The dark dimension scenario posits that an extra dimension of micron size is tied to the dark energy scale. In this context, the right-handed neutrino can propagate in the bulk rather than being localized on the brane, which naturally explains the smallness of neutrino masses. This work sits alongside studies of sterile neutrinos and neutrino oscillations, but differs in that it considers not a single sterile state but a whole tower of Kaluza-Klein excitations arising from the compactification of the extra dimension. This links the search for new neutrinos to fundamental questions in quantum gravity and dark energy.
The authors construct a five-dimensional model where three right-handed neutrinos live in the bulk, while the Standard Model is localized on a brane. They solve the mass equations for the Kaluza-Klein states, obtaining the spectrum and mixing matrices. They then compute the differential tritium beta decay spectrum including contributions from all excitations and compare it with predictions from the Standard Model and the 3+1 model with a sterile neutrino. For analytical analysis, they identify two limiting cases: small and large bulk neutrino mass relative to the compactification scale.
For a small bulk mass (compared to the inverse radius), the beta decay spectrum contains a series of kinks corresponding to Kaluza-Klein excitations. For a radius smaller than about 0.4 microns, several kinks could fall within KATRIN's sensitivity. For a large bulk mass, a single effective kink appears at an energy equal to the bulk mass, with an effective mixing angle that can be computed analytically. This allows direct use of existing KATRIN limits on sterile neutrinos to constrain the dark dimension parameters. For example, for a radius of 10 microns and a bulk mass around 1 eV, a significant portion of the parameter space remains allowed but could be probed in future measurements.
The work shows that KATRIN can not only search for sterile neutrinos but also test the dark dimension hypothesis. Unlike previous studies that mainly focused on small Yukawa couplings, this work considers the large coupling regime, opening a new window in parameter space. This makes the connection between neutrino physics and quantum gravity more concrete and testable.
The model can be extended to multiple extra dimensions or more complex geometries. It will also require accounting for finite spectral line widths and systematic uncertainties in KATRIN. Future TRISTAN data with improved resolution will allow probing the kiloelectronvolt mass region, potentially providing additional constraints.
The results will affect the interpretation of KATRIN data and future searches for sterile neutrinos, as well as theoretical models with extra dimensions.
Perform detailed numerical simulations of the full beta decay spectrum including all Kaluza-Klein states and compare with actual KATRIN data.
This work directly addresses the unresolved problem of neutrino mass origin and the possible role of extra dimensions. It also touches on whether neutrinos are Dirac or Majorana, which has implications for neutrinoless double beta decay.
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What the work does
The authors build a five-dimensional model with right-handed neutrinos in the bulk and compute signatures in the tritium beta-decay spectrum for KATRIN. Two limiting cases of bulk mass are considered: a small mass gives a series of kinks, while a large mass yields a single effective kink, resembling a sterile neutrino signal. The resulting constraints on the dark dimension parameters show that a significant portion of the parameter space is accessible to KATRIN.
Where its strength is
This work connects the search for new neutrinos with the fundamental dark dimension hypothesis, proposing a concrete experimental signal: kinks in the KATRIN spectrum. Especially valuable is the considered regime of large Yukawa couplings, which had not been analyzed before and opens a new window in the parameter space.
Where it could go next
Worth trying
Why this may matter
A test of the dark dimension at KATRIN would be the first experimental hint of extra dimensions related to dark energy, linking neutrino physics with quantum gravity.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.60, against a usual 0.64. It looks like few have drilled nearby.
If the extra dimension has a radius of 10 microns, the Kaluza-Klein excitations of the neutrino are separated in mass by only 0.02 eV, which is smaller than the mass differences between ordinary neutrinos.
The idea of extra dimensions where particles can live evokes the concept of hyperspace from science fiction, for instance in Greg Egan's novel 'Diaspora', where characters travel through extra dimensions.