arXiv subjects

Ran Huo

Publications and source records attributed to Ran Huo.

30 records · Page 2Linked to original sources

arXiv2016

Latent Voter Model on Locally Tree-Like Random Graphs

In the latent voter model, which models the spread of a technology through a social network, individuals who have just changed their choice have a latent period, which is exponential with rate $λ$, during which they will not buy a new device. We study site and edge versions of this model on random graphs generated by a configuration model in which the degrees $d(x)$ have $3 \le d(x) \le M$. We show that if the number of vertices $n \to\infty$ and $\log n \ll λ_n \ll n$ then the latent voter model has a quasi-stationary state in which each opinion has probability $\approx 1/2$ and persists in this state for a time that is $\ge n^m$ for any $m<\infty$. Thus, even a very small latent period drastically changes the behavior of the voter model.

math.PR↗

arXiv2016

The Minimal Scalar-Stealth Top Interpretation of the Diphoton Excess

A resonance near 750 GeV has been observed at both the ATLAS and CMS experiments in its diphoton decay channel. We propose a possible interpretation based on a simplified model: a 750 GeV singlet scalar resonance, and a minimal stealth vector quark are introduced. At the LHC, the scalar is produced through gluon fusion, and subsequently decays to diphoton final state. The light stealth quarks could enhance the diphoton rate significantly, while still keeping other channels below the current LHC search limits. We find that the stealth quarks around 400 ~ 500 GeV with its coupling to scalar around 0.5 ~ 1.5 could explain the diphoton excess, and avoid current di-jet constraints. We also comment on other potential signatures of this stealth model.

hep-ph↗

arXiv2015

Standard Model Effective Field Theory: Integrating out Vector-Like Fermions

We apply the covariant derivative expansion of the Coleman-Weinberg potential to vector-like fermion models, matching the UV theory to the relevant dimension-6 operators in the standard model effective field theory. The $γ$ matrix induced complication in the fermionic covariant derivative expansion is studied in detail, and all the contributing combinations are enumerated. From this analytical result we also provide numerical constraints for a generation of vector-like quarks and vector-like leptons.

hep-ph↗

arXiv2015

Standard Model Effective Field Theory: Integrating out a Generic Scalar

We consider renormalisable models extended in the scalar sector by a generic scalar field in addition to the standard model Higgs boson field, and work out the effective theory for the latter in the decoupling limit. We match the full theory onto the effective theory at tree and one-loop levels, and concentrate on dimension-6 operators of the Higgs and electroweak gauge fields induced from such matching. The Wilson coefficients of these dimension-6 operators from tree-level matching are further improved by renormalisation group running. For specific $SU(2)_L$ representations of the scalar field, some "accidental" couplings with the Higgs field are allowed and can lead to dimension-6 operators at tree and/or one-loop level. Otherwise, two types of interaction terms are identified to have only one-loop contributions, for the Wilson coefficients of which we have obtained a general formula. Using the obtained results, we analyse constraints from electroweak oblique parameters and the Higgs data on several phenomenological models.

hep-ph↗

arXiv2015

Dark Matter in Split SUSY with Intermediate Higgses

The searches for heavy Higgs bosons and supersymmetric (SUSY) particles at the LHC have left the minimal supersymmetric standard model (MSSM) with an unusual spectrum of SUSY particles, namely, all squarks are beyond a few TeV while the Higgs bosons other than the one observed at 125 GeV could be relatively light. In light of this, we study a scenario characterized by two scales: the SUSY breaking scale or the squark-mass scale $(M_S)$ and the heavy Higgs-boson mass scale $(M_A)$.We perform a survey of the MSSM parameter space with $M_S < 10^{10}$ GeV and $M_A < 10^4$ GeV such that the lightest Higgs boson mass is within the range of the observed Higgs boson as well as satisfying a number of constraints. The set of constraints include the invisible decay width of the $Z$ boson and that of the Higgs boson, the chargino-mass limit, dark matter relic abundance from Planck, the spin-independent cross section of direct detection by LUX, and gamma-ray flux from dwarf spheroidal galaxies and gamma-ray line constraints measured by Fermi LAT. Survived regions of parameter space feature the dark matter with correct relic abundance, which is achieved through either coannihilation with charginos, $A/H$ funnels, or both. We show that future measurements, e.g., XENON1T and LZ, of spin-independent cross sections can further squeeze the parameter space.

hep-ph↗

arXiv2014

Electroweak baryogenesis in the MSSM with vector-like superfields

Introducing heavy particles with strong couplings to the Higgs field can strengthen electroweak phase transition, through the entropy release mechanism from both bosons and fermions. We analyze the possibility of electroweak baryogenesis in the MSSM with new vector-like superfields. The new vector-like particles belong to the representation 5+5bar+10+10bar of SU(5). By analyzing in detail the effective potential at finite temperature, we show that a strongly first order electroweak phase transition in this model is ruled out by a combination of 125 GeV Higgs requirement, the bound for exotic quarks, the gluon fusion Higgs production rate and the Higgs diphoton decay rate as well as the electroweak precision measurement.

hep-ph↗

arXiv2013

Electroweak Baryogenesis with a Supersymmetric Sector

We study a model with an exotic new sector strongly coupled to the Higgs boson, in which supersymmetry is introduced to protect the quartic coupling from a large running and avoid potential vacuum stability problem. The fermionic components present vector like mass terms, through which the Higgs diphoton decay branching ratio can be tuned. The bosonic components trigger a strongly first order electroweak phase transition. We find a large parameter region of effective Yukawa coupling $y\gtrsim2$ and mass parameters $m_f\sim m_s$ of a few hundred GeV, that can simultaneously accommodate the diphoton excess and electroweak baryogenesis, without vacuum stability and electroweak precision measurement problems.

hep-ph↗

arXiv2013

$SU(2)\otimes SU(2)$ Gauge Extensions of the MSSM Revisited

We study an extension of the Minimal Supersymmetric Standard Model with a gauge group $SU(2)_1\otimes SU(2)_2$ breaking to $SU(2)_L$. The extra wino has an enhanced gauge coupling to the SM-like Higgs boson and, if light, has a relevant impact on the weak scale phenomenology. The low energy Higgs quartic coupling is modified both by extra $D$-term corrections and by a modification of its renormalization group evolution from high energies. At low values of $\tanβ$, the latter effect may be dominant. This leads to interesting regions of parameter space in which the model can accommodate a 125 GeV Higgs with relatively light third generation squarks and an increased $h \rightarrow γγ$ decay branching ratio, while still satisfying the constraints from electroweak precision data and Higgs vacuum stability.

hep-ph↗

arXiv2012

Searching for Low Mass Dark Portal at the LHC

Light dark matter with mass smaller than about 10 GeV is difficult to probe from direct detection experiments. In order to have the correct thermal relic abundance, the mediator of the interaction between dark matter and the Standard Model (SM) should also be relatively light, $\sim 10^2$ GeV. If such a light mediator couples to charged leptons, it would already be strongly constrained by direct searches at colliders. In this work, we consider the scenario of a leptophobic light $Z'$ vector boson as the mediator, and study the the prospect of searching for it at the 8 TeV Large Hadron Collider (LHC). To improve the reach in the low mass region, we perform a detailed study of the processes that the $Z'$ is produced in association with jet, photon, $W^\pm$ and $Z^0$. We show that in the region where the mass of $Z'$ is between 80 and 400 GeV, the constraint from associated production can be comparable or even stronger than the known monojet and dijet constraints. Searches in these channels can be complementary to the monojet search, in particular if the $Z'$ couplings to quarks ($g_{Z'}$) and dark matter ($g_D$) are different. For $g_D < g_{Z'}$, we show that there is a larger region of parameter space which has correct thermal relic abundance and a light $Z'$, $M_{Z'} \sim 100 $ GeV. This region, which cannot be covered by the mono-jet search, can be covered by the resonance searches described in this paper.

hep-ph↗

arXiv2011

An Alternative Yukawa Unified SUSY Scenario

Supersymmetric SO(10) Grand Unified Theories with Yukawa unification represent an appealing possibility for physics beyond the Standard Model. However Yukawa unification is made difficult by large threshold corrections to the bottom mass. Generally one is led to consider models where the sfermion masses are large in order to suppress these corrections. Here we present another possibility, in which the top and bottom GUT scale Yukawa couplings are equal to a component of the charged lepton Yukawa matrix at the GUT scale in a basis where this matrix is not diagonal. Physically, this weak eigenstate Yukawa unification scenario corresponds to the case where the charged leptons that are in the 16 of SO(10) containing the top and bottom quarks mix with their counterparts in another SO(10) multiplet. Diagonalizing the resulting Yukawa matrix introduces mixings in the neutrino sector. Specifically we find that for a large region of parameter space with relatively light sparticles, and which has not been ruled out by current LHC or other data, the mixing induced in the neutrino sector is such that $sin^2 2Θ_{23} \approx 1$, in agreement with data. The phenomenological implications are analyzed in some detail.

hep-ph↗

arXiv2011

Constraining Decaying Dark Matter

We revisited the decaying dark matter (DDM) model, in which one collisionless particle decays early into two collisionless particles, that are potentially dark matter particles today. The effect of DDM will be manifested in the cosmic microwave background (CMB) and structure formation. With a systematic modification of CMB calculation tool \texttt{camb}, we can numerically calculated this effect, and compare it to observations. Further Markov Chain Monte Carlo \texttt{cosmomc} runnings update the constraints in that model: the free streaming length $λ_{FS}\lesssim0.5$Mpc for nonrelativistic decay, and $((M_{DDM}/keV) Y)^2 (T_d/yr)\lesssim5\times10^{-5}$ for relativistic decay.

hep-ph↗

arXiv2006

M-Theory Brane as Giant Graviton and the Fractional Quantum Hall Effect

A small number of M-theory branes as giant gravitons in the M-theory sector of LLM geometry is studied as a probe. The abelian way shows that the low energy effective action for M-theory brane is exactly the 2d electron subject to a vertical magnetic field. We also briefly discuss the microscopic description of M2-brane giant graviton in this geometry, in the language of a combination of D0-branes as fuzzy 2-spheres. Then we go to the well-established Noncommutative Chern-Simons theory description. After quantization, well behaved Fractional Quantum Hall Effect is demonstrated. This goes beyond the original LLM description and should be some indication of novel geometry.

hep-th↗