arxiv: 2604.23955 · v1 · submitted 2026-04-27 · ⚛️ nucl-ex · nucl-th

Recognition: unknown

Hard Probes in Ultraperipheral Collisions at LHCb

Krista Smith (on behalf of the LHCb Collaboration)

Pith reviewed 2026-05-07 17:33 UTC · model grok-4.3

classification ⚛️ nucl-ex nucl-th

keywords ultraperipheral collisionsvector meson productionLHCbquarkoniaphoton-nucleus interactionsrho mesonphi mesonforward rapidity

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The pith

LHCb measures rho and phi vector mesons in ultraperipheral heavy-ion collisions at forward rapidity.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper presents high-precision quarkonium data from peripheral and ultraperipheral collisions and introduces new measurements of rho(770) and phi(1020) production at forward rapidity. These results are compared directly to theoretical models of photon-photon and photon-nucleus processes. The work shows how the LHCb detector isolates vector-meson photoproduction to probe nuclear parton structure without hadronic interference. Future runs with the upgraded detector are expected to increase the reach of these studies.

Core claim

The LHCb experiment has obtained the highest-precision quarkonium measurements to date in ultraperipheral collisions and reports new forward-rapidity cross sections for rho(770) and phi(1020) vector mesons that can be contrasted with existing data and photon-induced production calculations.

What carries the argument

Forward-rapidity vector-meson reconstruction in ultraperipheral heavy-ion events that selects photon-nucleus interactions.

If this is right

The data constrain nuclear gluon distributions at low momentum fractions.
Comparisons test the relative importance of different vector-meson photoproduction mechanisms.
Forward acceptance accesses kinematic regions complementary to central detectors.
Upgraded detector running will supply higher-statistics samples for the same observables.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The measurements could help discriminate between models that include gluon saturation effects in nuclei.
Similar techniques might be applied to other light vector mesons or to asymmetric collision systems.
Longer-term data sets could link photoproduction results to low-x QCD behavior seen in deep-inelastic scattering.

Load-bearing premise

The selected events are dominated by photon-induced production with only small contributions from hadronic interactions or detector effects.

What would settle it

Measured rho and phi yields that deviate substantially from photon-nucleus model predictions after all experimental corrections would falsify the interpretation.

Figures

Figures reproduced from arXiv: 2604.23955 by Krista Smith (on behalf of the LHCb Collaboration).

**Figure 1.** Figure 1: Left: The Q 2 vs x reach for HERA and the four major LHC experiments, with the LHCb collider (fixed target) mode shown by the red (orange) curve. Right: The LHCb detector configuration for Run 2 data taking [2]. 3 Centrally Produced, Peripheral, and UPC Results The results from hard probes in ultraperipheral collisions at the LHCb experiment are shown in Figs. 2-6 and organized into the following: proton-p… view at source ↗

**Figure 2.** Figure 2: Centrally produced J/ψϕ invariant mass spectrum in pp collisions at √ s = 13 TeV [3]. χc1 (4685), and χc1 (4700) also appear in the spectrum, confirming previous LHCb results in hadronic collisions [4]. With the use of the HeRSCheL detector [5], it was determined that more than two-thirds of the candidates in the spectrum are produced in events in which one or both protons dissociate. In results from per… view at source ↗

**Figure 3.** Figure 3: Left: The invariant yield as a function of pT for J/ψ photoproduction candidates [8]. Right: The differential cross section vs rapidity for ψ(2S ) mesons in UPC at √ sNN = 5.02 TeV [9]. 0 1 2 3 4 5 y ∗ 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 d σ J/ψ/d y ∗ [mb] LHCb PbPb √ sNN = 5.02 TeV Coherent J/ψ production Luminosity unc. : 4.4% data stat. unc. syst. unc. LO pQCD (GKSZ): NLO pQCD (FEGLP… view at source ↗

**Figure 4.** Figure 4: The differential cross section vs rapidity for J/ψ mesons in UPC at √ sNN = 5.02 TeV [9]. is described reasonably well by pQCD calculations, particularly at large rapidity. With its smaller statistical uncertainties, the J/ψ cross section measurement can discriminate between different predictions view at source ↗

**Figure 5.** Figure 5: The full π +π − [10] (left) and the low K +K − [11] (right) invariant mass distributions in ultraperipheral PbPb collisions at √ sNN = 5.02 TeV for pT < 100 MeV/c. 1000 2000 3000 4000 5000 ] 2 ) [MeV/c − K + m(K 1 10 2 10 3 10 4 10 ) 2 c Candidates / (12 MeV/ φ(1020) MISID 0 ρ(770) MISID J/ψ J/ψ LHCb Preliminary Data: UPC PbPb sNN = 5.02 TeV view at source ↗

**Figure 6.** Figure 6: The full K +K − invariant mass distribution in UPC at √ sNN = 5.02 TeV [11]. The ϕ(1020) → K +K − signal at the lower end of the spectrum was observed with > 5σ significance. differs between mid and forward rapidity, with a rich spectrum seen only in the forward region. During Long Shutdown II, LHCb tracking was fully upgraded with a new Vertex Locator, Upstream Tracker (UT), and Scintillating Fiber (Sci… view at source ↗

read the original abstract

Measurements of quarkonia production in peripheral and ultraperipheral heavy-ion collisions are sensitive to photon-photon and photon-nucleus interactions, the partonic structure of nuclei, and the mechanisms of vector-meson production. In this contribution, quarkonium measurements with the highest precision currently accessible will be compared with the latest theoretical models. Additionally, new studies of $\rho(770)$ and $\phi(1020)$ vector meson production at forward rapidity will be presented and contrasted with other experimental results. Future UPC measurements with the upgraded LHCb detector in Run 3 will also be discussed.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

LHCb's conference note adds new forward-rapidity rho and phi data in UPC but stays at the level of a status summary with limited analysis depth.

read the letter

This LHCb contribution mainly reports new forward-rapidity measurements of rho(770) and phi(1020) vector mesons in ultraperipheral PbPb collisions, alongside comparisons of existing quarkonia results to models. The central point is that these data probe photon-nucleus interactions and nuclear gluon distributions at low x, with LHCb's forward acceptance giving access to kinematics not fully covered elsewhere. The text also outlines plans for Run 3 with the upgraded detector. That forward-rapidity angle is the concrete addition over prior UPC work at other LHC experiments. The paper handles the physics framing cleanly by linking the measurements to photoproduction mechanisms and partonic structure without overclaiming. Standard event selection via rapidity gaps and zero-degree calorimeter activity is invoked, which aligns with established practice in the field. Model comparisons for quarkonia are shown at a level that lets readers see where data and theory line up or diverge. The future outlook section is practical and points to concrete improvements in luminosity and detector capabilities. The main limitation is the conference format itself. Details on background subtraction, systematic uncertainties, and fit procedures are not expanded, so the robustness of the new rho and phi results against possible hadronic contamination or detector effects cannot be judged from the text alone. No internal contradictions appear in the claims or selection strategy, and the citations follow the usual pattern for this topic. This is aimed at the heavy-ion and UPC community, especially theorists needing forward-rapidity constraints on nuclear PDFs and experimentalists planning similar measurements. Readers who want the latest LHCb numbers or the Run 3 roadmap will get direct value. It deserves peer review because the new data points are worth checking even if the analysis techniques are not novel.

Referee Report

0 major / 2 minor

Summary. The manuscript is a conference-style contribution from the LHCb collaboration summarizing measurements of quarkonia production in peripheral and ultraperipheral PbPb collisions. It highlights the sensitivity of these measurements to photon-photon and photon-nucleus interactions, the partonic structure of nuclei, and vector-meson production mechanisms. The text compares high-precision quarkonium results with theoretical models, presents new forward-rapidity studies of ρ(770) and φ(1020) vector meson production, contrasts them with other experimental results, and discusses prospects for Run 3 with the upgraded LHCb detector.

Significance. If the summarized results hold, this work adds value by providing LHCb's forward-rapidity coverage for vector-meson photoproduction in UPCs, which can constrain nuclear PDFs and production models. The new ρ and φ measurements extend the dataset beyond mid-rapidity experiments, and the Run 3 outlook indicates potential for higher-statistics studies of photon-induced processes.

minor comments (2)

The abstract claims 'highest precision currently accessible' for quarkonium measurements; this should be supported in the main text by explicit references to integrated luminosities, statistical uncertainties, or direct comparisons with prior results from other experiments.
The discussion of new ρ(770) and φ(1020) studies would benefit from a brief description of the event selection (e.g., rapidity gaps or ZDC requirements) and any preliminary cross-section values or ratios, even if the contribution is concise.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the careful reading and positive assessment of our manuscript. The summary accurately captures the scope of our contribution on quarkonia and vector-meson measurements in ultraperipheral collisions. We are pleased that the forward-rapidity coverage and Run 3 prospects are viewed as adding value. No specific major comments were provided in the report, so we have no points to address individually. We will incorporate any minor editorial suggestions in the revised version.

Circularity Check

0 steps flagged

No significant circularity: experimental summary only

full rationale

This is a conference-style experimental report summarizing LHCb measurements of vector-meson production in UPC and peripheral collisions. No derivation chain, theoretical model, or quantitative prediction is advanced within the manuscript; results are extracted from detector data using standard event-selection criteria (rapidity gaps, ZDC activity) that are field conventions rather than internally derived. Comparisons to external models are presented but not used to close any loop back to the paper's own inputs. The paper is therefore self-contained against external benchmarks with no load-bearing self-referential steps.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The work is purely experimental and relies on prior established frameworks for UPC kinematics and vector-meson photoproduction; no new free parameters, axioms, or entities are introduced in the abstract.

axioms (1)

domain assumption Standard assumptions of quantum chromodynamics and electromagnetic interactions govern photon-nucleus processes in ultraperipheral collisions
The claimed sensitivity to partonic structure and production mechanisms presupposes these established theoretical ingredients.

pith-pipeline@v0.9.0 · 5386 in / 1243 out tokens · 108097 ms · 2026-05-07T17:33:02.142161+00:00 · methodology

discussion (0)

Reference graph

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