Higgs mass determination from direct reconstruction at a Linear e=e- Collider

We study the feasibility of a precise measurement of the mass of a 120 GeV MSM Higgs boson through direct reconstruction of ZH->qqH events that would be achieved in a future e+e- linear collider operating at a center-of-mass energy of 500 GeV. Much effort has been put in a ``realistic simulation'' by including irreducible and reducible backgrounds, realistic detector effects and reconstruction procedures and sophisticated analysis tools involving Neural Networks and kinematical fitting. As a result, the Higgs mass is determined with a statistical accuracy of 50 MeV and the Z-Higgs Yukawa coupling measured to 0.7%, assuming 500 fb^-1 of integrated luminosity.Comment: LaTex, 29 pages, 18 Postscript figure

Top Quark Current Experimental Status

Ten years after its discovery at the Tevatron collider, we still know little about the top quark. Its large mass suggests it may play a key role in the mechanism of Electroweak Symmetry Breaking (EWSB), or open a window of sensitivity to new physics related to EWSB and preferentially coupled to it. To determine whether this is the case, precision measurements of top quark properties are necessary. The high statistics samples being collected by the Tevatron experiments during Run II start to incisively probe the top quark sector. This report summarizes the experimental status of the top quark, focusing in particular on the recent measurements from the Tevatron.Comment: 12 pages, 6 figures, 1 table. Talk presented at TOP 2006, International Workshop on Top Quark Physics, Coimbra, Portugal, January 12-15, 2006. To appear in the proceeding

Heavy Higgs Searches: Flavour Matters

We point out that the stringent lower bounds on the masses of additional electrically neutral and charged Higgs bosons crucially depend on the flavour structure of their Yukawa interactions. We show that these bounds can easily be evaded by the introduction of flavour-changing neutral currents in the Higgs sector. As an illustration, we study the phenomenology of a two Higgs doublet model with a Yukawa texture singling out the third family of quarks and leptons. We combine constraints from low-energy flavour physics measurements, LHC measurements of the 125 GeV Higgs boson rates, and LHC searches for new heavy Higgs bosons. We propose novel LHC searches that could be performed in the coming years to unravel the existence of these new Higgs bosons.Comment: 41 pages, 11 figures and 4 tables (v2: References added. Comment on associated production with a top quark added. Matched published version.

On the determination of probability density functions by using Neural Networks

It is well known that the output of a Neural Network trained to disentangle between two classes has a probabilistic interpretation in terms of the a-posteriori Bayesian probability, provided that a unary representation is taken for the output patterns. This fact is used to make Neural Networks approximate probability density functions from examples in an unbinned way, giving a better performace than ``standard binned procedures''. In addition, the mapped p.d.f. has an analytical expression.Comment: 13 pages including 3 eps figures. Submitted to Comput. Phys. Commu

Four-top as probe of light top-philic New Physics

We study the four-top final state at the LHC as a probe for New Physics (NP) effects due to new particles that couple predominantly to the top quark and whose masses are below the top-quark-pair production threshold. We consider simple NP models containing a new particle with either spin 0, spin 1, or spin 2, and find benchmark points compatible with current experimental results. We find that interference effects between NP and QED amplitudes can be large, pointing out the necessity of NLO contributions to be explicitly computed and taken into account when NP is present. We examine kinematic differences between these models and the Standard Model (SM) at the parton level and the reconstructed level. In the latter case, we focus on events selected requiring two same-sign leptons and multiple jets. We investigate how the different Lorentz structure of the light NP affects the kinematic hardness, the polarization, the spin correlations, and the angular distributions of the parton-level and/or final-state particles. We find that spin-2 light NP would be identified by harder kinematics than the SM. We also show that the angular separation between the same-sign leptons is a sensitive observable for spin-0 NP. The spin-0 and spin-2 NP cases would also yield a signal in $t\bar t \gamma\gamma$ with the invariant mass of the photons indicating the mass of the new particle. The spin-1 NP would be identified through an excess in four-top signal and slight or not modification in other observables, as for instance the lack of signal in $t\bar t \gamma\gamma$ due to the Landau-Yang theorem. We comment on the opportunities that would open from the kinematic reconstruction of some of the top quarks in the four-top state. Our results provide new handles to probe for light top-philic NP as part of the ongoing experimental program of searches for four-top production at the LHC Run 2 and beyond.Comment: 33 pages, 14 figures. JHEP accepted versio

Determination of the top quark mass circa 2013: methods, subtleties, perspectives

We present an up-to-date overview of the problem of top quark mass determination. We assess the need for precision in the top mass extraction in the LHC era together with the main theoretical and experimental issues arising in precision top mass determination. We collect and document existing results on top mass determination at hadron colliders and map the prospects for future precision top mass determination at e+e- colliders. We present a collection of estimates for the ultimate precision of various methods for top quark mass extraction at the LHC.Comment: 26 pages. Snowmass White Pape

Topping-up multilepton plus b-jets anomalies at the LHC with a Z′ boson

During the last years ATLAS and CMS have reported a number of slight to mild discrepancies in signatures of multileptons plus b-jets in analyses such as tt¯ H, tt¯ W±, tt¯ Z and tt¯ tt¯. Among them, a recent ATLAS result on tt¯ H production has also reported an excess in the charge asymmetry in the same-sign dilepton channel with two or more b-tagged jets. Motivated by these tantalizing discrepancies, we study a phenomenological New Physics model consisting of a Z′ boson that couples to up-type quarks via right-handed currents: tRγμt¯ R, tRγμc¯ R, and tRγμu¯ R. The latter vertex allows to translate the charge asymmetry at the LHC initial state protons to a final state with top quarks which, decaying to a positive lepton and a b-jet, provides a crucial contribution to some of the observed discrepancies. Through an analysis at a detector level, we select the region in parameter space of our model that best reproduces the data in the aforementioned tt¯ H study, and in a recent ATLAS tt¯ tt¯ search. We find that our model provides a better fit to the experimental data than the Standard Model for a New Physics scale of approximately ∼500 GeV, and with a hierarchical coupling of the Z′ boson that favours the top quark and the presence of FCNC currents. In order to estimate the LHC sensitivity to this signal, we design a broadband search featuring many kinematic regions with different signal-to-background ratio, and perform a global analysis. We also define signal-enhanced regions and study observables that could further distinguish signal from background. We find that the region in parameter space of our model that best fits the analysed data could be probed with a significance exceeding 3 standard deviations with just the full Run-2 dataset.Fil: Alvarez, Ezequiel. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Juste, Aurelio. Universitat Autònoma de Barcelona; España. Institució Catalana de Recerca I Estudis Avançats; EspañaFil: Szewc, Manuel. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Schroeder, Tamara Vazquez. Cern - European Organization for Nuclear Research; Suiz