arXiv subjects

John Ellis

Publications and source records attributed to John Ellis.

At least 469 records · Page 26Linked to original sources

arXiv1997

Signatures of Parton Exogamy in e+ e- -> W+ W- -> hadrons

We propose possible signatures of `exogamous' combinations between partons in the different W+ and W- hadron showers in e+e- -> W+W- events with purely hadronic final states. Within the space-time model for hadronic shower development that we have proposed previously, we find a possible difference of about 10 % between the mean hadronic multiplicity in such purely hadronic final states and twice the hadronic multiplicity in events in which one W decays hadronically and the other leptonically, i.e., \ne 2 , associated with the formation of hadronic clusters by `exogamous' pairs of partons. We discuss the dependence of this possible difference in multiplicity on the center-of-mass energy, on the hadron momenta, and on the angular separation between the $W^{\pm}$ dijets. If it were observed, any such multiplicity difference would indicate that the W's do not hadronize independently, and hence raise questions about the accuracy with which the W mass could be determined from purely hadronic final states.

hep-ph↗

arXiv1997

A Prediction for the 4-Loop βFunction

We predict that the four-loop contribution β_3 to the QCD βfunction in the MS-bar prescription is given by β_3\simeq 23,600(900) - 6,400(200) N_f + 350(70) N_f^2 + 1.5 N_f^3, where N_f is the number of flavours and the coefficient of N_f^3 is an exact result from large-N_f expansion. In the phenomenologically-interesting case N_f=3, we estimate β_3 = (7.6 \pm 0.1) x 10^3. We discuss our estimates of the errors in these QCD predictions, basing them on the demonstrated accuracy of our method in test applications to the O(N) Φ^4 theory, and on variations in the details of our estimation method, which goes beyond conventional Pade approximants by estimating and correcting for subasymptotic deviations from exact results.

hep-ph↗

arXiv1996

A Fresh Look at Axions and SN 1987A

We re-examine the very stringent limits on the axion mass based on the strength and duration of the neutrino signal from SN 1987A, in the light of new measurements of the axial-vector coupling strength of nucleons, possible suppression of axion emission due to many-body effects, and additional emission processes involving pions. The suppression of axion emission due to nucleon spin fluctuations induced by many-body effects degrades previous limits by a factor of about 2. Emission processes involving thermal pions can strengthen the limits by a factor of 3-4 within a perturbative treatment that neglects saturation of nucleon spin fluctuations. Inclusion of saturation effects, however, tends to make the limits less dependent on pion abundances. The resulting axion mass limit also depends on the precise couplings of the axion and ranges from 0.5x10**(-3) eV to 6x10**(-3) eV.

astro-ph↗

arXiv1996

Outlook on Neutrino Physics

Some of the hot topics in neutrino physics are discussed, with particular emphasis on neutrino oscillations. After proposing credibility criteria for assessing various claimed effects, particular stress is laid on the solar neutrino deficit, which seems unlikely to have an astrophysical explanation. Comments are also made on the possibility of atmospheric neutrino oscillations and on the LSND experiment, as well as cosmological aspects of neutrinos and neutralinos. Several of the central issues in neutrino physics may be resolved by the new generation of experiments now underway, such as CHORUS, NOMAD and Superkamiokande, and in preparation, such as SNO and a new round of accelerator- and reactor-based neutrino-oscillation experiments. At the end, there is a brief review of ways in which present and future CERN experiments may be able to contribute to answering outstanding questions in neutrino physics.

hep-ph↗

arXiv1996

The $ΔI = 1/2 $ Rule in the Light of Two-Dimensional QCD

We calculate in QCD$_2$ the ratios of baryonic matrix elements of $ΔI = 2$ and $ΔI = 0$ four-fermion operators, with a view to understanding better the mechanism of $ΔI = 1/2$ enhancement in QCD$_4$. We find relatively small suppressions of both the scalar-scalar and vector-vector $ΔI = 2$ four-fermion operators. We discuss the possible implications of these results, in view of a suggestion that gluon condensation may be an important contributing factor in the $ΔI = 1/2$ enhancement seen in QCD$_4$. At the technical level, our calculation of the vector-vector operator matrix element requires a treatment of the time dependence of the QCD$_2$ soliton which had not been developed in previous phenomenological calculations within this model.

hep-ph↗

arXiv1996

Current Issues in the Phenomenology of Particle Physics

The present status of the Standard Model and its experimental tests are reviewed, including indications on the likely mass of the Higgs boson. Also discussed are the motivations for supersymmetry and grand unification, searches for sparticles at LEP, neutrino oscillations, and the prospects for physics at the LHC.

hep-ph↗

arXiv1996

Problems and Prospects in Spin Physics

This talk reviews some of the hot topics in spin physics and related subjects, including perturbative QCD predictions for polarized parton distributions and their possible behaviours at small x, the Bjorken and singlet sum rules and the treatment of higher orders in perturbative QCD, different interpretations of the EMC spin effect including chiral solitons and the axial U(1) anomaly, other experimental indications for the presence of strange quarks in the nucleon wave function, implications for dark matter physics, and a few words about polarization as a tool in electroweak physics.

hep-ph↗

arXiv1996

Hadronic Antenna Patterns to Distinguish Production Mechanisms for Large-E_T Jets

Hadronic antenna patterns provide a tool able to diagnose different patterns of colour flow in large-E_T jet events. They reflect the underlying short-distance dynamics, and are sensitive to colour coherence and interference between the initial- and final-state partons. We discuss how hadronic antenna patterns may be used on large-E_T events from the Fermilab Tevatron or the CERN LHC to distinguish between conventional QCD and new physics production mechanisms such as a possible Z' boson or compositeness.

hep-ph↗

arXiv1996

Indications from Precision Electroweak Physics Confront Theoretical Bounds on the Mass of the Higgs Boson

An updated fit to the precision electroweak data and to the direct measurement of the top quark mass $m_t$ provides significant constraints on $m_t$ and on the Higgs boson mass $M_H$: $m_t/\text{GeV}=172\pm 6$ and $\log_{10}(M_H/\text{GeV})=2.16\pm 0.33$, with an error correlation $ρ=0.5$. We integrate the $(M_H, m_t)$ probability distribution found in this analysis over various zones of the $(M_H, m_t)$ plane defined by one-sided experimental and theoretical bounds on the Higgs boson mass, both in the Standard Model and in its minimal supersymmetric extension. The comparison of the cumulative probabilities gives interesting information on the likelihood that the true value of $M_H$ is compatible with different theoretical scenarios.

hep-ph↗

arXiv1996

CPT and Superstring

We discuss the possibility that CPT violation may appear as a consequence of microscopic decoherence due to quantum-gravity effects, that we describe using a density-matrix formalism motivated by our studies of non-critical string theory. The maximum possible order of magnitude of such decohering CPT-violating effects is not far from the sensitivity of present experiments on the neutral kaon system, and we review a simple parametrization for them. We also review a recent data analysis carried out together with the CPLEAR collaboration, which bounds any such decohering CPT-violating parameters to be $\lappeq 10^{-19}$ GeV.

hep-ph↗

arXiv1996

Renormalization-Scheme Dependence of Pade Summation in QCD

We study the renormalization-scheme (RS) dependence of Pade Approximants (PA's), and compare them with the Principle of Minimal Sensitivity (PMS) and the Effective Charge (ECH) approaches. Although the formulae provided by the PA, PMS and ECH predictions for higher-order terms in a QCD perturbation expansion differ in general, their predictions can be very close numerically for a wide range of renormalization schemes. Using the Bjorken sum rule as a test case, we find that Pade Summation (PS) reduces drastically the RS dependence of the Bjorken effective charge. We use these results to estimate the theoretical error due to the choice of RS in the extraction of $α_s$ from the Bjorken sum rule, and use the available data at $Q^2=3 GeV^2$ to estimate $α_s(M_Z) = 0.117^{+0.004}_{-0.007} \pm 0.002$, where the first error is experimental, and the second is theoretical.

hep-ph↗

arXiv1996

Supersymmetric Dark Matter in the Light of LEP 1.5

We discuss the lower limit on the mass of the neutralino $χ$ that can be obtained by combining data from $e^+e^-$ annihilation at LEP and elsewhere with astrophysical and theoretical considerations. Loopholes in the purely experimental analysis of ALEPH data from the Z peak and LEP 1.5, which appear when $μ<0$ for certain values of the sneutrino mass $m_{\tildeν}$ and the ratio $\tanβ$ of supersymmetric Higgs vacuum expectation values, may be largely or totally excluded by data from lower-energy $e^+e^-$ data, the hypothesis that most of the cosmological dark matter consists of $χ$ particles, and the assumption that electroweak symmetry breaking is triggered by radiative corrections due to a heavy top quark. The combination of these inputs imposes $m_χ \ge 21.4~\gev$, if soft supersymmetry-breaking masses are assumed to be universal at the grand-unification scale.

hep-ph↗

arXiv1996

Virtual-Sparticle Threshold Effects on Large E_T Jet Cross Sections

We discuss the one-loop virtual-sparticle corrections to QCD jet cross sections at large $E_T$ and large dijet invariant masses, with reference to present Tevatron and future LHC collider experiments. We find characteristic peaks and dips in the sparticle threshold region, due to interferences with tree-level QCD diagrams. Their magnitudes may be several per cent of the total jet cross section, so they might provide a useful search tool that is complementary to the usual missing-energy signature for supersymmetry.

hep-ph↗

arXiv1996

Deep-Inelastic Final States in a Space-Time Description of Shower Development and Hadronization

We extend a quantum kinetic approach to the description of hadronic showers in space, time and momentum space to deep-inelastic $ep$ collisions, with particular reference to experiments at HERA. We follow the history of hard scattering events back to the initial hadronic state and forward to the formation of colour-singlet pre-hadronic clusters and their decays into hadrons. The time evolution of the space-like initial-state shower and the time-like secondary partons are treated similarly, and cluster formation is treated using a spatial criterion motivated by confinement and a non-perturbative model for hadronization. We calculate the time evolution of particle distributions in rapidity, transverse and longitudinal space. We also compare the transverse hadronic energy flow and the distribution of observed hadronic masses with experimental data from HERA, and find encouraging results. The techniques developed in this paper may be applied in the future to more complicated processes such as eA, pp, pA and AA collisions.

hep-ph↗

arXiv1996

Geological Isotope Anomalies as Signatures of Nearby Supernovae

Nearby supernova explosions may cause geological isotope anomalies via the direct deposition of debris or by cosmic-ray spallation in the earth's atmosphere. We estimate the mass of material deposited terrestrially by these two mechanisms, showing the dependence on the supernova distance. A number of radioactive isotopes are identified as possible diagnostic tools, such as Be-10, Al-26, Cl-36, Mn-53, Fe-60, and Ni-59, as well as the longer-lived I-129, Sm-146, and Pu-244. We discuss whether the 35 and 60 kyr-old Be-10 anomalies observed in the Vostok antarctic ice cores could be due to supernova explosions. Combining our estimates for matter deposition with results of recent nucleosynthesis yields, we calculate the expected signal from nearby supernovae using ice cores back to $\sim 300$ kyr ago, and we discuss using deep ocean sediments back to several hundred Myr. In particular, we examine the prospects for identifying isotope anomalies due to the Geminga supernova explosion, and signatures of the possibility that supernovae might have caused one or more biological mass extinctions.

astro-ph↗

arXiv1996

Astroparticle physics - A Personal Outlook

At the request of the organizers, this talk surveys some of the hot topics discussed at this meeting, giving my {\it subjective views} on them. Subjects covered include the present age and Hubble expansion rate of the Universe - {\it inflation theorists need not yet abandon $Ω= 1$}, theories of structure formation in the light of COBE and other data - {\it my favourite is a flat spectrum of initial perturbations subsequently amplified by mixed hot and cold dark matter}, neutrino masses and oscillations - {\it the only experimental indication I take seriously at the moment is the persistent solar neutrino deficit}, the lightest supersymmetric particle - {\it which may behave differently if conventional assumptions are relaxed}, and the axion - {\it much of the window between limits from SN 1987a and cosmology will be explored in an ongoing experiment}. Finally, I present a chronology of some possible interesting future experiments.

astro-ph↗

arXiv1996

Quantum Decoherence in a Four-Dimensional Black Hole Background

We display a logarithmic divergence in the density matrix of a scalar field in the presence of an Einstein-Yang-Mills black hole in four dimensions. This divergence is related to a previously-found logarithmic divergence in the entropy of the scalar field, which cannot be absorbed into a renormalization of the Hawking-Bekenstein entropy of the black hole. As the latter decays, the logarithmic divergence induces a non-commutator term $\nd{δH}ρ$ in the quantum Liouville equation for the density matrix $ρ$ of the scalar field, leading to quantum decoherence. The order of magnitude of $\nd{δH}$ is $μ^2/M_P$, where $μ$ is the mass of the scalar particle.

gr-qc↗

arXiv1996

Implications of the Strange Spin of the Nucleon for the Neutron Electric Dipole Moment in Supersymmetric Theories

Supersymmetric model contributions to the neutron electric dipole moment arise via quark electric dipole moment operators, whose matrix elements are usually calculated using the Naive Quark Model (NQM). However, experiments indicate that the NQM does not describe well the quark contributions $Δq$ to the nucleon spin, and so may provide misleading estimates of electric dipole operator matrix elements. Taking the $Δq$ from experiment, we indeed find consistently smaller estimates of the neutron electric dipole moment for given values of the supersymmetric model parameters. This weakens previous constraints on CP violation in supersymmetric models, which we exemplify analytically in the case where the lightest supersymmetric particle (LSP) is a $U(1)$ gaugino $\tilde{B}$, and display numerically for other LSP candidates.

hep-ph↗