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Search for a new neutral gauge boson produced in association with one or two b jets and decaying into a pair of muons in proton-proton collisions at $\sqrt{s}$ = 13 TeV

This paper presents a search for a new neutral gauge boson (Z') produced in association with one or two b jets and decaying into a pair of muons using 138 fb $^{-1}$ of 13 TeV proton-proton collision data from the CMS detector, finding no significant deviation from background expectations and setting the most stringent limits to date in the 125–350 GeV mass range.

Original authors: CMS Collaboration

Published 2026-07-13

📖 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, high-speed racetrack where tiny particles zoom around at nearly the speed of light. For years, physicists have been watching this track, looking for a new kind of car—a mysterious, invisible vehicle called a Z' boson (pronounced "Z-prime"). This isn't just any car; it's a heavy, neutral particle that might explain some of the universe's biggest secrets, like why there is more matter than antimatter or what dark matter is made of.

In this latest race, the CMS Collaboration (a massive team of scientists working at CERN's Large Hadron Collider) acted like ultra-precise race officials. They scanned 138 fb⁻¹ of proton-proton collision data (that's a huge amount of racing footage from 2016 to 2018) to see if this new Z' car ever showed up.

The Hunt for the "Ghost" Car
The scientists had a specific theory about how this Z' car might behave. They thought it wouldn't just appear out of nowhere; it would likely be produced alongside one or two "b-jets" (jets of particles containing bottom quarks) and then immediately crash into a pair of muons (heavy cousins of electrons). It's like looking for a specific type of ghost that only appears when two other ghosts are dancing nearby.

To find this ghost, the team set up a clever trap using a method called the ABCD technique. Imagine you have four rooms in a house:

Room A (The Signal Room): This is where you expect to find the Z' ghost, but it's also where the background noise is loudest.
Rooms B, C, and D (The Control Rooms): These rooms are filled with "normal" background noise (like standard particle collisions) but are designed so the ghost shouldn't be there.

By carefully measuring how much noise is in Rooms B, C, and D, the scientists could mathematically predict exactly how much noise should be in Room A. If the actual noise in Room A was higher than the prediction, that extra bit would be the ghost!

The Result: No Ghost Found
After checking every corner of the track for Z' bosons with masses between 125 and 350 GeV, the scientists found nothing. The number of events they saw in the "Signal Room" matched the noise prediction from the control rooms perfectly.

This means that, within the mass range they checked, no new Z' boson was discovered. The data is consistent with the Standard Model (our current rulebook of physics) and does not show any signs of this new particle.

What This Means for the Rules
Even though they didn't find the particle, the search wasn't a failure. It's like checking a map and realizing, "Okay, the treasure isn't buried here." The team set strict upper limits on how often this Z' boson could exist. They determined that if it does exist, its production rate (cross section) multiplied by how often it turns into muons must be less than 0.2 to 2 fb (femtobarns).

This is a big deal because:

It rules out specific theories: Many models suggested this Z' boson would be easy to spot in this mass range (125–200 GeV). The paper explicitly states these are the only results to date in that lower mass range, effectively closing the door on those specific ideas.
It's the best so far: For the higher mass range (200–350 GeV), these are the most stringent limits yet for Z' bosons produced via bottom quark fusion. They are tighter than any previous search for this specific type of production.

The Bottom Line
The paper doesn't claim to have solved the mystery of the Z' boson. Instead, it says, "We looked very hard in this specific spot, using the best tools we have, and we didn't see it." The search continues, but for now, the Z' boson remains a ghost that refuses to be caught in the 125–350 GeV mass range. The scientists have successfully narrowed the search area, telling future explorers exactly where not to look next.

Technical Summary: Search for a New Neutral Gauge Boson ( $Z'$ ) Produced in Association with $b$ Jets and Decaying to Dimuons

Problem and Motivation
The Standard Model (SM) of particle physics fails to explain several observed phenomena, including neutrino masses, matter-antimatter asymmetry, and dark matter. Recent anomalies in $B$ meson decay rates, specifically the ratios $R(K^*)$ and $R(D^*)$ , have suggested potential violations of lepton flavor universality in $b \to s\ell\ell$ and $b \to c\ell\nu_\ell$ transitions. While updated LHCb measurements have alleviated some tensions in $b \to s\ell\ell$ transitions, theoretical models proposing a new neutral gauge boson ( $Z'$ ) with non-universal couplings to third-generation fermions remain viable. Specifically, models involving a spontaneously broken $U(1)_X$ gauge group with family-dependent charges suggest a $Z'$ boson that couples preferentially to bottom quarks and muons.

Previous searches for $Z'$ bosons at the LHC have largely focused on inclusive production via light-quark fusion or high-mass regions ( $>350$ GeV). However, these searches often assume flavor-universal couplings, which may not apply to models addressing the $B$ -physics anomalies. There is a specific need to probe the low-mass region ( $125–350$ GeV) for $Z'$ bosons produced via bottom-bottom ( $b\bar{b}$ ) or bottom-strange ( $b\bar{s}$ ) quark fusion, as this production mode is enhanced in models with non-universal couplings but has been less constrained by inclusive searches.

Methodology
The analysis utilizes proton-proton collision data collected by the CMS detector at $\sqrt{s} = 13$ TeV during the LHC Run 2 (2016–2018), corresponding to an integrated luminosity of $138 \text{ fb}^{-1}$ .

Signal Model: The search targets a narrow spin-1 $Z'$ resonance (width $<1\%$ of its mass) decaying into a dimuon pair ( $\mu^+\mu^-$ ). The $Z'$ is produced in association with one or two jets, where at least one jet is $b$ -tagged. The benchmark model assumes exclusive couplings to muons and muon neutrinos in the lepton sector and bottom quarks (and potentially strange quarks via flavor-changing neutral currents, $\delta_{bs}$ ) in the quark sector.
Event Selection: Events are selected requiring exactly two opposite-sign, same-flavor leptons (muons) with $p_T > 53$ GeV and $|\eta| < 2.4$ . The analysis categorizes events based on jet multiplicity (1-jet or 2-jet) and $b$ -tagging status. Signal regions (SRs) require at least one $b$ -tagged jet. Control regions (CRs) are defined using dielectron pairs or light-quark/gluon jets to estimate backgrounds.
Background Estimation: Due to observed discrepancies between simulation and data in Drell-Yan plus $b$ $b$ -jet cross-section measurements, the background estimation relies entirely on data-driven methods using an ABCD matrix technique.
- The method utilizes two uncorrelated variables: lepton flavor (muon vs. electron) and $b$ -tagging status (b-tagged vs. non-b-tagged).
- The signal yield in the dimuon $b$ -tagged region is predicted using yields from three control regions: dielectron $b$ -tagged, dimuon non- $b$ -tagged, and dielectron non- $b$ -tagged.
- Transfer factors derived from non- $b$ -tagged regions correct for differences in acceptance and resolution between electrons and muons.
Systematic Uncertainties: Uncertainties include those from trigger efficiency, lepton reconstruction/identification, jet energy scale/resolution, $b$ -tagging efficiency, pileup modeling, and parton distribution functions (PDFs). A specific uncertainty is assigned to the closure of the ABCD method based on fit variations in the control regions.
Statistical Analysis: A simultaneous maximum likelihood fit is performed across data-taking years and jet multiplicity categories. Limits are set on the product of the cross-section, branching fraction to dimuons, acceptance, and efficiency ( $\sigma \times \mathcal{B} \times A \times \epsilon$ ) using the asymptotic approximation at 95% confidence level (CL).

Key Contributions

First Results in Low-Mass Range: This paper presents the first specific limits on $Z'$ production via bottom quark fusion in the mass range of $125–200$ GeV.
Stringent Constraints on $b$ -Fusion: For the $200–350$ GeV mass range, the results provide the most stringent constraints to date on $Z'$ production modes involving bottom quark fusion ( $b\bar{b}$ and $b\bar{s}$ ), complementing inclusive $Z'$ searches.
Model-Independent Presentation: The results are provided in a model-independent manner, offering process-dependent acceptance and efficiency values for seven distinct production categories (e.g., $0b(b-s)$ , $0b(b-b)$ , $1b$ , $1s$ , $1b+1s$ , $2b$ , $2s$ ). This allows reinterpretation for various theoretical models beyond the specific benchmark.
Data-Driven Background: The analysis avoids reliance on potentially inaccurate Monte Carlo simulations for the dominant backgrounds by employing a robust, fully data-driven ABCD estimation strategy.

Results

Observation: No significant deviation from the background-only hypothesis was observed in the data. The observed data distributions in the signal regions are consistent with the ABCD background predictions within uncertainties.
Exclusion Limits: Upper limits at 95% CL on the product $\sigma \times \mathcal{B} \times A \times \epsilon$ were set for $Z'$ masses between 125 and 350 GeV. The excluded cross-section limits range from approximately $0.2$ to $2$ fb.
Local Significance: A local deviation of $3.1$ standard deviations was observed in the $1$-jet signal region ( $SR^\mu_\mu_b$ ) at a mass of $230$ GeV. However, after accounting for the look-elsewhere effect across both signal regions, the global significance is reduced to $0.5$ standard deviations, consistent with a background fluctuation.
Acceptance and Efficiency: Detailed tables (Tables 5–11) provide the acceptance times efficiency for each production category and signal region, facilitating the reinterpretation of these limits for specific theoretical models.

Significance
The paper claims that these results represent a crucial complement to existing inclusive $Z'$ searches. By targeting the specific production mechanism of bottom quark fusion, the analysis probes a region of parameter space that is less constrained by models assuming flavor universality. The results effectively rule out or constrain specific $Z'$ models that propose non-universal couplings to third-generation fermions in the $125–350$ GeV mass range, particularly those motivated by $B$ -physics anomalies. The provision of model-independent limits and acceptance factors ensures that these results remain valuable for testing future theoretical developments in physics beyond the Standard Model.

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