231 research outputs found
Fitting EMC structure functions with intrinsic charm
A detailed study of the impact of the data collected by the European Muon
Collaboration (EMC) on the parton distribution function (PDF) of the charm
quark is presented. The analysis is performed in the NNPDF framework, and the
charm PDF is freely parametrized on equal footing as light quark and gluon
distributions. We find that variations in the treatment of EMC data do not
modify the charm PDF and do not affect our previous conclusion on the presence
of an intrinsic component in the charm PDF.Comment: 6 pages, 3 figures. To appear in the proceedings of the XXIV
International Workshop on Deep-Inelastic Scattering and Related Subjects,
11-15 April, 2016, DESY Hamburg, Germany. v2: added one reference and
corrected figure
Towards small- resummed parton distribution functions
We present preliminary results for fits of parton distribution functions
(PDFs) which include the resummation of small- logarithms at NLLx accuracy,
performed in the NNPDF framework. We observe an improvement in the description
of DIS data at small values of Bjorken when resummation effects are
included. The improvement is more marked when comparing NNLO+NLLx fits to NNLO
ones, and is particularly noticeable for small- and small- HERA
inclusive structure functions. The main effect of the resummation is an
enhancement of the gluon and singlet PDFs at small-, which persists at high
scales.Comment: 6 pages, 2 figures. To appear in the proceedings of the XXV
International Workshop on Deep-Inelastic Scattering and Related Subjects, 3-7
April 2017, University of Birmingham, U
Precision QCD at the LHC: from the structure of the proton to all-order resummations
Experiments at the LHC are collecting a wealth of data with an unprecedented
level of precision. As a consequence, the theoretical error is now starting to
lag behind the experimental one, and a ceaseless effort is required to reduce
the theory uncertainty to match the precision of the data. At hadron colliders,
QCD predictions are obtained by convoluting perturbative parton-level results
with non-perturbative PDFs, whose precise determination is crucial to reach
percent-accurate theoretical predictions. At the parton level, cross sections
are obtained through a perturbative expansion in the strong coupling. In some
cases, large terms appear at all orders and spoil the convergence of the
series. The perturbative description is rescued by resumming the series to all
orders, thereby making theory calculations accurate in regions where a
fixed-order treatment is not sufficient. In this thesis I first present two
global PDF sets where fixed-order calculations are supplemented by threshold
and high-energy resummation, respectively. In the first case, it is found that
including resummation into PDFs can compensate for the enhancement in the
partonic cross sections, with implications for high-mass resonance searches. In
the second case, resummation quantitatively improves the description of the
HERA structure functions, thus providing evidence of the onset of a new
dynamical regime of QCD in the HERA data. I then focus on Higgs production in
gluon fusion. The effect of threshold resummation on the total cross section is
found to significantly improve the convergence of the perturbative series and
to provide a robust method for estimating missing higher order uncertainty.
Finally, I present predictions for the Higgs transverse-momentum spectrum both
in the inclusive case and within fiducial cuts, exploiting a novel approach
where transverse-momentum resummation is performed in direct space.Comment: 208 pages, 47 figures, DPhil Thesi
Constraints on the quartic Higgs self-coupling from double-Higgs production at future hadron colliders
We study the indirect constraints on the quartic Higgs self-coupling that
arise from double-Higgs production at future hadron colliders. To this purpose,
we calculate the two-loop contributions to the amplitudes that
involve a modified vertex. Based on our results, we estimate the reach of
a collider operating at and
centre-of-mass energy in constraining the cubic and quartic Higgs
self-couplings by measurements of double-Higgs and triple-Higgs production in
gluon-fusion.Comment: Final version, to be published in JHE
Resummation prescriptions and ambiguities in SCET vs. direct QCD: Higgs production as a case study
We perform a comparison of soft-gluon resummation in SCET vs. direct QCD
(dQCD), using Higgs boson production in gluon fusion as a case study, with the
goal of tracing the quantitative impact of each source of difference between
the two approaches. We show that saddle-point methods enable a direct
quantitative comparison despite the fact that the scale which is resummed in
the two approaches is not the same. As a byproduct, we put in one-to-one
analytic correspondence various features of either approach: specifically, we
show how the SCET method for treating the Landau pole can be implemented in
dQCD, and how the resummation of the optimal partonic scale of dQCD can be
implemented in SCET. We conclude that the main quantitative difference comes
from power-suppressed subleading contributions, which could in fact be freely
tuned in either approach, and not really characteristic of either. This
conclusion holds for Higgs production in gluon fusion, but it is in fact
generic for processes with similar kinematics. For Higgs production, everything
else being equal, SCET resummation at NNLL in the Becher-Neubert implementation
leads to essentially no enhancement of the NNLO cross-section, unlike dQCD in
the standard implementation of Catani et al..Comment: 21 pages, 4 figures; final version, to be published in JHEP. Eq. 2.39
and subsequent discussion added, fig.1 and corresponding discussion added,
discussion on sqrt{z} prefactor added on pag.1
On the Higgs cross section at NLO+NLL and its uncertainty
We consider the inclusive production of a Higgs boson in gluon-fusion and we
study the impact of threshold resummation at next-to-next-to-next-to-leading
logarithmic accuracy (NLL) on the recently computed fixed-order prediction
at next-to-next-to-next-to-leading order (NLO). We propose a conservative,
yet robust way of estimating the perturbative uncertainty from missing higher
(fixed- or logarithmic-) orders. We compare our results with two other
different methods of estimating the uncertainty from missing higher orders: the
Cacciari-Houdeau Bayesian approach to theory errors, and the use of algorithms
to accelerate the convergence of the perturbative series. We confirm that the
best convergence happens at , and we conclude that a
reliable estimate of the uncertainty from missing higher orders on the Higgs
cross section at 13 TeV is approximately %.Comment: 27 pages, 6 figures. Version to be published in JHE
The three loop soft function for NLL gluon fusion Higgs production in SCET
We derive the three loop soft function for inclusive Higgs production in
gluon fusion, and use it to perform the resummation of the Higgs cross section
at NLL in SCET. We improve the accuracy of the resummation by
including contributions of collinear origin. We include finite top, bottom and
charm mass effect where available. These results are available through the
public code ResHiggs.Comment: 6 pages, 1 figure. Final version to be published in PR
Momentum-space resummation for transverse observables and the Higgs at NLL+NNLO
We present an approach to the momentum-space resummation of global, recursive
infrared and collinear safe observables featuring kinematic zeros away from the
Sudakov limit. In the hadro-production of a generic colour singlet, we consider
the family of inclusive observables which do not depend on the rapidity of the
radiation, prime examples being the transverse momentum of the singlet, and
in Drell-Yan pair production. We derive a resummation formula valid up
to next-to-next-to-next-to-leading-logaritmic accuracy for the considered
observables. This formula reduces exactly to the customary resummation
performed in impact-parameter space in the known cases, and it also predicts
the correct power-behaved scaling of the cross section in the limit of small
value of the observable. We show how this formalism is efficiently implemented
by means of Monte Carlo techniques in a fully exclusive generator that allows
one to apply arbitrary cuts on the Born variables for any colour singlet, as
well as to automatically match the resummed results to fixed-order
calculations. As a phenomenological application, we present state-of-the-art
predictions for the Higgs-boson transverse-momentum spectrum at the LHC at
next-to-next-to-next-to-leading-logarithmic accuracy matched to fixed
next-to-next-to-leading order.Comment: Journal versio
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