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Register a new userCollider Implications of Models with Extra Dimensions
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We discuss the collider signals of large extra dimensions in which gravity as well as the SM particles propagate into the extra dimensions. These signals arise either from the production of Kaluza-Klein excitations of the SM particles and their subsequent decay, or from their off-shell exchanges. Depending on the scenario, the dominant signals are two high p_T jets + missing energy, two high p_T photons + missing energy and soft leptons, or a combination of photon + jet and missing energy. For the scenario in which only the gauge bosons propagate into the extra dimensions, Tevatron Run II (LHC) can observe such signals up to a compactification scale of about 2 TeV (7 TeV), while for the case of universal extra dimensions, the corresponding limits are about 600 GeV (3 TeV) respectively.
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We consider the universal extra dimensions scenario of Appelquist, Cheng, and Dobrescu, in which all of the SM fields propagate into one extra compact dimension, estimated therein to be as large as $sim (350$ GeV$)^{-1}$. Tree-level KK number conservation dictates that the associated KK excitations can not be singly produced. We calculate the cross sections for the direct production of KK excitations of the gluon, $gs$, and two distinct towers of quarks, qs and $qt$, in proton-antiproton collisions at the Tevatron Run I and II energies in addition to proton-proton collisions at the Large Hadron Collider energy. The experimental signatures for these processes depend on the stability of the lowest-lying KK excitations of the gluons and light quarks. We find that the Tevatron Run I mass bound for KK quark and gluon final states is about 350--400 GeV, while Run II can push this up to 450--500 GeV at its initial luminosity and 500--550 GeV if the projected final luminosity is reached. The LHC can probe much further: The LHC will either discover UED KK excitations of quarks and gluons or extend the mass limit to about 3 TeV.
Many models that include small extra space dimensions predict graviton states which are well separated in mass, and which can be detected as resonances in collider experiments. It has been shown that the ATLAS detector at the Large Hadron Collider can identify such narrow states up to a mass of 2080 GeV in the decay mode G->ee, using a conservative model. This work extends the study of the ee channel over the full accessible parameter space, and shows that the reach could extend as high as 3.5 TeV. It then discusses ways in which the expected universal coupling of the resonance can be confirmed using other decay modes. In particular, the mode G-> di-photons is shown to be measurable with good precision, which would provide powerful confirmation of the graviton hypothesis. The decays G-> mu mu, WW, ZZ and jet--jet are measurable over a more limited range of couplings and masses. Using information from mass and cross-section measurements, the underlying parameters can be extracted. In one test model, the size of the extra dimension can be determined to a precision in length of 7x10^-33 m.
We investigate preheating in a higher-dimensional generalized Kaluza-Klein theory with a quadratic inflaton potential $V(phi)=frac12 m^2phi^2$ including metric perturbations explicitly. The system we consider is the multi-field model where there exists a dilaton field $sigma$ which corresponds to the scale of compactifications and another scalar field $chi$ coupled to inflaton with the interaction $frac12 g^2phi^2chi^2+tilde{g}^2phi^3chi$. In the case of $tilde{g}=0$, we find that the perturbation of dilaton does not undergo parametric amplification while the $chi$ field fluctuation can be enhanced in the usual manner by parametric resonance. In the presence of the $tilde{g}^2phi^3chi$ coupling, the dilaton fluctuation in sub-Hubble scales is modestly amplified by the growth of metric perturbations for the large coupling $tilde{g}$. In super-Hubble scales, the enhancement of the dilaton fluctuation as well as metric perturbations is weak, taking into account the backreaction effect of created $chi$ particles. We argue that not only is it possible to predict the ordinary inflationary spectrum in large scales but extra dimensions can be held static during preheating in our scenario.
We compute the couplings of the zero modes and first excited states of gluons, $W$s, $Z$ gauge bosons, as well as the Higgs, to the zero modes and first excited states of the third generation quarks, in an RS Gauge-Higgs unification scenario based on a bulk $SO(5)times U(1)_X$ gauge symmetry, with gauge and fermion fields propagating in the bulk. Using the parameter space consistent with electroweak precision tests and radiative electroweak symmetry breaking, we study numerically the dependence of these couplings on the parameters of our model. Furthermore, after emphasizing the presence of light excited states of the top quark, which couple strongly to the Kaluza Klein gauge bosons, the associated collider phenomenology is analyzed. In particular, we concentrate on the possible detection of the first excited state of the top, $t^1$, which tends to have a higher mass than the ones accessible via regular QCD production processes. We stress that the detection of these particles is still possible due to an increase in the pair production of $t^1$ induced by the first excited state of the gluon, $G^1$.
We study the capability of the international linear collider (ILC) to probe extra dimensions via the seesaw mechanism. In the scenario we study, heavy Kaluza-Klein neutrinos generate tiny neutrino masses and, at the same time, have sizable couplings to the standard-model particles. Consequently, a Kaluza-Klein tower of heavy neutrinos (N) can be produced and studied at the ILC through the process: e+e- -> vN followed by N -> Wl decay. We show that the single lepton plus two-jets final states with large missing energy from this signal process will provide a good opportunity to measure the masses and cross sections of Kaluza-Klein neutrinos up to the third level. Furthermore, the neutrino oscillation parameters can be extracted from the flavor dependence of the lowest-mode signals, which give us information about the origin of low-energy neutrino masses.
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