<p>Decay pion spectroscopy provides a direct and model-independent approach to the precise mass measurement of light hypernuclei, such as the hypertriton (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$^3_\Lambda $</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow /> <mi mathvariant="normal">Λ</mi> <mn>3</mn> </mmultiscripts> </math></EquationSource> </InlineEquation>H), through the monochromatic momentum of pions emitted in two-body weak decays. In this experiment at the Mainz Microtron (MAMI), decay pions from stopped hypernuclei produced in the <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$p(e,e'K^+)\Lambda $</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>p</mi> <mo stretchy="false">(</mo> <mi>e</mi> <mo>,</mo> <msup> <mi>e</mi> <mo>′</mo> </msup> <msup> <mi>K</mi> <mo>+</mo> </msup> <mo stretchy="false">)</mo> <mi mathvariant="normal">Λ</mi> </mrow> </math></EquationSource> </InlineEquation> reaction on a natural lithium target were measured with a high-resolution magnetic spectrometer, Spectrometer A (SpecA). Since the achievable precision of the hypertriton mass is directly determined by the accuracy of the pion momentum, this contribution focuses on the momentum calibration essential for decay pion spectroscopy. A comprehensive calibration based on elastic electron scattering was performed using <InlineEquation ID="IEq3"> <EquationSource Format="TEX">$^{181}$</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow /> <mrow /> <mn>181</mn> </mmultiscripts> </math></EquationSource> </InlineEquation>Ta and <InlineEquation ID="IEq4"> <EquationSource Format="TEX">$^{12}$</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow /> <mrow /> <mn>12</mn> </mmultiscripts> </math></EquationSource> </InlineEquation>C targets. The reconstructed scattering angle, momentum linearity over the full spectrometer acceptance, and the dependence on the reaction point along the beam axis were systematically evaluated and corrected. These procedures constrained the systematic uncertainty of the elastic-scattering-based momentum calibration to below 10 keV, excluding the beam-energy contribution. The established calibration procedure provides the foundation for a high-precision determination of the hypertriton mass relying on decay pion spectroscopy.</p>

Spectrometer Momentum Calibration for a Precise Hypertriton Mass Measurement with Decay Pion Spectroscopy

期刊论文
发表日期：

2026年5月13日

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Ryoko Kino,
Patrick Achenbach,
Sho Nagao,
Satoshi N. Nakamura,
Kotaro Nishi,
Pascal Klag,
Josef Pochodzalla,
Tianhao Shao

摘要

Decay pion spectroscopy provides a direct and model-independent approach to the precise mass measurement of light hypernuclei, such as the hypertriton ( $^3_\Lambda $ $_{Λ}^{3}$ H), through the monochromatic momentum of pions emitted in two-body weak decays. In this experiment at the Mainz Microtron (MAMI), decay pions from stopped hypernuclei produced in the $p(e,e'K^+)\Lambda $ $p (e, e^{'} K^{+}) Λ$ reaction on a natural lithium target were measured with a high-resolution magnetic spectrometer, Spectrometer A (SpecA). Since the achievable precision of the hypertriton mass is directly determined by the accuracy of the pion momentum, this contribution focuses on the momentum calibration essential for decay pion spectroscopy. A comprehensive calibration based on elastic electron scattering was performed using $^{181}$ $^{181}$ Ta and $^{12}$ $^{12}$ C targets. The reconstructed scattering angle, momentum linearity over the full spectrometer acceptance, and the dependence on the reaction point along the beam axis were systematically evaluated and corrected. These procedures constrained the systematic uncertainty of the elastic-scattering-based momentum calibration to below 10 keV, excluding the beam-energy contribution. The established calibration procedure provides the foundation for a high-precision determination of the hypertriton mass relying on decay pion spectroscopy.