Standard Model couplings and collider signatures of a light scalar

TL;DR

The paper investigates the possibility that the electroweak symmetry breaking sector is nearly scale invariant, yielding a light dilaton as a pseudo-Goldstone boson of broken conformal symmetry. It uses conformal symmetry to constrain the dilaton’s couplings to SM fields, distinguishing cases where SM fields are embedded in the conformal sector from those where they are spectators, and analyzes how explicit scale-breaking operators control the dilaton mass $m_\chi$ and self-interactions. A key result is that dilaton couplings to SM fields can deviate from SM Higgs expectations: tree-level couplings to massive fields scale as $v/f$, while loop-induced couplings to massless gauge bosons depend on beta-function coefficients, and Yukawa couplings to light fermions can acquire mass-independent contributions, enabling new production modes such as $u\bar{u}\to \chi$ and decay channels like $\chi \to \mu^+\mu^-$ or $\chi \to e^+e^-$. The work shows that, depending on UV physics, these effects can yield observable signals at the Tevatron and LHC, and it provides a framework to interpret collider data in terms of a dilaton-like scalar arising from a near-conformal sector.

Abstract

The electroweak symmetry breaking (EWSB) sector of the Standard Model can be far richer and more interesting than the usual single scalar doublet model. We explore scenarios where the EWSB sector is nearly scale invariant and consequently gives rise to a light CP even scalar particle. The one-doublet SM is in that category, as are many other models with either weakly or strongly coupled sectors that trigger EWSB. We study the couplings of the light scalar to the SM particles that can arise from the explicit breaking of scale invariance focusing on the possible differences with the minimal SM. The couplings of the light scalar to light fermions, as well as to the massless gauge bosons, can be significantly enhanced. We find possible new discovery channels due to the decays of the conformal scalar into e^+e^- and mu^+mu^- pairs as well as new production channels via light quark annihilation.

Figures (7)

• Figure 1: Tevatron production cross sections for different channels at LO as a function of $m_\chi$. Shown are the cross sections $u\bar{u} \to \chi$ with $y^{(1)}_{u}=1$, $d\bar{d} \to \chi$ with $y^{(1)}_{d}=1$, and $gg \to \chi$ for two different coupling values. The $gg_1$ curve corresponds to a SM like $gg\chi$ coupling arising solely from the top quark loop while the $gg_2$ curve corresponds to the scenario where the SM is fully embedded in the CFT such that $c_G=11-2 n_f/3$.
• Figure 2: LHC production cross sections for different channels at LO as a function of $m_\chi$. The correspondence between line types and processes is as in Fig. \ref{['fig:production']}.
• Figure 3: Upper limits on $v/f$ as a function of $m_\chi$ implied by the Tevatron lepton pair data. We assume $y^{(1)}_\mu=1$ and all other $y^{(1)}_{\psi}=0$. The different curves correspond to bounds for several choices of the $\chi gg$ coupling $c_G$. Also shown is the line $m^2=f^2$ beyond which approximate scale symmetry no longer holds.
• Figure 4: Upper limits on $y_\mu^{(1)}$ as a function of $v/f$ implied by the Tevatron lepton pair data. We assume $m_\chi=150~\rm{GeV}$, $y^{(1)}_\mu\neq 0$, and all other $y^{(1)}_{\psi}=0$. From left to right, $c_G=11-2/3\, n_{f},4,2,1$.
• Figure 5: $95\%$ CL upper limit on the product $\lambda=(v/f)^2 y_e^{(1)} y_\mu^{(1)}$ implied by the LEP2 data, as a function of $m_\chi$. The plot only shows a small range of $m_\chi$ around the energies for which LEP2 collected data, where the bounds are most stringent.