Optimizing Higgs factories by modifying the recoil mass

It is difficult to measure the WW-fusion Higgs production process(e~+e~-→νh)at a lepton collider with a center of mass energy of 240-250 Ge V due to its small rate and the large background from the Higgsstrahlung process with an invisible Z(e~+e~-→h Z,Z→ν).We construct a modified recoil mass variable,m~p_(recoil),defined using only the 3-momentum of the reconstructed Higgs particle,and show that it can separate the WW-fusion and Higgsstrahlung events better than the original recoil mass variable m_(recoil).Consequently,the m~p_(recoil)variable can be used to improve the overall precisions of the extracted Higgs couplings,in both the conventional framework and the effective-field-theory framework.We also explore the application of the m~p_(recoil)variable in the inclusive cross section measurements of the Higgsstrahlung process,while a quantitive analysis is left for future studies.     

Next-to-leading order QCD corrections to the decay of Higgs to vector meson and Z boson

The exclusive decay of the Higgs boson to a vector meson(J/ψ or Υ(1 S)) and Z boson is studied in this work. The decay amplitudes are separated into two parts in a gauge invariant manner. The first part comes from the direct coupling of the Higgs boson to the charm(bottom) quark and the other from the HZZ*or the loop-induced HZγ*vertexes in the standard model. While the branching ratios from the direct channel are much smaller than those of the indirect channel, their interference terms give nontrivial contributions. We further calculate the QCD radiative corrections to both channels, which reduce the total branching ratios by around 20% for both J/ψ and Υ(1S) production. Our results provide a possible chance to check the SM predictions of the Hcc(Hbb)coupling and to seek for hints of new physics at the High Luminosity LHC or future hadron colliders.     

Measurement of the absolute branching fraction of D+→K0e+ve via K0→π0π0

By analyzing 2.93 fb^-1 data collected at the center-of-mass energy √s=3.773 GeV with the BESIII detector, we measure the absolute branching fraction of the semileptonic decay D⁺→K⁰e⁺v_e to be B(D⁺→K⁰e⁺v_e)=(8.59±0.14±0.21)% using K⁰→K_S⁰→π⁰π⁰, where the first uncertainty is statistical and the second systematic. Our result is consistent with previous measurements within uncertainties..     

The DD* interaction with isospin zero in an extended hidden gauge symmetry approach

The DD^* interaction via a ρ or ω exchange is constructed within an extended hidden gauge symmetry approach, where the strange quark is replaced by the charm quark in the SU(3) flavor space. With this DD^* interaction, a bound state slightly lower than the DD^* threshold is generated dynamically in the isospin zero sector by solving the Bethe-Salpeter equation in the coupled-channel approximation, which might correspond to the X(3872) particle announced by many collaborations. This formulism is also used to study the BB^* interaction, and a BB^* bound state with isospin zero is generated dynamically, which has no counterpart listed in the review of the Particle Data Group. Furthermore, the one-pion exchange between the D meson and the D^* is analyzed precisely, and we do not think the one-pion exchange potential need be considered when the Bethe-Salpeter equation is solved.     

Tuning and validation of hadronic event generator for R value measurements in the tau-charm region

To measure the R value in an energy scan experiment with e⁺e^- collisions, precise calculation of initial state radiation is required in the event generators. We present an event generator for this consideration, which incorporates initial state radiation effects up to second order accuracy. The radiative correction factor is calculated using the totally hadronic Born cross section. The measured exclusive processes are generated according to their cross sections, while the unknown processes are generated using the LUND Area Law model, and its parameters are tuned with data collected at √s=3.65 GeV. The optimized values are validated with data in the range √s=2.2324-3.671 GeV. These optimized parameters are universally valid for event generation below the DD threshold.     

Possible strange hidden-charm pentaquarks from ∑c(*)Ds* and Ξc(',*)* interactions

Using the one-boson-exchange model, we investigate the Λ_cD_s^*, ∑_cD_s^*, ∑_c^*D_s^*, Ξ_cD^*, Ξ'_cD^*, and Ξ_c^*D^* interactions by considering the one-eta-exchange and/or one-pion-exchange contributions. We further predict the existence of hidden-charm molecular pentaquarks. Promising candidates for hidden-charm molecular pentaquarks include a Ξ'_cD^* state with 0((1/2)^-) and the Ξ_c^*D^* states with 0((1/2)^-) and 0((3/2)^-). Experimental searches for these predicted hidden-charm molecular pentaquarks are an interesting future research topic for experiments like LHCb.     

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.     

An optimal scheme for top quark mass measurement near the tt threshold at future e+e- colliders

A top quark mass measurement scheme near the tt production threshold in future e⁺e^- colliders, e.g.the Circular Electron Positron Collider (CEPC), is simulated. A χ² fitting method is adopted to determine the number of energy points to be taken and their locations. Our results show that the optimal energy point is located near the largest slope of the cross section v. beam energy plot, and the most efficient scheme is to concentrate all luminosity on this single energy point in the case of one-parameter top mass fitting. This suggests that the so-called data-driven method could be the best choice for future real experimental measurements. Conveniently, the top mass statistical uncertainty can also be calculated directly by the error matrix even without any sampling and fitting. The agreement of the above two optimization methods has been checked. Our conclusion is that by taking 50 fb^-1 total effective integrated luminosity data, the statistical uncertainty of the top potential subtracted mass can be suppressed to about 7 MeV and the total uncertainty is about 30 MeV. This precision will help to identify the stability of the electroweak vacuum at the Planck scale.     

SUSY effects in Higgs production at high energy e+e- colliders

Considering the constraints from collider experiments and dark matter detection, we investigate the SUSY effects in the Higgs production channels e⁺e^-→Zh at an e⁺e^- collider with a center-of-mass energy above 240 GeV and γγ→h→bb 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^-→Zh can reach a few percent and the production rate of γγ→h→bb can be enhanced by a factor of 1.2 over the SM prediction. We also calculate the exotic Higgs production e⁺e^-→Zh₁ in the next-to-minimal supersymmetric model (NMSSM) (h is the SM-like Higgs, h₁ is the CP-even 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^-→Zh₁ can reach 60 fb.     

Model-independent analysis of Higgs spin and CP properties in the process \boldsymbol{e}^{+} \boldsymbol{e}^{-} \to\boldsymbol{t}\bar{\boldsymbol{t}} \boldsymbol{\Phi}

In this paper we investigate methods to study the $t\bar{t}$ Higgs coupling. The spin and CP properties of a Higgs boson are analysed in a model-independent way in its associated production with a $t\bar{t}$ pair in high-energy e ⁺ e ^? collisions. We study the prospects of establishing the CP quantum numbers of the Higgs boson in the CP-conserving case as well as those of determining the CP-mixing if CP is violated. We explore in this analysis the combined use of the total cross section and its energy dependence, the polarisation asymmetry of the top quark and the up-down asymmetry of the antitop with respect to the top–electron plane. We find that combining all three observables remarkably reduces the error on the determination of the CP properties of the Higgs Yukawa coupling. Furthermore, the top polarisation asymmetry and the ratio of cross sections at different collider energies are shown to be sensitive to the spin of the particle produced in association with the top-quark pair.     

Flavor non-universal gauge interactions and anomalies in B-meson decays

Motivated by flavor non-universaity and anomalies in semi-leptonic B-meson decays, we present a general and systematic discussion about how to construct anomaly-free U(1)' gauge theories based on an extended standard model with only three right-handed neutrinos. If all standard model fermions are vector-like under this new gauge symmetry, the most general family non-universal charge assignments,(a,b,c) for three-generation quarks and(d,e,f)for leptons, need satisfy just one condition to be anomaly-free, 3(a+b+c) =-(d+e+f). Any assignment can be linear combinations of five independent anomaly-free solutions. We also illustrate how such models can generally lead to flavor-changing interactions and easily resolve the anomalies in B-meson decays. Probes with B_s-B_s mixing, decay into τ~±, dilepton and dijet searches at colliders are also discussed.     

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.     

Probing HZγ and Hγγ anomalous couplings in the process e~+e~-→ Hγ

We propose to measure the HZγ and Hγγ anomalous couplings in the process e+e-→Hγ with the sequential decay of H→bˉb. The discovery potential of observing the anomalous couplings are explored in detail.Our study shows that future electron–positron colliders have great potential to test the HZγ and Hγγ couplings.Conservative bounds on the two anomalous couplings are also derived when no new physics signal is detected on top of the SM backgrounds.     

Probing scalar particle and unparticle couplings in e^{+}e^{-}\to t\bar{t} with transversely polarized beams

In searching for indications of new-physics scalar particle and unparticle couplings in $e^{+}e^{-}\to t\bar{t}$ , we consider the role of transversely polarized initial beams at e ⁺ e ^? colliders. By using a general relativistic spin density matrix formalism for describing the particles spin states, we find analytical expressions for the differential cross section of the process with t or $\bar{t}$ polarization measured, including the anomalous coupling contributions. Thanks to the transversely polarized initial beams these contributions are first order anomalous coupling corrections to the Standard Model (SM) contributions. We present and analyze the main features of the SM and anomalous coupling contributions. We show how differences between SM and anomalous coupling contributions provide means to search for anomalous coupling manifestations at future e ⁺ e ^? linear colliders.     

Top-quark mediated effects in hadronic Higgs-Strahlung

Novel contributions to the total inclusive cross section for Higgs-Strahlung in the Standard Model at hadron colliders are evaluated. Although formally of order a_s²\alpha_{s}^{2}, they have not been taken into account in previous NNLO predictions. The terms under consideration are induced by Higgs radiation off top-quark loops and thus proportional to the top-quark Yukawa coupling. At the Tevatron, their effects to WH production are below 1% in the relevant Higgs mass range, while for ZH production, we find corrections between about 1% and 2%. At the LHC, the contribution of the newly evaluated terms to the cross section is typically of the order of 1%–3%. Based on these results, we provide updated predictions for the total inclusive Higgs-Strahlung cross section at the Tevatron and the LHC.     

Search for W' signal in single top quark production at the LHC

Heavy charged gauge bosons are proposed in some theories beyond the standard model. We explore the discovery potential for W →tb with top quark semi-leptonic decay at the LHC. We concentrate on the new physics signal search with the deviation from the standard model prediction if the resonance peak of W cannot be observed directly. Signal events with two jets plus one charged lepton and missing energy are simulated, together with the dominant standard model backgrounds. In this paper, it is found that suitable cuts on the kinematic observables can effectively suppress the standard model backgrounds, so that it is possible to search for a W signal at the LHC if its mass is less than 6.6 Te V.     

Heavy Dirac and Majorana neutrino production ine + e − collisions

The production of heavy Dirac and Majorana neutrinos ine ⁺ e ^? collisions is investigated. The heavy Dirac and/or Majorana neutrinos can be produced in charged and neutral current processes $(e^ + e^ - \to N_1 \bar N_2 )$ . The production of a single heavy neutrino is possible if it mixes with the light neutrino species. The production of heavy neutrinos in Higgs channels is also studied, since in some specific models the Yukawa couplings could be large enough to make the production of heavy neutrinos through Higgs boson exchanges sufficiently large for detection. The most general left-right symmetric model with possibly complexV orA couplings is used in the analytic calculations of the production cross sections, but the numerical examples are given using simplified left-right symmetric model. The interference terms between different production channels have been studied in great detail and in some cases the interference terms are found to be non-negligible in wide range of production spectrum. The pair production cross section is larger in the Dirac case than it is in the Majorana case, but the single heavy Majorana neutrino production cross section is roughly twice as large as that of a Dirac neutrino.     

Searching for a light Higgs boson

The Lindé-Weinberg bound [1] on the mass of the Higgs boson does not apply if one of the fermions of the standard model is very massive [2] or in non-standard models with multiple Higgs particles [1]. We consider both the standard model and a common extension thereof to two or more Higgs doublets. If the Higgs responsible for lepton masses is very light, one reliable method for indirectly detecting it would be a more careful measurement of g − 2 for the muon. More direct is the search for Higgs particles detected in association with τ pairs in existing or defunct e⁺e⁻ colliders operating well below the Z mass. We analyze both methods in detail, and find that data from several existing colliders could eliminate large portions of parameter space - or, perhaps, find the Higgs boson.