arXiv subjects

Hooman Davoudiasl

Publications and source records attributed to Hooman Davoudiasl.

At least 109 records · Page 6Linked to original sources

arXiv2011

Testing the OPERA Superluminal Neutrino Anomaly at the LHC

The OPERA collaboration has reported the observation of superluminal muon neutrinos, whose speed $v_ν$ exceeds that of light $c$, with $(v_ν- c)/c \simeq 2.5 \times 10^{-5}$. In a recent work, Cohen and Glashow (CG) have refuted this claim by noting that such neutrinos will lose energy, by pair-emission of particles, at unacceptable rates. Following the CG arguments, we point out that pair-emissions consistent with the OPERA anomaly can lead to detectable signals from decays of highly boosted top quarks at the LHC, allowing an independent test of the superluminal neutrino hypothesis.

hep-ph↗

arXiv2011

Long-Range Lepton Flavor Interactions and Neutrino Oscillations

Recent results from the MINOS accelerator neutrino experiment suggest a possible difference between nu_mu and anti-nu_mu disappearance oscillation parameters, which one may ascribe to a new long-distance potential acting on neutrinos. As a specific example, we consider a model with gauged B - L_e - 2 L_tau number that contains an extremely light new vector boson m_Z' < 10^-18 eV and extraordinarily weak coupling alpha' < 10^-52. In that case, differences between nu_mu to nu_tau and anti-nu_mu to anti-nu_tau oscillations can result from a long-range potential due to neutrons in the Earth and the Sun that distinguishes nu_mu and nu_tau on Earth, with a potential difference of ~ 6*10^-14 eV, and changes sign for anti-neutrinos. We show that existing solar, reactor, accelerator, and atmospheric neutrino oscillation constraints can be largely accommodated for values of parameters that help explain the possible MINOS anomaly by this new physics, although there is some tension with atmospheric constraints. A long-range interaction, consistent with current bounds, could have very pronounced effects on atmospheric neutrino disappearance in the 20-50 GeV range that will be studied with the IceCube DeepCore array, currently in operation, and can have a significant effect on future high-precision long-baseline oscillation experiments that aim for +- 1% sensitivity, in nu_mu and anti-nu_mu disappearance, separately. Together, these experiments can extend the reach for new long-distance effects well beyond current bounds and test their relevance to the aforementioned MINOS anomaly. We also point out that long-range potentials originating from the Sun could lead to annual modulations of neutrino data at the percent level, due to the variation of the Earth-Sun distance. A similar phenomenology is shown to apply to other potential new gauge symmetries such as L - 3 L_tau and B - 3 L_tau.

hep-ph↗

arXiv2011

Baryon Destruction by Asymmetric Dark Matter

We investigate new and unusual signals that arise in theories where dark matter is asymmetric and carries a net antibaryon number, as may occur when the dark matter abundance is linked to the baryon abundance. Antibaryonic dark matter can cause {\it induced nucleon decay} by annihilating visible baryons through inelastic scattering. These processes lead to an effective nucleon lifetime of 10^{29}-10^{32} years in terrestrial nucleon decay experiments, if baryon number transfer between visible and dark sectors arises through new physics at the weak scale. The possibility of induced nucleon decay motivates a novel approach for direct detection of cosmic dark matter in nucleon decay experiments. Monojet searches (and related signatures) at hadron colliders also provide a complementary probe of weak-scale dark-matter--induced baryon number violation. Finally, we discuss the effects of baryon-destroying dark matter on stellar systems and show that it can be consistent with existing observations.

hep-ph↗

arXiv2010

Hylogenesis: A Unified Origin for Baryonic Visible Matter and Antibaryonic Dark Matter

We present a novel mechanism for generating both the baryon and dark matter densities of the Universe. A new Dirac fermion X carrying a conserved baryon number charge couples to the Standard Model quarks as well as a GeV-scale hidden sector. CP-violating decays of X, produced non-thermally in low-temperature reheating, sequester antibaryon number in the hidden sector, thereby leaving a baryon excess in the visible sector. The antibaryonic hidden states are stable dark matter. A spectacular signature of this mechanism is the baryon-destroying inelastic scattering of dark matter that can annihilate baryons at appreciable rates relevant for nucleon decay searches.

hep-ph↗

arXiv2010

On Taming the Warped Radion with Supersymmetry

In warped models that solve the hierarchy problem, there is generally no dynamical relation between the size of the fifth dimension and the scale of electroweak symmetry breaking (EWSB). The establishment of such a relation, without fine-tuning, requires that Casimir contributions to the radion potential not exceed the energy density associated with EWSB. Here, we examine the use of supersymmetry for controlling the Casimir energy density and making quantum contributions calculable. We compute the effects of supersymmetry breaking at the UV and IR boundaries of warped backgrounds, in the presence of brane localized kinetic terms. Various limits of supersymmetry breaking are examined. We find that when supersymmetry is broken on the UV brane, vacuum contributions to the radion potential can be controlled (as likely necessary for EWSB to govern the radion potential) via small soft masses as well as a "double volume suppression." Our formalism can also provide a setup for radion stabilization by bulk fields, when supersymmetry is broken on both the UV and the IR branes.

hep-ph↗

arXiv2010

The INTEGRAL/SPI 511 keV Signal from Hidden Valleys in Type Ia and Core Collapse Supernova Explosions

We examine under what circumstances the INTEGRAL/SPI 511 keV signal can originate from decays of MeV-scale composite states produced by: (A) thermonuclear (type Ia) or (B) core collapse supernovae (SNe). The requisite dynamical properties that would account for the observed data are quite distinct, for cases (A) and (B). We determine these requirements in simple hidden valley models, where the escape fraction problem is naturally addressed, due to the long lifetime of the new composite states. A novel feature of scenario (A) is that the dynamics of type Ia SNe, standard candles for cosmological measurements, might be affected by our mechanism. In case (A), the mass of the state mediating between the hidden sector and the SM $\ee$ could be a few hundred GeV and within the reach of a 500 GeV $\ee$ linear collider. We also note that kinetic mixing of the photon with a light vector state may provide an interesting alternate mediation mechanism in this case. Scenarios based on case (B) are challenged by the need for a mechanism to transport some of the produced positrons toward the Galactic bulge, due to the inferred distribution of core collapse sources. The mass of the mediator in case (B) is typically hundreds of TeV, leading to long-lived particles that could, under certain circumstances, include a viable dark matter candidate. The appearance of long-lived particles in typical models leads to cosmological constraints and we address how a consistent cosmic history may be achieved.

hep-ph↗

arXiv2009

Warped 5-Dimensional Models: Phenomenological Status and Experimental Prospects

Warped 5-dimensional models, based on the original Randall-Sundrum geometry, have been extended beyond their initial purpose of resolving the gauge hierarchy problem. Over the past decade, various ingredients have been added to their basic structure in order to provide natural and predictive models of flavor and also to address existing constraints from precision data. In this review, we examine the theoretical and experimental status of realistic models that accommodate current data, while addressing the hierarchy and flavor puzzles of the Standard Model. We also discuss the prospects for future discovery of the TeV-scale Kaluza-Klein states that are predicted to emerge in these models, and outline some of the challenges that the detection of such particles pose for experiments at the Large Hadron Collider.

hep-ph↗

arXiv2009

Signals of Warped Top-Condensation in B-Decays

A recently proposed warped top-condensation model generally predicts a light radion ϕ, with a mass of a few GeV or less, whose interactions are suppressed by a scale of order 100 TeV. In this talk, we present existing constraints and potential signals from the process b \to s ϕ, in the context of such a model. Astrophysical bounds, applicable to very light radions, are also briefly discussed.

hep-ph↗

arXiv2009

Echoes from a Warped Dimension

The Randall-Sundrum (RS) model, based on a slice of warped 5D spacetime, was originally introduced to explain the apparent hierarchy between the scales of weak and gravitational interactions. Over the past decade, this model has been extended to provide a predictive theory of flavor, as well as to address various constraints from precision data. In this talk, we will present a brief review of the RS model and some of its extensions. A survey of some key signatures of realistic warped models and the experimental challenges they pose will be presented. We will also consider truncated warped scenarios that address smaller hierarchies and discuss why their discovery can have improved prospects, at the LHC.

hep-ph↗

arXiv2009

Big Signals of Little Randall-Sundrum Models

We examine signals at the Large Hadron Collider (LHC) of Kaluza-Klein modes, in volume-truncated "Little Randall-Sundrum" (LRS) models of flavor, characterized by 5D cutoff scales M_5 that are small compared to the 4D Planck mass M_P ~ 10^{19} GeV. In particular, for the phenomenologically viable choice M_5 ~ 10^4 TeV, the discovery of a 2 (3)-TeV "Little" Z' at the LHC requires about 1 (4) 1/fb at \sqrt{s}=10 (14) TeV, in the clean di-lepton channel. Our results highlight the possibility of probing interesting values of M_5, starting with the early LHC data. With M_5 ~ 10^4 TeV, discovering the second KK mode Z'', at about 4.3 TeV, requires O(100) 1/fb at \sqrt{s}=14 TeV, providing a probe of the warped nature of the bulk that is encoded in the mass ratio of the first two KK modes, at design luminosity. By comparison, discovering a 3-TeV Z' of the Planck-weak hierarchy models (with M_5 ~ M_P), in any channel, would require upwards of O(300) 1/fb at \sqrt{s}=14 TeV. We also point out that discovery prospects markedly improve for Little KK gluons as well, but the challenging reconstruction of their t tbar decay products may not allow an appreciable advantage for early discovery, over the Little Z' case.

hep-ph↗

arXiv2009

Dark Matter with Time-Varying Leptophilic Couplings

Two general problems arise when interpreting the recent cosmic ray data as signals of Dark Matter (DM) annihilation: (i) the required cross section is too large by O(100), and (ii) the annihilation products seem to be mostly leptonic. We propose to address these two problems by assuming that the couplings of DM to leptons grow with time. This can be achieved by a dynamic localization of DM in extra dimensions. A possible outcome of this proposal is a time (red-shift) dependent annihilation signal, in terms of strength and dominant final states.

hep-ph↗

arXiv2009

Feasibility Study for Measuring Geomagnetic Conversion of Solar Axions to X-rays in Low Earth Orbits

We present a detailed computation of the expected rate for Geomagnetic Conversion of Solar Axions to X-rays (GECOSAX) along the orbit of an x-ray satellite. We use realistic satellite orbits and propagation in time. A realistic model for the Earth's magnetic field, which properly accounts for its spatial non-uniformity, is used. We also account for the effect of the Earth's atmosphere on the propagation of x-rays in our calculation of axion-photon conversion probability. To estimate possible sensitivities to the axion-photon coupling g_{aγ}, we use an actual measurement of the expected backgrounds by the SUZAKU satellite. Assuming a detector area of 10^3 cm^2 and about 10^6 s of data, we show that a 2 σlimit of g_{aγ} < (4.7-6.6) times 10^{-11} GeV^{-1} from GECOSAX is achievable, for axion masses m_a<10^{-4} eV. This significantly exceeds current laboratory sensitivities to g_{aγ}.

astro-ph↗

arXiv2009

The Little Randall-Sundrum Model at the LHC

We present a predictive warped model of flavor, cut off at an ultraviolet scale O(10^3) TeV, called the "Little Randall-Sundrum (LRS)" model. This model corresponds to a volume-truncation, by a factor y \approx 6, of the RS scenario and is holographically dual to dynamics with number of colors larger by y. With separate gauge and flavor dynamics, several unwanted contributions to precision electroweak, Z b \bar b, and flavor observables are suppressed in the LRS framework, compared with the corresponding RS case. The LRS truncation leads to a significant enhancement of the clean (golden) di-lepton LHC signals, by O(y^3).

hep-ph↗

arXiv2009

B-Decay Signatures of Warped Top-Condensation

We point out that the light radion phi in a recently proposed Warped Top-Condensation Model, can provide distinct signatures in b -> s phi, where the on-shell phi can decay with displaced vertices. We find that some of the parameter space of these models is constrained by B-meson and astrophysical data. Future B-decay measurements can lead to the discovery of the WTCM.

hep-ph↗

arXiv2009

Thermal Geo-axions

We estimate the production rate of axion-type particles in the core of the Earth, at a temperature T~5000K. We constrain thermal geo-axion emission by demanding a core-cooling rate less than 100K/Gyr, as suggested by geophysics. This yields a "quasi-vacuum" (unaffected by extreme stellar conditions) bound on the axion-electron fine structure constant α_a^{QV} < 10^{-18}, stronger than the existing accelerator (vacuum) bound by 4 orders of magnitude. We consider the prospects for measuring the geo-axion flux through conversion into photons in a geoscope; such measurements can further constrain α_a^{QV}.

hep-ph↗

arXiv2008

New Dimensions for Randall-Sundrum Phenomenology

We consider a 6D extension of the Randall-Sundrum (RS) model, RS6, where the Standard Model (SM) gauge fields are allowed to propagate in an additional dimension, compactified on $S^1$ or $S^1/Z_2$. In a minimal scenario, fermions propagate in the 5D RS subspace and their localization provides a model of flavor. New Kaluza-Klein (KK) states, corresponding to excitations of the gauge fields along the 6th dimension, appear near the TeV scale. The new gauge KK modes behave differently from those in the 5D warped models. These RS6 states have couplings with strong dependence on 5D field localization and, within the SM, only interact with heavy fermions and the Higgs sector, to a very good approximation. Thus, the collider phenomenology of the new gauge KK states sensitively depends on the 5D fermion geography. We briefly discuss inclusion of SM fermions in all 6 dimensions, as well as the possibility of going beyond 6D.

hep-ph↗

arXiv2008

The Little Randall-Sundrum Model at the Large Hadron Collider

We present a predictive warped model of flavor that is cut off at an ultraviolet scale O(10^3) TeV. This "Little Randall-Sundrum (LRS)" model is a volume-truncation, by a factor $y \approx 6$, of the RS scenario and is holographically dual to dynamics with number of colors larger by $y$. The LRS couplings between Kaluza-Klein states and the Standard Model fields, including the proton constituents, are explicitly calculable without ad hoc assumptions. Assuming separate gauge and flavor dynamics, a number of unwanted contributions to precision electroweak, $Z b\bar b$ and flavor observables are suppressed in the LRS framework, compared with the corresponding RS case. An important consequence of the LRS truncation, independent of precise details, is a significant enhancement of the clean (golden) di-lepton LHC signals, by O(y^3), due to a larger "$ρ$-photon" mixing and a smaller inter-composite coupling.

hep-ph↗

arXiv2007

On Direct Verification of Warped Hierarchy-and-Flavor Models

We consider direct experimental verification of warped models, based on the Randall-Sundrum (RS) scenario, that explain gauge and flavor hierarchies, assuming that the gauge fields and fermions of the Standard Model (SM) propagate in the 5D bulk. Most studies have focused on the bosonic Kaluza Klein (KK) signatures and indicate that discovering gauge KK modes is likely possible, yet challenging, while graviton KK modes are unlikely to be accessible at the LHC, even with a luminosity upgrade. We show that direct evidence for bulk SM fermions, {\it i.e.} their KK modes, is likely also beyond the reach of a luminosity-upgraded LHC. Thus, neither the spin-2 KK graviton, the most distinct RS signal, nor the KK SM fermions, direct evidence for bulk flavor, seem to be within the reach of the LHC. We then consider hadron colliders with $\sqrt{s} =$ 21, 28, and 60 TeV. We find that discovering the first KK modes of SM fermions and the graviton typically requires the Next Hadron Collider (NHC) with $\sqrt{s} \approx 60$ TeV and ${\cal O}(1)$ ab$^{-1}$ of integrated luminosity. If the LHC yields hints of these warped models, establishing that Nature is described by them, or their 4D CFT duals, requires an NHC-class machine in the post-LHC experimental program.

hep-ph↗