[Show abstract][Hide abstract] ABSTRACT: We show that in studies of light quark- and gluon-initiated jet
discrimination, it is important to include the information on softer
reconstructed jets (associated jets) around a primary hard jet. This is
particularly relevant while adopting a small radius parameter for
reconstructing hadronic jets. The probability of having an associated jet as a
function of the primary jet transverse momentum ($p_T$) and radius, the minimum
associated jet $p_T$ and the association radius is computed upto next-to-double
logarithmic accuracy (NDLA), and the predictions are compared with results from
Herwig++, Pythia6 and Pythia8 Monte Carlos (MC). We demonstrate the improvement
in quark-gluon discrimination on using the associated jet rate variable with
the help of a multivariate analysis. The associated jet rates are found to be
only mildly sensitive to the choice of parton shower and hadronization
algorithms, as well as to the effects of initial state radiation and underlying
event. In addition, the number of $k_T$ subjets of an anti-$k_T$ jet is found
to be an observable that leads to a rather uniform prediction across different
MC's, broadly being in agreement with predictions in NDLA, as compared to the
often used number of charged tracks observable.
Journal of High Energy Physics 01/2015; 2015(4). DOI:10.1007/JHEP04(2015)131 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We compute the resummed hadronic transverse-energy ($E_T$) distribution due
to initial-state QCD radiation in the production of a Standard Model Higgs
boson of mass 126 GeV by gluon fusion at the Large Hadron Collider, with
matching to next-to-leading order calculations at large $E_T$. Effects of
hadronization, underlying event and limited detector acceptance are estimated
using aMC@NLO with the Herwig++ event generator.
Journal of High Energy Physics 03/2014; 2014(9). DOI:10.1007/JHEP09(2014)056 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We point out that QCD coherence effects can help to identify the colour
structure of possible new physics contributions to the anomalously large
forward-backward asymmetry in top quark pair production. New physics models
that yield the same inclusive asymmetry make different predictions for its
dependence on the transverse momentum of the pair, if they have different
colour structures. From both a fixed-order effective field theory approach and
Monte Carlo studies of specific models, we find that an s-channel octet
structure is preferred.
Journal of High Energy Physics 09/2013; 2013(11). DOI:10.1007/JHEP11(2013)105 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The leading-order accurate description of top quark pair production, as
usually employed in standard Monte Carlo event generators, gives no rise to the
generation of a forward--backward asymmetry. Yet, non-negligible --
differential as well as inclusive -- asymmetries may be produced if coherent
parton showering is used in the hadroproduction of top quark pairs. In this
contribution we summarize the outcome of our study of this effect. We present a
short comparison of different parton shower implementations and briefly comment
on the phenomenology of the colour coherence effect at the Tevatron.
The European Physical Journal Conferences 02/2013; 49. DOI:10.1051/epjconf/20134917001
[Show abstract][Hide abstract] ABSTRACT: We derive generating functions, valid to next-to-double logarithmic accuracy,
for QCD jet rates according to the inclusive forms of the kt, Cambridge/Aachen
and anti-kt algorithms, which are equivalent at this level of accuracy. We
compare the analytical results with jet rates and average jet multiplicities
from the SHERPA event generator, and study the transition between Poisson-like
and staircase-like behaviour of jet ratios.
Journal of High Energy Physics 12/2012; 2013(4). DOI:10.1007/JHEP04(2013)089 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We propose a method for reconstructing the mass of a particle, such as the
Higgs boson, decaying into a pair of tau leptons, of which one subsequently
undergoes a 3-prong decay. The kinematics is solved using information from the
visible decay products, the missing transverse momentum, and the 3-prong tau
decay vertex, with the detector resolution taken into account using a
likelihood method. The method is shown to give good discrimination between a
125 GeV Higgs boson signal and the dominant backgrounds, such as Z decays to
tau tau and W plus jets production. As a result, we find an improvement,
compared to existing methods for this channel, in the discovery potential, as
well as in measurements of the Higgs boson mass and production cross section
times branching ratio.
Journal of High Energy Physics 10/2012; 2013(3). DOI:10.1007/JHEP03(2013)106 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Coherent QCD radiation in the hadroproduction of top quark pairs leads to a
forward--backward asymmetry that grows more negative with increasing transverse
momentum of the pair. This feature is present in Monte Carlo event generators
with coherent parton showering, even though the production process is treated
at leading order and has no intrinsic asymmetry before showering. In addition,
depending on the treatment of recoils, showering can produce a positive
contribution to the inclusive asymmetry. We explain the origin of these
features, compare them in fixed-order calculations and the Herwig++, Pythia and
Sherpa event generators, and discuss their implications.
Journal of High Energy Physics 05/2012; 2012(7). DOI:10.1007/JHEP07(2012)151 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We review the methods developed for combining the parton shower approximation
to QCD with fixed-order perturbation theory, in such a way as to achieve
next-to-leading-order (NLO) accuracy for inclusive observables. This has made
it possible to generate fully-simulated hadronic final states with the
precision and stability of NLO calculations. We explain the underlying theory
of the existing methods, MC@NLO and POWHEG, together with their similarities,
differences, achievements and limitations. For illustration we mainly compare
results on Higgs boson production at the LHC, with particular emphasis on the
residual uncertainties arising from the different treatment of effects beyond
NLO. We also briefly summarize the difference between these NLO + parton shower
methods and matrix-element + parton shower matching, and current efforts to
combine the two approaches.
Annual Review of Nuclear and Particle Science 02/2012; 62(1). DOI:10.1146/annurev-nucl-102711-094928 · 11.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: These are the proceedings of the "Workshop on Precision Measurements of
alphas" held at the Max-Planck-Institute for Physics, Munich, February 9-11,
2011. The workshop explored in depth the determination of alphas(mZ) in the
MS-bar scheme from the key categories where high precision measurements are
currently being made, including DIS and global PDF fits, tau-decays,
electroweak precision observables and Z-decays, event-shapes, and lattice QCD.
These proceedings contain a short summary contribution from the speakers, as
well as the lists of authors, conveners, participants, and talks.
[Show abstract][Hide abstract] ABSTRACT: We consider the problem of reconstructing energies, momenta, and masses in
collider events with missing energy, along with the complications introduced by
combinatorial ambiguities and measurement errors. Typically, one reconstructs
more than one value and we show how the wrong values may be correlated with the
right ones. The problem has a natural formulation in terms of the theory of
Riemann surfaces. We discuss examples including top quark decays in the
Standard Model (relevant for top quark mass measurements and tests of spin
correlation), cascade decays in models of new physics containing dark matter
candidates, decays of third-generation leptoquarks in composite models of
electroweak symmetry breaking, and Higgs boson decay into two tau leptons.
Journal of High Energy Physics 03/2011; 9(9). DOI:10.1007/JHEP09(2011)140 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We review the physics basis, main features and use of general-purpose Monte
Carlo event generators for the simulation of proton-proton collisions at the
Large Hadron Collider. Topics included are: the generation of hard-scattering
matrix elements for processes of interest, at both leading and next-to-leading
QCD perturbative order; their matching to approximate treatments of higher
orders based on the showering approximation; the parton and dipole shower
formulations; parton distribution functions for event generators;
non-perturbative aspects such as soft QCD collisions, the underlying event and
diffractive processes; the string and cluster models for hadron formation; the
treatment of hadron and tau decays; the inclusion of QED radiation and
beyond-Standard-Model processes. We describe the principal features of the
ARIADNE, Herwig++, PYTHIA 8 and SHERPA generators, together with the Rivet and
Professor validation and tuning tools, and discuss the physics philosophy
behind the proper use of these generators and tools. This review is aimed at
phenomenologists wishing to understand better how parton-level predictions are
translated into hadron-level events as well as experimentalists wanting a
deeper insight into the tools available for signal and background simulation at
the LHC.
[Show abstract][Hide abstract] ABSTRACT: Fermion masses may arise via mixing of elementary fermions with composite
fermions of a strong sector in scenarios of strongly-coupled electroweak
symmetry breaking. The strong sector may contain leptoquark states with masses
as light as several hundred GeV. In the present study we focus on the scalar
modes of such leptoquarks since their bosonic couplings are determined
completely and hence their production cross sections only depend on their
masses. We study all the possible gauge-invariant non-derivative and
single-derivative couplings of the scalar leptoquarks to the quarks and
leptons, which turn out to be, predominantly, of the third generation. We
examine their phenomenology and outline search strategies for their dominant
decay modes at the LHC.
Journal of High Energy Physics 10/2010; 2011(1). DOI:10.1007/JHEP01(2011)156 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This is the user's manual of MC@NLO 4.0. This package is a practical implementation, based upon the Fortran HERWIG and Herwig++ event generators, of the MC@NLO formalism, which allows one to incorporate NLO QCD matrix elements consistently into a parton shower framework. Processes available in this version include the hadroproduction of single vector and Higgs bosons, vector boson pairs, heavy quark pairs, single top, single top in association with a W, single top in association with a charged Higgs in type I or II 2HDM models, lepton pairs, and Higgs bosons in association with a W or Z. Spin correlations are included for all processes except ZZ production. This document is self-contained, but we emphasise the main differences with respect to previous versions. Comment: 36 pages, no figures
[Show abstract][Hide abstract] ABSTRACT: We present the calculations necessary to obtain next-to-leading order QCD
precision with the Herwig++ event generator using the MC@NLO approach, and
implement them for all the processes that were previously available from
Fortran HERWIG with MC@NLO. We show a range of results comparing the two
implementations. With these calculations and recent developments in the
automatic generation of NLO matrix elements, it will be possible to obtain NLO
precision with Herwig++ for a much wider range of processes
Journal of High Energy Physics 10/2010; 2011(1). DOI:10.1007/JHEP01(2011)053 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We re-evaluate the non-perturbative contribution to the thrust distribution in $e^+e^-\to$ hadrons, in the light of the latest experimental data and the recent NNLO perturbative calculation of this quantity. By extending the calculation to NNLO+NLL accuracy, we perform the most detailed study to date of the effects of non-perturbative physics on this observable. In particular, we investigate how well a model based on a low-scale QCD effective coupling can account for such effects. We find that the difference between the improved perturbative distribution and the experimental data is consistent with a $1/Q$-dependent non-perturbative shift in the distribution, as predicted by the effective coupling model. Best fit values of $\alpha_s(M_Z) = 0.1164^{+0.0028}_{-0.0026}$ and $\alpha_0(2 GeV)=0.59+/-0.03$ are obtained with $\chi^2/d.o.f.=1.09$. This is consistent with NLO+NLL results but the quality of fit is improved. The agreement in $\alpha_0$ is non-trivial because a part of the 1/Q-dependent contribution (the infrared renormalon) is included in the NNLO perturbative correction.
European Physical Journal C 09/2010; 69(1):341-341. DOI:10.1140/epjc/s10052-010-1378-3 · 5.08 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2158 new measurements from 551 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on neutrino mass, mixing, and oscillations, QCD, top quark, CKM quark-mixing matrix, V_(ud) & V_(us), V_(cb) & V_(ub), fragmentation functions, particle detectors for accelerator and non-accelerator physics, magnetic monopoles, cosmological parameters, and big bang cosmology. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: pdg.lbl.gov.
Journal of Physics G Nuclear and Particle Physics 07/2010; 86(1). DOI:10.1088/0954-3899/37/7A/075021 · 2.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A method is proposed for determining the masses of the new particles N,X,Y,Z in collider events containing a pair of effectively identical decay chains Z to Y+jet, Y to X+l_1, X to N+l_2, where l_1, l_2 are opposite-sign same-flavour charged leptons and N is invisible. By first determining the upper edge of the dilepton invariant mass spectrum, we reduce the problem to a curve for each event in the 3-dimensional space of mass-squared differences. The region through which most curves pass then determines the unknown masses. A statistical approach is applied to take account of mismeasurement of jet and missing momenta. The method is easily visualized and rather robust against combinatorial ambiguities and finite detector resolution. It can be successful even for small event samples, since it makes full use of the kinematical information from every event. Comment: 12 pages, 5 figures
Journal of High Energy Physics 05/2010; 2010(6). DOI:10.1007/JHEP06(2010)069 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We examine the effects of invisible particle emission in conjunction with QCD initial state radiation (ISR) on quantities designed to probe the mass scale of new physics at hadron colliders, which involve longitudinal as well as transverse final-state momenta. This is an extension of our previous treatment, arXiv:0903.2013, of the effects of ISR on global inclusive variables. We present resummed results on the visible invariant mass distribution and compare them to parton-level Monte Carlo results for top quark and gluino pair-production at the LHC. There is good agreement as long as the visible pseudorapidity interval is large enough (eta ~ 3). The effect of invisible particle emission is small in the case of top pair production but substantial for gluino pair production. This is due mainly to the larger mass of the intermediate particles in gluino decay (squarks rather than W-bosons). We also show Monte Carlo modelling of the effects of hadronization and the underlying event. The effect of the underlying event is large but may be approximately universal. Comment: 22 pages, expanded sections and other minor modifications. Version published in JHEP
Journal of High Energy Physics 04/2010; 2010(7). DOI:10.1007/JHEP07(2010)018 · 6.11 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We compute the resummed hadronic transverse energy (E_T) distribution due to initial-state QCD radiation in vector boson and Higgs boson production at hadron colliders. The resummed exponent, parton distributions and coefficient functions are treated consistently to next-to-leading order. The results are matched to fixed-order calculations at large E_T and compared with parton-shower Monte Carlo predictions at Tevatron and LHC energies. Comment: 24 pages, 15 figures
Journal of High Energy Physics 02/2010; 2010(4). DOI:10.1007/JHEP04(2010)084 · 6.11 Impact Factor