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arXiv2011

Vanilla Technicolor at Linear Colliders

We analyze the reach of Linear Colliders (LC)s for models of dynamical electroweak symmetry breaking. We show that LCs can efficiently test the compositeness scale, identified with the mass of the new spin-one resonances, till the maximum energy in the center-of-mass of the colliding leptons. In particular we analyze the Drell-Yan processes involving spin-one intermediate heavy bosons decaying either leptonically or into two Standard Model (SM) gauge bosons. We also analyze the light Higgs production in association with a SM gauge boson stemming also from an intermediate spin-one heavy vector.

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arXiv2011

Search for Massive Neutrinos in the Decay pi --> e nu

Evidence of massive neutrinos in the pi --> e nu decay spectrum was sought with the background pi --> mu --> e decay chain highly suppressed. Upper limits (90% C.L.) on the neutrino mixing matrix element |U_ei|^2 in the neutrino mass region 60--129 MeV/c^2 were set at the level of 10^-8.

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arXiv2011

Study of the K0(L) --> pi0 pi0 nu nu-bar decay

The rare decay K0(L) --> pi0 pi0 nu nu-bar was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on 9.4 x 10^9 K0L decays, an upper limit of 8.1 x 10^{-7} was obtained for the branching fraction at 90% confidence level. We also set a limit on the K0(L) --> pi0 pi0 X (X --> invisible particles) process; the limit on the branching fraction varied from 7.0 x 10^{-7} to 4.0 x 10^{-5} for the mass of X ranging from 50 MeV/c^2 to 200 MeV/c^2.

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arXiv2011

Evidence of theta(13)>0 from global neutrino data analysis

The neutrino mixing angle theta(13) is at the focus of current neutrino research. From a global analysis of the available oscillation data in a 3-neutrino framework, we previously reported [Phys. Rev. Lett. 101, 141801 (2008)] hints in favor of theta(13)>0 at the 90 % C.L. Such hints are consistent with the recent indications of nu(mu)-->nu(e) appearance in the T2K and MINOS long-baseline accelerator experiments. Our global analysis of all the available data currently provides >3 sigma evidence for nonzero theta(13), with 1-sigma ranges sin^2 theta(13) = 0.021+-0.007 or 0.025+-0.007, depending on reactor neutrino flux systematics. Updated ranges are also reported for the other 3-neutrino oscillation parameters (delta m^2, sin^2 theta(12)) and (Delta m^2, sin^2 theta(23)).

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arXiv2011

Two Higgs doublets with 4th generation fermions - models for TeV-scale compositeness

We construct a class of two Higgs doublets models with a 4th sequential generation of fermions that may effectively accommodate the low energy characteristics and phenomenology of a dynamical electroweak symmetry breaking scenario which is triggered by the condensates of the 4th family fermions. In particular, we single out the heavy quarks by coupling the "heavier" Higgs doublet (Φ_h) which possesses a much larger VEV only to them while the "lighter" doublet (Φ_l) couples only to the light fermions. We study the constraints on these models from precision electroweak data as well as from flavor data. We also discuss some distinct new features that have direct consequences on the production and decays of the 4th family quarks and leptons in high energy colliders; in particular the conventional search strategies for t' and b' may need to be significantly revised.

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arXiv2011

$A^t_{FB}$ Meets LHC

The recent Tevatron measurement of the forward-backward asymmetry of the top quark shows an intriguing discrepancy with Standard Model expectations, particularly at large $\ttbar$ invariant masses. Measurements of this quantity are subtle at the LHC, due to its $pp$ initial state, however, one can define a forward-central-charge asymmetry which captures the physics. We study the capability of the LHC to measure this asymmetry and find that within the SM a measurement at the $5σ$ level is possible with roughly 60 fb$^{-1}$ at $\sqrt{s} = 14$ TeV. If nature realizes a model which enhances the asymmetry (as is necessary to explain the Tevatron measurements), a significant difference from zero can be observed much earlier, perhaps even during early LHC running at $\sqrt{s} = 7$ TeV. We further explore the capabilities of the 7 TeV LHC to discover resonances or contact interactions which modify the $\ttbar$ invariant mass distribution using recent boosted top tagging techniques. We find that TeV-scale color octet resonances can be discovered, even with small coupling strengths and that contact interactions can be probed at scales exceeding 6 TeV. Overall, the LHC has good potential to clarify the situation with regards to the Tevatron forward-backward measurement.

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arXiv2011

Effective axial-vector coupling of gluon as an explanation to the top quark asymmetry

We explore the possibility that the large $t\bar t$ forward-backward asymmetry measured by the CDF detector at Tevatron could be due to a universal effective axial-vector coupling of gluon. Using an effective field theory approach we show model independently how such a log-enhanced coupling occurs at 1-loop level. The interference with QCD gluon vector coupling naturally induces the observed positive forward-backward $t\bar t$ asymmetry that grows with $t\bar t$ invariant mass and is consistent with the cross section measurements. This scenario does not involve new flavor changing couplings nor operators that interfere with QCD, and, therefore, is not constrained by the LHC searches for 4-quark contact interactions. We predict top quark polarization effects that grow with energy and allow to test this scenario at the LHC. Our proposal offers a viable alternative to new physics scenarios that explain the $t\bar t$ forward-backward asymmetry anomaly with the interference between QCD and tree level new physics amplitudes.

hep-ph↗

arXiv2011

Drell-Yan Production of Z' in the Three-Site Higgsless Model at the LHC

In the Higgsless models, there are extra gauge bosons which keep the perturbative unitarity of a longitudinally polarized gauge boson. The three-site Higgsless model is a minimal Higgsless model and contains three extra gauge bosons, $W^{\prime \pm}$ and Z'. In this paper, we report the discovery potential of the Z' gauge boson via Drell-Yan production with Z'(mass=380, 500, 600 GeV) $\rightarrow WW \rightarrow \ellνqq$ ($\ell=e$, $μ$) at the LHC ($\sqrt{s}$=14 TeV).

hep-ph↗

arXiv2011

Search for the disappearance of muon antineutrinos in the NuMI neutrino beam

We report constraints on antineutrino oscillation parameters that were obtained by using the two MINOS detectors to measure the 7% muon antineutrino component of the NuMI neutrino beam. In the Far Detector, we select 130 events in the charged-current muon antineutrino sample, compared to a prediction of 136.4 +/- 11.7(stat) ^{+10.2}_{-8.9}(syst) events under the assumption |dm2bar|=2.32x10^-3 eV^2, snthetabar=1.0. Assuming no oscillations occur at the Near Detector baseline, a fit to the two-flavor oscillation approximation constrains |dm2bar|<3.37x10^-3 eV^2 at the 90% confidence level with snthetabar=1.0.

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arXiv2011

Searches for Rare or Forbidden Semileptonic Charm Decays

We present searches for rare or forbidden charm decays of the form $X_c^+\to h^\pm\ell^\mp\ell^{(\prime)+}$, where $X_c^+$ is a charm hadron ($D^+$, $D^+_s$, or $Λ_c^+$), $h^\pm$ is a pion, kaon, or proton, and $\ell^{(\prime)\pm}$ is an electron or muon. The analysis is based on $384 fb^{-1}$ of $e^+e^-$ annihilation data collected at or close to the $Υ(4S)$ resonance with the BaBar detector at the SLAC National Accelerator Laboratory. No significant signal is observed for any of the 35 decay modes that are investigated. We establish 90% confidence-level upper limits on the branching fractions between $1 \times 10^{-6}$ and $44 \times 10^{-6}$ depending on the channel. In most cases, these results represent either the first limits or significant improvements on existing limits for the decay modes studied.

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arXiv2011

Model-independent Higgs coupling measurements at the LHC using the H \to ZZ \to 4l lineshape

We show that combining a direct measurement of the Higgs total width from the H \to ZZ \to 4l lineshape with Higgs signal rate measurements allows Higgs couplings to be extracted in a model-independent way from CERN Large Hadron Collider (LHC) data. Using existing experimental studies with 30 fb-1 at one detector of the 14 TeV LHC, we show that the couplings-squared of a 190 GeV Higgs to WW, ZZ, and gg can be extracted with statistical precisions of about 10%, and a 95% confidence level upper limit on an unobserved component of the Higgs decay width of about 22% of the SM Higgs width can be set. The method can also be applied for heavier Higgs masses.

hep-ph↗

arXiv2011

A Higgs Quadruplet for Type III Seesaw and Implications for $μ\to eγ$ and $μ- e$ Conversion

In Type III seesaw model the heavy neutrinos are contained in leptonic triplet representations. The Yukawa couplings of the triplet fermion and the left-handed neutrinos with the doublet Higgs field produce the Dirac mass terms. Together with the Majorana masses for the leptonic triplets, the light neutrinos obtain non-zero seesaw masses. We point out that it is also possible to have a quadruplet Higgs field to produce the Dirac mass terms to facilitate the seesaw mechanism. The vacuum expectation value of the quadruplet Higgs is constrained to be small by electroweak precision data. Therefore the Yukawa couplings of a quadruplet can be much larger than those for a doublet. We also find that unlike the usual Type III seesaw model where at least two copies of leptonic triplets are needed, with both doublet and quadruplet Higgs representations, just one leptonic triplet is possible to have a phenomenologically acceptable model because light neutrino masses can receive sizable contributions at both tree and one loop levels. Large Yukawa couplings of the quadruplet can induce observable effects for lepton flavor violating processes $μ\to e γ$ and $μ- e$ conversion. Implications of the recent $μ\to eγ$ limit from MEG and also limit on $μ- e $ conversion on Au are also given. Some interesting collider signatures for the doubly charged Higgs boson in the quadruplet are discussed.

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arXiv2011

Model independent determination of the axial mass parameter in quasielastic neutrino-nucleon scattering

Quasielastic neutrino-nucleon scattering is a basic signal process for neutrino oscillation studies. At accelerator energies, the corresponding cross section is subject to significant uncertainty due to the poorly constrained axial-vector form factor of the nucleon. A model-independent description of the axial-vector form factor is presented. Data from the MiniBooNE experiment for quasielastic neutrino scattering on carbon are analyzed under the assumption of a definite nuclear model. The value of the axial mass parameter, m_A=0.85^{+0.22}_{-0.07} +/- 0.09 GeV, is found to differ significantly from extractions based on traditional form factor models. Implications for future neutrino scattering and pion electroproduction measurements are discussed.

hep-ph↗

arXiv2011

Finding physics signals with shower deconstruction

We introduce shower deconstruction, a method to look for new physics in a hadronic environment. The method aims to be a full information approach using small jets. It assigns to each event a number chi that is an estimate of the ratio of the probability for a signal process to produce that event to the probability for a background process to produce that event. The analytic functions we derive to calculate these probabilities mimic what full event generators like Pythia or Herwig do and can be depicted in a diagrammatic way. As an example, we apply this method to a boosted Higgs boson produced in association with a Z-boson and show that this method can be useful to discriminate this signal from the Z+jets background.

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arXiv2012

Next-to leading order analysis of target mass corrections to structure functions and asymmetries

We perform a comprehensive analysis of target mass corrections (TMCs) to spin-averaged structure functions and asymmetries at next-to-leading order. Several different prescriptions for TMCs are considered, including the operator product expansion, and various approximations to it, collinear factorization, and xi-scaling. We assess the impact of each of these on a number of observables, such as the neutron to proton F_2 structure function ratio, and parity-violating electron scattering asymmetries for protons and deuterons which are sensitive to gamma-Z interference effects. The corrections from higher order radiative and nuclear effects on the parity-violating deuteron asymmetry are also quantified.

hep-ph↗

arXiv2011

Using t -> b \bar{b} c to Search for New Physics

We consider new-physics (NP) contributions to the decay t -> b \bar{b} c. We parameterize the NP couplings by an effective Lagrangian consisting of 10 Lorentz structures. We show that the presence of NP can be detected through the measurement of the partial width. A partial identification of the NP can be achieved through the measurements of a forward-backward-like asymmetry, a top-quark-spin-dependent asymmetry, the partial rate asymmetry, and a triple-product asymmetry. These observables, which vanish in the standard model, can all take values in the 10-20% range in the presence of NP. Since |V_{tb}V_{cb}| \simeq |V_{ts} V_{cs}|, most of our results also hold, with small changes, for t -> s \bar{s} c.

hep-ph↗

arXiv2011

Monotops at the LHC

We explore scenarios where top quarks may be produced singly in association with missing energy, a very distinctive signature, which in analogy with monojets, we dub monotops. We find that monotops can be produced in a variety of modes, typically characterized by baryon number violating or flavor changing neutral interactions. We build a simplified model that encompasses all the possible (tree-level) production mechanisms and study the LHC sensitiveness to a few representative scenarios by considering fully hadronic top decays. We find that constraints on such exotic models can already be set with one inverse femtobarn of integrated luminosity collected at seven TeV.

hep-ph↗

arXiv2011

Zγγproduction with leptonic decays and triple photon production at NLO QCD

We present a calculation of the O(alpha_s) QCD corrections to the production of a Z boson in association with two photons and to triple photon production at hadron colliders. All final-state photons are taken as real. For the Z boson, we consider the decays both into charged leptons and into neutrinos including all off-shell effects. Numerical results are obtained via a Monte Carlo program based on the structure of the VBFNLO program package. This allows us to implement general cuts and distributions of the final-state particles. We find that the NLO QCD corrections are sizable and significantly exceed the expectations from a scale variation of the leading-order result. In addition, differential distributions of important observables change considerably. The prediction of two-photon-associated Z production with Z decays into neutrinos from the charged-lepton rate works well, once we use an additional cut on the invariant mass of the charged-lepton pair.

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