Opening up the QCD axion window Agrawal, Prateek; Marques-Tavares, Gustavo; Xue, Wei
The journal of high energy physics,
03/2018, Letnik:
2018, Številka:
3
Journal Article
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A
bstract
We present a new mechanism to deplete the energy density of the QCD axion, making decay constants as high as
f
a
≃ 10
17
GeV viable for generic initial conditions. In our setup, the axion ...couples to a massless dark photon with a coupling that is moderately stronger than the axion coupling to gluons. Dark photons are produced copiously through a tachyonic instability when the axion field starts oscillating, and an exponential suppression of the axion density can be achieved. For a large part of the parameter space this dark radiation component of the universe can be observable in upcoming CMB experiments. Such dynamical depletion of the axion density ameliorates the isocurvature bound on the scale of inflation. The depletion also amplifies the power spectrum at scales that enter the horizon before particle production begins, potentially leading to axion miniclusters.
Rare kaon decays are excellent probes of light, new weakly coupled particles. If such particles X couple preferentially to muons, they can be produced in K→μνX decays. We evaluate the future ...sensitivity for this process at NA62 assuming X decays either invisibly or to dimuons. Our main physics target is the parameter space that resolves the (g-2)_{μ} anomaly, where X is a gauged L_{μ}-L_{τ} vector or a muonphilic scalar. The same parameter space can also accommodate dark matter freeze-out or reduce the tension between cosmological and local measurements of H_{0} if the new force decays to dark matter or neutrinos, respectively. We show that for invisible X decays, a dedicated single muon trigger analysis at NA62 could probe much of the remaining (g-2)_{μ} favored parameter space. Alternatively, if X decays to muons, NA62 can perform a dimuon resonance search in K→3μν events and greatly improve existing coverage for this process. Independently of its sensitivity to new particles, we find that NA62 is also sensitive to the standard model predicted rate for K→3μν, which has never been measured.
A
bstract
The low frequency part of the gravitational wave spectrum generated by local physics, such as a phase transition or parametric resonance, is largely fixed by causality, offering a clean ...window into the early Universe. In this work, this low frequency end of the spectrum is analyzed with an emphasis on a physical understanding, such as the suppressed production of gravitational waves due to the excitation of an over-damped harmonic oscillator and their enhancement due to being frozen out while outside the horizon. Due to the difference between sub-horizon and super-horizon physics, it is inevitable that there will be a distinct spectral feature that could allow for the direct measurement of the conformal Hubble rate at which the phase transition occurred. As an example, free-streaming particles (such as the gravity waves themselves) present during the phase transition affect the production of super-horizon modes. This leads to a steeper decrease in the spectrum at low frequencies as compared to the well-known causal
k
3
super-horizon scaling of stochastic gravity waves. If a sizable fraction of the energy density is in free-streaming particles, they even lead to the appearance of oscillatory features in the spectrum. If the universe was not radiation dominated when the waves were generated, a similar feature also occurs at the transition between sub-horizon to super-horizon causality. These features are used to show surprising consequences, such as the fact that a period of matter domination following the production of gravity waves actually increases their power spectrum at low frequencies.
A
bstract
We present models in which axions have large couplings to hadrons while remaining naturally light. By assigning U(1)
PQ
charges to Standard Model quarks such that U(1)
PQ
is not anomalous ...under QCD, the derivative couplings naturally arise while no potential is generated for the axion upon QCD confinement. We present simple models that implement this idea both for an axion-like particle and for the QCD axion. We show that both models lead to sizable flavor violations that could be probed by future experiments. Our construction shows that the axion coupling to hadrons can be essentially independent from its mass and thus motivates experimental searches in a wide range of axion parameter space.
A
bstract
The causal tail of stochastic gravitational waves can be used to probe the energy density in free streaming relativistic species as well as measure
g
*
(
T
) and beta functions
β
(
T
) as a ...function of temperature. In the event of the discovery of loud stochastic gravitational waves, we demonstrate that LISA can measure the free streaming fraction of the universe down to the the 10
−
3
level, 100 times more sensitive than current constraints. Additionally, it would be sensitive to
O
(1) deviations of
g
*
and the QCD
β
function from their Standard Model value at temperatures ~ 10
5
GeV. In this case, many motivated models such as split SUSY and other solutions to the Electroweak Hierarchy problem would be tested. Future detectors, such as DECIGO, would be 100 times more sensitive than LISA to these effects and be capable of testing other motivated scenarios such as WIMPs and axions. The amazing prospect of using precision gravitational wave measurements to test such well motivated theories provides a benchmark to aim for when developing a precise understanding of the gravitational wave spectrum both experimentally and theoretically.
A
bstract
The presence of a plethora of light spin 0 and spin 1 fields is motivated in a number of BSM scenarios, such as the axiverse. The study of the interactions of such light bosonic fields with ...the Standard Model has focused mostly on interactions involving only one such field, such as the axion (
ϕ
) coupling to photons,
, or the kinetic mixing between photon and the dark photon,
FF
D
. In this work, we continue the exploration of interactions involving two light BSM fields and the standard model, focusing on the mixed axion-photon-dark-photon interaction
. If either the axion or dark photon are dark matter, we show that this interaction leads to conversion of the CMB photons into a dark sector particle, leading to a distortion in the CMB spectrum. We present the details of these unique distortion signatures and the resulting constraints on the
coupling. In particular, we find that for a wide range of masses, the constraints from these effect are stronger than on the more widely studied axion-photon coupling.
A
bstract
A dark photon is a well-motivated new particle which, as a component of an associated dark sector, could explain dark matter. One strong limit on dark photons arises from excessive cooling ...of supernovae. We point out that even at couplings where too few dark photons are produced in supernovae to violate the cooling bound, they can be observed directly through their decays. Supernovae produce dark photons which decay to positrons, giving a signal in the 511 keV annihilation line observed by SPI/INTEGRAL. Further, prompt gamma-ray emission by these decaying dark photons gives a signal for gamma-ray telescopes. Existing GRS observations of SN1987a already constrain this, and a future nearby SN could provide a detection. Finally, dark photon decays from extragalactic SN would produce a diffuse flux of gamma rays observable by detectors such as SMM and HEAO-1. Together these observations can probe dark photon couplings several orders of magnitude beyond current constraints for masses of roughly 1-100 MeV.
In this Letter, we investigate the effects of single derivative mixing in massive bosonic fields. In the regime of large mixing, we show that this leads to striking changes of the field dynamics, ...delaying the onset of classical oscillations and decreasing, or even eliminating, the friction due to Hubble expansion. We highlight this phenomenon with a few examples. In the first example, we show how an axionlike particle can have its number abundance parametrically enhanced. In the second example, we demonstrate that the QCD axion can have its number abundance enhanced allowing for misalignment driven axion dark matter all the way down to fa of order astrophysical bounds. In the third example, we show that the delayed oscillation of the scalar field can also sustain a period of inflation. In the last example, we present a situation where an oscillating scalar field is completely frictionless and does not dilute away in time.
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