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Scalar Mesons Physics with the KLOE detector

This paper discusses the status of the KLOE experiment and its contribution to understanding scalar mesons, such as f0(980), f0(600), and a0(980), at a f-factory. The results of various analysis and measurements are presented, including the detection of radiative decays and the determination of relevant couplings. The study also explores the unreducible backgrounds and their impact on signal extraction.

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Scalar Mesons Physics with the KLOE detector

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  1. Scalar Mesons Physics with the KLOE detector C.Bini Universita’ “La Sapienza” and INFN Roma For the KLOE collaboration Outline 1. Status of the KLOE experiment 2. “Hadron Physics” at af- factory 3. Scalar Mesons at a f- factory : 4. Results 4.1p+p-g 4.2p0p0g 4.3hp0g 5. Summary and perspectives

  2. 1) Status of the KLOE experiment These days: end of f –peak run  2 fb-1 collected in the last two years. ANALYZED Program now: energy scan around the f + off-peak run s = 1 GeV: KLOE end by spring 2006

  3. 2) “Hadron Physics” at a f - factory Preview: analysis done and in progress (non Kaon physics)

  4. Mass (MeV/c2) f(1020) 1000 a0(980) f0(980) K*0(800) “k” 500 f0(600) “s” 0 I=0 I=1/2 I=1 3) Scalar Mesons at a f - factory How a f-factory can contribute to the understanding of the scalar mesons Scalar Mesons Spectroscopy: f0(980),f0(600) and a0(980) are accessible (knot accessible) through f  Sg; Questions: 1. Is f0(600) needed to describe the mass spectra ? 2. “couplings”of f0(980) and a0(980) to f |ss>and to KK, pp and hp.  4-quarkvs.2-quarkstates

  5. How to detect these radiative decays •  f0(980)g  p+p-g  p0p0g  K+K-g [ 2m(K)~m(f0)~m(f) ] expectedBR ~ 10-6  K0K0g ““ ~ 10-8 • a0(980)g  hp0g  K+K-g expected BR ~ 10-6  K0K0g  “ ~ 10-8 •  f0(600)g  p+p-g  p0p0g General Comments:  fits of mass spectra are needed to extract the signals: this requires a parametrization for the signal shape;  the unreducible background is not fully known: a parametrization is required and some parameters have to be determined from the data themselves;  sizeable interferences between signaland background;

  6. How to extract the signal: • 1. Electric Dipole Transitions: •  G(E1)  Eg3× |Mif(Eg)|2 • 2.Distortions due to KK • thresholds (Flatte’-like). Kaon-loop(by N.N.Achasov): for each scalar meson S: gSpp, gSKK, MS No-Structure (by G.Isidori and L.Maiani): a modified BW + a polynomial continuum: gfSg, gSpp, gSKK, MS + pol. cont. parameters Scattering Amplitudes (by M.E.Boglione and M.R.Pennington) A  (a1+b1m2+c1m4) T(pppp) + (a1+b1m2+c1m4) T(ppKK)  pole residual gf

  7. p+ g Kaon-loop No-structure f0,a0 p+ p- f0,a0 K+ f f K- g p- Definition of the relevant couplings (S=f0 or a0): S to fgfSg (GeV-1) S to kaons gSKK=gSK+K-=gSK0K0 (GeV) f0 to pp (I=0) gf0pp=√3/2 gf0p+p-=√3 gf0p0p0(GeV) a0 to hp (I=1) ga0hp(GeV) Coupling ratio Rf0=(gf0KK/ gf0p+p-)2 Ra0=(ga0KK/ ga0hp)2

  8. The unreducible backgrounds Unreducible backgrounds: (p+p-): huge backgrounds: Initial state radiation (ISR) Final state radiation (FSR) fr±p±with r ± p±g (p0p0): large backgrounds: e+e- wp0withw  p0g fr0p0withr 0 p0g (hp0): small backgrounds: e+e- wp0withw  hg fr0p0withr 0 hg (p+p-) vs. (p0p0): search for the “same amplitude” with a completely different background ! (hp0) is the “cleanest” sample

  9. (“trackmass”) pions muons 4.1) Thep+p-ganalysis I - Event selection: (a) 2 tracks withqt>45o; missing momentumqpp>45o (Large Angle); (b) Each track is pion-like (tracking, ToF and Shower shape): (c) 1 photon matching the missing momentum Particle identification: p vs. e and m (Likelihood: Tof and Shower shape) pions, muons electrons

  10. m(pp) spectra: (blue) “Small angle”qpp<15o; (red) “Large angle”qpp>45o; “Large angle”: clear f0(980) signal f0(980) region photon efficiency m(pp) (MeV) m(pp) (MeV) II – The data sample 6.7 ×105 events / 350 pb-1 @ √s = Mf 2.2 ×104 events / 11 pb-1 “off-peak”

  11. KL and NS fits: • Good description in both cases of signal and background (KS);  “negative” interference; • f0(600) doesn’t help. KL fit NS fit III - Fit to the m(pp) spectrum (491 bins, 1.2 MeV wide, 420 to 1009 MeV) F= ISR + FSR + rp + scalar ± interference

  12. Parameter uncertainties are dominated by the systematic errors: • Comments: • Mass value OK [ PDG 980 ± 10 MeV ] • R > 1 in both fits (in agreement with published values p0p0g) • KL couplings >> NS couplings: effect of polynomial continuum • NS suggests “large” coupling to the f (see following)

  13. Scattering Amplitude Fit gf= 6.6 ×10-4 BR(ff0(980)g) × BR(f0(980)p+p-) ~ 3 ×10-5 [ similar conclusion from BP analysis of p0p0g data (KLOE + SND)] Summarizing: The peak at ~980 MeV is well interpreted in both KL and NS approaches as due to the decay f f0(980)g with a negative interference with FSR. The couplings suggest the f0(980) to be strongly coupled to kaons and to the f. No space for f0(600). Scattering Amplitude gives a marginal agreement.

  14. Pion polar angle distributions (Red) =p+ (Blue) =p- Effect of the scalar amplitude on the charge asymmetry: Plot of A in slices ofm(pp); Comparison with simulation with and without the scalar amplitude. IV - The Forward-Backward asymmetry: A = (N(q+>90o) –N(q+<90o)) / sum p+p-system: A(ISR)C-odd A(FSR) & A(scalar) C-even Cross-section: |A(tot)|2 = |A(ISR)|2 + |A(FSR)|2 + |A(scalar)|2 + 2Re[A(ISR) A(FSR)] + 2Re[A(ISR) A(scalar)] + 2Re[A(FSR) A(scalar)]

  15. FB asymmetry vs. m(pp):  Clear signal ~ 980 MeV  Interesting comparison with simulation: Data Simulation FSR+ISR Simulation FSR+ISR+ scalar(KL) The simulation provides a “qualitative” description of: • f0(980) region behaviour (the signal is reproduced); • Low mass behaviour (low mass tail of the signal. Remarkable result: not a fit but an absolute prediction

  16. V –Cross section dependence on √s: Absolute prediction based on KL fit parameters Data: on-peak Data: off-peak KL absolute prediction Based on fit parameters Concluding remark: p+p-g is a powerful tool to test scalar production: mass spectrum, FB asym. and s dependence KLOE has now collected 2 fb-1 at f  factor 6 more. Is now starting a finer energy scan around the f

  17. 2002 analysis scheme: 1. Rejectwp0events  interference is neglected; 2. 1-dim analysis: fit with KL The spectrum is dominated by w  p0g KLOE PLB537 (2002) 21 4.2) Thep0p0ganalysis I - event selection: 5 photons withqg>21o; no tracks; Kinematic fit  energy-momentum conservation; Kinematic fit p0masses: choice of the pairing.  4 ×105 events / 450 pb-1

  18. New analysis scheme: • Removed the m(w) cut :wp0are nowin the sample • 2. Bi-dimensional analysis [ Dalitz-plotm(p0p0) –m(p0g) ] • New treatment of systematics [ pairing problem...] • Improved VDM parametrization ofwp0

  19. II –Fit of the Dalitz plot (still preliminary results) KL and NS fits in progress Residuals vs. DP position Data- fit comparison (on projections) Comments: 1. VDM part still not perfect (see residuals); 2. Scalar part ok BUT f0(600) is still needed in KL fit[p(c2) ~ 10-4  30% !]; 3. f0(980) parameters agree withp+p-ganalysis again R > 1 (gfKK > gfp+p-).

  20. 4.3) Thehp0ganalysis I –The data samples: out of 400 pb-1 : Statistics of PLB536 (2002) 209× 20 • (h gg) Improved reducible • background subtraction: • 2.2 ×104 events [ ½ are signal] (h  p+p-p0) almost “background free”  4100 events [ bck < 3%] Red = signal Other colors= bck Full pts. = “20 pb-1” data Empty pts. = “400 pb-1” data M(hp) (MeV)

  21. II – The combined fit Simultaneous fit ofhggand hp+p-p0channels: ratio of BR is fixed Pts. = data, hist = fit (including smearing) NS fit KL fit The spectra are dominated by the a0 production: both models are able to reproduce them

  22. Preliminary results of the fits (KL and NS): • Comments: • gfa0g ~ in agreement with f0 value • Ra0= 0.78 (KL) and 0.74 (NS) < 1

  23. 5) Summary and Perspectives The KLOE scalar analysis is not yet completed. However: • f0(600): required in the p0p0channel not in • the p+p- one: no clear answer by now. • 2.Couplings: • both NS and KL fits indicate • Rf0 =(gf0K+K-/ gf0p+p-)2 = 2 ÷ 4 • Ra0 = (ga0K+K-/ ga0hp-)2 = 0.7 ÷ 0.8 • from NS analysis: large couplings to the f • gff0g = 1.2 ÷ 2.0 • gfa0g ~ 1.9 (unc. evaluation in progress) • in any case >> gfpg gfhg gfh’g = (0.1  0.7).

  24. KLOE perspectives on scalar mesons 1. Conclude analysis on 2001-2002 data sample for f0(980) (neutral final states) and a0(980). 2.With 2000 pb-1 @ f peak: improvement expected for f0→p+p- combined fitp+p- ANDp0p0 search for f0, a0 KK 3.With new forthcoming energy scan data improved study of the √s-dependence of the cross-section; Off-peak: “test run” of gg  p0p0

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