Nuclear Experiment

A detailed study of event by event fluctuation of maximum particle density of the produced particles in narrow pseudo-rapidity interval in terms of the scaled variance {\omega} has been carried out for 16O-AgBr, 28Si-AgBr and 32S-AgBr interactions at an incident momentum of 4.5 AGeV/c. For all the interactions the values of scaled variance are found to be greater than zero indicating the presence of strong event by event fluctuation of maximum particle density values in the multiparticle production process. The event by event fluctuations are found to decrease with the increase of pseudo-rapidity interval. Experimental analysis has been compared with the results obtained from the analysis of events simulated by the Ultra Relativistic Quantum Molecular Dynamics (UrQMD) model. UrQMD model could not replicate the experimental results.

The genuine event-by-event correlations between three flow amplitudes are measured for the first time in Pb-Pb collisions at s NN ????????=2.76 TeV by the ALICE Collaboration at the Large Hadron Collider. The results are obtained with recently developed observables, the higher order Symmetric Cumulants (SC), in the midrapidity region |η|<0.8 and the transverse momentum range 0.2< p T <5.0 GeV/ c . These higher order observables show the same robustness against systematic biases arising from nonflow effects as the two-harmonic SC. The new results cannot be interpreted in terms of lower order flow measurements, since they are dominated by different patterns of event-by-event flow fluctuations. The results are compared with expectations from initial state models such as T R ENTo and next-to-leading order perturbative-QCD+saturation model of initial conditions, followed by iEBE-VISHNU and EKRT viscous hydrodynamic calculations. Model comparisons provide an indication of the development of genuine correlations between the elliptic v 2 , the triangular v 3 and the quadrangular v 4 flow amplitudes during the collective evolution of the medium. The comparison with the predictions for the correlations between v 2 , v 3 and the pentagonal flow magnitude v 5 illustrate the need for further tuning of model parameterizations. Therefore, these results can provide new and independent constraints for the initial conditions and system properties of nuclear matter created in heavy-ion collisions, complementary to previous flow measurements.

Background: The Efimov effect is a universal phenomenon in physics whereby three-body systems are stabilized via the interaction of an unbound two-body sub-systems. A hypothetical state in 12 C at 7.458 MeV excitation energy, comprising of a loose structure of three α -particles in mutual two-body resonance, has been suggested in the literature to correspond to an Efimov state in nuclear physics. The existence of such a state has not been demonstrated experimentally. Method: Using the combined data sets from two recent experiments, one with the TexAT TPC to measure α -decay and the other with Gammasphere to measure γ -decay of states in 12 C populated by 12 N and 12 B β -decay respectively, we achieve high sensitivity to states in close-proximity to the α -threshold in 12 C . Results: No evidence of a state at 7.458 MeV is seen in either data set. Using a likelihood method, the 95\% C.L. γ -decay branching ratio is determined as a function of the β -decay feeding strength relative to the Hoyle state. In parallel, calculations of the triple-alpha reaction rate show the inclusion of the Efimov corresponds to a large increase in the reaction rate around 5× 10 7 K. Conclusion: From decay spectroscopy - at the 95\% C.L., the Efimov state cannot exist at 7.458 MeV with any γ -decay branching ratio unless the β -strength is less than 0.7\% of the Hoyle state. This limit is evaluated for a range of different excitation energies and the results are not favorable for existence of the hypothetical Efimov state in 12 C . Furthermore, the triple-alpha reaction rate with the inclusion of a state between 7.43 and 7.53 MeV exceeds the rate required for stars to undergo the red giant phase.

The analysis of the nine 1-fold differential cross sections for the γ r,v p→ π + π − p photo- and electroproduction reactions obtained with the CLAS detector at Jefferson Laboratory was carried out with the goal to establish the contributing resonances in the mass range from 1.6~GeV to 1.8~GeV. In order to describe the photo- and electroproduction data with Q 2 -independent resonance masses and hadronic decay widths in the Q 2 range below 1.5~GeV 2 , it was found that an N ′ (1720)3/ 2 + state is required in addition to the already well-established nucleon resonances. This work demonstrates that the combined studies of π + π − p photo- and electroproduction data are vital for the observation of this resonance. The contributions from the N ′ (1720)3/ 2 + state and the already established N(1720)3/ 2 + state with a mass of 1.745~GeV are well separated by their different hadronic decays to the πΔ and ρp final states and the different Q 2 -evolution of their photo-/electroexcitation amplitudes. The N ′ (1720)3/ 2 + state is the first recently established baryon resonance for which the results on the Q 2 -evolution of the photo-/electrocouplings have become available. These results are important for the exploration of the nature of the ``missing'' baryon resonances.

The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited 116 Sn and 116 Te nuclei. These nuclei are populated via p + 115 In and 4 He + 112 Sn reactions in the excitation energy range of 18 - 26 MeV. Because of low excitation energy, the neutron spectra are predominantly contributed by the first-chance decay leading to the β -stable 115 Sn and neutron-deficient 115 Te as residues for the two cases. Theoretical analysis of the experimental spectra have been performed within the Hauser-Feshbach formalism by employing different models of the level density parameter. It is observed that the data could only be explained by the level density parameter that decreases monotonically when the proton number deviates from the β -stable value. This is also confirmed by performing a microscopic shell-model calculation with the Wood-Saxon mean field. The results have strong implication on the estimation of the level density of unstable nuclei, and calculation of astrophysical reaction rates relevant to r - and rp -processes.

Using 1.7±0.1n b −1 of lead-lead ( A=208 ) collision data recorded by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV, we report evidence of top quark pair ( t t ¯ ) production. The t t ¯ cross section ( σ t t ¯ ) is extracted from likelihood fits to a multivariate discriminator using lepton kinematic variables in dilepton final states and two methods. One method relies on the leptonic information alone, and the second one exploits, in addition, the presence of bottom quarks. The measured σ t t ¯ is 2.54 +0.84 −0.74 and 2.03 +0.71 −0.64 μb in the two cases, respectively, consistent with predictions from perturbative quantum chromodynamics. We demonstrate, for the first time, that top quark decay products (leptonically decaying W bosons and bottom quarks) can be identified, irrespective of any possible final-state interactions with the quark-gluon plasma.

The major goal of high-energy heavy-ion collisions is to study the properties of the deconfined quark gluon plasma (QGP), such as partonic collectivity. The collective motion of constituent quarks can be derived from the anisotropic flow measurements of identified hadrons within the coalescence framework. Based on published results of elliptic flow ( v 2 ), we shall test the coalescence sum rule using K ± , p , p ¯ , Λ and Λ ¯ , and further extract v 2 values for produced u ( d , u ¯ , d ¯ ), s and s ¯ quarks, as well as transported u(d) quarks in 10-40\% Au+Au collisions at s NN − − − √ = 7.7, 11.5, 14.5, 19.6, 27, 39 and 62.4 GeV. We also attempt to link the v 2 difference between π − and π + to the different numbers of u and d quarks in the initial gold ions, and to relate the v 2 measurements of multi-strange hadrons to the formation times of ϕ , Ω ± and Ξ + .

There is sparse direct experimental evidence that atomic nuclei can exhibit stable pear shapes arising from strong octupole correlations. In order to investigate the nature of octupole collectivity in radium isotopes, electric octupole ( E3 ) matrix elements have been determined for transitions in 222,228 Ra nuclei using the method of sub-barrier, multi-step Coulomb excitation. Beams of the radioactive radium isotopes were provided by the HIE-ISOLDE facility at CERN. The observed pattern of E 3 matrix elements for different nuclear transitions is explained by describing 222 Ra as pear-shaped with stable octupole deformation, while 228 Ra behaves like an octupole vibrator.

The FSU spsdfp cross-shell interaction for the shell model was successfully fitted to a wide range of mostly intruder negative parity states of the sd shell nuclei. This paper reports the application of the FSU interaction to systematically trace out the relative positions of the effective single-particle energies of the 0 f 7/2 and 1 p 3/2 orbitals, the evolution from normally ordered low-lying states to the "Island of Inversion" (IoI), and the behavior of a wide range of excited states with a 0 f 7/2 proton and neutron coupled to maximum spin of 7ℏ . Above a proton number of about 13 the 0 f 7/2 orbital lies below that of 1 p 3/2 , which is considered normal ordering, but systematically at Z=10 to 12 the orbitals cross. The calculations reproduce well the 2p2h - 0p0h inversion in the configurations of nuclei inside the IoI, they reproduce the absolute binding energies and the transition to normal ordering as the proton number approaches that of the neutrons. The important role of 1 p 3/2 neutron pairs in the IoI is also demonstrated. The calculations account well for the energies of the fully aligned states with 0, 1, or 2 individual sd nucleon aligned in spin with the aligned π0 f 7/2 - ν0 f 7/2 pair and reproduce well their systematic variation with A and number of aligned sd nucleons. The results presented in this paper give hope for the predictive power of the FSU interaction for more exotic nuclei to be explored in near future.

The dipole polarizability of stable even-mass tin isotopes 112,114,116,118,120,124 was extracted from inelastic proton scattering experiments at 295 MeV under very forward angles performed at RCNP. Predictions from energy density functionals cannot account for the present data and the polarizability of 208Pb simultaneously. The evolution of the polarizabilities in neighboring isotopes indicates a kink at 120Sn while all model results show a nearly linear increase with mass number after inclusion of pairing corrections.