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TeraZ Blind Spots How Precision Colliders Could Miss New Physics

Ever wondered if future particle colliders like TeraZ could miss new physics discoveries despite their incredible precision? Scientists discovered ‘blind directions’ in SMEFT where New Physics effects cancel out, making them invisible even to super precise measurements. Discover how these ‘blind spots’ challenge precision physics and why high-energy colliders are essential for BSM hunts.

Frequently Asked Questions (FAQ) | short

  1. What are TeraZ colliders and why are they important for particle physics? High-luminosity electron-positron colliders (like FCC-ee, CEPC) at the Z-boson threshold. Produce huge numbers of Z bosons for extremely precise electroweak measurements, testing the Standard Model and searching for new physics.

  2. How is new physics typically interpreted using electroweak precision observables? Using the Standard Model Effective Field Theory (SMEFT). Precise measurements constrain SMEFT operator coefficients, representing effects of high-scale new physics.

  3. What are “blind directions” in the context of SMEFT and EWPOs? Combinations of SMEFT parameters (operator coefficients) to which electroweak precision data are largely insensitive, often due to canceling or correlated effects.

  4. Are blind directions just theoretical artefacts or do they appear in realistic new physics models? They appear generically in realistic multi-field new physics models, arising from correlations when heavy fields are integrated out.

  5. How do multi-field extensions of the Standard Model contribute to the problem of blind directions? Multiple heavy fields generate correlated SMEFT operators, whose combined effects can fall into blind directions, unlike simpler single-field scenarios.

  6. What role do Renormalisation Group Evolution (RGE) and one-loop matching effects play in blind directions? RGE and one-loop effects modify blind directions but do not eliminate significant blind subspaces.

  7. Which specific combinations of four-fermion operators have been identified as contributing to blind directions? Combinations involving operators like Odd, O(8)qu, O(8)qd, O(8)ud, Ole, Oll, Oed, Old, especially with third-family couplings.

  8. Why are complementary high-energy collider probes essential alongside TeraZ? Blind directions limit TeraZ’s ability to constrain all new physics. High-energy colliders probe different regimes, breaking degeneracies and providing complementary searches.

Frequently Asked Questions (FAQ) | long

  1. What are TeraZ colliders and why are they important for particle physics? TeraZ refers to the next generation of high-luminosity electron-positron colliders, such as the proposed Future Circular Collider (FCC-ee) and the Circular Electron-Positron Collider (CEPC), specifically operating at the Z-boson threshold. These colliders are designed to produce an unprecedented number of Z bosons (around 109). This high-statistics environment allows for extremely precise measurements of electroweak precision observables (EWPOs), which are crucial for testing the Standard Model (SM) of particle physics with high sensitivity. Any observed deviations from the SM predictions in these precision measurements could indicate the presence of new physics beyond the Standard Model (BSM). Historically, similar precision measurements at the Large Electron-Positron Collider (LEP) provided sensitive probes of BSM physics scenarios.

  2. How is new physics typically interpreted using electroweak precision observables? The standard approach to interpreting potential small deviations from the Standard Model observed in electroweak precision observables (EWPOs) is through the framework of Effective Field Theory (EFT). Specifically, the Standard Model Effective Field Theory (SMEFT) provides a systematic, model-independent method to parameterise the effects of physics beyond the Standard Model (BSM) that occur at higher energy scales but manifest as modifications at lower energies accessible by experiments. By measuring EWPOs and comparing them to SM predictions, one can place constraints on the coefficients of the various operators in the SMEFT Lagrangian, which represent the new physics contributions.

  3. What are “blind directions” in the context of SMEFT and EWPOs? Blind directions in the SMEFT parameter space are combinations of parameters (specifically, the coefficients of SMEFT operators) to which electroweak precision data are largely insensitive. This insensitivity arises because multiple SMEFT operators can contribute to the same EWPOs in such a way that their effects cancel each other out or are highly correlated. Even with the high precision of TeraZ measurements, it becomes difficult to constrain these specific combinations of parameters, meaning that certain types of new physics could exist within these “blind” subspaces without being detected through indirect precision searches alone.

  4. Are blind directions just theoretical artefacts or do they appear in realistic new physics models? The analysis presented demonstrates that blind directions are not merely an artefact of performing agnostic scans across the entire SMEFT operator space. Instead, they are shown to arise generically in realistic ultraviolet (UV) completions of the Standard Model, particularly those involving multiple heavy fields. When these multiple heavy fields are integrated out to produce the low-energy effective theory (SMEFT), their contributions to the SMEFT operators can exhibit specific correlations that align with the known blind subspaces identified through bottom-up EFT analyses. This suggests that blind directions are a structural feature of many plausible BSM models, making it challenging to fully probe these models with precision electroweak measurements alone.

  5. How do multi-field extensions of the Standard Model contribute to the problem of blind directions? Realistic ultraviolet (UV) models of new physics generically require the introduction of many new degrees of freedom or fields. Multi-field extensions of the Standard Model are more likely to generate multiple SMEFT operators upon integrating out the heavy fields. Crucially, the contributions from different heavy fields to various SMEFT operators can be correlated. These correlations can lead to specific combinations of SMEFT operator coefficients that fall within the blind directions. This is in contrast to simplified scenarios where only a single heavy field is integrated out, which typically results in a small set of highly correlated EFT operators without significant blind directions. The complexity introduced by multiple fields in realistic models exacerbates the problem of blind directions at TeraZ.

  6. What role do Renormalisation Group Evolution (RGE) and one-loop matching effects play in blind directions? Renormalisation Group Evolution (RGE) describes how the coefficients of SMEFT operators change with energy scale. Including RGE effects can alter the sensitivity of EWPOs to different operators and, in some cases, can remove approximate “flat directions” that might appear at a single energy scale. Finite one-loop matching effects account for contributions from the heavy fields that are not captured at tree level. The analysis shows that while RGE can modify the landscape of constraints, and one-loop matching corrections slightly alter the blind directions, neither process eliminates the existence of significant blind subspaces, especially those involving multiple operators. This reinforces the conclusion that blind directions are robust features that persist even with improved theoretical calculations.

  7. Which specific combinations of four-fermion operators have been identified as contributing to blind directions? Focusing on the four-fermion sector of the SMEFT, particularly with third-family couplings, the analysis identifies several combinations of operators that contribute to blind directions. Some of the key combinations include:

  • Operators like Odd, O(8)qu, O(8)qd, O(8)ud, Ole, Oll, Oed, and Old.
  • The combination O(1)ud + O(1)qd.
  • Combinations involving ceu, cqe, clu, and c(1)lq, such as Oeu + Oqe + Olu + O(1)lq and Oeu + Oqe - Olu - O(1)lq. These combinations represent subspaces of the SMEFT parameter space where EWPO measurements are largely insensitive, even after accounting for RGE.
  1. Why are complementary high-energy collider probes essential alongside TeraZ? The presence and persistence of blind directions mean that relying solely on indirect searches through high-precision electroweak measurements at TeraZ is often insufficient to comprehensively probe and potentially rule out broad classes of new physics. While TeraZ excels at precision, it is fundamentally limited in its ability to distinguish between different combinations of SMEFT operators that fall within blind directions. High-energy collider experiments, such as those at the LHC and its potential upgrades, are essential because they can probe different kinematic regimes and energy scales. These different regimes can break the degeneracies present in precision electroweak measurements, allowing for a more complete exploration of the SMEFT parameter space and providing a complementary way to search for and constrain new physics that might be invisible to TeraZ.

The Problem

The core problem identified in the paper is that even with the unprecedented precision of electroweak measurements expected from future high-luminosity electron-positron colliders like TeraZ (FCC-ee and CEPC operating at the Z pole), new physics could remain effectively hidden or undetectable. This is referred to as the “TeraZ Mirage”.

  • While these precision measurements are interpreted within the Standard Model Effective Field Theory (SMEFT) to constrain new physics, the SMEFT framework involves a large number of independent operators.
  • This large parameter space allows for the possibility of “blind directions”, where specific combinations of SMEFT operator coefficients can cancel each other out, making the electroweak precision data largely insensitive to them. A historical example at LEP involved a blind direction in the S-T parameter plane, preventing the exclusion of a fourth generation of fermions using EWPOs alone.
  • Crucially, the paper demonstrates that these blind directions are not merely theoretical artifacts of agnostic SMEFT scans. Instead, they are a systematic and generic feature of realistic ultraviolet completions involving multiple heavy fields.
  • The authors identify concrete multi-field extensions of the Standard Model that project onto these known blind subspaces.
  • They show that these blind directions persist even after accounting for important higher-order effects like renormalisation group evolution (RGE) and finite one-loop matching corrections. Figures 6, 7, and 8 explicitly illustrate the persistence of blind directions in specific multi-field UV models after including these corrections.
  • This means that classes of new physics models, even those with energy scales not far above the reach of current colliders, might be invisible to TeraZ relying solely on indirect precision measurements. The apparent ability of TeraZ to constrain new physics may therefore be significantly overestimated.

The Solution

The paper concludes that relying solely on the indirect probes offered by TeraZ is often insufficient to rule out broad classes of ultraviolet physics.

  • To overcome the limitations imposed by blind directions and comprehensively probe the SMEFT parameter space, complementary high-energy collider probes are essential.
  • High-energy hadron colliders, for instance, possess the ability to break these blind directions by exploring different kinematic regions of the phase space. In these regions, operators with non-trivial energy dependence contribute more significantly, leading to tighter constraints.
  • Therefore, the proposed approach is to combine the high precision of TeraZ measurements with direct searches and measurements at high-energy colliders. This combination allows for a more complete exploration of potential new physics signals by leveraging the strengths of different experimental approaches.

In essence, the solution is not to abandon precision measurements, but to recognise their inherent limitations due to blind directions in realistic multi-field scenarios and to integrate them with high-energy searches to achieve a more complete picture of physics beyond the Standard Model.

Concept Exploration

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