New experimental approaches in the search for axion-like particles

Abstract

Axions and other very light axion-like particles appear in many extensions of the Standard Model, and are leading candidates to compose part or all of the missing matter of the Universe. They also appear in models of inflation, dark radiation, or even dark energy, and could solve some long-standing astrophysical anomalies. The physics case of these particles has been considerably developed in recent years, and there are now useful guidelines and powerful motivations to attempt experimental detection. Admittedly, the lack of a positive signal of new physics at the high energy frontier, and in underground detectors searching for weakly interacting massive particles, is also contributing to the increase of interest in axion searches. The experimental landscape is rapidly evolving, with many novel detection concepts and new experimental proposals. An updated account of those initiatives is lacking in the literature. In this review we attempt to provide such an update. We will focus on the new experimental approaches and their complementarity, but will also review the most relevant recent results from the consolidated strategies and the prospects of new generation experiments under consideration in the field. We will also briefly review the latest developments of the theory, cosmology and astrophysics of axions and we will discuss the prospects to probe a large fraction of relevant parameter space in the coming decade.

Introduction

The 20th century witnessed a spectacular revolution in our understanding of the fundamental laws of nature, that culminated with the establishment of the Standard Model (SM) of particle physics, the theory that describes with accuracy (at least as far as our experimental and computational accuracy goes) the results of every experiment performed so far in particle physics. There are however many reasons to believe the SM is not an ultimate theory of nature. Some decades ago it could have been argued that the SM does not include the gravitational interactions –so successfully described at the classical level by Einstein’s theory of general relativity – and so it has to be extended or embedded in a more complete theory. Nowadays we can count on a few other striking observations. Perhaps the most pressing come from cosmology, which seems to be also extremely well described by a classical solution of Einstein’s gravity equations, a homogeneously expanding Universe with some primordial inhomogeneities seeded by tiny quantum fluctuations during an exponential expansion phase, so-called primordial inflation. And this excellent description requires a few ingredients that are nowhere to be found in the SM: Dark Matter (DM) — a substance that behaves under gravity as cold gas of non-baryonic weakly interacting particles, Dark Energy (DE), which gravitates as Einstein’s famous cosmological constant, and at least a new field (not necessarily a fundamental field) whose potential energy drives inflation for some time and then transforms somehow into the radiation that will dominate the energy density of the Universe during Big Bang Nucleosynthesis. Amongst these three, the evidence for Cold DM is the most precious for particle physics as it is directly attributable to the existence of new species of particles, i.e. it has been convincingly proven that the majority of DM is not in the form of neutrinos or any other SM particle.

But the SM itself also provides compelling reasons to seek a more fundamental theory of nature. Most of them follow the same pattern: the lack of symmetry of the SM will be alleviated as we consider physics at higher energy scales. New particles/fields are expected to appear and restore symmetries that are not altogether evident in the SM. Couplings can be all related at high energies and still lose this unified character at low energies because they run with the energy scale. Electroweak and strong interactions could be two aspects of the same Grand Unified Theory (GUT) at a very high energy scale of $10^{15}$ GeV where quarks and leptons would also be different ingredients of the same multicomponent fundamental field. Other ideas consider the unification of the fermion generations into the framework of family symmetries. Finally, theories beyond the framework of quantum field theory have to be invoked to include gravity at the same quantum footing than the rest of known interactions. The most conspicuous framework in which this appears to be possible, at least in principle, is the framework of string theories in 10 dimensions.

Another avenue of speculation about possible extensions of the SM is concerned with the hierarchy problem and the concept of naturalness. A very well motivated scenario predicts the existence of a “supersymmetry” (SUSY) in nature between fermions and bosons. In addition of solving the hierarchy problem, SUSY partners contribute to the running of SM gauge couplings, providing a strong hint for GUTs. SUSY is also widely present in string theories. In addition, SUSY theories usually predict a stable weakly interacting massive particle (WIMP), typically the lightest SUSY particle (LSP). In a rather generic way, the expected relic density of LSPs thermally produced after the Big Bang falls in same ballpark as the observed DM density. This has been called the WIMP miracle, and has constituted a major motivation to invest large efforts to search for SUSY and WIMPs as DM candidates for the last few decades. Unfortunately, experiments at the Large Hadron Collider have not yet found any convincing signature of SUSY or any other new physics at the TeV scale and the many underground experiments searching for WIMP-nuclear recoils have borne any unambiguous signal either. Besides, cosmic rays from DM–DM annihilation (at the core of the WIMP miracle) above the accountable astrophysical backgrounds, have not yet been found.

We have a very strong prejudice towards nature accommodating more particles/fields and more symmetries at high energy scales. If this new physics is well above the electroweak scale, there is little hope to directly reach the needed energy scale at future accelerators. However, there are mechanisms by which physics associated with high energy scales have important measurable consequences at very low energy. There are well known examples of this. Gravity is associated with physics at the Planck scale and has very appreciable effects at low energies (thanks to the graviton being massless and its effects coherently summed up over a large amount of particles). Another example is the neutrino, whose properties are better studied not by producing the highest energies possible, but the highest luminosities and the most controlled environments for their experimental detection. Neutrinos offer more analogies with the topic of this review, regarding e.g. their role in astrophysics and cosmology. Let us mention that the smallness of the graviton and neutrino masses compared with the electroweak scale does not pose another hierarchy problem because quantum corrections to their mass are protected by symmetries. This inspires us to think about low mass particles associated with symmetries present at high energies and their effects in cosmology, in astrophysics, and in experiments at the high intensity, precision frontier. A discipline with its own taste sometimes called the low energy frontier of particle physics [1].

The paradigm of this low energy frontier is the QCD axion, a hypothetical spin 0 particle predicted by the Peccei–Quinn mechanism [[2], [3]] to solve dynamically the so-called strong CP problem (the absence of CP violation in the strong interactions) by using QCD dynamics itself. The axion was identified by Weinberg [4] and Wilczek [5] as the pseudo Nambu–Goldstone (pNG) boson of a new spontaneously broken global symmetry that Peccei and Quinn had postulated (and that since then bears the name of PQ symmetry). The axion is strongly related to mesons and indeed would mix with the known ${\pi }^0,\eta ,{\eta }^{\prime }$ obtaining a mass and featuring couplings to hadrons and two photons. Both the mass and the strength of these couplings are inversely proportional to $f_A$, an energy scale related to the spontaneous breaking of the PQ symmetry, so that the smaller the mass the weaker the couplings. Indeed, the first Weinberg [4] and Wilczek [5] models had $f_A$ of the order of the electroweak scale and were soon ruled out [[6], [7]]. However, very soon it was realised that $f_A$ could correspond to a much higher energy scale [[8], [9]] which implies very low mass and weakly interacting axions. These axions were so weakly coupled that they were dubbed invisible axions, a term which we shall not need henceforth as any other types of axions are ruled out. The experimental constraints we discuss later on force us to consider values of $f_A\gg 10^7$ GeV, which imply very small masses $m_A\ll \mathrm{eV}$. Indeed, the axion is the paradigm of the so-called weakly interacting slim particles WISP s [1] and its discovery would imply the identification of a new energy scale in particle physics.

Axions as pNG bosons are very easily embedded in extensions of the SM at high energies by invoking new fields and symmetries. Other NG bosons with similar properties to the axion have been proposed, like familons [[10], [11], [12]] (related to family symmetries), majorons [[13], [14]] (related to lepton number) or even axi-majorons (where the lepton and PQ symmetries are the same), see [[15], [16]] and Refs. therein. The PQ mechanism can be easily embedded also in SUSY [17], GUTs [18] and most notably it is built in string theory in a model-independent way [[19], [20]]. Indeed string theories predict the existence of many axion-like particle candidates [[21], [22]] one of which would be the QCD axion, but the rest could still play a similar phenomenological role [23]. In this review we will call axion-like particle (ALP) to any such low mass pNG with weak interactions to SM particles and denote it with the letter $a$. The particular ALP solving the strong CP problem by the PQ mechanism is called QCD axion and shown as $A$, being granted the uppercase distinction by the Review of Particle Physics [24].

The phenomenology of axions and ALPs is determined by their low mass and very weak interactions. ALPs (and other WISPs) could affect stellar evolution [[25], [26]] and cosmology [27] in a similar way to thermal neutrinos. These effects are responsible for the constraint $f_A\gg 10^7$ GeV mentioned above. Fortunately, the analogy with our standard WISPs (gravitons and neutrinos) does not stop here as axions could be discovered by experiments at the low-energy high-intensity frontier. This is because axions could mediate new long range forces [28], allow rare decays, appear after thick walls in beam-dump experiments (leading to the fascinating light-shining-through-walls (LSW) experiment [29]), and be thermally produced in copious amounts in the Sun to be detected on Earth [30]. The analogy stops, however, when we realise that axions are excellent DM candidates. Being very weakly interacting, their main production mechanisms in the early Universe are non-thermal: the vacuum realignment mechanism [[31], [32], [33]] and the decay of topological defects (axion strings and domain walls) [[34], [35]]. Therefore, these axions are produced with extremely small velocity dispersion and are therefore very cold DM, which perfectly fits the needs of the $\Lambda$CDM model of the Universe that so well describes the large scale structure of the Universe.

For these reasons, axions have been searched for in dedicated laboratory experiments since their proposal. The techniques employed for their detection are by no means common in the particle physics community, focused to a large extent on accelerators and high energy collisions. After the experimental exclusion of the first electroweak-scale axion models, the high values of $f_A$ needed to evade the astrophysical and cosmological bounds threatened to make the axion impossible to find. In 1983, Pierre Sikivie came up with a seminal paper in which he proposed two of the most fruitful techniques to search for invisible axions [30]: the axion helioscope to detect the copious flux of axions emitted from the Sun and the axion haloscope to detect axions from the hypothetical DM galactic halo. One idea is behind all these experiments: to use coherent effects over macroscopic distances/long times to boost the axion production or detection. As will be seen throughout this paper, this concept is crucial for the detectability of the axion, but Sikivie’s proposal had another very important point, we can use natural sources of axions which are extremely efficient, the Sun and the Big Bang, and concentrate the searches in the detection part. Because of the extremely large fluxes of natural axions, helioscopes and haloscopes are typically much more sensitive to axions and ALPs than their purely laboratory competitors, although their luminosities are also subject to larger uncertainties, especially in the case of DM axions.

Since those early days, there has been a small but continuous experimental activity attempting the detection of axions. Relevant pioneering experimental results in the 90s include: the Brookhaven–Rochester–Fermilab collaboration implementing the first haloscope [[36], [37]] and helioscope [38] setups with moderate sensitivity as well as, together with the Trieste group, also the first LSW setup [39]; the axion haloscope setup in Florida U. [40] (later to become ADMX [41]); a competing haloscope in Japan (CARRACK) [42] focused on R&D in photon counting [43]; the first polarisation experiment [44], precursor of PVLAS [45]; the Tokyo helioscope [46] and, towards the end of the decade, the start of the CAST helioscope at CERN.

During the last two decades, the efforts and size of the community have been steadily growing but the few last years are witnessing a real blooming phase. Many new groups have entered the field, new exciting detection concepts have been proposed and several demonstrative small-scale setups have been commissioned. Moreover, well established techniques appear now as clearly consolidated and face the upgrade to large scale experiments, entering the radar of more formal particle-physics roadmaps. The reason for this is, on the one hand, the development of theoretical and phenomenological aspects of axions (like their potential cosmological or astrophysical roles), that has helped sharpening their physics case and yielding further motivation and guidance for detection, and, on the other, the fact that detection technologies have reached levels that allow entering unexplored territory beyond current constraints. The lack of positive detection of SUSY at LHC and WIMPs in underground detectors has further contributed to the increased interest in axions too.

The proof that the experimental landscape is rapidly changing is that relatively recent reviews on the matter have become already obsolete [[47], [48]]. In spite of the risk of it being soon outdated too, we attempt here to provide a complete review of the experimental landscape that is lacking at the moment in the literature. We will describe the different detection strategies and their complementarity, with a focus on the novel concepts recently proposed, and a review of the future plans and prospects for the consolidated research lines. We start by presenting the theoretical motivations of the axion and ALPs in Section 2. We follow with a short update on the cosmology and astrophysics of these particles in Section 3, explaining their potential role as DM candidate, and including an account of the status of their astrophysical hints. We then proceed with the experimental review. Section 4 provides a small bridge between the theory and the experiment in which we describe the relevant features of the natural sources as well as the most theoretical elements of the detection. The experimental part is organised, as it is customary, in three sections according to the source of axions considered: laboratory (Section 5), solar (Section 6) and dark matter (Section 7) axions. We finish with our discussion and conclusions in Sections 8 Discussion, 9 Conclusions respectively.