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Achim Denig

Publications and source records attributed to Achim Denig.

15 recordsLinked to original sources
arXiv2026

HadroTOPS: A Monte Carlo Event Generator For Hadron Production In Two-Photon Scattering In Electron Positron Collisions

We present a Monte Carlo event generator specifically developed for the study of hadronic two-photon fusion events in two-photon scattering at electron-positron colliders. The code enables the generation of events with exact leading-order QED coupling and a flat phase space decay of the hadronic state into an arbitrary number of final state particles as selected by the user. Thus, this generator is well-suited for the use of partial wave analyses tools to study the two-photon production of higher-multiplicity final states across a wide range of energies and photon virtualities. Furthermore, the code integrates both experimental and theoretical inputs on the two-photon couplings of hadrons to simulate two-photon production processes. Motivated by the investigations of the BESIII collaboration, the final states $π^+π^-$, $π^0π^0$, $π^0η$, $K^+K^-$, $K^0_SK^0_S$, $ηη$, and $f_1(1285)\to ηπ^+π^-$ via $a_0^\pm(980)π^\mp$ and $f_0(500)η$ are currently included. The code is sufficiently flexible to easily add additional final states as well as quickly change the already included channels.

hep-ph↗
arXiv2025

Radiative corrections and Monte Carlo tools for low-energy hadronic cross sections in $e^+ e^-$ collisions

We present the results of Phase I of an ongoing review of Monte Carlo tools relevant for low-energy hadronic cross sections. This includes a detailed comparison of Monte Carlo codes for electron-positron scattering into a muon pair, pion pair, and electron pair, for scan and radiative-return experiments. After discussing the various approaches that are used and effects that are included, we show differential cross sections obtained with AfkQed, BabaYaga@NLO, KKMC, MCGPJ, McMule, Phokhara, and Sherpa, for scenarios that are inspired by experiments providing input for the dispersive evaluation of the hadronic vacuum polarisation.

hep-ph↗
arXiv2024

The MESA physics program

In the recent past, a comprehensive experimental program has been worked out at the Mainz Energy-Recovery Superconducting Accelerator, MESA, at the Institute of Nuclear Physics in Mainz. MESA is a high-intensity, low-energy electron accelerator presently under construction and will thereby provide great opportunities to perform a new generation of high-precision scattering experiments. The versatile MAGIX experiment will use MESA's innovative energy recovery technique, with a science focus on the study of hadron structure and few-body systems, dark sector searches, and investigations into reactions pertinent to nuclear astrophysics. An external beam line will supply spin-polarized electrons to the P2 experiment, enabling the performance of sensitive tests of the Standard Model through parity-violating electron scattering. The DarkMESA beam dump experiment, situated behind P2, is dedicated to the search for light dark matter particles.

nucl-ex↗
arXiv2022

50 Years of Quantum Chromodynamics

This paper presents a comprehensive review of both the theory and experimental successes of Quantum Chromodynamics, starting with its emergence as a well defined theory in 1972-73 and following developments and results up to the present day. Topics include a review of the earliest theoretical and experimental foundations; the fundamental constants of QCD; an introductory discussion of lattice QCD, the only known method for obtaining exact predictions from QCD; methods for approximating QCD, with special focus on effective field theories; QCD under extreme conditions; measurements and predictions of meson and baryon states; a special discussion of the structure of the nucleon; techniques for study of QCD at high energy, including treatment of jets and showers; measurements at colliders; weak decays and quark mixing; and a section on the future, which discusses new experimental facilities or upgrades currently funded. The paper is intended to provide a broad background for Ph.D. students and postdocs starting their career. Some contributions include personal accounts of how the ideas or experiments were developed.

hep-ph↗
arXiv2019

Dark Matter at the Intensity Frontier: the new MESA electron accelerator facility

Dark Matter is being searched with a variety of methods, each of which tackles this challenge focusing on different kinds of particles, masses and couplings. Here we describe Dark Matter searches conducted with accelerators and fixed targets. In particular, we highlight the contribution of the experiments being built at the new Mainz Energy-recovery Superconducting Accelerator (MESA) facility. MESA will provide intense electron beams for hadron and nuclear physics, as well as for Dark Matter searches with competitive sensitivities.

hep-ex↗
arXiv2019

The proton charge radius extracted from the Initial State Radiation experiment at MAMI

We report on a comprehensive reinterpretation of the existing cross-section data for elastic electron-proton scattering obtained by the initial-state radiation technique, resulting in a significantly improved accuracy of the extracted proton charge radius. By refining the external energy corrections we have achieved an outstanding description of the radiative tail, essential for a detailed investigation of the proton finite-size effects on the measured cross-sections. This development, together with a novel framework for determining the radius, based on a regression analysis of the cross-sections employing a polynomial model for the form factor, led us to a new value for the charge radius, which is $(0.870 \pm 0.014_\mathrm{stat.}\pm 0.024_\mathrm{sys.} \pm 0.003_\mathrm{mod.})\,\mathrm{fm}$.

nucl-ex↗
arXiv2018

Search for light dark matter with the MESA accelerator

At the Institute for Nuclear Physics of the Johannes Gutenberg University in Mainz, the MESA facility is currently being constructed. At its core there is a new superconducting energy-recovery linac which will provide intense electron beams for precision experiments in subatomic physics. An important part of the MESA physics program consists in the search for a "dark sector" which is a candidate explanation for the longstanding dark matter problem. This report will highlight the MESA dark sector program, and in particular two experiments will be described. The first one, MAGIX, is a two-spectrometer setup employing an internal gas-jet target installed on a recirculation arc of MESA. The second one is a beam-dump experiment for directly detecting dark matter particles. The experiments are in the R\&D phase and the current status and future prospects will be presented.

hep-ex↗
arXiv2016

Dark Sectors 2016 Workshop: Community Report

This report, based on the Dark Sectors workshop at SLAC in April 2016, summarizes the scientific importance of searches for dark sector dark matter and forces at masses beneath the weak-scale, the status of this broad international field, the important milestones motivating future exploration, and promising experimental opportunities to reach these milestones over the next 5-10 years.

hep-ph↗
arXiv2013

The Muon (g-2) Theory Value: Present and Future

This White Paper briefly reviews the present status of the muon (g-2) Standard-Model prediction. This value results in a 3 - 4 standard-deviation difference with the experimental result from Brookhaven E821. The present experimental uncertainty is $\pm 63 \times 10^{-11}$ (0.54~ppm), and the Standard-Model uncertainty is $\simeq \pm 49 \times 10^{-11}$. Fermilab experiment E989 has the goal to reduce the experimental error to $\pm 16 \times 10^{-11}$. Improvements in the Standard-Model value, which should be achieved between now and when the first results from Fermilab E989 could be available, should lead to a Standard-Model uncertainty of $\sim \,\pm 35 \times 10^{-11}$. These improvements would halve the uncertainty on the difference between experiment and theory, and should clarify whether the current difference points toward New Physics, or to a statistical fluctuation. At present, the (g-2) result is arguably the most compelling indicator of physics beyond the Standard Model and, at the very least, it represents a major constraint for speculative new theories such as supersymmetry, dark gauge bosons or extra dimensions.

hep-ph↗
arXiv2012

Nucleon Electromagnetic Form Factors in the Timelike Region

The electromagnetic form factors of the proton and the neutron in the timelike region are reviewed. In view of the forthcoming experimental projects devoted to investigate these observables, we present the current status of the field and we emphasize the relevant role, that accurate measurements, refined phenomenological analyses, as well as microscopic models will play with the goal of achieving deeper insights into the structure of the nucleon and its inner dynamics.

hep-ex↗
arXiv2006

The Radiative Return: A Review of Experimental Results

The radiative return is a new method for hadronic cross section measurements at electron-positron colliders, which are operated at a fixed center-of-mass energy. In order to lower the effective hadronic mass M_hadr only such events are taken, in which one of the electrons or positrons has emitted an initial state radiation photon. We present precision measurements of the pion form factor from the Frascati phi-factory DAPHNE with the KLOE experiment and measurements of higher particle multiplicities as well as a measurement of the timelike proton-antiproton form factor from the BaBar experiment at the B-factory PEP-II. These radiative return measurements are compared to results, which are obtained by means of an energy scan, i.e. by means of a systematic variation of the beam energy of the collider. We also report on the impact of these measurements on the hadronic contribution of the anomalous magnetic moment of the muon, which is obtained via a dispersion integral using hadronic cross section data as input.

hep-ex↗
arXiv2006

Measurement of the Pion Formfactor with KLOE and Study of the Reaction f0(980) to pi+pi-

At the Frascati phi-factory DAPHNE the pion formfactor is measured by means of the 'radiative return', i.e. by using events in which one of the collider electrons (positrons) has radiated an initial state radiation photon, lowering in such a way the invariant mass M_pipi of the two-pion-system. In a recent publication of the KLOE collaboration the initial state radiation photon had been required to be at small polar angles with respect to the beam axis. We are presenting results from a new and complementary analysis in which the photon is tagged at large polar angles. Only like this the threshold region M_pipi^2<0.35 GeV^2 becomes accessible. Moreover, the final state pi+pi-gamma allows to study the phi radiative decay into the scalar particle f_0(980) with f_0(980) \to pi+pi-. For the first time the two-pion mass spectrum could be fitted with different theoretical models for the description of this phi radiative decay.

hep-ex↗
arXiv2003

Measurement of the hadronic cross section at DAPHNE with the KLOE detector

We have measured the cross section sigma(e+e- \to pi+pi-gamma) as a function of the pi+pi- invariant mass, M_pipi, with the KLOE detector at DAPHNE (W=m_phi=1.02GeV). The photon in the above process is due to Initial State Radiation. Dividing by a theoretical radiator function we obtain the cross section sigma(e+e- \to pi+pi-) for the mass range 0.37<M_pipi^2<0.93GeV^2. We extract the pion form factor and the hadronic contribution to the muon anomaly, a_mu.

hep-ex↗