LEP Energies Research Articles

Probing top-quark operators with precision electroweak measurements* *Yiming Liu, Cen Zhang and Jiayin Gu are supported by National Natural Science Foundation of China (NSFC) (12035008), Yuhao Wang and Lei Zhang are supported by NSFC (12122507)

In the standard model effective field theory, operators involving the top quark are generally difficult to probe and can generate sizable loop contributions to electroweak precision observables measured by past and future lepton colliders. Could the high precision of electroweak measurements compensate for loop suppression and provide competitive reaches on these operators? Would the inclusion of these contributions introduce too many additional parameters for a meaningful global electroweak analysis to be performed? In this paper, we perform a detailed phenomenological study to address these two important questions. Focusing on eight dimension-6 operators that generate anomalous couplings between electroweak gauge bosons and third-generation quarks, we calculate their one loop contributions to processes, both on and off the Z-pole, and the process. A global analysis is performed with these eight operators and those that contribute to the above processes at tree level using measurements at the LEP, SLC, and several low energy experiments. We find that although current electroweak precision measurements are sensitive to the one-loop effects of top-quark operators, it is difficult to separate them from the operators that contribute at tree level, making a global analysis rather challenging. Under further assumptions (for instance, new physics contributes to only third generation quark operators and the S and T parameters), competitive reaches may be obtained in a global fit. Another important finding of our study is that the two operators that generate the dipole interactions of the bottom quark have a significant impact on the Z-pole measurements and should not be omitted. We also discuss the implications of the recently reported W-boson mass measurement at the CDF for our results. Finally, we estimate the reaches of future lepton colliders in probing top-quark operators with precision electroweak measurements.

Minimal embedding of the Standard Model into intersecting D-brane configurations with a bulk leptonic U(1)

It has been recently shown that the discrepancy between the theoretical and experimental values of the anomalous magnetic moment of the muon can be fully accommodated by considering the contribution of few Kaluza–Klein (KK) states of the gauged lepton number with masses lighter than the LEP energy, consistently with present experimental limits. In this article, we construct the minimal embedding of the Standard Model (SM) into D-brane configurations with a gauged lepton number. In order to give rise to such KK modes, the lepton number gauge boson must live on an abelian U(1)_L brane extended along at least one “large” extra dimension in the bulk, with a compactification scale M_Lsim {mathcal {O}}(10{-}10^2~mathrm{GeV}) for a string scale M_sgtrsim 10~mathrm{TeV}. As a consequence, U(1)_L cannot participate to the hypercharge linear combination. We show that the minimal realisation of this framework contains five stacks of branes: the SM color U(3)_c, weak U(2)_w and abelian U(1) stacks extended effectively only in four dimensions, the bulk U(1)_L, as well as a fifth U(1)^{'} brane. With these five abelian factors, one finds besides the hypercharge a second anomaly-free linear combination which does not couple to the SM spectrum, both in the non-supersymmetric case as well as in the minimal supersymmetric extension of the model. It is also shown how the right-handed neutrino can be implemented in the spectrum, and how fermions arising from the two non-SM branes and coupled to the SM through the U(1)_L KK modes can provide Dark Matter candidates. Finally, the possibility of breaking Lepton Flavour Universality is studied by replacing U(1)_L with a brane gauging only the muonic lepton number, avoiding most experimental constraints and enlarging the parameter space for explaining the discrepancy on the muon magnetic moment.

Addendum to “Leptophilic U(1) massive vector bosons from large extra dimensions: Reexamination of constraints from LEP data” [Phys. Lett. B 820 (2021) 136585

Very recently, we proposed an explanation of the discrepancy between the measured anomalous magnetic moment of the muon and the Standard Model (SM) prediction in which the dominant contribution to (g−2)μ originates in Kaluza-Klein (KK) excitations (of the lepton gauge boson) which do not mix with quarks (to lowest order) and therefore can be quite light avoiding LHC constraints. In this addendum we reexamine the bounds on 4-fermion contact interactions from precise electroweak measurements and show that the constraints on KK masses and couplings are more severe than earlier thought. However, we demonstrate that our explanation remains plausible if a few KK modes are lighter than LEP energy, because if this were the case the contribution to the 4-fermion scattering from the internal propagator would be dominated by the energy and not by the mass. To accommodate the (g−2)μ discrepancy we assume that the lepton number L does not partake in the hypercharge and propagates in one extra dimension (transverse to the SM branes): for a mass of the lowest KK excitation of 60 GeV (lower than the LEP energy), the string scale is roughly 10 TeV while the L gauge coupling is of order ∼10−1.

Spin alignment measurement of vector mesons produced in high energy collisions

This paper covers the recent experimental development on spin alignment measurements of [Formula: see text] and [Formula: see text] vector mesons in heavy-ion and [Formula: see text] collisions at RHIC and LHC energies. Measurements in [Formula: see text] collisions at LEP energies are also discussed. Spin alignment of vector mesons is studied by measuring the second diagonal element [Formula: see text] of spin density matrix. The spin density matrix element [Formula: see text] is obtained by measuring the angular distribution of vector meson decay daughter with respect to the quantization axis in vector meson rest frame. Measured [Formula: see text] values for vector mesons are found to be larger than [Formula: see text] at high momentum in [Formula: see text] collisions at LEP energies, suggesting the preferential production of vector meson with helicity zero state from the fragmentation process. The [Formula: see text] values are found to be smaller than [Formula: see text] ([Formula: see text] implies no spin alignment) for [Formula: see text] and [Formula: see text] vector mesons at low transverse momentum in Pb–Pb collisions at [Formula: see text] TeV. These observations are qualitatively consistent with the expectation from models which attribute the spin alignment effect due to polarization of quarks in the presence of large initial angular momentum in noncentral heavy-ion collisions and its subsequent hadronization by the process of recombination. No significant spin alignment effect is observed for [Formula: see text] [Formula: see text] in mid-central Pb–Pb collisions and for vector mesons in [Formula: see text] collisions. However, the preliminary results of [Formula: see text] for [Formula: see text] mesons are larger than [Formula: see text] at intermediate [Formula: see text] in Au–Au collisions at RHIC energies and can be attributed to the presence of [Formula: see text] meson field. Although there is evidence of spin alignment effect of vector mesons in heavy-ion collisions but the measured effect is surprisingly larger in context of hyperon polarization. Therefore, these results will trigger further theoretical study.

Strangeness enhancement and flow-like effects in $$e^+e^-$$ annihilation at high parton density

Strangeness enhancement and collective flow are considered signatures of the quark gluon plasma formation. These phenomena have been detected not only in relativistic heavy ion collisions but also in high energy, high multiplicity events of proton-proton and proton-nucleus ("small systems") scatterings. Indeed, a universal behavior emerges by considering the parton density in the transverse plane as the dynamical quantity to specify the initial condition of the collisions. On the other hand, $e^+ e^-$ annihilation data at LEP and lower energies indicate that there is no strangeness enhancement and no flow-like effect. We show that the parton density in the transverse plane generated in $e^+ e^-$ annihilation at the available energy is too low to expect such effects. The event-by-event multiplicity where strangeness suppression and flow-like phenomenon could show up in $e^+ e^-$ is evaluated.

Patterns of strong coupling for LHC searches

Even though the Standard Model (SM) is weakly coupled at the Fermi scale, a new strong dynamics involving its degrees of freedom may conceivably lurk at slightly higher energies, in the multi TeV range. Approximate symmetries provide a structurally robust context where, within the low energy description, the dimensionless SM couplings are weak, while the new strong dynamics manifests itself exclusively through higher-derivative interactions. We present an exhaustive classification of such scenarios in the form of effective field theories, paying special attention to new classes of models where the strong dynamics involves, along with the Higgs boson, the SM gauge bosons and/or the fermions. The IR softness of the new dynamics suppresses its effects at LEP energies, but deviations are in principle detectable at the LHC, even at energies below the threshold for production of new states. Our construction provides the so far unique structurally robust context where to motivate several searches in Higgs physics, diboson production, or WW scattering, which were so far poorly justified. Perhaps surprisingly, the interplay between weak coupling, strong coupling and derivatives, which is controlled by symmetries, can override the naive expansion in operator dimension, providing instances where dimension-8 dominates dimension-6, well within the domain of validity of the low energy effective theory. This result reveals the limitations of an analysis that is both ambitiously general and restricted to dimension-6 operators.

Supersymmetry with light dark matter confronting the recent CDMS and LHC results

We revisit MSSM scenarios with light neutralino as a dark matter candidate in view of the latest LHC and dark matter direct and indirect detection experiments. We show that scenarios with a very light neutralino (~ 10 GeV) and a scalar bottom quark close in mass, can satisfy all the available constraints from LEP, Tevatron, LHC, flavour and low energy experiments and provide solutions in agreement with the bulk of dark matter direct detection experiments, and in particular with the recent CDMS results.

Higher-order threshold resummation for semi-inclusive [formula omitted] annihilation

The complete soft-enhanced and virtual-gluon contributions are derived for the quark coefficient functions in semi-inclusive e+e− annihilation to the third order in massless perturbative QCD. These terms enable us to extend the soft-gluon resummation for the fragmentation functions by two orders to the next-to-next-to-next-to-leading logarithmic (N3LL) accuracy. The resummation exponent is found to be the same as for the structure functions in inclusive deep-inelastic scattering. This finding, together with known results on the higher-order quark form factor, facilitates the determination of all soft and virtual contributions of the fourth-order difference of the coefficient functions for these two processes. Unlike the previous (N2LL) order in the exponentiation, the numerical effect of the N3LL contributions turns out to be negligible at LEP energies.

Micro-canonical pentaquark production in [formula omitted] and pp collisions

The existence of pentaquarks became questionable, because the Θ + peak is observed in some p + p collisions, but not in e + e − annihilations. People think initial baryon number is necessary to produce pentaquarks. In this paper, we estimate and compare the production of Θ + ( 1540 ) and Ξ − − ( 1860 ) in e + e − and pp collisions at different energies using Fermi statistical model as originally proposed in its microcanonical form. We find that both Θ + ( 1540 ) and Ξ − − ( 1860 ) yield more in e + e − at LEP energies than in pp collisions at SPS and RHIC energies, if pentaquarks do exist. Initial baryon number is not necessary for pentaquark production.

Measurement of the tau lepton polarisation at LEP2

A first measurement of the average polarisation Pτ of tau leptons produced in e+e− annihilation at energies significantly above the Z resonance is presented. The polarisation is determined from the kinematic spectra of tau hadronic decays. The measured value Pτ=−0.164±0.125 is consistent with the Standard Model prediction for the mean LEP energy of 197 GeV.

HADRON FORMATION IN HIGH ENERGY ELEMENTARY AND NUCLEAR COLLISIONS

We consider the dynamical origin of the apparent statistical equilibrium that governs the yields, and yield ratios, of all hadron and resonance species (consisting of the three light quark flavours) produced in nucleus-nucleus collisions from AGS via SPS to RHIC energies [Formula: see text]. This hadro-chemical equilibrium state is well described, overall, by the grand canonical, quasi-classical Gibbs ensemble of all corresponding hadrons and resonances. In order to pin down the stochastic elements, featured by the dynamical evolution prior to hadron formation and hadronic "chemical" (i.e. species) freeze-out, and determining the eventual equilibrium state, we concentrate on the high energy domain, [Formula: see text], where a model of primordial perturbative QCD partonic shower evolution appears plausible. For guidance concerning a hadronization model we revisit the QCD description of jet-induced hadron formation in e+e- annihilation at LEP energy. At the end of the pQCD partonic shower evolution a stage of color neutralization and flavour recombination leads to transition into non perturbative QCD clusters or strings, that decay to hadrons/resonances under phase space dominance. The combination of stochastic shower multiplication and cluster decay to the phase space defined by the hadron/resonancemass and spin spectrum results in a hadronization output featuring statistical equilibrium of the species, which is well described by the canonical Gibbs ensemble. We then assume that hadronization in A + A collisions occurs from a similar stage of singlet cluster formation. However, owing to the extreme overall energy density these clusters should overlap spatially, giving rise to extended super-cluster formation, increasing with [Formula: see text], A and collision centrality. In the limit of an extended volume decaying coherently, hadronization is free of local quantum number conservation constraints. This leads to strangeness enhancement and explains the success of a grand canonical description.

Microcanonical pentaquark production ine+e−annihilations

The existence of pentaquarks, namely baryonic states made up of four quarks and one antiquark, became questionable, because the candidates, i.e. the $\Theta^+$ peak, are seen in certain reactions, i.e. p+p collisions, but not in others, i.e. $\ee$ annihilations. In this paper, we estimate the production of $\Theta ^{+}(1540)$ and $\Xi^{--} (1860)$ in $\ee$ annihilations at different energies using Fermi statistical model as originally proposed in its microcanonical form. The results is compared with that from pp collisions at SPS and RHIC energies. We find that, if pentaquark states exist, the production is highly possible in $\ee$ annihilations. For example, at LEP energy $\sqrt{s}$=91.2 GeV, both $\Theta ^{+}(1540)$ and $\Xi^{--} (1860)$ yield more than in pp collisions at SPS and RHIC energy.

Two-particle correlations inside one jet at ``modified leading logarithmic approximation'' of quantum chromodynamics I. Exact solution of the evolution equations at smallX

We discuss correlations between two particles in jets at high energy colliders and exactly solve the MLLA evolution equations in the small x limit. We thus extend the Fong-Webber analysis to the region away from the hump of the single inclusive energy spectrum. We give our results for LEP, Tevatron and LHC energies, and compare with existing experimental data.

A study of heavy flavoured meson fragmentation functions ine+e−annihilation

We compare QCD theoretical predictions for heavy flavoured mesons fragmentation spectra in e+ e- annihilation with data from CLEO, BELLE and LEP. We include several effects in our calculation: next-to-leading order initial conditions, evolution and coefficient functions. Soft-gluon effects are resummed at next-to-leading-log accuracy. A matching condition for the crossing of the bottom threshold in evolution is also implemented at next-to-leading order accuracy. Important initial-state electromagnetic radiation effects in the CLEO and BELLE data are accounted for. We find that, with reasonably simple choices of a non-perturbative correction to the fixed-order initial condition for the evolution, the data from CLEO and BELLE can be fitted with remarkable accuracy. The fitted fragmentation function, when evolved to LEP energies, does not however represent fairly the D* fragmentation spectrum measured by ALEPH. Large non-perturbative corrections to the coefficient functions of the meson spectrum are needed in order to reconcile CLEO/BELLE and ALEPH results. Non-perturbative parameters extracted from the fits to e+ e- fragmentation data for D/D* and B mesons are tabulated. They can be employed in the theoretical predictions for the production of charmed and bottomed mesons in hadron-hadron, photon-hadron and photon-photon collisions.

A determination of the centre-of-mass energy at LEP2 using radiative two-fermion events

Using e(+)e(-) -> mu(+)mu(-)(gamma) and e(+)e(-) -> q (q) over bar(gamma) events radiative to the Z pole, DELPHI has determined the centre-of-mass energy, root s, using energy and momentum constraint methods. The results are expressed as deviations from the nominal LEP centre-of-mass energy, measured using other techniques. The results are found to be compatible with the LEP Energy Working Group estimates for a combination of the 1997 to 2000 data sets.

Determination of αS using jet rates at LEP with the OPAL detector

Hadronic events produced in e+e- collisions by the LEP collider and recorded by the OPAL detector were used to form distributions based on the number of reconstructed jets. The data were collected between 1995 and 2000 and correspond to energies of 91 GeV, 130-136 GeV and 161-209 GeV. The jet rates were determined using four different jet-finding algorithms (Cone, JADE, Durham and Cambridge). The differential two-jet rate and the average jet rate with the Durham and Cambridge algorithms were used to measure alpha(s) in the LEP energy range by fitting an expression in which order alpah_2s calculations were matched to a NLLA prediction and fitted to the data. Combining the measurements at different centre-of-mass energies, the value of alpha_s (Mz) was determined to be alpha(s)(Mz)=0.1177+-0.0006(stat.)+-0.0012$(expt.)+-0.0010(had.)+-0.0032(theo.) \.

Gauge invariance, infrared/collinear singularitiesand tree level matrix element for $e^ + e^- \to \nu_e \bar{\nu}_e \gamma \gamma$

One of the necessary steps in constructing a high-precision option of KKMC, a Monte Carlo program for the high-precision simulation of fermion pair production at LEP and Linear Collider energies, was to make a careful study of the appropriate matrix elements calculated from QED and the complete standard model. In particular, the installation of the double bremsstrahlung matrix element for the process $e^ + e^- \to \nu_e \bar{\nu}_e $ into the scheme of coherent exclusive exponentiation (CEEX) was necessary. In the CEEX scheme one has to define an extrapolation and/or reduction procedure to enable the use of the matrix elements for kinematical configurations with a large number of outgoing particles. The process under study is particularly interesting because of the gauge cancellation of contributions for photon emission from incoming fermion lines and t-channel W. The QED U(1) gauge properties require terms of the triple and quartic gauge couplings to be taken into consideration as well. A natural separation of the complete amplitude into gauge invariant parts was found and is among the main results of the paper. Each part has a well defined physical interpretation, which after partial integration over phase space provides infrared singular, leading-log, next-to-leading-log, and other terms. Contributions related to the triple and quartic gauge coupling of W (extracted with the help of an expansion around the contact W-interaction) have been ordered as well. The separation can be of broader interest; it originates from the rigorous calculation of matrix elements; it visualizes, in the language of spin amplitudes, the properties of factorization necessary for the common multi-process picture. For example, the multiple photon algorithm of PHOTOS, based on the parton shower-like approach, profits from similar considerations. These somewhat speculative aspects of the calculation will be mentioned in the paper as well.

NEW RESULTS ON PRECISION STUDIES OF HEAVY VECTOR BOSON PHYSICS

We present new results for two important heavy vector boson physics processes: (1), virtual corrections to hard bremsstrahlung which are relevant to precision predictions for the radiative return process in Z boson production at and beyond LEP2 energies; and, (2), electric charge screening effects in single W production with finite pT, multiple photon radiation in high energy collider physics processes. In both cases we show that we improve the respective precision tag significantly. Phenomenological implications are discussed.

Resonant Raman scattering dispersion in poly(dithieno[3,4-b:3′,4-d]-thiophene): 2A g spectroscopy

Small band gap semiconducting π-conjugated polymers may, in principle, be more efficient as the active media in solar cells, due to their better adaptability to the solar spectrum. In luminescent π-conjugated polymers, interband photon absorption results in the radiative recombination of the photogenerated excitons, which competes with charge separation. The photoluminescence (PL) quantum efficiency is largely determined by the relative energies of the optically allowed lowest odd parity (LOP, e.g. 1B u) excited state and the optically forbidden lowest even parity (LEP, e.g. 2A g) excited state. If E(LEP) < E(LOP), the quantum efficiency is small and charge separation following photoexcitation is more likely than in efficient PL polymers. The determination of the relative position of LEP and LOP states is therefore essential for the possible utilization of small band gap π-conjugated polymers in practical devices. It was previously shown that resonant Raman scattering dispersion might serve as a spectroscopic tool for the determination of the LEP states, since the dispersion is solely determined by the dependence of the LEP state on the conjugation length, rather than that of the optical gap, or LOP state. In this work, we use the Raman spectra measured in poly(dithieno-thiophene) (PDTT1) in order to estimate the LEP energy level. We find that the LEP energy level is in the vicinity of the optical gap.

Determination of the LEP beam energy using radiative fermion-pair events

We present a determination of the LEP beam energy using “radiative return” fermion-pair events recorded at centre-of-mass energies from 183 to 209 GeV. We find no evidence of a disagreement between the OPAL data and the LEP Energy Working Group's standard calibration. Including the energy-averaged 11 MeV uncertainty in the standard determination, the beam energy we obtain from the OPAL data is higher than that obtained from the LEP calibration by 0±34(stat.)±27(syst.)MeV.