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Hadron Spectroscopy and Exotic States

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Hadron Spectroscopy and Exotic States EPJ Web of Conferences 60, 06003 (2013) DOI: 10.1051/epjconf/20136006003 © Owned by the authors, published by EDP Sciences, 2013 Hadron spectroscopy and exotic states A. Augusto Alves Jr.1;a on behalf of the LHCb collaboration and presenting results from CMS and ALICE collabora- tions. 1INFN and Università di Roma “Sapienza”, Rome, Italy Abstract. This contribution summarizes some of the main results from the LHC experiments on hadron spectroscopy and exotic states. 1 Introduction bers in the LHCb and CMS experiments. In Section 3, the results of the search for the X(4140) and X(4274) states in According to our current understanding, the forces re- B+ ! K+J= φ decays at LHCb and CMS are presented. sponsible to bind quarks into hadrons are described by In the Section 4, the results from ALICE on search for the non-Abelian field theory called Quantum Chromody- H-Dibaryon candidates is discussed. The conclusions are namics (QCD). In QCD-motivated quark potential mod- presented in Section 5. els, the quarkonia states are described as a quark-antiquark pair bound by an interquark force with a short- distance behavior that is approximately Coulombic, plus an in- 2 Studies on X(3872) creasing confining potential that dominates at large sepa- The X(3872) resonance was discovered in 2003 by the rations. In one of the simplest approaches, the energy lev- Belle collaboration in the B+ ! K+X(3872), X(3872) ! els can be determined by solving the corresponding non- J= π+π− decay chain [16]. Its existence was confirmed relativistic Schrodinger equation in order to obtain the ex- by the CDF[17], D; [18] and BaBar[19] collaborations. pected masses of the charmonium spectrum, characterized The X(3872) mass is currently known with < 1.0 MeV=c2 by the radial quantum number and the relative orbital an- precision and the dipion mass spectrum in the decay gular momentum between the quark and the antiquark. In X(3872) ! J= π+π− [20, 21] has been precisely studied. particular, all predicted states lying under the DD mass The X(3872) quantum numbers was recently measured by threshold have been observed[1–4]. LHCb as JPC = 1++. On the other hand, the possible existence of more However, despite the cumulated experimental and the- sophisticated states than mesons and baryons, like the oretical effort, the nature of the X(3872) remains uncer- multiquark states, hybrid mesons and mesonic molecules tain. Among the possible interpretations for this state has been discussed since the early days of the quark currently discussed in the literature, one can remark the model[2, 5–8]. mesonic molecule, the hybrid meson and the tetraquark In the last decade, considerable experimental evidence hypotesis. The conventional charmonium interpretation is has been collected about the existence of new states, ly- strongly unfavored. ing in the charmonium mass range, but not fitting well the charmonium mass spectrum picture[9–15]. Most of the observations also suggested that these candidates may be 2.1 Mass measurements at LHCb exotic. These studies have been performed at Babar and −1 Belle, two experiments which took data at the e+e− Beauty In LHCb the analysis is performed a 34:7 pb integratedp Factories at SLAC (Stanford Linear Accelerator Center, luminosity dataset collected in 2010 in pp collisions at s USA) and KEK (High-Energy Accelerator Research Or- = 7 TeV. The X(3872) signal has been isolated apply- ganization, Japan), respectively. Confirmations have also ing tight cuts in order to reduce the combinatorial back- come from the CDF experiment, collecting data from pp ground, generated when a correctly reconstructed J= me- + − interactions at Fermilab,USA. son is combined with a random π π pair from the pri- In this report the main results from the LHC experi- mary pp interaction. The selection cuts are optimized + − ments on exotic states candidates are summarized. Sec- using reconstructed (2S ) ! J= π π decays. The tion 2 is dedicated to the discussion of the measurements background shape is determined reconstructing “same- of the X(3872) mass and cross-section and quantum num- sign pion” candidates satisfying the same criteria as used for the X(3872) and (2S ) selection. A further back- ae-mail: [email protected] ground suppression is reached applying the requirement Q This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20136006003 EPJ Web of Conferences 2 1600 LHCb 0.12 R CMS s = 7 TeV 1400 s = 7 TeV 600 0.11 L = 4.8 fb-1 1200 |y| < 1.2 500 1000 0.1 Events / 2 MeV/c 3850 3900 800 0.09 600 0.08 400 200 0.07 0 3600 3700 3800 3900 0.06 M(J/ψ π+ π-) [MeV/c2] 0.05 Figure 1. The M(J= π+π−) mass spectrum. The shadowed area 0.04 10 20 30 40 50 represents the “same-sign pion” candidates satisfying the same ψ π+ π- p (J/ ) [GeV] criteria as used for the X(3872) and (2S ) selection.The (2S ) T and X(3872) signals are described with a non-relativistic Breit- Wigner function convolved with a Gaussian resolution function. Figure 2. The ratio of the cross section times branching fractions for the X(3872) and (2S ) in bins of pT bins as measured by the CMS collabo- ration. 2 < 300 MeV=c , where Q = Mµ+µ−π+π− − Mµ+µ− − Mπ+π− . See [22] for a detailed discussion on the selection proce- dure. The masses of the (2S ) and X(3872) mesons are branching fraction of the X(3872), σ(pp ! X(3872) + determined from an extended unbinned maximum likeli- ··· ) × B(X(3872) ! J= π+π−), is measured at LHCb to hood fit of the reconstructed J= π+π− mass in the interval be 5:4 ± 1:3(stat) ± 0:8(syst) nb: 2 3:60 < MJ= π+π− < 3:95 GeV=c . The (2S ) and X(3872) signals are described with a non-relativistic Breit-Wigner 2.3 Production studies at CMS function convolved with a Gaussian resolution function. The intrinsic width of the (2S ) is fixed to the PDG value The CMS collaboration studied the X(3872) ! J= π+π− and the X(3872) width is fixed to zero in the nominal decay channel using 4.8 fb−1 pp collision data collected fit. The ratio of the mass resolutions for the X(3872) and during 2011 [24]. The measurements are performed for the (2S ) is fixed to the value σX(3872)/σ (2S ) = 1:31. the X(3872) candidates with a rapidity η < 1:2 and with The background shape is described by the functional form 10-50 GeV transverse momentum range. The CMS col- 2 c0 (−c1M−c2M ) f(M) / (M − MJ= − 2Mπ) e . The fit result is laboration measured the ratio of the cross section times shown in Figure 1. The result is branching fractions for the X(3872) and (2S ). The ratio 2 without acceptance corrections for the exclusive pT bins M(X(3872)) = 3871:95 ± 0:48(stat) ± 0:12(syst) MeV=c is shown in Figure 2. The fraction of X(3872) originating from B hadron decay in bins of p is shown in Figure 3. The mass of X(3872) is a critical parameter for the T The cross section times the branching fractions for interpretation of this state as D0D¯0∗-bound state. In this the X(3872) ! J= π+π− prompt X(3872) production, context, it is important to draw the reader’s attention to the σprompt(pp ! X(3872) + ··· ) × B(X(3872) ! J= π+π−), recent precision measurement of the D0 mass, performed has been measured as 1:06 ± 0:11(stat) ± 0:15(syst) nb for by LHCb [23]. the X(3872) candidates with 10-30 GeV pT range. Finally, the comparison of the measured dipion mass 2.2 Production studies at LHCb spectrum of X(3872) ! J= π+π− with simulations where the samples are produced with and without intermediate At LHCb, the same sample used to measure the X(3872) ρ0 decay shows that the X(3872) ! J= π+π− proceed pre- mass has been used to perform X(3872) production stud- dominately through a ρ0 resonance. ies. See [22] for detailed discussion about the calibration procedure and the treatment of the different sources of sys- tematic uncertainty. The studies are performed just con- 2.4 Quantum number measurements at LHCb sidering candidates lying inside the fiducial region for the The LHCb collaboration also measured X(3872) quantum measurement defined by numbers. The measurement is performed using 313 ± 26 + ! + 2:5 < y < 4:5 and 5 < pT < 20 GeV=c candidates of the decay chain B K X(3872), with X(3872) ! J= π+π− and J= ! µ+µ− selected from −1 p where y and pT are the rapidity and transverse momentum 1:1 fb dataset collected in 2011 in pp collisions at s of the X(3872). The product of the inclusive production = 7 TeV. 06003-p.2 LHCP 2013 3 Search for structures in the J= φ spectrum The CDF collaboration has reported a 3:8σ evidence for the X(4140) ! J= φ state using data collectedp in proton-antiproton collisions at the Tevatron ( s = 1.96 TeV)[28]. In a preliminary update on the analysis [29], the CDF collaboration reported 115 ± 12 B+ ! K+J= φ events and 19 ± 6 X(4140) candidates with a statisti- cal significance of more than 5σ.
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