2,751 research outputs found
Separating Electroweak and Strong interactions in Drell-Yan processes at LHC: leptons angular distributions and reference frames
Among the physics goals of LHC experiments, precision tests of the Standard
Model in the Strong and Electroweak sectors play an important role. Because of
nature of the proton-proton processes, observables based on the measurement of
the direction and energy of leptons provide the most precise signatures. In the
present paper, we concentrate on the angular distribution of Drell-Yan process
leptons, in the lepton-pair rest-frame. The vector nature of the intermediate
state imposes that distributions are to a good precision described by spherical
polynomials of at most second order.
We show that with the proper choice of the coordinate frames, only one
coefficient in this polynomial decomposition remains sizable, even in the
presence of one or two high jets. The necessary stochastic choice of the
frames relies on probabilities independent from any coupling constants.
This remains true when one or two partons accompany the lepton pairs. In this
way electroweak effects can be better separated from strong interaction ones
for the benefit of the interpretation of the measurements.
Our study exploits properties of single gluon emission matrix elements which
are clearly visible if a conveniently chosen form of their representation is
used. We rely also on distributions obtained from matrix element based Monte
Carlo generated samples of events with two leptons and up to two additional
partons in test samples. Incoming colliding protons' partons are distributed
accordingly to PDFs and are strictly collinear to the corresponding beams.Comment: 22 pages 9 figure
Prospect for the Higgs searches with the ATLAS detector
The investigation of the electroweak symmetry breaking is one of the primary
tasks of the experiments at the CERN Large Hadron Collider (LHC). The potential
of the ATLAS experiment for the discovery of the Higgs boson(s) in Standard
Model and Minimal Supersymmetric Standard Model is presented, with emphasis on
studies which have been completed recently.Comment: Presented at Cracow Epiphany Conference on Hadron Interactions at the
Dawn of the LHC, dedicated to memory of J. Kwiecinski, Krakow, Poland, 5-7
January 2009, 22page
The tauola-photos_F environment for versioning the TAUOLA and PHOTOS packages
We present the system for versioning two packages: the TAUOLA of tau lepton
decay and PHOTOS for radiative corrections in decays. The following features
can be chosen in automatic or semi-automatic way: (1) format of the common
block HEPEVT; (2) version of the physics input (for TAUOLA): as published, as
initialized by CLEO collaboration, as initialized by ALEPH collaboration (it is
suggested to use this version only with the help of the collaboration advice);
(3) type of application: stand-alone, universal interface through HEPEVT,
interface for KKMC Monte Carlo; (4) random number generators; (5) compiler
options.Comment: nine pages, late
Production of tau tau jj final states at the LHC and the TauSpinner algorithm: the spin-2 case
The TauSpinner algorithm is a tool that allows to modify the physics model of
the Monte Carlo generated samples due to the changed assumptions of event
production dynamics, but without the need of re-generating events. With the
help of weights -lepton production or decay processes can be modified
accordingly to a new physics model. In a recent paper a new version TauSpinner
ver.2.0.0 has been presented which includes a provision for introducing
non-standard states and couplings and study their effects in the
vector-boson-fusion processes by exploiting the spin correlations of
-lepton pair decay products in processes where final states include also
two hard jets. In the present paper we document how this can be achieved taking
as an example the non-standard spin-2 state that couples to Standard Model
particles and tree-level matrix elements with complete helicity information
included for the parton-parton scattering amplitudes into a -lepton pair
and two outgoing partons. This implementation is prepared as the external (user
provided) routine for the TauSpinner algorithm. It exploits amplitudes
generated by MadGraph5 and adopted to the TauSpinner algorithm format.
Consistency tests of the implemented matrix elements, reweighting algorithm and
numerical results for observables sensitive to polarization are
presented.Comment: 17 pages, 6 figures; version published in EPJ
What is the W b anti-b, Z b anti-b or t anti-t b anti-b irreducible background to the light Higgs boson searches at LHC?
The W b anti-b, Z b anti-b and t anti-t b anti-b production at LHC are
irreducible backgrounds for possible observability of the Standard Model (SM)
and Minimal Supersymmetric Standard Model (MSSM) light Higgs boson respectively
in the associated WH, ZH and t anti-t H production followed by the H -> b
anti-b decay. We present a comparison of the background estimates as obtained
with (a) complete massive matrix element implemented in AcerMC Monte Carlo
generator and (b) PYTHIA implementation of the inclusive W, Z, and t anti-t
production, followed by the parton showering mechanism. Both approaches lead to
a production of the final state of interest, but differ in the approximations
used. We concentrate on the comparison of these approaches for the background
to the light Higgs boson searches at LHC.Comment: EPJC Referee's comments adde
Application of TauSpinner for studies on tau-lepton polarization and spin correlations in Z, W and H decays at LHC
The tau-lepton plays an important role in the physics program at LHC. Its
spin can be used for separation of signal from background or in measuring
properties of New Particles decaying to tau leptons.
The TauSpinner package represents a tool to modify tau spin effects in any
sample containing tau leptons. Generated events, featuring taus produced from
intermediate state W, Z, H bosons can be used as an input. The information on
the polarization and spin correlations is reconstructed from the kinematics of
the tau lepton(s) (nutau in case of W-mediated processes) and tau decay
products. By weights, attributed on the event-by-event basis, it enables
numerical evaluation and/or modification of the spin effects.
We review distributions to monitor spin effects in leptonic and hadronic tau
decays with up to three pions, to provide benchmarks for validation of spin
content of the event sample and to visualize the tau lepton spin polarization
and correlation effects. The demonstration examples for use of TauSpinner
libraries, are documented. New validation methods of such an approach are
provided. Other topics, like TauSpinner systematic errors or sensitivity of
experimental distributions to spin, are addressed in part only.
This approach is of interest for implementation of spin effects in embedded
tau lepton samples, where Z to mu mu events from data of muons are replaced by
simulated tau leptons. Embedding is used at LHC for estimating Z to tau tau
background to H to tau tau signatures.Comment: 1+41 pages, 5 figures in main text, multitude of figures in
appendice
Machine learning classification: case of Higgs boson CP state in H to tau tau decay at LHC
Machine Learning (ML) techniques are rapidly finding a place among the
methods of High Energy Physics data analysis. Different approaches are explored
concerning how much effort should be put into building high-level variables
based on physics insight into the problem, and when it is enough to rely on
low-level ones, allowing ML methods to find patterns without explicit physics
model.
In this paper we continue the discussion of previous publications on the CP
state of the Higgs boson measurement of the H to tau tau decay channel with the
consecutive tau^pm to rho^pm nu; rho^pm to pi^pm pi^0 and tau^pm to a_1^pm nu;
a_1^pm to rho^0 pi^pm to 3 pi^pm cascade decays. The discrimination of the
Higgs boson CP state is studied as a binary classification problem between
CP-even (scalar) and CP-odd (pseudoscalar), using Deep Neural Network (DNN).
Improvements on the classification from the constraints on directly
non-measurable outgoing neutrinos are discussed. We find, that once added, they
enhance the sensitivity sizably, even if only imperfect information is
provided. In addition to DNN we also evaluate and compare other ML methods:
Boosted Trees (BT), Random Forest (RF) and Support Vector Machine (SVN).Comment: 1+20 pages, 9 figures, 6 tables, extended content and improved
readabilit
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