Download presentation
Presentation is loading. Please wait.
1
0 5 Outline Event selection & analysis Background rejection Efficiencies Mass spectrum Comparison data-MC Branching ratio evaluation Systematics Comparison with SND and CMD 2 Conclusions (Search for a 0 ) P.Gauzzi Workshop on KLOE Physics La Biodola – May 23-25, 2001
2
0 Signal: (nb) expected ( =1) a 0 0 0.10 1700 0 0 0 0.037 630 Background: e + e - 0 0 0 0.74 12500 0 0 0 0 0.10 1700 f 0 0 0 0.37 6300 e + e - 0 0 5.6x10 -3 100 3 16.3 3500* 0 0 0 13.4 6000* Cross-sections from PDG-2000; a 0, f 0, and 0 from Novosibirsk data Large errors on cross sections 15% — 25 % * reconstructed as 5 photon events (acc. to MC)
3
Initial sample Runs 15174—17033 Ldt 17 pb -1 Events from radiative stream (selected by nrfilt) “ “ substream neu_min_5g (no tracks and at least 5 prompt ) First selection 5 prompt photons ( t.w.= min(5 t,, 2 ns) ) > 21 o for each photon total energy > 900 MeV (to reject background) Initial sample : 36979 events (tr.+nrfilt+ 5 pr. photons) signal 53 % ( 3%) 2084 expected evts 0 0 53 % 10865 3 3500 0 0 0 6000
4
Analysis scheme First kinematic fit: 30 parameters (E,x,y,z,t for each photon + X,Y,Z of the I.P., E e+, E e- ) 9 constraints (energy and momentum conservation + T-L/c=0 for each photon) 9 ndf cut: 2 /ndf < 10 Best photon pairing in the following hypotheses: 1) 0 2) 0 0 3) 0 0 0 ( mass, E 0 in the selection 2 ) 4) 3 ( mass, E rad in the selection 2 ) rejection 3 rejection Second kinematic fit : 30 parameters, 11 constraints ( 9 + and 0 masses for 1) or two 0 masses for 2) 3) ) For each event this fit is performed three times hyp. 1), 2) and 3) cut: 2 /ndf (hyp. 1) < 10 Final cuts: 0 0 rejection 7 rejection
5
MC samples Samples used to study cuts and evaluate efficiencies : a 0 0 40000 evts 0 (a 0 “flat”) 45000 e + e - 0 0 0 80000 e + e - 0 0 30000 0 0 0 0 70000 0 0 0 50000 f 0 0 0 ( old generation ) 5000 3 300000 0 0 0 285000 Cuts on the signal have been studied with 0 (a 0 “flat”) in order to avoid introducing any bias on the unknown a 0 shape
6
a 0 “flat”
7
0 0 rejection 00 00 Second fit in the 0 0 0 hypothesis = angle between the photon from and the 0 not coming from in the CoM system of ( 0 0 ) 0 0 0 0 f0f0 0 0 0
8
Sample composition Data – MC (without ) Agreement is not good, but expected number of events have large errors signal = 48 % 824 expected evts 0 + f 0 30 % 5719 0 0 37 % 635 0 0 0 3 x 10 -3 736 ?
9
Sample composition Fit the MC distributions to data: 1) 1 ( + ) + 2 ( ) + 3 (f 0 ) 2)( + ) fixed + 1 ( 7 ) + 2 ( ) + 3 (f 0 ) Data – fit Data – fit 1) 1 1.25 2 3 0.8 2) 1 0.95 2 3 0.8
10
Sample composition Check with the other variable of the scatter plot (E ) Data – MC With parameters from 1 st fit without 0 0 0 With parameters from 2 nd fit with 0 0 0 Sizeable content of 0 0 0 decrease 0 and f 0 cross-sections by 20% (see Simona’s results)
11
0 0 rejection Data – MC There are many events that have good probability to be both 0 0 and 0 2 - 2
12
0 0 0 rejection Cluster of maximum energy Data – MC invariant mass after the elliptic cut 2 cut Data – MC 7 E max (MeV) M /
13
Efficiencies a 0 0 27% 0 0 0 27% e + e - 0 0 27% e + e - 0 0 0 3.5 x 10 -3 0 0 0 0 5% f 0 0 0 1.4 x 10 -3 (depends on the shape) 3 2 x 10 -6 0 0 0 5 x 10 -4
14
Mass spectrum Data: 666 events Backgrounds (MC) –– 0 0 (129 3.7) –– 7 (118 10) –– 0 0 (21 0.2) –– total (268 10.7) M (MeV)
15
cos of rad. Data: 666 events –– background (268 events) –– signal (MC) + background
16
Comparison data-MC 2 of the first fit, ndf =9 (without mass contraints) 2 of the second fit, ndf =11 (with mass contraints) Data – MC Data – MC
17
Comparison data-MC Data – MC Data – MC M / M / Pion and eta mass (before the second fit)
18
Branching ratio M (MeV)
19
a 0 0 If interference in (a 0 + 0 ) 0 is negligible (Achasov-Gubin: Phys.Rev.Lett.D63,094007) N( 0 ) = 171 60(from cross-sect.) events
20
a 0 0 M (MeV)
21
Efficiency M (MeV) Not corrected for photon detection efficiency
22
Efficiency M (MeV) dBr/dM (MeV -1 ) Next step: M (MeV)
23
QCAL veto Veto = hit in QCAL on time with the event does not change ( 7 ) 3 x 10 -4 (40% less) 564 events selected (102 rej.) M (MeV)
24
Systematics Background subtraction (main contribution): B = 118( ) + 150(others) 20% uncertainty on cross-sections Br/Br = 8% uncertainty on to be evaluated Efficiency : uncertainty to be evaluated N : uncertainty on Ldt “ on : 5% (from Kloe memo 234)
25
Comparison with SND SND: 39 events with = 2.1% 2x10 7 (Phys. Lett. B 279 (2000),53) Br=(0.88 0.14 0.09)x10 -4 S/B 0.6 KLOE bckg subtracted and corrected for efficiency S/B = 1.5 M (MeV) dBr/dM (MeV -1 ) SND: from tab.1 of their paper KLOE spectrum before bckg subtraction, not corrected for eff. (No normaliz.) (Normalized) M (MeV)
26
Comparison with CMD-2 CMD-2: after bckg subtraction; = 6% 1.9 x10 7 Br = (0.90 0.24 0.10) x 10 -4 (Phys.Lett.B 462 (1999), 380) KLOE spectrum bckg subtracted (Normalized)
27
Conclusions Ldt 17 pb -1 from 2000 data analyzed 666 events survive the selection 268 background events estimated from MC 398 signal events Br( 0 ) = (0.71 0.05(stat)) x 10 -4 good agreement between data and MC Efficiency as a function of M : need a better evaluation Systematics: bckgd. subtraction is dominant( 8%) uncertainty on efficiencies to be evaluated Fit to the spectrum has to be performed in order to get the a 0 parameters Comparison with the Novosibirsk results: our Br is 20% less, BUT: we have much more statistics, greater efficiency, better S/N ratio.
28
Efficiencies a 0 0 27% 26.8% 0 0 0 27% 26.8% e + e - 0 0 26% 25.7% e + e - 0 0 0 3.5 x 10 -3 3.4 x 10 -3 0 0 0 0 5% 5% f 0 0 0 1.4 x 10 -3 1.4 x 10 -3 0 0 0 5.3 x 10 -4 4.1 x 10 -4 2 /ndf<10 2 /ndf<5
29
2 /ndf<5 0 0 0 63 (not bckg.) e + e - 0 0 8 e + e - 0 0 0 37 0 0 0 0 83 f 0 0 0 7 0 0 0 91 Expected events Total. 226 9 From data : N = 599 events
30
0 0 0 Cut e + e - 0 0 0 26.8% f 0 0 0 2.7% 166 f 0 0 0 (f0g_neu4) 5.1% 314 0 0 0 0 8.0% 133 0 2.5% 37 5x10 -4 226 From data : 3443 events Total : 562 – 710
Similar presentations
