Peter W. Graham

Stanford University, Palo Alto, California, United States

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Publications (38)171.13 Total impact

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    ABSTRACT: There are very few direct experimental tests of the inverse square law of gravity at distances comparable to the scale of the Solar System and beyond. Here we describe a possible space mission optimized to test the inverse square law at a scale of up to 100 AU. For example, sensitivity to a Yukawa correction with a strength of $10^{-7}$ times gravity and length scale of 100 AU is within reach, improving the current state of the art by over two orders of magnitude. This experiment would extend our understanding of gravity to the largest scale that can be reached with a direct probe using known technology. This would provide a powerful test of long-distance modifications of gravity including many theories motivated by dark matter or dark energy.
  • Peter W. Graham · Surjeet Rajendran · Jaime Varela
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    ABSTRACT: The transit of primordial black holes through a white dwarf causes localized heating around the trajectory of the black hole through dynamical friction. For sufficiently massive black holes, this heat can initiate runaway thermonuclear fusion causing the white dwarf to explode as a supernova. The shape of the observed distribution of white dwarfs with masses up to $1.25 M_{\odot}$ rules out primordial black holes with masses $\sim 10^{19}$ gm - $10^{20}$ gm as a dominant constituent of the local dark matter density. Black holes with masses as large as $10^{24}$ gm will be excluded if recent observations by the NuStar collaboration of a population of white dwarfs near the galactic center are confirmed. Black holes in the mass range $10^{20}$ gm - $10^{22}$ gm are also constrained by the observed supernova rate, though these bounds are subject to astrophysical uncertainties. These bounds can be further strengthened through measurements of white dwarf binaries in gravitational wave observatories. The mechanism proposed in this paper can constrain a variety of other dark matter scenarios such as Q balls, annihilation/collision of large composite states of dark matter and models of dark matter where the accretion of dark matter leads to the formation of compact cores within the star. White dwarfs, with their astronomical lifetimes and sizes, can thus act as large space-time volume detectors enabling a unique probe of the properties of dark matter, especially of dark matter candidates that have low number density. This mechanism also raises the intriguing possibility that a class of supernova may be triggered through rare events induced by dark matter rather than the conventional mechanism of accreting white dwarfs that explode upon reaching the Chandrasekhar mass.
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    Peter W. Graham · David E. Kaplan · Surjeet Rajendran
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    ABSTRACT: A new class of solutions to the electroweak hierarchy problem is presented that does not require either weak scale dynamics or anthropics. Dynamical evolution during the early universe drives the Higgs mass to a value much smaller than the cutoff. The simplest model has the particle content of the standard model plus a QCD axion and an inflation sector. The highest cutoff achieved in any technically natural model is 10^8 GeV.
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    Peter W. Graham · Jeremy Mardon · Surjeet Rajendran
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    ABSTRACT: We calculate the production of a massive vector boson by quantum fluctuations during inflation. This gives a novel dark-matter production mechanism quite distinct from misalignment or thermal production. While scalars and tensors are typically produced with a nearly scale-invariant spectrum, surprisingly the vector is produced with a power spectrum peaked at intermediate wavelengths. Thus dangerous, long-wavelength, isocurvature perturbations are suppressed. Further, at long wavelengths the vector inherits the usual adiabatic, nearly scale-invariant perturbations of the inflaton, allowing it to be a good dark matter candidate. The final abundance can be calculated precisely from the mass and the Hubble scale of inflation, H_I. Saturating the dark matter abundance we find a prediction for the mass m = 10^-5 eV (10^14 GeV/H_I)^4. High-scale inflation, potentially observable in the CMB, motivates an exciting mass range for recently proposed direct detection experiments for hidden photon dark matter. Such experiments may be able to reconstruct the distinctive, peaked power spectrum, verifying that the dark matter was produced by quantum fluctuations during inflation and providing a direct measurement of the scale of inflation. Thus a detection would not only be the discovery of dark matter, it would also provide an unexpected probe of inflation itself.
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    ABSTRACT: Merging galaxy clusters such as the Bullet Cluster provide a powerful testing ground for indirect detection of dark matter. The spatial distribution of the dark matter is both directly measurable through gravitational lensing and substantially different from the distribution of potential astrophysical backgrounds. We propose to use this spatial information to identify the origin of indirect detection signals, and we show that even statistical excesses of a few sigma can be robustly tested for consistency--or inconsistency--with a dark matter source. For example, our methods, combined with already-existing observations of the Coma Cluster, would allow the 3.55 keV line to be tested for compatibility with a dark matter origin. We also discuss the optimal spatial reweighting of photons for indirect detection searches. The current discovery rate of merging galaxy clusters and associated lensing maps strongly motivates deep exposures in these dark matter targets for both current and upcoming indirect detection experiments in the X-ray and gamma-ray bands.
    Physical Review D 02/2015; 91(10). DOI:10.1103/PhysRevD.91.103524 · 4.86 Impact Factor
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    ABSTRACT: We propose a resonant electromagnetic detector to search for hidden-photon dark matter over an extensive range of masses. Hidden-photon dark matter can be described as a weakly coupled "hidden electric field," oscillating at a frequency fixed by the mass, and able to penetrate any shielding. At low frequencies (compared to the inverse size of the shielding), we find that observable effect of the hidden photon inside any shielding is a real, oscillating magnetic field. We outline experimental setups designed to search for hidden-photon dark matter, using a tunable, resonant LC circuit designed to couple to this magnetic field. Our "straw man" setups take into consideration resonator design, readout architecture and noise estimates. At high frequencies,there is an upper limit to the useful size of a single resonator set by $1/\nu$. However, many resonators may be multiplexed within a hidden-photon coherence length to increase the sensitivity in this regime. Hidden-photon dark matter has an enormous range of possible frequencies, but current experiments search only over a few narrow pieces of that range. We find the potential sensitivity of our proposal is many orders of magnitude beyond current limits over an extensive range of frequencies, from 100 Hz up to 700 GHz and potentially higher.
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    Peter W. Graham · Jeremy Mardon · Surjeet Rajendran · Yue Zhao
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    ABSTRACT: Many theories beyond the Standard Model contain hidden photons. A light hidden photon will generically couple to the Standard Model through a kinetic mixing term, giving a powerful avenue for detection using "Light-Shining-Through-A-Wall"-type transmission experiments with resonant cavities. We demonstrate a parametric enhancement of the signal in such experiments, resulting from transmission of the longitudinal mode of the hidden photon. While previous literature has focused on the production and detection of transverse modes, the longitudinal mode allows a significant improvement in experimental sensitivity. Although optical experiments such as ALPS are unable to take useful advantage of this enhancement, the reach of existing microwave cavity experiments such as CROWS is significantly enhanced beyond their published results. Future microwave cavity experiments, designed with appropriate geometry to take full advantage of the longitudinal mode, will provide a powerful probe of hidden-photon parameter space extending many orders of magnitude beyond current limits, including significant regions where the hidden photon can be dark matter.
    Physical Review D 07/2014; 90(7). DOI:10.1103/PhysRevD.90.075017 · 4.86 Impact Factor
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    Peter W. Graham · Bart Horn · Surjeet Rajendran · Gonzalo Torroba
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    ABSTRACT: We construct nonsingular cyclic cosmologies that respect the null energy condition, have a large hierarchy between the minimum and maximum size of the universe, and are stable under linearized fluctuations. The models are supported by a combination of positive curvature, a negative cosmological constant, cosmic strings and matter that at the homogeneous level behaves as a perfect fluid with equation of state -1 < w < -1/3. We investigate analytically the stability of the perturbation equations and discuss the role of parametric resonances and nonlinear corrections. Finally, we argue that Casimir energy contributions associated to the compact spatial slices can become important at short scales and lift nonperturbative decays towards vanishing size. This class of models (particularly in the static limit) can then provide a useful framework for studying the question of the ultimate (meta)stability of an eternal universe.
  • Peter W. Graham · Bart Horn · Surjeet Rajendran · Gonzalo Torroba
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    ABSTRACT: We construct nonsingular cyclic cosmologies that respect the null energy condition, have a large hierarchy between the minimum and maximum size of the universe, and are stable under linearized fluctuations. The models are supported by a combination of positive curvature, a negative cosmological constant, cosmic strings and matter that at the homogeneous level behaves as a perfect fluid with equation of state -1 < w < -1/3. We investigate analytically the stability of the perturbation equations and discuss the role of parametric resonances and nonlinear corrections. Finally, we argue that Casimir energy contributions associated to the compact spatial slices can become important at short scales and lift nonperturbative decays towards vanishing size. This class of models (particularly in the static limit) can then provide a useful framework for studying the question of the ultimate (meta)stability of an eternal universe.
    Journal of High Energy Physics 04/2014; 2014(8). DOI:10.1007/JHEP08(2014)163 · 6.22 Impact Factor
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    Peter W. Graham · Surjeet Rajendran · Prashant Saraswat
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    ABSTRACT: Supersymmetry is under pressure from LHC searches requiring colored superpartners to be heavy. We demonstrate R-parity violating spectra for which the dominant signatures are not currently well searched for at the LHC. In such cases, the bounds can be as low as 800 GeV on both squarks and gluinos. We demonstrate that there are nontrivial constraints on squark and gluino masses with baryonic RPV (UDD operators) and show that in fact leptonic RPV can allow comparable or even lighter superpartners. The constraints from many searches are weakened if the LSP is significantly lighter than the colored superpartners, such that it is produced with high boost. The LSP decay products will then be collimated, leading to the miscounting of leptons or jets and causing such models to be missed even with large production cross-sections. Other leptonic RPV scenarios that evade current searches include the highly motivated case of a higgsino LSP decaying to a tau and two quarks, and the case of a long-lived LSP with a displaced decay to electrons and jets. The least constrained models can have SUSY production cross-sections of ~pb or larger, implying tens of thousands of SUSY events in the 8 TeV data. We suggest novel searches for these signatures of RPV, which would also improve the search for general new physics at the LHC.
    Physical Review D 03/2014; 90(7). DOI:10.1103/PhysRevD.90.075005 · 4.86 Impact Factor
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    Kurt Barry · Peter W. Graham · Surjeet Rajendran
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    ABSTRACT: The LHC has placed stringent limits on superpartner masses, in conflict with naturalness. R-parity violation is one of the few scenarios that allows for the reduction of these limits and is thus worth significant exploration at the LHC. We demonstrate that if the R-parity-violating operator UDD is used, we generically expect all supersymmetric events at the LHC to have displaced vertices. If a squark is the lightest supersymmetric particle (LSP), it will have a short displaced vertex. If any other particle is the LSP, the displaced vertex is naturally expected to be quite long, possibly even outside the detectors. These scenarios are already constrained by existing searches for missing energy. This arises because this operator efficiently washes out the baryon asymmetry in the early Universe, unless the squarks are heavy and the coupling is small. Avoiding displaced vertices is possible, but requires baryogenesis below the weak scale. Thus, for example, the use of sphalerons in baryogenesis does not avoid the requirement of displaced vertices. This motivates searching for hadronic displaced vertices at the LHC with decay lengths anywhere from tens of microns to meters.
    Physical Review D 02/2014; 89(5). DOI:10.1103/PhysRevD.89.054003 · 4.86 Impact Factor
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    Peter W. Graham · Surjeet Rajendran
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    ABSTRACT: We propose new signals for the direct detection of ultralight dark matter such as the axion. Axion or axion like particle (ALP) dark matter may be thought of as a background, classical field. We consider couplings for this field which give rise to observable effects including a nuclear electric dipole moment, and axial nucleon and electron moments. These moments oscillate rapidly with frequencies accessible in the laboratory, ~ kHz to GHz, given by the dark matter mass. Thus, in contrast to WIMP detection, instead of searching for the hard scattering of a single dark matter particle, we are searching for the coherent effects of the entire classical dark matter field. We calculate current bounds on such time varying moments and consider a technique utilizing NMR methods to search for the induced spin precession. The parameter space probed by these techniques is well beyond current astrophysical limits and significantly extends laboratory probes. Spin precession is one way to search for these ultralight particles, but there may well be many new types of experiments that can search for dark matter using such time-varying moments.
    Physical review D: Particles and fields 06/2013; 88(3). DOI:10.1103/PhysRevD.88.035023 · 4.86 Impact Factor
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    ABSTRACT: We propose an experiment to search for QCD axion and axion-like-particle (ALP) dark matter. Nuclei that are interacting with the background axion dark matter acquire time-varying CP-odd nuclear moments such as an electric dipole moment. In analogy with nuclear magnetic resonance, these moments cause precession of nuclear spins in a material sample in the presence of a background electric field. This precession can be detected through high-precision magnetometry. With current techniques, this experiment has sensitivity to axion masses m_a <~ 10^(-9) eV, corresponding to theoretically well-motivated axion decay constants f_a >~ 10^16 GeV. With improved magnetometry, this experiment could ultimately cover the entire range of masses m_a <~ 10^(-6) eV, just beyond the region accessible to current axion searches.
    Physical Review X 06/2013; 4(2). DOI:10.1103/PhysRevX.4.021030 · 8.39 Impact Factor
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    ABSTRACT: Laser frequency noise is a dominant noise background for the detection of gravitational waves using long-baseline optical interferometry. Amelioration of this noise requires near simultaneous strain measurements on more than one interferometer baseline, necessitating, for example, more than two satellites for a space-based detector or two interferometer arms for a ground-based detector. We describe a new detection strategy based on recent advances in optical atomic clocks and atom interferometry which can operate at long baselines and which is immune to laser frequency noise. Laser frequency noise is suppressed because the signal arises strictly from the light propagation time between two ensembles of atoms. This new class of sensor allows sensitive gravitational wave detection with only a single baseline. This approach also has practical applications in, for example, the development of ultrasensitive gravimeters and gravity gradiometers.
    Physical Review Letters 04/2013; 110(17):171102. DOI:10.1103/PhysRevLett.110.171102 · 7.51 Impact Factor
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    ABSTRACT: Laser frequency noise is a dominant noise background for the detection of gravitational waves using long-baseline optical interferometry. Amelioration of this noise requires near simultaneous strain measurements on more than one interferometer baseline, necessitating, for example, more than two satellites for a space-based detector, or two interferometer arms for a ground-based detector. We describe a new detection strategy based on recent advances in optical atomic clocks and atom interferometry which can operate at long-baselines and which is immune to laser frequency noise [1]. Laser frequency noise is suppressed because the signal arises strictly from the light propagation time between two ensembles of atoms. This new class of sensor allows sensitive gravitational wave detection with only a single baseline. This approach also has practical applications in, for example, the development of ultra-sensitive gravimeters and gravity gradiometers. We show that a space-based detector based on such principles can operate at LISA sensitivity levels and below with just a single measurement arm. We will present recent data from our 10 m ground-based prototype instrument which supports the preliminary instrument design concept. [4pt] [1] P. Graham, et. al., arXiv:1206.0818.
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    ABSTRACT: The apparent absence of light superpartners at the LHC strongly constrains the viability of the MSSM as a solution to the hierarchy problem. These constraints can be significantly alleviated by R-parity violation (RPV). Bilinear R-parity violation, with the single operator L H_u, does not require any special flavor structure and can be naturally embedded in a GUT while avoiding constraints from proton decay (unlike baryon-number-violating RPV). The LSP in this scenario can be naturally long-lived, giving rise to displaced vertices. Many collider searches, particularly those selecting b-jets or leptons, are insensitive to events with such detector-scale displaced decays owing to cuts on track quality and impact parameter. We demonstrate that for decay lengths in the window ~1-1000 mm, constraints on superpartner masses can be as low as ~450 GeV for squarks and ~40 GeV for LSPs. In some parts of parameter space light LSPs can dominate the Higgs decay width, hiding the Higgs from existing searches. This framework motivates collider searches for detector-scale displaced vertices. LHCb may be ideally suited to trigger on such events, while ATLAS and CMS may need to trigger on missing energy in the event.
    Journal of High Energy Physics 04/2012; 2012(7). DOI:10.1007/JHEP07(2012)149 · 6.22 Impact Factor
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    ABSTRACT: Dark matter with mass below about a GeV is essentially unobservable in conventional direct detection experiments. However, newly proposed technology will allow the detection of single electron events in semiconductor materials with significantly lowered thresholds. This would allow detection of dark matter as light as an MeV in mass. Compared to other detection technologies, semiconductors allow enhanced sensitivity because of their low ionization energy around an eV. Such detectors would be particularly sensitive to dark matter with electric and magnetic dipole moments, with a reach many orders of magnitude beyond current bounds. Observable dipole moment interactions can be generated by new particles with masses as great as 1000 TeV, providing a window to scales beyond the reach of current colliders.
    Physics of the Dark Universe 03/2012; 1(s 1–2). DOI:10.1016/j.dark.2012.09.001
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    Peter W. Graham · Kiel Howe · Surjeet Rajendran · Daniel Stolarski
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    ABSTRACT: Metastable particles are common in many models of new physics at the TeV scale. If charged or colored, a reasonable fraction of all such particles produced at the LHC will stop in the detectors and give observable out of time decays. We demonstrate that significant information may be learned from such decays about the properties (e.g. charge or spin) of this particle and of any other particles to which it decays, for example a dark matter candidate. We discuss strategies for measuring the type of decay (two- vs three-body), the types of particles produced, and the angular distribution of the produced particles using the LHC detectors. We demonstrate that with O(10-100) observed decay events, not only can the properties of the new particles be measured but indeed even the Lorentz structure of the decay operator can be distinguished in the case of three-body decays. These measurements can not only reveal the correct model of new physics at the TeV scale, but also give information on physics giving rise to the decay at energy scales far above those the LHC can probe directly.
    Physical review D: Particles and fields 11/2011; 86(3). DOI:10.1103/PhysRevD.86.034020 · 4.86 Impact Factor
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    ABSTRACT: We explore simple but novel bouncing solutions of general relativity that avoid singularities. These solutions require curvature k = +1, and are supported by a negative cosmological term and matter with −1 < w < −1/3. In the case of moderate bounces (where the ratio of the maximal scale factor a + to the minimal scale factor a − is \( \mathcal{O}(1) \)), the solutions are shown to be classically stable and cycle through an infinite set of bounces. For more extreme cases with large a + /a −, the solutions can still oscillate many times before classical instabilities take them out of the regime of validity of our approximations. In this regime, quantum particle production also leads eventually to a departure from the realm of validity of semiclassical general relativity, likely yielding a singular crunch. We briefly discuss possible applications of these models to realistic cosmology.
    Journal of High Energy Physics 09/2011; 2014(2). DOI:10.2172/1032771 · 6.22 Impact Factor
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    ABSTRACT: Contrary to the claims of P. Bender [preceding Comment, Phys. Rev. D 84, 028101 (2011).PRVDAQ1550-7998], we show that the effects discussed therein do not impose significant constraints on the design of atomic interferometer gravity wave detectors. These higher-order effects are subordinate to backgrounds discussed in S. Dimopoulos, et al. [Phys. Rev. DPRVDAQ1550-7998 78, 122002 (2008).10.1103/PhysRevD.78.122002], and can be mitigated with straightforward design approaches.
    Physical review D: Particles and fields 07/2011; 84(2). DOI:10.1103/PhysRevD.84.028102 · 4.86 Impact Factor