Science Friday

The Largest US Particle Collider Stops Its Collisions

February 9, 2026

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  • The Relativistic Heavy Ion Collider (RHIC) ceased operations with a sense of celebration for its achievements, not sadness, as scientists have a wealth of data to analyze for years to come. 
  • RHIC's primary goal was to study the quark-gluon plasma, but the collisions unexpectedly revealed this state of matter to be a nearly perfect liquid with the lowest shear viscosity ever achieved, rather than the expected gas. 
  • The future of U.S. nuclear physics research shifts from high-temperature plasma studies to probing cold nuclear matter using a new collider design that will collide small electrons with large nuclei, a project expected to take about a decade. 

Segments

RHIC Shutdown Sentiment
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(00:02:05)
  • Key Takeaway: The final collisions at RHIC were treated as a celebration of its successful lifetime, with the emotional impact centered on the long-term relationships among the collaborating scientists.
  • Summary: The conclusion of RHIC’s operations was marked by celebration due to its successful run since 2000. The emotional aspect of shutting down the facility is tied to the decades-long professional relationships formed by the researchers. Significant positive results and stored data remain for future analysis.
RHIC Design Goals
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(00:03:16)
  • Key Takeaway: RHIC was specifically designed to create and study the quark-gluon plasma, a hypothesized gaseous state of matter formed by extremely hot and dense nuclear material.
  • Summary: The initial purpose of RHIC was to investigate the quark-gluon plasma, a new state of matter hinted at in lower-energy collisions. This plasma is analogous to water turning into steam, where nuclear matter constituents (quarks and gluons) become liberated from the nucleus structure due to high energy and density. This phase change reveals crucial information about the fundamental interactions within matter.
Discovery of Perfect Liquid
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(00:05:36)
  • Key Takeaway: Contrary to expectations of a gaseous state, RHIC discovered the quark-gluon plasma behaves as a nearly perfect liquid, exhibiting the lowest shear viscosity ever achieved.
  • Summary: The experimental results showed that the quark-gluon plasma was not gaseous but a liquid plasma. This liquid possesses extremely low shear viscosity, meaning its movement is highly fluid and resists internal friction. This characteristic places it near the quantum mechanical limit for a perfect fluid.
Heavy Ion Collision Rationale
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(00:06:39)
  • Key Takeaway: Heavy ions, such as gold or lead nuclei, are used because their large mass provides sufficient nuclear material to observe bulk properties like liquid or gaseous states during collision.
  • Summary: The term ‘heavy ion’ refers to an atom with a very large nucleus. Colliding heavy nuclei is necessary to concentrate enough nuclear matter at the collision point to observe bulk phenomena. Studying only a few particles is insufficient to characterize liquid or gaseous plasma states.
Future Research Direction
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(00:08:21)
  • Key Takeaway: The nuclear physics community is pivoting from high-temperature plasma studies to better understanding normal, cold nuclear matter, necessitating a new electron-ion collider.
  • Summary: The closure of RHIC does not signify the end of all questions, but a strategic shift in focus toward understanding cold nuclear matter structure. The new facility will involve colliding small electrons with large nuclei to probe internal structure without generating extreme heat. This new collider is projected to take approximately a decade to construct, utilizing one of RHIC’s existing storage rings.
Post-Shutdown Activities
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(00:11:08)
  • Key Takeaway: U.S. physicists will spend the next decade analyzing the large volume of high-rate data collected in RHIC’s final years of operation.
  • Summary: Researchers have substantial data accumulated, especially from recent upgrades that allowed for faster data acquisition rates. Analyzing this stored data is expected to occupy about two-thirds of the next decade. The lack of a running U.S. collider places the entire global physics community at a temporary disadvantage.
Safety and Natural Collisions
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(00:12:22)
  • Key Takeaway: Concerns about RHIC creating dangerous anomalies like black holes were unfounded because similar high-energy collisions occur naturally throughout space without adverse effects on celestial bodies.
  • Summary: The facility was designed to study collisions in a controlled environment that naturally occur when cosmic rays strike Earth or the Moon. If these natural, high-energy collisions created detectable black holes, evidence would have already been observed across nature. RHIC simply allowed scientists to study these events when and where they chose.