Physics

Lattice QCD method suggests simpler spectra of exotic XYZ hadrons

This diagram shows the contours of a complex energy surface. Each color corresponds to a different pair of D mesons. Although their shapes are very different, they revolve around a common vertex. This means that there is a single resonance coupled to all three channels. Credit: Jefferson Lab Illustration/David Wilson

The elusive particles that first formed in the hot, dense maelstrom of the early universe have puzzled physicists for decades. Following the surprising discovery in 2003, scientists began observing a number of other strange objects associated with the millionth of a second after the Big Bang.

These signals, which appear as “bumps” in the data of high-energy experiments, became known as short-lived “XYZ states,” for lack of a better label. These defy the standard picture of particle behavior and are a major problem in modern physics, giving rise to several attempts to understand their mysterious properties.

But theorists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility suggest that fewer XYZ states, also known as resonances, may explain the experimental data than currently claimed.

The researchers, on behalf of the Jefferson Institute’s Center for Theoretical and Computational Physics and the University of Cambridge in the UK, used the field of quantum physics to study particles containing specific “flavors” of elementary particle building blocks known as quarks. The energy level (also called mass) of is calculated. .

The researchers discovered that multiple particle states that share the same degree of spin (angular momentum) are coupled, meaning that there is only one resonance in each spin channel. This fresh interpretation contradicts several other theoretical and experimental studies.

“We argue that by incorporating the data into coupled channel analysis, we have a better chance of understanding the experimental spectra,” said Joseph Dudek, a staff scientist at the Jefferson Institute and a professor at the College of William and Mary. . . “Our hypothesis is that this approach may ultimately reduce the number of resonances needed to explain all the data.”

Dudek and his colleagues at the Theory Center published their results in two companion papers published for the International Hadron Spectral Collaboration (HadSpec) in Physical Review Letters and Physical Review D. This exciting research may also soon yield clues about a complex particle with a mysterious name. :X(3872).

The charm of X(3872)

Charm quark, one of six quark “flavors,” was first observed experimentally in 1974. Charm quarks are discovered together with their antimatter counterparts, anticharms, and these paired particles are part of an energy field called “charmonium.”

Fast forward to 2003, and Japan’s High Energy Accelerator Research Organization (KEK)’s Belle experiment has discovered a new Charmonium candidate called X(3872). Although its enigmatic name suggests an interstellar object such as an exoplanet, X(3872) is a short-lived particle state that appears to contradict the current quark model.

Some scientists argue that X(3872) may be a tetraquark, a composite particle (hadron) made up of two quarks and two antiquarks. For comparison, protons and neutrons are hadrons with three quarks. Other possible explanations for X(3872) include a molecularly coupled system of two mesons, each containing two quarks, or some kind of quark-gluon hybrid.

“X(3872) is more than 20 years old, but we still don’t have a clear and simple explanation that everyone can support,” said David, a science user at the Jefferson Institute at the University of Cambridge, UK. Wilson said. Lead author of the HadSpec study.

Other exotic candidate states have also been observed, such as Y(4260) and Zc(3900). Therefore, it is labeled XYZ. In fact, so many states were being discovered that in 2017, Particle Data Group revamped its naming scheme.

This hodgepodge of excited states is the result of an explosion in the amount of data collected by modern particle accelerators.

“High-energy experiments have begun to measure processes that are hundreds of times fainter,” Dudek said. “They started seeing bumps that were interpreted as new particles almost everywhere. And very few of these states agreed with the previous model.”

The “XYZ” alphabet soup motivated the HadSpec group to begin using quantum field theory to classify spectra of states.

Lattice QCD method suggests simpler spectra of exotic XYZ hadrons

Spectrum of irreps Λ𝑃=𝐴+1, 𝐸+, 𝑇+2 with zero overall momentum. They have leading partial waves 𝐽𝑃⁢𝐶=0++, 2++, and 2++, respectively. The points are the computed energies of the finite volumes, colored according to the overlap of their dominant operators, with the colors specified in the keys on the right. The black dots overlap significantly with both the 𝑐⁢𝑐-like and 𝐷⁢ ̄𝐷-like operators. The solid line indicates the non-interacting meson energy and the dashed line indicates the kinetic threshold. Degenerate non-interaction levels are indicated by multiple parallel curves with energies slightly shifted for visual clarity. Credit: Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.241901

from the grid

Quantum chromodynamics (QCD) describes the interaction of gluons and quarks, which are photon-like carriers of strong forces. Supercomputers have the ability to process huge numbers and can apply the theory to QCD by putting it on a grid.

Think of a lattice as a small, tightly packed grid of points that represent space and time. Theorists can use the possible arrangements of quarks and gluons within this “box” to predict properties of hadrons, such as their mass and lifetime.

The lattice box is extremely small, measuring a few femtometers, nearly a million times smaller than a single atom. But sampling the possible behavior of quarks and gluons even in such a small volume requires extremely powerful computers. That’s why HadSpec Group used several high-performance computing clusters, including one at the Jefferson Institute, to perform mind-boggling calculations.

“If you hated calculus in school, imagine the worst calculus problem ever,” Dudek says.

The HadSpec group calculated the mass and lifetime of mesons in the charmonium region, where experiments have claimed an XYZ state. These mesons rapidly decay into “D mesons” and their antimatter.

D mesons consist of a heavy charm quark and a lighter antiquark with an “up”, “down”, or “strange” flavor. Anti-D mesons are just the opposite. Contains anti-attractive quarks and regular quarks with either a lighter flavor.

If you’re wondering where the extra quarks come from, QCD predicts vacuum fluctuations in which quark-antiquark pairs are constantly created and annihilated. These phenomena are long-theorized quantum oddities of empty space.

“There must have been at least one pair of light quarks somewhere,” Dudek says. “They didn’t exist when attraction and anti-attraction were just flying around, but they do exist in this well-separated pair of D mesons.”

The authors used the lattice results to infer all possible ways a charmonium meson could decay. To do this, they had to relate the results obtained from the small box to what would happen in a virtually infinite volume, the size of the universe.

“With our calculations, unlike in experiments, we can’t just fire two particles and measure the two particles that come out,” Wilson said. “Quantum mechanics will find the final state for you, so you need to calculate all possible final states at the same time.”

The results can be understood in terms of a single short-lived particle (resonance), whose appearance can vary depending on the possible decay state observed.

“We’re trying to use basic theory and the best methods available to simplify the situation as much as possible,” Wilson said. “Our goal is to explain what we learned in experiments.”

Now that the HadSpec team has proven that this type of calculation is feasible, they are ready to apply it to the mysterious particle X(3872).

“X(3872) is very interesting,” Wilson said. “Its origin is an open question. It seems very close to the threshold, but it could be a coincidence or an important part of the story. This is one of the things we will look into soon. It’s one.”

Further information: David J. Wilson et al, Scalar and tensor harmonium resonances in coupled channel scattering from lattice QCD, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.241901

David J. Wilson et al., Harmonium χc0 and χc2 resonances in coupled channel scattering from lattice QCDs, Physical Review D (2024). DOI: 10.1103/PhysRevD.109.114503

Provided by Thomas Jefferson National Accelerator Facility

Citation: Lattice QCD method suggests simpler spectra for exotic XYZ hadrons (November 12, 2024), https://phys.org/news/2024-11-lattice-qcd-method-simpler Retrieved November 12, 2024 from -spectrum.html

This document is subject to copyright. No part may be reproduced without written permission, except in fair dealing for personal study or research purposes. Content is provided for informational purposes only.

Related Articles

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button