Searcharxiv⌕ Search

arXiv subjects

David A. Kosower

Publications and source records attributed to David A. Kosower.

At least 55 records · Page 3Linked to original sources
arXiv2006

QCD at the Dawn of the LHC Era

Precision computations of QCD and QCD+electroweak emission scattering processes will play an important role in understanding experimental data from the LHC, and thereby in uncovering new physics beyond the Standard Model. Recent years have seen substantial progress in expanding our ability to perform and implement such calculations. I discuss several different topics: the resolution of seeming discrepancies between heavy-quark predictions and measurements; the ongoing program of next-to-next-to-leading order calculations; and also progress in merging the parton-shower and matrix-element calculations. I will also outline some of the techniques emerging from twistor-space string theory and its intellectual progeny.

hep-ph↗
arXiv2006

Bootstrapping Multi-Parton Loop Amplitudes in QCD

We present a new method for computing complete one-loop amplitudes, including their rational parts, in non-supersymmetric gauge theory. This method merges the unitarity method with on-shell recursion relations. It systematizes a unitarity-factorization bootstrap approach previously applied by the authors to the one-loop amplitudes required for next-to-leading order QCD corrections to the processes e^+e^- -> Z,γ^* -> 4 jets and pp -> W + 2 jets. We illustrate the method by reproducing the one-loop color-ordered five-gluon helicity amplitudes in QCD that interfere with the tree amplitude, namely A_{5;1}(1^-,2^-,3^+,4^+,5^+) and A_{5;1}(1^-,2^+,3^-,4^+,5^+). Then we describe the construction of the six- and seven-gluon amplitudes with two adjacent negative-helicity gluons, A_{6;1}(1^-,2^-,3^+,4^+,5^+,6^+) and A_{7;1}(1^-,2^-,3^+,4^+,5^+,6^+,7^+), which uses the previously-computed logarithmic parts of the amplitudes as input. We present a compact expression for the six-gluon amplitude. No loop integrals are required to obtain the rational parts.

hep-ph↗
arXiv2005

On-Shell Recurrence Relations for One-Loop QCD Amplitudes

We present examples of on-shell recurrence relations for determining rational functions appearing in one-loop QCD amplitudes. In particular, we give relations for one-loop QCD amplitudes with all legs of positive helicity, or with one leg of negative helicity and the rest of positive helicity. Our recursion relations are similar to the tree-level ones described by Britto, Cachazo, Feng and Witten. A number of new features arise for loop amplitudes in non-supersymmetric theories like QCD, including boundary terms and double poles. We show how to eliminate the boundary terms, which would interfere with obtaining useful relations. Using the relations we give compact explicit expressions for the n-gluon amplitudes with one negative-helicity gluon, up through n=7.

hep-th↗
arXiv2005

Antenna Factorization in Strongly-Ordered Limits

When energies or angles of gluons emitted in a gauge-theory process are small and strongly ordered, the emission factorizes in a simple way to all orders in perturbation theory. I show how to unify the various strongly-ordered soft, mixed soft-collinear, and collinear limits using antenna factorization amplitudes, which are generalizations of the Catani--Seymour dipole factorization function.

hep-ph↗
arXiv2005

Twistor-Inspired Construction of Electroweak Vector Boson Currents

We present an extension of the twistor-motivated MHV vertices and accompanying rules presented by Cachazo, Svrvcek and Witten to the construction of vector-boson currents coupling to an arbitrary source. In particular, we give rules for constructing off-shell vector-boson currents with one fermion pair and n gluons of arbitrary helicity. These currents may be employed directly in the computation of electroweak amplitudes. The rules yield expressions in agreement with previously-obtained results for Z,W,γ^* --> qbar q + n gluons (analytically up to n=3, beyond via the Berends--Giele recursion relations). We also confirm that the contribution to a seven-point amplitude containing the non-abelian triple vector-boson coupling obtained using the next-to-MHV currents matches the previous result in the literature.

hep-ph↗
arXiv2005

All Non-Maximally-Helicity-Violating One-Loop Seven-Gluon Amplitudes in N=4 Super-Yang-Mills Theory

We compute the non-MHV one-loop seven-gluon amplitudes in N=4 super-Yang-Mills theory, which contain three negative-helicity gluons and four positive-helicity gluons. There are four independent color-ordered amplitudes, (- - - + + + +), (- - + - + + +), (- - + + -+ +) and (- + - + - + +). The MHV amplitudes containing two negative-helicity and five positive-helicity gluons were computed previously, so all independent one-loop seven-gluon helicity amplitudes are now known for this theory. We present partial information about an infinite sequence of next-to-MHV one-loop helicity amplitudes, with three negative-helicity and n-3 positive-helicity gluons, and the color ordering (- - - + + ... + +); we give a new coefficient of one class of integral functions entering this amplitude. We discuss the twistor-space properties of the box-integral-function coefficients in the amplitudes, which are quite simple and suggestive.

hep-th↗
arXiv2005

Loops in Twistor Space

We elucidate the one-loop twistor-space structure corresponding to momentum-space MHV diagrams. We also discuss the infrared divergences, and argue that only a limited set of MHV diagrams contain them. We show how to introduce a twistor-space regulator corresponding to dimensional regularization for the infrared-divergent diagrams. We also evaluate explicitly the `holomorphic anomaly' pointed out by Cachazo, Svrcek, and Witten, and use the result to define modified differential operators which can be used to probe the twistor-space structure of one-loop amplitudes.

hep-th↗
arXiv2004

Two-Loop g -> gg Splitting Amplitudes in QCD

Splitting amplitudes are universal functions governing the collinear behavior of scattering amplitudes for massless particles. We compute the two-loop g -> gg splitting amplitudes in QCD, N=1, and N=4 super-Yang-Mills theories, which describe the limits of two-loop n-point amplitudes where two gluon momenta become parallel. They also represent an ingredient in a direct x-space computation of DGLAP evolution kernels at next-to-next-to-leading order. To obtain the splitting amplitudes, we use the unitarity sewing method. In contrast to the usual light-cone gauge treatment, our calculation does not rely on the principal-value or Mandelstam-Leibbrandt prescriptions, even though the loop integrals contain some of the denominators typically encountered in light-cone gauge. We reduce the integrals to a set of 13 master integrals using integration-by-parts and Lorentz invariance identities. The master integrals are computed with the aid of differential equations in the splitting momentum fraction z. The epsilon-poles of the splitting amplitudes are consistent with a formula due to Catani for the infrared singularities of two-loop scattering amplitudes. This consistency essentially provides an inductive proof of Catani's formula, as well as an ansatz for previously-unknown 1/epsilon pole terms having non-trivial color structure. Finite terms in the splitting amplitudes determine the collinear behavior of finite remainders in this formula.

hep-ph↗
arXiv2003

Multiple Singular Emission in Gauge Theories

I derive a class of functions unifying all singular limits for the emission of a given number of soft or collinear gluons in tree-level gauge-theory amplitudes. Each function is a generalization of the single-emission antenna function of ref. [1]. The helicity-summed squares of these functions are thus also generalizations to multiple singular emission of the Catani--Seymour dipole factorization function.

hep-ph↗
arXiv2003

Evolution Kernels from Splitting Amplitudes

We recalculate the next-to-leading order Altarelli-Parisi kernel using a method which relates it to the splitting amplitudes describing the collinear factorization properties of scattering amplitudes. The method breaks up the calculation of the kernel into individual pieces which have an independent physical interpretation.

hep-ph↗
arXiv2003

All-Orders Singular Emission in Gauge Theories

I present a class of functions unifying all singular limits for the emission of soft or collinear gluons in gauge-theory amplitudes at any order in perturbation theory. Each function is a generalization of the antenna functions of ref. [1]. The helicity-summed interferences these functions are thereby also generalizations to higher orders of the Catani--Seymour dipole factorization function.

hep-ph↗
arXiv2001

Parton Distribution Function Uncertainties

We present parton distribution functions which include a quantitative estimate of its uncertainties. The parton distribution functions are optimized with respect to deep inelastic proton data, expressing the uncertainties as a density measure over the functional space of parton distribution functions. This leads to a convenient method of propagating the parton distribution function uncertainties to new observables, now expressing the uncertainty as a density in the prediction of the observable. New measurements can easily be included in the optimized sets as added weight functions to the density measure. Using the optimized method nowhere in the analysis compromises have to be made with regard to the treatment of the uncertainties.

hep-ph↗
arXiv1999

QCD Corrections to Four-jet Production and Three-jet Structure in e+ e- annihilation

We report on the general purpose numerical program MERCUTIO, which can be used to calculate any infrared safe four-jet quantity in electron-positron annihilation at next-to-leading order. The program is based on the dipole formalism and uses a remapping of phase-space in order to improve the efficiency of the Monte Carlo integration. Numerical results are given for the four-jet fraction and the D-parameter. These results are compared with already existing ones in the literature and serve as a cross-check. The program can also be used to investigate the internal structure of three-jet events at NLO. We give results for previously uncalculated observables: the jet broadening variable and the softest-jet explanarity.

hep-ph↗
arXiv1999

One-Loop Splitting Amplitudes in Gauge Theory

We recompute the functions describing the collinear factorization of one-loop amplitudes using the unitarity-based method. We present the results in a form suitable for use as an ingredient in two-loop calculations. We also present a function summarizing the behavior at one loop in both the soft and collinear limits.

hep-ph↗
arXiv1998

Antenna Factorization of Gauge-Theory Amplitudes

I derive a single factorization formula which summarizes all soft and collinear divergences of a tree-level gauge theory amplitude. The singular factor squared is in a certain sense the generalization of the Catani-Seymour dipole factorization formula.

hep-ph↗
arXiv1997

Extracting Parton Densities from Collider Data

Collider data can play an important role in determining the parton distribution functions of the nucleon. I present a formalism which makes it possible to use next-to-leading order calculations in such a determination, while minimizing the amount of numerical computation required.

hep-ph↗