Antimo Palano INFN and University of Bari, Italy On behalf of the LHCb Collaboration

Transcription

1 Hadron spectroscopy in LHCb Outline: The LHCb experiment. Antimo Palano INFN and University of Bari, Italy On behalf of the LHCb Collaboration The observation of pentaquark candidates Observation of possible tetraquark states Observation of new Baryonic states EXA International Conference on Exotic Atoms and Related Topics, Wien, September 11-15, 017 1

2 The LHCb experiment High cross-section of heavy-quark production. Excellent decay time resolution. Excellent particle identification. Excellent momentum resolution. Flexible trigger.

3 The LHCb experiment Efficiency for b b production in LHCb is 7% of b or b and 5% of b b pair. Collected Luminosity. Most of the analyses presented here made use of Run1(78 TeV) (fb 1 ) dataset only. A few analyses make use also of the Run (1 TeV) (1.7fb 1 ) data.

4 Multiquark states In the original Gell-Mann paper ( A schematic model for baryons and mesons, Phys. Lett. 8, (1964)). Baryons can now be constructed from quarks by using combinations (qqq),(qqqq q), etc., while mesons are made out of (q q),(qq q q), etc. Today qqqq q baryons are called pentaquarks, qq q q mesons are called tetraquarks. Pentaquarks 4

5 The rise and fall of pentaquarks Low statistics evidences for pentaquarks were provided by several experiments around (see A. Dzierba, C. Mayer and A. Szczepaniak, hep-ex/0410). Evidences for Θ in the nk and pk 0 S. 00 Normalization region Candidates / (5 MeV/c ) ZEUS BaBar e ± p e Be candidates E CM 00 GeV 9.4 Q >0 GeV ~ pk short Mass (GeV/c ) Significances in these data were largely overestimated and high statistics searches gave negative results (See for example BaBar: Phys.Rev.Lett. 95 (005) 0400, FOCUS: Phys.Lett. B69 (006) 604). Around 007 pentaquarks were dead. 5

6 Observation of J/ψp resonances in Λ 0 b J/ψpK decays in LHCb Multivariate Analysis (MTVA) selection. 6,007 ± 166 Λ 0 b events with 94.6% purity. The Dalitz plot shows rich Λ s resonant structures along the pk axis. Unexpected structure along the J/ψp axis. 6 (PRL 115, (015)).

7 Amplitude analysis and mass projections Key point is a full amplitude analysis which also describes the complex resonant structure in the pk final state. The analysis requires the presence of two new resonances (labelled P c ). (PRL 115, (015)). 7

8 Resonances parameters and angular analysis Resonance Mass (MeV) Width (MeV) Significance Fit fraction (%) P c (480) 480 ± 8 ± 9 05 ± 18 ± 86 9σ 8.4 ± 0.7 ± 4. P c (4450) ± 1.7 ±.5 9 ± 5 ± 19 1σ 4.1 ± 0.5 ± 1.1 The best fit has J P = / and J P = 5/. Good description of the angular distributions. Measure the real and imaginary parts of the P c amplitudes (PRL 115, (015)). Argand Diagram consistent with expectations from a Breit-Wigner behaviour. Model independent analysis gives consistent results (Phys. Rev. Lett. 117, 0800 (016)). 8

9 Search for other P c decay modes Finding the same P c in other channels is helpful to understand P c production mechanism and internal structure. Two P c production mechanisms predicted. The two cases can be tested using the R π/k ratio which is expected to be very different. R π/k = B(Λ0 b π P c ) B(Λ 0 b K P c ) , R π/k = 0.58 ± 0.05 Cheng, Phys. Rev. D 9, (015),Hsiao, Phys. Lett. B 751, 57 (015) 9

10 Study of Λ 0 b J/ψpπ decays in LHCb Branching fraction for the Cabibbo suppressed Λ 0 b J/ψpπ is 8% of the Cabibbo favoured Λ 0 b J/ψpK decay mode. More complex because of the possible contribution of Z c (400) J/ψπ (observed by Belle in B 0 J/ψK π (PRD 90 (014) 11009)). Full amplitude analysis. Accurate description of the rich resonant structure in the pπ final state. Λ b J/ψN ( pπ ), Λ b π P c ( J/ψp), Λ b pz c (400) ( J/ψπ ) Phys. Rev. Lett. 117, 0800 (016) 10

11 Study of Λ 0 b J/ψpπ decays Z c (400), N and exotic states parameters fixed. Each Pc: 4 free parameters 6 fixed to that from Λ 0 b J/ψpK. Significance of the two P c is.1σ. The b c diagram strongly favoured. Phys. Rev. Lett. 117, 0800 (016) 11

12 Resonances decaying to J/ψφ in B J/ψφK The X(4140) state is first claimed by the CDF collaboration in 008. (PRL ). Narrow width: Γ = ±.7 MeV. Many experiments results. PRD Events / 0 MeV/c BaBar (b) 50 Summary of the experimental evidences m J/ψφ (GeV/c ) Experiment CDF Belle CDF LHCb CMS D0 BaBar year Significance (Nσ)

13 New results on B J/ψφK from LHCb Update of the analysis using Run1 data (fb 1 ) (PRL118, 000 (017), PRD95, 0100 (017)). Six dimensional amplitude analysis. The best fit requires the presence of four X states and a non-resonant term. 1

14 New results on B J/ψφK from LHCb Resonances parameters (PRL118, 000 (017)). The X(4140) is not a narrow resonance. A possible diagram for producing a 4-quark state. Lot of discussions. Interpretation of these states still open. 14

15 Study of B 0 ψ π K in LHCb First analysis from Belle: observation of a new Z c (440) ψ π in B Kπ ψ (PRL 100, (008)). Not confirmed by BaBar: data could be described without the presence of a Z c (440) resonance (PRD 79, (009)). Recent analysis from LHCb (PRL 11, 00 (014)). B 0 signal: 5,176 events (Belle:,010, BaBar:,01 events). Z c 15

16 Study of B 0 ψ π K Amplitude analysis confirms the presence of the Z c resonance (PRL 11, 00 (014)). Argand diagram shows typical resonance behaviour. Resonance parameters: M(Z c ) = 4475 ± MeV, Γ(Z c) = 17 ± MeV. In good agreement with Belle. Possible presence of an additional Z c at a mass of 49 MeV. Z c is a charged charmonium state. Multiquark state? 16

17 Baryon spectroscopy Heavy quark effective theory (HQET) predictions for Ω c states. ] Mass [GeV/c L j P J qq Ω c *(S) Ω c (S) Ω c S 1 1 Ω c * S 1 Ω c0 (P) Ω c1 (P) Ω Ω c1 (P) c (P) Ω c (P) Ω c0 P 0 1- Ωc1 P 1 1- Ωc1 P 1 - Ωc P - Ω c P 5 - Ω c1 (1D) D 1 1 Ω (1D) c1 D 1 Ω c (1D) D Ω c (1D) Ω c (1D) Ω c (1D) D 5 D 5 D 7 K Λ c Ξ D Ξ c * K Ξ c K π Ξ c K Ω c * π π Ω c π π K Ξ c 0 K Ω c quark content: ssc. Only 1/ and / ground states were known. 17

18 Observation of five new Ω C states in LHCb Explore excited Ω c states in their strong decay to Ξ c K (PRL 118 (017) 18001). Make use of data collected at 7,8 and 1 TeV (. fb 1 ). Ξ c reconstructed in the Cabibbo suppressed mode Ξ c pk π Ξ c reconstructed with a 8% purity. Ξ c combined with a prompt K : five narrow Ω C observed. No structure in the Ξ c sidebands or in the wrong sign Ξ c K mass spectrum. Candidates / (1 MeV) LHCb Candidates / ( MeV) LHCb Ξ c K Ξ K c m(pk π ) [MeV] m(ξ c K ) [MeV] 18

19 Observation of five new Ω C states Describe peaks with relativistic Breit-Wigner convoluted with Gaussian with σ from 0.7 to 1.7 MeV. Account for feed-down from Ω c K Ξ c( Ξ c γ). Model enhancement at 00 MeV with one Breit-Wigner. Resonances parameters. Candidates / (1 MeV) 400 LHCb Ξ c K Full fit Background Feed-downs sidebands Ξ c Resonance Mass ( MeV) Γ ( MeV) Ωc(000) ± 0. ± Ωc(050) ± 0.1 ± Ωc(066) ± 0.1 ± Ωc(090) ± 0. ± Ωc(119) ± 0. ± ± 0.6 ± ± 0. ± 0.1 < 1. MeV, 95% CL.5 ± 0.4 ± ± 1.0 ± ± 0.8 ± 0.4 <.6 MeV, 95% CL Ωc(188) ± 4.8 ± ± 15 ± m(ξ c K ) [MeV] Ω c (050) 0 and Ω c (119) 0 exceptionally narrow (PRL 118 (017) 18001). 19

20 Observation of five new Ω C states Comparison with theoretical expectations. ] Mass [GeV/c L j P J qq Ω c *(S) Ω c (S) Ω c S 1 1 Ω c * S 1 Ω c0 (P) Ω c1 (P) Ω Ω c1 (P) c (P) Ω c (P) Ω c0 P 0 1- Ωc1 P 1 1- Ωc1 P 1 - Ωc P - Ω c P 5 - Ω c1 (1D) D 1 1 Ω (1D) c1 D 1 Ω c (1D) D Ω c (1D) Ω c (1D) Ω c (1D) D 5 D 5 D 7 0 Ω c (119) Ω Ω c Ω c(050) (065) (090) c Ω c (000) D and P-wave states may be narrow (G. Chiladze, A. Falk arxiv: ). Need to measure the quantum numbers of these states. Many phenomenological interpretations, including the possible presence of pentaquarks. 0

21 The search for double charmed baryons Ξ cc states The first claim for observing the Ξ cc (dcc) state comes from SELEX experiment (PRL 89 (00) 11001, PLB 68 (005) 18) Not observed by BaBar (Phys.Rev. D74 (006) 01110), nor by Belle (Phys.Rev.Lett. 97 (006) 16001). Different production mechanisms? 1

22 Observation of the double charmed baryon Ξ cc in LHCb Search for the Ξ cc (ucc) using the decay (Phys. Rev. Lett. 111 (017) ). Ξ cc Λ c K π π, Λ c pk π (BR = 10%) Analyze 1.7 fb 1 of Run using a dedicated high efficiency trigger. First observation. No signal observed in the Λ c sidebands, no signal in the wrong sign Λ c K π π combination. Consistent signal also observed in the Run1 data.

23 Observation of the double charmed baryon Ξ cc Significance > 1σ (Phys. Rev. Lett. 111 (017) ). Yield 1 ± decays. The signal persists after a lifetime cut. Ξ cc parameters. m(ξ cc ) = ± 0.7(stat) ± 0.7(syst) ± 0.14(Λ c )MeV Mass difference with respect to the possible SELEX isospin partner: 10 ± MeV. Inconsistent with expected isospin splitting for Ξ cc.

24 Amplitude analysis of Λ b D 0 pπ in LHCb The Λ c spectrum needs to be completed. Explore the Λ c spectroscopy using the D 0 p final state (JHEP 05 (017) 0). The inclusive D 0 p was studied by BaBar (PRL 98 (007) 01). High statistics clean Λ b signal in LHCb (11,00 events, 86% purity). 4

25 Amplitude analysis of Λ b D 0 pπ Follow helicity formalism to describe 5D amplitude of D 0 p and pπ masses (JHEP 05 (017) 0). Dalitz plot and D 0 p mass projection. Λ c (860) parameters (first observation), J P = / : m = (stat) ± 0.5(syst)1.1(model) MeV 5.6 Γ = (stat) ± 1.4(syst) 5.9 (model) MeV 0.0 Λ c (880) parameters, J P = 5/ preferred: m = ±9(stat) ±0.07(syst) 0.14 (model) MeV 0.0 Γ = (stat) ± 0.9(syst)0.75(model) MeV 0.00 Λ c (940) parameters, J P = / preferred: m = (stat) ± 0.4(syst)0.1(model) MeV 4.6 Γ = (stat) ± 0.9(syst)5. (model) MeV

26 Conclusions LHCb is a flavor factory, exploring a large set of physics topics. In particular, in the spectroscopy field, many new unexplored regions are being studied. These studies are producing unexpected results, such as the discovery of exotic states, or the observation of many unexpected resonances and particles. Basic ingredients of these results are high statistics and purity of the final states and highly sophisticated and newly developed full amplitude analyses. This field is in rapid development and much more experimantal and theoretical work is needed to understand the full pattern. Many more analyses are underway, making use of the large amount of data which are being collected at LHC. 6