The photo-philic QCD axion
Received: December
Published for SISSA by Springer
Marco Farina 0 1 3 6 7
Duccio Pappadopulo 0 1 3 4 7
Fabrizio Rompineve 0 1 3 4 5 7
Andrea Tesi 0 1 2 3 7
Open Access 0 1 3 7
c The Authors. 0 1 3 7
0 Philosophenweg 19 , 69120 Heidelberg , Germany
1 New York , NY 10003 , U.S.A
2 Enrico Fermi Institute, Department of Physics, University of Chicago
3 136 Frelinghuisen Road , Piscataway, NJ 08854 , U.S.A
4 Center for Cosmology and Particle Physics, Department of Physics, New York University
5 Institute for Theoretical Physics, University of Heidelberg
6 New High Energy Theory Center, Department of Physics, Rutgers University
7 5620 S. Ellis Ave , Chicago, IL 60637 , U.S.A
We propose a framework in which the QCD axion has an exponentially large coupling to photons, relying on the \clockwork" mechanism. We discuss the impact of present and future axion experiments on the parameter space of the model. In addition to the axion, the model predicts a large number of pseudoscalars which can be light and observable at the LHC. In the most favorable scenario, axion Dark Matter will give a signal in multiple axion detection experiments and the pseudo-scalars will be discovered at the LHC, allowing us to determine most of the parameters of the model.
axion; Beyond Standard Model; CP violation
1 Introduction 2 3 4
A lightning review of axion physics
The standard QCD axion
Variations over KSVZ
The clockwork QCD axion
The clockwork mechanism
A photo-philic QCD axion
Clockwork axion window: constraints and signals
Phenomenology of clockwork pseudoscalars
Discussion and outlook
The strong CP problem arises from the experimental observation that CP symmetry is
respected by strong interactions with very high precision. In particular, bounds on the
neutron electric dipole moment (nEDM) at the level of dn < 3
the QCD theta angle to be tiny
QCD . 10 10. A very elegant solution to this puzzle is
embodied by the QCD axion [2{4]. The axion is a pseudo-Nambu-Goldstone boson with
a non vanishing potential generated by strong interactions, which dynamically relaxes the
-angle to 0. Furthermore, the energy density stored in the oscillations of the eld around
the minimum of its potential constitute, in an expanding universe, a viable cold Dark
Matter (DM) candidate. For all these reasons, the QCD axion represents a minimal and
10 26 e cm [1], require
compelling extension of the Standard Model (SM).
Many experiments have been built to detect axion DM, with many others that will be
operating in the near future. Interestingly, most of them rely on the existence of a coupling
between the axion eld a and the photons, described by the interaction lagrangian
The coupling ga
is generally non vanishing for the QCD axion, and typical models display
a strong correlation between the axion mass and the coupling to photons
Currently, only a very small fraction of the QCD axion DM window is being probed
experimentally by the ADMX experiment [5]. Existent proposals for future experiments,
like improvement of the ADMX setup [6] or new axion detection techniques like
ABRACADABRA [7] are expected to increase the sensitivity on the parameter space de ned by
eq. (1.2), but they are still only able to cover a fraction of the whole QCD axion DM
window. Most of the constraining power of axion DM detection experiments reside in the
region, outside of the range roughly de ned by eq. (1.2).
It is however important to keep in mind that the strong correlation between ma and
described by eq. (1.2) is not a generic consequence of solving the strong CP problem
with an axion. The purpose of this paper is to show that the coupling ga
can indeed be
arbitrarily large without spoiling CP conservation by strong interactions. We will show, for
the rst time, how this is indeed possible using a special realization of the clockwork
mechanism introduced by [8, 9] (see also [10] for a broad overview, and [11] for an application
to in ation).
The existence of models in which the QCD axion couples to photons with arbitrary
strength broadens the scope of experiments like ADMX and ABRACADABRA, and opens
up region in the axion parameter space covered by more than one experiments leading
to a rich phenomenology with multiple signals and possible collider smoking guns for our
The structure of the paper is as follows. In section 2 we review the axion solution
of the strong CP problem, its viability as a cold Dark Matter (DM) candidate, and the
existing and planned experiments to detect it. As the majority of these experiments relies
on the coupling of the axion to photons, in section 3 we review the predictions of the
simplest QCD axion models showing that they imply a very strong correlation between
this coupling and the axion mass. In section 4 we review the `clockwork mechanism' which
allows to obtain exponentially large axion decay constants and we show how this can be
used to get arbitrarily large couplings of the axion to photons without spoiling its solution
to th (...truncated)