arXiv subjects

Michael Melles

Publications and source records attributed to Michael Melles.

At least 19 recordsLinked to original sources

arXiv2003

Sudakov expansions at one loop and beyond for charged scalar and fermion pair production in SUSY models at future Linear Colliders

We consider the high energy behaviour of the amplitudes for pair production of charged leptons, quarks, Higgs bosons, sleptons, squarks and charginos at lepton colliders. We give the general expressions of the leading quadratic and subleading linear logarithms that appear at the one loop level, and derive the corresponding resummed expansions to \underline{subleading} logarithmic order accuracy. Under the assumption of a relatively light SUSY scenario and choosing the MSSM as a specific model, we compare the predictions of the one-loop and of the resummed expansions at variable energy. We show that the two predictions are very close in the one TeV regime, but drastically differ in the few ($2,3$) TeV range.

hep-ph↗

arXiv2003

Electroweak radiative corrections in high energy processes

Experiments at future colliders will attempt to unveil the origin of electroweak symmetry breaking in the TeV range. At these energies the Standard Model (SM) predictions have to be known precisely in order to disentangle various viable scenarios such as supersymmetry and its manifestations. In particular, large logarithmic corrections of the scale ratio $\sqrt{s}/M$, where $M$ denotes the gauge boson masses, contribute significantly up to and including the two loop level. In this paper we review recent progress in the theoretical understanding of the electroweak Sudakov corrections at high energies up to subleading accuracy in the SM and the minimal supersymmetric SM (MSSM). We discuss the symmetric part of the SM Lagrangian at high energies yielding the effective theory employed in the framework of the infrared evolution equation (IREE) method. Applications are presented for important SM and MSSM processes relevant for the physics program of future linear colliders including higher order purely electroweak angular dependent corrections. The size of the higher order subleading electroweak corrections is found to change cross sections in the several percent regime at TeV energies and their inclusion is thus mandatory for predictions of high energy processes at future colliders.

hep-ph↗

arXiv2002

Two-loop electroweak corrections at high energies

We discuss two-loop leading and angular-dependent next-to-leading logarithmic electroweak virtual corrections to arbitrary processes at energies above the electroweak scale. The relevant Feynman diagrams involving soft-collinear gauge bosons gamma, Z, W, have been evaluated in eikonal approximation. We present results obtained from the analytic evaluation of massive loop integrals. To isolate mass singularities we used the Sudakov method and alternatively the sector decomposition method in the Feynman-parameter representation.

hep-ph↗

arXiv2002

Supersymmetric Sudakov corrections

For superpartner masses not much heavier than the weak scale $M=M_{\rm W}$, large logarithmic corrections of the Sudakov type arise at TeV energies. In this paper we discuss the general structure of electroweak supersymmetric (susy) Sudakov corrections in the framework of the infrared evolution equation method. We discuss Yukawa sector Ward-identities which lead to the exponentiation of the subleading (SL) logarithmic Yukawa enhanced Sudakov corrections in both the Standard Model (SM) as well as in softly broken supersymmetric extensions. The results are given to SL accuracy to all orders in perturbation theory for arbitrary external lines in the ``light'' susy-mass scenario. The susy-QCD limit for virtual corrections is presented. Phenomenological applications regarding the precise determination of the important parameter $\tan β$ through virtual corrections are discussed which are independent of the soft susy breaking mechanism to sub-subleading accuracy to all orders.

hep-ph↗

arXiv2001

Resummation of angular dependent corrections in spontaneously broken gauge theories

Recent investigations of electroweak radiative corrections have revealed the importance of higher order contributions in high energy processes, where the size of typical corrections can exceed those associated with QCD considerably. Beyond one loop, only universal (angular independent) corrections are known to all orders except for massless $e^+ e^- \longrightarrow f {\overline f}$ processes where also angular dependent corrections exist in the literature. In this paper we present general arguments for the consistent resummation of angular dependent subleading (SL) logarithmic corrections to all orders in the regime where all invariants are still large compared to the gauge boson masses. We discuss soft isospin correlations, fermion mass and gauge boson mass gap effects, the longitudinal and Higgs boson sector as well as mixing contributions including CKM effects for massive quarks. Two loop arguments are interpreted in the context of the effective high energy effective theory based on the Standard Model Lagrangian in the symmetric basis with the appropriate matching conditions to include the soft QED regime. The result is expressed in exponentiated operator form in a CKM-extended isospin space in the symmetric basis. Thus, a full electroweak SL treatment based on the infrared evolution equation method is formulated for arbitrary high energy processes at future colliders. Comparisons with known results are presented.

hep-ph↗

arXiv2001

Electroweak renormalization group corrections in high energy processes

At energies ($\sqrt{s}$) much higher than the electroweak gauge boson masses ($M$) large logarithmic corrections of the scale ratio $\sqrt{s}/M$ occur. While the electroweak Sudakov type double (DL) and universal single (SL) logarithms have recently been resummed, at higher orders the electroweak renormalization group (RG) corrections are folded with the DL Sudakov contributions and must be included for a consistent subleading treatment to all orders. In this paper we derive first all relevant formulae for massless as well as massive gauge theories including all such terms up to order ${\cal O} (α^n β_0 \log^{2n-1} \frac{s}{M^2})$ by integrating over the corresponding running couplings. The results for broken gauge theories in the high energy regime are then given in the framework of the infrared evolution equation (IREE) method. The analogous QED-corrections below the weak scale $M$ are included by appropriately matching the low energy solution to the renormalization group improved high energy results. The corrections are valid for arbitrary external lines and largest in the scalar Goldstone and Higgs boson sector as well as for transverse gauge bosons. At TeV energies, these SL-RG terms change scattering cross sections in the percentile regime at two loops and are thus non-negligible for precision objectives at future linear colliders.

hep-ph↗

arXiv2000

The Standard Model Higgs in $γγ$ Collisions

For a Higgs boson below the $W^\pm$ threshold, the $γγ$ collider option of a future linear $e^+ e^-$ machine is compelling. In this case one can measure the ``gold-plated'' loop induced $Γ(H \longrightarrow γγ)$ partial width to high precision, testing various extensions of the Standard Model. With recent progress in the expected $γγ$ luminosity at TESLA, we find that for a Higgs of 115 GeV a statistical accuracy of the two photon partial width of 1.4 % is possible. The total width depends thus solely on the accuracy of $BR(H \longrightarrow γγ)$ and is of ${\cal O} (10 %)$.

hep-ph↗

arXiv2000

Electroweak Sudakov corrections

At energies much larger than the mass of the weak gauge bosons, electroweak radiative corrections can lead to significant corrections. At 1 TeV the one loop corrections can be of ${\cal O} (20 %)$ due to large contributions of the Sudakov type. We summarize recent progress in the evaluation and resummation of the double and single logarithmic corrections to general scattering amplitudes for fermions, transversely as well as longitudinally polarized external lines.

hep-ph↗

arXiv2000

Resummation of Yukawa enhanced and subleading Sudakov logarithms in longitudinal gauge boson and Higgs production

Future colliders will probe the electroweak theory at energies much larger than the gauge boson masses. Large double (DL) and single (SL) logarithmic virtual electroweak Sudakov corrections lead to significant effects for observable cross sections. Recently, leading and subleading universal corrections for external fermions and transverse gauge boson lines were resummed by employing the infrared evolution equation method. The results were confirmed at the DL level by explicit two loop calculations with the physical Standard Model (SM) fields. Also for longitudinal degrees of freedom the approach was utilized for DL-corrections via the Goldstone boson equivalence theorem. In all cases, the electroweak Sudakov logarithms exponentiate. In this paper we extend the same approach to both Yukawa enhanced as well as subleading Sudakov corrections to longitudinal gauge boson and Higgs production. We use virtual contributions to splitting functions of the appropriate Goldstone bosons in the high energy regime and find that all universal subleading terms exponentiate. The approach is verified by employing a non-Abelian version of Gribov's factorization theorem and by explicit comparison with existing one loop calculations. As a side result, we obtain also all top-Yukawa enhanced subleading logarithms for chiral fermion production at high energies to all orders. In all cases, the size of the subleading contributions at the two loop level is non-negligible in the context of precision measurements at future linear colliders.

hep-ph↗

arXiv2000

Subleading Sudakov logarithms in electroweak high energy processes to all orders

In future collider experiments at the TeV scale, large logarithmic corrections originating from massive boson exchange can lead to significant corrections to observable cross sections. Recently double logarithms of the Sudakov-type were resummed for spontaneously broken gauge theories and found to exponentiate. In this paper we use the virtual contributions to the Altarelli-Parisi splitting functions to obtain the next to leading order kernel of the infrared evolution equation in the fixed angle scattering regime at high energies where particle masses can be neglected. In this regime the virtual corrections can be described by a generalized renormalization group equation with infrared singular anomalous dimensions. The results are valid for virtual electroweak corrections to fermions and transversely polarized vector bosons with an arbitrary number of external lines. The subleading terms are found to exponentiate as well and are related to external lines, allowing for a probabilistic interpretation in the massless limit. For $Z$-boson and $γ$ final states our approach leads to exponentiation with respect to each amplitude containing the fields of the unbroken theory. For longitudinal degrees of freedom it is shown that the equivalence theorem can be used to obtain the correct double logarithmic asymptotics. At the subleading level, corrections to the would be Goldstone bosons contribute which should be considered separately. Explicit comparisons with existing one loop calculations are made.

hep-ph↗

arXiv2000

Two loop mass effects in the static position space QCD-potential

The perturbatively calculable short distance QCD potential is known to two loops including the effect of massive quarks. Recently, a simple approximate solution in momentum space was utilized to obtain the potential in coordinate space. The latter is important in several respects. A comparison with non-perturbative lattice results is feasible in the overlap regime using light $\bar{MS}$ masses. This might be even more promising employing the concept of the force between the heavy color singlet sources, which can be easily derived from the potential. In addition, the better than two percent accuracy bottom mass determination from $Υ$-mesons is sensitive to massive charm loops at the two loop order. We summarize recent results using exact one loop functions and explicit decoupling parametrizations.

hep-ph↗

arXiv2000

Precision Higgs physics at a $γγ$ collider

The loop induced coupling of an intermediate mass Higgs boson to two photons is a sensitive and unique measure for precision tests of physics beyond the Standard Model. In this work we summarize recent results on the expected precision of the partial $Γ(H \to γγ)$ width at the $γγ$ option of a future linear collider. Heavy particles do not decouple in general and differences between the SM and MSSM predictions or 2HD-models can differ in the percentile regime. Large non-Sudakov DL corrections need to be resummed and consistency requirements demand the use of the Sterman-Weinberg jet definition in order to avoid additional DL terms from three jet final states. We find that the well understood background process $γγ\to q \bar{q}$ allows for a ${\mathcal O}$(2%) determination of $Γ(H \to γγ)$ using conservative collider parameters. Recent improvements in the expected $γγ$ luminosity suggest that the precision for the diphoton partial Higgs width can be further improved and is dominated by the error in BR($H \to b \bar{b}$) from the $e^\pm$ mode, which is presently estimated to be in the one percent regime.

hep-ph↗

arXiv2000

Mass gap effects and higher order electroweak Sudakov logarithms

The infrared structure of spontaneously broken gauge theories is phenomenologically very important and theoretically a challenging problem. Various attempts have been made to calculate the higher order behavior of large double-logarithmic (DL) corrections originating from the exchange of electroweak gauge bosons resulting in contradictory claims. We present results from two loop electroweak corrections for the process $g \longrightarrow f_{\rm R} {\bar f}_{\rm L}$ to DL accuracy. This process is ideally suited as a theoretical model reaction to study the effect of the mass gap of the neutral electroweak gauge bosons at the two loop level. Contrary to recent claims in the literature, we find that the calculation performed with the physical Standard Model fields is in perfect agreement with the results from the infrared evolution equation method. In particular, we can confirm the exponentiation of the electroweak Sudakov logarithms through two loops.

hep-ph↗

arXiv2000

The static QCD potential in coordinate space with quark masses through two loops

The potential between infinitely heavy quarks in a color singlet state is of fundamental importance in QCD. While the confining long distance part is inherently non-perturbative, the short-distance (Coulomb-like) regime is accessible through perturbative means. In this paper we present new results of the short distance potential in coordinate space with quark masses through two loops. The results are given in explicit form based on reconstructed solutions in momentum space in the Euclidean regime. Thus, a comparison with lattice results in the overlap region between the perturbative and non-perturbative regime is now possible with massive quarks. We also discuss the definition of the strong coupling based on the force between the static sources.

hep-ph↗

arXiv1999

Precise measurement of $Γ(H \longrightarrow γγ)$ at a PLC and theoretical consequences

With the LEP II Higgs search approaching exclusion limits on low values of $\tan β\sim 2$ it becomes increasingly important to investigate physical quantities sensitive to large masses of a pseudoscalar Higgs mass. This regime is difficult and over a large range of $\tan β$ impossible to cover at the LHC proton proton collider. In this paper we focus on the achievable statistical precision of the Higgs decay into two photons at a future $γγ$ collider (PLC) in the MSSM mass range below 130 GeV. The MSSM and SM predictions for $Γ(H \longrightarrow γγ)$ can differ by up to 10 % even in the decoupling limit of large $m_A$. We summarize recent progress in both the theoretical understanding of the background process $γγ\longrightarrow q \bar{q}$, $q=\{b,c\}$, and in the expected detector performance allow for a high accuracy of the lightest MSSM or SM Higgs boson decay into a $b \bar{b}$ pair. We find that for optimized but still realistic detector and accelerator assumptions, statistically a 1.4% accuracy is feasible after about four years of collecting data for a Higgs boson mass which excludes $\tan β<2$.

hep-ph↗

arXiv1999

Exact gauge invariant mass dependence of α_s through two loops

A physically defined QCD coupling parameter naturally incorporates massive quark flavor thresholds in a gauge invariant, renormalization scale independent and analytical way. In this paper we summarize recent results for the finite-mass fermionic corrections to the heavy quark potential through two loops leading to the numerical solution of the physical and mass dependent Gell-Mann Low function. The decoupling-, massless- and Abelian-limits are reproduced and an analytical fitting function is obtained in the V-scheme. Thus the gauge invariant mass dependence of $α_V$ is now known through two loops. Possible applications in lattice analyses, heavy quark physics and effective charges are briefly discussed.

hep-ph↗

arXiv1999

Higgs Boson Production at the Compton Collider

The high precision determination of the partial width $Γ(H \longrightarrow γγ)$ of an intermediate mass Higgs boson is among the most important measurements at a future photon--photon collider. Recently it was shown that large non-Sudakov as well as Sudakov double logarithmic corrections can be summed to all orders in the background process $γγ\longrightarrow q \bar{q}$, $q=\{b,c\}$, from an initially polarized $J_z=0$ state. In addition, running coupling corrections were included exactly to all orders by employing the renormalization group. Thus all necessary theoretical results for calculating the Higgs signal and the non-Higgs continuum background contributions to the process $γγ\longrightarrow q \bar{q}$ are now known. We are therefore able to present for the first time precise predictions for the measurement of the partial width $Γ(H \longrightarrow γγ)$ at the Compton collider ($γγ$) option at a future linear $e^+e^-$ collider. The interplay between signal and background is very sensitive to the experimental cuts and the ability of the detectors to identify $b$-quarks in the final state. We investigate this in some detail using a Monte Carlo analysis, and conclude that a measurement with a 2 % statistical accuracy should be achievable. This could have important consequences for the discovery of physics beyond the Standard Model, in particular for large masses of a pseudoscalar Higgs boson as the decoupling limit is difficult and for a wide range of $\tan β$ impossible to cover at the LHC proton-proton collider.

hep-ph↗

arXiv1999

Higgs Physics at a γγCollider

High precision measurements of electroweak observables at $e^\pm$ colliders indicate the existence of a light Higgs boson below the $W^\pm$ threshold. If such a fundamental scalar should be found in the near future it is important to fully investigate the electroweak symmetry braking sector of the Standard Model. This is particularly important for an intermediate mass Higgs as its existence might indicate physics beyond the Standard Model, for instance in form of its minimal supersymmetric extension (MSSM). In this work we present first results on the expected precision of the partial $Γ(H \longrightarrow γγ)$ width at the $γγ$ option of a future linear collider. This quantity is sensitive to new physics as heavy particles do not decouple in general and differences between the SM and MSSM predictions can differ by up to 10% even in the decoupling limit of large pseudoscalar Higgs masses. This regime is difficult and for some values of $\tan β$ impossible to cover at the LHC. We find that the well understood background process $γγ\longrightarrow q \bar{q}$ allows for a ${\cal O} (2%)$ determination of $Γ(H \longrightarrow γγ)$ using conservative collider parameters.

hep-ph↗