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How EFT Interference Warps Top Quark Resonances at the LHC

Ever wondered how new physics might warp particle shapes? Discover how EFT interactions subtly alter top quark resonances at colliders like the LHC! We delve into interference effects that could distort particle ID, finding HL-LHC analyses robust but future colliders may reveal these key new physics signals. Learn why studying resonance shapes is vital for future precision experiments and the hunt for new physics.

Frequently Asked Questions (FAQ) | short

  1. What is Effective Field Theory (EFT) and how is it relevant to searching for new physics? EFT is a framework to describe potential new physics beyond current experimental reach by adding new terms to the Standard Model. It helps build a global picture of potential deviations.

  2. How can new physics interactions distort the ‘shape’ of known particles like the top quark? New physics interactions (e.g., from EFT) can interfere with Standard Model processes, distorting the characteristic “resonance shape” (like a Breit-Wigner) of unstable particles like the top quark during reconstruction.

  3. Why is the study of top-quark pair production particularly relevant for investigating these distortions? Top-quark pairs are abundantly produced at the LHC, providing large statistics ideal for searching for subtle deviations from the Standard Model, including resonance shape distortions.

  4. What are interference effects and how do they impact the sensitivity to new physics? Interference occurs when Standard Model and new physics amplitudes combine, subtly changing observed distributions. Near a resonance, this can “tilt” the expected shape, impacting sensitivity to the new physics signal.

  5. What are four-fermion operators and how do they relate to the distortions discussed? Four-fermion operators are specific EFT interactions involving four fermions. Certain types can interfere with Standard Model processes (like W boson decay in top reconstruction), causing the discussed resonance distortions.

  6. What did the study find about the relevance of resonance distortions at the High-Luminosity LHC (HL-LHC)? While four-fermion interactions do modify the on-shell region, current HL-LHC experimental strategies are unlikely to capture these subtle distortions due to resolution, statistics, and binning limitations.

  7. Why might resonance distortions be more significant at future lepton colliders like the FCC-ee? Future lepton colliders offer higher resolution and precision measurements, potentially making these subtle interference-induced resonance distortions observable and critical for analysis.

  8. What are the implications of these findings for current and future high-energy physics experiments? Current HL-LHC methods for Standard Model particle reconstruction remain robust. However, for future precision lepton colliders, resonance-shape measurements become important complementary probes in the search for new physics.

Frequently Asked Questions (FAQ) | long

  1. What is Effective Field Theory (EFT) and how is it relevant to searching for new physics? Effective Field Theory (EFT) is a theoretical framework used to frame the sensitivity to new physics under well-defined theoretical assumptions. It provides a way to describe potential new interactions that occur at energy scales beyond the reach of current experiments by adding higher-dimensional operators to the Standard Model Lagrangian. This approach is becoming a standard for analysing various processes to build a global picture of potential deviations from the Standard Model.

  2. How can new physics interactions distort the ‘shape’ of known particles like the top quark? New physics interactions, particularly non-resonant contributions like those from certain four-fermion operators in EFT, can interfere with the Standard Model amplitude of a process. This interference can distort the characteristic ‘resonance shape’ (like the Breit-Wigner distribution) of intermediate unstable particles such as the top quark or the W boson when they are reconstructed from their decay products. This distortion can change the line profile and subtly affect how these particles are identified and measured.

  3. Why is the study of top-quark pair production particularly relevant for investigating these distortions? Top-quark pair production is one of the most abundant processes at the Large Hadron Collider (LHC). This high abundance provides large statistical samples, making it a prime channel to search for subtle deviations from Standard Model predictions, including those caused by EFT-induced distortions of resonance shapes. The precise measurement of the top quark and its decay products is crucial for many Standard Model analyses and searches for new physics.

  4. What are interference effects and how do they impact the sensitivity to new physics? Interference effects occur when the Standard Model amplitude of a process and the amplitude from new physics contributions are both present. These amplitudes can either constructively or destructively interfere, leading to subtle changes in the observed distributions. For processes near a particle’s resonance, interference with a relatively constant background from new physics can ‘tilt’ the Breit-Wigner distribution, potentially impacting the sensitivity of experiments to detect the new physics signal, especially in methods that rely on precisely defined Standard Model expectations.

  5. What are four-fermion operators and how do they relate to the distortions discussed? Four-fermion operators are specific types of interactions in EFT that involve four fundamental fermionic particles. These operators can contribute to processes like top-quark pair production and decay. Certain four-fermion operators, such as the Q(3)lq operator discussed in the source, can interfere with the Standard Model processes involving the W boson decay, which is crucial for reconstructing the top quark. This interference is a key mechanism causing the resonance distortions investigated.

  6. What did the study find about the relevance of resonance distortions at the High-Luminosity LHC (HL-LHC)? The study found that while four-fermion interactions do modify the on-shell region (near the particle’s mass) comparably to enhancements in the high-momentum ‘tail’ regions, current experimental strategies at the HL-LHC are unlikely to capture these subtle interference-induced distortions. This is primarily due to the experimental resolutions and statistical limitations inherent to the hadron collider environment and the typical binning used in analyses.

  7. **Why might resonance distortions be more significant at future lepton colliders like the FCC-ee? Future lepton colliders, such as the proposed FCC-ee, offer a significantly different environment compared to hadron colliders. Running close to particle production thresholds (like the top-quark pair threshold), these facilities can provide higher resolution and precision measurements. This increased precision and potentially finer binning of experimental data could make the subtle interference-induced resonance distortions observable, making resonance-shape measurements critical for precision analyses and setting tighter constraints on EFT parameters.

  8. What are the implications of these findings for current and future high-energy physics experiments? The findings suggest that for current HL-LHC analyses, methods based on reconstructing Standard Model particles remain robust, and resonance distortion is not a major confounding factor within the limits observed from high-momentum tails. However, the study underscores the importance of considering these effects for future precision experiments at lepton colliders. Resonance-shape measurements could serve as valuable complementary probes in global EFT analyses, guiding the development of robust and self-consistent experimental strategies for future programmes.

The Problem

  • Effective field theory (EFT) analyses, which are becoming the standard for framing sensitivity to new physics, can be complicated by interference effects between Standard Model (SM) and Beyond the Standard Model (BSM) contributions.
  • These interference effects can lead to distortions of SM particle thresholds and changes in the “line profile” or shape of intermediate unstable particles like the top quark or W boson when they are reconstructed from their decay products.
  • Specifically, an additional, approximately constant BSM contribution can “tilt” the expected Breit-Wigner distribution near the resonance mass through interference, potentially mitigating the natural suppression. This is illustrated in Fig. 1.
  • This distortion could implicitly affect any EFT analysis at the Large Hadron Collider (LHC) and future facilities. The concern is whether effective contributions can affect the reconstruction of SM particles used to set constraints on these interactions.

The Solution/Findings

  • The study provides a detailed quantitative assessment of these resonance distortions using the specific example of four-fermion operators in top-quark pair production at the LHC.
  • The findings indicate that although four-fermion interactions do modify the on-shell region comparably to continuum enhancements, current experimental strategies at the High-Luminosity LHC (HL-LHC) are unlikely to capture these subtle interference-induced distortions.
  • At the HL-LHC, resonance distortion is found to be not a relevant effect in the hadron collider environment, and the existing SM reconstruction techniques remain robust.
  • This is because the coarse-graining (binning) of measured invariant mass distributions mitigates distortion effects, leaving only a slight asymmetry. The limits observed from the tails of distributions also suggest no significant on-shell distortion is observable within those constraints. This is supported by the comparison of “on-shell” and “off-shell” constraints shown in Fig. 3.

Future Potential

  • Nonetheless, such interference effects could become critical for precision analyses at future lepton colliders, such as the FCC-ee.
  • The precision environment of a lepton collider allows for more fine-grained measurements.
  • The shape change of distributions relative to the SM is more pronounced at lepton colliders than at hadron colliders, indicating a greater intrinsic sensitivity. This difference in sensitivity is illustrated by comparing Fig. 2 (LHC) and Fig. 4 (FCC-ee).
  • Projections indicate that future precision experiments could achieve significantly tighter constraints on EFT parameters by being sensitive to these distortions. For example, a refined sensitivity range of [-0.0059, 0.0059] for the O(3)lq operator could be obtained from analysing the invariant mass distribution of leptonic W decays at the FCC-ee, which surpasses what is achievable at the LHC. This is quantitatively shown in Fig. 5.
  • Therefore, while not a major problem for current LHC methods due to limitations in capturing the subtlety, resonance-shape measurements are highlighted as complementary probes that could guide future high-energy physics programmes, especially in precision environments.

Concept Exploration

Significance

This research explores how new, yet undiscovered, physics interactions, described by Effective Field Theory (EFT), can subtly warp the observed “shapes” or resonance profiles of known particles like the top quark at colliders such as the LHC. These distortions arise from interference effects between Standard Model processes and potential new physics. The study finds that while current analyses at the High-Luminosity LHC (HL-LHC) are largely robust to these subtle effects, future, more precise colliders (especially lepton colliders like FCC-ee) could be sensitive to them. This highlights the importance of carefully studying resonance shapes as a complementary avenue in the ongoing quest for new physics, potentially revealing signals that might otherwise be missed.

Resources & Further Watching

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Youtube Hashtags

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Youtube Keywords

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