Unveiling the Universe's Mass Mystery: Experiments Uncover Hadron Mass Emergence
The vast majority of the universe's visible mass remains a perplexing enigma. While protons and neutrons, the fundamental building blocks of matter, are composed of quarks bound by gluons, their masses significantly surpass those of their constituent quarks. This discrepancy presents a central puzzle in nuclear physics.
The solution lies in the intricate dynamics of quantum chromodynamics (QCD), the theory governing the strong interaction. QCD reveals that the strong interaction generates mass from the energy stored in the fields of strongly interacting quarks and gluons. This process, known as the emergence of hadron mass (EHM), is a key to understanding the universe's composition.
Over the past decade, scientists have made significant strides in unraveling the dominant portion of the universe's visible mass. This progress is attributed to studies employing the continuum Schwinger method (CSM), a QCD-based approach that examines the distance (or momentum) dependence of the strong interaction.
By bridging CSM with experiments through phenomenology, researchers analyzed nearly 30 years of data collected at Jefferson Lab. This comprehensive effort has provided the most detailed insights into the mechanisms driving EHM. The study, spanning from the 1990s to potential future upgrades of Jefferson Lab's high-intensity accelerator, was featured on the cover of the journal Symmetry.
Daniel Carman, an experimental nuclear physicist at Jefferson Lab, emphasized the significance of this collective effort, stating, 'This is the payoff from what we've been doing at Jefferson Lab for decades. We still have a lot of work ahead, but this marks a major milestone along the way.'
The quest to understand EHM is far from over. QCD, which describes the dynamics of quarks and gluons, the elementary constituents of matter, plays a pivotal role in generating all hadronic matter, including protons, neutrons, and atomic nuclei. A distinctive feature of the strong force is gluon self-interaction, which significantly influences the behavior of particles.
Victor Mokeev, a staff scientist and phenomenologist at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility, highlighted the profound impact of gluon self-interaction, stating, 'Without gluon self-interaction, the universe would be completely different. It creates beauty through different particle properties and makes real-world hadron phenomena through emergent physics.'
The experiments at Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF) have been instrumental in connecting theory and experiment. The CEBAF Large Acceptance Spectrometer for 12 GeV (CLAS12), an upgraded version of its predecessor, enables the identification of particles produced when electrons scatter off protons across a wide range of emission angles.
These experiments have conclusively demonstrated that dressed quarks with dynamically generated masses are the fundamental building blocks of the proton's structure and its excited states. This finding solidifies the case that experimental results from Jefferson Lab can be used to assess the mechanisms responsible for EHM.
Looking ahead, Mokeev anticipates further progress, stating, 'We see much more work ahead.' Experiments from the 6 GeV era of CEBAF have explored the dressed-quark momentum range where approximately 30% of hadron mass is generated. Data from the current 12 GeV era is extending this coverage to about 50%. Future experiments with higher-energy electron beams will enable full coverage of the distance domain where the dominant portion of hadron mass emerges.
As scientists continue to delve deeper into the mysteries of the universe's mass, the insights gained from Jefferson Lab's experiments will contribute significantly to our understanding of the fundamental building blocks of matter and the intricate dynamics that govern their behavior.
