2,647 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 $p_T$ 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

### The TauSpinner approach for electroweak corrections in LHC Z to ll observables

The LHC enters era of the Standard Model Z-boson couplings precise
measurements, to match precision of LEP. The calculations of electroweak (EW)
corrections in the Monte Carlo generators become of relevance. Precise
predictions of Z-boson production and decay require classes of QED/EW/QCD
corrections, preferably in the manner which allows for separation from the QCD
dynamics of the production. At LEP, calculations, genuine weak and lineshape
corrections were introduced into electroweak form-factors and Improved Born
Approximation. This was well suited for so-called doubly-deconvoluted
observables around the Z-pole; observables for which the initial- and
final-state QED real and virtual emissions are treated separately or integrated
over. This approach to EW corrections is followed for LHC pp collisions. We
focus on the EW corrections to doubly-deconvoluted observables of Z to ll
process, in a form of per-event weight and on numerical results. The
reweighting technique of TauSpinner package is revisited and the program is
enriched with the EW sector. The Dizet library, as interfaced to KKMC Monte
Carlo of the LEP era, is used to calculate O(alpha) weak loop corrections,
supplemented by some higher-order terms. They are used in the form of look-up
tables by the TauSpinner package. The size of the corrections is evaluated for
the following observables: the Z-boson resonance line-shape, the outgoing
leptons forward-backward asymmetry, effective leptonic weak mixing angles and
the lepton distribution spherical harmonic expansion coefficients. Evaluation
of the EW corrections for observables with simplified calculations based on
Effective Born of modified EW couplings, is also presented and compared with
the predictions of Improved Born Approximation where complete set of EW
form-factors is used.Comment: 27+1 pages, 13 tables, 12 figures, update with some misprints and
opaque statements improved, number of figures reduced to 9 and tables to 1

### 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 $\tau$-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
$\tau$-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 $\tau$-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 $\tau$ polarization are
presented.Comment: 17 pages, 6 figures; version published in EPJ

### 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

### Next Generation Higgs Bosons: Theory, Constraints and Discovery Prospects at the Large Hadron Collider

Particle physics model building within the context of string theory suggests
that further copies of the Higgs boson sector may be expected. Concerns
regarding tree-level flavor changing neutral currents are easiest to allay if
little or no couplings of next generation Higgs bosons are allowed to Standard
Model fermions. We detail the resulting general Higgs potential and mass
spectroscopy in both a Standard Model extension and a supersymmetric extension.
We present the important experimental constraints from meson-meson mixing,
loop-induced $b\to s\gamma$ decays and LEP2 direct production limits. We
investigate the energy range of valid perturbation theory of these ideas. In
the supersymmetric context we present a class of examples that marginally aids
the fine-tuning problem for parameter space where the lightest Higgs boson mass
is greater than the Standard Model limit of 114 GeV. Finally, we study collider
physics signatures generic to next generation Higgs bosons, with special
emphasis on $Ah\to hhZ\to 4b+2l$ signal events, and describe the capability of
discovery at the Large Hadron Collider.Comment: 43 pages, 12 figures; v3: minor corrections, published in Physical
Review

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