73619195
Probing the cosmographic parameters to distinguish between dark energy and modified gravity models
Aims. In this paper we investigate the deceleration, jerk and snap parameters to distinguish between the dark energy and modified gravity models using high redshift gamma-ray bursts (GRBs) and supernovae (SNe). Methods. We first derive the expressions of deceleration, jerk and snap parameters in dark energy and modified gravity models. In order to constrain the cosmographic parameters, we calibrate the GRB luminosity relations without assuming any cosmological models using SNe Ia. Then we constrain the model parameters (including dark energy and modified gravity models) using type Ia supernovae and gamma-ray bursts. Finally we calculate the cosmographic parameters. GRBs can extend the redshift-distance relation up to high redshifts, because they can be detected to high redshifts. Results. We find that the statefinder pair (r, s) could not be used to distinguish between some dark energy and modified gravity models, but these models could be differentiated by the snap parameter. Using the model-independent constraints on cosmographic parameters, we conclude that the ΛCDM model is consistent with current data.
124365356
Current constraints on early dark energy and growth index using latest observations
Aims. In this paper we study observational constraints on early dark energy model proposed by Doran & Robbers and growth index using the latest Union2 type Ia supernovae (SNe Ia), the baryon acoustic oscillations (BAO) in the large-scale correlation function of the Sloan Digital Sky Survey and Two-degree Field Galaxy Redshift Survey, cosmic microwave background (CMB) from the Wilkinson Microwave Anisotropy Probe seven-year data, the linear growth factors data and gamma-ray bursts (GRBs). Methods. By using the χ 2 statics method, we constrain the early dark energy model and growth factor from the above datasets. When including the GRB data, we use the cosmographic parameters to calibrate the GRB luminosity relations using SNe Ia. Results. At 2σ confidence level, we find that the fractional dark energy density at early times is Ω e < 0.05 using SNe Ia, CMB and BAO. When we include high-redshift probes, such as measurements of the linear growth factors and GRBs, the constraint is tightened considerably and becomes Ω e < 0.03 at 2σ confidence level. We also discuss the growth rate index γ .W e f indγ = 0.661 +0.302 −0.203 (1σ) using the SNe Ia, CMB, BAO and linear growth factor data. After including high-redshift GRB data, the growth index is γ = 0.653 +0.372 −0.363 .
118854953
High-redshift star formation rate up to z~8.3 derived from gamma-ray bursts and influence of background cosmology
The high-redshift star formation rate (SFR) is difficult to measure directly even by modern approaches. Long-duration gamma-ray bursts (GRBs) can be detected to the edge of the visible universe because of their high luminosities. The collapsar model of long GRBs indicates that they may trace the star formation history. So, long GRBs may be a useful tool of measuring the high-redshift SFR. Observations show that long GRBs prefer to form in a low-metallicity environment. We study the high-redshift SFR up to z∼ 8.3 considering the Swift GRBs tracing the star formation history and the cosmic metallicity evolution in different background cosmological models including Λ cold dark matter (ΛCDM), quintessence, quintessence with a time-varying equation of state and brane-world model. We use latest Swift GRBs including two highest-z GRBs, GRB 080913 at z= 6.7 and GRB 090423 at z= 8.3. We find that the SFR at z > 4 shows a steep decay with a slope of ∼−5.0 in ΛCDM. In the other three models, the high-redshift SFR is slightly different from ΛCDM model and also shows a steep decay.
119245048
An Evolving Stellar Initial Mass Function and the Gamma-ray Burst Redshift Distribution
Recent studies suggest that Swift gamma-ray bursts (GRBs) may not trace an ordinary star formation history (SFH). Here, we show that the GRB rate turns out to be consistent with the SFH with an evolving stellar initial mass function (IMF). We first show that the latest Swift sample of GRBs reveals an increasing evolution in the GRB rate relative to the ordinary star formation rate at high redshifts. We then assume only massive stars with masses greater than the critical value to produce GRBs and use an evolving stellar IMF suggested by Dave to fit the latest GRB redshift distribution. This evolving IMF would increase the relative number of massive stars, which could lead to more GRB explosions at high redshifts. We find that the evolving IMF can well reproduce the observed redshift distribution of Swift GRBs.
18159661
Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae
The property of dark energy and the physical reason for the acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernovae (SNe Ia) and anisotropy of cosmic microwave background (CMB). The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``Gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure Ω M = 0.28 −0.05 +0.04 , which is consistent with Riess et al. The transition redshift is z T = 0.60 −0.08 +0.06 . We also discuss several dark energy models that define w ( z ) of the parameterized equation of state of dark energy including one parameter and two parameters ( w ( z ) being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from z T = 0.29 −0.06 +0.07 to z T = 0.60 −0.08 +0.06 across these models. We also calculate the minimum redshift z c at which the current observations need the universe to accelerate.
44994799
Internal shock model for the X-ray flares of Swift J1644+57
Swift J1644+57 is an unusual transient event, likely powered by the tidal disruption of a star by a massive black hole. Multiple short time-scale X-ray flares were seen over a span of several days. We propose that these flares could be produced by internal shocks. In the internal shock model, the forward and reverse shocks are produced by collisions between relativistic shells ejected from a central engine. The synchrotron emission from the forward and reverse shocks could dominate in two quite different energy bands; under some conditions, the relativistic reverse shock dominates the X-ray emission and the Newtonian forward shock dominates the infrared and optical emission. We show that the spectral energy distribution of Swift J1644+57 could be explained by an internal shock model.
124213992
High performance computation on beam dynamics problems in high intensity compact cyclotrons
This paper presents the research progress in the beam dynamics problems for future high intensity compact cyclotrons by utilizing the state-of-the-art high performance computation technology. A “Start-to-Stop” model, which includes both the interaction of the internal particles of a single bunch and the mutual interaction of neighboring multiple bunches in the radial direction, is established for compact cyclotrons with multi-turn extraction. This model is then implemented in OPAL-CYCL, which is a 3D object-oriented parallel code for large scale particle simulations in cyclotrons. In addition, to meet the running requirement of parallel computation, we have constructed a small scale HPC cluster system and tested its performance. Finally, the high intensity beam dynamics problems in the 100 MeV compact cyclotron, which is being constructed at CIAE, are studied using this code and some conclusions are drawn.
118676242
Gravitational effects of condensate dark matter on compact stellar objects
We study the gravitational effect of non-self-annihilating dark matter on compact stellar objects. The self-interaction of condensate dark matter can give high accretion rate of dark matter onto stars. Phase transition to condensation state takes place when the dark matter density exceeds the critical value. A compact degenerate dark matter core is developed and alter the structure and stability of the stellar objects. Condensate dark matter admixed neutron stars is studied through the two-fluid TOV equation. The existence of condensate dark matter deforms the mass-radius relation of neutron stars and lower their maximum baryonic masses and radii. The possible effects on the Gamma-ray Burst rate in high redshift are discussed.
111221941
ASTROD and ASTROD I: Progress Report
Over the next decade the gravitational physics community will benefit from dramatic improvements in many technologies critical to the tests of gravity and gravitational-wave detection. The highly accurate deep space navigation, interplanetary laser ranging and communication, interferometry and metrology, high precision frequency standards, precise pointing and attitude control, together with the drag-free technologies will revolutionize the field of the experimental gravitational physics. Deep-space laser ranging will be ideal for gravitational-wave detection, and testing relativity and measuring solar-system parameter to an unprecedented accuracy. ASTROD I is such a mission with single spacecraft; it is the first step of ASTROD (Astrodynamical Space Test of Relativity using Optical Devices) with 3 spacecraft. In this paper, we will present the progress of ASTROD and ASTROD I with emphases on the acceleration noises, mission requirement, charging simulation, drag-free control and low-frequency gravitational-wave sensitivity.
185692670
Study on the High Order Modes of the 3.5cell Cavity at Peking University
As part of the updated DC-SC injector, a 3.5cell cavity has been fabricated at Peking University, which includes two Coaxial High Order Mode (HOM) couplers. The effect of the HOM couplers has been studied by numerical simulation and measurement. The results are highly uniform and show that the two couplers do effectively damp the HOMs. INTRODUCTION The 3.5cell cavity is part of the DC-SC injector, the feasibility of which is proved in 2004 [1] with beam done test. The updated 3.5cell cavity was designed and fabricated from large grain niobium, and the vertical test result (Eacc=23.4MV/m@Q0>10) at JLAB is exciting. The main RF parameters are listed in Table1. Table 1: RF Parameters of 3.5 Cell Cavity Type of accelerating structure Standing wave Accelerating mode TM010, pi mode fundamental frequency 1300MHz Quality factor ≥5x10 Active length 41.7cm Cell-to-cell coupling 1.8% Effective shunt impedance 418Ω Transit-time factor 0.74 Total longitudinal loss factor* 3.95(V/pC) transverse loss factor* 2.15(V/pC/cm) *the standard deviation length of the bunch is 2mm Among those RF parameters, the total longitudinal loss factor and transverse loss factor are from ABCI simulation; all the others are from Superfish. NUMERICAL CALCULATIONS OF HOMS The electron bunches passing through the cavity excite the eigenmodes of higher frequency which are HOMs. Here Microwave Studio is used to simulation the HOMs. To check whether there are trapped modes, two different boundary conditions at the end of beam pipes are used in two separate runs [2]. Large frequency shift of one mode indicates the mode depends on the boundary condition sensitivity; otherwise the mode could be trapped. The simulation shows TM110(2) and TM020(1)could be trapped. But they will not be excited by the beam as the beam bunch repetition frequency is 81.25MHz. All of the interested modes with ideal electric boundary condition below 3GHz are listed in Table 2. Table 2: HOMs of 3.5 Cell Cavity by Simulation Mode Mode f(MHz) *R/Q Mode1 TM010(1) 1278.69 9.75 Mode2 TM010(2) 1286.80 13.09 Mode3 TM010(3) 1296.83 16.63 Mode4 TM010(4) 1301.07 417.26 Mode5 TE111(1) 1634.35 0.07 Mode6 TE111(2) 1681.53 2.49 Mode7 TE111(3) 1747.03 10.07 Mode8 TM110(1) 1798.05 3.43 Mode9 TM110(2) 1849.61 3.47 Mode10 TM110(3) 1872.20 1.42 Mode11 TM110(4) 1883.84 0.77 Mode12 TM011(1) 2391.49 0.62 Mode13 TE111 (4) 2391.79 0.18 Mode14 TM011(2) 2421.67 12.65 Mode15 TM011(3) 2449.83 84.33 Mode16 TM020(1) 2658.30 23.97 Mode17 TM020(2) 2690.74 8.93 Mode18 TM020(3) 2728.82 4.26 Mode19 TM020(4) 2762.00 2.08 Mode20 TM111(1) 2819.80 0.71 Mode21 TM111(2) 2880.96 0.85 *unit of R/Q isΩ/cm; for monopole mode, n=0; dipole mode,n=2. The number next to the mode indicates the passband of the mode. For some modes, the electric field along the beam centre axis is almost zero, so the voltage is zero. Here we integrate the electric field along the axis 10 mm away from the beam centre and get the accelerating THPD009 Proceedings of IPAC’10, Kyoto, Japan 4296 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators voltage Vc , and the effective shunt impedance is ( ) ( ) U V Q R C ω / * / 2 * 0 = , where ω is the angular frequency of the mode, U is the stored energy. The effects of HOM couplers are simulated on a 2cell cavity also by CST Microwave Studio, which deduces the simulation time obviously [3]. The simulation shows Qe of HOM coupler on 2-cell TESLA cavity is 10 for fundamental mode, and 10~10 for HOMs. MEASUREMENTS OF THE RESONATE MODES In order to confirm the type of the modes, the electric field profile of the modes is measured by bead pulling. Two beads are used, one is a teflon cylinder with diameter of 4 mm and length of 4mm, the other is a copper cylinder with diameter of 5mm and length of 4mm. By comparing the measured electric fields with the simulated ones, 16 of all the 21 interested modes are obtained. The block diagram of the test bench is shown is Figure 1. Figure 1: Block diagram of the test bench. RF MEASUREMENTS OF THE HOM DAMPING ON THE CAVITY Before the HOM couplers are welded to the cavity, they are tested on a self-designed coaxial transmission line [4], to ensure the tuning range of the resonate frequency. The antenna of the HOM coupler is match to the load and with a round tip to increase the coupling as shown in Figure 2. Figure 2: The shape of the antenna. The antenna is made of copper in a series of length. With these antennas amounted, the gap from the antenna to the HOM coupler is 0.5mm, 1mm, 1.5mm, 2mm, 2.3mm. During this change, the Qext does not change much. Finally 1mm is chosen and the result is shown in Table 3. Table 3: Qe of the HOMs by Measurements mode f (MHz) Qe TE111(1) 1652.406 9.55x10 TE111(2) 1693.484 3.82x10 TE111(3) 1757.300 peak overlaps TM110(1) 1797.265 1.66x10 6 TM110(2) 1864.493 1.89x10 7 TM110(3) 1876.990 1.06x10 7 TM110(4) 1882.496 3.45x10 6 TE111(4) 2374.582 4.69x10 6 TM011(1) 2412.774 7.46x10 5 TM011(2) 2429.367 2.27x10 5 TM011(3) 2462.537 7.26x10 5 TM020(1) 2678.580 1.57x10 5 TM020(2) 2703.662 1.60x10 4 TM020(3) 2732.066 1.50x10 4 TM020(4) 2769.828 8.93x10 4 During the measurements, one HOM coupler is used because the available vector network analyzer has only two ports. For TE111(3) pass band, the two polarization modes are too close to measure directly by the vector network analyzer. Table 3 only lays out one polarization mode of each passband, the Qe of the other one is much bigger. Obviously one coupler is not enough for all the modes. CONCLUSION The test results of the coaxial HOM coupler on the 3.5cell large grain niobium cavity at room temperature indicate that the coupler works efficiently after a careful adjustment. The effects at 2K with beam loading are looking forward. REFERENCES [1] J. Hao et al., NIM A 557 (2006) 138-141. [2] F. Marhauser, P. Huelsmann, H. Klein, Proceedings of the 1999 Particle Accelerator Conference, New York, 1999. 3405-3407. [3] X. Luo, CPC (HEP & NP), to be published. [4] E. Wang, CPC (HEP & NP), 2009, 33(5): 401-405 Proceedings of IPAC’10, Kyoto, Japan THPD009 03 Linear Colliders, Lepton Accelerators and New Acceleration Techniques A08 Linear Accelerators 4297
109204818
Performance Limiting Effects in X‐Band Accelerators
Acceleration gradient is a critical parameter for the design of future TeV‐scale linear colliders. The major obstacle to higher gradient in room‐temperature accelerators is rf breakdown, which is still a very mysterious phenomenon that depends on the geometry and material of the accelerator as well as the input power and operating frequency. Pulsed heating has been associated with breakdown for many years however there have been no experiments that clearly separate field and heating effects on the breakdown rate. Recently, such experiments have been performed at SLAC with both standing‐wave and travelling‐wave structures. These experiments have demonstrated that pulsed heating is limiting the gradient. Also, a dual‐moded cavity has been designed to better distinguish the electric field, magnetic field and pulsed heating effects on breakdown.