arXiv subjects

Yasunori Nomura

Publications and source records attributed to Yasunori Nomura.

At least 73 records · Page 4Linked to original sources

arXiv2012

Complementarity Endures: No Firewall for an Infalling Observer

We argue that the complementarity picture, as interpreted as a reference frame change represented in quantum gravitational Hilbert space, does not suffer from the "firewall paradox" recently discussed by Almheiri, Marolf, Polchinski, and Sully. A quantum state described by a distant observer evolves unitarily, with the evolution law well approximated by semi-classical field equations in the region away from the (stretched) horizon. And yet, a classical infalling observer does not see a violation of the equivalence principle, and thus a firewall, at the horizon. The resolution of the paradox lies in careful considerations on how a (semi-)classical world arises in unitary quantum mechanics describing the whole universe/multiverse.

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arXiv2012

Spread Supersymmetry with Wino LSP: Gluino and Dark Matter Signals

The discovery of a Higgs boson near 125 GeV, together with the absence of LHC signals for supersymmetry or direct detection signals of dark matter, motivate further study of a particular theory of split supersymmetry. In arguably the theoretically simplest implementation of split, the superpartner spectrum is spread over several decades. The squarks and sleptons are heavier than the gravitino and Higgsinos by a factor M_Pl/M_*, where M_* is the mediation scale of supersymmetry breaking and is high, between unified and Planck scales. On the other hand the gaugino masses are 1-loop smaller than the gravitino and Higgsino masses, arising from both anomaly mediation and a Higgsino loop. Wino dark matter arises from three sources: gravitino production by scattering at high temperatures, gravitino production from squark decays, and thermal freeze-out. For reheating temperatures larger than the squark mass, these conspire to require that the squarks are lighter than about 10^4 TeV, while collider limits on gaugino masses require squarks to be heavier than about 100 TeV. Whether winos constitute all or just a fraction of the dark matter, a large fraction of the allowed parameter space has the gluino within reach of the LHC with 0.1 mm < cτ_{g} < 10 cm, leading to displaced vertices. In addition, events with cascades via W^+- lead to disappearing charged tracks with cτ_{W^+-} \sim 10 cm. The squarks and sleptons are predicted to be just heavy enough to solve the supersymmetric flavor and CP problems. Thus gluino decay modes may typically violate flavor and involve heavy quarks: [\bar{t}(t,c,u)+\bar{b}(b,s,d)] W^0 and [\bar{t}(b,s,d)+(\bar{t},\bar{c},\bar{u})b] W^+-. The electron electric dipole moment is expected to be of order 10^-29 ecm, two orders of magnitude below the current limit. The AMS-02 search for cosmic ray antiprotons will probe an interesting region of parameter space.

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arXiv2012

Quantum Mechanics, Gravity, and the Multiverse

The discovery of accelerating expansion of the universe has led us to take the dramatic view that our universe may be one of the many universes in which low energy physical laws take different forms: the multiverse. I explain why/how this view is supported both observationally and theoretically, especially by string theory and eternal inflation. I then describe how quantum mechanics plays a crucial role in understanding the multiverse, even at the largest distance scales. The resulting picture leads to a revolutionary change of our view of spacetime and gravity, and completely unifies the paradigm of the eternally inflating multiverse with the many worlds interpretation of quantum mechanics. The picture also provides a solution to a long-standing problem in eternal inflation, called the measure problem, which I briefly describe.

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arXiv2012

What can the observation of nonzero curvature tell us?

The eternally inflating multiverse provides a consistent framework to understand coincidences and fine-tuning in the universe. As such, it provides the possibility of finding another coincidence: if the amount of slow-roll inflation was only slightly more than the anthropic threshold, then spatial curvature might be measurable. We study this issue in detail, particularly focusing on the question: "If future observations reveal nonzero curvature, what can we conclude?" We find that whether an observable signal arises or not depends crucially on three issues: the cosmic history just before the observable inflation, the measure adopted to define probabilities, and the nature of the correlation between the tunneling and slow-roll parts of the potential. We find that if future measurements find positive curvature at Ω_k < -10^-4, then the framework of the eternally inflating multiverse is excluded with high significance. If the measurements instead reveal negative curvature at Ω_k > 10^-4, then we can conclude (1) diffusive (new or chaotic) eternal inflation did not occur in our immediate past; (2) our universe was born by a bubble nucleation; (3) the probability measure does not reward volume increase; and (4) the origin of the observed slow-roll inflation is an accidental feature of the potential, not due to a theoretical mechanism. Discovery of Ω_k > 10^-4 would also give us nontrivial information about the correlation between tunneling and slow-roll; e.g. a strong correlation favoring large N would be excluded in certain measures. We also ask whether the current constraint on Ω_k is consistent with multiverse expectations, finding that the answer is yes, except for certain cases. In the course of this work we were led to consider vacuum decay branching ratios, and found that it is more likely than one might guess that the decays are dominated by a single channel.

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arXiv2012

Higgs Descendants

We define a Higgs descendant $χ$ to be a particle beyond the standard model whose mass arises predominantly from the vacuum expectation value of the Higgs boson. Higgs descendants arise naturally from new physics whose intrinsic mass scale is unrelated to the electroweak scale. The coupling of $χ$ to the Higgs boson is fixed by the mass and spin of $χ$, yielding a highly predictive setup in which there may be substantial modifications to the properties of the Higgs boson. For example, if the decay of the Higgs boson to $χ$ is kinematically allowed, then this branching ratio is largely determined. Depending on the stability of $χ$, Higgs decays may result in a variety of possible visible or invisible final states. Alternatively, loops of $χ$ may affect Higgs boson production or its decays to standard model particles. If $χ$ is stable dark matter, then the mandatory coupling between $χ$ and the Higgs boson gives a lower bound on the direct detection cross section as a function of the $χ$ mass. We also present a number of explicit models which are examples of Higgs descendants. Finally, we comment on Higgs descendants in the context of the excesses near 125 GeV recently observed at ATLAS and CMS.

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arXiv2012

The Static Quantum Multiverse

We consider the multiverse in the intrinsically quantum mechanical framework recently proposed in Refs. [1,2]. By requiring that the principles of quantum mechanics are universally valid and that physical predictions do not depend on the reference frame one chooses to describe the multiverse, we find that the multiverse state must be static---in particular, the multiverse does not have a beginning or end. We argue that, despite its naive appearance, this does not contradict observation, including the fact that we observe that time flows in a definite direction. Selecting the multiverse state is ultimately boiled down to finding normalizable solutions to certain zero-eigenvalue equations, analogous to the case of the hydrogen atom. Unambiguous physical predictions would then follow, according to the rules of quantum mechanics.

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arXiv2012

Quantum Mechanics, Spacetime Locality, and Gravity

Quantum mechanics introduces the concept of probability at the fundamental level, yielding the measurement problem. On the other hand, recent progress in cosmology has led to the "multiverse" picture, in which our observed universe is only one of the many, bringing an apparent arbitrariness in defining probabilities, called the measure problem. In this paper, we discuss how these two problems are intimately related with each other, developing a complete picture for quantum measurement and cosmological histories in the quantum mechanical universe. On one hand, quantum mechanics eliminates the arbitrariness of defining probabilities in the multiverse, as discussed in arXiv:1104.2324. On the other hand, the multiverse allows for understanding why we observe an ordered world obeying consistent laws of physics, by providing an infinite-dimensional Hilbert space. This results in the irreversibility of quantum measurement, despite the fact that the evolution of the multiverse state is unitary. In order to describe the cosmological dynamics correctly, we need to identify the structure of the Hilbert space for a system with gravity. We argue that in order to keep spacetime locality, the Hilbert space for dynamical spacetime must be defined only in restricted spacetime regions: in and on the (stretched) apparent horizon as viewed from a fixed reference frame. This requirement arises from eliminating all the redundancies and overcountings in a general relativistic, global spacetime description of nature. It is responsible for horizon complementarity as well as the "observer dependence" of horizons/spacetime---these phenomena arise to represent changes of the reference frame in the relevant Hilbert space. This can be viewed as an extension of the Poincare transformation in the quantum gravitational context, as the Lorentz transformation is viewed as an extension of the Galilean transformation.

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arXiv2012

Supersymmetry with Light Stops

Recent LHC data, together with the electroweak naturalness argument, suggest that the top squarks may be significantly lighter than the other sfermions. We present supersymmetric models in which such a split spectrum is obtained through "geometries": being "close to" electroweak symmetry breaking implies being "away from" supersymmetry breaking, and vice versa. In particular, we present models in 5D warped spacetime, in which supersymmetry breaking and Higgs fields are located on the ultraviolet and infrared branes, respectively, and the top multiplets are localized to the infrared brane. The hierarchy of the Yukawa matrices can be obtained while keeping near flavor degeneracy between the first two generation sfermions, avoiding stringent constraints from flavor and CP violation. Through the AdS/CFT correspondence, the models can be interpreted as purely 4D theories in which the top and Higgs multiplets are composites of some strongly interacting sector exhibiting nontrivial dynamics at a low energy. Because of the compositeness of the Higgs and top multiplets, Landau pole constraints for the Higgs and top couplings apply only up to the dynamical scale, allowing for a relatively heavy Higgs boson, including m_h = 125 GeV as suggested by the recent LHC data. We analyze electroweak symmetry breaking for a well-motivated subset of these models, and find that fine-tuning in electroweak symmetry breaking is indeed ameliorated. We also discuss a flat space realization of the scenario in which supersymmetry is broken by boundary conditions, with the top multiplets localized to a brane while other matter multiplets delocalized in the bulk.

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arXiv2011

The Cosmological Constant in the Quantum Multiverse

Recently, a new framework for describing the multiverse has been proposed which is based on the principles of quantum mechanics. The framework allows for well-defined predictions, both regarding global properties of the universe and outcomes of particular experiments, according to a single probability formula. This provides complete unification of the eternally inflating multiverse and many worlds in quantum mechanics. In this paper we elucidate how cosmological parameters can be calculated in this framework, and study the probability distribution for the value of the cosmological constant. We consider both positive and negative values, and find that the observed value is consistent with the calculated distribution at an order of magnitude level. In particular, in contrast to the case of earlier measure proposals, our framework prefers a positive cosmological constant over a negative one. These results depend only moderately on how we model galaxy formation and life evolution therein.

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arXiv2011

Spread Supersymmetry

In the multiverse the scale of SUSY breaking, \tilde{m} = F_X/M_*, may scan and environmental constraints on the dark matter density may exclude a large range of \tilde{m} from the reheating temperature after inflation down to values that yield a LSP mass of order a TeV. After selection effects, the distribution for \tilde{m} may prefer larger values. A single environmental constraint from dark matter can then lead to multi-component dark matter, including both axions and the LSP, giving a TeV-scale LSP lighter than the corresponding value for single-component LSP dark matter. If SUSY breaking is mediated to the SM sector at order X^* X, only squarks, sleptons and one Higgs doublet acquire masses of order \tilde{m}. The gravitino mass is lighter by a factor of M_*/M_Pl and the gaugino masses are suppressed by a further loop factor. This Spread SUSY spectrum has two versions; the Higgsino masses are generated in one from supergravity giving a wino LSP and in the other radiatively giving a Higgsino LSP. The environmental restriction on dark matter fixes the LSP mass to the TeV domain, so that the squark and slepton masses are order 10^3 TeV and 10^6 TeV in these two schemes. We study the spectrum, dark matter and collider signals of these two versions of Spread SUSY. The Higgs is SM-like and lighter than 145 GeV; monochromatic photons in cosmic rays arise from dark matter annihilations in the halo; exotic short charged tracks occur at the LHC, at least for the wino LSP; and there are the eventual possibilities of direct detection of dark matter and detailed exploration of the TeV-scale states at a future linear collider. Gauge coupling unification is as in minimal SUSY theories. If SUSY breaking is mediated at order X, a much less hierarchical spectrum results---similar to that of the MSSM, but with the superpartner masses 1--2 orders of magnitude larger than in natural theories.

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arXiv2011

Physical Theories, Eternal Inflation, and Quantum Universe

We present a framework in which well-defined predictions are obtained in an eternally inflating multiverse, based on the principles of quantum mechanics. We show that the entire multiverse is described purely from the viewpoint of a single "observer," who describes the world as a quantum state defined on his/her past light cones bounded by the (stretched) apparent horizons. We find that quantum mechanics plays an essential role in regulating infinities. The framework is "gauge invariant," i.e. predictions do not depend on how spacetime is parametrized, as it should be in a theory of quantum gravity. Our framework provides a fully unified treatment of quantum measurement processes and the multiverse. We conclude that the eternally inflating multiverse and many worlds in quantum mechanics are the same. Other important implications include: global spacetime can be viewed as a derived concept; the multiverse is a transient phenomenon during the world relaxing into a supersymmetric Minkowski state. We also present a theory of "initial conditions" for the multiverse. By extrapolating our framework to the extreme, we arrive at a picture that the entire multiverse is a fluctuation in the stationary, fractal "mega-multiverse," in which an infinite sequence of multiverse productions occurs. The framework discussed here does not suffer from problems/paradoxes plaguing other measures proposed earlier, such as the youngness paradox, the Boltzmann brain problem, and a peculiar "end" of time.

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arXiv2010

Singlet Portal to the Hidden Sector

Ultraviolet physics typically induces a kinetic mixing between gauge singlets which is marginal and hence non-decoupling in the infrared. In singlet extensions of the minimal supersymmetric standard model, e.g. the next-to-minimal supersymmetric standard model, this furnishes a well motivated and distinctive portal connecting the visible sector to any hidden sector which contains a singlet chiral superfield. In the presence of singlet kinetic mixing, the hidden sector automatically acquires a light mass scale in the range 0.1 - 100 GeV induced by electroweak symmetry breaking. In theories with R-parity conservation, superparticles produced at the LHC invariably cascade decay into hidden sector particles. Since the hidden sector singlet couples to the visible sector via the Higgs sector, these cascades necessarily produce a Higgs boson in an order 0.01 - 1 fraction of events. Furthermore, supersymmetric cascades typically produce highly boosted, low-mass hidden sector singlets decaying visibly, albeit with displacement, into the heaviest standard model particles which are kinematically accessible. We study experimental constraints on this broad class of theories, as well as the role of singlet kinetic mixing in direct detection of hidden sector dark matter. We also present related theories in which a hidden sector singlet interacts with the visible sector through kinetic mixing with right-handed neutrinos.

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arXiv2010

A Definitive Signal of Multiple Supersymmetry Breaking

If the lightest observable-sector supersymmetric particle (LOSP) is charged and long-lived, then it may be possible to indirectly measure the Planck mass at the LHC and provide a spectacular confirmation of supergravity as a symmetry of nature. Unfortunately, this proposal is only feasible if the gravitino is heavy enough to be measured at colliders, and this condition is in direct conflict with constraints from big bang nucleosynthesis (BBN). In this work, we show that the BBN bound can be naturally evaded in the presence of multiple sectors which independently break supersymmetry, since there is a new decay channel of the LOSP to a goldstino. Certain regions of parameter space allow for a direct measurement of LOSP decays into both the goldstino and the gravitino at the LHC. If the goldstino/gravitino mass ratio is measured to be 2, as suggested by theory, then this would provide dramatic verification of the existence of multiple supersymmetry breaking and sequestering. A variety of consistent cosmological scenarios are obtained within this framework. In particular, if an R symmetry is imposed, then the gauge-gaugino-goldstino interaction vertices can be forbidden. In this case, there is no bound on the reheating temperature from goldstino overproduction, and thermal leptogenesis can be accommodated consistently with gravitino dark matter.

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arXiv2010

Goldstini

Supersymmetric phenomenology has been largely bound to the hypothesis that supersymmetry breaking originates from a single source. In this paper, we relax this underlying assumption and consider a multiplicity of sectors which independently break supersymmetry, thus yielding a corresponding multiplicity of goldstini. While one linear combination of goldstini is eaten via the super-Higgs mechanism, the orthogonal combinations remain in the spectrum as physical degrees of freedom. Interestingly, supergravity effects induce a universal tree-level mass for the goldstini which is exactly twice the gravitino mass. Since visible sector fields can couple dominantly to the goldstini rather than the gravitino, this framework allows for substantial departures from conventional supersymmetric phenomenology. In fact, this even occurs when a conventional mediation scheme is augmented by additional supersymmetry breaking sectors which are fully sequestered. We discuss a number of striking collider signatures, including various novel decay modes for the lightest observable-sector supersymmetric particle, gravitinoless gauge-mediated spectra, and events with multiple displaced vertices. We also describe goldstini cosmology and the possibility of goldstini dark matter.

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arXiv2009

Environmentally Selected WIMP Dark Matter with High-Scale Supersymmetry Breaking

We explore the possibility that both the weak scale and the thermal relic dark matter abundance are environmentally selected in a multiverse. An underlying supersymmetric theory containing the states of the MSSM and singlets, with supersymmetry and R symmetry broken at unified scales, has just two realistic low energy effective theories. One theory, (SM + \tilde{w}), is the Standard Model augmented only by the wino, having a mass near 3 TeV, and has a Higgs boson mass in the range of (127 - 142) GeV. The other theory, (SM + \tilde{h}/\tilde{s}), has Higgsinos and a singlino added to the Standard Model. The Higgs boson mass depends on the single new Yukawa coupling of the theory, y, and is near 141 GeV for small y but grows to be as large as 210 GeV as this new coupling approaches strong coupling at high energies. Much of the parameter space of this theory will be probed by direct detection searches for dark matter that push two orders of magnitude below the present bounds; furthermore, the dark matter mass and cross section on nucleons are correlated with the Higgs boson mass. The indirect detection signal of monochromatic photons from the galactic center is computed, and the range of parameters that may be accessible to LHC searches for trilepton events is explored. Taking a broader view, allowing the possibility of R symmetry protection to the TeV scale or axion dark matter, we find four more theories: (SM + axion), two versions of Split Supersymmetry, and the E-MSSM, where a little supersymmetric hierarchy is predicted. The special Higgs mass value of (141 \pm 2) GeV appears in symmetry limits of three of the six theories, (SM + axion), (SM + \tilde{w}) and (SM + \tilde{h}/\tilde{s}), motivating a comparison of other signals of these three theories.

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arXiv2009

A Finely-Predicted Higgs Boson Mass from A Finely-Tuned Weak Scale

If supersymmetry is broken directly to the Standard Model at energies not very far from the unified scale, the Higgs boson mass lies in the range 128-141 GeV. The end points of this range are tightly determined. Theories with the Higgs boson dominantly in a single supermultiplet predict a mass at the upper edge, (141 \pm 2) GeV, with the uncertainty dominated by the experimental errors on the top quark mass and the QCD coupling. This edge prediction is remarkably insensitive to the supersymmetry breaking scale and to supersymmetric threshold corrections so that, in a wide class of theories, the theoretical uncertainties are at the level of \pm 0.4 GeV. A reduction in the uncertainties from the top quark mass and QCD coupling to the level of \pm 0.3 GeV may be possible at future colliders, increasing the accuracy of the confrontation with theory from 1.4% to 0.4%. Verification of this prediction would provide strong evidence for supersymmetry, broken at a very high scale of ~ 10^{14 \pm 2} GeV, and also for a Higgs boson that is elementary up to this high scale, implying fine-tuning of the Higgs mass parameter by ~ 20-28 orders of magnitude. Currently, the only known explanation for such fine-tuning is the multiverse.

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arXiv2009

Multiverse Understanding of Cosmological Coincidences

There is a deep cosmological mystery: although dependent on very different underlying physics, the timescales of structure formation, of galaxy cooling (both radiatively and against the CMB), and of vacuum domination do not differ by many orders of magnitude, but are all comparable to the present age of the universe. By scanning four landscape parameters simultaneously, we show that this quadruple coincidence is resolved. We assume only that the statistical distribution of parameter values in the multiverse grows towards certain catastrophic boundaries we identify, across which there are drastic regime changes. We find order-of-magnitude predictions for the cosmological constant, the primordial density contrast, the temperature at matter-radiation equality, the typical galaxy mass, and the age of the universe, in terms of the fine structure constant and the electron, proton and Planck masses. Our approach permits a systematic evaluation of measure proposals; with the causal patch measure, we find no runaway of the primordial density contrast and the cosmological constant to large values.

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arXiv2009

Cosmic Signals from the Hidden Sector

Cosmologically long-lived, composite states arise as natural dark matter candidates in theories with a strongly interacting hidden sector at a scale of 10 - 100 TeV. Light axion-like states, with masses in the 1 MeV - 10 GeV range, are also generic, and can decay via Higgs couplings to light standard model particles. Such a scenario is well motivated in the context of very low energy supersymmetry breaking, where ubiquitous cosmological problems associated with the gravitino are avoided. We investigate the astrophysical and collider signatures of this scenario, assuming that dark matter decays into the axion-like states via dimension six operators, and we present an illustrative model exhibiting these features. We conclude that the recent data from PAMELA, FERMI, and H.E.S.S. points to this setup as a compelling paradigm for dark matter. This has important implications for future diffuse gamma ray measurements and collider searches.

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