118416483
Bifurcation and global dynamical behavior of the f(T) theory
Usually, in order to investigate the evolution of a theory, one may find the critical points of the system and then perform perturbations around these critical points to see whether they are stable or not. This local method is very useful when the initial values of the dynamical variables are not far away from the critical points. Essentially, the nonlinear effects are totally neglected in such kind of approach. Therefore, one cannot tell whether the dynamical system will evolute to the stable critical points or not when the initial values of the variables do not dose enough to these critical points. Furthermore, when there are two or more stable critical points in the system, local analysis cannot provide the information on which the system will finally evolute to. In this paper, we have further developed the nullcline method to study the bifurcation phenomenon and global dynamical behavior of the f(T) theory. We overcome the shortcoming of local analysis. And, it is very clear to see the evolution of the system under any initial conditions.
73579088
Towards a realistic solution of the cosmological constant fine-tuning problem by Higgs inflation
Why is the cosmological constant Lambda observed today so much smaller than the Planck scale, and why is the Universe accelerating at present? This is the so-called cosmological constant fine-tuning problem. In this paper, we find that this problem may be solved with the help of Higgs inflation by simply assuming a variable cosmological "constant" during the inflationary epoch. Meanwhile, it could predict a large tensor-to-scalar ratio r approximate to 0.20 and a large running of the spectral index n(s)' approximate to -0.028 with a red-tilt spectrum n(s) approximate to 0.96, as well as a big enough number of e-folds N approximate to 40, requiring that we solve the problems in big bang cosmology with the help of Lambda.
119114156
Thermodynamic Origin of the Cardassian Universe
In the Cadassian universe, one can explain the acceleration of the universe without introducing dark energy component. However, the dynamical equations of this model can not be directly obtained from the action principle. Recently, works on the relation between thermodynamics and gravity indicates that gravity force may not be the fundamental force. In this paper, we study the thermodynamics of the Cardassian universe, and regard it as the origin of this cosmological model. We find that the corresponding entropy obeys ordinary area law when the area of the trapping horizon is small, while it becomes a constant when area is going to be large in the original and modified polytropic Cardassian model, and it has a maximum value in the exponential one. It seems that the Cardassian universe only contains finite information according to the holographic principle, which states that all the information in the bulk should be encoded in the boundary of the bulk.
118651169
CASIMIR EFFECT WITH A HELIX TORUS BOUNDARY CONDITION
We use the generalized Chowla–Selberg formula to consider the Casimir effect of a scalar field with a helix torus boundary condition in the flat (D + 1)-dimensional spacetime. We obtain the exact results of the Casimir energy density and pressure for any D for both massless and massive scalar fields. The numerical calculation indicates that once the topology of spacetime is fixed, the ratio of the sizes of the helix will be a decisive factor. There is a critical value rc of the ratio r of the lengths at which the pressure vanishes. The pressure changes from negative to positive as the ratio r passes through rc increasingly. In the massive case, we find the pressure tends to the result of massless field when the mass approaches zero. Furthermore, there is another critical ratio of the lengths and the pressure is independent of the mass at in the D = 3 case.
15412072
Chaplygin gas and the cosmological evolution of alpha
The class of Chaplygin gas models regarded as a candidate of dark energy can be realized by a scalar field, which could drive the variation of the fine structure constant $\alpha$ during the cosmic time. This phenomenon has been observed for almost ten years ago from the quasar absorption spectra and attracted many attentions. In this paper, we reconstruct the class of Chaplygin gas models to a kind of scalar fields and confront the resulting $\Delta\alpha/\alpha$ with the observational constraints. We found that if the present observational value of the equation of state of the dark energy was not exactly equal to -1, various parameters of the class of Chaplygin gas models are allowed to satisfy the observational constraints, as well as the equivalence principle is also respected.
118321008
Ricci Dark Energy in Brans-Dicke theory
A holographic dark energy from Ricci scalar curvature called Ricci dark energy was proposed recently. In this model the future event horizon area is replaced by the inverse of the Ricci scalar curvature. We study the evolution of equation of state of the Ricci dark energy and the transition from decelerated to accelerated expansion of the universe in the Brans-Dicke theory, which is a natural extension of general relativity. We find that the current acceleration of our universe is well explained.
39350314
Study on FPGA SEU mitigation for readout electronics of DAMPE BGO calorimeter
The Dark Matter Particle Explorer (DAMPE) is being constructed as a scientific satellite to observe high energy cosmic rays in space. As a main detector of DAMPE, the Bismuth Germanium Oxide (BGO) calorimeter is in charge of measuring particle energy deposition, distinguishing positrons, electrons and gamma rays from hadron background and providing trigger information. The satellite is designed to fly on a near-earth orbit with an altitude of 500km, which makes demands on electronic components for the ability to work in radiation environment.A flash-based Field-Programmable Gate Array (FPGA) of Actel, ProASCI Plus (APA) is chosen as the control component of the Front-End Electronics (FEE). To study the radiation resistance, SEE test was performed at the Heavy Ion Research Facility in Lanzhou (HIRFL). The chip did not exhibit latchup to an effective LET of 90 MeV-cm2/mg. The Flash switches of APA, which provide nonvolatile, reconfigurable interconnect programming and connect routing nets, were immune to SEL. However, SEE experiment also showed that the registers implemented from logic tiles and RAM blocks were SEU sensitive. Therefore, appropriate solutions should be adopted to mitigate SEU effects and make sure that the chip can function well in the radiation environment of space. To achieve maximum SEU tolerance with minimum resource consumption and speed degradation, engineering tradeoffs should be taken into account. Firstly, to the crucial registers and RAMs, Triple Modular Redundancy (TMR) technique is used to ensure that the value after voter is correct and if the values in three modules are different, the voter result will be written back to modules to scrub SEU errors. Besides, the three modules are manually placed in different physical areas to avoid Multiple Bit Upset (MBU). Secondly, CRC checksum is used to check the correctness of configuration data stored in RAM, which can be used as judgment of RAM reconfiguration. And thirdly, the whole logic is divided into four parts with different reset signals, which makes it convenient to set the idle parts into sleep until the external wake-up signals come. After using these means in the logic design, an ion-beam testing is performed to evaluate the SEU tolerance of FPGA logic. The FPGA works well under the condition with the LET of 39.6 MeV-cm2/mg and the ion flux of 100 ions/cm2/s without any mistake. The radiation test verifies that the SEU mitigationstrategies implemented in APA FPGA can meet the space qualification.
137255349
Electronics of BESIII TOF Monitor System
This paper describes the control electronics for the time of flight (TOF) monitor system of BESIII. A 9U VME module is designed as the interface between the TOF monitor laser box and BESIII data acquisition system. It takes charge of enabling the laser source, sending trigger signals to the laser driver, controlling the optic path, and measuring the charge and arrival time of pulses from two reference phototubes. The module also sends a Level 1 trigger to the TOF readout system, after each trigger pulse to the laser source. The interface and control logic are implemented in one Altera field-programmable gate array, and signals from the reference PMTs of the laser system are digitized with the CERN HPTDC technique. A series of experimental tests show that the time resolution of the monitor electronics is better than 25 ps, and the relative methodical error of the charge measurement is less than 0.5% after quadratic fitting. The TOF monitor system was installed at the end of 2007 and has been operating continuously since then. A total time resolution of about 70 ps and a charge resolution of about 4% of the TOF counters have been obtained during field monitor tests.
140196480
TOF Clock System for BES III
In this paper, a system for providing a precisely timed clock to the time-of-flight (TOF) electronics at Beijing Spectrometer (BES III) is presented. System functions include radio-frequency reference signal transmission and low-jitter clock regeneration and distribution. A new clock synchronizing and monitoring mechanism is developed with off-the-shelf devices to synchronize the clock signal to the beam bunches. The measured short-term jitter performance of the system is 11 ps worst case, and robust synchronization to the beam bunches has also been demonstrated. The system has been installed and is operating reliably.
120054500
The non-gated charge-to-time converter for TOF detector in BES III
Abstract The TOF (Time-Of-Flight) detector in BES (Beijing Spectrometer) III experiment requires a total time resolution of better than 90 ps (for the barrel part) and 110 ps (for the endcap part). With the requirement of such a high precision time measurement, it is essential to measure the amplitude or charge of the PMT (photomultiplier tube) signals, in order to correct the amplitude dependent time-walk. A method of using non-gated QTC (Charge-to-Time Converter) to measure the input signal charge was developed for BES III TOF readout electronics. The non-gated QTC with differential inputs was designed to measure the input charge covering a dynamic range of about 30–927 pC. Test results showed that the rms resolution is better than 10 bit and the relative error is less than 0.5% after quadratic fitting. The designed converter is simple, stable and suitable for multi-channel systems. A total of 448 channels of QTCs have been developed for BES III TOF readout electronics, which help the TOF system achieve the expected time resolution after the time-walk correction.
30938289
A high density time to digital converter module for BESIII end-cap TOF upgrade
This paper describes the high precision time measurement for the end-cap time of flight (ETOF) upgrade of a Beijing Spectrometer (BESIII). After upgrade, ETOF will be built using Multigap Resistive Plate Chambers (MRPC) and will have 1728 readout channels which is a big challenge for the readout electronics. The time resolution of the readout electronics system is required to be better than 25 ps. A 9U VME module with 72 time measurement channels is designed to perform particles flight time measurement. The signals, which are produced by the detector and then amplified and discriminated by the frontend electronics, are sent to the time-to-digital converter (TDIG) module to be digitized. The TDIG module uses the CERN HPTDC technique to achieve high precision time-to-digital converter and totally applies nine HPTDC chips to realize 72 channels. The primary hit measurements from HPTDC chips are forwarded to one Altera field-programmable gate array (FPGA) to be processed. Finally, the data are sent to DAQ server by Ethernet. The VME interface logic is implemented in an Altera CPLD. The TDIG module can also accept an external trigger and pick out the expected events which are correlated with the given trigger. A series of experiment tests show that the time resolution of the TDIG module is better than 20 ps.