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Spin Physics at NICA SPD

This paper presents the physics program and detector design of the Spin Physics Detector (SPD) at the NICA collider, which aims to study the spin structure of protons and deuterons—particularly gluon transverse momentum-dependent distributions and tensor PDFs—using polarized beams and various complementary probes.

Original authors: Igor Denisenko

Published 2026-08-07

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Igor Denisenko

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

Imagine the universe is built from tiny, invisible Lego bricks called atoms. For a long time, scientists thought these bricks were made of even smaller, simpler pieces: protons, neutrons, and electrons. But as we zoomed in closer, we realized protons and neutrons aren't solid balls; they are more like bustling, chaotic cities filled with a swarm of tiny, energetic messengers called quarks and gluons. Quarks are the residents, but gluons are the glue that holds the whole city together, zipping around at incredible speeds.

Here is the mystery: We know these particles have a property called "spin," which is a bit like a tiny internal top spinning on its axis. If you could line up a whole crowd of these protons so they were all spinning in the same direction, you'd have a "polarized" beam. The big question scientists are asking is: How does this spinning motion affect the glue (gluons) inside? Does the spin of the proton come mostly from the quarks, or is the gluon glue doing most of the heavy lifting? Understanding this is like trying to figure out how a car engine works by watching the pistons move, but without ever seeing the gears. If we can't map out how the spin is distributed, we don't fully understand how matter gets its mass and structure.

This is where a new experiment called the Spin Physics Detector (SPD) comes in. It's a massive, high-tech camera being built at a particle collider in Dubna, Russia, known as NICA. Think of NICA as a giant racetrack where protons and deuterons (a type of heavy hydrogen nucleus) are smashed together at speeds up to 27 GeV. The SPD is the ultimate detective, designed to catch the debris from these crashes to see how the gluons are spinning.

The paper you are reading is a blueprint and a mission statement for this detector. It explains that the SPD is being built in two phases, like a video game where you unlock new levels as you go. In the first phase, the machine will run at lower speeds with a simpler version of the detector. This is the "training level," where scientists will study a wide variety of strange and interesting phenomena, like how particles bounce off each other or how strange forms of matter called hypernuclei are created. It's a time to test the waters and see what the machine can do with less power.

The real "boss battle" happens in the second phase, when the detector is fully built and the collider is running at its top speed. Here, the team has a very specific goal: to map out the "gluon structure" of the proton and deuteron. To do this, they plan to look at three specific types of "smash-ups" that act like flashlights shining into the dark corners of the proton:

Charmonium production: Creating heavy particles made of charm quarks.
Open charm: Creating particles that contain a charm quark but aren't bound in a pair.
Prompt photons: Catching high-energy flashes of light (photons) that are produced directly in the crash.

The paper suggests that by measuring how often these things happen and how they are distributed when the beams are spinning, scientists can finally pin down some very elusive numbers. For instance, they hope to measure the "gluon Sivers function," which is a fancy way of describing how the spin of a proton might make the gluons inside it drift to one side, like a spinning top wobbling. They also want to look at the "gluon transversity" in deuterons, which could reveal if there are weird, non-standard parts inside the deuteron that we haven't seen before.

The author is careful to note that this is a challenging job. The signals they are looking for are often buried under a mountain of background noise, like trying to hear a whisper in a rock concert. They have simulated the results and found that with their new detector, they could reduce the uncertainty in our knowledge of the gluon's spin by half in certain ranges. They also point out that different computer models predict different outcomes, so they need real data to figure out which model is right.

In short, this paper doesn't claim to have solved the mystery of the proton's spin yet. Instead, it lays out the plan for a powerful new tool that will finally let us take a clear, high-definition picture of the gluons inside. By the time the detector is fully operational, it promises to provide the missing pieces of the puzzle, helping us understand how the spin of the universe's building blocks actually works. It's a promise of discovery, not a finished story, but a very exciting chapter in the making.

Technical Summary: Spin Physics at NICA SPD

Problem and Motivation
The primary objective of the Spin Physics Detector (SPD) experiment at the Nuclotron-based Ion Collider Facility (NICA) is to conduct a comprehensive study of the polarized and unpolarized gluon structure of the proton and deuteron. While the unpolarized collinear gluon density, $f_g(x)$ , and the collinear helicity PDF, $\Delta g(x)$ , are relatively well-established, significant uncertainties remain, particularly at high Bjorken- $x$ . Furthermore, the leading-twist gluon Transverse Momentum Dependent (TMD) distributions—specifically the gluon Sivers function ( $f_{1T}^{\perp g}$ ), the gluon Boer-Mulders function ( $h_{1}^{\perp g}$ ), and the gluon transversity distribution ( $h_1^g$ ) in the deuteron—remain largely unconstrained. The SPD aims to address these gaps by operating in a unique energy range ( $\sqrt{s} = 3.5$ –$27$ GeV) between lower-energy facilities (COSY, SATURNE II, U-70) and higher-energy colliders (RHIC, LHC), utilizing polarized proton and deuteron beams with polarization levels exceeding 70%.

Methodology and Detector Design
The SPD is designed as a universal detector optimized for three complementary processes: charmonium production, open charm production, and prompt photon production. These processes serve as probes for gluon structure at high $x$ and relatively low energy scales.

Detector Configuration: The detector features a silicon vertex detector (based on MAPS or DSSD technology) for $D$ -meson decay reconstruction, followed by a straw tracker (ST) with 150 $\mu$ m spatial resolution. Particle identification is achieved through ionization losses in the ST, a time-of-flight (TOF) system (50 ps resolution), and FARICH detectors in the endcaps for $\pi/K$ separation up to 5.5 GeV. Photons are detected by an electromagnetic calorimeter (ECal) with a resolution of $5\%/\sqrt{E} \oplus 1\%$ . A superconducting magnet provides a field up to 1.2 T, and a range system (RS) serves for muon identification and as the structural support.
Operational Phases: The project is implemented in two phases.
- Phase I: Operates with reduced luminosity and energy. For polarized $pp$ collisions, the center-of-mass energy range is $3.5 < \sqrt{s} < 9.4$ GeV. Deuterons and light nuclei are accelerated to a kinetic energy of 4.5 GeV/A. The detector configuration is simplified, utilizing a Micromegas-based central tracker instead of the silicon vertex detector, and lacking FARICH, TOF, and ECal. This phase focuses on a broad range of QCD phenomena, including spin effects in elastic scattering, hyperon polarization, and light ion collisions.
- Phase II: Operates at full energy ( $10 < \sqrt{s} < 27$ GeV) and maximum luminosity ( $10^{32}$ cm $^{-2}$ s $^{-1}$ for $pp$). The detector is fully assembled to enable precise measurements of the primary physics goals.
Data Acquisition: To minimize bias, the detector operates in a triggerless mode, with a software trigger reducing the data stream by a factor of 20–50 to ensure computational manageability.

Key Contributions and Physics Program
The paper outlines the specific physics capabilities of the SPD, focusing on the extraction of gluon TMDs and collinear PDFs through single and double spin asymmetries.

Charmonium Production:
- Process: Inclusive $J/\psi$ production is dominated by gluon-gluon fusion. The experiment targets the measurement of longitudinal double spin asymmetries ( $A_{LL}$ ) and transverse single spin asymmetries ( $A_N$ ).
- Capabilities: The SPD expects to reconstruct approximately 5 million $J/\psi \to \mu^+\mu^-$ events per year. It will also measure heavier charmonia states ( $\chi_{c1}, \chi_{c2}, \psi(3686)$ ) via radiative and hadronic decays, allowing for the determination of feed-down contributions.
- Impact: Simulations indicate that SPD $J/\psi$ $A_{LL}$ measurements can reduce the uncertainty on $x\Delta g$ by a factor of 2 in the range $x \in [0.2, 0.3]$ . The experiment will also test TMD factorization breaking in inclusive $J/\psi$ production.
Prompt Photon Production:
- Process: Considered the cleanest theoretical probe of gluon structure, though experimentally challenging due to small cross-sections and large backgrounds from hadron decays.
- Capabilities: The SPD plans to measure photons with $p_T^\gamma > 4$ GeV/c.
- Impact: These measurements are projected to reduce DSSV14 uncertainties on $x\Delta g$ by a factor of 2 in the high $x_F$ region. The experiment will also probe the gluon Sivers function via transverse single spin asymmetries.
Open Charm Production:
- Process: Reconstruction of $D$ -mesons via decays to $K\pi$ and $K\pi\pi$ .
- Capabilities: Despite the strong combinatorial background at SPD energies, the large cross-section allows for aggressive cut strategies. The vertex detector resolution is sufficient for $D^0$ reconstruction.
- Impact: These measurements aim to precisely determine $g(x)$ at large $x$ , as well as gluon helicity and Sivers functions. $D$ -meson pair production will probe the Boer-Mulders function.
Deuteron Structure:
- The use of polarized deuteron beams allows for the investigation of non-nucleonic degrees of freedom. Specifically, the experiment aims to measure the gluon transversity distribution ( $h_1^g$ ) and tensor-polarized gluon distributions, which may be non-zero in the spin-1 deuteron, unlike in spin-1/2 hadrons.

Results and Claims of Significance
The paper presents a detailed physics program and detector design, supported by simulation studies (e.g., using NNPDFpol1.1 and DSSV14 sets) that predict significant improvements in the knowledge of gluon distributions.

Significance: The SPD is claimed to be a unique facility covering the transition region between hadronic and quark-gluon degrees of freedom. Its physics program is described as complementary to existing and proposed experiments at RHIC, the Electron-Ion Collider (EIC), LHC (AFTER, LHC-Spin), and EicC.
Modest Claims: The author acknowledges that the interpretation of charmonium results is model-dependent (relying on NRQCD or Color Evaporation Model) and requires comprehensive model validation. They also note that inclusive $J/\psi$ production is a TMD factorization breaking process, the size of which must be studied experimentally. For the deuteron, the non-zero nature of transversity is presented as a potential indicator of non-nucleonic degrees of freedom rather than a confirmed result.

In summary, the paper establishes the SPD as a critical facility for constraining the gluonic content of nucleons and nuclei, offering a unique energy window to explore spin-momentum correlations and the emergence of hadron properties from QCD degrees of freedom.

Technical Summary: Spin Physics at NICA SPD

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