741 research outputs found
Time-symmetric quantization in spacetimes with event horizons
The standard quantization formalism in spacetimes with event horizons implies
a non-unitary evolution of quantum states, as initial pure states may evolve
into thermal states. This phenomenon is behind the famous black hole
information loss paradox which provoked long-standing debates on the
compatibility of quantum mechanics and gravity. In this paper we demonstrate
that within an alternative time-symmetric quantization formalism thermal
radiation is absent and states evolve unitarily in spacetimes with event
horizons. We also discuss the theoretical consistency of the proposed
formalism. We explicitly demonstrate that the theory preserves the
microcausality condition and suggest a "reinterpretation postulate" to resolve
other apparent pathologies associated with negative energy states. Accordingly
as there is a consistent alternative, we argue that choosing to use
time-asymmetric quantization is a necessary condition for the black hole
information loss paradox.Comment: 9 page
Multi-Step Cascade Annihilations of Dark Matter and the Galactic Center Excess
If dark matter is embedded in a non-trivial dark sector, it may annihilate
and decay to lighter dark-sector states which subsequently decay to the
Standard Model. Such scenarios - with annihilation followed by cascading
dark-sector decays - can explain the apparent excess GeV gamma-rays identified
in the central Milky Way, while evading bounds from dark matter direct
detection experiments. Each 'step' in the cascade will modify the observable
signatures of dark matter annihilation and decay, shifting the resulting
photons and other final state particles to lower energies and broadening their
spectra. We explore, in a model-independent way, the effect of multi-step
dark-sector cascades on the preferred regions of parameter space to explain the
GeV excess. We find that the broadening effects of multi-step cascades can
admit final states dominated by particles that would usually produce too
sharply peaked photon spectra; in general, if the cascades are hierarchical
(each particle decays to substantially lighter particles), the preferred mass
range for the dark matter is in all cases 20-150 GeV. Decay chains that have
nearly-degenerate steps, where the products are close to half the mass of the
progenitor, can admit much higher DM masses. We map out the region of
mass/cross-section parameter space where cascades (degenerate, hierarchical or
a combination) can fit the signal, for a range of final states. In the current
work, we study multi-step cascades in the context of explaining the GeV excess,
but many aspects of our results are general and can be extended to other
applications.Comment: 18 pages, 15 figures, 2 tables; comments welcome. Updated to
published versio
Disentangling Heavy Flavor at Colliders
We propose two new analysis strategies for studying charm and beauty quarks
at colliders. The first strategy is aimed at testing the kinematics of
heavy-flavor quarks within an identified jet. Here, we use the SoftDrop
jet-declustering algorithm to identify two subjets within a large-radius jet,
using subjet flavor tagging to test the heavy-quark splitting functions of QCD.
For subjets containing a or , this declustering technique
can also help probe the mechanism for quarkonium production. The second
strategy is aimed at isolating heavy-flavor production from gluon splitting.
Here, we introduce a new FlavorCone algorithm, which smoothly interpolates from
well-separated heavy-quark jets to the gluon-splitting regime where jets
overlap. Because of its excellent ability to identify charm and beauty hadrons,
the LHCb detector is ideally suited to pursue these strategies, though similar
measurements should also be possible at ATLAS and CMS. Together, these SoftDrop
and FlavorCone studies should clarify a number of aspects of heavy-flavor
physics at colliders, and provide crucial information needed to improve
heavy-flavor modeling in parton-shower generators.Comment: 22 pages, 14 figures; v2: updated figures with new z_tag condition,
references and discussion adde
Listening to the Universe through Indirect Detection
Indirect detection is the search for the particle nature of dark matter with
astrophysical probes. Manifestly, it exists right at the intersection of
particle physics and astrophysics, and the discovery potential for dark matter
can be greatly extended using insights from both disciplines. This thesis
provides an exploration of this philosophy. On the one hand, I will show how
astrophysical observations of dark matter, through its gravitational
interaction, can be exploited to determine the most promising locations on the
sky to observe a particle dark matter signal. On the other, I demonstrate that
refined theoretical calculations of the expected dark matter interactions can
be used disentangle signals from astrophysical backgrounds. Both of these
approaches will be discussed in the context of general searches, but also
applied to the case of an excess of photons observed at the center of the Milky
Way. This galactic center excess represents both the challenges and joys of
indirect detection. Initially thought to be a signal of annihilating dark
matter at the center of our own galaxy, it now appears more likely to be
associated with a population of millisecond pulsars. Yet these pulsars were
completely unanticipated, and highlight that indirect detection can lead to
many new insights about the universe, hopefully one day including the particle
nature of dark matter.Comment: Ph.D. thesis, MIT, April 2018; based on the work appearing in
arXiv:1708.09385, arXiv:1612.05638, arXiv:1612.04814, arXiv:1511.08787,
arXiv:1503.01773, and arXiv:1402.670
A Search for Dark Matter Annihilation in Galaxy Groups
We use 413 weeks of publicly-available Pass 8 gamma-ray
data, combined with recently-developed galaxy group catalogs, to search for
evidence of dark matter annihilation in extragalactic halos. In our study, we
use luminosity-based mass estimates and mass-to-concentration relations to
infer the -factors and associated uncertainties for hundreds of galaxy
groups within a redshift range . We employ a conservative
substructure boost-factor model, which only enhances the sensitivity by an
factor. No significant evidence for dark matter annihilation
is found and we exclude thermal relic cross sections for dark matter masses
below 30 GeV to 95% confidence in the annihilation channel.
These bounds are comparable to those from Milky Way dwarf spheroidal satellite
galaxies. The results of our analysis increase the tension, but do not rule
out, the dark matter interpretation of the Galactic Center excess. We provide a
catalog of the galaxy groups used in this study and their inferred properties,
which can be broadly applied to searches for extragalactic dark matter.Comment: 5+18 pages, 1+14 figures, catalog available at:
https://github.com/bsafdi/DMCat; v2 updated to journal version with several
updates, results and conclusions unchange
Model-Independent Indirect Detection Constraints on Hidden Sector Dark Matter
If dark matter inhabits an expanded "hidden sector", annihilations may
proceed through sequential decays or multi-body final states. We map out the
potential signals and current constraints on such a framework in indirect
searches, using a model-independent setup based on multi-step hierarchical
cascade decays. While remaining agnostic to the details of the hidden sector
model, our framework captures the generic broadening of the spectrum of
secondary particles (photons, neutrinos, e+e- and antiprotons) relative to the
case of direct annihilation to Standard Model particles. We explore how
indirect constraints on dark matter annihilation limit the parameter space for
such cascade/multi-particle decays. We investigate limits from the cosmic
microwave background by Planck, the Fermi measurement of photons from the dwarf
galaxies, and positron data from AMS-02. The presence of a hidden sector can
change the constraints on the dark matter annihilation cross section by up to
an order of magnitude in either direction (although the effect can be much
smaller). We find that generally the bound from the Fermi dwarfs is most
constraining for annihilations to photon-rich final states, while AMS-02 is
most constraining for electron and muon final states; however in certain
instances the CMB bounds overtake both, due to their approximate independence
of the details of the hidden sector cascade. We provide the full set of cascade
spectra considered here as publicly available code with examples at
http://web.mit.edu/lns/research/CascadeSpectra.html.Comment: Published version. Added analysis on interplay between indirect
detection bounds and the Galactic Center GeV excess. Added antiproton ratio
bound
What the Milky Way's Dwarfs tell us about the Galactic Center extended excess
The Milky Way's Galactic Center harbors a gamma-ray excess that is a
candidate signal of annihilating dark matter. Dwarf galaxies remain
predominantly dark in their expected commensurate emission. In this work we
quantify the degree of consistency between these two observations through a
joint likelihood analysis. In doing so we incorporate Milky Way dark matter
halo profile uncertainties, as well as an accounting of diffuse gamma-ray
emission uncertainties in dark matter annihilation models for the Galactic
Center Extended gamma-ray excess (GCE) detected by the Fermi Gamma-Ray Space
Telescope. The preferred range of annihilation rates and masses expands when
including these unknowns. Even so, using two recent determinations of the Milky
Way halo's local density leave the GCE preferred region of single-channel dark
matter annihilation models to be in strong tension with annihilation searches
in combined dwarf galaxy analyses. A third, higher Milky Way density
determination, alleviates this tension. Our joint likelihood analysis allows us
to quantify this inconsistency. We provide a set of tools for testing dark
matter annihilation models' consistency within this combined dataset. As an
example, we test a representative inverse Compton sourced self-interacting dark
matter model, which is consistent with both the GCE and dwarfs.Comment: v2, 12 pages, 4 figures, tools online at:
https://github.com/rekeeley/GCE_error
Testable two-loop radiative neutrino mass model based on an effective operator
A new two-loop radiative Majorana neutrino mass model is constructed from the
gauge-invariant effective operator that violates lepton number conservation by two units. The
ultraviolet completion features two scalar leptoquark flavors and a color-octet
Majorana fermion. We show that there exists a region of parameter space where
the neutrino oscillation data can be fitted while simultaneously meeting
flavor-violation and collider bounds. The model is testable through lepton
flavor-violating processes such as , , and
conversion, as well as collider searches for the scalar
leptoquarks and color-octet fermion. We computed and compiled a list of
necessary Passarino-Veltman integrals up to boxes in the approximation of
vanishing external momenta and made them available as a Mathematica package,
denoted as ANT.Comment: 42 pages, 11 figures, typo in Eq. (4.9) as well as wrong chirality
structures in Secs. 4.5 and 5.2 corrected, final results unchange
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