Pentaquark baryons from lattice QCD
Shoichi Sasaki∗ Department of Physics, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japapn
This is an exploratory study of the pentaquark baryons in lattice QCD. We perform quenched lattice calculations at β = 6/g2 = 6.2 on a 323 × 48 lattice with the newly proposed operator for a pentaquark state Θ(uudds¯), which is based on an exotic description like diquark-diquark-antiquark. It is found that our simulations can accommodate a negative parity pentaquark rather than a positive parity pentaquark with a mass close to the experimental value of the Θ+(1540) state. We also explore the anti-charmed pentaquark Θc(uuddc¯) and find that the lowest Θc state is not to be expected as a bound state, in contrast to several model predictions.
I. INTRODUCTION channel indicates that possibility of I = 1 has been al- ready ruled out [3, 6] The quantum chromo-dynamics (QCD) may not pre- Many theoretical studies of pentaquarks are also trig- clude the presence of the multi-quark hadrons such gered by the discovery of the Θ+(1540). We introduce the as tetraquark (qqq¯q¯), pentaquark (qqqqq¯), dibaryon most reputed model proposed by Jaffe and Wilczek [8]. (qqqqqq) and so on, because of the color confinement. In the naive quark models, the low-lying pentaquark state Especially, we are let to be interested in exotic multi- should have spin-1/2 and negative parity. However, in quark hadrons, which should have exotic quantum num- this case, the pentaquark baryon just falls apart into bers. Now, we address ourself to the pentaquark state. KN in a S-wave. It is difficult to explain its very narrow Consider the SU(3) flavor case. Pentaquark states should width. Jaffe and Wilczek propose a simple idea to flip the form six different multiplets: parity of the low-lying pentaquark. Suppose there is the strong diquark correlation. The spin-0, color triplet and 3f ⊗3f ⊗3f ⊗3f ⊗3f =13 ⊕88 ⊕104 ⊕102 ⊕273 ⊕351 (1) flavor triplet diquark would be favored within the simple one gluon exchange. The pentaquark can be composed of where subscripts in the right hand side denote the num- two identical bosons (diquarks) and one antiquark. How- ber of degeneracy in each multiplet. The first three multi- ever, the anti-symmetrization in terms of color, requires plets are common in the case of usual baryons. However, relative odd number’s angular-momentum between the the last three multiplets, antidecuplet, 27-plet and 35- pairs of identical bosons. Otherwise, the wave function plet, are distinct irreducible representations since those of the pentaquark state should be vanished. Resulting multiplets have an apparent exotic quantum number as parity of the low-lying pentaquark is same as the chi- strangeness +1. Needless to say, S = +1 baryon can not ral soliton model [8]. In addition, S = +1 baryon are be accommodated by usual baryons. Other possible ex- uniquely assigned to antidecuplet in this description. otic numbers are represented as stars in Fig.1. Of course, i.e. one cannot predict which multiplet is preferred for the Consequently, correlated quark models, the di- possible S = +1 pentaquark baryon, within the group quark model, may accommodate the positive-parity isos- theoretical argument. The Skyrme model [1] and the inglet pentaquark Θ(uudds¯) as same as the chiral soliton chiral soliton model [2], however, predict that the lowest model. However, there are essential differences between S = +1 state appears uniquely in the antidecuplet and the diquark model and the chiral soliton model. In cor- its spin and parity is spin-half and positive parity. related quark models, we cannot fully discriminate the Recently, LEPS collaboration at Spring-8 has observed other multiplet such as octet. If apart from the SU(3) a very sharp peak resonance in the K− missing-mass flavor limit, the mixing between octet and antidecuplet spectrum of the γn → nK+K− reaction on 12C [3]. The should be taken into account. Jaffe and Wilczek advo- observed resonance should have strangeness +1. Thus, cate the ideal mixing case, which is favored in the vector Θ+(1540) cannot be a three quark state and should be meson spectrum [8]. The ideal mixing provides the dif- an exotic baryon state with the minimal quark content ferent prediction for the mass of possible exotic Ξ states. uudds¯. The peak position is located at 1540 MeV with a Furthermore, the Roper resonance can be accommodated very narrow width. Those are quite consistent with the in the diquark model owing to the ideal mixing [8]. chiral-soliton model’s prediction [2]. This discovery is A candidate of Ξ-type pentaquark has been reported subsequently confirmed by other experiments [4–7] [26]. by NA49 collaboration [9]. They observed the sharp Experimentally, spin, parity and isospin are not deter- peak resonance, which has exotic quantum numbers, in mined yet. Non-existence of a narrow resonance in pK+ Ξπ missing mass analysis. The observed mass is lo- cated somewhat between predictions by the chiral soli- ton model and the diquark model. In this experiment, the isospin partner is also observed. It should be pointed ∗Electronic address: [email protected] out that these discoveries are not confirmed yet by other 2
FIG. 1: Weight diagrams for possible pentaquark multiplets. Stars represent the states which have exotic quantum numbers (electric charge and strangeness). experiments [7]. A. Interpolating operators
For above mentioned purpose, we propose some lo- II. LATTICE SIMULATION cal interpolating operators of antidecuplet baryons based on an exotic description as diquark-diquark-antiquark. ¯ ¯ ¯ If the pentaquark baryons really exist, such states must There are basically two choices as 3c ⊗ 3c or 3c ⊗ 6c to emerge directly from first principles, QCD. Of course, construct a color triplet diquark-diquark cluster [15, 16]. what we should do is to confirm the presence of the pen- We adopt the former for a rather simple construction of taquarks from lattice QCD. Experimentally, it is rather diquark-diquark-antiquark. We first introduce the flavor ¯ ¯ difficult to determine the parity of the Θ+(1540). Thus, antitriplet (3f ) and color antitriplet (3c) diquark field lattice QCD has a chance to answer the undetermined i,a 1 T quantum numbers before experimental efforts. Lattice ΦΓ (x)= εijkεabcqj,b(x)CΓqk,c(x) (2) QCD has also a feasibility to predict the masses for undis- 2 covered pentaquark baryons. We stress that there is where C is the charge conjugation matrix, abc the color substantial progress in lattice study of excited baryons indices, and ijk the flavor indices. Γ is any of the six- recently [10]. Especially, the negative parity nucleon teen Dirac γ-matrices. Accounting for both color and ∗ + N (1535), which lies close to the Θ (1540), has become flavor antisymmetries, possible Γs are restricted within T an established state in quenched lattice QCD [10, 11]. 1, γ5 and γ5γµ which satisfy the relation (CΓ) = −CΓ. Here we report that quenched lattice QCD is capable of Otherwise, above defined diquark operator is identically + studying the Θ (1540) as well. zero. Hence, three types of flavor 3¯f and color 3¯c di- Indeed, it is not so easy to deal with the qqqqq¯ state quark; scalar (γ5), pseudoscalar (1) and vector (γ5γµ) di- rather than usual baryons (qqq) and mesons (qq¯) in lat- quarks are allowed [14, 17] The color singlet state can be tice QCD. The qqqqq¯ state can be decomposed into a pair constructed by the color antisymmetric parts of diquark- of color singlet states as qqq and qq¯, in other words, can diquark (3¯c ⊗ 3¯c)antisym =3c with an antiquark (3¯c). In decay into two-hadron states even in the quenched ap- terms of flavor, 3¯f ⊗ 3¯f ⊗ 3¯f =1f ⊕ 8f ⊕ 8f ⊕ 10f . Mani- proximation. For instance, one can start a study with a festly, in this description, the S=+1 state belongs to the simple minded local operator for the Θ+(1540), which is flavor antidecuplet [18]. Automatically, the S=+1 state constructed from the product of a neutron operator and should have isospin zero. Then, the interpolating opera- + T a K operator such as Θ = εabc(da Cγ5ub)dc(¯seγ5ue). tor of the Θ(uudds¯) is obtained as The two-point correlation function composed of this op- erator, in general, couples not only to the Θ state (single s,a s,b T Θ(x)= εabcΦΓ (x)ΦΓ0 (x)Cs¯c (x) (3) hadron) but also to the two-hadron states such as an in- teracting KN system [12, 13]. Even worse, when the for Γ 6=Γ0. The form Cs¯T for the strange antiquark field mass of the qqqqq¯ state is higher than the threshold of is responsible for giving the proper transformation prop- the hadronic two-body system, the two-point function erties of the resulting pentaquark operator under parity should be dominated by the two-hadron states. Thus, a and Lorentz transformations [19]. Remark that because specific operator with as little overlap with the hadronic of the color antisymmetry, the combination of the same two-body states as possible is desired in order to identify types of diquark is not allowed. Consequently, we have the signal of the pentaquark state in lattice QCD. three different types of exotic S=+1 baryon operators 3 through combination of two different types of diquarks, 3.0 which have different spin-parity [14, 17]: Θ (1/2−) c 1 T T T Θ+ = εabcεaef εbgh(ue Cdf )(ug Cγ5dh)Cs¯c , (4) 2.5 2 T T T Θ+,µ = εabcεaef εbgh(ue Cγ5df )(ug Cγ5γµdh)Cs¯c ,(5) 3 T T T Θ−,µ = εabcεaef εbgh(ue Cdf )(ug Cγ5γµdh)Cs¯c (6) 2.0
2 + where the subscript “ + (−)” refers to positive (negative) ) Θ(1/2 )
† 5Q parity since these operators transform as PΘ±(~x, t)P = 1.5 ±γ4Θ±(−~x, t) (for µ = 1, 2, 3) under parity. The first (aM operator of Eq. (4) is proposed for QCD sum rules in a recent paper [19] independently. 1.0 Θ(1/2−) In this description, the operator of exotic Ξ3/2 (ssddu¯ or uussd¯) states, which are members of the antidecu- 0.5 plet, can be treated by interchanging u and s or d and s in above operators. If a strange antiquark is simply 0.0 replaced by a charm antiquark, the proposed pentaquark 0.00 0.05 0.10 0.15 0.20 operators can be regarded as the anti-charmed analog of (aMπ)2 the isosinglet pentaquark state, Θc(uuddc¯). Recall that any of local type baryon operators can cou- ple to both positive- and negative-parity states since the FIG. 2: Masses of the spin-1/2 Θ(uudds¯) states with both parity assignment of an operator is switched by multiply- positive parity (open squares) and negative parity (open cir- ing the left hand side of the operator by γ5. The desired cles) as functions of pion mass squared in lattice units. The parity state is obtained by choosing the appropriate pro- charm analog Θc(uuddc¯) state (open diamonds) is also plot- ted. Horizontal short bar represents the KN(DN) threshold jection operator, 1 ± γ4, on the two-point function G(t) and direction of propagation in time. Details of the par- estimated by MN + MK (MN + MD) in the chiral limit. ity projection are described in Ref. [11]. We emphasize that the second and third operators, Eqs. (5) and (6), can couple to both spin-1/2 and spin-3/2 states. By using appropriate linear combination of propagators with pe- them, it is possible to study the spin-orbit partner of the riodic and anti-periodic boundary conditions in the time spin-1/2 Θ state, whose presence contradicts the Skyrme direction. This procedure yields a forward in time prop- picture of the Θ [16]. However, we will not pursue this agation in the time slice range 0
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