Searches for New Physics at High Object Masses with CMS

This paper summarizes recent CMS results from Moriond Electroweak 2026 regarding searches for new physics at high object masses, including heavy vector boson, $W^\prime$, and dijet resonance analyses, which observed no significant deviations from the Standard Model and extended sensitivity to multi-TeV scales.

The Gist

Imagine the universe is a giant, high-stakes game of Cosmic Jenga. For decades, physicists have been stacking blocks according to a rulebook called the Standard Model. This rulebook has been incredibly accurate, predicting how particles interact with almost perfect precision. But we know the tower isn't complete. There are missing blocks: What is Dark Matter? Why is there more matter than antimatter? Why is the "weight" of the universe's energy scale so strange?

To find these missing blocks, scientists at the CMS experiment (a massive detector at the Large Hadron Collider in Switzerland) are playing a game of "High-Stakes Smash." They smash protons together at incredible speeds, hoping to create heavy, new particles that have never been seen before.

This paper is a report card on their recent efforts, specifically looking for "heavy" things (particles with masses in the multi-TeV range, which is like trying to find a mountain hidden inside a grain of sand).

Here is the breakdown of their search, explained simply:

1. The "Heavy Weight" Hunt (Heavy Vector Bosons)

The Analogy: Imagine you are looking for a specific type of heavy, invisible truck (a new particle) that might be driving through a busy city. You can't see the truck directly, but you can look for the debris it leaves behind. Sometimes it drops off two heavy crates (leptons), sometimes it drops off a crate and a cloud of smoke (a lepton and missing energy), and sometimes it drops off a pile of bricks (quarks/jets).

• What they did: The team combined data from many different "debris" searches (Run 2 data). They looked for these heavy trucks in every possible configuration.
• The Result: They found no trucks.
• Why it matters: Even though they didn't find the new particles, they managed to say, "If these trucks exist, they must be heavier than 5.5 tons." This pushes the search into new, heavier territory. They also checked if the trucks behaved differently depending on who they were talking to (different types of quarks), and again, found nothing unusual.

2. The "New Run" Search (Run 3 W' Boson)

The Analogy: This is like upgrading your searchlight. The previous search (Run 2) used a flashlight; this new search (Run 3) uses a high-powered laser. They are looking for a specific charged truck (called a W') that decays into a visible particle and a "ghost" (missing energy).

• What they did: They analyzed fresh data from 2022–2023. Because the collisions are now slightly more energetic, they can see further.
• The Result: Still no W' trucks.
• The Win: They pushed the "heaviness limit" up to nearly 6 TeV. If these particles exist, they are even heavier than we thought possible before.

3. The "Shape Shifter" Hunt (Dijet Angular Analysis)

The Analogy: Sometimes, new physics doesn't show up as a single heavy truck. Instead, it's like a subtle change in the wind that pushes the debris in a slightly different direction than expected.

• The Search: They looked at pairs of jets (sprays of particles) and measured the angle at which they flew apart. They asked: "Is the scattering pattern perfectly smooth, or is there a weird bump or dip?"
• The Result: The wind blew exactly as the Standard Model predicted. There were a couple of tiny, wobbly spots in the data (like a slight breeze), but they weren't significant enough to claim a discovery.
• The Power: Even without finding a new particle, this method is sensitive to "invisible" forces. It tells us that if there are new forces acting on these particles, they must be incredibly weak or incredibly heavy (tens of thousands of TeV).

4. The "Double Trouble" Hunt (Pair-Produced Dijets)

The Analogy: Imagine looking for a factory that produces two trucks at once, which then break apart into four smaller boxes.

• The Mystery: In the previous data (Run 2), the scientists saw a weird blip—a potential signal of these double trucks at a very high mass. It was like seeing a shadow that looked like a monster.
• The Update: They went back with new data (Run 3) to see if the shadow was real.
• The Result: The shadow vanished. The new data showed no evidence of the monster. The "blip" from before was just a random statistical fluctuation (a coincidence).
• The Silver Lining: They did find a new small fluctuation, but it wasn't significant. They used this clean data to set strict limits: "If these double-truck factories exist, they must be heavier than 6.3 TeV."

5. The "Bottom-Heavy" Hunt (b-jets)

The Analogy: Some theories suggest the new particles decay into "heavy" bottom-quarks (b-jets). It's like looking for a specific type of heavy, dark-colored brick in the debris pile.

• The Result: They looked for these specific bricks in pairs and found nothing. They set new world-record limits on how heavy these hypothetical particles could be.

The Big Picture Conclusion

"No New Physics... Yet."

If you were hoping for a headline that says "New Particle Discovered," this paper might feel like a letdown. But in science, ruling things out is just as important as finding things.

Think of it like searching for a needle in a haystack.

• If you find the needle, you win.
• If you search the whole haystack and don't find it, you learn something vital: "The needle isn't in this part of the haystack."

This paper tells us that the "needle" (New Physics) isn't hiding in the heavy, resonant, or angular spots we just checked. This forces theorists to go back to the drawing board and invent new, more creative theories.

The Future:
The CMS team is just getting started. They have more data coming (Run 3 is ongoing), and their tools are getting sharper. They are essentially turning up the volume on the universe's radio, listening for a signal that hasn't been heard yet. Until then, the Standard Model remains the champion, but the hunt for the next level of reality continues.

Technical Summary

1. Problem Statement

The Standard Model (SM) of particle physics, while highly successful, fails to address fundamental questions such as the nature of dark matter, the origin of matter-antimatter asymmetry, and the stability of the electroweak scale. Theoretical extensions to the SM predict new heavy particles (e.g., heavy vector bosons, dark matter candidates, extra dimensions) that could manifest at the Large Hadron Collider (LHC).

The primary challenge is that the parameters of these models are generally unknown, meaning new physics signatures could appear in various final states (jets, leptons, missing transverse momentum) via both resonant production (new particles decaying into SM particles) and non-resonant distortions (deviations in the tails of SM spectra). The paper addresses the need to systematically probe these high-mass regimes using the full Run 2 dataset and early Run 3 data to constrain these theoretical models.

2. Methodology

The analysis utilizes data from the CMS experiment at $\sqrt{s} = 13$ TeV (Run 2, 138 fb$^{-1}$) and $\sqrt{s} = 13.6$ TeV (Run 3, 62 fb$^{-1}$ and 90 fb$^{-1}$). The methodology is divided into four distinct search strategies:

• Heavy Vector Boson (HVB) Combination (Run 2):

  • A statistical combination of 16 individual searches for spin-1 resonances ($V'$).
  • Channels: Includes bosonic decays ($WW, ZZ, WH, ZH$) and fermionic decays ($qq, bb, tt, tb, \ell\ell, \ell\nu$).
  • Framework: Interpreted using the Heavy Vector Triplet (HVT) model, which parameterizes couplings to fermions ($g_F$) and bosons ($g_H$).
  • Goal: To resolve local fluctuations ($2\sigma$–$3\sigma$) reported in individual channels and provide global exclusion limits on coupling parameters.

• $W' \to \ell\nu$ Search (Run 3):

  • Signature: One isolated high-$p_T$ lepton ($e$ or $\mu$) and large missing transverse momentum ($p_T^{miss}$).
  • Discriminant: Transverse mass ($m_T$).
  • Background Modeling: Dominated by SM $W$+jets, modeled with NNLO QCD and NLO electroweak corrections.
  • Strategy: Includes both a Sequential Standard Model (SSM) benchmark and a model-independent interpretation based on event counts above a minimum $m_T$ threshold.

• Dijet Angular Analysis (Run 2):

  • Objective: Probe non-resonant distortions in the scattering angle of dijet systems.
  • Variable: Uses $\chi_{dijet} \sim \frac{1 + \cos(\theta^*)}{1 - \cos(\theta^*)}$, which is robust against PDF and $\alpha_s$ uncertainties.
  • Comparison: Data distributions are compared against NNLO QCD + NLO EW predictions, smeared to match detector response.
  • Scope: Tests effective field theories (EFT) including contact interactions, virtual graviton exchange, and quantum black holes.

• Pair-Produced Dijet Resonances (Run 2 & Run 3):

  • Inclusive Search (Run 3): Targets events with $\ge 4$ jets and invariant mass $m_{4j} > 1.6$ TeV. Uses a binning strategy in $\alpha = m_{2j}/m_{4j}$ to avoid background sculpting.
  • $b$-tagged Search (Run 2): Targets final states with $b$-jets to probe R-parity violating (RPV) top-squark models and diquark-to-diquark resonances.

3. Key Contributions

• Run 2 HVB Combination: This is the first comprehensive combination of heavy vector boson searches covering both bosonic and fermionic final states. It clarifies previous local excesses by demonstrating they are statistical fluctuations when channels are combined.
• Run 3 Sensitivity Extension: The $W' \to \ell\nu$ search utilizes the higher collision energy (13.6 TeV) and lower trigger thresholds of Run 3 to push sensitivity limits beyond Run 2 capabilities.
• Model-Independent Limits: The $W'$ analysis provides limits based on event counts rather than specific signal shapes, allowing reinterpretation for scenarios with large widths or interference effects.
• Resolution of Run 2 Excesses: The Run 3 dijet analysis specifically investigates a previous excess observed in Run 2 ($m_{4j} \approx 8$ TeV), providing a definitive update with new data.
• First $b$-tagged Limits: Establishes the first LHC limits on resonant paired dijets containing $b$-jets.

4. Results

• Heavy Vector Bosons:

  • No significant deviation from the SM was observed.
  • Exclusions: Heavy vector resonances are excluded below 5.5 TeV (weakly coupled) and 4.8 TeV (strongly coupled). Vector boson fusion production is excluded up to 2 TeV.
  • Coupling Limits: The combination excludes significant portions of the $(g_F, g_H)$ and $(g_{q3}, g_{q12})$ parameter spaces, achieving complete exclusion of certain non-universal coupling scenarios that single channels could not.

• $W' \to \ell\nu$ (Run 3):

  • No evidence for new physics.
  • Exclusions: 95% CL exclusions set at 5.7 TeV (electron), 5.6 TeV (muon), and 5.9 TeV (combined). The expected sensitivity is 6.2 TeV.
  • This represents a significant improvement over Run 2, driven by higher energy and improved triggers.

• Dijet Angular Analysis:

  • Data generally agrees with NNLO QCD + NLO EW predictions.
  • A mild shape difference was noted for dijet masses between 2.4–4.8 TeV and $>6$ TeV, but this is consistent with alternative scale choices.
  • Exclusions: New physics scales are excluded up to tens of TeV for various BSM scenarios (contact interactions, quantum black holes, etc.).

• Pair-Produced Dijets:

  • Run 3 Update: The Run 2 excess at $m_{4j} \approx 8$ TeV is not confirmed; no events were observed in that region. A new fluctuation was observed at $m_{4j} = 4.7$ TeV (local significance 3.3$\sigma$, global 1.0$\sigma$), which is not significant.
  • Exclusions: In the scalar-diquark model, masses below 6.3–8 TeV are excluded.
  • $b$-tagged Results: Top squarks (RPV) are excluded for masses 0.5–0.85 TeV. Heavy diquark states are excluded for masses 2–7 TeV.

5. Significance

• Validation of the Standard Model: Across all high-mass regimes (resonant and non-resonant), the data remains consistent with SM predictions, placing stringent constraints on BSM theories.
• Resolution of Anomalies: The combination of channels and the arrival of Run 3 data have successfully resolved previous local fluctuations, preventing false claims of discovery and refining the understanding of statistical backgrounds.
• Extended Reach: The results extend the sensitivity of the CMS experiment to multi-TeV scales (up to ~6 TeV for direct resonance searches and tens of TeV for contact interactions), probing energy scales well beyond the direct kinematic reach of the LHC.
• Future Outlook: These results establish a robust baseline for future analyses. The continued accumulation of Run 3 data, combined with improvements in reconstruction, triggering, and theoretical modeling, will further push the boundaries of new physics discovery.