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Lattice QCD Study of the $ H $ Dibaryon Using Hexaquark and Two

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Lattice QCD Study of the $ H $ Dibaryon Using Hexaquark and Two CERN-TH-2018-098, DESY 18-066, HIM-2018-02, MITP/18-030, TIFR/TH/18-12 Lattice QCD study of the H dibaryon using hexaquark and two-baryon interpolators A. Francis,1 J. R. Green,2 P. M. Junnarkar,3 Ch. Miao,4, 5 T. D. Rae,4 and H. Wittig4, 5 1Theoretical Physics Department, CERN, CH-1211 Geneva 23, Switzerland 2NIC, Deutsches Elektronen-Synchrotron, D-15738 Zeuthen, Germany 3Tata Institute of Fundamental Research (TIFR), 1 Homi Bhabha Road, Mumbai 400005. India. 4PRISMA Cluster of Excellence and Institut f¨urKernphysik, University of Mainz, Becher Weg 45, D-55099 Mainz, Germany 5Helmholtz Institute Mainz, University of Mainz, D-55099 Mainz, Germany (Dated: April 16, 2019) We present a lattice QCD spectroscopy study in the isospin singlet, strangeness −2 sectors relevant for the conjectured H dibaryon. We employ both local and bilocal interpolating operators to isolate the ground state in the rest frame and in moving frames. Calculations are performed using two flavors of O(a)-improved Wilson fermions and a quenched strange quark. Our initial point-source method for constructing correlators does not allow for bilocal operators at the source; nevertheless, results from using these operators at the sink indicate that they provide an improved overlap onto the ground state in comparison with the local operators. We also present results, in the rest frame, using a second method based on distillation to compute a hermitian matrix of correlators with bilocal operators at both the source and the sink. This method yields a much more precise and reliable determination of the ground-state energy. In the flavor-SU(3) symmetric case, we apply L¨uscher's finite-volume quantization condition to the rest-frame and moving-frame energy levels to determine the S-wave scattering phase shift, near and below the two-particle threshold. For a pion mass of 960 MeV, we find that there exists a bound H dibaryon with binding energy ∆E = (19 ± 10) MeV. In the 27-plet (dineutron) sector, the finite-volume analysis suggests that the existence of a bound state is unlikely. I. INTRODUCTION formed more than thirty years ago [7], using a quenched ensemble with lattice size 62 12 18. Quenched studies × × The strong force between quarks and gluons produces a | which all used local interpolating operators with six rich spectrum of bound states and resonances, the color- quarks at the same point in keeping with the bag model neutral hadrons. Most of these can be described by con- picture, together with standard lattice spectroscopy tech- stituent quark models as either quark-antiquark mesons niques | produced inconclusive results: while some or three-quark baryons. The existence of exotic hadrons, found a bound state [8{10], others did not [7, 11{13]. which cannot be described as such, is an active field of in- Early studies of the H dibaryon using lattice QCD are summarized in Ref. [14]. quiry. Over the past several years, the so-called \X, Y, Z" 1 mesons have been studied intensively, both theoretically Aside from the present work , calculations with dy- and experimentally [1], and in recent years pentaquark namical fermions have been performed by two collab- baryons have also gained attention [2]. orations, both of which reported a bound H dibaryon at heavier-than-physical quark masses. The NPLQCD Nearly four decades ago, using the MIT bag model, collaboration performed lattice spectroscopy calculations Jaffe predicted a deeply bound dibaryon with quark con- using a setup based on clover fermions with local hexa- tent uuddss that is a scalar and a flavor singlet, the quark operators at the source and bilocal two-baryon H dibaryon [3]. In contrast with the only known stable operators at the sink. First results were obtained on dibaryon, the deuteron, which can be well described as a anisotropic ensembles with Nf = 2 + 1 dynamical loosely-bound proton-neutron state and is bound by just fermions [14, 18, 19], followed by isotropic ensembles 2.2 MeV, the bag model predicted the H dibaryon as an with three mass-degenerate (Nf = 3) quarks [20]. An exotic hexaquark state where all six quarks are in S-wave alternative approach, employed by the HAL QCD col- in the same hadronic bag, bound by about 80 MeV below laboration, is based on determining baryon-baryon po- the ΛΛ threshold. tentials from Nambu-Bethe-Salpeter wave functions com- arXiv:1805.03966v2 [hep-lat] 14 Apr 2019 Experimental evidence disfavors such a large binding puted on the lattice, followed by solving the Schr¨odinger energy. The strongest constraint is the \Nagara" event equation to study baryon-baryon scattering and bound provided by the E373 experiment at KEK [4], which states. This was done on ensembles with Nf = 3 6 found a ΛΛHe double-hypernucleus with ΛΛ binding en- clover fermions for a range of quark masses [21{23]. Al- ergy BΛΛ = 6:91 0:16 MeV [5] that decayed weakly. A though these two sets of calculations agreed on the pres- deeply bound H dibaryon± would enable the strong decay 6 4 ΛΛHe He + H; its absence implies mH > 2mΛ BΛΛ. There! was also no indication of an H dibaryon− from a high-statistics study of upsilon decays at Belle [6]. 1 Exploratory studies and preliminary results were previously re- The first lattice QCD study of the H dibaryon was per- ported in Refs. [15{17]. 2 ence of a bound state, they disagreed significantly on tering phase shifts at the SU(3)-symmetric point, and the binding energy: in the Nf = 3 case with pseu- identify the presence of a bound H dibaryon. Finally, doscalar meson mass near 800 MeV, the value reported our conclusions are presented in SectionV. by NPLQCD was 74:6 4:7 MeV, whereas HAL QCD reported 37:8 5:1 MeV.± Recently, HAL QCD have pub- ± lished a Nf = 2 + 1 study of coupled channel (ΛΛ II. LATTICE CALCULATION AND SETUP and NΞ) baryon-baryon interactions with near-physical quark masses, which claims that the H dibaryon may be A. Simulation details a ΛΛ resonance just below the NΞ threshold [24, 25]. Given that there are conflicting results for the binding Our study has been performed on a set of ensem- energy of the H dibaryon, we have started a new initia- bles with two mass-degenerate dynamical flavors of O(a)- tive which may help to resolve the issue. As a first step we improved Wilson quarks [33] and the Wilson plaquette present results from a study in two-flavor QCD, i.e. with action, which were generated as part of the CLS (Coordi- a mass-degenerate doublet of dynamical u and d quarks. nated Lattice Simulations) initiative, using the deflation- The mass of the (quenched) strange quark is either tuned accelerated DD-HMC [34, 35] and MP-HMC [36] algo- such that ms = md = mu or set to a heavier value, imply- rithms. The improvement coefficient csw multiplying the ing that the SU(3) flavor symmetry is broken. Clearly, Sheikholeslami-Wohlert term was tuned according to the SU(3) symmetry is significantly broken at the physical non-perturbative determination of Ref. [37]. An overview point [26, 27], which allows the three flavor multiplets, of the ensembles can be found in TableI. All our cal- i.e. the singlet, octet and 27-plet to couple. Therefore, culations were performed in the SU(3)-flavor symmetric it is advantageous to study the octet and 27-plet even limit, with the exception of ensemble E5 for which the in the case of exact SU(3) symmetry. Furthermore, the valence strange quark mass was tuned so that the com- 27-plet contains the two-nucleon I = 1 sector which has bination (2m2 m2 )=m2 takes its physical value. The K − π Ω a possible dineutron bound state. The nucleon-nucleon corresponding values of mπ and mK are provided in the sector has been studied extensively in experiment and table. The values of the lattice spacing in physical units may serve as a benchmark for lattice calculations. were determined using the kaon decay constant [38]. Our work is mainly focused on the methodology of de- Quark propagators were computed using the Schwarz termining the spectrum and the binding energy via the alternating procedure (SAP) domain-decomposed, de- computation of correlation matrices and their diagonal- flated generalized conjugate residual (GCR) solver of the ization [28{30]. In order to allow for a direct comparison DD-HMC package [34] with smeared point sources on a with the results from older quenched studies and from grid of source positions that was randomly displaced on NPLQCD we have chosen a similar setup. As we will each gauge configuration. For the distillation calculation, describe in more detail in the following sections, we have we used a similar solver in OpenQCD [39], computed used point sources to compute correlator matrices with low modes of the spatial Laplacian using PRIMME [40], local interpolating operators at the source and both lo- and contracted them to form \perambulators" and mode cal and bilocal interpolators at the sink. In addition, triplets using QDP++ [41]. Baryon and multi-baryon we report initial results from a follow-up study in which correlators computed in lattice QCD suffer from a severe we, for the first time, applied the distillation method [31] signal-to-noise problem, since the noise grows with a rate to the two-baryon sector. This allowed us to compute proportional to exp (mB 3=2mπ)t per baryon, where f − g a correlator matrix using operators made from products mB denotes the baryon mass. This makes it difficult to of two spatially displaced, momentum-projected baryon identify a \window" in which the asymptotic behavior interpolators at both the source and the sink.
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