2017 Vol. 41, No. 6
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By analyzing the large-angle Bhabha scattering events e+e-→(γ)e+e- and diphoton events e+e-→(γ)γγ for the data sets collected at center-of-mass (c.m.) energies between 2.2324 and 4.5900 GeV (131 energy points in total) with the upgraded Beijing Spectrometer (BESIII) at the Beijing Electron-Positron Collider (BEPCII), the integrated luminosities have been measured at the di erent c.m. energies, individually. The results are important inputs for the R value and J/Ψ resonance parameter measurements.
We investigate new physics effects on the Wtb effective couplings in a model-independent framework. The new physics effects can be parametrized by four independent couplings, f1L, f1R, f2L and f2R. We further introduce a set of parameters x0, xm, xp and x5 which exhibit a linear relation to the single top production cross sections. Using recent data for the t-channel single top production cross section σt, tW associated production cross section σtW, s-channel single top production cross section σs, and W-helicity fractions F0, FL and FR collected at the 8 TeV LHC and Tevatron, we perform a global fit to impose constraints on the top quark effective couplings. Our global fitting results show that the top quark effective couplings are strongly correlated. We show that (i) improving the measurements of σt and σtW is important in constraining the correlation of (f1R,f2R) and (f2L,f2R); (ii) f1L and f2R are anti-correlated, and are sensitive to all the four experiments; (iii) f1R and f2L are also anti-correlated, and are sensitive to the F0 and FL measurements; (iv) the correlation between f2L and f2R is sensitive to the precision of the σt, σtW and F0 measurements. The effective Wtb couplings are studied in three kinds of new physics models: the G(221)=SU(2)1⊗SU(2)2⊗U(1)X models, the vector-like quark models and the Littlest Higgs model with and without T-parity. We show that the Wtb couplings in the left-right model and the un-unified model are sensitive to the ratio of gauge couplings when the new heavy gauge boson's mass (MW') is less than several hundred GeV, but the constraint is loose if MW' >1 TeV. Furthermore, the Wtb couplings in vector-like quark models and the Littlest Higgs models are sensitive to the mixing angles of new heavy particles and SM particles.
The discovery of unexpected properties of the Higgs boson would o er an intriguing opportunity to shed light on some of the most profound puzzles in particle physics. Beyond Standard Model (BSM) decays of the Higgs boson could reveal new physics in a direct manner. Future electron-positron lepton colliders operating as Higgs factories, including CEPC, FCC-ee and ILC, with the advantages of a clean collider environment and large statistics, could greatly enhance sensitivity in searching for these BSM decays. In this work, we perform a general study of Higgs exotic decays at future e+e- lepton colliders, focusing on the Higgs decays with hadronic nal states and/or missing energy, which are very challenging for the High-Luminosity program of the Large Hadron Collider (HL-LHC). We show that with simple selection cuts, O(10-3-10-5) limits on the Higgs exotic decay branching fractions can be achieved using the leptonic decaying spectator Z boson in the associated production mode e+e-→ZH. We further discuss the interplay between detector performance and Higgs exotic decays, and other possibilities of exotic decays. Our work is a rst step in a comprehensive study of Higgs exotic decays at future lepton colliders, which is a key area of Higgs physics that deserves further investigation.
We calculate the contributions of a general non-vacuum conformal family to Rényi entropy in two-dimensional conformal field theory (CFT). The primary operator of the conformal family can be either non-chiral or chiral, and we denote its scaling dimension by Δ. For the case of two short intervals on a complex plane, we expand the Rényi mutual information by the cross ratio x to order x2Δ+2. For the case of one interval on a torus with low temperature, we expand the Rényi entropy by q=exp(-2πβ/L), with β being the inverse temperature and L being the spatial period, to order qΔ+2. To make the result meaningful, we require that the scaling dimension Δ cannot be too small. For two intervals on a complex plane we need Δ >1, and for one interval on a torus we need Δ >2. We work in the small Newton constant limit on the gravity side and so a large central charge limit on the CFT side, and find matches of gravity and CFT results.
The Eigenstate Method has been developed to deduce the fermion propagator with a constant external magnetic field. In general, we find its result is equivalent to other methods and this new method is more convenient, especially when one evaluates the contribution from the infinitesimal imaginary term of the fermion propagator. Using the Eigenstate Method we try to discuss whether the infinitesimal imaginary frequency of the fermion propagator in a strong magnetic field and Lorentz-violating extension of the minimal SU(3)×SU(2)×SU(1) Standard Model could have a significant influence on the dynamical mass. When the imaginary term of the fermion propagator in this model is not trivial (√ < σ < √ ), this model gives a correction to the dynamical mass. When one does not consider the influence from the imaginary term (σ > √ ), there is another correction from the conventional term. Under both circumstances, chiral symmetry is broken.
A recent experimental study of excited 8Be decay to its ground state revealed an anomaly in the angular distribution of the final states. This exceptional result is attributed to a new vector gauge boson X(16.7). We study the significance of this new boson, especially its effect in anomalies observed in long-lasting experimental measurements. By comparing the discrepancies between the Standard Model predictions and the experimental results, we find the values and regions of the couplings of X(16.7) to the muon and muon neutrino. In this work, we find that the newly observed boson X(16.7) may be the solution of both the NuTeV anomaly and the (g-2)μ puzzle.
We consider the systematics of α-decay half-lives of super-heavy nuclei versus the decay energy and the total α-kinetic energy. We calculate the half-lives using the experimental Qα values. The computed half-lives are compared with the experimental data and with existing empirical estimates and are found to be in good agreement. Also, we obtain α-preformation factors from the ratio between theoretical and experimental results for some super-heavy nuclei and evaluate the standard deviation. The results indicate the acceptability of the approach.
The role of positive Q-value neutron transfers in sub-barrier fusion reactions has been studied with a modified quantum coupled channels model with all order couplings (CCFULL model). Neutron rearrangement related only to the dynamical matching condition with no free parameters is implemented in the model, which provides a way to understand especially the Q-value dependence of sub-barrier fusion reactions. The fusion cross sections of the collision systems 40Ca+94, 96Zr have been calculated and analyzed. The general trend of experimental data can be reproduced well with additional channels for neutron rearrangement. We find that enhancement of sub-barrier fusion cross sections is closely related to the Q-value of the transferred neutrons, in particular for channels with sequential even number transferred neutrons.
Within the alpha-cluster model, we particularly investigate the alpha decay of exotic nuclei in the vicinity of the N=126 neutron shell plus the Z=82 proton shell. The systematics of alpha-preformation probability (Pα), as an indicator of the shell effect, is deduced from the ratio of the experimental decay width to the calculated one. Through the comparative analysis of the Pα trend in the N=124-130 isotonic chain, the N=126 and Z=82 shell closures are believed to strongly affect the formation of the alpha particle before its penetration. Additionally, the Pα variety in Po and Rn isotopes is presented as another proof for such an influence. More importantly, it may be concluded that the expected neutron (or proton) shell effect gradually fades away along with the increasing valence proton (or neutron) number. The odd-even staggering presented in the Pα value is also discussed.
Alpha-induced reactions on 154Sm, 233,235,236,238U, and 237Np deformed nuclei are studied theoretically. The effects of hexadecapole deformation, deformed surface diffuseness parameter, and orientation on barrier height and position, fusion cross-section at any angle, and fusion cross-section have been investigated. Both hexadecapole deformation and deformed surface diffuseness can affect barrier characteristics and enhance fusion cross-section. Good agreement between experimental data and theoretical calculations with quadrupole and hexadecapole deformation and deformed surface diffuseness were observed for the 4He+154Sm, 235U, 237Np reactions.
The nuclear structure of the actinide even-even thorium isotopes from A=230-234 have been investigated within the framework of the Interacting Boson Model (IBM-1). Predictions are given for the excited state energies for the ground state, β and γ -bands, the transition probabilities between these states, the rotational moment of inertia, and the energy staggering in the γ-band energies. The results of these calculations are compared with the experimental data on these isotopes.
It has recently been pointed out that, under certain conditions, the energy of particles accelerated by black holes in the center-of-mass frame can become arbitrarily high. In this paper, we study the collision of two particles in the case of four-dimensional charged nonrotating, extremal charged rotating and near-extremal charged rotating Kaluza-Klein black holes as well as the naked singularity case in Einstein-Maxwell-dilaton theory. We find that the center-of-mass energy for a pair of colliding particles is unlimited at the horizon of charged nonrotating Kaluza-Klein black holes, extremal charged rotating Kaluza-Klein black holes and in the naked singularity case.
Recent observations show that the electromagnetic fine-structure constant, αe, may vary with space and time. In the framework of Finsler spacetime, we propose here an anisotropic cosmological model, in which both spatial and temporal variations of αe are allowed. Our model naturally leads to the dipole structure of αe, and predicts that the dipole amplitude increases with time. We fit our model to the most up-to-date measurements of αe from the quasar absorption lines. It is found that the dipole direction points towards (l,b)=(330.2°±7.3°,-13.0°±5.6°) in galactic coordinates, and the anisotropic parameter is b0=(0.47±0.09) ×10-5, which corresponds to a dipole amplitude (7.2±1.4)×10-8 at redshift z=0.015. This is consistent with the upper limit of the variation of αe measured in the Milky Way. We also fit our model to Union2.1 type Ia supernovae, and find that the preferred direction of Union2.1 is consistent with the dipole direction of αe.
Hannay's angle is a classical analogue of the “geometrical phase factor” found by Berry in his research on the quantum adiabatic theorem. This classical analogue is defined if closed curves of constant action variables return to the same curves in phase space after an adaibatic evolution. Adiabatic evolution of Yang-Mills cosmology, which is described by a time-dependent quartic oscillator, is investigated. Phase properties of the Yang-Mills fields are analyzed and the corresponding Hannay's angle is derived from a rigorous evaluation. The obtained Hannay's angle shift is represented in terms of several observable parameters associated with such an angle shift. The time evolution of Hannay's angle in Yang-Mills cosmology is examined from an illustration plotted on the basis of numerical evaluation, and its physical nature is addressed. Hannay's angle, together with its quantum counterpart Berry's phase, plays a pivotal role in conceptual understanding of several cosmological problems and indeed can be used as a supplementary probe for cosmic inflation.
The statistical properties of the soft gamma repeater SGR J1550-5418 are investigated carefully. We find that the cumulative distributions of fluence, peak flux and duration can be well fitted by a bent power law, while the cumulative distribution of waiting time follows a simple power law. In particular, the probability density functions of fluctuations of fluence, peak flux, and duration have a sharp peak and fat tails, which can be well fitted by a q-Gaussian function. The q values keep approximately steady for different scale intervals, indicating a scale-invariant structure of soft gamma repeaters. Those results support that the origin of soft gamma repeaters is crustquakes of neutron stars with extremely strong magnetic fields.
Plastic scintillation detectors for Time-of-Flight (TOF) measurements are almost essential for event-by-event identification of relativistic rare isotopes. In this work, a pair of plastic scintillation detectors of dimensions 50×50 ×3t mm3 and 80×100×3t mm3 have been set up at the External Target Facility (ETF), Institute of Modern Physics (IMP). Their time, energy and position responses are measured with the 18O primary beam at 400 MeV/nucleon. After off-line corrections for walk effect and position, the time resolutions of the two detectors are determined to be 27 ps (σ) and 36 ps (σ), respectively. Both detectors have nearly the same energy resolution of 3.1% (σ) and position resolution of about 3.4 mm (σ). The detectors have been used successfully in nuclear reaction cross section measurements, and will be be employed for upgrading the RIBLL2 beam line at IMP as well as for the high energy branch at HIAF.
An ultrafast front-end ASIC chip has been developed for APD array detectors in X-ray time-resolved experiments. The chip has five channels: four complete channels and one test channel with an analog output. Each complete channel consists of a preamplifier, a voltage discriminator and an open-drain output driver. A prototype chip has been designed and fabricated using 0.13 μm CMOS technology with a chip size of 1.3 mm×1.9 mm. The electrical characterizations of the circuit demonstrate a very good intrinsic time resolution (rms) on the output pulse leading edge, with the test result better than 30 ps for high input signal charges (> 75 fC) and better than 100 ps for low input signal charges (30-75 fC), while keeping a low power consumption of 5 mW per complete channel.
The Jiangmen Underground Neutrino Observatory (JUNO) detector is designed to determine the neutrino mass hierarchy and precisely measure oscillation parameters. The general purpose design also allows measurements of neutrinos from many terrestrial and non-terrestrial sources. The JUNO Event Data Model (EDM) plays a central role in the offline software system. It describes the event data entities through all processing stages for both simulated and collected data, and provides persistency via the input/output system. Also, the EDM is designed to enable flexible event handling such as event navigation, as well as the splitting of MC IBD signals and mixing of MC backgrounds. This paper describes the design, implementation and performance of the JUNO EDM.
5-cell elliptical cavities have been selected for the main linac of the China Accelerator Driven sub-critical System (C-ADS) in the medium energy section. According to the design, each cavity should be driven with radio frequency (RF) energy up to 150 kW by a fundamental power coupler (FPC). As the cavities work with high quality factor and high accelerating gradient, the coupler should keep the cavity from contamination in the assembly procedure. To fulfil the requirements, a single-window coaxial type coupler was designed with the capabilities of handling high RF power, class 10 clean room assembly, and heat load control. This paper presents the coupler design and gives details of RF design, heat load optimization and thermal analysis as well as multipacting simulations. In addition, a primary high power test has been performed and is described in this paper.
Producing high-brightness and high-charge (>100 pC) electron bunches at blowout regime requires ultrashort laser pulses with high fluence. The effects of laser pulse heating of the copper photocathode are analyzed in this paper. The electron and lattice temperature is calculated using an improved two-temperature model, and an extended Dowell-Schmerge model is employed to calculate the thermal emittance and quantum efficiency. A time-dependent growth of the thermal emittance and the quantum efficiency is observed. For a fixed amount of charge, the projected thermal emittance increases with decreasing laser radius, and this effect should be taken into account in laser optimization at blowout regime. Moreover, laser damage threshold fluence is simulated, showing that the maximum local fluence should be less than 40 mJ/cm2 to prevent damage to the cathode.
In this paper, a new type of magnet is proposed and produced to give a uniform transverse beam profile. Compared to octupole magnets, the new type of magnet can provide a similar octupole magnet field in the middle, but the rise rate declines quickly at the edges, so that a beam of the same uniformity is obtained with less particle loss. Besides that, a mechanical structure is added to adjust the width of the middle region to satisfy different transverse dimensions, which would further reduce particle loss. Some numerical simulations have been done with the octupole and the new type of magnet to show the advantages of the new magnet.
The secondary neutron fields at the deep tumor therapy terminal at HIRFL (Heavy Ion Research Facility in Lanzhou) were investigated. The distributions of neutron ambient dose equivalent were measured with a FHT762 Wendi-II neutron ambient dose equivalent meter as 12C ions with energies of 165, 207, 270, and 350 MeV/u were bombarded on thick tissue-like targets. The thickness of targets used in the experiments was larger than the range of the carbon ions. The neutron spectra and dose equivalent were simulated by using FLUKA code, and the results agree well with the experimental data. The experiment results showed that the neutron dose produced by fragmentation reactions in tissue can be neglected in carbon-ion therapy, even considering their enhanced biological effectiveness. These results are also valuable for radiation protection, especially in the shielding design of high energy heavy ion medical machines.
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