Overview of recent UPC measurements

This paper presents a comprehensive overview of recent ALICE measurements from Run 2 and Run 3 on photon-induced processes in ultra-peripheral collisions, covering exclusive vector meson photoproduction, nuclear breakup mechanisms, and inclusive photonuclear interactions to probe gluon saturation, quantum interference, and particle production dynamics in nuclear environments.

The Gist

Imagine two massive, speeding trains (heavy atomic nuclei) passing each other on parallel tracks. Usually, if they get too close, they crash, sending sparks and debris everywhere. But in Ultra-Peripheral Collisions (UPCs), the trains pass just far enough apart that they don't crash. Instead, their powerful magnetic fields (which act like giant flashlights) flash at each other.

This paper is a report from the ALICE experiment at the Large Hadron Collider (LHC), describing what happens when these "flashlights" (photons) from the passing trains interact. It's like studying the light beams themselves without the trains ever touching.

Here is a breakdown of their recent discoveries, using simple analogies:

1. The "Ghost" of the Collision (Nuclear Breakup)

When the trains pass, the flash of light is so bright it can knock pieces off the train cars.

• The Discovery: Scientists found that sometimes, the light knocks off just a few tiny screws (protons) along with the usual debris (neutrons).
• The Analogy: Imagine shining a laser so bright it knocks a few screws off a passing car. By counting exactly how many screws fall off, scientists can figure out exactly how close the two trains passed each other. This helps them map the "shape" of the collision with incredible precision. They even saw a train car turn into a different type of metal (transmuting Lead into Gold) just by losing a few parts!

2. The "Shadow" Game (Gluons and Saturation)

Inside the atomic nuclei are tiny particles called gluons that hold everything together.

• The Discovery: Scientists looked at how the light creates heavy particles (like J/𝜓 mesons). They found that when the light hits the nucleus at a very specific angle, the production of these particles stops growing as fast as expected.
• The Analogy: Imagine trying to paint a wall. If you spray a little paint, the wall gets darker. But if you spray too much, the paint starts to clump and stop covering new areas. This "clumping" is called saturation. The data suggests that the gluons inside the nucleus are so crowded they start to "clump" together, behaving like a thick, saturated sponge rather than a loose collection of particles.

3. The "Quantum Dance" (Spin and Interference)

Sometimes, the light doesn't just hit the nucleus; it creates a wave-like pattern.

• The Discovery: When light creates a specific particle (a rho meson), the particles don't just fly out randomly. They wiggle in a specific pattern depending on how close the trains passed.
• The Analogy: Think of two people clapping in a large hall. If they clap at the exact same time, the sound waves combine to create a louder, specific echo. This is quantum interference. The ALICE team saw this "echo" pattern, proving that the light came from both trains simultaneously, acting as a single wave. They also checked the "spin" (how the particle rotates) and found it spins exactly as predicted by the laws of physics, like a perfectly balanced top.

4. The "New Window" (Run 3 and Inclusive Collisions)

In the past (Run 2), scientists only looked at "clean" collisions where nothing else happened. In the new phase (Run 3), they opened the curtains wider.

• The Discovery: They started looking at "messy" collisions where the nucleus breaks apart and creates many particles, including heavy "charm" quarks.
• The Analogy: Previously, they only watched a magic trick where a rabbit appeared from an empty hat. Now, they are watching the whole show, including the messy backstage where the rabbit might have been hiding. They found that the "messy" collisions produce heavy particles in ways that current computer models can't explain yet. It's like the magic trick is working, but the magician's manual is missing a few pages.

5. The "Tiny Fireball" (Collectivity)

Usually, we think of "collective behavior" (like a fluid flowing together) only in massive crashes.

• The Discovery: Even in these gentle, non-crashing light collisions, the particles produced seem to move together like a fluid.
• The Analogy: Imagine a drop of water hitting a puddle. You expect a splash. But here, the splash behaves like a tiny, organized wave. This suggests that even in these small, photon-induced systems, the particles are "talking" to each other and organizing themselves, challenging the idea that these are just simple, random events.

6. The "Future Flashlight" (Electroweak and Upgrades)

• The Discovery: They are using these collisions to study the tau lepton, a heavy cousin of the electron, to see if it has a weird magnetic personality (anomalous magnetic moment).
• The Future: They are building a new, giant "eye" (called FoCal) to look further down the track. This will let them see even smaller, faster particles and study the very edge of the universe's structure.

Summary

The ALICE team is using the "flashlights" of passing atomic trains to:

1. Map the nucleus by seeing what pieces fall off.
2. Test the limits of physics by seeing if gluons get too crowded (saturation).
3. Watch quantum waves interfere with each other.
4. Discover new behaviors in "messy" collisions that our current theories can't fully explain.

It's like using a strobe light to study the inside of a hurricane without ever getting wet. The results are rewriting our understanding of how matter is built and how it behaves at the smallest scales.

Technical Summary

Here is a detailed technical summary of the paper "Overview of recent UPC measurements" by Anisa Khatun on behalf of the ALICE Collaboration.

1. Problem and Motivation

Ultra-peripheral collisions (UPCs) of heavy ions at the Large Hadron Collider (LHC) offer a unique environment to study photon-induced interactions at unprecedented energies. In these collisions, the impact parameter exceeds the sum of the nuclear radii, suppressing hadronic interactions and allowing electromagnetic interactions to dominate.

• Scientific Gap: While exclusive processes (where the nucleus remains intact) have been well-studied, there is a need to explore inclusive photonuclear interactions and understand the transition from exclusive to inelastic regimes.
• Key Physics Questions:
  • What is the structure of gluon distributions in nuclei at very small Bjorken-$x$ (down to $10^{-5}$)?
  • Do nonlinear QCD effects (gluon saturation) dominate over standard nuclear shadowing?
  • Can collective effects (typically associated with Quark-Gluon Plasma in central heavy-ion collisions) emerge in small, photon-induced systems?
  • How do nuclear breakup mechanisms (electromagnetic dissociation) function, and can they be used to tag collision geometry?

2. Methodology and Experimental Setup

The study utilizes data from the ALICE experiment at the LHC, covering Run 2 (2015–2018) and Run 3 (2022–present).

• Collision Systems: Primarily Pb–Pb collisions at $\sqrt{s_{NN}} = 5.02$ TeV, with some p–Pb data.
• Event Selection & Tagging:
  • Electromagnetic Dissociation (EMD): Detection of forward neutrons and protons using Zero Degree Calorimeters (ZDC) and Proton Calorimeters (ZPC). This allows for the classification of events based on nuclear breakup (e.g., 0n, 1n, 1p channels).
  • Asymmetric Vetoing (Run 3): A major upgrade allowing the selection of exclusive, semi-inclusive, and inclusive topologies by vetoing activity in specific forward detectors.
  • Kinematic Variables: Analysis relies on transverse momentum ($p_T$) distributions to distinguish between coherent (nucleus intact) and incoherent (nucleus breaks up) processes, and the Mandelstam variable $|t|$ (momentum transfer).
• Detector Upgrades: Run 3 features continuous readout, an improved Inner Tracking System (ITS), and enhanced forward coverage (FIT), enabling the study of inclusive processes previously inaccessible due to trigger limitations.

3. Key Contributions and Results

A. Nuclear Structure and QCD Dynamics

• Incoherent $J/\psi$ Photoproduction:
  • Measurements of incoherent $J/\psi$ production as a function of energy and $|t|$ reveal a suppression of energy evolution at large $|t|$.
  • Significance: This suppression favors saturation-based models (Color Glass Condensate) over models based solely on nuclear shadowing, providing evidence for nonlinear gluon dynamics at subnucleonic scales.
• Proton Emission and Nuclear Breakup:
  • First measurement of proton emission accompanied by neutrons in Pb–Pb UPCs.
  • Result: Observation of gold ($^{195-197}\text{Au}$) production via the emission of three protons from $^{208}\text{Pb}$, with a cross-section $\sigma_{3p} = 6.8 \pm 2.2$ b.
  • Implication: This establishes "proton-tagged EMD" as a precise handle for determining collision geometry and studying isotope production, though current models (RELDIS) underestimate 1- and 2-proton emission yields.

B. Spin, Interference, and Light Vector Mesons

• Coherent $\rho^0$ Photoproduction:
  • Discovery of impact-parameter-dependent azimuthal anisotropy ($\cos(2\phi)$ modulation).
  • Significance: The modulation strength increases by an order of magnitude from large to small impact parameters, interpreted as a quantum interference effect arising from the indistinguishability of photon emission from either nucleus.
• $J/\psi$ Polarization:
  • First measurement of polarization parameters ($\lambda_\theta, \lambda_\phi, \lambda_{\theta\phi}$) for coherently photoproduced $J/\psi$.
  • Result: Consistent with transverse polarization and $s$-channel helicity conservation, confirming that coherent photoproduction off nuclei follows the same helicity dynamics as lepton–hadron interactions.
• Multi-hadron States: Measurements of four-pion and charged kaon pair final states probe resonance contributions ($\rho(1450)$, $\rho(1700)$) and light vector meson couplings.

C. Inclusive Photonuclear Interactions (Run 3 Breakthroughs)

• Inclusive Open Charm:
  • First systematic measurement of inclusive open charm ($D^0, D^+, D^{*+}, \Lambda_c^+$) down to $p_T = 0$.
  • Result: Significant deviations from Color Glass Condensate (CGC) predictions are observed. Current models fail to fully capture the dynamics of inelastic charm photoproduction.
• Collectivity in Photonuclear Systems:
  • Analysis of identified hadron spectra ($\pi, K, p$) and baryon-to-meson ratios ($K/\pi, p/\pi, \Lambda/K_S^0$) in inclusive $\gamma$–Pb collisions.
  • Result:
    • Models (STARlight+DPMJET) describe pion production but fail for heavier hadrons.
    • The $\Lambda/K_S^0$ ratio shows an enhancement around $p_T \approx 2$ GeV/$c$, similar to low-multiplicity p–Pb collisions.
    • Significance: These findings suggest that collective-like effects (strangeness enhancement, baryon-to-meson modifications) may emerge in photon-induced systems, challenging the view of UPCs as purely dilute systems.

D. Electroweak Measurements

• Anomalous Magnetic Moment of the Tau ($a_\tau$):
  • Study of $\gamma\gamma \to \tau^+\tau^-$ production.
  • Method: Focus on the $e^+\mu/\pi$ topology (one-prong decays) to suppress backgrounds.
  • Projection: With Run 3 luminosity ($2.3 \text{ nb}^{-1}$),$\sim 3.6 \times 10^4$events are expected, offering competitive sensitivity to$a_\tau$despite the short$\tau$ lifetime.

4. Significance and Future Outlook

• Physics Impact: The paper demonstrates that UPCs are a versatile laboratory for probing gluon saturation, nuclear structure, and spin dynamics. The observation of collective effects in photonuclear systems blurs the line between "small" and "large" collision systems, suggesting new QCD mechanisms.
• Technological Advancement: The transition from Run 2 (exclusive focus) to Run 3 (inclusive capabilities) represents a paradigm shift, allowing the study of inelastic photonuclear interactions and rare processes.
• Future Prospects:
  • Run 4 (FoCal): The installation of the Forward Calorimeter will extend measurements to extremely small Bjorken-$x$ ($x < 10^{-6}$) and allow detailed studies of dissociative $J/\psi$ and $\psi(2S)$.
  • Run 5 (ALICE 3): Future concepts aim to enhance sensitivity to photon-photon interactions (light-by-light scattering) and precision electroweak tests, potentially probing physics beyond the Standard Model.

Conclusion

This overview highlights ALICE's comprehensive progress in UPC physics. By combining precision exclusive measurements with new inclusive capabilities, the collaboration has constrained nuclear parton distribution functions, identified saturation effects, and uncovered potential collective behavior in photon-induced systems. These results solidify UPCs as a central pillar of the LHC heavy-ion physics program.