arXiv subjects

Tianjun Li

Publications and source records attributed to Tianjun Li.

At least 181 records · Page 10Linked to original sources

arXiv2015

The Supersymmetric Standard Models with a Pseudo-Dirac Gluino from Hybrid $F-$ and $D-$Term Supersymmetry Breakings

We propose the Supersymmetric Standard Models (SSMs) with a pseudo-Dirac gluino from hybrid $F-$ and $D-$term supersymmetry (SUSY) breakings. Similar to the SSMs before the LHC, all the supersymmetric particles in the Minimal SSM (MSSM) obtain the SUSY breaking soft terms from the traditional gravity mediation and have masses within about 1 TeV except gluino. To evade the LHC SUSY search constraints, the gluino also has a heavy Dirac mass above 3 TeV from $D-$term SUSY breaking. Interestingly, such a heavy Dirac gluino mass will not induce the electroweak fine-tuning problem. We realize such SUSY breakings via an anomalous $U(1)_X$ gauge symmetry inspired from string models. To maintain the gauge coupling unification and increase the Higgs boson mass, we introduce extra vector-like particles. We study the viable parameter space which satisfies all the current experimental constraints, and present a concrete benchmark point. This kind of models not only preserves the merits of pre-LHC SSMs such as naturalness, dark matter, etc, but also solves the possible problems in the SSMs with Dirac gauginos due to the $F$-term gravity mediation.

hep-ph↗

arXiv2015

Towards the Natural Gauge Mediation

The sweet spot supersymmetry (SUSY) solves the mu problem in the Minimal Supersymmetric Standard Model (MSSM) with gauge mediated SUSY breaking (GMSB) via the generalized Giudice-Masiero (GM) mechanism where only the mu-term and soft Higgs masses are generated at the unification scale of the Grand Unified Theory (GUT) due to the approximate PQ symmetry. Because all the other SUSY breaking soft terms are generated via the GMSB below the GUT scale, there exists SUSY electroweak (EW) fine-tuning problem to explain the 125 GeV Higgs boson mass due to small trilinear soft term. Thus, to explain the Higgs boson mass, we propose the GMSB with both the generalized GM mechanism and Higgs-messenger interactions. The renormalization group equations are runnings from the GUT scale down to EW scale. So the EW symmetry breaking can be realized easier. We can keep the gauge coupling unification and solution to the flavor problem in the GMSB, as well as solve the μ/B_μ-problem. Moreover, there are only five free parameters in our model. So we can determine the characteristic low energy spectra and explore its distinct phenomenology. The low-scale fine-tuning measure can be as low as 20 with the light stop mass below 1 TeV and gluino mass below 2 TeV. The gravitino dark matter can come from a thermal production with the correct relic density and be consistent with the thermal leptogenesis. Because gluino and stop can be relatively light in our model, how to search for such GMSB at the upcoming run II of the LHC experiment could be very interesting.

hep-ph↗

arXiv2015

NMSSM explanations of the Galactic center gamma ray excess and promising LHC searches

The Galactic Center Excess (GCE) is explained in the framework of the Next-to-Minimal Supersymmetric Standard Model (NMSSM) with a $Z_3$ discrete symmetry. We show that a resonant CP-odd Higgs boson with mass twice that of the Dark Matter (DM) candidate is favoured. Meanwhile, the DM candidate is required to have relatively large coupling with the $Z$ boson through its Higgsino component in order to obtain correct DM relic density. Its LHC discovery potential via four signatures is discussed in detail. We find that the most sensitive signals are provided by the Higgsino-like chargino and neutralino pair production with their subsequent decays into $W$ bosons, $Z$ bosons, and DM. The majority of the relevant parameter space can be probed at the Large Hadron Collider with a centre-of-mass energy of 14 TeV and an integrated luminosity 1000 fb$^{-1}$.

hep-ph↗

arXiv2015

The Electroweak Supersymmetry (EWSUSY) from the GmSUGRA in the MSSM

Considering the Generalized Minimal Supergravity Model (GmSUGRA) in the Minimal Supersymmetric Standard Model (MSSM), we study the Electroweak Supersymmetry (EWSUSY), where the squarks and/or gluino are heavy around a few TeVs while the sleptons, sneutrinos, Bino, Winos, and/or Higgsinos are light within one TeV. We resolves the $(g_μ-2)/2$ discrepancy for the muon anomalous magnetic moment in the Standard Model (SM) successfully and identifies a parameter space where such solutions also have the electroweak fine-tuning measures $Δ_{EW}~16.5$ (6$\%$) and $Δ_{EW}~25$ (4$\%$) without and with the WMAP bounds, respectively. We find that the allowed mass ranges consistent within 3$σ$ of the $g-2$ discrepancy for the lightest neutralino, charginos, stau, stau neutrinos, and first two-family sleptons are $[44,390]$ GeV, $[100,700]$ GeV, $[100,700]$, and $[52,700]$ GeV, respectively. Moreover, our solutions satisfy the latest bounds reported by the ATLAS and CMS Collaborations on electroweakinos and sleptons. The colored sparticles such as light stop, gluinos, and the first two-generation squark masses have been found in the mass ranges of $[500, 3000]$ GeV,[1300, 4300] GeV, and $[1800, 4200]$ GeV, respectively. To obtain the observed dark matter relic density for the Lightest Supersymmetric Particle (LSP) neutralino, we have the bino-wino, LSP neutralino-stau, and LSP neutralino-tau sneutrinos coannihilation scenarios, and the resonance solutions such as $A$-pole, Higgs-pole, and $Z$-pole. We identify the higgsino-like LSP neutralino and display its spin-independent and spin-dependent cross sections with nucleons. We present ten benchmark points which can be tested at the up coming collider searches as well.

hep-ph↗

arXiv2015

Two component dark matter with multi-Higgs portals

With the assistance of two extra groups, i.e., an extra hidden gauge group $SU(2)_D$ and a global $U(1)$ group, we propose a two component dark matter (DM) model. After the symmetry $SU(2)_D\times U(1)$ being broken, we obtain both the vector and scalar DM candidates. The two DM candidates communicate with the standard model (SM) via three Higgs as multi-Higgs portals. The three Higgs are mixing states of the SM Higgs, the Higgs of the hidden sector and real part of a supplement complex scalar singlet. We study relic density and direct detection of DM in three scenarios. The resonance behaviors and interplay between the two component DM candidates are represented through investigating of the relic density in the parameter spaces of the two DMs masses. The electroweak precision parameters constrains the two Higgs portals couplings ($λ_m$ and $δ_2$). The relevant vacuum stability and naturalness problem in the parameter space of $λ_m$ and $δ_2$ are studied as well. The model could alleviate these two problems in some parameter spaces under the constraints of electroweak precision observables and Higgs indirect search.

hep-ph↗

arXiv2015

Super-Natural MSSM

We point out that the electroweak fine-tuning problem in the supersymmetric Standard Models (SSMs) is mainly due to the high energy definition of the fine-tuning measure. We propose super-natural supersymmetry which has an order one high energy fine-tuning measure automatically. The key point is that all the mass parameters in the SSMs arise from a single supersymmetry breaking parameter. In this paper, we show that there is no supersymmetry electroweak fine-tuning problem explicitly in the Minimal SSM (MSSM) with no-scale supergravity and Giudice-Masiero (GM) mechanism. We demonstrate that the $Z$-boson mass, the supersymmteric Higgs mixing parameter $μ$ at the unification scale, and the sparticle spectrum can be given as functions of the universal gaugino mass $M_{1/2}$. Because the light stau is the lightest supersymmetric particle (LSP) in the no-scale MSSM, to preserve $R$ parity, we introduce a non-thermally generated axino as the LSP dark matter candidate. We estimate the lifetime of the light stau by calculating its 2-body and 3-body decays to the LSP axino for several values of axion decay constant $f_a$, and find that the light stau has a lifetime $τ_{\tilde τ_1}$ in $[10^{-4},100]$ s for an $f_a$ range $[10^{9},10^{12}]$ GeV. We show that our next to the LSP stau solutions are consistent with all the current experimental constraints, including the sparticle mass bounds, B-physics bounds, Higgs mass, cosmological bounds, and the bounds on long-lived charge particles at the LHC.

hep-ph↗

arXiv2015

Symmetry Breaking Indication for Supergravity Inflation in Light of the Planck 2015

The supergravity (SUGRA) theories with exact global $U(1)$ symmetry or shift symmetry in Kähler potential provide the natural frameworks for inflation. However, the quadratic inflation is disfavoured by the new results on primordial tensor fluctuations from the Planck Collaboration. To be consistent with the new Planck data, we point out that the explicit symmetry breaking is needed, and study these two SUGRA inflation in details. For the SUGRA inflation with global $U(1)$ symmetry, the symmetry breaking term leads to a trigonometric modulation on inflaton potential. The coefficient of the $U(1)$ symmetry breaking term is of the order $10^{-2}$, which is sufficient large to improve the inflationary predictions while its higher order corrections are negligible. Such models predict sizeable tensor fluctuations and highly agree with the Planck results. In particular, the model with a linear $U(1)$ symmetry breaking term predicts the tensor-to-scalar ratio around $\textbf{r}\sim0.01$ and running spectral index $α_s\sim-0.004$, which comfortably fit with the Planck observations. For the SUGRA inflation with breaking shift symmetry, the inflaton potential is modulated by an exponential factor. The modulated linear and quadratic models are consistent with the Planck observations. In both kinds of models the tensor-to-scalar ratio can be of the order $10^{-2}$, which will be tested by the near future observations.

hep-ph↗

arXiv2015

Modified Lyth bound and implications of BICEP2 results

To reconcile the BICEP2 measurement on the tensor-to-scalar ratio $r$ with Planck constraint, a large negative running of scalar spectral index $n_s$ is needed. So the inflationary observable such as $n_s$ should be expanded at least to the second-order slow-roll parameters for single-field inflationary models. The large value of $r$ and the Lyth bound indicate that it is impossible to obtain the sub-Planckian excursion for the inflaton. However, we derive an absolutely minimal bound $Δϕ/M_{\rm Pl}>\sqrt{r/2}$ on the inflaton excursion for single-field inflationary models, which can be applied to non-slow-roll inflationary models as well. This bound excludes the possibility of the small-field inflation with $Δϕ<0.1 M_{\rm Pl}$ if the BICEP2 result on $r$ stands and it opens the window of sub-Planckian excursion with $Δϕ<M_{\rm Pl}$ even if $r$ is as large as $0.1$. To get the sub-Planckian excursion with $Δϕ<0.1 M_{\rm Pl}$, our modified bound requires $r<0.02$. Using a fifth-order polynomial potential as an explicit example, we show that it not only agrees with the observational results, but also violates the Lyth bound.

gr-qc↗

arXiv2015

Natural $X$-ray Lines from the Low Scale Supersymmetry Breaking

In the supersymmetric models with low scale supersymmetry (SUSY) breaking where the gravitino mass is around keV, we show that the 3.5 keV $X-$ray lines can be explained naturally through several different mechanisms: (I) A keV scale dark gaugino plays the role of sterile neutrino in the presence of bilinear $R-$partiy violation. Because the light dark gaugino obtains Majorana mass only via gravity mediation, it is a decaying warm dark matter (DM) candidate; (II) Compressed cold DM states, whose mass degeneracy is broken by gavity mediated SUSY breaking, emit such a line via the heavier one decay into the lighter one plus photon(s). A highly supersymmetric dark sector may readily provide such kind of system; (III) Light axino, whose mass again is around the gravitino mass, decays to neutrino plus gamma in the $R-$partiy violating SUSY. Moreover, we comment on dark radiation from dark gaugino.

hep-ph↗

arXiv2014

Third Generation in Cascade Decays

In supersymmetric models with gluinos around 1000-2000 GeV, new physics searches based on cascade decay products of the gluino are viable at the next run of the LHC. We investigate a scenario where the light stop is lighter than the gluino and both are lighter than all other squarks, and show that its signal can be established using multi b-jet, multi W and/or multi lepton final state topologies. We then utilize both boosted and conventional jet topologies in the final state in conjunction with di-tau production as a probe of the stau-neutralino co-annihilation region responsible for the model's dark matter content. This study is performed in the specific context of one such phenomenologically viable model named No-Scale F-SU(5).

hep-ph↗

arXiv2014

Helical Phase Inflation and Monodromy in Supergravity Theory

We study helical phase inflation in supergravity theory in details. The inflation is driven by the phase component of a complex field along helical trajectory. The helicoid structure originates from the monodromy of superpotential with an singularity at origin. We show that such monodromy can be formed by integrating out heavy fields in supersymmetric field theory. The supergravity corrections to the potential provide strong field stabilizations for the scalars except inflaton, therefore the helical phase inflation accomplishes the "monodromy inflation" within supersymmetric field theory. The phase monodromy can be easily generalized for natural inflation, in which the super-Planckian phase decay constant is realized with consistent field stabilization. The phase-axion alignment is fulfilled indirectly in the process of integrating out the heavy fields. Besides, we show that the helical phase inflation can be naturally realized in no-scale supergravity with $SU(2,1)/SU(2)\times U(1)$ symmetry since the no-scale Kähler potential provides symmetry factors of phase monodromy directly. We also demonstrate that the helical phase inflation can reduce to the shift symmetry realization of supergravity inflation. The super-Planckian field excursion is accomplished by the phase component, which admits no dangerous polynomial higher order corrections. The helical phase inflation process is free from the UV-sensitivity problem, and it suggests that inflation can be effectively studied in supersymmetric field theory close to the unification scale in Grand Unified Theory and a UV-completed frame is not prerequisite.

hep-th↗

arXiv2014

Helical Phase Inflation

We show that the quadratic inflation can be realized by the phase of a complex field with helicoid potential. Remarkably, this helicoid potential can be simply realized in minimal supergravity. The global $U(1)$ symmetry of the Kähler potential introduces a flat direction and evades the $η$ problem automatically. So such inflation is technically natural. The phase excursion is super-Planckian as required by the Lyth bound, while the norm of the complex field can be suppressed in the sub-Planckian region. This model resolves the ultraviolet sensitive problem of the large field inflation, besides, it also provides a new type of monodromy inflation in supersymmetric field theory with consistent field stabilization.

hep-th↗

arXiv2014

Supersymmetry with a Heavy Lightest Supersymmetric Particle

To escape the current LHC supersymmetry (SUSY) search constraints while preserve the naturalness condition, we propose the heavy Lightest Supersymmetric Particle (LSP) SUSY. According to the different dependence on the LSP mass, we systematically classify the discriminating variables into three categories. We find the strong dependence of all current SUSY searches on variables in the first category render the weak sensitivity for the heavy LSP SUSY. Especially, all the current LHC SUSY search constraints can be evaded if the LSP mass is around 600 GeV or higher. In the Minimal Supersymmetric Standard Model (MSSM), we find that the heavy LSP SUSY does not induce more fine-tuning than Higgs boson mass. Moreover, the muon anomalous magnetic moment can be satisfied within 3-$σ$ level. We systematically study the viable parameter space for heavy LSP SUSY, and present four benchmark points which realize our proposal concretely. An improved collider search for those benchmark points, which mainly relies on the variable in the second category, is discussed in detail.

hep-ph↗

arXiv2014

Confronting Electroweak Fine-tuning with No-Scale Supergravity

Applying No-Scale Supergravity boundary conditions at a heavy unification scale to the Flipped $SU(5)$ grand unified theory with extra TeV-scale vector-like multiplets, $i.e.$ No-Scale ${\cal F}$-$SU(5)$, we express the $Z$-boson mass $M_Z$ as an explicit function of the boundary gaugino mass $M_{1/2}$, $M_Z^2 = M_Z^2 (M_{1/2}^2)$, with implicit dependence upon a dimensionless ratio $c$ of the supersymmetric Higgs mixing parameter $μ$ and $M_{1/2}$. Setting the top Yukawa coupling consistent with $m_t = 174.3$ GeV at $M_Z = 91.2$ GeV, the value of $c$ naturally tends toward $c \simeq 1$, which indirectly suggests underlying action of the Giudice-Masiero mechanism. Proportional dependence of all model scales upon the unified gaugino mass $M_{1/2}$ in the No-Scale ${\cal F}$-$SU(5)$ model suggests one possible mechanism of confronting the electroweak fine tuning problem.

hep-ph↗

arXiv2014

Fractional chaotic inflation in the lights of PLANCK and BICEP2

In the lights of current BICEP2 observations accompanied with the PLANCK satellite results, it has been observed that the simple single field chaotic inflationary models provide a good agreement with their spectral index n_s and large tensor-to-scalar ratio r (0.15 <r <0.26). To explore the other simple models, we consider the fractional-chaotic inflationary potentials of the form V_0 phi^(a/b) where a and b are relatively prime. We show that such kind of inflaton potentials can be realized elegantly in the supergravity framework with generalized shift symmetry and a nature bound a/b < 4 for consistency. Especially, for the number of e-folding from 50 to 60 and some a/b from 2 to 3, our predictions are nicely within at least 1 $σ$ region in the r-n_s plane. We also present a systematic investigation of such chaotic inflationary models with fractional exponents to explore the possibilities for the enhancement in the magnitude of running of spectral index (α_{n_s}) beyond the simplistic models.

hep-ph↗

arXiv2014

Combining Universal and Odd RR Axions for Aligned Natural Inflation

We successfully embed the Kim-Nilles-Peloso (KNP) alignment mechanism for enhancing the axion decay constant in the context of large volume type IIB orientifolds. The flat direction is generated in the plane of ($C_0$-$C_2$) axions corresponding to the involutively even universal axion $C_0$ and odd axion $C_2$, respectively. The moduli stabilization with large volume scheme has been established as well.

hep-th↗

arXiv2014

Cosmological observables in multi-field inflation with a non-flat field space

Using $δN$ formalism, in the context of a generic multi-field inflation driven on a non-flat field space background, we revisit the analytic expressions of the various cosmological observables such as scalar/tensor power spectra, scalar/tensor spectral tilts, non-Gaussianity parameters, tensor-to-scalar ratio, and the various runnings of these observables. In our backward formalism approach, the subsequent expressions of observables automatically include terms beyond the leading order slow-roll expansion correcting many of the expression at subleading order. To connect our analysis properly with the earlier results, we rederive the (well) known (single field) expressions in the limiting cases of our generic formulae. Further, in the light of PLANCK results, we examine for the compatibility of the consistency relations within the slow-roll regime of a two-field roulette poly-instanton inflation realized in the context of large volume scenarios.

hep-th↗

arXiv2014

Distinguishing the right-handed up/charm quarks from top quark via discrete symmetries in the standard model extensions

We propose a class of the two Higgs doublet Standard models (SMs) with a SM singlet and a class of supersymmetric SMs with two pairs of Higgs doublets, where the right-handed up/charm quarks and the right-handed top quark have different quantum numbers under extra discrete symmetries. Thus, the right-handed up and charm quarks couple to one Higgs doublet field, while the right-handed top quark couples to another Higgs doublet. The quark CKM mixings can be generated from the down-type quark sector. As one of phenomenological consequences in our models, we explore whether one can accommodate the observed direct CP asymmetry difference in singly Cabibbo-suppressed D decays. We show that it is possible to explain the measured values of CP violation under relevant experimental constraints.

hep-ph↗