Searching for Large Extra Dimensions

     The idea that the Universe may contain higher dimensions goes back over a century. However historically these dimensions were constrained to be on the subatomic scale, and therefore very difficult to detect. In 1998, several teams of researchers independently demonstrated that in certain models, these dimensions could be as large as a few millimeters without contadicting experiments (although in the decade since the bounds have been tightened to the micrometer or even nanometer scale depending on the model). This loophole in the study of extra dimensions arises by requiring all known forms of matter and all forces except for gravity to exist on a three dimensional surface called a brane, while gravity can exist in all dimensions.
     Since gravity exists in higher dimensions than the other forces, it is dilluted and therefore explains why it appears to us that gravity is orders of magnitude weaker than the other three forces. The existence of higher dimensions can also be used to solve a number of other problems in modern physics, and as such it has become an important field of study.

     My contribution to the field of extra dimensions is in two research articles:

"Galactic Positrons as a Probe of Large Extra Dimensions".C.Bird . Nov 2008. 12pp. e-Print: arXiv:0812.4572 [hep-ph]

Cosmological Bounds on Large Extra Dimensions from Non-Thermal Production of Kaluza-Klein Modes
  -R.Allahverdi, C. Bird, S. Groot-Nibbelink, M. Pospelov,
  TRI-PP-03-07, UVIC-TH-05-03, May 2003. 17pp. hep-ph/0305010, Phys.Rev.D69:045004,2004


which place strict constraints on the size of the higher dimensions. The first of these papers demonstrates that the effects of the extra dimensions in the very early universe can cause the destruction of lithium that was produced in the first few minutes after the Big Bang. By comparing the predictions of this model with observations of primordial lithium abundances, the properties of extra dimensions can be constrained. In the second article, I demonstrated that the extra dimensions will produce a significant abundance of positrons and electrons in the center of our galaxy. The positrons then annihilate into gamma-radiation which can be detected by satellite experiments. Again, comparison of the observed gamma-radiation with my predictions produces strong constraints on the size of the dimensions.

Minimal Models of Dark Matter

     For many decades, there has been growing evidence that most of the matter in the Universe is of some unknown form. Recent astrophysics experiments have placed the amount at ~84% of matter in the Universe being nonbaryonic (meaning it isn't formed of elements or atoms) and having littl or no interaction with light. However this dark matter can only be detected through its gravity, and not in any other way.
    Currently there are a large number of proposals for what dark matter could be, with some having other motivations in physics and some being just interesting ideas. As a result, it is difficult to study the generic properties of dark matter, since every model has different properties.
     One option is to develop a set of minimal models of dark matter, in which only a small amount of new physics is introduced - in fact only the minimal amount of new theory which can still explain all the data we have on dark matter. The simplest of these includes only one new particle, which is defined only by its mass and the strength of its interactions with a particle in the Standard Model of physics known as the Higgs boson. (This interaction is required to allow the dark matter abundance to match measurements from experiments)
   
     My contribution to developing these minimal models is in the three research papers,

"The Minimal Model of Radion Mediated Dark Matter". Chris Bird . Dec 2008. 12pp. e-Print: arXiv:0812.4563 [hep-ph]

"Dark matter pair-production in b -> s transitions"  -C. Bird, R. Kowalewski, M. Pospelov. UVIC-TH-06-01, 
      Jan 2006. 23pp.
 ,   hep-ph/0601090,  
Mod.Phys.Lett.A21:457-478,2006

"Search for dark matter in b -> s transitions with missing energy " - Chris Bird, Paul Jackson, Robert Kowalewski, Maxim Pospelov ,
     UVIC-TH-04-02, Jan 2004. 4pp.,   hep-ph/0401195,
 Phys.Rev.Lett.93:201803,2004


and in my doctoral dissertation,

The Early Universe as a Probe of New Physics   -PhD Dissertation, University of Victoria, October 2008


In these four publications, I introduced six new minimal models and updated the constraints and predicted experimental signatures of a seventh model (Model #1 in the following list). These models are:

   1. The Minimal Model of Dark Matter: This is the most basic model that can fully explain dark matter. A spinless, massive particle is introduced which can interact with the Standard Model through the exchange of a Higgs boson. In spite of its simplicity, this model has been successful in explaining the observed dark matter abundance, while also avoiding all existing constraints from particle collider experiments and dedicated dark matter searches.

   1b. The Next-to-Minimal Model: This model is a slight variation on Model #1, in which a second new particle is introduced to mediate the interactions between the dark matter and the Standard model particles. However for most reasonable values of the masses and other parameters, the two models are identical and therefore this model is not studied in much detail anymore.

   2. The Minimal Model with a Two-Higgs Doublet: The minimal model of dark matter assumes that the only interaction between the dark matter particle and the Standard Model particles is through a mediator, known as the Higgs boson. However the Higgs has never been observed directly, and there are motivations in physics for using two Higgs particles instead of one. For this reason, it is also possible and advisable to develop a minimal model in which the dark matter is coupled to one or both of these Higgs particles. (This minimal model is often treated as three minimal models, depending on which Higgs is coupled to. In general, it can couple to both Higgses but such a model becomes more difficult to study)

   3. The Minimal Model of Fermionic Dark Matter: One potential concern with the Minimal Models given above are that they all use a spinless particle for the dark matter candidate. However of the known fundamental particles in nature, there are 24 with a spin of 1/2, and 5 particles with a spin of 1. No spinless particles have ever been observed in nature. For that reason, we also introduce the minimal model of fermionic dark matter, in which the dark matter candidate is given a spin of 1/2.

   4. The Minimal Model of 2HDM Fermionic Dark Matter: This model is simply the combination of Model #2 and Model #3, with the modification inherent in each combined into a single model.

   5. The Minimal Model with a Higgsino:

   6. The Minimal Model with Warped Extra Dimensions: The other minimal models all require the dark matter to interact with the Standard Model through the exchange of the Higgs particle. In this model, the dark matter has no interactions with the Standard Model making it even more minimal. Instead, it relies on recent models of extra dimensions in which gravity can be made orders of magnitude stronger in small regions, and the dark matter abundance is maintained by allowing the particles to decay through this strong gravity.

Light Dark Matter

     In the field of dark matter research, it has become common to study very heavy particles with masses a few hundred times higher than that of a hydrogen atom. The reasons for this are primarily that experiments have already excluded most - but not all - lighter candidates. In fact in the seven minimal models listed above, only two are excluded for lighter particles.
     Light dark matter, with masses comparable to a hydrogen atom, actually have several benefits. A number of published articles have demonstrated how light dark matter could explain anomalous levels of positrons in our galaxy, explain the source of certain high energy gamma-radiation that has been detected in the galaxy, and can explain some of the preliminary data from dedicated dark matter searches such as the DAMA and CDMS experiments.

     In the two articles:

"Dark matter pair-production in b -> s transitions"  -C. Bird, R. Kowalewski, M. Pospelov. UVIC-TH-06-01, 
      Jan 2006. 23pp.
 ,   hep-ph/0601090,  
Mod.Phys.Lett.A21:457-478,2006

"Search for dark matter in b -> s transitions with missing energy " - Chris Bird, Paul Jackson, Robert Kowalewski, Maxim Pospelov ,
     UVIC-TH-04-02, Jan 2004. 4pp.,   hep-ph/0401195,
 Phys.Rev.Lett.93:201803,2004


we demonstrated that five of the seven minimal models allow for light dark matter, and that furthermore existing data from experiments at the SLAC collider can be used to probe for light dark matter.

Catalyzed Big Bang Nucleosynthesis

     In addition to dark matter, it is also possible that the early Universe contains a very small abundance of heavy charged particles which are currently undiscovered. Collider experiments and experiments searching for heavier versions of hydrogen atoms have effectively ruled out and significant amount of stable charged particles, but there are no constraints on very heavy particles that live for several hours before decaying.

     My work on this topic is published in the article:

In which we demonstrated that CHAMPs can also deplete the primordial lithium-7 abundance from the predicted levels to the observed levels, providing even stronger motivation for the existence of such massive charged particles.

Low Energy Effective Field Theories

     Although the fundamental properties of nuclei are understood in theory, there are problems with the calculations that cannot be easily resolved. In particular, at low energies the quarks that form into protons and neutrons interact so strongly that any calculation of the properties of the nucleons becomes untenable. Therefore it is common to work with effective theories instead, in which the quarks are ignored and the protons and neutrons treated as being fundamental particles with no internal structure.

    As my Master's thesis,

 I reviewed two methods of calculating the interactions of photons and of a class of particles called mesons, with neutrons and protons, and produced the most accurate calculation of the reaction rates for certain single nucleon reactions.


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