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John Ellis

Publications and source records attributed to John Ellis.

At least 487 records · Page 27Linked to original sources
arXiv1996

The Strange Spin of the Nucleon

The recent series of experiments on polarized lepton-nucleon scattering have provided a strange new twist in the story of the nucleon, some of whose aspects are reviewed in these lectures. In the first lecture, we review some issues arising in the analysis of the data on polarized structure functions, focusing in particular on the importance and treatment of high-order QCD perturbation theory. In the second lecture some possible interpretations of the ``EMC spin effect" are reviewed, principally in the chiral soliton (Skyrmion) approach, but also interpretations related to the axial $U(1)$ anomaly. This lecture also discusses other indications from recent LEAR data for an $\bar{s} s$ component in the nucleon wave function, and discusses test of a model for this component. Finally, the third lecture reviews the implications of polarized structure functions measurements for experiments to search for cold dark matter particles, such as the lightest supersymmetric particle and the axion, after reviewing briefly the astrophysical and cosmological evidence for cold dark matter.

hep-ph↗
arXiv1995

Supersymmetry and Grand Unification

Supersymmetry and Grand Unification are the two most promising directions for physics beyond the Standard Model. They receive indirect experimental support from the apparent lightness of the Higgs boson, the values of the gauge couplings measured at LEP and elsewhere, and the persistent solar neutrino deficit. Phenomenological constraints and theoretical models constrain predictions in interesting ways. All these ideas may be embedded in string theory, which is shown by newly-discovered dualities to possess previously-unsuspected richness and simplicity.

hep-ph↗
arXiv1995

Testing the Standard Model and Beyond

This paper is based on lectures presented to mathematical physicists and attempts to provide an overview of the present status of the Standard Model, its experimental tests, phenomenological and experimental motivations for going beyond the Standard Model via supersymmetry and grand unification, and ways to test these ideas with particle accelerators.

hep-th↗
arXiv1995

Space, Time and Color in Hadron Production Via e+e- -> Z0 and e+e- -> W+W-

The time-evolution of jets in hadronic e+e- events at LEP is investigated in both position- and momentum-space, with emphasis on effects due to color flow and particle correlations. We address dynamical aspects of the four simultanously-evolving, cross-talking parton cascades that appear in the reaction e+e- -> gamma/Z0 -> W+W- -> q1 q~2 q3 q~4, and compare with the familiar two-parton cascades in e+e- -> Z0 -> q1 q~2. We use a QCD statistical transport approach, in which the multiparticle final state is treated as an evolving mixture of partons and hadrons, whose proportions are controlled by their local space-time geography via standard perturbative QCD parton shower evolution and a phenomenological model for non-perturbative parton-cluster formation followed by cluster decays into hadrons. Our numerical simulations exhibit a characteristic `inside-outside' evolution simultanously in position and momentum space. We compare three different model treatments of color flow, and find large effects due to cluster formation by the combination of partons from different W parents. In particular, we find in our preferred model a shift of several hundred MeV in the apparent mass of the W, which is considerably larger than in previous model calculations. This suggests that the determination of the W mass at LEP2 may turn out to be a sensitive probe of spatial correlations and hadronization dynamics.

hep-ph↗
arXiv1995

Nucleon Spin

We review the theory of polarized deep inelastic scattering in light of the most recent experimental data. We discuss the nucleon spin decomposition and the Bjorken sum rule. The latter is used for extraction of $α_s(M_Z^2)=0.116^{{+}0.004}_{{-}0.006}$ and as a test case for a new method of analyzing divergent perturbation series in QCD.

hep-ph↗
arXiv1995

Pade Approximants, Borel Transforms and Renormalons: the Bjorken Sum Rule as a Case Study

We prove that Pade approximants yield increasingly accurate predictions of higher-order coefficients in QCD perturbation series whose high-order behaviour is governed by a renormalon. We also prove that this convergence is accelerated if the perturbative series is Borel transformed. We apply Pade approximants and Borel transforms to the known perturbative coefficients for the Bjorken sum rule. The Pade approximants reduce considerably the renormalization-scale dependence of the perturbative correction to the Bjorken sum rule. We argue that the known perturbative series is already dominated by an infra-red renormalon, whose residue we extract and compare with QCD sum-rule estimates of higher-twist effects. We use the experimental data on the Bjorken sum rule to extract $α_s(M_Z^2) = 0.116_{-0.006}^{+0.004}$, including theoretical errors due to the finite order of available perturbative QCD calculations, renormalization-scale dependence and higher-twist effects.

hep-ph↗
arXiv1995

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 O(M_Z): M_H = 76+152-50 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 alpha(M_Z) and alpha_s(M_Z).

hep-ph↗
arXiv1995

The Proton Spin Puzzle and $Λ$ Polarization in Deep--Inelastic Scattering

We point out that measurements of longitudinal $Λ$ polarization in the target fragmentation region of deep--inelastic $ν\,N$ and $μ\,N$ or $e \, N$ scattering may test dynamical mechanisms invoked to explain the proton spin puzzle. A previously-proposed model for polarized $\bar s s$ pairs in the proton wave function reproduces successfully the negative $Λ$ polarization found in the WA59 $\bar ν\, N$ experiment, and makes predictions that could be tested in future $μ\, N$ and $e \, N$ experiments.

hep-ph↗
arXiv1995

On the Thermal Regeneration Rate for Light Gravitinos in the Early Universe

We investigate the light gravitino regeneration rate in the early Universe in models based on $N=1$ supergravity. Motivated by a recent claim by Fischler, we evaluate finite-temperature effects on the gravitino regeneration rate due to the hot primordial plasma for a wide range of the supersymmetry-breaking scale $F$. We find that thermal corrections to the gravitino pole mass and to the Goldstino coupling are negligible for a wide range of temperatures, thereby justifying the extension of the equivalence theorem for the helicity-1/2 gravitino and Goldstino to a hot primordial plasma background. Utilizing the Braaten-Pisarski resummation method, the helicity-1/2 gravitino regeneration rate is found to be $0.25 α_s(T) \log(1/α_s(T))|{m_{\rm soft}/F}|^2 T^3(1 + α_s(T) \log(1/α_s(T)) + T^2 / |F|)$ up to a calculable, model-dependent ${\cal O}(1)$ numerical factor. We review the implications of this regeneration rate for supergravity cosmology, focusing in particular on scenaria for baryogenesis.

hep-ph↗
arXiv1995

REAL-TIME DESCRIPTION OF PARTON-HADRON CONVERSION AND CONFINEMENT DYNAMICS

We propose a new and universal approach to the hadronization problem that incorporates both partonic and hadronic degrees of freedom in their respective domains of relevance, and that describes the conversion between them within a kinetic field theory formulation in real time and full 7-dimensional phase space. We construct a scale-dependent effective theory that reduces to perturbative QCD with its scale and chiral symmetry properties at short space-time distances, but at large distances (r > 1 fm) yields symmetry breaking gluon and quark condensates plus hadronic excitations. The approach is applied to the evolution of fragmenting qq~ and gg jet pairs as the system evolves from the initial 2-jet configuration, via parton showering and cluster formation, to the final yield of hadrons. The phenomenological implications for e+e- -> hadrons are investigated, such as the time scale of the transition, and its energy dependence, cluster size and mass distributions. We compare our results for particle production and Bose-Einstein correlations with experimental data, and find an interesting possibility of extracting the basic parameters of the space-time evolution of the system from Bose enhancement measurements.

hep-ph↗
arXiv1995

New LEP constraints on some supersymmetric Yukawa interactions that violate R-parity

We consider one-loop corrections to partial widths of the $Z$ induced by supersymmetric Yukawa interactions that violate $R$-parity. The precise experimental values of the leptonic $Z$ partial widths bound these Yukawa couplings, with the most interesting constraints being those on couplings involving the $τ$, since previous constraints on them were very mild.

hep-ph↗
arXiv1995

REAL-TIME DYNAMICS OF PARTON-HADRON CONVERSION

We propose a new and universal approach to the hadronization problem that incorporates both perturbative QCD and effective field theory in their respective domains of validity and that models the transition between them in analogy to the finite temperature QCD phase transition. Using techniques of quantum kinetic theory, we formulate a real-time description in momentum and position space. The approach is applied to the evolution of fragmenting $q\bar q$ and $gg$ jets as the system evolves from the initial 2-jet, via parton multiplication and cluster formation, to the final yield of hadrons. We investigate time scale of the transition, energy dependence, cluster size and mass distributions, and compare our results for particle production and Bose-Einstein correlations with experimental data for $e^+e^-\rightarrow hadrons$. An interesting possibility to extract the space-time evolution of the system from Bose enhancement measurements is suggested.

hep-ph↗
arXiv1994

The Anomalous Magnetic Moments of the Electron and the Muon - Improved QED Predictions Using Pade Approximants

We use Pade Approximants to obtain improved predictions for the anomalous magnetic moments of the electron and the muon. These are needed because of the very precise experimental values presently obtained for the electron, and soon to be obtained at BNL for the muon. The Pade prediction for the QED contribution to the anomalous magnetic moment of the muon differs significantly from the naive perturbative prediction.

hep-ph↗
arXiv1994

Determination of $α_s$ and the Nucleon Spin Decomposition Using Recent Polarized Structure Function Data

New data on polarized $μ-p$ and $e-p$ scattering permit a first determination of $α_s$ using the Bjorken sum rule, as well as higher precision in determining the nucleon spin decomposition. Using perturbative QCD calculations to $O(α_s^4)$ for the non-singlet combination of structure functions, we find $α_s(2.5 GeV^2) = 0.375^{+0.062}_{-0.081}$, corresponding to $α_s(M_Z^2) =0.122^{+0.005}_{-0.009}$, and using calculations to $O(α_s^3)$ for the singlet combination we find $Δu = 0.83 \pm 0.03$, $Δd= -0.43 \pm 0.03$, $Δs =-0.10 \pm 0.03$, $ΔΣ\equiv Δu + Δd + \Ds = 0.31 \pm 0.07$, at a renormalization scale $Q^2=10 GeV^2$. Perturbative QCD corrections play an essential role in reconciling the interpretations of data taken using different targets. We discuss higher-twist uncertainties in these determinations. The $Δq$ determinations are used to update predictions for the couplings of massive Cold Dark Matter particles and axions to nucleons.

hep-ph↗
arXiv1994

Bounds on the Masses and Couplings of Leptoquarks from Leptonic Partial Widths of the Z

The vertex corrections to the leptonic partial widths of the $Z$ induced by leptoquarks that couple leptons to the top quark are considered. We obtain stringent bounds on the parameter space of the masses and Yukawa couplings of these leptoquarks, using the latest information on the $Z \rightarrow l^+l^-$ decay widths measured at LEP. Leptoquarks coupling with electroweak strength to top quarks are constrained to be heavier than several hundred GeV, at 95$\%$ C.L. As a consequence, such leptoquarks cannot make a significant contribution to lepton asymmetries, $τ$ polarisation asymmetries or $A_{LR}$.

hep-ph↗
arXiv1993

Spin Structure Functions

We review the theory and phenomenology of deep inelastic polarized lepton-nucleon scattering in the light of recent data with a deuteron target from the SMC at CERN and a Helium 3 target from the E142 experiment at SLAC. After including higher-order perturbative QCD corrections, mass corrections and updated estimates of higher-twist effects, we find good agreement with the basic Bjorken sum rule, and extract a consistent set of values for the quark contributions to the proton spin: $$ΔΣ\equiv Δu + Δd + Δs = 0.27 \pm 0.11 $$ $$Δu = 0.82 \pm 0.04, Δd = -0.44 \pm 0.04,Δs = -0.11 \pm 0.04 $$ which are consistent with chiral soliton models and indications from lattice estimates. We also mention the prospects for future experiments on the spin structure of the nucleon.

hep-ph↗
arXiv1993

Analysis of Data on Polarized Lepton-Nucleon Scattering

We re-analyze data on deep inelastic polarized lepton-nucleon scattering, with particular attention to testing the Bjorken sum rule and estimating the quark contributions to the nucleon spin. Since only structure function data at fixed $Q^2$ can be used to test sum rules, we use E142 asymmetry measurements and unpolarized structure function data to extract $g_1^n$ at fixed $Q^2$ = 2 GeV$^2$. When higher-twist effects, which are important at low $Q^2$, are included, both the E142 and SMC data are compatible with the Bjorken sum rule within one standard deviation. Assuming validity of the Bjorken sum rule, we estimate the quark contributions to the nucleon spin, finding that their total net contribution is small, with the strange quark contribution non-zero and negative. The quark spin content of the nucleon spin is in agreement with Skyrme model.

hep-ph↗