Experimental Review of New Results on Hadron Spectroscopy

Experimental review of new results on
hadron spectroscopy

Shan Jin[†]

Institute of High Energy Physics
Beijing 100049, P. R. China

In this talk, I will review the most important progress in the field of hadron spectroscopy in recent one year, especially on multi-quark candidates, including pentaquarks, X(3872), DS(2632) and resonant structures near , and mass thresholds. I will also review the new results on scalar mesons, including , f0(980), f0(1370), f0(1500) and possible f0(1790). This talk will also cover some other interesting results from BES and CLEO-c experiments.

1. Introduction

In the naive quark model, hadrons consist of 2 or 3 quarks. QCD allows the existence of new forms of hadrons, including multiquark states, hybrids and glueballs. These new forms of hadrons have been searched for experimentally for a very long time, but none has been established. However, during the past one year, a lot of surprising experimental evidences show the existence of hadrons that cannot be or cannot easily be explained by the conventional quark model.

In this talk, I will review the most important progress in the field of hadron spectroscopy in recent one year, especially on multiquark candidates, including pentaquarks, X(3872), DS(2632) and resonant structures near , and mass thresholds. I will also review the new results on scalar mesons, including , f0(980), f0(1370), f0(1500) and possible f0(1790). This talk will also cover some other interesting results from BES and CLEO-c experiments.

2. Multiquark Candidates

2.1. Pentaquarks

The first evidence of pentaquark called as was presented by LEPS experiment in the final states in process (Fig.1) [1]. Its mass and width are: MeV; MeV at 90% CL. The statistical significance of the signal is about 4.6 . The minimum quark content of is . Since its mass and width are consistent with the chiral soliton model prediction [2], it has triggered a lot of following experimental studies. The was observed at SAPHIR, DIANA, CLAS, HERMES, ZEUS, SVD-2, COSY-TOF and by Asratyan et al. either in the mode or mode with claimed statistical significances about 3~7.8 [3]. Close and Zhao noticed that the masses measured in the mode are somewhat systematically lower than those measured in the mode (Fig.2) [4]. The width of is estimated to be MeV by Cahn and Trilling [5] based on its production cross section from DIANA data, which is consistent with other estimation of the upper limit on the width (<1 MeV) based on the partial wave reanalysis [6]. Why the width is so narrow seems very hard to understand.

The second pentaqurk called was observed by NA49 experiment with a mass MeV and a width smaller than 18 MeV in the mode in the pp collision process (Fig.3) [7]. Its minimum quark content is .

The third pentaquark called was observed at H1 experiment with a mass MeV and a width smaller than 12 MeV in the mode in the deep inelastic process () (Fig.4) [8]. Its minimum quark content is .

There are a lot of experiments obtaining negative results in the pentaquark searches [9]: BES experiment did not observe in and decays; ALEPH experiment did not observe , or in Z decays; L3 experiment saw no evidence of in two photon collisions; No evidence of , or was observed at CDF experiments; HERA-B experiment did not see any evidence of or . was not observed in the ZEUS data which is 1.7 times of the H1 data sample. Babar experiment searched for , and other possible pentaquarks but no evidence was observed. Belle experiment did not see or . For these negative search results, the upper limits on the pentaquark production rates were reported.

There are some inconsistencies in the pentaquark searches:

(1) The width of : HERMES experiment reported a width MeV and ZEUS reported a width MeV which seem higher than the estimation from the production cross section ( MeV) and the upper limit (<1MeV) obtained from the partial wave reanalysis.

(2) As mentioned above, the masses measured in the mode are somewhat systematically lower than those measured in the mode.

(3) The most serious inconsistency is the production rates: the relative production rates of to in the “positive” experiments such as SAPHIR and HERMES are one or two orders higher than those upper limits obtained by the “negative” experiments such as Babar and Belle.

It is noticed that the “negative” experiments have much larger statistics, also are at relative higher energies (but Babar and Belle at low energies), so the conclusion would be that either the pentaquarks do not exist or they have very exotic production mechanisms. Let’s look forward to more experimental results at low energies with high statistics, especially those photo-production experiments.

2.2. X(3872)

The X(3872) was first observed by Belle experiment in the mode in B decays (Fig.5) [10]. The mass is MeV and the width is smaller than 2.3 MeV at 90 % C.L.

The X(3872) was quickly confirmed by the CDF and D0 experiments [11], and the mass and width measured at these two experiments are consistent with the Belle measurement. The production rate measured by Babar experiment is consistent with the Belle measurement although the statistical significance of the X(3872) at Babar is low (about 3.5 ) [12].

Babar experiment also searched for in and with process, but no evidence was observed [12]. So the isovector hypothesis of X(3872) is disfavored.

CLEO experiment searched for the X(3872) in the γγ fusion and ISR process, but no evidence was observed [13]. The following 90% C.L. upper limits were obtained:

So the is disfavored.

In this conference, Belle reported a new decay mode of with = [10].

So far, the X(3872) was only observed in and mode. The non-observation of mode suggests that ,…, are ruled out.

More decay modes are desirable to identify X(3872) as a conventional charmonium or a molecular state (see next talk [14]).

2.3.

Recently, SELEX experiment reported a new narrow resonance in the mode with a statistical significance 7.2 (Fig.6) and in mode with 5.3[15]. However, the statistical significance estimated seems too optimistic.

CLEO, Babar and Belle experiments searched for in both mode and mode with much larger data sample but they did not observed any evidence of it. So the existence of needs confirmation.

2.4. mass threshold enhancement

The BES Collaboration observed an anomalous near the threshold of mass spectrum in the process (Fig.7) [16]. It can be fit with either an S- or P- wave Breit-Wigner resonance function. In the case of the S-wave fit, the mass is MeV, and the width is smaller than 30 MeV at 90% C.L.

Now let us check whether there is any strong dynamical threshold enhancement in the collision data (such as LEAR data) [17, 18]. In order to compare the dynamical effects of different process, it is important to remove the kinematical contributions, especially near kinematical thresholds. With kinematical contribution removed, there are very smooth enhancement in the elastic “matrix element” and very small enhancement in the annihilation “matrix element”, which are much weaker than what BES observed (Fig.8). The original measured annihilation cross section does have strong threshold enhancement, but it is from kinematical effect, not dynamical effect. So there is no strong dynamical mass threshold enhancement in the collision process.

Is there any inconsistency between the BES observation and the cross section measurements? The answer is no. With M = 1859 MeV, Γ= 30 MeV and BR() = 10%, a very naive estimation of resonant cross section [17] near threshold at (i.e., PLab=150 MeV) is about 0.6 mb, which is much smaller than the continuum cross section 94±20mb. So it is very difficult to observe a resonance as BES observed in the cross section measurement.

The final state interaction (FSI) interpretation of the BES observation is disfavored. Zou and Chiang showed that the enhancement caused by one-pion-exchange FSI is too small to explain the BES structure (Fig.9) [19]. The threshold enhancement caused by the Coulomb interaction is even smaller than one-pion-exchange FSI.. Theoretical calculations might be unreliable, however, according to Watson’s theorem, we can use elastic scattering cross section data to check the FSI effect, i.e., if the BES structure were from FSI, it should be the same as the elastic scattering cross section data. But it is not the same (see Fig.8), so the FSI can hardly explain the BES mass threshold structure.

One possible interpretation of the BES threshold structure is a deuteron like bound state [20]. Since its mass is below the mass threshold, observations of this structure in other decay modes are desirable.

Belle experiment also observed some mass “threshold” enhancement in B decays (Fig.10) [21], however, compared with the BES structure, the enhancement observed at Belle is much more wider and it is not really at threshold, so it is not really the same as the BES structure. Actually the Belle structure can be explained by the fragmentation mechanism [22].

2.5. Mass threshold enhancements of and

In the Dalitz plot of process at BESII, there is an obvious clustering of events at the upper-right corner of the right plot of Fig.11, which corresponds to a mass threshold enhancement. It can be fit with an S-wave Breit-Wigner function and the mass is MeV and the width is MeV [23].

In the same Dalitz plot, there is a clear band near mass threshold, which corresponds to a mass threshold enhancement (upper plot of Fig.12). After efficiency and phase space correction, we do see a clear and strong threshold enhancement in the lower plot of Fig.12.

Preliminary partial wave analysis (PWA) [24] with various possible combinations of excited baryons N* and Λ* in the fits gives that the mass threshold structure has a mass around 1500~1650 MeV , a width around 70~110 MeV and its spin-parity favors . Its most important property is that it has large ~ , indicating > 20%. Since is very close or even below the mass threshold, its decaying phase space to is very small, so the large shows that has strong coupling to , suggesting it could be a molecular state (5-quark system).

2.6. Mass threshold enhancement of and

The Belle experiment observed a threshold enhancement in the mass spectrum (Fig.13) [21]. It can be fit with a Breit-Wigner function with a mass GeV and a width GeV.

An mass threshold enhancement was also observed at Belle. If it is fit with a Breit-Wigner funtion, its mass is MeV and its width is MeV (Fig.14) [10].

3. Light Scalar Mesons: σ, κ, f0(980), f0(1370), f0(1500), f0(1710) and possible f0(1790)

Light scalar mesons are of special interests because: (1) There have been hot debates on the existences of σ and κ; (2) σ, κ and f0(980) are also multiquark candidates and they are all near mass thresholds; (3) Lattice QCD predicts the scalar glueball mass around 1.6 GeV and f0(1500) and f0(1710) are good candidates.

The σ resonance was observed in at BES II (Fig. 15) [25,26]. The pole position from PWA is MeV.

The κ resonance was observed in process at BESII (Fig.16) [26]. The pole position from preliminary PWA is (760 ~ 840 ) – i ( 310 ~ 420 ) MeV.

The f0(980) was observed in both and modes in the and process at BES II (Fig. 17, 18) [26, 27]. From the combined PWA of these two channel, the following important parameters of f0(980) were obtained: (1) MeV ;(2) MeV; (3) . Its large coupling to KK indicates big components in f0(980).

The f0(980) was also observed in process at KLOE (Fig. 19) [28]. The result supports that f0(980) has large coupling to .

There has been some debates on the existence of f0(1370). In the BES II data, f0(1370) was clearly observed in the process (Fig.17) [26, 27], however, it was not seen in (Fig.15). The mass obtained from the PWA of is MeV, and the width is MeV.

BES II observed a clear signal of f0(1710) in process (Fig.20) [26, 29],but no signal in process, so the upper limit on the relative production rate is: < 0.13 at 95% C.L.

ZEUS experiment observed a narrow peak around 1730 MeV in the final state with a mass MeV and a width MeV [30]. Its width is much smaller than other observations of f0(1710).

A clear peak around 1790 MeV was observed in the mass spectrum in process. The PWA shows that this structure favors with a mass MeV and a width MeV [26, 27]. However, around 1.75GeV in the mass spectrum of process, there is no evident peak, so one can obtain ~ 1.5 which is much larger than the upper limit in . So the peak around 1790 MeV could be a possible new scalar f0(1790).

In the preliminary PWA of at BES II (Fig. 21) [26], there are two scalars observed in the mass below 2 GeV: one is around 1470 MeV, which might be f0(1500), and the other is around 1765 MeV, which might be from f0(1710) or a mixture of f0(1710) and possible f0(1790).

In the PWA of at BES II [31], there is only one scalar f0(1710) observed below 2 GeV.

At BES II, in the partial wave analysis of , the f0(1500) was included in the fits, however, no mass peak can be direct seen in the mass spectrum (Fig.15, 17, 18, 20).