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ALOHA: Automatic libraries of helicity amplitudes for Feynman diagram computations☆

We present an application that automatically writes the Helas library corresponding to the Feynman rules of any Lagrangian, renormalizable or not, in quantum field theory. The code, written in Python, takes the Universal FeynRules Output as an input and produces the complete set of routines (wave-functions and amplitudes) that are needed for the computation of Feynman diagrams at leading as well as at higher orders. The representation is language independent and outputs in Fortran, C++, Python are currently available. A few key sample applications implemented in the MadGraph5 framework are presented.

Simulating graviton production at hadron colliders

Spin-2 particles and in particular gravitons are predicted in many new physics scenarios at the TeV scale. Depending on the details of models such new states might show up as a continuum, massless particles, or TeV scale resonances. Correspondingly, very different discovery signatures should be exploited, from the search of excesses in events with multi jets and large missing transverse energy, to resonances in weak boson or jet pair productions. We present a very general and flexible implementation in Mad-Graph/MadEvent of spin-2 particles interacting with the standard model particles via the energy momentum tensor, which encompasses all of the most popular TeV scale models featuring gravitons. By merging matrix elements with parton shower, we can generate inclusive samples of graviton + jets at the hadron colliders in several scenarios (ADD, zero-mass graviton and RS). We compare and validate our results against the corresponding next-to-leading order QCD calculations.

Unravelling tt¯h via the Matrix Element Method

Associated production of the Higgs boson with a top-antitop pair is a key channel to gather further information on the nature of the newly discovered boson at the LHC. Experimentally, however, its observation is very challenging due to the combination of small rates, difficult multijet final states, and overwhelming backgrounds. In the standard model, the largest number of events is expected when h→bb, giving rise to a W+ W- bbbb signature, deluged in tt + jets. A promising strategy to improve the sensitivity is to maximally exploit the theoretical information on the signal and background processes by means of the matrix element method. We show how, despite the complexity of the final state, the method can be efficiently applied to discriminate the signal against combinatorial and tt + jets backgrounds. Remarkably, we find that a moderate integrated luminosity in the next LHC run will be enough to make the signature involving both Ws decaying leptonically as sensitive as the single-lepton one.

Unravelling

Associated production of the Higgs boson with a top-antitop pair is a key channel to gather further information on the nature of the newly discovered boson at the LHC. Experimentally, however, its observation is very challenging due to the combination of small rates, difficult multijet final states, and overwhelming backgrounds. In the standard model, the largest number of events is expected when h→bb, giving rise to a W+ W- bbbb signature, deluged in tt + jets. A promising strategy to improve the sensitivity is to maximally exploit the theoretical information on the signal and background processes by means of the matrix element method. We show how, despite the complexity of the final state, the method can be efficiently applied to discriminate the signal against combinatorial and tt + jets backgrounds. Remarkably, we find that a moderate integrated luminosity in the next LHC run will be enough to make the signature involving both Ws decaying leptonically as sensitive as the single-lepton one.

t\bar{t}h

We show that the LHC will be able to differentiate between a four-dimensional model with quantum gravity at ~1 TeV where the (massless) graviton becomes strongly coupled to standard model particles at 1 TeV and brane world type models with a large extra-dimensional volume and massive Kaluza-Klein gravitons. We estimate that the 14 TeV LHC could put a limit of the order of ~5 TeV on the four dimensional Planck mass in a model independent way. Comment: 9 pages

via the matrix element method

We study the production of massless gravitons at the LHC and compare our results to those obtained in extra-dimensional models. The signature in both cases is missing energy plus jets. In case of non-observation, the LHC could be used to put the tightest limit to date on the value of the Planck mass.

Massless versus Kaluza-Klein Gravitons at the LHC

The Standard Model (SM) is a successful description of particle physics nowadays. It is a quantum field theory that explains the dynamics of our universe through matter and forces. It incorporates three fundamental forces: the weak, the strong and the electromagnetic forces, and includes different types of particles.

Quantum gravity at the LHC

The Large Hadron Collider (or LHC for short) was built to collide protons at a very high center-of-mass energy, in order to test predictions of different theories in particle physics and to confirm the existence of the Higgs boson. For the comparison between experimental data against theory predictions, simulations play a crucial role. In this chapter, we present an overview of both the LHC itself and methods used to simulate events at hadron colliders.

The Standard Model

Supersymmetric theories can naturally solve the hierarchy problem, and at the same time, shed some light on different SM shortcomings, providing for example candidates suitable for being a dark matter particle. We start the chapter with a simple description of how quadratic divergences arise in the SM, and the possible solution for the hierarchy problem provided by Supersymmetric theories. We subsequently characterize such theories and describe some of its main branches. Finally, phenomenological analyses for the production of specific super-particles are exhibited.

The LHC and Collision Simulations

It has been shown that the SM is a successful theory that unifies three forces of Nature. However, general relativity is still left apart, and for many years several analyses seeking a general unification have been made. Throughout this chapter we shall briefly present the standard description used to quantize gravity, and assemble a quantum field theory that unifies all forces. The intention is also to display the limitation of the standard quantum gravity theory and introduce how new theories could well address SM shortcomings.