Axion

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In physics , Axion describes a hypothetical elementary particle that would be a solution to the problem that theoretical considerations demanded a violation of the CP symmetry in quantum chromodynamics (QCD), but this was not observed. Such a CP violation would  predict an electric dipole moment of up to d n  ≈ 10 −16 e · cm for the neutron , instead  none has been measured even with d n  ≤ 10 −25 e · cm.

So far (2020) the Axion, which is also a candidate for dark matter , could not be found. In 2019 the observation of a quasi-particle analog of an axion in a Weyl semimetal was reported.

Theoretical background

In contrast to the weak interaction , the discrete symmetries C ( charge reversal , the exchange of all particles by their antiparticles ), P ( parity , space reflection) and T ( time reversal ) are unbroken in the strong interaction . One consequence is the vanishing electric dipole moment of the neutron.

In particular, the combination CP is thus also  an unbroken symmetry. The fundamental theory of strong interaction, quantum chromodynamics, however, as Gerardus' t Hooft found in 1976 , predicts a CP- infringing component in the form of non- perturbative configurations of vacuum fields ( instantons ). More precisely, Hooft introduced this CP-violating, non-disorder theoretic term in the QCD effect in order to solve another problem ( called the problem by Steven Weinberg ), but with it he introduced a new problem known as the strong CP problem ) got known.

The additional term to 't Hooft's Lagrangian had a vacuum angle as a parameter, and the problem was to explain why it was so small. Roberto Peccei and Helen Quinn suggested the existence of an additional global, spontaneously broken chiral symmetry (Peccei-Quinn symmetry), which makes the vacuum angle disappear. This new symmetry leads via the Goldstone theorem to a new Nambu-Goldstone boson. Frank Wilczek named this new light, weakly interacting particle in 1978 after the American detergent Axion (Steven Weinberg also introduced it independently).

On the one hand there is the model of the more strongly interacting KSVZ axion and on the other hand that of the less strongly interacting DFSZ axion.

Dark Matter Candidate

In addition to neutrinos and the also only postulated WIMPs and MACHOs , axions are traded as possible candidates for solving the problem of dark matter (Frank Wilczek, Mark Wise, John Preskill 1983). If axions represent a large part of dark matter, lattice QCD calculations for the mass of the axions resulted in mass values ​​of 50 to 1500 microelectron volts - and thus up to ten billion times lighter than the electron.

Detection experiments

Various experiments try to detect axions with different methods.

Laboratory experiments

The laboratory experiments are “light through the wall” experiments in which a laser beam passes through a magnetic field and is then blocked by a wall. On the other side of the wall there is a magnetic field of equal strength that is perpendicular to the beam and at the end of this field there is a detector calibrated to the laser quantum ( photons ) .

The trick is that through the Primakoff effect, with the help of a virtual photon through the magnet in front of the wall, an axion is to be created, which on the other side of the wall changes back into a light quantum due to the reverse effect. The incoming light interacts with the magnetic field and fluctuates into another shape that can propagate through the wall. Behind the wall there are again fluctuations of the new state back to the original character. Parts of the photons could "bypass" the wall so that they could be detected. Detection of the photons behind the wall would prove the short-term presence of light in the form of axions. Changes to the fields affect the amount of light detected. This would allow conclusions to be drawn about the details of the Axion conversion.

Helioscopes

Helioscopes look for axions that could arise inside the sun.

Crystalline detectors

The axion-photon coupling is coherent within an electric field if the Bragg equation is fulfilled. Well-known experiments are SOLAX, COSME and DAMA.

Primakoff telescopes

The Primakoff telescopes are used to search for axions using the Primakoff effect (see CAST experiment at the CERN Research Center). Due to the Primakoff effect, an axion would be exposed to an external magnetic field, e.g. B. at CAST in the field of an LHC prototype magnet with 9  Tesla strength , converted into a photon with energies in the keV range. This can then be detected in particle detectors such as a CCD .

Mössbauer telescopes

Here the axion would be detected by resonant excitation of an atomic nucleus , similar to the excitation by photons in the Mössbauer effect . A first generation of the experiment is under construction.

Haloscopes

Haloscopes look for any axions from the halo of the Milky Way.

ADMX

The US Large-Scale Axion Search (ADMX) is a collaboration . Involved:

The experiment is set up at the Lawrence Livermore National Laboratory. Experience from the two previous experiments, the University of Florida Experiment and the Rochester Fermilab Brookhaven Experiment (RBF), was taken into account in its construction.

The goals of the experiment are

  • to increase the quality of the experiment so far that KSVZ axions can be detected from our halo and
  • to search the mass range of 1.3 μ eV / c² <m a <13 μeV / c² completely.

The ADMX experiment uses a so-called Sikivie detector (after Pierre Sikivie , who proposed many detection experiments on the Axion). Here an axion is generated within a static magnetic field via the Primakoff effect. The achievable wavelength of the photon is determined by the resonance frequency , i.e. H. the size of the container is limited: the cylinder used is 1 m long and has a diameter of 0.5 m. The magnet volume made available by a superconducting solenoid (electromagnet) is B 0 2 · V <11 T 2 m 3 .

Solid-state analog of the axion

As with other hypothetical elementary particles, analogues in solids were sought. A research group led by Johannes Gooth (Max Planck Institute for Chemical Physics of Solids, Tübingen) reported in Nature in 2019 on an analogue of the Axion . As a topological phase in a Weyl semimetal , it was predicted in 2010 by Shou-Cheng Zhang and colleagues. In Weyl semimetals, the electrons form quasiparticles that behave like Weyl fermions . They are similar to topological isolators . The Weyl semimetal was the tantalum - selenium compound (TaSe 4 ) 2 , in which the Weyl fermions collected in charge density waves when cooled to −11 degrees Celsius . A mode of this wave ( sliding mode , Phason) formed the analog of the axion, as was demonstrated by the similar behavior under electric and magnetic fields. A large positive contribution to the magnetic conductivity was shown in the case of bulging electrical and magnetic fields, corresponding to the Axion contribution to the chiral anomaly .

literature

Original essays:

  • Gerardus' t Hooft : Symmetry Breaking through Bell-Jackiw Anomalies , Phys. Rev. Lett., Vol. 37, 1976, p. 8
  • Gerardus' t Hooft: Computation of the quantum effects due to a four-dimensional pseudoparticle , Phys. Rev. D, Volume 14, 1976, p. 3432, Erratum Volume 18, 1978, p. 2199
  • Roberto Peccei , Helen Quinn : CP Conservation in the Presence of Pseudoparticles , Physical Review Letters, Volume 38, 1977, pp. 1440-1443
  • Roberto Peccei, Helen Quinn: Constraints imposed by CP conservation in the presence of pseudoparticles , Physical Review. D, Volume 16, 1977, pp. 1791-1797.
  • Steven Weinberg : A New Light Boson? , Physical Review Letters, Vol 40, 1978, pp 223-226
  • Frank Wilczek : Problem of Strong P and T Invariance in the Presence of Instantons , Physical Review Letters, Volume 40, 1978, pp. 279-282.
  • John Preskill , Mark Wise, Frank Wilczek: Cosmology of the invisible axion , Physics Letters B, Volume 120, 1983, pp. 127-132

Books:

  • Markus Kuster, et al .: Axions - theory, cosmology, and experimental searches. Springer, Berlin 2008, ISBN 978-3-540-73517-5

Web links

Individual evidence

  1. ↑ The value depends on a parameter, the vacuum angle, which varies from zero to ; the problem can also be formulated to explain why the vacuum angle disappears
  2. ↑ Axions: Desperately sought particle emerged in solid bodies. Retrieved October 12, 2019 .
  3. ↑ a b J. Gooth et al .: Axionic charge-density wave in the semi Weyl metal (TaSe 4 ) 2 I . In: Nature . No. 575 , October 2019, p. 315-319 , doi : 10.1038 / s41586-019-1630-4 (English).
  4. ↑ The Lagrangian of QCD has a symmetry in the case of the almost massless u, d quarks (chiral limit case) (U (2) is the unitary matrix in two dimensions that transforms the two quarks into one another, A stands for axial, V for vector) , the axial part of which is spontaneously broken by quark-antiquark condensates in a vacuum, which leads to four Nambu-Goldstone bosons, of which only three can be identified with the almost massless pions on the usual QCD mass scale , the fourth light goldstone boson is not observed
  5. ↑ Peccei, The strong CP problem and axions, pdf , lecture slides , Invisibles 2015 Workshop, Madrid
  6. ↑ The vacuum angle receives a contribution from the weak interaction from the diagonalization of the Kobayashi-Maskawa matrix, but the problem remains.
  7. ↑ Chiral here means chiral transformation of the quark fields, i.e. the same symmetry that was described above as axial symmetry, with the introduction of a new phase angle
  8. ↑ KSVZ is an acronym from the names of the authors of two scientific articles: J. Kim, Phys. Lett. 43 (1979) 103; MAShifman , AIVainshtein , VIZakharov , Nucl. Phys. B166 (1980) 493
  9. ↑ DFSZ is an acronym from the names of the authors of two scientific articles: M. Dine , W. Fischler , M. Srednicki , Phys. Lett. 104B (1981) 199; AP Zhitnitskii : Sov. J. Nucl. Phys. 31 (1980) 260
  10. ↑ S. Borsanyi, Z. Fodor, J. Gunther, K.-H. Kampert, SD Katz, T. Kawanai, TG Kovacs, SW Mages, A. Pasztor, F. Pittler, J. Redondo, A. Ringwald & KK Szabo: Calculation of the axion mass based on high-temperature lattice quantum chromodynamics, Nature, Volume 539, 2016, pp. 69-71, abstract
  11. ↑ Supercomputer provides profile of dark matter, Desy, November 2, 2016
  12. ↑ ADMX website (English)
  13. ↑ Rundong Li, Jing Wang, Xiao-Liang Qi, Shou-Cheng Zhang: Dynamical axion field in topological magnetic insulators, Nature Physics, Volume 6, 2010, pp. 284-288, abstract