Experiments at the CERN SPS: first signals of deconfinement

This paper reviews the history of heavy-ion experiments at the CERN SPS, detailing how studies with light and lead ions from the mid-1980s to 2000 culminated in the announcement of evidence for quark-gluon plasma formation and a subsequent energy scan to determine the creation threshold.

The Gist

Here is an explanation of the paper, translated from complex physics jargon into a story about cooking, traffic, and melting ice.

The Big Idea: Melting the Unmeltable

Imagine the universe right after the Big Bang. It wasn't made of atoms (like the ones in your body or a star); it was a super-hot, super-dense soup where the tiny building blocks of matter (quarks and gluons) were swimming freely, not stuck together. Scientists call this "Quark-Gluon Plasma" (QGP).

For decades, physicists at CERN (the European physics lab) asked: Can we recreate this ancient soup in a lab?

This paper tells the story of how they did it using the SPS (Super Proton Synchrotron), a giant particle accelerator in Switzerland. They didn't just smash rocks together; they smashed heavy atomic nuclei (like Lead and Sulphur) into each other at near-light speed to create a "mini-Big Bang."

Here is how they did it, broken down by the different "detectives" (experiments) they sent on the case.

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1. The Setup: The Heavy-Ion Factory

In the 1980s, CERN started with light ions (Oxygen and Sulphur). Think of this like throwing tennis balls at a wall to see what happens. It was a good start, but to melt the "ice" of normal matter, they needed a bigger hammer.

In the 1990s, they built a new machine (Linac 3) to shoot Lead ions. This was like switching from tennis balls to cannonballs. They smashed these lead nuclei together at energies up to 158 GeV. This created a fireball so hot and dense that, for a tiny fraction of a second, the protons and neutrons inside the nuclei "melted" into the QGP soup.

2. The Detectives: How They Found the Soup

You can't see the QGP directly; it evaporates in a trillionth of a second. So, the scientists built seven different "detectives" (experiments) to look for clues left behind, like footprints in the snow.

Detective A: The Traffic Cop (NA44)

The Clue: Radial Flow.
Imagine a crowd of people standing still in a stadium. If someone sets off a firework in the middle, everyone runs outward.

• The Analogy: When the lead nuclei collided, the resulting fireball expanded outward like an explosion. The NA44 experiment measured how fast the particles (pions, protons) were flying away.
• The Result: They found that the particles were moving faster than expected, and the heavier ones were pushed harder. This proved the fireball was expanding with a collective "flow," just like a hot gas expanding. It was the first sign that a new, fluid-like state of matter had been created.

Detective B: The Ghost Hunter (NA45/CERES)

The Clue: Dileptons (Electron-Positron Pairs).
Most particles interact with everything around them, like a car crashing through a crowd. But electrons and positrons are "ghosts"—they pass through the dense soup without bumping into anything.

• The Analogy: If you want to know what's happening inside a crowded room, you can't ask the people inside. But if you have a ghost that flies through the room and tells you what it saw, that's valuable.
• The Result: These "ghosts" carried a message from the very center of the collision. They showed an excess of energy that couldn't be explained by normal matter. It suggested the "soup" was so hot that it was radiating heat (thermal radiation) and that the rules of how particles stick together (chiral symmetry) were changing.

Detective C: The Librarian (NA35 & NA49)

The Clue: Strange Particles.
In normal matter, "strange" particles (like Kaons and Lambda baryons) are rare. They are like finding a rare book in a library.

• The Analogy: The scientists predicted that if you create a QGP soup, it's like turning the library into a factory that only makes these rare books. The "strangeness" should go up massively.
• The Result: When they smashed Sulphur and Lead, they found a huge surplus of these strange particles. It was like finding the library had suddenly started printing thousands of copies of the rare book. This was a massive hint that the matter had changed phase.

Detective D: The Shadow Tracker (NA50 & NA60)

The Clue: Quarkonium Suppression.
Some particles, like the J/psi, are like a heavy anchor made of a charm quark and an anti-charm quark holding hands. In normal matter, they stay together. In the hot QGP soup, the heat is so intense it rips them apart.

• The Analogy: Imagine holding hands with a friend in a cool room. You stay together. But if you jump into a boiling pot of lava, you will be forced apart.
• The Result: The NA50 experiment saw that in heavy collisions, fewer J/psi particles survived. They were "suppressed" (melted). Later, the NA60 experiment (using a super-precise silicon camera) confirmed this and also found that the "heat" of the soup was around 200 MeV—hotter than the temperature needed to melt the protons and neutrons.

Detective E: The Flashlight (WA80, WA93, WA98)

The Clue: Direct Photons.
When you heat a piece of metal, it glows. The QGP soup should glow with "direct photons" (light) from the very first moment of the collision.

• The Analogy: It's hard to see the light of a candle in a room full of fireworks (decay photons). But the WA98 experiment managed to filter out the fireworks and spot the candle.
• The Result: They found a small but significant excess of direct light, confirming the fireball reached temperatures of 200 MeV.

Detective F: The Multi-Strangeness Hunter (WA97 & NA57)

The Clue: Multi-Strange Hyperons.
This detective looked for particles with two or three "strange" units (like the Omega particle).

• The Analogy: If the "strange book factory" theory is true, you shouldn't just find a few rare books; you should find the rarest, most complex ones in huge numbers.
• The Result: They found exactly that. The more "strange" the particle, the more of them were produced. This matched the prediction that the matter had reached a state of thermal equilibrium (a balanced soup) typical of QGP.

3. The Grand Conclusion: February 2000

After years of data, the scientists gathered at CERN in February 2000. They put all the clues together:

1. The fireball expanded like a fluid (Radial Flow).
2. It was incredibly hot (Direct Photons & Dileptons).
3. It was full of strange particles (Strangeness Enhancement).
4. It melted heavy anchors (Quarkonium Suppression).

The Verdict: They announced that they had successfully created a new state of matter: the Quark-Gluon Plasma. They had proven that at high enough energy, normal matter melts into a soup of free quarks and gluons.

The Legacy

This wasn't the end; it was the beginning.

• The SPS at CERN proved the soup exists.
• The RHIC collider in the US later proved it behaves like a perfect liquid.
• The LHC at CERN is now studying its properties in extreme detail.

The paper ends by mentioning NA61/SHINE, the current experiment. It's like a "2D scanner" that is now mapping out exactly where and how this transition happens, looking for the "Critical Point"—the exact moment where matter changes from a gas to a liquid, like water boiling into steam.

In short: CERN smashed heavy atoms together, melted them into a primordial soup, and used a team of high-tech detectives to prove that the universe once looked like that soup, and that we can recreate it today.

Technical Summary

Based on the provided text, here is a detailed technical summary of the paper "Experiments at the CERN SPS: first signals of deconfinement."

1. Problem Statement and Scientific Goal

The primary objective of the CERN Super Proton Synchrotron (SPS) heavy-ion program, initiated in the mid-1980s, was to study nuclear matter under extreme temperatures and densities to observe the Quark-Gluon Plasma (QGP). The QGP is a state of matter where quarks and gluons are deconfined from hadrons, predicted by Quantum Chromodynamics (QCD).

The central challenge was to identify unambiguous experimental signatures ("smoking guns") of this phase transition. Key theoretical predictions included:

• Deconfinement: The liberation of color charges.
• Chiral Symmetry Restoration: The restoration of symmetry broken in the vacuum, leading to changes in hadron masses and spectral functions.
• Thermalization: The formation of a hot, dense medium emitting thermal radiation (photons and dileptons).
• Strangeness Enhancement: Increased production of strange quarks due to the lower mass of strange quarks in the QGP compared to hadronic matter.

2. Methodology and Experimental Evolution

The program evolved through three generations of experiments, utilizing increasingly heavy ion beams (Oxygen, Sulphur, and Lead) and advanced detector technologies to increase collision energy and multiplicity.

A. Beam Evolution

• First Generation (1986–1992): Used light ions (Oxygen at 60 $A$GeV, Sulphur at 200 $A$GeV).
• Second Generation (1994–2000): Utilized Lead (Pb) beams at 158 $A$GeV, enabled by the construction of Linac 3. Later included an energy scan (20, 30, 40, 80 $A$GeV) to locate the QGP threshold.
• Third Generation (Post-2000): Focused on precision measurements and energy scans (NA61/SHINE) and follow-ups to previous anomalies (NA60).

B. Key Experiments and Techniques

1. NA44 (Particle Correlations & Flow):

   • Method: Used a Threshold Imaging Cherenkov (TIC) detector and Time-of-Flight (TOF) for particle identification ($\pi, K, p$).
   • Innovation: First to use CsI photocathodes in a noisy environment; measured two-particle correlations and transverse mass spectra to detect radial flow.

2. NA45/CERES (Dileptons):

   • Method: A "hadron-blind" spectrometer using Ring Imaging Cherenkov (RICH) detectors and Silicon Drift Chambers (SiDC).
   • Innovation: Designed to detect $e^+e^-$ pairs (dileptons) which escape the medium without final-state interactions, carrying information about the early thermal stage. Upgraded with a Time Projection Chamber (TPC) to improve mass resolution.

3. NA35/NA49 (Strangeness & Fluctuations):

   • Method: Large acceptance tracking using Streamer Chambers (NA35) and four large-volume Time Projection Chambers (TPCs) (NA49).
   • Innovation: NA49 performed a systematic energy scan (20–158 $A$GeV) to study the collision energy dependence of hadron yields, specifically looking for non-monotonic behavior ("Horn") indicating a phase transition.

4. NA38/NA50 (Quarkonium Suppression):

   • Method: Muon spectrometers with toroidal magnets and hadron absorbers.
   • Innovation: Measured $J/\psi$ and $\psi(2S)$ production. The "anomalous suppression" of $J/\psi$ was hypothesized to result from color screening in the QGP.

5. NA60 (Dimuon Specialist):

   • Method: Combined the NA50 muon spectrometer with a silicon pixel vertex tracker inside a dipole magnet.
   • Innovation: The vertex tracker allowed for precise reconstruction of displaced vertices (distinguishing open charm from prompt thermal radiation) and significantly improved mass resolution (20 MeV) for the $\rho$ meson.

6. WA85/WA94/WA97/NA57 (Hyperon Enhancements):

   • Method: Utilized the Omega spectrometer with a high-field superconducting magnet. Pioneered the use of Silicon Pixel Detectors (Omega-Ion chips) for tracking in high-multiplicity environments.
   • Innovation: Measured multi-strange hyperons ($\Xi, \Omega$) to test predictions of strangeness equilibration in QGP.

7. WA80/WA93/WA98 (Direct Photons):

   • Method: Calorimetry (Lead-glass, Plastic Ball) and Photon Multiplicity Detectors (PMD).
   • Innovation: Attempted to isolate direct thermal photons from the overwhelming background of decay photons to measure the initial temperature.

8. NA61/SHINE (2D Scan):

   • Method: Evolved from NA49, utilizing the same TPCs but with a new trigger and data acquisition system.
   • Innovation: Conducted a two-dimensional scan varying both collision energy and the mass of colliding nuclei (from $p+p$ to $Pb+Pb$) to map the transition from string-dominated to resonance-dominated to QGP-dominated production.

3. Key Contributions and Results

A. Evidence for Deconfinement (The "Horn," "Step," and "Kink")

• NA49 observed a non-monotonic energy dependence in the ratio of strange to non-strange hadrons ($K/\pi$), known as the "Horn." This peak at $\sim 30 A$GeV aligns with the Statistical Model of the Early Stage (SMES) prediction for the onset of deconfinement.
• The "Step" (a plateau in the inverse slope parameter $T$ of transverse mass spectra) and the "Kink" (change in energy dependence of pion yield) were also observed, collectively suggesting a first-order phase transition.

B. Strangeness Enhancement

• WA97/NA57 demonstrated a clear hierarchy in the enhancement of multi-strange antibaryons ($\bar{\Lambda}, \bar{\Xi}, \bar{\Omega}$) in Pb-Pb collisions compared to $p$-Be. The enhancement factor increased with strangeness content (up to $\sim 20$ for $\Omega$), consistent with statistical hadronization from a QGP source.

C. Dilepton and Chiral Symmetry Restoration

• CERES and NA60 observed an excess of low-mass $e^+e^-$ and $\mu^+\mu^-$ pairs.
• NA60 provided definitive evidence that this excess originates from thermal radiation (partonic and hadronic).
• Crucially, NA60 resolved the spectral function of the $\rho$ meson, showing a broadening (rather than a mass shift) in the medium. This supports the scenario of chiral symmetry restoration via the broadening of vector and axial-vector mesons.

D. Quarkonium Suppression

• NA50 reported "anomalous" suppression of $J/\psi$ in Pb-Pb collisions beyond what was expected from cold nuclear matter absorption.
• NA60 refined these measurements, showing that the suppression pattern follows the sequential melting scenario (where $\psi(2S)$ melts at lower temperatures than $J/\psi$), though the interpretation of the "anomalous" component in Pb-Pb remains complex due to energy-dependent absorption cross-sections.

E. Radial Flow and Temperature

• NA44 and NA60 provided strong evidence for radial flow (collective expansion) in heavy-ion collisions, indicated by the mass-dependent slope of transverse momentum spectra.
• NA60 extracted a temperature of $T \approx 205 \pm 12$ MeV from the intermediate mass region of dimuons, significantly exceeding the QCD pseudo-critical temperature ($T_c \approx 155$ MeV), confirming the creation of a partonic medium.

F. Direct Photons and Jet Quenching

• WA98 provided the first compelling evidence for direct photon production at low $p_T$, hinting at thermal radiation.
• WA98 also observed the suppression of high-$p_T$ neutral pions in central collisions, providing early evidence for jet quenching (energy loss of partons in the dense medium).

4. Significance and Impact

• Discovery of a New State of Matter: On February 10, 2000, CERN officially announced that the SPS experiments had found evidence for the formation of a new state of matter characterized by color deconfinement and collective explosion.
• Technological Legacy: The program pioneered critical technologies, including:
  • Silicon Pixel Detectors: The Omega/NA57 pixel telescope was the first in high-energy physics and directly influenced the design of the ALICE Inner Tracking System (ITS) at the LHC.
  • Hadron-Blind Spectrometers: The CERES RICH design set a precedent for electron identification in heavy-ion collisions.
  • Vertex Detectors in Absorbers: The NA60 concept of placing a vertex tracker before a hadron absorber became a standard for precision dimuon physics.
• Scientific Foundation: The results established the existence of a strongly interacting QGP and provided the first experimental constraints on the QCD phase diagram at high baryochemical potential.
• Future Directions: The findings motivated the Beam Energy Scan (BES) at RHIC and the CBM experiment at FAIR, and continue to drive the NA61/SHINE program at the SPS, which aims to locate the Critical Point (CP) of the QCD phase transition.

In summary, the CERN SPS heavy-ion program successfully transitioned from initial exploratory studies with light ions to precision measurements with lead beams, providing the first experimental evidence for the deconfinement of quarks and gluons and characterizing the properties of the Quark-Gluon Plasma.