Search results for "massless"
showing 10 items of 191 documents
Dipole model at Next-to-Leading Order meets HERA data
2021
Deep inelastic scattering (DIS) total cross section data at small-x as measured by the HERA experiments is well described by Balitsky-Kovchegov (BK) evolution in the leading order dipole picture. Recently the full Next-to-Leading Order (NLO) dipole picture total cross sections have become available for DIS, and a working factorization scheme has been devised which subtracts the soft gluon divergence present at NLO. We report our recently published work in which we make the first comparisons of the NLO DIS total cross sections to HERA data. The non-perturbative initial condition to BK evolution is fixed by fitting the HERA reduced cross section data. As the NLO results for the DIS total cros…
Four-dimensional unsubtraction with massive particles
2016
We extend the four-dimensional unsubtraction method, which is based on the loop-tree duality (LTD), to deal with processes involving heavy particles. The method allows to perform the summation over degenerate IR configurations directly at integrand level in such a way that NLO corrections can be implemented directly in four space-time dimensions. We define a general momentum mapping between the real and virtual kinematics that accounts properly for the quasi-collinear configurations, and leads to an smooth massless limit. We illustrate the method first with an scalar toy example, and then analyse the case of the decay of a scalar or vector boson into a pair of massive quarks. The results pr…
Search for long-lived particles decaying into electron or photon pairs with the D0 detector
2008
In this Letter we report on a search for long-lived particles that decay into final states with two electrons or photons. Such long-lived particles arise in a variety of theoretical models, like hidden valleys and supersymmetry with gauge-mediated breaking. By precisely reconstructing the direction of the electromagnetic shower we are able to probe much longer lifetimes than previously explored. We see no evidence of the existence of such long-lived particles and interpret this search as a quasi model-independent limit on their production cross section, as well as a limit on a long-lived fourth generation quark.
New contributions to heavy quark sum rules
2002
We analyse new contributions to the theoretical input in heavy quark sum rules and we show that the general theory of singularities of perturbation theory amplitudes yields the method to handle these specific features. In particular we study the inclusion of heavy quark radiation by light quarks at O(alpha_s^2) and non-symmetric correlators at O(alpha_s^3). Closely related, we also propose a solution to the construction of moments of the spectral densities at O(alpha_s^3) where the presence of massless contributions invalidates the standard approach. We circumvent this problem through a new definition of the moments, providing an infrared safe and consistent procedure.
Quark and gluon form factors to four-loop order in QCD: TheNf3contributions
2017
We calculate the four-loop massless QCD corrections with three closed quark lines to quark and gluon form factors. We apply a novel integration by parts algorithm based on modular arithmetic and compute all relevant master integrals for arbitrary values of the space-time dimension. This is the first calculation of a gluon form factor at this perturbative order in QCD.
O(αs) longitudinal spin polarization in heavy-quark production
1995
We present the massive one-loop QCD corrections to the production cross sections of polarized quarks in the annihilation process ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}qq\ifmmode\bar\else\textasciimacron\fi{}(g) for bottom, top, and charm quarks. From the full analytical expressions for the production cross sections, Schwinger-type interpolation formulas for all parity-parity combinations (VV, VA, AA) are derived. The parity-odd interpolation formula contains the correct limit for vanishing quark masses taking into account a residual coupling of left- and right-chiral states in the massless theory. Numerical results for the total cross section and …
Scalar diagrammatic rules for Born amplitudes in QCD
2005
We show that all Born amplitudes in QCD can be calculated from scalar propagators and a set of three- and four-valent vertices. In particular, our approach includes amplitudes with any number of quark pairs. The quarks may be massless or massive. The proof of the formalism is given entirely within quantum field theory.
On the Structure of Infrared Singularities of Gauge-Theory Amplitudes
2009
A closed formula is obtained for the infrared singularities of dimensionally regularized, massless gauge-theory scattering amplitudes with an arbitrary number of legs and loops. It follows from an all-order conjecture for the anomalous-dimension matrix of n-jet operators in soft-collinear effective theory. We show that the form of this anomalous dimension is severely constrained by soft-collinear factorization, non-abelian exponentiation, and the behavior of amplitudes in collinear limits. Using a diagrammatic analysis, we demonstrate that these constraints imply that to three-loop order the anomalous dimension involves only two-parton correlations, with the possible exception of a single c…
One-loop corrections to four-point functions with two external massive fermions and two external massless partons
2002
We present a complete set of one-loop matrix elements relevant for the hadroproduction of heavy quarks in next-to-leading order employing dimensional regularization to isolate ultraviolet and soft divergences. All results of the perturbative calculation are given in detail. These one-loop matrix elements can also be used as input in the determination of the corresponding next-to-leading order cross sections for heavy flavor photoproduction and in photon-photon reactions, as well as for any of the relevant crossed processes. Our results are tested against the results of other related studies in which unpolarized and longitudinally polarized processes were considered.
Dimensional reduction methods in QCD
1994
We apply the technique of dimensional reduction to massless quantum chromodynamics. It is shown that compared to conventional dimensional regularization methods calculations of radiative corrections at the one-loop level are less involved. We discuss the use of helicity methods within this framework and as an application we evaluate the one-loop corrections to the parity-violating cross sections and to the quark forwardbackward asymmetric polarization in\(e^ + e^ - \to V \to q\bar q(g)\). Finally, we demonstrate that further simplifications occur in the computation of structure functions including the parity-violating structure function in quark- and gluoninitiated electroproduction process…