271 research outputs found
Hadronic final states in deep-inelastic scattering with Sherpa
We extend the multi-purpose Monte Carlo event generator Sherpa to include processes in deeply inelastic leptonânucleon scattering. Hadronic final states in this kinematical setting are characterised by the presence of multiple kinematical scales, which were up to now accounted for only by specific resummations in individual kinematical regions. Using an extension of the recently introduced method for merging truncated parton showers with higher-order tree-level matrix elements, it is possible to obtain predictions which are reliable in all kinematical limits. Different hadronic final states, defined by jets or individual hadrons, in deep-inelastic scattering are analysed and the corresponding results are compared to HERA data. The various sources of theoretical uncertainties of the approach are discussed and quantified. The extension to deeply inelastic processes provides the opportunity to validate the merging of matrix elements and parton showers in multi-scale kinematics inaccessible in other collider environments. It also allows to use HERA data on hadronic final states in the tuning of hadronisation models
High multiplicity W+jets predictions at NLO
In these proceedings we present results from a recent calculation for the
production of a W boson in conjunction with five jets at next-to-leading order
in perturbative QCD. We also use results at lower multiplicities to extrapolate
the cross section to the same process with six jets.Comment: 5 pages, Proceedings for the DIS2013 conferenc
Matching Tree-Level Matrix Elements with Interleaved Showers
We present an implementation of the so-called CKKW-L merging scheme for
combining multi-jet tree-level matrix elements with parton showers. The
implementation uses the transverse-momentum-ordered shower with interleaved
multiple interactions as implemented in PYTHIA8. We validate our procedure
using e+e--annihilation into jets and vector boson production in hadronic
collisions, with special attention to details in the algorithm which are
formally sub-leading in character, but may have visible effects in some
observables. We find substantial merging scale dependencies induced by the
enforced rapidity ordering in the default PYTHIA8 shower. If this rapidity
ordering is removed the merging scale dependence is almost negligible. We then
also find that the shower does a surprisingly good job of describing the
hardness of multi-jet events, as long as the hardest couple of jets are given
by the matrix elements. The effects of using interleaved multiple interactions
as compared to more simplistic ways of adding underlying-event effects in
vector boson production are shown to be negligible except in a few sensitive
observables. To illustrate the generality of our implementation, we also give
some example results from di-boson production and pure QCD jet production in
hadronic collisions.Comment: 44 pages, 23 figures, as published in JHEP, including all changes
recommended by the refere
Interference effects in the H(â γγ) + 2 jets channel at the LHC
We compute the interference between the resonant process pp â H(â γγ) + 2 jets and the corresponding continuum background at leading order in QCD. For the Higgs signal, we include gluon fusion (GF) and vector boson fusion (VBF) production channels, while for the background we consider all tree-level contributions, including pure EW effects (O(α 4 QED)) and QCD contributions (O(α 2 QEDα 2 s )), plus the loopinduced gluon-initiated process. After convolution with the experimental mass resolution, the main effect of the interference is to shift the position of the mass peak, as in the inclusive GF case studied previously. The apparent mass shift is small in magnitude but strongly dependent on the Higgs width, potentially allowing for a measurement of, or bound on, the width itself. In the H(â γγ) + 2 jets channel, the VBF and GF contributions generate shifts of opposite signs which largely cancel, depending on the sets of cuts used, to as little as 5 MeV (toward a lower Higgs mass). The small magnitude of the shift makes this channel a good reference mass for measuring the inclusive mass shift of around 60 MeV in the Standard Model.Fil: Coradeschi, F.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentina. UniversitĂ degli Studi di Firenze; ItaliaFil: de Florian, Daniel Enrique. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Ciudad Universitaria. Instituto de FĂsica de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de FĂsica de Buenos Aires; ArgentinaFil: Dixon, L. J.. University of Stanford; Estados Unidos. Walter Burke Institute for Theoretical Physics; Estados UnidosFil: Fidanza Romani, Nerina Andrea. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; ArgentinaFil: Höche, S.. University of Stanford; Estados UnidosFil: Ita, H.. Albert Ludwigs UniversitĂ€t Freiburg; AlemaniaFil: Li, Y.. University of Stanford; Estados UnidosFil: Mazzitelli, Javier SebastiĂĄn. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de FĂsica; Argentin
The Sigma 13 (10-14) twin in alpha-Al2O3: A model for a general grain boundary
The atomistic structure and energetics of the Sigma 13 (10-14)[1-210]
symmetrical tilt grain boundary in alpha-Al2O3 are studied by first-principles
calculations based on the local-density-functional theory with a mixed-basis
pseudopotential method. Three configurations, stable with respect to
intergranular cleavage, are identified: one Al-terminated glide-mirror twin
boundary, and two O-terminated twin boundaries, with glide-mirror and two-fold
screw-rotation symmetries, respectively. Their relative energetics as a
function of axial grain separation are described, and the local electronic
structure and bonding are analysed. The Al-terminated variant is predicted to
be the most stable one, confirming previous empirical calculations, but in
contrast with high-resolution transmission electron microscopy observations on
high-purity diffusion-bonded bicrystals, which resulted in an O-terminated
structure.
An explanation of this discrepancy is proposed, based on the different
relative energetics of the internal interfaces with respect to the free
surfaces
Phenomenology of iron-assisted ion beam pattern formation on Si(001)
Pattern formation on Si(001) through 2 keV Kr+ ion beam erosion of Si(001) at an incident angle of # = 30° and in the presence of sputter codeposition or co-evaporation of Fe is investigated by using in situ scanning tunneling microscopy, ex situ atomic force microscopy and electron microscopy. The phenomenology of pattern formation is presented, and experiments are conducted to rule out or determine the processes of relevance in ion beam pattern formation on Si(001) with impurities. Special attention is given to the determination of morphological phase boundaries and their origin. Height fluctuations, local flux variations, induced chemical inhomogeneities, silicide formation and ensuing composition-dependent sputtering are found to be of relevance for pattern formation
Phenomenology of iron-assisted ion beam pattern formation on Si(001)
Pattern formation on Si(001) through 2 keV Kr+ ion beam erosion of Si(001) at an incident angle of # = 30° and in the presence of sputter codeposition or co-evaporation of Fe is investigated by using in situ scanning tunneling microscopy, ex situ atomic force microscopy and electron microscopy. The phenomenology of pattern formation is presented, and experiments are conducted to rule out or determine the processes of relevance in ion beam pattern formation on Si(001) with impurities. Special attention is given to the determination of morphological phase boundaries and their origin. Height fluctuations, local flux variations, induced chemical inhomogeneities, silicide formation and ensuing composition-dependent sputtering are found to be of relevance for pattern formation
Extrapolating W-associated jet-production ratios at the LHC
Electroweak vector-boson production, accompanied by multiple jets, is an important background to searches for physics beyond the standard model. A precise and quantitative understanding of this process is helpful in constraining deviations from known physics. We study four key ratios in W+n-jet production at the LHC. We compute the ratio of cross sections for W+n- to W+(nâ1)-jet production as a function of the minimum jet transverse momentum. We also study the ratio differentially, as a function of the W-boson transverse momentum; as a function of the scalar sum of the jet transverse energy, HjetsT; and as a function of certain jet transverse momenta. We show how to use such ratios to extrapolate differential cross sections to W+6-jet production at next-to-leading order, and we cross-check the method against a direct calculation at leading order. We predict the differential distribution in HjetsT for W+6 jets at next-to-leading order using such an extrapolation. We use the BlackHat software library together with SHERPA to perform the computations
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