arXiv subjects

Hubert Spiesberger

Publications and source records attributed to Hubert Spiesberger.

At least 37 records · Page 2Linked to original sources

arXiv2021

Beam-normal single-spin asymmetry in elastic scattering of electrons from a spin-0 nucleus

We study the beam-normal single-spin asymmetry (BNSSA) in high-energy elastic electron scattering from several spin-0 nuclei. Existing theoretical approaches work in the plane-wave formalism and predict the BNSSA to scale as $\sim A/Z$ with the atomic number $Z$ and nuclear mass number $A$. While this prediction holds for light and intermediate nuclei, a striking disagreement in both the sign and the magnitude of BNSSA was observed by the PREX collaboration for $^{208}$Pb, coined the "PREX puzzle". To shed light on this disagreement, we go beyond the plane-wave approach which neglects Coulomb distortions known to be significant for heavy nuclei. We explicitly investigate the dependence of BNSSA on $A$ and $Z$ by i) including inelastic intermediate states' contributions into the Coulomb problem in the form of an optical potential, ii) by accounting for the experimental information on the $A$-dependence of the Compton slope parameter, and iii) giving a thorough account of the uncertainties of the calculation. Despite of these improvements, the PREX puzzle remains unexplained. We discuss further strategies to resolve this riddle.

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arXiv2021

CFNS Ad-Hoc meeting on Radiative Corrections Whitepaper

Current precision scattering experiments and even more so many experiments planed for the Electron Ion Collider will be limited by systematics. From the theory side, a fundamental source of systematic uncertainty is the correct treatment of radiative effects. To gauge the current state of technique and knowledge, help the cross-pollination between different direction of nuclear physics, and to give input to the yellow report process, the community met in an ad-hoc workshop hosted by the Center for Frontiers in Nuclear Science, Stony Brook University. This whitepaper is a collection of contributions to this workshop.

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arXiv2020

Mixed EW-QCD two-loop amplitudes for $q\bar{q} \to \ell^+\ell^-$ and $γ_5$ scheme independence of multi-loop corrections

We perform a dedicated study of the $q \bar{q}$-initiated two-loop electroweak-QCD Drell-Yan scattering amplitude in dimensional regularization schemes for vanishing light quark and lepton masses. For the relative order $α$ and $α_s$ one-loop Standard Model corrections, details of our comparison to the original literature are given. The infrared pole terms of the mixed two-loop amplitude are governed by a known generalization of the dipole formula and we show explicitly that exactly the same two-loop polarized hard scattering functions are obtained in both the standard 't Hooft-Veltman-Breitenlohner-Maison $γ_5$ scheme and Kreimer's anticommuting $γ_5$ scheme.

hep-ph↗

arXiv2020

Weak charge and weak radius of ${}^{12}$C

We present a feasibility study of a simultaneous sub-percent extraction of the weak charge and the weak radius of the ${}^{12}$C nucleus using parity-violating electron scattering, based on a largely model-independent assessment of the uncertainties. The corresponding measurement is considered to be carried out at the future MESA facility in Mainz with $E_{\rm beam} = 155$ MeV. We find that a combination of a $0.3\%$ precise measurement of the parity-violating asymmetry at forward angles with a $10\%$ measurement at backward angles will allow to determine the weak charge and the weak radius of ${}^{12}$C with $0.4\%$ and $0.5\%$ precision, respectively. These values could be improved to $0.3\%$ and $0.2\%$ for a $3\%$ backward measurement. This experimental program will have impact on precision low-energy tests in the electroweak sector and nuclear structure.

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arXiv2020

Electroweak Physics in Inclusive Deep Inelastic Scattering at the LHeC

The proposed electron-proton collider LHeC is a unique facility where electroweak interactions can be studied with a very high precision in a largely unexplored kinematic regime of spacelike momentum transfer. We have simulated inclusive neutral- and charged-current deep-inelastic lepton proton scattering cross section data at center-of-mass energies of 1.2 and 1.3 TeV including their systematic uncertainties. Based on simultaneous fits of electroweak physics parameters and parton distribution functions, we estimate the uncertainties of Standard Model parameters as well as a number of parameters describing physics beyond the Standard Model, for instance the oblique parameters $S$, $T$, and $U$. An unprecedented precision at the sub-percent level is expected for the measurement of the weak neutral-current couplings of the light-quarks to the $Z$ boson, $g_{A/V}^{u/d}$, improving their present precision by more than an order of magnitude. The weak mixing angle can be determined with a precision of about $Δ\sin^2θ_\textrm{W} = \pm 0.00015$, and its scale dependence can be studied in the range between about 25 and 700 GeV. An indirect determination of the $W$-boson mass in the on-shell scheme is possible with an experimental uncertainty down to $Δm_{W}=\pm6\,\textrm{MeV}$. We discuss how the uncertainties of such measurements in deep-inelastic scattering compare with those from measurements in the timelike domain, e.g. at the $Z$-pole, and which aspects of the electroweak interaction are unique to measurements at the LHeC, for instance electroweak parameters in charged-current interactions. We conclude that the LHeC will determine electroweak physics parameters, in the spacelike region, with unprecedented precision leading to thorough tests of the Standard Model and possibly beyond.

hep-ph↗

arXiv2019

Mixed QCD-EW two-loop corrections to Drell-Yan production

The Drell-Yan production of charged lepton pairs is one of the key processes measured at hadron colliders. The QCD corrections to the cross section are known to order $α_s^2$ and electroweak corrections are known to order $α$. The next important step for a better theoretical understanding is the complete calculation of the mixed QCD-EW corrections of order $α_sα$. In my talk, I report on the first complete calculation of the virtual two-loop corrections of order $αα_s$ to the lepton-pair production cross section. The calculation is carried out analytically using tensor reduction, integration-by-parts relations and the method of differential equations. We validate a previous calculation of the subset of mixed QCD-QED corrections and show how the jet and soft functions of that reference can be used to subtract the infrared divergences of the complete mixed QCD-electroweak virtual corrections.

hep-ph↗

arXiv2019

Reduced uncertainty of the axial $γZ$-box correction to the proton's weak charge

We present the fully up-to-date calculation of the $γZ$-box correction which needs to be taken into account to determine the weak mixing angle at low energies from parity-violating electron proton scattering. We make use of neutrino and antineutrino inclusive scattering data to predict the parity-violating structure function $F_3^{γZ}$ by isospin symmetry. Our new analysis confirms previous results for the axial contribution to the $γZ$-box graph, and reduces the uncertainty by a factor of~2. In addition, we note that the presence of parity-violating photon-hadron interactions induces an additional contribution via $F_3^{γγ}$. Using experimental and theoretical constraints on the nucleon anapole moment we are able to estimate the uncertainty associated with this contribution. We point out that future measurements are expected to significantly reduce this latter uncertainty.

hep-ph↗

arXiv2019

QED radiative corrections for Polarized Lepton-Proton Scattering

We discuss QED radiative corrections at first and second order for the parity violating asymmetry $A_\text{PV}$ in elastic electron proton scattering. A measurement of $A_\text{PV}$ is planned at low energies and with high precision by the forthcoming P2 experiment at the new MESA facility in Mainz. Bremsstrahlung leads to a shift of the momentum transfer $Q^2$ which affects the determination of the weak charge of the proton.

hep-ph↗

arXiv2018

The P2 Experiment - A future high-precision measurement of the electroweak mixing angle at low momentum transfer

This article describes the future P2 parity-violating electron scattering facility at the upcoming MESA accelerator in Mainz. The physics program of the facility comprises indirect, high precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclear physics, single-spin asymmetries stemming from two-photon exchange and a possible future extension to the measurement of hadronic parity violation. The first measurement of the P2 experiment aims for a high precision determination of the weak mixing angle to a precision of 0.14% at a four-momentum transfer of Q^2 = 4.5 10^{-3} GeV^2. The accuracy is comparable to existing measurements at the Z pole. It comprises a sensitive test of the standard model up to a mass scale of 50 TeV, extendable to 70 TeV. This requires a measurement of the parity violating cross section asymmetry -39.94 10^{-9} in the elastic electron-proton scattering with a total accuracy of 0.56 10^-9 (1.4 %) in 10,000 h of 150 \micro A polarized electron beam impinging on a 60 cm liquid H_2 target allowing for an extraction of the weak charge of the proton which is directly connected to the weak mixing angle. Contributions from gamma Z-box graphs become small at the small beam energy of 155 MeV. The size of the asymmetry is the smallest asymmetry ever measured in electron scattering with an unprecedented goal for the accuracy. We report here on the conceptual design of the P2 spectrometer, its Cherenkov detectors, the integrating read-out electronics as well as the ultra-thin, fast tracking detectors. There has been substantial theory work done in preparation of the determination of the weak mixing angle. The further physics program in particle and nuclear physics is described as well.

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arXiv2017

Charm Quark Mass with Calibrated Uncertainty

We determine the charm quark mass ${\hat m}_c({\hat m}_c)$ from QCD sum rules of moments of the vector current correlator calculated in perturbative QCD. Only experimental data for the charm resonances below the continuum threshold are needed in our approach, while the continuum contribution is determined by requiring self-consistency between various sum rules, including the one for the zeroth moment. Existing data from the continuum region can then be used to bound the theoretical error. Our result is ${\hat m}_c({\hat m}_c) = 1272 \pm 8$ MeV for $\hatα_s(M_Z) = 0.1182$. Special attention is given to the question how to quantify and justify the uncertainty.

hep-ph↗

arXiv2016

Charm quark mass determined from a pair of sum rules

In this paper, we present preliminary results of the determination of the charm quark mass $\hat{m}_c$ from QCD sum rules of moments of the vector current correlator calculated in perturbative QCD at ${\cal O} (\hat α_s^3)$. Self-consistency between two different sum rules allow to determine the continuum contribution to the moments without requiring experimental input, except for the charm resonances below the continuum threshold. The existing experimental data from the continuum region is used, then, to confront the theoretical determination and reassess the theoretic uncertainty.

hep-ph↗

arXiv2016

Charm Quark Mass with Calibrated Uncertainty

We determine the charm quark mass $\hat{m}_c$ from QCD sum rules of moments of the vector current correlator calculated in perturbative QCD at ${\cal O} (\hatα_s^3)$. Only experimental data for the charm resonances below the continuum threshold are needed in our approach, while the continuum contribution is determined by requiring self-consistency between various sum rules. Existing data from the continuum region can then be used to bound the theoretic uncertainty. Our result is $\hat{m}_c(\hat{m}_c) = 1272 \pm 8$~MeV for $\hatα_s(M_Z) = 0.1182$.

hep-ph↗

arXiv2016

Hadronic weak charges and parity-violating forward Compton scattering

Parity-violating elastic electron-nucleon scattering at low momentum transfer allows one to access the nucleon's weak charge, the vector coupling of the $Z$-boson to the nucleon. In the Standard Model and at tree level, the weak charge of the proton is related to the weak mixing angle and accidentally suppressed, $Q_W^{p,\,{\rm tree}}=1-4\sin^2θ_W\approx0.07$. Modern experiments aim at extracting $Q_W^p$ at $\sim1\%$ accuracy. Similarly, parity non-conservation in atoms allows to access the weak charge of atomic nuclei. We consider a novel class of radiative corrections, an exchange of two photons with parity violation in the hadronic/nuclear system. These corrections may affect the extraction of $\sin^2θ_W$ from the experimental data at the relevant level of precision because they are affected by long-range interactions similar to other parity-violating radiative corrections, such as, e.g., the $γZ$-exchange, which has obtained much attention recently. We show that the significance of this new correction increases with the beam energy in parity-violating electron scattering, but the general properties of the parity-violating forward Compton amplitude protect the formal definition of the weak charge as a limit at zero-momentum transfer and zero-energy. We also discuss the relevance of the new correction for upcoming experiments.

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arXiv2015

Measuring the weak mixing angle with the P2 experiment at MESA

The P2 experiment in Mainz aims to measure the weak mixing angle in electron- proton scattering to a precision of 0.13 %. In order to suppress uncertainties due to proton structure and contributions from box graphs, both a low average momentum transfer $Q^2$ of $4.5\cdot 10^{-3}$ GeV$^2/c^2$ and a low beam energy of 155 MeV are chosen. In order to collect the enormous statistics required for this measurement, the new Mainz Energy Recovery Superconducting Accelerator (MESA) is being constructed. These proceedings describe the motivation for the measurement, the experimental and accelerator challenges and how we plan to tackle them.

physics.ins-det↗

arXiv2015

How strange is pion electroproduction?

We consider pion production in parity-violating electron scattering (PVES) in the presence of nucleon strangeness in the framework of partial wave analysis with unitarity. Using the experimental bounds on the strange form factors obtained in elastic PVES, we study the sensitivity of the parity-violating asymmetry to strange nucleon form factors. For forward kinematics and electron energies above 1 GeV, we observe that this sensitivity may reach about 20\% in the threshold region. With parity-violating asymmetries being as large as tens p.p.m., this study suggests that threshold pion production in PVES can be used as a promising way to better constrain strangeness contributions. Using this model for the neutral current pion production, we update the estimate for the dispersive $γZ$-box correction to the weak charge of the proton. In the kinematics of the Qweak experiment, our new prediction reads Re$\,\Box_{γZ}^V(E=1.165\,{\rm GeV}) = (5.58\pm1.41)\times10^{-3}$, an improvement over the previous uncertainty estimate of $\pm2.0\times10^{-3}$. Our new prediction in the kinematics of the upcoming MESA/P2 experiment reads Re$\,\Box_{γZ}^V(E=0.155\,{\rm GeV}) = (1.1\pm0.2) \times 10^{-3}$.

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arXiv2014

Top Quark Charge Asymmetry: Searching for Light Axigluons in top pair + jet Production at the LHC

We investigate the discovery potential of light color-octet bosons in the mass range of 100 - 400 GeV in exclusive top-pair plus jet production at the LHC. We study the impact of such bosons on the incline, the energy and the rapidity asymmetries. We show that light axigluons with large couplings to quarks can be discovered at the LHC with a luminosity of a few inverse fb. Almost all of the considered axigluon parameter space can be probed using the already available 2011/2012 LHC data. In a small-coupling scenario, axigluons could be discovered using the charge asymmetry with 65 inverse fb at the LHC and a center of mass energy of 14 TeV. We furthermore show that top pair+jet production could reveal the existence of scenarios where axigluons couple with a different strength to up- and down-type quarks.

hep-ph↗

arXiv2013

Higgs CP properties using the tau decay modes at the ILC

We investigate the prospects of determining the CP nature of the 126 GeV neutral spin-0 (Higgs) boson h, discovered at the LHC, at a future linear collider. We consider the production of h by the Higgsstrahlung process and its subsequent decays to tau leptons. We investigate how precisely a possible pseudoscalar component of h can be detected by the measurement of a suitably defined angular distribution, if all major decay modes of the tau lepton are used. From our numerical simulations, we estimate the expected precision to the scalar-pseudoscalar mixing angle, including estimates of the background and of measurement uncertainties, to be 2.8 degree for Higgs-boson production at a center-of-mass energy of 250 GeV and for a collider with integrated luminosity of 1 inverse attobarn.

hep-ph↗

arXiv2013

Theory Uncertainty in Extracting the Proton's Weak Charge: White Paper

We review the state-of-the-art and address open questions relative to the theory uncertainty of the $γ-Z$ box contribution to the $A_{PV}$ measurement within the QWEAK experiment at Jefferson Lab. This white paper summarizes the contributions by participants and discussion sessions on this topic within the MITP Workshop on Precision Electroweak Physics held in Mainz, Germany, September 23 - October 11, 2013 \url{http://indico.cern.ch/conferenceDisplay.py?confId=248072

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