We report a study of ν(μ) charged-current quasielastic events in the segmented scintillator inner tracker of the MINERvA experiment running in the NuMI neutrino beam at Fermilab. The events were selected by requiring a μ- and low calorimetric recoil energy separated from the interaction vertex. We measure the flux-averaged differential cross section, dσ/dQ², and study the low energy particle content of the final state. Deviations are found between the measured dσ/dQ² and the expectations of a model of independent nucleons in a relativistic Fermi gas. We also observe an excess of energy near the vertex consistent with multiple protons in the final state.

We have isolatedνµ charged-current quasi-elastic interactions occurring in the segmented scintillator tracking region of the MINERvA detector running in the NuMI neutrino beam at Fermilab. We measure the flux-averaged differential cross-section, dσ/dQ 2 , and compare to several theoretical models of quasi-elastic scattering. Good agreement is obtained with a model where the nucleon axial mass, MA, is set to 0.99 GeV/c 2 but the nucleon vector form factors are modified to account for the observed enhancement, relative to the free nucleon case, of the cross-section for the exchange of transversely polarized photons in electron-nucleus scattering. Our data at higher Q 2 favor this interpretation over an alternative in which the axial mass is increased.

We present results from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. We report the first mass measurements of 48 Ar and 49 Ar and find atomic mass excesses of -22.28(31) MeV and -17.8(1.1) MeV, respectively. These masses provide strong evidence for the closed shell nature of neutron number N = 28 in argon, which is therefore the lowest even-Z element exhibiting the N = 28 closed shell. The resulting trend in binding-energy differences, which probes the strength of the N = 28 shell, compares favorably with shell-model calculations in the sd-pf shell using SDPF-U and SDPF-MU Hamiltonians.The "magic" numbers of protons and neutrons, which enhance nuclear binding for isotopes near the valley of β-stability, can evolve for more neutron-rich or neutrondeficient nuclei [1][2][3]. The neutron magic number N = 28 has been the subject of extensive recent experimental and theoretical investigations [4][5][6][7][8]. Since neutron-rich N = 28 nuclei are within experimental reach and are computationally tractable for shell-model calculations, they are ideal candidates for illuminating the fundamental forces at work in exotic nuclei. It is known that the N = 28 shell gap, which stabilizes doubly magic Mg 28 suggests it has a prolate deformed ground state [18], which would be consistent with the absence of a neutron shell gap.The existence of the N = 28 shell gap for argon is a matter of some controversy. Several previous experimental studies have assessed the shell structure of neutronrich argon [19][20][21][22][23][24][25][26][27][28][29] [19,20] and one at Coulomb-barrier beam energy [29], deduce a low B(E2), corresponding to a reduced quadrupole collectivity. In this case quadrupole collectivity reflects a propensity for neutrons to be excited across the N = 28 shell gap, and thus a low B(E2) may be expected for a semi-magic nucleus. State-of-the-art shell-model calculations that properly account for the breakdown of the N = 28 magic number in silicon and sulfur isotopes predict a markedly higher B(E2) for 46 Ar [28]. A low-statistics lifetime measurement of the 2 + 1 state of 46 Ar deduced a high B(E2) value in agreement with theory [27], but at odds with the three consistent, independent Coulomb excitation measurements [19,20,29].However, B(E2) measurements are not necessarily unambiguous probes of neutron shell structure, since they are sensitive to proton degrees of freedom and proton-neutron interactions. In contrast, mass measurements, and the neutron separation energies derived from them, directly probe the neutron shell gap in a modelindependent way.We report here results from the first [31] mass measurements of 48 Ar and 49 Ar, which provide robust evidence for the persistence of the N = 28 shell gap for argon. These results were obtained with the time-offlight (TOF) technique at the National Superconducting Cyclotron Laboratory (NSCL) [32][33][34]. Neutron-rich isotopes of silicon to zinc were produced by fragmentation of a 140 ...

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Time-of-flight mass measurements of neutron-rich chromium isotopes up to N=40 and implications for the accreted neutron star crust We present the mass excesses of 59−64 Cr, obtained from recent time-of-flight nuclear mass measurements at the National Superconducting Cyclotron Laboratory at Michigan State University. The mass of 64 Cr was determined for the first time with an atomic mass excess of −33.48(44) MeV. We find a significantly different two-neutron separation energy S2n trend for neutron-rich isotopes of chromium, removing the previously observed enhancement in binding at N = 38. Additionally, we extend the S2n trend for chromium to N = 40, revealing behavior consistent with the previously identified island of inversion in this region. We compare our results to state-of-the-art shell-model calculations performed with a modified Lenzi-Nowacki-Poves-Sieja interaction in the f p-shell, including the g 9/2 and d 5/2 orbits for the neutron valence space. We employ our result for the mass of 64 Cr in accreted neutron star crust network calculations and find a reduction in the strength and depth of electron capture heating from the A = 64 isobaric chain, resulting in a cooler than expected accreted neutron star crust. This reduced heating is found to be due to the over 1 MeV reduction in binding for 64 Cr with respect to values from commonly used global mass models. I. INTRODUCTION 22The evolution of nuclear structure away from the val- production of more neutron-rich nuclides of interest were 106 used alternately, keeping Bρ of the A1900 and S800 fixed. 107Fragments were transmitted through the A1900 fragment spectively. 124The relationship between TOF and nuclear rest mass 125 m rest is obtained from the equation of motion for a 126 charged massive particle through a magnetic system. 127Equating the two counteracting forces, the Lorentz force 128 F L and the centripetal force F c , results in the followingwhere the Lorentz factor γ is a function of velocity v, the Bρ-corrected data were fit with a Gaussian distribu-158 tion in order to determine a mean TOF for each nuclide. 159The relationship between mass over charge m rest /q and 160TOF was fit to the data of reference nuclides in order to