2,089 research outputs found
PDFs, , and quark masses from global fits
The strong coupling constant and the heavy-quark masses, ,
, are extracted simultaneosly with the parton distribution functions
(PDFs) in the updated ABM12 fit including recent data from CERN-SPS, HERA,
Tevatron, and the LHC. The values of \begin{eqnarray} \nonumber
\alpha_s(M_Z)&=&0.1147\pm0.0008~({\rm exp.)},\\ \nonumber m_c(m_c)&=&1.252\pm
0.018~({\rm exp.})~{\rm GeV},\\ \nonumber m_b(m_b)&=&3.83\pm0.12~({\rm
exp.})~{\rm GeV},\\ \nonumber m_t(m_t)&=&160.9\pm1.1~({\rm exp.})~{\rm GeV}
\end{eqnarray} are obtained with the heavy-quark mass
definition being employed throughout the analysis.Comment: 7 pages, 4 figures; preprint number correcte
Iso-spin asymmetry of quark distributions and implications for single top-quark production at the LHC
We present an improved determination of the up- and down-quark distributions
in the proton using recent data on charged lepton asymmetries from
gauge-boson production at the LHC and Tevatron. The analysis is performed in
the framework of a global fit of parton distribution functions. The fit results
are consistent with a non-zero iso-spin asymmetry of the sea, , at small values of Bjorken indicating a delayed onset of
the Regge asymptotics of a vanishing -asymmetry at
small-. We compare with up- and down-quark distributions available in the
literature and provide accurate predictions for the production of single
top-quarks at the LHC, a process which can serve as a standard candle for the
light quark flavor content of the proton.Comment: 21 pages, 13 figure
Determination of Strange Sea Quark Distributions from Fixed-target and Collider Data
We present an improved determination of the strange sea distribution in the
nucleon with constraints coming from the recent charm production data in
neutrino-nucleon deep-inelastic scattering by the NOMAD and CHORUS experiments
and from charged current inclusive deep-inelastic scattering at HERA. We
demonstrate that the results are consistent with the data from the ATLAS and
the CMS experiments on the associated production of -bosons with
-quarks. We also discuss issues related to the recent strange sea
determination by the ATLAS experiment using LHC collider data.Comment: 23 pages, 14 figure
Prompt neutrino fluxes in the atmosphere with PROSA parton distribution functions
Effects on atmospheric prompt neutrino fluxes of present uncertainties
affecting the nucleon composition are studied by using the PROSA fit to parton
distribution functions (PDFs). The PROSA fit extends the precision of the PDFs
to low x, which is the kinematic region of relevance for high-energy neutrino
production, by taking into account LHCb data on charm and bottom
hadroproduction. In the range of neutrino energies explored by present Very
Large Volume Neutrino Telescopes, it is found that PDF uncertainties are far
smaller with respect to those due to renormalization and factorization scale
variation and to assumptions on the cosmic ray composition, which at present
dominate and limit our knowledge of prompt neutrino fluxes. A discussion is
presented on how these uncertainties affect the expected number of atmospheric
prompt neutrino events in the analysis of high-energy events characterized by
interaction vertices fully contained within the instrumented volume of the
detector, performed by the IceCube collaboration.Comment: 36 pages, 17 figures, 1 tabl
A Critical Appraisal and Evaluation of Modern PDFs
We review the present status of the determination of parton distribution
functions (PDFs) in the light of the precision requirements for the LHC in Run
2 and other future hadron colliders. We provide brief reviews of all currently
available PDF sets and use them to compute cross sections for a number of
benchmark processes, including Higgs boson production in gluon-gluon fusion at
the LHC. We show that the differences in the predictions obtained with the
various PDFs are due to particular theory assumptions made in the fits of those
PDFs. We discuss PDF uncertainties in the kinematic region covered by the LHC
and on averaging procedures for PDFs, such as advocated by the PDF4LHC15 sets,
and provide recommendations for the usage of PDF sets for theory predictions at
the LHC.Comment: 70 pages pdflatex, 19 figures, 17 tables; final versio
Fast shower simulation in the ATLAS calorimeter
The time to simulate pp collisions in the ATLAS detector is largely dominated by the showering of electromagnetic particles in the heavy parts of the detector, especially the electromagnetic barrel and endcap calorimeters. Two procedures have been developed to accelerate the processing time of electromagnetic particles in these regions: (1) a fast shower parameterisation and (2) a frozen shower library. Both work by generating the response of the calorimeter to electrons and positrons with Geant 4, and then reintroduce the response into the simulation at runtime.
In the fast shower parameterisation technique, a parameterisation is tuned to single electrons and used later by simulation. In the frozen shower technique, actual showers from low-energy particles are used in the simulation. Full Geant 4 simulation is used to develop showers down to ~1 GeV, at which point the shower is terminated by substituting a frozen shower. Judicious use of both techniques over the entire electromagnetic portion of the ATLAS calorimeter produces an important improvement of CPU time. We discuss the algorithms and their performance in this paper
Parton distribution functions, alpha(s), and heavy-quark masses for LHC Run II
We determine a new set of parton distribution functions (ABMP16), the strong
coupling constant and the quark masses , and in a
global fit to next-to-next-to-leading order (NNLO) in QCD. The analysis uses
the scheme for and all quark masses and is
performed in the fixed-flavor number scheme for . Essential new
elements of the fit are the combined data from HERA for inclusive
deep-inelastic scattering (DIS), data from the fixed-target experiments NOMAD
and CHORUS for neutrino-induced DIS, and data from Tevatron and the LHC for the
Drell-Yan process and the hadro-production of single-top and top-quark pairs.
The theory predictions include new improved approximations at NNLO for the
production of heavy quarks in DIS and for the hadro-production of single-top
quarks. The description of higher twist effects relevant beyond the leading
twist collinear factorization approximation is refined. At NNLO we obtain the
value .Comment: 70 pages pdflatex, 34 figures, 15 table
The photon PDF from high-mass Drell Yan data at the LHC
Achieving the highest precision for theoretical predictions at the LHC requires the calculation of hard-scattering cross-sections that include perturbative QCD corrections up to (N)NNLO and electroweak (EW) corrections up to NLO. Parton distribution functions (PDFs) need to be provided with matching accuracy, which in the case of QED effects involves introducing the photon parton distribution of the proton, . In this work a determination of the photon PDF from fits to recent ATLAS measurements of high-mass Drell-Yan dilepton production at TeV is presented. This analysis is based on the xFitter framework, and has required improvements both in the APFEL program, to account for NLO QED effects, and in the aMCfast interface to account for the photon-initiated contributions in the EW calculations within MadGraph5_aMC@NLO. The results are compared with other recent QED fits and determinations of the photon PDF, consistent results are found
The Geant4-Based ATLAS Fast Electromagnetic Shower Simulation
We present a three-pronged approach to fast electromagnetic shower simulation in ATLAS. Parameterisation is used for high-energy, shower libraries for medium-energy, and an averaged energy deposition for very low-energy particles. We present a comparison between the fast simulation and full simulation in an ATLAS Monte Carlo production
- âŠ