Resonating protons could shed light on early universe

Each nucleon consists of three quarks which are held together by gluons. The lowest energy state is also known as the ground state of a nucleon. When excited into a higher energy state, the quarks inside the nucleon vibrate against each other and begin rotating. This is referred to as nucleon resonance.

A collaboration between physicists from Justus Liebig Universitat (JLU) Giessen in Germany and the University of Connecticut conducted experiments at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Jefferson National Accelerator Facility to further explore nucleon resonances.

Structure of resonating nucleons

For their experiments, the team targeted a high-energy electron beam in a chamber of cooled hydrogen gas. The quarks inside the protons were excited and produced nucleon resonance along with a quark-antiquark state, referred to as a meson.

The excited state is short-lived. However, the evidence of their existence is left behind by the remaining particles that can be detected for a long enough period that a detector can pick them up. The team used this data to reconstruct the resonance, the first time this has been achieved.

"This is the first time we have some measurement, some observation, which is sensitive to the 3D characteristics of such an excited state," said Stefan Diehl, a professor at the University of Connecticut.

Going forward, the team plans to carry out more experiments using different targets and polarizations. Scattering electrons from polarized protons, the researchers aim to study different characteristics of the scattering process. Such experimentation can provide researchers with information about the properties of the chaotic early periods of the universe after the Big Bang.

Scientists are of the view that the early universe consisted of plasma where in quarks and gluons were spinning around due to very high energy. Later, the matter was formed and it existed in high-energy nucleon states. As the universe continued to expand, the energy levels lowered, and matter in its ground states began to exist.

Studies such as those conducted at CEBAF help scientists understand how matter was formed early on in the universe and why it exists in its present form today. "In principle, this is just the beginning, and this measurement is opening a new field of research," added Diehl.

The study findings were published in the journal Physical Review Letters.

Abstract

The polarized cross-section ratio σLT′/σ0 from hard exclusive π−Δ++ electroproduction off an unpolarized hydrogen target has been extracted based on beam-spin asymmetry measurements using a 10.2  GeV/10.6  GeV incident electron beam and the CLAS12 spectrometer at Jefferson Lab. The study, which provides the first observation of this channel in the deep-inelastic regime, focuses on very forward-pion kinematics in the valence regime, and photon virtualities ranging from 1.5  GeV2 up to 7  GeV2. The reaction provides a novel access to the d-quark content of the nucleon and to p→Δ++ transition generalized parton distributions. A comparison to existing results for hard exclusive π+n and π0p electroproduction is provided, which shows a clear impact of the excitation mechanism, encoded in transition generalized parton distributions, on the asymmetry.