Search for production of single top quarks via tcg and tug flavor-changing-neutral-current couplings.
We search for the production of single top quarks via flavor-changing-neutral-current couplings of a gluon to the top quark and a charm (c) or up (u) quark. We analyze 230 pb{-1} of lepton+jets data from pp[over] collisions at a center of mass energy of 1.96 TeV collected by the D0 detector at the Fermilab Tevatron Collider. We observe no significant deviation from standard model predictions, and hence set upper limits on the anomalous coupling parameters kappa{g}{c}/Lambda and kappa{g}{u}/Lambda, where kappa{g} define the strength of tcg and tug couplings, and Lambda defines the scale of new physics. The limits at 95% C.L. are kappa{g}{c}/Lambda<0.15 TeV-1 and kappa{g}{u}/Lambda<0.037 TeV-1.

title: Precision measurement of the top quark mass in lepton + jets final States.
abstract: We measure the mass of the top quark in lepton+jets final states using the full sample of pp collision data collected by the D0 experiment in Run II of the Fermilab Tevatron Collider at sqrt[s] = 1.96 TeV, corresponding to 9.7 fb(-1) of integrated luminosity. We use a matrix element technique that calculates the probabilities for each event to result from tt production or background. The overall jet energy scale is constrained in situ by the mass of the W boson. We measure m(t) = 174.98 ± 0.76 GeV. This constitutes the most precise single measurement of the top-quark mass.

title: Jet energy scale determination in the D0 experiment
abstract: The calibration of jet energy measured in the DO detector is presented, based on p (p) over bar collisions at a center-of-mass energy of 1.96 TeV at the Fermilab Tevatron collider, jet energies are measured using a sampling calorimeter composed of uranium and liquid argon as the passive and active media, respectively. This paper describes the energy calibration of jets performed with gamma+jet, Z+jet and dilet events, with jet transverse momentum p(T) > 6 GeV and pseudorapidity range vertical bar eta vertical bar < 3.6, The corrections are measured separately for data and simulation, achieving a precision of 1.4-1.8% for jets in the central part of the calorimeter ancl up to 3.5% for the jets with pseudorapidity vertical bar eta vertical bar = 3.0. Specific corrections are extracted to enhance the description of jet energy in simulation and in particular of the effects due to the flavor of the parton originating the jet, correcting biases up to 3-4% in jets with low PT originating from gluons and up to 6-8% in jets from b quarks.

title: Measurement of angular correlations of jets at s=1.96 TeV and determination of the strong coupling at high momentum transfers
abstract: We present a measurement of the average value of a new observable at hadron colliders that is sensitive to QCD dynamics and to the strong coupling constant, while being only weakly sensitive to parton distribution functions. The observable measures the angular correlations of jets and is defined as the number of neighboring jets above a given transverse momentum threshold which accompany a given jet within a given distance Delta-R in the plane of rapidity and azimuthal angle. The ensemble average over all jets in an inclusive jet sample is measured and the results are presented as a function of transverse momentum of the inclusive jets, in different regions of Delta-R and for different transverse momentum requirements for the neighboring jets. The measurement is based on a data set corresponding to an integrated luminosity of 0.7 fb-1 collected with the D0 detector at the Fermilab Tevatron Collider in pp-bar collisions at sqrt(s)=1.96 The results are well described by a perturbative QCD calculation in next-to-leading order in the strong coupling constant, corrected for non-perturbative effects. From these results, we extract the strong coupling and test the QCD predictions for its running over a range of momentum transfers of 50-400 GeV.

title: Observation and properties of the orbitally excited B_(s2)(*) meson.
abstract: We report the direct observation of the excited L=1 state B_(s2)(*) in fully reconstructed decays to B+K-. The mass of the B_(s2)(*) meson is measured to be 5839.6+/-1.1(stat)+/-0.7(syst) MeV/c(2), and its production rate relative to the B+ meson is measured to be [1.15+/-0.23(stat)+/-0.13(syst)]%.

title: Search for long-lived particles decaying into electron or photon pairs with the D0 detector.
abstract: In this Letter we report on a search for long-lived particles that decay into final states with two electrons or photons. Such long-lived particles arise in a variety of theoretical models, such as hidden valleys and supersymmetry with gauge-mediated breaking. By precisely reconstructing the direction of the electromagnetic shower we are able to probe much longer lifetimes than previously explored. We see no evidence of the existence of such long-lived particles and interpret this search as a quasi model-independent limit on their production cross section, as well as a limit on a long-lived fourth generation quark.

title: Explicit non-Abelian monopoles and instantons in SU ( N ) pure Yang-Mills theory
abstract: It is well known that there are no static non-Abelian monopole solutions in pure Yang-Mills theory on Minkowski space ${\mathbb{R}}^{3,1}$. I show that such solutions exist in $\mathrm{SU}(N)$ gauge theory on the spaces ${\mathbb{R}}^{2}\ifmmode\times\else\texttimes\fi{}{S}^{2}$ and $\mathbb{R}\ifmmode\times\else\texttimes\fi{}{S}^{1}\ifmmode\times\else\texttimes\fi{}{S}^{2}$ with Minkowski signature $(\ensuremath{-}+++)$. In the temporal gauge they are solutions of pure Yang-Mills theory on $\mathbb{T}\ifmmode\times\else\texttimes\fi{}{S}^{2}$, where $\mathbb{T}$ is $\mathbb{R}$ or ${S}^{1}$. Namely, imposing SO(3) invariance and some reality conditions, I consistently reduce the Yang-Mills model on the above spaces to a non-Abelian analog of the ${\ensuremath{\phi}}^{4}$ kink model whose static solutions give $\mathrm{SU}(N)$ monopole (-antimonopole) configurations on the space ${\mathbb{R}}^{1,1}\ifmmode\times\else\texttimes\fi{}{S}^{2}$ via the above-mentioned correspondence. These solutions can also be considered as instanton configurations of Yang-Mills theory in $2+1$ dimensions. The kink model on $\mathbb{R}\ifmmode\times\else\texttimes\fi{}{S}^{1}$ admits also periodic sphaleron-type solutions describing chains of $n$ kink-antikink pairs spaced around the circle ${S}^{1}$ with arbitrary $ng0$. They correspond to chains of $n$ static monopole-antimonopole pairs on the space $\mathbb{R}\ifmmode\times\else\texttimes\fi{}{S}^{1}\ifmmode\times\else\texttimes\fi{}{S}^{2}$ which can also be interpreted as instanton configurations in $2+1$ dimensional pure Yang-Mills theory at finite temperature (thermal time circle). I also describe similar solutions in Euclidean $\mathrm{SU}(N)$ gauge theory on ${S}^{1}\ifmmode\times\else\texttimes\fi{}{S}^{3}$ interpreted as chains of $n$ instanton\char21{}anti-instanton pairs.

title: Integrability of vortex equations on Riemann surfaces
abstract: The Abelian Higgs model on a compact Riemann surface Σ of genus g is considered. We show that for g>1 the Bogomolny equations for multi-vortices at critical coupling can be obtained as compatibility conditions of two linear equations (Lax pair) which are written down explicitly. These vortices correspond precisely to SO(3)-symmetric Yang–Mills instantons on the (conformal) gravitational instanton Σ×S2 with a scalar-flat Kahler metric. Thus, the standard methods of constructing solutions and studying their properties by using Lax pairs (twistor approach, dressing method, etc.) can be applied to the vortex equations on Σ. In the twistor description, solutions of the integrable vortex equations correspond to rank-2 holomorphic vector bundles over the complex 3-dimensional twistor space of Σ×S2. We show that in the general (nonintegrable) case there is a bijection between the moduli spaces of solutions to vortex equations on Σ and of pseudo-holomorphic bundles over the almost complex twistor space.

title: Sigma models with N=8 supersymmetries in 2+1 and 1+1 dimensions
abstract: We introduce an N = 8 supersymmetric extension of the Bogomolny-type model for Yang–Mills–Higgs fields in 2 + 1 dimensions related with twistor string theory. It is shown that this model is equivalent to an N = 8 supersymmetric U ( n ) chiral model in 2 + 1 dimensions with a Wess–Zumino–Witten-type term. Further reduction to 1 + 1 dimensions yields N = ( 8 , 8 ) supersymmetric extensions of the standard U ( n ) chiral model and Grassmannian sigma models.

title: Yang-Mills Flows on Nearly Kähler Manifolds and G2-Instantons
abstract: We consider Lie(G)-valued G-invariant connections on bundles over spaces $${G/H,\, \mathbb{R}\times G/H\, {\rm and}\, \mathbb{R}^2\times G/H}$$, where G/H is a compact nearly Kähler six-dimensional homogeneous space, and the manifolds $${\mathbb{R}\times G/H}$$ and $${\mathbb{R}^2\times G/H}$$ carry G2- and Spin(7)-structures, respectively. By making a G-invariant ansatz, Yang-Mills theory with torsion on $${\mathbb{R}\times G/H}$$ is reduced to Newtonian mechanics of a particle moving in a plane with a quartic potential. For particular values of the torsion, we find explicit particle trajectories, which obey first-order gradient or hamiltonian flow equations. In two cases, these solutions correspond to anti-self-dual instantons associated with one of two G2-structures on $${\mathbb{R}\times G/H}$$. It is shown that both G2-instanton equations can be obtained from a single Spin(7)-instanton equation on $${\mathbb{R}^2\times G/H}$$.

title: Instantons in six dimensions and twistors
abstract: Recently, conformal field theories in six dimensions were discussed from the twistorial point of view. In particular, it was demonstrated that the twistor transform between chiral zero-rest-mass fields and cohomology classes on twistor space can be generalized from four to six dimensions. On the other hand, the possibility of generalizing the correspondence between instanton gauge fields and holomorphic bundles over twistor space is questionable. It was shown by Samann and Wolf that holomorphic line bundles over the canonical twistor space Tw ( X ) (defined as a bundle of almost complex structures over the six-dimensional manifold X ) correspond to pure-gauge Maxwell potentials, i.e. the twistor transform fails. On the example of X = C P 3 we show that there exists a twistor correspondence between Abelian or non-Abelian Yang–Mills instantons on C P 3 and holomorphic bundles over complex submanifolds of Tw ( C P 3 ) , but it is not so efficient as in the four-dimensional case because the twistor transform does not parametrize instantons by unconstrained holomorphic data as it does in four dimensions.