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arxiv: 2606.14166 · v2 · pith:CL734RP4new · submitted 2026-06-12 · ✦ hep-ph · hep-ex· nucl-th

Thermal dileptons to probe the baryon-rich QCD matter in the forward region of LHC energy heavy-ion collisions

Motomi Oya , Nicholas J. Benoit , Chiho Nonaka , Azumi Sakai , Yorito Yamaguchi This is my paper

Pith reviewed 2026-06-27 05:05 UTC · model grok-4.3

classification ✦ hep-ph hep-exnucl-th
keywords thermal dileptonsbaryon chemical potentialheavy-ion collisionsquark-gluon plasmaforward rapidityhydrodynamicsLHC
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The pith

Finite baryon density suppresses thermal dilepton yields by 3-4% in the forward rapidity region of LHC heavy-ion collisions.

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

The paper calculates thermal dilepton production from quark-gluon plasma that carries a nonzero baryon chemical potential in central lead-lead collisions at 5.02 TeV. It incorporates finite μ_B into a full three-dimensional hydrodynamic evolution and shows that this potential exceeds 500 MeV near space-time rapidity 6, producing a modest 3-4% reduction in dilepton rates at detector rapidities between 5.2 and 7.2. The reduction arises because finite baryon density lowers the available quark-antiquark pairs. The effective temperature extracted from the intermediate-mass dilepton spectrum remains tightly correlated with the underlying hydrodynamic temperature and continues to reflect the earliest, hottest stage of the medium.

Core claim

In central Pb-Pb collisions at √s_NN = 5.02 TeV a (3+1)-dimensional hydrodynamic model with finite baryon chemical potential predicts μ_B > 500 MeV near η_s = 6. This density reduces the quark-antiquark abundance and therefore suppresses the thermal dilepton yield by 3-4% in the forward interval 5.2 < y < 7.2. The effective temperature obtained from the dilepton mass spectrum in 1.2 < M_ℓℓ < 2.6 GeV stays strongly correlated with the hydrodynamic temperature and retains sensitivity to the early high-temperature phase of the evolution.

What carries the argument

A (3+1)-dimensional hydrodynamic framework that evolves finite baryon chemical potential μ_B and feeds the resulting space-time profiles into the calculation of thermal dilepton spectra across a wide rapidity window.

If this is right

  • μ_B exceeds 500 MeV near η_s = 6 throughout the medium evolution.
  • Thermal dilepton production is suppressed by 3-4% in the forward rapidity window 5.2 < y < 7.2.
  • The effective temperature extracted from intermediate-mass dileptons remains correlated with the early hydrodynamic temperature.
  • Forward-rapidity dileptons continue to function as thermometers while acquiring sensitivity to finite baryon density.

Where Pith is reading between the lines

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

  • Forward dilepton data already collected at the LHC could be reanalyzed to extract baryon-density information without new lower-energy runs.
  • The same hydrodynamic profiles could be used to predict finite-μ_B corrections to other electromagnetic probes such as real photons.
  • The small size of the suppression indicates that existing mid-rapidity calculations remain a good first approximation but can be refined by including forward μ_B effects.

Load-bearing premise

The hydrodynamic evolution accurately reproduces the space-time profile of baryon density at forward rapidities in these collisions.

What would settle it

A precision measurement of the dilepton yield ratio between the forward interval 5.2 < y < 7.2 and mid-rapidity that shows either zero suppression or a suppression outside the 3-4% range would falsify the predicted effect of finite μ_B.

Figures

Figures reproduced from arXiv: 2606.14166 by Azumi Sakai, Chiho Nonaka, Motomi Oya, Nicholas J. Benoit, Yorito Yamaguchi.

Figure 1
Figure 1. Figure 1: FIG. 1: Comparison of the charged particle multiplicity [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Comparison of the initial baryon density (fm [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Temperature and baryon chemical potential of [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: The space-time rapidity distributions of dilepton [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: The rapidity dependence of thermal dilepton [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 7
Figure 7. Figure 7: The mass range is divided into three regions in [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: The comparison of the effective temperature and [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Fit mass range dependence of the effective [PITH_FULL_IMAGE:figures/full_fig_p008_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: Rapidity distribution of the effective [PITH_FULL_IMAGE:figures/full_fig_p009_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: Fit mass range dependence of the effective [PITH_FULL_IMAGE:figures/full_fig_p009_9.png] view at source ↗
read the original abstract

We investigate thermal dilepton production from a quark-gluon plasma (QGP) with finite baryon chemical potential ($\mu_{\text{B}}$) in central Pb-Pb collisions at $\sqrt{s_{\text{NN}}}=5.02~\text{TeV}$. Recent studies suggest that sizable baryon densities can be achieved at forward rapidity even at LHC energies. We incorporate finite $\mu_{\text{B}}$ into a (3+1)-dimensional hydrodynamic framework and find that $\mu_{\text{B}}$ exceeds 500 MeV around $\eta_\text{s} = 6$ during the medium evolution. Using this framework, we calculate thermal dilepton spectra over a wide rapidity range and evaluate the impact of finite $\mu_{\text{B}}$ on dilepton production. A suppression of 3-4% is observed in the forward-rapidity region $5.2 < y < 7.2$ due to the reduced quark-antiquark abundance at finite baryon density. We further examine the effective temperature extracted from dilepton mass spectra in the intermediate-mass region $1.2 < M_{\ell \ell} < 2.6~\text{GeV}$ . The effective temperature remains strongly correlated with the underlying hydrodynamic temperature and retains sensitivity to the early high-temperature stage of the QGP evolution. These results demonstrate that forward-rapidity dileptons remain effective thermometers while providing sensitivity to finite baryon density at the LHC.

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.

Referee Report

1 major / 1 minor

Summary. The paper claims that incorporating finite μ_B into a (3+1)D hydrodynamic model for central Pb-Pb collisions at √s_NN=5.02 TeV yields μ_B >500 MeV near η_s=6, producing a 3-4% suppression of thermal dilepton yields in 5.2<y<7.2 from reduced q q-bar abundance; it further claims that the effective temperature extracted from intermediate-mass dilepton spectra (1.2<M_ℓℓ<2.6 GeV) remains strongly correlated with the hydrodynamic temperature and retains sensitivity to the early high-T stage of the QGP.

Significance. If the hydrodynamic prediction of sizable forward μ_B is robust, the work provides concrete, falsifiable predictions for how forward-rapidity dileptons can simultaneously probe baryon-rich matter and serve as early-stage thermometers at LHC energies. The small quantified suppression and temperature correlation constitute specific, testable outputs from the model.

major comments (1)
  1. [Hydrodynamic framework] Hydrodynamic framework section: the 3-4% suppression and the claimed temperature sensitivity both rest on the specific prediction that μ_B exceeds 500 MeV around η_s=6; the manuscript presents no variation over initial conditions, transport coefficients, or comparison to other (3+1)D hydro codes or baryon-stopping data to establish robustness of this μ_B value.
minor comments (1)
  1. [Abstract] Abstract and results: numerical outcomes (3-4% suppression, effective-temperature correlation) are stated without explicit reference to the dilepton rate formula, hydrodynamic parameters, or uncertainty quantification used to obtain them.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive feedback. We address the major comment on the hydrodynamic framework below.

read point-by-point responses

  1. Referee: Hydrodynamic framework section: the 3-4% suppression and the claimed temperature sensitivity both rest on the specific prediction that μ_B exceeds 500 MeV around η_s=6; the manuscript presents no variation over initial conditions, transport coefficients, or comparison to other (3+1)D hydro codes or baryon-stopping data to establish robustness of this μ_B value.

    Authors: We agree that additional checks would strengthen the robustness claim for μ_B > 500 MeV at η_s ≈ 6. The present work employs a standard (3+1)D hydrodynamic setup calibrated to existing LHC data, with the forward μ_B arising from baryon stopping as reported in recent studies. In the revised manuscript we will add a dedicated paragraph (or short subsection) that (i) varies the initial baryon density profile within the range consistent with lower-energy stopping data, (ii) shows the effect of modest changes in shear viscosity, and (iii) compares the resulting μ_B(η_s) profiles to those obtained with other (3+1)D codes. These additions will demonstrate that the 3–4 % suppression remains stable while the temperature–dilepton correlation is largely insensitive to the precise μ_B value. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper computes μ_B(η_s) as an output of its (3+1)D hydrodynamic evolution, then feeds the resulting space-time profiles into standard thermal dilepton rate integrals that depend on local T and μ_B. The quoted 3-4% suppression and the effective-temperature correlation are direct numerical consequences of those integrals evaluated on the hydro output; neither quantity is inserted by hand nor recovered by fitting the same data that the model was tuned on. No self-citation, ansatz, or uniqueness theorem is invoked to force the result, and the derivation chain remains a forward model calculation rather than a tautology.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

Review based on abstract only; typical free parameters in hydrodynamic models of heavy-ion collisions are not enumerated here.

free parameters (1)
  • hydrodynamic initial conditions and transport coefficients
    Standard in (3+1)D hydro models; values are chosen or fitted to reproduce bulk observables.
axioms (1)
  • domain assumption The hydrodynamic evolution with finite baryon chemical potential accurately describes the medium at forward rapidities.
    Invoked to obtain μ_B > 500 MeV and the dilepton spectra.

pith-pipeline@v0.9.1-grok · 5816 in / 1341 out tokens · 30075 ms · 2026-06-27T05:05:46.677499+00:00 · methodology

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