title: Breakdown of the isobaric multiplet mass equation for the A = 20 and 21 multiplets.
abstract: Using the Penning trap mass spectrometer TITAN, we performed the first direct mass measurements of (20,21)Mg, isotopes that are the most proton-rich members of the A = 20 and A = 21 isospin multiplets. These measurements were possible through the use of a unique ion-guide laser ion source, a development that suppressed isobaric contamination by 6 orders of magnitude. Compared to the latest atomic mass evaluation, we find that the mass of (21)Mg is in good agreement but that the mass of (20)Mg deviates by 3 σ. These measurements reduce the uncertainties in the masses of (20,21)Mg by 15 and 22 times, respectively, resulting in a significant departure from the expected behavior of the isobaric multiplet mass equation in both the A = 20 and A = 21 multiplets. This presents a challenge to shell model calculations using either the isospin nonconserving universal sd USDA and USDB Hamiltonians or isospin nonconserving interactions based on chiral two- and three-nucleon forces.
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 Canada.
authors.forename: A T
authors.initials: AT
authors.lastname: Gallant
authors.affiliation: Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA.
authors.forename: M
authors.initials: M
authors.lastname: Brodeur
authors.affiliation: Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, V5A 1S6 Canada.
authors.forename: C
authors.initials: C
authors.lastname: Andreoiu
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and École des Mines de Nantes, La Chantrerie, 4, rue Alfred Kastler, B.P. 20722, F-44307 Nantes Cedex 3, France.
authors.forename: A
authors.initials: A
authors.lastname: Bader
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada.
authors.forename: A
authors.initials: A
authors.lastname: Chaudhuri
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canada.
authors.forename: U
authors.initials: U
authors.lastname: Chowdhury
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada.
authors.forename: A
authors.initials: A
authors.lastname: Grossheim
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Klawitter
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada.
authors.forename: A A
authors.initials: AA
authors.lastname: Kwiatkowski
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, V5A 1S6 Canada.
authors.forename: K G
authors.initials: KG
authors.lastname: Leach
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Institut für Kernphysik, Westfälische Wilhelms-Universität, D-48149 Münster, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Lennarz
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 Canada.
authors.forename: T D
authors.initials: TD
authors.lastname: Macdonald
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada.
authors.forename: B E
authors.initials: BE
authors.lastname: Schultz
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canada.
authors.forename: J
authors.initials: J
authors.lastname: Lassen
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada.
authors.forename: H
authors.initials: H
authors.lastname: Heggen
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada.
authors.forename: S
authors.initials: S
authors.lastname: Raeder
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canada.
authors.forename: A
authors.initials: A
authors.lastname: Teigelhöfer
authors.affiliation: Department of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824-1321, USA.
authors.forename: B A
authors.initials: BA
authors.lastname: Brown
authors.affiliation: Department of Physics, Florida State University, Tallahassee, Florida 32306, USA.
authors.forename: A
authors.initials: A
authors.lastname: Magilligan
authors.affiliation: Department of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824-1321, USA and Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany.
authors.forename: J D
authors.initials: JD
authors.lastname: Holt
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Menéndez
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Simonis
authors.affiliation: Institut für Kernphysik, Technische Universität Darmstadt, 64289 Darmstadt, Germany and ExtreMe Matter Institute EMMI, GSI Helmholtzzentrum für Schwerionenforschung GmbH, 64291 Darmstadt, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Schwenk
authors.affiliation: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, V6T 2A3 Canada and Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1 Canada.
authors.forename: J
authors.initials: J
authors.lastname: Dilling