arXiv subjects
Publications and source records attributed to Kingman Cheung.
In this talk, we summarize the collider phenomenology and recent experimental results for various models of extra dimensions, including the large extra dimensions (ADD model), warped extra dimensions (Randall-Sundrum model), TeV$^{-1}$-sized extra dimensions with gauge bosons in the bulk, universal extra dimensions, and an 5D SU(5) SUSY GUT model in AdS space.
Based on the observed neutrino mass-square differences and mixings, we derive a few general patterns for the neutrino mass matrix in the flavor basis, which can correctly describe the atmospheric and solar neutrino data. Upon the most recent result on the absolute neutrino mass bound from WMAP, we can rule out a large portion of the parameter space that claimed by the positive signal in the Heidelberg-Moscow $0νββ$ experiment. In the case when one of the majorana phases is $π$, the effective neutrino mass obtained is inconsistent with the claim.
The conventional signature for grand unified theories (GUT) is the proton decay. Recently, some models in extra dimensions or with specific discrete symmetries, which aim at solving the doublet-triplet problem, allow the color-triplet in the TeV mass region by suppressing the Yukawa couplings of the triplets to matter fermions. We study the hadronic production and detection of these TeV colored Higgs bosons as an alternative signature for GUT, which would behave like massive stable charged particles in particle detectors producing a striking signature of a charged track in the central tracking system and being ionized in the outer muon chamber. We found that the LHC is sensitive to a colored Higgs boson up to about 1.5 TeV. If the color-triplets are stable in cosmological time scale, they may constitute an interesting fraction of the dark matter.
In the Randall-Sundrum model, the radion-Higgs mixing is weakly suppressed by the effective electroweak scale. A novel feature of the existence of gravity-scalar mixing would be a sizable three-point vertex among the KK graviton, Higgs and radion. We study this vertex in the process e^+ e^- -> h phi, which is allowed only with a non-zero radion-Higgs mixing. It is shown that the angular distribution is a unique characteristic of the exchange of massive spin-2 gravitons, and the total cross section at the future e^+ e^- collider is big enough to cover a large portion of the parameter space where the LEP/LEP II data cannot constrain.
The novel feature of a Higgs-triplet representation is a nonzero tree-level coupling of $H^+ W^- Z$, which is absent in all Higgs-doublet models. We study the associated production of a singly-charged Higgs boson of the Higgs-triplet representation with a $W$ or $Z$ boson at hadron colliders, followed by the $H^+ \to W^+ Z$ decay. We find that the $2\ell+4j$ final state gives an interesting level of signal with a negligible background, plus it allows a full mass reconstruction of the charged-Higgs boson. The cover range of the charged-Higgs mass is between 110 and 200 GeV.
The scenario of light gluinos and light sbottoms was advocated to explain the discrepancy between the measured and theoretical production of $b$ quarks at the Tevatron. This scenario will have model-independent predictions for $Z\to q\bar q \tilde{g} \tilde{g}$ at the $Z^0$-pole, and $e^+ e^- \to q\bar q \tilde{g} \tilde{g}$ at LEPII. We show that the data for $Z\to q \bar q g^* \to q\bar q b \bar b$ at LEPI cannot constrain the scenario, because the ratio $Γ(Z \to q\bar q \tilde{g} \tilde{g})/ Γ(Z \to q \bar q g^* \to q\bar q b \bar b) = 0.15 - 0.04$ for $m_{\tilde{g}}= 12-16$ GeV is smaller than the uncertainty of the data. However, at LEPII the ratio $σ(e^+ e^- \to q\bar q \tilde{g} \tilde{g})/ σ(e^+ e^- \to q \bar q g^* \to q\bar q b \bar b) \simeq 0.4 - 0.2$ for $m_{\tilde{g}}= 12-16$ GeV, which may give an observable excess in $q\bar q b\bar b$ events; especially, the $4b$ events.
In models of large extra dimensions, the string and Planck scales become accessible at future colliders. When the energy scale is above the string scale or Planck scale a number of interesting phenomena occur, namely, production of stringy states, $p$-branes, string balls, black hole, etc. In this Proceedings, we summarize a recent work \cite{me0} on the production of black holes, string balls, and $p$-branes at hadronic supercolliders, and discuss their signatures.
In models of large extra dimensions, the string and Planck scales become accessible at future colliders. When the energy scale is above the string scale or Planck scale a number of interesting phenomena occur, namely, production of stringy states, p-branes, string balls, black hole, etc. In this work, we systematically study the production cross sections of black holes, string balls, and p-branes at hadronic supercolliders. We also discuss their signatures. At the energy scale between the string scale M_s and M_s/g_s^2, where g_s is the string coupling, the production is dominated by string balls, while beyond M_s/g_s^2 it is dominated by black holes. The production of a p-brane is only comparable to black holes when the p-brane wraps entirely on small extra dimensions. Rough estimates on the sensitivity reaches on the fundamental Planck scale M_D are also obtained, based on the number of raw events.
In models of large extra dimensions, the fundamental Planck scale can be as low as TeV. Thus, in hadronic collisions interesting objects like black holes, string balls, or $p$-branes can be produced. In scenarios of fat brane or universal extra dimensions, the SM particles are allowed to propagate in the extra spatial dimensions, which leads to the enhancement of production cross sections of black holes and $p$-branes. Especially, the ratio of $p$-brane cross section to the black hole cross section increases substantially, in comparison with the original confined scenario. The ratio can be as large as 105 (for the case $n=7,m=5=p=r=k$).
The original Zee model can easily accommodate a bi-maximal mixing solution for the atmospheric and solar neutrino problems. From the most recent fit to the data we obtain a set of parameters for the Zee model. However, the recent claim on a positive signal observed in a neutrinoless double beta decay, which requires a nonzero $m_{ee}$, cannot be accounted for by the Zee model. Based on the observed neutrino mass-square differences and mixings, we derive a few general patterns for the neutrino mass matrix in the flavor basis, which can correctly describe the atmospheric, solar, and $0νββ$ data. Finally, we investigate a few possible extensions to the Zee model, which can give the required mass patterns.
We compare the tau neutrino flux arising from the galaxy and the earth atmosphere for 10^3 < E/GeV < 10^11. The intrinsic and oscillated tau neutrino fluxes from both sources are calculated. The intrinsic galactic ν_τ flux (E > 10^3 GeV) is calculated by considering the interactions of high-energy cosmic-rays with the matter present in our galaxy, whereas the oscillated galactic ν_τ flux is coming from the oscillation of the galactic ν_μ flux. For the intrinsic atmospheric ν_τ flux, we extend the validity of a previous calculation from E < 10^6 GeV up to E < 10^11 GeV. The oscillated atmospheric ν_τ flux is, on the other hand, rather suppressed. We find that, for 10^3 < E/GeV < 5\cdot 10^7, the oscillated ν_τ flux along the galactic plane dominates over the maximal intrinsic atmospheric ν_τ flux, i.e., the flux along the horizontal direction. We also briefly mention the presently envisaged prospects for observing these high-energy tau neutrinos.
Black hole (BH) production at colliders is possible when the colliding energy is above the Planck scale, which can effectively be at TeV scale in models of large extra dimensions. In this work, we study the production of black holes at colliders and discuss the possible signatures. We point out the ``$ij\to {\rm BH}+ $ others'' subprocesses, in which the BH and other SM particles are produced with a large transverse momentum. When the BH decays, it gives a signature that consists of particles of high multiplicity in a boosted spherical shape on one side of the event and a few number of high $p_T$ partons on the other side, which provide very useful tags for the event.
In theories with the standard model gauge bosons propagating in inverse-TeV-size extra dimensions, their Kaluza-Klein states interact with the rest of the SM particles confined to the 3-brane. We look for possible signals for this interaction in the present high-energy collider data, and estimate the sensitivity offered by the next generation of collider experiments. Based on the present data from the LEP 2, Tevatron, and HERA experiments, we set a lower limit on the extra dimension compactification scale Mc > 6.8 TeV at the 95% confidence level (dominated by the LEP 2 results) and quote expected sensitivities in the Tevatron Run 2 and at the LHC.
The recent measurement on the muon anomalous magnetic moment $a_μ$ shows a $2.6σ$ deviation from the standard model value. We show that it puts strong constraints on the parameter space of the two-higgs doublet model (2HDM) II. The dominant contribution of the Higgses comes at the two-loop level, and in order to explain the data it favors a pseudoscalar $A$ with a light mass range and a large $\tanβ$. At 95% C.L., upper limit for $m_{\scriptscriptstyle A}$ is $29 (55) (85)$ GeV for $\tanβ=30 (45) (60)$, and $\tan β$ is bounded below at 17. This is in sharp contrast to the conclusion one draws from considering one-loop Higgs contributions alone. Finally, we also discuss the role of the Higgs contributions in the minimal supersymmetric standard model.
Starting from the new sources of $LL$- and $LR$- scalar (slepton) mixings due to R-parity violation, we discuss the structure of lepton-flavor violation focusing on the radiative decay of muon into electron. Using an optimal parametrization, we give the general formulae for the one-loop contributions. from which we discuss all combinations of R-parity violating parameters having possibly a substantial contribution to the $μ\to e γ$ decay width. An exact numerical study is performed to obtain explicit bounds on the parameters under the present experimental limit. The most interesting one involves a combination of bilinear and trilinear couplings.
We update the global constraint on four-fermion $ee q q$ contact interactions. In this update, we included the published data of H1 and ZEUS for the 94--96 run in the $e^+ p$ mode and the newly published data of H1 for the 1999 run in the $e^- p$ mode. Other major changes are the new LEPII data on hadronic cross sections above 189 GeV, and the atomic parity violation measurement on Cesium because of a new and improved atomic calculation, which drives the data within $1σ$ of the standard model value. The global data do not show any evidence for contact interactions, and we obtain 95% C.L. limits on the compositeness scale. A limit of $Λ^{eu}_{LL+(-)} > 23 (12.5)$ TeV is obtained. Implications to models of leptoquarks and extra Z bosons are examined.
We study the sensitivity reach of $0.5-2$ TeV $e^+ e^-$ linear colliders in the context of the ADD model, in which gravity becomes strong at TeV scale. We include the real emission channel $e^+ e^- \to γG$, as well as the virtual-graviton-exchange channels $e^+ e^- \to γγ, q\bar q, μ^+μ^-, τ^+ τ^-, e^+ e^-$. Assuming no excess of events over the standard model predictions, we obtain the lower 95% C.L. limits on the effective Planck scale. These limits are better than those obtained in the Run II's of the Tevatron and comparable to those of the LHC.
The recent measurement on the muon anomalous magnetic moment $a_μ$ shows a $2.6σ$ deviation from the standard model value. We show that it puts an interesting bound on the mass of the second generation leptoquarks. To account for the data the leptoquark must have both the left- and right-handed couplings to the muon. Assuming that the couplings have electromagnetic strength, the mass is restricted in the range $0.7 {\rm TeV} < M_{\rm LQ} < 2.2$ TeV at 95% C.L. We also discuss constraints coming from other low energy and high energy experiments. If the first-second-generation universality is assumed, constraints come from the atomic parity violation and charged-current universality. We show that coexistence with other leptoquarks can satisfy these additional constraints and at the same time do not affect the $a_μ$.