In a recent paper¹ published in Phys. Rev. C, NSD Senior Scientist Spencer Klein pointed out an intriguing puzzle associated with the observation of coherent photonuclear reactions.

The concept of coherence is well known to most physicists, from lasers, coherent light sources, and diffraction patterns. One adds the amplitudes for the identical process to occur on different targets and then squares; the cross-section scales as the square of the number of targets. This squaring has an important consequence for relativistic heavy-ion collisions: the cross-sections are large. Coupled with the charge-squared photon flux, the cross-sections for coherent photonuclear interactions in ultra-peripheral collisions (UPCs), interactions without hadronic interactions, are large – 7 barns for ⁰ photoproduction on lead at the LHC,  similar to the total hadronic cross-section.  UPCs have become an important technique for studying high-energy photoproduction. At the LHC, UPCs reach higher -p energies than the HERA ep collider, and orders of magnitude higher -Pb energies than fixed-target experiments.   Measurements of J/ production in Pb collisions, like the event in Fig. 1, show that gluon saturation is moderate at Bjorken-x 10^-3, comparable to the predictions from the leading-twist approximation and near the mid-point of nuclear parton distribution parameterizations. 

The new ALICE Run-3 streaming data acquisition system will greatly boost UPC data collection by eliminating the previous bottleneck – a trigger to collect low-multiplicity UPC events, while rejecting most backgrounds.  For STAR, the main UPC triggers had to require neutrons in their zero-degree calorimeters in addition to low–multiplicity in the central detector.   The neutrons come from nuclear excitation via the exchange of additional photons, as shown in Fig. 2.  Multiphoton exchange is expected at relatively small impact parameters.  

And therein lies the puzzle highlighted by Klein. Quantum mechanically, coherence requires that the target nucleus remain in the ground state – the Good-Walker paradigm.   Nuclear excitation violates the ground state requirement, but a coherent enhancement is still observed.  Coherence can be explained semi-classically, by adding the amplitudes to interact with each target,  with the usual propagator, but not in a fully quantum approach. This puzzle remains unresolved, but might be explained with a higher-order calculation. 

¹ The arXiv preprint of the article can be found here.

In a January 2023 publication by nuclear theory postdoctoral fellow Jennifer Rittenhouse West, a quantum chromodynamics (QCD) bond between quarks from different nucleons is proposed as the cause of short-range correlated pairs in nuclei – a fundamental particle physics explanation for roughly 20% of nuclear structure.

Nuclei were found to have their own internal structure composed of neutrons and protons arranged in shells of quantized energy by Nobel Laureate Maria Goeppert Mayer in 1948.  Decades later it was shown, surprisingly, that not all nucleons are contained within shells. Nearly 20% of nucleons are paired into highly overlapped states called short-range correlations (SRC) and the cause of this pairing is unknown.

The uncertainty principle of quantum physics allows nucleons to suddenly come very close together in the nucleus – forcing their wavefunctions to strongly overlap – but the overlap lasts only momentarily before the nucleons must fly apart again.  How the nucleon pairs in SRC stick together is a longstanding mystery.  This is where the new work by Rittenhouse West comes into play.

At a fundamental particle physics level, QCD describes interactions between quarks, antiquarks and gluons, all of which have “color charge,” in analogy to the electric charge of quantum electrodynamics (QED).  Two color-charged quarks can bind together into a diquark if they come into very close range, a process that typically only occurs within the close quarters of a single nucleon; for example, an up quark and a down quark inside a proton forming a diquark. However, the large nucleon-nucleon overlap due to the quantum uncertainty principle allows quarks from different nucleons to come close enough to form a diquark (see Fig.1 for a plot of the large wavefunction overlap between SRC nucleons in the nucleus).  It is this attractive short-range diquark bond across a nucleon pair that is proposed to cause the correlation between nucleons.  Rittenhouse West’s calculation of the diquark binding energy shows that certain diquarks – those made of up and down quarks whose spins cancel out – are energetically favored to form in the nuclear environment.  In her model, the energy “borrowed” by the quantum uncertainty principle in order to force nucleons very close together is, at least temporarily, paid back in the form of significant diquark binding energy (see Fig.2 for a schematic of the diquark bond process).

Diquark formation between nucleons is poised to be a major leap forward in understanding the structure of nuclei in terms of the underlying particle theory of QCD if proven to be correct.  Recent experimental results acquired at Jefferson Lab by a collaboration led by NSD scientists including Shujie Li and John Arrington are favorable to Rittenhouse West’s model. 

[1] Jennifer Rittenhouse West, Nuclear Physics A 1029 (2023)

Diquark induced short-range nucleon-nucleon correlations & the EMC effect

[2] S. Li, R. Cruz-Torres, et al. with J. Arrington, Nature 609 (2022)

Revealing the short-range structure of the mirror nuclei 3H and 3He

Figure 2: A diquark bond formed across a nucleon pair. Quarks are represented by “color-charged” circles in red, green and blue. Gluons, the mediators of the strong force, are represented by black spirals. The panels evolve from left to right as the nucleons begin as neighbors, undergo a quantum fluctuation into an overlapped pair, followed by gluon exchange between a blue quark from the proton and a red quark from the neutron, ending with the diquark bond formed as represented by the lime green oval in the final panel. The mathematical transformation of 2 quarks into a diquark changes the color-charge of the final state. The diquark is no longer a simple red quark-blue quark object, but an anti-green object, by the mathematical rules of the SU(3) group theory that QCD is based on. (Lime green was chosen to represent the anti-color charge called anti-green.)

Two Postdoctoral Researchers from NSD competed in the final of the Berkeley Lab Research SLAM.  Fatima Garcia from the Low Energy Nuclear Physics Program and Emil Rofors from The Applied Nuclear Physics Program represented the Division in the SLAM competition where contestants are challenged to present a three-minute presentation of their research to a non-specialist audience.  

Fatima presented a talk entitled “The Great Periodic Table Debate” in which she provided an insight into how the Superheavy Elements Group is exploring new regions of the periodic table.  Emil’s presentation was entitled “Hey Drone, Should I Eat that Banana?” and described how he is combining radiation detectors, robotics, and advanced algorithms to develop new capabilities for the detection and mapping of nuclear radiation.

Twelve presenters competed in the final, delivering their presentations to an audience from across the Lab, and a panel of judges.  First place went to Sarah Gleeson, ESA. Second place went to Mohammed Ombadi, EESA. Third place went to Aparajitha Srinivasan, BSA.  They will now advance to the regional Bay Area Research SLAM to compete against the finalists from Lawrence Livermore National Lab, Sandia National Labs, and SLAC National Lab.

The Electron Ion Collider (EIC) will be the next major facility in Nuclear Physics and LBNL’s Relativistic Nuclear Collisions (RNC) group is playing a leading role in its development. The EIC will be constructed at Brookhaven National Laboratory (BNL) and begin data taking in the 2030s, enabling detailed studies of protons, neutrons and atomic nuclei. Key science questions the EIC will address include: How do the nucleonic properties such as mass and spin emerge from partons and their underlying interactions? How do the quark-gluon interactions create nuclear binding? What are the properties of the dense gluonic matter that exists deep inside heavy nuclei?

While the EIC complex will have two interaction regions with the potential to host two large-scale detectors, the initial scope of the EIC construction project includes only one day-one detector.  In December 2021, BNL solicited proposals from collaborations interested in participating in the program and three proposals were received:  ECCE, ATHENA, and CORE. The proposals were reviewed by the Detector Proposal Advisory Panel (DPAP) in March and a primary recommendation of the DPAP was that each of these collaborations was too small for the final task and they encouraged all three collaborations to merge into a single “Detector One” collaboration. 

LBNL took a major role in the design, optimization, and tracking implementation for the silicon tracking and vertexing system in ATHENA, and in the optimization of the far-forward detector systems.  It is likely that a derivative of these systems will be used in the new detector.  In addition, the DPAP endorsed the re-use of the existing BaBar magnet (1.5 Tesla) and the recently completed sPHENIX hadronic calorimeter as proposed by the ECCE collaboration.  Additional detector elements are expected to be designed and optimized according to the experience retained by the three former collaborations. 

The RNC group took a major role in developing the ATHENA concept and proposal. Barbara Jacak was a member of the ATHENA steering committee and proposal writing committee, Ernst Sichtermann was chair of the Institutional Board, and John Arrington, Spencer Klein & Ernst were conveners for different detector and physics working groups.  Extensive simulations were carried out by Rey Cruz Torres, Shujie Li, Wenqing Fan, and other RNC postdocs, students and staff members. Going forward, RNC will be a major contributor to the final design and construction of Detector One.