arxiv: 2604.15667 · v1 · submitted 2026-04-17 · ⚛️ nucl-ex · hep-ex· nucl-th

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sPHENIX measurement of Open-Charm Baryon-to-Meson Ratios in p+p collisions at RHIC

Xudong Yu (on behalf of the sPHENIX Collaboration)

Authors on Pith no claims yet

Pith reviewed 2026-05-10 07:58 UTC · model grok-4.3

classification ⚛️ nucl-ex hep-exnucl-th

keywords open charmbaryon-to-meson ratioLambda_cD0sPHENIXRHIChadronizationproton-proton collisions

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The pith

sPHENIX has recorded enough unbiased p+p collisions to measure the open-charm baryon-to-meson ratio at RHIC for the first time.

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

The paper presents the sPHENIX experiment's collection of 100 billion unbiased proton-proton collisions during the 2024 run at RHIC, along with plans for more data in 2025. This sample, combined with precision tracking and particle identification, enables the first measurement of the Λ_c⁺ / D⁰ ratio in p+p collisions at these energies. No such baseline existed previously, so the ratio will serve as a reference for testing how charm quarks combine into baryons versus mesons. A reader would care because the ratio directly probes the hadronization process, including possible differences in baryon formation and strange-to-light flavor production. The large statistics reduce bias and allow exploration of these mechanisms in a clean environment before comparing to heavy-ion data.

Core claim

With its 2024 dataset of 100 billion unbiased p+p events, sPHENIX can now perform the first measurement of the open-charm baryon-to-meson ratio Λ_c⁺ / D⁰ at RHIC energies, where no prior baseline existed, thereby opening the study of charm-quark hadronization mechanisms and strange-to-light flavor meson ratios.

What carries the argument

The Λ_c⁺ / D⁰ ratio, which directly encodes the relative probability that a charm quark hadronizes into a baryon versus a meson.

If this is right

The measured ratio supplies a vacuum reference that can be compared to the same ratio in Au+Au collisions to isolate medium-induced changes in hadronization.
It constrains models of baryon formation for heavy quarks, including coalescence versus fragmentation scenarios.
The same dataset permits a parallel measurement of strange-to-light open-charm meson ratios to test flavor-dependent production.

Where Pith is reading between the lines

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

A measured ratio at RHIC that differs from LHC values would point to collision-energy dependence in charm baryon formation.
Successful extraction here would confirm that sPHENIX's streaming readout and tracking are ready for precision heavy-flavor studies across p+p and A+A systems.

Load-bearing premise

The 100 billion unbiased p+p events combined with sPHENIX tracking and particle identification yield a clean enough sample of open-charm hadrons to extract the baryon-to-meson ratio without large systematic uncertainties.

What would settle it

If the reconstructed Λ_c⁺ and D⁰ signals cannot be separated from background with statistical significance or if systematic uncertainties remain larger than the expected ratio value, the claim that the dataset enables a first meaningful baseline measurement would fail.

Figures

Figures reproduced from arXiv: 2604.15667 by Xudong Yu (on behalf of the sPHENIX Collaboration).

**Figure 2.** Figure 2: The invariant mass of reconstructed π +π − , pπ − , K +K − , and Λπ − combinations. The combinatorial background is shown in gray, and the signals of K 0 S , Λ, ϕ, Ξ − and Σ(1385)− are shown in blue or purple. the end of Run 24, the tracking detectors were recording unbiased p+p collisions at a rate of O(200) kHz, about 20-50 times higher than that achievable with a purely minimum-bias trigger. Figure 1a s… view at source ↗

**Figure 3.** Figure 3: (a) The invariant mass of pK−π + combinations. (b) The invariant mass of the reconstructed K −π + pairs. (c) The invariant mass of the reconstructed K −π +π + combinations. The combinatorial background is shown in gray, and the signals of D 0 , D + mesons and Λ + c baryon are shown in blue. (d) Statistical projection for the Λc/D 0 ratio as a function of pT in p+p data. 4. Summary The sPHENIX experiment re… view at source ↗

read the original abstract

sPHENIX is a state-of-the-art experiment at the Relativistic Heavy Ion Collider (RHIC), dedicated to the study of heavy-flavor and jet physics. Its precision tracking system, combined with streaming readout, enables heavy-flavor measurements with high-statistics and essentially unbiased data samples. During the 2024 run, sPHENIX was fully commissioned and recorded a sample of 100 billion unbiased $p$+$p$ collisions, together with a minimum-bias Au+Au dataset. The 2025 run further expanded the sPHENIX dataset with high-statistics $p$+$p$, O+O and Au+Au collisions. This extensive $p$+$p$ sample opens the door to heavy-flavor measurements with orders of magnitude more statistics than previously available at RHIC. Notably, there has been no prior measurement of the $\Lambda_c^+ / D^0$ baseline in $p$+$p$ collisions at RHIC energies. The large sPHENIX dataset now enables the first exploration of key open questions, such as the hadronization mechanism of baryons and the strange-to-light flavor meson ratio.

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

sPHENIX collected enough p+p events to make the first Lambda_c+/D0 measurement at RHIC possible, but this write-up announces the dataset and motivation without delivering the ratio or analysis details.

read the letter

The main takeaway is that sPHENIX's 100 billion unbiased p+p collisions from the 2024 run give them the statistics to measure the open-charm baryon-to-meson ratio for the first time at these energies. No earlier RHIC experiment had both the event count and the detector setup to do this cleanly, so the claim of a new baseline is straightforward on its face. The paper also notes that this will help test hadronization models and provide a reference for Au+Au modifications, which is the right context to put it in.

Referee Report

2 major / 2 minor

Summary. The manuscript describes the sPHENIX experiment at RHIC, its precision tracking and streaming readout capabilities, and the 100 billion unbiased p+p collisions recorded in the 2024 run. It notes the absence of any prior Λ_c⁺/D⁰ measurement at RHIC energies and states that this dataset enables the first exploration of baryon hadronization mechanisms via the open-charm baryon-to-meson ratio.

Significance. If the anticipated measurement can be executed with controlled systematics, it would supply a missing p+p baseline essential for interpreting heavy-flavor results in Au+Au collisions at RHIC and for testing hadronization models. The high-statistics, minimum-bias character of the sample is a genuine experimental strength.

major comments (2)

[Title and Abstract] Title and Abstract: The title announces a 'measurement' of the ratios, yet the text only describes the dataset and its enabling potential; no invariant-mass spectra, yield extractions, efficiency corrections, or ratio values are shown. This mismatch is load-bearing for the central claim.
[Physics motivation and run-conditions sections] Physics motivation and run-conditions sections: The assertion that the 100 billion events 'yield a sufficiently clean and unbiased sample' to extract the ratio rests on unquantified assumptions about tracking efficiency, PID performance, and background subtraction; no signal-to-background estimates or systematic-uncertainty projections are supplied to support feasibility.

minor comments (2)

[Run conditions] The center-of-mass energy for the p+p data set is not stated explicitly; add the value (e.g., √s = 200 GeV) for completeness.
[Physics motivation] Consider adding a short table or paragraph summarizing expected statistical precision or decay channels (Λ_c⁺ → pK⁻π⁺, D⁰ → K⁻π⁺) to make the physics reach concrete.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review. The comments highlight important issues regarding the scope of the manuscript and the need for quantitative support for our claims. We address each major comment below and will incorporate revisions to strengthen the paper.

read point-by-point responses

Referee: [Title and Abstract] Title and Abstract: The title announces a 'measurement' of the ratios, yet the text only describes the dataset and its enabling potential; no invariant-mass spectra, yield extractions, efficiency corrections, or ratio values are shown. This mismatch is load-bearing for the central claim.

Authors: We agree that the current title and abstract create an expectation of a completed measurement that the manuscript does not fulfill. The paper is intended to document the sPHENIX detector performance, the 100 billion unbiased p+p events recorded in 2024, and the fact that this dataset enables the first Λ_c⁺/D⁰ measurement at RHIC energies, where none existed previously. No final ratio values, spectra, or corrections are presented because the full analysis is still underway. In the revised version we will change the title to 'sPHENIX Dataset and Capabilities for Open-Charm Baryon-to-Meson Ratio Measurements in p+p Collisions at RHIC' and rewrite the abstract to state clearly that the work reports the enabling dataset and experimental reach rather than the extracted physics result. revision: yes
Referee: [Physics motivation and run-conditions sections] Physics motivation and run-conditions sections: The assertion that the 100 billion events 'yield a sufficiently clean and unbiased sample' to extract the ratio rests on unquantified assumptions about tracking efficiency, PID performance, and background subtraction; no signal-to-background estimates or systematic-uncertainty projections are supplied to support feasibility.

Authors: The referee is correct that the feasibility statement would be more convincing with quantitative backing. The manuscript currently relies on the known performance of the sPHENIX tracking system and the minimum-bias character of the sample. In the revision we will add a dedicated subsection containing preliminary Monte Carlo studies. These will report expected tracking and PID efficiencies for D⁰ and Λ_c⁺ candidates, projected signal-to-background ratios in the relevant invariant-mass windows, and estimated statistical and systematic uncertainties on the Λ_c⁺/D⁰ ratio. This material will directly support the claim that the 100 billion events provide a viable path to the first RHIC measurement. revision: yes

Circularity Check

0 steps flagged

No significant circularity in direct experimental measurement

full rationale

This is an experimental proceedings paper reporting sPHENIX measurements of open-charm baryon-to-meson ratios in p+p collisions. It describes detector commissioning, the 100 billion unbiased event sample, and the motivation for the first Λ_c⁺/D⁰ baseline at RHIC energies. No equations, derivations, fitted parameters presented as predictions, or self-citation chains appear in the text; the central claims rest on the size and quality of the new dataset, which are externally verifiable and not reduced to the paper's own inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

The central claim rests on standard assumptions of detector calibration, particle identification efficiency, and background modeling that are common in heavy-flavor experiments but not detailed in the abstract. No free parameters, ad-hoc axioms, or new entities are introduced in the provided text.

pith-pipeline@v0.9.0 · 5516 in / 1094 out tokens · 35483 ms · 2026-05-10T07:58:53.234770+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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