16041279
Holographic Cosmological Constant and Dark Energy
A general holographic relation between UV and IR cutoff of an effective field theory is proposed. Taking the IR cutoff relevant to the dark energy as the Hubble scale, we find that the cosmological constant is highly suppressed by a numerical factor and the fine tuning problem seems alleviative. We also use different IR cutoffs to study the case in which the universe is composed of matter and dark energy.
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.
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.
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.
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.
119116661
Global behavior of cosmological dynamics with interacting Veneziano ghost
A bstractIn this paper, we shall study the dynamical behavior of the universe accelerated by the so called Veneziano ghost dark energy component locally and globally by using the linearization and nullcline method developed in this paper. The energy density is generalized to be proportional to the Hawking temperature defined on the trapping horizon instead of Hubble horizon of the Friedmann-Robertson-Walker (FRW) universe. We also give a prediction of the fate of the universe and present the bifurcation phenomenon of the dynamical system of the universe. It seems that the universe could be dominated by dark energy at present in some region of the parameter space.
33505335
Design of the readout electronics for the Qualification Model of DAMPE BGO calorimeter
The DAMPE (DArk Matter Particle Explorer) is a scientific satellite aimed at cosmic ray study, gamma ray astronomy, and searching for the clue of dark matter particles, with a planned mission period of more than 3 years. The calorimeter, which is composed of 308 BGO (Bismuth Germanate Oxid) crystal bars, is a critical sub-detector for measuring the energy of cosmic particles from 5 GeV to 10TeV, distinguishing positrons/electrons and gamma rays from hadron background, and providing trigger information to the whole payload. In the calorimeter each BGO crystal bar is coupled by two Hamamatsu R5610A PMTs (photomultiplier tubes) from both ends respectively, while each PMT send out signals from three dynodes (2, 5, 8) to cover a large dynamic range from about 11 MeV to 2 TeV for each crystal unit. Thus 616 PMTs and 1848 signal channels are used in the BGO calorimeter. The large amount of detector components and signal channels, as well as large dynamic range requirement, greatly challenge the design of readout electronics, because of the confined area for PCB (Printed Circuit Board) layout and cable layout, the potential crosstalk between high density signal channels especially from dynode 8 to dynode 2, and limited power budget. Besides there are other particular requirements derived from hash space environment, such as good radiation tolerance, high reliability, etc. To conquer above challenges, a serial of methods are taken into the design. Two ASICs with low power consumption, low noise, high dynamic range and high integrity, named VA160 and VATA160, are chosen as the front end chips to read out the charge signal from PMT dynodes, and to provide fast trigger information. The Flash-based FPGA (Field Programmable Gate Array) from Actel ProASIC Plus family is applied, which is considered to be less sensitive to SEU (Single Event Upset) and more power-saving, compared to regular SRAM-based FPGAs. In order to estimate the performances of the ASICs and FPGA chips in radiative space environment, both single event effect (SEE) and total ionizing dose (TID) tests are conducted, and SEU and SEL (Single Event Latch-up) protecting measures are adopted, based on the test results. Right now the DAMPE mission is in QM (Qualification Model) stage, all the readout electronics for the qualification model of BGO calorimeter were successfully designed and produced in the end of year 2013. The DAMPE BGO calorimeter was later integrated at the beginning of 2014 and right now the environmental tests, such as vibration test, thermal cycling and thermal-vacuum test, have been performed. Currently the BGO calorimeter is under integration tests with other sub-detectors, and a beam test will be carried out at CERN, in the autumn of 2014.
204935814
A new method of waveform digitization based on time-interleaved A/D conversion
Time interleaved analog-to-digital conversion (TIADC) based on parallelism is an effective way to meet the requirement of the ultra-fast waveform digitizer beyond Gsps. Different methods to correct the mismatch errors among different analog-to-digital conversion channels have been developed previously. To overcome the speed limitation in hardware design and to implement the mismatch correction algorithm in real time, this paper proposes a fully parallel correction algorithm. A 12-bit 1-Gsps waveform digitizer with ENOB around 10.5 bit from 5 MHz to 200 MHz is implemented based on the real-time correction algorithm.
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.
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.