The relative yields of Υ mesons produced in pp and PbPb collisions at √ s NN = 5.02 TeV and reconstructed via the dimuon decay channel are measured using data collected by the CMS experiment. Double ratios are formed by comparing the yields of the excited states, Υ(2S) and Υ(3S), to the ground state, Υ(1S), in both PbPb and pp collisions at the same center-of-mass energy. The double ratios, [Υ(nS)/Υ(1S)] PbPb /[Υ(nS)/Υ(1S)] pp , are measured to be 0.308 ± 0.055 (stat) ± 0.019 (syst) for the Υ(2S) and less than 0.26 at 95% confidence level for the Υ(3S). No significant Υ(3S) signal is found in the PbPb data. The double ratios are studied as a function of collision centrality, as well as Υ transverse momentum and rapidity. No significant dependencies are observed.
The dynamics of fission has been studied by solving Euler-Lagrange equations with dissipation generated through one and two body nuclear friction. The average kinetic energies of the fission fragments, prescission neutron multiplicities and the mean energies of the prescission neutrons have been calculated and compared with experimental values and they agree quite well. A single value of friction coefficient has been used to reproduce the experimental data for both symmetric and asymmetric splitting of the fissioning systems over a wide range of masses and excitation energies. It has been observed that a stronger friction is required in the saddle to scission region as compared to that in the ground state to saddle region.
Constraining excitation energy at which nuclear shell effect washes out has important implications on the production of super heavy elements and many other fields of nuclear physics research. We report the fission fragment mass distribution in alpha induced reaction on an actinide target for wide excitation range in close energy interval and show direct evidence that nuclear shell effect washes out at excitation energy ∼ 40 MeV. Calculation shows that second peak of the fission barrier also vanishes around similar excitation energy. One of the major areas that have generated unprecedented interest among contemporary nuclear physicists and chemists is the synthesis of super heavy elements (SHE). It is known from the Liquid Drop Model (LDM) of the nucleus [1] that if the two fundamental nuclear parameters, the attractive nuclear surface potential and the repulsive coulomb forces are taken into account, then our nuclear chart may end at around element number 104. This is simply because, nuclei with Z ≥ 104 immediately fission as there is no barrier to prevent their decay. However, elements have been synthesized beyond that atomic number [2].The observed stability of these heavy elements is believed to originate from the microscopic shell effects in nuclei. While LDM predicts the bulk properties of nuclei and explains their collective behavior, nuclear Shell Model [3] explains these shell gaps and the single-particle nature of nuclear states. Both the bulk properties and the shell properties of nuclei can be incorporated by adding a shell-correction term to the liquid-drop model energy. Strutinsky [4,5] considered the shell effect as a deviation from uniform liquid drop model prediction and used the shell averaged single particle energy as a correction term to the liquid drop model energy. The liquid drop barrier height diminishes smoothly with the increase in atomic number as the nuclear fissility increases. However, as the shell correction term retains the fluctuations in the shell model energy, it is found that the incorporation of this shell correction alters the fission barrier and in fact, causes to develop large barrier to decay that can increase alpha or fission half-lives by several orders of magnitude for the heavy elements. Thus shell effects play a central role in determining the stability of the super heavy elements. Many important nuclear phenomena such as the fission isomers [6], super deformed nuclei [7] and new magic numbers in the exotic nuclei [8] are the consequences of the shell effect. * tilak@vecc.gov.in It is generally believed that shell effects are washed out at higher excitation energy [9]. For the production of the super heavy elements by heavy ion bombardment on actinides targets, the compound nuclei are always formed with an excitation energy exceeding a few tens of MeV. Judicious choice of the excitation energy is critical as the production cross section of the SHE may be increased by a few orders of magnitude if the beam energy is increased by few MeV. Therefore, constraining the exci...
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