Experimental Review of New Results on Hadron Spectroscopy
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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), D S(2632) and resonant structures near pp, p, p c , K and J / 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 The first evidence of pentaquark called as In the naive quark model, hadrons consist of 2 (1540) was presented by LEPS experiment in the nK final states in or 3 quarks. QCD allows the existence of new forms of hadrons, including multiquark states, n K K n process (Fig.1) [1]. Its mass hybrids and glueballs. These new forms of and width are: M 1540 10 5MeV; hadrons have been searched for experimentally 20 MeV at 90% CL. The statistical for a very long time, but none has been significance of the signal is about 4.6 . The 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 pp, p, p c , K and J / mass thresholds. I will also review the new results on scalar mesons, including , , f0(980), Figure 1. Mass spectrum of nK (from missing mass of f0(1370), f0(1500) and possible f0(1790). This K ) in n K K n at LEPS. talk will also cover some other interesting results from BES and CLEO-c experiments. minimum quark content of (1540) is 2. Multiquark Candidates uudds . Since its mass and width are consistent with the chiral soliton model prediction [2], it 2.1. Pentaquarks has triggered a lot of following experimental † Work partially supported by the Chinese Academy of Sciences under the contract No.KJCX2- SW-N10.
1 2
studies. The (1540) was observed at 12 MeV in the D* p mode in the deep SAPHIR, DIANA, CLAS, HERMES, ZEUS, inelastic process (Q 2 1GeV 2 ) (Fig.4) [8]. SVD-2, COSY-TOF and by Asratyan et al. Its minimum quark content is uuddc . 0 either in the nK mode or pK S mode with There are a lot of experiments obtaining claimed statistical significances about 3~7.8 negative results in the pentaquark searches [9]: [3]. Close and Zhao noticed that the masses BES experiment did not observe (1540) in 0 measured in the pK S mode are somewhat J / and ' decays; ALEPH experiment did systematically lower than those measured in the not observe (1540) , (1862) or nK mode (Fig.2) [4]. The width of c (3099) in Z decays; L3 experiment saw no
Figure 4. Mass spectrum of D * p at H1. Figure 2. (1540) mass measurements: black dots 0 represent the nK mode and open dots for pK S mode.
Figure 3. Mass spectrum of at NA49. (1540) is estimated to be 0.9 0.3 MeV by Cahn and Trilling [5] based on its production cross section from DIANA data, which is consistent with other estimation of the upper evidence of (1540) in two photon limit on the width (<1 MeV) based on the collisions; No evidence of (1540) , K N partial wave reanalysis [6]. Why the (1862) or c (3099) was observed at width is so narrow seems very hard to CDF experiments; HERA-B experiment did not understand. see any evidence of (1540) or The second pentaqurk called (1862) (1862) . c (3099) was not observed in was observed by NA49 experiment with a mass the ZEUS data which is 1.7 times of the H1 data 1862 2 MeV and a width smaller than 18 sample. Babar experiment searched for MeV in the mode in the pp collision (1540) , (1862) and other possible process (Fig.3) [7]. Its minimum quark content pentaquarks but no evidence was observed. is ssddu . Belle experiment did not see (1540) or The third pentaquark called c (3099) c (3099) . For these negative search results, was observed at H1 experiment with a mass the upper limits on the pentaquark production 3099 3 5 MeV and a width smaller than rates were reported. 3
There are some inconsistencies in the pentaquark searches: (1) The width of (1540) : HERMES experiment reported a width 17 9 2 MeV and ZEUS reported a width 8 4 Figure 5. Mass spectrum of M (J / ) M (J / ) at MeV which seem higher than the Belle. estimation from the production cross section ( 0.9 0.3 MeV) and the upper limit (<1MeV) obtained from the K N partial wave reanalysis. X(3872) at Babar is low (about 3.5 ) [12]. (2) As mentioned above, the masses measured Babar experiment also searched for X (3872) 0 0 0 in the pK mode are somewhat in B X K and B X K S with S 0 systematically lower than those measured X J / process, but no evidence in the nK mode. was observed [12]. So the isovector hypothesis (3) The most serious inconsistency is the of X(3872) is disfavored. production rates: the relative production CLEO experiment searched for the rates of (1540) to (1520) in the X(3872) in the γγ fusion and ISR process, but “positive” experiments such as SAPHIR no evidence was observed [13]. The following and HERMES are one or two orders higher 90% C.L. upper limits were obtained: than those upper limits obtained by the (2J 1) (X (3872))B(X (3872) J / ) 12.9eV “negative” experiments such as Babar and
Belle. ee (X (3872))B(X (3872) J / ) 8.0eV It is noticed that the “negative” PC experiments have much larger statistics, also are So the J 1 is disfavored. at relative higher energies (but Babar and Belle In this conference, Belle reported a new * at low energies), so the conclusion would be decay mode of X (3872) J / with that either the pentaquarks do not exist or they BR(X J / ) / BR(X J / ) = have very exotic production mechanisms. Let’s 0.8 0.3 0.1 [10]. look forward to more experimental results at So far, the X(3872) was only observed in low energies with high statistics, especially J / and J / mode. The non-observation those photo-production experiments.
2.2. X(3872) The X(3872) was first observed by Belle experiment in the J / mode in B decays (Fig.5) [10]. The mass is 3872.0 0.6 0.5 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 M ( pp) 2m are consistent with the Belle measurement. The Figure 7 Mass spectrum of p at BES production rate measured by Babar experiment II. is consistent with the Belle measurement although the statistical significance of the of DD 4
p 35 mode suggests that J 0 ,1 ,2 ,…, are fit, the mass is 18591025 MeV, and the width ruled out. is smaller than 30 MeV at 90% C.L. More decay modes are desirable to identify Now let us check whether there is any X(3872) as a conventional charmonium or a strong dynamical threshold enhancement in the DD * molecular state (see next talk [14]). pp collision data (such as LEAR data) [17, 18]. In order to compare the dynamical effects
2.3. DS (2632) of different process, it is important to remove the kinematical contributions, especially near Recently, SELEX experiment reported a new kinematical thresholds. With kinematical narrow resonance DS (2632) in the DS contribution removed, there are very smooth mode with a statistical significance 7.2 enhancement in the elastic “matrix element” 0 (Fig.6) and in D K mode with 5.3 [15]. and very small enhancement in the annihilation However, the statistical significance estimated “matrix element”, which are much weaker than seems too optimistic. what BES observed (Fig.8). The original measured pp 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 pp collision process. Is there any inconsistency between the BES observation and the pp cross section measurements? The answer is no. With M = 1859 MeV, Γ= 30 MeV and BR( pp ) = 10%, a very naive estimation of resonant cross section [17] near threshold at Ecm 2m p 6MeV
(i.e., PLab=150 MeV) is about 0.6 mb, which is much smaller than the continuum cross section Figure 6. Mass spectrum of D at SELEX. S 94±20mb. So it is very difficult to observe a resonance as BES observed in the pp cross section measurement. CLEO, Babar and Belle experiments 0 searched for DS (2632) in both D K mode and DS mode with much larger data sample but they did not observed any evidence of it. So the existence of DS (2632) needs confirmation.
2.4. pp mass threshold enhancement The BES Collaboration observed an anomalous near the threshold of pp mass spectrum in the J / pp 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 5
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 M ( pp) 2m data.Figure But 8 Dynamical it is not “matrix the same element (see” square Fig.8), as a function so the of p . The left plot is for the elastic process and the right plot for the annihilation process. FSI can hardly explain the BES pp mass threshold structure.
Figure 9 Comparison between the pp mass threshold structure observed at BES and theoretical calculations. The left plot is for the one-pion-exchangeFigure 11FSI Mass and right spectrum plot for of thep Coulomb (left) and interaction. Dalitz plot (right) in J / pK process at BES II
One possible interpretation of the BES pp threshold structure is a deuteron like pp bound state [20]. Since its mass is below the pp mass threshold, observations of this structure in other decay modes are desirable. Belle experiment also observed some pp mass “threshold” enhancement in B decays (Fig.10) [21], however, compared with the BES Figure 10. Differential Branching Fraction as a structure, the enhancement observed at Belle is function of M ( pp) in B ppK . much more wider and it is not really at threshold, so it is not really the same as the BES structure. Actually the Belle pp structure can 6
be explained by the fragmentation mechanism [22].
2.5. Mass threshold enhancements of p and K In the Dalitz plot of J / pK process at Figure 12. Mass plots from BES II. Upper plot: BESII, there is an obvious clustering of events mass spectrum of K . Lower Plot: mass Figure 13. Mass spectrum of p c at Belle. at the upper-right corner of the right plot of spectrum of M (K ) M K M with efficiency Fig.11, which corresponds to a p mass and phase space factor corrected. threshold enhancement. It can be fit with an S- wave Breit-Wigner function and the mass is 2075 12 5 MeV and the width is 90 35 9 MeV [23]. In the same Dalitz plot, there is a clear band near K mass threshold, which corresponds to a K mass threshold enhancement (upper plot of Fig.12). After efficiency and phase space correction, we do see a clear and strong threshold enhancement in Figure 16. Mass spectrum of K in the lower plot of Fig.12. J / K *K KK at BESII. Preliminary partial wave analysis (PWA) [24] with various possible combinations of excited baryons N* and Λ* in the fits gives that An J / * the K mass threshold structure N X has a mass threshold enhancement was also observed mass around 1500~1650 MeV , a width around at Belle. If it is fit with a Breit-Wigner funtion, 70~110 MeV and its spin-parity favors 1/ 2 . its mass is 3941 11 MeV and its width is Its most important property is that it has large 92 4 MeV (Fig.14) [10]. * * BR(J / pN X )BR(N X K ) ~ 4 * 2 10 , indicating BR(N X K ) > * 20%. Since N X is very close or even below the K mass threshold, its decaying phase space to K is very small, so the large * * BR(N X K ) shows that N X has strong coupling to K , suggesting it could be a K molecular state (5-quark system).
2.6. Mass threshold enhancement of p J / c and Figure 14. Mass spectrum of J / at Belle. The Belle experiment observed a threshold enhancement in the p c mass spectrum (Fig.13) [21]. It can be fit with a Breit-Wigner 0.02 function with a mass 3.340.03 0.02 GeV and a 0.09 0.04 0.04 width GeV. BES II
Figure 15. Mass spectrum of in J / at BESII. 7
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 J / at BES II (Fig. 15) [25,26]. The pole position from PWA is (541 39) i(252 42) MeV. The κ resonance was observed in J / K *K KK process at BESII (Fig.16) [26]. The pole position from preliminary PWA is (760 ~ 840 ) – i ( 310 ~ 420 ) MeV. 8
The f0(980) was observed in both and K K modes in the J / and J / KK 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) M 965 8 6 MeV ;(2) g 165 10 15 MeV; (3) gKK / g 4.21 0.25 0.21. Its large FigureFigure 20. 17. Mass Mass spectrum spectrum of of K K in coupling to KK indicates big ss components in J /J /KKat atBESII. BESII. f0(980).
Figure 18. Mass spectrum of K K in J / K K at BESII.
Figure 19. Mass spectrum of in
f0 (980) at KLOE. Background from ISR, FSR and .
The f0(980) was also observed in
f0 (980) process at KLOE
(Fig. 19) [28]. The result supports that f0(980) has large coupling to ss . There has been some debates on the
existence of f0(1370). In the BES II data,
f0(1370) was clearly observed in the J / process (Fig.17) [26, 27], however, it was not seen in J / (Fig.15). The mass obtained from the PWA of 9
Figure 21. Mass spectrum of in J / at BESII. J / is 1350 50 MeV, and the width is 265 40 MeV.
BES II observed a clear signal of f0(1710) in J / K K process (Fig.20) [26, 29],but produced recoiling against , which favors no signal in J / process, so the large ss component; f0(1710), on the other upper limit on the relative production rate is: hand, it dominantly decays to KK, but it is BR( f0 (1710) ) / BR( f 0 (1710) KK) < produced recoiling against , i.e., these three 0.13 at 95% C.L. scalar mesons show quite different behavior in ZEUS experiment observed a narrow peak the decay and production properties – I would 0 0 around 1730 MeV in the K S K S final state with call it as a “scalar puzzle”. 20 a mass 1726 7 MeV and a width 3814 MeV [30]. Its width is much smaller than other 4. Other interesting results from BES observations of f0(1710). and CLEO-c A clear peak around 1790 MeV was A possible new excited baryon N*(2050) was observed in the mass spectrum in observed in the p mass spectrum in J / process. The PWA shows that J / pn process at BES II (Fig.22) this structure favors J PC 0 with a mass 15 40 60 [32]. The mass is 2065 330 MeV, and the 179030 MeV and a width 27030 MeV [26, width is 175 12 40 MeV. 27]. However, around 1.75GeV in the K K In the study of charmonium decays, there mass spectrum of J / KK process, is a so-called “12% rule”: the ratio Qh of the there is no evident peak, so one can obtain hadronic production rates between ' decays BR( f (1790) ) / BR( f (1710) KK) 0 0 ~ and J / decays is expected to be the same as 1.5 which is much larger than the upper limit in the ratio of the e e production rates in these J / ,KK . So the peak around two charmonium decays, i.e., about 12%. It was 1790 MeV could be a possible new scalar found that in many decays, Qh is roughly equal f0(1790). to 12%, but in some decays, especially in VP In the preliminary PWA of J / and VT decay modes, Qh could be much 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 J / KK at BES II
[31], there is only one scalar f0(1710) observed below 2 GeV. At BES II, in the partial wave analysis of J / ,KK, ,KK , the f0(1500) was included in the fits, however, no mass peak can be direct seen in the mass Figure 22. Mass spectrum of p in spectrum (Fig.15, 17, 18, 20). J / pn at BESII.
The properties of f0(1370), f0(1710) and possible f0(1790) are quite unusual: f0(1370) and f0(1790) dominantly decay to , which favors large uu dd component, but they are smaller than 12%, which is called as “ puzzle”. Now since BES II and CLEO-c 10
collected large ' sample, so this rule has been Asratyan, A. Dogolenko and M. A. test at BES II and CLEO-c in many hadronic Kubantsev, Phys. Atom. Nucl. 67 (2004) decays [33] and some suppressed decay modes 682. such as and K*K were first observed at 4. F. Close and Q. Zhao, hep-ph/0404075. these two experiments recently [34]. 5. R. N. Cahn and G. H. Trilling, Phys. Rev. D69 (2004) 011501. 5. Summary 6. R. Arndt, I. Strakovsky and R. Workman, There were many surprising new observations Phys. Rev. C68 (2003) 042201. in the field of hadron spectroscopy during the 7. NA49 Coll., Phys. Rev. Lett. 92 (2004) past one year, especially on the multiquark 042003. candidates, although some of them still need 8. H1 Coll., Phys. Lett. B588 (2004) 17. confirmation. No matter what the interpretations 9. BES Coll., Phys. Rev D70 (2004) 012004; are for them, these discoveries will certainly ALEPH Coll., DELPHI Coll., L3 Coll., open a new window for understanding strong CDF Coll., HERA-B Coll., ZEUS Coll., interactions and hadron spectroscopy. Babar Coll., Belle Coll., conf. notes We need to have a global picture if there contributed to ICHEP 2004. are new forms of hadrons beyond naive quark 10. Belle Coll., Phys. Rev. Lett. 91 (2003) model. Any pentaquarks, tetraquarks, molecular 262001. Belle Coll., conf. notes contributed states or other multiquark states cannot stay to ICHEP 2004. alone. Let’s look forward to more and more 11. CDF Coll., Phys. Rev. Lett. 93 (2004) exciting and surprising experimental results on 072001; D0 Coll., hep-ex/0405004. hadron spectroscopy. 12. Babar Coll., hep-ex/0406022. 13. CLEO Coll., conf. note contributed to Acknowledgments ICHEP 2004, CONF 04-07. 14. F. Close, invited plenary talk at ICHEP I would like to thank a lot of friends from the 2004. experiments I mentioned above for their kind 15. SELEX Coll., hep-ex/0406045. help on preparing my talk, especially Dr. X.B. 16. BES Coll., Phys. Rev. Lett. 91 (2003) Ji. 022001. References 17. Particle Data Group, K. Hagiwara et al., Phys. Rev D66 (2002) 010001. 1. T. Nakano et al., Phys. Rev. Lett. 91 (2003) 18. E. Klempt et al., Phys. Rep. 368 (2002) 012002. 119. 2. D. Diakonov, V. Petrov and M. Polyakov, 19. B. S. Zou and H. C. Chiang, Phys. Rev. D Z. Phys. A359 (1997) 305. 69 (2003) 034004. 3. SAPHIR Coll., Phys. Lett. B572 (2003) 20. I. S. Shapiro, Phys. Rep. 35 (1978) 129; C. 127; DIANA Coll., Phys. Atom. Nucl. 66, B. Dover and M. Goldhaber, Phys. Rev. (2003) 1715; Yad. Fiz. 66 (2003) 1763; D15 (1977) 1997; A. Datta and P. J. CLAS Coll., Phys. Rev. Lett. 91 (2003) O’Donnell, Phys. Lett. B 567 (2003) 273; 252001; CLAS Coll., Phys. Rev. Lett. 92 M. L. Yan et al., hep-ph/0405087. (2004) 032001, Erratum-ibid 049902; 21. M.Z. Wang for Belle Coll., talk given at HERMES Coll., Phys. Lett. B585 (2004) ICHEP 2004. 213; ZEUS Coll., Phys. Lett. B591 (2004) 22. J. L. Rosner, Phys. Rev. D 68 (2003) 7; SVD Coll., hep-ex/0401024; COSY- 014004. TOF Coll., hep-ex/0403011; A. E. 23. BES Coll., Phys. Rev. Lett. 93 (2004) 11
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