[Paper Review] Implications of the 750 GeV gamma-gamma Resonance as a Case Study for the International Linear Collider

H. Fujii, Christophe Grojean+23 more|arXiv (Cornell University)|Jul 13, 2016

Particle physics theoretical and experimental studies58 references6 citations

TL;DR

This paper investigates the implications of a hypothetical 750 GeV gamma-gamma resonance, as suggested by early LHC data, for the physics program of the International Linear Collider (ILC). It argues that the ILC’s 500 GeV precision program can distinguish among leading theoretical models of the resonance, while a 1 TeV energy upgrade enables direct production of the resonance in e⁺e⁻ and γγ collisions, offering a unique pathway to probe its properties and associated new physics beyond the Standard Model.

ABSTRACT

If the gamma-gamma resonance at 750 GeV suggested by 2015 LHC data turns out to be a real effect, what are the implications for the physics case and upgrade path of the International Linear Collider? Whether or not the resonance is confirmed, this question provides an interesting case study testing the robustness of the ILC physics case. In this note, we address this question with two points: (1) Almost all models proposed for the new 750 GeV particle require additional new particles with electroweak couplings. The key elements of the 500 GeV ILC physics program---precision measurements of the Higgs boson, the top quark, and 4-fermion interactions---will powerfully discriminate among these models. This information will be important in conjunction with new LHC data, or alone, if the new particles accompanying the 750 GeV resonance are beyond the mass reach of the LHC. (2) Over a longer term, the energy upgrade of the ILC to 1 TeV already discussed in the ILC TDR will enable experiments in gamma-gamma and e+e- collisions to directly produce and study the 750 GeV particle from these unique initial states.

Motivation & Objective

• To assess how the discovery of a 750 GeV γγ resonance would impact the physics case for the International Linear Collider (ILC).
• To evaluate whether the ILC’s 500 GeV precision program can distinguish among competing theoretical models of the resonance.
• To determine the feasibility of directly producing the 750 GeV resonance at an upgraded 1 TeV ILC via e⁺e⁻ and γγ initial states.
• To demonstrate that precision measurements at the ILC provide complementary and robust information on new physics, even if associated particles are beyond the LHC’s reach.

Proposed method

• Analyzing a wide range of theoretical models for the 750 GeV resonance, including vectorlike fermions, extended Higgs sectors, and Randall-Sundrum scenarios.
• Evaluating the imprint of these models on precision observables measurable at the ILC, such as Higgs boson couplings, top quark properties, and 4-fermion interactions.
• Estimating cross sections for direct resonance production in e⁺e⁻ and γγ collisions at 1 TeV center-of-mass energy using theoretical models.
• Assessing the sensitivity of the ILC to invisible and multi-particle decay modes of the resonance through processes like e⁺e⁻ → Φ + γ.
• Comparing the ILC’s reach to that of the LHC, particularly for models with new particles beyond the LHC’s kinematic reach.
• Using the ILC TDR framework to model the upgrade path to 1 TeV and its implications for resonance detection and characterization.

Experimental results

Research questions

• RQ1 Can the ILC’s 500 GeV precision program distinguish between competing theoretical models of a 750 GeV γγ resonance?
• RQ2 What is the sensitivity of the ILC to direct production of the 750 GeV resonance in e⁺e⁻ and γγ initial states at 1 TeV center-of-mass energy?
• RQ3 How do precision measurements of the Higgs boson, top quark, and W boson at the ILC constrain the properties of the 750 GeV resonance and its associated new physics?
• RQ4 To what extent can the ILC probe invisible or multi-particle decays of the resonance, especially when such decay modes are inaccessible at the LHC?
• RQ5 How does the ILC’s physics program provide complementary and robust information on new physics, even if the associated particles are beyond the LHC’s energy reach?

Key findings

• The 500 GeV ILC program can powerfully discriminate among models of the 750 GeV resonance through precision measurements of the Higgs boson, top quark, and 4-fermion interactions.
• The 1 TeV upgraded ILC can directly produce the 750 GeV resonance in both e⁺e⁻ and γγ initial states, enabling full reconstruction of its decay modes.
• Cross sections for γγ → Φ production are sufficiently large to allow observation of the resonance in a γγ collider mode, even with moderate luminosity.
• The process e⁺e⁻ → Φ + γ provides high sensitivity to invisible decays of the resonance, offering a unique probe for dark matter or other weakly interacting particles.
• Precision measurements at the ILC provide critical information on the nature of the new physics associated with the resonance, even if the new particles are beyond the LHC’s discovery reach.
• The ILC offers a robust, technology-ready path to explore the 750 GeV resonance and its underlying theory, providing complementary insights to those from higher-energy proton colliders.