The role of beam polarization for radiative neutralino production at the ILC

We analyze the impact of electron and positron beam polarization on radiative neutralino production at the international linear collider (ILC). We focus on three different mSUGRA scenarios in turn at the Higgs strahlung threshold, the top pair production threshold, and at 500 GeV. In these scenarios at the corresponding , radiative neutralino production is the only supersymmetric production mechanism that is kinematically allowed. The heavier neutralinos and charginos as well as the sleptons, squarks and gluinos are too heavy to be pair produced. We calculate the signal cross section and also the standard model background from radiative neutrino production. For our scenarios, we obtain significances larger than 10 and signal to background ratios between 2% and 5%, if we have electron beam polarization –0.8 and positron beam polarization –0.3. If we have electron beam polarization of , then the signal is observable with but both the significance and the signal to background ratio are significantly improved for .     

CP asymmetries in the supersymmetric trilepton signal at the LHC

In the CP-violating Minimal Supersymmetric Standard Model, we study the production of a neutralino–chargino pair at the LHC. For their decays into three leptons, we analyze CP asymmetries which are sensitive to the CP phases of the neutralino and chargino sector. We present analytical formulas for the entire production and decay process, and identify the CP-violating contributions in the spin correlation terms. This allows us to define the optimal CP asymmetries. We present a detailed numerical analysis of the cross sections, branching ratios, and the CP observables. For light neutralinos, charginos, and squarks, the asymmetries can reach several 10%. We estimate the discovery potential for the LHC to observe CP violation in the trilepton channel.     

Measurement of CP-violating asymmetries in B0 decays to CP eigenstates

We present measurements of time-dependent CP-violating asymmetries in neutral B decays to several CP eigenstates. The measurement uses a data sample of 23x10(6) Upsilon(4S)-->BbarB decays collected by the BABAR detector at the PEP-II asymmetric B Factory at SLAC. In this sample, we find events in which one neutral B meson is fully reconstructed in a CP eigenstate containing charmonium and the flavor of the other neutral B meson is determined from its decay products. The amplitude of the CP-violating asymmetry, which in the standard model is proportional to sin2beta, is derived from the decay time distributions in such events. The result is sin2beta = 0.34+/-0.20 (stat)+/-0.05 (syst).     

CP asymmetries in chargino production and decay: the three-body decay case

We study CP violation in chargino production and decay in the minimal supersymmetric standard model (MSSM) with complex parameters at an e⁺e^- linear collider with longitudinally polarised beams. We investigate CP-sensitive asymmetries by means of triple product correlations and study their dependence on the complex parameters M₁ and μ. We give numerical predictions for the asymmetries and their measurability at the future International Linear Collider. Our results show that the CP asymmetries can be measured in a large region of the MSSM parameter space.     

CP asymmetries in Strange Baryon Decays

While indirect and direct C P violation(CPV) has been established in the decays of strange and beauty mesons, no CPV has yet been found for baryons. There are different paths to finding C P asymmetry in the decays of strange baryons; they are all highly non-trivial. The Hyper CP Collaboration has probed CPV in the decays of singleΞ and Λ [1]. We discuss future lessons from e~+e~-collisions at BESⅢ/BEPCⅡ: probing decays of pairs of strange baryons, namely Λ, Σ and ■. Realistic goals are to learn about non-perturbative QCD. One can hope to find CPV in the decays of strange baryons; one can also dream of finding the impact of New Dynamics. We point out that an important new era will start with the BESIII/BEPCII data accumulated by the end of 2018. This also supports new ideas to trigger J/ψ→■Λ at the LHCb collaboration.     

Beam polarization effects on top-pair production at the ILC

Full one-loop electroweak corrections for an \(e^-e^+\rightarrow t \bar{t}\) process associated with sequential \(t\rightarrow b \mu \nu _\mu \) decay are discussed. At the one-loop level, the spin-polarization effects of the initial electron and positron beams are included in the total and differential cross sections. A narrow-width approximation is used to treat the top-quark production and decay while including full spin correlations between them. We observed that the radiative corrections due to the weak interaction have a large polarization dependence on both the total and the differential cross sections. Therefore, experimental observables that depend on angular distributions such as the forward–backward asymmetry of the top-production angle must be treated carefully including radiative corrections. We also observed that the energy distribution of bottom quarks is largely affected by the radiative corrections.     

NLO event generation for chargino production at the ILC

We present a Monte Carlo event generator for simulating chargino pair production at the International Linear Collider (ILC) at next-to-leading order in the electroweak couplings. By properly resumming photons in the soft and collinear regions, we avoid negative event weights, so the program can simulate physical (unweighted) event samples. Photons are explicitly generated throughout the range where they can be experimentally resolved. Inspecting the dependence on the cutoffs separating the soft and collinear regions, we evaluate the systematic errors due to soft and collinear approximations. In the resummation approach, the residual uncertainty can be brought down to the per-mil level, coinciding with the expected statistical uncertainty at the ILC. PACS 12.15.Lk; 13.40.Ks; 13.66.Hk; 14.80.Ly     

CP violating asymmetries in the neutral kaon decays

We study the time dependence of the integrated asymmetries in strangeness of kaons in the decays in two and three pions and in two photons with the purpose of disentangling theCP violating effects in the decay amplitudes from the ones related to the mass matrix. We find two cases where they are not reduced by the I=1/2 rule or hidden byCP violation in the mass matrix, namely the strangeness-energy correlation of the charged pions in the prompt decay ( _S ) in ^+-0, and the decays in two photons. The semileptonic decays are also briefly studied.     

Gluino and neutralino contribution to the direct CP violation parameter έ

I present a detailed and complete calculation of the gluino and neutralino contribution to the direct CP violating parameter ? within the MSSM. I include the complete mixing matrices of the neutralino and of the scalar partners of the left and right handed down quarks. I find that the neutralino contribution is generally small but can be larger than the gluino contribution for small values m _s ≤ 400 GeV of the supersymmetric breaking scale.     

CP violating rate asymmetries in B decays

We briefly discuss measurements of angles β and α of the unitarity triangle. We then review rate asymmetries usingSU(3) relationships in the standard model (SM). Some methods to measure angle y usingSU(3) are then discussed. We note that rate forb → sγ can be used to set limits on extra dimensions in which standard model particles propagate.     

Corrections to from CP asymmetries in decays

Neglecting smaller amplitudes the time-dependent CP asymmetry in penguin-dominated transitions (such as B→K_S) is expected to equal ±sin(2β), an expectation not borne out by the present average experimental data. I compute and discuss the correction due to the smaller amplitudes in the framework of QCD factorization.     

Wedgebox analysis of four-lepton events from neutralino pair production at the LHC

A ‘wedgebox’ plot is a two-dimensional scatter-plot of two invariant mass quantities. Here pp→e⁺e^-μ⁺μ^-+≠E signature LHC events are analyzed by plotting the di-electron invariant mass versus the di-muon invariant mass. Data sets of such events are obtained across the MSSM input parameter space in realistic event-generator simulations, including cuts designed to remove SM backgrounds. Their study reveals several general features. Firstly, regions in the MSSM input parameter space where a sufficient number of events are expected so as to be able to construct a clear wedgebox plot are delineated. Secondly, the presence of box shapes on a wedgebox plot either indicates the presence of heavy Higgs bosons’ decays or restricts the location to a quite small region of low μ and M₂ values, ≲ 200 GeV, a region denoted as the ‘lower island’. In this region, wedgebox plots can be quite complicated and change in pattern rather quickly as one moves around in the (μ,M₂) plane. Thirdly, direct neutralino pair production from an intermediate Z^0* may only produce a wedge shape since only decays can contribute significantly. And fourthly, a double-wedge or wedge-protruding-from-a-box pattern on a wedgebox plot, which results from combining a variety of MSSM production processes, yields three distinct observed endpoints, almost always attributable to decays, which can be utilized to determine a great deal of information about the neutralino and slepton mass spectra and related MSSM input parameters. Wedge and double-wedge patterns are seen in wedgebox plots in another region of higher μ and M₂ values, denoted as the ‘upper island’. Here the pattern is simpler and more stable as one moves across the (μ,M₂) input parameter space.     

Scalar top quarks production in polarized photon-photon collisions at ILC

We study pair production of scalar top quarks (stop, \(\tilde t_1\)) in polarized photon-photon collisions with the subsequent decay of the top squarks into b quarks and charginos \(\tilde t_1 \to b\tilde \chi _1^ \pm\). We simulate this process by using PYTHIA6.4 for an electron beam energy \(2E_{beam}^e = \sqrt {s_{ee} } = 1000 GeV\). The energy spectrum of back-scattered photons is generated by CIRCE2 program. A set of criteria for physical variables is proposed which leads to a good separation of stop signal events from top quark pair production, being the main background. These criteria allow us to reconstruct the mass of the top squark, provided that the neutralino mass is known.     

Dark-matter pair production at the ILC in the littlest Higgs model with T-parity

In the littlest Higgs model with T-parity, the heavy photon (A_H) is supposed to be a possible dark matter (DM) candidate. The direct proof of the validity of this model is to produce the heavy photon at an accelerator. In this paper, we study the production rate of e~+ e~- → AH AH at the international e~+ e~- linear collider (ILC) in the littlest Higgs model with T-parity, and show the distributions of the transverse momenta of A H . The numerical results indicate that the heavy photon production rate could reach the 10~-1 fb level at some parameter space, so this could be a good chance to observe the heavy photon via the pair production process with high luminosity at the ILC (500 fb~-1). We know that DM is composed of weakly interacting massive particles, so the interactions with standard model particles are weak. How to detect heavy photons at a collider and distinguish them from other DM candidates are discussed in the final section of the paper.     

Measurement of time-dependent CP asymmetries in B0-->D(*)+/-D+/- decays

We present a first measurement of CP asymmetries in neutral B decays to D+D-, and updated CP asymmetry measurements in decays to D(*+)D- and D(*-)D+. We use fully reconstructed decays collected in a data sample of (232+/-3) x 10(6) gamma(4S)-->BB events in the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We determine the time-dependent asymmetry parameters to be SD(*+)(D-)=-0.54+/-0.35+/-0.07, CD(*+)(D-)=0.09+/-0.25+/-0.06, SD(*-)(D+)=-0.29+/-0.33+/-0.07, CD(*-)(D+)=0.17+/-0.24+/-0.04, SD+(D-)=-0.29+/-0.63+/-0.06, and CD+(D-)=0.11+/-0.35+/-0.06, where in each case the first error is statistical and the second error is systematic.     

ILC上Z玻色子与荷电top-pion对联合产生过程的研究

在国际直线对撞机(ILC)上顶色辅助的人工色(TC2)模型下对Z玻色子与荷电top-pion对联合产生过程e⁺e^- → Zπ_t⁺π_t^-进行了研究. 计算了过程e⁺e^- → Zπ_t⁺π_t^-的产生截面, 发现在TC2模型所允许的参数空间范围之内该过程的截面最大可达到1 fb. 考虑到荷电top-pion的主要的衰变模式π_t⁺ → tb, 发现该截面在ILC能产生很可观的事例数.