Testing Invisible Momentum Ansatze in Missing Energy Events at the LHC

We consider SUSY-like events with two decay chains, each terminating in an invisible particle, whose true energy and momentum are not measured in the detector. Nevertheless, a useful educated guess about the invisible momenta can still be obtained by optimizing a suitable invariant mass function. We review and contrast several proposals in the literature for such ansatze: four versions of the M_T2-assisted on-shell reconstruction (MAOS), as well as several variants of the on-shell constrained M_2 variables. We compare the performance of these methods with regards to the mass determination of a new particle resonance along the decay chain from the peak of the reconstructed invariant mass distribution. For concreteness, we consider the event topology of dilepton ttbar events and study each of the three possible subsystems, in both a ttbar and a SUSY example. We find that the M_2 variables generally provide sharper peaks and therefore better ansatze for the invisible momenta. We show that the performance can be further improved by preselecting events near the kinematic endpoint of the corresponding variable from which the momentum ansatz originates.Comment: 38 pages, 15 figure

Search for SUSY in Top Final States in the mSUGRA Scenario at CMS

A search for SUSY is performed in a low mass mSUGRA test point by looking for top quark enriched in the final states. A robust algorithm is developed to suppress the electrons faking a jet and a two constraints kinematic fit is utilized to improve the top quark extraction. It is found that the signal over background ratio is 11.0 and a 5 sigma excess can be observed with 0.2 fb^-1. The fast simulation of the detector response is used to find the CMS potential discovery for SUSY in this channel. The reach contours for 1, 10 and 30 fb^-1 are presented

Search for a light Higgs boson in SUSY cascades

This note presents the potential of the CMS experiment to discover a light supersymmetric Higgs boson (h0) produced at the end of a cascade of supersymmetric particles starting with the strong production of squarks and gluinos. Because of this production mechanism, the events can be efficiently triggered using inclusive SUSY triggers such as jet+MET and the dominant h0 -> b bbar decay mode of the Higgs boson can be exploited. The Higgs mass can be extracted from the reconstructed di-b-jet effective mass distribution. The present investigation for the so-called LM5 test point has been done for 1, 3 and 10 fb-1

Improving the sensitivity of stop searches with on-shell constrained invariant mass variables

The search for light stops is of paramount importance, both in general as a promising path to the discovery of beyond the standard model physics and more specifically as a way of evaluating the success of the naturalness paradigm. While the LHC experiments have ruled out much of the relevant parameter space, there are "stop gaps", i.e., values of sparticle masses for which existing LHC analyses have relatively little sensitivity to light stops. We point out that techniques involving on-shell constrained M_2 variables can do much to enhance sensitivity in this region and hence help close the stop gaps. We demonstrate the use of these variables for several benchmark points and describe the effect of realistic complications, such as detector effects and combinatorial backgrounds, in order to provide a useful toolkit for light stop searches in particular, and new physics searches at the LHC in general.Comment: 49 pages, 28 figures, revised version published in JHEP, references adde

On-shell constrained M2M_2 variables with applications to mass measurements and topology disambiguation

We consider a class of on-shell constrained mass variables that are 3+1 dimensional generalizations of the Cambridge $M_{T2}$ variable and that automatically incorporate various assumptions about the underlying event topology. The presence of additional on-shell constraints causes their kinematic distributions to exhibit sharper endpoints than the usual $M_{T2}$ distribution. We study the mathematical properties of these new variables, e.g., the uniqueness of the solution selected by the minimization over the invisible particle 4-momenta. We then use this solution to reconstruct the masses of various particles along the decay chain. We propose several tests for validating the assumed event topology in missing energy events from new physics. The tests are able to determine: 1) whether the decays in the event are two-body or three-body, 2) if the decay is two-body, whether the intermediate resonances in the two decay chains are the same, and 3) the exact sequence in which the visible particles are emitted from each decay chain.Comment: 44pages, 17 figures. revised version, published in JHEP. Minor addition: a paragraph discussing the effect on the background at the end of section 5.

OPTIMASS: A Package for the Minimization of Kinematic Mass Functions with Constraints

Reconstructed mass variables, such as $M_2$, $M_{2C}$, $M_T^\star$, and $M_{T2}^W$, play an essential role in searches for new physics at hadron colliders. The calculation of these variables generally involves constrained minimization in a large parameter space, which is numerically challenging. We provide a C++ code, OPTIMASS, which interfaces with the MINUIT library to perform this constrained minimization using the Augmented Lagrangian Method. The code can be applied to arbitrarily general event topologies and thus allows the user to significantly extend the existing set of kinematic variables. We describe this code and its physics motivation, and demonstrate its use in the analysis of the fully leptonic decay of pair-produced top quarks using the $M_2$ variables.Comment: 39 pages, 12 figures, (1) minor revision in section 3, (2) figure added in section 4.3, (3) reference added and (4) matched with published versio

Data Preservation at LEP

The four LEP experiments ALEPH, DELPHI, L3 and OPAL successfully recorded e+e- collision data during the years 1989 to 2000. As part of the ordinary evolution in High Energy Physics, these experiments can not be repeated and their data is therefore unique. This article briefly reviews the data preservation efforts undertaken by the four experiments beyond the end of data taking. The current status of the preserved data and associated tools is summarised.Comment: 7 pages, contribution to proceedings of the "First Workshop on Data Preservation and Long Term Analysis in HEP

Propagation de fissure et endommagement dans un milieu viscoélastique linéaire

La propagation d'une fissure est associée à un changement de nature des conditions aux limites sur la portion de frontière nouvellement créée. Cette situation interdit le recours à la transformation de Laplace pour la résolution des problèmes viscoélastiques linéaires. On développe donc une approche numérique en éléments finis permettant d'évaluer le taux de restitution d'énergie élastique, puis d'analyser la propagation. On aborde ensuite une extension à l'endommagement par microfissuration

Evaluation d'un système d'assistance à la conduite de fauteuil roulant électrique

National audienceL'utilisation d'un fauteuil roulant électrique est pour nombre de personnes en situation de handicap le seul mode de déplacement autonome leur donnant la possibilité d'une liberté de déplacement en extérieur ou en intérieur. La maîtrise de ce type d'aide technique peut être rendue difficile du fait de handicaps sensoriels, notamment visuels, ou de troubles de commande du membre supérieur. L'objectif de cette étude est d'évaluer un système électronique embarquée d'assistance à la conduite mis au point par l'INSA. Ce système permet de détecter les obstacles puis de corriger vitesse et trajectoire pour éviter les collisions et améliorer la qualité de la trajectoire