CMS-PAS-HIN-19-005

CMS-PAS-HIN-19-005
Evidence for $\chi_{c1}\text{(3872)}$ in PbPb collisions and studies of its prompt production at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 5.02 TeV
CMS Collaboration
November 2019

Abstract: The first evidence for $\chi_{c1}\text{(3872)}$ in relativistic heavy ion collisions is reported. Prompt $\chi_{c1}\text{(3872)}$ production is studied in PbPb collisions at a center-of-mass energy of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV per nucleon pair, using the decay chain $\chi_{c1}\text{(3872)} \rightarrow {\rm J}/\psi\pi^+\pi^-\rightarrow \mu^+\mu^-\pi^+\pi^-$. The data were recorded by the CMS detector in 2018 and correspond to an integrated luminosity of 1.7 $\pm$ 0.1 nb$^{-1}$. The measurement is performed in a kinematic range in which the $\chi_{c1}\text{(3872)}$ candidates have a transverse momentum 15 $ < p_{\mathrm{T}} < $ 50 GeV/$c$ and rapidity $\|y\| < $ 1.6. The statistical significance of the $\chi_{c1}\text{(3872)}$ signal is in excess of 3 standard deviations. The production and survival of the prompt $\chi_{c1}\text{(3872)}$ in the quark-gluon plasma is expected to depend upon its internal structure. The ratio of the $\chi_{c1}\text{(3872)}$ and $\psi\text{(2S)}$ yields is found to be $R = $ 1.10 $\pm$ 0.51(stat) $\pm$ 0.53 (syst). This results is compared to the similar ratio measured in pp collisions at $\sqrt{s} = $ 7 and 8 TeV. An indication of larger $R$ in PbPb collisions with respect to the pp reference is reported.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: Invariant mass distribution of $m_{\mu \mu \pi \pi}$ in PbPb collisions. The mass of the dimuon pair is forced to be equal to the nominal ${\mathrm{J}/\psi}$ mass [35]. The red line presents the unbinned maximum-likelihood fit result. The signal components of $\psi\text{(2S)}$ and $\chi_{c1}\text{(3872)}$ are shown by the blue and the green line respectively. The pull distribution is presented by the red boxes.
png pdf	Figure 2: The yield ratio $R$ of prompt $\chi_{c1}\text{(3872)}$ over $\psi\text{(2S)}$ in PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The yield ratio $R$ of prompt and nonprompt $\chi_{c1}\text{(3872)}$ over $\psi\text{(2S)}$ in pp collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8 TeV from the ATLAS experiment [22] and $R$ of inclusive $\chi_{c1}\text{(3872)}$ over $\psi\text{(2S)}$ in pp collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 7 TeV from the CMS experiment [7] is also shown.

Summary

In summary, the first evidence for $\chi_{c1}\text{(3872)}$ production in heavy ion collisions is presented, using PbPb collisions at a center-of-mass energy of ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV per nucleon pair. The $\chi_{c1}\text{(3872)}$ state is reconstructed using the decay chain $\chi_{c1}\text{(3872)}\rightarrow {\rm J}/\psi\pi^+\pi^-\rightarrow \mu^+\mu^-\pi^+\pi^-$. The measurement is performed in a kinematic range in which the $\chi_{c1}\text{(3872)}$ candidates have a transverse momentum15 $ < p_{\mathrm{T}} < $ 50 GeV/$c$ and rapidity $ |y| < $ 1.6. The ratio $R$ of the prompt $\chi_{c1}\text{(3872)}$ and $\psi\text{(2S)}$ yields times their branching fractions into ${\rm J}/\psi\pi^+\pi^-$ is found to be 1.10 $\pm$ 0.51 (stat) $\pm$ 0.48 (syst). This result provides a unique experimental input to the theory community, towards elucidating the production mechanism and the nature of the $\chi_{c1}\text{(3872)}$ state.

Additional Figures
png pdf	Additional Figure 1: Pseudo-proper decay length ${l_{xy}}$ distributions of prompt and nonprompt signal X(3872) and $\psi$(2S) in PYTHIA+HYDJET Monte Carlo samples.
png pdf	Additional Figure 2: Invariant mass distribution of $m_{\mu \mu \pi \pi}$ in PbPb collisions with a nonprompt X(3872) enriched selection $ {l_{xy}} > $ 1 mm. The mass of dimuon pair is forced to be equal to the nominal ${{\mathrm {J} / \psi}} $ mass. The red line presents the unbinned maximum-likelihood fit result. The signal components of $\psi$(2S) and X(3872) are shown by the blue and the green line respectively. The pull distribution is presented by the red boxes.
png pdf	Additional Figure 3: The variation of the likelihood as a function of the number of X(3872) signals. The red lines indicate the values in the profile likelihood at the confidence level intervals of 68% and 95%.

References
1	Belle Collaboration	Observation of a narrow charmonium - like state in exclusive $ B^{\pm} \to K^{\pm} \pi^+ \pi^- J/\psi $ decays	PRL 91 (2003) 262001	hep-ex/0309032
2	BaBar Collaboration	Study of the $ B \to J/\psi K^- \pi^+ \pi^- $ decay and measurement of the $ B \to X(3872) K^- $ branching fraction	PRD71 (2005) 071103	hep-ex/0406022
3	BESIII Collaboration	Observation of $ e^+e^-\to X $(3872) at BESIII	PRL 112 (2014), no. 9, 092001	1310.4101
4	CDF Collaboration	Observation of the narrow state $ X(3872) \to J/\psi \pi^+ \pi^- $ in $ \bar{p}p $ collisions at $ \sqrt{s} = $ 1.96 TeV	PRL 93 (2004) 072001	hep-ex/0312021
5	D0 Collaboration	Observation and properties of the $ X(3872) $ decaying to $ J/\psi \pi^+ \pi^- $ in $ p\bar{p} $ collisions at $ \sqrt{s} = $ 1.96 TeV	PRL 93 (2004) 162002	hep-ex/0405004
6	LHCb Collaboration	Observation of $ X(3872) $ production in $ pp $ collisions at $ \sqrt{s}= $ 7 TeV	EPJC72 (2012) 1972	1112.5310
7	CMS Collaboration	Measurement of the $ X $(3872) Production Cross Section Via Decays to $ J/\psi \pi^+ \pi^- $ in $ pp $ collisions at $ \sqrt{s} = $ 7 TeV	JHEP 04 (2013) 154	CMS-BPH-11-011 1302.3968
8	ATLAS Collaboration	Measurements of $ \psi(2S) $ and $ X(3872) \to J/\psi\pi^+\pi^- $ production in $ pp $ collisions at $ \sqrt{s} = $ 8 TeV with the ATLAS detector	JHEP 01 (2017) 117	1610.09303
9	CDF Collaboration	Analysis of the quantum numbers $ J^{PC} $ of the $ X(3872) $	PRL 98 (2007) 132002	hep-ex/0612053
10	LHCb Collaboration	Determination of the X(3872) meson quantum numbers	PRL 110 (2013) 222001	1302.6269
11	N. A. Tornqvist	Isospin breaking of the narrow charmonium state of Belle at 3872-MeV as a deuson	PLB590 (2004) 209--215	hep-ph/0402237
12	L. Maiani, F. Piccinini, A. D. Polosa, and V. Riquer	Diquark-antidiquarks with hidden or open charm and the nature of X(3872)	PRD71 (2005) 014028	hep-ph/0412098
13	C. Hanhart, \relax Yu. S. Kalashnikova, and A. V. Nefediev	Interplay of quark and meson degrees of freedom in a near-threshold resonance: multi-channel case	EPJA47 (2011) 101--110	1106.1185
14	ExHIC Collaboration	Multi-quark hadrons from Heavy Ion Collisions	PRL 106 (2011) 212001	1011.0852
15	ExHIC Collaboration	Studying Exotic Hadrons in Heavy Ion Collisions	PRC84 (2011) 064910	1107.1302
16	A. Martinez Torres et al.	On $ X(3872) $ production in high energy heavy ion collisions	PRD90 (2014), no. 11, 114023	1405.7583
17	J. C. Collins and M. J. Perry	Superdense Matter: Neutrons Or Asymptotically Free Quarks?	PRL 34 (1975) 1353
18	E. V. Shuryak	Quark-Gluon Plasma and Hadronic Production of Leptons, Photons and Psions	PL78B (1978) 150, .[Yad. Fiz.28,796(1978)]
19	T. Matsui and H. Satz	$ J/\psi $ Suppression by Quark-Gluon Plasma Formation	PLB178 (1986) 416--422
20	LHCb Collaboration	Evidence for the decay $ X(3872)\rightarrow\psi(2S)\gamma $	NPB886 (2014) 665--680	1404.0275
21	A. Andronic, P. Braun-Munzinger, and J. Stachel	Hadron production in central nucleus-nucleus collisions at chemical freeze-out	NP A772 (2006) 167--199	nucl-th/0511071
22	The ATLAS collaboration	Measurements of $ \psi $(2S) and X(3872) $ \rightarrow $ J/$ \psi \pi \pi $ production in pp collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 8 TeV with the atlas detector	Journal of High Energy Physics 2017 (Jan, 2017) 117
23	CMS Collaboration	Suppression and azimuthal anisotropy of prompt and nonprompt $ {\mathrm{J}}/\psi $ production in PbPb collisions at $ \sqrt{{s_{_{\text {NN}}}}} = $ 2.76 TeV	EPJC77 (2017), no. 4, 252	CMS-HIN-14-005 1610.00613
24	CMS Collaboration	Measurement of prompt and nonprompt charmonium suppression in PbPb collisions at 5.02 TeV	EPJC78 (2018), no. 6, 509	CMS-HIN-16-025 1712.08959
25	CMS Collaboration	Studies of Beauty Suppression via Nonprompt $ D^0 $ Mesons in Pb-Pb Collisions at $ Q^2 = $ 4 GeV$^{2}$	PRL 123 (2019), no. 2, 022001	CMS-HIN-16-016 1810.11102
26	CMS Collaboration	Measurement of the $ {B}^{\pm} $ Meson Nuclear Modification Factor in Pb-Pb Collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 5.02 TeV	PRL 119 (2017), no. 15, 152301	CMS-HIN-16-011 1705.04727
27	CMS Collaboration	Measurement of B$ ^0_\mathrm{s} $ meson production in pp and PbPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}} = $ 5.02 TeV	PLB796 (2019) 168--190	CMS-HIN-17-008 1810.03022
28	CMS Collaboration	Evidence of b-Jet Quenching in PbPb Collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 2.76 TeV	PRL 113 (2014), no. 13, 132301	CMS-HIN-12-003 1312.4198
29	CMS Collaboration	Description and performance of track and primary-vertex reconstruction with the CMS tracker	JINST 9 (2014), no. 10, P10009	CMS-TRK-11-001 1405.6569
30	CMS Collaboration	Performance of CMS Muon Reconstruction in $ pp $ Collision Events at $ \sqrt{s}= $ 7 TeV	JINST 7 (2012) P10002	CMS-MUO-10-004 1206.4071
31	CMS Collaboration	The CMS Experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
32	CMS Collaboration	Charged-particle nuclear modification factors in PbPb and pPb collisions at $ {\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}= $ 5.02 TeV	JHEP 04 (2017) 039	CMS-HIN-15-015 1611.01664
33	CMS Collaboration	Transverse momentum and pseudorapidity distributions of charged hadrons in pp collisions at $ \sqrt{s} = $ 0.9 and 2.36 TeV	JHEP 02 (2010) 041	CMS-QCD-09-010 1002.0621
34	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
35	Particle Data Group Collaboration	Review of Particle Physics	PRD98 (2018), no. 3, 030001
36	A. Hoecker et al.	TMVA: Toolkit for Multivariate Data Analysis	PoS ACAT (2007) 040	physics/0703039