Dark matter and supersymmetry in the tera-electronvolt era
Abstract/Contents
- Abstract
- The current era of fundamental particle physics research is driven by experimental data to a degree that has not been seen in decades. The flagship experiments in our field are being performed by the ATLAS and CMS collaborations whose detectors record the results of proton-proton collisions provided by the CERN Large Hadron Collider (LHC) at center-of-mass energies of 7 or 8 tera-electron volts (TeV). This has allowed the first direct test of the Standard Model of Particle Physics (SM) at/above weak-scale energies ($\sim 100\gev$-$1\tev$) in a laboratory setting. At the same time the massive and coherent body of gravitational evidence for the existence of dark matter has prompted a host of experimentental efforts to uncover its possible identity as a fundamental particle. In particular, a program of searches for dark matter scattering off of targets in cryogenic detectors situated deep underground (``direct detection'') and for the anomalous energetic annihilation products of dark matter annihilation in astro-particle physics experimental measurements (``indirect detection'') have gone beyond prototypes to reaching their designed sensitivities. In the preceding decades the work of particle theorists\footnote{Building, of course, off of the previous efforts of particle experimentalists} has lead to a picture of the weak-scale as a special scale in physics, and to expectations for further discoveries related to the weak-scale. Perhaps the most influential facet of this picture is the Gauge Hierarchy Problem: in the Standard Model the Higgs vacuum expectation value, the quantity that determines the weak scale, is sensitive to physical processes ranging all of the way up to the Planck scale\footnote{The Planck scale refers to the energy scale at which gravity becomes important even in the interactions of fundamental particles} in such a way that its value should naturally be on the order of Planck scale, $10^{19}\gev$, as opposed to its observed value, $\sim 10^2\gev$. This apparent fine-tuning can be eliminated provided there are new particles whose contributions cancel the dominant sources of Planck scale sensitivity, leaving behind terms that scale only logarithmically with the largest scale in the theory and polynomially with the mass of the new particle/s. In practice fine-tuning better than percent-level can be achieved as long as the new particle masses are not much heavier than the weak-scale. Theoretical proposals for a solution of the hierarchy problem, frameworks such as supersymmetry, thus typically posit new particles with masses near the weak-scale that could be discovered in experiments at the LHC. Another theoretical motivation for new physics at the weak scale is the apparent near-unification of the Standard Model gauge couplings as their values run (in the sense of the renormalization group) up to energies $\sim 10^{16}\gev$ (the GUT scale), which has been interpreted as possible evidence for an enlarged symmetry at energies above the GUT scale. With the Standard Model particle content alone the gauge couplings, run up to the GUT scale, do not unify, however the addition of new particle content at the weak scale has been shown (for instance, in supersymmetric models) to achieve this unification. While the gauge hierarchy problem and grand-unification are largely aesthetic problems with the Standard Model as we know it, our ignorance of the nature of the dark matter (DM) in our universe is an issue that is being forced upon us by experimental data \cite{p-dm}. Cosmological data such as the detailed measurements of the CMB anisotropy and matter power spectra \cite{p-Komatsu:2008hk}, large scale structure measurements \cite{p-lss}, and related data can all be consistently interpreted in terms of a universe whose bulk energy density is partitioned roughly as: $73$\% dark energy, $23$\% dark matter and $4$\% ordinary matter. Astronomical data such as stellar rotation curves (the oldest evidence of dark matter) and measurements of the gravitational potential of objects via gravitational lensing all suggest that the dominant matter density in galaxies (such as our own Milky Way) is dark. Recent lensing analyses, the ``Bullet Cluster'' \cite{Clowe:2006eq} in particular, seem to provide solid dynamical proof of the \emph{particulate} nature of the dark matter component of these galaxy clusters\footnote{Such evidence has been particularly difficult to explain in theories without dark matter, such as Modified Newtonian Dynamics (MOND) \cite{p-mond}.}. Despite the depth and breadth of the evidence for dark matter, none of this evidence suggests stronger-than-gravitational interactions between dark matter and the particles of the Standard Model. Possibilities range from the (plausible but extremely difficult to study) case of having \emph{only} gravitational-strength DM-SM interactions to having DM-SM and SM-SM interactions of roughly the strength of the weak-nuclear force (above which there are indirect bounds from the gravitational observations of the effects of dark matter). A very natural expectation then is that the dark matter particle is a particle with roughly weak scale mass coupling to the Standard Model via the weak interaction by exchanging mediating particles that also have mass near the weak scale. This could arise simply via the electroweak interaction if the DM and the mediator come in electroweak multiplets of the SM gauge group $SU(2)_L \otimes U(1)_Y$, as happens in many common examples (like supersymmetry), but may also arise from one of a variety of other possible scenarios (for example, secluded sector models \cite{p-pospelov}, in which the dark sector includes its own force carrier/s and DM-SM interactions can arise via kinetic mixing between these ``dark photons" and the SM photon). Despite the depth and breadth of the evidence for dark matter, none of this evidence suggests stronger-than-gravitational interactions between dark matter and the particles of the Standard Model. Possibilities range from the (plausible but extremely difficult to study) case of having \emph{only} gravitational-strength DM-SM interactions to having DM-SM and SM-SM interactions of roughly the strength of the weak-nuclear force (above which there are indirect bounds from the gravitational observations of the effects of dark matter). A very natural expectation then is that the dark matter particle is a particle with roughly weak scale mass coupling to the Standard Model via the weak interaction by exchanging mediating particles that also have mass near the weak scale. This could arise simply via the electroweak interaction if the DM and the mediator come in electroweak multiplets of the SM gauge group $SU(2)_L \otimes U(1)_Y$, as happens in many common examples (like supersymmetry), but may also arise from one of a variety of other possible scenarios (for example, secluded sector models \cite{p-pospelov}, in which the dark sector includes its own force carrier/s and DM-SM interactions can arise via kinetic mixing between these ``dark photons" and the SM photon). While the study of weakly-interacting massive particle (WIMP) dark matter has flourished because of the expectations for discoveries of new weak-scale particles at the LHC and of signals in direct- and indirect-detection experiments, the WIMP scenario is also motivated theoretically. The ``WIMP Miracle'' is the statement that in a universe with WIMP dark matter ($\ie$, particles having an annihilation cross-section that is $\sim\:10^{-26}\:\rm{cm}^3\:\rm{s}^{-1}$), evolving in thermal equilibrium, more or less naturally generates a relic density of dark matter that is in accord with the measured value $\Omega_{\mathrm{WMAP}}h^2\approx0.11$. This is a non-trivial accomplishment, as evidenced by how difficult it is to actually achieve in realistic WIMP models, but may certainly also be mere coincidence\footnote{A similar line of reasoning points out that the closeness of the dark matter and baryon energy densities (they differ only by a factor of $\sim 5$) suggests that the baryon asymmetry of the universe is related to (perhaps generated by) a dark matter/anti-dark matter asymmetry in the dark sector \cite{p-adm}.}. The best-studied example of Beyond the Standard Model physics that can simultaneously solve the hierarchy problem, provide for grand unification and that includes a WIMP dark matter candidate particle, is supersymmetry (SUSY) \cite{p-mssmrev}. Supersymmetric theories are built out of multiplets of particles having the same mass and gauge quantum numbers, but differing in spin. In a supersymmetric universe we would, for example, observe that there is a negatively charged scalar particle weighing $511\mev$, the supersymmetric partner of the electron. This is not what we observe, of course, so that phenomenological models that incorporate supersymmetry must also involve supersymmetry breaking terms that lift the mass of these super-partner particles beyond experimental bounds ($\sim 100\gev$ for most new particles prior to the running of the LHC but as much as $\sim 1.5 \tev$ for many such particles at the time of this writing\footnote{The LHC data set is currently $\sim 5\infb$ for collisions at $7\tev$ and $\sim 10\infb$ at $8\tev$, with many published SUSY analyses based primarily on the $7\tev$ data.}). The minimal\footnote{In the sense that the anomalies generated by the addition of chiral fermions can be cancelled.} phenomenological supersymmetric model is the Minimal Supersymmetric Standard Model (MSSM). After the inclusion of the most general set of supersymmetry breaking terms (which still allow for a satisfactory solution to the hierarchy problem) the MSSM is described by more than $100$ unknown parameters describing the details of the superpartner masses and couplings \cite{p-Chung:2003fi}. Of the new neutral particles of the MSSM, the lightest neutralino \cite{p-Jungman:1995df} is the ... .
Description
| Type of resource | text |
|---|---|
| Form | electronic; electronic resource; remote |
| Extent | 1 online resource. |
| Publication date | 2012 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Associated with | Cotta, Randel Chase |
|---|---|
| Associated with | Stanford University, Department of Physics |
| Primary advisor | Hewett, JoAnne L |
| Thesis advisor | Hewett, JoAnne L |
| Thesis advisor | Peskin, Michael Edward, 1951- |
| Thesis advisor | Silverstein, Eva, 1970- |
| Advisor | Peskin, Michael Edward, 1951- |
| Advisor | Silverstein, Eva, 1970- |
Subjects
| Genre | Theses |
|---|
Bibliographic information
| Statement of responsibility | Randel C. Cotta. |
|---|---|
| Note | Submitted to the Department of Physics. |
| Thesis | Thesis (Ph.D.)--Stanford University, 2012. |
| Location | electronic resource |
Access conditions
- Copyright
- © 2012 by Randel Chase Cotta
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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