Christian Appelt
?Extending the limits in the?hunt for long‐lived heavy?neutral leptons with the?ATLAS experiment at the?Large Hadron Collider at?CERN“
The standard model of elementary particle physics defines our current
knowledge of the smallest constituents of matter. However, a few
crucial observations point to the existence of physics beyond the
standard model. Studies in astrophysics and cosmology revealed that
additional matter of unknown type, so-called dark matter, is needed to
explain the rotation patterns of galaxies, effects seen in gravitational lensing,
the large-scale structure of the observable universe, and cosmic microwave
background anisotropies. Various measurements show that dark matter is
almost six times more abundant in the universe than ordinary matter. A
second fact that the standard model fails to explain is the abundance of
matter over antimatter that remained after the Big Bang. Life could not exist
without matter-antimatter asymmetry. Understanding the processes behind
this imbalance is essential for comprehending the foundation of life as we
know it. Finally, neutrinos are assumed to have zero mass in the standard
model. Neutrinos are elementary particles that interact only weakly. It has
been observed that one type (or ‘flavour’) of neutrino can turn into another
over time via a process called neutrino oscillations, which can only occur if
neutrinos have nonzero mass.
knowledge of the smallest constituents of matter. However, a few
crucial observations point to the existence of physics beyond the
standard model. Studies in astrophysics and cosmology revealed that
additional matter of unknown type, so-called dark matter, is needed to
explain the rotation patterns of galaxies, effects seen in gravitational lensing,
the large-scale structure of the observable universe, and cosmic microwave
background anisotropies. Various measurements show that dark matter is
almost six times more abundant in the universe than ordinary matter. A
second fact that the standard model fails to explain is the abundance of
matter over antimatter that remained after the Big Bang. Life could not exist
without matter-antimatter asymmetry. Understanding the processes behind
this imbalance is essential for comprehending the foundation of life as we
know it. Finally, neutrinos are assumed to have zero mass in the standard
model. Neutrinos are elementary particles that interact only weakly. It has
been observed that one type (or ‘flavour’) of neutrino can turn into another
over time via a process called neutrino oscillations, which can only occur if
neutrinos have nonzero mass.
?
All those observations call for an extension of the standard model. A
possible explanation could be the existence of new, massive, right-handed
neutrinos, also called Heavy Neutral Leptons (HNLs), that complement the
left-handed standard model neutrinos. The ‘handedness’ is a quantum state
that characterises all fundamental spin-half particles in two groups: left-
handed and right-handed. The weak interaction is known to affect only left-
handed particles, which is why we only know of the existence of left-handed
neutrinos.
possible explanation could be the existence of new, massive, right-handed
neutrinos, also called Heavy Neutral Leptons (HNLs), that complement the
left-handed standard model neutrinos. The ‘handedness’ is a quantum state
that characterises all fundamental spin-half particles in two groups: left-
handed and right-handed. The weak interaction is known to affect only left-
handed particles, which is why we only know of the existence of left-handed
neutrinos.
?
In the ATLAS experiment at the Large Hadron Collider at CERN, we are
sensitive to HNLs whose addition to the standard model could give rise to
the smallness of neutrino masses and the observed matter-antimatter
asymmetry in the universe. In my thesis, ATLAS colleagues and I searched
HNLs using proton-proton collision data recorded between 2015 and 2018.
The search was published in the high-impact journal ‘Physical Review
Letters’. The small coupling values and masses of the considered HNLs
make them long-lived, which requires dedicated techniques to reconstruct
particle tracks far away from the beamline and secondary vertices that arise
from the HNL decay signature. The results were consistent with the
standard model expectation. Although no HNL was observed, we could set
the tightest bounds on the HNL parameters to date. We collaborated with
prominent theorists in the field to interpret a real physics HNL model, which
I then incorporated into the statistical analysis of the search. For the first
time in the history of HNL searches, limits were given for both single-flavour
and multi-flavour mixing scenarios motivated by neutrino-flavour oscillation
results.
sensitive to HNLs whose addition to the standard model could give rise to
the smallness of neutrino masses and the observed matter-antimatter
asymmetry in the universe. In my thesis, ATLAS colleagues and I searched
HNLs using proton-proton collision data recorded between 2015 and 2018.
The search was published in the high-impact journal ‘Physical Review
Letters’. The small coupling values and masses of the considered HNLs
make them long-lived, which requires dedicated techniques to reconstruct
particle tracks far away from the beamline and secondary vertices that arise
from the HNL decay signature. The results were consistent with the
standard model expectation. Although no HNL was observed, we could set
the tightest bounds on the HNL parameters to date. We collaborated with
prominent theorists in the field to interpret a real physics HNL model, which
I then incorporated into the statistical analysis of the search. For the first
time in the history of HNL searches, limits were given for both single-flavour
and multi-flavour mixing scenarios motivated by neutrino-flavour oscillation
results.
Thesis publication: https://doi.org/10.18452/28639
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