204 research outputs found
Jet energy calibration at the LHC
Jets are one of the most prominent physics signatures of high energy proton
proton (p-p) collisions at the Large Hadron Collider (LHC). They are key
physics objects for precision measurements and searches for new phenomena. This
review provides an overview of the reconstruction and calibration of jets at
the LHC during its first Run. ATLAS and CMS developed different approaches for
the reconstruction of jets, but use similar methods for the energy calibration.
ATLAS reconstructs jets utilizing input signals from their calorimeters and use
charged particle tracks to refine their energy measurement and suppress the
effects of multiple p-p interactions (pileup). CMS, instead, combines
calorimeter and tracking information to build jets from particle flow objects.
Jets are calibrated using Monte Carlo (MC) simulations and a residual in situ
calibration derived from collision data is applied to correct for the
differences in jet response between data and Monte Carlo. Large samples of
dijet, Z+jets, and photon+jet events at the LHC allowed the calibration of jets
with high precision, leading to very small systematic uncertainties. Both ATLAS
and CMS achieved a jet energy calibration uncertainty of about 1% in the
central detector region and for jets with transverse momentum pT>100 GeV. At
low jet pT, the jet energy calibration uncertainty is less than 4%, with
dominant contributions from pileup, differences in energy scale between quark
and gluon jets, and jet flavor composition.Comment: Article submitted to the International Journal of Modern Physics A
(IJMPA) as part of the special issue on the "Jet Measurements at the LHC",
editor G. Dissertor
Jet-Images: Computer Vision Inspired Techniques for Jet Tagging
We introduce a novel approach to jet tagging and classification through the
use of techniques inspired by computer vision. Drawing parallels to the problem
of facial recognition in images, we define a jet-image using calorimeter towers
as the elements of the image and establish jet-image preprocessing methods. For
the jet-image processing step, we develop a discriminant for classifying the
jet-images derived using Fisher discriminant analysis. The effectiveness of the
technique is shown within the context of identifying boosted hadronic W boson
decays with respect to a background of quark- and gluon- initiated jets. Using
Monte Carlo simulation, we demonstrate that the performance of this technique
introduces additional discriminating power over other substructure approaches,
and gives significant insight into the internal structure of jets
Fuzzy Jets
Collimated streams of particles produced in high energy physics experiments
are organized using clustering algorithms to form jets. To construct jets, the
experimental collaborations based at the Large Hadron Collider (LHC) primarily
use agglomerative hierarchical clustering schemes known as sequential
recombination. We propose a new class of algorithms for clustering jets that
use infrared and collinear safe mixture models. These new algorithms, known as
fuzzy jets, are clustered using maximum likelihood techniques and can
dynamically determine various properties of jets like their size. We show that
the fuzzy jet size adds additional information to conventional jet tagging
variables. Furthermore, we study the impact of pileup and show that with some
slight modifications to the algorithm, fuzzy jets can be stable up to high
pileup interaction multiplicities
Jets from Jets: Re-clustering as a tool for large radius jet reconstruction and grooming at the LHC
Jets with a large radius and grooming algorithms are widely used
to fully capture the decay products of boosted heavy particles at the Large
Hadron Collider (LHC). Unlike most discriminating variables used in such
studies, the jet radius is usually not optimized for specific physics
scenarios. This is because every jet configuration must be calibrated, insitu,
to account for detector response and other experimental effects. One solution
to enhance the availability of large- jet configurations used by the LHC
experiments is {\it jet re-clustering}. Jet re-clustering introduces an
intermediate scale at which jets are calibrated and used as the inputs to
reconstruct large radius jets. In this paper we systematically study and
propose new jet re-clustering configurations and show that re-clustered large
radius jets have essentially the same jet mass performance as large radius
groomed jets. Jet re-clustering has the benefit that no additional large-R
calibration is necessary, allowing the re-clustered large radius parameter to
be optimized in the context of specific precision measurements or searches for
new physics
Simplified Models for LHC New Physics Searches
This document proposes a collection of simplified models relevant to the
design of new-physics searches at the LHC and the characterization of their
results. Both ATLAS and CMS have already presented some results in terms of
simplified models, and we encourage them to continue and expand this effort,
which supplements both signature-based results and benchmark model
interpretations. A simplified model is defined by an effective Lagrangian
describing the interactions of a small number of new particles. Simplified
models can equally well be described by a small number of masses and
cross-sections. These parameters are directly related to collider physics
observables, making simplified models a particularly effective framework for
evaluating searches and a useful starting point for characterizing positive
signals of new physics. This document serves as an official summary of the
results from the "Topologies for Early LHC Searches" workshop, held at SLAC in
September of 2010, the purpose of which was to develop a set of representative
models that can be used to cover all relevant phase space in experimental
searches. Particular emphasis is placed on searches relevant for the first
~50-500 pb-1 of data and those motivated by supersymmetric models. This note
largely summarizes material posted at http://lhcnewphysics.org/, which includes
simplified model definitions, Monte Carlo material, and supporting contacts
within the theory community. We also comment on future developments that may be
useful as more data is gathered and analyzed by the experiments.Comment: 40 pages, 2 figures. This document is the official summary of results
from "Topologies for Early LHC Searches" workshop (SLAC, September 2010).
Supplementary material can be found at http://lhcnewphysics.or
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