Evidence for in-medium modification of the phi meson at normal nuclear density

Area of Science:

• Nuclear Physics
• Particle Physics
• Hadron Spectroscopy

Background:

• Vector mesons are crucial probes for understanding the properties of nuclear matter.
• Previous studies suggested modifications to rho and omega mesons in nuclear environments.
• Investigating in-medium vector meson properties provides insights into the strong interaction.

Purpose of the Study:

• To experimentally verify in-medium mass modification of vector mesons, specifically the phi meson.
• To study the nuclear-size dependence of vector meson spectral shapes.
• To provide evidence for changes in vector meson properties at normal nuclear density.

Main Methods:

• Measurement of invariant mass spectra of e(+)e(-) pairs from 12 GeV p+A reactions.
• Utilizing copper and carbon targets to probe nuclear-size effects.
• Analysis of phi meson peak shapes in different betagamma regions.

Main Results:

• A significant low-mass excess was observed for phi mesons in the low betagamma region (< 1.25) with copper targets.
• The spectral shape of phi mesons in the high betagamma region (> 1.25) was consistent with the Breit-Wigner shape, accounting for experimental effects.
• Experimental verification of vector meson mass modification at normal nuclear density was achieved.

Conclusions:

• The study provides experimental evidence for in-medium mass modification of vector mesons.
• The observed modification is dependent on the nuclear environment and meson kinematics (betagamma).
• These findings contribute to the understanding of hadron properties within nuclear matter.