Can the minimal supersymmetric standard model with light bottom squark and light gluino survive Z-peak constraints?

In the framework of the minimal supersymmetric model we examine the Z-peak constraints on the scenario of one light bottom squark (sbottom) ( approximately 2-5.5 GeV) and light gluino (approximately 12-16 GeV), which has been successfully used to explain the excess of bottom quark production in hadron collisions. Such a scenario is found to be severely constrained by the CERN LEP Z-peak observables, especially by R(b), due to the large effect of gluino-sbottom loops. To account for the R(b) data in this scenario, the other mass eigenstate of sbottom, i.e., the heavier one, must be lighter than 125 (195) GeV at 2sigma(3sigma) level, which, however, is disfavored by CERN LEP II experiments.     

Do electroweak precision data and Higgs-mass constraints rule out a scalar bottom quark with mass of order 5 GeV?

We study the implications of a scalar bottom quark, with a mass of O (5 GeV), within the minimal supersymmetric standard model. Light sbottoms may naturally appear for large tan(beta) and, depending on the decay modes, may have escaped experimental detection. We show that a light sbottom cannot be ruled out by electroweak precision data and the bound on the lightest CP-even Higgs-boson mass. We infer that a light b scenario requires a relatively light scalar top quark whose mass is typically about the top-quark mass. In this scenario the lightest Higgs boson decays predominantly into b pairs and obeys the mass bound m(h) less, similar 123 GeV.     

Stop decays in SUSY-QCD

The partial widths are determined for stop decays to top quarks and gluinos, and gluino decays to stop particles and top quarks (depending on the masses of the particles involved). The widths are calculated including one-loop SUSY-QCD corrections. The radiative corrections for these strong-interaction decays are compared with the SUSY-QCD corrections for electroweak stop decays to quarks and neutralinos/charginos and top-quark decays to stops and neutralinos.     

Phenomenology of the minimal supergravity<Emphasis Type="Italic">SU</Emphasis>(5) model

The minimal grand unified supergravity model is discussed. Requiring radiative breaking of the electroweak gauge symmetry, the unification ofb and τ Yukawa couplings, a sufficiently stable nucleon, and not too large a relic density of neutralinos produced in the Big Bang constrains the parameter space significantly. In particular, the soft breaking parameterm _1/2 has to be less than about 130 GeV, and the top quark Yukawa coupling has to be near its (quasi) fixed point. The former condition implies\(m_{\bar g} \leqslant 400\) GeV and hence very large production rates for gluino pairs at the LHC, while the latter constraint implies that the lighter stop and sbottom eigenstates are significantly lighter than the other squarks, leading to characteristic signatures for gluino pair events.     

Leading Electroweak Corrections to the Neutral Higgs Boson Production at the Fermilab Tevatron

We calculate the leading electroweak corrections to the light neutral Higgs boson production via qq → WH at the Fermilab Tevatron in both the standard model and the minimal supersymmetric model, which arise from the top-quark and Higgs boson loop diagrams. We found that the leading electroweak corrections can exceed the QCD corrections for favorable values of the parameters in the MSSM, but such corrections are only about -2%~-4% in the SM, which are much smaller than the QCD corrections. For the mass region of 90 < mh, < 120 GeV, the leading electroweak corrections can reach -20% for large tan β, and these corrections may be observable at a high luminosity Tevatron; at the least, new constraints on the tan β can be established.     

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.     

Two-photon-exchange and gammaZ-exchange corrections to parity-violating elastic electron-proton scattering

Leading electroweak corrections play an important role in precision measurements of the strange form factors. We calculate the two-photon-exchange (TPE) and gammaZ-exchange corrections to the parity-violating asymmetry of the elastic electron-proton scattering in a simple hadronic model including the finite size of the proton. We find both can reach a few percent and are comparable in size with the current experimental measurements of strange-quark effects in the proton neutral weak current. The effect of gammaZ exchange is in general larger than that of TPE, especially at low momentum transfer Q2相似文献   

Limits on a composite Higgs boson

Precision electroweak data are generally believed to constrain the Higgs boson mass to lie below approximately 190 GeV at 95% confidence level. The standard Higgs model is, however, trivial and can only be an effective field theory valid below some high energy scale characteristic of the underlying nontrivial physics. Corrections to the custodial isospin violating parameter T arising from interactions at this higher energy scale dramatically enlarge the allowed range of Higgs mass. We perform a fit to precision electroweak data and determine the region in the (m(H),delta T) plane that is consistent with experimental results. Overlaying the estimated size of corrections to T arising from the underlying dynamics, we find that a Higgs mass up to 500 GeV is allowed.     

Indirect bounds on heavy scalar masses of the two-Higgs-doublet model in light of recent Higgs boson searches

We study an upper bound on masses of additional scalar bosons from the electroweak precision data and theoretical constraints such as perturbative unitarity and vacuum stability in the two-Higgs-doublet model taking account of recent Higgs boson search results. If the mass of the Standard-Model-like Higgs boson is rather heavy and is outside the allowed region by the electroweak precision data, such a discrepancy should be compensated by contributions from the additional scalar bosons. We show the upper bound on masses of the additional scalar bosons to be about 2 (1) TeV for the mass of the Standard-Model-like Higgs boson to be 240 (500) GeV.     

Complete two-loop electroweak fermionic corrections to the effective leptonic weak mixing angle sin2theta(lept)eff and indirect determination of the Higgs Boson Mass

We present a complete calculation of the contributions to the effective leptonic weak mixing angle, sin((2)theta;(lept)(eff), generated by closed fermion loops at the two-loop level of the electroweak interactions. This quantity is the source of the most stringent bound on the mass M(H) of the standard model Higgs boson. The size of the corrections with respect to known partial results varies between -4 x 10(-5) and -8 x 10(-5) for a realistic range of M(H) from 100 to 300 GeV. This translates into a shift of the predicted (from sin((2)theta;(lept)(eff) alone) central value of M(H) by +19 GeV, to be compared with the shift induced by a recent change in the measured top quark mass which amounts to +36 GeV.     

On the electroweak and gluonic corrections to the hadronic width of the Z boson

The electroweak corrections to the Z-boson hadronic width are presented in a simple analytical form for the width itself, Λ_h, for its ratio to the leptonic width, R_l = Λ_h/Λ_l, for the Λ_b/Λ_h ratio, and for the total width Λ_Z. The rational parametrization, advocated in our recent papers on leptonic decays, simplifies the analysis of the hadronic decays too. There are two main results of this analysis: (1) All electroweak precision measurements agree within 1 σ with the electroweak Born approximation if the gluon coupling constant . Thus the electroweak radiative corrections have not been observed experimentally. The unexpected smallness of electroweak radiative corrections is caused by the mutual cancellation of the large positive contribution of a heavy top quark and the large negative contribution of all other virtual particles. (2) With electroweak radiative corrections being taken into account, the value of extracted from the experimental value of Λ_Z differs by 3σ from that obtained from R_l, if m_t 200 GeV; they agree within 1σ if m_t 150 GeV. Thus a low upper limit on m_t can be obtained from hadronic decays of the Z-boson alone, even without data on its leptonic decays.     

NLO electroweak corrections to Higgs boson production at hadron colliders

Results for the complete NLO electroweak corrections to Standard Model Higgs production via gluon fusion are included in the total cross section for hadronic collisions. Artificially large threshold effects are avoided working in the complex-mass scheme. The numerical impact at LHC (Tevatron) energies is explored for Higgs mass values up to 500 GeV (200 GeV). Assuming a complete factorization of the electroweak corrections, one finds a +5% shift with respect to the NNLO QCD cross section for a Higgs mass of 120 GeV both at the LHC and the Tevatron. Adopting two different factorization schemes for the electroweak effects, an estimate of the corresponding total theoretical uncertainty is computed.     

DKP oscillator in the presence of magnetic field in (1+2)-dimensions for spin-zero and spin-one particles in noncommutative phase space

In view of the discovery of a new boson by the ATLAS and CMS Collaborations at the LHC, we present an update of the global Standard Model (SM) fit to electroweak precision data. Assuming the new particle to be the SM Higgs boson, all fundamental parameters of the SM are known allowing, for the first time, to overconstrain the SM at the electroweak scale and assert its validity. Including the effects of radiative corrections and the experimental and theoretical uncertainties, the global fit exhibits a p-value of 0.07. The mass measurements by ATLAS and CMS agree within 1.3σ with the indirect determination $M_{H}=94^{\,+25}_{\,-22}~\mathrm{GeV}$ . Within the SM the W boson mass and the effective weak mixing angle can be accurately predicted to be M _W =80.359±0.011 GeV and $\sin ^{2}\theta ^{\ell }_{{\rm eff}}= 0.23150\pm 0.00010$ from the global fit. These results are compatible with, and exceed in precision, the direct measurements. For the indirect determination of the top quark mass we find $m_{t}= 175.8^{\:+2.7}_{\:-2.4}~ \mathrm {GeV}$ , in agreement with the kinematic and cross-section-based measurements.     

Gluino reach and mass extraction at the LHC in radiatively-driven natural SUSY

Radiatively-driven natural SUSY (RNS) models enjoy electroweak naturalness at the 10% level while respecting LHC sparticle and Higgs mass constraints. Gluino and top-squark masses can range up to several TeV (with other squarks even heavier) but a set of light Higgsinos are required with mass not too far above \(m_h\sim 125\) GeV. Within the RNS framework, gluinos dominantly decay via \(\tilde{g}\rightarrow t\tilde{t}_1^{*},\ \bar{t}\tilde{t}_1 \rightarrow t\bar{t}\widetilde{Z}_{1,2}\) or \(t\bar{b}\widetilde{W}_1^-+c.c.\), where the decay products of the higgsino-like \(\widetilde{W}_1\) and \(\widetilde{Z}_2\) are very soft. Gluino pair production is, therefore, signaled by events with up to four hard b-jets and large \(\not \!\!{E_T}\). We devise a set of cuts to isolate a relatively pure gluino sample at the (high-luminosity) LHC and show that in the RNS model with very heavy squarks, the gluino signal will be accessible for \(m_{\tilde{g}} < 2400 \ (2800)\) GeV for an integrated luminosity of 300 (3000) fb\(^{-1}\). We also show that the measurement of the rate of gluino events in the clean sample mentioned above allows for a determination of \(m_{\tilde{g}}\) with a statistical precision of 2–5% (depending on the integrated luminosity and the gluino mass) over the range of gluino masses where a 5\(\sigma \) discovery is possible at the LHC.     

The Higgs boson mass from precision electroweak data

We present a new global fit to precision electroweak data, including new low- and high-energy data and analyzing the radiative corrections arising from the minimal symmetry breaking sectors of the Standard Model (SM) and its supersymmetric extension (MSSM). It is shown that present data favor a Higgs mass of ${cal O}(M_Z)$: $$M_{H}=76 {+ 152 ?op -50}{? GeV}.$$ We confront our analysis with (meta) stability and perturbative bounds on the SM Higgs mass, and the theoretical upper bound on the MSSM Higgs mass. Present data do not discriminate significantly between the SM and MSSM Higgs mass ranges. We comment in passing on the sensitivity of the Higgs mass determination to the values of $←pha (M_Z)$ and ${←pha_s} (M_Z)$.     

Light gauginos: Masses and instabilities

Within the framework of supergravity models without grand unified steps, we analyse in detail the consequences of the hypothesis that gauginos have no bare masses due to supergravity interactions. To this purpose we have made a one-loop calculation of wino, zino, and photino masses and a renormalization group improved two-loop calculation of the gluino masses.We find that: (i) the non-observation of charged winos is compatible either with a gravitino mass m ? 300 GeV or $\text{m ?3}\text{TeV}$; (ii) with a top quark mark of about 40 GeV, gluino and photino have very similar masses ranging from O(1 GeV) to O(20 GeV). In most cases consistency with cosmology requires that the gauge singlet needed to break the SU(2) × U(1) symmetry, be the lightest stable supersymmetric particle, with a mass as low as 1 keV or less. In such cases photino (or gluino) lifetimes into one photon (gluon) and one light singlet fermion (zerino), are typically between 10^?3 and 1 sec.We discuss the problem of the experimental detection of gauginos, which, according to the various options, require rather different approaches.     

Updated status of the global electroweak fit and constraints on new physics

We present an update of the Standard Model fit to electroweak precision data. We include newest experimental results on the top-quark mass, the W mass and width, and the Higgs-boson mass bounds from LEP, Tevatron and the LHC. We also include a new determination of the electromagnetic coupling strength at the Z pole. We find for the Higgs-boson mass $91^{+30}_{-23}~\mbox{GeV}$ and $120^{+12}_{-5}~\mbox{GeV}$ when not including and including the direct Higgs searches, respectively. From the latter fit we indirectly determine the W mass to be $(80.360^{+0.014}_{-0.013})~\mbox{GeV}$ . We exploit the data to determine experimental constraints on the oblique vacuum polarisation parameters, and confront these with predictions from the Standard Model (SM) and selected SM extensions. By fitting the oblique parameters to the electroweak data we derive allowed regions in the BSM parameter spaces. We revisit and consistently update these constraints for a fourth fermion generation, two Higgs doublet, inert Higgs and littlest Higgs models, models with large, universal or warped extra dimensions and technicolour. In most of the models studied a heavy Higgs boson can be made compatible with the electroweak precision data.     

Z'-mediated supersymmetry breaking

We consider a class of models in which supersymmetry breaking is communicated dominantly via a U1' gauge interaction, which also helps solve the mu problem. Such models can emerge naturally in top-down constructions and are a version of split supersymmetry. The spectrum contains heavy sfermions, Higgsinos, exotics, and Z' approximately 10-100 TeV, light gauginos approximately 100-1000 GeV, a light Higgs boson approximately 140 GeV, and a light singlino. A specific set of U1' charges and exotics is analyzed, and we present five benchmark models. The implications for the gluino lifetime, cold dark matter, and the gravitino and neutrino masses are discussed.     

Status and prospects of top-quark physics

The top quark is the heaviest elementary particle observed to date. Its large mass of about 173 GeV/c² makes the top quark act differently than other elementary fermions, as it decays before it hadronises, passing its spin information on to its decay products. In addition, the top quark plays an important role in higher order loop corrections to standard model processes, which makes the top-quark mass a crucial parameter for precision tests of the electroweak theory. The top quark is also a powerful probe for new phenomena beyond the standard model.During the time of discovery at the Tevatron in 1995 only a few properties of the top quark could be measured. In recent years, since the start of Tevatron Run II, the field of top-quark physics has changed and entered a precision era. This report summarises the latest measurements and studies of top-quark properties and gives prospects for future measurements at the Large Hadron Collider (LHC).     

Search for an LSP gluino at LEP with the DELPHI detector

In some supersymmetric models, the gluino () is predicted to be light and stable. In that case, it would hadronize to form R-hadrons. In these models, the missing energy signature of the lightest supersymmetric particle is no longer valid, even if R-parity is conserved. Therefore, such a gluino is not constrained by hadron collider results, which looked for the decay . Data collected by the DELPHI detector in 1994 at 91.2 GeV have been analysed to search for events. No deviation from Standard Model predictions is observed and a gluino mass between 2 and 18 GeV/c² is excluded at the 95% confidence level in these models. Then, R-hadrons produced in the squark decays were searched for in the data collected by DELPHI at the centre-of-mass energies of 189 to 208 GeV, corresponding to an overall integrated luminosity of 609 Pb^-1. The observed number of events is in agreement with the Standard Model predictions. Limits at 95% confidence level are derived on the squark masses from the excluded regions in the plane () GeV/c², and GeV/c² for purely left squarks. GeV/c², and GeV/c² independent of the mixing angle. Received: 6 November 2002 / Published online: 15 January 2003