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The future fixed-target program at the CERN SPS

This paper reviews the historical achievements of the CERN SPS fixed-target program in Quark-Gluon Plasma studies and outlines the future physics program involving NA61 and the newly approved NA60+/DiCE experiment to further explore the QCD phase diagram at finite baryon chemical potential.

Original authors: Enrico Scomparin

Published 2026-07-27

📖 7 min read🧠 Deep dive

CC BY 4.0

Original authors: Enrico Scomparin

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Soup and the Time Machine

Imagine the universe just a split second after the Big Bang. It wasn't made of stars, planets, or even atoms. Instead, it was a super-hot, super-dense soup where the tiny building blocks of matter—quarks and gluons—were swimming freely, not stuck together inside protons or neutrons. Scientists call this state of matter "Quark-Gluon Plasma" (QGP). It's like the difference between ice cubes (frozen matter) and boiling water (free-flowing plasma). To understand how our universe cooled down and formed everything we see today, physicists need to recreate this soup in the lab.

But there's a twist. We know how to make this soup at extremely high temperatures (like in the Large Hadron Collider), but we also want to know what happens when the soup is "thicker" or has more "stuff" packed into it. In physics terms, this is called a high "baryochemical potential" ( $\mu_B$ ). Think of it like the difference between a crowded concert where everyone is squished together versus a sparse park. The rules of how matter behaves might change depending on how crowded it is. The paper you are about to read is about a special machine at CERN, called the SPS, which acts like a time machine and a pressure cooker, allowing scientists to smash heavy atoms together to create this specific type of crowded, hot soup and study how it behaves.

The CERN SPS: A Heavy-Ion Time Machine

The CERN Super Proton Synchrotron (SPS) is a giant particle accelerator that has been a star player in the world of nuclear physics since 1986. While other machines might focus on the highest possible energies, the SPS has a special superpower: it can smash heavy ions (like lead atoms) together at a wide range of speeds. This allows scientists to explore a specific "corner" of the universe's history where the density of matter is high, but the temperature is just right to see how the "soup" changes.

The paper, written by Enrico Scomparin, is a roadmap for the future of this program. It argues that even though the SPS has been running for decades, we haven't fully explored the "middle ground" of its energy range. Most past experiments ran at the top speed (158 GeV/nucleon for lead-lead collisions), but the real secrets might be hiding at lower speeds. The author suggests that with new, sharper tools, we can finally answer big questions: Does the transition from normal matter to the Quark-Gluon Plasma happen smoothly, or is there a sudden "phase change" like water turning to ice? Is there a "critical point" where the rules of physics get weird?

The New Players: NA61/SHINE and NA60+/DiCE

To tackle these questions, the paper introduces two main characters: an upgraded veteran and a brand-new rookie.

1. The Upgraded Veteran: NA61/SHINE
The NA61/SHINE experiment has been around since 2009, studying how particles are born in collisions. It's famous for finding a "horn" in the data—a weird spike in the number of particles produced that hints at the moment matter switches from being "hadronic" (normal) to "partonic" (the QGP soup).

The paper outlines a new plan for NA61/SHINE to smash lighter nuclei (like oxygen, magnesium, and boron) together at three different speeds: 13, 30, and 150 A GeV/c.

The Analogy: Imagine testing how a sponge behaves when you squeeze it lightly, moderately, and hard. By using different sizes of "sponges" (light nuclei) and squeezing them at different speeds, scientists hope to see exactly when the "sponge" starts acting like a fluid (the QGP).
The Upgrade: They plan to replace a big detector with a super-fast silicon camera system. This will let them take pictures 10,000 times a second, capturing details they missed before. They also want to look for "charm" particles (heavy versions of normal particles) in lead collisions, something they haven't done directly before.

2. The Brand-New Rookie: NA60+/DiCE
Approved in June 2026, NA60+/DiCE is the paper's big new proposal. It is designed to be a high-precision microscope for the "hard" and "electromagnetic" probes of the QGP.

The Analogy: If the QGP is a hot, foggy room, most particles are like people walking through the fog; they get bumped around and lose energy. But "hard probes" (like heavy quarks) and "electromagnetic probes" (like light particles called dileptons) are like ghosts. They pass right through the fog without getting bumped, carrying a perfect message from the very center of the explosion.
The Setup: This experiment uses a clever two-part detector. The first part (the vertex spectrometer) uses ultra-modern silicon sensors to track where particles come from with incredible precision (down to 20 micrometers!). The second part (the muon spectrometer) catches particles that make it all the way through a thick wall of concrete.
The Goal: By matching the tracks from both sides, they hope to measure the temperature of the soup with a projected uncertainty of only ~4%. They are specifically looking for a "caloric curve"—a graph that shows how the temperature of the soup changes as you add more energy. If this curve flattens out, it would be a smoking gun for a first-order phase transition (a sudden change of state).

What the Simulations Say

The paper doesn't just talk about ideas; it runs simulations to see if these new detectors can actually do the job.

The "Ghost" Signal: In simulations of lead-lead collisions at 8.8 GeV, the team expects to see a clear signal of "thermal radiation" (light emitted by the hot soup) in the mass range between 1.5 and 2.5 GeV/c². They predict they can measure the temperature slope with high precision.
The "Chiral" Mystery: They are also hunting for a sign that "chiral symmetry" is being restored. This is a fancy way of saying the particles are losing their "mass" and becoming more like the free-roaming quarks of the early universe. The simulations suggest that if this happens, there will be a 20–30% increase in a specific type of particle signal.
The Heavy Hitters: For the heavy "charm" particles, the simulations show that even with a lot of background noise, the new detectors should be able to clearly identify particles like the $D^0$ meson. They also simulated a measurement of the $J/\psi$ particle (a type of heavy quark pair) and predicted they could see a 30% suppression (a sign the soup is melting the particle) in the most violent collisions.

The Bottom Line

The paper concludes that the CERN SPS is still a powerhouse, capable of running until around 2041. It is uniquely positioned to explore the "finite $\mu_B$ " region of the universe's phase diagram—a place where other machines can't reach. While the NA61/SHINE experiment will continue to map out the general landscape of particle production, the new NA60+/DiCE experiment promises to take high-definition photos of the most elusive signals.

The authors are confident that with these new tools, we will finally be able to see if the transition to Quark-Gluon Plasma is a smooth slide or a sudden cliff, and whether a "critical point" exists in the laws of nature. It's not a solved mystery yet, but the paper argues that the SPS is the perfect place to find the answer, provided we give it the right equipment to look.

Technical Summary: The Future Fixed-Target Program at the CERN SPS

Problem and Motivation
The CERN Super Proton Synchrotron (SPS) remains a unique facility capable of accelerating heavy ions in the center-of-mass energy range of $5 < \sqrt{s_{NN}} < 17$ GeV, corresponding to a baryochemical potential ( $\mu_B$ ) of approximately 220–450 MeV. While the initial phase of SPS experiments (concluded in 2000) established compelling evidence for the formation of a new state of matter (Quark-Gluon Plasma, QGP) through discoveries such as strangeness enhancement and $J/\psi$ suppression, the energy domain below the top SPS energy (158 GeV/nucleon) remains under-explored. Specifically, there is a lack of data on hard and electromagnetic probes at lower energies, and the region potentially hosting the QCD critical point (CP) and the onset of a first-order phase transition requires further investigation. The paper argues that despite the conclusion of the first generation of experiments, the SPS can continue operations until approximately 2041. This continuation is justified by the need to map the QCD phase diagram more thoroughly in the finite $\mu_B$ region and by the advancements in detection technology over the last two decades, which allow for overcoming previous accuracy limitations.

Methodology and Proposed Experimental Programs
The paper outlines two primary initiatives to address these physics gaps: the upgraded NA61/SHINE experiment and the newly approved NA60+/DiCE experiment.

NA61/SHINE Upgrade:
- Objective: To investigate the onset of deconfinement by studying the transition from hadronic to partonic matter across different system sizes and energies.
- Approach: The program proposes measurements using light-ion beams (Oxygen, Magnesium, Boron) at momenta of 13, 30, and 150 A GeV/c. This allows for the study of resonance-dominated dynamics at low energy, the interplay of hadronic and partonic mechanisms at intermediate energy, and the identification of QGP onset at top energy.
- Hardware: The upgrade involves replacing the first large-volume Time Projection Chamber (TPC) with a fast tracking system based on Silicon-pixel detectors (near the target) and larger gas detectors (downstream). This system is designed to handle interaction rates up to $\sim$ 10 kHz.
- Scope: In addition to light-ion collisions, the upgraded program aims to perform the first direct measurements of charm-hadron production in central Pb-Pb collisions and study multi-strangeness production.
NA60+/DiCE Experiment:
- Objective: A high-precision study of hard (open charm, charmonia) and electromagnetic probes (dileptons) in Pb-Pb collisions across the SPS energy range.
- Detector Design: Inspired by the former NA60 detector, the setup comprises a vertex spectrometer and a muon spectrometer separated by a thick hadron absorber, each equipped with a dipole magnet.
  - Vertex Spectrometer: Utilizes Monolithic Active Pixel Sensors (MAPS) organized in five stations. The sensors use a stitching technique (Repeated Sensor Units) to create large areas ( $\sim$ 13.5 $\times$ 13.5 cm $^2$ ) with minimal dead zones.
  - Muon Spectrometer: Consists of six Multi-Wire Proportional Chamber (MWPC) tracking stations. It utilizes the MNP33 magnet (previously used by NA62) and includes a secondary concrete absorber to stop punch-through hadrons.
- Performance: The system is designed to match tracks between the vertex and muon spectrometers to achieve high resolution. Expected mass resolutions are 11 MeV for the $\omega$ meson and 33 MeV for the $J/\psi$ (via dimuon decay), with an impact parameter resolution of $\sim$ 20 $\mu$ m at $p_T = 1$ GeV/c.

Key Results and Physics Prospects
The paper presents detailed performance simulations and physics projections for the proposed experiments:

Electromagnetic Probes (Dileptons):
- Simulations for Pb-Pb collisions at $\sqrt{s_{NN}} = 8.8$ GeV (with an integrated luminosity of $0.6 \times 10^{12}$ ions on target) indicate the ability to measure the thermal radiation yield dominated by in-medium $\rho$ decay for $m_{\mu\mu} < 1$ GeV/c $^2$ .
- The thermal spectrum is expected to be measurable up to 2.5–3 GeV/c $^2$ .
- By fitting the spectrum in the interval $1.5 < m_{\mu\mu} < 2.5$ GeV/c $^2$ , the experiment projects a total uncertainty of $\sim$ 4% on the extracted average temperature ( $T_{slope}$ ). This precision aims to detect structures in the $\sqrt{s_{NN}}$ -dependence of $T_{slope}$ , potentially revealing the flattening of the "caloric curve" associated with a first-order phase transition.
- The setup is also sensitive to chiral symmetry restoration, with the potential to detect a $\sim$ 20–30% increase in dilepton yield in the 0.9–1.4 GeV/c $^2$ region due to $\rho$ - $a_1$ mixing.
Heavy Quark Probes:
- Open Charm: The experiment plans to reconstruct open charm hadrons ( $D^0$ , $D_s$ , $\Lambda_c$ ) via their decays into charged hadrons in the vertex telescope. Geometric selections based on displaced decay vertices (mean proper decay lengths of 60–310 $\mu$ m) are expected to reduce combinatorial background sufficiently to identify these particles without explicit particle identification.
- Charmonium: Simulations for $J/\psi$ suppression ( $R_{AA}$ ) in Pb-Pb collisions at $E_{lab} = 50$ A GeV assume a 30% suppression beyond cold nuclear matter effects. The detector's resolution allows for precise centrality dependence studies.
Strangeness and Fluctuations:
- NA61/SHINE has previously mapped the "horn" structure in the $K^+/\pi^+$ ratio. Future measurements will extend this to light-ion collisions to better define the transition threshold.
- Fluctuation studies (cumulant ratios of net charge) will continue to search for non-monotonic behavior indicative of the QCD critical point, filling the energy gap (4.5–7.5 GeV) between current SPS data and the FAIR/CBM domain.

Significance and Claims
The paper asserts that the CERN SPS offers a unique and flexible environment for heavy-ion physics that is not fully replicated by other facilities in the same energy range. While the NICA facility is a potential competitor, the paper notes that NICA's interaction rates are expected to be more than an order of magnitude lower than the SPS's capability (which can exceed $10^6$ Pb ions/s).

The significance of the proposed program lies in its ability to:

Map the QCD Phase Diagram: Specifically targeting the finite $\mu_B$ region where the critical point and the nature of the phase transition (crossover vs. first-order) are expected to manifest.
Leverage Technological Advances: Utilizing modern Si-pixel detectors and high-rate capabilities to overcome the accuracy limitations of previous generations of experiments (e.g., NA38/NA50/NA60).
Provide Complementary Data: Offering crucial low-energy data on hard and electromagnetic probes that are currently unavailable for Pb-Pb collisions, thereby complementing high-energy collider results and future FAIR data.

The authors conclude that the continuation of this physics program, supported by the NA61/SHINE upgrade and the new NA60+/DiCE experiment, is technically feasible and scientifically vital for a comprehensive understanding of the QGP and the QCD phase diagram.

The Cosmic Soup and the Time Machine

The CERN SPS: A Heavy-Ion Time Machine

The New Players: NA61/SHINE and NA60+/DiCE

What the Simulations Say

The Bottom Line

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