.

Mesons, neutrinos and gamma rays

Rick Van Kooten of Indiana gave an overview of the recent evidence from the DØ collaboration at Fermilab for anomalous B-system CP violation differing at the level of 3.2 σ from the prediction of the Standard Model (CERN Courier July/August 2010 p6). He and Kostelecký have recently demonstrated that this result also yields the first sensitivity to CPT violation in the Bs system. The analysis interprets the anomaly in terms of CPT violation, placing a first limit on a CPT-breaking SME-coefficient combination at the level of 10–12, a result that could be improved in the LHCb experiment at the LHC at CERN. Top-quark experiments at Fermilab have reached sufficient statistical power to produce first-time bounds on SME coefficients for the third generation of matter, while improved sensitivities may be possible with LHC statistics, as Fermilab’s Gaston Gutierrez described in his discussion of prospects for future results.

In Europe, new accelerator-based results have come from the GRAAL beamline at the European Synchrotron Radiation Facility (ESRF). Dominique Rebreyend of the Laboratory for Subatomic Physics and Cosmology, Grenoble, presented recent results published in Physical Review Letters, which improve the limits on parity-violating SME coefficients in the photon sector by an order of magnitude. Ralf Lehnert of the National University in Mexico City presented the theoretical underpinnings of this work.


Conventional theory attributes neutrino oscillations to mass and predicts that oscillations are controlled by the baseline-to-beam-energy ratio, L/E. A variety of other dependences arising from Lorentz and CPT violation occur in the SME framework, and these have the potential to model some of the anomalous behaviour seen in recent oscillation experiments. The MINOS collaboration at Fermilab has recently published the results of their search for sidereal variations in neutrino oscillation probabilities using the MINOS near detector. Brian Rebel of Fermilab presented this and the latest related work, in which the collaboration also searched for effects at the far detector located about 700 km away in northern Minnesota. Teppei Katori of Massachusetts Institute of Technology gave an account of preliminary results from an analysis using the SME coefficients for Lorentz violation to reconcile the data from the LSND experiment at Los Alamos and MiniBooNE at Fermilab. Jorge Diaz of Indiana provided a complementary theoretical overview of SME neutrino physics.

Gamma-ray bursts (GRBs) are particularly suited to providing limits on some of the nonminimal SME coefficients in the photon sector, owing to their high energies, long baselines and high variability. Vlasios Vasileiou of the NASA Goddard Space Flight Center presented the first bounds from GRB 090510 on certain SME coefficients of mass dimensions 6 and 8. These results are from data taken with the Large-Area Telescope and the Gamma-Burst Monitor on the Fermi Gamma-Ray Space Telescope. High sensitivity to Lorentz violation in the photon sector has also been achieved in laboratory experiments with resonators. The cavity-oscillator groups of Achim Peters at Humboldt University and Mike Tobar of Western Australia have plans to start a new international collaboration based in Germany. Recent years have seen the development of a full theory of higher-order SME operators for Lorentz violation in the photon sector. This work, by Kostelecký and Matt Mewes of Swarthmore College, systematically enumerates and classifies Lorentz-violating operators of arbitrary dimension in electrodynamics. More formal developments were described by Luis Urrutia of the National Autonomous University of Mexico, who discussed spontaneous Lorentz breaking in models of nonlinear electrodynamics that maintain gauge invariance.

Experimental advances in the neutron sector, reports on the steady progress of antihydrogen technology, intriguing developments in the meson and neutrino sectors, experimental results in the nonminimal photon sector and new work in the theory of Lorentz-breaking matter-gravity couplings are some of the highlights from this meeting. While there is still no compelling sign of Lorentz violation, hints of effects have appeared in several sectors. The lively exchange of ideas and information at CPT ’10 shows that the resolve of physicists in this field to dig more deeply into fundamental symmetries is stronger than ever.

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