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News from the Nuclear Science Division at Berkeley Lab
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Nuclear Science Division Newsletter
In this issue:July, 2020

New newsletter section: Diversity, Equity and Inclusion Moments

With this issue, we are proud to introduce a new regular newsletter section “Diversity, Equity and Inclusion Moments,” highlighting recent Division DEI activities. We hope you will find it a useful resource for all things DEI.

Baryons show charm in relativistic heavy-ion collisions

The STAR collaboration has observed for the first time the short-lived charm baryon Λc in heavy-ion collisions produced at the Relativistic Heavy Ion Collider (RHIC). Observing the Λc baryon is difficult because of its very short life time (cτ ~ 60 microns). This measurement was only possible because of the excellent pointing resolution of the STAR Heavy Flavor Tracker (HFT). As Fig. 1 shows, this enabled a clear observation of the Λc mass peak containing 215 ± 21 events

Two mechanisms for charmed baryon formation have been proposed. In the first, Λc baryons are produced from the fragmentation of energetic charm quark, via string fragmentation. This mechanism is embodied in the PYTHIA event generator. In the second mechanism, Λc baryons are produced through the coalescence of nearby (in phase space) charm quark and light quarks. In relativistic heavy-ion collisions, the coalescence mechanism had been expected to be dominant due to the extreme high parton density environment created in these collisions.

Figure 2 shows the ratio of Λc yield to D0 meson yield as a function of particle transverse momentum (pT) compared to various model calculations. The data show a factor of ~5 enhancement compared to the p+p collision baseline from the PYTHIA model with charm fragmentation (green solid line) constrained by previous ee, ep and pp collisions. Other colored lines in the figure depict model calculations incorporating the coalescence hadronization mechanism for charm quarks in the hot and dense medium. These calculations show comparable magnitude and pT dependence as seen in the data while they differ in the medium and coalescence parameters. Our new result suggests that the coalescence mechanism plays an important role for charm quark hadronization in heavy-ion collisions.

The analysis was led by NSD scientists from the RNC group together with collaborators from Czech Technical University, University of Illinois Chicago, University of Science and Technology of China and other STAR institutions.

Reference
[1] J. Adam et al. [STAR Collaboration], "Observation of enhancement of charmed baryon-to-meson ratio in Au+Au collisions at √sNN = 200 GeV, Phys. Rev. Lett. 124, 172301 (2020).

Lightest mendelevium isotope discovered and weighed

One of the goals of nuclear science is to understand how the structure of the nucleus changes with each combination of protons and neutrons. Each new isotope that is discovered is a chance to study a mixture of protons and neutrons that has never before been seen. At the edges of the chart of nuclides, even the smallest piece of information on a new isotope could impact how we think about nuclear structure. In a recent experiment performed at the 88-inch cyclotron facility, NSD's Heavy Elements Group discovered a new isotope of mendelevium [1]. This isotope, 244Md, is the lightest isotope of mendelevium yet observed.

Typically, a new isotope is identified because its decay properties are different from those of other previously-observed isotopes. However, it is often the case that neighboring isotopes can have very similar decay properties and it can be difficult to confirm that a new isotope had been produced. In this measurement, researchers took advantage of the FIONA (for the identification of Nuclide A) apparatus to directly measure the mass number of the new isotope and to erase any ambiguity in its discovery. FIONA is a new addition to the 88-inch cyclotron facility and has recently made headlines for making the first ever direct mass number identification of a super heavy element [2].

In FIONA, ions take special looped trajectories that are dependent on their mass number. Ions with different mass numbers take different trajectories and arrive at different locations in the detector. In this measurement, FIONA was calibrated with ions of astatine isotopes and then setup such that mass-244 ions of 244Md would arrive for detection at the same position as ions of 198At. As Fig. 2 shows, the produced ions of mendelevium arrived where ions of mass number 244 were expected. This gives direct confirmation that a new isotope of mendelevium with mass number 244 has been discovered.

The half-life of the new isotope was measured to be roughly 0.4s, in agreement with theoretical calculations that assume this isotope decays primarily through alpha-particle emission. Given this information researchers believe it is unlikely that 244MD has a large beta-decay branch.

The Heavy Elements Group is now preparing similar experiments to search for even more isotopes on the edge of the nuclidic chart.

References
[1] J. L. Pore, J. M. Gates, R. Orford, et al., "Identification of the new isotope 244," Physical Review Letters 124, 252502 (2020).
[2]. J. M. Gates, G. K. Pang, J. L. Pore, et al., "First Direct Measurements of Superheavy-Element Mass Numbers," Phys. Rev. Lett. 121, 222501 (2018).

Tantalum decays set limits on dark matter

Tantalum-180m is one the most interesting isotopes a nuclear physicist could wish for. In nature, it does not exist in its ground state but is spin-trapped in a 77 keV excited state, storing a little bit of nuclear energy in every nucleus. Using this unique quality to their advantage, a group of physicists, led by Björn Lehnert (LBL) and Harikrishnan Ramani (LBL, UCB), is setting world-leading limits on exotic Dark Matter. The results of their work were recently published in Physical Review Letters [1].

But Ta180m is even more unique: Its meta-stable state (m) is by far the most stable in nature. Its decay has never been observed, living longer than 4.5x1016 years (90% credibility). This was determined in 2016 by low-background gamma-spectroscopy at the underground laboratory, HADES, Belgium [2]. Lehnert and Ramani re-analyzed this data to search for Dark Matter they hypothesized could interact more strongly than previously thought. Under this assumption, Dark Matter loses its galactic velocities when colliding with the atmosphere and earth. Upon reaching underground laboratories, the thermalized Dark Matter does not have enough kinetic energy to create a signal in traditional experiments. However, it can de-excite Ta180m creating an observable gamma-ray signature. These gamma-rays, undetected in the analysis, are used to set the most stringent constraint for this Dark Matter scenario.

Because this project represents a very successful collaboration between theorists and experimentalists and also between the Physics and Nuclear Science divisions at Berkeley Lab, the scientists are now looking toward future experiments. Alternative detector technologies such as thick, segmented large-area semiconductors or a tantalum crystal operated as a bolometer are being discussed with the local experts at NSDs Semiconductor Detector Laboratory and bolometer experts from the CUORE experiment.

Further work in this area, however, comes with inherent obstacles. With only 0.012% isotopic abundance in the rare earth element tantalum, Ta180m is the rarest stable isotope known in nature. While this makes it "famous" even outside nuclear physics circles [3], it is difficult to acquire large target masses. Isotopically enriching a sample in Ta180m could be a game-changer to improve the sensitivity of future Dark Matter searches.

References
[1] B. Lehnert et al., "Search for Dark Matter Induced Deexcitation of 180Ta-m," Phys. Rev. Lett. 124, 181802 (2020)
[2] B. Lehnert et al., "Search for the decay of nature's rarest isotope 180mTa," Phys. Rev. C 95, 044306 (2017).
[3] Science News, Rarest nucleus reluctant to decay https://www.sciencenews.org/article/rarest-nucleus-reluctant-decay (2016)

Diversity Equity and Inclusion Moments

The Nuclear Science Division's Diversity, Equity, and Inclusion Council is kicking off a new DEI Moments section in the NSD Newsletter. DEI Moments will include a digest of recent Division DEI activities, recognition of DEI champions in the Division or those recognized by Division members, and an article on various DEI topics.

Staying connected while social distancing

When the order to shelter-in-place came down, the Division was faced with the new reality of working remotely. While we have had many DEI topics presented at our twice-monthly NSD Staff Meetings over the past year, none showed how much the Division stepped up and came together like the presentation on April 7, 2020. The topic "NSD Initiatives to Stay Connected" highlighted the various ways the Programs have come together while working remotely.

Each program, and many groups and individuals, spontaneously found ways to informally keep in contact, reduce social isolation, and encourage group cohesiveness. From "always active" Google hangouts and Slack channels to scheduled virtual coffee gatherings via Zoom, our Division continues to come together in these challenging times. We are thankful to all of you.

Recognizing efforts of our staff

The DEI council would like to introduce a new aspect of the Luminary Card program. In response to the "new normal" of remote work, the Luminary Card is now available in digital form. The program is otherwise unchanged. If you feel someone has made a difference for Inclusion, Diversity, Equity and Accountability at the Laboratory, you can recognize them with a (virtual) Luminary Card. The virtual card has the added benefit of a few pointers for the recipient. Recipients have the possibility to log their card via the Division website and can opt-in to be mentioned in future editions of the newsletter.

The DEI Moments section in the newsletter will feature a list of all Luminary Card Recipients, who have opted to be listed in the newsletter, given out between issues as shown by our inaugural list below.

Recent DEI topics @ NSD Staff Meetings

April 21, 2020 - Tips for Inclusive & Accessible Meetings (especially during the shelter in place)
April 7, 2020 - NSD Initiatives to Stay Connected

Luminary Card Recipients

To recognize their efforts in the area(s) of Inclusion, Diversity, Equity and Accountability, the following people received a Luminary Card:

Kymba A'Hearn, Kathryn Meehan, Joey Curtis, Mark Bandstra, Rosa Rodriguez-Flores, and Henrik von der Lippe

Fragments

NSDer Bethany Goldblum is the 2020 winner of the James Corones award in Leadership, Community Building and Education. Goldblum was recognized for her exceptional achievements in nuclear physics and nonproliferation research and on training future nuclear experts, in connection with her role as the executive director of the Nuclear Science and Security Consortium. She will receive the award, along with a $2,000 honorarium at a ceremony later this year. Bethany's portfolio includes a diverse set of projects, including studies at the 88-inch cyclotron to measure the light yield of scintillators undergoing proton recoil.

New NSD postdoc Reynier (Rey) Cruz Torres received the MIT Physics Department Sergio Vazquez Prize for his dissertation. Working under Prof. Or. Hen, Dr. Cruz Torrez was recognized "for his novel experimental study of the strong nuclear interaction in few-body mirror nuclei and his theoretical study of short-distance factorization in many-body nuclear wave functions using the new Generalized Contact Formalism."

Newsletter Notes
Please send any comments, including story suggestions to Spencer Klein at srklein@lbl.gov.
 
Previous issues of the newsletter are available at:
https://commons.lbl.gov/display/nsd/NSD+Newsletter.






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