Observation of vertical betatron sideband due to electron clouds in the KEKB low energy ring

The effects of electron clouds on positively charged beams have been an active area of research in recent years at particle accelerators around the world. Transverse beam-size blowup due to electron clouds has been observed in some machines and is considered to be a major limiting factor in the development of higher-current, higher-luminosity electron-positron colliders. The leading proposed mechanism for beam blowup is the excitation of a fast head-tail instability due to short-range wakes within the electron cloud. We present here observations of betatron oscillation sidebands in bunch-by-bunch spectra that may provide direct evidence of such head-tail motion in a positron beam.     

The VEPP-2000 electron-positron collider: First experiments

In 2007, at the Institute of Nuclear Physics (Novosibirsk), the construction of the VEPP-2000 electron-positron collider was completed. The first electron beam was injected into the accelerator structure with turned-off solenoids of the final focus. This mode was used to tune all subsystems of the facility and to train the vacuum chamber using synchrotron radiation at electron currents of up to 150 mA. The VEPP-2000 structure with small beta functions and partially turned-on solenoids was used for the first testing of the “round beams” scheme at an energy of 508 MeV. Beam-beam effects were studied in strong-weak and strong-strong modes. Measurements of the beam sizes in both cases showed a dependence corresponding to model predictions for round colliding beams. Using a modernized SND (spherical neutral detector), the first energy calibration of the VEPP-2000 collider was performed by measuring the excitation curve of the phimeson resonance; the phi-meson mass is known with high accuracy from previous experiments at VEEP-2M. In October 2009, a KMD-3 (cryogenic magnetic detector) was installed at the VEPP-2000 facility, and the physics program with both the SND and LMD-3 particle detectors was started in the energy range of 1–1.9 GeV. This first experimental season was completed in summer 2010 with precision energy calibration by resonant depolarization.     

Status and prospects of VEPP-2000 electron-positron collider

High energy physics experiments were started at VEPP-2000 at the end of 2010; the third experimental run was finished in July 2013. The last run was devoted to the energy range 160–510 MeV in a beam. Compton backscattering energy measurements were used for the regular energy calibration of the VEPP-2000, together with resonance depolarization and NMR methods. The conception of the round colliding beam lattice along with precise orbit and lattice correction yielded a record high peak luminosity of 1.2 × 10³¹ cm^?2 s^?1 at 510 MeV and an average luminosity of 0.9 × 10³¹ cm^?2 s^?1 per run. A total betatron tune shift of 0.174 was achieved at 392.5 MeV. This corresponds to the beam-beam parameter ξ = 0.125 in terms of the collision point. The injection system is currently modernized to allow injection of particles at the VEPP-2000 energy maximum and the elimination of the existing lack of positrons.     

Recommissioning the Modernized VEPP-2000 Electron–Positron Collider

The VEPP-2000 electron–positron collider has been operating at the Budker Institute of Nuclear Physics (BINP) since 2010. Applying the concept of round colliding beams allowed the record value of the beam–beam parameter ξ ～ 0.12 to be reached. The upgrading of the VEPP-2000 complex, including the connection to the new BINP Injection Complex and modification of the electron–position booster and the BEP–VEPP-2000 transfer channels to work at 1 GeV, resulted in a significant increase in luminosity. Work on statistical data collection using detectors is in progress.     

Analysis of e + e −→π+π−π+π− and e + e −→π+π−π0π0 processes in the energy range of \sqrt 2 = 0.98 - 1.38 GeV in experiments with a spherical neutral detector

The cross sections of the e ⁺ e ^?→π⁺π^?π ⁺π^?, e ⁺ e ^?→π⁺π^?π⁰π ⁰, and e ⁺ e ^?→ωπ→ π⁺π^?π ⁰π⁰ processes, as well as the e ⁺ e ^?→π⁺π^?π⁰π⁰ process with subtracted contribution of the ωπ intermediate state, are measured in experiments with a spherical neutral detector on the VEPP-2M collider in the energy range 0.98–1.38 GeV. About 41 000 events of the e ⁺ e ^?→π⁺π^?π⁺π ^? process and more than 54 000 events of the e ⁺ e ^?→π⁺π ^?π⁰π⁰ process have been selected in experiment. The statistical error in determining the cross section is 2–20%, while the systematic error is 7 and 8%.     

Measurement of the ω, ρ → π0 e + e − branching fractions

The e ⁺ e ^? → ω, ρ → π⁰ e ⁺ e ^? processes have been investigated in the experiments with Spherical Neutral Detector at the VEPP-2M e ⁺ e ^? collider. The measurements provide the probability ?(ω → π⁰ e ⁺ e ^?) = (0.761 ± 0.053 ± 0.064) × 10^?3 of the ω → π⁰ e ⁺ e ^g- conversion decay and the upper limit ?(ρ → π⁰ e ⁺ e ^?) < 1.2 × 10^?5 (at 90% CL) for the ρ → π⁰ e ⁺ e ^? decay. The transition form factor was measured at three values of the 4-momentum transfer squared.     

Solution‐crystallization and related phenomena in 9,9‐dialkyl‐fluorene polymers. II. Influence of side‐chain structure

Solution‐crystallization is studied for two polyfluorene polymers possessing different side‐chain structures. Thermal analysis and temperature‐dependent optical spectroscopy are used to clarify the nature of the crystallization process, while X‐ray diffraction and scanning electron microscopy reveal important differences in the resulting microstructures. It is shown that the planar‐zigzag chain conformation termed the β‐phase, which is observed for certain linear‐side‐chain polyfluorenes, is necessary for the formation of so‐called polymer‐solvent compounds for these polymers. Introduction of alternating fluorene repeat units with branched side‐chains prevents formation of the β‐phase conformation and results in non‐solvated, i.e. melt‐crystallization‐type, polymer crystals. Unlike non‐solvated polymer crystals, for which the chain conformation is stabilized by its incorporation into a crystalline lattice, the β‐phase conformation is stabilized by complexation with solvent molecules and, therefore, its formation does not require specific inter‐chain interactions. The presented results clarify the fundamental differences between the β‐phase and other conformational/crystalline forms of polyfluorenes. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1492–1506     

Solution‐crystallization and related phenomena in 9,9‐dialkyl‐fluorene polymers. I. Crystalline polymer‐solvent compound formation for poly(9,9‐dioctylfluorene)

Polymer‐solvent compound formation, occurring via co‐crystallization of polymer chains and selected small‐molecular species, is demonstrated for the conjugated polymer poly(9,9‐dioctylfluorene) (PFO) and a range of organic solvents. The resulting crystallization and gelation processes in PFO solutions are studied by differential scanning calorimetry, with X‐ray diffraction providing additional information on the resulting microstructure. It is shown that PFO‐solvent compounds comprise an ultra‐regular molecular‐level arrangement of the semiconducting polymer host and small‐molecular solvent guest. Crystals form following adoption of the planar‐zigzag β‐phase chain conformation, which, due to its geometry, creates periodic cavities that accommodate the ordered inclusion of solvent molecules of matching volume. The findings are formalized in terms of nonequilibrium temperature–composition phase diagrams. The potential applications of these compounds and the new functionalities that they might enable are also discussed. © 2015 The Authors. Journal of Polymer Science Part B: Polymer Physics published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1481–1491     

Search for the ϕ → π+π−γγ and ϕ → π+π−η decays with the CMD-2 detector

The ? → π⁺π^?γγ and ? → π⁺π^?η decays are sought in the experimental data obtained with the CMD-2 detector in the region of the ?-meson resonance. Upper limits ?(? → ππγγ)< 1.2 × 10^?4 and ?(? → ππη) < 6.1 × 10^?5 (both at a C.L. of 90%) are determined for the branching ratios of these decays.     

Spectroscopic properties of poly(9,9‐dioctylfluorene) thin films possessing varied fractions of β‐phase chain segments: enhanced photoluminescence efficiency via conformation structuring

Poly(9,9‐dioctylfluorene) (PFO) is a widely studied blue‐emitting conjugated polymer, the optoelectronic properties of which are strongly affected by the presence of a well‐defined chain‐extended “β‐phase” conformational isomer. In this study, optical and Raman spectroscopy are used to systematically investigate the properties of PFO thin films featuring a varied fraction of β‐phase chain segments. Results show that the photoluminescence quantum efficiency (PLQE) of PFO films is highly sensitive to both the β‐phase fraction and the method by which it was induced. Notably, a PLQE of ～69% is measured for PFO films possessing a ～6% β‐phase fraction induced by immersion in solvent/nonsolvent mixtures; this value is substantially higher than the average PLQE of ～55% recorded for other β‐phase films. Furthermore, a linear relationship is observed between the intensity ratios of selected Raman peaks and the β‐phase fraction determined by commonly used absorption calibrations, suggesting that Raman spectroscopy can be used as an alternative means to quantify the β‐phase fraction. As a specific example, spatial Raman mapping is used to image a mm‐scale β‐phase stripe patterned in a glassy PFO film, with the extracted β‐phase fraction showing excellent agreement with the results of optical spectroscopy. © 2016 The Authors. Journal of Polymer Science Part B: Polymer Physics Published by Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1995–2006