Scalar Mesons Physics with the KLOE detector

C.BiniUniversita’ “La Sapienza” and INFN Roma

For the KLOE collaboration

Outline1. Status of the KLOE experiment2. “Hadron Physics” at a - factory3. Scalar Mesons at a - factory :4. Results

4.1 4.2 4.3

5. Summary and perspectives

1) Status of the KLOE experiment

These days: end of –peak run 2 fb-1 collected in the last two years.

6 ×109

K+ - K- 3 ×109

KL - KS 2 ×109

6 ×107

f0, a0 6 ×105

’ 5 ×105Program now: energy scan around the + off-peak run s = 1 GeV:

KLOE end by spring 2006

ANALYZED

2) “Hadron Physics” at a - factoryPreview: analysis done and in progress (non Kaon physics)

Study of a0(980)

Study of f0(980) + f0(600)

, ’ / ’ mixing

+,-,0 line shapes + “direct term”

Upper limit on C, P, CP violating decays

e+e- leptonic widths: lepton universality

e+e-(rad. ret,) Hadronic contribution to g-2

Study of f0(980) + f0(600)

Fit of Dalitz plot: ChPT test

Fit of Dalitz plot: ChPT test

ChPT test @ p6

, K0 masses

How a -factory can contribute to the understanding of the scalar mesons

(1020)

Mass (MeV/c2)

0

500

1000a0(980)

I=0 I=1/2 I=1

f0(980)

f0(600)“”

K*0(800)“”

3) Scalar Mesons at a - factory

Scalar Mesons Spectroscopy:f0(980), f0(600) and a0(980) are accessible (not accessible)through S; Questions:1. Is f0(600) needed to describe

the mass spectra ?2. “couplings”of f0(980) and

a0(980) to |ss> and to KK, and .

4-quark vs. 2-quark states

How to detect these radiative decays

f0(980)K+K-[ 2m(K)~m(f0)~m() ] expected BR ~ 10-6

K0K0““ ~ 10-8

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 signal and background;

a0(980)K+K-expected BR ~ 10-6

K0K0 “ ~ 10-8

f0(600)

How to extract the signal:

1. Electric Dipole Transitions: (E1) E

3 × |Mif(E)|2

2. Distortions due to KK thresholds (Flatte’-like).

Kaon-loop (by N.N.Achasov): for each scalar meson S: gS, gSKK, MS

No-Structure (by G.Isidori and L.Maiani): a modified BW + a polynomial continuum: gS, gS, gSKK, MS + pol. cont. parameters

Scattering Amplitudes (by M.E.Boglione and M.R.Pennington) A (a1+b1m2+c1m4) T() + (a1+b1m2+c1m4) T()

pole residual g

S to gS (GeV-1)S to kaons gSKK=gSK+K-=gSK0K0 (GeV)f0 to (I=0) gf0=√3/2 gf0=√3 gf0 (GeV)a0 to (I=1) ga0(GeV)Coupling ratioRf0=(gf0KK/ gf0)2

Ra0=(ga0KK/ ga0)2

K+

K-

+

-

+

-Kaon-loop No-structure

f0,a0f0,a0

Definition of the relevant couplings (S=f0 or a0):

Unreducible backgrounds:

( ): huge backgrounds:Initial state radiation (ISR)Final state radiation (FSR) ±± with ± ±

(): large backgrounds: e+e- 0 with 0 00 with 0 0

(): small backgrounds:e+e- 0 with 00 with 0

The unreducible backgrounds

() vs. (): search for the “same amplitude” with a completely different background !

() is the “cleanest” sample

I - Event selection:(a) 2 tracks with t>45o; missing momentum >45o (Large Angle); (b) Each track is pion-like (tracking, ToF and Shower shape):(c) 1 photon matching the missing momentum

4.1) The analysis

(“trackmass”)

muons

pions

(Likelihood: Tof and Shower shape)

electrons

pions, muons

Particle identification: vs. e and

II – The data sample6.7 ×105 events / 350 pb-1 @ √s = M

2.2 ×104 events / 11 pb-1 “off-peak”

photon efficiency

f0(980) region

m() (MeV)

“Large angle”: clear f0(980) signal

m() spectra:(blue) “Small angle” <15o;(red) “Large angle” >45o;

m() (MeV)

III - Fit to the m() spectrum (491 bins, 1.2 MeV wide, 420 to 1009 MeV)

F= ISR + FSR + + scalar ± interferenceKL and NS fits:Good description in both casesof signal and background (KS); “negative” interference;f0(600) doesn’t help.

KL fit NS fit

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 )KL couplings >> NS couplings: effect of polynomial

continuumNS suggests “large” coupling to the (see following)

Scattering Amplitude Fit

g = 6.6 ×10-4 BR(f0(980)) × BR(f0(980)) ~ 3 × 10-5

[ similar conclusion from BP analysis of data (KLOE + SND)]

Summarizing:The peak at ~980 MeV is well interpreted in both KL and NS approaches as due to the decay f0(980) with anegative interference with FSR.The couplings suggest the f0(980) to be strongly coupled to kaons and to the No space for f0(600).Scattering Amplitude gives a marginal agreement.

IV - The Forward-Backward asymmetry:A = (N(+>90o) – N(+<90o)) / sum

+- 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)]

Pion polar angle distributions

(Red) = +

(Blue) = -

Effect of the scalar amplitude on the charge asymmetry: Plot of A in slices of m(); Comparison with simulation with and without the scalar amplitude.

DataSimulation FSR+ISRSimulation FSR+ISR+ scalar(KL)

FB asymmetry vs. m(): Clear signal ~ 980 MeV Interesting comparison with simulation:

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

V – Cross section dependence on √s:Absolute prediction based on KL fit parameters

Data: on-peakData: off-peak

KL absolute predictionBased on fit parameters

Concluding remark: is a powerful tool to test scalar production: mass spectrum, FB asym. and s dependenceKLOE has now collected 2 fb-1 at factor 6 more. Is now starting a finer energy scan around the

I - event selection:5 photons with >21o ; no tracks;Kinematic fit energy-momentum conservation;Kinematic fit 0 masses: choice of the pairing. 4 ×105 events / 450 pb-1

4.2) The analysis

KLOE PLB537 (2002) 21

2002 analysis scheme:1. Reject events interference isneglected;2. 1-dim analysis: fit with KL

The spectrum is dominatedby

New analysis scheme:1. Removed the mcut : 0 are now in the sample2. Bi-dimensional analysis [ Dalitz-plot m() – m() ]3. New treatment of systematics [ pairing problem...]4. Improved VDM parametrization of 0

II – Fit of the Dalitz plot (still preliminary results)KL and NS fits in progress

Comments: 1. VDM part still not perfect (see residuals);2. Scalar part ok BUT f0(600) is still needed in KL fit [p(2) ~ 10-4 30% !];3. f0(980) parameters agree with analysis again R > 1 (gfKK > gf).

Residuals vs. DPposition

Data- fit comparison (on projections)

I – The data samples: out of 400 pb-1 : Statistics of PLB536 (2002) 209 × 20

( ) Improved reducible background subtraction:2.2 ×104 events [ ½ are signal]

Red = signalOther colors= bck

M() (MeV)

( ) almost “backgroundfree” 4100 events [ bck < 3%]

Full pts. = “20 pb-1” dataEmpty pts. = “400 pb-1” data

4.3) The analysis

II – The combined fit

Simultaneous fit of and channels: ratio of BR is fixedPts. = data, hist = fit (including smearing)

KL fit NS fit

The spectra are dominated by the a0 production: both models are able to reproduce them

KL fit NS fit

ga0 (GeV-1) 1.9

ga0K+K- (GeV) 2.3 1.9

Ga0 (GeV) 2.6 2.2

Preliminary results of the fits (KL and NS):

Comments:ga0 ~ in agreement with f0 valueRa0= 0.78 (KL) and 0.74 (NS) < 1

5) Summary and Perspectives

1. f0(600): required in the channel not in the one: no clear answer by now.

2. Couplings:both NS and KL fits indicate

Rf0 =(gf0K+K-/ gf0)2 = 2 ÷ 4Ra0 = (ga0K+K-/ ga0)2 = 0.7 ÷ 0.8

from NS analysis: large couplings to the gf0 = 1.2 ÷ 2.0

ga0 ~ 1.9 (unc. evaluation in progress)in any case >> ggg’= (0.1 0.7).

The KLOE scalar analysis is not yet completed. However:

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 @ peak: improvement expected for f0 →

combined fit AND search for f0, a0 KK

3. With new forthcoming energy scan dataimproved study of the √s-dependence of the cross-section;Off-peak: “test run” of