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Posts related to particle and nuclear physics research

Professor Puckett’s Group Prepares New Measurements of “femtoscopic” Neutron Structure at Jefferson Lab

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UConn group on the floor of Hall A
The UConn group on the floor of Hall A during the SBS installation. From left to right: Postdoctoral Research Associate Dr. Eric Fuchey, Professor Andrew Puckett, and Graduate Research Assistants Provakar Datta and Sebastian Seeds. Click the image for a slideshow of additional installation photos and for more details about the experiment.

Professor Andrew Puckett’s research group is currently leading, as part of a collaboration of approximately 100 scientists from approximately 30 US and international institutions, the installation in Jefferson Lab’s Experimental Hall A of the first of a series of planned experiments known as the Super BigBite Spectrometer (SBS) Program, with beam to Hall A tentatively scheduled to begin in early September of 2021. Jefferson Lab, located in Newport News, Virginia, is a national user facility operated by the US Department of Energy, and is the world’s premiere laboratory for imaging the subatomic (and subnuclear) quark-gluon structure of protons, neutrons, and nuclei using its continuous, polarized electron beam. In addition to Professor Puckett, the UConn researchers involved in this effort are Postdoctoral Research Associate Eric Fuchey, and Graduate Research Assistants Provakar Datta and Sebastian Seeds. The first set of experiments in the SBS program, slated to run during Fall 2021, is focused on the measurement of neutron electromagnetic form factors at very large values of the momentum transfer Q2, which essentially probe the spatial distributions of electric charge and magnetism inside the neutron at very small distance scales of order 0.05-0.1 fm (1 fm = one femtometer = 10-15 m = 0.000 000 000 000 001 m), approximately 10-20 times smaller than the size of the proton and approximately 1 million times smaller than the size of a typical atom.

Electrons from Jefferson Lab’s Continuous Electron Beam Accelerator Facility (CEBAF), with energies of up to 10 GeV (=10 billion electron-volts), will scatter elastically from protons and neutrons in a liquid deuterium target in Hall A. Scattered electrons will be detected in the BigBite Spectrometer, located on the left side of the beam, while the high-energy protons and neutrons recoiling from the “hard” collisions with the beam electrons will be detected in the SBS by the newly constructed Hadron Calorimeter (HCAL), located on the right side of the beam. The SBS dipole magnet will provide a small vertical deflection of the scattered protons, which allows HCAL to distinguish them from scattered neutrons, which are undeflected by the magnetic field, but produce otherwise identical signals in HCAL.

The first group of SBS experiments, collectively known as the “GMN run group”, will answer several important questions about the “femtoscopic” structure of the neutron, including:

  • What is the behavior of the neutron’s magnetic form factor at large momentum transfers? The SBS experiment will dramatically expand the Q2 reach of neutron magnetic form factor data compared to all previously existing measurements, from approximately 4 –> 14 (GeV/c)2. See original experiment proposal here.
  • How is the charge and magnetism of the proton shared among its “up” and “down” quark constituents as a function of Q2? The proton magnetic form factor has been measured over a much wider range of Q2 than the neutron, and combined proton and neutron measurements can be used to disentangle the contributions of “up” and “down” quarks (and diquark correlations) to the proton’s structure, under the assumption of charge symmetry of the strong interactions (see, e.g., https://inspirehep.net/literature/1812076)
  • How important and/or significant is the contribution of two-photon-exchange to elastic electron-neutron scattering? The first SBS experiment group will perform measurements of the electric/magnetic form factor ratio for the neutron using two different techniques known as “Rosenbluth Separation” and “Polarization Transfer”, at a Q2 where these two techniques have shown significant disagreement for the proton. Both measurements will be the first of their kind for the neutron at such large Q2 values (see, e.g., Polarization Transfer Proposal and Rosenbluth Separation Proposal)

The GMN run group will start in early September and run through the fall of 2021. The broader SBS program will continue in Hall A through at least 2023, and will drastically improve our understanding of the femtoscopic quark-gluon structure of protons, neutrons, and atomic nuclei. Professor Puckett’s research in the SBS and Hall A Collaborations is supported by the US Department of Energy, Office of Science, Office of Nuclear Physics. Stay tuned!

Standard model challenged by new measurement

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The following article appeared in UConn Today on May 20, 2021 under by-line  Elaina Hancock – UConn Communications

Physicists are one step closer to describing an anomaly, called the Muon g-2, that could challenge the fundamental laws of physics. It seems the muon may be breaking what have been understood as the laws of physics, and the findings announced on April 7th were met with much excitement and speculation at what this might mean. UConn physics researchers Professor Thomas Blum and Assistant Professor Luchang Jin helped pioneer the theoretical physics behind the findings, and they recently met with UConn Today to help explain the excitement.

What is a muon, and how do you study them?

Blum: A muon is a “fundamental particle,” meaning it’s an elementary particle like an electron or a photon. Muons are unstable, so they don’t live very long. Unlike an electron, where we can focus on them as long as we want and do measurements, we only have a little bit of time to take measurements of muons.

The way researchers perform the experiment is by slamming particles into other particles to create the muons, and they eventually collect them into a beam. This beam of muons travels at almost the speed of light where they live a little bit longer than they would if they were at rest. That’s Einstein’s theory of relativity in action.

The researchers put the muons into what’s called a storage ring where, eventually, they decay into other particles, and it’s those other particles that are detected in the experiment.

Muons have a property called a magnetic moment, which is like a little compass that points in the direction of the magnetic field that it’s in. In the storage ring, there’s a uniform magnetic field, and as the muons are going around in the storage ring, their magnetic moment, which would be perfectly aligned with their direction of travel if there were no anomaly, actually precesses with respect to the direction of travel as it goes around the ring, because of the interaction with the magnetic field.

It’s that precession that they’re measuring, because the precession is proportional to the strength of the magnetic moment. We can measure this magnetic moment extremely precisely in experiments, and we can calculate its value theoretically very precisely, to less than one-half part per million. Then we can compare the two and see how well they agree.

Can you explain the excitement surrounding these results?

Blum: For a long time — almost 20 years — the best measurement had been done at Brookhaven National Lab on Long Island, where they measured this magnetic moment very precisely, and found that it didn’t agree with our best fundamental theory, which is called the Standard Model of particle physics. The discrepancy wasn’t big enough to say that there was definitely something wrong with the Standard Model or not.

The new results are from a new experiment done to measure the magnetic moment even more precisely. That effort has been going on at Fermilab outside of Chicago for a few years now, and they just announced these results in early April. Their measurement is completely compatible with the Brookhaven value, and if you take the two together, then the disagreement with the Standard Model gets even worse: it now stands at 4.2 standard deviations.

People are very excited, because this could possibly signal that there is new physics in the universe that that we don’t know about yet. The new physics could be new particles that we’ve never seen before, or new interactions beyond the ones we know about already and that could explain the difference between what’s measured and what’s calculated. So that’s what everybody’s excited about.

Can you tell us about the Standard Model?

Jin: The Standard Model describes electromagnetic interactions between charged particles. It also describes the so called weak interactions, which is responsible for nuclear decay. The weak interactions become more important in high energy collisions, and unifies with the electromagnetic interactions. Lastly, the Standard Model describes the strong interactions, which bind quarks into nucleons and nuclei.

Basically, the Standard Model describes everything around us, ranging from things happening in our daily lives to the high-energy proton collisions in the Large Hadron Collider, with the major exception being gravity, which is only sort of visible, but we can feel it because gravity forces always add up, and there are a lot of other massive objects around us. It also doesn’t include dark matter, if we actually do have that in the universe.

People believe, and I think this is really true, that the Standard Model cannot possibly describe everything to extremely high precision, especially when we accelerate subatomic particles to very high energies. However, it was not very clear how high the energy or the precision has to be before we can see some discrepancies. We know the upper bound — usually referred to as the Planck scale, where the Standard Model has to fail due to the omission of gravity. But the Planck scale is so high that there is little hope to be able to perform experiments at that high energy. It is very nice to find a concrete example that the Standard Model actually misses something, and the g-2 anomaly is a very good candidate.

What roles did you each have in this research?

Jin: Theoretically, we decomposed the g-2 into contributions from the different types of interactions. At present, most of the values are obtained by analytic calculations of the various contributions. Other experimentally measurable quantities that have little to do with the muon magnetic moment experiments in terms of what they measure can be related through the Standard Model to the Muon g-2 value. So, to a large extent, this can still be viewed as a theory prediction. Blum pioneered the first lattice calculation for a certain g-2 contribution called the hadronic vacuum polarization, which doesn’t use experimental data at all.

Blum: Jin came up with new methods to compute the Hadronic Light-by-Light contribution which allowed us – with colleagues at Brookhaven National Lab, Columbia University, and Nagoya University – to compute it completely for the first time without experimental input. What Jin and I are doing, along with a host of other theorists around the world, is trying to better calculate the value of this magnetic moment from the theory side, so that we can have an even better comparison with the experimental measurements.

Jin: The Standard Model itself has a few parameters, which for most, we know very, very precisely. This includes the masses of the fundamental particles. In principle, as one might imagine, the theory prediction of the Muon g-2 is a very complicated expression just in terms of these numbers. We are not able to do that yet, but maybe soon we can. We expect that if we continue to improve our calculations, and as computers continue to get faster, the last digit determination may become more accurate.

To dig deeper into the science behind the findings, read Blum and Jin’s feature article on the findings in CERN Courier.

An anomalous moment for the muon

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Mark Rayner/CERN

The Fermilab E989 experiment announced the first new result on the muon’s anomalous magnetic moment in almost 20 years. The new measurement, combined with Brookhaven’s E821, has increased the discrepancy with the Standard Model value to 4.2 standard deviations. UConn Professors Tom Blum and Luchang Jin explain the theory calculations in a feature story in the Cern Courier.

Prof. Kyungseon Joo named Chair of CLAS Collaboration at Jefferson Lab

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Kyungseon Joo, a professor of physics, has been named Chair of the CLAS Collaboration, one of the largest international collaborations in nuclear physics.  CLAS involves 50 institutions from 9 countries and has about 250 collaborators.  The collaboration recently completed the upgrade of the CEBAF Large Acceptance Spectrometer (CLAS12) for operation at 11 GeV beam energy in Hall B at Jefferson National Laboratory in Newport News, VA, funded by the United States Department of Energy.

CLAS12 is based on a dual-magnet system with a superconducting torus magnet that provides a largely azimuthal field distribution that covers the forward polar angle range up to 35°, and a solenoid magnet and detector covering the polar angles from 35° to 125° with full azimuthal coverage. Trajectory reconstruction in the forward direction using drift chambers and in the central direction using a vertex tracker results in momentum resolutions of 1% and 3%, respectively. Cherenkov counters, time-of-flight scintillators, and electromagnetic calorimeters provide good particle identification. Fast triggering and high data-acquisition rates allow operation at a luminosity of 1035 cm−2s−1. These capabilities are being used in a broad scientific program to study the structure and interactions of nucleons, nuclei, and mesons, using polarized and unpolarized electron beams and targets for beam energies up to 11 GeV.

As Chair, Joo represents the collaboration in scientific, technical, and managerial concerns, while he closely works with the Lab management on scheduling experiments, organizing collaboration activities and expanding the reach of the collaboration. He currently focuses on collaboration-wide efforts to timely make first publications from CLAS12 with high impact science.

CLAS detector in Hall B at Jefferson Lab
The CLAS12 detector in the Hall B beamline. The beam enters from the right near the upstream end of the solenoid magnet and the cryogenic service tower, followed by the High Threshold Cherenkov Counter and the torus magnet with the drift chambers. The Low Threshold Cherenkov Counter, Forward Time-of-Flight, and the electromagnetic calorimeters are seen at the downstream end to the left.

Professor Luchang Jin receives prestigious DOE Early Career Award

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Assistant Professor of Physics Luchang Jin has been chosen to receive a prestigious Early Career Award from the US Department of Energy’s Office of High Energy Physics (HEP) for 2020. The amount of the award is $750,000 to be used over five years. The DOE Early Career Award is extremely competitive: this year only 16 scientists in HEP in the US were awarded such grants, and only 76 scientists across the entire DOE. Dr. Jin will use the grant to support his research using numerical methods to study how electromagnetic interactions affect the decays of mesons, subatomic particles composed of a quark and anti-quark pair. This study, carried out within the framework of the fundamental Standard Model of Particle Physics, is expected to improve our knowledge of the interactions between quarks and the Weak gauge bosons. With some luck, Professor Jin’s research may provide evidence of new interactions or particles yet to be discovered.

New result for part of muon anomaly

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Professors Tom Blum and Luchang Jin, along with colleagues at BNL and Columbia, Nagoya, and Regensburg universities have completed a first-ever calculation of the hadronic light-by-light scattering contribution to the muon’s anomalous magnetic moment with all errors controlled. The work is published in Physical Review Letters as an Editor’s Suggestion and also appeared in Physics Magazine. A recent press release from Argonne National Lab described the calculation, which was performed on Mira, Argonne’s peta-scale supercomputer.

The team found the contribution is not sufficient to explain the longstanding difference between the Standard Model value of the anomalous magnetic moment and the BNL experiment that measured it. The discrepancy, which could indicate new physics, should be resolved soon by a new experiment at Fermilab (E989) and improved theory calculations, including the one described here, both with significantly reduced errors. E989 is set to release their first results later this year.

Ron Mallett Featured on NBC Connecticut

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Could traveling into the past be part of our future? Quite possibly, says Ron Mallett, a UConn emeritus professor of physics who has studied the concept of time travel for decades. Earlier this month, he spoke with NBC Connecticut reporter Kevin Nathan about his life and work as a theoretical physicist, and discussed how time travel may be possible someday.

View the video on nbc.com

Research Spotlight: Exploring the nature of the universe with Dr. Thomas Blum

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The Daily Campus published an article highlighting the research of Prof. Thomas Blum about Quantum Chromodynamics, a theory which describes the interactions between elementary particles. The development of this theory could help further understanding of the Standard Model of particle physics. The Standard Model is what physicists use to describe the fundamental building blocks of everything in the universe.

For more information follow the link.

International Summer School “Strong interactions beyond simple factorization”

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May 27-June 5 UConn Physics Department hosted an international summer school Strong interactions beyond simple factorization: collectivity at high energy from initial to final state. The school was supported by an NSF grant to Prof. Kovner and was devoted to modern approaches to the physics of high energy hadronic and heavy ion collisions.

Workshop ‘Dynamic Quantum Matter’ organized by UConn faculty

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Dynamic Quantum Matter, Entangled orders and Quantum Criticality Workshop
Dates: June 18- June 19, 2018

Sponsors

UConn, NSF, Nordita, Villum Center for Dirac Materials, Institute for Materials Science – Los Alamos, Wiley Publishers
           

Scope

The conference will focus on entangled and non-equilibrium orders in quantum materials. The 21st century marked the revolution of probing matter at the nano- to mesoscale and these developments continue to be the focus of active research. We now witness equally powerful developments occurring in our understanding, ability to probe, and manipulate quantum matter, in entangled orders and novel states, in the time domain. Recent progress in experimental techniques including x-ray optics, optical pumping, time resolved spectroscopies (ARPES optics), and in cold-atom systems has led to a resurgence of interest in the non-equilibrium aspect of quantum dynamics. The novel entangled orders that have nonzero “overlap” with more than one order parameter also have emerged as an exciting new direction for research in quantum matter. Entangled orders go beyond the conventional orders such as density and spin, and significantly expand the possible condensates we can observe. It is only because of the lack of experimental control, resolution, theoretical framework, and computational power, that the realm of entangled and quantum non-equilibrium remained largely unexplored until now. The time has come for us to turn full attention to these phenomena. Specific topics include: superconductivity and dynamics near quantum criticality, composite orders in correlated materials, effects of strain on quantum critical points, and superconductivity in STO. This conference will have a format of topical lectures, while leaving ample time for discussions.

Venue

Gurney’s Resorts | Newport, RI