One-loop QCD Research Articles

One-loop QCD corrections to the thermal Wilson line model

We calculate the time independent four-point function in high temperature (T) QCD and obtain the leading momentum dependent terms. Furthermore, we relate these derivative interactions to derivative terms in a recently proposed finite T effective action based on the SU(3) Wilson Line and its trace, the Polyakov Loop. By this procedure we thus obtain a perturbative matching at finite T between QCD and the effective model. In particular, we calculate the leading perturbative QCD-correction to the kinetic term for the Polyakov Loop.

SupersymmetricQCDdecoupling properties inH+→tb¯decay

The supersymmetric (SUSY) QCD radiative corrections to the $\ensuremath{\Gamma}{(H}^{+}\ensuremath{\rightarrow}t\overline{b})$ partial decay width are analyzed within the minimal supersymmetric standard model at the one-loop level, $\mathcal{O}({\ensuremath{\alpha}}_{s}),$ and in the decoupling limit. We present the analytical expressions of these corrections in the large SUSY masses limit and study the decoupling behavior of these corrections in various limiting cases. We find that if the SUSY mass parameters are large and of the same order, the one-loop SUSY QCD corrections do not decouple. The nondecoupling contribution is enhanced by $\mathrm{tan}\ensuremath{\beta}$ and therefore large corrections are expected in the large $\mathrm{tan}\ensuremath{\beta}$ limit. In contrast, we also find that the SUSY QCD corrections decouple if the masses of either the squarks or the gluinos are separately taken large.

The structure of large logarithmic corrections at small transverse momentum in hadronic collisions

We consider the region of small transverse momenta in the production of high-mass systems in hadronic collisions. By using the current knowledge on the infrared behaviour of tree-level and one-loop QCD amplitudes at O(α S 2) , we analytically compute the general form of the logarithmically-enhanced contributions up to next-to-next-to-leading logarithmic accuracy. By comparing the results with q T -resummation formulae we extract the coefficients that control the resummation of the large logarithmic contributions for both quark and gluon channels. Our results show that within the conventional resummation formalism the Sudakov form factor is actually process-dependent.

One-loop QCD corrections to gluon-gluon scattering at NNLO

We present the O(alphas^4) virtual QCD corrections to gluon-gluon scattering due to the self-interference of the one-loop amplitude. We give analytic expressions renormalised in the MSbar scheme and work in conventional dimensional regularisation. We write the structure of the infrared divergences from direct Feynman diagram evaluation in terms of the Catani formalism for infrared divergences. Formulae for the finite remainder are given in terms of logarithms that are real in the physical region. These results, together with those previously obtained for quark-quark and quark-gluon scattering complete the one-loop matrix elements for 2 to 2 processes needed for the next-to-next-to-leading order contribution to inclusive jet production at hadron colliders.

One-loop QCD corrections to massless quark scattering at NNLO

We present the O(α s 4) virtual QCD corrections to massless unlike-quark q q ̄ →q′ q ̄ ′ and massless like-quark scattering q q ̄ →q q ̄ due to the interference of one-loop amplitudes with one-loop amplitudes. The structure of the infrared divergences agrees with that predicted by Catani. The results are expressed in an analytic form so that the relevant expressions for crossed scattering processes can be obtained in a straightforward manner. The one-loop contributions presented here, together with the interference of tree with two-loop amplitudes given recently, complete the 2→2 virtual QCD corrections at NNLO for the massless quark scattering processes.

Non-diagrammatic calculation of QCD one-loop β-function based on the renormalization group equation

Using the higher covariant derivatives regularization of gauge theories in the framework of the background field method, supplemented with one-loop Pauli-Villars regulator fields, we obtain a version of the renormalization group equation for the regulator fields, whose vacuum energy depends on the background gauge field. It is evaluated using an anomalous Ward-Takahashi identity, which is related to the rescaling anomaly of the auxiliary fields, obtained by the Fujikawa approach. In this way the anomalous origin of the one-loop beta-function in QCD is clearly shown in terms of scaling of effective Lagrangians without the use of any Feynman diagram. The simplicity of the method is due to the preservation of the background and quantum gauge invariance in any step of the calculation.

Heavy quark pair production in polarized photon-photon collisions

We present the next-to-leading-order cross sections of the heavy quark-antiquark pair production in polarized photon-photon collision for the general case of photon polarizations. The numerical results for top-antitop production cross sections together with production asymmetries are obtained for linearly polarized photon-photon collisions, including one-loop QCD radiative corrections.

The soft-gluon current at one-loop order

We study the soft limit of one-loop QCD amplitudes and we derive the process-independent factorization formula that controls the singular behaviour in this limit. This is obtained from the customary eikonal factorization formula valid at tree (classical) level by introducing a generalized soft-gluon current that embodies the quantum corrections. We compute the explicit expression of the soft-gluon current at one-loop order. It contains purely non-abelian correlations between the colour charges of each pair of hard-momentum partons in the matrix element. This leads to colour correlations between (two and) three hard partons in the matrix element squared. Exploiting colour conservation, we recover QED-like factorization for the square of the matrix elements with two and three hard partons.

QCD heavy-quark potential to orderv2:One loop matching conditions

The one-loop QCD heavy quark potential is computed to order v^2 in the color singlet and octet channels. Several errors in the previous literature are corrected. To be consistent with the velocity power counting, the full dependence on |p' + p|/|p' - p| is kept. The matching conditions for the NRQCD one-loop potential are computed by comparing the QCD calculation with that in the effective theory. The graphs in the effective theory are also compared to terms from the hard, soft, potential, and ultrasoft regimes in the threshold expansion. The issue of off-shell versus on-shell matching and gauge dependence is discussed in detail for the 1/(m k) term in the potential. Matching on-shell gives a 1/(m k) potential that is gauge independent and does not vanish for QED.

GAUGE CONSISTENT WILSON RENORMALIZATION GROUP II: THE NON-ABELIAN CASE

We give a Wilsonian formulation of non-Abelian gauge theories explicitly consistent with axial gauge Ward identities. The issues of unitarity and dependence on the quantization direction are carefully investigated. A Wilsonian computation of the one-loop QCD beta function is performed.

Infrared behavior of one-loop QCD amplitudes at next-to-next-to-leading order

We present universal factorization formulas describing the behavior of one-loop QCD amplitudes as external momenta become either soft or collinear. Our results are valid to all orders in the dimensional regularization parameter, $\eps$. Terms through $\Ord(\eps^2)$ can contribute in infrared divergent phase space integrals associated with next-to-next-to-leading order jet cross-sections.

Radiative corrections to the structure functions and sum rules in polarized dis

The one-loop NLO radiative corrections (RC) to the observables in polarized DIS using assumption that a quark is an essential massive particle are considered. If compared with classical QCD formulae the obtained results are identical for the unpolarized and different for polarized sum rules, that can be explained as the influence of the finite quark mass effects on NLO QCD corrections. The explicit expression for one-loop NLO QCD contribution to the structure function g 2 is presented.

One-loop QCD interconnection effects in pair production of top quarks

We calculate the one-loop non-factorizable QCD corrections to the production and decay of pairs of top quarks at various collider experiments. These non-factorizable corrections interconnect the different production and decay stages of the off-shell top-pair production processes. This in particular affects the invariant-mass distributions of the off-shell top quarks, resulting in a shift of the maximum of the distorted Breit–Wigner distributions. Although the non-factorizable corrections can be large, the actual shift in the mass as determined from the peak position of the corrected Breit–Wigner line-shape is below 100 MeV .

Complete helicity decomposition of the $\gamma^*/Z^*t\bar{t}$ vertex including higher order QCD corrections and applications to $e^+e^-\rightarrow t \bar t$

The complete density matrix for all polarization configurations in the process B--> t tbar, where B is an off--shell Z or photon and t is the top quark, is calculated numerically including oneloop QCD corrections, i.e. virtual and real gluon contributions in O(alphas). The analysis is done in the framework of the helicity formalism. The results are particularly suited for top quark production at the Linear Collider, but may be useful in other circumstances as well. Relations to LEP and Tevatron physics are pointed out.

The supersymmetric QCD radiative corrections to top quark semileptonic decays

The one-loop supersymmetric QCD corrections to the top quark semileptonic decays t→b l ̄ ν l are considered. The corrections are found to reduce the decay width. In the still acceptable parameter space the corrections to the percentage change in top quark semileptonic decay width can reach the level of −0.7%.

Helicity amplitudes for O( αs) production of W+W−, W±Z, ZZ, W±γ, or Zγ pairs at hadron colliders

We present the one-loop QCD corrections to the helicity amplitudes for the processes q q → W +W −, Z Z, W ±Z, W ±γ, or Zγ , including the subsequent decay of each massive vector boson into a pair of leptons. We also give the corresponding tree-level amplitudes with an additional gluon radiated off the quark line. Together, these amplitudes provide all the necessary input for the calculation of the next-to-leading order QCD corrections to the production of any electroweak vector boson pair at hadron colliders, including the full spin and decay-angle correlations.

QCD corrections to at Lep 2 and the Next Linear Collider: CC11 at

QCD one-loop corrections to the full gauge invariant set of electroweak diagrams describing the hadronic process $e^+ e^- \to u~\bar d~s~\bar c$ are computed. Four-jet shape variables for $WW$ events are studied at next-to-leading order and the effects of QCD corrections on the determination of the $W$--mass in the hadronic channel at Lep 2 and NLC is discussed. We compare the exact calculation with a ``naive''approach to strong radiative corrections which has been widely used in the literature.

One-loop QCD corrections to deeply-virtual Compton scattering: The parton helicity-independent case

We show that the one-loop QCD correction to deeply virtual Compton scattering can be factorized into finite perturbative contributions and collinearly divergent terms, which correspond to the matrix elements of the off-forward parton distributions. As a by-product, we obtain the next-to-leading order coefficient functions in the generalized operator product expansion of two vector currents.

O(αs) spin-spin correlations for top and bottom quark production in e+e− annihilation

We present the full O(alpha_s) longitudinal spin-spin correlations for heavy-quark pair production at e+ e- high-energy colliders in closed analytical form. In such reactions, quark and antiquark have strongly correlated spins, and the longitudinal components are dominant. For the explicit computation of the QCD bremsstrahlung contributions, new phase-space integrals are derived. Explicit numerical estimates are given for t t_bar and b b_bar production. Around the Z-peak, QCD one-loop corrections depolarize the spin-spin asymmetry for bottom quark pairs by approximately -4%. For top pair production, we find at 350GeV a 0.6% increased polarization over a value of 0.4 in the longitudinal correlation. For more than 1 TeV the O(alpha_s) corrections enhance depolarization to -2% in the top-pair case.

The one-loop QCD corrections for

The one-loop QCD corrections for