2015 Vol. 39, No. 12
- PARTICLES AND FIELDS
- NUCLEAR PHYSICS
- ASTROPHYSICS AND COSMOLOGY RELATED TO PARTICLES AND NUCLEI
- Detectors, Related Electronics and Experimental Methods
- DETECTORS, RELATED ELECTRONICS AND EXPERIMENTAL METHODS
- SYNCHROTRON RADIATION, FREE ELECTRON LASER, APPLICATION OF NU-CLEAR TECHNOLOGY, ETC.
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Cross sections for the production of pairs of photons plus two additional jets produced from double parton scattering in high-energy proton-proton collisions at the LHC are calculated for the first time. The estimates are based on the theoretical perturbative QCD predictions for the productions of γγ at next-to-next-to-leading-order, jet+jet and γ +jet at next-to-leading-order, for their corresponding single-scattering cross sections. The cross sections and expected event rates for γγ +2 jets from double parton scattering, after typical acceptance and selections, are given for proton-proton collisions with the collision energy √ =13 TeV and integrated luminosity of 100 fb-1 planned for the following years, and also √ =14 TeV with 3000 fb-1 of integrated luminosity as the LHC design.
The D-brane superpotential is very important in the low energy effective theory. As the generating function of all disk instantons from the worldsheet point of view, it plays a crucial role in deriving some important properties of the compact Calabi-Yau manifolds. By using the generalized GKZ hypergeometric system, we will calculate the D-brane superpotentials of two non-Fermat type compact Calabi-Yau hypersurfaces in toric varieties, respectively. Then according to the mirror symmetry, we obtain the A-model superpotentials and the Ooguri-Vafa invariants for the mirror Calabi-Yau manifolds.
We discuss how to determine and combine upper limits based on observed events and estimated backgrounds with a Bayesian method, when insignificant signals are observed in independent measurements. In addition to some general features deduced from the analytical formulae, systematic numerical results are obtained by a C++ program (CULBA) for low-count experiments, which can be used as a reference to combine two upper limits.
The electromagnetic radiative transition widths for heavy quarkonia, as well as digamma and digluon decay widths, are computed in the framework of the extended harmonic confinement model (ERHM) and Coulomb plus power potential (CPPv) with varying potential index v. The outcome is compared with the values obtained from other theoretical models and experimental results. While the mass spectra, digamma and digluon widths from ERHM as well as CPPv=1 are in good agreement with experimental data, the electromagnetic transition widths span over a wide range for the potential models considered here making it difficult to prefer a particular model over the others because of the lack of experimental data for most transition widths.
Following the nonrelativistic QCD factorization scheme, by taking the latest available measurements of χbJ(3P) into consideration, we present an updated study on the yield and polarization of Y(1S, 2S, 3S) hadroproduction, and the fractions of χbJ(mP) feed-down in Y(nS) production at QCD next-to-leading order. In the fitting, three schemes are applied with different choices of χbJ(mP) feed-down ratios and NRQCD factorization scale. The results can explain the measurements of yield very well. The polarization puzzle to (3S) is now solved by considering the χbJ(3P) feed-down contributions. The ratio of σ[χb2(1P)]/σ[χb1(1P)] measured by the CMS experiment can also be reproduced in our prediction. Among the different schemes, the results show little difference, but there are sizeable differences for the fitted long-distance color-octet matrix elements. This may bring large uncertainties when the values are applied in theoretical predictions for other experiments such as those at ee, ep colliders.
Based on a modified quantum molecular dynamics model, we calculate the neutron-proton ratio and the nuclear stopping of reaction systems with different symmetry potentials and collision cross sections. We perform correlations of several probes using the covariance data processing method. It is shown that the correlation between the nuclear stopping and the isospin-dependent nucleon-nucleon cross sections is strong, but the nuclear stopping and symmetry potentials have a weak correlation. The correlation between neutron-proton ratio and symmetry potentials in the case of low energy is stronger. The correlation between neutron-proton ratio and isospin-dependent collision cross sections is enhanced with the increase of energy, but remains weak. In addition, the correlations of the emission numbers of the deuteron with the symmetry potentials and collision cross sections at different beam energies are not obvious compared to two prior physical quantities. In this paper, we define a parameter to quantitatively describe the sensitivity of isospin-dependent probes. By analyzing this parameter, one can extract more information about the isospin effects of the physical quantity.
The knee phenomenon of the cosmic ray spectrum, which plays an important role in studying the acceleration mechanism of cosmic rays, is still an unsolved mystery. We try to reconcile the knee spectra measured by ARGO-YBJ and Tibet-III. A simple broken power-law model fails to explain the experimental data. Therefore a modified broken power-law model with non-linear acceleration effects is adopted, which can describe the sharp knee structure. This model predicts that heavy elements dominate at the knee.
The fluorescence quantum yield of bis-MSB, a widely used liquid scintillator wavelength shifter, was measured to study the photon absorption and re-emission processes in a liquid scintillator. The re-emission process affects the photoelectron yield and distribution, especially in a large liquid scintillator detector, thus must be understood to optimize the liquid scintillator for good energy resolution and to precisely simulate the detector with Monte Carlo. In this study, solutions of different bis-MSB concentration were prepared for absorption and fluorescence emission measurements to cover a broad range of wavelengths. Harmane was used as a standard reference to obtain the absolution fluorescence quantum yield. For the first time we measured the fluorescence quantum yield of bis-MSB up to 430 nm as inputs required by Monte Carlo simulation, which is 0.926±0.053 at λex=350 nm.
The Large High Altitude Air Shower Observatory (LHAASO) project has been proposed for the survey and study of cosmic rays. In the LHAASO project, the Water Cherenkov Detector Array (WCDA) is one of the major detectors for searching for gamma ray sources. A Charge-to-Time Convertor (QTC) ASIC (Application Specification Integrated Circuit), fabricated with Global Foundry 0.35 μm CMOS technology, has been developed for readout of photomultiplier tubes (PMTs) in the WCDA. This ASIC provides both time and charge measurement of PMT signals. The input charge is converted to a pulse width based on the Time-Over-Threshold (TOT) technique and linear discharge method; as for time measurement, leading edge discrimination is employed. This paper focuses on the evaluation of this front-end readout ASIC performance. Test results indicate that the time resolution is better than 400 ps and the charge resolution is better than 1% with large input signals and remains better than 15% @ 1 photoelectron (P.E.), both beyond the application requirement. Moreover, this ASIC has a weak ambient temperature dependence, low input rate dependence and high channel-to-channel isolation.
An automatic clock synchronization method implemented in a field programmable gate array (FPGA) is proposed in this paper. It is developed for the clock system which will be applied in the end-cap time of flight (ETOF) upgrade of the Beijing Spectrometer (BESIII). In this design, an FPGA is used to automatically monitor the synchronization circuit and deal with signals coming from the external clock synchronization circuit. By testing different delay time of the detection signal and analyzing the signal state returned, the synchronization windows can be found automatically by the FPGA. The new clock system not only retains low clock jitter which is less than 20ps root mean square (RMS), but also demonstrates automatic synchronization to the beam bunches. So far, the clock auto-synchronizing function has been working successfully under a series of tests. It will greatly simplify the system initialization and maintenance in the future.
In an electron storage ring, the bunch length can be increased or decreased by using harmonic cavities. Taking the High Energy Photon Source as an example, we test the bunch length manipulation with harmonic cavities in a diffraction-limited storage ring (DLSR). The most important collective effects in a DLSR, intra-beam scattering and Touschek effects, are evaluated for different bunch-length patterns. Our study shows that it is feasible to produce long and short bunches simultaneously in a DLSR, without causing severe emittance growth and reduction in lifetime.
Dual harmonic systems have been widely used in high intensity proton synchrotrons to suppress the space charge effect, as well as reduce the beam loss. To investigate the longitudinal beam dynamics in a dual rf system, the potential well, the sub-buckets in the bunch and the multi-solutions of the phase equation are studied theoretically in this paper. Based on these theoretical studies, optimization of bunching factor and rf voltage waveform are made for the dual harmonic rf system in the upgrade phase of the China Spallation Neutron Source Rapid Cycling Synchrotron (CSNS/RCS). In the optimization process, the simulation with space charge effect is done using a newly developed code, C-SCSIM.
The upgrade project of the Hefei Light Source storage ring is under way. In this paper, the broadband impedances of resistive wall and coated ceramic vacuum chamber are calculated using the analytic formula, and the wake fields and impedances of other designed vacuum chambers are simulated by CST code, and then a broadband impedance model is obtained. Using the theoretical formula, longitudinal and transverse single bunch instabilities are discussed. With the carefully-designed vacuum chamber, we find that the thresholds of the beam instabilities are higher than the beam current goal.
In PASER (particle acceleration by stimulated emission of radiation), in the presence of an active medium incorporated in a Penning trap, moving electrons can become bunched, and as they get enough energy, they escape the trap forming an optical injector. These bunched electrons can enter the next PASER section filled with the same active medium to be accelerated. In this paper, electron dynamics in the presence of a gas mixture active medium incorporated in a Penning trap is analyzed by developing an idealized 1D model. We evaluate the energy exchange occurring as the train of electrons traverses into the next PASER section. The results show that the oscillating electrons can be bunched at the resonant frequency of the active medium. The influence of the trapped time and population inversion are analyzed, showing that the longer the electrons are trapped, the more energy from the medium the accelerated electrons get, and with the increase of population inversion, the decelerated electrons are virtually unchanged but the accelerated electrons more than double their peak energy values. The simulation results show that the gas active medium needs a lower population inversion to bunch the electrons compared to a solid active medium, so the experimental conditions can easily be achieved.
The robust conjugate direction search (RCDS) method has high tolerance to noise in beam experiments. It has been demonstrated that this method can be used to optimize the machine performance of a light source online. In our study, taking BEPCII as an example, the feasibility of online tuning of the luminosity in a circular collider is explored, through numerical simulation and preliminary online experiments. It is shown that the luminosity that is artificially decreased by a deviation of beam orbital offset from optimal trajectory can be recovered with this method.
TiZrV film is mainly applied in the ultra-high vacuum pipes of storage rings. Thin film coatings of palladium, which are added onto the TiZrV film to increase the service life of nonevaporable getters and enhance H2 pumping speed, were deposited on the inner face of stainless steel pipes by dc magnetron sputtering using argon gas as the sputtering gas. The TiZrV-Pd film properties were investigated by atomic force microscope (AFM), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and X-Ray Diffraction (XRD). The grain size of TiZrV and Pd films were about 0.42-1.3 nm and 8.5-18.25 nm respectively. It was found that the roughness of TiZrV films is small, about 2-4 nm, but for Pd film it is large, about 17-19 nm. The PP At. % of Pd in TiZrV/Pd films varied from 86.84 to 87.56 according to the XPS test results.
An electron cooler is used to improve the quality of the ion beam in a synchrotron; however it also introduces a nonlinear electromagnetic field to the accelerator, which causes tune shift, tune spread and may drive resonances leading to ion beam loss. In this paper the tune shift and the tune spread caused by the nonlinear electromagnetic field of a hollow electron beam is investigated, and the resonance driving terms of the nonlinear electromagnetic field are analysed. The differences are presented compared with a solid electron beam. Calculations are performed for m 238U32+ ions of energy 1.272 MeV stored in the main Cooler Storage Ring (CSRm) at the Institute of Modern Physics, Lanzhou. It is found that in this situation the nonlinear field caused by the hollow electron beam does not lead to serious resonances.
We designed and fabricated a multilayer Laue lens (MLL) as a hard X-ray focusing device. WSi2/Si multilayers were chosen owing to their excellent optical properties and relatively sharp interface. The multilayer sample was fabricated by using direct current (DC) magnetron sputtering technology and then was sliced and thinned to form an MLL. The thickness of each layer was determined by scanning electron microscopy (SEM) image analysis with marking layers. The focusing property of the MLL was measured at Beamline 15U, Shanghai Synchrotron Facility (SSRF). One-dimensional (1D) focusing resolutions of 92 nm are obtained at photon energy of 14 keV.
Positron Emission Tomography (PET) using time-of-flight information, which can greatly improve the quality of the reconstructed image, has recently become an exciting topic. In this work, 3.2 mm×3.2 mm× 25 mm finger-like LYSO crystals were used to make a 5×5 array, coupled to the Hamamatsu H8500 photomultiplier tube (PMT) as a detector. A fast leading-edge discriminator was designed for the LYSO-H8500 detector. Average coincidence time resolution FWHM of 330 ps was obtained for the LYSO detector with a reference BaF2 detector, whose time resolution for 511 keV γ -rays was FWHM 150 ps. Time resolution FWHM of 294 ps was calculated for the LYSO detector, and coincidence time resolution of FHWM 415 ps can be expected for two identical LYSO detectors.
A conduction-cooled superconducting magnet producing a transverse field of 4 T has been designed for a new generation multi-field coupling measurement system, which will be used to study the mechanical behavior of superconducting samples at cryogenic temperatures and intense magnetic fields. A compact cryostat with a two-stage GM cryocooler is designed and manufactured for the superconducting magnet. The magnet is composed of a pair of flat racetrack coils wound by NbTi/Cu superconducting composite wires, a copper and stainless steel combinational former and two Bi2Sr2CaCu2Oy superconducting current leads. The two coils are connected in series and can be powered with a single power supply. In order to support the high stress and attain uniform thermal distribution in the superconducting magnet, a detailed finite element (FE) analysis has been performed. The results indicate that in the operating status the designed magnet system can sufficiently bear the electromagnetic forces and has a uniform temperature distribution.
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