X(3872)

{{Short description|Subatomic particle}}

The X(3872) is an exotic meson candidate with a mass of 3871.68 MeV/c²[http://pdglive.lbl.gov Particle Data Group Live listings] which does not fit into the quark model. It was first discovered in 2003 by the Belle experiment

{{Cite journal

|last1=Choi |first1=S.-K.

|collaboration=Belle Collaboration

|year=2003

|title=Observation of a Narrow Charmoniumlike State in Exclusive B±→K±π+π-J/ψ Decays

|journal=Physical Review Letters

|volume=91 |issue=26 |pages=262001

|arxiv=hep-ex/0308029

|bibcode=2003PhRvL..91z2001C

|doi=10.1103/PhysRevLett.91.262001

|pmid=14754041

|s2cid=1017547

}} in Japan and later confirmed by several other experimental collaborations. Several theories have been proposed for its nature,{{cite journal

|last1=Swanson |first1=E. S.

|year=2006

|title=The new heavy mesons: A status report

|journal=Physics Reports

|volume=429 |issue=5 |pages=243–305

|arxiv=hep-ph/0601110

|bibcode=2006PhR...429..243S

|doi=10.1016/j.physrep.2006.04.003

|s2cid=54704359

}} such as a mesonic molecule or a diquark-antidiquark pair (tetraquark).

The quantum numbers of X(3872) have been determined by the LHCb experiment at CERN in March 2013. The values for J^PC are 1⁺⁺.

{{Cite journal

|last1=Aaij|first1=R.

|collaboration=LHCb collaboration

|year=2013

|title=Determination of the X(3872) meson quantum numbers

|journal=Physical Review Letters

|volume=110 |issue=22 |page=222001

|arxiv=1302.6269

|bibcode=2013PhRvL.110v2001A

|doi=10.1103/PhysRevLett.110.222001 |doi-access=free |pmid=23767712

}}

The first evidence of X(3872) production in the quark–gluon plasma have been reported by the CMS experiment at CERN in January 2022.

{{Cite journal

|last1=Sirunyan|first1=A. M.

|collaboration=CMS collaboration

|year=2022

|title=Evidence for X(3872) in Pb-Pb Collisions and Studies of its Prompt Production at 5.02 TeV

|journal=Physical Review Letters

|volume=128 |issue=3 |page=032001

|arxiv=2102.13048

|doi=10.1103/PhysRevLett.128.032001 |pmid=35119878

|doi-access=free

}}