Light Axion-like Particles Research Articles

Axion-like particle dark matter in the linear regime of structure formation

If axion-like particles (ALPs) constitute a major part of dark matter (DM), due to their bosonic nature and a relatively small mass, they could behave differently from the pointlike dark matter particles on the formation of the cosmic structures. When studying the structure formation, it is often useful to consider DM as a special fluid with a given density and a given velocity. ALP fluid obeys the same continuity equation compared to the pointlike collisionless DM, but has a different first-order velocity equation. In the linear regime of structure formation, the resulted observational differences are negligible for the QCD axions but can be interesting for very light ALPs.

Diphotons at the Z-pole in models of the 750 GeV resonance decaying to axion-like particles

Models in which the 750 GeV resonance ($S$) decays to two light axion-like particles (ALPs $a$), which in turn decay to collimated photons mimicking the observed signal, are motivated by Hidden Valley scenarios and could also provide a mechanism by which a $S \to \gamma \gamma$ signal persists while $S \to Z \gamma,\; ZZ$ and $WW$ remain subdued in the near future. We point out that these Hidden Valley like models invoking $S \to aa \to 4 \gamma$ must also contend with $Z \to a (\to \gamma \gamma) \gamma$ constraints coming from CDF and ATLAS. Within an effective field theory framework, we work out the constraints on the couplings of $S$ to $a$ and gauge bosons coming from photonic $Z$ decays and ensuring that the ALPs decay inside the electromagnetic calorimeter, in two regimes - where $a$ decays primarily to photons, and where $a$ also has hadronic branchings. The analysis is done for both when $S$ has a large as well as a narrow width, and for different relative contributions to the signal coming from $S \to \gamma \gamma$ and $a \to \gamma \gamma$. Results for the particular case where $S$ and $a$ belong to the same complex field are also presented. A $\gamma\gamma$ resonance at the $Z$-pole coming from $Z \to a \gamma$ is expected in this class of models. Taking benchmark ALP masses below around 0.4 GeV and, assuming reasonable values for the fake jet rate and the identification efficiency of the photon-jet, we find the prospects for the discovery of diphotons at the $Z$-pole.

Evidence for an axion-like particle from PKS1222+216?

The surprising discovery by MAGIC of an intense, rapidly varying emission in the energy range 70 - 400 GeV from the flat spectrum radio quasar PKS 1222+216 represents a challenge for all interpretative scenarios. Indeed, in order to avoid absorption of \gamma rays in the dense ultraviolet radiation field of the broad line region (BLR), one is forced to invoke some unconventional astrophysical picture, like for instance the existence of a very compact (r\sim 10^{14} cm) emitting blob at a large distance (R \sim10^{18} cm) from the jet base. We offer the investigation of a scenario based on the standard blazar model for PKS 1222+216 where \gamma rays are produced close to the central engine, but we add the new assumption that inside the source photons can oscillate into axion-like particles (ALPs), which are a generic prediction of several extensions of the Standard Model of elementary particle interactions. As a result, a considerable fraction of very-high-energy photons can escape absorption from the BLR through the mechanism of photon-ALP oscillations much in the same way as they largely avoid absorption from extragalactic background light when propagating over cosmic distances in the presence of large-scale magnetic fields in the nG range. In addition we show that the above MAGIC observations and the simultaneous Fermi/LAT observations in the energy range 0.3 - 3 GeV can both be explained by a standard spectral energy distribution for experimentally allowed values of the model parameters. In particular, we need a very light ALP just like in the case of photon-ALP oscillations in cosmic space. Moreover, we find it quite tantalizing that the most favorable value of the photon-ALP coupling happens to be the same in both situations. Although our ALPs cannot contribute to the cold dark matter, they are a viable candidate for the quintessential dark energy. [abridged]

The type IIB string axiverse and its low-energy phenomenology

Abstract We study closed string axions in type IIB orientifold compactifications. We show that for natural values of the background fluxes the moduli stabilisation mechanism of the LARGE Volume Scenario (LVS) gives rise to an axiverse characterised by the presence of a QCD axion plus many light axion-like particles whose masses are logarithmically hierarchical. We study the phenomenological features of the LVS axiverse, deriving the masses of the axions and their couplings to matter and gauge fields. We also determine when closed string axions can solve the strong CP problem, and analyse the first explicit examples of semi-realistic models with stable moduli and a QCD axion candidate which is not eaten by an anomalous Abelian gauge boson. We discuss the impact of the choice of inflationary scenario on the LVS axiverse, and summarise the astrophysical, cosmological and experimental constraints upon it. Moreover, we show how models can be constructed with additional light axion-like particles that could explain some intriguing astrophysical anomalies, and could be searched for in the next generation of axion helioscopes and light-shining-through-a-wall experiments.

New Cosmological Constraints on Axions and Axion-Like Particles

We present constraints on the axion-photon coupling scale M for light axion-like particles by combining recent Supernova Type Ia data with the latest measurements of the Hubble expansion at redshifts between 0 and 2. Allowing for a coupling between axions and photons leads to a modification of the inferred luminosity distances for supernovae due to the conversion of photons to axions in the presence of intergalactic magnetic fields. We constrain such couplings by considering deviations from the luminosity-angular diameter distance relation dL = dA(1+z)2. We find that for intergalactic magnetic fields of order 1 nG, current supernova and Hubble expansion data rule out a region in the coupling scale M between 1010 and 1011 GeV.

Axion-like particle effects on the polarization of cosmic high-energy gamma sources

Various satellite-borne missions are being planned to measure the polarization of a large number of gamma-ray bursts (GRBs). We show that the polarization pattern resulting from the current models of GRB emission can be drastically modified by the existence of very light axion-like particles (ALPs), which are predicted by many extensions of the Standard Model of particle physics. Basically, the propagation of photons emitted by a GRB through cosmic magnetic fields with a domain-like structure induces photon-ALP mixing, which is expected to produce a strong modification of the initial photon polarization. Because of the random orientation of the magnetic field in each domain, this effect strongly depends on the orientation of the line of sight. As a consequence, photon-ALP conversion considerably broadens the initial polarization distribution. Searching for such a peculiar feature through future high-statistics polarimetric measurements therefore offers a new opportunity to discover very light ALPs.

Have Cherenkov telescopes detected a new light boson?

Recent observations by H.E.S.S. and MAGIC strongly suggest that the Universe is more transparent to very-high-energy gamma rays than previously thought. We show that this fact can be reconciled with standard blazar emission models provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect indeed explains the observed data and in particular the spectrum of blazar 3C279.

Searching for axion-like-particles in the sky

If dark energy couples to the fields of the standard model we can hope to detect or constrain it through non-gravitational effects. If the dark energy field couples to photons it behaves as an Axion-Like-Particle (ALP). ALPs mix with photons in the presence of magnetic fields and hence affect astronomical observations. We show that empirically established luminosity relations can be used as a new test for ALPs and that when applied to observations of active galactic nuclei this is highly suggestive of the existence of a very light ALP.

Active Galactic Nuclei Shed Light on Axionlike Particles

We demonstrate that the scatter in the luminosity relations of astrophysical objects can be used to search for axionlike particles. This analysis is applied to observations of active galactic nuclei, where we find evidence highly suggestive of the existence of a very light axionlike particle.

Photon propagation and the very high energy γ-ray spectra of blazars: how transparent is the Universe?

Abstract Recent findings by γ-ray Cherenkov telescopes suggest a higher transparency of the Universe to very high energy (VHE) photons than expected from current models of the extragalactic background light. It has been shown that such transparency can be naturally explained by the DARMA scenario, in which the photon mixes with a new, very light, axion-like particle predicted by many extensions of the Standard Model of elementary particles. We discuss the implications of DARMA for observations of blazar VHE γ-ray spectra, and show that it successfully accounts for the observed correlation between spectral slope and redshift when the same intrinsic emission spectrum is adopted for faraway sources and for nearby ones. DARMA also predicts the observed blazar spectral index to become asymptotically independent of redshift for faraway sources. Our prediction can be tested with the satellite-borne Fermi/LAT (Large Area Telescope) detector as well as with the ground-based Cherenkov telescopes HESS, MAGIC, CANGAROO III, VERITAS and the Extensive Air Shower arrays ARGO-YBJ and Milagro.

A new light boson from MAGIC observations?

Recent detection of blazar 3C279 by MAGIC has confirmed previous indications by H.E.S.S. that the Universe is more transparent to very-high-energy gamma rays than currently thought. This circumstance can be reconciled with observations of nearby blazars provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. The emerging "DARMA scenario" can be tested in the near future by the satellite-borne Fermi LAT detector as well as by the ground-based Imaging Atmospheric Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III, VERITAS and by the Extensive Air Shower arrays ARGO-YBJ and MILAGRO.

Cosmology with light axions from technicolor

Abstract We consider cosmological consequences of the spontaneous breaking of a global symmetry that is anomalous under technicolor interactions, leading to the emergence of a light axion-like particle. Avoiding overclosure of the universe by such axions yields the upper bound f a ≲ 10 10 GeV on the symmetry breaking scale, corresponding to keV-scale axions. However, diffuse X-ray background data typically require larger values of f a . The overclosure and X-ray bounds can be reconciled if the axion initial amplitude of oscillations A i ∼ f a / 10 . In this case, a viable axionic dark matter candidate with a mass in the 50–100 eV range emerges. The detection of this type of dark matter may pose a challenge.

Compatibility of Cast Search with Axionlike Interpretation of PVLAS Results

The PVLAS Collaboration has results that may be interpreted in terms of a light axionlike particle, while the CAST Collaboration has not found any signal of such particles. We propose a particle physics model with paraphotons and with a low energy scale in which this apparent inconsistency is circumvented.

Evading astrophysical constraints on axion-like particles

Stellar energy loss arguments lead to strong constraints on the couplingϕγγ of a light axion-like particle to two photons. Helioscopes, like CAST, are able to imposecompetitive bounds. The PVLAS experiment has recently observed a rotation of the polarizationof a laser propagating in a magnetic field that can be interpreted as the effect of a quite strongϕγγ coupling. We present scenarios where the astrophysical and CAST bounds can be evaded,and we show that the PVLAS result can be accommodated in one of the models, providedthat the new physics scale is at very low energies.

Brane-worlds and theta vacua

Reductions from odd to even dimensionalities ($5\to 4$ or $3\to 2$), for which the effective low-energy theory contains chiral fermions, present us with a mismatch between ultraviolet and infrared anomalies. This applies to both local (gauge) and global currents; here we consider the latter case. We show that the mismatch can be explained by taking into account a change in the spectral asymmetry of the massive modes--an odd-dimensional analog of the phenomenon described by the Atiyah-Patodi-Singer theorem in even dimensionalities. The result has phenomenological implications: we present a scenario in which a QCD-like $\theta$-angle relaxes to zero on a certain (possibly, cosmological) timescale, despite the absence of any light axion-like particle.