QCD, electroweak physics, and searches for exotic signatures in the forward region at LHCb

This paper outlines LHCb's competitive forward physics program, detailing complementary heavy-flavour jet measurements, electroweak studies involving top and W bosons, and searches for exotic new physics states such as axion-like particles, heavy-neutral leptons, and rare B-meson decays.

The Gist

Imagine the Large Hadron Collider (LHC) as a massive, high-speed particle racetrack where protons zoom around and crash into each other, creating a shower of new particles. Most detectors are like wide-angle cameras placed in the center of the track, trying to catch everything.

But the LHCb detector is different. It's like a specialized telescope pointed at the very edge of the track (the "forward" region). Because it looks at a specific angle, it has a unique superpower: it can see particles that fly off to the side with incredible detail, acting like a high-speed camera with a super-sharp focus.

This paper, written by Emilio X. Rodríguez Fernández on behalf of the LHCb team, is a report card on what this special telescope has been seeing. Here is the breakdown in simple terms:

1. The Detective's Toolkit

Think of the LHCb detector as a master detective. It doesn't just count how many particles it sees; it cares about who they are and how they behave.

• The "Low Pile-up" Advantage: Usually, when protons crash, they create a messy pile of debris (like a crowded party). LHCb operates during times when the party is less crowded, allowing it to spot the quiet, subtle clues that other detectors might miss in the chaos.
• The Trigger System: It has a two-step security guard system. The first guard (Hardware) quickly scans the crowd, and the second guard (Software) does a deep background check on the interesting suspects. This ensures they don't miss anything weird.

2. Studying the "Heavy" Stuff (Jet Measurements)

When protons collide, they sometimes spit out "jets" of particles. Some of these jets are made of "heavy" ingredients (like bottom and charm quarks).

• The Analogy: Imagine throwing a bag of marbles (protons) together. Sometimes, a heavy, special marble (a heavy quark) gets kicked out and breaks into a spray of smaller marbles (a jet).
• What they did: LHCb mapped out exactly how these heavy marbles break apart. They measured the speed and direction of the pieces to understand the "recipe" (PDFs) of the proton itself.
• The Higgs Hunt: They also tried to find the Higgs boson decaying into heavy quarks. It's like looking for a needle in a haystack where the needle looks exactly like the hay. They haven't found it yet, but they've narrowed down where it could be hiding.

3. Weighing the Giants (Electroweak Physics)

The team also looked at the "heavyweights" of the particle world: the Top quark and the W boson.

• The Charge Asymmetry: Imagine a dance floor where Top quarks and anti-Top quarks are dancing. The team checked if they dance more to the left or the right. This helps test if the rules of physics (the Standard Model) are perfectly balanced.
• The W Boson Mass: They measured the weight of the W boson (a particle that carries the weak force) with extreme precision. It's like weighing a feather with a scale designed for an elephant, but doing it so precisely that they can tell if the feather is slightly heavier than expected.

4. Hunting for Ghosts (Exotic Signatures)

This is the most exciting part: looking for things that shouldn't exist according to current rules.

• Axion-Like Particles (ALPs): These are hypothetical, ghostly particles that might explain dark matter. LHCb looked for them by seeing if they turn into pairs of light particles (photons). It's like looking for a ghost by seeing if it leaves a trail of glowing footprints. They found no ghosts yet, but they set very strict "no trespassing" signs for where these ghosts could be.
• Heavy Neutral Leptons (HNLs): These are "invisible" heavy cousins of neutrinos. The team looked for them appearing in the decay of B-mesons (unstable particles). They are looking for a "displaced vertex," which is like seeing a firework explode a few feet away from where it was lit, rather than right at the spark.
• Multi-Muon Decays: They looked for B-mesons decaying into four or six muons at once. This is a rare event, like a magician pulling six rabbits out of a hat that usually only holds one. Finding this would be a huge clue for new physics theories like Supersymmetry.

5. The Future: LHCb Upgrade I

The detector is getting a major makeover, like upgrading a sports car with a turbo engine.

• The Change: They are increasing the "crowd" (luminosity) at the party. To handle the chaos, they are replacing old tracking sensors with new, faster ones (SciFi detectors) and removing the first hardware guard to let a super-fast computer do all the work.
• The Goal: This will let them see even rarer events. They hope to finally catch the Higgs decaying into heavy quarks and measure the W boson's weight with even greater precision.

The Bottom Line

This paper is a progress report from a highly specialized detective agency. They are using their unique "side-view" perspective to:

1. Map the internal structure of protons.
2. Test the fundamental laws of physics to see if they break.
3. Hunt for invisible, exotic particles that could explain the mysteries of the universe.

Even though they haven't found the "new physics" yet, they have drawn a much tighter map of where to look next, proving that looking at the edge of the track is just as important as looking at the center.

Technical Summary

Based on the provided proceedings paper by Emilio X. Rodríguez Fernández on behalf of the LHCb collaboration, here is a detailed technical summary covering the problem, methodology, key contributions, results, and significance.

1. Problem and Motivation

The Standard Model (SM) of particle physics remains incomplete, particularly regarding the nature of dark matter, the matter-antimatter asymmetry, and the hierarchy problem. While the Large Hadron Collider (LHC) generally focuses on high-mass, central-rapidity physics, there is a critical need to explore:

• Small and high Bjorken-$x$ parton distribution functions (PDFs): Crucial for understanding QCD dynamics in unexplored kinematic regions.
• Electroweak (EW) precision: Testing the SM to its limits, specifically regarding top quark and W boson properties.
• Beyond the Standard Model (BSM) signatures: Many BSM scenarios (e.g., Axion-like particles, Heavy Neutral Leptons, Supersymmetry) predict low-mass, displaced vertices, or rare decays that are difficult to detect in central detectors due to trigger thresholds or background noise.

The LHCb detector, with its unique forward single-arm spectrometer geometry ($2 < \eta < 5$), offers a complementary perspective to ATLAS and CMS, enabling high-precision measurements in these specific kinematic regimes.

2. Methodology

The paper outlines a multi-faceted approach utilizing the LHCb detector's capabilities during Run 1 and Run 2, with projections for the Upgrade I.

• Detector Strategy:

  • Tracking & Vertexing: Exceptional performance in reconstructing complex topologies and displaced vertices.
  • Particle Identification (PID): Powerful tools to distinguish between pions, kaons, protons, and muons.
  • Trigger System: A two-level system (Hardware L0 and Software HLT1/HLT2) optimized for low pile-up ($\langle\mu\rangle \approx 1.1$) to capture rare events rather than just high-abundance ones.
  • Data Acquisition: Utilization of Long tracks (all subdetectors) and Downstream tracks (downstream of VELO) for displaced signatures.

• Analysis Techniques:

  • Jet Physics: Reconstruction using the anti-$k_T$ algorithm. Heavy-flavour jets are tagged using a Secondary Vertex (SV) algorithm and a Boosted Decision Tree (BDT) classifier.
  • Unfolding: Application of Bayesian Unfolding to correct detector-level distributions for finite resolution, inefficiencies, and migration effects to recover true particle-level distributions.
  • Machine Learning:
    • DNN (Deep Neural Networks): Used for jet-flavour identification in $t\bar{t}$ production and for suppressing combinatorial backgrounds in HNL searches.
    • Gradient Boosting Regressors: Tested for energy calibration in Higgs-to-bb/cc studies.
  • Background Suppression: Specific kinematic cuts (e.g., $m(\mu^+\mu^-) < 40$ GeV, $p_T > 20$ GeV) and isolation requirements to reduce QCD multijet and $Z/\gamma^*$ backgrounds.

3. Key Contributions and Results

A. Jet Measurements (QCD)

• Objective: Study longitudinal momentum fraction ($z$), transverse momentum ($j_T$), and radial distribution ($r$) of charged particles in heavy-flavour jets to constrain PDFs.
• Results:
  • Presented distributions of charged-hadron $z$ across different $p_T$ bins (Figure 1a).
  • Studied $b$-quark PDFs via $B^+ \to J/\psi(\mu^+\mu^-)K^\pm$ decays, providing 2D PDFs in the $(j_T, z)$ plane (Figure 1b).
  • Higgs Constraints: Using inclusive $H \to b\bar{b}, c\bar{c}$ events, limits were set on the cross-sections: $\sigma_{H\to b\bar{b}} = 11.1 \times \sigma_{SM}$ and $\sigma_{H\to c\bar{c}} = 1834 \times \sigma_{SM}$. This constrains the charm Yukawa coupling to $y_c = 43 \times y_c^{SM}$.

B. Electroweak Measurements

• Top Quark Charge Asymmetry:
  • Measured using $t \to W^+(\mu^+\nu_\mu)b$ decays with DNN-based jet-flavour identification.
  • Results: Integrated cross-sections of $\sigma_t = 0.95 \pm 0.04 \pm 0.08 \pm 0.02$ pb and $\sigma_{\bar{t}} = 0.81 \pm 0.03 \pm 0.07 \pm 0.02$ pb.
• W Boson Properties:
  • Measured the $W \to \mu\nu_\mu$ cross-section and mass using 100 pb$^{-1}$ at $\sqrt{s} = 5.02$ TeV.
  • Results: Provided the unfolded $d\sigma/dp_T$ distribution for the $W^+$ boson (Figure 2b), utilizing template fits for signal and backgrounds (including misidentified hadrons and $\tau$ decays).

C. Exotic Signatures (BSM Searches)

• Axion-Like Particles (ALPs):
  • Searched for $a \to \gamma\gamma$ via gluon fusion in the mass range $m_{\gamma\gamma} \in [4.9, 19.4]$ GeV.
  • Results: Set stringent limits for prompt ALPs in the mass range $[4.9, 10]$ GeV, suppressing the dominant $B^0 \to \pi^0\pi^0$ background via PID cuts (Figure 3a).
• Heavy Neutral Leptons (HNLs):
  • Searched for HNLs from $B$-meson decays ($m_N \in [1.6, 5.5]$ GeV), distinguishing between Majorana (same-sign dimuons) and Dirac (opposite-sign) topologies.
  • Results: Provided the most stringent constraints on $|U_{\mu N}|^2$ at large masses and short lifetimes (Figure 3b).
• Multi-muon Decays:
  • Analyzed $B_s \to (4,6)\mu$ and $B^+ \to (4,6)\mu K^+$ decays mediated by pseudo Nambu-Goldstone bosons ($a_1, a_2$).
  • Results: Normalized to $B^0_s \to J/\psi\phi$ to cancel systematics, setting limits for $B_{u,d}$ decays with specific mediator masses (Figure 3c).

4. Significance and Future Outlook (Upgrade I)

The paper highlights the transition to LHCb Upgrade I, which fundamentally changes the data-taking strategy:

• Increased Luminosity: Pile-up will rise to $\langle\mu\rangle = 5.2$.
• Hardware Changes: Replacement of inner/outer trackers with the SciFi detector and removal of the L0 hardware trigger in favor of a full software trigger. This allows for higher-rate data acquisition and lower thresholds.
• Projected Improvements:
  • Higgs Physics: Projected sensitivity improvements to $\sigma_{H\to b\bar{b}} = 0.38 \times \sigma_{SM}$ and $\sigma_{H\to c\bar{c}} = 45 \times \sigma_{SM}$, potentially constraining $y_c$ to $6.7 \times y_c^{SM}$.
  • EW Precision: Significant reduction in statistical errors for $t\bar{t}$ charge asymmetry and W mass measurements.
  • BSM Sensitivity: Enhanced capabilities for dielectron and dimuon modes due to looser thresholds and improved vertexing for displaced physics.

Conclusion

This work demonstrates the LHCb collaboration's unique ability to perform precision QCD and Electroweak measurements while simultaneously acting as a premier facility for discovering low-mass, displaced, and rare BSM phenomena. The combination of forward coverage, advanced machine learning techniques, and the upcoming Upgrade I positions LHCb as a critical component in the global effort to complete the Standard Model and uncover new physics.