CMS-PAS-HIN-25-013

CMS-PAS-HIN-25-013
Study of the charm-quark dead cone in heavy-ion collisions
CMS Collaboration
2026-07-06

Abstract: This note presents the first measurement of the dead-cone effect for the charm quark in heavy-ion collisions. PbPb data recorded by the CMS detector at the CERN LHC in 2018 is used. The data correspond to a center-of-mass energy of 5.02 TeV and to an integrated luminosity of 1.7 $ \mathrm{nb}^{-1} $. The observable under study is the angular distribution of hard, collinear intrajet emissions from inclusive and prompt $ \mathrm{D}^0 $- meson-tagged jets in heavy-ion collisions. The emissions are selected by the late-$ k_\mathrm{T} $ grooming algorithm. The angular distribution of late-$ k_\mathrm{T} $ emissions in heavy ions is shifted towards large angles in $ \mathrm{D}^0 $-jets compared to inclusive jets. This is consistent with a suppression of collinear splittings for $ \mathrm{D}^0 $-jets as expected from the dead-cone effect. The shift is consistent with that observed in a previous measurement in pp collisions. This indicates that, within uncertainties, and within the kinematic region covered by the analysis, the dead cone of the charm quark is not filled by quark-gluon-plasma-induced radiation in heavy-ion collisions.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Left: invariant mass distribution of K$ \pi $ pairs for $ \mathrm{D^0} $ jet candidates and fit (red line) for jets with momentum 100 $ \leq p_{\mathrm{T}}^{\text{jet}} < $ 120 GeV and late-$ k_{\mathrm{T}} $ splitting angle 2 $ \leq \ln(1/\theta_{\text{l}}) < $ 2.3. The different components of the fit are the signal (orange-shaded area), K$ \pi $ swaps (green-shaded area), KK and $ \pi\pi $ (gray- and yellow-shaded areas), combinatorial background (blue line), and the non-signal (pink line) fit component, taking into account all the different background sources just mentioned. Right: splitting angle distribution for D$ ^0 $ meson-tagged jets (black points), and the prompt component (red points) after the non-prompt component (cyan-shaded area) subtraction, as well as the untagged fraction of jets not satisfying late-$ k_{\mathrm{T}} $ groomer, in 100 $ \le p_{\mathrm{T}}^{\text{jet}} < $ 120 GeV.
png pdf	Figure 1-a: Left: invariant mass distribution of K$ \pi $ pairs for $ \mathrm{D^0} $ jet candidates and fit (red line) for jets with momentum 100 $ \leq p_{\mathrm{T}}^{\text{jet}} < $ 120 GeV and late-$ k_{\mathrm{T}} $ splitting angle 2 $ \leq \ln(1/\theta_{\text{l}}) < $ 2.3. The different components of the fit are the signal (orange-shaded area), K$ \pi $ swaps (green-shaded area), KK and $ \pi\pi $ (gray- and yellow-shaded areas), combinatorial background (blue line), and the non-signal (pink line) fit component, taking into account all the different background sources just mentioned. Right: splitting angle distribution for D$ ^0 $ meson-tagged jets (black points), and the prompt component (red points) after the non-prompt component (cyan-shaded area) subtraction, as well as the untagged fraction of jets not satisfying late-$ k_{\mathrm{T}} $ groomer, in 100 $ \le p_{\mathrm{T}}^{\text{jet}} < $ 120 GeV.
png pdf	Figure 1-b: Left: invariant mass distribution of K$ \pi $ pairs for $ \mathrm{D^0} $ jet candidates and fit (red line) for jets with momentum 100 $ \leq p_{\mathrm{T}}^{\text{jet}} < $ 120 GeV and late-$ k_{\mathrm{T}} $ splitting angle 2 $ \leq \ln(1/\theta_{\text{l}}) < $ 2.3. The different components of the fit are the signal (orange-shaded area), K$ \pi $ swaps (green-shaded area), KK and $ \pi\pi $ (gray- and yellow-shaded areas), combinatorial background (blue line), and the non-signal (pink line) fit component, taking into account all the different background sources just mentioned. Right: splitting angle distribution for D$ ^0 $ meson-tagged jets (black points), and the prompt component (red points) after the non-prompt component (cyan-shaded area) subtraction, as well as the untagged fraction of jets not satisfying late-$ k_{\mathrm{T}} $ groomer, in 100 $ \le p_{\mathrm{T}}^{\text{jet}} < $ 120 GeV.
png pdf	Figure 2: Upper panel: late-$ k_{\mathrm{T}} $ angular distribution for Prompt-$ \mathrm{D^0} $ mesons in jets (red line) and inclusive jets (cyan line). Middle panel: ratio prompt $ D $ meson-tagged to inclusive jets in PbPb collisions (blue line), ratio prompt $ \mathrm{D^0} $ meson-tagged to inclusive jets in proton-proton collisions (purple line), and quenched models predictions for the ratio with diffusion effects -- orange-shaded area, and the vacuum prediction -- green-shaded area of Hybrid model, and LIDO medium -- dark-blue-shaded area and vacuum -- red-shaded area. Lower panel: PbPb-to-pp double ratio. The ratio is consistent with unity within uncertainties, indicating no significant modification of the suppression of small-angle emissions in PbPb collisions, this suggests that the dead cone is intact in PbPb collisions as predicted by theory. The fraction of late-$ k_{\mathrm{T}} $ untagged jets, for $ \mathrm{D^0} $ meson-tagged jets is about 22% and for inclusive jets is about 16%.

Tables
png pdf	Table 1: Summary of systematic uncertainties (relative) in each bin of $ \ln(1/\theta_{l}) $ in percent for prompt $ \mathrm{D}^0 $-tagged jets.
png pdf	Table 2: Summary of systematic uncertainties (relative) in each bin of $ \ln(1/\theta_{l}) $ in percent for Inclusive jets.

Summary

This note presents the first measurement of the dead-cone effect for charm quarks in heavy-ion collisions. Jet substructure techniques allow inspection of the jet radiation pattern and enhance sensitivity to quark mass effects, compared with measurements that treat jets as monolithic objects. This enables measurements of mass effects even for quarks with masses as small as that of the charm quark, and for high-energy jets. The angular distribution of hard, collinear emissions selected by the late-$ k_{\mathrm{T}} $ grooming algorithm is measured for inclusive and prompt $ \mathrm{D^0} $-tagged jets. Compared to inclusive jets, $ \mathrm{D^0} $-jets exhibit a suppression of small-angle emissions, consistent with the expected dead-cone effect. The observed angular distribution is compatible with the corresponding measurement in pp collisions, yielding a double ratio consistent with unity within uncertainties. These results provide no evidence that the quark-gluon plasma modifies the suppression of radiation in the charm dead-cone region within the kinematic reach and experimental precision of this measurement.

References
1	W. Busza, K. Rajagopal, and W. van der Schee	Heavy ion collisions: the big picture and the big questions	Ann. Rev. Nucl. Part. Sci. 68 (2018) 339	1802.04801
2	A. Adams et al.	Strongly correlated quantum fluids: ultracold quantum gases, quantum chromodynamic plasmas, and holographic duality	New J. Phys. 14 (2012) 115009	1205.5180
3	CMS Collaboration	Overview of high-density QCD studies with the CMS experiment at the LHC	Phys. Rep. 1115 (2025) 219	CMS-HIN-23-011 2405.10785
4	S. Cao and X.-N. Wang	Jet quenching and medium response in high-energy heavy-ion collisions: a review	Rept. Prog. Phys. 84 (2021) 024301	2002.04028
5	M. Gyulassy and M. Plumer	Jet quenching in dense matter	PLB 243 (1990) 432	hep-ph/9408344
6	R. Baier et al.	Radiative energy loss of high-energy quarks and gluons in a finite-volume quark-gluon plasma	NPB 483 (1997) 291	hep-ph/9607355
7	R. Baier et al.	Radiative energy loss and $ p_{\mathrm{T}} $ broadening of high-energy partons in nuclei	NPB 484 (1997) 265	hep-ph/9608322
8	J. Casalderrey-Solana, Y. Mehtar-Tani, C. A. Salgado, and K. Tywoniuk	New picture of jet quenching dictated by color coherence	PLB 725 (2013) 357	1210.7765
9	Y. L. Dokshitzer, V. A. Khoze, and S. I. Troian	On specific QCD properties of heavy quark fragmentation ('dead cone')	JPG 17 (1991) 1602
10	ALICE Collaboration	Direct observation of the dead-cone effect in quantum chromodynamics	[Erratum: Nature 607, E22 ()], 2022 Nature 605 (2022) 440	2106.05713
11	CMS Collaboration	Exploring small-angle emissions in charm quark jets in proton-proton collisions at $ \sqrt{s} = $ 5.02 TeV	JHEP 05 (2025) 176	CMS-HIN-24-007 2507.13469
12	LHCb Collaboration	Measurement of the Lund plane for light- and beauty-quark jets	PRD 112 (2025) 072015	2505.23530
13	CMS Collaboration	Jet fragmentation function and groomed substructure of bottom quark jets in proton-proton collisions at 5.02 TeV	JHEP 04 (2026) 147	CMS-HIN-24-005 2511.10666
14	S. Marzani, G. Soyez, and M. Spannowsky	Looking inside jets: an introduction to jet substructure and boosted-object phenomenology	volume 958. Springer, 2019 link
15	G.-Y. Qin and X.-N. Wang	Jet quenching in high-energy heavy-ion collisions	Int. J. Mod. Phys. E 24 (2015) 1530014	1511.00790
16	N. Armesto, C. A. Salgado, and U. A. Wiedemann	Medium induced gluon radiation off massive quarks fills the dead cone	PRD 69 (2004) 114003	hep-ph/0312106
17	L. Cunqueiro, D. Napoletano, and A. Soto-Ontoso	Dead cone searches in heavy-ion collisions using the jet tree	PRD 107 (2023) 094008	2211.11789
18	C. Andres et al.	Seeing beauty in the quark-gluon plasma with energy correlators	PRD 110 (2024) L031503	2307.15110
19	A. Lifson, G. P. Salam, and G. Soyez	Calculating the primary Lund jet plane density	JHEP 10 (2020) 170	2007.06578
20	F. A. Dreyer, G. P. Salam, and G. Soyez	The Lund jet plane	JHEP 12 (2018) 064	1807.04758
21	Y. L. Dokshitzer, G. D. Leder, S. Moretti, and B. R. Webber	Better jet clustering algorithms	JHEP 08 (1997) 001	hep-ph/9707323
22	L. Cunqueiro and M. P\l osko\'n	Searching for the dead cone effects with iterative declustering of heavy-flavor jets	PRD 99 (2019) 074027	1812.00102
23	CMS Collaboration	Luminosity measurement for lead-lead collisions at $ \sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV in 2015 and 2018 at CMS		CMS-LUM-20-002 2503.03946
24	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004
25	CMS Collaboration	Development of the CMS detector for the CERN LHC Run 3	JINST 19 (2024) P05064	CMS-PRF-21-001 2309.05466
26	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
27	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
28	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
29	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
30	CMS Collaboration	Description and performance of track and primary-vertex reconstruction with the CMS tracker	JINST 9 (2014) P10009	CMS-TRK-11-001 1405.6569
31	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
32	C. Loizides, J. Kamin, and D. d'Enterria	Improved Monte Carlo Glauber predictions at present and future nuclear colliders	Phys. Rev. C 97 (2018) 054910	1710.07098
33	CMS Collaboration	Observation and studies of jet quenching in PbPb collisions at nucleon-nucleon center-of-mass energy $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV	Phys. Rev. C 84 (2011) 024906	CMS-HIN-10-004 1102.1957
34	T. Sj$\text ö $strand et al.	An introduction to PYTHIA8.2	Comput. Phys. Commun. 191 (2015) 159	1410.3012
35	CMS Collaboration	Extraction and validation of a new set of CMS PYTHIA8 tunes from underlying-event measurements	EPJC 80 (2020) 4	CMS-GEN-17-001 1903.12179
36	I. P. Lokhtin and A. M. Snigirev	A model of jet quenching in ultrarelativistic heavy ion collisions and high-$ p_{\mathrm{T}} $ hadron spectra at RHIC	EPJC 45 (2006) 211	hep-ph/0506189
37	GEANT4 Collaboration	GEANT 4---a simulation toolkit	NIM A 506 (2003) 250
38	M. Cacciari, G. P. Salam, and G. Soyez	The anti-$ k_{\mathrm{T}} $ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
39	M. Cacciari, G. P. Salam, and G. Soyez	FastJet user manual	EPJC 72 (2012) 1896	1111.6097
40	H. A. Andrews et al.	Novel tools and observables for jet physics in heavy ion collisions	JPG 47 (2020) 065102	1808.03689
41	P. Berta, M. Spousta, D. W. Miller, and R. Leitner	Particle-level pileup subtraction for jets and jet shapes	JHEP 06 (2014) 092	1403.3108
42	O. Kodolova, I. Vardanyan, A. Nikitenko, and A. Oulianov	The performance of the jet identification and reconstruction in heavy ions collisions with CMS detector	EPJC 50 (2007) 117
43	ALICE Collaboration	Measurement of event background fluctuations for charged particle jet reconstruction in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV	JHEP 03 (2012) 053	1201.2423
44	CMS Collaboration	Determination of jet energy calibration and transverse momentum resolution in CMS	JINST 6 (2011) P11002	CMS-JME-10-011 1107.4277
45	CMS Collaboration	Jet energy scale and resolution in the CMS experiment in pp collisions at 8 TeV	JINST 12 (2017) P02014	CMS-JME-13-004 1607.03663
46	Particle Data Group , S. Navas et al.	Review of particle physics	PRD 110 (2024) 030001
47	CMS Collaboration	Studies of charm quark diffusion inside jets using PbPb and pp collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV	PRL 125 (2020) 102001	CMS-HIN-18-007 1911.01461
48	T. Skwarnicki	A study of the radiative cascade transitions between the Upsilon-prime and Upsilon resonances	PhD thesis, Institute of Nuclear Physics, Krakow, DESY-F31-86-02, 1986
49	CMS Collaboration	Measurements of azimuthal anisotropy of nonprompt $ \mathrm{D^0} $ mesons in PbPb collisions at 5.02TeV	PLB 850 (2024) 138389	CMS-HIN-21-003 2212.01636
50	G. D'Agostini	A multidimensional unfolding method based on Bayes' theorem	NIM A 362 (1995) 487	1010.0632
51	T. Adye	Unfolding algorithms and tests using RooUnfold	in Proc. Workshop on Statistical Issues Related to Discovery Claims in Search Experiments and Unfolding, H. Prosper and L. Lyons, eds., Geneva, Switzerland, 2011 PHYSTAT 2011 (2011) 313	1105.1160
52	CMS Collaboration	Single-particle response in the CMS calorimeters	CMS Physics Analysis Summary, 2010 link
53	J. Casalderrey-Solana et al.	A hybrid strong/weak coupling approach to jet quenching	JHEP 10 (2014) 019	1405.3864
54	J. Casalderrey-Solana et al.	Jet wake from linearized hydrodynamics	JHEP 05 (2021) 230	2010.01140
55	CMS Collaboration	Girth and groomed radius of jets recoiling against isolated photons in lead-lead and proton-proton collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV	PLB 861 (2025) 139088	CMS-HIN-23-001 2405.02737
56	CMS Collaboration	Nuclear modification factor of D$ ^0 $ mesons in PbPb collisions at $ \sqrt {\smash [b]{s_{_{\mathrm {NN}}}}} = $ 5.02 TeV	PLB 782 (2018) 474	CMS-HIN-16-001 1708.04962
57	A. Beraudo, J. F. Du Plessis, D. Pablos, and K. Rajagopal	Heavy Quark Energy Loss in the Hybrid Model		2510.24847
58	W. Ke, Y. Xu, and S. A. Bass	Linearized Boltzmann-Langevin model for heavy quark transport in hot and dense QCD matter	Phys. Rev. C 98 (2018) 064901	1806.08848
59	W. Ke, Y. Xu, and S. A. Bass	Modified Boltzmann approach for modeling the splitting vertices induced by the hot QCD medium in the deep Landau-Pomeranchuk-Migdal region	Phys. Rev. C 100 (2019) 064911	1810.08177
60	M. Cacciari, M. Greco, and P. Nason	The $ p_T $ spectrum in heavy-flavour hadroproduction.	JHEP 05 (1998) 007	hep-ph/9803400