Photoproduction of the f 1 (1285) Meson

Transcription

1 Photoproduction of the f 1 (1285) Meson Reinhard Schumacher Ph.D. work of Ryan Dickson, completed 2011 October 21, 2015, CLAS Collaboration meeting

2 Outline What are the f 1 (1285) and η(1295) mesons? Identification of the state in CLAS/g11 Final results for: Mass and Width Differential cross sections model comparisons Branching ratios ηππ, γρ 0, Κ Κ π Dalitz plot analysis spin and parity determination clue to production mechanism (only really new topic since 2011) CLAS publication October 2015 R. A. Schumacher, Carnegie Mellon University 2

3 The Players: f 1 (1285) I G (J PC ) = 0 + (1 + + ) Well-established axial-vector meson seen in several hadronic reactions; ~7000 events reported world-wide seen in PWA analyses Possible dynamically generated KK* c.c. state η(1295) I G (J PC ) = 0 + (0 - + ) A controversial state seen in π - pà π π π n ~12000 events analyzed Seen only in PWA, e.g. J. Manak et al., E852/BNL October 2015 R. A. Schumacher, Carnegie Mellon University 3

4 Quark Model for Mesons PDG 2014 October 2015 R. A. Schumacher, Carnegie Mellon University 4

5 Dynamically Generated Mesons The f 1 (1285) as a {KK* + c.c.} composite state Chiral Lagrangian + unitarization of the pseudoscalar vector meson nonet interaction Lattice calculations Expect non-standard production mechanisms, if true M. F. M. Lutz and E. E. Kolomeitsev Nucl Phys A , (2004) L. Roca, E. Oset, J. Singh Phys Rev D72, (2005) F. Aceti, Ju-Jun Xie, E. Oset, arxiv: (2015) October 2015 R. A. Schumacher, Carnegie Mellon University 5

6 Properties of f 1 (1285) and η(1295) f 1 (1285) η(1295) CLAS m x I G (J PC ) 0 + (1 + + ) 0 + (0 - + ) TBD Mass (MeV) ± ± 4 TBD Width, Γ (MeV) 24.2 ± ± 5 TBD Decays: 4 π 33 ± 2% - - η π π 52 ± 2% Seen TBD à a 0 (980)π, noa 0 KK 36 ± 7% Seen TBD à η π π, Excluding a 0 π 16 ± 7% - TBD KKπ 9.0 ± 0.4% - TBD γρ ± 1.3% Not Seen TBD October 2015 R. A. Schumacher, Carnegie Mellon University 6

7 One Reaction Topology (example) γ π + p m x a 0 - p (η) π - red = measured particles October 2015 R. A. Schumacher, Carnegie Mellon University 7

8 First Observation of the f 1 (1285) and/or η(1295) in Photoproduction η x(1280) Which state is it? f 1 (1285), η(1295), or both? Great statistics in CLAS g11 data: ~1.5 x 10 5 x(1280) events October 2015 R. A. Schumacher, Carnegie Mellon University 8

9 Two yield extraction methods W = 2.35 GeV Voigtian lines shape using known CLAS resolution Convolution of BW and Gaussian W = 2.35 GeV Monte Carlo fitting using signal and estimated multi-pion backgrounds p ρ π π (green) p φ 1 (1370) (purple) p x(1280) (red) - signal Total (blue) MM(γ,p) GeV October 2015 R. A. Schumacher, Carnegie Mellon University 9

10 Two x à K K π decay modes Number of Events MM(γ,p) GeV Number of Events MM(γ,p) GeV Number of Events < Cos Θ x CM < W = 2.45 GeV MM(γ,p) GeV Detect p K + π - (K 0 ) (left) Detect p K - π + (K 0 ) (right) Combine channels prior to yield extraction using Voigtian + polynomial October 2015 R. A. Schumacher, Carnegie Mellon University 10

11 Looking for γρ 0 decays Number of Events MM(γ,p) GeV γ ρ,ω η - p p π + π? (γ) Kinematic fit to γpà pπ + π - (γ) Select p perp > 40 MeV/c 2 nd kin. fit to γpà pπ + π - (π 0 ) to reject π 0 background Very small signal: only extract branching ratio to η π + π - Sum over all kinematics Number of Events Number of Events MM(γ,p) GeV MM(γ,p) GeV October 2015 R. A. Schumacher, Carnegie Mellon University f 1 (1285)

12 Cross-check η cross section π - ) + dσ / dω (nb/sr) ( η π π - ) + π dσ / dω (nb/sr) ( η W=2.35 GeV cos(θ CM ) W=2.65 GeV Compare two CLAS analyses of η photoproduction Same data set using different methods Red: Krahn & Williams Black: Dickson (this work) Good agreement Use difference as a measure of systematic uncertainty (histogram) (Note log scale) cos(θ CM ) October 2015 R. A. Schumacher, Carnegie Mellon University 12

13 Results October 2015 R. A. Schumacher, Carnegie Mellon University 13

14 Mass & Width Measurement October 2015 R. A. Schumacher, Carnegie Mellon University 14

15 Mass & Width Measurement Mass consistent with PDG value for f 1 (1285) Width is smaller than PDG by several σ but we are right October 2015 R. A. Schumacher, Carnegie Mellon University 15

16 Cross Section vs.angle and W Differential cross-section to ηπ + π - final state not scaled since we don t measure total rate Systematics Very weak forward peaking seen drop at most forward angles October 2015 R. A. Schumacher, Carnegie Mellon University 16

17 Comparison with Models N. Kochelev model Effective Lagrangian t-channel ρ and ω exchange Solid: f 1 (1285) Dashed: η(1295) Dotted: sum Poor match to data N. Kochelev, M. Battaglieri, R. DeVita, Phys. Rev. C 80, (2009) October 2015 R. A. Schumacher, Carnegie Mellon University 17

18 More models Solid red: Effective Lagrangian with meson exch. (same as previous page) Dashed: Effective Lagrangian with meson exch. Uncontrolled hadronic form factor cut-offs J-J. Xie (unpublished, private comm.) Dotted: Holographic QCD model S. Domokos: meson exchange with specific recipe to compute couplings S. Domokos et al., Phys. Rev. D 80, (2009) N. Kochelev et al., Phys. Rev. C 80, (2009) October 2015 R. A. Schumacher, Carnegie Mellon University 18

19 Compare Mesons: f 1 and η Compare f 1 (1285) and η W=2.55 GeV Red: CLAS η Black: CLAS f 1 (1285) Note logarithmic scale f 1 (1285) is produced flatter than the η Clue about production: not meson-exchange dominated like the η October 2015 R. A. Schumacher, Carnegie Mellon University 19

20 Branching Ratios Κ Κ π / η π π ratio agrees with PDG average (isospin factors applied) γ ρ 0 / η π π ratio smaller than PDG average by 55% October 2015 R. A. Schumacher, Carnegie Mellon University 20

21 From decay: find spin-parity x a 0 ± + π π ± x J P L 1L f 1 : p-wave decay η: s-wave decay x ˆx Θ a 0 π ŷ ẑ Adair system a 0 γ cm.. Θ x proton proton ˆx ŷ ẑ October 2015 R. A. Schumacher, Carnegie Mellon University 21

22 From decay: find spin-parity x a 0 ± + π J P L 1L f 1 : p-wave decay η: s-wave decay γ ˆxʹ ŷʹ N * ẑʹ x N π ± * cm.. Θ x xˆ ' ŷ ẑʹ proton x Θ H p a 0 Helicity system proton ˆx ŷ ẑ - tests s-channel helicity conservation October 2015 R. A. Schumacher, Carnegie Mellon University 22

23 From decay: find spin-parity x a 0 ± + π J P L 1L γ x γ π ± xˆ '' ŷ zˆ '' x Θ GJ a 0 Gottfried-Jackson system ρ ρ ˆx ŷ ẑ proton proton - tests t-channel helicity conservation October 2015 R. A. Schumacher, Carnegie Mellon University 23

24 Dalitz analysis of x à η π + π - 2 ) GeV 1.4 Raw Event Distribution (η,π 2 M MM(γ,p) GeV M (η,π + ) GeV Event-by-event, we must rescale meson sidebands to lie within Dalitz plot contour Algebraic method developed to do this projection October 2015 R. A. Schumacher, Carnegie Mellon University 24

25 Dalitz analysis of x à η π + π - 2 ) GeV - (η,π 2 M Background Subtracted a ) GeV - (η,π 2 M Acceptance Corrected - a a M (η,π + ) GeV a M 2 (η,π + ) GeV Subtract huge multi-pion background to reveal dominance of decay via a ± 0 π intermediate state. Strong interference of bands seen. Amplitude analysis! October 2015 R. A. Schumacher, Carnegie Mellon University 25

26 Dalitz analysis of x à η π + π - π 2 ) GeV - (η,π 2 M a 0 + a cos! Θ = +1 π + π - back-to-back cos! Θ = 1 π + π - together M 2 (η,π + ) GeV π + π x x!θ η!θ η a 0 π + a + 0 -two amplitudes A m=±1 (m a0,m + π a0 ) π + to sum for each event October 2015 R. A. Schumacher, Carnegie Mellon University 26

27 Dalitz analysis of x à η π + π - A m=±1 (m a0 + π,m a0 π + )= BW(m a0 + π )W 1,±1 (Θ a0 + π,φ a0 + π )+BW(m a0 π + )W 1,±1 (Θ a0 π +,φ a0 π + ) A m=0 (m + a0,m π a0 )= BW(m π + + a0 )( W π 1,0 (Θ + a0,φ π + a0 )+W ) +BW(m π 0,0 )( W a 0 π + 1,0 (Θ a0,φ π + a0 )+W ) π + 0,0 f 1 : BW (m m 0,Γ 0 ) = m 0 Γ 0 m 0 2 m 2 im 0 Γ 0 q(m) q(m 0 ) W L=1, m=0,±1 (Θ H,φ) = a Y 1,+1 (Θ H,φ) + b Y 1,0 (Θ H,φ) + a Y 1, 1 (Θ H,φ) η : W L=0, m=0 (Θ H,φ) = c Y 0,0 - a Breit Wigner for each a 0 - angular distribution in the selected system ( ) = q(m a0 T m a0,m + π a0 π +! + π ) q(m 0 ) q(m a0 ) π + q(m 0 ) -total decay-weighted magnitude squared ( A m=±1 m + a0,m π a0 π + October 2015 R. A. Schumacher, Carnegie Mellon University 27 ) 2 ( + A m=0 m + a0,m π a0 π + ) 2

28 Dalitz analysis of x à η π + π - Fold data on symmetry axis Generate phase space Monte Carlo events with finite width of meson and CLAS resolution included Weight the events with amplitudebased intensity October 2015 R. A. Schumacher, Carnegie Mellon University 28

29 Dalitz analysis of x à η π + π - Fit to full plot did not converge, so trim data to focus on bands. Structure in unweighted Monte Carlo due to finite width and resolution effects October 2015 R. A. Schumacher, Carnegie Mellon University 29

30 Helicity system fit DATA MONTE CARLO October 2015 R. A. Schumacher, Carnegie Mellon University 30 s-channel helicity system Components: Blue: L=1, m=0 Green: L=1, m=±1 Red: Total a 0 ± interference reproduced p-wave decay and negative parity demonstrated Decaying meson is definitely the f 1 (1285)

31 Helicity system fit The f 1 (1285) is aligned in the helicity system. The mix of m = 0 and m= ± 1 is a property of the production mechanism in the range 2.30 < W < 2.80 GeV. Discuss later We also measure the ratio Agrees with PDG value October 2015 R. A. Schumacher, Carnegie Mellon University 31

32 Gottfried-Jackson system fit t-channel helicity system Components: Blue: L=1, m=0 Green: L=1, m=±1 Red: Total Cyan: L=0 fit a 0 interference NOT reproduced Decaying meson is not aligned in this system October 2015 R. A. Schumacher, Carnegie Mellon University 32

33 Properties of f 1 (1285) and η(1295) f 1 (1285) η(1295) CLAS m x I G (J PC ) 0 + (1 ++ ) 0 + (0 -+ ) J P = 1 + Mass (MeV) ± ± ± 0.8 Width, Γ (MeV) 24.2 ± ± ± 1.4 Decays: 4π 33 ± 2% - - η π π 52 ± 2% Seen - Γ(a 0 π (nokk)) Γ(ηππ (total)) Γ(KKπ ) Γ(ηππ ) Γ(γρ 0 ) Γ(ηππ ) 69 ± 13% - 74 ± 9% 17.1 ± 1.3% ± 3.1% 10.5 ± 2.2% Not seen 4.7 ± 1.8% October 2015 R. A. Schumacher, Carnegie Mellon University 33

34 Speculation re f 1 (1285) production Alignment in helicity system suggests s- channel N* production decays to f 1 (1285) p Can we infer J P of the N* baryon resonance? 3/2 + à ½ + in s-wave leads to P ± : P 0 = 1 : 2 as seen in the data ½ + à ½ + in s-wave leads to P ± : P 0 = 2 : 1, opposite to what data show But there are no known N* states with low J at W~ 2.5 GeV, so the question remains open October 2015 R. A. Schumacher, Carnegie Mellon University 34

35 Conclusions The photoproduced meson CLAS sees at 1281 MeV is the f 1 (1285). The production mechanism is more consistent with N*-decay (s-channel) than mesonexchange (t-channel) Cross section is much flatter than η production The f 1 (1285) is aligned in the s-channel helicity system, as seen in the η π + π - Dalitz-plot amplitude analysis Γ ~ 18.2 MeV; narrower than PDG average Branching ratios measured: K K π / η π π, a 0 π / η π π and γ ρ 0 / η π π October 2015 R. A. Schumacher, Carnegie Mellon University 35

36 Backup Slides October 2015 R. A. Schumacher, Carnegie Mellon University 36

37 Compare meson types At equal W October 2015 R. A. Schumacher, Carnegie Mellon University 37

38 Compare meson types At equal excess energy above threshold October 2015 R. A. Schumacher, Carnegie Mellon University 38