3,492 research outputs found

### An NNLO subtraction formalism in hadron collisions and its application to Higgs boson production at the LHC

We consider higher-order QCD corrections to the production of colourless
high-mass systems (lepton pairs, vector bosons, Higgs bosons,...) in hadron
collisions. We propose a new formulation of the subtraction method to
numerically compute arbitrary infrared-safe observables for this class of
processes. To cancel the infrared divergences, we exploit the universal
behaviour of the associated transverse-momentum (q_T) distributions in the
small-q_T region. The method is illustrated in general terms up to the
next-to-next-to-leading order (NNLO) in QCD perturbation theory. As a first
explicit application, we study Higgs boson production through gluon fusion. Our
calculation is implemented in a parton level Monte Carlo program that includes
the decay of the Higgs boson in two photons. We present selected numerical
results at the LHC.Comment: References adde

### Cross section of isolated prompt photons in hadron-hadron collisions

We consider the production of isolated prompt photons in hadronic collisions.
We present a general discussion in QCD perturbation theory of the isolation
criterion used by hadron collider experiments. The isolation criterion is
implemented in a computer programme of the Monte Carlo type, which evaluates
the production cross section at next-to-leading order accuracy in perturbative
QCD. The calculation includes both the direct and the fragmentation components
of the cross section, without any approximation of the dependence on the radius
R of the isolation cone. We examine the scale dependence of the isolated cross
section, the sensitivity of the cross section to the values of the isolation
parameters, and we provide a quantitative comparison between the full R
dependence and its small-R approximation.Comment: 29 pages, 1 figure, few comments slightly expanded, results
unchanged, misprints correcte

### Approximate NNLO Threshold Resummation in Heavy Flavour Decays

We present an approximate NNLO evaluation of the QCD form factor resumming
large logarithmic perturbative contributions in semi-inclusive heavy flavour
decays.Comment: 16 pages, 3 figures, Latex; minor changes; 2 figures adde

### A next-to-next-to-leading order calculation of soft-virtual cross sections

We compute the next-to-next-to-leading order (NNLO) soft and virtual QCD
corrections for the partonic cross section of colourless-final state processes
in hadronic collisions. The results are valid to all orders in the dimensional
regularization parameter \ep. The dependence of the results on a particular
process is given through finite contributions to the one and two-loop
amplitudes. To evaluate the accuracy of the soft-virtual approximation we
compare it with the full NNLO result for Drell-Yan and Higgs boson production
via gluon fusion. We also provide a universal expression for the hard
coefficient needed to perform threshold resummation up to
next-to-next-to-leading logarithmic (NNLL) accuracy.Comment: 25 pages, 4 figure

### Subtraction terms at NNLO

Perturbative calculations at next-to-next-to-leading order for multi-particle
final states require a method to cancel infrared singularities. I discuss the
subtraction method at NNLO. As a concrete example I consider the leading-colour
contributions to e+ e- --> 2 jets. This is the simplest example which exhibits
all essential features. For this example, explicit subtraction terms are given,
which approximate the four-parton and three-parton final states in all double
and single unresolved limits, such that the subtracted matrix elements can be
integrated numerically.Comment: 41 page

### Azimuthal asymmetries in QCD hard scattering: infrared safe but divergent

We consider high-mass systems of two or more particles that are produced by
QCD hard scattering in hadronic collisions. We examine the azimuthal
correlations between the system and one of its particles. We point out that the
perturbative QCD computation of such azimuthal correlations and asymmetries can
lead to divergent results at fixed perturbative orders. The fixed-order
divergences affect basic (and infrared safe) quantities such as the total cross
section at fixed (and arbitrary) values of the azimuthal-correlation angle
$\varphi$. Examples of processes with fixed-order divergences are heavy-quark
pair production, associated production of vector bosons and jets, dijet and
diboson production. A noticeable exception is the production of high-mass
lepton pairs through the Drell--Yan mechanism of quark-antiquark annihilation.
However, even in the Drell--Yan process, fixed-order divergences arise in the
computation of QED radiative corrections. We specify general conditions that
produce the divergences by discussing their physical origin in fixed-order
computations. We show lowest-order illustrative results for $\cos(n\varphi)$
asymmetries (with $n=1,2,4,6$) in top-quark pair production and associated
production of a vector boson and a jet at the LHC. The divergences are removed
by a proper all-order resummation procedure of the perturbative contributions.
Resummation leads to azimuthal asymmetries that are finite and computable. We
present first quantitative results of such a resummed computation for the
$\cos(2\varphi)$ asymmetry in top-quark pair production at the LHC.Comment: 43 pages, 5 eps figure

### Measurement of triple differential photon plus jet cross section by D0

We report on a new measurement of triple differential cross section for the
process ppbar -> photon + jet + X in ppbar collisions at sqrt(s)=1.96 TeV by
the D0 Collaboration at Fermilab based on a dataset corresponding to an
integrated luminosity of 1.1 fb-1.Comment: 3 pages, 1 figure, To appear in the Proceedings of the 2007
Europhysics Conference on High Energy Physics, Manchester, UK. Accepted for
publication in Journal of Physics : Conference Serie

### Transverse-momentum resummation for heavy-quark hadroproduction

We consider the production of a pair of heavy quarks ($Q{\bar Q}$) in
hadronic collisions. When the transverse momentum $q_T$ of the heavy-quark pair
is much smaller than its invariant mass, the QCD perturbative expansion is
affected by large logarithmic terms that must be resummed to all-orders. This
behavior is well known from the simpler case of hadroproduction of colourless
high-mass systems, such as vector or Higgs boson(s). In the case of $Q{\bar Q}$
production, the final-state heavy quarks carry colour charge and are
responsible for additional soft radiation (through direct emission and
interferences with initial-state radiation) that complicates the evaluation of
the logarithmically-enhanced terms in the small-$q_T$ region. We present the
all-order resummation structure of the logarithmic contributions, which
includes colour flow evolution factors due to soft wide-angle radiation.
Resummation is performed at the completely differential level with respect to
the kinematical variables of the produced heavy quarks. Soft-parton radiation
produces azimuthal correlations that are fully taken into account by the
resummation formalism. These azimuthal correlations are entangled with those
that are produced by initial-state collinear radiation. We present explicit
analytical results up to next-to-leading order and next-to-next-to-leading
logarithmic accuracy.Comment: Some comments expanded and references added. Version published on NP

### Threshold resummation for high-transverse-momentum Higgs production at the LHC

We study the resummation of large logarithmic QCD corrections for the process
pp ->H+ X when the Higgs boson H is produced at high transverse momentum. The
corrections arise near the threshold for partonic reaction and originate from
soft gluon emission. We perform the all-order resummation at next-to-leading
logarithmic accuracy and match the resummed result with the next-to-leading
order perturbative predictions. The effect of resummation on the Higgs
transverse momentum distribution at the LHC is discussed.Comment: 19 pages, 3 figure

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