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Search for a top-philic Z' boson decaying into a $\mathrm{t\bar{t}}$ pair in a final state with jets and an electron or muon in proton-proton collisions at $\sqrt{s}$ = 13.6 TeV

Using 138 fb $^{-1}$ of proton-proton collision data at $\sqrt{s}$ = 13 TeV collected by the CMS experiment, this study presents a search for a top-philic Z' boson decaying into a top-antitop pair in a final state with jets and a lepton, setting the most stringent constraints to date on its production cross section across a mass range of 0.5–3 TeV without observing any significant deviation from the Standard Model.

Original authors: CMS Collaboration

Published 2026-08-04

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: CMS Collaboration

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, cosmic Lego set. For decades, scientists have been building a model called the "Standard Model" to explain how all the tiny pieces—particles like electrons and quarks—snap together to make everything we see. It's a brilliant model that has passed almost every test thrown at it. But, like any good mystery novel, it has some glaring plot holes. It doesn't explain what "dark matter" is (the invisible glue holding galaxies together), why neutrinos have mass, or why there is more matter than antimatter in the universe. Because of these missing pieces, physicists suspect there are hidden "secret agents" lurking in the shadows, particles that don't play by the usual rules.

One popular theory suggests these secret agents might be "top-philic" particles. Think of the "top quark" as the heavyweight champion of the particle world; it's the heaviest known particle. A "top-philic" particle is like a celebrity bodyguard that only cares about the heavyweight champion and ignores everyone else. If these bodyguards exist, they might be the key to unlocking the secrets of dark matter or why the universe is built the way it is. But here's the catch: nobody has ever seen one. They are hypothetical, meaning they exist in our math but haven't been caught in the act yet. This paper is the latest attempt by a team of cosmic detectives to find these elusive bodyguards, or at least prove they aren't hiding in the places we thought they might be.

The Great Top-Quark Hunt

The scientists behind this study, known as the CMS Collaboration, decided to play a high-stakes game of "hide and seek" using the world's most powerful particle microscope: the Large Hadron Collider (LHC) at CERN. They smashed protons together at mind-blowing speeds (13 TeV) to create a chaotic explosion of energy, hoping that out of the debris, a heavy "Z′ boson" (our top-philic bodyguard) would pop into existence.

Here's the tricky part: if this Z′ boson exists, it doesn't just hang out alone. It loves to party with top quarks. The team was looking for a specific scenario where a Z′ boson is created alongside a pair of top quarks, and then the Z′ boson immediately decays into another pair of top quarks. That's four top quarks in total! It's like looking for a specific four-car train wreck in a massive pile of scrap metal.

Because these top quarks are so heavy and energetic, they move so fast that their decay products (the smaller particles they break into) get squished together into giant, messy blobs. To spot these blobs, the team used a super-smart computer algorithm called PARTICLENET. Think of PARTICLENET as a highly trained sniffer dog that can look at a messy pile of trash and instantly say, "Hey, that specific pile smells like a top quark!" This allowed the scientists to filter out the noise and focus on the most promising signals.

They analyzed a massive amount of data collected between 2016 and 2018, equivalent to 138 "inverse femtobarns" of collisions (a fancy way of saying they looked at a huge number of particle crashes). They scanned for Z′ bosons with masses ranging from 0.5 to 3 TeV (thousands of times heavier than a proton) and tested different scenarios for how "wide" or "narrow" these particles might be.

The Verdict: No Bodyguards Found (Yet)

After running the numbers and comparing their data to the predictions of the Standard Model, the result was clear: no sign of the top-philic Z′ boson.

The data looked exactly like what the Standard Model predicted would happen if these new particles didn't exist. There were no unexpected spikes or bumps in the data that would scream, "We found it!" Instead, the universe remained stubbornly quiet.

Because they didn't find the particle, the team didn't just throw up their hands; they set new boundaries. They calculated the strictest limits to date on how likely it is for these particles to exist. They essentially said, "If this Z′ boson exists, it must be heavier than 560 GeV (if it's narrow), 850 GeV (if it's medium-width), or 1,130 GeV (if it's very wide)." They also set limits on how often these particles could be produced, ruling out a wide range of possibilities.

In short, this paper is a victory for the Standard Model, but a bummer for the searchers. They didn't find the new physics they were hoping for, but they successfully narrowed the search area, telling future explorers exactly where not to look. The hunt for the top-philic Z′ boson continues, but now the detectives know the suspect is either hiding in a much heavier, more elusive corner of the universe, or perhaps doesn't exist at all.

Technical Summary: Search for a Top-Philic Z′ Boson in Proton-Proton Collisions at $\sqrt{s} = 13$ TeV

Problem and Motivation
While the Standard Model (SM) successfully explains a vast array of experimental results, it leaves open fundamental questions regarding dark matter, neutrino masses, matter-antimatter asymmetry, and the integration of gravity. Theoretical models addressing these issues often predict Beyond-the-Standard-Model (BSM) particles. While many such models involve new particles produced via light quark-antiquark annihilation, current exclusion limits on their masses have pushed into the multi-TeV range. Consequently, hypotheses involving new particles that favor higher-generation fermions—specifically "top-philic" resonances coupling exclusively to top quarks—have gained relevance. These models are motivated by the large Yukawa coupling of the top quark to the Higgs boson, which contributes significantly to quantum corrections of the Higgs mass, necessitating a BSM mechanism to address the naturalness problem. This analysis investigates a simplified model featuring a color-singlet vector $Z'$ boson that couples solely to top quarks and antiquarks.

Methodology
The search utilizes proton-proton collision data collected by the CMS experiment at the CERN LHC during 2016–2018 at a center-of-mass energy of $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of 138 fb $^{-1}$ . The analysis focuses on a specific production mechanism where a $Z'$ boson is produced in association with a top-antitop ( $t\bar{t}$ ) pair, followed by the decay of the $Z'$ into another $t\bar{t}$ pair, resulting in a final state of four top quarks. The study targets the single-lepton channel (electron or muon), where one top quark decays leptonically and the others hadronically.

Key methodological features include:

Signal Topology: The analysis targets scenarios where the $Z'$ boson is sufficiently massive to produce Lorentz-boosted top quarks. In such cases, the hadronic decay products of the top quarks merge into single large-radius jets.
Machine Learning: The PARTICLENET algorithm is employed to identify these large-radius jets originating from hadronically decaying boosted top quarks ( $t \to Wb \to q\bar{q}'b$ ). This approach utilizes jet substructure variables, jet constituents, and secondary vertices to enhance discrimination against SM backgrounds.
Event Selection: Events are required to contain exactly one tight lepton (electron or muon), at least one $b$ -tagged small-radius jet, and at least two large-radius jets ($tWh+b$ candidates) with transverse momentum ( $p_T$ ) greater than 300 GeV. Additional requirements include a scalar sum of jet transverse momenta ( $H_T$ ) exceeding 700 GeV and missing transverse momentum ( $p_T^{miss}$ ) greater than 60 GeV.
Discriminant Variable: The invariant mass ( $m_{rec}^{Z'}$ ) of the two highest- $p_T$ top quark candidates is used as the primary discriminant variable to search for a resonance.
Background Estimation: The dominant background is SM $t\bar{t}$ + jets production. A signal-depleted control region (CR), defined by requiring zero $b$ -tagged jets, is used to validate background modeling and constrain normalization via a simultaneous maximum likelihood fit with the signal region (SR).
Systematic Uncertainties: Uncertainties affecting both signal and background shapes and normalizations are propagated, including those related to jet energy scale/resolution, $b$ -tagging, lepton reconstruction, pileup, parton distribution functions (PDFs), and theoretical scale variations.

Key Contributions and Results
The analysis sets upper limits on the production cross section of the $Z'$ boson in association with a $t\bar{t}$ pair, multiplied by the branching fraction for $Z' \to t\bar{t}$ , denoted as $\sigma(pp \to t\bar{t}Z') \times \mathcal{B}(Z' \to t\bar{t})$ . The search covers a $Z'$ mass range of 0.5 to 3 TeV and considers intrinsic widths ( $\Gamma_{Z'}/m_{Z'}$ ) of 4%, 10%, 20%, and 50%.

Observation: The observed data are found to be in agreement with the SM background predictions across the entire mass range and for all tested widths. No significant excess is observed.
Exclusion Limits: At 95% confidence level (CL), the observed upper limits on the cross section range from 170 fb to 3 fb for a relative width of 4% over the mass range of 500 to 3000 GeV. For widths of 10%, 20%, and 50%, the ranges are 160–6 fb, 160–9 fb, and 110–17 fb, respectively.
Mass Limits: These cross-section limits translate into lower limits on the $Z'$ boson mass. For widths of 10%, 20%, and 50%, the analysis excludes $Z'$ masses below 560 GeV, 850 GeV, and 1130 GeV, respectively.

Significance
The paper claims that these results represent the most stringent constraints to date on the existence of a top-philic $Z'$ boson. By utilizing the associated production channel ( $pp \to t\bar{t}Z'$ ) and advanced machine-learning techniques for boosted top tagging, the analysis significantly enhances sensitivity to heavy resonances that decay into top quarks, providing tighter bounds on models predicting such particles in the context of dark matter, composite Higgs scenarios, and naturalness problems.

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