Baryons at BESIII LIU Beijiang Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS) June 7, 2017 GSI
• Introduction • Λ 𝑑 physics at BESIII • Baryon spectroscopy at BESIII
Beijing Electron Positron Collider (BEPCII)
Beijing Spectroscopy (BESIII) Detector ETOF (MRPC) upgraded ( σ T =55ps)
Features of the BEPC Energy Region Rich of resonances: charmonia and charmed mesons Threshold characteristics (pairs of , D, D s , …) Transition between smooth and resonances, perturbative and non-perturbative QCD Energy location of the gluonic excitations and multi-quark states
Physics at BESIII Charmonium physics: Open charm physics: - spectroscopy - charmed mesons – decay constant, form - transitions and decays factors – CKM matrix: Vcd, Vcs Light hadron physics: – D 0 -D 0 bar mixing and CP violation - meson & baryon – rare/forbidden decays spectroscopy - 𝚳 𝒅 - glueball, hybrid, multiquark Tau and QCD physics - two-photon physics - e.m. form factors of New physics nucleon
Data collected at BESIII
+ PHYSICS AT BESIII 𝜧 𝒅
Quark Model picture + : a heavy quark (c) with a unexcited spin-zero diquark (u-d) 𝜧 𝒅 u d c s c Charmed baryon ( Λ c [udc]) Charmed meson (D + [c 𝒆 ]) Strange baryons ( Λ [uds]) m u , m d << m c diquark + quark m u , m d ≈ m s (qqq) uniform m d << m c quark + heavy quark (qq) (Q) (q) (Q) Heavy Quark Effective Theory : diquark correlation is enhanced by weak Color Magnetic Interaction with a heavy quark More reliable prediction of heavy-light quark transition without dealing with light degrees of freedom that have net spin or isospin. + may provide complementary powerful test on internal 𝜧 𝒅 dynamics to charmed meson does
Cornerstone of charmed baryon Spectroscopy The lightest charmed baryon 5/2 + c (3080) Most of the charmed baryons c (2980) ? ? c (2940) + will eventually decay to 𝜧 𝒅 c (2880) c (2815) + is one of important The 𝜧 𝒅 3 c (2800) c (2790) c (2770) / tagging hadrons in c-quark 2 - counting in the productions at c (2625) c (2645) c (2595) high energies and bottom ’ c c (2520) baryon decays + → 𝐪𝐋 − 𝛒 + ) : dominant 𝑪(𝚳 𝒅 error for V ub via baryon decay
+ Decays The 𝚳 𝐝 PDG2014
+ weak Decays 𝚳 𝐝 Contrary to charm meson, receive sizable non-factorization W- exchange contribution Chau, HYC, Tseng 96 Two distinct internal W emission diagrams, three different W exchange diagrams Need information of decay asymmetry to extract s-wave and p- wave amplitudes separately 𝟏 → 𝑲/𝝎𝐪𝑳 − Exotic search in 𝚳 𝒅 + → 𝝔𝐪𝛒 𝟏 : an analog to Pc in 𝚳 𝒄
+ Data at BESIII 𝚳 𝐝 First time to run around 4.6 GeV in 2014, marvelous achievement of BEPCII available data set at BESIII PRL 101 (2008) 172001 Energy(GeV) lum.(1/pb) 4.575 ~48 4.580 ~8.5 4.590 ~8.1 4.600 ~567 + Measurement using the threshold pair-productions via e + e - Λ c annihilations is unique: the most simple and straightforward First time to systematically study charmed baryon at threshold!
Analysis Technique − + 𝛭 𝑑 − pair production at e + e - collision at mass threshold, 𝛭 𝑑 no additional hadron in final states Tagging method : − Single tag (ST) : reconstruct one 𝜧 𝒅 + − − pair − Double tag (DT) : fully reconstruct 𝜧 𝒅 + 𝜧 𝒅 Two important variables: Advantages: − Clean environment − Straightforward and model independent absolute BRs measurement − Some systematic uncertainties canceled in DT method
+ → l + n l Semi-Leptonic decay 𝚳 𝐝 ARGUS first measurement : Phys. Lett. B 269, 234 (1991). CLEO improved measurement : Phys. Lett. B 323, 219 (1994). Combined with the ( 𝛭 𝑑 + ) and the assumption of form factors PDG 2015 Not a direct measurement! Theoretical calculations on the BF ranges from 1.4% to 9.2%
+ → l + n l The measurement of 𝚳 𝐝 Double tag method 11 tag modes : ST yields: 14415 ± 159 events with 11 ST modes
+ → l + n l decay BFs of 𝚳 𝐝 First direct measurement, optimized variables : e n B[ c + m + n m ]=(3.49 0.46 0.26)% B ( ) (3.63 0.38 0.20)% c e Phys. Lett. B 767(2017)42 PRL115(2015)221805 Events /0.010 (GeV) 104 ± 11 79 ± 11 Events Events U miss (GeV) G [ c + m + n m ]/G [ c + e + n e ] 0.96 ± 0.16 ± 0.04 Important for test and calibrate the LQCD and lepton universality .
+ Cabibbo-Favored Absolute BFs of 𝚳 𝐝 Hadronic decays Signal Tag Variable : DT yields ST yields Very clean backgrounds!!! PRL 116, 052001 (2016)
Results of 12 CF hadronic BFs Straightforward and model independent PRL 116, 052001 (2016) A least square global simultaneous fit : [CPC 37, 106201 (2013)] + → 𝐪𝐋 − 𝛒 + : BESIII precision comparable with Belle’s 𝐂 𝜧 𝒅 + → 𝐪𝐋 − 𝛒 + is compatible with BELLE's with 2 𝞽 BESIII 𝐂 𝜧 𝒅 Improved precisions of the other 11 modes significantly
HFAG Fit to world BF data A fitter to constrain the 12 hadronic BFs and 1 SL BF, based on all the existing experimental data, overall 𝝍 2 /ndf=30.0/23=1.3 Correlated systematics are fully taken into account Precise 𝑪 𝒒𝑳 − 𝝆 + is useful for V ub measurement via baryonic mode
0 p + → nK S Observation of 𝚳 𝐝 + decays involving the neutron in final states . First observation of 𝛭 𝑑 PRL118(2016)112001 83 ± 11 The phase difference between I (0) and I (1) : B[ c 0 p + ]=(1.82 0.23 0.11)% + → nK S B[ c + → nK 0 p + ]/B[ c + → pK - p + ]=0.62 0.09 cos d = - 0.24 0.08 B[ c + → nK 0 p + ]/B[ c + → pK 0 p 0 ]=0.97 0.16 and relative strength: |I (1) |/|I (0) |= 1.14 0.11 The relative BF of neutron-involved mode to proton-involved mode is essential to test the isospin symmetry and extract the strong phases of different final states.
+ → - p p (p 0 ) Measurement of 𝚳 𝐝 + decay BFs is 65%, searching for more decay The total measured 𝜧 𝒅 modes are important + decay involved - is observed, B( 𝜧 𝒅 + → - p + p - )=(2.3 0.4)%, Only one 𝜧 𝒅 where - dominantly decay to n p - + tagged candidates 11 ST modes, 11415 ± 159 𝜧 𝒅 + - p + p + + - p + p + p 0 𝚳 𝐝 𝚳 𝐝 161.3 ± 15.2 88.1 ± 13.9 Events Events + - p p ] =(1.81 0.17)% [Improved precision] B[ 𝚳 𝐝 Statistical only, + - p p p 0 ]=(2.11 0.33)% [first observation] totally uncertainty <5% B[ 𝚳 𝐝
Single-Cabibbo-Suppressed decay of + → p p p - /K K - Λ c Sensitive to non-factorizable contributions from W-exchanged process PRL117(2016)232002 first observation improved precision
+ p p 0 and c SCS Decays c + p h • Their relative size essential to understand the interference of different non-factorizable diagrams arXiv:1702.05279 • It is expected that G ( c + p h )>> G ( c + p p 0 )
+ → +X The measurement of 𝚳 𝐝 The measurement is useful to test of HQET + → + 𝒀 ) = 35 11% PDG2016 B( 𝜧 𝒅 − − − → pK + p - − → pK S 𝚳 𝐝 𝚳 𝐝 0 + → 𝚳 + 𝒀 = 𝟒𝟕. 𝟘𝟗 ± 𝟑. 𝟐𝟗 % 𝓒 𝜧 𝑫
BARYON SPECTROSCOPY AT BESIII
Spectrum of Nucleon Resonances Particle Data Group **** *** ** * (Phys. Rev. D 86 , 010001 (2012)) N Spectrum 10 5 7 3 Many open questions left Δ Spectrum 7 3 7 5
Where are the “missing” baryons? Quark models predict many more baryons than have been observed ??? ??? 1 1 0 2 3 2 1 2 2 1 4 5 3 1
Where are the “missing” baryons? Are the states missing in the predicted spectrum because our models do not capture the correct degrees of freedom? … 4, multi quarks 3, quark-diquark 1, 3 quarks 2, quarks and flux tubes N predictied : N 4 >N 2 >N 1 >N 3 , N observed << N 1 Or have the resonances simply escaped detection? Nearly all existing data result from 𝜌𝑂 experiments
Excited state baryon spectroscopy from lattice QCD PR D84 074508 (2011)
Charmonium decays can provide novel insights into baryons and complementary information to other experiments Pure isospin 1/2 filter: 𝜔 → 𝑂 𝑂𝜌 , 𝜔 → 𝑂 𝑂𝜌𝜌 Missing N* with small couplings to 𝜌𝑂 & 𝛿𝑂 , but large coupling to gggN : 𝜔 → 𝑂 𝑂𝜌/𝜃/𝜃 ′ /𝜕/𝜚 , 𝑞Σ𝜌 , 𝑞Λ𝐿 … Not only N*, but also Λ ∗ , Σ ∗ , Ξ ∗ Gluon-rich eviroment: a favorable place for producing hybrid (qqqg) baryons Interference between N* and 𝑂 * bands in 𝜔 → 𝑂 𝑂𝜌 Dalitz plots may help to distinguish some ambiguities in PWA of 𝜌𝑂 High statistics of charmonium @ BES III
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