arXiv subjects
Publications and source records attributed to Matthias Steinhauser.
Recent developments in higher order calculations within the framework of Dimensional Reduction, the preferred regularization scheme for supersymmetric theories, are reported on. Special emphasis is put on the treatment of evanescent couplings, the equivalence to Dimensional Regularization, and the evaluation of $α_s(M_GUT)$ from $α_s(M_Z)$.
New data for the total cross section sigma(e^+e^- --> hadrons) in the charm and bottom threshold region are combined with an improved theoretical analysis, which includes recent four-loop calculations, to determine the short distance \bar{MS} charm and bottom quark masses. The final result for the \bar{MS}-masses, m_c(3 GeV)=0.986(13) GeV and m_b(10 GeV)=3.609(25) GeV is consistent with but significantly more precise than a similar previous study.
New data for the total cross section $σ(e^+e^-\to{hadrons})$ in the charm and bottom threshold region are combined with an improved theoretical analysis, which includes recent four-loop calculations, to determine the short distance $\bar{\rm MS}$ charm and bottom quark masses. A detailed discussion of the theoretical and experimental uncertainties is presented. The final result for the $\bar{\rm MS}$-masses, $m_c(3 {GeV})=0.986(13)$ GeV and $m_b(10 {GeV})=3.609(25)$ GeV, can be translated into $m_c(m_c)=1.286(13)$ GeV and $m_b(m_b)=4.164(25)$ GeV. This analysis is consistent with but significantly more precise than a similar previous study.
One of the most troublesome contributions to the NNLO QCD corrections to B -> X_s gamma originates from three-loop matrix elements of four-quark operators. A part of this contribution that is proportional to the QCD beta-function coefficient beta_0 was found in 2003 as an expansion in m_c/m_b. In the present paper, we evaluate the asymptotic behaviour of the complete contribution for m_c >> m_b/2. The asymptotic form of the beta_0-part matches the small-m_c expansion very well at the threshold m_c = m_b/2. For the remaining part, we perform an interpolation down to the measured value of m_c, assuming that the beta_0-part is a good approximation at m_c=0. Combining our results with other contributions to the NNLO QCD corrections, we find BR(B -> X_s gamma) = (3.15 +_ 0.23) x 10^-4 for E_gamma > 1.6 GeV in the B-meson rest frame. The indicated error has been obtained by adding in quadrature the following uncertainties: non-perturbative (5%), parametric (3%), higher-order perturbative (3%), and the interpolation ambiguity (3%).
The next-to-leading order (NLO) QCD corrections to the production and decay rate of a Higgs boson are computed within the framework of the Minimal Supersymmetric Standard Model (MSSM). The calculation is based on an effective theory for light and intermediate mass Higgs bosons. We provide a Fortran routine for the numerical evaluation of the coefficient function. For most of the MSSM parameter space, the relative size of the NLO corrections is typically of the order of 5% smaller than the Standard Model value. We exemplify the numerical results for two scenarios: the benchmark point SPS1a, and a parameter region where the gluon-Higgs coupling at leading order is very small due to a cancellation of the squark and quark contributions.
Recent developments in the treatment of a system of heavy quarks close to their production threshold are discussed. Particular emphasis is put on the next-to-next-to-next-to-leading order (N$^3$LO) corrections to the energy levels and the wave function and their effect on the cross section $σ(e^+e^-\to t\bar{t})$. Furthermore, the resummation of logarithmic corrections within the framework of potential non-relativistic QCD (pNRQCD) are considered.
We calculate the O(alpha alpha_s) corrections to the relationships between the MS-bar Yukawa couplings and the pole masses of the first five quark flavours in the standard model. We also present the corresponding relationships between the MS-bar and pole masses, which emerge as by-products of our main analysis. The occurring self-energies are evaluated using the method of asymptotic expansion.
In this contribution the first next-to-next-to-leading order corrections to the branching ratio ${BR}(\bar{B}\to X_sγ)$ are discussed. This includes the completion of the matching part and the fermionic corrections to the most important operator matrix elements.
We compute the dominant two-loop electroweak correction, of O(G_F M_t^2), to the partial width of the decay of an intermediate-mass Higgs boson into a pair of photons. We use the asymptotic-expansion technique in order to extract the leading dependence on the top-quark mass plus four expansion terms that describe the dependence on the W- and Higgs-boson masses. This correction reduces the Born result by approximately 2.5%. As a by-product of our analysis, we also recover the O(G_F M_t^2) correction to the partial width of the Higgs-boson decay to two gluon jets.
We evaluate the three-loop matching conditions for the dimension-five operators that are relevant for the b -> s gamma decay. Our calculation completes the first out of three steps (matching, mixing and matrix elements) that are necessary for finding the next-to-next-to-leading QCD corrections to this process. All such corrections must be calculated in view of the ongoing accurate measurements of the B -> Xs gamma branching ratio.
An estimate of the NNLO supersymmetric QCD effects for Higgs production at hadron colliders is given. Assuming an effective gluon-Higgs interaction, these corrections enter only in terms of process-independent, factorizable terms. We argue that the current knowledge of these terms up to NLO is sufficient to derive the NNLO hadronic cross section within the limitations of the standard theoretical uncertainties arising mainly from renormalization and factorization scale variations. The SUSY contributions are small with respect to the QCD effects, which means that the NNLO corrections to Higgs production are very similar in the Standard Model and the MSSM.
We calculate the O(alpha_s^3 ln(alpha_s)) contributions to the heavy-quarkonium production and annihilation rates. Our result sheds new light on the structure of the high-order perturbative corrections and opens a new perspective for a high-precision theoretical analysis. We also determine the three-loop anomalous dimensions of the nonrelativistic vector and pseudoscalar currents.
In this paper we take the first step towards a complete next-to-next-to-leading logarithmic (NNLL) calculation of the inclusive decay rate for $B \to X_sγ$. We consider the virtual corrections of order $\alphas^2 n_f$ to the matrix elements of the operators ${O}_1$, ${O}_2$ and ${O}_8$ and evaluate the real and virtual contributions to ${O}_7$. These corrections are expected to be numerically important. We observe a strong cancelation between the contributions from the current-current operators and $O_7$ and obtain, after applying naive non-abelianization, a reduction of the branching ratio of 3.9% (for $μ=3.0$ GeV) and an increase of 3.4% (for $μ=9.6$ GeV).
This paper describes the fortran program rhad which performs a numerical evaluation of the photon-induced hadronic R-ratio, R(s), related to the cross section for electron-positron annihilation, for a given center-of-mass energy \sqrt{s}. In rhad the state-of-the-art perturbative corrections to R(s) are implemented and the running and decoupling of the strong coupling constant and the quark masses is automatically treated consistently. Several options allow for a flexible use of the program.
We calculate the QCD corrections to the cross section of e^+ e^- -> t anti-b H^- and its charge-conjugate counterpart within the minimal supersymmetric extension of the Standard Model. This process is particularly important if m_t < m_H + m_b and root(s) < 2 m_H, so that t -> b H^+ and e^+ e^- -> H^+ H^- are not allowed kinematically. Large logarithmic corrections that arise in the on-mass-shell scheme of quark mass renormalization, especially from the t anti-b H^- Yukawa coupling for large values of tan(beta), are resummed by adopting the modified minimal-subtraction scheme, so that the convergence behavior of the perturbative expansion is improved. The inclusion of the QCD corrections leads to a significant reduction of the theoretical uncertainties due to scheme and scale dependences.
We calculate the leading mass-suppressed two-loop contribution, proportional to alpha_s^3/(m_q r^2), to the potential of a heavy quark-antiquark system. This contribution originates in the nonabelian nature of quantum chromodynamics (QCD) and has no analogue in quantum electrodynamics (QED). For this purpose, we elaborate a technique based on the implementation of the threshold expansion within the effective theory of nonrelativistic QCD (NRQCD). We discuss phenomenological implications of our result for heavy-quarkonium physics. We also confirm a previous result for the two-loop O(alpha_s^2) correction to the static potential.
The decay of the Standard Model Higgs boson in the intermediate-mass range into gluons is considered where special emphasis is put on the influence of the leading electroweak corrections proportional to G_F m_t^2. An effective Lagrangian approach is used where the top quark is integrated out. The evaluation of the coefficient function is performed using two different methods. The first one is concerned with the direct evaluation of the vertex diagrams and the second method is based on a low-energy theorem. In a first step the tools needed for the computation are provided namely the renormalization constants of the QCD Lagrangian are computed up to O(α_s^2 G_F m_t^2). Also the decoupling constants for the strong coupling constant α_s and the light quark masses relating the quantities of the full theory to the corresponding quantities of the effective one are evaluated up to order α_s^2 G_F m_t^2.
The QCD correction of $O(α_s^2)$ to the decay of the Standard Model Higgs boson into two photons is presented. We consider the contribution coming from diagrams with a heavy top quark as virtual particle. The first three terms of the expansion in the inverse top mass is calculated. Expressing the result through the on-shell top mass M_t, we find large coefficients for the power-suppressed terms whereas in the $\overline{MS}$ scheme the coefficients are tiny.