arXiv subjects

Tianjun Li

Publications and source records attributed to Tianjun Li.

At least 217 records · Page 12Linked to original sources

arXiv2013

No-Scale F-SU(5) in the Light of LHC, Planck and XENON

We take stock of the No-Scale F-SU(5) model's experimental status and prospects in the light of results from LHC, Planck, and XENON100. Given that no conclusive evidence for light Supersymmetry (SUSY) has emerged from the 7, 8 TeV collider searches, the present work is focused on exploring and clarifying the precise nature of the high-mass cutoff enforced on this model at the point where the stau and neutralino mass degeneracy becomes so tight that cold dark matter relic density observations cannot be satisfied. This hard upper boundary on the model's mass scale constitutes a top-down theoretical mandate for a comparatively light (and testable) SUSY spectrum which does not excessively stress natural resolution of the gauge hierarchy problem. The overlap between the resulting model boundaries and the expected sensitivities of the future 14 TeV LHC and XENON 1-Ton direct detection SUSY / dark matter experiments is described.

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arXiv2013

Implications of Higgs Sterility for the Higgs and Stop Sectors

The LHC data implies that the newly discovered Higgs boson h may be sterile (highly SM-like). In supersymmetric SMs (SSMs), Higgs couplings are often modified by Higgs mixing and stop loop corrections, so we study the Higgs sterility in the Higgs and stop sectors in two SSMs: (I) The Minimal SSM (MSSM). In the nearly decoupling region, the doublet-doublet mixing effect can only enhance C_{hb\bar b} by 2m_Z^2/M_A^2. Sterility places M_A\gtrsim 900 GeV. But it hardly constrains the stop sector due to the heaviness of Higgs boson mass m_h; (II) The next to MSSM(NMSSM). In the presence of doublet-singlet mixing, the mixing structure is complicated. We find a simple approximation to understand Higgs sterility and its implications, says the amount of pushing-up m_h\lesssim 5 GeV while the pulling-down scenario is favored. Stops can be light here, so Higgs sterility significantly constrains them directly and indirectly except for blind spots. We also study the LHC features of the whole stop sector facing a sterile Higgs and find that, in virtue of decays between stops and sbottom, characteristic signatures like same-sign leptons and multi b-jets are promising probes.

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arXiv2013

Correlated Event Excesses in LHC SUSY Searches at 7 & 8 TeV: New Physics or Conspiring Noise?

We examine the ATLAS and CMS 7 & 8 TeV multijet supersymmetry (SUSY) searches requiring the incidence of a single lepton in the framework of the supersymmetric grand unified model No-Scale Flipped SU(5) with extra vector-like flippon multiplets derived from F-Theory, or F-SU(5) for short. Investigated are five multijet+lepton SUSY searches: 4.7/fb ATLAS 7 TeV gluino and light stop searches, as well as 13/fb ATLAS and 9.7/fb CMS 8 TeV light stop searches. Most significantly, all five leptonic SUSY searches represent statistically independent data samples. Findings show that all five orthogonal sets of leptonic LHC observations give a lower bound to the gaugino mass scale at M_{1/2} >= 680 GeV, with all the current best fits correlating within a narrow region. Furthermore, eight statistically independent LHC SUSY search regions (leptonic + all-hadronic) accessible to the No-Scale F-SU(5) model space intersect with all the currently operating beyond the Standard Model experiments within the range of M_{1/2} = 680-850 GeV, with the upper bound established by the lower experimental limit of the anomalous magnetic moment (g_{mu}-2)/2 of the muon. We emphasize that this region of the F-SU(5) model space may not be fully probed by leptonic SUSY searches at the LHC until the 13 TeV LHC energizes in 2015. Additionally, we describe an efficient technique for the effective statistical disentanglement of searches sensitive to mutually overlapping event spaces.

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arXiv2013

Electroweak Supersymmetry (EWSUSY) in the NMSSM

To explain all the available experimental results, we have proposed the Electroweak Supersymmetry (EWSUSY) previously, 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. In the Next to Minimal Supersymmetric Standard Model (NMSSM), we perform the systematic χ^2 analyses on parameter space scan for three EWSUSY scenarios: (I) R-parity conservation and one dark matter candidate; (II) R-parity conservation and multi-component dark matter; (III) R-parity violation. We obtain the minimal χ^2/(degree of freedom) of 10.2/15, 9.6/14, and 9.2/14 respectively for Scenarios I, II, and III. Considering the constraints from the LHC neutralino/chargino and slepton searches, we find that the majority of viable parameter space prefered by the muon anomalous magnetic moment has been excluded except for the parameter space with moderate to large \tanβ(\ge 8). Especially, the most favorable parameter space has relatively large \tanβ, moderate λ, small μ_{eff}, heavy squarks/gluino, and the second lightest CP-even neutral Higgs boson with mass around 125 GeV. In addition, if the left-handed smuon is nearly degenerate with or heavier than Wino, there is no definite bound on Wino mass. Otherwise, the Wino with mass up to \sim 450 GeV has been excluded. Furthermore, we present several benchmark points for Scenarios I and II, and briefly discuss the prospects of the EWSUSY searches at the 14 TeV LHC and ILC.

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arXiv2013

Top and Bottom Seesaw from Supersymmetric Strong Dynamics

We propose a top and bottom seesaw model with partial composite top and bottom quarks. Such composite quarks and topcolor gauge bosons are bound states from supersymmetric strong dynamics by Seiberg duality. Supersymmetry breaking also induces the breaking of topcolor into the QCD gauge coupling. The low energy description of our model reduces to a complete non-minimal extension of the top seesaw model with bottom seesaw. The non-minimal nature is crucial for Higgs mixings and the appearance of light Higgs fields. The Higgs fields are bound states of partial composite particles with the lightest one compatible with a 125 GeV Higgs field which was discovered at the LHC.

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arXiv2013

Top SU(5) Models: Baryon and Lepton Number Violating Resonances at the LHC

We propose the minimal and renormalizable non-supersymmetric top SU(5) models where the $SU(5)\times SU(3)'_C \times SU(2)'_L \times U(1)'_Y$ gauge symmetry is broken down to the Standard Model (SM) gauge symmetry at the TeV scale. The first two families of the SM fermions are charged under $SU(3)'_C \times SU(2)'_L \times U(1)'_Y$ while the third family is charged under SU(5). In the minimal top SU(5) model, we show that the quark CKM mixing matrix can be generated via dimension-five operators, and the proton decay problem can be solved by fine-tuning the coefficients of the higher dimensional operators at the order of $10^{-4}$. In the renormalizable top SU(5) model, we can explain the quark CKM mixing matrix by introducing vector-like particles, and we do not have proton decay problem. The models give rise to leptoquark and diquark gauge bosons which violate both lepton and baryon numbers involving the third family quarks and leptons. The current experimental limits for these particles is well below the TeV scale. We also discuss the productions and decays of these new gauge bosons, and their ensuing signals, as well as their reach at the LHC.

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arXiv2013

Natural NMSSM confronting with the LHC7-8

The natural supersymmetry (SUSY) requires that stop, sbottom, and gluino be around one TeV or lighter. By using the direct SUSY search bounds from both ATLAS and CMS Collaborations, we examine the constraints on the natural SUSY in the Next to Minimal Supersymmetric Standard Model (NMSSM). We consider two cases of interpretations for the Higgs boson data: (1) the Standard Model (SM) like Higgs boson is the lightest CP-even Higgs boson; (2) the SM like Higgs boson is the second lightest CP-even Higgs boson. We find that the direct SUSY searches at the LHC impose a strong constraint on the light gluino scenarios, and in both cases the gluino can not be lighter than 1.1 TeV with sbottom mass up to 600 GeV and stop mass up to 550 GeV.

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arXiv2013

Probing the CP-even Higgs Sector via $H_3\to H_2H_1$ in the Natural NMSSM

After the discovery of a Standard Model (SM) like Higgs boson, naturalness strongly favors the next to the Minimal Supersymmetric SM (NMSSM). In this letter, we point out that the most natural NMSSM predicts the following CP-even Higgs $H_i$ sector: (A) $H_2$ is the SM-like Higgs boson with mass pushed-upward by a lighter $H_1$ with mass overwhelmingly within $[m_{H_2}/2,m_{H_2}]$; (B) $m_{H_3}\simeq 2μ/\sin2β\gtrsim300$ GeV; (C) $H_3$ has a significant coupling to top quarks and can decay to $H_1H_2$ with a large branching ratio. Using jet substructure we show that all the three Higgs bosons can be discovered via $gg\to H_3 \to H_1H_2\to b\bar b \ellνjj$ at the 14 TeV LHC. Especially, the LEP-LHC scenario with $H_1\simeq98$ GeV has a very good discovery potential.

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arXiv2013

Electroweak Supersymmetry around the Electroweak Scale

Inspired by the phenomenological constraints, LHC supersymmetry and Higgs searches, dark matter search as well as string model building, we propose the electroweak supersymmetry around the electroweak scale: the squarks and/or gluinos are around a few TeV while the sleptons, sneutrinos, bino and winos are within one TeV. The Higgsinos can be either heavy or light. We consider bino as the dominant component of dark matter candidate, and the observed dark matter relic density is achieved via the neutralino-stau coannihilations. Considering the Generalized Minimal Supergravity (GmSUGRA), we show explicitly that the electroweak supersymmetry can be realized, and the gauge coupling unification can be preserved. With two Scenarios, we study the viable parameter spaces that satisfy all the current phenomenological constraints, and we present the concrete benchmark points. Furthermore, we comment on the fine-tuning problem and LHC searches.

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arXiv2012

Octonion and Split Octonion Representation of SO(8) Symmetry

The 8 $\times$ 8 matrix representation of SO(8) Symmetry has been defined by using the direct product of Pauli matrices and Gamma matrices. These 8 $\times$ 8 matrices are being used to describe the rotations in SO(8) symmetry. The comparison of 8$\times$8 matrices with octonions has also been shown. The transformations of SO(8) symmetry are represented with the help of Octonions and split Octonions spinors.

math-ph↗

arXiv2012

Asymmetric Origin for Gravitino Relic Density in the Hybrid Gravity-Gauge Mediated Supersymmetry Breaking

We propose the hybrid gravity-gauge mediated supersymmetry breaking where the gravitino mass is about several GeV. The strong constraints on supersymmetry viable parameter space from the CMS and ATLAS experiments at the LHC can be relaxed due to the heavy colored supersymmetric particles, and it is consistent with null results in the dark matter (DM) direct search experiments such as XENON100. In particular, the possible maximal flavor and CP violations from the relatively small gravity mediation may naturally account for the recent LHCb anomaly. In addition, because the gravitino mass is around the asymmetric DM mass, we propose the asymmetric origin of the gravitino relic density and solve the cosmological coincident problem on the DM and baryon densities Ω_{\rm DM}:Ω_{B}\approx 5:1. The gravitino relic density arises from asymmetric metastable particle (AMP) late decay. However, we show that there is no AMP candidate in the minimal supersymmetric Standard Model (SM) due to the robust gaugino/Higgsino mediated wash-out effects. Interestingly, AMP can be realized in the well motivated supersymmetric SMs with vector-like particles or continuous U(1)_R symmetry. Especially, the lightest CP-even Higgs boson mass can be lifted in the supersymmetric SMs with vector-like particles.

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arXiv2012

Unity of elementary particles and forces for the third family

We propose a non-supersymmetric SU(5) model in which only the third family of fermions are unified. The model remedies the non-unification of the three Standard Model couplings in non-supersymmetric SU(5). It also provides a mechanism for baryon number violation which is needed for the baryon asymmetry of the Universe and is not present in the Standard Model. Current experimental constraints on the leptoquark gauge bosons, mediating such baryon and lepton violating interactions in our model, allow their masses to be at the TeV scale. These can be searched for as a (bτ) or (tt) resonance at the Large Hadron Collider as predicted in our model.

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arXiv2012

The Superluminal Neutrinos from Deformed Lorentz Invariance

We study two superluminal neutrino scenarios where δv\equiv (v-c)/c is a constant. To be consistent with the OPERA, Borexino, and ICARUS experiments and with the SN1987a observations, we assume that δv_ν on the Earth is about three order larger than that on the interstellar scale. To explain the theoretical challenges from the Bremsstrahlung effects and pion decays, we consider the deformed Lorentz invariance, and show that the superluminal neutrino dispersion relations can be realized properly while the modifications to the dispersion relations of the other Standard Model particles can be negligible. In addition, we propose the deformed energy and momentum conservation laws for a generic physical process. In Scenario I the momentum conservation law is preserved while the energy conservation law is deformed. In Scenario II the energy conservation law is preserved while the momentum conservation law is deformed. We present the energy and momentum conservation laws in terms of neutrino momentum in Scenario I and in terms of neutrino energy in Scenario II. In such formats, the energy and momentum conservation laws are exactly the same as those in the traditional quantum field theory with Lorentz symmetry. Thus, all the above theoretical challenges can be automatically solved. We show explicitly that the Bremsstrahlung processes are forbidden and there is no problem for pion decays.

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arXiv2012

The Race for Supersymmetric Dark Matter at XENON100 and the LHC: Stringy Correlations from No-Scale F-SU(5)

The discovery of supersymmetry (SUSY) via action of the cold dark matter candidate is being led on the indirect collider production front by the LHC, and correspondingly by the XENON100 collaboration on the direct detection front. We undertake a dual case study of the specific SUSY signatures which the No-Scale flipped SU(5) x U(1)_X grand unified theory with TeV-scale vector-like particles (No-Scale F-SU(5)) would exhibit at each of these experiments. We demonstrate a correlation between the near-term prospects of these two distinct approaches. We feature a dark matter candidate which is over 99% bino due to a comparatively large Higgs bilinear mass μ-term around the electroweak scale, and thus automatically satisfy the current constraints from the XENON100 and CDMS/EDELWEISS experiments. We do however expect that the ongoing extension of the XENON100 run may effectively probe our model. Likewise, our model is also currently being probed by the LHC via a search for events with ultra-high multiplicity hadronic jets, which are a characteristic feature of the distinctive No-Scale F-SU(5) mass hierarchy.

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arXiv2012

A 125.5 GeV Higgs Boson in F-SU(5): Imminently Observable Proton Decay, A 130 GeV Gamma-ray Line, and SUSY Multijets & Light Stops at the LHC8

We establish that the light Higgs boson mass in the context of the No-Scale Flipped SU(5) GUT with TeV scale vector-like matter multiplets (flippons) is consistent with m_h = 125.5+-0.5 GeV in the region of the best supersymmetry (SUSY) spectrum fit to low statistics data excesses observed by ATLAS in multijet and light stop 5/fb SUSY searches at the LHC7. Simultaneous satisfaction of these disparate goals is achieved by employing a minor decrease in the SU(5) partial unification scale M_{32} to lower the flippon mass, inducing a larger Higgs boson mass shift from the flippon loops. The reduction in M_{32}, which is facilitated by a phenomenologically favorable reduction of the low-energy strong coupling constant, moreover suggests an imminently observable (e|mu)^+ pi^0 proton decay with a central value time scale of 1.7x10^34 years. At the same point in the model space, we find a lightest neutralino mass of m_χ = 145 GeV, which is suitable for the production of 130 GeV monochromatic gamma-rays through annihilations yielding associated Z-bosons; a signal with this energy signature has been identified within observations of the galactic center by the FERMI-LAT Space Telescope. In conjunction with direct correlations to the fate of the ATLAS multijet and light stop production channels presently being tested at the LHC8, we suggest that the reality of a 125.5 GeV Higgs boson affords a particularly rich company of specific and imminently testable associated observables.

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arXiv2012

On Naturalness of the MSSM and NMSSM

With a bottom-up approach, we consider naturalness in the MSSM and NMSSM. Assuming the light stops, the LHC gluino search implies that the degree of fine tuning in both models is less than 2.5%. Taking the LHC hints for the SM-like Higgs boson mass m_h\sim125 GeV seriously, we find that naturalness will favor the NMSSM. We study the Higgs boson mass for several scenarios in the NMSSM: (1) A large λand the doublet-singlet Higgs boson mixing effect pushing upward or pulling downward m_h. The former case can readily give the di-photon excess of the Higgs boson decay whereas the latter case can not. However, we point out that the former case has a new large fine-tuning related to strong λ-RGE running effect and vacuum stability. (2) A small λand the mixing effect pushing m_h upward. Naturalness status becomes worse and no significant di-photon excess can be obtained. In these scenarios, the lightest supersymmetric particle (LSP) as a dark matter candidate is strongly disfavored by the XENON100 experiment. Even if the LSP can be a viable dark matter candidate, there does exist fine-tuning. The above naturalness evaluation is based on a high mediation scale for supersymmetry breaking, whereas for a low mediation scale, fine-tuning can be improved by about one order.

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arXiv2012

Origins of the Isospin Violation of Dark Matter Interactions

Light dark matter (DM) with a large DM-nucleon spin-independent cross section and furthermore proper isospin violation (ISV) $f_n/f_p\approx-0.7$ may provide a way to understand the confusing DM direct detection results. Combing with the stringent astrophysical and collider constraints, we systematically investigate the origin of ISV first via general operator analyses and further via specifying three kinds of (single) mediators: A light $Z'$ from chiral $U(1)_X$, an approximate spectator Higgs doublet (It can explain the $W+jj$ anomaly simultaneously) and color triplets. In addition, although $Z'$ from an exotic $U(1)_X$ mixing with $U(1)_Y$ generating $f_n=0$, we can combine it with the conventional Higgs to achieve proper ISV. As a concrete example, we propose the $U(1)_X$ model where the $U(1)_X$ charged light sneutrino is the inelastic DM, which dominantly annihilates to light dark states such as $Z'$ with sub-GeV mass. This model can address the recent GoGeNT annual modulation consistent with other DM direct detection results and free of exclusions.

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arXiv2012

One-Parameter Model for the Superworld

We review the No-Scale F-SU(5) model with extra TeV-scale vector-like flippon multiplets and its associated collider phenomenology in 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 SUSY spectrum that is consistent with emerging low-statistics excesses beyond the Standard Model expectation in the ATLAS and CMS multijet data.

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