HIEPA2018, Beijing March 19-21, Two-photon Physics. Simon Eidelman

Transcription

1 Two-photon Physics Simon Eidelman Budker Institute of Nuclear Physics SB RAS and Novosibirsk State University, Novosibirsk, Russia Outline 1. Belle results for e + e e + e X 2. Transition form factors 3. Conclusions p.1/27

2 Basic Features of Two-Photon Collisions I e + e + γ X e - γ e - W γγ (X) c.m. energy, q 2 1, q momenta squared of virtual photons θ X polar c.m. angle with respect to e + e p.2/27

3 Basic Features of Two-Photon Collisions II σ(e + e e + e γ γ e + e f) α 4 log 2 E/m e compared to σ(e + e γ f) α 2 /E 2 Particles produced in γγ collisions have C = +1 and J P = 0,2 while those in single-photon annihilation have C = 1 and J P = 1 Special kinematics: Initial electrons tend to fly in their original directions and lose a small part of their energy The produced system of particles f has E tot s = 2E and tends to have small transverse momentum p.3/27

4 Balance of Transverse Momenta for γγ K 0 S K0 S Events No. of entries/0.01 GeV/c W= GeV Data before background subtraction MC γγ K 0 SK 0 S Background Data after background subtraction Σp ee t (GeV/c) p.4/27

5 Classification of γγ Experiments There are three different types of γγ experiments depending on whether or not initial electrons are detected: Both e ± not detected no tag (small q1,2, 2 quasireal photons) One e ± is detected single tag Both e ± detected double tag In some cases experiments have a dedicated tagging system (tagger) to detect outgoing e ± s (TPC-2γ, MD-1 in the past, KEDR in Novosibirsk, KLOE-2 in Frascati - today) Detectors with a large solid angle (CLEO, BaBar, Belle) can perform single-tag experiments, when one final e ± is detected p.5/27

6 γγ Studies before Belle In the 80-s SPEAR, PEP at SLAC, PETRA at DESY In the 90-s LEP at CERN, TRISTAN at KEK ARGUS at DESY and CLEO: high luminosity for γγ (0.5-1 fb 1 ) Resonance Γ γγ (B f ) for light mesons and charmonia First single-tag and σ(γγ hadrons) measurements Scaling laws for processes with large Q 2 using quark counting rules; predictions for σ(γγ M M) in pqcd For π + π the prediction is dσ W 6, for p p W 10 dcosθ Predictions of pqcd are asymptotic, but what energy is high enough? p.6/27

7 QCD Studies at Belle Final state Ldt, fb 1 W, GeV cosθ Reference π + π, < 0.6 H. Nakazawa et al., K + K PLB 615, 39 (2005) p p < 0.6 C.C.Kuo et al., PLB 621, 41 (2005) π 0 π < 0.8 S.Uehara et al., PRD 79, (2009) ηπ < 0.8 S.Uehara et al., PRD 80, (2009) ηη < 0.9 S.Uehara et al., < 1.0 PRD 82, (2010) KSK 0 S < 0.8 S.Uehara et al., PTEP 2013 (2013) 123C01 p.7/27

8 γγ π + π, K + K <0.6 ) * 1 (a)π + π - Belle ALEPH <0.6 ) * (b)k K Belle ALEPH σ 0[nb] ( cos θ 10-1 σ 0[nb] ( cos θ W[GeV] W[GeV] n = 7.9±0.4±1.5 for π + π and n = 7.3±0.3±1.5 for K + K, n = 6 possible Absolute cross sections not predicted! p.8/27

9 Conclusions of QCD Tests State n W, GeV π + π 7.9±0.4± K + K 7.3±0.3± p p K 0 S K0 S 11.0±0.4± π 0 π 0 8.0±0.5± ηπ ±1.2± ηη 7.8±0.6± n = 10 does not work for π 0 π 0, one more puzzle, nor for ηη, but here the W range is limited p.9/27

10 Charmonia in γγ K 0 S K0 S Events/10MeV χ c0 Belle fb -1 ( cosθ * <0.6) 40 χ c W (GeV) Events of γγ charmonia are selected without background p.10/27

11 Observation of New Charmonium-like States at Belle Final state Ldt, fb 1 W, GeV Reference D + D, D 0 D S. Uehara et al., PRL 96, (2006) J/ψω S. Uehara et al., PRL 104, (2010) J/ψφ C.P. Shen et al., PRL 104, (2010) p.11/27

12 γγ J/ψω Events/10 MeV W (GeV) Also observed by Belle and BaBar in B J/ψωK decays, the same as χ c2 (2P)? S. Uehara et al. (Belle Collab.), Phys. Rev. Lett. 104, (2010) J.P. Lees et al. (BaBar Collab.), Phys. Rev. D 86, (2012) p.12/27

13 Conclusions on Charmonium Studies Various decay modes of η c (1S), χ c0 (1P), χ c2 (1P) into two-, four- and six-body final states studied Two-photon width Γ 2γ B((c c) f), M, Γ measured, interference effects very important More precise branching fractions determined For η c (2S) new decay modes, in addition to K Kπ, found New charmonium and charmonium-like states seen p.13/27

14 Final state Studies of Light-Quark Mesons at Belle Ldt, fb 1 W, GeV Reference K + K K. Abe et al., EPJC 32, 323 (2003) f 0 (980) π + π T. Mori et al., PRD 75, (2007) ωω, ωφ, φφ Z.-Q. Liu et al., PRL 108, (2012) η π + π C.C. Zhang et al., PRD 86, (2012) η π + π Q.N. Xu et al., Preliminary In addition, systematic partial wave analysis was performed for the π + π, π 0 π 0, ηπ 0, ηη final states p.14/27

15 γγ K + K Belle (this experiment) TPC/Two-Gamma ARGUS ( cos q* <1) f 2(1525) and 3 more states at 1.7, 2.0 and 2.3 GeV (tensors?) K. Abe et al., Eur. Phys. J. C 32, 323 (2003) p.15/27

16 γγ ωω, ωφ, φφ Events / 0.04 GeV/ M(ω φ) (GeV/c ) Events / 0.03 GeV/c M(φφ) (GeV/c ) Events / 0.02 GeV/c M(ωω) (GeV/c ) Structures at 1.91 (ωω), 2.2 (ωφ), 2.35 GeV (φφ), 0 ++ or 2 ++ Z.-Q. Liu et al.(belle Collab.), Phys. Rev. Lett. 108, (2012) Theory: 1/W 6, experiment steeper Theory: σ(ωω) σ(φφ) σ(ωφ), correctly predicts σ(φφ), σ(ωφ) at 4 GeV, but in experiment σ(ωω) is too high, V. Chernyak, arxiv: p.16/27

17 γγ η π + π at Belle 2 Events / 45 MeV/c Data Total η (1S) η f (980) c 0+ - η (1S) η π π /two-body c η (1S) η f (1270) c 2 η (1S) η f (2080) c 0 (b) Events / Data PC J = 0 PC J = M(π + π )[GeV/c 2 2 ] cosθ hel Belle studied γγ η π + π and observed η c (1S), η c (2S) + f 0 (980), f 2 (1270), f 0 (2080) M = ±32 MeV, Γ = ±55 MeV Preliminary, to be submitted to Phys. Rev. D p.17/27

18 Transition Form Factors - I (General) P γγ, γe + e, e + e e + e, e + e, e + e Pγ, Pe + e, γe Pe, γγ P All of them probe F(q 2 1,q 2 2) in different q 2 i regions p.18/27

19 Transition Form Factors - II (π 0 ) Belle data do not confirm fast rise observed at BaBar B. Aubert et al. (BaBar Collab.), Phys. Rev. D 80, (2009), S. Uehara et al. (Belle Collab.), Phys. Rev. D 86, (2012) p.19/27

20 Transition Form Factors - III (η, η and π 0 ) (3/5)Q 2 F n (Q 2 ) (GeV) BABAR (γ γ π 0 ) CLEO* (γ γ η,η / ) CLEO (e + e - γη,γη / ) BABAR (γ γ η,η / ) BABAR (e + e - γη,γη / ) Q 2 (GeV 2 ) The u, d part of the meson distribution amplitude, η and η transition f/f follow QCD Belle is completing analysis of γγ π 0 π 0, Q 2 < 30 GeV 2, f 0 (980) and f 2 (1270) clearly seen p.20/27

21 Transition Form Factors - IV (γγ π 0 π 0 at Belle) TFF of f 0 (980) (1270) 2 helicity 0 TFF of f Q (GeV ) Q (GeV ) (1270) 2 1 (1270) 2 1 helicity 1 TFF of f 1 10 helicity 2 TFF of f Q (GeV ) Q (GeV ) Belle studied γγ π 0 π 0 at Q 2 < 30 GeV 2 and 0.5GeV < W < 2.1GeV in M. Masuda et al., Phys. Rev. D93 (2016) Theory I G.A.Schuler et al., Nucl. Phys. B523 (1998) 423 Theory II V. Pascalutsa et al., Phys. Rev. D85 (2012) p.21/27

22 Transition Form Factors - V (γγ K 0 S K0 S at Belle) 1 helicity (1525) 2 helicity-i TFF of f helicity-1 helicity-2 Belle studied γγ K 0 S K0 S at Q 2 < 30 GeV 2 and 1.0 < W < 2.6GeV in M. Masuda et al., Phys. Rev. D97 (2018) Theory G.A.Schuler et al., Nucl. Phys. B523 (1998) Q (GeV ) p.22/27

23 Status of P l + l Decay Searches Decay mode B exp Events Group B unit.bound π 0 e + e (6.46±0.33) KTEV, η e + e < HADES, η µ + µ (5.7±0.9) SATURNEII, η e + e < CMD-3, K 0 L e + e ( ) B871, K 0 L µ + µ (6.84±0.11) B871, B s can be enhanced by photon virtuality and transition f/f All results but CMD-3 were obtained using hadron beams CMD-3 searched for the inverse reaction e + e η ηπ + π p.23/27

24 Search for C-even resonances in e + e Direct production of C-even states in e + e is possible via a γγ: The unitarity bound (UB) assuming 2 real photons is B P l + l = B P γγ α2 2β ( m e m P ) 2 [ln( 1+β 1 β )]2,β = 1 4( m e m P ) 2. Standard mechanism via e + e e + e P involves two almost real photons and provides Γ(P γγ) only p.24/27

25 Search for e + e η with CMD-3 I CMD-3 repeated a search for the process e + e η (958) ηπ + π, η 2γ using Ldt = 2.69 pb 1 collected with the CMD-3 detector at the VEPP-2000 c.m. energy E c.m. m η = ±0.06 MeV/c 2 The total width of the η is rather small, (198±9) kev, it is very important to have c.m. energy close to this value. The collider beam energy was continuously monitored during the whole period of data taking (12 days) using the Back-Scattering-Laser-Light system providing the accuracy of R.R. Akhmetshin et al., Phys. Lett. B 740, 273 (2015) p.25/27

26 From the absence of the signal Search for e + e η with CMD-3 II Γ η e + e B η ππηb η γγ < ev at 90% C.L.. and with B η ππη and B η γγ from PDG: Γ η e + e < ev Group ND, 1988 CMD-3, 2014 Γ η e + e, ev < 0.06 < Γ η, kev ±9 B η e + e,10 8 < 21 < 1.2 Much more stringent than that of ND, but still 300 times higher than the unitarity bound p.26/27

27 Conclusions e + e e + e + hadrons is easily studied at e + e colliders γγ physics is quite rich: two-photon widths, spectroscopy of light-quark mesons and charmonia, QCD tests, transition f/f in γγ R, J PC (R) = 0 +, 0 ++, 1 +, 2 ++ Resonance studies are very sensitive to interference with non-resonant continuum Studies of TFF are in progress at MAMI, JLAB, VEPP-2000, BEPC-II, Julich,..., can be also studied via R e + e, e.g. in e + e cc f Taggers provide much broader possibilities for γγ and γ γ γγ physics is very promising for various QCD studies: resonance studies test various models (potential, tetraquark, molecule), energy and angular dependence of cross sections pqcd Further theoretical and experimental efforts needed p.27/27

28 Backup slides p.28/27

29 γγ π 0 π 0 at Belle p.29/27

30 Muon (g 2) 2 I (Comparison to Experiment) Contribution a µ,10 10 Experiment ±5.4±3.3(6.3) tot QED ± Electroweak 15.4 ± 0.1 ± 0.2 Hadronic 692.3±4.2±2.6±0.2(4.9) tot Theory ± 4.9 Exp. Theory 28.7 ± 8.0 (3.6σ) The difference between experiment and theory is ( )σ! Experiment: G.W. Bennett et al., Phys. Rev. D 73, (2006) p.30/27

31 e + e η ηπ + π E beam, MeV time, days Measurements of the beam energy show good stability of the collider energy. The average value of the c.m. energy is E av. c.m. = ±0.014 MeV with a few deviations of up to 0.2 MeV, corresponding to less than 5% of the integrated luminosity, which are still within an energy spread of the collider The collider beams have an energy spread mainly due to the quantum effects. For VEPP-2000 the c.m. energy spread σ Ec.m. = (0.246±0.030) MeV p.31/27

32 Search for e + e η with CMD-3 III p.32/27

33 Particle Production at B Factories Production from B-decay (broadd, D sj, X(3872), Y(3940)) e - e + Υ(4S) b u _,d _, b _ B - u,d, B X Production from continuum (D sj, η c (2S), X(3940), Σ(2800)) e + u _,d _,s _,c _ e - u,d,s,c e + e + X Two-photon production (η c (2S), χ c2 (2P)) e - γ γ X e - γ e + Initial state radiation (Y(4260), Y(4360), Y(4660)) e - X p.33/27

34 γ VP I σ (fb) a) b) σ (fb) c) d) s (GeV 2 ) s (GeV 2 ) (a) φη, (b) φη, (c) ρη, (d) ρη Solid 1/s 4, dashed 1/s 3 p.34/27

35 γ VP II Mode Belle [3] [4] [5] BaBar [2] φη 1.4±0.4± ±0.5±0.1 φη 5.3±1.1± φη 3.1 ± 0.5 ± φη 3.3±0.6± G.S. Adams et al. (CLEO) Phys. Rev. D 73, (2006) [1] B. Aubert et al. (BaBar) Phys. Rev. D 74, (2006) [2] K. Belous et al. (Belle) Phys. Lett. B 681, 400 (2009) [3] C.D.Lu et al. (Light cone) Phys. Rev. D 75, (2007) [4] V.V. Braguta et al. (Light cone) Phys. Rev. D 78, (2008) [5] p.35/27

36 γγ π + π, K + K I For all final states Belle surpassed the previous measurements both in statistics and the quality of detection For γγ π + π, K + K Belle used a data sample of 87.7 fb 1 to study angular dependence, energy behavior and the ratio σ(π+ π ) σ(k + K ) H. Nakazawa et al., Phys. Lett. B 615, 39 (2005) The best previous experiment (ALEPH) had a data sample of pb 1 A. Heister et al., Phys. Lett. B 569, 140 (2003) p.36/27

37 γγ π + π, K + K II * -1 dσ/d cosθ σ GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV GeV cosθ * π + π - + K - K sin θ * p.37/27

38 γγ π + π, K + K III <0.6 ) * σ 0[nb] ( cos θ (a)π + π - Belle ALEPH <0.6 ) * σ 0[nb] ( cos θ (b)k K Belle ALEPH σ 0 (KK) /σ 0 (ππ) (c) W[GeV] W[GeV] W[GeV] p.38/27

39 γγ p p II Power corrections are still significant Diquark and handbag models need improvement p.39/27

40 γγ 4 Charged Tracks I Events/10 MeV/c 2 Events/10 MeV/c 2 (a) 4p M(4p) (GeV/c 2 ) (c) 4K (b) 2K2p M(2K2p) (GeV/c 2 ) M(4K) (GeV/c 2 ) S. Uehara et al. (Belle Collab.), Eur. Phys. J. 53, 1 (2007) p.40/27

41 γγ 4 Charged Tracks II Events/10 MeV/c 2 Events/10 MeV/c2 Events/10 MeV/c 2 c 4 (a) c0 4 (b) c2 4 M(4 ) (GeV/c 2 ) c 2K2 (d) c0 2K2 (e) c2 2K2 M(2K2 ) (GeV/c 2 ) c 4K (g) c0 4K (h) c2 4K (c) (f) (i) M(4K) (GeV/c 2 ) p.41/27

42 Four-prong Final States Events/10 MeV/c 2 Events/10 MeV/c 2 (a) 4p M(4p) (GeV/c 2 ) (c) 4K (b) 2K2p M(2K2p) (GeV/c 2 ) M(4K) (GeV/c 2 ) In each of the three final states 2(π + π ), π + π K + K, 2(K + K ) three charmonia η c, χ c0 and χ c2 are clearly seen They also study dynamics and see: η c K 0 K 0, f 2 f 2, φφ, f 2 f 2 But η c (2S) not seen, the only mode observed is K Kπ p.42/27

43 Six-prong Final States and η c (2S) I Preliminary Belle results with 923 fb 1 : 3(π + π ) 6π, K + K 2(π + π ) 2K4π, 2(K + K )π + π 4K2π, K 0 S K± π π + π K 0 S K3π η c (1S), χ c0 and χ c2 are also clearly seen p.43/27

44 Six-prong Final States and η c (2S) - II Mode M, MeV/c 2 Γ, MeV N ev S, σ Γ γγ B, ev 6π ±1.6± ± ± ±3.7±3.2 2K4π ±1.6± (< 13) 407± ±2.3±3.4 K S K3π ±1.8± ± ± ±3.9±3.7 K + K 3π ±3.2± (fixed) 1201± ±6.0±5.0 Averaging Belle results over 3 modes of η c (2S): Mass = ±1.1±2.5±5.0 MeV, Width = 9.9±3.2±2.6±2.0 MeV consistent with Belle results on B ± K ± (K S Kπ) 0 : Mass = MeV, Width = MeV and with BaBar results on K + K 3π BaBar: P. del Amo Sanchez et al., Phys. Rev. D84, (2011) p.44/27

45 γγ J/ψφ Entries/25 MeV/c M(φJ/ψ) (GeV/c 2 ) The Y(4140) and Y(4274) states of CDF not seen (also disproved by LHCb) C.P. Shen et al., Phys. Rev. Lett..104, (2010) p.45/27

46 Observation of New Charmonium-like States at Belle Final state Ldt, fb 1 W, GeV Reference D + D, D 0 D S. Uehara et al., PRL 96, (2006) J/ψω S. Uehara et al., PRL 104, (2010) J/ψφ C.P. Shen et al., PRL 104, (2010) p.46/27

47 γγ D + D, D 0 D0 and χ c2 (2P) (a) D 0 D 0 Events/10 MeV/c 2 Events/10 MeV/c 2 (b) D + D - (c) combined M(DD) (GeV/c 2 ) Compatible with χ c2 (2P), confirmed by BaBar p.47/27

48 γγ J/ψω Events/10 MeV W (GeV) Also observed by Belle and BaBar in B J/ψωK decays, the same as χ c2 (2P)? S. Uehara et al. (Belle Collab.), Phys. Rev. Lett..104, (2010) p.48/27

49 γγ J/ψφ Entries/25 MeV/c M(φJ/ψ) (GeV/c 2 ) The Y(4140) and Y(4274) states of CDF not seen (also disproved by LHCb) C.P. Shen et al., Phys. Rev. Lett..104, (2010) p.49/27

50 γγ ωω, ωφ, φφ II Events/0.01 GeV/c M(ω φ ) (GeV/c ) Events / 0.01 GeV/c M(φφ) (GeV/c ) Events / 0.02 GeV/c M(ωω) (GeV/c ) p.50/27