LHCb Probes Beyond the Standard Model

Author: Denis Avetisyan

New results from the LHCb experiment are pushing the boundaries of electroweak precision measurements and the search for exotic particles.

The LHCb experiment reconstructs diverse particle tracks, each representing the aftermath of high-energy collisions and providing crucial data for exploring the fundamental constituents of matter and the forces governing their interactions.

This review details recent LHCb analyses focusing on electroweak bosons, top quark production, and searches for heavy neutral leptons and axion-like particles.

While the Standard Model of Particle Physics remains remarkably successful, its inability to explain phenomena like dark matter motivates searches for new physics. This contribution, based on recent results from the LHCb experiment presented in ‘Electroweak physics and long-lived particles at LHCb’, details precision measurements of electroweak processes and explorations beyond the Standard Model. Analyses of $W$ and $Z$ boson properties, top quark production, and searches for axion-like particles and heavy neutral leptons provide complementary probes of fundamental interactions and parton distribution functions. Could these investigations at LHCb reveal subtle discrepancies hinting at a more complete understanding of the universe?

Mapping the Boundaries of Known Physics

Despite its remarkable success in describing fundamental particles and forces, the Standard Model of particle physics is not considered a final theory. Its inherent limitations – such as its inability to incorporate gravity or explain dark matter – motivate the search for physics beyond its established framework. Consequently, increasingly precise tests of the Standard Model’s predictions are crucial; even minuscule deviations from expected behavior in electroweak interactions – those governing the weak force and the unification of electromagnetic and weak forces – could signal the existence of new particles or forces. These high-precision measurements don’t aim to disprove the Standard Model outright, but rather to meticulously map its boundaries, identifying areas where its predictions falter and thereby guiding the development of more comprehensive theories. The search for such subtle discrepancies represents a cornerstone of modern particle physics, with experiments continually pushing the limits of precision to reveal potential glimpses of the universe beyond our current understanding.

The LHCb experiment has undertaken exacting measurements of the masses of the ZZ and WW bosons, fundamental particles mediating the weak force, as a means of rigorously testing the Standard Model of particle physics. Achieving a precision measurement of the ZZ boson mass – determined to be 91185.7 ± 8.3 (stat) ± 3.9 (sys) MeV – marks the first dedicated analysis of this kind performed at the Large Hadron Collider. This accomplishment relies on sophisticated data analysis techniques, meticulously calibrating the detector to minimize uncertainties, and efficiently reconstructing the decay products of these fleeting particles. Subtle deviations from predicted values, even within the margin of error, could hint at the existence of new, undiscovered physics beyond the established framework, making these high-precision measurements a crucial frontier in particle physics research.

Achieving the stated precision in measurements of the WW and ZZ bosons demands sophisticated methodologies centered on minimizing systematic uncertainties. The LHCb collaboration employed robust calibration techniques, carefully characterizing detector response to ensure accurate energy and momentum measurements of decay products. Efficient particle reconstruction algorithms were also crucial, enabling the precise identification and association of particles originating from the boson decays. This culminated in a WW-boson mass measurement of 80369 ± 130 (exp) ± 33 (th) MeV, notable not only for its precision but also as a proof-of-concept utilizing a novel analytical approach to data analysis – a significant step towards even more refined electroweak measurements at the energy frontier.

A comparison of differential <span class="katex-eq" data-katex-display="false"> ext{W} ext{W}</span>-boson production cross sections, separated by muon charge and displayed as a function of muon pseudo-rapidity, reveals good agreement between measurements and various theoretical generators and PDF sets within statistical and total uncertainties of 68% confidence level. — A comparison of differential $ext{W} ext{W}$ -boson production cross sections, separated by muon charge and displayed as a function of muon pseudo-rapidity, reveals good agreement between measurements and various theoretical generators and PDF sets within statistical and total uncertainties of 68% confidence level.

The Foundation of Precision: Detector Calibration

Precise determination of the ZZ and WW boson masses relies fundamentally on accurate detector calibration. Systematic uncertainties in these measurements are directly proportional to inaccuracies in the momentum and energy scales established by the detector; therefore, rigorous calibration procedures are essential. The detector’s ability to accurately reconstruct the decay products of these bosons-primarily leptons and quarks-is paramount. Any deviations in the measured momenta or energies of these particles will propagate into an erroneous calculation of the boson mass. Consequently, the calibration process focuses on establishing a precise relationship between the detector’s response and the true particle momenta and energies, minimizing systematic biases and ensuring the validity of the experimental results.

The LHCb experiment relies on the precise measurement of known particle resonances, specifically the Upsilon (Υ) and J/Psi ( $J/\Psi$ ), to calibrate its momentum scale. These particles decay into well-reconstructed final states, allowing for accurate determination of their invariant mass and, consequently, a precise momentum reference point. By comparing measured momenta of decay products to the known masses of these resonances – calculated using $E^2 = (pc)^2 + (mc^2)^2$ – systematic offsets in the detector’s momentum measurement can be identified and corrected. This process ensures the accurate reconstruction of particle trajectories and energies, which is fundamental for precise measurements of quantities like the masses of the W and Z bosons.

The Pseudomass method represents a refinement of detector calibration techniques utilized to improve the identification of b-quarks and c-quarks. This method analyzes the invariant mass of displaced secondary vertices, providing a more precise measurement of the flight path length and thus, the momentum of the originating heavy-flavor hadrons. Implementation of the Pseudomass method has demonstrated an 11-53% increase in bb- and cc-tagging efficiency when compared to prior tagging algorithms that relied solely on secondary vertex information. This improvement in tagging efficiency directly contributes to enhanced statistical power in analyses focused on the precise measurement of boson masses and other physics observables.

At the 95% confidence level, upper limits on the coupling strength between heavy neutral leptons (HNLs) and muon neutrinos, expressed as <span class="katex-eq" data-katex-display="false">|U_{\mu N}|^{2}</span>, are shown as a function of HNL mass <span class="katex-eq" data-katex-display="false">m_{N}</span> for both Dirac-like (left) and Majorana-like (right) scenarios. — At the 95% confidence level, upper limits on the coupling strength between heavy neutral leptons (HNLs) and muon neutrinos, expressed as $|U_{\mu N}|^{2}$ , are shown as a function of HNL mass $m_{N}$ for both Dirac-like (left) and Majorana-like (right) scenarios.

Probing the Unknown: Direct Searches for New Particles

The LHCb experiment, while primarily focused on b-quark decays, also conducts direct searches for physics beyond the Standard Model, specifically targeting Axion-Like Particles (ALPs) and Heavy Neutral Leptons (HNLs). ALPs are hypothetical particles predicted by extensions to the Standard Model that would couple to photons and other particles, potentially explaining dark matter or anomalies in existing measurements. HNLs, also known as sterile neutrinos, are heavier counterparts to the known neutrinos and could account for neutrino masses and oscillations, as well as provide a portal to other new physics. These searches utilize the LHC’s high luminosity and LHCb’s unique detector capabilities to identify potential production and decay signatures of these particles.

The ‘Bump Hunt’ strategy utilized in direct searches for new particles at the LHCb experiment involves analyzing particle decay distributions for statistically significant excesses, or ‘bumps’, at specific invariant mass values. These excesses would indicate the resonant production and subsequent decay of a new, short-lived particle. The method relies on reconstructing the decay products of potential new particles and plotting their combined mass; a peak above the expected background distribution suggests a signal. Background contributions are carefully modeled and subtracted to enhance sensitivity to potential signals, and stringent statistical criteria are applied to assess the significance of any observed excess, minimizing the probability of a false positive discovery.

Recent analyses conducted by the LHCb collaboration have significantly refined the constraints on Heavy Neutral Lepton (HNL) interactions with muon neutrinos. Specifically, the upper limits on the coupling strength between HNLs and muon neutrinos have been improved by a factor of ten – representing a one order of magnitude improvement – compared to results obtained during LHC Run 1. This advancement stems from increased integrated luminosity and improvements in detector performance and analysis techniques, allowing for a more sensitive search for the subtle signatures of HNL production and decay.

Differential cross sections for top (<span class="katex-eq" data-katex-display="false">t</span>) and antitop (<span class="katex-eq" data-katex-display="false">t\bar{t}</span>) quark production as a function of muon pseudorapidity (<span class="katex-eq" data-katex-display="false">\eta\_{\mu}</span>) are presented with uncertainty bands and compared to predictions from various generators and parton distribution function sets, revealing the top charge asymmetry. — Differential cross sections for top ( $t$ ) and antitop ( $t\bar{t}$ ) quark production as a function of muon pseudorapidity ( $\eta\_{\mu}$ ) are presented with uncertainty bands and compared to predictions from various generators and parton distribution function sets, revealing the top charge asymmetry.

Refining the Search: Advanced Analysis Techniques

The pursuit of Axion-Like Particles (ALPs) relies heavily on a thorough understanding of how these hypothetical particles are created. A primary production mechanism involves gluon-gluon fusion, a process where two gluons – the force carriers of the strong nuclear force – collide to form an ALP. This interaction is particularly relevant at the Large Hadron Collider, where high-energy collisions frequently produce gluon pairs. By precisely modeling the rate and characteristics of this gluon-gluon fusion process, physicists can better predict the expected signal strength of ALPs and distinguish it from background noise. Consequently, detailed theoretical calculations and simulations of this production pathway are crucial for optimizing search strategies and maximizing the potential for discovering these elusive particles, which could resolve several mysteries in particle physics and cosmology.

The search for heavy neutral leptons (HNLs) employs a distinctive strategy centered on the observation of decay signatures originating from $B\overline{B}$ meson pairs. These particles, produced prolifically in high-energy collisions, offer a unique window into HNL existence because HNLs can be produced in $B$ -meson decays. Researchers meticulously analyze the decay products of these mesons, seeking anomalies in energy, momentum, or particle composition that deviate from standard model predictions. Specifically, the detection of displaced vertices – points where decay products originate some distance from the collision point – provides compelling evidence for the existence of these weakly interacting particles, as HNLs possess relatively long lifetimes. This technique allows scientists to probe beyond the standard model, potentially revealing new physics through subtle deviations in observed decay patterns.

The precision of searches for new physics relies heavily on accurately characterizing known processes, and recent analyses have benefited from substantial datasets. Specifically, researchers leveraged an integrated luminosity of 5.1 fb^-1 collected at a center-of-mass energy of 13 TeV to refine measurements of top quark production cross-sections. Simultaneously, a separate dataset with an integrated luminosity of 100 pb^-1, gathered at 5.02 TeV, enabled a precise extraction of the W⁺W^– boson mass. These refined standard model measurements are crucial; they reduce uncertainties in background estimations and allow for more sensitive searches for deviations indicative of new particles or interactions, ultimately expanding the scope and potential of beyond-the-Standard-Model investigations.

The pursuit of electroweak precision measurements, as detailed in this study, echoes a fundamental tenet of rigorous inquiry. It isn’t simply about confirming existing models, but relentlessly probing for discrepancies. This mirrors the sentiment expressed by Friedrich Nietzsche: “There are no facts, only interpretations.” The LHCb collaboration doesn’t seek absolute proof of the Standard Model; instead, it meticulously gathers data, acutely aware that any deviation-any outlier in the observed ZZ and WW boson masses or top quark production-could reveal a more accurate interpretation of reality. The devil, predictably, resides not in the established norms, but in the anomalies that challenge them.

Where Do We Go From Here?

The precision with which the LHCb collaboration now probes electroweak interactions is, in a sense, a testament to the Standard Model’s resilience. Each increasingly stringent test of W and Z boson masses, top quark properties, and related observables doesn’t necessarily bring discovery closer, but rather clarifies the contours of what isn’t there. The sensitivity of these measurements, however, demands a continuing, skeptical assessment of underlying assumptions. How sensitive are the derived limits on Beyond the Standard Model (BSM) parameters to uncertainties in Parton Distribution Functions, for instance? Or to the subtle, often-ignored correlations between systematic effects?

The searches for long-lived particles – heavy neutral leptons and axion-like particles – present a different, perhaps more profound, challenge. Negative results aren’t failures, but rather increasingly detailed maps of the parameter space where such particles don’t reside. The theoretical landscape is populated with a multitude of BSM candidates; the experimental challenge lies in narrowing the possibilities, not by confirming a single model, but by systematically excluding large swaths of it. The focus must remain on developing innovative search strategies that are less model-dependent and more broadly sensitive to new physics.

Ultimately, the value of these investigations isn’t just in finding something new, but in refining the questions. Each null result forces a re-evaluation of theoretical frameworks, encourages the development of more sophisticated analysis techniques, and-perhaps most importantly-reminds one that the most interesting discoveries often lie just beyond the limits of current understanding. The pursuit of precision, then, is not a dead end, but a continuous, self-correcting process of approximation.

Original article: https://arxiv.org/pdf/2605.10243.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/

Mapping the Boundaries of Known Physics

The Foundation of Precision: Detector Calibration

Probing the Unknown: Direct Searches for New Particles

Refining the Search: Advanced Analysis Techniques

Where Do We Go From Here?

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