The hunt for axions

Many theoretically well-motivated extensions of the Standard Model of particle physics predict the existence of the axion and further ultralight axion-like particles. They may constitute the mysterious dark matter in the universe and solve some puzzles in stellar and high-energy astrophysics. There are new, relatively small experiments around the globe, which started to hunt for these elusive particles and complement the accelerator based search for physics beyond the Standard Model.Comment: 11 pages, invited talk at XVI International Workshop on Neutrino Telescopes, 2-6 March 2015, Palazzo Franchetti, Istituto Veneto, Venice, Ital

Multi-W(Z) Production in High-Energy Collisions?

There exists the possibility that the cross-section for the nonperturbative production of many, ${\cal O}(\alpha_W^{-1} \simeq 30 )$, weak gauge bosons may be as large as ${\cal O}(100\ {\rm pb}\ -\ 10\ \mu$b$)$ above a parton-parton center-of-mass threshold in the range $2.4\ -\ 30$ TeV. We review the theoretical considerations which lead to this suggestion and outline its phenomenological implications, both for present cosmic ray as well as for future collider experiments. (Lecture presented at the 4th Hellenic School on Elementary Particle Physics, September 2-20, 1992, Corfu, Greece) (Figures not included, available by ordinary mail)Comment: 15 pages, CERN-TH.6862/9

Axion dark matter in the post-inflationary Peccei-Quinn symmetry breaking scenario

We consider extensions of the Standard Model in which a spontaneously broken global chiral Peccei-Quinn (PQ) symmetry arises as an accidental symmetry of an exact $Z_N$ symmetry. For $N = 9$ or $10$, this symmetry can protect the accion - the Nambu-Goldstone boson arising from the spontaneous breaking of the accidental PQ symmetry - against semi-classical gravity effects, thus suppressing gravitational corrections to the effective potential, while it can at the same time provide for the small explicit symmetry breaking term needed to make models with domain wall number $N_{\rm DW}>1$, such as the popular Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model ($N_{\rm DW}=6$), cosmologically viable even in the case where spontaneous PQ symmetry breaking occurred after inflation. We find that $N=10$ DFSZ accions with mass $m_A \approx 3.5$-$4.2\,\mathrm{meV}$ can account for cold dark matter and simultaneously explain the hints for anomalous cooling of white dwarfs. The proposed helioscope International Axion Observatory - being sensitive to solar DFSZ accions with mass above a few meV - will decisively test this scenario.Comment: 12 pages, 4 figures; revised version of the manuscript, accepted for publication in PR

Axion mass in the case of post-inflationary Peccei-Quinn symmetry breaking

The axion not only solves the strong CP puzzle, but it also may be the main constituent of cold dark matter. We review the axion dark matter predictions for the case that the Peccei-Quinn symmetry is restored after inflation.Comment: 4 pages, contribution to the Proceedings of the 53rd Rencontres de Morion

Photon Regeneration from Pseudoscalars at X-ray Laser Facilities

Recently, the PVLAS collaboration has reported an anomalously large rotation of the polarization of light in the presence of a magnetic field. As a possible explanation they consider the existence of a light pseudoscalar particle coupled to two photons. In this note, we propose a method of independently testing this result by using a high-energy photon regeneration experiment (the X-ray analogue of "invisible light shining through walls") using the synchrotron X-rays from a free-electron laser (FEL). With such an experiment the region of parameter space implied by PVLAS could be probed in a matter of minutes.Comment: 3 pages, 2 figure

Axion dark matter (theory & experiment)

We review the motivation for the axion as a solution of the strong CP puzzle and as a candidate for cold dark matter. Then we discuss benchmark axion models and present their predictions concerning axion couplings to the Standard Model and axion dark matter abundance. Finally, we give an overview on the discovery potential of current and planned axion experiments, reaching from axion dark matter direct detection, over searches for solar axions, to direct production and detection of axions in the laboratory.Comment: 11 pages, 5 figures, contribution to the proceedings of the XVIII International Conference on Topics in Astroparticle and Underground Physics (TAUP2023

Limits on Electroweak Instanton-Induced Processes with Multiple Boson Production

Recently, the CMS collaboration has reported their search for electroweak instanton-like processes with anomalous $B+L$ violation assuming multi-fermion but zero-boson final states. On the other hand, many theoretical studies suggest that anomalous $B+L$ processes may have an observably large production rate only if their final state contains a large number of electroweak gauge bosons. In this paper, we compare collider signatures of zero- and multi-boson events of anomalous $B+L$ violation at the LHC and derive an upper limit on the cross-section for the multi-boson process by recasting the CMS analysis.Comment: 14 pages, 6 figures; JHEP version with typographic errors fixe