Scientists measure the spin-parity of charm baryons for the first time

In a new development at CERN, researchers at the LHCb collaboration have determined the spin-parity of singly heavy charm baryons for the first time, addressing a long-standing mystery in baryon research.

Singly heavy baryons are particles containing one heavy quark—which in this case is a charm quark—and two light quarks. While the existence of these particles is not new, the exact nature of their excitation modes has remained elusive.

The study, published in Physical Review Letters, determined the nature by measuring the spin-parity of these charm baryons. Phys.org spoke to co-author Guanyue Wan, a Ph.D. Candidate at Peking University, China.

“Our study focuses on measuring a fundamental property of the Ξ_c(3,055)⁺ and Ξ_c(3,055)⁰ i.e., Ξ_c(3,055)^+,0 baryons—its spin-parity, which describes how these particles behave under certain symmetry transformations,” explained Wan.

“The study of charm baryons like Ξ_c(3,055) is particularly intriguing because it helps us test theoretical models of quantum chromodynamics (QCD), the theory that describes the strong interaction.”

Scientists have long discussed the configuration and interactions of constituent quarks, exploring multiple theoretical models with spin-parity assignments spanning from 1/2^± to 7/2⁺. Without knowing the spin-parity measurements, the excitation modes of the particles cannot be determined.

Spin-parity, excitations, and orbital angular momentum

Hadrons are particles composed of quarks, and baryons are a type of hadron consisting of three quarks. This makes them an ideal setting for studying strong interactions and their confinement within particles.

Spin and parity quantum numbers provide fundamental information about subatomic particles. Spin refers to the particle’s inherent angular momentum, whereas parity describes the symmetry of the particle’s wavefunction.

Knowing the spin-parity of a baryon provides critical information about the arrangement and orientation of the constituent quarks. This information is revealed through the orbital angular momentum between the quarks and the specific excitation modes present in the system.

In singly heavy baryons like Ξ_c(3,055), the excitation modes characterize the distribution of energy within the system. There are two primary excitation modes. The λ-mode describes the excitation between the heavy quark and the diquark system formed by the two lighter quarks.

The second excitation mode is the ρ-mode between the two lighter quarks.

These different modes create distinct patterns of orbital angular momentum, representing both specific energy distributions and spatial arrangements of the quark wavefunctions.

“Measuring the spin-parity provides crucial insight into the internal quark dynamics and interaction mechanisms within the baryon,” said Wan.

High precision measurements

The researchers began by analyzing proton-proton collision data collected between 2016 and 2018 at the LHCb experiment.

In particular, the researchers focused on the weak decay reaction of bottom baryons (Ξ_b) to charm baryons (Ξ_c), allowing them to exploit the instability of one (Ξ_b) to study the properties of the more stable one (Ξ_c).

They used sophisticated amplitude analysis techniques to examine the angular distributions and kinematics of the decay products emitted in different directions.

“Using the data from LHCb, we determined that the spin-parity of these states is 3/2⁺ with high significance. This result provides critical insights into the internal structure of this basic particle and helps refine theoretical models in hadron spectroscopy,” said Wan.

The high significance indicated strong statistical confidence in the result, confirming the hypothesis that Ξ_c(3,055)^+,0 baryons correspond to the “D-wave λ-mode excitation” of the Ξ_c flavor triplet.

This excitation mode means that the angular momentum (L) between the charm quark and the diquark system is two and the flavor triplet refers to the antisymmetric configuration of the lighter quarks.

The researchers also measured another parameter called the up-down asymmetry of transitions. This gives information about how the baryon’s spin orientation affects the decay process, finding values consistent with maximal parity violation, i.e., it is strongly affected by the spin orientation.

This provides evidence in favor of theoretical predictions on bottom baryon decays to charm baryons, enhancing confidence in the models used to explain them.

Ruling out alternatives and future work

These results are the first experimental confirmation of the spin-parity of Ξ_c(3,055) and effectively eliminate several competing theories about the nature of these particles.

“To our knowledge, no previous experiment has directly determined the spin-parity of Ξ_c(3,055). While ATLAS and CMS primarily focus on high-energy searches and have limited studies in heavy-flavor spectroscopy, Ξ_c(3,055) was first observed by Belle.

“Both Belle and BaBar have studied its mass and width, with which our results show consistency, but no spin-parity measurements were conducted,” explained Wan.

In the future, a similar approach could be used to establish the nature of other poorly understood baryons, helping scientists to map the complex spectrum of baryons.

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