Discovering the Hidden Mass of Protons Through Gravitational Form Factors
Scientists have conducted experiments and uncovered part of the hidden mass in a proton by measuring its gravitational form factors.
Scientists have just discovered the part of a proton which makes it have a lot of mass. This experiment was done at an American energy research center and the results were published in a magazine called Nature. They found out that the part giving the proton its mass is created by something called the strong force, which holds together quarks – little building blocks inside protons.
One of the major unsolved puzzles is why protons have so much mass. It looks like the three tiny pieces that make up a proton–called valence quarks–aren’t enough to explain its mass.
Sylvester Joosten, an experimental physicist from Argonne National Laboratory, said: “If you add together all of the masses of the valence quarks in a proton, it’s still not enough to reach its actual total mass.”
For quite a few years, scientists have been learning about why protons have mass. It turns out that the mass is due to 3 things: the weights of its separate pieces (called quarks), energy released by those quarks binding together (called gluons), and interactions between the quarks and gluons.
Scientists discovered that the matter created by tiny particles called gluons is located in the middle of protons. They also found out that this matter has a different size than the commonly measured charge radius which describes how big protons are.
Mark Jones, leader of Jefferson Lab’s Halls A&C, says that the mass structure of something is smaller than its charge radius. Zein-Eddine Meziani from Argonne National Laboratory was surprised by this finding.
Meziani said, “We discovered something excitingly different from what we initially expected! Our experiment was originally tracking a pentaquark noticed by scientists at CERN.”
The experiment was done in a special lab called the Continuous Electron Beam Accelerator Facility. Here, electrons with a lot of energy (10.6 GeV) were fired into a block of copper. When they hit it, they made some kind of radiation called bremsstrahlung radiation, which then went on to hit some protons inside a liquid hydrogen target. After this collision happened, detectors measured what was left behind as particles called electrons and positrons.
The scientists wanted to know what happened when protons in hydrogen atoms interacted. This brings about J/ Ψ particles, which are very small but quickly break down into an electron and a positron.
Out of all the interactions, the experimenters found about 2,000 instances where the J/ Ψ particle happened by looking for pairs of electrons and positrons together at the same time.
Jones said: “We’ve been doing something very similar to what we’ve been doing all this time. We usually fire an electron at the proton and measure how much it reacts – this gives us information on how the proton is charged. But now, we’re using a different method where we use light instead of electrons and that will give us information about how gluons are distributed in the proton.”
Researchers were able to put their measurements of the inside of a proton into models that help explain how it works. These models looked at the proton’s mass and pressure to give us an idea of what sort of mechanical characteristics it had.
We looked into two special numbers, called gravitational form factors, by using two different models of calculations. The first was a generalized parton distributions model and the second was a holographic quantum chromodynamics (QCD) model. Then we compared our results with an additional calculation model known as lattice QCD.
The experimenters used two different combinations of numbers to discover that gluons have both a mass radius which is mainly caused by graviton-like gluons and a larger attractive radius which stretches beyond the moving quarks and holds them together.
“Something strange came up in our experiment,” said Joosten. “It looks like a force called gluon goes far beyond what we thought it could do. To really understand this new discovery, and why it matters to us, more sophisticated experiments need to be done.”
Scientists are working on a new and exciting experiment program called SoLID, which is still in the proposal stage. If it’s approved, this new project could lead us to discover more about J/ Ψ production with SoLID detector. This will help us make very accurate measurements of things like transverse momentum distribution and data from deep inelastic scattering experiments.
Jones, Joosten and Meziani are a group of more than 50 nuclear physicists from 10 different institutions who made an experiment together. They particularly want to mention Burcu Duran, the lead author and researcher at the University of Tennessee, Knoxville. Before that, she studied this experiment as part of her Ph.D. dissertation while studying at Temple University and was in charge of studying the data.
The experiment was done for a month during February and March 2019, which gave some interesting results. Everyone involved is excited for what the future will bring as this new result could show us something special about the world we live in!
“I’m so excited! Can we find out if this new information we’re seeing is true? If it is, then it means we understand more about a proton than ever before. Isn’t that awesome?!” said Meziani.
Scientists from lots of countries have come together to study how the proton (a subatomic particle) is affected by gravity. Their results have been published in a Nature journal article on March 29th, 2023. This research helps us understand more about the physical universe around us!
The U.S. Department of Energy funded three research projects. The first project looks at why some people don’t stay on their HIV medicine in African countries, the second is about a type of worm found in Ethiopia and finally, the third is about germs called “Nontuberculous Mycobacteria” and how quickly they spread in China.
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