2018 Vol. 42, No. 3
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We analyze the transverse momentum (pT) and rapidity (y) spectra of top quark pairs, hadronic top quarks, and top quarks produced in proton-proton (m pp) collisions at center-of-mass energy √ =8 TeV. For pT spectra, we use the superposition of the inverse power-law suggested by the QCD (quantum chromodynamics) calculus and the Erlang distribution resulting from a multisource thermal model. For y spectra, we use the two-component Gaussian function resulting from the revised Landau hydrodynamic model. The modelling results are in agreement with the experimental data measured at the detector level, in the fiducial phase-space, and in the full phase-space by the ATLAS Collaboration at the Large Hadron Collider (LHC). Based on the parameter values extracted from pT and y spectra, the event patterns in three-dimensional velocity (βx-βy-βz), momentum (px-py-pz), and rapidity (y1-y2-y) spaces are obtained, and the probability distributions of these components are also obtained.
It is difficult to measure the m WW-fusion Higgs production process (e+e-→ννh) at a lepton collider with a center of mass energy of 240-250GeV due to its small rate and the large background from the Higgsstrahlung process with an invisible Z (e+e-→hZ,Z→νν). We construct a modified recoil mass variable, mrecoilp, defined using only the 3-momentum of the reconstructed Higgs particle, and show that it can separate the m WW-fusion and Higgsstrahlung events better than the original recoil mass variable mrecoil. Consequently, the mrecoilp variable can be used to improve the overall precisions of the extracted Higgs couplings, in both the conventional framework and the effective-field-theory framework. We also explore the application of the mrecoilp variable in the inclusive cross section measurements of the Higgsstrahlung process, while a quantitive analysis is left for future studies.
Heavy charged gauge bosons are proposed in some theories beyond the standard model. We explore the discovery potential for m W'→tb with top quark semi-leptonic decay at the LHC. We concentrate on the new physics signal search with the deviation from the standard model prediction if the resonance peak of m W' cannot be observed directly. Signal events with two jets plus one charged lepton and missing energy are simulated, together with the dominant standard model backgrounds. In this paper, it is found that suitable cuts on the kinematic observables can effectively suppress the standard model backgrounds, so that it is possible to search for a m W' signal at the LHC if its mass is less than 6.6 TeV.
Motivated by flavor non-universality and anomalies in semi-leptonic B-meson decays, we present a general and systematic discussion about how to construct anomaly-free U(1)' gauge theories based on an extended standard model with only three right-handed neutrinos. If all standard model fermions are vector-like under this new gauge symmetry, the most general family non-universal charge assignments, (a,b,c) for three-generation quarks and (d,e,f) for leptons, need satisfy just one condition to be anomaly-free, 3 (a+b+c)=-(d+e+f). Any assignment can be linear combinations of five independent anomaly-free solutions. We also illustrate how such models can generally lead to flavor-changing interactions and easily resolve the anomalies in B-meson decays. Probes with Bs-Bs mixing, decay into τ±, dilepton and dijet searches at colliders are also discussed.
The exclusive decay of the Higgs boson to a vector meson (J/ψ or Υ(1S)) and m Z boson is studied in this work. The decay amplitudes are separated into two parts in a gauge invariant manner. The first part comes from the direct coupling of the Higgs boson to the charm (bottom) quark and the other from the m HZZ* or the loop-induced m HZγ* vertexes in the standard model. While the branching ratios from the direct channel are much smaller than those of the indirect channel, their interference terms give nontrivial contributions. We further calculate the QCD radiative corrections to both channels, which reduce the total branching ratios by around 20% for both J/ψ and Υ(1S) production. Our results provide a possible chance to check the SM predictions of the m H cc(H bb) coupling and to seek for hints of new physics at the High Luminosity LHC or future hadron colliders.
The properties of the low-lying states, especially the relevant shape coexistence in 80Ge, close to one of most neutron-rich doubly magic nuclei at N=50 and Z=28, have been investigated within the framework of the proton-neutron interacting model (IBM-2). Based on the fact that the relative energy of the d neutron boson is different from that of the proton boson, the calculated energy levels of low-lying states and E2 transition strengths can reproduce the experimental data very well. Particularly, the first excited state 0+2, which is intimately related to the shape coexistence phenomenon, is reproduced quite nicely. The ρ2(E2, 02+→01+) transition strength is also predicted. The experimental data and theoretical results indicate that both collective spherical and γ-soft vibration structures coexist in 80Ge.
The ratio of pairing-energy coefficient to temperature (ap/T) of neutron-rich fragments produced in spallation reactions has been investigated by adopting an isobaric yield ratio method deduced in the framework of a modified Fisher model. A series of spallation reactions, 0.5A and 1A GeV 208Pb + p, 1A GeV 238U + p, 0.5A GeV 136Xe + d, 0.2A, 0.5A and 1A GeV 136Xe + p, and 56Fe + p with incident energy ranging from 0.3A to 1.5A GeV, has been analysed. An obvious odd-even staggering is shown in the fragments with small neutron excess (I≡ N-Z), and in the relatively small-A fragments which have large I. The values of ap/T for the fragments, with I from 0 to 36, have been found to be in a range from -4 to 4, and most values of ap/T fall in the range from -1 to 1. It is suggested that a small pairing-energy coefficient should be considered in predicting the cross sections of fragments in spallation reactions. It is also concluded that the method proposed in this article is not good for fragments with A/As > 85% (where As is the mass number of the spallation system).
A classical analysis of shape phase transitions and phase coexistence in odd-even nuclei has been performed in the framework of the interacting boson-fermion model. The results indicate that the effects of a single particle may influence different types of transitions in different ways. Especially, it is revealed that phase coexistence can clearly emerge in the critical region and thus be taken as a indicator of the shape phase transitions in odd-even nuclei.
We investigate mixing of the lowest-lying qqq configurations with JP=1/2- caused by the hyperfine interactions between quarks mediated by Goldstone Boson Exchange, One Gluon Exchange, and both Goldstone Boson and One Gluon exchange, respectively. The first orbitally excited nucleon, Σ, Λ and Ξ states are considered. Contributions of both the contact term and tensor term are taken into account. Our numerical results show that mixing of the studied configurations in the two employed hyperfine interaction models are very different. Therefore, the present results, which should affect the strong and electromagnetic decays of baryon resonances, may be used to examine the present employed hyperfine interaction models.
We propose to interpret the DAMPE electron excess at 1.5 TeV through scalar or Dirac fermion dark matter (DM) annihilation with doubly charged scalar mediators that have lepton-specific Yukawa couplings. The hierarchy of such lepton-specific Yukawa couplings is generated through the Froggatt-Nielsen mechanism, so that the dark matter annihilation products can be dominantly electrons. Stringent constraints from LEP2 on intermediate vector boson production can be evaded in our scenarios. In the case of scalar DM, we discuss one scenario with DM annihilating directly to leptons and another scenario with DM annihilating to scalar mediators followed by their decays. We also discuss the Breit-Wigner resonant enhancement and the Sommerfeld enhancement in the case where the s-wave annihilation process is small or helicity-suppressed. With both types of enhancement, constraints on the parameters can be relaxed and new ways for model building can be opened in explaining the DAMPE results.
In recent years there has been a lot of interest in discussing frame dependences/independences of the cosmological perturbations under the conformal transformations. This problem has previously been investigated in terms of the covariant approach for a single component universe, and it was found that the covariant approach is very powerful to pick out the perturbative variables which are both gauge and conformal invariant. In this work, we extend the covariant approach to a universe with multicomponent fluids. We find that similar results can be derived, as expected. In addition, some other interesting perturbations are also identified to be conformal invariant, such as entropy perturbation between two different components.
We have carried out an approximate analytical solution to precisely consider the influence of magnetic field on the transverse oscillation of particles in a cyclotron. The differential equations of transverse oscillation are solved from the Lindstedt-Poincare method. After careful deduction, accurate first-order analytic solutions are obtained. The analytical solutions are applied to the magnetic field from an isochronous cyclotron with four spiral sectors. The accuracy of these analytical solutions is verified and confirmed from comparison with a numerical method. Finally, we discussed the transverse oscillation at v0=N/2, using the same analytical solution.
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