arXiv subjects

Savas Dimopoulos

Publications and source records attributed to Savas Dimopoulos.

At least 19 recordsLinked to original sources

arXiv2025

Superradiant Interactions for Relic Detection with Entangled Nuclear Spins

We recently showed that macroscopic nuclear spin ensembles prepared in coherent spin states can dramatically enhance the interaction rates of weakly interacting cosmic relics-such as dark matter and the cosmic neutrino background-through collective quantum effects analogous to Dicke superradiance, where the de-excitation and excitation rates scale as the square of the number of spins, $N^2$. We thus coined these processes superradiant interactions. In this paper, we propose a protocol to realize this enhancement and boost the discovery potential for such relics. We show how concepts from quantum optics can be adapted to nuclear spins coupled to superconducting circuits, enabling high-sensitivity systems. The spins are first initialized into a coherent spin state via a $\pi/2$ Rabi pulse from the ground state. When the circuit is sufficiently detuned from resonance, the spin-circuit interaction implements a squeezing Hamiltonian. Because squeezing must outpace spin relaxation and dephasing, the protocol favors macroscopic ensembles and high-quality superconducting circuits. During this squeezing phase, the standard quantum variance is reduced by up to 4.8 orders of magnitude-equivalent to 48 dB of squeezing-for circuits with quality factors $Q \sim 10^8$-$10^9$. The signal imprinted on the spins during the squeezing protocol can be magnified by further utilizing the squeezing interactions, easing the requirement for shot-noise-limited readout. This protocol has the potential to significantly accelerate axion and dark photon dark matter searches and extend the reach of existing axion experiments to probe QCD axion-nuclear spin couplings. More broadly, it paves the way for detecting coherent inelastic interactions from other cosmic relics-most notably the cosmic neutrino background-and establishes nuclear-spin-based systems as a new class of quantum, ultra-low-threshold detectors.

hep-ph↗

arXiv2024

Superradiant Interactions of the Cosmic Neutrino Background, Axions, Dark Matter, and Reactor Neutrinos

In this paper we do three things. First, we outline the conditions under which the interaction rate of inelastic processes that change the internal state of a system of $N$ targets scales as $N^2$. This is an effect distinct from coherent elastic scattering, but with the same scaling. Second, we compute rates for such processes for various weakly interacting particles. Finally, we point to potential quantum observables for these processes, beyond energy exchange. Maximal coherence in inelastic processes is achieved when the targets are placed in an equal superposition of the ground and excited states. These coherent inelastic processes are analogous to Dicke superradiance, and we thus refer to them as superradiant interactions. We compute the superradiant interaction rates for the Cosmic Neutrino Background (C$\nu$B), dark matter scattering and absorption, and late-universe particles, such as reactor neutrinos, when the two-level system is realized by nuclear or electron spins in a magnetic field. The rates can be sizeable on macroscopic yet small targets. For example, the C$\nu$B interacts with a rate of $\mathcal{O}(\text{Hz})$ when scattering off a 10~cm liquid or solid-state density spin-polarized sphere, a $\mathcal{O}(10^{21})$ enhancement compared to the incoherent contribution. For QCD axion dark matter, similar rates can be achieved with much smaller samples, $N \sim \mathcal{O}(10^{15})\left(\frac{m}{2\times 10^{-8}~\text{eV}}\right)^{-1/2}$, where $m$ is the axion mass. Using the Lindblad formalism, we show that these superradiant interactions can manifest as a source of noise on the system. This points to new observables, sensitive to the sum of the excitation and de-excitation rates, and can be viewed as introducing diffusion and decoherence to the system. The effects presented in this paper may point to a new class of ultra-low threshold detectors.

hep-ph↗

arXiv2023

A Diffraction Grating for the Cosmic Neutrino Background and Dark Matter

We propose structures of size between $\sim 1$ meter to 100 meters that drastically alter the local distribution of the Cosmic Neutrino Background ($C\nu B$). These structures have a shape reminiscent of a sea urchin: They consist of rods of width $w$ and length $L \gg w$ periodically arranged on the surface of sphere of radius $R\sim L$. Such a structure functions as a diffraction phase grating and produces a region around its center where the fractional neutrino-antineutrino asymmetry is $\sim k\delta_\nu L$, where $k$ is the neutrino momentum, and $\delta_\nu$ the deviation of the neutrino index of refraction from unity. The asymmetry has a gradient set by the rod width. We find that the local neutrino asymmetry can be enhanced by $\mathcal{O}(\text{few}\times 10^6)$ relative to the naive Standard Model expectation, for reasonably sized structures. This results in a force $\mathcal{O}(10^3)$ times bigger than the one we recently pointed out due to the neutrinos' reflection on the surface of the Earth. While in this paper we do not propose a concrete detection setup, we estimate that the $\mathcal{O}(G_F)$ force on a test mass can be close to the Standard Quantum Limit of a torsion balance or a low frequency harmonic oscillator. Finally, we show that this $C \nu B$ diffractor can be used as a Dark Matter diffractor. For example, the QCD axion Dark Matter with decay constant $f_a$ around $10^9$ GeV can be sufficiently diffracted to produce a gradient force that is up to $\mathcal{O}(10^2)$ times larger than the one from the $C \nu B$. This is the first setup of this kind and the simplicity of our design suggests that there could be significant improvements that escape us.

hep-ph↗

arXiv2022

The Cosmic Neutrino Background on the Surface of the Earth

We argue that the reflection of relic neutrinos from the surface of the Earth results in a significant local $\nu-\bar{\nu}$ asymmetry, far exceeding the expected primordial lepton asymmetry. The net fractional electron neutrino number $\frac{n_{\nu_e}-n_{\bar{\nu}_e}}{n_{\nu_e}}$ is up to $\mathcal{O}(10^5) \sqrt{\frac{m_\nu}{0.1~\text{eV}}}$ larger than that implied by the baryon asymmetry. This enhancement is due to the weak 4-Fermi repulsion of the $\nu_e$ from ordinary matter which slows down the $\nu_e$ near the Earth's surface, and to the resulting evanescent neutrino wave that penetrates below the surface. This repulsion thus creates a net $\nu_e$ overdensity in a shell $\sim 7~\text{meters} \sqrt{\frac{0.1~\text{eV}}{m_\nu}}$ thick around the Earth's surface. Similarly the repulsion between $\bar{\nu}_\mu$ or $\bar{\nu}_\tau$ and ordinary matter creates an overdensity of $\bar{\nu}_{\mu, \tau}$ of similar size. These local enhancements increase the size of $\mathcal{O}(G_F)$ torques of the $C\nu B$ on spin-polarized matter by a factor of order $10^5$. In addition, they create a gradient of the net neutrino density which naturally provides a way out of the forty-year-old ``no-go'' theorems on the vanishing of $\mathcal{O}(G_F)$ forces. The torque resulting from such a gradient force can be $10^8$ times larger than that of earlier proposals. Although the size of these effects is still far from current reach, they may point to new directions for $C\nu B$ detection.

hep-ph↗

arXiv2021

Dark QED from Inflation

One contribution to any dark sector's abundance comes from its gravitational production during inflation. If the dark sector is weakly coupled to the inflaton and the Standard Model, this can be its only production mechanism. For non-interacting dark sectors, such as a free massive fermion or a free massive vector field, this mechanism has been studied extensively. In this paper we show, via the example of dark massive QED, that the presence of interactions can result in a vastly different mass for the dark matter (DM) particle, which may well coincide with the range probed by upcoming experiments. In the context of dark QED we study the evolution of the energy density in the dark sector after inflation. Inflation produces a cold vector condensate consisting of an enormous number of bosons, which via interesting processes - Schwinger pair production, strong field electromagnetic cascades, and plasma dynamics - transfers its energy to a small number of "dark electrons" and triggers thermalization of the dark sector. The resulting dark electron DM mass range is from 50 MeV to 30 TeV, far different from both the $10^{-5}$ eV mass of the massive photon dark matter in the absence of dark electrons, and from the $10^9$ GeV dark electron mass in the absence of dark photons. This can significantly impact the search strategies for dark QED and, more generally, theories with a self-interacting DM sector. In the presence of kinetic mixing, a dark electron in this mass range can be searched for with upcoming direct detection experiments, such as SENSEI-100g and OSCURA.

hep-ph↗

arXiv2021

Matter-wave Atomic Gradiometer Interferometric Sensor (MAGIS-100)

MAGIS-100 is a next-generation quantum sensor under construction at Fermilab that aims to explore fundamental physics with atom interferometry over a 100-meter baseline. This novel detector will search for ultralight dark matter, test quantum mechanics in new regimes, and serve as a technology pathfinder for future gravitational wave detectors in a previously unexplored frequency band. It combines techniques demonstrated in state-of-the-art 10-meter-scale atom interferometers with the latest technological advances of the world's best atomic clocks. MAGIS-100 will provide a development platform for a future kilometer-scale detector that would be sufficiently sensitive to detect gravitational waves from known sources. Here we present the science case for the MAGIS concept, review the operating principles of the detector, describe the instrument design, and study the detector systematics.

physics.atom-ph↗

arXiv2019

The Large-Misalignment Mechanism for the Formation of Compact Axion Structures: Signatures from the QCD Axion to Fuzzy Dark Matter

Axions are some of the best motivated particles beyond the Standard Model. We show how the attractive self-interactions of dark matter (DM) axions over a broad range of masses, from $10^{-22}$ eV to $10^7$ GeV, can lead to nongravitational growth of density fluctuations and the formation of bound objects. This structure formation enhancement is driven by parametric resonance when the initial field misalignment is large, and it affects axion density perturbations on length scales of order the Hubble horizon when the axion field starts oscillating, deep inside the radiation-dominated era. This effect can turn an otherwise nearly scale-invariant spectrum of adiabatic perturbations into one that has a spike at the aforementioned scales, producing objects ranging from dense DM halos to scalar-field configurations such as solitons and oscillons. We call this class of cosmological scenarios for axion DM production "the large-misalignment mechanism." We explore observational consequences of this mechanism for axions with masses up to $10$ eV. For axions heavier than $10^{-5}$ eV, the compact axion halos are numerous enough to significantly impact Earth-bound direct detection experiments, yielding intermittent but coherent signals with repetition rates exceeding one per decade and crossing times less than a day. These episodic increases in the axion density and kinematic coherence suggest new approaches for axion DM searches, including for the QCD axion. Dense structures made up of axions from $10^{-22}$ eV to $10^{-5}$ eV are detectable through gravitational lensing searches, and their gravitational interactions can also perturb baryonic structures and alter star formation. At very high misalignment amplitudes, the axion field can undergo self-interaction-induced implosions long before matter-radiation equality, producing potentially-detectable low-frequency stochastic gravitational waves.

astro-ph.CO↗

arXiv2017

A small weak scale from a small cosmological constant

We propose a framework in which Weinberg's anthropic explanation of the cosmological constant problem also solves the hierarchy problem. The weak scale is selected by chiral dynamics that controls the stabilization of an extra dimension. When the Higgs vacuum expectation value is close to a fermion mass scale, the radius of an extra dimension becomes large, and develops an enhanced number of vacua available to scan the cosmological constant down to its observed value. At low energies, the radion necessarily appears as an unnaturally light scalar, in a range of masses and couplings accessible to fifth-force searches as well as scalar dark matter searches with atomic clocks and gravitational-wave detectors. The fermion sector that controls the size of the extra dimension consists of a pair of electroweak doublets and several singlets. These leptons satisfy approximate mass relations related to the weak scale and are accessible to the LHC and future colliders.

hep-ph↗

arXiv2017

Resonant absorption of bosonic dark matter in molecules

We propose a new class of bosonic dark matter (DM) detectors based on resonant absorption onto a gas of small polyatomic molecules. Bosonic DM acts on the molecules as a narrow-band perturbation, like an intense but weakly coupled laser. The excited molecules emit the absorbed energy into fluorescence photons that are picked up by sensitive photodetectors with low dark count rates. This setup is sensitive to any DM candidate that couples to electrons, photons, and nuclei, and may improve on current searches by several orders of magnitude in coupling for DM masses between 0.2 eV and 20 eV. This type of detector has excellent intrinsic energy resolution, along with several control variables---pressure, temperature, external electromagnetic fields, molecular species/isotopes---that allow for powerful background rejection methods as well as precision studies of a potential DM signal. The proposed experiment does not require usage of novel exotic materials or futuristic technologies, relying instead on the well-established field of molecular spectroscopy, and on recent advances in single-photon detection. Cooperative radiation effects, which arise due to the large spatial coherence of the nonrelativistic DM field in certain detector geometries, can tightly focus the DM-induced fluorescence photons in a direction that depends on the DM's velocity, possibly permitting a detailed reconstruction of the full 3D velocity distribution in our Galactic neighborhood, as well as further background rejection.

hep-ph↗

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.

hep-ph↗

arXiv2016

A collider observable QCD axion

We present a model where the QCD axion is at the TeV scale and visible at a collider via its decays. Conformal dynamics and strong CP considerations account for the axion coupling strongly enough to the standard model to be produced as well as the coincidence between the weak scale and the axion mass. The model predicts additional pseudoscalar color octets whose properties are completely determined by the axion properties rendering the theory testable.

hep-ph↗

arXiv2016

Sound of Dark Matter: Searching for Light Scalars with Resonant-Mass Detectors

The fine-structure constant and the electron mass in string theory are determined by the values of scalar fields called moduli. If the dark matter takes on the form of such a light modulus, it oscillates with a frequency equal to its mass and an amplitude determined by the local dark-matter density. This translates into an oscillation of the size of a solid that can be observed by resonant-mass antennas. Existing and planned experiments, combined with a dedicated resonant-mass detector proposed in this Letter, can probe dark-matter moduli with frequencies between 1 kHz and 1 GHz, with much better sensitivity than searches for fifth forces.

hep-ph↗

arXiv2015

Auto-Concealment of Supersymmetry in Extra Dimensions

In supersymmetric (SUSY) theories with extra dimensions the visible energy in sparticle decays can be significantly reduced and its energy distribution broadened, thus significantly weakening the present collider limits on SUSY. The mechanism applies when the lightest supersymmetric particle (LSP) is a bulk state-- e.g. a bulk modulino, axino, or gravitino-- the size of the extra dimensions larger than ~$10^{-14}$ cm, and for a broad variety of visible sparticle spectra. In such cases the lightest ordinary supersymmetric particle (LOSP), necessarily a brane-localised state, decays to the Kaluza-Klein (KK) discretuum of the LSP. This dynamically realises the compression mechanism for hiding SUSY as decays into the more numerous heavier KK LSP states are favored. We find LHC limits on right-handed slepton LOSPs evaporate, while LHC limits on stop LOSPs weaken to ~350-410 GeV compared to ~700 GeV for a stop decaying to a massless LSP. Similarly, for the searches we consider, present limits on direct production of degenerate first and second generation squarks drop to ~450 GeV compared to ~800 GeV for a squark decaying to a massless LSP. Auto-concealment typically works for a fundamental gravitational scale of $M_*$~10-100 TeV, a scale sufficiently high that traditional searches for signatures of extra dimensions are mostly avoided. If superpartners are discovered, their prompt, displaced, or stopped decays can also provide new search opportunities for extra dimensions with the potential to reach $M_*$~$10^9$ GeV. This mechanism applies more generally than just SUSY theories, pertaining to any theory where there is a discrete quantum number shared by both brane and bulk sectors.

hep-ph↗

arXiv2014

Maximally Natural Supersymmetry

We consider 4D weak scale theories arising from 5D supersymmetric (SUSY) theories with maximal Scherk-Schwarz breaking at a Kaluza-Klein (KK) scale of several TeV. Many of the problems of conventional SUSY are avoided. Apart from 3rd family sfermions the SUSY spectrum is heavy, with only ~50% tuning at a gluino mass of ~2TeV and a stop mass of ~650 GeV. A single Higgs doublet acquires a vacuum expectation value, so the physical Higgs is automatically Standard-Model-like. A new U(1)' interaction raises the Higgs mass to 126 GeV. For minimal tuning the associated Z', as well as the 3rd family sfermions, must be accessible to LHC13. A gravitational wave signal consistent with BICEP2 is possible if inflation occurs when the extra dimensions are small.

hep-ph↗

arXiv2012

Mini-Split

The lack of evidence for new physics beyond the standard model at the LHC points to a paucity of new particles near the weak scale. This suggests that the weak scale is tuned and that supersymmetry, if present at all, is realized at higher energies. The measured Higgs mass constrains the scalar sparticles to be below 10^5 TeV, while gauge coupling unification favors Higgsinos below 100 TeV. Nevertheless, in many models gaugino masses are suppressed and remain within reach of the LHC. Tuning the weak scale and the renormalization group evolution of the scalar masses constrain Split model building. Due to the small gaugino masses, either the squarks or the up-higgs often run tachyonic; in the latter case, successful electroweak breaking requires heavy higgsinos near the scalar sparticles. We discuss the consequences of tuning the weak scale and the phenomenology of several models of Split supersymmetry including anomaly mediation, U(1)_(B-L) mediation, and Split gauge mediation.

hep-ph↗

arXiv2012

Split families unified

We present a simple supersymmetric model of split families consistent with flavor limits that preserves the successful prediction of gauge coupling unification and naturally accounts for the Higgs mass. The model provides an intricate connection between the Standard Model flavor hierarchy, supersymmetric flavor problem, unification and the Higgs mass. In particular unification favors a naturally large Higgs mass from D-term corrections to the quartic couplings in the Higgs potential. The unification scale is lowered with a stable proton that can account for the success of b-tau Yukawa coupling unification. The sparticle spectrum is similar to that of natural supersymmetry, as motivated by the supersymmetric flavor problem and recent LHC bounds, with a heavy scalar particle spectrum except for a moderately light stop required for viable electroweak symmetry breaking. Finally, Higgs production and decays, NLSP decays, and new states associated with extending the Standard Model gauge group above the TeV scale provide signatures for experimental searches at the LHC.

hep-ph↗

arXiv2011

Phenomenology of TeV Little String Theory from Holography

We study the graviton phenomenology of TeV Little String Theory by exploiting its holographic gravity dual five-dimensional theory. This dual corresponds to a linear dilaton background with a large bulk that constrains the Standard Model fields on the boundary of space. The linear dilaton geometry produces a unique Kaluza-Klein graviton spectrum that exhibits a ~ TeV mass gap followed by a near continuum of narrow resonances that are separated from each other by only ~ 30 GeV. Resonant production of these particles at the LHC is the signature of this framework that distinguishes it from large extra dimensions where the KK states are almost a continuum with no mass gap, and warped models where the states are separated by a TeV.

hep-ph↗

arXiv2009

String Axiverse

String theory suggests the simultaneous presence of many ultralight axions possibly populating each decade of mass down to the Hubble scale 10^-33eV. Conversely the presence of such a plenitude of axions (an "axiverse") would be evidence for string theory, since it arises due to the topological complexity of the extra-dimensional manifold and is ad hoc in a theory with just the four familiar dimensions. We investigate how upcoming astrophysical experiments will explore the existence of such axions over a vast mass range from 10^-33eV to 10^-10eV. Axions with masses between 10^-33eV to 10^-28eV cause a rotation of the CMB polarization that is constant throughout the sky. The predicted rotation angle is of order α~1/137. Axions in the mass range 10^-28eV to 10^-18eV give rise to multiple steps in the matter power spectrum, that will be probed by upcoming galaxy surveys. Axions in the mass range 10^-22eV to 10^-10eV affect the dynamics and gravitational wave emission of rapidly rotating astrophysical black holes through the Penrose superradiance process. When the axion Compton wavelength is of order of the black hole size, the axions develop "superradiant" atomic bound states around the black hole "nucleus". Their occupation number grows exponentially by extracting rotational energy from the ergosphere, culminating in a rotating Bose-Einstein axion condensate emitting gravitational waves. This mechanism creates mass gaps in the spectrum of rapidly rotating black holes that diagnose the presence of axions. The rapidly rotating black hole in the X-ray binary LMC X-1 implies an upper limit on the decay constant of the QCD axion f_a<2*10^17GeV, much below the Planck mass. This reach can be improved down to the grand unification scale f_a<2*10^16GeV, by observing smaller stellar mass black holes.

hep-th↗