arXiv subjects

Jin Min Yang

Publications and source records attributed to Jin Min Yang.

At least 217 records · Page 12Linked to original sources

arXiv2006

Dark Matter Constraints on Gaugino/Higgsino Masses in Split Supersymmetry and Their Implications at Colliders

In split supersymmetry, gauginos and Higgsinos are the only supersymmetric particles which are possibly accessible at foreseeable colliders. While the direct experimental searches, such as LEP and Tevatron experiments, gave robust lower bounds on the masses of these particles, the cosmic dark matter can give some upper bounds and thus have important implications for the searches at future colliders. In this work we scrutinize such dark matter constraints and show the allowed mass range for charginos and neutralinos (the mass eigenstates of gauginos and Higgsinos). We find that the lightest chargino must be lighter than about 1 TeV under the popular assumption M_1=M_2/2 and about 2 or 3 TeV in other cases. The corresponding production rates of the lightest chargino at the CERN Large Hadron Collider (LHC) and the International Linear Colldier (ILC) are also shown. While in some parts of the allowed region the chargino pair production rate can be larger than 1 pb at LHC and 100 fb at the ILC, other parts of the region correspond to very small production rates and thus there is no guarantee to find the charginos of split supersymmetry at future colliders.

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arXiv2005

Gravitino dark matter from gluino late decay in split supersymmetry

In split-supersymmetry (split-SUSY), gluino is a metastable particle and thus can freeze out in the early universe. The late decay of such a long-life gluino into the lightest supersymmetric particle (LSP) may provide much of the cosmic dark matter content. In this work, assuming the LSP is gravitino produced from the late decay of the metastable gluino, we examine the WMAP dark matter constraints on the gluino mass. We find that to provide the full abundance of dark matter, the gluino must be heavier than about 14 TeV and thus not accessible at the CERN Large Hadron Collider (LHC).

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arXiv2005

Flavor-changing Top Quark Decays In R-Parity Violating SUSY

The flavor changing top quark decays t -> cV (V=Z,photon,gluon) induced by R-parity-violating couplings in the minimal supersymmetric standard model (MSSM) are evaluated. We find that the decays t -> cV can be significantly enhanced relative to those in the R-parity conserving SUSY model. Our results show that the top quark FCNC decay can be as large as Br(t->cg)=10^-3, Br(t->cZ)=10^-4 and Br(t-> cγ)= 10^-5, which may be observable at the upgraded Tevatron and/or the LHC.

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arXiv2005

Late-decaying Q-ball with BBN lifetime

In the Affleck-Dine mechanism of baryogenesis, non-topological solitons called Q-balls can be formed. In this work we study the Q-balls which decay during the Big Bang Nucleosythesis (BBN) era. We show that such late-decaying baryonic Q-balls with lifetime of about 10^3 s can provide a new developing mechanism for the BBN through a rolling baryon to photon ratio η, which can naturally explain the discrepancy of BBN prediction with the WMAP data on ^7Li abundance. For the late-decaying leptonic Q-balls with lifetime of about 10^6 s, we find that their decay product, gravitino, can serve as a dark matter candidate without overclosing the universe and, at the same time, can give an explanation of the approximate equality of dark and baryon matter densities.

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arXiv2005

Lightest Higgs Boson Mass in Split Supersymmetry with See-saw Mechanism

In the minimal supersymmetric standard model extended by including right-handed neutrinos with see-saw mechanism, the neutrino Yukaka couplings can be as large as the top-quark Yukawa couplings and thus the neutrino/sneutrino may cause sizable effects in Higgs boson self-energy loops. Our explicit one-loop calculations show that the neutrino/sneutrino effects may have an opposite sign to top/stop effects and thus lighten the lightest Higgs boson. If the soft-breaking mass of the right-handed neutrino is very large (at the order of Majorana mass scale), such as in the split-SUSY scenario, the effects can lower the lightest Higgs boson mass by a few tens of GeV. So the Higgs mass bound of about 150 GeV in split-SUSY may be lowered significantly if right-handed neutrinos come into play with see-saw mechanism.

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arXiv2005

Probing New Physics from Top-charm Associated Productions at Linear Colliders

The top-charm associated productions via $e^+ e^-$, $e^- γ$ and $γγ$ collisions at linear colliders, which are extremely suppressed in the Standard Model (SM), could be significantly enhanced in some extensions of the SM. In this article we calculate the full contribution of the topcolor-assisted technicolor (TC2) to these productions and then compare the results with the existing predictions of the SM, the general two-Higgs-doublet model and the Minimal Supersymmetric Model. We find that the TC2 model predicts much larger production rates than other models and the largest-rate channel is $γγ\to t \bar{c}$, which exceeds 10 fb for a large part of the parameter space. From the analysis of the observability of such productions at the future linear colliders, we find that the predictions of the TC2 model can reach the observable level for a large part of the parameter space while the predictions of other models are hardly accessible.

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arXiv2005

Heavy Supersymmetric Particle Effects in Higgs Boson Production Associated with a Bottom Quark Pair at LHC and Tevatron

If all the supersymmetry particles (sparticles) except a light Higgs boson are too heavy to be directly produced at the Large Hadron Collider (LHC) and Tevatron, a possible way to reveal evidence for supersymmetry is through their virtual effects in other processes. We examine such supersymmetric QCD effects in bottom pair production associated with a light Higgs boson at the LHC and Tevatron. We find that if the relevant sparticles (gluinos and squarks) are well above the TeV scale, too heavy to be directly produced, they can still have sizable virtual effects in this process. For large $\tanβ$, such residual effects can alter the production rate by as much as 40 percent, which should be observable in future measurements of this process.

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arXiv2005

Some studies on dark energy related problems

In this work we perform some studies related to dark energy. Firstly, we propose a dynamical approach to explain the dark energy content of the universe. We assume that a massless scalar field couples to the Hubble parameter with some Planck-mass suppressed interactions. This scalar field developes a Hubble parameter-dependent (thus time-dependent) vacum expectation value, which renders a time-independent relative density for dark energy and thus can explain the coincidence of the dark energy density of the universe. Secondly, we assume the dark matter particle is meta-stable and decays very lately into the dark energy scalar field. Such a conversion of matter to dark energy can give an explanation for the starting time of the accelerating expansion of the universe. Thirdly, we introduce multiple Affleck-Dine fields to the landscape scenario of dark energy in order to have the required baryon-asymmetry universe.

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arXiv2004

Probing new physics from top quark FCNC processes at linear colliders : a mini review

We briefly review the studies on the top quark FCNC processes at a next-generation linear collider. Such processes, including various FCNC top quark rare decays and top-charm associated productions, are extremely suppressed in the standard model (SM) but could be significantly enhanced in some extensions. We compared the predictions from different typical new physics models: the SM, the minimal supersymmetric model (MSSM), the general two-higgs-doublet model (2HDM) and the topcolor-assisted technicolor (TC2) model. Our conclusion is: (1) While all the new physics models can enhance the rates by several orders relative to the SM predictions, the TC2 model predicts much larger rates than other models; (2) The optimal channel for probing the top quark FCNC is the top-charm associated production in γ-γcollision, which occurs at a much higher rate than e^+ e^- or e^- γcollision and can reach the detectable level for a large part of the parameter space.

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arXiv2004

Probing Topcolor-Assisted Technicolor from Like-sign Top Pair Production at LHC

The topcolor-assisted technicolor (TC2) theory predicts tree-level flavor-changing neutral-current (FCNC) top quark Yukawa couplings with top-pions. Such FCNC interactions will induce like-sign top quark pair productions at CERN Large Hadron Collider (LHC). While these rare productions are far below the observable level in the Standard Model and other popular new physics models such as the Minimal Supersymmetric Model, we find that in a sound part of parameter space the TC2 model can enhance the production cross sections to several tens of fb and thus may be observable at the LHC due to rather low backgrounds. Searching for these productions at the LHC will serve as an excellent probe for the TC2 model.

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arXiv2004

SuperWIMP dark matter scenario in light of WMAP

The heavy gravitinos in minimal supergravity (mSUGRA) models couple gravitionally and can naturally be the Superweakly Interacting Massive Particle (SuperWIMP). As plausible candidates for the cold dark matter in the universe, such gravitino SuperWIMPs can naturally evade the current dark matter detection experiments due to their superweak couplings. However, this scenario must subject to the constraints from the Big Bang nucleosynthesis (BBN) predictions for light element abundances as well as the Wilkinson Mircrowave Anisotropy Probe (WMAP) data for the relic density. Our study shows that requiring BBN predictions to agree with the WMAP data of light element abundances can impose upper and lower mass bounds for both the gravitino and the next-to-lightest supersymmetric particle (NLSP). From a scan over the mSUGRA parameter space subjected to the BBN constraints and the WMAP data, we find that in the neutralino NLSP case, the low tan-beta (< 40) region as well as the region accessible at CERN Large Hadron Colldier (LHC) will be severely restrained.

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arXiv2004

The FCNC top-squark decay as a probe of squark mixing

In supersymmetry (SUSY) the flavor mixing between top-squark (stop) and charm-squark (scharm) induces the flavor-changing neutral-current (FCNC) stop decay $\tilde t_1 \to c \tilde χ^0_1$. Searching for this decay serves as a probe of soft SUSY breaking parameters. Focusing on the stop pair production followed by the FCNC decay of one stop and the charge-current decay of the other stop, we investigate the potential of detecting this FCNC stop decay at the Fermilab Tevatron, the CERN Large Hadron Collider (LHC) and the next-generation $e^+e^-$ linear collider (LC). We find that this decay may not be accessible at the Tevatron, but could be observable at the LHC and the LC with high sensitivity.

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arXiv2003

Lepton flavor violating Z-decays in supersymmetric see-saw model

In supersymmetric see-saw model, the large flavor mixings of sleptons induce the lepton flavor violating (LFV) interactions $\ell_I \bar\ell_J V$ ($V=γ, Z$), which give rise to various LFV processes. In this work we examine the induced LFV decays $Z\to\ell_I \bar\ell_J$. Subject to the constraints from the existing neutrino oscillation data and the experimental bounds on the decays $\ell_J\to\ell_Iγ$, these LFV $Z$-decays are found to be sizable, among which the largest-rate channel $Z\to τ\barμ$ can ocuur with a branching ratio of $10^{-8}$ and may be accessible at the LHC or GiagZ experiment.

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arXiv2003

Supersymmetric effects in top quark decay into polarized W-boson

We investigate the one-loop supersymmetric QCD (SUSY-QCD) and electroweak (SUSY-EW) corrections to the top quark decay into a b-quark and a longitudinal or transverse W-boson. The corrections are presented in terms of the longitudinal ratio Γ(t-->W_L b)/Γ(t--> W b) and the transverse ratio Γ(t-->W_- b)/Γ(t--> W b). In most of the parameter space, both SUSY-QCD and SUSY-EW corrections to these ratios are found to be less than 1% in magnitude and they tend to have opposite signs. The corrections to the total width Γ(t-->W b) are also presented for comparison with the existing results in the literature. We find that our SUSY-EW corrections to the total width differ significantly from previous studies: the previous studies give a large correction of more than 10% in magnitude for a large part of the parameter space while our results reach only few percent at most.

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arXiv2003

Higgs-boson production associated with a bottom quark at hadron colliders with SUSY-QCD corrections

The Higgs boson production p p (p\bar p) -> b h +X via b g -> b h at the LHC, which may be an important channel for testing the bottom quark Yukawa coupling, is subject to large supersymmetric quantum corrections. In this work the one-loop SUSY-QCD corrections to this process are evaluated and are found to be quite sizable in some parameter space. We also study the behavior of the corrections in the limit of heavy SUSY masses and find the remnant effects of SUSY-QCD. These remnant effects, which are left over in the Higgs sector by the heavy sparticles, are found to be so sizable (for a light CP-odd Higgs and large \tanβ) that they might be observable in the future LHC experiment. The exploration of such remnant effects is important for probing SUSY, especially in case that the sparticles are too heavy (above TeV) to be directly discovered at the LHC.

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arXiv2003

Probing Topcolor-Assisted Technicolor from Top-Charm Associated Production at LHC

We propose to probe the topcolor-assisted technicolor (TC2) model from the top-charm associated productions at the LHC, which are highly suppressed in the Standard Model. Due to the flavor-changing couplings of the top quark with the scalars (top-pions and top-Higgs) in TC2 model, the top-charm associated productions can occur via both the s-channel and t-channel parton processes by exchanging a scalar field at the LHC. We examined these processes through Monte Carlo simulation and found that they can reach the observable level at the LHC in quite a large part of the parameter space of the TC2 model.

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arXiv2002

Measuring CP violation and mass ordering in joint long baseline experiments with superbeams

We propose to measure the CP phase $δ_{\rm CP}$, the magnitude of the neutrino mixing matrix element $|U_{e3}|$ and the sign of the atmopheric scale mass--squared difference $Δ{\rm m}^2_{31}$ with a superbeam by the joint analysis of two different long baseline neutrino oscillation experiments. One is a long baseline experiment (LBL) at 300 km and the other is a very long baseline (VLBL) experiment at 2100 km. We take the neutrino source to be the approved high intensity proton synchrotron, HIPA. The neutrino beam for the LBL is the 2-degree off-axis superbeam and for the VLBL, a narrow band superbeam. Taking into account all possible errors, we evaluate the event rates required and the sensitivities that can be attained for the determination of $δ_{\rm CP}$ and the sign of $Δm^2_{31}$. We arrive at a representative scenario for a reasonably precise probe of this part of the neutrino physics.

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arXiv2002

The Spectrum of a Binding System for a Heavy Quark with an Anti-Sbottom or for a Sbottom and Anti-Sbottom Pair

Since long-lived light bottom squark (sbottom) and its anti-particle with a mass close to the bottom quark have not been excluded by experiments so far, we consider such a sbottom to combine with its anti-particle to form a color singlet meson-like bound state or to combine with a common anti-quark to form a fermion-like one, or accordingly their anti-particles to form an anti-particle bound system. Namely we calculate the low-lying spectrum of the systems based on QCD inspired potential model. To be as relativistic as possible, we start with the framework of Bethe-Salpeter (BS) equation even for non-relativistic binding systems. Finally, we obtain the requested spectrum by constructing general forms of the BS wave functions and solving the BS equations under instantaneous approximation.

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