[Triumf-seminars] TRIUMF Upcoming Seminars

[TRIUMF Seminars]

Date/Time: Mon 2021-03-29 at 10:00 (Special Seminar)
Location:  Remote
Speaker:   Ragnar Stroberg (U Washington/INT)
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: Wed 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 (U Washington/INT)
"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)