Evidence for a BoundHDibaryon from Lattice QCD

Abstract

We present evidence for the existence of a bound H-dibaryon, an I = 0, J = 0, s = −2 state with valence quark structure uuddss, at a pion mass of mπ ∼ 389 MeV. Using the results of Lattice QCD calculations performed on four ensembles of anisotropic clover gauge-field configurations, with spatial extents of L ∼ 2.0, 2.5, 3.0 and 3.9 fm at a spatial lattice spacing of bs ∼ 0.123 fm, we find an H-dibaryon bound by B H ∞ = 16.6 ± 2.1 ± 4.6 MeV at a pion mass of mπ ∼ 389 MeV.It is now well established that quantum chromodynamics (QCD), the theory describing the dynamics of quarks and gluons, and the electroweak interactions, underlie all of nuclear physics, from the hadronic mass spectrum to the synthesis of heavy elements in stars. To date, there have been few quantitative connections between nuclear physics and QCD, but fortunately, Lattice QCD is entering an era in which precise predictions for hadronic quantities with quantifiable errors are being made. This development is particularly important for processes which are difficult to explore in the laboratory, such as hyperon-hyperon and hyperon-nucleon interactions for which knowledge is scarce, primarily due to the short lifetimes of the hyperons, but which may impact the late-stages of supernovae evolution. In this letter we report strong evidence for a bound H-dibaryon, a six-quark hadron with valence structure uuddss, from n f = 2 + 1 Lattice QCD calculations at light-quark masses that give the pion a mass of m π ∼ 389 MeV.The prediction of a relatively deeply bound system with the quantum numbers of ΛΛ (called the H-dibaryon) by Jaffe [1] in the late 1970s, based upon a bag-model calculation, started a vigorous search for such a system, both experimentally and also with alternate theoretical tools. Experimental constraints on, and phenomenological models of, the H-dibaryon can be found in Refs. [2,3,4]. While experimental studies of doublystrange hypernuclei restrict the H-dibaryon to be unbound or to have a small binding energy, the most recent constraints on the existence of the H-dibaryon come from heavy-ion collisions at RHIC, from which it is concluded that the H-dibaryon does not exist in the mass region 2.136 < M H < 2.231 GeV [5], effectively eliminating the possibility of a loosely-bound H-dibaryon at the physical light-quark masses. Recent experiments at KEK suggest there is a resonance near threshold in the H-dibaryon channel [6].The first study of baryon-baryon interactions with Lattice QCD was performed more than a decade ago [7,8]. This calculation was quenched and with m π > ∼ 550 MeV. The NPLQCD collaboration performed the first n f = 2+ 1 QCD calculations of baryon-baryon interactions [9,10] at low-energies but at unphysical pion masses. Quenched and dynamical calculations were subsequently performed by the HALQCD collaboration [11,12]. A number of quenched Lattice QCD calculations [13,14,15,16,17,18] have searched for the H-dibaryon, but to date no definitive results have been reported. Earlier work concluded that the H-dibaryon does not exi...

Statistics of baryon correlation functions in lattice QCD

“…In particular StN growth in the GW has been shown to be consistent with an energy scale close to zero, as is expected from a variance correlation function dominated by baryon, as opposed to meson, states. Despite the ongoing successes of GW analyses of few-baryon correlation functions, the GW shrinks with increasing baryon number [7,29,30] and calculations of larger nuclei may require different analysis strategies suitable for correlation function without a GW.…”

“…Optimization of the interpolating operators for high signal overlap contributes to this. Another contribution arises from momentum projection, which suppresses the variance overlap factor by ∼ 1/(m 3 π V ) [29]. A large hierarchy between the signal and noise overlap factors provides a GW visible at intermediate times 10 t 25.…”

Calm Multi-Baryon Operators

“…[24] and subsequently applied to two and three hadron systems in Refs. [6,[25][26][27]. The great utility of the method stems from its simplicity to formulate and implement, as well as an ansatz which is imposed that is easy to verify or disprove.…”

“…We note that the NPLQCD collaboration has previously investigated the application of MP to twonucleon correlation functions [6,27] by constructing sinks with all possible different combinations of single-nucleon operator smearings [31], gaining benefits from the reduction of single-nucleon excited states, but possibly leading to the difficulties which can occur when trying to tune more than two operators. Only by imposing the selection of the single-nucleon MP combination that eliminates the first excited state explicitly before constructing two-nucleon operators do we gain the full advantage of our method.…”

WWscattering parameters via pseudoscalar phase shifts

“…For QCD (N f = 2), there are two such parameters (l 1 and l 2 ). Lattice methods have so far constrained only linear combinations of these terms via pion form factors [19] or the extraction of effective range parameters in I=2 ππ scattering [20].…”