Pentaquarks Facts and Mysteries

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1 Pentaquarks Facts and Mysteries Quark Model Quark model in SU(3) (u, d and s quark have similar mass): three valence quarks define flavour content of a baryon Octet and Decuplet S n p S I 3 Introduction Experimental facts: Θ (154) Λ I3 * * * Theoretical situation * * Experimental facts: (186) Quintessence Ω Success: prediction of Ω - (Gell-Mann, 1964) and subsequent observation (V.E. Barnes et al., 1964) Why only 2 and three valence quarks? Beyond Meson and Baryons GLUON NUMBER GLUEBALLS HYBRIDS M B QUARK NUMBER PENTAQUARKS DI-BARYONS Until a year ago many signals of narrow exotic resonances have appeared, but all disappeared after detailed studies again! for example the famous U-particle at 31MeV/c 2 (diquonium) 1. Experimental facts Part 1: Θ (154) "The important thing is not to stop questioning. Curiosity has its own reason for existing." A. Einstein

2 How to make a Pentaquark Θ (154) SPring-8 exclusive: γn K - Θ K - K (n) K - K (p) γp K Θ K K (n) K K (p) pp Θ pk ( ) COSY baryon can be detected (easy: p) or can be reconstructed from missing mass (diffcult: n) inclusive: γd Θ X K p (X) pp Θ X K p (X) K Xe Θ Xe K p (Xe ) Spring-8 JLAB ELSA HERMES CERN ITEP Jan. 3 T. Nakano et al., Phys. Rev. Lett. 91, 122 (23) Compton backscattering of photons, E γ = GeV plastic scintillator (C:H=1:1) p or n from missing mass of N(γ,K K - )X Cuts total K K - pair 8 SC target 4 E γ <2.35GeV 32 [.9<M(N)<.98] 18 M(KK) [1.,1.4] 27 γp K K - p 19 M(Θ ) = 154 MeV Γ 25 MeV 4.6σ significance Θ ITEP CLAS TNAF Apr. 3 V.V. Barmin et al., Phys. Atom. Nucl. 66, 1715 (23) bubble chamber, K Xe K pxe p K =47MeV/c Kn Θ Kp Θ Jul. 3 S. Stepanyan et al., Phy. Rev. Lett. 91, 2521 (23) γd K K - p(n); n from missing mass M(Θ) = 1539±2 MeV Γ 9 MeV 4.4σ significance M(Θ ) = 1542 MeV Γ 21 MeV 5.8σ significance

3 Is the Θ a reflection? ELSA Reflections von f 2, ρ 3 A.R. Dzierba et al.,hep-ph/ total energy w of the KKN System in CLAS experiment: w= gev w=2.6gev Jul. 3 J. Barth et al., Phys. Lett. B 572, 127 (23) γ p K Θ K K n s s angular distribution w-distribution CLAS data M(Θ) = 154±6 MeV Γ 25 MeV 5.2σ significance no signal in Θ K p suggests isoscalar state σ(θ ): σ(λ 152 )=6nb : (8-12)nb 1:15 νcc CERN Sep. 3 A.E. Asratyan et al., hep-ex/3942 reanalysis of neutrino data collected at CERN in bubble chambers (WA21, WA25, WA59, E18, E632) targets: p, d, Ne dominated by Ne CLAS TNAF γ Nov. 3 π π K - proton N* Θ K n shifted bins M(Θ) = 1533±5 MeV Γ 2 MeV 6.7σ significance

4 DESY Dec. 3 A. Airapetian et al., hep-ex/31244 γd K p X SVD Jan. 4 A.Aleev et al., hep-ex/ 7GeV pc, Si, Pb pk X events/8mev M(Θ) = 1528±2.6±2.1 MeV Γ=19±5±2 MeV 4-6 σ significance no Θ seen σ(θ ): σ(λ 152 )= (1-22)nb : (62±11)nb 2:1 COSY-TOF Mar. 4 external beam at COSY; 2 22 data pp pk ( ) p p = 2.85, 2.95 and 3.2 GeV/c² σ tot (2.95GeV/c 2 )=12.7µb no significant A dependence: consistent with A.7 ZEUS (preliminary) Feb. 4 S. Chekanov, YITP workshop, Kyoto, Feb. 24 e pand e - p c.m. energy GeV Q 2 >1GeV GeV/c 2 Γ Θ 25MeV σ=.4±.1(stat)±.1(sys) µb signals in K p and K pbar 372±75 candidates 1527±2(stat)±few(sys) MeV Γ=1±2(stat) MeV (from Monte Carlo expected 4MeV)

5 PHENIX (preliminary) Mar. 4 C. Pinkenburg, Quark Matter 24 dau, s=2gev Summary of published Observation Excess at 1.54 GeV in nk - invariant mass Γ Θ 6MeV Γ Λ We ain t saying it s there and we ain t saying it s not there. Experimental status of Θ (154) Presently 12 experiments have seen a signal around 1.54 GeV/c 2 9 published 3 preliminary The unseen Θ (154)??? V.D. Burkert, Pentaquark 23 Workshop peak 65 background MeV/c K p interactions at p K = GeV/c A. Berthon, Nucl. Phys. B 63, 54 (1973)

6 The unseen Θ (154)! A. Berthon, Nucl. Phys. B 63, 54 (1973) The most immediate concern must be to verify that it indeed exists and is not some combination of statistical fluctuations, some complex novel dynamical background effect that has been overlooked, or psychological desire to be attracted by small positive signals while arguing away any compensating negative results. F. Close, hep-ph/31187 Lesson to learn: beware of low statistics! see also comment of M. Zavertyaev, hep-ph/31125 Poor Mans High Statistics Experiment Adding all spectra fine binning (.25MeV) because of different bin limits equal distribution of counts within an original bin adding corresponding sub-bins re-binning in 1 resp. 15 MeV bins HERA-B (preliminary) T. Knöpfle, Quark Matter 4, Oakland, January 11-17, 24 pc, Ti, W; p p =92GeV/c expected mass resolution for δm(θ )=3.2±.2MeV at mid rapidity Θ (154)/Λ(152)<.2 F. Becattini et al., hep-ph/3149: Θ (154)/Λ(152)~.6

7 Status of Width Γ Θ Θ -Experiments in most experiments the observed width is compatible with the experimental resolution (FWHM) SPring8 <25 DIANA <9 CLAS (d) <21 SAPHIR <25 ITEP (n) <2 CLAS (p) <26 SVD <24 COSY-TOF <18 some indications for width 1MeV HERMES: Γ =19±5±2MeV MC: 14.3 MeV ZEUS: Γ =1±2(stat) MeV MC: 4 MeV From KN scattering data R.A. Arndt et al., nucl-th/3113 J. Haidenbauer et al., hep-ph/39243 examined K pand K d scattering database no structure in present data at p Lab,K.44GeV/c Compatible with a resonance around 154MeV only if Γ Θ <1MeV 2. Theoretical situation "Everything should be made as simple as possible, but not simpler." A. Einstein Boom of theoretical papers about 2.5 resubmissions/paper What are these Peaks? KN molecular interpretation unlikely Θ is above the KN threshold by 15 MeV; width <1 MeV assume simple potential scattering width and depth of a potential is related to position and width of resonance for illustration: p-wave width of potential.5fm but typical scale of strong interaction 1fm no mechanism known to produce a resonance at r.5fm unless high L waves involved D.E. Kahana and S.H. Kahana, hep-ph/3126 even if possible: kaon and nucleon would loose their identity at r=.5fm KπN molecule P. Bicudo and M. Marques, hep-ph/3878 m(k )m(π)m(n) 157MeV binding energy of 3 MeV typical possible but: implies bound πk system (not observed so far) T. Kishimoto and T. Sato, hep-ex/3123 V centrifugal barrier r

8 What are these Peaks? What can it be: K ( us) n( udd) decay into a baryon It must be a baryonic system thesmallwidth<1mev must decay via strong interaction strong decay conserves strangeness particle must contain strange antiquark minimal quark configuration uudds is the mass of Θ (154) consistent with a pentaquark state? naïve quark model: m(θ )= = 19 MeV 3. Experimental facts Part 2: (186) "The whole of science is nothing more than a refinement of everyday thinking. A. Einstein need additional interaction between quarks solitons diquarks NA49: Observation of -- Pentaquark ssd d u π Observation of the (1862) by NA49 Θ (153) S N (171) N (171) (189) (27) ddssu (27) (189) (27) dussd I 3 (189) (27) - combined with primary π , 551 n( ) n( ) 1 n( ) n( ) 5 m T = = MeV 14MeV ln5 ln5

9 The WA89 Experiment - and π- beamof 34 GeV/c, n-beam of 26 GeV/c C, Cu 1993, 1994 data taking interactions (NA49: events) TRD: beam identification Si-µ-strip: vertex near target MWPC: tracking RICH: π/k separation Calorimeter: e,γ; n Cross sections more than 2 different strange and charmed hadrons are analyzed under identical conditions typical statistical distribution with slope ~ 15 MeV exp(- m/t) T=15MeV Counts/.5 MeV/c 2 production Λ π - p π - x F =-.2 x F = (169) π (169) - π Λ π - p π - M( * ) = 1685 ± 4 MeV/c 2 Γ= 1 ± 6 MeV/c 2 σ BR=6.8±.2µb Euro. Phys. J. C 5, 621 (1998) 1 3 (153) (186)? (169) NA49 m(λ p - )-m( - )

10 - π WA89 Limits for cross section ΝΑ49 tried x F -cuts, p t -cuts, angle-cuts extrapolation to cross section per nucleon σ nucl = σ A 2/ <x F <1 C target: σ nucl <2µb σ <4µb Cu target: σ nucl <75µb σ <5µb Other cross sections (<x<1) - (132): σ =1µb (153): σ =2µb - (182): σ BR=2µb - (195): σ BR =12µb x F <.4 F 1862 NA49 vs. WA89 What is different between NA49 and WA89? target: p C or Cu beam energy: 158GeV 34GeV all known cross section have smoth beam momentum dependence in this energy regime beam: p {uud} {dds} has probably [ds] diquark structure [also possible for (186)] no penalty factor compared to - production expected x F range for observed - : [-.25,.25] [.1,1] HERA-B (preliminary) T. Knöpfle, Quark Matter 4, Oakland, January 11-17, 24 pc, Ti, W; p p =92GeV/c C: 76M events Ti: 16M events (153) W: 72M events Interesting situation! if the -- (186) exists it has an exotic production mechanism what about other experiments? 11 - resolution: 2.6 MeV

11 Last but not least H1 collaboration, hep-ex/4317 predicted mass M(Θ c )=274MeV/c 2 Bin Wu & Bo-Qiang Ma, hep-ph/42244 Quintessence On the experimental side The number of experiments, which have seen signatures of the Θ (154) is quite impressive but So far only low statistics experiments have seen signals for pentaquarks No experiment has seen both decay channels of the Θ (154) Masses of the observed peaks are barely consistent Consistency with KN scattering data not clear All present high statistics searches for the observed structures have failed so far The existence of a pentaquark is not yet established and needs unequivocal verification by high statistics experiments New experiments are scheduled to confirm the Θ Spring-8, JLAB, ELSA, COSY typical improvement: factor 1 or being considered COMPASS? (significant improvement over NA49 feasible)