Review of flavour physics at ATLAS and CMS

This paper presents a comprehensive review of recent flavour physics results from the ATLAS and CMS experiments using Run 2 and partial Run 3 LHC data, covering measurements of quarkonia, charm production, B meson properties, all-charm tetraquarks, and rare heavy-flavour decays to advance the precision frontier in the field.

The Gist

Imagine the Large Hadron Collider (LHC) as a massive, high-speed particle racetrack where tiny subatomic particles are smashed together at nearly the speed of light. The ATLAS and CMS experiments are like two giant, ultra-sensitive cameras positioned around this track, snapping billions of photos to see what happens when these particles collide.

This paper is a "photo album review" from these two cameras, focusing specifically on a special group of particles called heavy-flavour particles. Think of these as the "heavyweights" of the particle world—particles made of heavy quarks (like bottom and charm) that are much heavier than the ones making up the atoms in your body.

Here is a breakdown of what the scientists found, explained simply:

1. Weighing the Heavyweights (Production Cross Sections)

The scientists wanted to know how often these heavy particles are created and how they behave.

• The "Bottomonium" Family: They looked at a family of particles called $\Upsilon$ (Upsilon), which are like heavy, bound pairs of bottom quarks. For the first time, they measured how often these appear at a record-breaking energy level (13.6 TeV). It's like checking how many heavy trucks are produced on a factory line when you crank the machine up to maximum power. They found the numbers matched the "blueprints" predicted by quantum physics (QCD) very well.
• The "Charm" Messengers: They also tracked particles containing "charm" quarks. They measured how these particles spread out across the detector (like rain falling at different angles). The results matched the theoretical models, confirming our understanding of how these particles form.

2. Timing the Ticking Clocks (Lifetimes and Masses)

• The $B^0$ Stopwatch: One specific particle, the $B^0$ meson, is known to live for a tiny fraction of a second before decaying. The ATLAS experiment measured this "lifespan" with incredible precision—more precise than any measurement before. It's like timing a sprinter so accurately that you can see the difference in their stride down to the millimeter.
• The "Excited" vs. "Ground" State: They also looked at "excited" versions of B mesons (particles that are vibrating with extra energy) and measured the tiny difference in mass between these excited states and their calm, "ground" states. This is like measuring the tiny weight difference between a calm guitar string and one that is vibrating loudly.

3. Hunting for Exotic "Four-Quark" Clusters

For a long time, we thought particles were made of either two quarks (like a pair) or three quarks (like a trio). But recently, physicists started looking for "tetraquarks"—particles made of four quarks stuck together.

• The All-Charm Mystery: The scientists searched for a specific type of tetraquark made entirely of four charm quarks. They looked for these by watching how they decay into pairs of "J/$\psi$" particles.
• The Findings: They found strong evidence for three new "resonances" (clumps of particles) at specific energy levels (6.6, 6.9, and 7.1 GeV). It's like hearing a specific chord played on a piano and realizing there are three new, previously unknown notes being played. The data suggests these are indeed four-quark clusters, a rare and exotic form of matter.

4. Looking for "Ghostly" Decays (Rare Events)

The final section of the paper is about looking for "forbidden" or extremely rare events that shouldn't happen according to our current rules (the Standard Model). Finding them would be like seeing a ghost—it would mean the rules of physics need to be rewritten.

• Lepton Flavor Violation: They searched for a tau particle turning into three muons ($\tau \to 3\mu$). This is like watching a cat suddenly turn into three mice. They didn't find any, which is good news for the current rules, but they set strict limits on how often this could happen.
• The "Four-Muon" Search: They also looked for B mesons decaying into four muons. They improved the sensitivity of this search, making it harder for these rare events to hide.
• The $B_s \to \phi \mu\mu$ Tension: They studied a specific decay where a B meson turns into a phi particle and two muons. While the results mostly agree with theory, there is a small "tension" (a slight disagreement) of up to 4.2 standard deviations. Think of this as a slight wobble in the data that might hint at new physics, but isn't strong enough yet to declare a discovery.

The Bottom Line

The ATLAS and CMS experiments are proving that they aren't just great at finding the Higgs boson; they are also becoming world-class detectives for heavy-flavour physics. By using their massive detectors and clever triggers (which act like smart filters to catch rare events), they are measuring particle properties with record-breaking precision and hunting for the exotic and the rare.

While they haven't found a "smoking gun" for new physics yet, they have tightened the screws on our current theories, making the search for what lies beyond even more exciting.

Technical Summary

Technical Summary: Review of Flavour Physics at ATLAS and CMS

Problem and Context
Flavour physics serves as a critical domain for testing the Standard Model (SM) with high precision and probing for potential new physics. While dedicated flavour experiments have historically dominated this field, the ATLAS and CMS general-purpose detectors at the Large Hadron Collider (LHC) have established themselves as competitive facilities. Leveraging their nearly full solid-angle coverage, excellent vertexing performance, and superior muon reconstruction capabilities, these experiments complement dedicated efforts. This paper reviews recent results in flavour physics obtained by ATLAS and CMS using proton-proton collision data from LHC Run 2 and partial Run 3. The analyses specifically target heavy-flavour decays, utilizing triggers that require at least two muons in the event to enhance acceptance for low-energy processes.

Methodology
The review encompasses a diverse set of analyses employing differential measurements, lifetime determinations, and resonance searches:

• Cross-Section Measurements: CMS measured production cross sections for $\Upsilon(nS)$ states ($n=1,2,3$) at a center-of-mass energy of 13.6 TeV using 37.4 fb$^{-1}$ of Run 3 data. These measurements are differential in transverse momentum ($p_T$) up to 200 GeV. ATLAS measured cross sections for $D^\pm$ and $D^\pm_s$ mesons using 137 fb$^{-1}$ of Run 2 data, extending the $p_T$ reach to 100 GeV via the $D \to \phi(\mu^+\mu^-)\pi^\pm$ decay channel.
• Lifetime and Mass Splitting: ATLAS determined the $B^0$ meson lifetime ($\tau(B^0)$) using 140 fb$^{-1}$ of Run 2 data. CMS performed the first exclusive reconstruction of excited $B^*$ mesons ($B^{*+}, B^{*0}, B^{*0}_s$) using 140 fb$^{-1}$ of Run 2 data, reconstructing photons via conversion ($\gamma \to e^+e^-$) to measure hyperfine mass splittings.
• Exotic State Searches: Both collaborations searched for all-charm tetraquarks in the double-charmonium mass spectra ($J/\psi J/\psi$ and $J/\psi \psi(2S)$). CMS utilized combined Run 2 (135 fb$^{-1}$) and Run 3 (180 fb$^{-1}$) data, while ATLAS used 140 fb$^{-1}$ of Run 2 data, incorporating the $\psi(2S) \to J/\psi \pi^+ \pi^-$ decay mode for the first time in this channel.
• Rare Decay Studies: Searches for lepton flavour violating decays ($\tau \to 3\mu$) were conducted by both experiments. CMS also reported on the first observation of $\eta \to \mu^+\mu^-e^+e^-$ and searches for $B^0_s(B^0) \to \mu^+\mu^-\mu^+\mu^-$. Furthermore, CMS performed a differential measurement of the rare $B^0_s \to \phi \mu^+\mu^-$ decay using 138 fb$^{-1}$ of Run 2 data, normalizing against $B^0_s \to \phi J/\psi(\mu^+\mu^-)$.

Key Contributions and Results

• Heavy-Flavour Production: The differential cross sections for $\Upsilon(nS)$ production at 13.6 TeV agree well with non-relativistic QCD (NRQCD) theoretical predictions. Similarly, ATLAS measurements of $D^\pm_s$ cross sections up to $p_T = 100$ GeV are consistent with general-mass variable-flavour-number scheme (GM-VFNS) predictions.
• Precision Parameters: The ATLAS measurement of the $B^0$ lifetime yields $\tau(B^0) = 1.5053 \pm 0.0012 \text{ (stat)} \pm 0.0035 \text{ (syst)}$ ps, representing the most precise measurement to date. CMS measurements of hyperfine splittings ($m(B^*) - m(B)$) achieved precision one order of magnitude better than previous results.
• All-Charm Tetraquarks: CMS observed three resonances in the $J/\psi J/\psi$ spectrum—$X(6600)$, $X(6900)$, and $X(7100)$—with significances exceeding $5\sigma$. The $X(6900)$ was also observed in the $J/\psi \psi(2S)$ spectrum. Angular analyses suggest these states are compatible with all-charm tetraquarks having $J^{PC} = 2^{++}$. ATLAS confirmed the $X(6900)$ resonance but reported no significant structure at 7.1 GeV.
• Rare Decays: No significant excess was found in searches for $\tau \to 3\mu$, leading to 90% confidence level upper limits on the branching fraction of $B(\tau \to 3\mu) < 8.7 \times 10^{-8}$ (ATLAS) and $6.7 \times 10^{-8}$ (CMS). CMS observed the $\eta \to \mu^+\mu^-e^+e^-$ decay and improved limits on $B^0_s(B^0) \to \mu^+\mu^-\mu^+\mu^-$ by approximately 30%.
• $B^0_s \to \phi \mu^+\mu^-$ Analysis: The differential cross section for this decay, measured in bins of $q^2$, agrees well with recent LHCb measurements. However, comparisons with various theoretical predictions reveal tensions of up to $4.2\sigma$. The longitudinal polarisation fraction ($F_L$) of the $\phi$ meson was found to be in good agreement with theoretical expectations.

Significance
The paper asserts that ATLAS and CMS have successfully established themselves as competitive facilities for heavy-flavour physics, providing measurements that are complementary to those from dedicated flavour experiments. The results demonstrate the capability of these general-purpose detectors to push the precision frontier in flavour physics through advanced trigger strategies and reconstruction techniques. The observations of all-charm tetraquarks and the precise measurements of heavy-flavour properties contribute significantly to the characterization of QCD dynamics and the search for physics beyond the Standard Model. The ongoing analysis of Run 3 data is expected to further enhance the flavour physics programme of both collaborations.