CMS-PAS-HIN-24-020

CMS-PAS-HIN-24-020
Energy-dependent measurement of coherent $ \rho(770) $ photoproduction in ultraperipheral PbPb Collisions at the LHC
CMS Collaboration
2026-07-25

Abstract: We report the first energy-dependent measurement of exclusive $ \rho(770) $ photoproduction at 5.36 TeV PbPb ultraperipheral collisions, covering a wide photon-nuclei centre-of-mass energy range of 26 $ < W < $ 158 GeV. The coherent $ \rho(770) $ photoproduction cross section is measured as a function of rapidity, bridging the gap between existing measurements by ALICE and LHCb. In such symmetric heavy-ion collisions, both nuclei can act as photon emitters or targets, making it impossible to uniquely identify the photon source. As a result, contributions from different photon energies are superimposed in the measured cross section. To resolve this two-fold ambiguity, an impact-parameter dependent analysis is performed, exploiting the dependence of the photon flux on the collision geometry. As a light vector meson, $ \rho(770) $ can serve as a probe of the non-perturbative regime of QCD. The measured discrepancies between data and model predictions highlight limitations of current descriptions and provide new constraints on theoretical models.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass and transverse momentum fitting of $ \rho(770) $ without any neutron selection in one of the 10 rapidity windows is shown in left and right. The red mass fit curve of data(black markers) in left consists of signal $ \rho(770) $, QED bkg and direct pion. The blue $ p_{\mathrm{T}} $ fit in the right contains the coherent, incoherent, and dissociative contributions using which the $ f_{I} $ is calculated.
png pdf	Figure 2: Differential cross section for the coherent photoproduction of $ \rho(770) $ as a function of rapidity with no neutron selection in left. The results from the ALICE and LHCb experiments are also displayed [15,16]. GKZ based theoretical predictions are added for low ($ W_{\gamma N}^{-} $) and high ($ W_{\gamma N}^{+} $) photon energy solutions. The right plot shows the differential cross section separated according to the neutron multiplicity measured in the ZDCs.
png pdf	Figure 3: The total coherent $ \rho(770) $ photoproduction cross section as a function of $ W^{\rm{A}}_{\gamma\rm{N}} $ from the CMS measurement in PbPb UPCs at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.36 TeV. Results from STAR and ALICE measurements at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 0.2 and 5.02 TeV are shown [20,21,22,23]. Additionally the Regge based and VMD model based theoretical predictions are added [1,19].

Tables
png pdf	Table 1: Systematic uncertainty values (%) for different neutron classes as a function of rapidity. The first column represents the midpoint of rapidity range and the remaining three columns represent the uncertainties for differential cross section values with different neutron configurations

Summary

In this note, the measurement of exclusive $ \rho(770) $ photoproduction off lead nuclei in ultraperipheral PbPb collisions at 5.36 TeV data corresponding to an integrated luminosity of 1.32 $ \mu $b$ ^{-1} $ is performed. The ambiguity regarding the incoming photon source in symmetric collisions is addressed by measuring the differential cross section as a function of rapidity by controlling the impact parameter dependence using the ZDCs. The calculated differential cross sections presented in this note covers a phase space unexplored in previous measurements from ALICE and LHCb experiments [15,16]. STARLIGHT and GKZ based theoretical predictions are added, where the STARLIGHT under estimates whereas GKZ prediction [18] describes the data comparatively well in the central rapidity region where we are focused. The total coherent $ \rho(770) $ photoproduction cross section is measured in a wide photon-nuclei centre of mass energy interval 26 $ < W^{\rm{A}}_{\gamma\rm{N}} < $ 158 GeV and the trend matches the most with the modified VMD model. These results demonstrate that coherent $ \rho(770) $ photoproduction in heavy-ion collisions is governed by soft QCD dynamics, where nuclear shadowing and multiple scattering effects are essential. The data favour a modified VMD description incorporating Gribov-Glauber corrections [1], while indicating additional suppression beyond current model expectations, potentially pointing towards enhanced shadowing or saturation effects at low Bjorken-$ x $.

References
1	L. Frankfurt, V. Guzey, M. Strikman, and M. Zhalov	Nuclear shadowing in photoproduction of $ \rho^{0} $ mesons in ultraperipheral nucleus collisions at RHIC and the LHC	Physics Letters B 752 (2016) 51
2	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004
3	CMS Collaboration	Development of the CMS detector for the CERN LHC Run 3	JINST 19 (2024) P05064
4	CMS Collaboration	Performance of the CMS Level-1 trigger in proton-proton collisions at $ \sqrt{s} = $ 13 TeV	JINST 15 (2020) P10017	CMS-TRG-17-001 2006.10165
5	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
6	CMS Collaboration	Performance of the CMS high-level trigger during LHC run 2	JINST 19 (2024) P11021	CMS-TRG-19-001 2410.17038
7	CMS Collaboration	Electron and photon reconstruction and identification with the CMS experiment at the CERN LHC	JINST 16 (2021) P05014	CMS-EGM-17-001 2012.06888
8	CMS Collaboration	Performance of the CMS muon detector and muon reconstruction with proton-proton collisions at $ \sqrt{s}= $ 13 TeV	JINST 13 (2018) P06015	CMS-MUO-16-001 1804.04528
9	CMS Collaboration	Description and performance of track and primary-vertex reconstruction with the CMS tracker	Journal of Instrumentation 9 (2014) P10009
10	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
11	CMS Collaboration Collaboration	Measurement of D$ ^0 $ meson photoproduction in ultraperipheral heavy ion collisions	PRL 136 (2026) 122303
12	CMS Collaboration	Observation of coherent $ \phi $(1020) meson photoproduction in ultraperipheral $ \mathrm{PbPb} $ collisions at $ \sqrt{s_\text{NN}} = $ 5.36 tev	Physical Review Letters 135 (2025)
13	P. S$\ddot o $ding	On the apparent shift of the rho meson mass in photoproduction	PLB 19 (1966) 702
14	ATLAS Collaboration	Exclusive dimuon production in ultraperipheral $ \mathrm{Pb}+\mathrm{Pb} $ collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}=5.02\phantom{\rule{0.16em}{0ex}}\mathrm{TeV} $ with ATLAS	Phys. Rev. C 104 (2021)
15	ALICE Collaboration	First measurement of coherent \ensuremath\rho^0 photoproduction in ultra-peripheral Xe-Xe collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}= $ 5.44 TeV	PLB 820 (2021) 136481	2101.02581
16	LHCb Collaboration	Coherent photoproduction of $ \rho^0 $, $ \omega $ and excited vector mesons in ultraperipheral $ \mathrm{PbPb} $ collisions	JHEP 2025 (2025)
17	V. Guzey, E. Kryshen, and M. Zhalov	Coherent photoproduction of vector mesons in ultraperipheral heavy ion collisions: Update for run 2 at the CERN Large Hadron Collider	Physical Review C 93 (2016)
18	S. R. Klein et al.	Starlight: A monte carlo simulation program for ultra-peripheral collisions of relativistic ions	Computer Physics Communications 212 (2017) 258
19	L. Jenkovszky, E. S. Rocha, and M. V. T. Machado	Investigating exclusive $ \rho^{0} $ photoproduction within the regge phenomenology approach	Physics Letters B 835 (2022) 137585
20	STAR Collaboration	Coherent $ \rho^{0} $ production in ultraperipheral heavy-ion collisions	Physical Review Letters 89 (2002)
21	STAR Collaboration	$ \rho^{0} $ photoproduction in ultraperipheral relativistic heavy ion collisions with STAR	Physical Review C 77 (2008)
22	STAR Collaboration	$ \rho^{0} $ photoproduction in AuAu collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 62.4 GeV measured with STAR	Physical Review C 85 (2012)
23	ALICE Collaboration	Coherent photoproduction of $ \rho^{0} $ vector mesons in ultra-peripheral PbPb collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV	Journal of High Energy Physics 2020 (2020)