Search Results (103 total)

We study the potential LHC discovery of the Z₁ Kaluza-Klein gauge boson unitarizing W_L⁺W_L^- scattering amplitude. In particular, we explore the decay mode Z₁→tt̅ , along with Z₁→W⁺W^-, without specifying the branching fractions. We propose to exploit the associated production pp→WZ₁ and select the final state of like-sign dileptons plus multijets and large missing energy. We conclude that it is possible to observe the Z₁ resonance at a 5σ level with an integrated luminosity of 100  fb^-1 at the LHC up to 650 GeV for a dominant WW channel, and 560 GeV for a dominant tt̅ channel.

We study the possibility to test the type I seesaw mechanism for neutrino masses at the CERN Large Hadron Collider. The inclusion of three generations of right-handed neutrinos (N_i) provides an attractive option of gauging the B-L accidental symmetry in the standard model (as well as an extended symmetry X=Y-5(B-L)/4). The production mechanisms for the right-handed neutrinos through the Z^′ gauge boson in the U(1)_B-L and U(1)_X extensions of the standard model are studied. We discuss the flavor combinations of the charged leptons from the decays of N_i in the ΔL=2 channels. We find that the clean channels with dilepton plus jets and possible secondary vertices of the N decay could provide conclusive signals at the LHC in connection with the hierarchical pattern of the light neutrino masses and mixing properties within the type I seesaw mechanism.

We study signals at the LHC for the Kaluza-Klein (KK) excitations of electroweak charged gauge bosons in the framework of the standard model (SM) fields propagating in the bulk of a warped extra dimension. Such a scenario can solve both the Planck-weak and flavor hierarchy problems of the SM. There are two such charged states in this scenario with couplings to light quarks and leptons being suppressed relative to those in the SM, whereas the couplings to top/bottom quarks are enhanced, similar to the case of electroweak neutral gauge bosons previously studied. However, unlike the case of electroweak neutral gauge bosons, there is no irreducible QCD background (including pollution from possibly degenerate KK gluons) for decays to top+bottom final states so that this channel is useful for the discovery of the charged states. Moreover, decays of electroweak charged gauge bosons to longitudinal W, Z and Higgs are enhanced just as for the neutral bosons. However, unlike for the neutral gauge bosons, the purely leptonic (and hence clean) decay mode of the WZ is fully reconstructible so that the ratio of the signal to the SM (electroweak) background can potentially be enhanced by restricting to the resonance region more efficiently. We show that such final states can give sensitivity to 2(3) TeV masses with an integrated luminosity of 100(300)  fb^-1. We emphasize that improvements in discriminating a QCD jet from a highly boosted hadronically decaying W, and a highly boosted top jet from a bottom jet will enhance the reach for these KK particles, and that the signals we study for the warped extra dimensional model might actually be applicable also to a wider class of nonsupersymmetric models of electroweak symmetry breaking.

Recent developments in models with warped extra dimensions have opened new possibilities for vectorlike quark studies at hadron colliders. These new vectorlike quarks can mix sizably with light standard model quarks without violating low energy constraints. We perform a model-independent analysis to determine the Tevatron reach in the search for new quarks. We find that the Tevatron has great potential to observe such quarks via their electroweak single production due to their mixing with valence quarks. With 4(8)  fb^-1 integrated luminosity, one may reach a 5σ statistical significance for a heavy quark of mass 580(630)  GeV if the heavy quark-Standard Model quark mixing parameter is order one.

We have investigated scanning tunneling microscope-induced luminescence (STML) from porphyrin molecules by varying the tip (PtIr, Ag, and Au)/substrate (Pt, Ag, Au, and indium tin oxide) combinations. Strong molecular fluorescence by highest-occupied molecular orbital and lowest-unoccupied molecular orbital transition comparable to plasmon-mediated light is emitted only when both the substrate and the tip are metals but not in other cases. Along with calculations of relative electromagnetic-field powers in the tip-substrate gaps, the enhancement of STML from molecules can be interpreted in terms of the strong plasmon field and its confinement in an STM cavity. We also find rather strong energy-forbidden fluorescence of porphyrin in an Au-tip/porphyrin/Au cavity that occurs under the extremely strong field in the plasmonic nanocavity.

We report neutron-scattering studies of the spin correlations of the geometrically frustrated pyrochlore Tb₂Mo₂O₇ using single-crystal samples. This material undergoes a spin-freezing transition below T_g≃24 K, similar to Y₂Mo₂O₇, and has little apparent chemical disorder. Diffuse elastic peaks are observed at low temperatures, indicating short-range ordering of the Tb moments in an arrangement where the Tb moments are slightly rotated from the preferred directions of the spin-ice structure. In addition, a Q⃗-independent signal is observed which likely originates from frozen but completely uncorrelated Tb moments. Inelastic measurements show the absence of sharp peaks due to crystal-field excitations. These data show how the physics of the Tb sublattice responds to the glassy behavior of the Mo sublattice with the associated effects of lattice disorder.

The Large Hadron Collider could be a discovery machine for the neutrino mass pattern and its Majorana nature in the context of a well-motivated TeV scale Type II seesaw model. This is achieved by identifying the flavor structure of the lepton-number violating decays of the charged Higgs bosons. The observation of either H⁺→τ⁺ν̅ or H⁺→e⁺ν̅ will be particularly robust to determine the neutrino spectra since they are independent of the unknown Majorana phases, which could be probed via the H⁺⁺→e_i⁺e_j⁺ decays. In a less favorable scenario when the leptonic channels are suppressed, one needs to observe the decays H⁺→W⁺H₁ and H⁺→tb̅ to confirm the triplet-doublet mixing that implies the Type II relation. The associated production H^±±H^∓ is crucial in order to test the triplet nature of the Higgs field.

We demonstrate how to systematically test a well-motivated mechanism for neutrino mass generation (type II seesaw) at the LHC, in which a Higgs triplet is introduced. In the optimistic scenarios with a small Higgs triplet vacuum expectation value v_Δ<10^-4  GeV, one can look for clean signals of lepton-number violation in the decays of doubly charged (H^±±) and singly charged (H^±) Higgs bosons to distinguish the normal hierarchy (NH), the inverted hierarchy (IH), and the quasidegenerate (QD) spectrum for the light neutrino masses. The observation of either H⁺→τ⁺ν̅ or H⁺→e⁺ν̅ will be particularly robust for the spectrum test since they are independent of the unknown Majorana phases. The H⁺⁺ decays moderately depend on a Majorana phase Φ₂ in the NH, but sensitively depend on Φ₁ in the IH. In a less favorable scenario v_Δ>2×10^-4  GeV, when the leptonic channels are suppressed, one needs to observe the decays H⁺→W⁺H₁ and H⁺→tb̅ to confirm the triplet-doublet mixing which in turn implies the existence of the same gauge-invariant interaction between the lepton doublet and the Higgs triplet responsible for the neutrino mass generation. In the most optimistic situation, v_Δ∼10^-4  GeV, both channels of the lepton pairs and gauge boson pairs may be available simultaneously. The determination of their relative branching fractions would give a measurement for the value of v_Δ.

We consider a new interaction between a heavy Majorana neutrino (N) and a charged Higgs boson (H^±), and show that it can have drastic implications on lepton-number-violating (LNV) signal of same-sign dileptons at the CERN LHC. The LNV signal of heavy Majorana neutrinos previously considered at the LHC, pp→ℓ⁺N→ℓ⁺ℓ⁺W^-, may be overwhelmed by pp→ℓ⁺N→ℓ⁺ℓ⁺H^-. With the subsequent decays H^-→t̅ b or H^-→W^-H⁰, the heavy Majorana neutrino production leads to the spectacular events of ℓ⁺ℓ⁺bb̅ +2 jets. We also explore the case m_N<m_H⁺, where the decay H⁺→ℓ⁺N can become the dominant N-production mechanism at the LHC. In particular, we show that the process gb→tH^- followed by t→bW⁺ and H^-→ℓ^-N→ℓ^-ℓ^-W⁺ could lead to another type of spectacular events of ℓ^-ℓ^-b+4 jets.

We study top-quark pair production to probe new physics at the CERN Large Hadron Collider. We propose reconstruction methods for tt̅ semileptonic events and use them to reconstruct the tt̅ invariant mass. The angular distribution of top quarks in their c.m. frame can determine the spin and production subprocess for each new physics resonance. Forward-backward asymmetry and CP-odd variables can be constructed to further delineate the nature of new physics. We parametrize the new resonances with a few generic parameters and show high invariant mass top pair production may provide an early indicator for new physics beyond the standard model.

We study signals at the Large Hadron Collider (LHC) for Kaluza-Klein (KK) excitations of the electroweak gauge bosons in the framework with the standard model (SM) gauge and fermion fields propagating in a warped extra dimension. Such a framework addresses both the Planck-weak and flavor hierarchy problems of the SM. Unlike the often studied Z^′ cases, in this framework, there are three neutral gauge bosons due to the underlying SU(2)_L×SU(2)_R×U(1)_X gauge group in the bulk. Furthermore, couplings of these KK states to light quarks and leptons are suppressed, whereas those to top and bottom quarks are enhanced compared to the SM gauge couplings. Therefore, the production of light quark and lepton states is suppressed relative to other beyond the SM constructions, and the fermionic decays of these states are dominated by the top and bottom quarks, which are, though, overwhelmed by KK gluons dominantly decaying into them. However, as we emphasize in this paper, decays of these states to longitudinal W, Z and Higgs are also enhanced similarly to the case of top and bottom quarks. We show that the W, Z and Higgs final states can give significant sensitivity at the LHC to ∼2(3)  TeV KK scale with an integrated luminosity of ∼100  fb^-1 (∼1  ab^-1). Since current theoretical framework(s) favor KK masses ≳3  TeV, a luminosity upgrade of LHC is likely to be crucial in observing these states.

We study the pair production of doubly charged Higgs bosons at the CERN Large Hadron Collider (LHC), assuming the doubly charged Higgs to be part of an SU(2)_L triplet which generates Majorana masses for left-handed neutrinos. Such pair production has the advantage that it is not constrained by the triplet vacuum expectation value, which tends to make the single production rate rather small. We point out that, in addition to the Drell-Yan production mechanism, two-photon processes also contribute to H⁺⁺H^-- production at a level comparable to the QCD corrections to the Drell-Yan channel. Decays of the doubly charged Higgs into both the ℓ⁺ℓ⁺ and W⁺W⁺ modes are studied in detail to optimize the signal observation over the backgrounds. Doubly charged scalars should be observable at the LHC with 300  fb^-1 integrated luminosity in the ℓ^±ℓ^± channel up to the mass range of 1 TeV even with a branching fraction of about 60%, and in the W^±W^± channel up to a mass of 700 GeV. Such a doubly charged Higgs, if it is a member of a triplet that generates neutrino masses, cannot be long-lived on the scale of collider detectors although it might lead to a displaced secondary vertex during its decay if it is lighter than about 250 GeV.

The Majorana nature of neutrinos may only be experimentally verified via lepton-number violating processes involving charged leptons. We explore the ΔL=2 like-sign dilepton production at hadron colliders to search for signals of Majorana neutrinos. We find significant sensitivity for resonant production of a Majorana neutrino in the mass range of 10–80 GeV at the current run of the Tevatron with 2  fb^-1 integrated luminosity and in the range of 10–400 GeV at the CERN LHC with 100  fb^-1.

We study the sensitivity of testing the anomalous gauge couplings g_HVV’s of the Higgs boson in the formulation of linearly realized gauge symmetry via the processes γγ→ZZ and γγ→WWWW at polarized and unpolarized photon colliders based on e⁺e^- linear colliders of c.m. energies 500 GeV, 1 TeV, and 3 TeV. Signals beyond the standard model (SM) and SM backgrounds are carefully studied. We propose certain kinematic cuts to suppress the standard model backgrounds. For an integrated luminosity of 1  ab^-1, we show that (a) γγ→ZZ can provide a test of g_Hγγ to the 3σ sensitivity of O(10^-3–10^-2)  TeV^-1 at a 500 GeV ILC, and O(10^-3)  TeV^-1 at a 1 TeV ILC and a 3 TeV CLIC, and (b) γγ→WWWW at a 3 TeV CLIC can test all the anomalous couplings g_HVV’s to the 3σ sensitivity of O(10^-3–10^-2)  TeV^-1.

We investigate the sources of neutrino mass generation in little Higgs theories, by confining ourselves to the “littlest Higgs” scenario. Our conclusion is that the most satisfactory way of incorporating neutrino masses is to include a lepton-number violating interaction between the scalar triplet and lepton doublets. The tree-level neutrino masses generated by the vacuum expectation value of the triplet are found to dominate over contributions from dimension-five operators so long as no additional large lepton-number violating physics exists at the cutoff scale of the effective theory. We also calculate the various decay branching ratios of the charged and neutral scalar triplet states, in regions of the parameter space consistent with the observed neutrino masses, hoping to search for signals of lepton-number violating interactions in collider experiments.

We consider four lepton-number violating (L̸) processes: (a) neutrinoless double-beta decay (0νββ), (b) ΔL=2 tau decays, (c) ΔL=2 rare meson decays and (d) nuclear muon-positron conversion. In the absence of exotic L̸ interactions, the rates for these processes are determined by effective neutrino masses ⟨m⟩_ℓ1ℓ2, which can be related to the sum of light neutrino masses, the neutrino mass-squared differences, the neutrino mixing angles, a Dirac phase and two Majorana phases. We sample the experimentally allowed ranges of ⟨m⟩_ℓ1ℓ2 based on neutrino oscillation experiments as well as cosmological observations, and obtain a stringent upper bound ⟨m⟩_ℓ1ℓ2≲0.14  eV. We then calculate the allowed ranges for ⟨m⟩_ℓ1ℓ2 from the experimental rates of direct searches for the above ΔL=2 processes. Comparing our calculated rates with the currently or soon available data, we find that only the 0νββ experiment may be able to probe ⟨m⟩_ee with a sensitivity comparable to the current bound. Muon-positron conversion is next in sensitivity, while the limits of direct searches for the other ΔL=2 processes are several orders of magnitude weaker than the current bounds on ⟨m⟩_ℓ1ℓ2. Any positive signal in those direct searches would indicate new contributions to the L̸ interactions beyond those from three light Majorana neutrinos.

We compute the inclusive and differential cross sections for the associated production of a top quark along with a charged Higgs boson at hadron colliders to next-to-leading order (NLO) in perturbative quantum chromodynamics (QCD) and in supersymmetric QCD. For small Higgs boson masses we include top-quark pair production diagrams with subsequent top-quark decay into a bottom quark and a charged Higgs boson. We compare the NLO differential cross sections obtained in the bottom parton picture with those for the gluon-initiated production process and find good agreement. The effects of supersymmetric loop contributions are explored. Only the corrections to the Yukawa coupling are sizable in the potential discovery region at the CERN Large Hadron Collider (LHC). All expressions and numerical results are fully differential, permitting selections on the momenta of both the top quark and the charged Higgs boson.

A Higgs boson lighter than 2m_W that decays mostly into invisible channels (e.g., dark matter particles) is theoretically well-motivated. We study the prospects for discovery of such an invisible Higgs, h_inv, at the LHC and the Tevatron in three production modes: (1) in association with a Z, (2) through weak boson fusion (WBF), and (3) accompanied by a jet. In the Z+h_inv channel, we show that the LHC can yield a discovery signal above 5σ with 10   fb^-1 of integrated luminosity for a Higgs mass of 120 GeV. With 30   fb^-1 the discovery reach extends up to a Higgs mass of 160 GeV. We also study the extraction of the h_inv mass from production cross sections at the LHC, and find that combining WBF and Z+h_inv allows a relatively model-independent determination of the h_inv mass with an uncertainty of 35–50 GeV (15–20 GeV) with 10   (100)   fb^-1. At the Tevatron, a 3σ observation of a 120 GeV h_inv in any single channel is not possible with less than 12   fb^-1 per detector. However, we show that combining the signal from WBF with the previously studied Z+h_inv channel allows a 3σ observation of h_inv with 7   fb^-1 per detector. Because of overwhelming irreducible backgrounds, h_inv+j is not a useful search channel at either the Tevatron or the LHC, despite the larger production rate.

We construct tree-level four-particle open-string amplitudes relevant to dilepton and diphoton production at hadron colliders. We expand the amplitudes into string resonance (SR) contributions and compare the total cross-section through the first SR with the Z^′ search at the Tevatron. We establish a current lower bound based on the CDF Run I results on the string scale to be about 1.1-2.1 TeV, and it can be improved to about 1.5-3  TeV with 2 fb^-1. At the LHC, we investigate the properties of signals induced by string resonances in dilepton and diphoton processes. We demonstrate the unique aspects of SR-induced signals distinguishable from other new physics, such as the angular distributions and forward-backward asymmetry. A 95% C.L. lower bound can be reached at the LHC for M_S>8.2-10 TeV with an integrated luminosity of 300 fb^-1. We emphasize the generic features and profound implications of the amplitude construction.

We consider the Higgs sector in an extension of the minimal supersymmetric standard model (MSSM) with extra SM singlets, involving an extra U(1)^′ gauge symmetry, in which the domain-wall problem is avoided and the effective μ parameter is decoupled from the new gauge boson Z^′ mass. The model involves a rich Higgs structure very different from that of the MSSM. In particular, there are large mixings between Higgs doublets and the SM singlets, significantly affecting the Higgs spectrum, production cross sections, decay modes, existing exclusion limits, and allowed parameter range. Scalars considerably lighter than the CERN LEP2 bound (114 GeV) are allowed, and the range tan⁡β∼1 is both allowed and theoretically favored. Phenomenologically, we concentrate our study on the lighter (least model-dependent, yet characteristic) Higgs particles with significant SU(2)-doublet components to their wave functions, for the case of no explicit CP violation in the Higgs sector. We consider their spectra, including the dominant radiative corrections to their masses from the top/stop loop. We computed their production cross sections and reexamine the existing exclusion limits at LEP2. We outline the searching strategy for some representative scenarios at a future linear collider. We emphasize that gaugino, Higgsino, and singlino decay modes are indicative of extended models and have been given little attention. We present a comprehensive list of model scenarios in the Appendices.

In supersymmetry, the flavor mixing between top-squark (stop) and charm-squark (scharm) induces the flavor-changing neutral-current (FCNC) stop decay t˜₁→cχ˜₁⁰. Searching for this decay serves as a probe of soft supersymmetry breaking parameters. Focusing on the stop pair production followed by the FCNC decay of one stop and the charge-current decay of the other stop, we investigate the potential of detecting this FCNC stop decay at the Fermilab Tevatron, the CERN Large Hadron Collider and the next-generation e⁺e^- linear collider. We find that this decay may not be accessible at the Tevatron, but could be observable at the Large Hadron Collider and the linear collider with high sensitivity for some part of parameter space.

One of the main goals for hadron colliders is the study of the properties of the third generation quarks. We study the signatures for new TeV resonances that couple to top or bottom quarks both at the Tevatron Run II and at the CERN LHC. We find that in the simplest production processes of Drell-Yan type at the Tevatron, the signals are overwhelmed by QCD backgrounds. We also find that it is possible to study these resonances when they are produced in association with a pair of heavy quarks or in association with a single top at the LHC. In particular, with an integrated luminosity of 300 fb^-1 at the LHC, it is possible to probe resonance masses up to around 2 TeV.

Based on tree-level open-string scattering amplitudes in the low string-scale scenario, we derive the massless fermion scattering amplitudes. The amplitudes are required to reproduce those of the standard model at the tree level in the low energy limit. We then obtain four-fermion contact interactions by expanding in inverse powers of the string scale and explore the constraints on the string scale from low energy data. The Chan-Paton factors and the string scale are treated as free parameters. We find that data from the neutral and charged current processes at DESY HERA, Drell-Yan process at the Fermilab Tevatron, and from CERN LEP-II put lower bounds on the string scale M_S, for typical values of the Chan-Paton factors, in the range M_S>~0.9-1.3 TeV, comparable to Tevatron bounds on Z^′ and W^′ masses.