arXiv subjects

Chengcheng Han

Publications and source records attributed to Chengcheng Han.

At least 91 records · Page 5Linked to original sources

arXiv2017

Top-squark in natural SUSY under current LHC run-2 data

We utilize the recent LHC-13 TeV data to study the lower mass bound on top-squark (stop) in natural supersymmetry. We recast the LHC sparticle inclusive search of $(\ge 1){\rm jets} + E^{miss}_T$ with $α_T$ variable, the direct stop pair search (1-lepton channel and all-hadronic channel) and the monojet analyses. We find that these searches are complementary depending on stop and higgsino masses: for a heavy stop the all-hadronic stop pair search provides the strongest bound, for an intermediate stop the inclusive SUSY analysis with $α_T$ variable is most efficient, while for a compressed stop-higgsino scenario the monojet search plays the key role. Finally, the lower mass bound on a stop is: (i) 320 GeV for compressed stop-higgsino scenario (mass splitting less than 20 GeV); (ii) 765 (860) GeV for higgsinos lighter than 300 (100) GeV.

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arXiv2016

Surviving scenario of stop decays for ATLAS $\ell+jets+E^{miss}_T$ search

Recently ATLAS reported a $3.3σ$ excess in the stop search with $\ell+jets+E_T^{miss}$ channel. We try to interpret the signal by a light stop pair production in the MSSM. We find: (1) simple models where stop decays into a higgsino or a bino are not favored. (2) an extension of them can explain the data at $2σ$ level without conflicting with the other search channels. A surviving possibility includes a light stop and a light higgsino, which is expected in a natural SUSY scenario.

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arXiv2016

Apparent unitarity violation in top quark's mass off-shell region from a new physics at high energy colliders

Perturbative unitarity conditions have been playing an important role by estimating the energy scale of new physics, including the Higgs mass as one of the most important examples. In this letter, we show that there is a possibility to see the hint of a new physics (top quark partner) indirectly by observing an "apparent" unitarity violation in $M_{bw}$ distribution well above top quark mass in a process of a heavy resonance decaying into a pair of top quarks.

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arXiv2016

Revealing the jet substructure in a compressed spectrum

The physics beyond the Standard Model with parameters of the compressed spectrum is well motivated both in a theory side and with phenomenological reasons, especially related to dark matter phenomenology. In this letter, we propose a method to tag soft final state particles from a decaying process of a new particle in this parameter space. By taking a supersymmetric gluino search as an example, we demonstrate how the Large Hadron Collider experimental collaborations can improve a sensitivity in these non-trivial search regions.

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arXiv2016

Implications of the 750 GeV Diphoton Excess in Gaugino Mediation

The 750 GeV diphoton excess reported by ATLAS and CMS indicates the presence of several pairs of the vector-like matter multiplets around TeV scale. If that is the case, radiative corrections from the $SU(3)$ gauge interaction significantly change from those of MSSM, and the infrared-free nature of the gauge interaction leads to characteristic SUSY mass spectra: a ratio of a squark mass to the gluino mass, and scalar trilinear couplings are enhanced at the low-energy scale. Consequently, even in gaugino mediation models, the Higgs boson mass of 125 GeV is explained with the fairly light gluino of 2-3 TeV, which can be accessible at the LHC.

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arXiv2016

The diphoton resonance as a gravity mediator of dark matter

We consider the possibility of interpreting the recently reported diphoton excess at 750 GeV as a spin-two massive particle (such as a Kaluza-Klein graviton in warped extra-dimensions) which serves as a mediator to Dark Matter via its gravitational couplings to the dark sector and to the Standard Model (SM). We model non-universal couplings of the resonance to gauge bosons in the SM and to Dark Matter as a function on their localization in the extra dimension. We find that scalar, fermion or vector dark matter can saturate the dark matter relic density by the annihilation of dark matter into a pair of the SM particles or heavy resonances, in agreement with the diphoton resonance signal strength. We check the compatibility of our hypothesis with other searches for the KK graviton. We show that the invisible decay rate of the resonance into a pair of dark matter is subdominant in the region of the correct relic density, hence leading to no constraints from the mono-jet bound at 8 TeV via the gluon coupling. We also discuss the kinematic features of the decay products of a KK graviton to distinguish the KK graviton from the SM backgrounds or a scalar particle interpretation of the diphoton resonance.

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arXiv2016

Heavy Fermion Bound States for Diphoton Excess at 750GeV $\sim$ Collider and Cosmological Constraints $\sim$

A colored heavy particle with sufficiently small width may form non-relativistic bound states when they are produced at the large hadron collider\,(LHC), and they can annihilate into a diphoton final state. The invariant mass of the diphoton would be around twice of the colored particle mass. In this paper, we study if such bound state can be responsible for the 750 GeV diphoton excess reported by ATLAS and CMS. We found that the best-fit signal cross section is obtained for the SU(2)$_L$ singlet colored fermion $X$ with $Y_X=4/3$. Having such an exotic hypercharge, the particle is expected to decay through some higher dimensional operators, consistent with the small width assumption. The decay of $X$ may involve a stable particle $χ$, if both $X$ and $χ$ are odd under some conserved $Z_2$ symmetry. In that case, the particle $X$ suffers from the constraints of jets + missing $E_T$ searches by ATLAS and CMS at 8 TeV and 13 TeV. We found that such a scenario still survives if the mass difference between $X$ and $χ$ is above $\sim$ 30 GeV for $m_X \sim 375$ GeV. Even assuming pair annihilation of $χ$ is small, the relic density of $χ$ is small enough if the mass difference between $X$ and $χ$ is smaller than $\sim$ 40 GeV.

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arXiv2016

Interpreting the 750 GeV diphoton excess by the singlet extension of the Manohar-Wise Model

The evidence of a new scalar particle $X$ from the 750 GeV diphoton excess, and the absence of any other signal of new physics at the LHC so far suggest the existence of new colored scalars, which may be moderately light and thus can induce sizable $X g g$ and $X γγ$ couplings without resorting to very strong interactions. Motivated by this speculation, we extend the Manohar-Wise model by adding one gauge singlet scalar field. The resulting theory then predicts one singlet dominated scalar $ϕ$ as well as three kinds of color-octet scalars, which can mediate through loops the $ϕgg$ and $ϕγγ$ interactions. After fitting the model to the diphoton data at the LHC, we find that in reasonable parameter regions the excess can be explained at $1σ$ level by the process $ g g \to ϕ\to γγ$, and the best points predict the central value of the excess rate with $χ_{min}^2=2.32$, which corresponds to a $p$-value of $0.68$. We also consider the constraints from various LHC Run I signals, and we conclude that, although these constraints are powerful in excluding the parameter space of the model, the best points are still experimentally allowed.

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arXiv2015

Trail of the Higgs in the primordial spectrum

We study the effects of the Higgs directly coupled to the inflaton on the primordial power spectrum. The quadratic coupling between the Higgs and the inflaton stabilizes the Higgs in the electroweak vacuum during inflation by inducing a large effective mass for the Higgs, which also leads to oscillatory features in the primordial power spectrum due to the oscillating classical background. Meanwhile, the features from quantum fluctuations exhibit simple monotonic k-dependence and are subleading compared to the classical contributions. We also comment on the collider searches.

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arXiv2015

$\mathcal{O}$(1) GeV dark matter in SUSY and a very light pseudoscalar at the LHC

We analyze the prospects of the detection of a $\mathcal{O}(1)$ GeV neutralino dark matter, $\widetildeχ^0_{1}$, in the Next-to-Minimal Supersymmetric Standard Model at the 14 TeV LHC. We perform dedicated scans of the relevant parameter space of the model and find a large number of points where the thermal relic abundance due to such a dark matter is consistent with the PLANCK measurement. We note that this dark matter is highly singlino-dominated and is always accompanied by a pseudoscalar, $A_1$, with a mass around twice its own, which is responsible for its resonant annihilation. For two benchmark points from our scan, we then carry out a detector-level signal-to-background analysis of the pair production of a heavier higgsino neutralino and a chargino. The higgsino thus produced decays into the dark matter and either the $Z$ boson or the $A_1$. For the $Z$-associated production of $\widetildeχ^0_{1}$, we investigate the scope of the trilepton search channel. For the $A_1$-associated production mode, in order to identify the two collimated muons coming from the decay of the $A_1$, we employ an unconventional method of clustering them together into one jet-like object. Using this method, for certain parameter space configurations, a much larger sensitivity can be obtained at the 14 TeV LHC for the $A_1\widetildeχ^0_{1}$ channel compared to the $Z\widetildeχ^0_{1}$ channel, with an integrated luminosity of 300 fb$^{-1}$.

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arXiv2015

Accessing the core of naturalness, nearly degenerate higgsinos, at the LHC

The presence of two light higgsinos nearly degenerate in mass is one of the important characteristics of suspersymmetric models meeting the naturalness criteria. Probing such higgsinos at the LHC is very challenging, in particular when the mass-splitting between them is less than 5 GeV. In this study, we analyze such a degenerate higgsino scenario by exploiting the high collinearity between the two muons which originate from the decay of the heavier higgsino into the lighter one and which are accompanied by a high-$p_T$ QCD jet. Using our method, we can achieve a statistical significance $\sim 2.9\,σ$ as well as a $S/B \sim 17\%$ with an integrated luminosity of 3000 fb$^{-1}$ at the 14 TeV LHC, for the pair production of higgsinos with masses 124 GeV and 120 GeV. A good sensitivity can be achieved even for a smaller mass-splitting when the higgsinos are lighter.

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arXiv2014

Constraining Top partner and Naturalness at the LHC and TLEP

We investigate indirect constraints on the top partner within the minimal fermionic top partner model. By performing a global fit of the latest Higgs data, $B_s \to μ^+μ^-$ measurements and the electroweak precision observables we find that the top partner with the mass up to 830 GeV is excluded at $2σ$ level. Our bound on the top partner mass is much stronger than the bounds obtained from the direct searches at the LHC. Under the current constraints the fine-tuning measure is less than 9% and the branching ratio of $T \to tZ$ is bounded between 14% and 25%. We also find that precise measurements of Higgs couplings at 240 GeV TLEP will constrain the top partner mass in multi-TeV region.

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arXiv2014

Probing light bino and higgsinos at the LHC

Motivated by the naturalness, we study a simplified MSSM scenario where only the bino-like LSP and higgsino-like NLSP are light. We first scan the parameter space of this scenario, considering the constraints from the Higgs mass, flavor physics, electroweak precision measurements and dark matter experiments. Then in the allowed parameter space, we perform a Monte Carlo simulation for the $\tildeχ^\pm_1 \tildeχ^0_{2,3}$ production followed by $\tildeχ^\pm_1 \to W^\pm \tildeχ^0_1$ and $\tildeχ^0_{2,3} \to Z\tildeχ^0_1$. By examining the presently available trilepton bounds on the wino-like chargino/neutralino, we find that only a narrow region $40\,\rm{GeV} \lesssim m_{\tildeχ^0_1} \lesssim 50\,\rm{GeV}$ and $160\,\rm{GeV} \lesssim m_{\tildeχ^0_{2,3}} \lesssim 170\,\rm {GeV}$ on the plane of $m_{\tildeχ^0_1}-m_{\tildeχ^0_{2,3}}$ can be excluded. Finally, we explore the potential of trilepton signature in probing such a scenario at 14 TeV LHC and find that the region with $40\,\rm{GeV} \lesssim m_{\tildeχ^0_1} \lesssim 60\,\rm {GeV}$ and $160 \rm {GeV}\,\lesssim m_{\tildeχ^0_{2,3}} \lesssim 300\,\rm{GeV}$ can be covered at $3σ$ level with luminosity ${\cal L}=300$ fb$^{-1}$.

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arXiv2014

SUSY effects in Higgs productions at high energy $e^+e^-$ colliders

Considering the constraints from collider experiments and dark matter detections, we investigate the SUSY effects in the Higgs productions $e^+e^- \to Zh$ at an $e^+e^-$ collider with a center-of-mass energy above 240 GeV and $γγ\to h \to b\bar{b}$ at a photon collider with a center-of-mass energy above 125 GeV. In the parameter space allowed by current experiments, we find that the SUSY corrections to $e^+e^- \to Zh$ can reach a few percent and the production rate of $γγ\to h \to b\bar{b}$ can be enhanced by a factor of 1.2 over the SM prediction. We also calculate the exotic Higgs productions $e^+e^-\to Zh_1$ and $e^+e^-\rightarrow A_1h$ in the next-to-minimal supersymmetric model (NMSSM) ($h$ is the SM-like Higgs, $h_1$ and $A_1$ are respectively the CP-even and CP-odd singlet-dominant Higgs bosons which can be much lighter than $h$). We find that at a 250 GeV $e^+e^-$ collider the production rates of $e^+e^-\rightarrow Zh_1$ and $e^+e^-\to A_1h$ can reach 60 fb and 0.1 fb, respectively.

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arXiv2014

A new approach for detecting compressed bino/wino at the LHC

In some supersymmetric models like split supersymmetry or models with non-universal gaugino mass, bino (LSP) and winos (NLSP) may have rather small mass splitting in order to provide the correct dark matter relic density through bino/wino co-annihilation. Such a scenario with the compressed bino/wino is difficult to explore at the LHC. In this work we propose to probe this scenario from $pp \to j \tildeχ^0_2 \tildeχ^\pm_1$ followed by $\tildeχ^0_2 \to γ\tildeχ^0_1$ and $\tildeχ^\pm_1 \to W^{*}\tildeχ^0_1\to \ell^\pm ν\tildeχ^0_1$ (this method is also applicable to the compressed bino/higgsino scenario). Through a detailed Monte Carlo simulation for both the signal and the backgrounds, we find that for a mass splitting $ΔM \sim 5-15$ GeV between bino (LSP) and wino (NLSP), the 14 TeV LHC with luminosity of 500$fb^{-1}$ can probe the wino up to 150 GeV (the sensitivity can reach $5σ$ for $ΔM = 5$ GeV and $2σ$ for $ΔM = 15$ GeV). We also investigate the dark matter detection sensitivities for this scenario and find that the planned XENON-1T(2017) cannot fully cover the parameter space with wino below 150 GeV allowed by relic density and the LUX limits.

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arXiv2014

SUSY induced top quark FCNC decay $t \to c h$ after Run I of LHC

In light of the Higgs discovery and the nonobservation of sparticles at the LHC, we revisit the SUSY induced top quark flavor changing decay into the Higgs boson. We perform a scan over the relevant SUSY parameter space by considering the constraints from the Higgs mass measurement, the LHC search for SUSY, the vacuum stability, the precision electro-weak observables as well as $B \to X_s γ$. We have the following observations: (1) In the MSSM, the branching ratio of $ t \to c h$ can only reach $3.0\times10^{-6}$, which is about one order smaller than previous results obtained before the advent of the LHC. Among the considered constraints, the Higgs mass and the LHC search for sparticles are found to play an important role in limiting the prediction. (2) In the singlet extension of the MSSM, since the squark sector is less constrained by the Higgs mass, the branching ratio of $t \to c h$ can reach the order of $10^{-5}$ in the allowed parameter space. (3) The chiral-conserving mixings $δ_{LL}$ and $δ_{RR}$ may have remanent effects on $t \to c h$ in heavy SUSY limit. In the MSSM with squarks and gluino above 3 TeV and meanwhile the CP-odd Higgs boson mass around 1 TeV, the branching ratio of $t\to c h$ can still reach the order of $10^{-8}$ under the constraints.

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arXiv2014

A light SUSY dark matter after CDMS-II, LUX and LHC Higgs data

In SUSY, a light dark matter is usually accompanied by light scalars to achieve the correct relic density, which opens new decay channels of the SM like Higgs boson. Under current experimental constraints including the latest LHC Higgs data and the dark matter relic density, we examine the status of a light neutralino dark matter in the framework of NMSSM and confront it with the direct detection results of CoGeNT, CDMS-II and LUX. We have the following observations: (i) A dark matter as light as 8 GeV is still allowed and its scattering cross section off the nucleon can be large enough to explain the CoGeNT/CDMS-II favored region; (ii) The LUX data can exclude a sizable part of the allowed parameter space, but still leaves a light dark matter viable; (iii) The SM-like Higgs boson can decay into the light dark matter pair with an invisible branching ratio reaching 30% under the current LHC Higgs data, which may be tested at the 14 TeV LHC experiment.

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arXiv2014

Higgs pair production with SUSY QCD correction: revisited under current experimental constraints

We consider the current experimental constraints on the parameter space of the MSSM and NMSSM. Then in the allowed parameter space we examine the Higgs pair production at the 14 TeV LHC via $b\bar{b}\to hh$ ($h$ is the 125 GeV SM-like Higg boson) with one-loop SUSY QCD correction and compare it with the production via $gg\to hh$. We obtain the following observations: (i) For the MSSM the production rate of $b\bar{b} \to hh$ can reach 50 fb and thus can be competitive with $gg \to hh$, while for the NMSSM $b\bar{b} \to hh$ has a much smaller rate than $gg \to hh$ due to the suppression of the $hb\bar{b}$ coupling; (ii) The SUSY-QCD correction to $b\bar{b} \to hh$ is sizable, which can reach $45\%$ for the MSSM and $15\%$ for the NMSSM within the $1σ$ region of the Higgs data; (iii) In the heavy SUSY limit (all soft mass parameters become heavy), the SUSY effects decouple rather slowly from the Higgs pair production (especially the $gg\to hh$ process), which, for $M_{\rm SUSY}=5$ TeV and $m_A<1$ TeV, can enhance the production rate by a factor of 1.5 and 1.3 for the MSSM and NMSSM, respectively. So, the Higgs pair production may be helpful for unraveling the effects of heavy SUSY.

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