“Charming” experiments find gluon mass in protons
Experimental determination of the proton’s gluonic gravitational form factors has revealed part of the proton’s hidden mass.
Nuclear physicists can finally pinpoint where a large fraction of the proton’s mass resides. A recent experiment conducted at the United States Department of Energy’s Thomas Jefferson National Accelerator Facility has revealed the radius of mass of protons produced by the strong force due to the binding together of quarks, the building blocks of protons. The results were published March 29 in the journal the nature.
One of the greatest mysteries of the proton is the origin of its mass. It turns out that the proton’s measured mass comes not just from its physical building blocks, but from its three so-called valence quarks.
“If you add the standard model mass of the quark to the proton, you get a small fraction of the proton’s mass,” explained test co-author Sylvester Joosten, an experimental physicist at DOE’s Argonne National Laboratory.
Over the past few decades, nuclear physicists have tentatively pieced together that the proton’s mass comes from several sources. First, it gets some mass from the mass of its quarks and some more from their motion. After that, it gains mass from the strong force that glues those quarks together, manifesting this force as a ‘gluon’. Finally, it gains mass from the kinetic interactions of the proton’s quarks and gluons.
This new measurement may finally shed some light on the mass produced by the proton gluon by determining the location of the object produced by this gluon. The radius of this center of matter has been found to lie at the center of the proton. The result also indicates that protons in this core have a different shape than the well-measured charge radius, a quantity often used as a proxy for proton size.
“The radius of this mass structure is smaller than the charge radius, and so it kind of gives us an idea of the mass stratification versus the charge structure of the nucleon,” said experiment co-author Mark Jones, Halls A&C leader at Jefferson Lab.
According to test co-author Jean-Edin Mezziani, a staff scientist at DOE’s Argonne National Laboratory, this result was actually somewhat surprising.
“What we got was something that we didn’t really expect to come out this way. The main goal of this experiment was to find a pentaquark that the researchers reported CERN” Mezziani said
The experiment was performed in Experimental Hall C of Jefferson Lab’s Continuous Electron Beam Accelerator Facility, a DOE Office of Science user facility. In the experiment, electrons from the CEBAF accelerator with an energy of 10.6 GeV (billion electron-volts) were sent into a small block of copper. The electrons were slowed or deflected by the block, causing them to emit bremsstrahlung radiation as photons. This beam of photons then strikes protons inside a liquid hydrogen target. The detectors measured the remnants of these interactions as electrons and positrons.
The experimenters were interested in the interactions that produce the J/ Ψ particle between the hydrogen proton nucleus. J/ Ψ is a short-lived meson made up of charm/anti-charm quarks. Once formed, it rapidly decays into an electron/positron pair.
Among the billions of interactions, the experimenters found about 2,000 J/Ψ particles by measuring the cross-sections of these interactions confirming coincident electron/positron pairs.
“It’s the same thing we’ve been doing all along. “By elastic scattering of the electron into the proton, we get the charge distribution of the proton,” Jones said. “In this case, we’ve photo-produced a single J/Ψ from the proton, and we get a gluon distribution instead of a charge distribution.”
The collaborators were then able to insert these cross-section measurements into theoretical models that describe the gluonic gravitational form factors of protons. The gluonic form factors describe the mechanical properties of the proton, such as its mass and stress.
“There were two quantities, known as gravitational form factors, that we were able to derive because we had access to these two models: the generalized parton distribution model and the holographic quantum chromodynamics (QCD) model. And we compared the results of each of these models to lattice QCD calculations. I did,” Mezziani added.
From two different combinations of these quantities, the experimenters determined the aforementioned gluonic mass radius influenced by graviton-like gluons, as well as a larger radius of attractive scalar gluons that extend beyond the moving quarks and confine them.
“One of the more surprising results from our experiments is that in one of the theoretical model approaches, our data hint at a scalar gluon distribution that extends well beyond the electromagnetic proton radius,” Justen said. “To fully understand these new observations and their impact on our understanding of confinement, we will need a new generation of high-precision J/Ψ experiments.”
One possibility for further exploration of these exciting new results is the Solenoidal Large Intensity Device Experiment Program, called SOLID. The SOLID program is still in the proposal stage. If allowed to proceed, experiments conducted with solid devices will provide new insights into J/Ψ production with solid detectors. It will indeed be able to make high-precision measurements in this region. One of the main pillars of that program is J/Ψ production, along with transverse momentum distribution measurements and parity-violating deep inelastic scattering measurements,” Jones said.
Jones, Joosten and Mezziani represent an experimental collaboration that includes more than 50 nuclear physicists from 10 institutions. The spokesperson also wants to highlight Burku Duran, lead author and postdoctoral research associate at the University of Tennessee, Knoxville. Duran presented this experiment in his Ph.D. thesis as a graduate student at Temple University, and he was a driving force behind the data analysis.
The collaboration conducted the experiment for about 30 days in February-March 2019. They agree that this new result is interesting, and they say they all look forward to future results that will shed additional light on the glimpses of new physics it implies.
“Bottom line for me – there’s an excitement right now. Can we find a way to confirm what we are seeing? What information will stick to this new picture? Dr. Mejiani. “But to me it’s really exciting. Because if I think about the proton now, we have more information about it than ever before.”
Reference: “Determining the gluonic gravitational form factor of protons” by B. Duran, Z.-E Meziani, S. Joosten, MK Jones, S. Prasad, C. Peng, W. Armstrong, H. Atac, E. Chudakov, H. .Bhatt, D. Bhetuwal, M. Boer, A. Camsonne, J.-P. Chen, MM Dalton, N. Deokar, M. Diefenthaler, J. Dunne, L. El Fassi, E. Fouche, H. Gao, D. Gaskell, O. Hansen, F. Hauenstein, D. Higginbotham, S. Zia, A. Karki, C. Keppel, P. King, HS Co, X. Lee, R. Lee, D. Mack, S. Malles, M. McCaughan, RE McClellan, R. Michaels, D. Meekins, Michael Paulone, L. Penchev, E. Pusher, A. Puckett, R. Rudolph, M. Rehfuss, PE Reimer, S. Riordan, B. Swatzky, A. Smith, N. Sparveris, H. Sumila-Vance, S. Wood, J. Xie, Z. Ye, C. Yero and Z. Zhao, 29 March 2023, the nature.
Funding: DOE/US Department of Energy