Testing Electroweak Baryogenesis with Future Colliders (1409.0005v4)

Abstract: Electroweak Baryogenesis (EWBG) is a compelling scenario for explaining the matter-antimatter asymmetry in the universe. Its connection to the electroweak phase transition makes it inherently testable. However, completely excluding this scenario can seem difficult in practice, due to the sheer number of proposed models. We investigate the possibility of postulating a "no-lose" theorem for testing EWBG in future e+e- or hadron colliders. As a first step we focus on a factorized picture of EWBG which separates the sources of a stronger phase transition from those that provide new sources of CP violation. We then construct a "nightmare scenario" that generates a strong first-order phase transition as required by EWBG, but is very difficult to test experimentally. We show that a 100 TeV hadron collider is both necessary and possibly sufficient for testing the parameter space of the nightmare scenario that is consistent with EWBG.

• The paper demonstrates that extending the Standard Model with a singlet scalar enables probing strong first-order electroweak phase transitions essential for baryogenesis.
• It employs detailed perturbative analysis and collider sensitivity studies to outline viable parameter regions under the SM + S framework.
• The study indicates that direct searches and precise measurements like triple-Higgs coupling shifts and Zh production deviations could reveal baryogenesis signatures.

Essay: Evaluating Future Collider Potential for Testing Electroweak Baryogenesis

The paper under discussion presents a comprehensive investigation into the feasibility of testing Electroweak Baryogenesis (EWBG) through future collider experiments. EWBG is a theoretical model proposed to explain the matter-antimatter asymmetry observed in the universe. The authors propose tackling the EWBG testability challenge by introducing a "nightmare scenario" within the SM + S extension—adding a real singlet scalar to the Standard Model (SM) to investigate strong electroweak phase transitions.

Framework and Model Considerations

The SM + S model formulated by the authors introduces a singlet scalar field with a $\mathbb{Z}_2$ symmetry to minimize direct experimental signatures such as Higgs-singlet mixing or exotic Higgs decays. The parameter space is defined in terms of the singlet mass $m_S$ and the singlet-Higgs coupling $\lambda_{HS}$. The model emphasizes regions where strong first-order phase transitions can occur, either as one-step transitions via zero-temperature loop effects or two-step transitions via tree-level modifications. These parameters underline the potential for a EWBG-favorable phase transition without immediately apparent collider signatures.

Phenomenological Implications and Collider Sensitivity

The authors dissect the potential of future collider experiments to probe the parameter space of the SM + S model. They find that future 100 TeV hadron colliders, such as SPPC or FCC, could be pivotal. These colliders could detect signatures through direct searches for scalar production (VBF and AP channels) and through precision measurement channels like shifts in the triple-Higgs coupling ($\lambda_3$) or deviations in the associated $Zh$ production cross-section. The indirect measurements, particularly at high-energy future lepton colliders like TLEP, offer additional sensitivity to smaller deviations caused by the singlet's presence.

Theoretical and Practical Considerations

The paper highlights that for a large portion of the EWBG-compatible parameter space, defining the viable regions requires meticulous perturbative analysis. The perturbative validity threshold is around $\lambda_{HS} \approx 5$, beyond which theoretical control may be compromised. Moreover, investigations into the running of couplings suggest that while a high-energy breakdown may occur, the model remains perturbatively reliable up to tens of TeV scales—reassuring for practical cosmic history calculations.

Future Developments

While current technology might limit comprehensive probing of this model, forthcoming experimental capabilities could reach the necessary sensitivity. The authors' analysis points to an optimistic future where a "no-lose" theorem could potentially be established—an assertion that specific collider experiments would decisively validate or invalidate EWBG within the SM + S framework.

Conclusion

This paper constructs a detailed map of where future collider physics might intersect with cosmological phenomena like baryogenesis, providing a pathway forward in what is often a challenging bridge between theoretical postulation and empirical verification. Continued developments in both theoretical extensions of the SM and future high-energy collider lineages offer rich prospects for illuminating one of the most profound mysteries in cosmology: the origin of matter's predominance over antimatter.