Heavy flavour spectroscopyat LHCb
Moriond QCD, La Thuile
23rd March 2016
Paolo GandiniUniversity of Oxford
on behalf of the LHCb collaboration
23rd March 2016 Paolo Gandini Moriond QCD 2
Outline● Several new results produced for winter conferences● I will concentrate only on newest results available● Heavy flavour spectroscopy is a rich and active field
Observation of a resonance in the Ξb-π+ mass spectrum
Measurement of the mass and lifetime of the Ωb baryon
Modelindependent analysis of Λb → J/Ψ p K-
New searches for interesting final states
Tetraquark searches at LHCb in Bsπ
New!
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LHCb Detector
● Forward spectrometer● Good Vertex measurements● Precise Tracking● Excellent PID up to 100 GeV● Versatile Trigger
Fully instrumented: 2 < η < 5
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Observation of a resonance in the Ξb-π+ mass spectrum
Ξb ∗→ Ξb
-π+
LHCb-PAPER-2016-010
New!
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Introduction● LHCb has successfully observed two Ξb
∗ charged states● Now look for the neutral ● CMS published in 2012 the observation of Ξb
0∗
● Aim is to confirm CMS observation and measure width
● Full Run1 dataset● Use a fully hadronic decay of the Ξb● Look near threshold● Fit with RBW convolved with resolution function from MC (BKG is a threshold function)
No other significant structure at higher mass
PRL 114, 062004 (2015)
PRL 108, 252002 (2012)
Preliminary
LHCb-PAPER-2016-010
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Results• We observe one resonant state with the following properties:
Systematics
• We also perform a relative production measurement• Relative efficiency is taken from MC• Several crosschecks correct for MC/Data differences→
Need to take into account only the efficiency of the “slow” pionMost systematics cancel out
Most Ξb mesons produced come from higher resonances
LHCb-PAPER-2016-010
CMS PRL 108, 252002 (2012)
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Conclusions● We see a consistent picture with CMS (10x precision)● The state is consistent with the JP = 3/2+ resonance expected in the quark model● Most precise determination of the mass and the first measurement of the width
● We see only ONE state above Ξb
-π+ threshold● But TWO are expected in this region (Ξb
' Ξb* )
● Therefore lower state is below the → Ξb-π+ threshold
● Has to decay via neutral particle
→ Ξb0π0
→ Ξb0γ
Depending on how below threshold
LHCb-PAPER-2016-010
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Ωb → Ωc π
New!
LHCb-PAPER-2016-008
Measurement of the mass and lifetime of the Ωb baryon
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Introduction● Recent measurement of Λb Ξb
- Ξb0 lifetimes by LHCb have reached < 12% precision
● Naive Heavy Quark Effective Theory predicts lifetime hierarchy:
● Current experimental results dominated by LHCb:
● Ωb events are still rare all measurement statistically limited→• Low production in hadronic collisions• Presence of flying hyperons• So far only studied by LHCb in Ωb J/→ Ψ Ω
• Look for fully hadronic decays• All daughters in the VELO
● Perform a relative lifetime measurement with respect to similar decay
20142014
2014
Phys.Lett. B736 (2014) 154-162
LHCb-PAPER-2016-008
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Lifetime
● Idea: measure signal yield in lifetime bins● Used before in many analyses performed by LHCb
● Signal decay distribution: Exponentials ⊗ Acceptance function● Shape parameters fixed to full fit ● The efficiency corrected yield ratio is then:
● Fit data with a simple exponential ● Use known lifetime of the control mode to get an absolute value
PRL 109, 152002 (2012)PLB, 734, (2014) 122PRL 112, 111802 (2014)PRL 113, 172001 (2014)
LHCb-PAPER-2016-008
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Lifetime● Same selection for signal and control mode ● MVA selection● Reconstruct 60 signal event● Binning scheme optimized with pseudoexper● Check any possible bias● Ratio of efficiencies taken from MC
Ratio of yields
Preliminary
Preliminary
Phys.Lett. B736 (2014) 154-162
LHCb-PAPER-2016-008
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New!
Modelindependent analysis of Λb → J/Ψ p K-
LHCb-PAPER-2016-009
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Introduction● LHCb recently observed pentaquarks candidates in Λb → J/Ψ p K-
● Rich spectrum of * resonancesΛ● Pentaquark amplitudes required to describe the spectra
PRL 115 072001(arXiv:1507.03414)
● Can structures in J/ p be due to * resonances not accounted for in the model?ψ Λ● Same dataset as previous model dependent analysis model independent analysis→● 26000 signal events with 5.4% background● Essentially a 2D Dalitz plot analysis working under the assumption that only contributions from resonances decaying to Kp are present● Referred to as the H0 (i.e. “null”) hypothesis.● One can then demonstrate compatibility/incompatibility with the data.
LHCb-PAPER-2016-009
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Methodology● We work with the “rectangular” Dalitz variables mKp and cosθΛ ● Describe resonance structure in (mKp and cosθΛ) with Legendre polynomials● Detailed description of the analysis in the paper● Summarise here only the key points
Use PDG to get lmax
● Data is efficiency corrected● Same efficiency parametrization as in the amplitude analysis
● We generate a high statistics toy data sets to illustrate possible contributions to the moments from:
– statesΛ∗– Pc states
● No interpretations are imposed on the mKp distribution: interpolation
Preliminary
LHCb-PAPER-2016-009
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Null hypothesis● Null hypothesis pseudo experiments
● We can project the “null hypothesis” on mJ/Ψp distribution● Projection on mJ/ΨK also shows clear discrepancies
Preliminary
Preliminary
Preliminary
H0 null hyp H0 null hyp
LHCb-PAPER-2016-009
23rd March 2016 Paolo Gandini Moriond QCD 16
Summary● We have demonstrated that conventional Kp contributions fail to reproduce the data with minimal assumptions on their spins, and without any assumptions on their number, shapes or how they interfere.
● NonKp contributions must be present in the data, and can be either of – the exotic hadron type– or due to rescattering effects among ordinary hadrons.
● Quantitatively, we can reject this H0 hypothesis at over 9 sigma
● This analysis adds to the previous model dependent evidence for exotic pentaquark states from the amplitude analysis of the same data set
LHCb-PAPER-2016-009
23rd March 2016 Paolo Gandini Moriond QCD 17
New!
Observation ofΛb → Ψ(2S) p K- and Λb → J/Ψ π π p K-
and precise measurement of the Λb baryon massLHCb-PAPER-2015-060
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LHCb-PAPER-2015-060
Λb → Ψ(2S) p K- and Λb → J/Ψ π π p K-
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New!
Search for Bc decays to ppπ final stateLHCb-PAPER-2016-001
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● Charmless should decay via annihilation diagram→● Look below the charmonium threshold mpp < 2.85GeV● Use B+ as control mode, same final state● BDT selection fit in bins of BDT response→● B signal:
LHCb-PAPER-2016-001
Search for Bc decays to ppπ final state
Preliminary Preliminary
● No significant Bc signal is found● Set an upper limit
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New!
Tetraquark searches at LHCb in BsπLHCb-CONF-2016-004
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Tetraquark searches at LHCb in Bsπ
● D0 announced a new state on 24 Feb: X(5568) B→ sπ arXiv:1602.07588● CERN seminar 21 March (Monday)● High production rate and significance of 5.1sigma● Generated several citations investigation by other collaborations→
Nev = 5582 ± 100
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Tetraquark searches at LHCb in BsπLHCb-CONF-2016-004● Very large, clean Bs samples, different final states
46k66k
● Slightly different selection compared to what shown at Moriond QCD● Add a pion no peak observed at 5568 MeV→● Several cross check with B plus pion modes● We can put an UL on production
Resolution on mass < 1MeVBW + poly bkg
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Conclusions
● Presented only newest analyses performed by LHCb
● Confirmed the observation of Ξb∗0 , measured its width and production
● Performed lifetime analysis of Ωb baryon in hadronic final state
● Performed modelindependent analysis of Λb → J/Ψ p K-
conventional Kp contributions fail to reproduce the data
● No evidence of a tetraquark state in Bsπ at LHCb with 10x D0 statistics
New!
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Backup Slides
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Ξb ∗→ Ξb
-π+
Preliminary
23rd March 2016 Paolo Gandini Moriond QCD 27
Modeldependent analysis of Λb → J/Ψ p K-
● Λb → J/Ψ p K- used for precise lifetime measurement● Full dataset 26k events with 5% background→● Unexpected peak in J/Ψ p
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Modeldependent analysis of Λb → J/Ψ p K-
● 6D amplitude fit● Amplitude model includes all known Λ* states● Tried to fiit without J/ψp states● Adding further higher resonances and other cross checks● Fit without Pq:
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Modeldependent analysis of Λb → J/Ψ p K-
● Adding two pentaquarks● LHCb analysis added 2 Pc states● Good fit even with reduced Λ* set●
● Resonant behaviour● Preference for JP =(3/2-,5/2+)● Phase rotation visible● Need to confirm quantum numbers
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Crosschecks on Tetraquark