2015 Vol. 39, No. 12

Letters
Production of γγ+2 jets from double parton scattering in proton-proton collisions at the LHC
TAO Jun-Quan, ZHANG Si-Jing, SHEN Yu-Qiao, FAN Jia-Wei, CHEN Guo-Ming, CHEN He-Sheng
2015, 39(12): 121001. doi: 10.1088/1674-1137/39/12/121001
Abstract:
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 √s=13 TeV and integrated luminosity of 100 fb-1 planned for the following years, and also √s=14 TeV with 3000 fb-1 of integrated luminosity as the LHC design.
Mirror symmetry, D-brane superpotentials and Ooguri-Vafa invariants of Calabi-Yau manifolds
ZHANG Shan-Shan, YANG Fu-Zhong
2015, 39(12): 121002. doi: 10.1088/1674-1137/39/12/121002
Abstract:
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.
PARTICLES AND FIELDS
Combining upper limits with a Bayesian approach
YANG Liu, ZHU Kai, ZHU Yong-Sheng, CAI Hao
2015, 39(12): 123001. doi: 10.1088/1674-1137/39/12/123001
Abstract:
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.
Decay rates and electromagnetic transitions of heavy quarkonia
J. N. Pandya, N. R. Soni, N. Devlani, A. K. Rai
2015, 39(12): 123101. doi: 10.1088/1674-1137/39/12/123101
Abstract:
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.
An updated study of Y production and polarizationat the Tevatron and LHC
FENG Yu, GONG Bin, WAN Lu-Ping, WANG Jian-Xiong
2015, 39(12): 123102. doi: 10.1088/1674-1137/39/12/123102
Abstract:
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.
NUCLEAR PHYSICS
Covariance analysis of an isospin-dependent probe
GUO Wen-Jun, LI Xian-Jie, HUANG Jiang-Wei, WANG Kuo, ZHANG Xiao-Ji
2015, 39(12): 124101. doi: 10.1088/1674-1137/39/12/124101
Abstract:
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.
ASTROPHYSICS AND COSMOLOGY RELATED TO PARTICLES AND NUCLEI
Reconciling the light component and all-particle cosmicray energy spectra at the knee
ZHAO Yi, JIA Huan-Yu, ZHU Feng-Rong
2015, 39(12): 125001. doi: 10.1088/1674-1137/39/12/125001
Abstract:
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.
Detectors, Related Electronics and Experimental Methods
Measurement of the fluorescence quantum yield of bis-MSB
DING Xue-Feng, WEN Liang-Jian, ZHOU Xiang, DING Ya-Yun, YE Xing-Chen, ZHOU Li, LIU Meng-Chao, CAI Hao, CAO Jun
2015, 39(12): 126001. doi: 10.1088/1674-1137/39/12/126001
Abstract:
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.
DETECTORS, RELATED ELECTRONICS AND EXPERIMENTAL METHODS
Evaluation of a front-end ASIC for the readout of PMTs over a large dynamic range
WU Wei-Hao, ZHAO Lei, LIANG Yu, YU Li, LIU Jian-Feng, LIU Shu-Bin, AN Qi
2015, 39(12): 126101. doi: 10.1088/1674-1137/39/12/126101
Abstract:
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.
Clock auto-synchronization method for BESIII ETOF upgrade
WANG Si-Yu, CAO Ping, LIU Shu-Bin, AN Qi
2015, 39(12): 126102. doi: 10.1088/1674-1137/39/12/126102
Abstract:
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.
ACCELERATORS
Bunch length manipulation in a diffraction-limited storage ring
TIAN Sai-Ke,