Physics

High-precision measurement of the kaonic hydrogen 1s level shift and width with SIDDHARTA-2

M. Bazzi, F. Clozza, C. Guaraldo, M. A. Iliescu, A. Scordo, F. Sgaramella, D. Sirghi, F. Sirghi, J. Zmeskal

Featured July 20, 2026

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Simply

A new experiment precisely measured how the strong force changes the energy and lifetime of a special 'kaonic hydrogen' atom, helping scientists better understand tiny particle interactions and a mysterious particle called the $\Lambda(1405)$ resonance.

In depth

The SIDDHARTA-2 experiment achieved a factor-of-two improvement in precision for measuring the strong-interaction-induced shift (

e p s i l o n_{1 s}

) and width (

G amm a_{1 s}

) of kaonic hydrogen's 1s level. This was accomplished through an upgraded detector system and reduced background, providing significantly tighter experimental constraints on theoretical models of the low-energy antikaon-nucleon interaction and the

L amb d a (1405)

resonance.

Key Takeaways

1
The SIDDHARTA-2 experiment provides the most precise determination to date of the kaonic hydrogen 1s level shift ($\\epsilon_{1s}$) and width ($\\Gamma_{1s}$), improving precision by approximately a factor of two.
2
This enhanced precision is attributed to an upgraded silicon drift detector (SDD) system with a larger active area, improved timing resolution, and reduced electronic noise, alongside a factor-of-three reduction in DA$\\Phi$NE machine background.
3
The new measurements significantly tighten experimental constraints on low-energy antikaon-nucleon ($\\bar{K}N$) interaction models and refine the theoretical description of the $\\Lambda(1405)$ resonance and few- and many-body antikaonic systems.

Conceptual Flow

HIGH LEVEL

Methodology: How was it done?

Scientists used a special machine to make exotic atoms and then measured tiny X-ray flashes from them with super sensitive cameras.

Particle Collider

Hydrogen Gas

Create Exotic Atoms

X-ray Flashes

Results: What did they find?

They found the X-ray flashes were shifted and broadened in a very specific way, giving the most accurate numbers yet for how the strong force acts on these atoms.

Old Measurement Precision

New Detector System

Measure X-ray Shifts & Widths

Much Higher Precision