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M. Maul

Publications and source records attributed to M. Maul.

At least 19 recordsLinked to original sources
arXiv2004

Search for Chargino and Neutralino Production at sqrt(s) = 192-209 GeV at LEP

Approximately 438 pb-1 of e+e- data from the OPAL detector, taken with the LEP collider running at centre-of-mass energies of 192-209 Gev, are analyzed to search for evidence of chargino pair production, e+e- -> tilde chi^+_1 tilde chi^-_1, or neutralino associated production, e+e- -> tilde chi^0_2 tilde chi^0_1. Limits are set at the 95% confidence level on the product of the cross-section for the process e+e- -> tilde chi^+_1 tilde chi^-_1 and its branching ratios to topologies containing jets and missing energy, of jest with a lepton and missing energy, and on the product of the cross-section for e+e- -> tilde chi^0_2 tilde chi^0_1 and its branching ratio to jets. R-parity conservation is assumed throughout this paper. When these results are interpreted in the context of the Constrained Minimal Supersymmetric Standard Model, limits are also set on the masses of the tilde chi^+-_1, tilde chi^0_1 and tilde chi^0_2, and regions of the parameter space of the model are ruled out. Nearly model-independent limits are also set at the 95% confidence level on sigma(e+e- -> tilde chi^+_1 tilde chi^-_1) with the assumption that each chargino decays via a W boson, and on sigma(e+e- -> tilde chi^0_2 tilde chi^0_1) with the tilde chi^0_2 assumed to decay via a Z^0.

hep-ex↗
arXiv2001

QCD Signatures of Narrow Graviton Resonances in Hadron Colliders

We show that the characteristic p_\perp spectrum yields valuable information for the test of models for the production of narrow graviton resonances in the TeV range at LHC. Furthermore, it is demonstrated that in those scenarios the parton showering formalism agrees with the prediction of NLO matrix element calculations.

hep-ph↗
arXiv2000

Higher-twist contributions to F_L and F_2 from instantons

We study the twist-4 contribution to the unpolarized nucleon structure functions, F_L and F_2, in an approach where the vacuum structure of QCD is described by a dilute medium of instantons ($\barρ/ \bar R \ll 1$). The dominant power corrections come from the twist-4 quark-gluon operators and are of the order $1/(\barρ^2 Q^2)$. The contributions from four-quark operators are suppressed. The approach predicts large 1/Q^2-corrections to F_L, in agreement with the results of QCD fits to the data.

hep-ph↗
arXiv2000

Do Instantons of the CP(N-1) Model Melt?

In the two-dimensional CP(N-1) model one can parametrize exact many-instanton solutions via N `constituents' (called `zindons'). This parameterization allows, in principle, a complete `melting' of individual instantons. The model is therefore well suited to study whether dynamics prefers a dilute or a strongly overlapping ensemble of instantons. We study the statistical mechanics of instantons both analytically and numerically. We find that at N=2 the instanton system collapses into zero-size instantons. At N=3,4 we find that well-isolated instantons are dynamically preferred though 15-25% of instantons have a considerable overlap with others.

hep-lat↗
arXiv1999

Pion Form Factor in QCD at Intermediate Momentum Transfers

We present a quantitative analysis of the electromagnetic pion form factor in the light-cone sum rule approach, including radiative corrections and higher-twist effects. The comparison to the existing data favors the asymptotic profile of the pion distribution amplitude and allows to estimate the deviation: $(\int du/u ϕ_π(u))/ (\int du/u ϕ^{\rm as}_π(u))=$ 1.1$\pm$ 0.1 at the scale 1 GeV. Special attention is payed to the precise definition and interplay of soft and hard contributions at intermediate momentum transfer, and to matching of the sum rule to the perturbative QCD prediction. We observe a strong numerical cancellation between the soft (end point) contribution and power suppressed hard contributions of higher twist, so that the total nonperturbative correction to the usual pQCD result turns out to be of order 30% for $Q^2\sim 1$ GeV$^2$.

hep-ph↗
arXiv1999

On Statistical Mechanics of Instantons in the CP^{N_c-1} Model

We introduce an explicit form of the multi-instanton weight including also instanton--anti-instanton interactions for arbitrary $N_c$ in the two-dimensional $CP^{N_c-1}$ model. To that end, we use the parametrization of multi-instantons in terms of instanton `constituents' which we call `zindons' for short. We study the statistical mechanics of zindons analytically (by means of the Debye-Hückel approximation) and numerically (running a Metropolis algorithm). Though the zindon parametrization allows for a complete `melting' of instantons we find that, through a combination of dynamical and purely geometric factors, a dominant portion of topological charge is residing in well-separated instantons and anti-instantons.

hep-th↗
arXiv1999

Twist-4 contribution to unpolarized structure functions F_L and F_2 from instantons

We compute in the instanton vacuum the nucleon matrix elements of the twist-4 QCD operators describing power corrections to the second moments of the unpolarized structure functions, F_L and F_2. Our approach takes into account the leading contribution in the packing fraction of the instanton medium, rho / R << 1. Parametrically leading are the matrix elements of a twist-4 quark-gluon operator, which are of the order of the inverse instanton size, 1/rho^2 = (600 MeV)^2. The matrix elements of the four-fermion (diquark) operators are suppressed by a factor (rho / R)^4 and numerically small. These results are in agreement with the pattern of phenomenological 1/Q^2-corrections to R = sigma_L / sigma_T and F_2 found in QCD fits to the data. In particular, the rise of R at low Q^2 can be obtained from instanton-type vacuum fluctuations at a low scale.

hep-ph↗
arXiv1999

Diffractive charged meson pair production

We investigate the possibility to measure the nonforward gluon distribution function by means of diffractively produced π^+π^- and K^+K^- pairs in polarized lepton nucleon scattering. The resulting cross sections are small and are dominated by the gluonic contribution. We find relatively large spin asymmetries, both for π^+π^- and for K^+K^- pairs.

hep-ph↗
arXiv1998

Stark quenching for the $1s^22s2p^3P_0$ level in beryllium-like ions and parity-violating effects

In this paper we present some concepts in heavy ion atomic physics for the extraction of parity violating effects. We investigate the effects of the so-called Stark-quenching, i.e., the fast decay of a meta stable state induced by a Stark field, and the superposition of one- and two-photon transitions in beryllium-like heavy ions. It turns out that the discussed theoretical phenomena for heavy ions with few electrons are beyond the scope of present day experimental possibilities because one has to require beam energies of up to 1 TeV/A, laser intensities of up to $10^{17} {\rm W/cm^2}$ and ion currents of up to $10^{11}$ ions per second in beryllium-like uranium. However, especially the superposition of one- and two-photon transitions is a very interesting phenomenon that could provide the germ of an idea to be applied in a more favorable system.

physics.atom-ph↗
arXiv1998

Asymmetry distributions and mass effects in dijet events at a polarized HERA

The asymmetry distributions for several kinematic variables are considered for finding a systematic way to maximize the signal for the extraction of the polarized gluon density. The relevance of mass effects for the corresponding dijet cross section is discussed and the different approximations for including mass effects are compared. We also compare via the programs Pepsi and Mepjet two different Monte Carlo (MC) approaches for simulating the expected signal in the dijet asymmetry at a polarized HERA.

hep-ph↗
arXiv1997

The polarised gluon density from di-jet events in DIS at a polarised HERA

We present a possible direct measurement of the polarised gluon density $ΔG(x)$ in LO from di-jet production in polarised deep inelastic ep scattering, assuming the kinematics of the HERA collider. We show the sensitivity to the x-dependence of $ΔG(x)$ and to the first moment $\int ΔG(x) dx$ in the range $0.002< x <0.2$, assuming the electron and proton beam of HERA being polarised to 70% and an integrated luminosity of at least 200 pb$^{-1}$. We include in our study hadronisation and higher order effects, as well as realistic detector smearing and acceptance. We find that the statistical and systematic uncertainties are small enough to distinguish between different parametrizations for $ΔG(x)$, which all are in accordance with present data. We stress that at HERA an x-range could be measured, that is not accessible to any other present or proposed experiment.

hep-ph↗
arXiv1997

Semi-inclusive asymmetries with polarized proton beams at HERA

The prospects of semi-inclusive measurements with polarized proton beams at HERA are discussed. Detailed simulations show that one can disentangle the valence-quark and sea-quark contribution to the polarized structure function g_1(x) in the small x-domain, if the equivalent of 1000 pb-1 of data are collected. It is also shown how semi-inclusive charged- current events can provide information on the relative importance of the spin contribution of the anti-s and anti-d sea quarks. Moreover, various methods to determine the fragmentation functions in this kinematical domain are presented.

hep-ph↗
arXiv1997

Recent developments of PEPSI and SPHINX

Recent developments of the two MC-generators SPHINX and PEPSI are discussed. In SPHINX the process of polarized photo production was included to simulate the potential of a polarized HERA to measure the photon structure function. In PEPSI the CC polarized cross sections on tree level have been added together with the electroweak corrections to simulate their effect on CC signals proposed for a polarized HERA.

hep-ph↗
arXiv1997

Perturbative Part of the Non-Singlet Structure Function F_2 in the Large-N_F Limit

We have calculated $\bar{MS}$ Wilson coefficients and anomalous dimensions for the non-singlet part of the structure function F_2 in the large-N_F limit. Our result agrees with exact two and three loop calculations and gives the leading N_F dependence of the perturbative non-singlet Wilson coefficients to all orders in $α_S$.

hep-ph↗
arXiv1997

Phenomenology of IR-renormalons in inclusive processes

We have compared the existing experimental data on the leading power corrections to the structure functions $F_2(x,Q^2), F_3(x,Q^2)$, and $F_L(x,Q^2)$ with the IR-renormalon model predictions for higher-twist contributions. Our analysis shows that the model properly describes the x-dependence, but typically falls short by a factor 2 or 3 as far as the magnitude of higher twist corrections is concerned.

hep-ph↗