[Paper Review] Phenomenology of heavy fermion and vector resonances in composite Higgs models

Natascia Vignaroli|arXiv (Cornell University)|Dec 1, 2011

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper investigates composite Higgs models (CHMs) with heavy fermions and vector resonances, focusing on flavor constraints and LHC discovery potential. Using two-site effective Lagrangians, it identifies a flavor-independent bound on heavy fermion masses from the $b\to s\gamma$ decay, and proposes that $G^*$ and heavy fermions can be discovered early at the LHC via $G^* \to t' \bar{b}$ and $t' \to Wb$ decays, with the top quark's compositeness extractable from invariant mass reconstruction.

ABSTRACT

In this project we study the phenomenology of models where the Higgs is a bound state of a strongly interacting dynamics at the TeV scale and we assess the LHC potential to discover new heavy colored states. In the first part of the thesis we analyze the bounds on the spectrum of Composite Higgs Models (CHM) that come from flavor observables. In the second part we propose a novel strategy to discover a heavy gluon ($G^*$) and heavy fermions at the LHC. We do so by means of simple two-site effective Lagrangians, which could also be used in further phenomenological studies on CHM. From the analysis of the bounds on the CHM spectrum, we derive an important constraint on the masses of the heavy fermions which does not depend on the flavor structure of the sector beyond the SM. This bound is obtained from the infrared contribution to $b o sγ$ induced by the flavor-conserving effective vertex $Wt_Rb_R$. We find that the presence of a custodial symmetry can play a role in protecting this effective coupling. Studying the heavy colored vectors phenomenology, we find that heavy composite fermions have a great impact on the phenomenology of heavy composite gluon at the LHC. If the composite gluon is heavier than the composite fermions, as flavor observables seem to suggest, the search in the channels where $G^*$ decays into one heavy fermion plus its Standard Model partner is very promising, with the possibility for both the $G^*$ and heavy fermions to be discovered at the early stage of the LHC. These channels offer also the possibility to extract important information on model's parameters, such as the top degree of compositeness.

Motivation & Objective

• To assess the LHC's potential to discover new heavy colored states in composite Higgs models (CHMs), particularly heavy fermions and a heavy gluon ($G^*$).
• To derive flavor-independent bounds on heavy fermion masses using $b\to s\gamma$ decay, focusing on the infrared contribution to the $Wt_Rb_R$ effective vertex.
• To propose a novel search strategy for $G^*$ and heavy fermions at the LHC based on $G^* \to t' \bar{b}$ and $t' \to Wb$ decay chains.
• To demonstrate that the top quark's degree of compositeness can be extracted from invariant mass reconstruction in $Wtb$ final states.

Proposed method

• Constructs two-site effective Lagrangians (TS5 and TS10) to model the interactions of $G^*$, heavy fermions, and the Higgs sector in CHMs.
• Calculates the effective $Wt_Rb_R$ coupling via infrared and ultraviolet contributions to $b\to s\gamma$, using perturbative diagonalization in the small mixing parameter $x \sim Y_*v/\sqrt{2}m_*$.
• Evaluates the $G^*$ production cross-section and branching ratios, focusing on $G^* \to t' \bar{b}$ and $t' \to Wb$ decay channels.
• Performs signal and background simulations for the $Wtb$ final state, applying acceptance cuts and invariant mass reconstruction to isolate $G^*$ and $t'$ resonances.
• Uses the $Wtb$ channel to reconstruct the $t'$ and $G^*$ masses, enabling extraction of the top quark's compositeness parameter.
• Analyzes the discovery reach in CHM parameter space, identifying viable regions for early LHC discovery.

Experimental results

Research questions

• RQ1 What are the flavor constraints on the spectrum of composite Higgs models, particularly from $b\to s\gamma$?
• RQ2 Can a flavor-conserving effective coupling $Wt_Rb_R$ induced by infrared dynamics impose a universal bound on heavy fermion masses independent of flavor structure?
• RQ3 How does the presence of a custodial symmetry protect the $Wt_Rb_R$ coupling in CHMs?
• RQ4 What is the optimal LHC search strategy for discovering a heavy gluon ($G^*$) and its associated heavy fermions?
• RQ5 Can the top quark's degree of compositeness be extracted from $G^*$ and $t'$ resonance reconstruction in $Wtb$ final states?

Key findings

• A flavor-independent bound on heavy fermion masses arises from the infrared contribution to $b\to s\gamma$, with the bound scaling as $m_{t_R} \lesssim 1.5\,\text{TeV}$ in the absence of custodial symmetry.
• The $Wt_Rb_R$ effective coupling is protected by $P_C$ parity, which suppresses large contributions from the $t_R$-bilinear vertex.
• The $G^* \to t' \bar{b}$ decay channel is highly promising for early LHC discovery if $m_{G^*} > m_{t'}$, as suggested by flavor constraints.
• The $Wtb$ final state allows for successful reconstruction of both $G^*$ and $t'$ resonances, enabling extraction of the top quark's compositeness parameter.
• The $G^* \to \psi\chi$ channel offers a complementary discovery channel, with significant signal-to-background ratios after cuts.
• The analysis shows that both $G^*$ and heavy fermions can be discovered at the early LHC if $m_{G^*} > m_{t'}$, with discovery reach extending to $m_{G^*} \sim 2.5\,\text{TeV}$ and $m_{t'} \sim 1.5\,\text{TeV}$.