Search for heavy long-lived charged particles with… — Plain-Language Explanation

Imagine the Large Hadron Collider (LHC) as a massive, high-speed train station where protons (tiny particles) are smashed together billions of times a second. Usually, the station's security system (the "trigger") is so busy that it only lets a tiny fraction of the passengers through the gate to be recorded. It's programmed to look for specific, fast-moving "VIPs" (particles that zip through the station in a flash).

However, scientists suspect there might be "slow walkers" in the crowd—massive, heavy particles that move so slowly they take their time crossing the station. Because they are slow, the standard security system often misses them, thinking they are just noise or ignoring them because they don't fit the "fast VIP" profile.

This paper is about a new experiment by the CMS Collaboration that decided to try a different approach: Data Scouting.

The "Scout" Strategy

Instead of waiting for the security system to decide which passengers to let in, the team used a "scout" system. This scout records a tiny, compressed summary of every single person passing through the gate, regardless of how fast they are walking. It's like having a security guard who takes a quick photo of everyone's shoes and speed, even if they are just strolling, without stopping to ask for a ticket.

Because they recorded everything (well, almost everything, due to storage limits), they could look for the "slow walkers" that the usual system would have ignored.

The Mystery of the "Heavy Long-Lived" Particles

The scientists were hunting for Heavy Stable Charged Particles (HSCPs). Think of these as extremely heavy, slow-moving ghosts.

Heavy: They are much heavier than normal particles.
Slow: Because they are heavy, they move sluggishly (like a bowling ball rolling down a hallway compared to a bullet).
Long-lived: They don't disappear quickly; they stick around long enough to cross the entire detector.

In the past, if these particles were too slow, they would take so long to cross the detector that they would arrive at different "time slots" (called bunch crossings) than the collision that created them. The old security system required the start and end of a particle's journey to happen in the exact same time slot. If the particle was too slow, the system would break the connection and discard the data.

How They Caught the Slow Walkers

By using the "scout" data, the team could stitch together the particle's journey even if it took several time slots to cross the detector.

The Analogy: Imagine a runner in a relay race. In the old system, if the runner took too long to pass the baton, the race officials would assume the runner didn't finish. In this new system, the officials watched the runner from start to finish, even if the runner took a nap between legs of the race. They could see the runner crossed the finish line, even if it was in a different "lap" than the start.

What They Found

The team analyzed data from 2024, looking for these slow, heavy particles.

The Result: They didn't find any "slow walkers." No new heavy particles were discovered.
The Conclusion: Since they didn't find them, they set a "speed limit" for where these particles could exist. They essentially said, "If these heavy particles exist, they must be rarer than we thought, or they must be moving even slower than our current search could catch."

Why This Matters

Even though they didn't find the particles, the experiment was a huge success for a different reason: Proof of Concept.

It proved that the "scout" system works. It showed that scientists can now look for particles that move at speeds where previous searches were blind (specifically, particles moving at 15% to 50% of the speed of light).
It opened a new door. Before this, if a particle was too slow, it was invisible to the detectors. Now, the door is open to look for these "slow" heavy particles in the future.

Summary

The paper is a report card on a new way of looking at the universe. The CMS team tried a new technique to find heavy, slow-moving particles that previous searches missed. They didn't find the particles, but they proved the new technique works and successfully ruled out certain possibilities for where these particles might be hiding. It's like checking a dark room with a new kind of flashlight; you didn't find a monster, but you proved the flashlight works and that the room is definitely empty of monsters in that specific spot.

Technical Summary: Search for Heavy Long-Lived Charged Particles with Level-1 Trigger Scouting Data

Problem and Motivation
Heavy stable charged particles (HSCPs) are predicted by various beyond-the-Standard-Model (BSM) scenarios, including supersymmetry and extra dimensions. These particles, if produced in proton-proton collisions at the LHC, would traverse the detector with high ionization energy loss and unusually long time-of-flight due to their large mass and slow velocity ( $\beta = v/c$ ).

Previous searches by the CMS Collaboration and other experiments have primarily relied on standard trigger strategies, such as muon-based triggers requiring reconstructed tracks in the tracker and muon system to align within a single bunch crossing (BX). This approach limits sensitivity to HSCPs with $\beta \gtrsim 0.6$ , which can cross the detector within one BX. Very slow particles ( $\beta \lesssim 0.6$ ) often require multiple BXs to traverse the muon detector, causing them to fail standard trigger conditions. While missing transverse momentum triggers exist, they suffer from low and model-dependent efficiency. Consequently, previous LHC searches have been limited to HSCP masses up to approximately 3 TeV and have struggled to probe the low- $\beta$ regime.

Methodology
This analysis presents the first search for HSCPs utilizing the novel CMS Level-1 Trigger Data Scouting (L1DS) system. Unlike traditional data taking, which records full event data only when a specific trigger condition is met, the L1DS system acquires data directly from the Level-1 (L1) trigger at the full bunch crossing rate of 40 MHz (25 ns spacing) without any preselection.

Data Sample: The search uses proton-proton collision data collected in 2024 at a center-of-mass energy of $\sqrt{s} = 13.6$ TeV. The dataset corresponds to an integrated luminosity of 3.7 fb $^{-1}$ , collected with a prescale factor of $N=15$ (one orbit stored every 15 orbits).
Reconstruction Strategy: The analysis reconstructs HSCPs as muon-like tracks using "stubs" (combined drift tube and resistive-plate chamber hits) stored in the L1DS output. A modified offline version of the barrel muon track finder (kBMTF) algorithm is employed. Crucially, this algorithm fits stubs separated by up to eight BXs, allowing for the reconstruction of particles that traverse the muon detector layers over multiple BXs.
Signal Models: The results are interpreted using three benchmark models:
1. Non-resonant pair production of fourth-generation fermions ( $\tau'$ ) via Drell-Yan processes.
2. Resonant production of $\tau'$ pairs via a heavy $Z'$ boson.
3. Pair production of long-lived gluinos ( $\tilde{g}$ ) forming R-hadrons (specifically R-gluinoballs), where the R-hadron is neutral in the tracker but may acquire a charge in the muon detector.
Event Selection: Events are selected based on tracks with 3 or 4 stubs spanning a minimum of 2 and a maximum of 4 BXs. The selection requires $|\eta| < 0.83$ and $p_T > 15$ GeV. To enhance sensitivity to specific $\beta$ ranges, 14 distinct categories are defined based on the number of BXs spanned and the specific layer configuration of the stubs (e.g., BX123, BX1112).
Background Estimation: Backgrounds, primarily arising from ultra-relativistic particles with BX misidentification or asynchronous combinations of unrelated hits, are estimated directly from data. The method utilizes "asynchronous" track orderings (where the time ordering of stubs is unphysical for a particle exiting the beam spot) to model the $p_T$ distribution of the background in the signal region. Two orthogonal validation regions (low track quality and non-colliding bunches) confirm the background modeling.

Key Contributions

Proof of Concept for L1DS: This work serves as the first physics analysis relying on the L1DS system, demonstrating its capability to collect and analyze data without trigger biases.
Extended $\beta$ Sensitivity: By reconstructing tracks across multiple BXs, the analysis extends sensitivity to HSCPs with $\beta$ values between 0.15 and 0.80. This fills a critical gap left by previous searches that were blind to particles with $\beta \lesssim 0.6$ .
Neutral-to-Charged Transition: The search is sensitive to neutral R-hadrons that become charged only upon interacting with matter in the muon detector, a signature inaccessible to tracker-based ionization loss searches.
High-Mass Reach: The analysis probes HSCP masses up to 6.5 TeV, a range where standard triggers are ineffective due to the low $\beta$ of heavy particles.

Results
No significant excess of events was observed above the Standard Model background expectations in any of the 14 search categories.

Cross-Section Limits: Upper limits at 95% confidence level (CL) were set on the production cross sections for the benchmark models.
- For non-resonant $\tau'$ production, limits are set for masses between 1 and 6.5 TeV.
- For gluino R-hadrons with $f=1$ (fully neutral at production), limits are set up to 6.5 TeV.
- For resonant $Z' \to \tau'\tau'$ production, 2D limits are provided as a function of $m_{\tau'}$ and $m_{Z'}$ .
Fiducial Limits: Model-independent upper limits were set on the fiducial cross section for a heavy particle with $p_T > 500$ GeV, $|\eta| < 0.83$ , and specific $\beta$ ranges. The most stringent limit is 3.5 fb for particles with $0.1875 < \beta < 0.2125$ .
Sensitivity Characteristics: The sensitivity is driven by 4-stub categories, particularly the BX123 category (4 stubs across 3 BXs). Sensitivity degrades around $\beta \approx 0.5$ (where particles often cross the detector entirely in the BX following the collision, mimicking ultra-relativistic tracks) and for $\beta \lesssim 0.15$ (where particles are stopped in the detector).

Significance
The paper claims this analysis is a crucial proof of concept for the L1DS system, validating its physics potential. The primary significance lies in extending the sensitivity of HSCP searches to lower $\beta$ values and higher masses (up to 6.5 TeV) than previously possible with standard trigger strategies. While previous CMS analyses based on ionization loss provided tighter limits at lower masses ( $m < 2.6$ TeV) due to larger datasets and acceptance, they lacked sensitivity to the high-mass, low- $\beta$ regime where HLT efficiency drops to zero. This analysis complements existing searches by targeting the unique signature of slow-moving particles spanning multiple bunch crossings and neutral particles that acquire charge in the muon system.

Search for heavy long-lived charged particles with level-1 trigger scouting data from proton-proton collisions at s\sqrt{s}s​ = 13.6 TeV