Babette Döbrich

Axion-like-particle search with high-intensity lasers

We study ALP -photon-conversion within strong inhomogeneous electromagnetic fields as provided by contemporary high-intensity laser systems. We observe that probe photons traversing the focal spot of a superposition of Gaussian beams of a single high-intensity laser at fundamental and frequency-doubled mode can experience a frequency shift due to their intermittent propagation as axion-like-particles. This process is strongly peaked for resonant masses on the order of the involved laser frequencies. Purely laser-based experiments in optical setups are sensitive to ALPs in the eV mass range and can thus complement ALP searches at dipole magnets.

Interferometry of light propagation in pulsed fields

We investigate the use of ground-based gravitational-wave interferometers for studies of the strong-field domain of QED. Interferometric measurements of phase velocity shifts induced by quantum fluctuations in magnetic fields can become a sensitive probe for nonlinear self-interactions among macroscopic electromagnetic fields. We identify pulsed magnets as a suitable strong-field source, since their pulse frequency can be matched perfectly with the domain of highest sensitivity of gravitational-wave interferometers. If these interferometers reach their future sensitivity goals, not only strong-field QED phenomena can be discovered but also further parameter space of hypothetical hidden-sector particles will be accessible.

Magnetically amplified tunneling of the third kind as a probe of minicharged particles.

We show that magnetic fields significantly enhance a new tunneling mechanism in quantum field theories with photons coupling to fermionic minicharged particles (MCPs). We propose a dedicated laboratory experiment of the light-shining-through-walls type that can explore a parameter regime comparable to and even beyond the best model-independent cosmological bounds. With present-day technology, such an experiment is particularly sensitive to MCPs with masses in and below the meV regime as suggested by new-physics extensions of the standard model.

Magnetically amplified light-shining-through-walls via virtual minicharged particles

We show that magnetic fields have the potential to significantly enhance a recently proposed light-shining-through-walls scenario in quantum-field theories with photons coupling to minicharged particles. Suggesting a dedicated laboratory experiment, we demonstrate that this particular tunneling scenario could provide access to a parameter regime competitive with the currently best direct laboratory limits on minicharged fermions below the

Looking for dark matter on the light side

\mathrm{meV}

High-Intensity Probes of Axion-Like Particles

regime. With present day technology, such an experiment has the potential to even overcome the best model-independent cosmological bounds on minicharged fermions with masses below

Can we see quantum gravity? Photons in the asymptotic-safety scenario

\mathcal{O} (10^{-4}) \mathrm{eV}

Scalar Casimir-Polder forces for uniaxial corrugations

.

Optical probes of the quantum vacuum: The photon polarization tensor in external fields

Among the prominent low-mass dark matter candidates is the QCD axion but also other light and weakly interacting particles beyond the Standard Model. We review briefly the case for such dark matter and give an overview on most recent experimental efforts within laboratory searches, where we focus on experiments exploiting a potential electromagnetic coupling of such particles. 1. Three ultra-light dark matter candidates It would be a huge break-through to find out what dark matter (DM) is made of. Whilst its constituents could be rather heavy and, e.g., leave an ‘imprint’ at the LHC, show up in astrophysics signatures or reveal themselves through recoil energy at direct detection setups [1], there are well-motivated candidates also on the ‘light side’. Our main concern in this article are henceforth DM candidates below the eV mass-scale. Of such ultra-light particles, the QCD axion is the most prominent dark matter candidate, see, e.g., [2] for a review. On the theoretical side, the axion is a pseudo-scalar pseudo-Goldstone boson that is a consequence of the Peccei-Quinn solution [3] to the strong CP problem. The strong CP problem amounts to the question why CP violation in QCD is unmeasurably small (or even absent). The effective parameter for CP violation receives contributions from the θangle of QCD, being essentially unconstrained a priori, and the quark mass matrix. As these parameters are unrelated from the outset, it arises the question for a natural explanation on why the CP-violating parameter is so close to zero. In essence, the axion solution to the strong CP-problem makes the parameter a dynamical variable which naturally relaxes to zero. From an experimental viewpoint, it is most interesting that axions in certain parameter regimes constitute a perfect candidate to make up the cold dark matter (CDM) in our universe. Although the axion is very light, it can be non-thermally produced in the early universe [4, 5]. A particularly attractive feature of axion dark matter is that its viable parameter range is comparatively small and it is thus a realistic aim to confirm or exclude axions as main dark matter component with current and near-future technology. For axions as cold DM, two natural cosmological windows exist, see, e.g., [6]: In the post-inflation scenario, the spontaneous breaking of the Peccei-Quinn symmetry at a scale f , which gives rise to the axion as its pseudo Goldstone boson, takes place only after inflation. In the other scenario, the Peccei-Quinn phase transition happens before inflation. The former scenario is typically related to axions with higher masses than the latter because the decay of axionic topological defects (absent in the latter) also produces DM axions and generically the DM abundance grows with the decay constant (i.e. decreases with the axion mass). ar X iv :1 50 1. 03 27 4v 2 [ he pph ] 2 A pr 2 01 5 The situation for the axion is sketched in Fig. 1 where performed experiments and the foreseen reach of some experiments that are planned for current and near future are shown. In addition, some astrophysical bounds are shown, for more insight on this matter see, e.g. [7, 8]. In Fig. 1, for the black line labeled ‘axion’ we have implemented g = α 2πf as relation of the axion-to-photon coupling and the symmetry breaking scale. For specific axion models, a O(1) factor enters in the relation between g and f , cf. [8]. Axions are a good cold dark matter candidate roughly between masses of 10−6eV and 10−3eV [9, 10] (indicated by thickening the axion line in Fig. 1. Below 10−6eV, axions tend to produce too much DM and they are disfavored, although some models can justify the selection of suitable initial conditions. Note that at very low masses, there might be also effects on black hole dynamics [11]. In laboratory searches, to probe the very tiny couplings that correspond to high values of the axion symmetry breaking scale, resonant search strategies are mostly employed and will be briefly sketched in Sect. 2.1 The axion is very light and very weakly interacting. Thus it exhibits the properties of a more general class of particles dubbed ‘Weakly Interacting Slim Particles’ (WISPs) [12, 13, 14]. Among the WISPs that could be cold dark matter [15] are also axion-like particles (ALPs). Such general ultralight pseudo-scalars could be pseudo-Nambu Goldstone bosons (pNGB) associated with a symmetry breaking scale different from the Peccei-Quinn scale [16, 17]. Effectively they can have a coupling to photons similarly to the QCD axion, but a ‘relaxed’ mass-coupling relation (i.e., they are not confined to the parameter region around the axion line in Fig. 1). Their existence is motivated in Standard Model extensions [12], and they have been also evoked to explain some astrophysical puzzles such as the observed transparency of the universe to high-energetic photons, see, e.g., [18]. Similar to axions, ALP CDM [15] can be realized through a misalignment mechanism: The axion mass m is strongly temperature dependent (same might apply to ALPs as pNGBs). For axions above the symmetry breaking scale (in the early universe), the axion is essentially massless. Thus at very high energies, the initial angle is not fixed (for the axion it needs not to be at its CP conserving value), i.e., it can be mis-aligned from its minimum. The equation of motion for the field in the expanding universe is that of a damped harmonic oscillator (where 3H quantifies the damping term and H is the Hubble expansion parameter) and initially the field is frozen when 3H m. At later times t1, when the particle mass m1 = 3H, the field starts to oscillate and behaves as cold dark matter fluid. The situation for ALP DM is reviewed in part through the dotted black lines in Fig. 1: The upper line, labeled m1 > 3H(Teq) is an upper bound for any such ALP DM model: The mass m1 at the which the oscillations start should be attained latest at matter-radiation equality. The lower black dotted line labeled m1 = m0 denotes ‘Standard ALP DM’ and is the simplest ALP DM model in which m1 is the same as the ALP mass today (m0). Models in which m1 m0 can in principle create a sufficient DM abundance at slightly higher coupling values [15], but most model-building efforts favor ALP DM within roughly an order of magnitude above this line. Beyond ALPs, hidden photons (HPs, reviewed, e.g., in [19]), which are hidden sector U(1)s coupled kinetically to the photons of the visible sector, are WISPy cold dark matter candidates [20, 15]. Such particles would with a hidden Higgs or Stückelberg generated mass term be manifest in photon hidden-photon oscillations similar to what is observed with neutrinos. Fig. 2 shows the viable parameter space for HP DM according to [15], in which the orange region labeled ‘Xenon’ denotes limits inferred from the XENON10 experiment [21] (see also [22] for novel bounds on the longitudinal HP component). In summary: For both ALPs and hidden photons the parameter regime in which they can constitute DM is much larger than for QCD axions. Thus, in laboratory searches, also nonresonant techniques have become attractive as they allow faster scanning (albeit at reduced overall sensitivity). This is discussed in Sect 2.2. In the following we give a brief overview on laboratory searches for axion and WISP DM. We focus on searches exploiting coupling toAmong the prominent low-mass dark matter candidates is the QCD axion but also other light and weakly interacting particles beyond the Standard Model. We review briefly the case for such dark matter and give an overview on most recent experimental efforts within laboratory searches, where we focus on experiments exploiting a potential electromagnetic coupling of such particles.

Axion searches with microwave filters: the RADES project

With continuously increasing intensities, modern laser systems can become a valuable tool for the search for axions and axion-like particles. As conventional setups of axion searches cannot easily accommodate the usage of a high-intensity laser system, we propose a novel, purely laser-based setup in which the occurrence of a frequency shift is an observable for the axion-photon interaction.