Phenomenology of Universal Extra Dimensions

In this proceeding, the phenomenology of Universal Extra Dimensions (UED), in which all the Standard Model fields propagate, is explored. We focus on models with one universal extra dimension, compactified on an $S_1/Z_2$ orbifold. We revisit calculations of Kaluza-Klein (KK) dark matter without an assumption of the KK mass degeneracy including all possible coannihilations. We then contrast the experimental signatures of low energy supersymmetry and UED.Comment: 4 pages, 6 figures, to appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 200

Higgs and Z-boson Signatures of Supersymmetry

In supersymmetric theories of nature the Higgsino fermionic superpartner of the Higgs boson can arise as the lightest standard model superpartner depending on the couplings between the Higgs and supersymmetry breaking sectors. In this letter the production and decay of Higgsino pairs to the Goldstone fermion of supersymmetry breaking and the Higgs boson, h, or gauge bosons, Z or $\gamma$ are considered. Relatively clean di-boson final states, hh, $h \gamma$, hZ, $Z \gamma$, or ZZ, with a large amount of missing energy result. The latter channels provide novel discovery modes for supersymmetry at high energy colliders since events with Z bosons are generally rejected in supersymmetry searches. In addition, final states with real Higgs bosons can potentially provide efficient channels to discover and study a Higgs signal at the Fermilab Tevatron Run II.Comment: 11 pages, LaTeX, 3 figure

A general method for determining the masses of semi-invisibly decaying particles at hadron colliders

We present a general solution to the long standing problem of determining the masses of pair-produced, semi-invisibly decaying particles at hadron colliders. We define two new transverse kinematic variables, $M_{CT_\perp}$ and $M_{CT_\parallel}$, which are suitable one-dimensional projections of the contransverse mass $M_{CT}$. We derive analytical formulas for the boundaries of the kinematically allowed regions in the $(M_{CT_\perp},M_{CT_\parallel})$ and $(M_{CT_\perp},M_{CT})$ parameter planes, and introduce suitable variables $D_{CT_\parallel}$ and $D_{CT}$ to measure the distance to those boundaries on an event per event basis. We show that the masses can be reliably extracted from the endpoint measurements of $M_{CT_\perp}^{max}$ and $D_{CT}^{min}$ (or $D_{CT_\parallel}^{min}$). We illustrate our method with dilepton $t\bar{t}$ events at the LHC.Comment: thoroughly revised; all new figures; new results on pages 3 and 4; new illustrative example; includes detector simulation. 4 pages, 6 figures, uses revtex and axodra

How to prove that the LHC did not discover dark matter

If the LHC is able to produce dark matter particles, they would appear at the end of cascade decay chains, manifesting themselves as missing transverse energy. However, such "dark matter candidates" may decay invisibly later on. We propose to test for this possibility by studying the effect of particle widths on the observable invariant mass distributions of the visible particles seen in the detector. We consider the simplest non-trivial case of a two-step two-body cascade decay and derive analytically the shapes of the invariant mass distributions, for generic values of the widths of the new particles. We demonstrate that the resulting distortion in the shape of the invariant mass distribution can be significant enough to measure the width of the dark matter "candidate", ruling it out as the source of the cosmological dark matter.Comment: 5 pages, 5 figure

Extra Dimensions at the LHC

We discuss the motivation and the phenomenology of models with either flat or warped extra dimensions. We describe the typical mass spectrum and discovery signatures of such models at the LHC. We also review several proposed methods for discriminating the usual low-energy supersymmetry from a model with flat (universal) extra dimensions. (For the official website of the book, see http://cambridge.org/us/catalogue/catalogue.asp?isbn=9780521763684 .)Comment: 33 pages, 17 figure

Light Axion within the Next-to-Minimal Supersymmetric Standard Model

We analyze the Higgs sector in the Next-to-Minimal Supersymmetric Standard Model, emphasizing the possibility of a light CP-odd scalar (axion) in the spectrum. We compute the coupling of the Standard-Model-like Higgs boson to a pair of axions, and show that it can be large enough to modify the Higgs branching fractions, with a significant impact on the Higgs searches. We delineate the range of parameters relevant for this scenario, and also derive analytic expressions for the scalar masses and couplings in two special cases - a decoupling limit where all scalars other than the axion are heavier than the Standard-Model-like Higgs boson, and the large tan beta limit.Comment: 28 pages, 6 figure