arXiv subjects

Edward Hardy

Publications and source records attributed to Edward Hardy.

At least 37 records · Page 2Linked to original sources

arXiv2021

More Axions from Strings

We study the contribution to the QCD axion dark matter abundance that is produced by string defects during the so-called scaling regime. Clear evidence of scaling violations is found, the most conservative extrapolation of which strongly suggests a large number of axions from strings. In this regime, nonlinearities at around the QCD scale are shown to play an important role in determining the final abundance. The overall result is a lower bound on the QCD axion mass in the post-inflationary scenario that is substantially stronger than the naive one from misalignment.

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arXiv2021

Observing Invisible Axions with Gravitational Waves

If the Peccei-Quinn symmetry associated to an axion has ever been restored after inflation, axion strings inevitably produce a contribution to the stochastic gravitational wave background. Combining effective field theory analysis with numerical simulations, we show that the resulting gravitational wave spectrum has logarithmic deviations from a scale invariant form with an amplitude that is significantly enhanced at low frequencies. As a result, a single ultralight axion-like particle with a decay constant larger than $10^{14}~{\rm GeV}$ and any mass between $10^{-18}~{\rm eV}$ and $10^{-28}~{\rm eV}$ leads to an observable gravitational wave spectrum and is compatible with constraints on the post-inflationary scenario from dark matter overproduction, isocurvature and dark radiation. Since the spectrum extends over a wide range of frequencies, the resulting signal could be detected by multiple experiments. We describe straightforward ways in which the Peccei-Quinn symmetry can be restored after inflation for such decay constants. We also comment on the recent possible NANOgrav signal in light of our results.

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arXiv2020

Exploring Hamiltonian Truncation in $\bf{d=2+1}$

We initiate the application of Hamiltonian Truncation methods to solve strongly coupled QFTs in $d=2+1$. By analysing perturbation theory with a Hamiltonian Truncation regulator, we pinpoint the challenges of such an approach and propose a way that these can be addressed. This enables us to formulate Hamiltonian Truncation theory for $ϕ^4$ in $d=2+1$, and to study its spectrum at weak and strong coupling. The results obtained agree well with the predictions of a weak/strong self-duality possessed by the theory. The $ϕ^4$ interaction is a strongly relevant UV divergent perturbation, and represents a case study of a more general scenario. Thus, the approach developed should be applicable to many other QFTs of interest.

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arXiv2019

Thermal Dark Energy

We present a novel source of dark energy, which is motivated by the prevalence of hidden sectors in string theory models and is consistent with all of the proposed swampland conjectures. Thermal effects hold a light hidden sector scalar at a point in field space that is not a minimum of its zero temperature potential. This leads to an effective "cosmological constant", with an equation of state $w=-1$, despite the scalar's zero temperature potential having only a 4D Minkowski or AdS vacuum. For scalar masses $\lesssim μ$eV, which could be technically natural via sequestering, there are large regions of phenomenologically viable parameter space such that the induced vacuum energy matches the measured dark energy density. Additionally, in many models a standard cosmological history automatically leads to the scalar having the required initial conditions. We study the possible observational signals of such a model, including at fifth force experiments and through $ΔN_{\rm eff}$ measurements. Similar dynamics that are active at earlier times could resolve the tension between different measurements of $H_0$ and can lead to a detectable stochastic gravitational wave background.

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arXiv2019

Hearing without seeing: gravitational waves from hot and cold hidden sectors

We study the spectrum of gravitational waves produced by a first order phase transition in a hidden sector that is colder than the visible sector. In this scenario, bubbles of the hidden sector vacuum can be nucleated through either thermal fluctuations or quantum tunnelling. If a cold hidden sector undergoes a thermally induced transition, the amplitude of the gravitational wave signal produced will be suppressed and its peak frequency shifted compared to if the hidden and visible sector temperatures were equal. This could lead to signals in a frequency range that would otherwise be ruled out by constraints from big bang nucleosynthesis. Alternatively, a sufficiently cold hidden sector could fail to undergo a thermal transition and subsequently transition through the nucleation of bubbles by quantum tunnelling. In this case the bubble walls might accelerate with completely negligible friction. The resulting gravitational wave spectrum has a characteristic frequency dependence, which may allow such cold hidden sectors to be distinguished from models in which the hidden and visible sector temperatures are similar. We compare our results to the sensitivity of the future gravitational wave experimental programme.

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arXiv2018

Axions from Strings: the Attractive Solution

We study the system of axion strings that forms in the early Universe if the Peccei-Quinn symmetry is restored after inflation. Using numerical simulations, we establish the existence of an asymptotic solution to which the system is attracted independently of the initial conditions. We study in detail the properties of this solution, including the average number of strings per Hubble patch, the distribution of loops and long strings, the way that different types of radiation are emitted, and the shape of the spectrum of axions produced. We find clear evidence of logarithmic violations of the scaling properties of the attractor solution. We also find that, while most of the axions are emitted with momenta of order Hubble, most of the axion energy density is contained in axions with energy of order the string core scale, at least in the parameter range available in the simulation. While such a spectrum would lead to a negligible number density of relic axions from strings when extrapolated to the physical parameter region, we show that the presence of small logarithmic corrections to the spectrum shape could completely alter such a conclusion. A detailed understanding of the evolution of the axion spectrum is therefore crucial for a reliable estimate of the relic axion abundance from strings.

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arXiv2018

Higgs portal dark matter in non-thermal cosmologies

A scalar particle with a relic density set by annihilations through a Higgs portal operator is a simple and minimal possibility for dark matter. However, assuming a thermal cosmological history this model is ruled out over most of parameter space by collider and direct detection constraints. We show that in theories with a non-thermal cosmological history Higgs portal dark matter is viable for a wide range of dark matter masses and values of the portal coupling, evading existing limits. In particular, we focus on the string theory motivated scenario of a period of late time matter domination due to a light modulus with a decay rate that is suppressed by the Planck scale. Dark matter with a mass < GeV is possible without additional hidden sector states, and this can have astrophysically relevant self-interactions. We also study the signatures of such models at future direct, indirect, and collider experiments. Searches for invisible Higgs decays at the high luminosity LHC or an e+ e- collider could cover a significant proportion of the parameter space for low mass dark matter, and future direct detection experiments will play a complementary role.

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arXiv2017

Symmetric and Asymmetric Reheating

We study models in which the inflaton is coupled to two otherwise decoupled sectors, and the effect of preheating and related processes on their energy densities during the evolution of the universe. Over most of parameter space, preheating is not disrupted by the presence of extra sectors, and even comparatively weakly coupled sectors can get an order 1 fraction of the total energy at this time. If two sectors are both preheated, the high number densities could also lead to inflaton mediated thermalisation. If only one sector is preheated, Bose enhancement of the late time inflaton decays may cause significant deviations from the perturbative prediction for their relative reheat temperatures. Meanwhile, in Non-Oscillatory inflation models resonant effects can result in exponentially large final temperature differences between sectors that have similar couplings to the inflaton. Asymmetric reheating is potentially relevant for a range of beyond the Standard Model physics scenarios. We show that in dark matter freeze-in models, hidden sector temperatures a factor of 10 below that of the visible sector are typically needed for the relic abundance to be set solely by freeze-in dynamics.

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arXiv2017

Non-perturbative reheating and Nnaturalness

We study models in which reheating happens only through non-perturbative processes. The energy transferred can be exponentially suppressed unless the inflaton is coupled to a particle with a parametrically small mass. Additionally, in some models a light scalar with a negative mass squared parameter leads to much more efficient reheating than one with a positive mass squared of the same magnitude. If a theory contains many sectors similar to the Standard Model coupled to the inflaton via their Higgses, such dynamics can realise the Nnaturalness solution to the hierarchy problem. A sector containing a light Higgs with a non-zero vacuum expectation value is dominantly reheated and there is little energy transferred to the other sectors, consistent with cosmological constraints. The inflaton must decouple from other particles and have a flat potential at large field values, in which case the visible sector UV cutoff can be raised to 10 TeV in a simple model.

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arXiv2017

Glueball dark matter in non-standard cosmologies

Hidden sector glueball dark matter is well motivated by string theory, compactifications of which often have extra gauge groups uncoupled to the visible sector. We study the dynamics of glueballs in theories with a period of late time primordial matter domination followed by a low final reheating temperature due to a gravitationally coupled modulus. Compared to scenarios with a high reheating temperature, the required relic abundance is possible with higher hidden sector confinement scales, and less extreme differences in the entropy densities of the hidden and visible sectors. Both of these can occur in string derived models, and relatively light moduli are helpful for obtaining viable phenomenology. We also study the effects of hidden sector gluinos. In some parts of parameter space these can be the dominant dark matter component, while in others their abundance is much smaller than that of glueballs. Finally, we show that heavy glueballs produced from energy in the hidden sector prior to matter domination can have the correct relic abundance if they are sufficiently long lived.

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arXiv2017

Stellar cooling bounds on new light particles: plasma mixing effects

Strong constraints on the coupling of new light particles to the Standard Model (SM) arise from their production in the hot cores of stars, and the effects of this on stellar cooling. For new light particles which have an effective in-medium mixing with the photon, plasma effects can result in parametrically different production rates to those obtained from a naive calculation. Taking these previously-neglected contributions into account, we make updated estimates for the stellar cooling bounds on light scalars and vectors with a variety of SM couplings. In particular, we improve the bounds on light (m <~ keV) scalars coupling to electrons or nucleons by up to 3 orders of magnitude in the coupling squared, significantly revise the supernova cooling bounds on dark photon couplings, and qualitatively change the mass dependence of stellar bounds on new vectors. Scalars with mass <~ 2 keV that couple through the Higgs portal are constrained to mixing angle <~ 3*10^-10, which gives the dominant bound for scalar masses above ~ 0.2 eV.

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arXiv2017

Miniclusters in the Axiverse

If dark matter is an axion-like-particle a significant fraction of the present day relic abundance could be concentrated in compact gravitationally bound miniclusters. We study the minicluster masses compatible with the dark matter relic density constraint. If they form from fluctuations produced by PQ symmetry breaking, minicluster masses up to hundreds of solar masses are possible, although over most of the parameter space they are much lighter. The size of these objects is typically within a few orders of magnitude of an astronomical unit. We also show that miniclusters can form if an axion gets mass from a hidden sector with a first order phase transition that takes a relatively long time to complete. Therefore they can appear in models where PQ symmetry is broken before inflation, compatible with large axion decay constants and string theory UV completions.

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arXiv2016

The Heterotic Superpotential and Moduli

We study the four-dimensional effective theory arising from ten-dimensional heterotic supergravity compactified on manifolds with torsion. In particular, given the heterotic superpotential appropriately corrected at $\mathcal{O}(α')$ to account for the Green-Schwarz anomaly cancellation mechanism, we investigate properties of four-dimensional Minkowski vacua of this theory. Considering the restrictions arising from F-terms and D-terms we identify the infinitesimal massless moduli space of the theory. We show that it agrees with the results that have recently been obtained from a ten-dimensional perspective where supersymmetric Minkowski solutions including the Bianchi identity correspond to an integrable holomorphic structure, with infinitesimal moduli calculated by its first cohomology. As has recently been noted, interplay of complex structure and bundle deformations through holomorphic and anomaly constraints can lead to fewer moduli than may have been expected. We derive a relation between the number of complex structure and bundle moduli removed from the low energy theory in this way, and give conditions for there to be no complex structure moduli or bundle moduli remaining in the low energy theory. The link between Yukawa couplings and obstruction theory is also briefly discussed.

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arXiv2016

Dirac vs Majorana gauginos at a 100 TeV collider

We compare the prospects for observing theories with Majorana or Dirac gauginos at a future 100 TeV proton-proton collider. Calculating the expected discovery and exclusion regions, we find that for heavy gluino masses the squark discovery reach is significantly reduced in Dirac gluino models relative to the Majorana case. However, if the squark and gluino masses are close the reach is similar in both scenarios. We also consider the electroweak fine tuning of theories observable at such a collider, and the impact of existing constraints from flavour and CP violating observables. Models with Majorana gluinos that are fine tuned to less than one part in 10,000 can typically be discovered or excluded, and Dirac models with tuning of one part in 1,000 can be probed. The flavour structure of Majorana models is highly constrained if they have observable squarks, while O(1) violation is possible in accessible Dirac models. In both cases new sources of CP violation must be very suppressed. Future collider searches can also give important information on possible dark matter candidates. We study the relation of this to indirect and direct detection searches, and find that if dark matter is a neutralino, a 100 TeV collider could probe the viable dark matter candidates in large classes of both Dirac and Majorana models.

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arXiv2016

Diphotons from Diaxions

We study models in which the 750 GeV diphoton excess is due to a scalar decaying to two pseudo-Nambu-Goldstone bosons that subsequently decay into two pairs of highly boosted photons, misidentified as individual photons. Performing a model independent analysis we find that, with axion mass around $200$ MeV, this class of theories can naturally explain the observed signal with a large width, without violating monojet constraints. At the same time the requirement of a prompt axion decay can be satisfied only with a relatively large axion-photon coupling, leading to many new charged particles at the TeV scale. However, the required multiplicities of such fields is still moderately reduced relative to models with direct diphoton production.

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arXiv2016

The QCD axion, precisely

We show how several properties of the QCD axion can be extracted at high precision using only first principle QCD computations. By combining NLO results obtained in chiral perturbation theory with recent Lattice QCD results the full axion potential, its mass and the coupling to photons can be reconstructed with percent precision. Axion couplings to nucleons can also be derived reliably, with uncertainties smaller than ten percent. The approach presented here allows the precision to be further improved as uncertainties on the light quark masses and the effective theory couplings are reduced. We also compute the finite temperature dependence of the axion potential and its mass up to the crossover region. For higher temperature we point out the unreliability of the conventional instanton approach and study its impact on the computation of the axion relic abundance.

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arXiv2015

Electroweak relaxation from finite temperature

We study theories which naturally select a vacuum with parametrically small Electroweak Scale due to finite temperature effects in the early universe. In particular, there is a scalar with an approximate shift symmetry broken by a technically natural small coupling to the Higgs, and a temperature dependent potential. As the temperature of the universe drops, the scalar follows the minimum of its potential altering the Higgs mass squared parameter. The scalar also has a periodic potential with amplitude proportional to the Higgs expectation value, which traps it in a vacuum with a small Electroweak Scale. The required temperature dependence of the potential can occur through strong coupling effects in a hidden sector that are suppressed at high temperatures. Alternatively, it can be generated perturbatively from a one-loop thermal potential. In both cases, for the scalar to be displaced, a hidden sector must be reheated to temperatures significantly higher than the visible sector. However this does not violate observational constraints provided the hidden sector energy density is transferred to the visible sector without disrupting big bang nucleosynthesis. We also study how the mechanism can be implemented when the visible sector is completed to the Minimal Supersymmetric Standard Model at a high scale. Models with a UV cutoff of 10 TeV and no fields taking values over a range greater than 10^12 GeV are possible, although the scalar must have a range of order 10^8 times the effective decay constant in the periodic part of its potential.

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arXiv2015

The Massless Spectrum of Heterotic Compactifications

We discuss the four-dimensional massless spectrum of supersymmetric Minkowski compactifications of ten-dimensional heterotic supergravity, including the anomaly cancelation condition. This can be calculated from restrictions arising from F-term conditions in a four-dimensional effective theory. The results agree with computations of the infinitesimal moduli space recently performed from a ten-dimensional perspective. The paper is based on a talk given by Eirik Eik Svanes in Leuven for the workshop on "The String Theory Universe".

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