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Observation of Structure in the -Pair Mass Spectrum Zurich Open Repository and Archive University of Zurich Main Library Strickhofstrasse 39 CH-8057 Zurich www.zora.uzh.ch Year: 2020 Observation of structure in the J/ψ-pair mass spectrum LHCb Collaboration ; Bernet, R ; Müller, K ; Owen, P ; Serra, N ; Steinkamp, O ; et al DOI: https://doi.org/10.1016/j.scib.2020.08.032 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-196709 Journal Article Published Version The following work is licensed under a Creative Commons: Attribution 4.0 International (CC BY 4.0) License. Originally published at: LHCb Collaboration; Bernet, R; Müller, K; Owen, P; Serra, N; Steinkamp, O; et al (2020). Observation of structure in the J/ψ-pair mass spectrum. Science Bulletin, 2020:65. DOI: https://doi.org/10.1016/j.scib.2020.08.032 Science Bulletin 65 (2020) 1983–1993 Contents lists available at ScienceDirect Science Bulletin journal homepage: www.elsevier.com/locate/scib Article Observation of structure in the J/w-pair mass spectrum LHCb collaboration 1 article info abstract Article history: Using proton-proton collision data at centre-of-mass energies of ps 7; 8 and 13 TeV recorded by the ffiffi ¼ 1 Received 1 July 2020 LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 9 fbÀ , the Received in revised form 28 July 2020 invariant mass spectrum of J/w pairs is studied. A narrow structure around 6:9 GeV=c2 matching the line- Accepted 19 August 2020 shape of a resonance and a broad structure just above twice the J/w mass are observed. The deviation of Available online 29 August 2020 the data from nonresonant J/w-pair production is above five standard deviations in the mass region between 6:2 and 7:4 GeV=c2, covering predicted masses of states composed of four charm quarks. The Keywords: mass and natural width of the narrow X 6900 structure are measured assuming a Breit-Wigner QCD ðÞ lineshape. Exotics Ó Tetraquark 2020 Science China Press. Published by Elsevier B.V. and Science China Press. This is an open access Spectroscopy article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Quarkonium Particle and resonance production 1. Introduction charmed baryon Nccþþ [36,37] and helps to explain the relative rates of bottom baryon decays [38]. The strong interaction is one of the fundamental forces of nat- Tetraquark states comprising only bottom quarks, Tbbbb, have ure and it governs the dynamics of quarks and gluons. According been searched for by the LHCb and CMS collaborations in the to quantum chromodynamics (QCD), the theory describing the Ç lþlÀ decay [39,40], with the Ç state consisting of a bb pair. How- strong interaction, quarks are confined into hadrons, in agreement ever, the four-charm states, have not yet been studied in detail with experimental observations. The quark model [1,2] classifies experimentally. A state could disintegrate into a pair of charmo- hadrons into conventional mesons (qq) and baryons (qqq or qqq), nium states such as J/w mesons, with each consisting of a cc pair. and also allows for the existence of exotic hadrons such as tetra- Decays to a J/w meson plus a heavier charmonium state, or two quarks (qqqq) and pentaquarks (qqqqq). Exotic states provide a heavier charmonium states, with the heavier states decaying sub- unique environment to study the strong interaction and the con- sequently into a J/w meson and accompanying particles, are also finement mechanism [3]. The first experimental evidence for an possible. Predictions for the masses of states vary from 5:8to exotic hadron candidate was the v 3872 state observed in c1ðÞ 7:4 GeV=c2 [10–26], which are above the masses of known charmo- 2003 by the Belle collaboration [4]. Since then a series of novel nia and charmonium-like exotic states and below those of bot- states consistent with containing four quarks have been discovered tomonium hadrons. 2 This mass range guarantees a clean [5]. Recently, the LHCb collaboration observed resonances inter- experimental environment to identify possible states in the J/w- preted to be pentaquark states [6–9]. All hadrons observed to date, pair (also referred to as di-J/w) invariant mass (Mdi-J=w) spectrum. including those of exotic nature, contain at most two heavy charm In proton-proton (pp) collisions, a pair of J/w mesons can be pro- (c) or bottom (b) quarks, whereas many QCD-motivated phe- duced in two separate interactions of gluons or quarks, named nomenological models also predict the existence of states consist- double-parton scattering (DPS) [41–43], or in a single interaction, ing of four heavy quarks, i.e. T , where Q i is a c or a b quark Q 1Q 2Q 3Q 4 named single-parton scattering (SPS) [44–51]. The SPS process [10–35]. Theoretically, the interpretation of the internal structure includes both resonant production via intermediate states, which of such states usually assumes the formation of a Q 1Q 2 diquark could be tetraquarks, and nonresonant production. Within the and a Q 3Q 4 antidiquark attracting each other. Application of this DPS process, the two J/w mesons are usually assumed to be pro- diquark model successfully predicts the mass of the doubly duced independently, thus the distribution of any di-J/w observ- 2 Energy units MeV 106 eV, GeV 109 eV and TeV 1012 eV are used in this ¼ ¼ ¼ 1 Authors are listed at the end of this paper. paper. https://doi.org/10.1016/j.scib.2020.08.032 2095-9273/Ó 2020 Science China Press. Published by Elsevier B.V. and Science China Press. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1984 LHCb collaboration / Science Bulletin 65 (2020) 1983–1993 able can be constructed using the kinematics from single J/w pro- for about 0.8% of the total sample, only one pair is randomly duction. Evidence of DPS has been found in studies at the Large chosen. Hadron Collider (LHC) experiments [52–61] and the AFS experi- The di-J/w signal yield is extracted by performing an extended ment [62] in pp collisions, and at the Tevatron experiments [63– unbinned maximum-likelihood fit to the two-dimensional distri- 67] and the UA2 experiment [68] in proton-antiproton collisions. 1 2 bution of J=w1 and J=w2 invariant masses, MllðÞ; MllðÞ , as displayed The LHCb experiment has measured the di-J/w production in pp in Fig. 1, where projections of the data and the fit result are shown. collisions at centre-of-mass energies of ps 7 [69] and 13 TeV ffiffi ¼ For both J/w candidates, the signal mass shape is modelled by a [70]. The DPS contribution is found to dominate the high M di-J=w Gaussian kernel with power-law tails [81]. Each component of region, in agreement with expectation. combinatorial background, consisting of random combinations of In this paper, fully charmed tetraquark states are searched for in muon tracks, is described by an exponential function. The total the di-J/w invariant mass spectrum, using pp collision data col- di-J/w signal yield is measured to be 33:57 0:23 103, where lected by LHCb at ps 7; 8 and 13 TeV, corresponding to an inte- ðÞÆ Â ffiffi ¼ 1 the uncertainty is statistical. grated luminosity of 9 fbÀ . The two J/w candidates in a pair are di-J=w The di-J/w transverse momentum (pT ) in SPS production is reconstructed through the J=w lþlÀ decay, and are labelled ran- ! expected to be, on average, higher than that in DPS [50]. The domly as either J=w1 or J=w2. di-J=w high-pT region is thus exploited to select a data sample with 2. Detector and data set enhanced SPS production, which could include contributions from states. Two different approaches are applied. In the first approach di-J=w The LHCb detector is designed to study particles containing b or (denoted as pT -threshold), J/w-pair candidates are selected with di-J=w c quarks at the LHC. It is a single-arm forward spectrometer cover- the requirement pT > 5:2 GeV=c, which maximises the statisti- ing the pseudorapidity range 2 < g < 5, described in detail in Refs. cal significance of the SPS signal yield, NSPS=pNtotal. NSPS and Ntotal ffiffiffiffiffiffiffiffiffiffiffi [71,72]. The online event selection is performed by a trigger, which are yields of the SPS component and total di-J/w candidates in consists of a hardware stage, based on information from the 2 the Mdi-J=w range between 6:2 and 7:4 GeV=c , respectively. This calorimeter and muon systems, followed by a software stage, mass region covers the predicted masses of states decaying into which applies a full event reconstruction. At the hardware stage, a J/w pair. In the second approach (denoted as pdi-J=w-binned), di- events are required to have at least one muon with high momen- T di-J=w J/w candidates are categorised into six pT intervals with bound- tum transverse to the beamline, pT. At the software stage, two aries 0; 5; 6; 8; 9:5; 12; 50 GeV=c, defined to obtain equally popu- oppositely charged muon candidates are required to have high pT fg 2 and to form a common vertex. Events are retained if there is at lated bins of SPS signal events in the 6:2 < Mdi-J=w < 7:4 GeV=c least one J/w candidate passing both the hardware and software range.
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