arXiv subjects

Tianjun Li

Publications and source records attributed to Tianjun Li.

At least 199 records · Page 11Linked to original sources

arXiv2014

Adhesion energy of single wall carbon nanotube loops on various substrates

The physics of adhesion of one-dimensional nano structures such as nanotubes, nano wires, and biopolymers on different material substrates is of great interest for the study of biological adhesion and the development of nano electronics and nano mechanics. In this paper, we present force spectroscopy experiments of a single wall carbon nanotube loop using our home-made interferometric atomic force microscope. Characteristic force plateaux during the peeling process allows us to access to quantitative values of the adhesion energy per unit length on various substrates: graphite, mica, platinum, gold and silicon. By combining a time-frequency analysis of the deflexion of the cantilever, we access to the dynamic stiffness of the contact, providing more information on the nanotube configurations and its intrinsic mechanical properties.

cond-mat.mtrl-sci↗

arXiv2014

No Naturalness or Fine-tuning Problems from No-Scale Supergravity

We compute the electroweak fine-tuning in No-Scale Supergravity for a representative supersymmetric Grand Unification Theory (GUT) model, flipped $SU(5)$ with extra vector-like $flippons$, dubbed $\cal{F}$-$SU(5)$. We find that there is no problematic electroweak fine-tuning in No-Scale $\cal{F}$-$SU(5)$, due to an elegant proportional rescaling of the full mass spectrum with respect to just the unified gaugino mass $M_{1/2}$, as well as a dynamic equivalence enforced between $M_{1/2}$ and the supersymmetric Higgs mixing parameter $μ$ at the heavy unification scale. We demonstrate both analytically and numerically that the No-Scale $\cal{F}$-$SU(5)$ fine-tuning parameter is consequently of unit order, $Δ_{\rm EENZ} \simeq {\cal O}(1)$, at the electroweak scale.

hep-ph↗

arXiv2014

Natural Inflation with Natural Trans-Planckian Axion Decay Constant from Anomalous $U(1)_X$

We propose a natural inflation model driven by an imaginary or axionic component of a Kähler modulus in string-inspired supergravity. The shift symmetry of the axion is gauged under an anomalous $U(1)_X$ symmetry, which leads to a modulus-dependent Fayet-Iliopoulos (FI) term. The matter fields are stabilized by F-terms, and the real component of the modulus is stabilized by the $U(1)_X$ D-term, while its axion remains light. Therefore, the masses of real and imaginary components of the modulus are separated at different scales. The scalar potential for natural inflation is realized by the superpotential from the non-perturbative effects. The trans-Planckian axion decay constant, which is needed to fit with BICEP2 observations, can be obtained naturally in this model.

hep-th↗

arXiv2014

Aligned Natural Inflation and Moduli Stabilization from Anomalous $U(1)$ Gauge Symmetries

To obtain natural inflation with large tensor-to-scalar ratio in string framework, we need a special moduli stabilization mechanism which can separate the masses of real and imaginary components of Kähler moduli at different scales, and achieve a trans-Planckian axion decay constant from sub-Planckian axion decay constants. In this work, we stabilize the matter fields by F-terms and the real components of Kähler moduli by D-terms of two anomalous $U(1)_X\times U(1)_A$ symmetries strongly at high scales, while the corresponding axions remain light due to their independence on the Fayet-Iliopoulos (FI) term in moduli stabilization. The racetrack-type axion superpotential is obtained from gaugino condensations of the hidden gauge symmetries $SU(n)\times SU(m)$ with massive matter fields in the bi-fundamental respresentations. The axion alignment via Kim-Nilles-Pelroso (KNP) mechanism corresponds to an approximate $S_2$ exchange symmetry of two Kähler moduli in our model, and a slightly $S_2$ symmetry breaking leads to the natural inflation with super-Planckian decay constant.

hep-th↗

arXiv2014

A Realistic Intersecting D6-Brane Model after the First LHC Run

With the Higgs boson mass around 125 GeV and the LHC supersymmetry search constraints, we revisit a three-family Pati-Salam model from intersecting D6-branes in Type IIA string theory on the $\mathbf{T^6/(\Z_2 \times \Z_2)}$ orientifold which has a realistic phenomenology. We systematically scan the parameter space for $μ<0$ and $μ>0$, and find that the gravitino mass is generically heavier than about 2 TeV for both cases due to the Higgs mass low bound 123 GeV. In particular, we identify a region of parameter space with the electroweak fine-tuning as small as $Δ_{EW} \sim$ 24-32 (3-4$\%$). In the viable parameter space which is consistent with all the current constraints, the mass ranges for gluino, the first two-generation squarks and sleptons are respectively $[3, ~18]$ TeV, $[3, ~16]$ TeV, and $[2, ~7]$ TeV. For the third-generation sfermions, the light stop satisfying $5σ$ WMAP bounds via neutralino-stop coannihilation has mass from 0.5 to 1.2 TeV, and the light stau can be as light as 800 GeV. We also show various coannihilation and resonance scenarios through which the observed dark matter relic density is achieved. Interestingly, the certain portions of parameter space has excellent $t$-$b$-$τ$ and $b$-$τ$ Yukawa coupling unification. Three regions of parameter space are highlighted as well where the dominant component of the lightest neutralino is a bino, wino or higgsino. We discuss various scenarios in which such solutions may avoid recent astrophysical bounds in case if they satisfy or above observed relic density bounds. Prospects of finding higgsino-like neutralino in direct and indirect searches are also studied. And we display six tables of benchmark points depicting various interesting features of our model.

hep-ph↗

arXiv2014

The Simple Single Field Inflation Models and the Running of Spectral Index

The BICEP2 experiment confirms the existence of primordial gravitational wave with the tensor-to-scalar ratio $r=0$ ruled out at $7σ$ level. The consistency of this large value of $r$ with the {\em Planck} data requires a large negative running $n'_s$ of the scalar spectral index. Herein we propose two types of the single field inflation models with simple potentials to study the possibility of the consistency of the models with the BICEP2 and {\em Planck} observations. One type of model suggested herein is realized in the supergravity model building. These models fail to provide the needed $n'_s$ even though both can fit the tensor-to-scalar ratio and spectral index.

hep-th↗

arXiv2014

Chaotic Inflation in No-Scale Supergravity with String Inspired Moduli Stabilization

The simple chaotic inflation is highly consistent with the BICEP2 experiment, and no-scale supergravity can be realized naturally in various string compactifications. Thus, we construct a chaotic inflation model in no-scale supergravity inspired from Type IIB string compactification with an anomalous U(1)_X gauged symmetry. We introduce two moduli T_1 and T_2 which transform non-trivially under U(1)_X, and some pairs of fundamental quarks charged under the SU(N) x U(1)_X gauge group. The non-trivial transformations of moduli under U(1)_X lead to a moduli-dependent Fayet-Iliopoulos (FI) term. The modulus T_2 and the real component of T_1 are stabilized by the non-perturbative effect from quark condensation and the U(1)_X D-term. In particular, the stabilization from the anomalous U(1)_X D-term with moduli-dependent FI term is crucial for inflation since it gives heavy mass to the real component of the modulus T_1 while keeping its axionic part light. Choosing the proper parameters, we obtain a global Minkowski vacuum where the imaginary part of T_1 has a quadratic potential for chaotic inflation.

hep-th↗

arXiv2014

Supergravity Inflation with Broken Shift Symmetry and Large Tensor-to-Scalar Ratio

We propose a class of inflation models with potential V(ϕ)=αϕ^n exp(-β^m ϕ^m). We show that such kind of inflaton potentials can be realized in supergravity theory with a small shift symmetry breaking term in the Kähler potential. We find that the models with (m=1, n=1), (m=1, n=2), (m=2, n=1), and (m=2, n=3/2) can easily accommodate the Planck results. In particular, the tensor-to-scalar ratio is larger than 0.01 in the 1σregion. Thus, these models generate the typical large field inflation, and can be tested at the future Planck and QUBIC experiments.

hep-ph↗

arXiv2014

No-Scale Ripple Inflation Revisited

We revisit the no-scale ripple inflation model, where no-scale supergravity is modified by an additional term for the inflaton field in the Kahler potential. This term not only breaks one SU(N,1) symmetry explicitly, but also plays an important role for inflation. We generalize the superpotential in the no-scale ripple inflation model slightly. There exists a discrete Z_2 symmetry/parity in the scalar potential in general, which can be preserved or violated by the non-canonical nomalized inflaton kinetic term. Thus, there are three inflation paths: one parity invariant path, and the left and right paths for parity violating scenario. We show that the inflations along the parity invariant path and right path are consistent with the Planck results. However, the gavitino mass for the parity invariant path is so large that the inflation results will be invalid if we consider the inflaton supersymmetry breaking soft mass term. Thus, only the inflation along the right path gives the correct and consistent results. Notably, the tensor-to-scalar ratio in such case can be large, with a value around 0.05, which may be probed by the future Planck experiment.

hep-ph↗

arXiv2014

Higgs Boson Mass and Complex Snuetrino Dark Matter in the Supersymmetric Inverse Seesaw Models

The discovery of a relatively heavy Standard Model (SM) -like Higgs boson challenges naturalness of the minimal supersymmetric standard model (MSSM) from both Higgs and dark matter (DM) sectors. We study these two aspects in the MSSM extended by the low-scale inverse seesaw mechanism. Firstly, it admits a sizable radiative correction on the Higgs boson mass m_h, up to \sim 4 GeV in the case of an IR-fixed point of the coupling Y_νLH_uν^c and a large sneutrino mixing. Secondly, the lightest sneutrino, highly complex as expected, is a viable thermal DM candidate. Owing to the correct relic density and XENON100, two cases survive: a Higgs-portal complex DM with mass lying around the Higgs pole or above $W$ threshold, and a coannihilating DM with slim prospect of detection. Given an extra family of sneutrinos, both scenarios naturally work when we attempt to suppress the DM left-handed sneutrino component, confronting with enhancing m_h.

hep-ph↗

arXiv2014

Measurements of mechanical thermal noise and energy dissipation in optical dielectric coatings

In recent years an increasing number of devices and experiments are shown to be limited by mechanical thermal noise. In particular sub-Hertz laser frequency stabilization and gravitational wave detectors, that are able to measure fluctuations of 1E-18 m/rtHz or less, are being limited by thermal noise in the dielectric coatings deposited on mirrors. In this paper we present a new measurement of thermal noise in low absorption dielectric coatings deposited on micro-cantilevers and we compare it with the results obtained from the mechanical loss measurements. The coating thermal noise is measured on the widest range of frequencies with the highest signal to noise ratio ever achieved. In addition we present a novel technique to deduce the coating mechanical losses from the measurement of the mechanical quality factor which does not rely on the knowledge of the coating and substrate Young moduli. The dielectric coatings are deposited by ion beam sputtering. The results presented here give a frequency independent loss angle of (4.70 $\pm$ 0.2)x1E-4 with a Young's modulus of 118 GPa for annealed tantala from 10 Hz to 20 kHz. For as-deposited silica, a weak frequency dependence (~ f^{-0.025}) is observed in this frequency range, with a Young's modulus of 70 GPa and an internal damping of (6.0 $\pm$ 0.3)x1E-4 at 16 kHz, but this value decreases by one order of magnitude after annealing and the frequency dependence disappears.

physics.ins-det↗

arXiv2014

Testing No-Scale Supergravity with the Fermi Space Telescope LAT

We describe a methodology for testing No-Scale Supergravity by the LAT instrument onboard the Fermi Space Telescope via observation of gamma ray emissions from lightest supersymmetric (SUSY) neutralino annihilations. For our test vehicle we engage the framework of the supersymmetric grand unified model No-Scale Flipped $SU(5)$ with extra vector-like flippon multiplets derived from F-Theory, known as $\cal{F}$-$SU(5)$. We show that through compression of the light stau and light bino neutralino mass difference, where internal bremsstrahlung (IB) photons give a dominant contribution, the photon yield from annihilation of SUSY dark matter can be elevated to a number of events potentially observable by the Fermi-LAT in the coming years. Likewise, the increased yield in No-Scale $\cal{F}$-$SU(5)$ may also have rendered the existing observation of a 133 GeV monochromatic gamma ray line visible, if additional data should exclude systematic or statistical explanations. The question of intensity aside, No-Scale $\cal{F}$-$SU(5)$ can indeed provide a natural weakly interacting massive particle (WIMP) candidate with a mass in the correct range to yield $γγ$ and $γZ$ emission lines at $m_χ \sim 133$ GeV and $m_χ \sim 145$ GeV, respectively. Additionally, we elucidate the emerging empirical connection between recent Planck satellite data and No-Scale Supergravity cosmological models which mimic the Starobinsky model of inflation. Together, these experiments furnish rich alternate avenues for testing No-Scale $\cal{F}$-$SU(5)$, and similarly structured models, the results of which may lend independent credence to observations made at the LHC.

hep-ph↗

arXiv2014

Radial quantization of the 3d CFT and the higher spin/vector model duality

We study the radial quantization of the 3d O(N) vector model. We calculate the higher spin charges whose commutation relations give the higher spin algebra. The Fock states of higher spin gravity in AdS_{4} are realized as the states in the 3d CFT. The inner products between these states encode dynamical information. The construction of bulk operators from CFT is discussed as well. This serves as the simplest explicit demonstration of the CFT definition for the quantum gravity.

hep-th↗

arXiv2014

Focus Point Supersymmetry in Extended Gauge Mediation

We propose a small extenion of the minimal gauge mediation through the combination of extended gauge mediation and conformal sequestering. We show that the focus point supersymmetry can be realized naturally, and the fine-tuning is signifcantly reduced compared to the minimal gauge mediation and extended gauge mediation without focus point. The Higgs boson mass is around 125 GeV, the gauginos remain light, and the gluino is likely to be detected at the next run of the LHC. However, the multi-TeV squarks is out of the reach of the LHC. The numerical calculation for finetuning shows that this model remains natural.

hep-ph↗

arXiv2014

The Nelson-Seiberg theorem revised

The well-accepted Nelson-Seiberg theorem relates R-symmetries to supersymmetry (SUSY) breaking vacua, and provides a guideline for SUSY model building which is the most promising physics beyond the Standard Model. In the case of Wess-Zumino models with perturbative superpotentials, we revise the theorem to a combined necessary and sufficient condition for SUSY breaking which can be easily checked before solving the vacuum. The revised theorem provides a powerful tool to construct either SUSY breaking or SUSY vacua, and offers many practicable applications in low energy SUSY model building and string phenomenology.

hep-th↗

arXiv2013

No-Scale Supergravity in the Light of LHC and Planck

We review the No-Scale F-SU(5) model with extra TeV-scale vector-like flippon multiplets and associated collider phenomenology relating to the search for supersymmetry at the LHC. 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 further elucidate the emerging empirical connection between recent Planck satellite data and No-Scale Supergravity cosmological models that mimic the Starobinsky model of inflation, drawing upon work conducted by the conference presenter (DVN) with John Ellis (King's College London/CERN) and Keith Olive (Minnesota).

hep-ph↗

arXiv2013

Simplified Supersymmetry with Sneutrino LSP at 8 TeV LHC

The current searches of supersymmetry (SUSY) are based on the neutralino lightest sparticle (LSP). In this article we instead focus on SUSY with sneutrino LSP. It is well motivated in many contexts, especially in which sneutrino services as a dark matter candidate. We first develop a simplified model, which contains the stop, chagino/neutralino and sneutrino, to describe the LHC phenomenologies of a large class of models with sneutrino LSP. Then we investigate bounds on the model using the SUSY searches at the 8 TeV LHC. Strong exclusion limits are derived, e.g., masses of stop and chargino can be excluded up to about 900 GeV and 550 GeV, respectively. We also propose optimizations for some searches without turning to higher energy and luminosity.

hep-ph↗

arXiv2013

Constricted SUSY from No-Scale F-SU(5): A "Blind Spot" for LHC Top Quark Reconstruction?

We describe a region of the No-Scale F-SU(5) supersymmetric model with TeV-scale vector-like matter multiplets that is potentially opaque to LHC observations based upon top quark reconstruction. Possessing a 100% on-shell branching fraction for gluino to light stop decays, the gluino-light stop mass splitting in F-SU(5) lies very close to the top quark rest mass in the region 810 (650) <~ M(gluino) (M(light stop)) <~ 1170 (975) GeV. For unboosted gluino pair-production, this could render two soft top quarks in the dominant 4-top signature, greatly reducing the likelihood for discrimination from the combinatoric background. We therefore suggest that caution is warranted when gluino exclusion bounds on models possessing a similar spectral character (gluino heavier than the light stop but lighter than all other squarks) using multijet SUSY search tactics. Alternate search methods are highlighted, including those based upon same sign dileptons and multiple heavy flavor tags, which are more sensitive to this delicate kinematic "blind spot" crease.

hep-ph↗