Speaker
Prof.
Jouni Suhonen
(Department of Physics, University of Jyvaskyla)
Description
We still do not know if the neutrino is a Majorana or a Dirac particle, i.e. if
the neutrino is its own antiparticle or not. Also the absolute mass scale of
the neutrino is unknown, only the relative scale is known from the
neutrino-oscillation experiments. These unknown features of the neutrino can be
tackled by experiments trying to detect the neutrinoless double beta
($0\nu\beta\beta$) decay. The rate of $0\nu\beta\beta$ decay can be schematically
written as
\begin{equation}
0\nu\beta\beta-\mathrm{rate} \sim \left\vert M^{(0\nu)}_{\rm GTGT}\right\vert^2 =
g_{{\rm A},0\nu}^4 \left\vert \sum_{J^{\pi}}
(0^+_f||\mathcal{O}^{(0\nu)}_{\rm GTGT}(J^{\pi})||0^+_i)\right\vert^2 \,,
\end{equation}
where $M^{(0\nu)}_{\rm GTGT}$ is the double Gamow-Teller nuclear matrix element,
$\mathcal{O}^{(0\nu)}_{\rm GTGT}$ denotes the transition operator mediating the
$0\nu\beta\beta$ transition through the various multipole states $J^{\pi}$,
$0^+_i$ denotes the initial ground state,
and the final ground state is denoted by $0^+_f$ (for simplicity, we
neglect the smaller double Fermi and tensor contributions). Here
$g^{\rm eff}_{{\rm A},0\nu}$ denotes the effective (quenched) value of the weak axial-vector
coupling for $0\nu\beta\beta$ decay and it plays an extremely important
role in determining the $0\nu\beta\beta$-decay rate since the rate is proportional
to its 4$th$ power. The amount of quenching has become an important issue in the
neutrino-physics community due to its impact on the sensitivities of the present
and future large-scale $0\nu\beta\beta$-decay experiments [1].
The quenching of $g_{\rm A}$ is traditionally related to shell-model calculations of
Gamow-Teller $\beta$-decay rates. Similar quenchings can also be obtained in some other
nuclear-model frameworks, like the proton-neutron quasiparticle random-phase
approximation (pnQRPA) and the microscopic interacting boson model (IBM-2).
The quenching of $g_{\rm A}$ has also been addressed in calculations of the rates of
two-neutrino double beta ($2\nu\beta\beta$) decays where the $g_{\rm A}^4$ dependence
is present like in the $0\nu\beta\beta$ decays but the quenching can be of
different magnitude since the scale of the exchanged momentum between the nucleons
and the neutrino is different. For a recent review on this topic, see [2].
Novel ways to address the quenching problem are offered by the studies of
forbidden non-unique $\beta$ decays. Rates of the
forbidden non-unique $\beta$ transitions are complex combinations of lepton
phase-space factors and many nuclear matrix elements. The shapes of the corresponding
spectra of the emitted electrons ($\beta$ spectra) can, however, be very sensitive
to the value of $g_{\rm A}$, and thus the measured $\beta$ spectra can give information
on the effective value of $g_{\rm A}$. In addition, the shapes of $\beta$
spectra play a role in the context of the reactor-antineutrino anomaly which
is currently of great interest in the neutrino-physics community.
REFERENCES
[1] J. Suhonen, Impact of the quenching of $g_{\rm A}$ on the sensitivity of
$0\nu\beta\beta$ experiments, Phys. Rev. C 96 (2007) 055501.
[2] J. Suhonen, Value of the axial-vector coupling strength in $\beta$ and
$\beta\beta$ decays: A review, Front. Phys. 5 (2017) 55.
Primary author
Prof.
Jouni Suhonen
(Department of Physics, University of Jyvaskyla)
