Evidence of Axions via the Axion-Photon Coupling
Chiara Salemi
There are many unsolved problems in physics, but only a few which are as popularly known as dark matter. The mysterious-sounding dark matter is nothing but a theory which explains why there appears to be more gravity in the universe than the normal matter that we see can explain. Physicists and astronomers have been searching for evidence of dark matter for many several decades, and one possible candidate for a ‘dark particle’ that has been hypothesized is the axion. The axion not only helps to solve the dark matter mystery, it also gives an explanation for another: the lack of CP violation in strong interactions (interactions involving the strong force, one of the four fundamental forces) (Carosi et al. 1). Lack of CP violation, also termed CP symmetry, means that reactions involving antiparticles occur exactly the same way that the same reactions occur with their corresponding particles. There was no obvious reason why CP symmetry should happen in strong interactions until Peccei and Quinn incorporated their newly theorized axions into the reactions. The addition of axions forces the reactions not to violate CP symmetry (Eleftheriadis et al. 2).
It is difficult to search for axions because they do not interact with matter in any of the usual ways. One way that researchers have recently found to look for axions is through their coupling to photons. Axion-photon coupling is the process in which an axion converts into a photon and vice versa. There are actually two different particles that undergo this same coupling: classical “Peccei-Quinn” axions and axion-like particles (ALPs). ALPs are just axions with a slightly different mass relationship (Carosi et al. 1). The two particles are often studied together because they share exactly the same photon coupling. This coupling is characterized a coupling constant, gaγ, which describes the likelihood of conversion in units of couplings per unit energy. Many studies on the topic are focused on narrowing down the possible values of this constant, which could help prove or disprove the existence of axions. Other studies are focused on using axions to explain anomalous observations. A difficulty in finding evidence for axions through the axion-photon coupling is that the likelihood of conversion is very low (Eleftheriadis et al. 1). In order to have enough conversions to study, an extremely intense source of photons must be used. Most studies take data on distant stars, but other sources such as the Universe’s background radiation and our own sun have also been researched recently.
Extragalactic sources of axions are the most well-studied because, in addition to being a plentiful source of light radiation, there are several available techniques which allow indirect observations which do not require expensive equipment. The procedure is to compare actual observations with models that include (or do not include) axions. White dwarf stars are one source which have been studied extensively using this procedure. A comparison can be made between the observed cooling rate and the predicted one using axion-based models. The addition of axions into the cooling model makes a big difference with white dwarf stars because of their strong magnetic fields (Gill, Hey 2). The mechanism (called the Primakoff process) which couples axions and photons is much more likely under the presence of a magnetic field. The reason that researchers can tell that so many axions are being converted is because the axions leaving the star carry a lot of energy away with them (Melendez, Bertolami, Althaus 4). Axions, unlike photons, do not interact with matter. Therefore, axions can leave the star uninhibited and carry all of their energy away with them, unlike photons which leave some behind when they interact with the matter in the star. This process greatly increases the cooling rate of white dwarf stars, and a 2013 study by Melendez, Bertolami, and Althaus showed that axion losses can in fact be the primary method of cooling (4).
Massive stars are another highly-studied extragalactic source of axions. Axion-photon coupling is important in these stars not because of their magnetic fields, like in white dwarves but because they are so large that even with the low likelihood of conversion, there are enough photons being emitted that even the small percentage which are converted are significant. In fact, an upper limit on the coupling constant can be found by observing these massive stars because if the constant were too large, the stars would lose too much energy to the escaping axions (Friedland, Giannotti, Wise 1). If the constant were greater than about 0.8*10-10 GeV-1, massive stars would lose energy so quickly to axions that they would skip an entire phase in their evolution, the blue loop phase. Observations of blue loop stars and Cepheid stars (a type of star which can only be formed during the blue loop phase) make higher coupling constants an impossibility (Friedland, Giannotti, Wise 2). This qualitative evidence is a strong support for the limitation on the coupling constant.
One well-researched source of axions is not a star at all but rather the Universe’s background radiation. The Universe is more transparent than it was expected to be, as in we can see more light from farther away than would be expected (Meyer, Horns, Raue 1). What was expected is that the Universe would be highly opaque due to photon-photon coupling. This coupling occurs when two photons combine to form an electron and a positron (the electron’s antiparticle). In effect, this reaction prevents the photons from propagating towards our eyes and telescopes, making the Universe seem opaque. However, the opacity we see is significantly less than calculations predict, until axions are included in the model (Meyer, Horns, Raue 2). If a photon propagating through space converts into an axion before it has a chance to couple with another photon, it can travel unimpeded until it converts back into a photon. This means that the light from distant sources is more likely to reach us. Based on models of how much light actually reaches us (versus how much would otherwise be expected with axion-less models), a lower limit on the coupling constant can be found, gaγ≥2*10-11 GeV-1 according to the data of Meyer, Horns, and Raue (13).
Studies of axions from our own sun are limited to a few large-scale experiments because of the expensiveness of the equipment needed to detect axions (Carosi et al. 1). So far, no tricks such as those used on extragalactic axion sources have been discovered to look for solar axions indirectly. Despite the limitations on their study, much headway has been made in the search for solar axions. The CERN Axion Solar Telescope (CAST) is the third and most recent generation of helioscopes made to look for axions. It uses recycled superconducting magnets from the Large Hadron Collider also located at CERN to set up a strong enough magnetic field to induce the Primakoff process (Eleftheriadis 4). This time, instead of converting photons into axions as occurs in stars, the magnetic field induces the axions to convert back into photons which can be detected. Using this procedure, CAST measurements were made which allowed scientists to determine that the coupling constant must be less than 8.8*10-11 GeV-1 (Eleftheriadis 7). Unfortunately, even through this method of detection, the results are not conclusive in proving the existence of axions. This is because photons, not axions, are the particle being directly detected–the magnetic field could be doing something other than axion conversion to create an increase in detected photons that scientists have not theorized about yet. Although these results are fairly old, updates to the helioscope are underway and plans for a new, more powerful helioscope have been proposed which will be able to narrow the constant down even further.
Recently, attentions have been turned to new sources of axions. Neutron stars, for example, supply both an intense source of light radiation and a strong magnetic field. Models of neutron star radiation and cooling patterns are more complex because of the unique polarization of their escaping radiation. Light radiation escapes from neutron stars in two forms: O-mode and X-mode. These two categories describe the plane of the radiation’s polarization (which are perpendicular to each other). Axions can only couple to O-mode polarized photons (Perna et al. 3). In their study, Perna et al. found several factors that indicated a likelihood of axion conversion, including less O-mode radiation in comparison to X-mode (9). A possible explanation for this phenomenon is that some of the lost O-mode radiation was converted to axions (which we cannot detect). Another factor that indicated possible axion losses is that the complete radiation spectrum from the neutron stars is different than what would be expected based on other factors such as the stars’ chemical content and temperature (Perna et al. 17). Neutron stars need further study to substantiate the claims of the early research and develop the current set of knowledge on this new axion source.
The research on axions has progressed significantly in the past few years, with discoveries based on the axion-photon coupling at the forefront. This characteristic coupling allows scientists to make observations on an otherwise undetectable particle. New modeling techniques allow greater precision when studying extragalactic sources and new technology allows direct observations of the sun to make an impact on the body of knowledge. It is important as research moves forward not to get stuck in the same patterns of observation. The recent study by Melendez, Bertolami, and Althaus on neutrons stars opens up another source of axions to research. Another possible category of axion observation is Earth-based; nuclear reactors have a high light radiation flux, which could be studied for axion-photon interactions. The search for axions via the axion-photon coupling is a blossoming field of research meriting more intense study in future years, not only because of the new technologies, which make research possible, but also because of the mysteries proving (or disproving) the existence of axions could solve (or create).
Works Cited
Carosi, G., A. Friedland, M. Giannotti, M. J. Pivovaroff, J. Ruz, and J. K. Vogel. “Probing the Axion-photon Coupling: Phenomenological and Experimental Perspectives. A Snowmass White Paper.” Snowmass White Papers (2013): n. pag. ArXiv.org. Web. 15 Jan. 2014.
Eleftheriadis, Christos, and CAST Collaboration. “Results on Axion Physics from the CAST Experiment at CERN.” Frascati Physics Series (2007): n. pag. ArXiv.org. Web. 25 Jan. 2014.
Friedland, Alexander, Maurizio Giannotti, and Michael Wise. “Constraining the Axion-Photon Coupling with Massive Stars.” Physical Review Letters 110.6 (2013): n. pag. ArXiv.org. Web. 26 Jan. 2014.
Gill, Ramandeep, and Jeremy Heyl. “Constraining the Photon-axion Coupling Constant with Magnetic White Dwarfs.” Physical Review D 84.8 (2011): n. pag. ArXiv.org. Web. 16 Jan. 2014.
Melendez, Brenda, Marcello M. Bertolami, and Leandro Althaus. “Revisiting the Impact of Axions in the Cooling of White Dwarfs.” 18th European White Dwarf Workshop Astronomical Society of the Pacific Conference Series 469 (2013): n. pag. ArXiv.org. Web. 26 Jan. 2014.
Meyer, Manuel, Dieter Horns, and Martin Raue. “First Lower Limits on the Photon-axion-like Particle Coupling from Very High Energy Gamma-ray Observations.” Physical Review D 87.3 (2013): n. pag. ArXiv.org. Web. 18 Jan. 2014.
Perna, Rosalba, Wynn C. G. Ho, Licia Verde, Matthew Van Adelsberg, and Raul Jimenez. “Signatures Of Photon-Axion Conversion In The Thermal Spectra And Polarization Of Neutron Stars.” The Astrophysical Journal 748.2 (2012): 116. ArXiv.org. Web. 17 Jan. 2014.