The effects of gravity are evident throughout the observable universe. Its effects can be observed in the synchronous orbits of satellites around planets, in comets that deviate from their paths due to the gravitational pull of large stars, and in the majestic spirals of huge galaxies. These spectacular phenomena highlight the role of gravity on the grandest scale of matter. At the same time, nuclear physicists are discovering a significant contribution of gravity to the smallest scale of matter.
A new study conducted by nuclear physicists at the Thomas Jefferson National Accelerator Facility at the U.S. Department of Energy is using a method to link the theory of gravity to the interactions between particles of the smallest matter to reveal new details on this smaller scale. This study now reveals for the first time a snapshot of the distribution of strong forces inside protons. This snapshot details the shear stresses that the force may exert on the quark particles that make up the protons. The findings were recently published in the Journal of Modern Physics.
According to the study's lead author, Volker Burkert, chief scientist at Jefferson Labs, the measurements reveal insight into the environment experienced by proton building blocks. Protons are made up of three quarks, and the quarks are held together by a powerful force.
At its peak, it's not just a four-ton force, one has to apply a quark to pull it out of the proton," Burkett explained. "Of course, nature doesn't allow us to separate just one quark from a proton, because quarks have something called'Color'characteristics. There are three colors that mix the quarks in the proton to make it look colorless from the outside, which is a requirement for its existence in space. Trying to pull a colored quark out of a proton creates a colorless quark Antiquark pair, a meson that uses the energy you put in to try to separate the quark, leaving behind a colorless proton (or neutron). Thus, 4 tons is an illustration of the intrinsic force strength of a proton.
The Bootstrap Program results are only the second of the proton mechanical properties to be measured. The mechanical properties of a proton include its internal pressure (measured in 2018), mass distribution (physical size), angular momentum, and shear stress (as shown in the figure). The results are made possible by data from half a century ago** and twenty years ago.
In the mid-1960s, it was theorized that if nuclear physicists could see how gravity interacted with subatomic particles, such as protons, such experiments could directly reveal the mechanical properties of protons.
But at the time, there was no way. For example, if you compare gravity to electromagnetic force, there is a difference of 39 orders of magnitude - so this is completely unpromising, right? Latifa Elouadhriri, a scientist at the Jefferson Laboratory and co-author of the study, explained.
A series of experiments were conducted at the Jefferson Laboratory's Continuous Electron Beam Accelerator Facility (CEBAF) decades ago. This equipment is an important user facility for the U.S. Department of Energy's Science Office. A typical CEBAF experiment is like a dance of high-energy electrons with another particle, exchanging a packet of energy and a unit of angular momentum called a virtual photon. The energy of the electrons determines the rhythm and pace of this dance. In the experiment, the high-energy electron beam interacted closely with the protons in the liquefied hydrogen target, exerting even more force than the four tons needed to pull out the quark anti-quark pair. "We developed this program to delve into virtual Compton scattering," Elouadhriri shared, "and this is a fantastic moment when electrons exchange virtual photons with protons. In the final state, the proton remains as it is, but the recoil is enormous. You produce a real, very high-energy photon, and that scattered electron. ”
When they collected the data, they didn't know that, in addition to intending to image it in 3D, they had inadvertently captured the critical data needed to access the proton's mechanical properties. It turns out that this particular process – Deep Virtual Compton Scattering (DVCS) – may be closely related to the interaction of gravity with matter. In 1973, Charles W. MisnerMisner), Kip SThorne and John Archibald Wheeler, in their classic work Gritation, elaborate on a general version of this concept. "Any massless spin-2 field produces a force that is indistinguishable from gravity because a massless spin-2 field couples to the stress-energy tensor in the same way that it interacts with gravity," they write in the book. Thirty years later, the theorist Maxim Polyakov further developed this idea, establishing a theoretical basis for connecting DVCS processes and gravitational interactions. This theoretical breakthrough establishes the relationship between the measurement of deep virtual Compton scattering and the gravitational form factor. For the first time, we were able to use it and extract the pressures that we did in 2018, published in Nature, are now normal and shear," Burkett explained. A more detailed description of the connection between the DVCS process and gravitational interactions can be found in this article. This article provides an in-depth look at the first results obtained from this study, adding another valuable contribution to the exploration of the mysteries of the universe. The researchers say their next step is to work to extract the information they need from the existing DVCS data to determine the mechanical size of the protons for the first time. They also hope to take advantage of newer, more statistical, and higher-energy experiments that are continuing DVCS research in protons.
At the same time, the study's co-authors were surprised by the sheer number of new theoretical efforts, detailed in hundreds of theoretical publications, that have begun to use this newly discovered avenue to explore the mechanical properties of protons.
And, now that we are in this new era of discovery, the 2023 Nuclear Science Long-Term Plan was recently released. This will be the main pillar of the scientific direction, including the development of new facilities and new detectors. We look forward to seeing more that can be done," Burkett said.
Elouadhriri agrees.
In my opinion, this is just the beginning of something bigger in the future. It has changed the way we think about proton structure," she said.
Now, we can express the structure of subnuclear particles with force, pressure, and physical size, which are not understandable to physicists," Burkett added.
Reference: v d. burkert、l. elouadrhiri、f. x. girod、c. lorcé、p.Schweitzer and P e."Symposium: The Gravitational Formal Factors of Protons," Shanahan, December 22, 2023, Modern Physics Review.
doi： 10.1103/revmodphys.95.041002
The study was conducted by the U.S. Department of Energy, the National Academy of Sciences, Carl Gand the Shirley Sontheimer Research** grant.