After nearly two decades at the helm of NA61/SHINE, Marek Gazdzicki reflects in this interview on a lifetime at the SPS — from proposing the world’s first two-dimensional scan of nuclear collisions to the unexpected discovery of a large isospin symmetry violation. In conversation, he shares the origins of the experiment, its most surprising results, and why, after forty years, the SPS still holds the power to surprise.

When Marek Gaździcki stepped down as spokesperson of NA61/SHINE on 1 December 2024, it marked the close of an extraordinary chapter — not just for the collaboration, but for the entire SPS physics programme. His 17-year tenure at the helm of NA61/SHINE, and his role in its very creation, have left a legacy of bold ideas, unexpected discoveries, and meticulous exploration of the strong interaction.

When Marek Gazdzicki thinks back to where it all began, the image is not of a detector hall or a conference stage, but of a school desk in Warsaw. “Already in school, I was somehow fascinated by physics,” he says. “Reading the university textbooks… this was my passion.” Good teachers nurtured his curiosity and never discouraged him. His parents, however, were less enthusiastic. “At that time, being a physicist essentially meant your career was limited to being a schoolteacher. That was not something they dreamt for me.” Yet the attraction for Gazdzicki was clear. “Scientists are somehow outside of the politics… you are more free.”

By the early 1980s, as Poland was gripped by the Solidarity movement, Gazdzicki had completed his master’s degree and was facing the reality of compulsory military service. “My main worry… was to dodge the draft” he says frankly. His research offered a way out. At the time, he was working on an experiment at the Joint Institute for Nuclear Research (JINR) in Dubna — the Soviet Union’s answer to CERN. The rector of the University of Warsaw wrote a letter to the military administration, arguing that Gazdzicki’s scientific work was “much more important… than taking me to other problems.” The appeal succeeded, and instead of joining the army, he and his wife moved to Dubna for three years.

In this way, Marek’s journey into high-energy physics began far from Geneva. Dubna was, as he recalls, “an international environment, but related to the Soviet bloc. It was created as a response to CERN.” There, he worked on heavy-ion collisions at 4–5 GeV/c per nucleon — the highest ion energies available in the world at the time. It was a formative experience, sharpening his skills in experimental nuclear physics and introducing him to the culture of international collaborations.

After returning to Warsaw to complete his PhD, he took a permanent position at the university — a fixture of the Communist system that offered security but limited mobility. Then came an unexpected opportunity. Physicists with experience from Berkeley and Dubna were proposing a heavy-ion programme at CERN’s Super Proton Synchrotron (SPS), and Gazdzicki was invited to Heidelberg University to contribute his Dubna expertise. “The first SPS experiments used light-ion beams — oxygen and sulphur — in the mid-1980s,” he remembers. “Then in the mid-1990s we moved to lead, and you had to really revolutionise experimental technique because of the very high multiplicity.”

At SPS he initially joined NA35, led by Reinhard Stock, studying strange hadron production. The results were striking. “The discovery we made was that there are more strange hadrons than expected. It looked like we had a quark–gluon plasma at the top SPS energy.” This signature of QGP motivated NA49, which confirmed the strangeness enhancement in lead–lead collisions but left an open question: where exactly does the transition occur between the smooth collision-energy dependence of proton–proton interactions and the anomalies of heavy-ion collisions?

The iconic image of an S+Au collision recorded by the NA35 streamer chamber in the mid-eighties – the first years of the heavy-ion programme at the CERN SPS.

By 2003, Marek (CERN and Frankfurt) had an answer: explore systematically. At the NA49 collaboration meeting in March that year, he proposed a two-dimensional scan in both collision energy and the size of the colliding nuclei at the SPS. The idea was seeded by NA49’s energy-scan results with Pb+Pb collisions, which led to the confirmation that the “Marek’s horn”, the most popular signal of the onset of deconfinement, is located at low SPS energies. NA49 spokesperson Peter Seyboth immediately backed the proposal, as did senior colleagues Zoltan Fodor (Budapest), Wojtek Dominik (Warsaw), Rainer Renfordt (Frankfurt), Herbert Ströbele (Frankfurt) and George Vesztergombi (Budapest).

From left to right: Peter Seyboth (Munich, former spokesperson of NA49), Mark Gorenstein (Kyiv, pioneer of statistical models of strong interactions), Edward Shuryak (Stony Brook, pioneer of QGP theory), Marek Gazdzicki (Frankfurt) and Viktor Begun (Kyiv) at the second workshop on Critical Point and Onset of Deconfinement (CPOD), Bergen, 2005. The CPOD series was initiated by Marek, Edward and Peter in 2004 at ECT Trento. The next workshop will be held in April 2026 at CERN. Marek and Mark have collaborated on the phenomenology of strong interactions for over 40 years.

At the CERN Research Board in June 2003, Chair Agnieszka Zalewska noted that the low-energy region would be difficult to explore at RHIC and encouraged development of the programme beyond 2005. Around the same time, cosmic-ray physicists Markus Roth and Ralph Engel began discussions that would later form NA61’s cosmic-ray programme. By November 2003, an Expression of Interest — A New Experimental Programme with Nuclei and Proton Beams at the CERN SPS — had been submitted to the SPSC. This included the energy–system size scan and high-transverse-momentum hadron production studies from Vesztergombi’s group.

A two-year preparation led to the January 2006 Letter of Intent, now expanded to include hadron-production measurements for the T2K neutrino experiment at J-PARC (proposed by Jaap Panman and developed by Alain Blondel). A short five-day test run in August 2006, led by Fodor, Dominik and Renfordt, showed that the Time Projection Chambers inherited from NA49 were still in excellent condition. The “NA49-future” proposal received strong community support, with endorsement letters from theorists including Edward Shuryak, Misha Stephanov, Krishna Rajagopal, and Nobel laureate Frank Wilczek. On 21 February 2007, the CERN Research Board approved the experiment that would become NA61/SHINE, and pilot data-taking for T2K began the same year.

From its inception, NA61/SHINE was designed as “the first in the world systematic 2D scan, in collision energy and the size of colliding nuclei, from p+p to Pb+Pb.” Over the years, this two-dimensional scan revealed unexpected structures, while together with other NA61/SHINE data, also providing vital reference data for neutrino and cosmic-ray physics.

Marek Gazdzicki in the target area of the NA61/SHINE detector during the two-dimensional scan period, CERN, 2012.

Studying p+p, Be+Be, Ar+Sc, Xe+La and Pb+Pb collisions at various energies, the collaboration could map how the dynamics of particle production changed across this space. The results uncovered a rich and sometimes surprising landscape. The kaon-to-pion ratio, K⁺/π⁺, showed a distinct “break” in p+p interactions at around √sₙₙ ≈ 10 GeV — a transition from resonance-dominated to string-dominated hadron production. “Subsequent results for Be+Be collisions overlapped with p+p, which was unexpected,” Marek explains. “The statistical models had predicted they would behave more like Pb+Pb.” Then came the data from central Ar+Sc collisions. At the top SPS energy, the K⁺/π⁺ ratio matched Pb+Pb, but at lower energies it fell between the p+p and Pb+Pb values, with no “horn” structure. The Xe+La data at the top energy were consistent with Pb+Pb and Ar+Sc.

The Marek’s horn in collision energy dependence of the positively charged kaon to pion ratio in Pb+Pb collisions (NA49) together with the results from the NA61/SHINE two-dimensional scan (p+p, Be+Be, Ar+Sc and Xe+La). 

These findings, combined with NA49, STAR, and ALICE results, allowed NA61/SHINE to propose a diagram of high-energy nuclear collisions — a kind of map showing where resonance, string, and quark–gluon plasma (QGP) domains dominate. “I don’t even remember exactly when the idea of the diagram emerged,” Marek laughs. “The first sketch I found was from a talk I gave in Corfu in 2017. Since then, it’s been evolving, and it will keep evolving as new data come in.”

While NA61/SHINE’s scan programme was already reshaping the understanding of the strong interaction, an unexpected twist arrived in 2023. In autumn 2023, master’s student Wojciech Bryliński from Warsaw University of Technology was analysing Ar+Sc collisions for his thesis when he noticed a striking imbalance between charged and neutral kaons. Instead of roughly equal numbers, charged kaons were 18.4% more abundant — far larger than the few-percent deviations expected from known effects. “When Wojciech got started, we thought it would be a trivial verification of the symmetry,” says Gazdzicki. “We expected it to be closely obeyed – though we had previously measured discrepancies at NA49, they had large uncertainties and were not significant.” The finding, with 4.7σ significance, suggests a possible breakdown of isospin symmetry — a cornerstone of the strong interaction.

The ratio of charged to neutral kaons produced in nucleus-nucleus collisions as a function of collision energy indicates violation of the isospin symmetry beyond the known effects (model lines).

Such surprises are part of what keeps the SPS programme alive after more than four decades. Gazdzicki likens the energy range to a physical phase change: “We are in the range where, when you change collision energy with heavy ions, you cross the threshold for quark–gluon plasma creation… something really special happens.” This “sweet spot” was never foreseen when SPS was built, but it has made the facility the longest-running heavy-ion programme in the world.

The past year was especially productive. In 2024, NA61/SHINE recorded first data with the LBNF DUNE prototype target, unique measurements of positively and negatively charged pions interacting with carbon, a new dataset on open charm production in lead–lead collisions, and extensive nuclear-fragmentation cross-section data using secondary light nuclei beams from lithium to phosphorus.

On 1 December 2024, after nearly 20 years as spokesperson, Marek (Kielce) handed over leadership of NA61/SHINE to Eric Zimmerman (Colorado) and Seweryn Kowalski (Silesia) as co-spokespersons, with Kasia Grebieszkow (Warsaw) and Yoshikazu Nagai (Budapest) as deputies. “I would like to express my heartfelt gratitude to my colleagues and friends from NA61/SHINE, CERN, and the SPSC,” he said. “It has been a privilege to work with you. Thanks to your excellence and commitment, we have uncovered previously unknown properties of strong interactions and performed critical measurements for neutrino and cosmic-ray physics. I’m delighted to pass on the leadership of the collaboration to my younger colleagues during this SHINing period — the future is bright.”

That future includes completing the Run 3 physics programme in 2025–2026, followed by major detector upgrades. These will enable the first charm–anti-charm correlation measurements in collisions producing a single c–c̄ pair — a unique test of charm-creation locality — and an extended mapping of the quark–gluon plasma onset with light ions. The collaboration will also perform essential hadron-production measurements for neutrino experiments in Japan and the US, potentially enhanced by very low-momentum hadron beams from a proposed branch of the H2 beamline.

During the data-taking on O+O collisions in 2025, Marek is taking a photo of the event browser with the reflected image of Seweryn Kowalski (together with Eric Zimmerman, co-spokesperson of NA61/SHINE, starting from December 2024).

As the conversation turns to broader issues, his tone becomes reflective. Having started his career navigating the politics of the Soviet era, he has again seen geopolitics intrude into science. “We lost many physicists from Russia… excellent colleagues, excellent people,” he says. “CERN was created after the Second World War to unify Europe and the world under the umbrella of basic science. Recently, we have witnessed the reverse process, and we’re collapsing here.” Despite this, he insists that scientists must “stay together as much as possible.”

Four decades after his first experiments, Gazdzicki still sees “a very interesting future” for the SPS and NA61/SHINE. The isospin anomaly is a reminder that QCD can still surprise us — and that in the interplay of quarks and gluons, there is always more to discover.

As he steps away from the spokesperson role, Marek’s legacy is not only in the data or the diagrams, but in the experiment’s spirit: curiosity-driven, collaborative, and unafraid to challenge expectations. “We’ve built something special here,” he says. “And the next chapter could be even more exciting.”

2024 NA61/SHINE Highlights

• LBNF DUNE Prototype Target – First data with 120 GeV/c protons.
• Charge Symmetry Studies – Unique data from π⁺ and π⁻ interactions with carbon at 158A GeV/c.
• Open Charm in Pb+Pb – New dataset at 150A GeV/c.
• Cosmic Ray Physics – Nuclear-fragmentation cross-sections from lithium to phosphorus at 13.5A GeV/c.
• Isospin Symmetry Violation – Striking 18.4% imbalance in kaon yields in Ar+Sc collisions, challenging QCD expectations.