[Triumf-seminars] TRIUMF Upcoming Seminars

[TRIUMF Seminars]

Date/Time: Tue 2021-03-29 at 10:00 (Special Seminar)
Location:  Remote
Speaker:   Ragnar Stroberg
Title:     Are you sure? The quest for error bars in nuclear theory
Meeting URL: https://ca01web.zoom.us/j/66543046629?pwd=aVh4aWs4ZjlxOUFPOGhVNWVoM3Fxdz09
Meeting ID:  665 4304 6629
Passcode:    531539

Date/Time: Thu 2021-03-31 at 10:00 (Special Seminar)
Location:  Remote
Speaker:   Gaute Hagen (ORNL/U Tennessee)
Title:     Advances in ab initio computations of nuclei
Meeting URL: https://ca01web.zoom.us/j/67452392983?pwd=T1AzSVlmSFoxL25wRjFlaTdNV0wrQT09
Meeting ID:  674 5239 2983
Passcode:    650356


Ragnar Stroberg (Argonne)
"Are you sure? The quest for error bars in nuclear theory"
Perhaps the greatest benefit of the recent advances in ab initio nuclear
theory is the possibility to make predictions with meaningful
uncertainties. This is especially valuable for quantities that are
difficult or impossible to access experimentally, such as those that
arise in astrophysics or in searches for physics beyond the standard
model. However, just because uncertainty quantification is possible does
not mean that it is easy. In many cases, converged ab initio
calculations of the quantities of interest, in the nuclei of interest,
have only recently been achieved and efforts at rigorous uncertainty
quantification are still in early stages. I will discuss some
developments which enabled the ab initio calculations to be done at all
and some initial efforts to make good on the promise of error bars for
nuclei.

Gaute Hagen (ORNL/U Tennessee)
"Advances in ab initio computations of nuclei"
High performance computing, machine learning techniques, many-body
methods with polynomial scaling, and ideas from effective-field-theory
have revolutionized computations of nuclei. In addition, with a lot of
interest and funding coming from industry and research institutions,
fault tolerant quantum computers could potentially transform the way
ab-initio computations are performed in the future. Here I report on
recent advances in ab-initio coupled-cluster computations of nuclei
starting from chiral Hamiltonians with two- and three-nucleon forces.
Using state-of-the-art ab-initio methods we addressed a 50-year old
quenching puzzle of beta-decays in nuclei [1]. Performing a systematic
study of this decay in light and medium-mass nuclei including the heavy
nucleus 100Sn, we showed that this quenching can be explained by a
proper treatment of two-body currents and many-body correlations.
Continuing along these lines we have computed the neutrino-less
double-beta decay in 48Ca [2]. We found a relatively small matrix
element for the neutrino-less double-beta decay, while the two-neutrino
matrix element is consistent with data. I will also show predictions for
  radii and binding energies of exotic neon and magnesium isotopes using a
  deformed coupled-cluster approach with chiral interactions with delta
degrees of freedom [3]. Using this approach, we provided support for the
  discovery of a two-neutron halo in 29F [4], and recent charge radii
measurements in neutron-rich potassium isotopes [5]. The measurements
challenge ab initio theory and the magic character of the neutron shell
closures at N = 20 and N = 32. Last, but not least, I will present a new
  method that emulates exact coupled-cluster computations and allows for
the computation of nuclear properties for millions of different model
parameters in less than one hour on a standard laptop, once the emulator
  is trained. The equivalent set of ab-initio coupled-cluster
computations would require about 20 years. This speedup enables
statistical computing of the chiral nuclear Hamiltonian, and entirely
new ways to use experimental data across the nuclear chart to generate
new knowledge about the strong nuclear interaction [6].

[1] P. Gysbers, et al, Nature Physics 15, 428–431 (2019)

[2] S. J. Novario, et al., arXiv:2008.09696 (2020)

[3] S. J. Novario, et al., Phys. Rev. C 102, 051303(R) (2020)

[4] S. Bagchi, et al., Phys. Rev. Lett. 124, 222504 (2020)

[5] Á. Koszorús, et al., Nature Physics, Open Access (2021)

[6] A. Ekström and G. Hagen, Phys. Rev. Lett. 123, 252501 (2019)