Latest Results


GRAND white paper

The Giant Radio Array for Neutrino Detection (GRAND): Science and Design
GRAND Collaboration (editor: Mauricio Bustamante)
arXiv: 1810.09994


The GRAND white paper contains the science goals, expected performance, design, and construction plans of GRAND.

Unitarity bounds of astrophysical neutrinos

Unitarity Bounds of Astrophysical Neutrinos
Markus Ahlers, Mauricio Bustamante, Siqiao Mu
arXiv: 1810.00893


At high energies, new physics might affect the flavor composition of astrophysical neutrinos.  In a large class of new-physics models, the total number of neutrinos of all flavors is conserved.  In other words, the connection between the neutrino flavor states and the neutrino propagation states is represented by a unitary mixing matrix, whose elements have unknown or weakly bounded values.  The new physics is expected to affected the flavor composition of high-energy astrophysical neutrinos, possibly significantly.

In a new paper, led by Markus Ahlers, we show that it is not necessary to know the values of the elements of the new mixing matrix in order to map the effect of new physics on the flavor composition.  Based solely on the unitary of the new matrix, we derive the boundaries of regions that enclose all of the possible flavor compositions expected at Earth.

Neutrinos feeling all of the electrons in the Universe

A Universe’s Worth of Electrons to Probe Long-Range Interactions of High-Energy Astrophysical Neutrinos
Mauricio Bustamante, Sanjib Kumar Agarwalla
arXiv: 1808.02042


Here, we look for new flavored long-ranged neutrino-electron interactions, using the flavor composition of the IceCube high-energy astrophysical neutrinos.  For the first time, we reach the ultimate sensitivity to probe these new interactions by considering the aggregated effect of all of the electrons in the local and distant Universe.

Measuring high-energy neutrino cross sections

Measurement of the Energy-Dependent Neutrino-Nucleon Cross Section Above 10 TeV Using IceCube Showers
Mauricio Bustamante, Amy Connolly
arXiv: 1711.11043


Neutrinos are key to probing the deep structure of matter and the high-energy Universe.  Yet, until recently, their interactions had only been measured at laboratory energies up to about 350 GeV.  An opportunity to measure their interactions at higher energies opened up with the detection of high-energy neutrinos in IceCube, partially of astrophysical origin.  Scattering off matter inside the Earth affects the distribution of their arrival directions — from this, we extract the neutrino-nucleon cross section at energies from 18 TeV to 2 PeV, in spite of uncertainties in the neutrino flux.  We show for the first time that the energy dependence of the high-energy cross section agrees with the predicted softer-than-linear dependence, and reaffirm the absence of new physics that would make the cross section rise sharply, up to a center-of-mass energy sqrt(s) ≈ 1 TeV.

Cosmogenic neutrinos challenge the cosmic ray proton dip model

Cosmogenic neutrinos challenge the cosmic ray proton dip model
Jonas Heinze, Denise Boncioli, Mauricio Bustamante, Walter Winter
Astrophys. J. 825, 122 (2016) [arXiv:1512.05988]


The origin and composition of ultra-high-energy cosmic rays (UHECRs) remain a mystery. In the simplest model –the proton dip model– they are mainly protons of extragalactic origin above 109 GeV and their spectral features can be explained by interactions on the cosmic microwave background. These interactions also produce cosmogenic neutrinos, with ~109 GeV. We test whether this model is still viable using recent UHECR spectrum measurements from the Telescope Array and upper limits on the cosmogenic neutrino flux from IceCube. […] The predicted cosmogenic neutrino flux exceeds the IceCube limit for any parameter combination. As a result, they challenge the proton dip model at more than 95% C.L. This is the first strong evidence against this model independently of mass composition measurements.

Flavor composition of high-energy astrophysical neutrinos

Theoretically palatable flavor combinations of high-energy astrophysical neutrinos
Mauricio Bustamante, John F. Beacom, Walter Winter
Phys. Rev. Lett. 115, 161302 (2015) [arXiv:1506.02645]

After decades of searching, IceCube has finally observed the first high-energy astrophysical neutrino events; recently, they have published the first determination of the flavor composition of the flux. This is one of the richest and most rewarding physical observables: its determination will reveal important information about production conditions at the sources and about neutrino properties, including whether there is new physics. It is therefore timely to study its theoretical expectations. We have done so and found that, contrary to naive expectations, the allowed region of flavor composition is quite small, even under a general class of new physics models. Furthermore, a possible volume upgrade of IceCube will likely achieve excellent determination of the flavor composition, thus constraining astrophysical production scenarios.

High-energy neutrino emission from gamma-ray bursts

High-energy neutrino emission from gamma-ray bursts
Mauricio Bustamante, Philipp Baerwald, Kohta Murase, Walter Winter
Nat. Commun. 6, 6783 (2015) [arXiv:1409.2874]


In the classical theory of gamma-ray bursts, it is expected that particles are accelerated at mildly relativistic shocks generated by the collisions of material ejected from a central engine. We consider neutrino and cosmic-ray emission from multiple emission regions since these internal collisions must occur at very different radii, from below the photosphere all the way out to the circumburst medium, as a consequence of the efficient dissipation of kinetic energy. We demonstrate that the different messengers originate from different collision radii, which means that multi-messenger observations open windows for revealing the evolving GRB outflows. We find that, even in the internal shock model, the neutrino production can be dominated by emission from around the photosphere, i.e., the radius where the ejecta become transparent to gamma-ray emission.