arXiv subjects

Hooman Davoudiasl

Publications and source records attributed to Hooman Davoudiasl.

At least 73 records · Page 4Linked to original sources

arXiv2018

Variation of $α$ from a Dark Matter Force

We consider a long range scalar force that mainly couples to dark matter and unstable Standard Model states, like the muon, with tiny strength. Probing this type of force would present a challenge to observations. We point out that the dependence of the induced background scalar field on dark matter number density can cause the mass of the unstable particles to have spatial and temporal variations. These variations, in turn, leave an imprint on the value of the fine structure constant $α$, through threshold corrections, that could be detected in astronomical and cosmological measurements. Our mechanism can accommodate the mild preference of the Planck data for such a deviation, $(α_{_{\rm CMB}}-α_{\rm present})/α_{\rm present} = (-3.6\pm 3.7)\times 10^{-3}$. In this case, the requisite parameters typically imply that violations of Equivalence Principle may be within reach of future experiments.

hep-ph↗

arXiv2018

A Tale of Two Anomalies

A recent improved determination of the fine structure constant, $α= 1/137.035999046(27)$, leads to a $\sim 2.4 σ$ negative discrepancy between the measured electron anomalous magnetic moment and the Standard Model prediction. That situation is to be compared with the muon anomalous magnetic moment where a positive $\sim 3.7 σ$ discrepancy has existed for some time. A single scalar solution to both anomalies is shown to be possible if the two-loop electron Barr-Zee diagrams dominate the scalar one-loop electron anomaly effect and the scalar couplings to the electron and two photons are relatively large. We also briefly discuss the implications of that scenario.

hep-ph↗

arXiv2018

Galactic Dark Matter Population as the Source of Neutrino Masses

We propose that neutrino masses can be zero $in ~vacuo$ and may be generated by the local distribution of dark matter through a feeble long range scalar force. We discuss potential phenomenological constraints and implications of this framework. Our model typically implies that the relic neutrino background left over from the Big Bang is mostly absent in our Galactic neighborhood. Hence, a positive detection signal from future proposed experiments, such as PTOLEMY, could in principle falsify our scenario.

hep-ph↗

arXiv2018

Dark matter repulsion could thwart direct detection

We consider a feeble repulsive interaction between ordinary matter and dark matter, with a range similar to or larger than the size of the Earth. Dark matter can thus be repelled from the Earth, leading to null results in direct detection experiments, regardless of the strength of the short-distance interactions of dark matter with atoms. Generically, such a repulsive force would not allow trapping of dark matter inside astronomical bodies. In this scenario, accelerator-based experiments may furnish the only robust signals of asymmetric dark matter models, which typically lack indirect signals from self-annihilation. Some of the variants of our hypothesis are also briefly discussed.

hep-ph↗

arXiv2017

Unified Scenario for Composite Right-Handed Neutrinos and Dark Matter

We entertain the possibility that neutrino masses and dark matter (DM) originate from a common composite dark sector. A minimal effective theory can be constructed based on a dark $SU(3)_D$ interaction with three flavors of massless dark quarks; electroweak symmetry breaking gives masses to the dark quarks. By assigning a $\mathbb Z_2$ charge to one flavor, a stable "dark kaon" can provide a good thermal relic DM candidate. We find that "dark neutrons" may be identified as right handed Dirac neutrinos. Some level of "neutron-anti-neutron" oscillation in the dark sector can then result in non-zero Majorana masses for light Standard Model neutrinos. A simple ultraviolet completion is presented, involving additional heavy $SU(3)_D$-charged particles with electroweak and lepton Yukawa couplings. At our benchmark point, there are "dark pions" that are much lighter than the Higgs and we expect spectacular collider signals arising from the UV framework. This includes the decay of the Higgs boson to $ττ\ell \ell^{\prime}$, where $\ell$($\ell'$) can be any lepton, with displaced vertices. We discuss the observational signatures of this UV framework in dark matter searches and primordial gravitational wave experiments; the latter signature is potentially correlated with the $H \to ττ\ell \ell^{\prime}$ decay.

hep-ph↗

arXiv2017

Asymmetric Dark Matter in Extended Exo-Higgs Scenarios

The exo-Higgs model can accommodate a successful baryogenesis mechanism that closely mirrors electroweak baryogenesis in the Standard Model, but avoids its shortcomings. We extend the exo-Higgs model by the addition of a singlet complex scalar $χ$. In our model, $χ$ can be a viable asymmetric dark matter (ADM) candidate. We predict the mass of the ADM particle to be $m_χ\approx1.3\, \textrm{GeV}$. The leptophilic couplings of $χ$ can provide for efficient annihilation of the ADM pairs. We also discuss the LHC signals of our scenario, and in particular the production and decays of exo-leptons which would lead to "lepton pair plus missing energy" final states. Our model typically predicts potentially detectable gravitational waves originating from the assumed strong first order phase transition at a temperature of $\sim$ TeV. If the model is further extended to include new heavy vector-like fermions, {\it e.g.} from an ultraviolet extension, $χ$ couplings could explain the $\sim 3.5σ$ muon $g-2$ anomaly.

hep-ph↗

arXiv2017

Parity Violation and Rare Higgs Decays from a Dark Force

We outline the phenomenology of the "dark" $Z$, denoted by $Z_d$, which is a generalization of the "dark" photon hypothesis. Whereas the dark photon interacts with the Standard Model through kinetic mixing, $Z_d$ is assumed also to have mass-mixing with the $Z$ boson. In particular, we highlight the possibility of $Z_d$ contributions to low $Q^2$ parity violation measurements and rare Higgs decays $H\to Z Z_d \to 4 \ell$, where $\ell$ is a charged lepton. The parity violation effects of a $Z_d$ with an intermediate mass $\sim 10-35$ GeV can in principle relieve the mild $\sim 1.8σ$ tension among various measurements of the weak mixing angle $θ_W$. We briefly comment on the prospects for future parity violation experiments at low $Q^2$ to probe this scenario, which could have correlated signals in rare Higgs decays at the LHC.

hep-ph↗

arXiv2017

Fuzzy Dark Matter from Infrared Confining Dynamics

A very light boson of mass $\mathcal{O}(10^{-22})$ eV may potentially be a viable dark matter (DM) candidate which can avoid phenomenological problems associated with cold DM. Such "fuzzy DM (FDM)" may naturally be an axion with a decay constant $f_a \sim 10^{16} ÷10^{18}$ GeV, and a mass $m_a \sim μ^2/f_a$ with $μ\sim 10^2$ eV. Here we propose a concrete model where $μ$ arises as a dynamical scale from infrared confining dynamics, analogous to QCD. Our model is an alternative to the usual approach of generating $μ$ through string theoretic instanton effects. We outline the features of this scenario that result from various cosmological constraints. We find that those constraints are suggestive of a period of mild of inflation, perhaps from a strong first order phase transition, that reheats the Standard Model (SM) sector only. A typical prediction of our scenario, broadly speaking, is a larger effective number of neutrinos compared to the SM value $N_{\text{eff}} \approx 3$, as inferred from precision measurements of the cosmic microwave background. Some of the new degrees of freedom may be identified as "sterile neutrinos," which may be required to explain certain neutrino oscillation anomalies. Hence, aspects of our scenario could be testable in terrestrial experiments, which is a novelty of our FDM model.

hep-ph↗

arXiv2017

Gravitational Waves from Primordial Black Holes and New Weak Scale Phenomena

We entertain the possibility that primordialblack holes of mass $\sim (10^{26}$--$10^{29})$~g, with Schwarzschild radii of $\mathcal{O}{\text{cm}}$, constitute $\sim 10\%$ or more of cosmic dark matter, as allowed by various constraints. These black holes would typically originate from cosmological eras corresponding to temperatures $\mathcal{O}{10-100}$~GeV, and may be associated with first order phase transitions in the visible or hidden sectors. In case these small primordial black holes get captured in orbits around neutron stars or astrophysical black holes in our galactic neighborhood, gravitational waves from the resulting "David and Goliath (D\&G)" binaries could be detectable at Advanced LIGO or Advanced Virgo for hours or more, possibly over distances of $\mathcal{O}{10}$~Mpc encompassing the Local Supercluster of galaxies. The proposed Einstein Telescope would further expand the reach for these signals. A positive signal could be further corroborated by the discovery of new particles in the $\mathcal{O}{10-100}$~GeV mass range, and potentially also the detection of long wavelength gravitational waves originating from the first order phase transition era.

gr-qc↗

arXiv2016

Dark Sectors 2016 Workshop: Community Report

This report, based on the Dark Sectors workshop at SLAC in April 2016, summarizes the scientific importance of searches for dark sector dark matter and forces at masses beneath the weak-scale, the status of this broad international field, the important milestones motivating future exploration, and promising experimental opportunities to reach these milestones over the next 5-10 years.

hep-ph↗

arXiv2016

Exo-Higgs at 750 GeV and Genesis of Baryons

We propose that the diphoton excess at 750 GeV reported by ATLAS and CMS is due to the decay of an ${\it exo-Higgs}$ scalar $η$ associated with the breaking of a new $SU(2)_e$ symmetry, dubbed ${\it exo-spin}$. New fermions, ${\it exo-quarks}$ and ${\it exo-leptons}$, get TeV-scale masses through Yukawa couplings with $η$ and generate its couplings to gluons and photons at 1-loop. The matter content of our model yields a $B-L$ anomaly under $SU(2)_e$, whose breaking we assume entails a first order phase transition. A non-trivial $B-L$ asymmetry may therefore be generated in the early universe, potentially providing a baryogenesis mechanism through the Standard Model (SM) sphaleron processes. The spontaneous breaking of $SU(2)_e$ can in principle directly lead to electroweak symmetry breaking, thereby accounting for the proximity of the mass scales of the SM Higgs and the exo-Higgs. Our model can be distinguished from those comprising a singlet scalar and vector fermions by the discovery of TeV scale exo-vector bosons, corresponding to the broken $SU(2)_e$ generators, at the LHC.

hep-ph↗

arXiv2016

Implications of a Light "Dark Higgs" Solution to the $g_μ-2$ Discrepancy

A light scalar $ϕ$ with mass $\lesssim 1$ GeV and muonic coupling $\mathcal{O}(10^{-3})$ would explain the 3.5 $σ$ discrepancy between the Standard Model (SM) muon $g-2$ prediction and experiment. Such a scalar can be associated with a light remnant of the Higgs mechanism in the "dark" sector. We suggest $ϕ\to l^+l^-$ bump hunting in $μ\to eν\barνϕ$, $μ^-p\toν_μnϕ$ (muon capture), and $K^\pm\to μ^\pmνϕ$ decays as direct probes of this scenario. In a general setup, a potentially observable muon electric dipole moment $\lesssim 10^{-23}\ e \cdot\textrm{cm}$ and lepton flavor violating decays $τ\toμ(e) ϕ$ or $μ\to e ϕ$ can also arise. Depending on parameters, a deviation in BR($H\toμ^+μ^-$) from SM expectations, due to Higgs coupling misalignment, can result. We illustrate how the requisite interactions can be mediated by weak scale vector-like leptons that typically lie within the reach of future LHC measurements.

hep-ph↗

arXiv2016

A 750 GeV Messenger of Dark Conformal Symmetry Breaking

The tentative hints for a diphoton resonance at a mass of $\sim 750$ GeV from the ATLAS and CMS experiments at the LHC may be interpreted as first contact with a "dark" sector with a spontaneously broken conformal symmetry. The implied TeV scale of the dark sector may be motivated by the interaction strength required to accommodate a viable thermal relic dark matter (DM) candidate. We model the conformal dynamics using a Randall-Sundrum type 5D geometry whose IR boundary is identified with the dynamics of the composite dark sector, while the Standard Model (SM) matter content resides on the UV boundary, corresponding to "elementary" fields. We allow the gauge fields to reside in the 5D bulk, which can be minimally chosen to be $SU(3)_c\times U(1)_Y$. The "dark" radion is identified as the putative 750 GeV resonance. Heavy vector-like fermions, often invoked to explain the diphoton excess, are not explicitly present in our model and are not predicted to appear in the spectrum of TeV scale states. Our minimal setup favors scalar DM of $\mathcal{O}(\text{TeV})$ mass. A generic expectation in this scenario, suggested by DM considerations, is the appearance of vector bosons at $\sim$ few TeV, corresponding to the gluon and hypercharge Kaluza-Klein (KK) modes that couple to UV boundary states with strengths that are suppressed uniformly compared to their SM values. Our analysis suggests that these KK modes could be within the reach of the LHC in the coming years.

hep-ph↗

arXiv2015

Implications of a Running Dark Photon Coupling

For an "invisible" dark photon $Z_d$ that dominantly decays into dark states, the running of its fine structure constant $α_d$ with momentum transfer $q > m_{Z_d}$ could be significant. A similar running in the kinetic mixing parameter $\varepsilon^2$ can be induced through its dependence on $α_d(q)$. The running of couplings could potentially be detected in "dark matter beam" experiments, for which theoretical considerations imply $α_d (m_{Z_d}) \lesssim 0.5$.

hep-ph↗

arXiv2015

Low Q^2 Weak Mixing Angle Measurements and Rare Higgs Decays

A weighted average weak mixing angle theta_W derived from relatively low Q^2 experiments is compared with the Standard Model prediction obtained from precision measurements. The approximate 1.8 sigma discrepancy is fit with an intermediate mass (~ 10-35 GeV) "dark" Z boson Z_d, corresponding to a U(1)_d gauge symmetry of hidden dark matter, which couples to our world via kinetic and Z-Z_d mass mixing. Constraints on such a scenario are obtained from precision electroweak bounds and searches for the rare Higgs decays H -> Z Z_d -> 4 charged leptons at the LHC. The sensitivity of future anticipated low Q^2 measurements of sin^2 theta_W(Q^2) to intermediate mass Z_d is also illustrated. This dark Z scenario can provide interesting concomitant signals in low energy parity violating measurements and rare Higgs decays at the LHC, over the next few years.

hep-ph↗

arXiv2015

Running in the Dark Sector

The "dark photon" $γ_d$ of a gauged $U(1)_d$ can become practically invisible if it primarily decays into light states from a dark sector. We point out that, in such scenarios, the running of the $U(1)_d$ "fine structure constant" $α_d$, with momentum transfer $q^2$, can be significant and potentially measurable. The $γ_d$ kinetic mixing parameter $\varepsilon^2$ is also expected to run with $q^2$, through its dependence on $α_d$. We show how the combined running of $\varepsilon^2 α_d$ may provide a probe of the spectrum of dark particles and, for $α_d\gtrsim {\rm few}\times 0.1$, substantially modify predictions for "beam dump" or other intense source experiments. These features are demonstrated in simple models that contain light dark matter and a scalar that breaks $U(1)_d$. We also discuss theoretic considerations, regarding the $U(1)_d$ model in the ultraviolet regime, that may suggest the infrared upper bound $α_d \lesssim 0.1$.

hep-ph↗

arXiv2015

Inflatable Dark Matter

We describe a general scenario, dubbed "Inflatable Dark Matter", in which the density of dark matter particles can be reduced through a short period of late-time inflation in the early universe. The overproduction of dark matter that is predicted within many otherwise well-motivated models of new physics can be elegantly remedied within this context, without the need to tune underlying parameters or to appeal to anthropic considerations. Thermal relics that would otherwise be disfavored can easily be accommodated within this class of scenarios, including dark matter candidates that are very heavy or very light. Furthermore, the non-thermal abundance of GUT or Planck scale axions can be brought to acceptable levels, without invoking anthropic tuning of initial conditions. A period of late-time inflation could have occurred over a wide range of scales from ~ MeV to the weak scale or above, and could have been triggered by physics within a hidden sector, with small but not necessarily negligible couplings to the Standard Model.

hep-ph↗

arXiv2015

Baryon Number Violation via Majorana Neutrinos in the Early Universe, at the LHC, and Deep Underground

We propose and investigate a novel, minimal, and experimentally testable framework for baryogenesis, dubbed dexiogenesis, using baryon number violating effective interactions of right-handed Majorana neutrinos responsible for the seesaw mechanism. The distinct LHC signature of our framework is same-sign top quark final states, possibly originating from displaced vertices. The region of parameters relevant for LHC phenomenology can also yield concomitant signals in nucleon decay experiments. We provide a simple ultraviolet origin for our effective operators, by adding a color-triplet scalar, which could ultimately arise from a grand unified theory.

hep-ph↗