arXiv subjects

Tianjun Li

Publications and source records attributed to Tianjun Li.

At least 163 records · Page 9Linked to original sources

arXiv2016

Supersoft Supersymmetry, Conformal Sequestering, and Single Scale Supersymmetry Breaking

Supersymmetric Standard Models (SSMs) with Dirac gauginos have the appealing supersoft property that they only cause finite contributions to scalar masses. Considering gauge mediated SUSY breaking with conformal sequestering and assuming there is one and only one fundamental parameter with dimension mass arising from supersymmetry breaking, we find a cancellation between the dominant terms that contribute to the EWFT. The resulting EWFT measure can be of order one even for supersymmetric particle masses and $μ$-terms in the TeV range.

hep-ph↗

arXiv2016

The Maximal $U(1)_L$ Inverse Seesaw from $d=5$ Operator and Oscillating Asymmetric Sneutrino Dark Matter

The maximal $U(1)_L$ supersymmetric inverse seesaw mechanism (M$L$SIS) provides a natural way to relate asymmetric dark matter (ADM) with neutrino physics. In this paper we point out that, M$L$SIS is a natural outcome if one dynamically realizes the inverse seesaw mechanism in the next-to minimal supersymmetric standard model (NMSSM) via the dimension-five operator $(N)^2S^2/M_*$, with $S$ the NMSSM singlet developing TeV scale VEV; it slightly violates lepton number due to the suppression by the fundamental scale $M_*$, thus preserving $U(1)_L$ maximally. The resulting sneutrino is a distinguishable ADM candidate, oscillating and favored to have weak scale mass. A fairly large annihilating cross section of such a heavy ADM is available due to the presence of singlet.

hep-ph↗

arXiv2016

No-Scale $μ$-Term Hybrid Inflation

To solve the fine-tuning problem in $μ$-Term Hybrid Inflation, we will realize the supersymmetry scenario with the TeV-scale supersymmetric particles and intermediate-scale gravitino from anomaly mediation, which can be consistent with the WMAP and Planck experiments. Moreover, we for the first time propose the $μ$-term hybrid inflation in no-scale supergravity. With four Scenarios for the $SU(3)_C\times SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ model, we show that the correct scalar spectral index $n_s$ can be obtained, while the tensor-to-scalar ratio $r$ is prediced to be tiny, about $10^{-10}-10^{-8}$. Also, the $SU(2)_R\times U(1)_{B-L}$ symmetry breaking scale is around $10^{14}$ GeV, and all the supersymmetric particles except gravitino are around TeV scale while gravitino mass is around $10^{9-10}$ GeV. Considering the complete potential terms linear in $S$, we for the first time show that the tadpole term, which is the key for such kind of inflationary models to be consistent with the observed scalar spectral index, vanishes after inflation. Thus, to obtain the $μ$ term, we need to generate the supersymmetry breaking soft term $A^{S ΦΦ'}_κ κS ΦΦ'$ due to $A^{S ΦΦ'}_κ=0 $ in no-scale supergravity, where $Φ$ and $Φ'$ are vector-like Higgs fields at high energy. We show that the proper $A^{S ΦΦ'}_κ κS ΦΦ'$ term can be obtained in the M-theory inspired no-scale supergravity. We also point out that $A^{S ΦΦ'}_κ$ around 700 GeV can be generated via the renormalization group equation running from string scale.

hep-ph↗

arXiv2016

Interpretation of the diphoton excess at CMS and ATLAS

We consider the diphoton resonance at the 13 TeV LHC in a consistent model with new scalars and vector-like fermions added to the Standard Model (SM), which can be constructed from orbifold grand unified theories and string models. The gauge coupling unification can be achieved, neutrino masses can be generated radiatively, and electroweak vacuum stability problem can be solved. To explain the diphoton resonance, we study a spin-0 particle, and discuss various associated final states.

hep-ph↗

arXiv2016

Systematic Study of Diphoton Resonance at 750 GeV from Sgoldstino

The ATLAS and CMS Collaborations of the Large Hadron Collider (LHC) have reported an excess of events in diphoton channel with invariant mass of about 750 GeV. With low energy supersymmetry breaking, we systematically consider the sgoldstino scalar $S$ as the new resonance, which is a linear combination of the CP-even scalar $s$ and CP-odd pseudoscalar $a$. Because we show that $s$ and $a$ can be degenerated or have large mass splitting, we consider two cases for all the following three scenarios: (1) Single resonance. $s$ is the 750 GeV resonance decays to a pair of 1 GeV pseudoscalar $a$. With suitable decay length, these two $a$ decay into collimated pair of photons which cannot be distinguished at the LHC and may appear as diphotons instead of four photons. (2) Twin resonances. $m_{s}\approx m_{a}$ with a mass difference of about 40 GeV and both $s$ and $a$ decay into diphoton pairs. For productions, we consider three scenarios: (I) vector boson fusion; (II) gluon gluon fusion; (III) $q{\bar q}$ pair production. In all these scenarios with two kinds of resonances, we find the parameter space that satisfies the diphoton production cross section from 3 to 13 ${\rm fb}$ and all the other experimental constraints. And we address the decay width as well. In particular, in the third scenario, we observe that the production cross section is small but the decay width of $s$ or $a$ can be from 40 to 60 GeV.

hep-ph↗

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↗

arXiv2016

Constraining Natural SUSY via the Higgs Coupling and the Muon Anomalous Magnetic Moment Measurements

We use the Higgs coupling and the muon anomalous magnetic moment measurements to constrain the parameter space of the natural supersymmetry (SUSY) in the Generalized Minimal Supergravity (GmSUGRA) model. We scan the parameter space of the GmSUGRA model with small electroweak fine-tuning measure ($Δ_{\rm EW} \leq 100$). The parameter space after applying various sparticle mass bounds, Higgs mass bounds, B-physics bounds, the muon magnetic moment constraint, and the Higgs coupling constraint from measurements at HL-LHC, ILC, and CEPC, is shown in the planes of various interesting model parameters and sparticle masses. Our study indicates that the Higgs coupling and muon anomalous magnetic moment measurements can constrain the parameter space effectively. It is shown that $Δ_{\rm EW}\sim$ 30, consistence with all constraints, and having supersymmetric contributions to the muon anomalous magnetic moment within 1$σ$ can be achieved. The precision of $k_b$ and $k_τ$ measurements at CEPC can bound $m_A$ to be above 1.2 TeV and 1.1 TeV respectively. The combination of the Higgs coupling measurement and muon anomalous magnetic moment measurement constrain $\tilde{e}_R$ mass to be in the range from 0.6 TeV to 2 TeV. The range of both $\tilde{e}_L$ and $\tildeν_e$ masses is 0.4 TeV $\sim$ 1.2 TeV. In all cases, the $\tildeχ_1^0$ mass needs to be small (mostly $\leq$ 400 GeV). The comparison of bounds in the $\tanβ- m_A$ plane shows that the Higgs coupling measurement is complementary to the direct collider searches for heavy Higgs when constraining the natural SUSY. A few mass spectra in the typical region of parameter space after applying all constraints are shown as well.

hep-ph↗

arXiv2015

A Promising Interpretation of Diphoton Resonance at 750 GeV

Recently, an excess of events in diphoton channel with invariant mass of about 750 GeV has been reported by the ATLAS and CMS Collaborations. Considering it as a tantalizing hint for new physics beyond the Standard Model (SM), we propose a simple extension of the SM with an additional doublet Higgs $H^{'}$ and a singlet $s$. We consider the neutral component $H^{\prime }_0$ of $H'$ as the 750 GeV resonance, and assume that $s$ is lighter than 2.6 GeV. In particular, $H^{\prime }_0$ can be produced at tree level via $q{\bar q}$ production, and decay into a pair of $s$ at tree level. And then $s$ can decay into a pair of collimated photons, which cannot be distinguished at the LHC. We show that the diphoton production cross section can be from 3 to 13 ${\rm fb}$, the decay width of $H^{\prime }_0$ can be from 30 to 60 GeV, and all the current experimental constraints including dijet constraint can be satisfied.

hep-ph↗

arXiv2015

The Diboson Excesses in an Anomaly Free Leptophobic Left-Right Model

The resonant excesses around 2 TeV reported by the ATLAS Collaboration can be explained in the left-right model, and the tight constraints from lepton plus missing energy searches can be evaded if the $SU(2)_R$ gauge symmetry is leptophobic. We for the first time propose an anomaly free leptophobic left-right model with gauge symmetry $SU(3)_C \times SU(2)_L \times SU(2)_R \times U(1)_{X}$ where the SM leptons are singlets under $SU(2)_R$. The gauge anomalies are cancelled by introducing extra vector-like quarks. The mass of $Z^\prime$ gauge boson, which cannot be leptophobic, is assumed to be around or above 2.5 TeV so that the constraint on dilepton final state can be avoided. Moreover, we find that the $W^\prime \to WZ$ channel cannot explain the ATLAS diboson excess due to the tension with the constraint on $W^\prime \to jj$ decay mode. We solve this problem by considering the mixings between the SM quarks and vector-like quarks. We show explicitly that the ATLAS diboson excess can be explained in the viable parameter space of our model, which is consistent with all the current experimental constraints.

hep-ph↗

arXiv2015

Light Doubly Charged Higgs Boson via the $WW^*$ Channel at LHC

The doubly charged Higgs bosons $H^{\pm\pm}$ searches at the Large Hadron Collider (LHC) have been studied extensively and strong bound is available for $H^{\pm\pm}$ dominantly decaying into a pair of same-sign di-leptons. In this paper we point out that there is a large cavity in the light $H^{\pm\pm}$ mass region left unexcluded. In particular, $H^{\pm\pm}$ can dominantly decay into $WW$ or $WW^*$ (For instance, in the type-II seesaw mechanism the triplet acquires a vacuum expectation value around 1 GeV.), and then it is found that $H^{\pm\pm}$ with mass even below $2m_W$ remains untouched by the current collider searches. Searching for such a $H^{\pm\pm}$ at the LHC is the topic of this paper. We perform detailed signal and background simulation, especially including the non-prompt $t\bar{t}$ background which is the dominant one nevertheless ignored before. We show that such $H^{\pm\pm}$ should be observable at the 14 TeV LHC with 10-30 fb$^{-1}$ integrated luminosity.

hep-ph↗

arXiv2015

The Super-Natural Supersymmetry and Its Classic Example: M-Theory Inspired NMSSM

We briefly review the super-natural supersymmetry (SUSY), which provides a most promising solution to the SUSY electroweak fine-tuning problem. In particular, we address its subtle issues as well. Unlike the Minimal Supersymmetric Standard model (MSSM), the Next to MSSM (NMSSM) can be scale invariant and has no mass parameter in its Lagrangian before SUSY and gauge symmetry breakings. Therefore, the NMSSM is a perfect framework for super-natural SUSY. To give the SUSY breaking soft mass to the singlet, we consider the moduli and dilaton dominant SUSY breaking scenarios in M-theory on $S^1/Z_2$. In these scenarios, SUSY is broken by one and only one $F$-term of moduli or dilaton, and the SUSY breaking soft terms can be determined via the Kähler potential and superpotential from Calabi-Yau compactification of M-theory on $S^1/Z_2$. Thus, as predicted by super-natural SUSY, the SUSY electroweak fine-tuning measure is of unity order. In the moduli dominant SUSY breaking scenario, the right-handed sleptons are relatively light around 1 TeV, stau can be even as light as 580 GeV and degenerate with the lightest neutralino, chargino masses are larger than 1 TeV, the light stop masses are around 2 TeV or larger, the first two-generation squark masses are about 3 TeV or larger, and gluinos are heavier than squarks. In the dilaton dominant SUSY breaking scenario, the qualitative picture remain the same but we have heavier spectra as compared to moduli dominant SUSY breaking scenario. In addition to it, we have Higgs $H_{2}/A_{1}$-resonance solutions for dark matter (DM). In both scenarios, the minimal value of DM relic density is about 0.2. To obtain the observed DM relic density, we can consider the dilution effect from supercritical string cosmology or introduce the axino as the lightest supersymmetric particle.

hep-ph↗

arXiv2015

The Renormalizable Three-Term Polynomial Inflation with Large Tensor-to-Scalar Ratio

We systematically study the renormalizable three-term polynomial inflation in the supersymmetric and non-supersymmetric models. The supersymmetric inflaton potentials can be realized in supergravity theory, and only have two independent parameters. We show that the general renormalizable supergravity model is equivalent to one kind of our supersymmetric models. We find that the spectral index and tensor-to-scalar ratio can be consistent with the Planck and BICEP2 results, but the running of spectral index is always out of the $2σ$ range. If we do not consider the BICEP2 experiment, these inflationary models can be highly consistent with the Planck observations and saturate its upper bound on the tensor-to-scalar ratio ($r \le 0.11$). Thus, our models can be tested at the future Planck and QUBIC experiments.

hep-ph↗

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

Group manifold approach to higher spin theory

We consider the group manifold approach to higher spin theory. The deformed local higher spin transformation is realized as the diffeomorphism transformation in the group manifold $\textbf{M}$. With the suitable rheonomy condition and the torsion constraint imposed, the unfolded equation can be obtained from the Bianchi identity, by solving which, fields in $\textbf{M}$ are determined by the multiplet at one point, or equivalently, by $(W^{[a(s-1),b(0)]}_μ,H)$ in $AdS_{4}\subset \textbf{M}$. Although the space is extended to $\textbf{M}$ to get the geometrical formulation, the dynamical degrees of freedom are still in $AdS_{4}$. The $4d$ equations of motion for $(W^{[a(s-1),b(0)]}_μ,H)$ are obtained by plugging the rheonomy condition into the Bianchi identity. The proper rheonomy condition allowing for the maximum on-shell degrees of freedom is given by Vasiliev equation. We also discuss the theory with the global higher spin symmetry, which is in parallel with the WZ model in supersymmetry.

hep-th↗

arXiv2015

The ATLAS Leptonic-$Z$ Excess from Light Squark Productions in the NMSSM Extension with a Heavy Dirac Gluino

The ATLAS Collaboration announced a 3$σ$ excess in the leptonic-$Z+jets+\met$ channel. We show that such an excess can be interpreted in the extension of the Next-to-Minimal Supersymmetric Standard Model (NMSSM) with a heavy Dirac gluino and light squarks. The abundant $Z$ bosons can be produced by light squark pair productions with the subsequent decays $\tilde{q} \to q \tildeχ_2^0 \to q Z \tildeχ_1^0$. We investigate the relevant parameter space by considering the constraints from both the ATLAS and CMS direct SUSY searches. Our model can provide sufficient $Z$-signal events in large parameter space if only the ATLAS searches are considered. After combining the ATLAS and CMS searches, the maximal number of signal events can still reach about 15, which is within the $1σ$ region of the observed excess. For comparison, we study the conventional low energy NMSSM with a Majorana gluino and its maximal number of signal events are about 11, although we cannot realize such model in the known supersymmetry breaking scenarios.

hep-ph↗

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

The Analytical One-Loop Contributions to Higgs Boson Mass in the Supersymmetric Standard Model with Vector-like Particles

In the Minimal Supersymmetric Standard Model (MSSM) with additional vector-like particles (VLPs), we for the first time derive the particle mass spectra and the Feynman rules, as well as analytically calculate the one-loop contributions to the Higgs boson mass from the fermions and sfermions. After discussing and numerically analysing a cases without bilinear terms and a case with a (partial) decoupling limit, we find: (i) The corrections depend on the mass splittings between quarks and squarks and between vector-like fermions and their sfermions; (ii) There exists the (partial) decoupling limit, where the VLPs decouple from the electrwoeak (EW) energy scale, even when one of the VLPs is light around the EW scale. The reason is that the contributions to Higgs mass can be suppressed by the (or partial) decoupling effects, which can make the EW phenomenology very different from the MSSM; (iii) The SM-like Higgs boson with mass around 125 GeV gives strong constraints on the VLPs if the top squarks are around 1~TeV. Moreover, we present some numerical analyses to understand these unique features.

hep-ph↗

arXiv2015

The Leptoquark Implication from the CMS and IceCube Experiments

The recent excess in the CMS measurements of $eejj$ and $eνjj$ channels and the emergence of PeV comsic neutrino events at the IceCube experiment share an intriguing implication for a leptoquark with a 600-650 GeV mass. We investigate the CMS constraints on the flavor structure of a scenario with the minimal leptoquark Yukawa couplings and correlate such a scenario to the resonant enhancement in the very high energy shower event rates at the IceCube. We find for a single leptoquark, the CMS signals require large couplings to the third generation leptons. This leads to an enhancement in the $ν_τ$-nucleon scattering cross-section and subsequently more $ν_τ$ events at PeV energies. However, a visible enhancement above the Standard Model scattering would require a leptoquark Yukawa coupling larger than one that can be easily tested at the upcoming LHC runs.

hep-ph↗