Phenomenological consequences of right-handed down squark mixings

The mixings of d(R) quarks, hidden from view in the standard model (SM), are naturally the largest if one has an Abelian flavor symmetry. With supersymmetry (SUSY) their effects can surface via d(R) squark loops. Squark and gluino masses are at TeV scale, but they can still induce effects comparable to SM in B(d) (or B(s)) mixings, while D0 mixing could be close to recent hints from data. In general, CP phases would be different from SM, as may be indicated by recent B factory data. Presence of nonstandard soft SUSY breakings with large tanbeta could enhance b-->dgamma (or sgamma) transitions.     

Single slepton production associated with a top quark at LHC in NLO QCD

Single slepton production in association with a top quark at the CERN Large Hadron Collider (LHC) is one of the important processes in probing the R-parity violation couplings. We calculate the QCD next-to-leading order (NLO) corrections to the $pp \to t\tilde{\ell}^{-}(\bar{t}\tilde{\ell}^{+})+X$ process at the LHC and discuss the impacts of the QCD corrections on kinematic distributions. We investigate the dependence of the leading order (LO) and the NLO QCD corrected integrated cross section on the factorization/renormalization energy scale, slepton, stop-quark and gluino masses. We find that the uncertainty of the LO cross section due to the energy scale is obviously improved by the NLO QCD corrections, and the exclusive jet event selection scheme keeps the convergence of the perturbative series better than the inclusive scheme. The results show that the polarization asymmetry of the top-quark will be reduced by the NLO QCD corrections, and the QCD corrections generally increase with the increment of the $\tilde{t}_{1}$ or $\tilde {g}$ mass value.     

The b \to X_s \gamma rate and Higgs boson limits in the constrained minimal supersymmetric model

New NLO calculations have become available using resummed radiative corrections. Using these calculations we perform a global fit of the supergravity inspired constrained minimal supersymmetric model. We find that the resummed calculations show similar constraints as the LO calculations, namely that only with a relatively heavy supersymmetric mass spectrum of (1 TeV) the b– Yukawa unification and the rate can coexist in the large scenario. The resummed calculations are found to reduce the renormalization scale uncertainty considerably. The low scenario is excluded by the present Higgs limits from LEP II. The constraint from the Higgs limit in the plane is severe, if the trilinear coupling at the GUT scale is fixed to zero, but is considerably reduced for . The relatively heavy SUSY spectrum required by corresponds to a Higgs mass of GeV in the CMSSM. Received: 14 February 2001 / Revised version: 22 March 2001 / Published online: 29 June 2001     

Top-quark pair production via polarized and unpolarized protons in the supersymmetric QCD

The QCD corrections to the top-quark pair production via both polarized and unpolarized gluon fusion in pp collisions are calculated in the minimal supersymmetric model (MSSM). We find that the MSSM QCD corrections can reach 4% and may be observable in future precise experiments. Furthermore, we studied CP violation in the MSSM. Our results show that the CP-violating parameter is sensitive to the masses of SUSY particles (it becomes zero when the c.m. energy is less than twice the masses of both the gluino and the stop quarks) and may reach . Received: 30 September 1998 / Revised version: 21 January 1999 / Published online: 28 May 1999     

W production at large transverse momentum at the CERN Large Hadron Collider

We study the production of W bosons at large transverse momentum in pp collisions at the CERN Large Hadron Collider. We calculate the complete next-to-leading order (NLO) corrections to the differential cross section. We find that the NLO corrections provide a large increase to the cross section but, surprisingly, do not reduce the scale dependence relative to leading order (LO). We also calculate next-to-next-to-leading-order (NNLO) soft-gluon corrections and find that, although they are small, they significantly reduce the scale dependence thus providing a more stable result.     

QCD corrections to double J/psi production in e+e- annihilation at sqrt[s]=10.6 GeV

Next-to-leading-order (NLO) QCD corrections to double J/psi production in e+e- annihilation at sqrt[s]=10.6 GeV are calculated. We find that they greatly decrease the cross section, with a K factor (NLO/LO) ranging from -0.31 to 0.25 depending on the renormalization scale. Although the renormalization scale dependence indicates a large uncertainty, when combined with the NLO QCD corrections to J/psi+etac production, it can explain why the double J/psi production could not be found at B factories while the J/psi+etac production could, despite the fact that cross section of the former is larger than that of the latter at LO by a factor of 1.8.     

SUSY—QCD Corrections to B^0—B^0 Mixing

We study the SUSY-QCD corrections to B^0-B^0 mixing with a reasonable SUSY parameter space and find that contribution from gluio is proportional to log(mg/μω) where μω is the weak interaction energy scale and by no means negligible.     

Soft supersymmetry breaking, scalar top-charm mixing and Higgs signatures

The squark mass-matrix from the soft supersymmetry (SUSY) breaking sector contains a rich flavor-mixing structure that allows O(1) mixings among top- and charm-squarks while being consistent with all the existing theoretical and experimental bounds. We formulate a minimal flavor-changing-neutral current scheme in which the squark mixings arise from the non-diagonal scalar trilinear interactions. This feature can be realized in a class of new models with a horizontal U(1)_H symmetry which generates realistic quark-mass matrices and provides a solution to the SUSY μ-problem. Finally, without using the mass-insertion approximation, we analyze SUSY radiative corrections to the H^±bc and h⁰tc couplings, and show that these couplings can reveal exciting new discovery channels for the Higgs boson signals at the Tevatron and the LHC.     

Precision Calculations for the T-Odd Quark Pair Production at the CLIC e+e- Linear Collider

We perform the precision calculations for the e⁺e^-→q_q_(q_q_=u_u_, c_c_, d_d_,s_s_) processes up to the QCD next-to-leading order (NLO) including full weak decays for the final T-odd mirror quarks in the littlest Higgs model with T-parity (LHT) at the Compact Linear Collider (CLIC). We show the dependence of the leading order (LO) and NLO QCD corrected cross sections on the colliding energy √s, and provide the LO and QCD NLO kinematic distributions of final particles. The results show that the LO cross section can be enhanced by the NLO QCD correction and the K-factor increases obviously when the threshold of the on-shell q_q_-pair production approaches the colliding energy √s. The K-factor value varies in the range of 1.04 ～ 1.41 in our chosen parameter space. We find that a simple approximation of multiplying the LO kinematic distribution with the integrated K-factor is not appropriate for precision study of the e⁺e^-→q_q_(q_q_=u_u_, c_c_, d_d_,s_s_) processes, since the NLO QCD corrections are phase space dependent. It is necessary to calculate the differential cross sections including full NLO QCD corrections to get reliable results.     

Gluino decays in the complex MSSM: a full one-loop analysis

We evaluate all two-body decay modes of the gluino, in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM). This constitutes an important step in the cascade decays of SUSY particles at the LHC. The evaluation is based on a full one-loop calculation of all two-body decay channels, also including hard QED and QCD radiation. The dependence of the gluino decay to a scalar quark and a quark on the relevant cMSSM parameters is analyzed numerically. We find sizable contributions to the decay widths and branching ratios. They are, roughly of O(±5%)\mathcal{O}({\pm 5\%)}, but can go up to ±10% or higher, where the pure SUSY QCD contributions alone can give an insufficient approximation to the full one-loop result. Therefore the full corrections are important for the correct interpretation of gluino decays at the LHC. The results will be implemented into the Fortran code FeynHiggs.     

Theory of two-proton radioactivity with application to 19Mg and 48Ni

If the soft supersymmetry (SUSY) breaking masses and couplings are complex and cancellations do take place in the SUSY induced contributions to the fermionic electric dipole moments, then the CP-violating soft phases can drastically modify much of the known phenomenological pattern of the minimal supersymmetric standard model. In particular, the squark loop content of the dominant Higgs production mechanism at the Large Hadron Collider, the gluon-gluon fusion mode, could be responsible for large corrections to the known cross sections.     

Flavor-dependent supersymmetric phases and CP asymmetry in B --> X(s)gamma decays

We study the direct CP asymmetry in B-->X(s)gamma decay in supersymmetric models with nonuniversal A terms. We show that the flavor-dependent phases of the A terms, unlike the flavor-independent ones, can give rise to a large contribution to the CP asymmetry while respecting the experimental bounds on the branching ratio of B-->X(s)gamma decay and the electric dipole moments of the electron and neutron. The CP asymmetry of this decay is predicted to be much larger than the standard model prediction in a wide region of the parameter space. In particular, it can be of order 10%-15% which can be accessible at B factories.     

From flavour to SUSY flavour models

If supersymmetry (SUSY) will be discovered, successful models of flavour not only have to provide an explanation of the flavour structure of the Standard Model fermions, but also of the flavour structure of their scalar superpartners. We discuss aspects of such “SUSY flavour” models, towards predicting both flavour structures, in the context of supergravity (SUGRA). We point out the importance of carefully taking into account SUSY-specific effects, such as 1-loop SUSY threshold corrections and canonical normalisation, when fitting the model to the data for fermion masses and mixings. This entangles the flavour model with the SUSY parameters and leads to interesting predictions for the sparticle spectrum. We demonstrate these effects by analyzing an example class of flavour models in the framework of an SU(5) Grand Unified Theory with a family symmetry with real triplet representations. For flavour violation through the SUSY soft breaking terms, the class of models realises a scheme we refer to as “Trilinear Dominance”, where flavour violation effects are dominantly induced by the trilinear terms.     

Anomalous enhancement of a penguin hadronic matrix element in B-->K(eta')

We estimate the density matrix element for the pi(0), eta, and eta(') production from the vacuum in the large-N(c) limit. As a consequence, we find that the QCD axial anomaly leads to highly nontrivial corrections to the usual flavor SU(3) relations between B(0)-->K(0)pi(0), B(0)-->K(0) eta, and B(0)-->K(0)eta(') decay amplitudes. These corrections may explain why the B-->Keta(') branching ratio is about 6 times larger than the B-->Kpi one.     

B-->pipi, new physics in B-->piK, and implications for rare K and B decays

The measured B-->pipi,piK branching ratios (BRs) exhibit puzzling patterns. We point out that the B-->pipi hierarchy can be nicely accommodated in the standard model (SM) through nonfactorizable hadronic interference effects, whereas the B-->piK system may indicate new physics (NP) in the electroweak (EW) penguin sector. Using the B-->pipi data and the SU(3) flavor symmetry, we fix the hadronic B-->piK parameters and show that any currently observed feature of the B-->piK system can be easily explained through enhanced EW penguin diagrams with a large CP-violating NP phase. This in turn implies in particular an enhancement of the K(L)-->pi(0)nunu rate by 1 order of magnitude, with BR(K(L)-->pi(0)nunu) approximately 4BR(K+-->pi(+)nunu), BR(K(L)-->pi(0)e(+)e(-))=O(10(-10)), and (sin(2beta)(pinunu)<0. We address also other rare K and B decays and B(d)-->phiK(S).     

SUSY QCD Impact on Top-Pair Production Associated with a Z0-Boson at a Photon-Photon Collider

The top-pair production in association with a Z⁰-boson at a photon-photon collider is an important process in probing the coupling between top-quarks and vector boson and discovering the signature of possible new physics. We describe the impact of the complete supersymmetric QCD (SQCD) next-to-leading order (NLO)radiative corrections on this process at a polarized or unpolarized photon collider, and make a comparison between the effects of the SQCD and the standard model (SM) QCD. We investigate the dependence of the lowest-order (LO) and QCD NLO corrected cross sections in both the SM and minimal supersymmetric standard model (MSSM) on colliding energy s^1/2 in different polarized photon collision modes. The LO, SM NLO, and SQCD NLO corrected distributions of the invariantmass of t\bar{t}-pair and the transverse momenta of final Z⁰-boson are presented. Our numerical results show that the pure SQCD effects in γγ→t\bar{t}z⁰ process can be more significant in the + + polarized photon collision mode than in other collision modes, and the relative SQCD radiative correction in unpolarized photon collision mode varies from 32.09\% to -1.89\% when s^1/2 goes up from 500 GeV to 1.5 TeV.     

Two-body decays of gluino at full one-loop level in the quark-flavour violating MSSM

We study the two-body decays of the gluino at full one-loop level in the Minimal Supersymmetric Standard Model with quark-flavour violation (QFV) in the squark sector. The renormalisation is done in the \(\overline{\mathrm{DR}}\) scheme. The gluon and photon radiations are included by adding the corresponding three-body decay widths. We discuss the dependence of the gluino decay widths on the QFV parameters. The main dependence stems from the \(\tilde{c}_R \)–\( \tilde{t}_R\) mixing in the decays to up-type squarks, and from the \(\tilde{s}_R \)–\( \tilde{b}_R\) mixing in the decays to down-type squarks due to the strong constraints from B-physics on the other quark-flavour-mixing parameters. The full one-loop corrections to the gluino decay widths are mostly negative and of the order of about ?10%. The QFV part stays small in the total width but can vary up to ?8% for the decay width into the lightest \(\tilde{u}\) squark. For the corresponding branching ratio the effect is somehow washed out by at least a factor of two. The electroweak corrections can be as large as 35% of the SUSY QCD corrections.     

NLO QCD corrections to graviton induced deep inelastic scattering

We consider Next-to-Leading-Order QCD corrections to ADD graviton exchange relevant for Deep Inelastic Scattering experiments. We calculate the relevant NLO structure functions by calculating the virtual and real corrections for a set of graviton interaction diagrams, demonstrating the expected cancellation of the UV and IR divergences. We compare the NLO and LO results at the centre-of-mass energy relevant to HERA experiments as well as for the proposed higher energy lepton–proton collider, LHeC, which has a higher fundamental scale reach.     

Quasi-degenerate neutrinos and lepton flavor violationin supersymmetric models

In supersymmetric models the misalignment between fermion and sfermion families introduces unsuppressed flavor-changing processes. Even if the mass parameters are chosen to give no flavor violation, family dependent radiative corrections make this adjustment not stable. We analyze the rate of in SUSY-GUT models with three quasi-degenerate neutrinos and universal scalar masses at the Planck scale. We pay special attention to a recently proposed scenario where the low-energy neutrino mixings are generated from identical quark and lepton mixings at large scales. We show the following. (i) To take universal slepton masses at the GUT scale is a very poor approximation, even in no-scale models. (ii) For large neutrino Yukawa couplings the decay would be observed in the planned experiment at PSI. (iii) For large values of the tau coupling gives important corrections, pushing and to accessible rates. In particular, the non-observation of these processes in the near future would exclude the scenario with unification of quark and lepton mixing angles. (iv) The absence of lepton flavor violating decays in upcoming experiments would imply a low value of , small neutrino couplings, and large ( GeV) SUSY-breaking masses.     

Minimal SUSY SO(10), b–τ unification and large neutrino mixings

We show that the assumption of type II seesaw mechanism for small neutrino masses coupled with b–τ mass unification in a minimal SUSY SO(10) model leads not only to a natural understanding of large atmospheric mixing angle (θ₂₃) among neutrinos, as recently noted, but also to large solar angle (θ₁₂) and a small θ₁₃≡U_e3 as required to fit observations. No additional symmetries are required to obtain large neutrino mixings. The proposed long baseline neutrino experiments will provide a crucial test of this model since it predicts U_e30.16.