Research Program

Since the twin revolution of quantum mechanics (QM) and relativity, the progress of fundamental physics is aligned with unifying their radical concepts. The unification of QM with special relativity resulted in the Standard Model (SM) of particle physics, which consistently explains most observations. However, it also leaves open questions, such as the unsolved hierarchy problem, the nature of the neutrino, dark matter, and baryon asymmetry, pointing out that the SM may be an effective theory of a more fundamental underlying theory which, in particular, has to include gravity. The tremendous effort of theoretical physicists in reconciling gravity and QM led to the establishment of profound relationships between spacetime and QM. In particular, revealing the emergent character of spacetime and its link to quantum entanglement [1-5] points to the unique role of quantum information science (QIS) in elevating our understanding of nature to a higher level. 

In the physics of atomic nuclei, microscopic calculations reveal the emergent nature of superfluid, vibrational, rotational, and other kinds of nucleonic collective motion. They form regular patterns in the excitation spectra and the spatial distributions of nuclear matter, connecting the realms of strong correlations and geometry [6]. The success of early phenomenological nuclear models operating collective variables established the notion of collective degrees of freedom, which can be introduced in the effective Hamiltonians independently of the single-nucleon degrees of freedom. However, even the most advanced phenomenological models based on the latest energy density functionals (EDFs) [7-12] and beyond-mean-field (BMF) techniques [13-19] still need to achieve the spectroscopic accuracy required by modern applications. Since the exact many-body solutions are far beyond the reach of the existing computational capabilities, it remains to be seen to what degree the lack of accuracy should be attributed to the imperfections of the EDFs, truncation schemes, or principal limitations of the BMF methods. Hope to resolve some issues is associated with “ab initio” approaches supposed to use the only input of the nucleon-nucleon (NN) interaction in the vacuum. Currently, such approaches are dominated by the chiral perturbation theory (χPT) combined with standard many-body techniques [20-30], leaving it unclear to what extent the emergent collectivity, crucial for medium-heavy nuclei, can be addressed. Presently available χPT calculations for such systems [20,21] require re-adjustments of the NN interaction to the properties of finite nuclei [29,30], thereby partly absorbing the emergent collectivity in the parameters or preprocessing. Such calculations demonstrate success on the quantitative level for the static properties and a few low-lying states in medium-light nuclei. However, the frontier applications require calculations for heavier systems in broad energy regions, including the giant resonances (up to 30-50 MeV).

Addressing the latter problematics, we constructed the relativistic nuclear field theory (RNFT) from the covariant meson-nucleon Lagrangian and exact equations of motion (EOM) for fermionic propagation in strongly correlated systems [18,19,31-36]. The apparent advantages of RNFT are its covariance, connection to particle physics, non-perturbative character, and transparent treatment of sub-leading contributions to NN forces in nuclear media. The only input to RNFT is the meson masses and coupling constants, slightly renormalized by EDF compared to their vacuum values and universal across the nuclear chart. Already in the leading approximation to emergent collectivity, RNFT has enabled the resolution of several major long-standing controversies in the structure of ordinary and exotic nuclei, such as the electric dipole polarizability [18,19,37], neutron skin oscillations [38-40], beta decay [41-43] and nuclear compressibility [44], essential for establishing the nuclear equation of state. Due to its unique ability to non-perturbatively derive and efficiently treat collective effects across the energy scales via a general class of correlation functions [45-47], RNFT has launched a research line of the consistent and reliable nuclear structure input for astrophysical modeling of neutron star mergers (kilonovae) and supernovae [48-50]. Another new direction that has grown from the recent progress of RNFT is the EOM-based quantum algorithms, promising a substantial quantum benefit in the computation of strongly coupled systems [51,52]. 

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[12]  T. Nikšić, D. Vretenar, and P. Ring, Relativistic Nuclear Energy Density Functionals: Mean-Field and Beyond, Prog. Part. Nucl. Phys. 66, 519 (2011). 

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[17] D. Gambacurta, M. Grasso, and F. Catara, Collective nuclear excitations with Skyrme-second random-phase approximation, Phys. Rev. C 81, 054312 (2010).
[18]  E. Litvinova, P. Ring, and V. Tselyaev, Relativistic quasiparticle time blocking approximation: Dipole response of open-shell nuclei, Phys. Rev. C 78, 014312 (2008).
[19]  E. Litvinova, P. Ring, and V. Tselyaev, Mode Coupling and the Pygmy Dipole Resonance in a Relativistic Two-Phonon Model, Phys. Rev. Lett. 105, 022502 (2010). 

[20] H. Hergert, S. K. Bogner, T. D. Morris, A. Schwenk, and K. Tsukiyama, The In-Medium Similarity Renormalization Group: A Novel Ab Initio Method for Nuclei, Phys. Rep. 621, 165 (2016).

[21] G. Hagen, T.  Papenbrock, M. Hjorth-Jensen, and D. Dean, Coupled-cluster computations of atomic nuclei, Rep. Prog. Phys. 77, 096302 (2014).

[22] V. Soma, C. Barbieri, and T. Duguet, Ab initio Gorkov-Green's function calculations of open-shell nuclei, Phys. Rev. C 87, 011303 (2013).
[23] A. Cipollone, C. Barbieri, and P. Navrátil, Isotopic Chains Around Oxygen from Evolved Chiral Two- and Three-Nucleon Interactions, Phys. Rev. Lett. 111, 062501 (2013).

[24] J. W. Holt, M. Kawaguchi, and N. Kaiser, Implementing chiral three-body forces in terms of medium-dependent two-body forces, Front. in Phys. 8, 100 (2020).

[25] M. Piarulli, L. Girlanda, R. Schiavilla, R. Navarro Pérez, J.E. Amaro, and E. Ruiz Arriola, Minimally non-local nucleon-nucleon potentials with chiral two-pion exchange including Δ’s, Phys. Rev. C 91, 024003 (2015).

[26] U. Van Kolck, The Problem of Renormalization of Chiral Nuclear Forces, Front. Phys. 05 May 2020, 00079.

[27] B. R. Barrett, P. Navrátil, and J. P. Vary, Ab initio no core shell model, Prog. Part. Nucl. Phys. 69, 131 (2013).

[28] J. E. Lynn, I. Tews, S. Gandolfi, and A. Lovato, Quantum Monte Carlo Methods in Nuclear Physics: Recent Advances, Ann. Rev. of  Nucl. and Part. Sci. 69, 279 (2019).

[29] A. Ekström, G. R. Jansen, K. A. Wendt, G. Hagen, T. Papenbrock, B. D. Carlsson, C. Forssén, M. Hjorth-Jensen, P. Navrátil, and W. Nazarewicz, Accurate nuclear radii and binding energies from a chiral interaction, Phys. Rev. C 91, 051301(R) (2015).

[30] T. Miyagi, S. R. Stroberg, J. D. Holt, and N. Shimizu, Ab initio multishell valence-space Hamiltonians and the island of inversion, Phys. Rev. C 102, 034320 (2020). 

[31] E. Litvinova, Quasiparticle-vibration coupling in a relativistic framework: Shell structure of Z=120 isotopes, Phys. Rev. C 85, 021303(R) (2012). 

[32] E. Litvinova, A. Afanasjev, Dynamics of nuclear single-particle structure in covariant theory of particle-vibration coupling: from light to superheavy nuclei, Phys. Rev. C 84, 014305 (2011).

[33] E. Litvinova and H. Wibowo, Finite-temperature relativistic nuclear field theory: an application to the dipole response, Phys. Rev. Lett. 121, 082501 (2018).

[34] E. Litvinova, Pion-nucleon correlations in finite nuclei in a relativistic framework: effects on the shell structure, Phys. Lett. B 755, 138 (2016).

[35] E. Litvinova, C. Robin, and I.A. Egorova, Soft modes in the proton-neutron pairing channel as precursors of deuteron condensate in N = Z nuclei, Phys. Lett. B 776, 72 (2018).

[36] V. Vacuero, A. Jungclaus, T. Aumann, J. Tscheuschner, E.V. Litvinova, J.A. Tostevin, H. Baba, D.S. Ahn, R. Avigo, K. Boretzky et al., Fragmentation of Single-Particle Strength around the Doubly Magic Nucleus 132-Sn and the Position of the 0f5/2 Proton Hole State in 131-In, Phys. Rev. Lett. 124, 022501 (2020).

[37] A. Tamii et al., Complete electric dipole response and the neutron skin in 208Pb, Phys. Rev. Lett. 107, 062502 (2011).

[38] E. Litvinova, P. Ring, V. Tselyaev, K. Langanke, Relativistic quasiparticle time blocking approximation. II. Pygmy dipole resonance in neutron-rich nuclei, Phys. Rev. C 79, 054312 (2009). 

[39] J. Endres, E. Litvinova, D. Savran, P. A. Butler, M. N. Harakeh, S. Harissopulos, R.-D. Herzberg, R. Krücken, A. Lagoyannis, N. Pietralla, V. Yu. Ponomarev, L. Popescu, P. Ring, M. Scheck, K. Sonnabend, V. I. Stoica, H. J. Wörtche, and A. Zilges, Isospin character of the pygmy dipole resonance in 124-Sn, Phys. Rev. Lett. 105, 212503 (2010).

[40] E. Lanza, A. Vitturi, E. Litvinova, D. Savran, Population of dipole states via isoscalar probes: the splitting of pygmy dipole resonance in 124-Sn, Phys. Rev. C 89, 041601(R) (2014).

[41] C. Robin and E. Litvinova, Nuclear response theory for spin-isospin excitations in a relativistic quasiparticle-phonon coupling framework, Eur. Phys. J. A 52, 205 (2016). 

[42] C. Robin and E. Litvinova, Coupling charge-exchange vibrations to nucleons in a relativistic framework: effect on Gamow-Teller transitions and beta-decay half-lives, Phys. Rev. C 98, 051301(R) (2018).

[43] C. Robin and E. Litvinova, Time-Reversed Particle-Vibration Loops and Nuclear Gamow-Teller Response, Phys. Rev. Lett. 123, 202501 (2019).

[44] E. Litvinova, Relativistic approach to the nuclear breathing mode, Phys. Rev. C 107, L041302 (2023), Letter & Editor’s Suggestion.

[45] E. Litvinova and Y. Zhang, Many-body theory for quasiparticle states in superfluid fermionic systems, Phys. Rev. C 104, 044303 (2021).

[46] E. Litvinova and Y. Zhang, Microscopic response theory for strongly-coupled superfluid fermionic systems, Phys. Rev. C 106, 064316 (2022).

[47] Y. Zhang, A. Bjelčić, T. Nikšić, E. Litvinova, P. Ring, and P. Schuck,  Many-body approach to superfluid nuclei in axial geometry, Phys. Rev. C 105, 044326 (2022).

[48] E. Litvinova, H.P. Loens, K. Langanke, G. Martinez-Pinedo, T. Rauscher, P. Ring, F.-K. Thielemann, V. Tselyaev, Low-lying dipole response in the relativistic quasiparticle time blocking approximation and its influence on neutron capture cross section, Nucl. Phys. A 823, 26 (2009). 

[49] E. Litvinova, C. Robin, and H. Wibowo, Temperature dependence of nuclear spin-isospin response and beta decay in hot astrophysical environments, Phys. Lett. B800, 135134 (2020).

[50]  E. Litvinova and C. Robin, Phys. Rev. C 103, 024326 (2021).

[51] M.Q. Hlatshwayo, Y. Zhang, H. Wibowo, R. LaRose, D. Lacroix and E. Litvinova, Simulating excited states of the Lipkin model on a quantum computer, Phys. Rev. C 106, 024319 (2022).

[52] M.Q. Hlatshwayo, J. Novak, and E. Litvinova, Quantum benefit of the quantum equation of motion for the strongly coupled many-body problem, Phys. Rev. C 109, 014306 (2024).