Double-pole Approximation Research Articles

One-loop electroweak factorizable correctionsfor the Higgsstrahlung at a linear collider

We present standard model predictions for a four fermion reaction e+ e- -> mu+ mu- bar_b b being one of the best detection channels of a low mass Higgs boson produced through the Higgsstrahlung mechanism at a linear collider. We include leading virtual and real QED corrections to all the lowest order Feynman diagrams of e+ e- -> mu+ mu- bar_b b and the complete O(alpha) electroweak corrections to the Z-Higgs production and to the Z and Higgs boson decay widths in the double pole approximation.

Electroweak corrections to charged-current [formula omitted] fermion processes: Technical details and further results

The complete electroweak O ( α ) corrections have been calculated for the charged-current four-fermion production processes e + e − → ν τ τ + μ − ν ¯ μ , u d ¯ μ − ν ¯ μ , and u d ¯ s c ¯ . Here, technical details of this calculation are presented. These include the algebraic reduction of spinor chains to a few standard structures and the consistent implementation of the finite width of the W boson. To this end, a generalization of the complex-mass scheme to the one-loop level is proposed, and the practical application of this method is described. Finally, the effects of the complete O ( α ) corrections to various differential cross sections of physical interest are discussed and compared to predictions based on the double-pole approximation, revealing that the latter approximation is not sufficient to fully exploit the potential of a future linear collider in an analysis of W-boson pairs at high energies.

Complete electroweak [formula omitted] corrections to charged-current [formula omitted] fermion processes

The complete electroweak O(α) corrections are calculated for the charged-current four-fermion production processes e+e−→νττ+μ−ν¯μ, ud¯μ−ν¯μ, and ud¯sc¯. The calculation is performed using complex gauge-boson masses, supplemented by complex couplings to restore gauge invariance. The evaluation of the occurring one-loop tensor integrals, which include 5- and 6-point functions, requires new techniques. Explicit numerical results are presented for total cross sections in the energy range from the W-pair-production threshold region up to a scattering energy of 2 TeV. A comparison with the predictions based on the “double-pole approximation” (DPA) provided by the generator RacoonWW reveals corrections beyond DPA of ≲0.5% in the energy range 170–300 GeV, in agreement with previous estimates for the intrinsic DPA uncertainty. The difference to the DPA increases to 1–2% for s∼1–2 TeV. At threshold, where the DPA becomes unreliable, the full O(α) calculation corrects an improved Born approximation (IBA) by about 1.6%, also consistent with an error estimate of the IBA.

Logarithmic electroweak corrections to gauge-boson pair production at the LHC

We have studied the effects of the complete logarithmic electroweak O ( α ) corrections on the production of vector-boson pairs WZ, ZZ, and WW at the LHC. These corrections are implemented into a Monte Carlo program for p p → 4 f ( + γ ) with final states involving four or two leptons using the double-pole approximation. We numerically investigate purely leptonic final states and find that electroweak corrections lower the predictions by 5–30% in the physically interesting region of large di-boson invariant mass and large angle of the produced vector bosons.

RacoonWW1.3: A Monte Carlo program for four-fermion production at e +e − colliders

We present the Monte Carlo generator RacoonWW that computes cross sections to all processes e +e −→4 f and e +e −→4 fγ and calculates the complete O(α) electroweak radiative corrections to e +e −→WW→4 f in the electroweak Standard Model in double-pole approximation. The calculation of the tree-level processes e +e −→4 f and e +e −→4 fγ is based on the full matrix elements for massless (polarized) fermions. When calculating radiative corrections to e +e −→WW→4 f, the complete virtual doubly-resonant electroweak corrections are included, i.e. the factorizable and non-factorizable virtual corrections in double-pole approximation, and the real corrections are based on the full matrix elements for e +e −→4 fγ. The matching of soft and collinear singularities between virtual and real corrections is done alternatively in two different ways, namely by using a subtraction method or by applying phase-space slicing. Higher-order initial-state photon radiation and naive QCD corrections are taken into account. RacoonWW also provides anomalous triple gauge-boson couplings for all processes e +e −→4 f and anomalous quartic gauge-boson couplings for all processes e +e −→4 fγ.

QCD loop corrections to top quark production and decay ate+e−colliders

We present a computation of QCD next-to-leading order virtual corrections to the top quark production and decay process at linear colliders. The top quarks are allowed to be off-shell and the production and decay subprocesses are treated together, thus allowing for interference effects. The framework employed for our computation is the double pole approximation (DPA). We describe the implementation of this approximation for the top quark production and decay process and compare it with the implementation of DPA for the evaluation of QED corrections to the W pair production at CERN LEP II. Similarities and differences between the two cases are pointed out. One result of interest is the incomplete cancellation of interference corrections. Other results include values for the total NLO top quark production cross section, and the impact the nonfactorizable (interference) corrections have on the top quark invariant mass distribution.

Impact of ${\cal O}(\alpha)$ radiative corrections on CC03 physics at LEP

In this work we report about the effect that the improved knowledge of electroweak radiative corrections, via the recent calculations of four-fermion processes with \({\cal O}(\alpha)\) corrections in double pole approximation, has on WW physics at LEP2. Particular emphasis is given to the effects on differential distributions and their impact on the experimental observables. This study is based on generator comparisons using the new codes from the 2000 LEP2 Montecarlo workshop and allows new, important insights on the effects full radiative corrections have on W physics observables. The results presented here explain why it is so important to take these new calculations into account for precision measurements at LEP2.

Non-factorizable corrections and effective field theories

We analyze the structure of higher-order radiative corrections for processes with unstable particles. In particular, we study a model with an unstable, scalar boson decaying into fermions that are subject to gauge interactions. We neglect effects from real radiation. By subsequently integrating out the various scales that are induced by the presence of unstable particles we obtain a hierarchy of effective field theories. In the effective field theory framework the separation of physically different effects is achieved naturally. In particular, we automatically obtain a separation of factorizable and non-factorizable corrections to all orders in perturbation theory. At one loop this treatment is equivalent to the double-pole approximation (DPA). It is known that one-loop non-factorizable corrections to invariant mass distributions are suppressed at high energy. We study the mechanism of this suppression and obtain estimates of higher-order non-factorizable corrections at high energy.

E+e−→W+W−→4f(+γ) at LEP2

The results on e+e- --> W+W- --> 4f(+gamma) obtained by different groups are compared with each other. Differences in the results for the total cross section of up to about 0.6% are traced back to different ways of implementing the double-pole approximation.

Electroweak radiative corrections to e+e−→WW→4 fermions in double-pole approximation — the RacoonWW approach

Abstract We calculate the complete O (α) electroweak radiative corrections to e + e − →WW→4 f in the electroweak Standard Model in the double-pole approximation. We give analytical results for the non-factorizable virtual corrections and express the factorizable virtual corrections in terms of the known corrections to on-shell W-pair production and W decay. The calculation of the bremsstrahlung corrections, i.e., the processes e + e − →4 fγ in lowest order, is based on the full matrix elements. The matching of soft and collinear singularities between virtual and real corrections is done alternatively in two different ways, namely by using a subtraction method and by applying phase-space slicing. The O (α) corrections as well as higher-order initial-state photon radiation are implemented in the Monte Carlo generator RacoonWW . Numerical results of this program are presented for the W-pair-production cross section, angular and W-invariant-mass distributions at LEP2. We also discuss the intrinsic theoretical uncertainty of our approach.

Radiative corrections to e+e− → WW → 4 f with RacoonWW

RacoonWW is the first Monte Carlo generator for e + e − → WW → 4 f(+ γ) that includes the electroweak O(α) radiative corrections in the double-pole approximation completely. Some numerical results for LEP2 energies are discussed, and the predictions for the total W-pair cross section are confronted with LEP2 data.

Precise Predictions for Four-Fermion Production in Electron-Positron Annihilation

The complete O(alpha) electroweak radiative corrections to e+e- --> WW --> 4f in the electroweak Standard Model are presented in the double-pole approximation. We give analytical and numerical results for the non-factorizable corrections and express the factorizable virtual corrections in terms of the known corrections to on-shell W-pair production and W decay. The full matrix elements to the lowest order processes e+e- --> 4f+gamma are taken into account for the real corrections. The matching of soft and collinear singularities between virtual and real corrections is done using the subtraction method. A Monte Carlo generator is constructed which corresponds to the subtraction branch of RacoonWW. Various numerical results are presented.

W-Pair Production at Future e+e? Colliders: Precise Predictions from RACOONWW

We present numerical results for total cross sections and various distributions for e+e− → WW → 4f(+γ) at a future 500 GeV linear collider, obtained from the Monte Carlo generator RACOONWW. This generator is the first one that includes $$\cal O$$ (α) electroweak radiative corrections in the double-pole approximation completely. Owing to their large size the corrections are of great phenomenological importance.

Formula omitted] corrections to [formula omitted]: first numerical results from RacoonWW

First numerical results of the Monte Carlo generator RacoonWW for e +e −→WW→4 f(+ γ) in the electroweak Standard Model are presented. This event generator is the first one that includes O(α) electroweak radiative corrections in the double-pole approximation completely. We briefly describe the strategy of the calculation and give numerical results for total cross sections, including CC03, and various distributions.

Radiative corrections to pair production of unstable particles: Results for E+E− → 4 fermions

Radiative corrections to processes that involve the production and subsequent decay of unstable particles are complex due to various theoretical and practical problems. The so-called double-pole approximation offers a way out of these problems. This method is applied to the reaction e + e − → W + W − → 4 fermions, which allows us to address all the key issues of dealing with unstable particles, like gauge invariance, interactions between different stages of the reaction, and overlapping resonances. Within the double-pole approximation the complete O (α) electroweak corrections are evaluated for this off-shell W-pair production process. Examples of the effect of these corrections on a number of distributions are presented. These comprise mass and angular distributions as well as the photon-energy spectrum.

Non-factorizable photonic corrections to e +e − → WW → 4 fermions

We study the non-factorizable corrections to W-pair-mediated four-fermion production in e +e − annihilation in double-pole approximation. We show how these corrections can be combined with the known corrections to the production and the decay of on-shell W-boson pairs, and how the full off-shell Coulomb singularity is included. Moreover, we find that the actual form of the real non-factorizable corrections depends on the parametrization of phase space, more precisely, on the definition of the invariant masses of the resonant W bosons. For the usual parametrization the full analytical results for the non-factorizable corrections are presented. Our analytical and numerical results for the non-factorizable corrections agree with a recent calculation, which was found to differ from a previous one. The detailed numerical discussion covers the invariant-mass distribution, various angular distributions, and the lepton-energy distribution for leptonic final states.