Nuclear Experiment

Recent results on the analysis of event-by-event net-baryon number fluctuations in Pb--Pb collisions at s NN − − − √ =2.76 and 5.02 TeV are presented. The cumulants of the net-proton distributions, proxies for the net-baryon distributions, up to third order are discussed. The experimental results are compared with HIJING and EPOS model calculations and the dependence of fluctuation measurements on the phase-space coverage is addressed in the context of calculations from Lattice QCD (LQCD) and the Hadron Resonance Gas (HRG) model.

Precise constraints on the equation of state (EoS) of dense matter can be obtained through comparison of data from heavy-ion collisions (HIC) with transport models employing different effective interactions. An essential input for such comparisons is a reliable estimate of the impact parameter distributions P(b) which are representative of the data. For HIC in the intermediate energy range (20-150 MeV/A), there was no way up to now to extract such distributions from data in a model-independent way and it is well known that the only existing method for experimental impact parameter estimation underestimates those of the most central collisions, but not by how much. We adopt a method first developed for ultra-relativistic HIC in which a monotonic relationship is assumed between the mean value of a given observable X and b, whose parameters are adjusted in order to reproduce the b-integrated inclusive distribution P(X), taking into account fluctuations of X around <X(b)>. Using Bayes' theorem, the resulting conditional probability distribution P(X|b) can then be used to deduce both the impact parameter dependence of the observable X and the impact parameter distributions P(b|S) of any subset of events S represented by the corresponding experimental distribution P(X|S). We perform a survey of the bombarding energy, total mass and mass asymmetry dependence of the deduced impact parameter dependence for the most common observables used for centrality estimation and/or selections. A consistent picture of the evolution of reaction mechanisms in this energy range towards the participant-spectator regime emerges. Evaluating the effective centrality of commonly-used selections of "central collisions" we show that it is largely independent of the colliding system, decreasing in a very similar way with the available energy in the center of mass frame.

Inelastic 6 Li scattering at 100 MeV/u on 12 C and 93 Nb have been measured with the high-resolution magnetic spectrometer Grand Raiden. The magnetic-rigidity settings of the spectrometer covered excitation energies from 10 to 40 MeV and scattering angles in the range 0 ∘ < θ lab. < 2 ∘ . The isoscalar giant monopole resonance was selectively excited in the present data. Measurements free of instrumental background and the very favorable resonance-to-continuum ratio of 6 Li scattering allowed for precise determination of the E0 strengths in 12 C and 93 Nb. It was found that the monopole strength in 12 C exhausts 52± 3 (stat.) ± 8 (sys.) \% of the energy-weighted sum rule (EWSR), which is considerably higher than results from previous α -scattering experiments. The monopole strength in 93 Nb exhausts 92± 4 (stat.) ± 10 (sys.) \% of the EWSR, and it is consistent with measurements of nuclei with mass number of A≈90 . Such comparison indicates that the isoscalar giant monopole resonance distributions in these nuclei are very similar, and no influence due to nuclear structure was observed.

In our previous publications we discussed various manifestations of a new decay channel of the low excited heavy nuclei called collinear cluster tri-partition (CCT). The most populated CCT mode was revealed in the mass correlation distribution of fission fragments (FFs) as a local region ("bump") of increased yields below the loci linked to the conventional binary fission. The bump was dubbed "Ni-bump" because it is centered at the masses associated with the magic isotopes of Ni. Intriguing features of the CCT, especially high collinearity of the CCT partners and relatively high probability comparable with that typical for conventional ternary fission, have caused rather wide discussion. In the majority of dedicated publications, the FFs partitions from the Ni-bump have been analyzed from the different points of view. In our publications, we have underlined that Ni-bump manifests itself at the detectable level only in the spectrometer arm that faces the source backing. So far, this fact has been left beyond the scope of all known theoretical considerations, while the backing likely plays a crucial role in the observation of the CCT experimental pattern.

In 1963, a proton radius of 0.805(11) fm was extracted from electron scattering data and this classic value has been used in the standard dipole parameterization of the form factor. In trying to reproduce this classic result, we discovered that there was a sign error in the original analysis and that the authors should have found a value of 0.851(19) fm . We additionally made use of modern computing power to find a robust function for extracting the radius using this 1963 data's spacing and uncertainty. This optimal function, the Padé (0,1) approximant, also gives a result which is consistent with the modern high precision proton radius extractions.

The nucleus is an extraordinarily complex object where fundamental forces are at work. The solution of this many-body problem has challenged physicists for decades: several models with complementary virtues and flaws have been adopted, none of which has a universal predictive capability. Double beta decay is a second-order weak nuclear decay whose precise measurement might steer fundamental improvements in nuclear theory. Its knowledge paves the way to a much better understanding of many-body nuclear dynamics and clarifies, in particular, the role of multiparticle states. This is a useful input to a complete understanding of the dynamics of neutrino-less double beta decay, the chief physical process whose discovery may shed light to the matter-antimatter asymmetry of the universe and unveil the true nature of neutrinos. Here, we report the study of 2νββ -decay in 82 Se with the CUPID-0 detector, an array of ZnSe crystals maintained at a temperature close to 'absolute zero' in an ultralow background environment. Thanks to the unprecedented accuracy in the measurement of the two electrons spectrum, we prove that the decay is dominated by a single intermediate state. We obtain also the most precise value for the 82 Se 2νββ -decay half-life of T 2ν 1/2 =[ 8.6 +0.2 ??.1 ]? 10 19 yr.

Ultrarelativistic heavy ion collisions at the laboratory provide a unique chance to study quantum chromodynamics (QCD) under extreme temperature ( ≈150MeV ) and density ( ≈1GeV/ fm 3 ) conditions. Over the past decade, experimental results from LHC have shown further evidence for the formation of the quark-gluon plasma (QGP), a phase that is thought to permeate the early Universe and is formed in the high-density neutron-star cores. Various QCD predictions that model the behavior of the low- x gluon nuclear density, a poorly explored region, are also tested. Since the photon flux per ion scales as the square of the emitting electric charge Z 2 , cross sections of so far elusive photon-induced processes are extremely enhanced as compared to nucleon-nucleon collisions. Here, we review recent progress on CMS measurements of particle production with large transverse momentum or mass, photon-initiated processes, jet-induced medium response, and heavy quark production. These high-precision data, along with novel approaches, offer stringent constraints on initial state, QGP formation and transport parameters, and even parametrizations beyond the standard model.

The principal component analysis (PCA), a mathematical tool commonly used in statistics, has recently been employed to interpret the p T -dependent fluctuations of harmonic flow v n in terms of leading and subleading flow modes in heavy ion collisions. Using simulated data from AMPT and HIJING models, we show that the PCA modes are not fixed, but depend on the choice of the particle weight and the p T range. Furthermore, the shape of the leading mode is affected by the presence of non-flow correlations, and fake subleading mode may arise from the mixing of non-flow correlations with leading flow mode with a magnitude that could be larger than the genuine subleading flow mode. Therefore, the meaning of PCA modes and their relations to physical leading and subleading flow modes associated initial state eccentricities need to be further clarified/validated in realistic model simulations.

A direct measurement of the decay width of the excited 0 + 1 state of 6 Li using the relative self-absorption technique is reported. Our value of Γ γ, 0 + 1 → 1 + 1 =8.17(14 ) stat. (11 ) syst. eV provides sufficiently low experimental uncertainties to test modern theories of nuclear forces. The corresponding transition rate is compared to the results of ab initio calculations based on chiral effective field theory that take into account contributions to the magnetic dipole operator beyond leading order. This enables a precision test of the impact of two-body currents that enter at next-to-leading order.

The transparency of metal foils for ultracold neutrons (UCNs) plays an important role in the design of future high-density UCN sources, which will feed a number of fundamental physics experiments. In this work, we describe and discuss the measured transmission of a collimated beam of very slow neutrons (UCNs and very cold neutrons) through foils of Al, Cu, and Zr of various thicknesses at room temperature. Our goal was to separate scattering and absorption in the sample bulk from surface scattering, and to quantify the contribution of the surface. We were able to demonstrate that the surface roughness of these foils caused a significant fraction of UCN scattering. The surface roughness parameter b extracted from UCN measurements was shown to be of the same order of magnitude as the surface parameter determined by atomic-force microscopy. They lie in the order of several hundreds of angstroms. Using the formalism developed here, transmission data from previous neutron-optical experiments were re-analyzed and their surface roughness parameter b was extracted.