arXiv subjects

Robert Lasenby

Publications and source records attributed to Robert Lasenby.

25 records · Page 2Linked to original sources

arXiv2017

Black Hole Mergers and the QCD Axion at Advanced LIGO

In the next few years Advanced LIGO (aLIGO) may see gravitational waves (GWs) from thousands of black hole (BH) mergers. This marks the beginning of a new precision tool for physics. Here we show how to search for new physics beyond the standard model using this tool, in particular the QCD axion in the mass range ma ~ 10^-14 to 10^-10 eV. Axions (or any bosons) in this mass range cause rapidly rotating BHs to shed their spin into a large cloud of axions in atomic Bohr orbits around the BH, through the effect of superradiance (SR). This results in a gap in the mass vs. spin distribution of BHs when the BH size is comparable to the axion's Compton wavelength. By measuring the spin and mass of the merging objects observed at LIGO, we could verify the presence and shape of the gap in the BH distribution produced by the axion. The axion cloud can also be discovered through the GWs it radiates via axion annihilations or level transitions. A blind monochromatic GW search may reveal up to 10^5 BHs radiating through axion annihilations, at distinct frequencies within ~3% of $2 ma. Axion transitions probe heavier axions and may be observable in future GW observatories. The merger events are perfect candidates for a targeted GW search. If the final BH has high spin, a SR cloud may grow and emit monochromatic GWs from axion annihilations. We may observe the SR evolution in real time.

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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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arXiv2015

Twin Higgs WIMP Dark Matter

Dark matter (DM) without a matter asymmetry is studied in the context of Twin Higgs (TH) theories in which the LHC naturalness problem is addressed. These possess a twin sector related to the Standard Model (SM) by a (broken) $\mathbb{Z}_2$ symmetry, and interacting with the SM via a specific Higgs portal. We focus on the minimal realisation of the TH mechanism, the Fraternal Twin Higgs, with only a single generation of twin quarks and leptons, and $SU(3)'\times SU(2)'$ gauge group. We show that a variety of natural twin-WIMP DM candidates are present (directly linked to the weak scale by naturalness), the simplest and most attractive being the $τ^\prime$ lepton with a mass $m_{τ^\prime} > m_{\rm Higgs}/2$, although spin-1 $W^{\prime\pm}$ DM and multicomponent DM are also possible (twin baryons are strongly disfavoured by tuning). We consider in detail the dynamics of the possibly (meta)stable glueballs in the twin sector, the nature of the twin QCD phase transition, and possible new contributions to the number of relativistic degrees of freedom $ΔN_{\rm eff}$. Direct detection signals are below current bounds but accessible in near future experiments. Indirect detection phenomenology is rich and requires detailed studies of twin hadronization and fragmentation to twin glueballs and quarkonia and their subsequent decay to SM, and possible light twin sector states.

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arXiv2015

Twin Higgs Asymmetric Dark Matter

We study Asymmetric Dark Matter (ADM) in the context of the minimal (Fraternal) Twin Higgs solution to the little hierarchy problem, with a twin sector with gauged $SU(3)' \times SU(2)'$, a twin Higgs, and only third generation twin fermions. Naturalness requires the QCD$^\prime$ scale $Λ'_{\rm QCD} \simeq 0.5 - 20 \ {\rm GeV}$, and $t'$ to be heavy. We focus on the light $b'$ quark regime, $m_{b'} \lesssim Λ'_{\rm QCD}$, where QCD$^\prime$ is characterised by a single scale $Λ'_{\rm QCD}$ with no light pions. A twin baryon number asymmetry leads to a successful DM candidate: the spin-3/2 twin baryon, $Δ' \sim b'b'b'$, with a dynamically determined mass ($\sim 5 Λ'_{\rm QCD}$) in the preferred range for the DM-to-baryon ratio $Ω_{\rm DM}/Ω_{\rm baryon} \simeq 5$. Gauging the $U(1)'$ group leads to twin atoms ($Δ'$ - $\bar {τ'}$ bound states) that are successful ADM candidates in significant regions of parameter space, sometimes with observable changes to DM halo properties. Direct detection signatures satisfy current bounds, at times modified by dark form factors.

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arXiv2015

Big Bang Synthesis of Nuclear Dark Matter

We investigate the physics of dark matter models featuring composite bound states carrying a large conserved dark "nucleon" number. The properties of sufficiently large dark nuclei may obey simple scaling laws, and we find that this scaling can determine the number distribution of nuclei resulting from Big Bang Dark Nucleosynthesis. For plausible models of asymmetric dark matter, dark nuclei of large nucleon number, e.g. > 10^8, may be synthesised, with the number distribution taking one of two characteristic forms. If small-nucleon-number fusions are sufficiently fast, the distribution of dark nuclei takes on a logarithmically-peaked, universal form, independent of many details of the initial conditions and small-number interactions. In the case of a substantial bottleneck to nucleosynthesis for small dark nuclei, we find the surprising result that even larger nuclei, with size >> 10^8, are often finally synthesised, again with a simple number distribution. We briefly discuss the constraints arising from the novel dark sector energetics, and the extended set of (often parametrically light) dark sector states that can occur in complete models of nuclear dark matter. The physics of the coherent enhancement of direct detection signals, the nature of the accompanying dark-sector form factors, and the possible modifications to astrophysical processes are discussed in detail in a companion paper.

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arXiv2015

Signatures of Large Composite Dark Matter States

We investigate the interactions of large composite dark matter (DM) states with the Standard Model (SM) sector. Elastic scattering with SM nuclei can be coherently enhanced by factors as large as A^2, where A is the number of constituents in the composite state (there exist models in which DM states of very large A > 10^8 may be realised). This enhancement, for a given direct detection event rate, weakens the expected signals at colliders by up to 1/A. Moreover, the spatially extended nature of the DM states leads to an additional, characteristic, form factor modifying the momentum dependence of scattering processes, altering the recoil energy spectra in direct detection experiments. In particular, energy recoil spectra with peaks and troughs are possible, and such features could be confirmed with only O(50) events, independently of the assumed halo velocity distribution. Large composite states also generically give rise to low-energy collective excitations potentially relevant to direct detection and indirect detection phenomenology. We compute the form factor for a generic class of such excitations - quantised surface modes - finding that they can lead to coherently-enhanced, but generally sub-dominant, inelastic scattering in direct detection experiments. Finally, we study the modifications to capture rates in astrophysical objects that follow from the elastic form factor, as well as the effects of inelastic interactions between DM states once captured. We argue that inelastic interactions may lead to the DM collapsing to a dense configuration at the centre of the object.

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arXiv2014

Annihilation Signals from Asymmetric Dark Matter

In the simplest models of asymmetric dark matter (ADM) annihilation signals are not expected, since the DM is non-self-conjugate and the relic density of anti-DM is negligible. We investigate a new class of models in which a symmetric DM component, in the `low-mass' 1-10 GeV regime favoured for linking the DM and baryon asymmetries, is repopulated through decays. We find that, in models without significant velocity dependence of the annihilation cross section, observational constraints generally force these decays to be (cosmologically) slow. These late decays can give rise to gamma-ray signal morphologies differing from usual annihilation profiles. A distinctive feature of such models is that signals may be absent from dwarf spheroidal galaxies.

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