Research – Prof. Schmidt's personal page

29/09/202229/09/2022

Town Hall on QCD: why we should accelerate positrons

This past weekend in Cambridge, MA, was one of several town hall meetings being held around the country to get input from the community as NSAC (Nuclear Science Advisory Committee) undertakes the next “Long-Range Plan.” These plans are developed every seven years or so and set the priorities for nuclear physics research funding by government agencies such as the U.S. Department of Energy and the National Science Foundation. This weekend’s town hall was on Quantum Chromodynamics (QCD), and I had the privilege to be invited to talk about physics that would be made possible with an accelerator that could accelerate positrons.

Accelerating positrons isn’t hard. An existing electron accelerator, such as Jefferson Lab’s CEBAF, can do the job, provided the polarities of all the steering magnets are reversed. But it’s making the positrons in the first place that requires some investment. It’s not cheap (in the ball park of tens of millions of dollars), but it’s not nearly as expensive as, say, upgrading CEBAF’s terminal energy from 12 GeV to 24 GeV. And it’s no where near as expensive as building a new accelerator, such as the upcoming electron ion collider. As part of the Jefferson Lab Positron Working Group, I am advocating that a positron source be built at Jefferson Lab, so that positrons could be accelerated using th CEBAF accelerator, and experiments could be conducted with all of the amazing detectors and equipment that are already part of the laboratory’s infrastructure.

There are many great experiments one can do with a positron beam, and among them, the application I find to be most critical is determining how much two-photon exchange occurs in electron scattering. Electron scattering is a great probe for learning about how quarks are distributed within protons, but in analyzing such measurements, we assume that the electron exchanges one and only one virtual photon with the quark that it collides with. It can exchange more than one photon (with each additional photon being significantly less likely), but calculating the probability of a second exchanged photon is not straightforward, and we typically assume that it has a negligible impact on the results. This may not have gotten us into trouble everywhere, but it likely has gotten us into trouble in our efforts to measure the proton’s charge distribution (its so-called “electromagnetic form factors.”)

Positrons can help clear up this mess because two-photon exchange has the opposite effect on positron scattering as it does on electron scattering. If we can measure a difference between, say, a positron-proton scattering reaction, and an electron-proton scattering reaction, then it tells us exactly what the effects of two-photon exchange are. This could help clear up the proton’s form factors, and tell us what to watch out for as we embark on measuring challenging new reactions like deeply virtual compton scattering or deeply virtual meson production.

Positrons are great for answering lots of other questions too, such as helping to map out the protons 3D structure (via its Generalized Parton Distributions), probing for “strange quarks” in the proton, or even searching for light dark matter. I think the benefits are really clear, and cost is not exorbitant, and I hope that a positron source at Jefferson Lab is endorsed in the Long-Range Plan.

If you’d like to learn more, I highly recommend checking out the JLab Positron Working Group’s White Paper, published as a special topical issue of the European Physical Journal A.

29/02/2020

Seminar at Stony Brook’s Center for Frontiers in Nuclear Science

On Feb. 28th, I had the privilege to give the bi-weekly seminar at Stony Brook’s Center for Frontiers in Nuclear Science. The timing could not have been better for me to talk about our group’s latest paper, in Nature, about the force between protons and neutrons at extremely short distances. One of the best things about the trip was getting to see all of the impressive R&D projects going on at Stony Brook, relating to the future sPHENIX detector, and the future Electron-Ion Collider.

Axel in front of sPHENIX TPC prototype — I’m standing in front of the prototype of the sPHENIX TPC’s inner ring.

Prototype EIC polarized e- gun — This is the prototype of the polarized electron gun that will inject into the future Electron-Ion Collider.

Photograph of Stony Brook University's Tandem Accelerator — Stony Brook’s Tandem Van de Graaff electrostatic accelerator

The highlights include seeing the prototype “Time Projection Chamber” (TPC) for sPHENIX, which will be able to simultaneously image the trajectories of thousands of charged particles. I also got to see the prototype of the polarized electron gun which will inject electrons into the future EIC. Lastly, it always warms my heart to see a Tandem Van de Graaff accelerator, since that’s the kind of machine run by Yale’s Wright Nuclear Structure Laboratory, where I did my first undergraduate research projects.

26/02/2020

“Probing the core of the strong nuclear interaction” published in Nature

My latest research article, “Probing the core of the strong nuclear interaction” has been published in the February 26th issue of the journal, Nature. The article describes a study of the force between protons and neutrons (collectively called “nucleons”) at very short distance scales, that is, less than a femtometer or 10^-15 m. Nuclear forces have traditionally been studied by shooting one accelerated nucleon at another, and looking at the distributions of scattering angles as a function of momentum. To study forces at shorter distance scales, one needs to shoot nucleons with higher and higher momenta. At very short-distance scales, this technique becomes unwieldy, not because we can’t build particle accelerators of sufficient size, but because the collisions start producing copious amounts of other particles, complicating the interpretation in terms of nuclear forces. In this paper, my collaborators and I show how a new approach can work. Nucleons inside the nucleus are constantly moving around, and at any given moment, some will find themselves a very short-distance away from a partner nucleon. By using high-energy electron scattering, we can knock this “short-range correlated pair” out of the nucleus and study it. In doing so, we found clear evidence of a transition to a “repulsive core” at extremely short distances! This can have big implications for the structure of the cores of neutron stars, in which neutrons are packed at even higher density than inside a nucleus.

Figure 2 from the paper — Fig. 2 from the paper shows how the data support a transition from a primarily tensor force at low relative momentum to a hard isospin-independent repulsive core at high relative momentum.

If you’re interested to learn more, I invite you to read the excellent “News and Views” companion piece by Prof. Alexandra Gade of Michigan State.

07/02/202007/02/2020

Check out my Nuclear Physics Seminar on Feb. 11!

The Physics Department’s Nuclear Physics Seminars are starting up for the semester, and I’m giving the first, this coming Tuesday, February 11th. Come hear about my research, at 4PM in Corcoran 413.

21/01/202021/01/2020

Check out my WoW Talk on February 5th

I’ll be giving an SEH WoW (What’s our Work) seminar on February 5th, at 4PM in SEH B1220. This will be my first talk at GW as a professor. Come check it out.

11/12/2019

BAND is taking data!

The Backward Angle Neutron Detector (BAND), part of the CLAS-12 Spectrometer in Jefferson Lab’s Experimental Hall B is back up and running and collecting data as part of the winter run. There have been a couple of bumpy days as the accelerator has come back online, but right now, CEBAF is delivering 40 nA of 10 GeV electrons, and BAND is “seeing” particles.

The BAND detector is live and collecting data. The left plot (green) shows the rates in individual scintillator bars, the center plot shows the energy spectrum, and the right panel shows the high voltage control.

To start, CLAS-12 is collecting data with a reversed magnetic field polarity in the torus magnet, which will be useful for calibrating how the magnetic field bends charged particles. The neutrons that hit BAND, however, will have straight trajectories regardless of the orientation of the magnetic field.

MIT Graduate Student Efrain Segarra in the Hall B “Counting House”

From this station, shift workers monitor the experiment, and make adjustments to the spectrometer and the data acquisition system.

05/09/201905/09/2019

Published in Phys. Lett. B: “Comparing proton momentum distributions in A = 2 and 3 nuclei via 2H, 3H, and 3He (e,e’p) measurements”

The Journal “Physics Letters B” has just published our paper measuring (e,e’p) cross section ratios in deuterium, tritium, and Helium-3 using the Jefferson Lab Hall A high resolution spectrometers. This experiment was part of the “Tritium Run Group” at Jefferson Lab, a once in a generation opportunity to perform electron-scattering experiments on the radioactive isotope of hydrogen with one proton and two neutrons. Tritium decays with a half life of about 12 years, which means it hangs around long enough to perform experiments, but it’s decay (via beta decay) means that it is constantly giving off radiation and is a significant safety hazard. It’s pretty rare that a National Lab will go through the rigorous steps to make a safe tritium target, and it was fantastic opportunity to do some very interesting science. The cool thing about tritium is that it’s isospin partner Helium-3 (stable with two protons and one neutron) can be studied together to disentangle the behavior of protons and neutrons in asymmetric nuclei.

We found that while most models for nucleon momentum distributions can explain the measured He3/H3 proton momentum distribution ratio up to about 250 MeV/c, at higher momentum there are still some significant unknown effects.

This paper was largely the work of MIT graduate student, Reynier Cruz-Torres, for whom this will be a big part of his thesis. This was the first experiment I got to run with Rey, and he did an amazing job coordinating the experiment’s run plan and working some very late nights trying to analyze the data as it was coming in.