arXiv subjects

J. H. Kuhn

Publications and source records attributed to J. H. Kuhn.

At least 19 recordsLinked to original sources

arXiv2015

Charm and Bottom Quark Masses: an Update

Using new four-loop results for the heavy quark vacuum polarization and new data for bottom quark production in electron-positron annihilation, an update on the determination of charm- and bottom-quark masses through sum rules has been performed. The previous result for the charm-quark mass, $m_c(3 {\rm GeV})= 0.986(13) {\rm GeV}$, based on the lowest moment, is supported by the new results from higher moments which lead to consistent values with comparable errors. The new value for the bottom quark, $m_b(10 {\rm GeV})= 3.610(16) {\rm GeV}$, corresponding to $m_b(m_b)= 4.163(16) {\rm GeV}$, makes use both of the new data and the new perturbative results and is consistent with the earlier determination.

hep-ph↗

arXiv2012

The relation between the QED charge renormalized in MSbar and on-shell schemes at four loops, the QED on-shell beta-function at five loops and asymptotic contributions to the muon anomaly at five and six loops

In this paper we compute the four-loop corrections to the QED photon self-energy Pi(Q^2) in the two limits of q=0 and Q^2->infinity. These results are used to explicitly construct the conversion relations between the QED charge renormalized in on-shell(OS) and MSbar scheme. Using these relations and results of Baikov et al. [1] we construct the momentum dependent part of Pi(Q^2,m,alpha) at large Q^2 at five loops in both MSbar and OS schemes. As a direct consequence we arrive at the full result for the QED beta-function in the OS scheme at five loops. These results are applied, in turn, to analytically evaluate a class of asymptotic contributions to the muon anomaly at five and six loops.

hep-ph↗

arXiv2011

Next-to-Next-to-Leading Electroweak Logarithms for W-Pair Production at LHC

We derive the high energy asymptotic of one- and two-loop corrections in the next-to-next-to-leading logarithmic approximation to the differential cross section of $W$-pair production at the LHC. For large invariant mass of the W-pair the (negative) one-loop terms can reach more than 40%, which are partially compensated by the (positive) two-loop terms of up to 10%.

hep-ph↗

arXiv2010

Electroweak Sudakov Logarithms and Real Gauge-Boson Radiation in the TeV Region

Electroweak radiative corrections give rise to large negative, double-logarithmically enhanced corrections in the TeV region. These are partly compensated by real radiation and, moreover, affected by selecting isospin-noninvariant external states. We investigate the impact of real gauge boson radiation more quantitatively by considering different restricted final state configurations. We consider successively a massive abelian gauge theory, a spontaneously broken SU(2) theory and the electroweak Standard Model. We find that details of the choice of the phase space cuts, in particular whether a fraction of collinear and soft radiation is included, have a strong impact on the relative amount of real and virtual corrections.

hep-ph↗

arXiv2009

Quest for precision in hadronic cross sections at low energy: Monte Carlo tools vs. experimental data

We present the achievements of the last years of the experimental and theoretical groups working on hadronic cross section measurements at the low energy e+e- colliders in Beijing, Frascati, Ithaca, Novosibirsk, Stanford and Tsukuba and on tau decays. We sketch the prospects in these fields for the years to come. We emphasise the status and the precision of the Monte Carlo generators used to analyse the hadronic cross section measurements obtained as well with energy scans as with radiative return, to determine luminosities and tau decays. The radiative corrections fully or approximately implemented in the various codes and the contribution of the vacuum polarisation are discussed.

hep-ph↗

arXiv2009

Weak effects in b-jet production at hadron colliders

One of the main challenges of the Tevatron at Fermilab and the Large Hadron Collider (LHC) at CERN is the determination of Standard Model (SM) parameters at the TeV scale. In this context various processes will be investigated which involve bottom-quark jets in the final state, for example decays of top-quarks or gauge bosons of the weak interaction. Hence the theoretical understanding of processes with bottom-quark jets is necessary. In this paper analytic results will be presented for the weak corrections to bottom-quark jet production -- neglecting purely photonic corrections. The results will be used to study differential distributions, where sizeable effects are observed.

hep-ph↗

arXiv2009

Next-to-Next-to-Leading Electroweak Logarithms in W-pair Production at ILC

We derive the high energy asymptotic behavior of gauge boson production cross section in a spontaneously broken SU(2) gauge theory in the next-to-next-to-leading logarithmic approximation. On the basis of this result we obtain the logarithmically enhanced two-loop electroweak corrections to the differential cross section of W-pair production at ILC/CLIC up to the second power of the large logarithm.

hep-ph↗

arXiv2008

High-Precision Charm-Quark Mass and QCD Coupling from Current-Current Correlators in Lattice and Continuum QCD

We use lattice QCD simulations, with MILC gluon configurations and HISQ c-quark propagators, to make very precise determinations of moments of charm-quark pseudoscalar, vector and axial-vector correlators. These moments are combined with new four-loop results from continuum perturbation theory to obtain several new determinations of the MSbar mass of the charm quark and of the MSbar coupling. We find m_c(3GeV)=0.986(10)GeV, or, equivalently, m_c(m_c)=1.268(9)GeV, both for n_f=4 flavors; and alpha_msb(3GeV,n_f=4)=0.251(6), or, equivalently, alpha_\msb(M_Z,n_f=5)=0.1174(12). The new mass agrees well with results from continuum analyses of the vector correlator using experimental data for e+e- annihilation (instead of using lattice QCD simulations). These lattice and continuum results are the most accurate determinations to date of this mass. Ours is also one of the most accurate determinations of the QCD coupling by any method.

hep-lat↗

arXiv2007

Two-Loop Formfactors in Theories with Mass Gap and Z-Boson Production

The two-loop formfactor both for a U(1)xU(1) and a SU(2)xU(1) gauge theory with massive and massless gauge bosons respectively is evaluated at arbitrary momentum transfer q^2. The asymptotic behaviour for q^2->\infinity is compared to a recent calculation of Sudakov logarithms. The result is an important ingredient for the calculation of radiative corrections to Z-boson production at hadron and lepton colliders.

hep-ph↗

arXiv2006

Electroweak effects in top-quark pair production at Hadron Colliders

Top-quark physics plays an important role at hadron colliders such as the Tevatron collider at Fermilab or the upcoming Large Hadron Collider (LHC) at CERN. Given the planned experimental precision, detailed theoretical predictions are mandatory. In this article we present analytic results for the complete electroweak corrections to gluon induced top-quark pair production, completing our earlier results for the quark-induced reaction. As an application we discuss top-quark pair production at Tevatron and at LHC. In particular we show that, although small for inclusive quantities, weak corrections can be sizeable for differential distribution.

hep-ph↗

arXiv2005

Electroweak corrections to top-quark pair production in quark--antiquark annihilation

Top-quark physics plays an important role at hadron colliders such as the Tevatron at Fermilab or the LHC at CERN. Given the planned precision at these colliders, precise theoretical predictions are required. In this paper we present the complete electroweak corrections to QCD-induced top-quark pair production in quark--antiquark annihilation. In particular we provide compact analytic expressions for the differential partonic cross section, which will be useful for further theoretical investigations.

hep-ph↗

arXiv2005

One-loop weak corrections to hadronic production of Z bosons at large transverse momenta

To match the precision of present and future measurements of Z-boson production at hadron colliders, electroweak radiative corrections must be included in the theory predictions. In this paper we consider their effect on the transverse momentum ($p_T$) distribution of Z bosons, with emphasis on large $p_T$. We evaluate, analytically and numerically, the full one-loop corrections for the parton scattering reaction $q\bar q \to Z g$ and its crossed variants. In addition we derive compact approximate expressions which are valid in the high-energy region, where the weak corrections are strongly enhanced by logarithms of $\hat s/M_W^2$. These expressions include quadratic and single logarithms as well as those terms that are not logarithmically enhanced. This approximation, which confirms and extends earlier results obtained to next-to-leading logarithmic accuracy, permits to reproduce the exact one-loop corrections with high precision. Numerical results are presented for proton-proton and proton-antiproton collisions. The corrections are negative and their size increases with $p_T$. For the Tevatron they amount up to -7% at 300 GeV. For the LHC, where transverse momenta of 2 TeV or more can be reached, corrections up to -40% are observed. We also include the dominant two-loop effects of up to 8% in our final LHC predictions.

hep-ph↗

arXiv2005

Heavy-quark vacuum polarization: first two moments of the O(alphas^3 nf^2) contribution

The vacuum polarization due to a virtual heavy quark pair and specifically the coefficients of its Taylor expansion in the external momentum are closely related to moments of the cross section for quark-antiquark pair production in electron-positron annihilation. Relating measurement and theoretically calculated Taylor coefficients, an accurate value for charm- and bottom-quark mass can be derived, once corrections from perturbative QCD are sufficiently well under control. Up to three-loop order these have been evaluated previously. We now present a subset of four-loop contributions to the lowest two moments, namely those from diagrams which involve two internal loops from massive and massless fermions coupled to virtual gluons, hence of order alphas^3 nf^2. The calculation demonstrates the applicability of Laporta's algorithm to four-loop vacuum diagrams with both massive and massless propagators and should be considered a first step towards the full evaluation of the order alphas^3 contribution.

hep-ph↗

arXiv2004

Charge asymmetry and radiative phi decays

The study of radiative phi decays into scalar mesons, with subsequent decay into pi pi gamma, constitutes an important topic at the electron-positron collider DAPHNE. The interference of the respective amplitude with the one for pi+ pi- gamma production, where the photon originates from initial state radiation, will allow for unambiguous tests of models for the phi -> gamma f0(-> pi pi) amplitude. The forward-backward asymmetry of charged pions, which is a clear signal of such an interference, amounts up to 30 % and is at the same time quite sensitive to the details of the various amplitudes. The results for several characteristic cases are presented and their implementation into the Monte Carlo generator PHOKHARA is described.

hep-ph↗

arXiv2004

Pole- versus MS-mass definitions in the electroweak theory

Two different two-loop relations between the pole- and the MS-mass of the top quark have been derived in the literature which were based on different treatments of the tadpole diagrams. In addition, the limit M_W^2/m_t^2 \to 0 was employed in one of the calculations. It is shown that, after appropriate transformations, the results of the two calculations are in perfect agreement. Furthermore we demonstrate that the inclusion of the non-vanishing mass of the W-boson leads to small modifications only.

hep-ph↗

arXiv2003

Perspectives for the radiative return at meson factories

The measurement of the pion form factor and, more generally, of the cross section for electron-positron annihilation into hadrons through the radiative return has become an important task for high luminosity colliders such as the Phi- or B-meson factories. This quantity is crucial for predictions of the hadronic contributions to the anomalous magnetic moment of the muon, and to the running of the electromagnetic coupling. But the radiative return opens the possibility of many other physical applications. The physics potential of this method at high luminosity meson factories is discussed, the last upgraded version of the event generator PHOKHARA is presented, and future developments are highlighted.

hep-ph↗

arXiv2001

Radiative return at NLO and the measurement of the hadronic cross-section in electron-positron annihilation

Electron-positron annihilation into hadrons plus an energetic photon from initial state radiation allows the hadronic cross-section to be measured over a wide range of energies. The full next-to-leading order QED corrections for the cross-section for e^+ e^- annihilation into a real tagged photon and a virtual photon converting into hadrons are calculated where the tagged photon is radiated off the initial electron or positron. This includes virtual and soft photon corrections to the process e^+ e^- \to γ+γ^* and the emission of two real hard photons: e^+ e^- \to γ+ γ+ γ^*. A Monte Carlo generator has been constructed, which incorporates these corrections and simulates the production of two charged pions or muons plus one or two photons. Predictions are presented for centre-of-mass energies between 1 and 10 GeV, corresponding to the energies of DAPHNE, CLEO-C and B-meson factories.

hep-ph↗

arXiv2000

Summing up Subleading Sudakov Logarithms

We apply the strategy of regions within dimensional regularization to find functions involved in evolution equations which govern the asymptotic dynamics of the Abelian form factor and four-fermion amplitude in the SU(N) gauge theory in the Sudakov limit up to the next-to-leading logarithmic approximation. The results are used for the analysis of the dominant electroweak corrections to the fermion-antifermion pair production in the $e^+e^-$ annihilation at high energy.

hep-ph↗