Top quark forward-backward asymmetry in the little Higgs model

A sizable difference in top quark pair forward backward asymmetry （AFB） is observed at Tevatron. The discrepancy triggers many new physics beyond the standard model （SM） and then constrains the parameter spaces in them. In this article we calculate the AFB of the top-pair production at Tevatron up to next to leading order （NLO） in the little Higgs model （LHM）. We find that the contribution of ZH can be large enough to make up the gap between SM prediction and experimental data. Then, the parameter space for the couplings between ZH and quarks are constrained. Thus, this model can fulfill the experimental data, both in AFB and in cross section.

Two Higgs Doublet Model Ⅲ and Double-Lepton Polarization Asymmetries in Λ_b →Λμ^+μ^- Decays

Using the form factors calculated both in lattice QCD and HQET, we analyze the branching ratio, doublelepton polarization asymmetries and averaged double-lepton polarization asymmetries of Λ_b →Λμ^+μ^- decay in the2 HDM Ⅲ, respectively. Combining the experimental constrains on the 2HDM Ⅲ parameters, we take two set of representative parameter spaces in our calculations. For the branching ratio of Λ+b →Λμμ-decay, considering the uncertainties of the form factors and input parameters, we find the results of both SM and 2HDM Ⅲ can satisfy the current experimental data in the framework of lattice QCD, but not in HQET. Then we calculate the double-lepton polarization asymmetries Pij and their averaged values Pij, we find the double-lepton polarization asymmetries PLT,PNN, and PTT are sensitive to the 2HDM Ⅲ. However, PLN, PNL, PNT, and PTN show the opposite results. The contribution of the 2HDM Ⅲ to their averaged values have similar results.

Impact of lepton pT threshold on charge asymmetry predictions for inclusive Wboson production in pp collisions at 13 TeV

This paper presents the impact of the lepton transverse momentum p^(l)_(T) threshold on the W boson charge asymmetry predictions in perturbative QCD for the inclusive W^(±)+X→l^(±)v+X production in proton-proton(pp)collisions.The predictions are obtained at various low-p^(l)_(T) thresholds p^(l)_(T)>20,25,30,and 40 GeV in a fiducial region encompassing both the central and forward detector acceptances in terms of the lepton pseudorapidity 0≤ηl≤<4.5.The predicted distributions for the lepton charge asymmetry,which is defined by ηl(Aηl),at the next-to-next-to-leading order(NNLO)accuracy are compared with the CMS and LHCb data at 8 TeV center-of-mass collision energy.The 8 TeV predictions reproduce the data fairly well within the quoted uncertainties.The predictions from the CT14 parton distribution function(PDF)model are in a slightly better agreement with the data over the other PDF sets that are tested.The 13 TeV predictions using various p^(l)_(T) thresholds are reported for An,and the charge asymmetries that are defined in terms of the differential cross sections in bins of the W boson rapidity yw(A_(yw))and transverse momentum(Apw).The NNLO predictions for the Ayw,and Apw distributions are assessed to be in close correlation with the plT value.The An,and A_(yw) distributions are particularly shown to be more correlated at a higher ^(l)_(T) threshold.The Apw distributions are also reported from the merged predictions with improved accuracy by the inclusion of the next-to-next-to-next-to-leading logarithm(N^(3)LL)corrections,i.e.,at NNLO+N^(3)LL.The predicted distributions from various ^(l)_(T) thresholds represent a finer probe in terms of the capability to provide more constraints on the ratio of u and d quark distribution functions in the parton momentum fraction range 10^(-4)<x<1.

Broken S_(3L)×S_(3R) flavor symmetry and leptonic CP violation

In the framework of the canonical seesaw model,we present a simple but viable scenario to explicitly break an S3L×S3R flavor symmetry in the leptonic sector.It turns out that the leptonic flavor mixing matrix is completely determined by the mass ratios of the charged leptons（i.e.,me/mμand mμ/mτ） and those of light neutrinos（i.e.,m1/m2 and m2/m3）.The latest global-fit results of the three neutrino mixing angles {θ12,θ13,θ23}and two neutrino mass-squared differences {？m212,？m312} at the 3σ level are used to constrain the parameter space of {m1/m2,m2/m3}.The predictions for the mass spectrum and flavor mixing are highlighted：（1） the neutrino mass spectrum shows a hierarchical pattern and a normal ordering,e.g.,m1≈2.2meV,m2≈8.8 meV and m3≈52.7 meV;（2） only the first octant of θ23 is allowed,namely,41.8？ θ23 43.3？;（3） the Dirac C P-violating phase δ ≈-22？deviates significantly from the maximal value-90？.All these predictions are ready to be tested in ongoing and forthcoming neutrino oscillation experiments.Moreover,we demonstrate that the cosmological matter-antimatter asymmetry can be explained via resonant leptogenesis,including the individual lepton-flavor effects.In our scenario,leptonic C P violation at low-and high-energy scales is closely connected.

Search for new physics in the B→K^((*))l^+l^- decays at BABAR

Based on a data sample of 384 million BB pairs collected with the BABAR detector at the PEP-Ⅱ asymmetric e ＋ e ￣ storage ring,we measure branching fractions,direct CP,isospin and lepton-flavor asymmetries for the rare decays B → K （＊） l ＋ l￣ in two di-lepton mass bins above and below the J/ψ resonance.For the B → K＊ l ＋ l￣ decay,we also measure the K＊ longitudinal polarization fraction and the di-lepton forwardbackward asymmetry.

The study of B→K^(*)l^+l^- decays in family non-universal Z′ models

In a combined investigation of B → K （ ＊ ） l ＋ l - decays, constraints on the related couplings in family non-universal Z′ models are derived. We find that within the allowed parameter space, the recently observed forward-backward asymmetry in the B → K ＊ l ＋ l - decay can be explained by flipping the signs of the Wilson coefficients C eff 9 and C 10 . With the obtained constraints, we also calculate the branching ratio of the B s →μ ＋ μ - decay. The upper bound of our prediction is nearly an order of magnitude smaller than the upper bound given by the CDF Collaboration recently.

Constraining parameter space of the little Higgs model using data from tera-Z factory and ILC

STYLE=“word-wrap： break-word”〉The Standard Model（SM） prediction on the forward-backward asymmetry for bˉb production（A^bFB） is well consistent with the data of LEP at the Z-pole, but deviates from the data at√s =89.55 and 92.95 Ge V which are slightly away from the pole. This deviation implies that there is still room for new physics. We calculate the A^bFB at the vicinity of the Z-pole in the little Higgs model as well as other measurable parameters such as R b and R c,by which we may constrain the parameter space of the little Higgs model. This can be further tested in the newly proposed tera-Z factory. With the fitted parameters we further make predictions on A^bFB and A t FB for tˉt production at the International Linear Collider（ILC）.

Analysis of B_s→Φμ^+μ^- decay within supersymmetry

Motivated by the first measurement on B（Bs→ Ф μ^＋ μ^-by the CDF Collaboration, we study the supersymmetric effects in semi-leptonic Bs→ Ф μ^＋ μ^-ecay. In our evaluations, we analyze the dependences of the dimuon invariant mass spectrum and the forward-backward asymmetry on relevant supersymmetric couplings in the MSSM with and without R-parity. The analyses show that the new experimental upper limits of B（Bs→ Ф μ^＋ μ^-from the LHCb Collaboration could further improve the bounds on sneutrino exchange couplings and （δ^u LL）23 as well as （δ^d LL,RR）23 mass insertion couplings. In addition, within the allowed ranges of relevant couplings under the constraints from B（Bs→ Ф μ^＋ μ^- B（B → K^（＊） μ^＋ μ^-and B（Bs → μ^＋ μ^-, the dimuon forward-backward asymmetry and the differential dimuon forward-backward asymmetry of Bs→Ф μ^＋ μ^-re highly sensitive to the squark exchange contribution and the （ LL）23 mass insertion contribution. The results obtained in this work will be very useful in searching for supersymmetric signals at the LHC.

Recent results from the HERMES experiment

Results are presented from the Hermes experiment which uses semi-inclusive deep inelastic lepton scattering to study the flavor structure of the nucleon.Data have been accumulated for pion and kaon double spin asymmetries,single-spin azimuthal asymmetries for meson electroproduction,deep virtual Compton scattering （DVCS）,and meson multiplicities.These results provide information on the properties of the strange sea in the proton,constraints on transverse momentum dependent quark parton distributions,and demonstrate the promise of DVCS for isolating the total angular momentum carried by the quarks in the proton.

基于协变重子手征微扰理论的弱辐射超子衰变研究

弱辐射超子衰变是少有的同时涉及强、弱和电磁3种相互作用的独特物理过程.这个看似简单的两体衰变,由于长久以来实验测量和理论预言存在较大差异,一直是粒子物理研究的难点问题之一,被称为弱辐射超子衰变疑难.近期,北京正负电子对撞机上的BESIII合作组首次测量了Λ→nγ衰变的不对称参数,更新了其衰变分支比,发现已有的理论预言都不能解释新的实验数据,这进一步加剧了弱辐射超子衰变疑难问题.本文在协变手征微扰理论的框架下,采用拓展的质量在壳重整化方案实现自洽的记阶规则,首次考虑了接触项树图贡献,通过拟合最新超子非轻衰变的实验数据确定理论的相关输入,系统研究了全部6个弱辐射(八重态)超子衰变道,结果显示协变手征微扰理论不仅能够很好地描述最新的BESIII实验数据,并且对4个有观测结果的弱辐射超子衰变道也能描述的较好.理论预言仅在∑^(+)→pγ衰变道与实验数据存在一定偏差.考虑到手征微扰理论的模型无关特性,作者鼓励实验物理学家在未来精确测量Ξ^(-)→Σ~-γ衰变道,以检验当前关于∑^(+)→pγ衰变的实验数据是否可靠,从而进一步加深对弱辐射超子衰变疑难的理解.此外,本研究也为超子稀有半轻衰变B→Bll中的新物理研究提供了必要的标准模型输入.