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Friday, February 22nd, 2019
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Event |
| 12:00a |
Physicists calculate proton’s pressure distribution for first time Neutron stars are among the densest-known objects in the universe, withstanding pressures so great that one teaspoon of a star’s material would equal about 15 times the weight of the moon. Yet as it turns out, protons — the fundamental particles that make up most of the visible matter in the universe — contain even higher pressures.
For the first time, MIT physicists have calculated a proton’s pressure distribution, and found that the particle contains a highly pressurized core that, at its most intense point, is generating greater pressures than are found inside a neutron star.
This core pushes out from the proton’s center, while the surrounding region pushes inward. (Imagine a baseball attempting to expand inside a soccer ball that is collapsing.) The competing pressures act to stabilize the proton’s overall structure.
The physicists’ results, published today in Physical Review Letters, represent the first time that scientists have calculated a proton’s pressure distribution by taking into account the contributions of both quarks and gluons, the proton’s fundamental, subatomic constituents.
“Pressure is a fundamental aspect of the proton that we know very little about at the moment,” says lead author Phiala Shanahan, assistant professor of physics at MIT. “Now we’ve found that quarks and gluons in the center of the proton are generating significant outward pressure, and further to the edges, there’s a confining pressure. With this result, we’re driving toward a complete picture of the proton’s structure.”
Shanahan carried out the study with co-author William Detmold, associate professor of physics at MIT.
Remarkable quarks
In May 2018, physicists at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility announced that they had measured the proton’s pressure distribution for the first time, using a beam of electrons that they fired at a target made of hydrogen. The electrons interacted with quarks inside the protons in the target. The physicists then determined the pressure distribution throughout the proton, based on the way in which the electrons scattered from the target. Their results showed a high-pressure center in the proton that at its point of highest pressure measured about 1035 pascals, or 10 times the pressure inside a neutron star.
However, Shanahan says their picture of the proton’s pressure was incomplete.
“They found a pretty remarkable result,” Shanahan says. “But that result was subject to a number of important assumtions that were necessary because of our incomplete understanding.”
Specifically, the researchers based their pressure estimates on the interactions of a proton’s quarks, but not its gluons. Protons consist of both quarks and gluons, which continuously interact in a dynamic and fluctuating way inside the proton. The Jefferson Lab team was only able to determine the contributions of quarks with its detector, which Shanahan says leaves out a large part of a proton’s pressure contribution.
“Over the last 60 years, we’ve built up quite a good understanding of the role of quarks in the structure of the proton,” she says. “But gluon structure is far, far harder to understand since it is notoriously difficult to measure or calculate.”
A gluon shift
Instead of measuring a proton’s pressure using particle accelerators, Shanahan and Detmold looked to include gluons’ role by using supercomputers to calculate the interactions between quarks and gluons that contribute to a proton’s pressure.
“Inside a proton, there’s a bubbling quantum vacuum of pairs of quarks and antiquarks, as well as gluons, appearing and disappearing,” Shanahan says. “Our calculations include all of these dynamical fluctuations.”
To do this, the team employed a technique in physics known as lattice QCD, for quantum chromodynamics, which is a set of equations that describes the strong force, one of the three fundamental forces of the Standard Model of particle physics. (The other two are the weak and electromagnetic force.) The strong force is what binds quarks and gluons to ultimately make a proton.
Lattice QCD calculations use a four-dimensional grid, or lattice, of points to represent the three dimensions of space and one of time. The researchers calculated the pressure inside the proton using the equations of Quantum Chromodynamics defined on the lattice.
“It’s hugely computationally demanding, so we use the most powerful supercomputers in the world to do these calculations,” Shanahan explains.
The team spent about 18 months running various configurations of quarks and gluons through several different supercomputers, then determined the average pressure at each point from the center of the proton, out to its edge.
Compared with the Jefferson Lab results, Shanahan and Detmold found that, by including the contribution of gluons, the distribution of pressure in the proton shifted significantly.
“We’ve looked at the gluon contribution to the pressure distribution for the first time, and we can really see that relative to the previous results the peak has become stronger, and the pressure distribution extends further from the center of the proton,” Shanahan says.
In other words, it appears that the highest pressure in the proton is around 1035 pascals, or 10 times that of a neutron star, similar to what researchers at Jefferson Lab reported. The surrounding low-pressure region extends farther than previously estimated.
Confirming these new calculations will require much more powerful detectors, such as the Electron-Ion Collider, a proposed particle accelerator that physicists aim to use to probe the inner structures of protons and neutrons, in more detail than ever before, including gluons.
“We’re in the early days of understanding quantitatively the role of gluons in a proton,” Shanahan says. “By combining the experimentally measured quark contribution, with our new calculation of the gluon piece, we have the first complete picture of the proton’s pressure, which is a prediction that can be tested at the new collider in the next 10 years.”
This research was supported, in part, by the National Science Foundation and the U.S. Department of Energy. | | 5:00a |
New MRI sensor can image activity deep within the brain Calcium is a critical signaling molecule for most cells, and it is especially important in neurons. Imaging calcium in brain cells can reveal how neurons communicate with each other; however, current imaging techniques can only penetrate a few millimeters into the brain.
MIT researchers have now devised a new way to image calcium activity that is based on magnetic resonance imaging (MRI) and allows them to peer much deeper into the brain. Using this technique, they can track signaling processes inside the neurons of living animals, enabling them to link neural activity with specific behaviors.
“This paper describes the first MRI-based detection of intracellular calcium signaling, which is directly analogous to powerful optical approaches used widely in neuroscience but now enables such measurements to be performed in vivo in deep tissue,” says Alan Jasanoff, an MIT professor of biological engineering, brain and cognitive sciences, and nuclear science and engineering, and an associate member of MIT’s McGovern Institute for Brain Research.
Jasanoff is the senior author of the paper, which appears in the Feb. 22 issue of Nature Communications. MIT postdocs Ali Barandov and Benjamin Bartelle are the paper’s lead authors. MIT senior Catherine Williamson, recent MIT graduate Emily Loucks, and Arthur Amos Noyes Professor Emeritus of Chemistry Stephen Lippard are also authors of the study.
Getting into cells
In their resting state, neurons have very low calcium levels. However, when they fire an electrical impulse, calcium floods into the cell. Over the past several decades, scientists have devised ways to image this activity by labeling calcium with fluorescent molecules. This can be done in cells grown in a lab dish, or in the brains of living animals, but this kind of microscopy imaging can only penetrate a few tenths of a millimeter into the tissue, limiting most studies to the surface of the brain.
“There are amazing things being done with these tools, but we wanted something that would allow ourselves and others to look deeper at cellular-level signaling,” Jasanoff says.
To achieve that, the MIT team turned to MRI, a noninvasive technique that works by detecting magnetic interactions between an injected contrast agent and water molecules inside cells.
Many scientists have been working on MRI-based calcium sensors, but the major obstacle has been developing a contrast agent that can get inside brain cells. Last year, Jasanoff’s lab developed an MRI sensor that can measure extracellular calcium concentrations, but these were based on nanoparticles that are too large to enter cells.
To create their new intracellular calcium sensors, the researchers used building blocks that can pass through the cell membrane. The contrast agent contains manganese, a metal that interacts weakly with magnetic fields, bound to an organic compound that can penetrate cell membranes. This complex also contains a calcium-binding arm called a chelator.
Once inside the cell, if calcium levels are low, the calcium chelator binds weakly to the manganese atom, shielding the manganese from MRI detection. When calcium flows into the cell, the chelator binds to the calcium and releases the manganese, which makes the contrast agent appear brighter in an MRI image.
“When neurons, or other brain cells called glia, become stimulated, they often experience more than tenfold increases in calcium concentration. Our sensor can detect those changes,” Jasanoff says.
Precise measurements
The researchers tested their sensor in rats by injecting it into the striatum, a region deep within the brain that is involved in planning movement and learning new behaviors. They then used potassium ions to stimulate electrical activity in neurons of the striatum, and were able to measure the calcium response in those cells.
Jasanoff hopes to use this technique to identify small clusters of neurons that are involved in specific behaviors or actions. Because this method directly measures signaling within cells, it can offer much more precise information about the location and timing of neuron activity than traditional functional MRI (fMRI), which measures blood flow in the brain.
“This could be useful for figuring out how different structures in the brain work together to process stimuli or coordinate behavior,” he says.
In addition, this technique could be used to image calcium as it performs many other roles, such as facilitating the activation of immune cells. With further modification, it could also one day be used to perform diagnostic imaging of the brain or other organs whose functions rely on calcium, such as the heart.
The research was funded by the National Institutes of Health and the MIT Simons Center for the Social Brain. | | 1:20p |
3 Questions: Josh Moss on tackling urban pollution Josh Moss is a PhD student in the lab of Professor Jesse Kroll, where he studies atmospheric chemistry and examines the chemistry of gases and particles in the atmosphere that humans are releasing and their interactions with existing particles in the atmosphere. He focuses on organic chemical reactions that occur in the atmosphere which contribute significantly to smog formation. In the laboratory, he uses a controlled atmospheric chamber to conduct physical experiments on the gas phase reactions that originate from smog particles. Moss also works on computer models for chemical reaction generation and predictions. His research is concerned with chemicals commonly found in large urban cities such as Los Angeles, Houston, and Mexico City, and he is interested in the implications that these micro-particles have on human health and climate change.
Q: What are the real-world implications of your research?
A: Primarily, much of what I study is related to urban pollution. My work is currently centered around understanding the impacts that gasoline, car emissions, and power-plant emissions have on smog formation, and what impacts they may have on the future of the environment. Due to the complexity of the atmosphere, it is difficult to break down all of the chemical reactions leading to smog formation, which is why this has become the focus of our research now.
I deal mostly with urban chemicals because they have generally been researched less than biologically emitted chemicals, and urban smog has adverse effects on human health in densely populated cities. In terms of human health, small smog particles are generally harmful for people to inhale because it can lead to various diseases such as heart failure, stroke, lung disease, and certain types of cancer. The largest source of uncertainty in global climate models is in these small particles. Right now, we are unsure exactly to what degree the particles are affecting the Earth’s temperature and climate. What we do know is that some particles can scatter sunlight, which cools the Earth. On the other hand, darker particles absorb sunlight and can actually warm the Earth. In addition to this, many particles lead to cloud formation, which contributes to both the cooling and warming of the Earth as well.
Humans are increasing the concentration of particles in the atmosphere on a regular basis. For instance, tiny particles can originate from burning or from particles that are formed from chemicals that have reacted in the atmosphere, known as secondary organic aerosol. Over the course of their reactions, they tend to stick together with other chemicals. Even though they are not emitted as particles, due to the chemical reactions that they undergo, particles are formed. Understanding secondary organic aerosol is really the core of my research. For example, if you look at a photo of LA, the smog formation over the city is extremely visible because they have an abundance of people in a concentrated area with countless cars. The combination of gas emissions together with the warm, sunny weather creates the perfect condition to form a great deal of smog particles. This is what I am really interested in when it comes to my research.
Q: What opportunities have you had to delve deeper into your research?
A: I was offered the opportunity to go to Paris last summer, which has led into the next exciting phase of my research, computer modeling. We are collaborating with a lab in Paris who has developed a unique software called GECKO-A that can predict chemical reactions in the atmosphere, giving me a new avenue to pursue in my research. Professor Kroll wrote a grant, funded by MIT International Science and Technology Initiatives with the lab in Paris, that enabled me to travel to France for almost a month in order to learn how to use the software. The software is very complex, relying on quantum chemistry knowledge to predict reactions. Jesse and I are excited for what this can tell us about the atmosphere that experiments cannot.
The atmosphere is arguably the most complex chemical system on Earth which makes it incredibly difficult to study. After several hours of reaction, a single chemical species can transform into millions of different chemicals. Even though we perform experiments in a controlled atmospheric chamber in our lab, it is impossible to measure and quantify every chemical that is generated during a reaction sequence. In order to dive further into my research, Jesse and I think the best course of action is to compare our experimental results to the model simulation results to improve both data sets. The models can give us detailed insight into the different chemical pathways related to smog formation, and the experimental data can serve to ground the model results in our observable reality.
Q: What is the next step for you?
A: I am still working to finish my thesis, however my long-term goals include becoming a professor. I love teaching and conducting research, so pursuing a career as a professor is a perfect fit for me. I have had several opportunities to TA classes here at MIT, including the Traveling Research Environmental Experience class (TREX), where I went to Hawaii with undergraduates to study volcanic emissions. TREX was one of the most fulfilling teaching experiences, and I hope to carry the excitement and joy I felt from TREX into all of my future teaching endeavors.
More recently, I have been mulling over a few other potential career paths. I am interested in environmental law and public policy because it would allow me to apply my research and knowledge in order to help shape the policies that protect our environment. I am very passionate about politics, and I have been concerned by the United States’ decreasing leadership role on the global stage, specifically in issues regarding climate change. I believe that scientists should take a more direct role in shaping critical policies, and I would be happy to contribute in any way that I can. My main passion lies in educating and informing people about the difficult and often highly nuanced environmental challenges that we face. I’ve given several public talks in the Boston area, and hosted a variety of classes for middle and high school students which I think is of vital importance for our collective future. I believe that the path to improving our environment, and more broadly our world, lies in education. If I can communicate my enthusiasm for environmental science and chemistry to others, I will consider it as a job well done. |
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