Searching for New Physics Inside Jets with the Herwig 7 Generalised Parton Shower

This paper utilizes the Herwig 7 framework to demonstrate that massive $Z'$ bosons produced via BSM parton shower radiation, rather than direct hard processes, populate non-isolated regions within jets, offering a novel and previously overlooked avenue for discovering new physics through jet substructure analysis at the LHC and future colliders.

The Gist

The Big Idea: Looking for Ghosts in the Crowd

Imagine you are at a massive, chaotic music festival (the Large Hadron Collider or LHC). The crowd is so dense and loud that it's impossible to hear a single person whispering.

For decades, physicists have been looking for "New Physics" (mysterious new particles like the $Z'$ boson) by waiting for them to pop out of the crowd like a VIP celebrity walking down a red carpet. They look for clean, isolated signals: a particle appearing alone, clearly separated from the noise.

This paper says: "Stop looking at the red carpet! The VIP might be hiding inside the crowd, wearing a disguise."

The authors are proposing a new way to search for new physics: looking for particles that are born inside a jet of other particles (a "jet" is just a spray of debris from a high-energy collision). Specifically, they are looking for a heavy particle ($Z'$) that decays into two muons (a type of heavy electron) right inside a spray of other particles, making them look "non-isolated" or "dirty."

The Problem: The "Fake" Lepton Blind Spot

In the past, if you saw two muons stuck inside a spray of other particles, physicists would usually ignore them. They assumed these were just "fake" signals caused by the messy debris of the collision (like a random fan in the crowd bumping into someone). They only looked for muons that were standing alone, far away from the crowd.

But what if the new physics prefers to hide in the crowd? If we only look at the VIPs walking alone, we might miss the ones hiding in the mosh pit.

The Tool: A New Camera Lens (Herwig 7)

To prove this is possible, the authors needed a better camera. They used a new version of a computer simulation program called Herwig 7.

Think of particle collisions like a game of billiards, but the balls are made of energy and can split into smaller balls infinitely.

• Old Simulations: These programs were great at simulating the main collision (the "Hard Process") but were bad at simulating the messy "aftermath" where particles keep splitting and radiating energy (the "Parton Shower"). They mostly assumed new particles only appeared in the main collision.
• The New Simulation: The authors updated Herwig 7 to allow new particles (like the $Z'$) to be created during the messy aftermath. It's like upgrading the billiard game to allow the balls to spontaneously split into new, invisible balls while they are still rolling across the table.

The Experiment: The $Z'$ in the Jet

They tested a specific theory called $U(1)_{B-L}$, which predicts a new particle called a $Z'$.

1. The Setup: They simulated collisions where two jets of particles are created.
2. The Twist: Inside one of those jets, a $Z'$ particle is born (via radiation) and immediately splits into two muons.
3. The Result: These two muons are now "non-isolated." They are huddled together inside a jet, surrounded by other particles.

The Detective Work: How to Spot Them

The paper asks: "How do we tell the difference between a $Z'$ hiding in a jet and just random junk?"

They found a few "tells" (clues):

• The Angle: The two muons from the $Z'$ are very close together, almost touching, because the $Z'$ was moving fast and didn't have time to spread out.
• The Energy: The muons carry a specific amount of energy relative to the rest of the jet.
• The "Scouting" Strategy: Usually, detectors have a "bouncer" (a trigger) that only lets in muons with high energy. This paper suggests using a "Scouting" mode, which is more lenient and lets in lower-energy muons. This is crucial because the $Z'$ might be light and produce softer muons that the standard bouncer would kick out.

The Results: What Did They Find?

They ran the numbers to see if the LHC could actually find this.

• The Good News: Yes! If the $Z'$ exists and has the right properties, the LHC (specifically the CMS detector) could find it, even with the data they have right now (Run 2 + Run 3).
• The Future: If they wait for the "High-Luminosity LHC" (a supercharged version of the collider in the future), they could find these hidden particles even more easily.
• The Catch: The standard "bouncer" (trigger) might miss the lighter versions of these particles. But if they use the "Scouting" mode (letting in softer particles), their chances of discovery go up significantly.

The Bottom Line

This paper is a call to action for physicists. It says: "Don't just look for the clean, isolated signals. Start looking in the messy, crowded places inside the jets."

By using a new, more realistic simulation tool, they showed that new physics might be hiding in plain sight, disguised as "dirty" muons inside a jet. If we change our search strategy to look for these "non-isolated" signatures, we might finally solve some of the biggest mysteries of the universe, like what Dark Matter is made of.

In short: They built a better flashlight to look into the dark corners of the particle collision, and they found that the monsters (new physics) might be hiding there all along.

Technical Summary

1. Problem Statement

The Standard Model (SM) is incomplete, yet direct searches for Beyond-the-Standard-Model (BSM) physics at the Large Hadron Collider (LHC) have yielded no direct evidence using standard objects (e.g., isolated leptons, high-mass resonances). Consequently, attention is shifting to "hidden" regions of phase space, such as non-isolated leptons embedded within jets.

• The Gap: Traditional BSM searches assume new particles (like a $Z'$ boson) are produced directly in the hard scattering process. However, new physics could also manifest via radiation during the parton shower evolution, where a neutral BSM particle is emitted inside a jet and decays into a collimated pair of muons.
• The Challenge: These "non-isolated" di-muon signatures are typically discarded as fake leptons or background (e.g., from heavy-flavor decays) because they are buried in QCD jets. Furthermore, simulating BSM radiation within the parton shower has historically been limited, with few general frameworks available to handle the interplay between SM and BSM emissions.

2. Methodology

The authors utilize the Herwig 7 Monte Carlo event generator, specifically its newly developed Generalised Parton Shower framework, to simulate BSM radiation.

• Theoretical Framework:

  • Model: A minimal $U(1)_{B-L}$ extension of the SM featuring a light $Z'$ boson. The $Z'$ couples vector-like to quarks and leptons and decays exclusively to $\mu^+\mu^-$.
  • Shower Algorithm: The study employs the angular-ordered parton shower in Herwig 7. This formalism preserves soft-gluon coherence and handles massive particle emission correctly. The splitting function for $f \to f'Z'$ is implemented based on UFO model inputs, preserving spin correlations and helicity amplitudes.
  • Regime: The focus is on the logarithmically enhanced regime (soft and collinear emissions), where the $Z'$ is radiated from a quark or gluon line within the jet.

• Event Generation Strategy:

  • Signal: Hard di-jet events are generated at the Matrix Element (ME) level using MadGraph 5. The subsequent evolution, including simultaneous SM QCD radiation and BSM $Z'$ radiation, is handled by Herwig 7.
  • Validation: To validate the shower algorithm, the authors compare the fully showered signal against:
    1. Fixed-order (FO) $Z' + 2$-jet samples generated in MadGraph 5 (with SM shower added).
    2. Samples where only the $Z'$ shower is active (no SM shower).
  • Backgrounds: Dominant backgrounds include QCD processes (heavy-flavor decays, gluon splitting $g \to b\bar{b}/c\bar{c}$), top-quark production ($t\bar{t}$, $tW$), and Drell-Yan (DY) with jets. These are modeled using PYTHIA 8 (QCD) and POWHEG+PYTHIA 8 (Top) with appropriate tunes (CP5).

• Analysis Strategy:

  • Selection: Events require a high-$p_T$ jet containing an oppositely charged muon pair.
  • Triggers: Two strategies are compared:
    1. Standard Trigger: Requires a leading muon with $p_T > 52$ GeV (CMS standard).
    2. Scouting Trigger: Allows lower thresholds ($p_T > 5$ GeV for both muons), crucial for probing low-mass $Z'$ bosons ($5 < m_{\mu\mu} < 50$ GeV).
  • Kinematics: Key observables include the angular separation $\Delta R(j, Z')$ and the muon energy fraction inside the jet ($E_\mu / E_{jet}$).
  • Statistics: Sensitivity is estimated using the Asimov approximation for statistical significance ($Z_A$) and 95% confidence-level (CL) exclusion limits, assuming integrated luminosities of 500 fb$^{-1}$ (Run 2+3) and 3 ab$^{-1}$ (HL-LHC).

3. Key Contributions

1. First Phenomenological Application: This is the first study to utilize the full Herwig 7 BSM parton shower framework with simultaneous SM and BSM evolution. Previous studies often isolated BSM emissions or treated them at fixed order.
2. Validation of Shower Mechanism: The authors demonstrate that the angular-ordered shower correctly reproduces the logarithmically enhanced collinear phase space. They show that while fixed-order calculations describe wide-angle emissions, the shower provides a superior description of the soft/collinear $Z'$ radiation, with yields exceeding fixed-order predictions as $\Delta R \to 0$ due to higher-order resummation.
3. Identification of Kinematic Features: The study establishes that $Z'$ bosons produced via shower radiation populate non-isolated regions inside jets. The muons from the $Z'$ decay are highly collimated with the jet axis ($\Delta R < 0.3$) and carry a significant fraction of the jet's energy.
4. Scouting Trigger Viability: The paper quantifies the critical role of "scouting" data strategies. Standard triggers lose sensitivity to low-mass $Z'$ bosons due to high $p_T$ thresholds, whereas scouting triggers retain high acceptance for soft, non-isolated muons.

4. Results

• Kinematic Distributions:
  • The angular separation $\Delta R(q, Z')$ peaks sharply at small values for shower-induced events, confirming the collinear nature of the radiation.
  • The muon energy fraction inside the jet shows a distinct distribution compared to SM backgrounds, particularly when the SM shower is active, as it softens the partner parton.
• Sensitivity Projections:
  • Standard Trigger: With 500 fb$^{-1}$, evidence-level sensitivity ($Z_A \approx 3$) is achievable for $Z'$ masses up to $\sim 25$ GeV for a coupling $\alpha_{qZ'} \times \text{BR} \approx 10^{-7}$.
  • Scouting Trigger: Provides systematically higher sensitivity across the full mass range (5–50 GeV). It closes the "gap" in the low-mass region where standard triggers fail.
  • HL-LHC: Projections for 3 ab$^{-1}$ suggest that the entire mass range of $O(10)$ GeV could become accessible for discovery or exclusion.
• Backgrounds: The dominant background in the low-mass region is QCD (heavy-flavor decays), while top-quark processes become relevant at higher masses. The scouting trigger is particularly effective because the signal (symmetric muon $p_T$) differs kinematically from the top background (asymmetric $p_T$).

5. Significance

• New Search Avenue: This work opens a complementary avenue for BSM searches that is currently overlooked. It demonstrates that new physics can hide inside jets via parton shower radiation, a mechanism not covered by traditional isolated-lepton analyses.
• Model Independence: While using a $U(1)_{B-L}$ benchmark, the methodology is model-agnostic and can be applied to other scenarios (e.g., Dark Shelters, Hidden Valleys, or models with flavor-changing neutral currents).
• Experimental Impact: The results provide a strong motivation for LHC experiments (ATLAS/CMS) to utilize scouting data and refine reconstruction strategies for non-isolated leptons. It suggests that significant new physics discovery potential remains in the low-mass, high-jet-multiplicity regime, which will become even more relevant at the High-Luminosity LHC (HL-LHC) due to increased pileup and jet activity.
• Theoretical Validation: It validates the capability of modern event generators (Herwig 7) to handle complex BSM radiation scenarios, bridging the gap between theoretical predictions and experimental reconstruction.

In conclusion, the paper argues that BSM parton shower radiation is a realistic and phenomenologically rich mechanism for uncovering new physics, offering a viable path to discover light $Z'$ bosons that would otherwise remain invisible to standard search strategies.