Recent results of the searches for Supersymmetry in final states with one or two leptons at CMS are presented. Many Supersymmetry scenarios, including the Constrained Minimal Supersymmetric extension of the Standard Model (CMSSM), predict a substantial amount of events containing leptons, while the largest fraction of Standard Model background events -which are QCD interactions -gets strongly reduced by requiring isolated leptons. The analyzed data was taken in 2011 and corresponds to an integrated luminosity of approximately L = 1 fb −1 . The center-of-mass energy of the pp collisions was √ s = 7 TeV.

Models of superconductivity in unconventional materials can be experimentally differentiated by the predictions they make for the symmetries of the superconducting order parameter. In the case of the heavy-fermion superconductor UPt3, a key question is whether its multiple superconducting phases preserve or break time-reversal symmetry (TRS). We tested for asymmetry in the phase shift between left and right circularly polarized light reflected from a single crystal of UPt3 at normal incidence and found that this so-called polar Kerr effect appears only below the lower of the two zero-field superconducting transition temperatures. Our results provide evidence for broken TRS in the low-temperature superconducting phase of UPt3, implying a complex two-component order parameter for superconductivity in this system.

He, that anisotropic disorder, engineered from highly porous silica aerogel, stabilizes a chiral superfluid state that otherwise would not exist. Furthermore, we find that the chiral axis of this state can be uniquely oriented with the application of a magnetic field perpendicular to the aerogel anisotropy axis. At sufficiently low temperature we observe a sharp transition from a uniformly oriented chiral state to a disordered structure consistent with locally ordered domains, contrary to expectations for a superfluid glass phase 6 . Superconducting states with non-zero orbital angular momentum, L = 0, are characterized by a competitive, but essential, relationship with magnetism, strong normal-state anisotropy or both [1][2][3]5 . Moreover, these states are strongly suppressed by disorder, an important consideration for applications 7 and a signature of their unconventional behaviour 3,8,9 . Although liquid 3 He in its normal phase is perfectly isotropic, it becomes a p-wave superfluid at low temperatures with non-zero orbital and spin angular momenta, L = S = 1 (ref. 10). One of its two superfluid phases in zero magnetic field is anisotropic with chiral symmetry, where the handedness results from the orbital motion of the bound 3 He pairs about an axis . This chiral superfluid, called the A phase or axial state, is stable at high pressure near the normal-to-superfluid transition, Fig. 1a-c, whereas the majority of the phase diagram is the non-chiral B phase, with isotropic physical properties. The stability of the A phase is attributed to strong-coupling effects arising from collisions between 3 He quasiparticles 10 . However, in the presence of isotropic disorder these strong-coupling effects are reduced and the stable chiral phase disappears 11,12 , Fig. 1a. Here we show that anisotropic disorder can reverse this process and stabilize an anisotropic phase over the entire phase diagram, Fig. 1c.For many years it was thought to be impossible to introduce disorder into liquid 3 He because it is intrinsically chemically and isotopically pure at low temperatures. Then it was discovered 13,14 that 3 He imbibed in ∼ 98% porosity silica aerogel, Fig. 1d, is a superfluid with a transition temperature that is sharply defined 12 , but reduced from that of pure 3 He. To test predictions that isotropic disorder favours isotropic states and anisotropic disorder favours anisotropic states 15 , we have grown a 97.5% porosity anisotropic aerogel with growth-induced radial compression 16 , effectively stretching it along its cylinder axis by 14.3%. Experiments using uncharacterized stretched aerogels have been previously reported 17,18 and are in disagreement with the work presented here. Silica aerogels, as in Fig. 1d, are formed by silica particles ≈ 3 nm in diameter, precipitated from a tetramethylorthosilicate solution, and aggregated in a diffusion-limited process. After supercritical drying we obtain a Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208, USA. *e-mail: w-halperin@northw...

Superfluid3 He confined to high porosity silica aerogel is the paradigm system for understanding impurity effects in unconventional superconductors. However, a crucial first step has been elusive; exact identification of the microscopic states of the superfluid in the presence of quenched disorder. Using a new class of highly uniform aerogel materials, we report pulsed nuclear magnetic resonance experiments that demonstrate definitively that the two observed superfluid states in aerogel are impure versions of the isotropic and axial p-wave states. The theoretically predicted destruction of long-range orbital order (Larkin-Imry Ma effect) in the impure axial state is not observed.PACS numbers: 67.30. Hm, 67.30.Er, 67.30.Hj, 74.20.Rp The discovery of effects of quenched disorder on superfluid 3 He using high porosity silica aerogel [1, 2] has created an opportunity for systematic study of the role of impurities on unconventional pairing. Although the two observed superfluid phases in aerogel have characteristics similar to those of pure 3 He (where the A-phase is the axial state and the B-phase is the isotropic state), the identification of the states is lacking. Theory indicates that the presence of elastic quasiparticle scattering reduces strong coupling [3], which is known to be responsible for the axial state in pure 3 He. This should favor the isotropic state in aerogel, consistent with susceptibility and acoustic experiments [4][5][6]. However, a metastable phase is observed at high pressure on cooling, stabilized by magnetic field [6], and its identity is more in question. In this regard we note that without strong coupling, the planar and axial states are degenerate [7]. There are predictions that local or global anisotropy in the scattering rates favor various possible anisotropic states, e.g. axial [3], polar [8], or possibly a family of robust states [9]. Furthermore, random local disorder is predicted to lead to an orbitally disordered superfluid glass [10,11] or Larkin-Imry-Ma (LIM) state [12,13]. To resolve this problem, we have grown a new type of highly homogeneous aerogel and developed methods for its characterization [14]. In this Letter we present results on 3 He in a 98.2% porosity uniformly-isotropic aerogel sample in which we precisely determine the order parameter structure and identify the microscopic states of the superfluid phases to be the impurity suppressed axial and isotropic states. Additionally, we find no evidence for the existence of the predicted LIM superfluid glass.Pulsed NMR is a powerful technique for identifying the superfluid states of 3 He, where the frequency shifts of the spectrum are directly related to the amplitude of the order parameter, ∆, and the dependence of the shift on tip angle is a fingerprint of the microscopic state [7]. However, to date the interpretation of pulsed NMR experiments in aerogel has been complicated by distributions in the frequency shifts owing to spatially non-uniform directions of the angular momentum, called orbital textures, that can b...

Order parameter symmetry is one of the basic characteristics of a superconductor. The heavy fermion compound UPt3 provides a rich system for studying the competition between superconductivity and other forms of electronic order and exhibits two distinct superconducting phases that are characterized by different symmetries. We fabricated a series of Josephson tunnel junctions on the as-grown surfaces of UPt3 single crystals spanning the a-b plane. By measuring their critical current, we mapped out the magnitude of the superconducting order parameter as a function of the momentum-space direction and temperature. In the high-temperature phase, we observed a sharp node in the superconducting gap at 45 degrees with respect to the a axis; an out-of-phase component appeared in the low-temperature phase, creating a complex order parameter.

Despite intense studies the exact nature of the order parameter in superconducting Sr2RuO4 remains unresolved. We have used small-angle neutron scattering to study the vortex lattice in Sr2RuO4 with the field applied close to the basal plane, taking advantage of the transverse magnetization. We measured the intrinsic superconducting anisotropy between the c axis and the Ru-O basal plane (~60), which greatly exceeds the upper critical field anisotropy (~20). Our result imposes significant constraints on possible models of triplet pairing in Sr2RuO4 and raises questions concerning the direction of the zero spin projection axis.

It is generally believed that fractional quantum excitations such as spinons in one-dimensional (1D) spin chains only proliferate and govern magnetism in systems with small and isotropic atomic magnetic moments, such as spin−1/2 Cu 2+ . In contrast, large and anisotropic orbital-dominated moments, such as those produced by strong spin-orbit coupling in the rare earths, are considered to be classical, becoming static as T → 0 since the conventional Heisenberg-Dirac exchange interaction [1, 2] cannot reverse their directions. We present here the results of neutron scattering measurements on Yb 2 Pt 2 Pb that completely negate this common wisdom. A diffuse continuum of magnetic excitations is observed in Yb 2 Pt 2 Pb, direct evidence that the elementary excitations carry a fractional spin quantum number, S = 1/2. The excitations disperse in only one direction, showing that the Yb moments form spin chains that are embedded in, but effectively decoupled from the three-dimensional conduction electron bands in metallic Yb 2 Pt 2 Pb. The spectrum of magnetic excitations strongly resembles the spinon continuum found in S = 1/2 Heisenberg spin chains, and indeed comparison to the 1D XXZ Hamiltonian indicates only a moderate exchange anisotropy, ∆ = J zz /J xx ∼ 3. Here we show how the orbital physics of 4f -electron exchange interactions can reconcile this moderately-anisotropic quantum Hamiltonian with the extreme anisotropy of the putatively classical Yb (J = 7/2) magnetic moments with respect to magnetic fields. We find that the unexpected quantum behavior emerges at low energies from the competition of interactions that act on much higher energy scales, i.e. the strong on-site Coulomb and spin-orbit interactions, as well as the crystal fields, and the inter-site hopping. Our findings thus provide a unique and a hitherto unforeseen manifestation of emergence [3] of quantum physics in the system of semi-classical electronic orbitals.The unusual properties of Yb 2 Pt 2 Pb derive in part from its crystal structure (Fig. 1A,B), where the Yb 3+ ions form ladders along the c−axis, with rungs on the orthogonal bonds of the ShastrySutherland Lattice (SSL) [5] in the ab-planes. Equally important is the strong spin-orbit coupling, which combines spin and orbital degrees of freedom into a large, J = 7/2 Yb moment.The absence of a Kondo effect indicates minimal coupling of Yb to the conduction electrons of this excellent metal [6, 7]. A point-charge model (Supplementary Information) indicates that the crystal electric field (CEF) lifts the eightfold degeneracy of the Yb 3+ moments, producing a Kramers doublet ground state that is a nearly pure state of the total angular momentum, J , |J, m J = |7/2, ±7/2 . The estimated anisotropy of the Landé g-factor is in good agreement with that of the measured magnetization, g /g ⊥ = 7.5(4) [6][7][8], implying strong Ising anisotropy in Yb 2 Pt 2 Pb, which confines the individual Yb moments to two orthogonal sublattices in the The reality is, in fact, completely different.The neutron scatt...

The topological superconductor UPt3, has three distinct vortex phases, a strong indication of its unconventional character. Using small-angle neutron scattering we have probed the vortex lattice in the UPt3 B phase with the magnetic field along the crystal c-axis. We find a difference in the vortex lattice configuration depending on the sign of the magnetic field relative to the field direction established upon entering the B phase at low temperature in a field sweep, showing that the vortices in this material posses an internal degree of freedom. This observation is facilitated by the discovery of a field driven non-monotonic vortex lattice rotation, driven by competing effects of the superconducting gap distortion and the vortex-core structure. From our bulk measurements we infer that the superconducting order parameter in the UPt3 B phase breaks time reversal symmetry and exhibits chiral symmetry with respect to the c-axis.