3,870 research outputs found

    A Z' Model for the CDF Dijet Anomaly

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    We adopt a bottom-up approach to constructing a new physics model to explain the CDF excess seen in dijets with an associated lepton and missing transverse energy. We find that the 145 GeV broad feature seen by CDF in the dijet invariant mass distribution can be explained by a Z' boson with a mass of 145 GeV that couples only to first generation quarks. After dijet resonance constraints are considered, a sizeable region of the parameter space favored by the CDF anomaly remains viable.Comment: 16 pages, 3 figures; v2, added references; v3, fixed reference

    Collider constraints and new tests of color octet vectors

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    We analyze the collider sensitivity for new colored resonances in ttΛ‰t\bar{t}, bbΛ‰b \bar{b}, and jjjj final states. While searches in the single production channel are model-dependent, the pair production rate is model independent and the existing (JJ)(JJ)(JJ)(JJ) and 4t4t searches impose strong constraints on the relevant branching fractions, where J=jJ = j or bb. We point out the missing, complementary searches in the mixed decay modes, ttΛ‰(jj)t\bar{t}(jj), ttΛ‰(bbΛ‰)t\bar{t}(b\bar{b}), and (bbΛ‰)(jj)(b\bar{b})(jj). We propose analysis strategies for the ttΛ‰(jj)t\bar{t}(jj) and ttΛ‰(bbΛ‰)t\bar{t}(b\bar{b}) decays and find their sensivity surpasses that of existing searches when the decay widths to tops and light jets are comparable. If no other decays are present, collective lower limits on the resonance mass can be set at 1.5~TeV using 37~fbβˆ’1^{-1} of 13~TeV data.Comment: 22 pages, 3 figures, 2 table

    A Tale of Two Portals: Testing Light, Hidden New Physics at Future e+eβˆ’e^+ e^- Colliders

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    We investigate the prospects for producing new, light, hidden states at a future e+eβˆ’e^+ e^- collider in a Higgsed dark U(1)DU(1)_D model, which we call the Double Dark Portal model. The simultaneous presence of both vector and scalar portal couplings immediately modifies the Standard Model Higgsstrahlung channel, e+eβˆ’β†’Zhe^+ e^- \to Zh, at leading order in each coupling. In addition, each portal leads to complementary signals which can be probed at direct and indirect detection dark matter experiments. After accounting for current constraints from LEP and LHC, we demonstrate that a future e+eβˆ’e^+ e^- Higgs factory will have unique and leading sensitivity to the two portal couplings by studying a host of new production, decay, and radiative return processes. Besides the possibility of exotic Higgs decays, we highlight the importance of direct dark vector and dark scalar production at e+eβˆ’e^+ e^- machines, whose invisible decays can be tagged from the recoil mass method.Comment: 47 pages, 9 figures, 1 table. v2: references added, version matched to JHE

    Hardening and Strain Localisation in Helium-Ion-Implanted Tungsten

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    Tungsten is the main candidate material for plasma-facing armour components in future fusion reactors. In-service, fusion neutron irradiation creates lattice defects through collision cascades. Helium, injected from plasma, aggravates damage by increasing defect retention. Both can be mimicked using helium-ion-implantation. In a recent study on 3000 appm helium-implanted tungsten (W-3000He), we hypothesized helium-induced irradiation hardening, followed by softening during deformation. The hypothesis was founded on observations of large increase in hardness, substantial pile-up and slip-step formation around nano-indents and Laue diffraction measurements of localised deformation underlying indents. Here we test this hypothesis by implementing it in a crystal plasticity finite element (CPFE) formulation, simulating nano-indentation in W-3000He at 300 K. The model considers thermally-activated dislocation glide through helium-defect obstacles, whose barrier strength is derived as a function of defect concentration and morphology. Only one fitting parameter is used for the simulated helium-implanted tungsten; defect removal rate. The simulation captures the localised large pile-up remarkably well and predicts confined fields of lattice distortions and geometrically necessary dislocation underlying indents which agree quantitatively with previous Laue measurements. Strain localisation is further confirmed through high resolution electron backscatter diffraction and transmission electron microscopy measurements on cross-section lift-outs from centre of nano-indents in W-3000He

    Orientation-dependent indentation response of helium-implanted tungsten

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    A literature review of studies investigating the topography of nano-indents in ion-implanted materials reveals seemingly inconsistent observations, with report of both pile-up and sink-in. This may be due to the crystallographic orientation of the measured sample point, which is often not considered when evaluating implantation-induced changes in the deformation response. Here we explore the orientation dependence of spherical nano-indentation in pure and helium-implanted tungsten, considering grains with , and out-of-plane orientations. Atomic force microscopy (AFM) of indents in unimplanted tungsten shows little orientation dependence. However, in the implanted material a much larger, more localised pile-up is observed for grains than for and orientations. Based on the observations for grains, we hypothesise that a large initial hardening due to helium-induced defects is followed by localised defect removal and subsequent strain softening. A crystal plasticity finite element model of the indentation process, formulated based on this hypothesis, accurately reproduces the experimentally-observed orientation-dependence of indent morphology. The results suggest that the mechanism governing the interaction of helium-induced defects with glide dislocations is orientation independent. Rather, differences in pile-up morphology are due to the relative orientations of the crystal slip systems, sample surface and spherical indenter. This highlights the importance of accounting for crystallographic orientation when probing the deformation behaviour of ion-implanted materials using nano-indentation

    A Viable Flavor Model for Quarks and Leptons in RS with T' Family Symmetry

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    We propose a Randall-Sundrum model with a bulk family symmetry based on the double tetrahedral group, T', which generates the tri-bimaximal neutrino mixing pattern and a realistic CKM matrix. The T' symmetry forbids tree-level flavor-changing-neutral-currents in both the quark and lepton sectors, as different generations of fermions are unified into multiplets of T'. This results in a low first KK mass scale and thus the model can be tested at collider experiments.Comment: 4 pages; based on talk presented at the 17th International Conference on Supersymmetry and the Unification of Fundamental Interactions (SUSY09), Boston, MA, June 5-10, 200
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