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Boris Kayser

Publications and source records attributed to Boris Kayser.

At least 19 recordsLinked to original sources
arXiv2021

Z-Boson Decays into Majorana or Dirac (Heavy) Neutrinos

We computed the kinematics of Z-boson decay into a heavy-light neutrino pair when the Z-boson is produced at rest in electron-positron collisions, including the subsequent decay of the heavy neutrino into a visible final state containing a charged-lepton. We concentrated on heavy-neutrino masses of order dozens of GeV and the issue of addressing the nature of the neutrinos - Dirac fermions or Majorana fermions. We find that while it is not possible to tell the nature of the heavy and light neutrinos on an event-by-event basis, the nature of the neutrinos can nonetheless be inferred given a large-enough sample of heavy-light neutrino pairs. We identify two observables sensitive to the nature of neutrinos. One is the forward-backward asymmetry of the daughter-charged-leptons. This asymmetry is exactly zero if the neutrinos are Majorana fermions and is non-zero (and opposite) for positively- and negatively-charged daughter-leptons if the neutrinos are Dirac fermions. The other observable is the polarization of the heavy neutrino, imprinted in the laboratory-frame energy distribution of the daughter-charged-leptons. Dirac neutrinos and antineutrinos produced in electron-positron collisions at the Z-pole are strongly polarized while Majorana neutrinos are at most as polarized as the $Z$-bosons.

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arXiv2018

Addressing the Majorana vs. Dirac Question with Neutrino Decays

The Majorana versus Dirac nature of neutrinos remains an open question. This is due, in part, to the fact that virtually all the experimentally accessible neutrinos are ultra-relativistic. Noting that Majorana neutrinos can behave quite differently from Dirac ones when they are non-relativistic, we show that, at leading order, the angular distribution of the daughters in the decay of a heavy neutrino into a lighter one and a self-conjugate boson is isotropic in the parent's rest frame if the neutrinos are Majorana, independent of the parent's polarization. If the neutrinos are Dirac fermions, this is, in general, not the case. This result follows from CPT invariance and is independent of the details of the physics responsible for the decay. We explore the feasibility of using these angular distributions -- or, equivalently, the energy distributions of the daughters in the laboratory frame -- in order to address the Majorana versus Dirac nature of neutrinos if a fourth, heavier neutrino mass eigenstate reveals itself in the current or next-generation of high-energy colliders, intense meson facilities, or neutrino beam experiments.

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arXiv2018

Addressing the Majorana vs. Dirac Question Using Neutrino Decays

We explain why it is so hard to determine whether neutrinos are Majorana or Dirac particles as long as the only neutrinos we study are ultra-relativistic. We then show how non-relativistic neutrinos could help, and focus on the angular distributions in the decays of an as-yet-to-be-discovered heavy neutrino $N$. We find that these angular distributions could very well tell us whether neutrinos are Majorana or Dirac particles.

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arXiv2018

On the Properties of Neutrinos

Our present understanding of neutrino properties is reviewed with a particular emphasis on observable differences between Majorana and Dirac neutrinos. Current and future experimental efforts towards measuring neutrino properties are summarized. Consequences of the Majorana vs.Dirac nature of neutrinos on neutrino masses, neutrino decays, and neutrino electromagnetic properties are described.

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arXiv2017

What measurements of neutrino neutral current events can reveal

We show that neutral current (NC) measurements at neutrino detectors can play a valuable role in the search for new physics. Such measurements have certain intrinsic features and advantages that can fruitfully be combined with the usual well-studied charged lepton detection channels in order to probe the presence of new interactions or new light states. In addition to the fact that NC events are immune to uncertainties in standard model neutrino mixing and mass parameters, they can have small matter effects and superior rates since all three flavours participate. We also show, as a general feature, that NC measurements provide access to different combinations of CP phases and mixing parameters compared to CC measurements at both long and short baseline experiments. Using the Deep Underground Neutrino Experiment (DUNE) as an illustrative setting, we demonstrate the capability of NC measurements to break degeneracies arising in CC measurements, allowing us, in principle, to distinguish between new physics that violates three flavour unitarity and that which does not. Finally, we show that NC measurements can enable us to restrict new physics parameters that are not easily constrained by CC measurements.

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arXiv2016

Capabilities of long-baseline experiments in the presence of a sterile neutrino

Assuming that there is a sterile neutrino, we ask what then is the ability of long-baseline experiments to i) establish that neutrino oscillation violates CP, ii) determine the three-neutrino mass ordering, and iii) determine which CP-violating phase or phases are the cause of any CP violation that may be observed. We find that the ability to establish CP violation and to determine the mass ordering could be very substantial. However, the effects of the sterile neutrino could be quite large, and it might prove very difficult to determine which phase is responsible for an observed CP violation. We explain why a sterile neutrino changes the long-baseline sensitivities to CP violation and to the mass ordering in the ways that it does. We note that long-baseline experiments can probe the presence of sterile neutrinos in a way that is different from, and complementary to, the probes of short-baseline experiments. We explore the question of how large sterile-active mixing angles need to be before long-baseline experiments can detect their effects, or how small they need to be before the interpretation of these experiments can safely disregard the possible existence of sterile neutrinos.

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arXiv2015

The impact of sterile neutrinos on CP measurements at long baselines

With the Deep Underground Neutrino Experiment (DUNE) as an example, we show that the presence of even one sterile neutrino of mass $\sim$1 eV can significantly impact the measurements of CP violation in long baseline experiments. Using a probability level analysis and neutrino-antineutrino asymmetry calculations, we discuss the large magnitude of these effects, and show how they translate into significant event rate deviations at DUNE. Our results demonstrate that measurements which, when interpreted in the context of the standard three family paradigm, indicate CP conservation at long baselines, may, in fact hide large CP violation if there is a sterile state. Similarly, any data indicating the violation of CP cannot be properly interpreted within the standard paradigm unless the presence of sterile states of mass O(1 eV) can be conclusively ruled out. Our work underscores the need for a parallel and linked short baseline oscillation program and a highly capable near detector for DUNE, in order that its highly anticipated results on CP violation in the lepton sector may be correctly interpreted.

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arXiv2013

Neutrino Oscillation Physics

To complement the neutrino-physics lectures given at the 2011 International School on Astro Particle Physics devoted to Neutrino Physics and Astrophysics (ISAPP 2011; Varenna, Italy), at the 2011 European School of High Energy Physics (ESHEP 2011; Cheila Gradistei, Romania), and, in modified form, at other summer schools, we present here a written description of the physics of neutrino oscillation. This description is centered on a new way of deriving the oscillation probability. We also provide a brief guide to references relevant to topics other than neutrino oscillation that were covered in the lectures.

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arXiv2013

Tensions with the Three-Neutrino Paradigm

We review the tensions with the standard three-neutrino framework for neutrino oscillation. These tensions hint at the possible existence of sterile neutrinos. We briefly describe some of the diverse ideas for probing the existence of such neutrinos in a definitive way through future experiments.

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arXiv2011

B-Meson and Neutrino Oscillation: A Unified Treatment

We present a unified treatment of the quantum mechanics of B-factory and neutrino oscillation experiments. While our approach obtains the usual phenomenological predictions for these experiments, it does so without having to invoke perplexing Einstein-Podolsky-Rosen correlations or non-intuitive kinematical assumptions.

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arXiv2011

Leptogenesis at the Electroweak Scale

In this note we propose a model of leptogenesis in which the scale for the mass of the necessary heavy neutral lepton is similar to the scale of electroweak symmetry breaking.

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arXiv2011

A Scalar Doublet at the Tevatron?

We propose a particularly simple explanation of the W+dijet excess reported by the CDF collaboration. No symmetries beyond those of the standard model are necessary, and the only new particles involved are a spin-zero weak SU(2) doublet with no vacuum expectation value. Possible tests of the model and comparisons to other proposals are discussed.

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arXiv2010

Two Questions About Neutrinos

We explain why the see-saw picture and leptogenesis make it particularly interesting to find out whether neutrinos are their own antiparticles, and whether their oscillations violate CP.

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arXiv2010

Testing the wave packet approach to neutrino oscillations in future experiments

When neutrinos propagate over long distances, the mass eigenstate components of a flavor eigenstate will become spatially separated due to their different group velocities. This can happen over terrestrial distance scales if the neutrino energy is of order MeV and if the neutrino is localized (in a quantum mechanical sense) to subatomic scales. For example, if the Heisenberg uncertainty in the neutrino position is below 0.01 Angstrom, neutrino decoherence can be observed in reactor neutrinos using a large liquid scintillator detector.

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arXiv2010

On a theory of neutrino oscillations with entanglement

We show that, despite appearances, a theoretical approach to neutrino oscillation in which the neutrino and its interaction partners are entangled yields the standard result for the neutrino oscillation wavelength. We also shed some light on the question of why plane-wave approaches to the neutrino oscillation problem can yield the correct oscillation wavelength even though they do not explicitly account for the localization of the neutrino source and the detector.

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arXiv2009

Leptogenesis and its Electromagnetic Variant

We briefly explain how the present baryon-antibaryon asymmetry of the universe could have arisen through leptogenesis, and then discuss a new version of leptogenesis in which CP violation in electromagnetic decays plays the central role.

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arXiv2009

Are Neutrinos Their Own Antiparticles?

We explain the relationship between Majorana neutrinos, which are their own antiparticles, and Majorana neutrino masses. We point out that Majorana masses would make the neutrinos very distinctive particles, and explain why many theorists strongly suspect that neutrinos do have Majorana masses. The promising approach to confirming this suspicion is to seek neutrinoless double beta decay. We introduce a toy model that illustrates why this decay requires nonzero neutrino masses, even when there are both right-handed and left-handed weak currents.

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