arXiv subjects

Dimitri V. Nanopoulos

Publications and source records attributed to Dimitri V. Nanopoulos.

At least 55 records · Page 3Linked to original sources

arXiv2017

Probing the No-Scale ${\cal F}$-$SU(5)$ One-Parameter Model via Gluino Searches at the LHC2

In our recent paper entitled "The return of the King: No-Scale ${\cal F}$-$SU(5)$", we showed that the model space supporting the most favorable phenomenology should have been probed in 2016 at the LHC2, with an even further reach into this region of the model in 2017-18. This ideal realm of the one-parameter version of No-Scale ${\cal F}$-$SU(5)$ yields a 1.9-2.3 TeV gluino mass at the very same point where the light Higgs boson mass enters its rather narrow experimentally determined range of $m_h = 125.09 \pm 0.24$ GeV. Given the recent results reported at Moriond 2017 for 36 ${\rm fb}^{-1}$ of luminosity collected in 2016 at the 13 TeV LHC2, we now update the status of the No-Scale ${\cal F}$-$SU(5)$ model space in light of the gluino mass exclusion limits presented by the ATLAS and CMS Collaborations. We illustrate that a resolution could be reached soon as to whether supersymmetry lives in this most critical region of the model space.

hep-ph↗

arXiv2017

Starobinsky-like Inflation, Supercosmology and Neutrino Masses in No-Scale Flipped SU(5)

We embed a flipped ${\rm SU}(5) \times {\rm U}(1)$ GUT model in a no-scale supergravity framework, and discuss its predictions for cosmic microwave background observables, which are similar to those of the Starobinsky model of inflation. Measurements of the tilt in the spectrum of scalar perturbations in the cosmic microwave background, $n_s$, constrain significantly the model parameters. We also discuss the model's predictions for neutrino masses, and pay particular attention to the behaviours of scalar fields during and after inflation, reheating and the GUT phase transition. We argue in favor of strong reheating in order to avoid excessive entropy production which could dilute the generated baryon asymmetry.

hep-ph↗

arXiv2017

A Novel Foamy Origin for Singlet Fermion Masses

We show how masses for singlet fermions can be generated by interactions with a D-particle model of space-time foam inspired by brane theory. It has been shown previously by one of the authors (N.E.M.) such interactions may generate generate dynamically small masses for charged fermions via the recoils of D-particle defects interacting with photons. In this work we consider the direct interactions of D-particle with uncharged singlet fermions such as right-handed neutrinos. Quantum fluctuations of the lattice of D-particles have massless vector (spin-one) excitations that are analogues of phonons. These mediate forces between the singlet fermions, generating large dynamical masses that may be communicated to light neutrinos via the seesaw mechanism.

gr-qc↗

arXiv2017

General No-Scale Supergravity: An ${\cal F}$-$SU(5)$ Tale

We study the grand unification model flipped $SU(5)$ with additional vector-like particle multiplets, or ${\cal F}$-$SU(5)$ for short, in the framework of General No-Scale Supergravity. In our analysis we allow the supersymmetry (SUSY) breaking soft terms to be generically non-zero, thereby extending the phenomenologically viable parameter space beyond the highly constrained one-parameter version of ${\cal F}$-$SU(5)$. In this initial inquiry, the mSUGRA/CMSSM SUSY breaking terms are implemented. We find this easing away from the vanishing SUSY breaking terms enables a more broad mass range of vector-like particles, dubbed flippons, including flippons less than 1 TeV that could presently be observed at the LHC2, as well as a lighter gluino mass and SUSY spectrum overall. This presents heightened odds that the General No-Scale ${\cal F}$-$SU(5)$ viable parameter space can be probed at the LHC2. The phenomenology comprises both bino and higgsino dark matter, including a Higgs funnel region. Particle states emerging from the SUSY cascade decays are presented to experimentally distinguish amongst the diverse phenomenological regions.

hep-ph↗

arXiv2017

No-Scale SU(5) Super-GUTs

We reconsider the minimal SU(5) Grand Unified Theory (GUT) in the context of no-scale supergravity, assuming that the soft supersymmetry-breaking parameters satisfy universality conditions at some input scale M_in above the GUT scale M_GUT. When setting up such a no-scale super-GUT model, special attention must be paid to avoiding the Scylla of rapid proton decay and the Charybdis of an excessive density of cold dark matter, while also having an acceptable mass for the Higgs boson. We do not find consistent solutions if none of the matter and Higgs fields are assigned to twisted chiral supermultiplets, even in the presence of Giudice-Masiero terms. However, consistent solutions may be found if at least one fiveplet of GUT Higgs fields is assigned to a twisted chiral supermultiplet, with a suitable choice of modular weights. Spin-independent dark matter scattering may be detectable in some of these consistent solutions.

hep-ph↗

arXiv2016

The Return of the King: No-Scale ${\cal F}$-$SU(5)$

We revisit the viable parameter space in No-Scale ${\cal F}$-$SU(5)$, examining the Grand Unified Theory within the context of the prevailing gluino mass limits established by the LHC. The satisfaction of both the No-Scale boundary condition and the experimentally measured Standard Model (SM) like Higgs boson mass requires a lower limit on the gluino mass in the model space of about 1.9 TeV, which maybe not coincidentally is the current LHC supersymmetry search bound. This offers a plausible explanation as to why a supersymmetry signal has thus far not been observed at the LHC. On the contrary, since the vector-like flippon particles are relatively heavy due to the strict condition that the supersymmetry breaking soft term $B_μ$ must vanish at the unification scale, we also cannot address the recently vanished 750 GeV diphoton resonance at the 13 TeV LHC. Therefore, No-Scale ${\cal F}$-$SU(5)$ returns as a King after the spurious 750 GeV diphoton excess was gone with the wind.

hep-ph↗

arXiv2016

Starobinsky-Like Inflation and Neutrino Masses in a No-Scale SO(10) Model

Using a no-scale supergravity framework, we construct an SO(10) model that makes predictions for cosmic microwave background observables similar to those of the Starobinsky model of inflation, and incorporates a double-seesaw model for neutrino masses consistent with oscillation experiments and late-time cosmology. We pay particular attention to the behaviour of the scalar fields during inflation and the subsequent reheating.

hep-ph↗

arXiv2016

A Flippon Related Singlet at the LHC II

We consider the 750 GeV diphoton resonance at the 13 TeV LHC in the ${\cal F}$-$SU(5)$ model with a Standard Model (SM) singlet field which couples to TeV-scale vector-like particles, dubbed $flippons$. This singlet field assumes the role of the 750 GeV resonance, with production via gluon fusion and subsequent decay to a diphoton via the vector-like particle loops. We present a numerical analysis showing that the observed 8 TeV and 13 TeV diphoton production cross-sections can be generated in the model space with realistic electric charges and Yukawa couplings for light vector-like masses. We further discuss the experimental viability of light vector-like masses in a General No-Scale ${\cal F}$-$SU(5)$ model, offering a few benchmark scenarios in this consistent GUT that can satisfy all experimental constraints imposed by the LHC and other essential experiments.

hep-ph↗

arXiv2016

The $750$ GeV Diphoton Excesses in a Realistic D-brane Model

We study the diphoton excesses near $750$ GeV recently reported by the ATLAS and CMS collaborations within the context of a phenomenologically interesting intersecting/magnetized D-brane model on a toroidal orientifold. It is shown that the model contains a SM singlet scalar as well as vector-like quarks and leptons. In addition, it is shown that the singlet scalar has Yukawa couplings with vector-like quarks and leptons such that it may be produced in proton-proton collisions via gluon fusion as well as decay to diphotons through loops involving the vector-like quarks. Moreover, the required vector-like quarks and leptons may appear in complete $SU(5)$ multiplets so that gauge coupling unification may be maintained. Finally, it is shown that the diphoton signal may be explained within the model.

hep-ph↗

arXiv2016

Comments on Graviton Propagation in Light of GW150914

The observation of gravitational waves from the Laser Interferometer Gravitational-Wave Observatory (LIGO) event GW150914 may be used to constrain the possibility of Lorentz violation in graviton propagation, and the observation by the Fermi Gamma-Ray Burst Monitor of a transient source in apparent coincidence may be used to constrain the difference between the velocities of light and gravitational waves: $c_g - c_γ< 10^{-17}$.

gr-qc↗

arXiv2016

$W_\infty$ Algebras, Hawking Radiation and Information Retention by Stringy Black Holes

We have argued previously, based on the analysis of two-dimensional stringy black holes, that information in stringy versions of four-dimensional Schwarzschild black holes (whose singular regions are represented by appropriate Wess-Zumino-Witten models) is retained by quantum $W$-symmetries when the horizon area is not preserved due to Hawking radiation. It is key that the exactly-marginal conformal world-sheet operator representing a massless stringy particle interacting with the black hole requires a contribution from $W_\infty$ generators in its vertex function. The latter correspond to delocalised, non-propagating, string excitations that guarantee the transfer of information between the string black hole and external particles. When infalling matter crosses the horizon, these topological states are excited via a process: (Stringy black hole) + infalling matter $\rightarrow $ (Stringy black hole)$^\star$, where the black hole is viewed as a stringy state with a specific configuration of $W_\infty$ charges that are conserved. Hawking radiation is then the reverse process, with conservation of the $W_\infty$ charges retaining information. The Hawking radiation spectrum near the horizon of a Schwarzschild or Kerr black hole is specified by matrix elements of higher-order currents that form a phase-space $W_{1+\infty}$ algebra. We show that an appropriate gauging of this algebra preserves the horizon two-dimensional area classically, as expected because the latter is a conserved Noether charge.

hep-th↗

arXiv2016

The Diboson Excesses in Leptophobic $U(1)_{\rm LP}$ Models from String Theories

The ATLAS Collaboration has reported excesses in the search for resonant diboson production with decay modes to hadronic final states at a diboson invariant mass around 2 TeV in boosted jets from $WZ$, $W^+W^-$, and $ZZ$ channels. Given potential contamination, we investigate the anomalies in leptophobic $U(1)_{\rm LP}$ models. We show that leptophobic models can be constructed in flipped $SU(5)\times U(1)_X$ models from free fermionic string constructions and Pati-Salam models from D-brane constructions. Additionally, we perform a collider phenomenological analysis to study production cross sections for $pp \to Z' \to jj/t \bar{t}/WW/Zh$ and discover the excess can be interpreted in both the leptophobic flipped $SU(5)\times U(1)_X$ models and intersecting D-branes.

hep-ph↗

arXiv2015

Post-Inflationary Gravitino Production Revisited

We revisit gravitino production following inflation. As a first step, we review the standard calculation of gravitino production in the thermal plasma formed at the end of post-inflationary reheating when the inflaton has completely decayed. Next we consider gravitino production prior to the completion of reheating, assuming that the inflaton decay products thermalize instantaneously while they are still dilute. We then argue that instantaneous thermalization is in general a good approximation, and also show that the contribution of non-thermal gravitino production via the collisions of inflaton decay products prior to thermalization is relatively small. Our final estimate of the gravitino-to-entropy ratio is approximated well by a standard calculation of gravitino production in the post-inflationary thermal plasma assuming total instantaneous decay and thermalization at a time $t \simeq 1.2/Γ_ϕ$. Finally, in light of our calculations, we consider potential implications of upper limits on the gravitino abundance for models of inflation, with particular attention to scenarios for inflaton decays in supersymmetric Starobinsky-like models.

astro-ph.CO↗

arXiv2015

Helical Phase Inflation via Non-Geometric Flux Compactifications: from Natural to Starobinsky-like Inflation

We show that a new class of helical phase inflation models can be simply realized in minimal supergravity, wherein the inflaton is the phase component of a complex field and its potential admits a deformed helicoid structure. We find a new unique complex-valued index $χ$ that characterizes almost the entire region of the $n_s-r$ plane favored by new Planck observations. Continuously varying the index $χ$, predictions interpolate from quadratic/natural inflation parameterized by a phase/axion decay constant to Starobinsky-like inflation parameterized by the $α$-parameter. We demonstrate that the simple supergravity construction realizing Starobinsky-like inflation can be obtained from a more microscopic model by integrating out heavy fields, and that the flat phase direction for slow-roll inflation is protected by a mildly broken global $U(1)$ symmetry. %, which is mildly broken at the inflation energy scale. We study the geometrical origin of the index $χ$, and find that it corresponds to a linear constraint relating \kah moduli. We argue that such a linear constraint is a natural result of moduli stabilization in Type \MyRoman{2} orientifold compactifications on Calabi-Yau threefolds with geometric and non-geometric fluxes. Possible choices for the index $χ$ are discrete points on the complex plane that relate to the distribution of supersymmetric Minkowski vacua on moduli space. More precise observations of the inflationary epoch in the future may provide a better estimation of the index $χ$. Since $χ$ is determined by the fluxes and vacuum expectation values of complex structure moduli, such observations would characterize the geometry of the internal space as well.

hep-th↗

arXiv2015

The 14 TeV LHC Takes Aim at SUSY: A No-Scale Supergravity Model for LHC Run 2

The Supergravity model named No-Scale ${\cal F}$-$SU(5)$, which is based upon the flipped $SU$(5) Grand Unified Theory (GUT) with additional TeV-scale vector-like flippon multiplets, has been partially probed during the LHC Run 1 at 7-8 TeV, though the majority of its model space remains viable and should be accessible by the 13-14 TeV LHC during Run 2. The model framework possesses the rather unique capacity to provide a light CP-even Higgs boson mass in the favored 124-126 GeV window while simultaneously retaining a testably light supersymmetry (SUSY) spectrum. We summarize the outlook for No-Scale ${\cal F}$-$SU(5)$ at the 13-14 TeV LHC and review a promising methodology for the discrimination of its long-chain cascade decay signature. We further show that proportional dependence of all model scales upon the unified gaugino mass $M_{1/2}$ minimizes electroweak fine-tuning, allowing the $Z$-boson mass $M_Z$ to be expressed as an explicit function of $M_{1/2}$, $M_Z^2 = M_Z^2 (M_{1/2}^2)$, with implicit dependence upon a dimensionless ratio $c$ of the supersymmetric Higgs mixing parameter $μ$ and $M_{1/2}$. Finally, we elucidate an empirical connection between recent scalar tensor measurements and No-Scale Supergravity cosmological models that mimic the Starobinsky model of inflation.

hep-ph↗

arXiv2015

No-Scale Inflation

Supersymmetry is the most natural framework for physics above the TeV scale, and the corresponding framework for early-Universe cosmology, including inflation, is supergravity. No-scale supergravity emerges from generic string compactifications and yields a non-negative potential, and is therefore a plausible framework for constructing models of inflation. No-scale inflation yields naturally predictions similar to those of the Starobinsky model based on $R + R^2$ gravity, with a tilted spectrum of scalar perturbations: $n_s \sim 0.96$, and small values of the tensor-to-scalar perturbation ratio $r < 0.1$, as favoured by Planck and other data on the cosmic microwave background (CMB). Detailed measurements of the CMB may provide insights into the embedding of inflation within string theory as well as its links to collider physics.

hep-ph↗

arXiv2015

Calculations of Inflaton Decays and Reheating: with Applications to No-Scale Inflation Models

We discuss inflaton decays and reheating in no-scale Starobinsky-like models of inflation, calculating the effective equation-of-state parameter, $w$, during the epoch of inflaton decay, the reheating temperature, $T_{\rm reh}$, and the number of inflationary e-folds, $N_*$, comparing analytical approximations with numerical calculations. We then illustrate these results with applications to models based on no-scale supergravity and motivated by generic string compactifications, including scenarios where the inflaton is identified as an untwisted-sector matter field with direct Yukawa couplings to MSSM fields, and where the inflaton decays via gravitational-strength interactions. Finally, we use our results to discuss the constraints on these models imposed by present measurements of the scalar spectral index $n_s$ and the tensor-to-scalar perturbation ratio $r$, converting them into constraints on $N_*$, the inflaton decay rate and other parameters of specific no-scale inflationary models.

hep-ph↗

arXiv2015

An Updated Historical Profile of the Higgs Boson

The Higgs boson was postulated in 1964, and phenomenological studies of its possible production and decays started in the early 1970s, followed by studies of its possible production in electron-positron, antiproton-proton and proton-proton collisions, in particular. Until recently, the most sensitive searches for the Higgs boson were at LEP between 1989 and 2000, which were complemented by searches at the Fermilab Tevatron. Then the LHC experiments ATLAS and CMS entered the hunt, announcing on July 4, 2012 the discovery of a "Higgs-like" particle with a mass of about 125~GeV. This identification has been supported by subsequent measurements of its spin, parity and coupling properties. It was widely anticipated that the Higgs boson would be accompanied by supersymmetry, although other options, like compositeness, were not completely excluded. So far there are no signs any new physics, and the measured properties of the Higgs boson are consistent with the predictions of the minimal Standard Model. This article reviews some of the key historical developments in Higgs physics over the past half-century.

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