LHCb Spectroscopy: Results and Plans

Transcription

1 LHCb Spectroscopy: Results and Plans Tadeusz Lesiak on behalf of the LHCb collaboration Institute of Nuclear Physics Polish Academy of Sciences, Kraków EINN2017, Paphos, Nov 2,

2 Outline 1. LHCb spectrometer as a tool for heavy hadron spectroscopy 2. Few introductory words about standard and exotic heavy hadron states Exotic states: 3a. Pentaquarks in Λ b J/ψ p K decay: P c (4380) and P c (4450) 3b. Search for P c states in Λ b J/ψ p π decay 3c. Search for P c states in Λ b0 χ c(1,2) p K decays 3d. Observation of Ξ b J/ψ Λ K Decay 4a. The puzzling states X J/ψ φ 4b. Tetraquark X(5568)? 5. Charged Exotic State Z(4430) Standard states: 6. Observation of the doubly charmed baryon Ξ cc Observation of five new narrow Ω c states 8. Measurements of the χ c production and decays 9. Summary and prospects for future studies T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

3 LHCb (Forward) Spectrometer The first hadron collider experiment that is dedicated to heavy flavour (HF) physics JINST 3 (2008) S08005; IJMPA 30 (2015) RICH: Separation of K, p from π: (K K) 95 % (π K) 5 % (p p) 95 % (π p) 5 % Vertex Detector: Impact parameter resolution: Decay time resolution: heavy hadrons: 50 fs Dipole magnet: Bending power: 4 Tm Precise tracking system: (trk) 96 % Momentum resolution: Electromagnetic and hadronic calorimeters ECAL: Muon system: (μ μ) 97 % (π μ ) (1 3) % Spectrometer: very good mass resolution Trigger: Highly flexible, currently have offline quality T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

4 Pros and Cons of Heavy Flavour Spectroscopy with LHCb + General advantages (pp interaction): High production cross sections for HF (at the LHC are 10 3 larger than at the e + e B factories) Simultaneous accumulation of huge B d, B s, B c and b baryons data samples The decay vertices are well separated from the production point (high boost of the b and c hadrons) + LHCb specific advantages (single arm forward spectrometer: 0.8 o < Θ < 15.4 o ) : LHCb captures a HF production cross section (comparable to that in the high p T range (ATLAS, CMS)) in MUCH SMALLER SOLID ANGLE smaller number of electronic channels smaller event size larger trigger bandwith to store LHCb forward detector (p >> p T ): efficient muon identification for lower P T values Space to accomodate excellent RICH detectors (flavour tagging, background suppression) General drawbacks: The instantaneous luminosity is limited ( 4x10 32 cm 2 s 1 ) The efficiencies of γ, π 0 and η reconstruction are much lower to compare with the e + e T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

5 LHCb Data Sample Almost all results presented here correspond to Run 1 data: 4 x b hadrons produced Run 2 vs Run 1: more abundant production of b hadrons improvements in trigger and selection efficiencies T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

6 Standard states: Standard vs Exotic States Exotic states: Area : km 2 (6th in EU, 8th in Europe) Population: (6th in EU, 8th in Europe) Male/female: 0.94 Natural increase: 0.7 (per 1000) As from: GDP: 475 G$ (rank: 23st) GDP per capita: $ (rank: 44th) T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

7 Exotic states (with heavy quarks) About 30 heavy, potentially exotic states observed (in 21st century) Most of them are charmonium (cc) or bottomonium (bb) like Quantum numbers: Unconventional J PC Conventional e.g. 0 Supernumerary atypical production and/or decays Unconventional charges may occur (e.g. baryon with S=1 or meson with electric charge +2) Angular distributions, Dalitz and Argand plots are of much help Taxonomy (general, not universally accepted, guidelines): P X Y Z pentaquark Neutral charge resonance (most of them observed in B decays), positive parity State produced in the Initial State Radiation (ISR) processes, negative parity Charmonium like, charged state (and its isospin partners) T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

8 Exotic States T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

9 Pentaquarks in Λb0 J/ψ p K Decays Pentaquarks stay with us from the onset of the quark model Phys. Lett. 8 (1964) Intense experimental searches (rumour about the Θ+(1540) state ) PRL 115 (2015) LHCb (2015, Run 1 data, 3fb 1) : study of the decay Λb [J/ψ p] K Huge, very clean sample of Λbs: signal events, background fraction: 5.4% Standard contribution: Exotic contribution: Surprising structure in m(j/ψ p) T.Lesiak LHCb Spectroscopy Pc+ the minimal valence quark content: uudcc EINN2017 Nov. 2nd

10 Pentaquarks in Λb0 J/ψ p K Decays discovery of two hidden charm pentaquark like states Pc(4380)+ and Pc(4450)+ PRL 115 (2015) The full 6D amplitude analysis using the helicity formalism and Breit Wigner (BW) amplitudes (observables: resonance mass, three helicity angles, two angles between decay planes; takes into account 14 well defined Λ* states and interference between both decay sequences) The satisfactory description only after including two additional BW amplitudes of Pc states: Opposite parities T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

11 Pentaquarks in Λ b0 J/ψ p K Decays Argand Diagrams (for 3/2 and 5/2 + hypothesis) PRL 115 (2015) Data points in the six equidistant bins of m(j/ψp) in the range from Γ to +Γ P c (4450) + : Consistent with resonant behaviour (rapid counter clockwise change of the phase around the maximum) BW fit P c (4380) + : needs more statistics The amplitude method: powerful, but require the Λ* model (spectroscopy of these sates is complicated) + can provide the detailed info about new states (mass width, J PC ) T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

12 Pentaquarks in Λ b0 J/ψ p K Decays LHCb: fortification of the P c states observation: PRL 117 (2016) Model independent approach: no need for the Λ* model; can only indicate the presence of exotic states 2D analysis in terms of ( helicity angle of the K p system) The ang. distr. is expanded in Legendre polynomials (in bins of m(kp)): Legendre moments: contain all the information of the angular structure of the system as well as the spin of resonances in question Λ * resonances can contribute only to low order moments LHCb The (Kp) mass and angular distributions are projected as reflection into the J/ψ p system 9σ discrepancy with data, assuming only Λ * contributions (H0 hypothesis) The discrepancy concentrated in the region of mass corresponding to the P c states (best seen on the m(j/ψ p) distribution) T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

13 Pentaquarks in Λb0 J/ψ p π Decays The Cabibbo suppressed mode The background fraction is higher by a factor of three The amplitude analysis again Four contributions three of them exotic considered : Λb candidates PRL 117 (2016) N* Reported by Belle in J/ψ π (2014) Pc The masses and widths of N* and Pc states are fixed; Zc(4200) parameters set free The data favour the existence of exotic contributions 3.1σ significance if both types of exotic resonances are included 3.3σ for the Pcs, assuming that the Zc(4200) contribution is negligible T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

14 Motivation: The First Observation of Λ b0 χ c(1,2) p K PRL 119 (2017) Test the kinematic rescattering effect (KRE) hypothesis The mass of P c (4450) is very close to the [χ c1 p] threshold If P c (4450) + is due to KRE should NOT be observed as a narrow enhancement near the [χ c1 p] threshold in the mode PRD 92 (2015) Test the factorisation approach: the decay with χ c2 is expected to be suppressed w.r.t. χ c1 LHCb: both observed for the first time and Ad 1. distributions of m(χ c1 p) and m(pk) studied more statistics needed for the amplitude analysis Run 1 & 2: >1000 χ c1 candidates Ad. 2. no χ c2 suppression, differs from arxiv: T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

15 Observation of Ξ b J/ψ Λ K Decay Motivation: search for possible (udscc) states, decaying to (J/ψ Λ ) pair LHCB: the first observation of the decay PRL 117 (2016) decays LL (Λ candidates): both tracks combined for the are long include segments in VELO DD (Λ candidates): both tracks are downstream reconstructed using only the tracking stations downstream of the VELO decays Results: The amplitude analysis, in search for (udscc) states, feasible with the full data sample of Run 2 T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

16 Reminder: the X(3872) revolution Belle (2003): PRL 91 (2003) observation of X in The most studied exotic state The Puzzling States X J/ψφ PRL 110 (2013) LHCb: X is most probably a mixture of and of molecule The X(4140) evidence for a narrow near threshold structure in B + (J/ψ φ) K + decays: CDF, D0, CMS The X(4274) the second relatively narrow [J/ψ φ] state evidence from CDF and CMS Negative results from other experiments CDF: PRL 102 (2009) arxiv: [hep-ex] CMS: PL 734 (2014) 261 D0: PR D89 (2014) PRL115 (2015) T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

17 The Puzzling States X J/ψφ LHCb: the first amplitude analysis of B + [J/ψ φ] K + decays candidates; nearly background free PR D95 (2017) PRL 118 (2017) D phase space composed of m(φk), helicity angles and Δφ angles The amplitude analysis able to resolve K*s K φ and X J/ψ φ resonances The model with excited K*s ( Kφ) does not describe the data Good description upon inclusion of four broad exotic resonances Reflections from K* states The X(4140) width is substantially larger than previously determined (average of other meas.: MeV) X(4140) and X(4274) J PC incompatible with cusps and molecular bound states possible interpretation as tetraquark (no light valence quarks) or χ c1 (3P) X(4500) and X(4700) state or χ c1 (4P), χ c1 (5P) T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

18 A Tetraquark X(5568)? D0 (2016): reports a narrow structure X(5568) in the B s0 π + spectrum ( ) PRL 117 (2016) A system (tetraquark or molecule) containing valence (anti)quarks with four different flavours (b, s, d, u)??? LHCb (2016): 20x more D s than D0 collab. lack of observation of any X(5568) like signal PRL 117 (2016) Similar result from CMS: CMS-PAS-BPH (2016) T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

19 Charged Exotic State Z(4430) Belle (2008): evidence for Z(4430) in ψ π mass distribution ( LHCb (2014): tenfold increase in signal yield (~25000 decays); decays) PRL 112 (2014) D amplitude analysis: PRD 92 (2015) Model independent approach: The Kπ angular distributions are extracted from data with Legendre polynomial moments and projected as reflection of the m(ψ π) spectrum The Kπ reflections with excluded (>8σ) Moments up to assignment established relative to: Positive parity hypothesis of threshold effect and ruled out the most plausible interpretation: a tetraquark Stat. uncertainty The minimal quark content of Z(4430) : T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

20 New Standard States T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

21 Observation of the Doubly Charmed Baryon Ξ cc ++ SU(3) Triplets (ccq), q=u,d,s Expectations: SU(4) 20 plets SELEX claim: observation of Ξ cc+ (ccd) NOT confirmed by FOCUS, Belle, BaBar and LHCb PRL 89 (2002) , PLB 628 (2005) 18 JHEP 12 (2013) 090 T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

22 Observation of the Doubly Charmed Baryon Ξ cc ++ LHCb: search for Data sample: 1.7 fb 1, Run 13 TeV full and efficient event reconstruction at trigger level Run 1 data (2012) analyzed for a cross check PRL 119 (2017) RS: clear structure visible at ~3620 MeV; no excess in WS nor SB spectra The peaking structure fully survives ( > 12σ) after requiring minimum decay time (t > 5 σ w.r.t. PV). It is indeed a weak decay! T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

23 Observation of the Doubly Charmed Baryon Ξcc++ decays decays PRL 119 (2017) Inconsistent (far too big) with the statement that both states (SELEX s and LHCb s) are isospin partners Further studies: additional decay modes, lifetime and production cross section measurement, search for isospin partner of the Ξcc++, search for heavier states e.g. Ξbc T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

24 Observation of Five New Narrow Ω c States Only two Ω c (css) ground states observed so far (Belle 2009): Ω c0 (m 2695 MeV, J P = 1/2 + ) and Ω c0 (2770) with J P = 3/2 + ground states Measurements of the excited charmstrange baryons valuable test of HQET LHCb: search for (10 6 Ξ c candidates) Data: 3fb 1 (Run 1) fb 1 (Run 2) PRL 118 (2017) fit Fitted bckg. Ξ c+ sidebands Feed down from partially rec. Ω c * resonances Five narrow Ω c states are observed; their J PC to be determined A broad structure around 3188 MeV: The 6 th wider state? A superposition of several states? T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

25 χ c Production and Decays LHCb: search for with The measurements of inclusive branching fractions of for the first time in the harsh hadronic environment: Eur. Phys. J. C Data sample: 3 fb 1 Run 1 No evidence for ; upper limits set LHCb: observation of decays Valuable test of QCD Data sample: 3 fb 1 Run 1 and 1.9 pb 1 Run 2 Results: arxiv: [hep-ex] Agreement with and comparable precision to the current world averages T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

26 Conclusions The renaissance of heavy flavour spectroscopy in recent 15 years: Observation of numerous new states: many of them exotic like A vivid field of research with strong liasons between theory and experiment LHCb has recently provided valuable contributions to heavy flavour spectroscopy: Observation of hidden charm pentaquarks Pc Observation of candidates for tetraquark states in X J/ψφ Non confirmation of a tetraquark X(5568) Confirmation and extensive studies of Z(4430) in ψ π mass distribution Observation of doubly charmed baryon Ξ cc ++ Observation of five new narrow Ω c states Studies of the χ c production and decays More precice spectroscopic measurements from the LHCb experiment and, hopefully, some discoveries should follow with the analysis of Run 2 data LHC and LHCb upgrade: 50 fb 1 of integrated luminosity expected by T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd

27 Backup T.Lesiak LHCb Spectroscopy EINN2017 Nov. 2nd