Studies of pentaquarks at LHCb

Transcription

1 Studies of pentaquarks at Mengzhen Wang Center of High Energy Physics, Tsinghua University (For the collaboration) CLHCP 17, Dec nd China 1

2 Outline The experiment Studies about pentaquarks from Discovery of pentaquarks in Λ " # decays First observation of Λ " # χ &{(,*} pk. decays First observation of Ξ ". J/ψΛK. decays Summary

3 The experiment The LHC is a beauty and charm factory In acceptance, σ "" 4~7 14 µμb at s = 7(13) TeV PRL. 118, 5 (17) The detector Single- arm forward spectrometer, < η < 5 Designed for the study of heavy flavor physics High- precision vertexing and tracking Excellent particle identification Versatile trigger system Int. J. Mod. Phys. A 3, 153 (15) JINST 3 (8) S85 3

4 Resonance in Λ " # J/ψpK. decays Two bands in the Dalitz plot, caused by shortlived resonances ] [GeV ψp J/ m PRL. 115, 71 (15) mkp [GeV ] 4

5 Λ " # J/ψpK. amplitude analysis Λ decay chain: Λ " # J/ψ μ G μ. Λ ( pk) 5 angles and 1 mass θ JK L, θ V, φ V, θ P, φ P, m TV P & G decay chain: Λ " # P & G ( J/ψ μ G μ. p)k 6 angles and 1 mass M θ N M L JK, φ MN, φ N M P, θ MN, φ N M Q, θ N P, φ P, m S/PT + μ rest frame Λb rest frame φ = Λ θ μ z μ θ φ ψ Λ ψ b * Λ x p + μ * φ μ ψ ψ Λ b lab frame * Λ K K p θ * Λ Λ rest frame K Not independent to variables in Λ decay chain ψ rest frame P c μ φ π P c μ θ Pc ψ p ψ P c rest frame Λb rest frame P c ψ * Λb lab frame Pc θ Λ b p Pc φ π Fit the angular distributions. Each node contributes to a helicity coupling and angular structures No float parameters in angular structure Helicity couplings are float in fit μ + θ ψ μ + μ ψ Λ * Λ K 5 φ π P c

6 Λ " # J/ψpK. amplitude analysis Partial wave resonance function R X (m) =B L X b (p, p,d) Blatt- Weisskopf p M b! L X b BW(m MX, X) BL X (q, q,d) Relativistic Breit- Wigner The fit projection including only Λ states acceptable PRL. 115, 71 (15) Blatt- Weisskopf q M X LX Not satisfactory Events/(15 MeV) (a) data total fit background Λ(145) Λ(15) Λ(16) Λ(167) Λ(169) Λ(18) Λ(181) Λ(18) Λ(183) Λ(189) Λ(1) Λ(11) Λ(35) Λ(385) Events/(15 MeV) (b) m K p [GeV] m J/ψp [GeV] 6

7 Λ " # J/ψpK. amplitude analysis Two P & G states are required to get acceptable fits 6D amplitude analysis allows to measure the resonance parameters Table 1: Pentaquark parameters Events/(15 MeV) 8 7 (b) PRL. 115, 71 (15) m J/ψp [GeV] State Mass(MeV/c ) Width(MeV/c ) J P P c (438) ± 8 ± 9 5 ± 18 ± 86 P c (445) ± 1.7 ±.5 39 ± 5 ± 19 Table The : Relative best solution PID e of spin- parity is. XG WG X., ), while (, ) and ciency with di erent solutions PID r ( bglobalt IS) PID r( + c HadronTOS &! b Globa Solution 1 ( W *.71 * ±.5 * *.739 ±.7 Solution.7 ± ±.3 ( X *., W * G ) are not excluded The existence of these two particles is confirmed in another Table 3: Systematic uncertainties of relative branching fraction source uncertainty of branching fraction p T model- independent,y reweight analysis.73% PRL. 117, 8 (16) 7 TISTOS correction.11% PID e ciency 1.4%

8 Branching fraction of Λ " # P & G (J/ψp)K. Absolute branching fraction of Λ " # J/ψpK. measured R JK L /Z 4 L = σ Λ " # B(Λ " # J/ψpK. ) σ(b4# )B(B4# J/ψK\ # ) From data Previous results PRD 85, 38 (1) JHEP 8(14)143 B! B Result from Belle PLB538 () 11! B( b! P c + K c B)B(P b! c +! J/ p) ( (.66 ±. ± = /B Λ b R Chinese Physics C 4 (16) TeV data 7 TeV fit 8 TeV data 8 TeV fit p [GeV/c] T Fraction of P & G from the Λ " # J/ψpK. amplitude analysis = f(p + c )B( b! J/ pk ) (.38) 1 5 for ± P ± c (438) +, (1.3 ±.16 ± ) 1 5 for P c (445) +, 8

9 Λ " # J/ψpπ. amplitude analysis One way to examine the existence of P & G resonance states: search them in other decay channels Z(J K L S/PT^_) Z(J K L S/PTV _ ).8 due to Cabibbo suppression effect PRL. 117, 83 (16) Candidates / ( 5 MeV ) Data Fit Signal Λ b J/ψ pk Cmb. bkg. - Obtained 1885 ± 5 Λ " # J/ψpπ. candidates with run- I data. Use them to examine the exotic hadron contribution from the P & G J/ψp states. The amplitude model includes several N pπ. resonances m J/ψ pπ [GeV] 9

10 Λ " # J/ψpπ. amplitude analysis If P & 438 G, P e 445 G and Z & 4. are included in fit model, the total significance for them is 3.1σ PRL. 117, 83 (16) Yields/ (5 MeV) 1 1 Data RM N*+Z c +P c EM N* P c (445) P c (438) Z c (4) m pπ [GeV] The rate is consistent with the results of Λ " # J/ψpK. decays, taking into account the Cabibbo suppression. Yields cosθ Λb cosθ Λb cosθ N* φ K Data RM N*+Z c +P c P c (445) P c (438) Z c (4) 1.5.5cosθ * N cosθ J/ψ φ µ cos θ φ [rad] 1

11 Λ " # J/ψpπ. amplitude analysis If assume the contribution of Z & 4. is negligible, the model with two P G & resonance yields a significance of 3.3σ PRL. 117, 83 (16) Yields/ (5 MeV) 4 (b) m J/ψ p [GeV] Yields/ (5 MeV) 1 1 Data RM N*+P c EM N* P c (445) P c (438) m pπ [GeV] 11

12 Observation of Λ " # χ &{(,*} pk. The P & 445 G mass is just above the [χ &( p] threshold. m MN iix# j m l Nm m T ~.9 MeV Real resonance or kinematic re- scattering? PRD 9, 715 (15) (a) Kinematic re- scattering would not lead to P & 445 G peaking in [χ &( p] invariant mass The initial stage: observe these decays Λ " # χ &{(,*} pk. (b) 1

13 Observation of Λ " # χ &{(,*} pk. ) Events / ( 5 MeV/c First observation with Run- I data. Λ " # J/ψpK. as control mode to measure the branching fractions Reconstruct χ &{(,*} with J/ψγ, J/ψ with μ G μ. Gradient- boosted Decision Tree to subtract background Λ " # mass fit with J/ψ and χ &( mass constrained: Mass peak of χ &* pk. is shifted (wrong mass hypothesis). Λ b χ Λ b χ Comb. N(χ &( )~453 N(χ &* )~85 c1 c pk pk m(χ pk c1 ) [MeV/c ] Weighted cands / (1 MeV/c ) PRL. 119, 61 (17) Background subtracted χ &S mass distribution, with no χ &( mass- constraint Λ b Λ b χ χ pk c1 pk c m + m [MeV/c J/ψ ] 13

14 Observation of Λ " # χ &{(,*} pk. Measure the branching fraction: Z(J K L l Nm TV _ ) Z(J K L S/PTV _ ) =.4 ±.1, Z(J K L lnp TV _ ) Z(J K L S/PTV _ ) =.48 ±.6 Some unexpected difference with B # decays: Z(J K L l Np TV _ ) Z(J L = 1. ±.11, Z(ZL l Np V ) K l Nm TV _ ) Z(Z L l Nm V ) PRL. 119, 61 (17) =.17 ±.5 Obtain 453 ± 5 χ &( candidates. Need more data for the m(χ &( p) investigation Number of χ &( candidates with Run- I and Run- II data is expected to be 4 times compared to the Run- I data 14

15 PLB 77 (17) Observation of Ξ ". J/ψΛK. Open strangeness pentaquark P &r predicted in the J/ψΛ structure (m~465 MeV, Γ~1 MeV) [PhysRevC ] Should be seen in Ξ ". J/ψΛK. : similar topology as Λ " # J/ψpK, with a u- quark replaced by an s- quark b ( b ) b d s (u) W (a) c J/ c s ū K u d (p) s (u) b b d s W (b) c c s d u ū s J/ K First observation of this decay with entire Run- I data 15

16 Observation of Ξ ". J/ψΛK. Candidates/(6 MeV/c ) Use Λ " # J/ψΛ as control channel. Reconstruct J/ψ with μ G μ., reconstruct Λ with pπ Data Total fit - Ξ b signal - J/ψ Σ K Comb. bkg LL 99 ± DD 9 ± PLB 77 (17) Separate analyses for Λ that decays inside (LL) or outside (DD) the Vertex Detector Gradient- boosted Decision Tree for event selection t u K t vk L Z(w K _ S/PJV _ ) Z(J K L S/PJ) =.419 ±.9 ±.14 f {wk,j K L } are the b {Ξ ", Λ " # } fragmentation functions. An amplitude analysis is expected with Run- I and Run- II data m(j/ψ ΛK ) [MeV/c ] 16

17 Summary is a huge factory of heavy quark baryons Discovery of two pentaquark states in Λ " # J/ψpK. decays Search for P & G resonances in other decay channels Analysis in Λ " # J/ψpπ. decays shows consistent results First observation of Λ " # χ &{(,*} pk. decays Amplitude analysis is under way for possible χ &( p structure Search for new kind of pentaquarks First observation of Ξ ". J/ψΛK. decays Search for open strangeness pentaquark P &r expected Thank you for your attention! 17