New paper in PRL on the quark structure of neutrons

06/03/2020

New paper in PRL on the quark structure of neutrons

Tyler and I, along with our collaborators at MIT, Old Dominion, Penn State, Tel Aviv, and Jefferson Lab have just published an article in Physical Review Letters on the quark structure of free neutrons. Whereas free protons are easy to study (using my favorite technique, electron scattering!), neutrons decay with a lifetime of about 15 minutes, and can’t be practically made into a target. Instead, we learn about the neutron by scattering from nuclei, like deuterium or helium, and try to account for any effects that come from nuclear binding. One problematic effect is that the nuclear environment seems to change the structure of protons and neutrons, which we’ve given the name “The EMC Effect.”

In this work, first-authored by MIT graduate student Efrain Segarra, we looked at electron-scattering data from lots of different nuclei, including heavy nuclei, where the EMC Effect is large. Then, using the model developed in one of our recent papers, in which the EMC Effect stems only from short-range correlated proton-neutron pairs, we inferred what the free neutron structure must be to explain the data. This has consequences for the results of the MARATHON Experiment, which will hopefully be published soon.

If short-range correlations are responsible for the EMC Effect, then heavy nuclei would indicate a larger F2n/F2p ratio at large x.

Our paper is a quick read, and I hope you enjoy it.

03/03/202003/03/2020

Tyler gives MIT’s Laboratory for Nuclear Science Colloquium

Tyler delivered this week’s MIT Laboratory for Nuclear Science Colloquium, speaking on his work with the MARATHON Experiment, as well as a giving a preview of results on extracting free neutron structure from deep inelastic scattering from nuclei. These will appear in PRL soon, stay tuned!

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.

13/12/2019

Neutrons incoming!

The BAND Detector is seeing neutrons!

A hit time spectrum for all of the BAND Scintillators. Gamma rays, traveling at the speed of light, hit BAND prior to slower neutrons. We also have a calibration laser that is delayed by 200 ns relative to the trigger.

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.

03/11/2019

Electromagnetic Interactions of Nucleons and Nuclei Conference

I just returned from the 13th European Research Conference on Electromagnetic Interactions of Nucleons and Nuclei, a biannual conference held in Cyprus. There were some fascinating results presented, including a new determination of the proton’s charge radius by the PRad Experiment (it’s small!), a calculation of the “gluon EMC Effect” using Lattice QCD by MIT’s Phiala Shanahan, and lots of discussion about Large Momentum Effective Theory (LaMET), a new technique which may finally allow calculations of parton distribution functions on the Lattice.

The 2019 Frontiers and Careers Workshop participants

One of the best things about this conference is that it is preceded by a two-day workshop called “Frontiers and Careers in Photonuclear Physics,” which is specifically geared for students and early-career physicists. I feel honored that I was invited to give a pedagogical lecture about the EMC Effect at this year’s Frontiers, and I was so impressed by the job the organizers, Lena Heikenskjöld (Mainz) and Afroditi Papadopoulou (MIT), to put a wonderful workshop together. I highly encourage students to attend the 2020 edition of Frontiers, which will be held in August in New England in advance of the Gordon Research Conference on Photonuclear Reactions. You will learn a ton and meet a bunch of very interesting young physicists.

Afro Papadopoulou and I overlooking St. George’s Beach, near Chloraka, Cyprus.

Not only did Afroditi co-organize a fantastic Frontiers workshop, she also gave an outstanding talk on one of her research projects, “Electrons for Neutrinos,” in which analysis techniques from neutrino-nucleus scattering are benchmarked on electron-scattering data. Neutrino scattering experiments face the challenge of reconstructing the incoming neutrino energy event-by-event while simultaneously having to model the wide range of nuclear effects that can lead to the event’s measured final state. It’s a tough game, but Afroditi and others are showing how electron data can help.

From Afro’s talk: trying to reconstruct the beam energy from just one out-going electron, as is done in Cherenkov-based neutrino experiments is really hard, as shown by electron scattering data in which the beam energy is known precisely. Using a coincident proton can help, but it’s still far from perfect.

01/11/201901/11/2019

“Quasi-Free Scattering with Radioactive Beams” Workshop

It was a pleasure to be invited to Maresias, Brazil, for the 4th International Workshop on “Quasi-Free Scattering with Radioactive Beams” to talk about some on-going work on short-range nuclear correlations. I primarily use electron-scattering as my experimental technique of choice, and to hear about all of the incredible work being done using nuclear reactions, especially with unstable nuclei in inverse kinematics was fascinating. I will be thinking about ways to incorporate these methods into my own research.

Nothing like a tropical location to catalyze productive discussions. I’m here with my collaborator, Ronen Weiss, from Hebrew University.

My collaborator, MIT graduate student Efrain Segarra gave an outstanding talk about the hypothesis that the EMC Effect may stem from the modification of nucleons in short-range correlated pairs. Specifically, he presented results on how we can use measurements of the EMC Effect in heavy nuclei to constrain what we might expect when looking at light nuclei (specifically helium-3 and tritium!) and infer some information about free neutron structure. You can read our paper on the arXiv.