Search

A search for nonresonant new phenomena, originating from either contact interactions or large extra spatial dimensions, has been carried out using events with two isolated electrons or muons. These events, produced at the LHC in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$, were recorded by the ATLAS detector. The data sample, collected throughout 2011, corresponds to an integrated luminosity of 4.9 and $5.0{\mathrm{fb}}^{-1}$ in the $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ channels, respectively. No significant deviations from the Standard Model expectation are observed. Using a Bayesian approach, 95% confidence level lower limits ranging from 9.0 to 13.9 TeV are placed on the energy scale of $\ell \ell qq$ contact interactions in the left-left isoscalar model. Lower limits ranging from 2.4 to 3.9 TeV are also set on the string scale in large extra dimension models. After combining these limits with results from a similar search in the diphoton channel, slightly more stringent limits are obtained.

A search for the standard model Higgs boson is performed in the diphoton decay channel. The data used correspond to an integrated luminosity of $4.9{\mathrm{fb}}^{-1}$ collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=7\mathrm{TeV}$. In the diphoton mass range 110–150 GeV, the largest excess with respect to the background-only hypothesis is observed at 126.5 GeV, with a local significance of 2.8 standard deviations. Taking the look-elsewhere effect into account in the range 110–150 GeV, this significance becomes 1.5 standard deviations. The standard model Higgs boson is excluded at 95% confidence level in the mass ranges of 113–115 GeV and 134.5–136 GeV.

Planar and nonorientable tadpole operators are evaluated in the Becchi-Rouet-Stora-Tyutin- (BRST-) invariant formalism. It is shown that the insertion of ${\left(\mathrm{-}1\right)}^{N_c+1/2\mathrm{}}$ ($N_c$=ghost-number operator) is needed for keeping the BRST invariance. We shall also discuss the extension of the BRST-invariant diagrammatic calculation to arbitrary diagrams.

Negative pion spectra emitted in the reactions of 775 MeV/nucleon ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$${+}^{12}$C and ${}_{}{}^{139}{}_{}{}^{}\mathrm{La}$${+}^{139}$La reactions have been measured in coincidence with the projectile fragments using the HISS spectrometer at the Bevalac. Prominent peaks near the beam velocity were observed in the pion spectra. Position and widths of the peaks were studied as a function of the ‘‘sum charge’’ of projectile fragments which is a good measure of impact parameter; the smaller the ‘‘sum charge,’’ the smaller the impact parameter. The peak position down shifts with the smaller ‘‘sum charge.’’ The pion peak is wider in the transverse than in the longitudinal direction, possibly mirroring the velocity dispersions of projectile fragments in the early stage of reactions.

We investigate nonplanar operators in the open-bosonic-string theory by means of the covariant operator formalism based on the Becchi-Rouet-Stora-Tyutin (BRST) formulation. The detailed evaluations of two nonplanar primitive operators, that is, nonplanar self-energy and nonplanar two-loop tadpole, are presented. At the one-loop level, we show that the ghost contribution yields a right measure factor to the physical amplitude. For the nonplanar two-loop tadpole operator, we discuss the factorization of the nonplanar self-energy. In order to avoid multiple countings, the integration regions are suitably restricted by investigating the duality of the 5-Reggeon vertex and the periodicity of the nonplanar self-energy operator. We also argue that only one singularity associated with the nonplanar self-energy part occurs in the nonplanar two-loop tadpole.

The vector analyzing power has been measured for ${\pi }^{+}$d elastic scattering at 0.74 GeV/c in the angular range of ${\mathrm{theta}}_{\mathrm{c}.\mathrm{m}.\mathrm{}=40\mathrm{}\mathrm{?}(\mathrm{de}–}$105°, using a polarized deuteron target in a large aperture spectrometer. A comparison with calculations based on the Glauber model was made.

It is shown that taking into account a symmetry breaking induced by quark mass differences in the transition form factor, the total cross sections for neutrino production of the three lightest charmed baryons ($C_0^{+}$, $C_1^{++}$, $C_1^{{*}_{++}}$) which were calculated by Avilez, Kobayashi, and Körner have to be reduced by a factor of $\frac{1}{4}-\frac{1}{7}$.

A $\mathrm{Si}\left(100\right)\mathrm{}c(8\mathrm{}\times 2)$ surface phase has been fabricated using atomic-hydrogen-induced self-organization followed by hydrogen extraction by the tip of scanning tunneling microscope. Similar to a conventional $\mathrm{Si}\left(100\right)2\times 1$ surface, the new phase is built of Si dimer rows, but with every second dimer row being missing. The second-layer Si atoms in the troughs between dimer rows are also dimerized. The second-layer Si dimers at the opposite sides of a top Si dimer row demonstrate a tendency to be out of phase. This configuration induces buckling of the top Si dimer with a $c(8\mathrm{}\times 2)$ periodicity.

A search for a Higgs boson has been performed in the $H\to WW\to \ell \nu jj$ channel in $1.04{\mathrm{fb}}^{-1}$ of $pp$ collision data at $\sqrt{s}=7\mathrm{TeV}$ recorded with the ATLAS detector at the Large Hadron Collider. No significant excess of events is observed over the expected background and limits on the Higgs boson production cross section are derived for a Higgs boson mass in the range $240\mathrm{GeV}<m_H<600\mathrm{GeV}$. The best sensitivity is reached for $m_H=400\mathrm{GeV}$, where the 95% confidence level upper bound on the cross section for $H\to WW$ production is 3.1 pb, or 2.7 times the standard model prediction.

We report ${}^{31}$P-NMR and specific-heat measurements on an iron (Fe) based superconductor SrFe${}_2$(As${}_{0.65}$P${}_{0.35}$)${}_2$ with $T_c=26$ K, which have revealed the development of antiferromagnetic correlations in the normal state and unconventional superconductivity (SC) with the nodal gap dominated by the gapless low-lying quasiparticle excitations. The results are consistently argued with an unconventional multiband SC state with the gap-size ratio of different bands being significantly large; the large full gaps in the $s_{\pm }$-wave state keep $T_c$ high, whereas a small gap with a nodal structure causes a gapless feature under magnetic field. The present results will help develop insight into the strong material dependence of the SC gap structure in Fe-based superconductors.

A search for new phenomena in $t\overline{t}$ events with large missing transverse momentum in proton-proton collisions at a center-of-mass energy of 7 TeV is presented. The measurement is based on $1.04{\mathrm{fb}}^{-1}$ of data collected with the ATLAS detector at the LHC. Contributions to this final state may arise from a number of standard model extensions. The results are interpreted in terms of a model where new top-quark partners are pair produced and each decay to an on-shell top (or antitop) quark and a long-lived undetected neutral particle. The data are found to be consistent with standard model expectations. A limit at 95% confidence level is set excluding a cross section times branching ratio of 1.1 pb for a top-partner mass of 420 GeV and a neutral particle mass less than 10 GeV. In a model of exotic fourth generation quarks, top-partner masses are excluded up to 420 GeV and neutral particle masses up to 140 GeV.

The results of a search for pair production of the scalar partners of bottom quarks in $2.05{\mathrm{fb}}^{-1}$ of $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ using the ATLAS experiment are reported. Scalar bottom quarks are searched for in events with large missing transverse momentum and two jets in the final state, where both jets are identified as originating from a bottom quark. In an $R$-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom quark decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits are obtained in the ${\tilde{b}}_1-{\tilde{\chi }}_1^0$ mass plane such that for neutralino masses below 60 GeV scalar bottom masses up to 390 GeV are excluded.

A search for contact interactions has been performed using dimuon events recorded with the ATLAS detector in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The data sample corresponds to an integrated luminosity of $42{\mathrm{pb}}^{-1}$. No significant deviation from the standard model is observed in the dimuon mass spectrum, allowing the following 95% C.L. limits to be set on the energy scale of contact interactions: $\Lambda >4.9\mathrm{TeV}$ (4.5 TeV) for constructive (destructive) interference in the left-left isoscalar compositeness model. These limits are the most stringent to date for $\mu \mu qq$ contact interactions.

The measurement of charged-particle event shape variables is presented in inclusive inelastic $pp$ collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The observables studied are the transverse thrust, thrust minor, and transverse sphericity, each defined using the final-state charged particles’ momentum components perpendicular to the beam direction. Events with at least six charged particles are selected by a minimum-bias trigger. In addition to the differential distributions, the evolution of each event shape variable as a function of the leading charged-particle transverse momentum, charged-particle multiplicity, and summed transverse momentum is presented. Predictions from several Monte Carlo models show significant deviations from data.

The energy and angular distributions of positive photopions emitted following the (e,${\pi }^{+}$) reaction on ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ has been studied for the spin-dipole isovector states in the highly excited residual nuclei. These states are found at ∼7 MeV in ${}_{}{}^{12}{}_{}{}^{}\mathrm{B}$ and 13.6, 17.7, and 24.0 MeV in ${}_{}{}^6{}_{}{}^{}\mathrm{He}$, in close agreement with the theoretical estimates.

Third-order nonlinear optical spectra ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ of conjugated polymers are theoretically studied within the treatment of double-excitation configuration interaction for the Pariser-Parr-Pople model. We consider a realistic situation of dephasing rates being much larger than depopulation rates. It is shown that, on the low-energy side of the one-photon resonance, the absolute value of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω,-ω,ω) is approximately proportional to the linear absorption spectrum α(ω). Additional peaks show up in the ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ spectrum at frequencies corresponding to energy differences among excited states. Their intensities can become comparable to and even stronger than ordinary two-photon absorption peaks in the same energy region. The same is true for the electromodulation spectrum ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω,0,0). In this situation the product of ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω,0,0) and α(ω) is shown to be approximately proportional to ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$(-ω;ω-ω,ω). © 1996 The American Physical Society.

The spectra of the third-order optical nonlinearity, $|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}(\omega ;\omega ,\omega ,-\omega )|,$ around excitonic resonance in a spin-coated film and a Langmuir-Blodgett (LB) film of polythiophene derivative, poly(3-[2-((S)-2-methylbutoxy)ethyl]thiophene), were measured using the degenerate four-wave-mixing method. The obtained value of $|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}(\omega ;\omega ,\omega ,-\omega )|$ at the excitonic resonance was $\sim {10}^{-7}\mathrm{esu},$ which was the largest value ever obtained in conjugated polymers. From the absorption spectra and $|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}|$ spectra, the average effective conjugation lengths of the spin-coated film and the LB film were estimated to be approximately 20- and 36-thiophene rings, respectively.

Shock-recovery experiments have been conducted on $6H$ polytype silicon carbide to 135 GPa and post-shock samples have been analyzed by Raman spectroscopy, x-ray diffraction, and electron microscopy. Results indicate that at up to 25–30 GPa, crystallite size is reduced faster than above this pressure, when reduction slows. Shock-induced inelastic deformation of $6H-\mathrm{SiC}$ is known to develop fully for stress above 25 GPa, coinciding with the pressure range observed where crystallite size reduction decreases. Inelastic deformation appears to be responsible for this slowed crystallite size reduction.

The isovalent-substituted iron-pnictide superconductor ${\mathrm{SrFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$ ($x=0.35$) has a slightly higher optimum critical temperature than the similar system ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$, and its parent compound ${\mathrm{SrFe}}_2$${\mathrm{As}}_2$ has a much higher Néel temperature than ${\mathrm{BaFe}}_2$${\mathrm{As}}_2$. We have studied the band structure and the Fermi surfaces of optimally doped ${\mathrm{SrFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$ by angle-resolved photoemission spectroscopy (ARPES). Three holelike Fermi surfaces (FSs) around $(0,0)$ and two electronlike FSs around $(\pi ,\pi )$ have been observed as in the case of ${\mathrm{BaFe}}_2$(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$. Measurements with different photon energies have revealed that the outermost hole FS is more strongly warped along the $k_z$ direction than the corresponding one in BaFe(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$, and that the innermost one is an ellipsoidal pocket. The electron FSs are almost cylindrical, unlike the corrugated ones in BaFe(${\mathrm{As}}_{1\text{−}x}$${\mathrm{P}}_x$)${}_2$. A comparison of the ARPES data with a first-principles band-structure calculation revealed that the quasiparticle mass renormalization factors are different not only between bands of different orbital character, but also between the hole and electron FSs of the same orbital character. By examining the nesting conditions between the hole and electron FSs, we conclude that magnetic interactions between FeAs layers rather than FS nesting play an important role in stabilizing the antiferromagnetic order. The insensitivity of superconductivity to the FS nesting can be explained if only the $d_{xy}$ and/or $d_{xz/yz}$ orbitals are active in inducing superconductivity or if FS nesting is not important for superconductivity.

An experimental study of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$+$p$ collisions at $75A$ MeV was performed. The excited state of ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ at $E^{*}=1.25\mathrm{}\pm 0.15$ MeV was observed, which is a candidate for a neutron halo excitation. Four other ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ states were manifested (at ∼ 3.0, 4.9, 6.4, and 11.3 MeV). Angular distributions were measured for elastic scattering ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$+$p$ and 8He+$p$. Structure of the ${}_{}{}^{11}{}_{}{}^{}\mathrm{Li}$ halo was investigated using the eikonal approach.