arXiv subjects

Tianjun Li

Publications and source records attributed to Tianjun Li.

At least 127 records · Page 7Linked to original sources

arXiv2018

A model of vector leptoquarks in view of the $B$-physics anomalies

Lepton number as a fourth color is an intriguing theoretical idea which is combined with a possible left-right symmetry within the famous Pati-Salam (PS) model. In the conventional PS model, a spontaneous breaking of the PS gauge group down to the SM one can only take place at very high scales (above the PeV scale) due to the stringent bounds from $K_L\toμe$ and $K\toπμe$ induced by the resulting vector leptoquarks. In this paper, we show that these constraints can be avoided once additional vector-like fermions are introduced and thus a breaking at the TeV scale is possible. We consider the flavor phenomenology of this model in the context of the intriguing hints for new physics in semileptonic $B$ decays. The necessary violation of lepton flavor universality is induced by mixing SM and vector-like fermions. Concerning $R(D)$ and $R(D^*)$ we find that sizable effects are possible while respecting the bounds from other flavor observables but predicting a large enhancement of $B_s\toτ^+τ^-$. Furthermore, also in $b\to s\ell^+\ell^-$ transitions the observed deviations from the SM predictions (including $R(K)$ and $R(K^*)$) can be explained with natural values for the free parameters of the model without any fine-tuning, predicting sizable decay rates for $b\to sτμ$. Finally, the anomaly in anomalous magnetic moment of the muon can be accounted for by a loop-contribution involving the vector leptoquark and vector-like leptons.

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arXiv2018

Probing the dark matter-electron interactions via hydrogen-atmosphere pulsating white dwarfs

In this work, we propose a novel scenario to probe the interactions between dark matter (DM) particles and electrons, via hydrogen-atmosphere pulsating white dwarfs (DAVs) in globular clusters. In this special configuration, the DM particles, which are predominantly captured by multiple scattering with the electrons in a DAV, would annihilate by pairs and provide extra energy source to the DAV. This mechanism slows down the natural cooling evolution which can be presented by the period variation rates of pulsation modes. The differences between the secular rates predicted by the precise asteroseismology and the secular rates obtained from observation can reveal the DM-electron interactions. An important observable has been proposed and corresponding estimations have been made. According to the estimation, if this scenario could be implemented in the near future, the potential sensitivity on $m_χ$ (DM particle's mass) and $σ_{χ,e}$ (elastic scattering cross section between DM and electron) could be hopefully extended to a region $5 \mathrm{GeV} \lesssim m_χ \lesssim 10^{4} \mathrm{GeV}$ and $σ_{χ,e} \gtrsim 10^{-40} \mathrm{cm}^{2}$. Combining with indirect DM detection results, this could give us a cross check on the existence of such leptonphilic DM particles to some extent.

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arXiv2018

Deep learning for the R-parity violating supersymmetry searches at the LHC

Supersymmetry with hadronic R-parity violation in which the lightest neutralino decays into three quarks is still weakly constrained. This work aims to further improve the current search for this scenario by the boosted decision tree method with additional information from jet substructure. In particular, we find a deep neural network turns out to perform well in characterizing the neutralino jet substructure. We first construct a Convolutional Neutral Network (CNN) which is capable of tagging the neutralino jet in any signal process by using the idea of jet image. When applied to pure jet samples, such a CNN outperforms the N-subjettiness variable by a factor of a few in tagging efficiency. Moreover, we find the method, which combines the CNN output and jet invariant mass, can perform better and is applicable to a wider range of neutralino mass than the CNN alone. Finally, the ATLAS search for the signal of gluino pair production with subsequent decay $\tilde{g} \to q q \tildeχ^0_1 (\to q q q)$ is recasted as an application. In contrast to the pure sample, the heavy contamination among jets in this complex final state renders the discriminating powers of the CNN and N-subjettiness similar. By analyzing the jets substructure in events which pass the ATLAS cuts with our CNN method, the exclusion limit on gluino mass can be pushed up by $\sim200$ GeV for neutralino mass $\sim 100$ GeV.

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arXiv2018

Inspiration from Intersecting D-branes: General Supersymmetry Breaking Soft Terms in No-Scale ${\cal F}$-$SU(5)$

Motivated by D-brane model building, we evaluate the $\cal{F}$-$SU(5)$ model with additional vector-like particle multiplets, referred to as flippons, within the framework of No-Scale Supergravity with non-vanishing general supersymmetry breaking soft terms at the string scale. The viable phenomenology is uncovered by applying all current experimental constraints, including but not limited to the correct light Higgs boson mass, WMAP and Planck relic density measurements, and several LHC constraints on supersymmetric particle spectra. Four interesting regions of the parameter space arise, as well as mixed scenarios, given by: (i) light stop coannihilation; (ii) pure Higgsino dark matter; (iii) Higgs funnel; and (iv) light stau coannihilation. All regions can generate the observed value of the relic density commensurate with a 125 GeV light Higgs boson mass, with the exception of the relatively small relic density value for the pure Higgsino lightest supersymmetric particle (LSP). This work is concluded by gauging the model against present LHC search constraints and derivation of the final states observable at the LHC for each of these scenarios.

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arXiv2018

Natural explanation for 21cm absorption signals via axion-induced cooling

The EDGES Collaboration has reported an anomalously strong 21cm absorption feature corresponding to the era of first star formation, which may indirectly betray the influence of dark matter during this epoch. We demonstrate that, by virtue of the ability to mediate cooling processes whilst in the condensed phase, a small amount of axion dark matter can explain these observations within the context of standard models of axions and axion-like-particles. The EDGES best-fit result favours an axion-like-particles mass in the (10, 450) meV range, which can be compressed for the QCD axion to (100, 450) meV in the absence of fine tuning. Future experiments and large scale surveys, particularly the International Axion Observatory (IAXO) and EUCLID, should have the capability to directly test this scenario.

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arXiv2018

The Generalized Abelian and Non-Abelian Gauge Theories and their Particle Physics Applications

Gauge theory is the foundation of the particle physics Standard Model (SM). Considering the multiple gauge sectors for one gauge transformation, we study the generalized Abelian and non-Abelian (Yang-Mills theory) gauge theories. We first point out that the U(1) gauge theory has a few unique properties, which provide the motivations for the generalized Yang-Mills theory. Also, we consider the generalized Abelian gauge theory, and study the Higgs mechanism with new interesting properties. In addition, we propose the simple and generic generalizations of Yang-Mills theory. In the simple generalization, we realize two specific properties in the Abelian gauge theory. For applications in particle physics, we propose the invisible axion model with TeV-scale Peceei-Quinn symmetry breaking. We can solve the strong CP problem, and obtain the effective decay constant around the intermediate scale. Moreover, we study the SM electroweak symmetry breaking induced from the symmetry breaking in the other gauge Sector. In particular, we can easily obtain the strong first order electroweak phase transition in the SM, which is important for electroweak baryogenesis and gravitational wave.

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arXiv2018

Natural Higgs Inflation, Gauge Coupling Unification, and Neutrino Masses

We present a class of non-supersymmetric models in which so-called critical Higgs inflation ($ξ<100$) naturally can be realized without using specific values for Higgs and top quark masses. In these scenarios, the Standard Model (SM) vacuum stability problem, gauge coupling unification, neutrino mass generation and Higgs inflation mechanism are linked to each other. We adopt in our models Type I seesaw mechanism for neutrino masses. An appropriate choice of the Type I Seesaw scale allows us to have an arbitrarily small but positive value of SM Higgs quartic coupling around the inflation scale. We present a few benchmark points where we show that the scalar spectral indices are around 0.9626 and 0.9685 for the number of e-folding $N=50$ and $N=60$ respectively. The tensor-to-scalar ratios are order of $10^{-3}$. The running of the scalar spectral index is negative and is order of $10^{-4}$.

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arXiv2018

Direct Detection of Axion-Like Particles in Bismuth-Based Topological Insulators

In recent years a new field emerged in dark matter community and immediately attracted a multitude of theoriests and experimentalists, that of light dark matter direct detection in electronic systems. The phenomenon is similar with nuclear recoil in elastic scattering between dark matter and nucleus but with different kinematics. Due to the small energy gap, the electronic system can probe sub-GeV dark matter rather than nucleus target. In particular the absorption into materials can even detect ultra-light dark matter within mass around meV. In terms of the equivalence between optical conductivity and absorption cross section, axion detection can be computed in Bismuth-based topological insulators. It is found that topological insulator has strong sensitivity on axion and provides a complementary direct detection to superconductor and semiconductors. The novelty of topological insulator is that the thin film could even obtain the same sensitivity as superconductor.

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arXiv2018

Non-locality and late-time cosmic acceleration from an ultraviolet complete theory

A local phenomenological model that reduces to a non-local gravitational theory giving dark energy is proposed. The non-local gravity action is known to fit the data as well as $Λ$-CDM thereby demanding a more fundamental local treatment. It is seen that the scale-invariant higher-derivative scalar-tensor theory of gravity, which is known to be ultraviolet perturbative renormalizable to all loops and where ghosts become innocuous, generates non-locality at low energies. The local action comprises of two real scalar fields coupled non-minimally with the higher-derivative gravity action. When one of the scalar acquiring the Vacuum Expectation Value (VEV) induces Einstein--Hilbert gravity, generates mass for fields, and gets decoupled from system, it leaves behind a residual theory which in turn leads to a non-local gravity generating dark energy effects.

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arXiv2018

Ultraviolet complete dark energy model

We consider a local phenomenological model to explain a non-local gravity scenario which has been proposed to address dark energy issues. This non-local gravity action has been seen to fit the data as well as $Λ$-CDM and therefore demands a more fundamental local treatment. The induced gravity model coupled with higher-derivative gravity is exploited for this proposal, as this perturbatively renormalizable model has a well-defined ultraviolet (UV) description where ghosts are evaded. We consider a generalised version of this model where we consider two coupled scalar fields and their non-minimal coupling with gravity. In this simple model, one of the scalar field acquires a Vacuum Expectation Value (VEV), thereby inducing a mass for one of the scalar fields and generating Newton's constant. The induced mass however is seen to be always above the running energy scale thereby leading to its decoupling. The residual theory after decoupling becomes a platform for driving the accelerated expansion under certain conditions. Integrating out the residual scalar generates a non-local gravity action. The leading term of which is the non-local gravity action used to fit the data of dark energy.

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arXiv2018

Bayesian analysis of the break in DAMPE lepton spectra

Recently, DAMPE has released its first results on the high-energy cosmic-ray electrons and positrons (CREs) from about $25$ GeV to $4.6$ TeV, which directly detect a break at $\sim 1$ TeV. This result gives us an excellent opportunity to study the source of the CREs excess. In this work, we used the data fo proton and helium flux (from AMS-02 and CREAM), $\bar{\mathrm{p}}/\mathrm{p}$ ratio (from AMS-02), positron flux (from AMS-02) and CREs flux (from DAMPE without the peak signal point at $\sim 1.4$ TeV) to do global fitting simultaneously, which can account the influence from the propagation model, the nuclei and electron primary source injection and the secondary lepton production precisely. For extra source to interpret the excess in lepton spectrum, we consider two separate scenarios (pulsar and dark matter annihilation via leptonic channels) to construct the bump ($\gtrsim 100$ GeV) and the break at $\sim 1$ TeV. The result shows: (i) in pulsar scenario, the spectral index of the injection should be $ν_{\mathrm{psr}} \sim 0.65$ and the cut-off should be $R_{c} \sim 650$ GV; (ii) in dark matter scenario, the dark matter particle's mass is $m_χ \sim 1208$ GeV and the cross section is $\langle σv \rangle \sim 1.48 \times 10^{-23} \mathrm{cm}^{3} \mathrm{s}^{-1}$. Moreover, in the dark matter scenario, the $τ\barτ$ annihilation channel is highly suppressed, and a DM model is built to satisfy the fitting results.

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arXiv2018

A new solar neutrino channel for grand-unification monopole searches

We identify a previously untapped discovery channel for grand-unification monopoles, arising from their ability to catalyse the direct decay of protons into monoenergetic 459 MeV antineutrinos within the Sun. Previous analyses omit this possibility as it necessarily involves an electroweak suppression factor, and instead search for the unsuppressed 20-50 MeV neutrinos produced via two-stage proton decays. By accounting for the relative difference in interaction cross section and experimental background at typical neutrino detection experiments, we demonstrate that this new channel in fact possesses greater discovery potential. As a case in point, using 5326 live days of Super-Kamiokande exposure we find that $2\;σ$ ($3\;σ$) deviations in the 20-50 MeV channel are amplified to $3\;σ$ ($4.6\;σ$) deviations in the 459 MeV case. Exploiting correlations between these two channels may also offer even greater statistical power.

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arXiv2018

Galactic cosmic-ray model in the light of AMS-02 nuclei data

Cosmic ray (CR) physics has entered a precision-driven era. With the latest AMS-02 nuclei data (boron-to-carbon ratio, proton flux, helium flux and antiproton-to-proton ratio), we perform a global fitting and constrain the primary source and propagation parameters of cosmic rays in the Milky Way by considering 3 schemes with different data sets (with and without $\bar{\mathrm{p}}/\mathrm{p}$ data) and different propagation models (diffusion-reacceleration and diffusion-reacceleration-convection models). We find that the data set with $\bar{\mathrm{p}}/\mathrm{p}$ data can remove the degeneracy between the propagation parameters effectively and it favors the model with a very small value of convection (or disfavors the model with convection). The separated injection spectrum parameters are used for proton and other nucleus species, which reveal the different breaks and slopes among them. Moreover, the helium abundance, antiproton production cross sections and solar modulation are parametrized in our global fitting. Benefited from the self-consistence of the new data set, the fitting results show a little bias, and thus the disadvantages and limitations of the existed propagation models appear. Comparing to the best fit results for the local interstellar spectra ($ϕ= 0$) with the VOYAGER-1 data, we find that the primary sources or propagation mechanisms should be different between proton and helium (or other heavier nucleus species). Thus, how to explain these results properly is an interesting and challenging question.

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arXiv2018

The Minimal GUT with Inflaton and Dark Matter Unification

Giving up the solutions to the fine-tuning problems, we propose the non-supersymmetric flipped $SU(5)\times U(1)_X$ model based on the minimal particle content principle, which can be constructed from the four-dimensional $SO(10)$ models, five-dimensional orbifold $SO(10)$ models, and local F-theory $SO(10)$ models. To achieve gauge coupling unification, we introduce one pair of vector-like fermions, which form complete $SU(5)\times U(1)_X$ representation. Proton lifetime is around $5\times 10^{35}$ years, neutrino masses and mixing can be explained via seesaw mechanism, baryon asymmetry can be generated via leptogenesis, and vacuum stability problem can be solved as well. In particular, we propose that inflaton and dark matter particle can be unified to a real scalar field with $Z_2$ symmetry, which is not an axion and does not have the non-minimal coupling to gravity. Such kind of scenarios can be applied to the generic scalar dark matter models. Also, we find that the vector-like particle corrections to the $B_s^0$ masses can be about 6.6%, while their corrections to the $K^0$ and $B_d^0$ masses are negligible.

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arXiv2018

Naturalness, dark matter, and the muon anomalous magnetic moment in supersymmetric extensions of the standard model with a pseudo-Dirac gluino

We study the naturalness, dark matter, and muon anomalous magnetic moment in the Supersymmetric Standard Models (SSMs) with a pseudo-Dirac gluino (PDGSSMs) from hybrid $F-$ and $D-$term supersymmetry (SUSY) breakings. To obtain the observed dark matter relic density and explain the muon anomalous magnetic moment, we find that the low energy fine-tuning measures are larger than about 30 due to strong constraints from the LUX and PANDAX experiments. Thus, to study the natural PDGSSMs, we consider multi-component dark matter and then the relic density of the lighest supersymmetric particle (LSP) neutralino is smaller than the correct value. We classify our models into six kinds: (i) Case A is a general case, which has small low energy fine-tuning measure and can explain the anomalous magnetic moment of the muon; (ii) Case B with the LSP neutralino and light stau coannihilation; (iii) Case C with Higgs funnel; (iv) Case D with Higgsino LSP; (v) Case E with light stau coannihilation and Higgsino LSP; (vi) Case F with Higgs funnel and Higgsino LSP. We study these Cases in details, and show that our models can be natural and consistent with the LUX and PANDAX experiments, as well as explain the muon anomalous magnetic moment. In particular, all these cases except the stau coannihilation can even have low energy fine-tuning measures around 10.

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arXiv2017

Naturalness and Dark Matter in a Realistic Intersecting D6-Brane Model

We revisit a three-family Pati-Salam model with a realistic phenomenology from intersecting D6-branes in Type IIA string theory compactified on a $\mathbf{T}^6/(\mathbb{Z}_2 \times \mathbb{Z}_2)$ orientifold, and study its naturalness in view of the current LHC and dark matter searches. We discuss spectrum and phenomenological features of this scenario demanding fine tuning better than 1\%. This requirement restricts the lightest neutralino to have mass less than about 600 GeV. We observe that the viable parameter space is tightly constrained by the requirements of naturalness and consistency with the observed dark matter relic density, so that it is fully testable at current and future dark matter searches, unless a non-thermal production mechanism of dark matter is at work. We find that $Z$-resonance, $h$-resonance, $A$-funnel and light stau/stop-neutralino coannihilation solutions are consistent with current LHC and dark matter constraints while the "well-tempered" neutralino scenario is ruled out in our model. Moreover, we observe that only Bino, Higgsinos, right-handed staus and stops can have mass below 1 TeV.

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arXiv2017

GUT monopoles, the Witten effect and QCD axion phenomenology

Taken individually, magnetic monopoles and axions are both well-motivated aspects of physics beyond the Standard Model. We demonstrate that by virtue of the Witten effect, their interplay is furthermore nontrivial; monopoles break the corresponding axial symmetries explicitly and result in an axion potential lacking the usual instanton-derived exponential suppression factor. As axions are distinguished by their extremely weak interactions, any regime where their effects are enhanced may be of interest. As an application of these findings, we demonstrate that a phenomenologically acceptable population of grand-unification monopoles in the early Universe can efficiently suppress both the QCD axion dark matter abundance and CMB isocurvature contribution.

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arXiv2017

Status of Natural Supersymmetry from the GmSUGRA in Light of the current LHC Run-2 and LUX data

We study natural supersymmetry in the Generalized Minimal Supergravity (GmSUGRA). For the parameter space with low energy electroweak fine-tuning measures less than 50, we are left with only the $Z$-pole, Higgs-pole and Higgsino LSP scenarios for dark matter (DM). We perform the focused scans for such parameter space and find that it satisfies various phenomenological constraints and is compatible with the current direct detection bound on neutralino DM reported by the LUX experiment. Such parameter space also has solutions with correct DM relic density besides the solutions with DM relic density smaller or larger than 5$σ$ WMAP9 bounds. We present five benchmark points as examples. In these benchmark points, gluino and the first two generations of squarks are heavier than 2 TeV, stop $\tilde t_{1,2}$ are in the mass range $[1,2]$ TeV, while sleptons are lighter than 1 TeV. Some part of the parameter space can explain the muon anomalous magnetic moment within 3$σ$ as well. We also perform the collider study of such solutions by implementing and comparing with relevant studies done by the ATLAS and CMS Collaborations. We find that the points with Higgsino dominant $\tildeχ_2^0/\tilde{χ_1}^\pm$ mass up to $300$ GeV are excluded in $Z$-pole scenario while for Higgs-pole scenario, the points with $\tildeχ_2^0$ mass up to $460$ GeV are excluded. We also notice that the Higgsino LSP points in our present scans are beyond the reach of present LHC searches. Next, we show that for both the $Z$-pole and Higgs-pole scenarios, the points with electroweak fine-tuning measure around 20 do still survive.

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