High-precission measurement of light hypernuclear masses

• Applicant: Professor Dr. Josef Pochodzalla
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2018 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 397465087

The lightest hypernucleus, the hypertriton, is a loosely bound system of a proton, a neutron and a lambda hyperon. Its understanding presents an important benchmark for strangeness nuclear physics. The combination of its unexpected short lifetime and its small lambda binding energy deduced from emulsion studies more than 40 years ago presents one of the most intriguing puzzles in hypernuclear physics. For this weakly bound strange system a lifetime comparable to the one of the free lambda hyperon is expected. Indeed, recent analyses of hypernuclear binding energies suggest, that the systematic uncertainties in nuclear emulsion studies may be larger than anticipated in the past. The project aims at a determination of ground state masses of light hypernuclei, in particular of the hypertriton, with unprecedented precision. MAMI is the only place worldwide where such a measurement is possible. The project uses a new method to determine the masses of weakly decaying hypernuclei by precision pion spectroscopy, which was pioneered at MAMI. Firstly, in order to reduce the systematic uncertainty of this method we will perform a novel high precision beam energy measurement based on interfering undulator radiation. Secondly, making use of the excellent beam quality at MAMI we will employ a new target geometry to reach a higher luminosity. The proposed method for a precise energy calibration of the MAMI beam utilizes the interference of coherent radiation in the optical regime from two undulators driven by the electron beam of MAMI. In a feasibility study, we have successfully demonstrated this new method. Optimizing the experimtal setup, we aim at a relative precision of better than 10-4 for beam energies around 200 MeV. Furthermore, we will explore the precision limitations of this novel method by studying the systematic effects of this method. To reach the desired luminosity we will design and develop low-density targets for the high resolution pion spectroscopy experiment at MAMI. In the first experiment, we will use a 50 mm thick lithium-6 target sheet. The use of such a target geometry is only possible because of the excellent beam quality and stability of MAMI. Besides the detection of the hypertriton, this measurement will also significantly increase the number of observed events of the A=4 hypernucleus consisting of 1 proton, 2 neutrons and 1 Lambdahyperon.The better precision will enable to address the question of the sign of the charge symmetry breaking effect of the excited statesof A=4 hypernuclei. Furthermore, the relative production yields of several hypernuclei will lead to a better understanding of thehypernucleus production process in electron induced reactions. This observation will allow to test the production model and to make more reliable predictions for future studies. Thus we will be able to optimize the experimental conditions for studies on p-shell hypernuclei or searches for e.g. exotic hypernuclei.

• DFG Programme: Research Grants
• Co-Investigators: Professor Dr. Patrick Achenbach; Dr. Werner Lauth