Now Pentaquarks, What Next?? Ken Hicks (Ohio U./NSF) Reimei Workshop Jan. 19, 2016
How MathemaJcians Count • 1,2,3,4,5,6,7,8,9,10,11…
Pre-2000: how hadron physicists count q q q � 1 2, 3.
2003: how hadron physicists count 2, 3, 4.
Z(4430) confirmed by LHCb B à K π + ψ ’ Model-ind. Argand plot 4-dim amplitude analysis arXiv:1404.1903 >13.9 σ BW-like resonant phase moJon P = 1 + J + 15 MeV M = 4475 ± 7 − 25 Good agreement with Belle, (with smaller errors) + 37 MeV Γ = 172 ± 13 − 34 0 → Z (4430) − → π − & − 1.1 ) × 10 − 5 − K + ) × Bf ( Z (4430) Bf ( B ψ ) ≈ (3.4 − 2.3
2015: how hadron physicists count + P c c u c d u (?) 2, 3, 4, 5.
Pentaquarks: some history
The Anti-decuplet in the χ SM D. Diakonov, V. Petrov, M. Polyakov, Z.Phys.A359, 305 (1997) S=+1 updated version D. Diakonov, V. Petrov, Δ m = 108 MeV arXiv:hep-ph/0310212 S=0 S=-1 S=-2 Theoretical prediction that spurred several experimental searches.
Θ + (1540) published (mistakes?) LEPS/SPring8 PRL91, 012002 (2003) CLAS/JLab PRL91, 252001 (2003) γ D K - pK + (n) γ C K + K - (n)
Aaempt to reproduce CLAS result n Restricted photon energy range n Two distributions statistically consistent with each other: n 26% c.l. for null hypothesis from the Kolmogorov test (two histograms are compatible). n G10 mass distribution can be used as a background for refitting the published spectrum.
2009: Results of Θ + analysis at LEPS nK + invariant mass with MMSA: Fermi motion effect corrected. Simple ( γ ,K - ) missing mass: No correction on Fermi motion effect. 2 Peak position: 1.527 0.002 GeV/ c ± Signal yeild: 116 21 events ± Differential cross-section: 12 2 nb/sr ± PRC 79, 025210 (2009) “ The narrow peak appears only after Fermi motion correction . ” Δ (-2ln L ) =31.1 for Δ ndf =2 5.2 σ 7 Prob(5.2 ) 2 10 − σ = ×
Unpublished CLAS result Energy of photon (2.0-2.4 GeV) fquaenergycmLEP0 fquaenergycmLEP0 Counts 12 nb Entries Entries 19087 19087 300 Mean Mean 1.561 1.561 RMS RMS 0.06258 0.06258 Proton veto 250 2 2 / ndf / ndf 90.99 / 98 90.99 / 98 χ χ p5 p5 1.993 1.993 6.265 6.265 − − ± ± 200 150 100 50 Proton detected 0 1.45 1.5 1.55 1.6 1.65 1.7 2 Gev/c Using the same analysis cuts and procedures as for LEPS (2009).
Pentaquarks: the LHCb results (Some slides borrowed from Sheldon Stone)
Physics 8 , 77 (2015) Viewpoint Elusive Pentaquark Comes into View Kenneth Hicks Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA Published August 12, 2015 A new type of particle containing five quarks has been observed by the LHCb experiment. Subject Areas: Particles and Fields A Viewpoint on: Observation of J / p Resonances Consistent with Pentaquark States in 0 b Ω J / K − p Decays R. Aaij et al. (LHCb Collaboration) Physical Review Letters 115 , 072001 2015 – Published August 12, 2015
Λ b → J/ ψ K p Λ b à J/ ψ K - p ! First looked for in LHCb as 26,000 signal a potential background for + 5.4% bkgrnd B 0 → J/ ψ K + K - within ±2 σ of peak ! Large signal found, used m(J/ ψ K - p) [MeV] for Λ b lifetime [arXiv:1402.6242] … Λ *’s ! Dalitz plot showed an m 2 (J/ ψ p) unusual feature [arXiv:1507.03414] BNL August 2015 ! m 2 (K - p) 4
Projections ProjecJons arXiv:1507.03414
Prejudices Be Careful! Previous History: ! No convincing states 51 years after Gell-mann & Zweig proposed qqq and qqqqq baryonic states ! Previous “observations” of several pentaquark states have been refuted ! These included " Θ + → K 0 p, K + n, mass=1.54 GeV, Γ ~10 MeV " Resonance in D* - p at 3.10 GeV, Γ =12 MeV " Ξ -- → Ξ - π - , mass=1.862 GeV, Γ <18 MeV ! Generally they were found/debunked by looking for “bumps” in mass spectra circa 2004 [see Hicks Eur. Phys. J. H37 (2012) 1.] BNL August 2015 !
Λ b → K − pJ/ ψ Can the s-channel band originate from a non-s-channel pole Λ (1520) m 2 pJ ψ s m 2 t Kp
Two PWA: Reduced and Extended ! Consider all Λ * states & all allowed L values Flatte´ BW ↓ # parameters 64 146 BNL August 2015 ! 10
PWA using only known Λ * states c ! Use extended model, so all possible known Λ * amplitudes. m Kp looks fine, but not m J/ ψ p ! Additions of non-resonant, extra Λ *’s doesn’t help
Fit with Λ * and one P c signal ! Try all J P up to 7/2 ± ! Best fit has J P =5/2 ± . Still not a good fit
Reduced PWA with two P c states c ! Best fit has J P =(3/2 - , 5/2 + ), also (3/2 + , 5/2 - ) & (5/2 + , 3/2 - ) are preferred
Argand diagrams: phase moJon Argand diagrams Breit- Wigner Breit- Wigner Amplitudes for 6 bins between + Γ & - Γ
Why two P c states are needed: Data demands 2 states ! Interference between opposite parity states needed to explain P c decay angle distribution ! Fit projections Corrected events/(0.1) 500 Combined P c P (4450) LHCb c P (4380) 400 c 300 Small m(Kp) region Large 200 positive m(Kp) interference region 100 negative interference 0 -1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 � cos( ) P c BNL August 2015 !
CauJon! Strong interference of Λ * gular distributions 8 14
Some TheoreJcal Models Meson-Baryon Molecule 5-quark “bag”
Why Heavy Quarks? • Karliner and Rosner: arXiv:1506.06386 – Predict a host of P c and P b states corresponding to various meson-baryon thresholds. – Based on meson-meson molecule model for XYZ – Note conspicuous absence of DD and BB states. • Pion exchange is suppressed b/w two pseudoscalars. – The “repulsive kineJc energy is inversely proporJonal to the reduced mass” (Ericson, Karl). • Requires heavy quarks to provide sufficient aaracJon.
What comes aqer 5? Dibaryons?
Dibaryons (some slides borrowed from Reinhard Schumacher)
� 6 quarks in a bag � The deuteron 3 S 1 2.2 MeV bound I(J P )=0(1 + ) � � The only clear-cut “dibaryonic molecule” � Recall the nn , pp , and np 1 S 0 I(J P )=1(0 + ) strong spin singlet states are unbound… � … by only ~100 keV � One of the great “fine-tuning” mysteries of nature!!
� Bound N � , bound �� , I (J P ) �� (Jaffe’s “H-particle”) � Binding? � Width: ‘narrow’ or ‘wide’? � Spin, Isospin ? N � I (J P )=1 (2 + ) (?) � CLAS study: new observations � Recent WASA@COSY � I (J P )=0 (3 + ) � claim of discovery
� “3-body model of N � and �� dibaryons” � A. Gal, H. Garcilazo, arXiv:1402.3171 (2014) � Three-body model with separable pairwise interactions � Solve � NN and � N � Faddeev equations � �� found below threshold for I(J P ) = 1(2 + ) & 2(1 + ) � �� found below threshold for I(J P ) = 0(3 + ) & 3(0 + )
�� ΔΔ resonance with I (J P ) = 0 (3 + ) � The WASA@COSY result for �� � M ~ 2370 MeV = 2m � – 90 MeV � � ~ 70 MeV < 1/3 � �� m �� � “ABC effect”: enhancement of low- mass pion pairs � Dibaryon interpretation is controversial (D. Bugg) P. Adlarson et al, Phys Rev Lett 106, 242302 (2011)
� � � 190 MeV � W=2.18 GeV 3 P 2 wave in � d elastic scattering � is most prominent � SAID analysis: “resonance-like” behavior in several partial waves R. Arndt, I. Strakovsky, R. Workman, Phys Rev C 50, 1796(1994) CLAS collab. 11-14-14 R. A. Schumacher, Carnegie Mellon University 11
PhotoproducJon at CLAS � � � Off- shell pion d* d d � Resembles � d elastic scattering but with an off-shell pion.
R. Schumacher, P. Masone (CMU): CLAS Preliminary!!
ProjecJons: CLAS preliminary!!
Background from ρ -meson removed: Preliminary!!
Fit to resonance shape: Preliminary! Fit to Resonance-like Shapes W=2.75 GeV � � � � � � �
Next Steps • Study of coherent ρ -meson photoproducJon – PhD of T. Chetry, Ohio U. grad student. • PWA that includes all possible final states: – First steps being done by CMU (P. Masone) • In addiJon to d* ++ and d* 0 , look for d* + . – Also part of T. Chetry’s PhD. – Interferes with ω -meson photoproducJon. – Clearly shows isospin 1 triplet of dibaryon states.
What’s Next??
Summary • Pre-2000: only mesons and baryons – N = 2 or 3 (except for nuclei, such as deuterium) • Post-2000: many new mulJ-quark hadrons: – Tetraquark (XYZ mesons), Pentaquark (P c states), Dibaryon ( ΔΔ and N Δ ). – N = 4, 5 and 6. • What’s next? Probably hybrid mesons. – Predicted already by Lasce Gauge theory.
Backup Slides
Some theoreJcal references • Roca, Nieves, Oset: arXiv:1507.04249 – Pentaquarks as a D* Σ c and D* Σ c * molecule. – Explains the best-fit spin and parity of P c states. • Guo, Meissner, Wang, Yang: arXiv:1507.04950 – Possible rescaaering from χ c1 to J/ ψ p. • Maiani, Polosa, Riquer: arXiv: 1507.04980 – Compact 5-q structure using diquarks. • R. Lebed: arXiv: 1507.05867 – Compact (cu) diquark and color triplet c(ud).
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