LEP Data Research Articles

Analysis of the consistency of kaon photoproduction data with Λ in the final state

The recent CLAS 2005, SAPHIR 2003, LEPS, and the old, pre-1972, data on ${K}^{+}\ensuremath{\Lambda}$ photoproduction are compared with theoretical calculations in the energy region of ${E}_{\ensuremath{\gamma}}^{\mathrm{lab}}<2.6$ GeV in order to learn about their mutual consistency. The isobaric models Kaon-Maid and Saclay-Lyon, along with new fits to the CLAS data, are utilized in this analysis. The SAPHIR 2003 data are shown to be coherently shifted down with respect to the CLAS, LEPS, and pre-1972 data, especially at forward kaon angles. The CLAS, LEPS, and pre-1972 data in the forward hemisphere can be described satisfactorily by using the isobaric model without hadronic form factors. The inclusion of the hadronic form factors yields a strong suppression of the cross sections at small kaon angles and c.m. energies larger than 1.9 GeV, which is not observed in the existing experimental data. We demonstrate that the discrepancy between the CLAS and SAPHIR data has a significant impact on the predicted values of the mass and width of the ``missing resonance'' ${D}_{13}(1895)$ in the Kaon-Maid model.

Unparticle physics and [formula omitted] on the Z pole

An attempt has been made to address the 3σ anomaly of the forward–backward asymmetry of b quark in LEP data via an unparticle sector. For most part of the parameter space except certain particular regions, the anomaly could not be explained away plausibly, when constraints from other LEP observables are taken into account.

Anomalous Higgs Couplings in the SO(5) x U(1)B-L Gauge-Higgs Unification in Warped Spacetime

The gauge couplings WWZ, WWWW , and WWZZ in the gauge-Higgs unification scenario in the Randall-Sundrum warped spacetime remain almost universal as in the standard model, but substantial deviation results for the Higgs couplings. In the SO(5) × U(1)B−L model, the couplings WWH and ZZH are suppressed by a factor cos θH from the values in the standard model, while the bare couplings WWHH and ZZHH are suppressed by a factor 1− 2 3 sin θH. Here θH is the Yang-Mills AB phase (Wilson line phase) along the fifth dimension, which characterizes the electroweak symmetry breaking. The suppression can be used to test the gauge-Higgs unification scenario at LHC and ILC. It is also shown that the WWZ coupling in flat spacetime deviates from the standard model value at moderate values of θH, contradicting with the LEP2 data.

Hint of aZ′boson from the CERN LEP II data

The many-parametric fits of the LEP2 data on ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}$, ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, ${\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ processes are performed with the goal to estimate the signals of the Abelian ${Z}^{\ensuremath{'}}$ boson. Four independent parameters must be fitted, if the derived already low-energy relations between the ${Z}^{\ensuremath{'}}$ couplings to the standard model fermions are taken into consideration. No signals are found when the complete LEP2 data set for these processes is treated. In the fit of the backward bins, the hint at the $1.3\ensuremath{\sigma}$ confidence level is detected. The ${Z}^{\ensuremath{'}}$ couplings to the vector and axial-vector lepton currents are constrained. The comparisons with the one-parameter fits and with the corresponding LEP1 experiments are fulfilled.

Collider phenomenology of unparticle physics

Low-energy phenomenology of the unparticle physics associated with an exact scale invariant sector possessing a nontrivial infrared fixed point at a higher energy scale is explored for both electron-positron and hadronic colliders. Feynman rules for a spin 0, 1, or 2 unparticle coupled to a variety of standard model gauge invariant operators that are relevant to many low-energy processes involving either real emissions of unparticles or their virtual propagator effects are presented. Missing energy and/or recoil mass distributions of the unparticle in the associated production of the unparticle together with a photon or Z boson at LEP2 and ILC as well as in Z decay into an unparticle plus a fermion-antifermion pair are studied. In addition, monojet production with missing energy from the unparticle at hadronic collisions are explored. The complex phase in the unparticle propagator that can give rise to interesting interference effects between an unparticle-exchange diagram and the standard model amplitudes are studied in details for the Drell-Yan process as well as muon pair and diphoton production in electron-positron annihilation. These energy and/or recoil mass distributions (with the exception in hadron colliders) and interference effects are found sensitively depending not only on the scaling dimension but also onmore » the spin of the unparticle. For the spin-2 unparticle, its physical effects are found to resemble that of a tower of Kaluza-Klein gravitons, which strongly indicates that the underlying unparticle physics may have root in a higher dimensional theory. A connection between unparticle physics and theories of large extra dimensions is speculated. Experimental constraints on the unparticle scale are deduced from the LEP2 data on monophoton production and from the 4-fermion contact interactions.« less

Signals of Z′ boson in the Bhabha process within the LEP2 data set

The LEP2 data set on the Bhabha process is analyzed with the aim to detect the signals of the heavy virtual Z' gauge bosons. The state interacting with the left-handed standard-model doublets and called the Chiral Z' is investigated. This particle was introduced already as the low-energy state allowed by the renormalizability of the model. The contribution of the Chiral Z' state to the Bhabha process is described by two parameters: the coupling to electrons and the Z-Z' mixing angle. The sign-definite one-parameter observable is proposed to measure the Z' coupling to the electron current. The one-parameter fit of the data shows no signals of the particle. The alternative two-parameter fit of the differential cross-sections is also performed. It also shows no Chiral Z' signals. The comparisons with other fits are discussed.

StueckelbergZ′extension with kinetic mixing and millicharged dark matter from the hidden sector

An analysis is given of the Stueckelberg extension of the Standard Model with a hidden sector gauge group $U(1)_X$ where the mass growth for the extra gauge boson occurs via the Stueckelberg mechanism, and where the kinetic mixing in the $U(1)_X\times U(1)_Y$ sector is included. Such a kinetic mixing is generic in a broad class of supergravity and string models. We carry out a detailed global fit of the model with the precision LEP data on and off the $Z$ pole, with $\chi^2$ within 1% of the $\chi^2$ of the Standard Model fit. Further, it is shown that in the absence of matter in the hidden sector, there is a single effective parameter that controls the deviations from the Standard Model predictions, and the dependence on the kinetic mixing emerges only when matter in the hidden sector is included. An analysis is also given of milli-charged dark matter arising from the hidden sector, where it is shown that such dark matter from the Stueckelberg extension can satisfy WMAP-3 data while allowing for a sharp $Z'$ resonance which can be detected at the Tevatron and at the LHC via a dilepton signal generated by the Drell-Yan process.

Modelling non-perturbative corrections to bottom-quark fragmentation

We describe B-hadron production in e + e − annihilation at the Z 0 pole by means of a model including non-perturbative corrections to b-quark fragmentation as originating, via multiple soft emissions, from an effective QCD coupling constant, which does not exhibit the Landau pole any longer and includes absorptive effects due to parton branching. We work in the framework of perturbative fragmentation functions at NLO, with NLL DGLAP evolution and NNLL large- x resummation in both coefficient function and initial condition of the perturbative fragmentation function. We include hadronization corrections via the effective coupling constant in the NNLO approximation and do not add any further non-perturbative fragmentation function. As part of our model, we perform the Mellin transforms of our resummed expressions exactly. We present results on the energy distribution of b-flavored hadrons, which we compare with LEP and SLD data, in both x- and N-spaces. We find that, within the theoretical uncertainties on our calculation, our model is able to reasonably reproduce the data at x ≲ 0.92 and the first five moments of the B cross section.

Measuring the Running of Alpha in Small Angle Bhabha Scattering

We propose a method to determine the running of α QED from a measurement of the small angle Bhabha scattering. The method is suited to high statistics experiments at e + e − colliders equipped with luminometers in the appropriate angular region. We discuss a recent application using LEP data.

Kaon photoproduction in a multipole approach

The recently published experimental data on K{sup +}{lambda} photoproduction by the SAPHIR, CLAS, and LEPS Collaborations are analyzed by means of a multipole approach. For this purpose the background amplitudes are constructed from appropriate Feynman diagrams in a gauge-invariant and crossing-symmetric fashion. The results of our calculation emphasize the lack of mutual consistency between the SAPHIR and CLAS data previously found by several independent research groups, whereas the LEPS data are found to be more consistent with those of CLAS. The use of SAPHIR and CLAS data, individually or simultaneously, leads to quite different resonance parameters that, therefore, could lead to different conclusions on 'missing resonances'. Fitting to the SAPHIR and LEPS data simultaneously indicates that the S{sub 11}(1650),P{sub 13}(1720),D{sub 13}(1700),D{sub 13}(2080),F{sub 15}(1680), and F{sub 15}(2000) resonances are required, whereas fitting to the combination of CLAS and LEPS data leads alternatively to the P{sub 13}(1900),D{sub 13}(2080),D{sub 15}(1675),F{sub 15}(1680), and F{sub 17}(1990) resonances. Although yielding different results in most cases, both SAPHIR and CLAS data indicate that the second peak in the cross sections at W{approx}1900 MeV originates from the D{sub 13}(2080) resonance with a mass between 1911 and 1936 MeV. Furthermore, in contrast to the results of currently available modelsmore » and the Table of Particle Properties, both data sets do not exhibit the need for a P{sub 11}(1710) resonance. The few data points available for target asymmetry cannot be described by the models proposed in the present work.« less

Anomalous Higgs boson couplings in vector boson fusion at the CERN LHC

Deviations from standard model expectations in the Higgs sector can be parametrized by an effective Lagrangian. The corresponding anomalous couplings have been implemented in a Monte Carlo program for Higgs production in vector boson fusion, at NLO QCD accuracy. It allows to study anomalous coupling effects for production and decay of the Higgs boson. We analyze deviations allowed by LEP data and study a new azimuthal angle variable which directly measures the interference between $CP$-even, $CP$-odd and standard model couplings.

Lepton non-universality at LEP and charged Higgs

A recent analysis of the LEP data shows an interesting deviation from lepton universality in W boson decays. An excess at the level of 2.8 sigma is found in the tau mode branching ratio with respect to the other two modes. It is suggested that this seeming lepton non-universality might stem from pair production of charged Higgs bosons almost degenerate with W, that preferentially decay to heavy fermions. It is shown that the deviation can be reduced to 1.4 sigma in two Higgs doublet model I without any conflict with the existing direct or indirect constraints. This conclusion is largely independent of tan beta, the ratio of Higgs vacuum expectation values. This scenario can be tested at the forthcoming international linear collider.

Naturalness and Higgs decays in the MSSM with a singlet

The simplest extension of the supersymmetric standard model — the addition of one singlet superfield — can have a profound impact on the Higgs and its decays. We perform a general operator analysis of this scenario, focusing on the phenomenologically distinct scenarios that can arise, and not restricting the scope to the narrow framework of the NMSSM. We reexamine decays to four b quarks and four τ's, finding that they are still generally viable, but at the edge of LEP limits. We find a broad set of Higgs decay modes, some new, including those with four gluon final states, as well as more general six and eight parton final states. We find the phenomenology of these scenarios is dramatically impacted by operators typically ignored, specifically those arising from D-terms in the hidden sector, and those arising from weak-scale colored fields. In addition to sensitivity of mZ, there are potential tunings of other aspects of the spectrum. In spite of this, these models can be very natural, with light stops and a Higgs as light as 82 GeV. These scenarios motivate further analyses of LEP data as well as studies of the detection capabilities of future colliders to the new decay channels presented.

Search for neutral MSSM Higgs bosons at LEP

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses of neutral Higgs bosons.

CERN LEP indications for two light Higgs bosons and theU(1)′model

Reanalyses of LEP data have shown preference to two light $CP$-even Higgs bosons. We discuss implications of such a Higgs boson spectrum for the minimal supersymmetric model extended by a standard model singlet chiral superfield and an additional Abelian gauge invariance [the $\mathrm{U}(1{)}^{\ensuremath{'}}$ model]. We, in particular, determine parameter regions that lead to two light $CP$-even Higgs bosons while satisfying existing bounds on the mass and mixings of the extra vector boson. In these parameter regions, the pseudoscalar Higgs is found to be nearly degenerate in mass with either the lightest or next-to-lightest Higgs boson. Certain parameters of the $\mathrm{U}(1{)}^{\ensuremath{'}}$ model such as the effective $\ensuremath{\mu}$ parameter are found to be significantly bounded by the LEP two light Higgs signal.

Radiative decay ofZ′boson

In the framework of the extended $SU(2)_{h}\times SU(2)_{l}$ model we study the radiative decay of an extra gauge boson $Z^{\prime}$, $Z^{\prime}\to\gamma Z$. The data analysis results of the CDF collaboration at the Fermilab Tevatron allow one to establish not only the lower bound of the decay probability $Z^{\prime}\to\gamma Z$ normalized to Drell-Yan process $Z^{\prime}\to \mu^{+}\mu^{-}$, $R(Z^{\prime}\to\gamma Z/\mu^{+}\mu^{-})> 1.4\times 10^{-5}\cot^{2}\phi$, but also to estimate the mixing angle $\xi$ between the states $Z^{\prime}$ and $Z$, $|\xi| < 5\times 10^{-2}$, taking into account the additional mixing angle $\phi$ related to the extended gauge group $SU(2)_{h}\times SU(2)_{l}$. The comparison with the decays $Z^{\prime}\to\tau^{+}\tau^{-}$ and $Z^{\prime}\to \bar b b$, $Z^{\prime}\to \bar t t$ based on the LEP data is also presented.

Simultaneous recording of laser-evoked brain potentials and continuous, high-field functional magnetic resonance imaging in humans

Simultaneous recording of event-related electroencephalographic (EEG) and functional magnetic resonance imaging (fMRI) responses has the potential to provide information on how the human brain reacts to an external stimulus with unique spatial and temporal resolution. However, in most studies combining the two techniques, the acquisition of functional MR images has been interleaved with the recording of evoked potentials. In this study we investigated the feasibility of recording pain-related evoked potentials during continuous and simultaneous collection of blood oxygen level-dependent (BOLD) functional MR images at 3 T. Brain potentials were elicited by selective stimulation of cutaneous Aδ and C nociceptors using brief radiant laser pulses (laser-evoked potentials, LEPs). MR-induced artifacts on EEG data were removed using a novel algorithm. Latencies, amplitudes, and scalp distribution of LEPs recorded during fMRI were not significantly different from those recorded in a control session outside of the MR scanner using the same equipment and experimental design. Stability tests confirmed that MR-image quality was not impaired by the evoked potential recording, beyond signal loss related to magnetic susceptibility differences local to the electrodes. fMRI results were consistent with our previous studies of brain activity in response to nociceptive stimulation. These results demonstrate the feasibility of recording reliable pain-related LEPs and fMRI responses simultaneously. Because LEPs collected during fMRI and those collected in a control session show remarkable similarity, for many experimental designs the integration of LEP and fMRI data collected in separate, single-modality acquisitions may be appropriate. Truly simultaneous recording of LEPs and fMRI is still desirable in specific experimental conditions, such as single-trial, learning, and pharmacological studies.

New NLO parameterizations of the parton distributionsin real photons

We present new NLO sets of parton distributions in real photons based on a scheme invariant definition of the non-perturbative input. We compare the theoretical predictions with LEP data and a best fit allows us to constrain the parameters of the distributions. The shape of the gluon distribution is poorly constrained and we consider the possibility to measure it in photoproduction experiments. Three parametrizations which aim to take into account the scattering of LEP data are proposed. They are compared to other NLO parametrizations.

Difficult scenarios for NMSSM Higgs discovery at the LHC

We identify scenarios not ruled out by LEP data in which NMSSM Higgs detection at the LHC will be particularly challenging. We first review the `no-lose' theorem for Higgs discovery at the LHC that applies if Higgs bosons do not decay to other Higgs bosons - namely, with L=300 fb^-1, there is always one or more `standard' Higgs detection channel with at least a 5 sigma signal. However, we provide examples of no-Higgs-to-Higgs cases for which all the standard signals are no larger than 7 sigma implying that if the available L is smaller or the simulations performed by ATLAS and CMS turn out to be overly optimistic, all standard Higgs signals could fall below 5 sigma even in the no-Higgs-to-Higgs part of NMSSM parameter space. In the vast bulk of NMSSM parameter space, there will be Higgs-to-Higgs decays. We show that when such decays are present it is possible for all the standard detection channels to have very small significance. In most such cases, the only strongly produced Higgs boson is one with fairly SM-like couplings that decays to two lighter Higgs bosons (either a pair of the lightest CP-even Higgs bosons, or, in the largest part of parameter space, a pair of the lightest CP-odd Higgs bosons). A number of representative bench-mark scenarios of this type are delineated in detail and implications for Higgs discovery at various colliders are discussed.

Unitarity violation of the CKM matrix in a nonuniversal gauge interaction model

We explore the unitarity violation of the Cabibbo-Kobayashi-Maskawa (CKM) matrix in the model where the third generation fermions are subjected to the separate SU(2){sub L} gauge interaction. With the recent LEP and SLAC linear collider (SLC) data at Z-pole and low-energy neutral current interaction data, the analysis on the parameter space of the model is updated, and the unitary violation is predicted under the constraint.