Searches for Dark Matter via Mono-W Production in Inert Doublet Model at the LHC

The Inert Doublet Model(IDM) is one of the many beyond Standard Model scenarios with an extended scalar sector, which provide a suitable dark matter particle candidate. Dark matter associated visible particle production at high energy colliders provides a unique way to determine the microscopic properties of the dark matter particle. In this paper, we investigate that the mono-W + missing transverse energy production at the Large Hadron Collider(LHC),where W boson decay to a lepton and a neutrino. We perform the analysis for the signal of mono-W production in the IDM and the Standard Model(SM) backgrounds, and the optimized criteria employing suitable cuts are chosen in kinematic variables to maximize signal significance. We also investigate the discovery potential in several benchmark scenarios at the 14 TeV LHC. When the light Z_2 odd scalar higgs of mass is about 65 GeV, charged Higgs is in the mass range from 120 GeV to 250 GeV, it provides the best possibility with a signal significance of about 3σ at an integrated luminosity of about 3000 fb~(-1).     

Associted Production of Z Boson and a Pair of New Quarks at LHC

The associated production of Z boson and a pair of new quarks at the Large Hadron Collider （LHC） is studied. The cross sections for both sequential fermions and vector-like fermions are presented. It is found that for sequential fermions the cross sections can reach 1 - 10^2 /b for heavy quark mass mQ from 1000 GeV to 200 GeV. For vector-like quarks, the cross sections are suppressed by mixing parameter sin OL. Focusing on process pp → b＇b＇, we investigate the possibility of detecting the 6l 4- 2j signal. For a b＇ with light mass and a large branching ratio of b＇ → bZ, it is found that only several signal events （ parton level ） can be produced with 1000 fb^-1 integrated luminosity. Although the signal events are rare, all the final states are produced centrally and multi lepton final states are clear at hadron collider, which could be easily detected.     

Mono-top signature from stop decay at the HE-LHC

Searching for the top squark(stop)is a key task to test the naturalness of SUSY.Different from stop pair production,single stop production relies on its electroweak properties and can provide some unique signatures.Following the single production process pp→t^~1X(~)1→tX^~1⁰X^~1^-,the top quark has two decay channels:leptonic channel and hadronic channel.In this paper,we probe the observability of these two channels in a simplified minimal supersymmetric standard model scenario.We find that,at the 27 TeV LHC with the integrated luminosity of L=15 ab^-1,mt^-1<1900 GeV andμ<750 GeV can be excluded at 2σthrough the leptonic mono-top channel,while m_t^-1<1200 GeV andμ<350 GeV can be excluded at 2σthrough the hadronic channel.     

Same-Sign Dilepton Signature in the Inert Doublet Model

In this study,we perform a detailed analysis on the same-sign dilepton signature in the inert doublet model.Focusing on the low dark matter mass region,we randomly scan the corresponding parameter space.Viable samples allowed by various constraints are obtained,and among them are twenty benchmark points that are selected for further study on collider signature.At hadron colliders,the same-sign dilepton signature is produced via pp→W~(±*)W~(±*)jj→H~±H~±jj with the leptonic decay mode H~±→HW~±(→l~±v),where H represents the dark matter candidate.We investigate the testability of this signal at the high-luminosity LHC(HL-LHC) and the proposed 27 TeV high-energy LHC(HE-LHC).According to our simulation,the HL-LHC with L=3 ab~(-1) can barely probe this signal.Meanwhile,for the HE-LHC with L=15 ab~(-1),it is promising to obtain a 5σ significance when ■ with dark matter mass m_H～60 or 71 GeV.     

Single Production of Top Quark via e-γ Collision in Left-Right Twin Higgs Model

In the context of the left-right twin Higgs （LRTH） model, we fist study single production of the standard model （SM） top quark via e-γ collisions. We find that the corrections of the LRTH model to the cross section of the process e-γ→vebt might be observed only for f ≤750 GeV and the heavy top quark mass scale M ≥500 GeV in future high energy linear e^＋e^- collider （LC） experiment with the center-of-mass （CM） energy √s = 500 GeV and a yearly integrated luminosity of ￡ = 100 fb^-1. We also consider single production of the heavy top quark T via e-γ collisions. Our numerical results show that the possible signals of the heavy top quark T might be observed via the decay channel T →Ф^＋b→tbb in future LC experiment with √s = 3 TeV and ￡ = 500 fb^-1.     

A dark vector resonance at CLIC

One of the main problems in particle physics is to understand the origin and nature of dark matter. An exciting possibility is to consider that dark matter belongs to a new complex but hidden sector. In this paper, we assume the existence of a strongly interacting dark sector consisting of a new scalar doublet and new vector resonances,in accordance with the model recently proposed by our group. Since it was found in the previous work that it is very challenging to find the new vector resonances at the LHC, here we study the possibility of finding them at the future Compact Linear Collider(CLIC) running at■= 3 TeV. We consider two distinct scenarios. In the first, when the non-standard scalars are heavy, the dark resonance is intense enough to make its discovery possible at CLIC when the resonance mass is in the range [2000, 3000] GeV. In the second scenario, when the non-standard scalars are light, the new vector boson is too broad to be recognized as a resonance, and is not detectable except when the mass of the scalars is close to(but smaller than) half of the resonance mass and the scale of the dark sector is high. In all positive cases, less than a tenth of the maximum integrated luminosity is needed to reach the discovery level. Finally, we also comment on the mono-Z production.     

T-odd top partner pair production in the dilepton final states at the LHC in the littlest Higgs Model with T-parity

In the littlest Higgs model with T-parity, we discuss the pair production of the T-odd top partner(T_) that decays almost 100% into the top quark and the lightest T-odd particle(A_H). Considering the current constraints,we investigate the observability of the T-odd top partner pair production through the process pp→T_T_→tt A_HA_H in the final states with two leptons at 14 TeV LHC. We analyze the signal significance and found that the lower limits on the T-odd top partner mass are approximately 1.2 TeV, 1.3 TeV, and 1.4 TeV at the 2σ confidence level at 14 TeV LHC with an integrated luminosities of 30 fb~(-1), 100 fb~(-1), and 300 fb~(-1), respectively. This lower limit can be increased to approximately 1.5(1.6) TeV if we used 1000(3000) fb~(-1) of the integrated luminosity.     

SUSY-QCD Effects in Top Quark Pair Production in Association with a Gluon at the ILC

Given the null results of searches for new physics at the LHC,we investigate the one-loop effects SUSY QCD in the process e~+e~-→tg at the ILC in Minimal Supersymmetric Standard Model(MSSM).We find that the relative SUSY-QCD corrections to the cross section of e~+e~-→ tg can maximally reach 6.5%(3.2%) at the ILC with (1/2)s= 1000 GeV when m__1= 313.4 GeV and m_g =500(1500) GeV.     

The semi-constrained NMSSM in light of muon g-2,LHC,and dark matter constraints

The semi-constrained NMSSM(scNMSSM) extends the MSSM by a singlet field, and requires unification of the soft SUSY breaking terms in the squark and slepton sectors, while it allows that in the Higgs sector to be different. We try to interpret the muon g-2 in the sc NMSSM, under the constraints of 125 Ge V Higgs data, B physics,searches for low and high mass resonances, searches for SUSY particles at the LHC, dark matter relic density by WMAP/Planck, and direct searches for dark matter by LUX, XENON1T, and PandaX-II. We find that under the above constraints, the sc NMSSM can still(i) satisfy muon g-2 at 1σ level, with a light muon sneutrino and light chargino;(ii) predict a highly-singlet-dominated 95 GeV Higgs, with a diphoton rate as hinted at by CMS data,because of a light higgsino-like chargino and moderate λ;(iii) get low fine tuning from the GUT scale with smallμeff, M_0, M_(1/2), and A_0, with a lighter stop mass which can be as low as about 500 GeV, which can be further checked in future studies with search results from the 13 TeV LHC;(iv) have the lightest neutralino be singlino-dominated or higgsino-dominated, while the bino and wino are heavier because of high gluino bounds at the LHC and universal gaugino conditions at the GUT scale;(v) satisfy all the above constraints, although it is not easy for the lightest neutralino, as the only dark matter candidate, to get enough relic density. Several ways to increase relic density are discussed.     

Probing tqZ anomalous couplings in the trilepton signal at the HL-LHC,HE-LHC,and FCC-hh

We investigate the prospect of discovering the Flavour Changing Neutral Current(FCNC)tqZ couplings via two production processes yielding trilepton signals:top quark pair production pp→tt with one top quark decaying to the Z boson and one light jet and the anomalous single top quark plus Z boson production process pp→tZ.We study these channels at various successors of the Large Hadron Collider(LHC),i.e,the approved High-Luminosity LHC(HL-LHC)as well as the proposed High-Energy LHC(HE-LHC)and Future Circular Collider in hadron-hadron mode(FCC-hh).We perform a full simulation for the signals and the relevant Standard Model(SM)backgrounds and obtain limits on the Branching Ratios(BRs)of t→qZ(q=u,c),eventually yielding a trilepton final state through the decay modes t→bW+→bl⁺V_land Z→l⁺l^-.The upper limits on these FCNC BRs at 95%Confidence Level(CL)are obtained at the HL-LHC with√s=14 TeV and 3 ab^-1,at the HE-LHC with√s=27TeV and 15 ab^-1,and at the FCC-hh with√s=100 TeV and 30ab^-1.     

Single Production of Vector-Like Top Quark in the Littlest Higgs Model at TeV Energy e-γ Colliders

The existence of a new coloured vector-like heavy fermion T is a crucial prediction in little Higgs models, which play a key role in breaking the electroweak symmetry. The littlest Higgs model is the most economical one among various little Higgs models. In the context of the littlest Higgs model, we study single production of the new heavy vector-like quark and discuss the possibility of detecting this new particle in the future LC experiment. It is found thai, the production cross section is in the range of 1.7× 10^-3 - 30 fb at TeV energy electron-photon collider with √s- 3 TeV and a yearly integrated luminosity of ,￡ = 500 fb^-1.     

Production and constraints for a massive dark photon at electron-positron colliders

Dark sector may couple to the Standard Model via one or more mediator particles.We discuss two types of mediators:the dark photon A’and the dark scalar mediatorΦ.The total cross-sections and various differential distributions of the processes e+e-→qqA’and e+e→qqφ(q=u,d,c,s and b quarks)are discussed.We focus on the study of the invisible due to the cleaner background at future e+e-colliders.It is found that the kinematic distributions of the two-jet system could be used to identify(or exclude)the dark photon and the dark scalar mediator,as well as to distinguish between them.We further study the possibility of a search for dark photons at a future CEPC experiment with s1/2=91.2 GeV and 240 GeV.With CEPC running at s1/2=91.2 GeV,it would be possible to perform a decisive measurement of the dark photon(20 GeV相似文献   

Single vector-like top partner production in the left-right twin Higgs model at TeV energy eγ colliders

The left-right twin Higgs model contains a new vector-like heavy top quark, which mixes with the SM-like top quark. In this work, we studied the single vector-like top partner production via process e^-γ→v_eTb at the International Linear Collider. We calculated the production cross section at tree level and displayed the relevant di erential distributions. The result shows that there will be 125 events produced each year with √s=2 TeV and the integrated luminosity L_int≈500 fb^-1, and the b-quark tagging and the relevant missing energy E/T cut will be helpful to detect this new e ect.     

Electroweak vacuum stability and diphoton excess at 750 GeV

Recently,both ATLAS and CMS collaborations at the CERN Large Hadron Collider(LHC) announced their observations of an excess of diphoton events around the invariant mass of 750 GeV with a local significance of 3.6σ and 2.6σ,respectively.In this paper,we interpret the diphoton excess as the on-shell production of a real singlet scalar in the pp →S→γγ channel.To accommodate the observed production rate,we further introduce a vector-like fermion F,which carries both color and electric charges.The viable regions of model parameters are explored for this simple extension of the Standard Model(SM).Moreover,we revisit the problem of electroweak vacuum stability in the same scenario,and find that the requirement for the electroweak vacuum stability up to high energy scales imposes serious constraints on the Yukawa coupling of the vector-like fermion and the quartic couplings of the SM Higgs boson and the new singlet scalar.Consequently,a successful explanation for the diphoton excess and the absolute stability of electroweak vacuum cannot be achieved simultaneously in this economical setup.     

Search for heavy sterile neutrinos in trileptons at the LHC

We present a search strategy for both Dirac and Majorana sterile neutrinos from the purely leptonic decays of W~±→e~±e~±μ~?ν and μ~±μ~± e~?ν at the 14 TeV LHC. The discovery and exclusion limits for sterile neutrinos are shown using both the Cut-and-Count(CC) and Multi-Variate Analysis(MVA) methods. We also discriminate between Dirac and Majorana sterile neutrinos by exploiting a set of kinematic observables which differ between the Dirac and Majorana cases. We find that the MVA method, compared to the more common CC method, can greatly enhance the discovery and discrimination limits. Two benchmark points with sterile neutrino mass m N =20 GeV and 50 GeV are tested. For an integrated luminosity of 3000 fb~(-1), sterile neutrinos can be found with 5σ significance if heavy-to-light neutrino mixings |U_(Ne)|~2~|U_(Nμ)|~2~10~(-6), while Majorana vs. Dirac discrimination can be reached if at least one of the mixings is of order 10~(-5).     

General scan in flavor parameter space in models with vector quark doublets and an enhancement in the B→X_sγ process

In models with vector-like quark doublets, the mass matrices of up and down type quarks are related.Precise diagonalization of the mass matrices has become an obstacle in numerical studies. In this work we first propose a diagonalization method. As its application, in the Standard Model with one vector-like quark doublet we present the quark mass spectrum and Feynman rules for the calculation of B → X_sγ. We find that i) under the constraints of the CKM matrix measurements, the mass parameters in the bilinear term are constrained to a small value by the small deviation from unitarity; ii) compared with the fourth generation extension of the Standard Model, there is an enhancement to the B → X_sγ process in the contribution of vector-like quarks, resulting in a non-decoupling effect in such models.     

Pair production of Higgs boson in NMSSM at the LHC with the next-to-lightest C P-even Higgs boson being SM-like

The next-to-minimal supersymmetric standard model(NMSSM)more naturally accommodates a Higgs boson with a mass of approximately 125 Ge V than the minimal supersymmetric standard model(MSSM).In this work,we assume that the next-to-lightest CP-even Higgs boson h2 is the SM-like Higgs boson h,whereas the lightest CP-even Higgs boson h_(1)is dominantly singlet-like.We discuss the h_1h_1,h_2h_2,and h_1h_(2)pair production processes via gluon-gluon fusion at the LHC for an collision energy of 14 Te V,and we consider the cases in which one Higgsboson decays to bˉb and the other decays toγγorτ~+τ~-.We find that,for m_(h1) 62 GeV,the cross section of the gg→h_1h_(1)process is relatively large and maximally reaches 5400 fb,and the production rate of the h_1h_1→bˉbτ~+τ~-final state can reach 1500 fb,which make the detection of this final state possible for future searches of an integrated luminosity of 300 and 3000 fb~(-1).This is mainly due to the contributions from the resonant production process pp→h_2→h_1h_(1)and the relatively large branching ratio of h_1→bb and h_1→τ~+τ~-.The cross sections of the pp→h_2h_2and pp→h_1h_2 production processes maximally reach 28 fb and 133 fb,respectively.     

What can a heavy U(1)_(B-L) Z' boson do to the muon(g-2)_μ anomaly and to a new Higgs boson mass?

The minimal U(1)_(B-L) extension of the Standard Model(B-L-SM) offers an explanation for neutrino mass generation via a seesaw mechanism;it also offers two new physics states,namely an extra Higgs boson and a new Z' gauge boson.The emergence of a second Higgs particle as well as a new Z' gauge boson,both linked to the breaking of a local U(1)_(B-L) symmetry,makes the B-L-SM rather constrained by direct searches in Large Hadron Collider(LHC) experiments.We investigate the phenomenological status of the B-L-SM by confronting the new physics predictions with the LHC and electroweak precision data.Taking into account the current bounds from direct LHC searches,we demonstrate that the prediction for the muon(g-2)_μ anomaly in the B-L-SM yields at most a contribution of approximately 8.9 × 10~(-12),which represents a tension of 3.28 standard deviations,with the current1σ uncertainty,by means of a Z' boson if its mass is in the range of 6.3 to 6.5 TeV,within the reach of future LHC runs.This means that the B-L-SM,with heavy yet allowed Z' boson mass range,in practice,does not resolve the tension between the observed anomaly in the muon(g-2)_μ and the theoretical prediction in the Standard Model.Such a heavy Z' boson also implies that the minimal value for the new Higgs mass is of the order of 400 GeV.     

Rare decays B_(s,d)~O→e~+e~- in a top quark two-Higgs-doublet model

In the framework of T2HDM, we calculated the new physics contributions involving neutral Higgs bosons to the branching ratios of B_(s,d)~0→e~+e~-(e=e,μ) decays. Comparing the theoretical predictions with the experimental upper-limits, we found that (a) The data of Br(B_d~0→e~+e~-)give the upper bound on tanβ: tanβ≤ 22, while Br(B_s~0→e~+e~-)give tanβ≤12 for fixed δ = 0°, m_H+=350 GeV, m_Ho = 160 GeV, m_Ho= 115 GeV and m_Ao=120 GeV; (b) A light neutral Higgs boson mass m_Ho (m_Ao) less than 50 GeV (120 GeV) is excluded by the data of branching ratios for B_(s,d)~0→e~+e~-(e=μ) decays with tanβ=10; (c) The bounds on m_(h~0) and tanβ, or m_(A~0) and tanβ are strongly correlated: a smaller (larger) tanβ means a lighter (heavier) neutral Higgs boson.     

Probing the sign-changeable interaction between dark energy and dark matter with current observations

We consider the models of vacuum energy interacting with cold dark matter in this study, in which the coupling can change sigh during the cosmological evolution. We parameterize the running coupling b by the form b(a) = b_0 a + b_e(1-a), where at the earlytime the coupling is given by a constant b_e and today the coupling is described by another constant b_0. We explore six specific models with(i) Q = b(a)H_0ρ_0,(ii) Q = b(a)H_0ρ_(de),(iii) Q = b(a)H_0ρ_c,(iv) Q = b(a)Hρ_0,(v) Q = b(a)Hρ_(de), and(vi) Q = b(a)Hρ_c.The current observational data sets we use to constrain the models include the JLA compilation of type Ia supernova data, the Planck 2015 distance priors data of cosmic microwave background observation, the baryon acoustic oscillations measurements,and the Hubble constant direct measurement. We find that, for all the models, we have b_0 0 and b_e 0 at around the 1σ level,and b_0 and b_e are in extremely strong anti-correlation. Our results show that the coupling changes sign during the evolution at about the 1σ level, i.e., the energy transfer is from dark matter to dark energy when dark matter dominates the universe and the energy transfer is from dark energy to dark matter when dark energy dominates the universe.