Observation of the decays $B^{+} \to \Sigma_{c}(2455)^{++} \bar{\Xi}_{c}^{\prime-}$ and $B^{0} \to \Sigma_{c}(2455)^{0} \bar{\Xi}_{c}^{\prime0}$

Using a combined dataset of over 1.29 billion $\Upsilon(4S)$ decays from the Belle and Belle II experiments, researchers report the first observation of the $B$-meson decays $B^{+} \to \Sigma_{c}(2455)^{++} \bar{\Xi}_{c}^{\prime-}$ and $B^{0} \to \Sigma_{c}(2455)^{0} \bar{\Xi}_{c}^{\prime0}$ with statistical significances of $6.4\,\sigma$ and $5.3\,\sigma$, respectively, and measure their branching fractions.

The Gist

Imagine the universe as a giant, high-speed particle factory. In this factory, heavy particles called B-mesons are constantly being created and then instantly breaking apart into smaller pieces. Physicists are like detectives trying to figure out exactly how these breakups happen and what pieces are left behind.

This paper reports a major breakthrough by the Belle and Belle II collaborations (a team of scientists using massive detectors in Japan). They have successfully spotted two very specific, rare types of "breakups" that had never been seen before.

Here is the story of their discovery, broken down into simple concepts:

1. The Mystery: A Rare Family Reunion

Usually, when a B-meson breaks apart, it might split into a mix of different particles. But sometimes, it splits into two heavy "cousins" called charmed baryons.

Think of these baryons as members of a large extended family. In the world of particle physics, families are organized into groups based on their "personality traits" (scientifically called flavor multiplets).

• The scientists were looking for a specific scenario: A B-meson breaking into two charmed baryons that belong to the exact same family group (specifically, the "sextet" family).
• Before this paper, no one had ever seen this specific "family reunion" happen. It was like looking for a needle in a haystack, or finding two specific twins in a crowd of billions.

2. The Investigation: Sifting Through the Noise

To find these rare events, the scientists used data from two massive particle colliders (KEKB and SuperKEKB). They collected data from over 1.2 billion B-meson decays.

• The Challenge: Most of the time, the detectors see "noise"—random debris from other collisions that looks similar to what they are looking for. It's like trying to hear a specific whisper in a stadium full of cheering fans.
• The Strategy: The team built a sophisticated "filter" (using computer algorithms and statistical models) to sort through the billions of events. They looked for a very specific chain of events:
  1. A B-meson splits.
  2. One piece turns into a $\Sigma_c(2455)$ particle.
  3. The other piece turns into a $\bar{\Xi}'_c$ particle.
  4. These particles then decay further into even smaller, recognizable pieces (like protons, pions, and photons) that the detectors can catch.

3. The Discovery: Finding the Signal

After filtering out the noise, the scientists found what they were looking for:

• The First Case: They found 62 clear examples of the charged version of this decay (B^+ \to \Sigma_c^{++} \bar{\Xi}'_c^-).
• The Second Case: They found 31 clear examples of the neutral version (B^0 \to \Sigma_c^{0} \bar{\Xi}'_c^0).

In the world of particle physics, finding a handful of events out of a billion isn't enough; you need to be sure it's not just a random fluke. The team calculated the "significance" of their find:

• The first discovery was 6.4 times more likely to be real than a random fluke.
• The second was 5.3 times more likely.
• (Scientists generally need a score of 5 to claim a "discovery," so they have officially found these new decays!)

4. The Results: How Often Does It Happen?

The team measured how often these rare breakups occur (called the branching fraction).

• For the charged version, it happens about 1.68 times out of every 1,000 B-meson decays.
• For the neutral version, it happens about 1.28 times out of every 1,000 decays.

Interestingly, these numbers are actually higher than expected compared to similar decays involving different types of baryons. This suggests that the "internal forces" holding these particles together are behaving in a way that makes this specific family reunion more likely than previously thought.

5. Why This Matters

This paper doesn't just add a new line to a list of known particles. It opens a new window into understanding the strong force (the glue that holds atomic nuclei together).

• By seeing how these specific "family members" interact, physicists can test their theories about how the universe works at the smallest scales.
• It confirms that our current models of particle physics can predict these complex interactions, even though the math is incredibly difficult.

In summary: The Belle and Belle II teams acted as cosmic detectives, sifting through over a billion particle collisions to find two very rare, specific "family reunions" of subatomic particles. They not only found them but proved they are real, giving us a new clue about how the fundamental forces of nature operate.

Technical Summary

Technical Summary: Observation of $B^+ \to \Sigma_c(2455)^{++}\bar{\Xi}'^{-}_c$ and $B^0 \to \Sigma_c(2455)^{0}\bar{\Xi}'^{0}_c$

Problem and Motivation
B-meson decays serve as a critical laboratory for investigating the interplay between weak and strong interactions within the nonperturbative regime of quantum chromodynamics (QCD). While two-body baryonic B decays have been studied for decades, specific modes involving charmed baryon-antibaryon pairs remain under-explored, particularly those where both daughter particles belong to the same SU(3) flavor sextet. Previous observations, such as $B \to \Sigma_c(2455)\bar{\Xi}_c$, involve pairs from different multiplets. The decays $B^+ \to \Sigma_c(2455)^{++}\bar{\Xi}'^{-}_c$ and $B^0 \to \Sigma_c(2455)^{0}\bar{\Xi}'^{0}_c$ are theoretically expected to proceed via internal W-emission topologies, similar to their $\bar{\Xi}_c$ counterparts. However, the $\bar{\Xi}'_c$ and $\bar{\Xi}_c$ baryons differ in the spin configuration of their light diquarks despite sharing valence-quark content. A comparison of the branching fractions between these modes ($B \to \Sigma_c \bar{\Xi}'_c$ vs. $B \to \Sigma_c \bar{\Xi}_c$) offers a potential probe into the underlying dynamics of baryonic decays and the role of SU(3) flavor symmetry. Prior to this work, no observations existed for B decays producing a pair of charmed baryons both belonging to a flavor sextet.

Methodology
The analysis utilizes data samples collected by the Belle and Belle II detectors at the KEKB and SuperKEKB $e^+e^-$ colliders, respectively. The datasets correspond to $771.6 \times 10^6$ and $520.6 \times 10^6$ $\Upsilon(4S)$ decays, collected at a center-of-mass energy of 10.58 GeV.

• Reconstruction Strategy: The signal decay chains are fully reconstructed. The $\Sigma_c(2455)^{++,0}$ baryons are identified via $\Lambda_c^+ \pi^\pm$, where the $\Lambda_c^+$ is reconstructed in $pK^-\pi^+$ and $pK_S^0$ modes. The $\bar{\Xi}'_c$ baryons are reconstructed via $\gamma \bar{\Xi}_c$, with the $\bar{\Xi}_c$ decaying into various modes (e.g., $\bar{\Xi}^+\pi^-\pi^-$, $\bar{p}K^+\pi^-$, $\bar{\Xi}^+\pi^-$, $\bar{\Lambda}K^+\pi^-$).
• Selection Criteria: Standard track selection and particle identification (PID) based on likelihood ratios are applied. Kinematic constraints, including the beam-constrained mass ($M_{bc}$) and energy difference ($\Delta E$), are used to isolate B-meson candidates. Specific mass windows are applied to intermediate resonances ($\Lambda_c^+$, $\bar{\Xi}_c$, $\Sigma_c$, $\bar{\Xi}'_c$).
• Background Handling:
  • Multiple Candidates: Due to the low energy of photons from $\bar{\Xi}'_c$ decays, events often contain multiple candidate combinations. A Best-Candidate Selection (BCS) strategy is employed, retaining the candidate with the smallest total $\chi^2$ from vertex and mass-constrained fits.
  • Self-Cross-Feed (SCF): Approximately 50% of selected candidates are incorrectly reconstructed (SCF events). These exhibit a broad $\Delta E$ distribution overlapping with combinatorial background. To constrain this, sideband regions of the $\bar{\Xi}'_c$ mass ($M(\gamma\bar{\Xi}_c)$) are defined.
  • Fit Model: A simultaneous unbinned extended maximum-likelihood fit is performed on the $\Delta E$ distributions for both signal and sideband regions in both Belle and Belle II datasets. The signal PDF is modeled by a double-Gaussian function, while SCF and combinatorial backgrounds are modeled using kernel density estimation and first-order polynomials, respectively.

Key Contributions and Results
This paper reports the first observation of the decays $B^+ \to \Sigma_c(2455)^{++}\bar{\Xi}'^{-}_c$ and $B^0 \to \Sigma_c(2455)^{0}\bar{\Xi}'^{0}_c$.

• Significances: The statistical significances are $8.6\sigma$ and $6.9\sigma$ for the charged and neutral channels, respectively. When systematic uncertainties are included, the significances are $6.4\sigma$ and $5.3\sigma$, satisfying the criteria for observation.
• Branching Fractions: The measured branching fractions are:
  • $\mathcal{B}(B^+ \to \Sigma_c(2455)^{++}\bar{\Xi}'^{-}_c) = (1.68 \pm 0.31 \text{ (stat)} \pm 0.12 \text{ (syst)} ^{+1.49}_{-0.54} \text{ (ext)}) \times 10^{-3}$
  • $\mathcal{B}(B^0 \to \Sigma_c(2455)^{0}\bar{\Xi}'^{0}_c) = (1.28 \pm 0.32 \text{ (stat)} \pm 0.10 \text{ (syst)} ^{+0.30}_{-0.21} \text{ (ext)}) \times 10^{-3}$
    The third uncertainty arises from the absolute branching fractions of the intermediate $\bar{\Xi}^-_c$ and $\bar{\Xi}^0_c$ decays.
• Systematic Uncertainties: Major sources include detection efficiency, simulation statistics, intermediate branching fractions, $\Upsilon(4S)$ yield, BCS strategy, and fit model variations. The total systematic uncertainties are approximately 7.2% and 7.8% for the charged and neutral channels, respectively.

Significance
The authors state that this result represents the first observation of B-meson decays into a pair of charmed baryon-antibaryon states where both daughters belong to the same SU(3) flavor sextet. The measured branching fractions are found to be two to three times larger than those of the previously observed $B \to \Sigma_c(2455)\bar{\Xi}_c$ modes, despite the latter having a larger available phase space. The paper suggests that this discrepancy may indicate nontrivial dynamical effects in baryonic B decays that enhance or suppress specific decay modes, though it refrains from making definitive claims about the specific nature of these dynamics beyond the observation of the enhancement.