1,721,229 research outputs found
Matter power spectrum in hidden neutrino interacting dark matter models: a closer look at the collision term
No full textDark Matter (DM) models providing possible alternative solutions to the smallscale crisis of the standard cosmology are nowadays of growing interest. We consider DM interacting with light hidden fermions via well-motivated fundamental operators showing the resultant, matter power spectrum is suppressed on subgalactic scales within a plausible parameter region. Our basic description of the evolution of cosmological perturbations relies on a fully consistent first principles derivation of a perturbed Fokker-Planck type equation, generalizing existing literature. The cosmological perturbation of the Fokker-Planck equation is presented for the first time in two different gauges, where the results transform into each other according to the rules of gauge transformation. Furthermore, our focus lies on a, derivation of a broadly applicable and easily computable collision term showing important phenomenological differences to other existing approximations. As one of the Main results and concerning the small-scale crisis, we show the equal importance of vector and scalar boson mediated interactions between the DM and the light fermions
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Mass, Spin, and Physics Beyond the Standard Model at Colliders
Full text linkThe Standard Model of particle physics has thus far proven extremely effective at describing the composition and interactions of matter we observe. However, theoretical considerations, such as the large hierarchy between the weak and Planck scales, and experimental evidence, such as the observation of non-baryonic dark matter, suggest the possibility of new physics beyond the Standard Model (BSM). In many scenarios, such new physics would occur around the TeV scale, and therefore has an excellent chance of being seen at current and future collider experiments.Following a review of the standard model, its problems, and some new physics scenarios, we explore a number of ways in which colliders may be used to study such new physics. We first discuss a technique for determining the masses of new particles in single-step decay chains, a task which is typically complicated by missing energy associated with discrete symmetries prevalent in BSM models. We then address the determination of the spins of new particles at colliders, developing a model-independent technique and demonstrating how it could be used to distinguish two specific models, supersymmetry and universal extra dimensions, at a future linear collider. We further demonstrate that the effectiveness of this technique could be realized experimentally using existing data from both e+e- and hadron colliders. Finally, we turn away from model-independent techniques and propose a search for color sextet scalars, which could be copiously produced at the Large Hadron Collider. Pair production of such particles could potentially be seen in the relatively clean same-sign dilepton + jets + missing energy channel, for which we propose an effective reconstruction of the sextet pair
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Spin Determination and Physics Beyond the Standard Model at the LHC and ILC
Full text linkMany of the proposed solutions to the hierarchy and naturalness problems postulate new ``partner'' fields to the Standard Model (SM) particles. Determining the spins of these new particles will be critical in distinguishing among the various possible SM extensions, yet proposed methods rely on the underlying models. We propose a new model-independent method for spin measurements which takes advantage of quantum interference among helicity states. By looking at the azimuthal angular dependence of the differential cross section in the production followed by decay of a new particle species one can determine its spin by looking at the various cosine modes. We demonstrate that this method will be able to discriminate scalar particles from higher spin states at the ILC, and discuss application to higher spins and possible uses at the LHC. Supersymmetry and Universal Extra Dimensions models prove problematic at the LHC because missing energy signatures result in too many unknowns while reconstructing events. However, warped extra dimension models in certain setups allow for events whose kinematics can be fully reconstructed. In such scenarios, the heavy spin-2 Kaluza-Klein (KK) graviton provides a unique signature with a mode.We study the feasibility of this approach to measuring the spin of the KK graviton in the warped Randall-Sundrum Model at the LHC.In chapter 5 of this thesis, taking a phenomenological approach, we study a color sextet scalar at the LHC. We focus on the QCD production of a color sextet pair through fusion and annihilation. Its unique coupling to allows the color sextet scalar to decay into same-sign diquark states, such as . We propose a new reconstruction in the multijet plus same sign dilepton with missing transverse energy samples (, ) to search for on-shell final states from sextet scalar pair production. Thanks to the large QCD production, the search covers the sextet mass range up to 1 TeV for 100 fb integrated luminosity
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Searching for the invisible: how dark forces shape our Universe
Full text linkAstrophysical observations on a wide range of scales indicate that the majority of matter in our Universe seems to be approximately inert and non-luminous. The existence of this dark matter implies the existence of an undiscovered particle, since there is no viable dark matter candidate within the Standard Model. Many terrestrial searches for dark matter particles are underway; however, there is no evidence to date that the dark matter interacts with particles in the Standard Model except through gravity. It highly conceivable that the dark matter exists as part of a rich hidden sector with diverse matter content and its own dark forces (in analogy to the Standard Model) which would imply that terrestrial searches may not pose an optimal path to discovering dark matter. Instead, observing astrophysical systems — where dark matter is known to be present through its gravitational influence — would be the best available way to test theories of dark matter where dark forces play a role in altering the properties of those systems. The complementarity of observing various astrophysical systems is a powerful asset for exploring the physics of dark sectors: one can explore broad classes of theories with dark forces in different environments, on different length scales, and at different epochs in the history of our Universe. This dissertation explores several scenarios where astrophysical observations inform our understanding of dark forces in a way that would not necessarily be possible on Earth. In particular, we consider dark sector energy dissipation, dark matter self-interaction, and the early Universe production of dark matter through dark channels. We propagate the implications of these effects for stars, supernovae, the Milky Way stellar disk, dwarf galaxies, galaxy clusters, large-scale structure, the epoch of reionization, and the cosmic microwave background
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Discoverable Matter: an Optimist’s View of Dark Matter and How to Find It
Full text linkAn abundance of evidence from diverse cosmological times and scales demonstrates that85% of the matter in the Universe is comprised of nonluminous, non-baryonic dark matter.Discovering its fundamental nature has become one of the greatest outstanding problemsin modern science. Other persistent problems in physics have lingered for decades, amongthem the electroweak hierarchy and origin of the baryon asymmetry. Little is known aboutthe solutions to these problems except that they must lie beyond the Standard Model. Thefirst half of this dissertation explores dark matter models motivated by their solution tonot only the dark matter conundrum but other issues such as electroweak naturalness andbaryon asymmetry. The latter half of this dissertation approaches the dark matter enigmafrom a different vantage point inspired by the null results at dark matter direct detectionexperiments. The theory community has explored alternative dark matter candidates andproduction mechanisms while the experimental program has made progress on larger andmore sensitive experiments. In this dissertation, we take a complementary approach byinvestigating signals of novel dark matter models which may have been overlooked in currentexperiments
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Phase Transitions in the Early Universe
Full text linkFrom the moment after the Big Bang to the present day, the universe has continually cooled and expanded. The Standard Model of particle physics predicts that as the temperature dropped, the universe underwent a series of phase transitions including the development of mass, the confinement of quarks and gluons into composite particles, and the formation of nuclei and atoms. But new physics beyond the Standard Model may also contribute to this rich thermal tapestry, whether by altering predicted Standard-Model phenomena or by introducing entirely new phase transitions earlier in the universe's history. By studying the early thermal history of the universe, we therefore hope not only to look into the past, but also to search for evidence of new physics beyond the reach of our current particle detectors.Space-based gravitational wave observatories planned for the coming decades propose to make this search a reality. In the first half of this dissertation I explore the landscape of new physics models with phase transitions that could potentially give rise to detectable gravitational wave signals, highlighting particularly interesting cases that also shed light on baryogenesis and the nature of dark matter. But identifying models from a signal depends, conversely, on our ability to accurately predict the gravitational wave signals that they would produce. In the second half of this paper, I detail a novel, non-perturbative method for predicting these signals, offering the chance to dramatically improve on existing perturbative methods for strongly-coupled or super-cooled cases where non-perturbative effects dominate. Together, these two pieces form a theoretical programme that is essential to make sense of experimental results, and in doing so lead us from signal to new insights into the fundamental laws of physics
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Aspects of Particle Physics Beyond the Standard Model
Full text linkThis dissertation describes a few aspects of particles beyond the Standard Model, with a focus on the remaining questions after the discovery of a Standard Model-like Higgs boson. In specific, three topics are discussed in sequence: neutrino mass and baryon asymmetry, naturalness problem of Higgs mass, and placing constraints on theoretical models from precision measurements.\\First, the consequence of the neutrino mass anarchy on cosmology is studied. Attentions are paid in particular to the total mass of neutrinos and baryon asymmetry through leptogenesis. With the assumption of independence among mass matrix entries in addition to the basis independence, Gaussian measure is the only choice. On top of Gaussian measure, a simple approximate flavor symmetry makes leptogenesis highly successful. Correlations between the baryon asymmetry and the light-neutrino quantities are investigated. Also discussed are possible implications of recently suggested large total mass of neutrinos by the SDSS/BOSS data.\\Second, the Higgs mass implies fine-tuning for minimal theories of weak-scale supersymmetry (SUSY). Non-decoupling effects can boost the Higgs mass when new states interact with the Higgs, but new sources of SUSY breaking that accompany such extensions threaten naturalness. I will show that two singlets with a Dirac mass can increase the Higgs mass while maintaining naturalness in the presence of large SUSY breaking in the singlet sector. The modified Higgs phenomenology of this scenario, termed ``Dirac NMSSM'', is also studied.\\Finally, the sensitivities of future precision measurements in probing physics beyond the Standard Model are studied. A practical three-step procedure is presented for using the Standard Model effective field theory (SM EFT) to connect ultraviolet (UV) models of new physics with weak scale precision observables. With this procedure, one can interpret precision measurements as constraints on the UV model concerned. A detailed explanation is given for calculating the effective action up to one-loop order in a manifestly gauge covariant fashion. This covariant derivative expansion method dramatically simplifies the process of matching a UV model with the SM EFT, and also makes available a universal formalism that is easy to use for a variety of UV models. A few general aspects of RG running effects and choosing operator bases are discussed. Mapping results are provided between the bosonic sector of the SM EFT and a complete set of precision electroweak and Higgs observables to which present and near future experiments are sensitive. Many results and tools which should prove useful to those wishing to use the SM EFT are detailed in several appendices
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The Accelerating Universe, the Landscape, and the Swampland
Full text linkThe accelerating expansion of the universe is a portal for us to understand the physics at the very fundamental level. It’s a phenomena that is apparently IR but intrinsically UV. In this dissertation we investigate various aspects of the accelerating universe in the context of the string landscape and swampland. In the first part of the dissertation, we investigate if the observed small and nearly scale-invariant primordial cosmic perturbation is typical in the landscape of vacua after imposing anthropic selections on them. We propose a scenario that combines new-inflation-type models with the landscape, in which our universe had been trapped at a meta-stable vacuum and underwent a precedent inflation. We argue that the initial inflaton field value is typically non-zero because of the quantum fluctuation created during the precedent inflation. Imposing anthropic constraint on the initial condition, together with certain distributions of inflation model parameters that are physically well- motivated, makes the observed small and nearly scale-invariant spectrum typical. In a latter part of the dissertation, we discuss the quintessence model building in supergravity, in light of the recently proposed de Sitter swampland conjecture. Particularly, the conjecture claims that the scalar potential V in any consistent theory of quantum gravity should satisfy the constraint |∇V | ≥ cV where c is a positive number of order one. If true, positive cosmological constant (even metastable one) cannot be obtained in string theory and dark energy needs to be described by an evolving scalar field, i.e. quintessence, within supergravity. We demonstrate that by imposing a shift symmetry on the Kahler potential, one can embed any quintessence models into supergravity while avoiding the fifth force constraint and protecting the flatness of the quintessence potential from supersymmetry breaking, which are the two main obstacles when constructing quintessence models in supergravity. In addition, the small energy scale of quintessence is technically natural in this setup. In the last part of the dissertation, we discuss the phenomenological implications of swampland conjectures on both inflation and dark energy, using the fact that the conjectures are universal throughout the whole field space. We show that the refined de Sitter conjecture, along with multi-field inflation, opens up the opportunity for observations to determine if the dark energy equation of state deviates from that of a cosmological constant
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Precision Higgs Physics, Effective Field Theory, and Dark Matter
Full text linkThe recent discovery of the Higgs boson calls for detailed studies of its properties. As precision measurements are indirect probes of new physics, the appropriate theoretical framework is effective field theory. In the first part of this thesis, we present a practical three-step procedure of using the Standard Model effective field theory (SM EFT) to connect ultraviolet (UV) models of new physics with weak scale precision observables. With this procedure, one can interpret precision measurements as constraints on the UV model concerned. We give a detailed explanation for calculating the effective action up to one-loop order in a manifestly gauge covariant fashion. The covariant derivative expansion dramatically simplifies the process of matching a UV model with the SM EFT, and also makes available a universal formalism that is easy to use for a variety of UV models. A few general aspects of renormalization group running effects and choosing operator bases are discussed. Finally, we provide mapping results between the bosonic sector of the SM EFT and a complete set of precision electroweak and Higgs observables to which present and near future experiments are sensitive.With a detailed understanding of how to use the SM EFT, we then turn to applications and study in detail two well-motivated test cases. The first is singlet scalar field that enables the first-order electroweak phase transition for baryogenesis; the second example is due to scalar tops in the MSSM. We find both Higgs and electroweak measurements are sensitive probes of these cases.The second part of this thesis centers around dark matter, and consists of two studies. In the first, we examine the effects of relic dark matter annihilations on big bang nucleosynthesis (BBN). The magnitude of these effects scale simply with the dark matter mass and annihilation cross-section, which we derive. Estimates based on these scaling behaviors indicate that BBN severely constrains hadronic and radiative dark matter annihilation channels in the previously unconsidered dark matter mass region MeV < m < 10 GeV. Interestingly, we find that BBN constraints on hadronic annihilation channels are competitive with similar bounds derived from the cosmic microwave background.Our second study of dark matter concerns a possible connection with supersymmetry and the keV scale. Various theoretical and experimental considerations motivate models with high scale supersymmetry breaking. While such models may be difficult to test in colliders, we propose looking for signatures at much lower energies. We show that a keV line in the X-ray spectrum of galaxy clusters (such as the recently disputed 3.5 keV observation) can have its origin in a universal string axion coupled to a hidden supersymmetry breaking sector. A linear combination of the string axion and an additional axion in the hidden sector remains light, obtaining a mass of order 10 keV through supersymmetry breaking dynamics. In order to explain the X-ray line, the scale of supersymmetry breaking must be about 10^{11-12} GeV. This motivates high scale supersymmetry as in pure gravity mediation or minimal split supersymmetry and is consistent with all current limits. Since the axion mass is controlled by a dynamical mass scale, this mass can be much higher during inflation, avoiding isocurvature (and domain wall) problems associated with high scale inflation. In an appendix we present a mechanism for dilaton stabilization that additionally leads to O(1) modifications of the gaugino mass from anomaly mediation
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