Model-Independent Evidence for Exotic Hadron Contributions to#[subscript b][superscript 0]→J/#pπ−Decays The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Aaij, R., C. Abellán Beteta, B. Adeva, M. Adinolfi, Z. Ajaltouni, S. Akar, J. Albrecht, et al. "Model-Independent Evidence for Exotic Hadron Contributions toΛ[subscript b][superscript 0]→J/ψpπ−Decays." Physical Review Letters 117, 082002 (August 2016): 1-9 © 2016 CERN for the LHCb Collaboration As Published http://dx.doi.org/10.1103/PhysRevLett.117.082002 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/110294 Terms of Use Creative Commons Attribution Detailed Terms http://creativecommons.org/licenses/by/3.0 week ending PRL 117, 082002 (2016) PHYSICAL REVIEW LETTERS 19 AUGUST 2016 0 − Model-Independent Evidence for J=ψp Contributions to Λb → J=ψpK Decays R. Aaij et al.* (LHCb Collaboration) (Received 19 April 2016; published 18 August 2016) 0 − The data sample of Λb → J=ψpK decays acquired with the LHCb detector from 7 and 8 TeV pp collisions, corresponding to an integrated luminosity of 3 fb−1, is inspected for the presence of J=ψp or J=ψK− contributions with minimal assumptions about K−p contributions. It is demonstrated at more than 0 − − nine standard deviations that Λb → J=ψpK decays cannot be described with K p contributions alone, and that J=ψp contributions play a dominant role in this incompatibility. These model-independent results þ support the previously obtained model-dependent evidence for Pc → J=ψp charmonium-pentaquark states in the same data sample. DOI: 10.1103/PhysRevLett.117.082002 From the birth of the quark model, it has been anticipated excitations than is established experimentally [12]. The high that baryons could be constructed not only from three quarks, density of predicted states, presumably with large widths, but also from four quarks and an antiquark [1,2], hereafter would make it difficult to identify them experimentally. referred to as pentaquarks. The distribution of J=ψp mass Nonresonant contributions with nontrivial K−p mass 0 − þ − (mJ=ψp)inΛb →J=ψpK , J=ψ →μ μ decays observed dependence may also be present. Therefore, it is worth 0 − with the LHCb detector at the LHC shows a narrow peak inspecting the Λb → J=ψpK data with an approach that is suggestive of uudcc¯ pentaquark formation, amidst the model independent with respect to K−p contributions. Such dominant formation of various excitations of the Λ ½uds a method was introduced by the BABAR Collaboration [13] baryon (ΛÃ) decaying to K−p [3]. (The inclusion of charge and later improved upon by the LHCb Collaboration [14]. conjugate states is implied in this Letter.) Amplitude analyses There it was used to examine B¯ 0 → ψð2SÞπþK− decays, were performed on all relevant masses and decay angles of which are dominated by kaon excitations decaying to K−πþ, the six-dimensional (6D) data, using the helicity formalism in order to understand whether the data require the presence and Breit-Wigner amplitudes to describe all resonances. In of the tetraquark candidate decay, Zð4430Þþ → ψð2SÞπþ.In ΛÃ 0 − addition to the previously well established resonances, this Letter, this method is applied to the same Λb → J=ψpK þ two pentaquark resonances Pcð4380Þ (9σ significance) and sample previously analyzed in the amplitude analysis [3]. þ Pcð4450Þ (12σ) were required in the model for a good The sensitivity of the model-independent approach to exotic description of the data. The mass, width, and fit fractions resonances is investigated with simulation studies. were determined to be 4380Æ8Æ29MeV, 205Æ18Æ The LHCb detector is a single-arm forward spectrometer 86 MeV, 8.4%Æ0.7%Æ4.3%,and4450Æ2Æ3 MeV, covering the pseudorapidity range 2 < η < 5, described in 39Æ5Æ19MeV, 4.1%Æ0.5%Æ1.1%, respectively. The detail in Ref. [15]. The data selection is described in 0 − Cabibbo suppressed Λb → J=ψpπ decays are consistent Ref. [3]. A mass window of Æ2σ (σ ¼ 7.5 MeV) around with the presence of these resonances [4]. the Λ0 mass peak is selected, leaving nsig ¼ 27469 Λ0 ΛÃ b cand b The addition of further states beyond the well- candidates for further analysis, with background fraction established ones, and of nonresonant contributions, did (β) equal to 5.4%. The background is subtracted using not remove the need for two pentaquark states in the model nside ¼ 10 259 candidates from the Λ0 sidebands, which to describe the data. Yet ΛÃ spectroscopy is a complex cand b extend from Æ38 to Æ140 MeV from the peak (see the problem, as pointed out in a recent reanalysis of KN¯ Λð1800Þ Supplemental Material [16]). scattering data [5], in which the well-established The aim of this analysis is to assess the level of state was not seen, and evidence for a few previously 0 Ã consistency of the data with the hypothesis that all Λb → unidentified states was obtained. Theoretical models of Λ − 0 Ã Ã − J=ψpK decays proceed via Λ → J=ψΛ , Λ → pK , baryons [6–11] predict a much larger number of higher mass b with minimal assumptions about the spin and line shape of possible ΛÃ contributions. This will be referred to as the * Ã Full author list given at the end of the article. null hypothesis H0. Here, Λ denotes not only excitations of the Λ baryon, but also nonresonant K−p contributions or Published by the American Physical Society under the terms of Σ the Creative Commons Attribution 3.0 License. Further distri- excitations of the baryon. The latter contributions are bution of this work must maintain attribution to the author(s) and expected to be small [17]. The analysis method is two the published article’s title, journal citation, and DOI. dimensional and uses the information contained in the 0031-9007=16=117(8)=082002(9) 082002-1 © 2016 CERN, for the LHCb Collaboration week ending PRL 117, 082002 (2016) PHYSICAL REVIEW LETTERS 19 AUGUST 2016 ðm2 ;m2 Þ Dalitz variables, Kp J=ψp , or equivalently, in 2600 ðmKp; cos θΛÃ Þ, where θΛÃ is the helicity angle of the − K p system, defined as the angle between the p~K and 2400 − −p~Λ0 (or −p~J=ψ ) directions in the K p rest frame. b 2200 The ðmKp; cos θΛÃ Þ plane is particularly suited for H implementing constraints stemming from the 0 hypoth- 2000 esis by expanding the cos θΛÃ angular distribution in Legendre polynomials Pl, [MeV] 1800 Kp l m Xmax 1600 U dN=d cos θΛÃ ¼ hPl iPlðcos θΛÃ Þ; l¼0 1400 1+ 1- 3+ 3- 5+ 5- 7+ 7- 9+ 9- 11+ where N is the efficiency-corrected and background- 1200 2 2 2 2 2 2 2 2 2 2 2 U subtracted signal yield, and hPl i is an unnormalized 1000 Legendre moment of rank l, 0246810 Z l ( m ) þ1 max Kp U hPl i¼ d cos θΛÃ Plðcos θΛÃ ÞdN=d cos θΛÃ : −1 FIG. 1. Excitations of the Λ baryon. States predicted in Ref. [8] H K−p are shown as short horizontal bars (black) and experimentally Under the 0 hypothesis, components cannot con- ΛÃ 2J J well-established states are shown as green boxes covering the tribute to moments of rank higher than max, where max is M − Γ M þ Γ m − mass ranges from 0 0 to 0 0. The Kp mass range the highest spin of any K p contribution at the given mKp 0 − probed in Λb → J=ψpK decays is shown by long horizontal value. This requirement sets the appropriate l value, l ðm Þ max lines (blue). The max Kp filter is shown as a stepped line (red). which can be deduced from the lightest experimentally ΛÃ JP Ã All contributions from states with values to the left of the known Λ resonances for each J, or from the quark model, red line are accepted by the filter. The filter works well also for l ðm Þ as in Fig. 1.An max Kp function is formed, guided by the excitations of the Σ baryon [8,12] (not shown). the values of resonance masses (M0) lowered by two units Γ l ¼ 3 m of their widths ( 0): max for Kp up to 1.64 GeV, 5 up and background-subtracted histogram of mKp is used. to 1.70 GeV, 7 up to 2.05 GeV, and 9 for higher masses as To obtain a continuous probability density function, visualized in Fig. 1. 0 þ − þ FðmKpjH0Þ, a quadratic interpolation of the histogram Reflections from other channels, Λb → Pc K , Pc → J=ψp Λ0 → Z− p Z− → J=ψK− is performed, as shown in Fig. 2. The essential part of or b cs , cs , would introduce both l≤l ðm Þ low and high rank moments (see the Supplemental Material this analysis method is to incorporate the max Kp ΛÃ [16] for an illustration). The narrower the resonance, constraint on the helicity angle distribution: Fðm ; θ Ã jH Þ¼Fðm jH ÞFð θ Ã jH ;m Þ the narrower the reflection, and the higher the rank l of Kp cos Λ 0 Kp 0 cos Λ 0 Kp , Fð θ Ã jH ;m Þ Legendre polynomials required to describe such a structure. where cos Λ 0 Kp is obtained via linear inter- Selection criteria and backgrounds can also produce polation between neighboring mKp bins of high-l structures in the cos θΛÃ distribution. Therefore, the l ðm kÞ data are efficiency corrected and the background is sub- maxXKp H k N k tracted.