1

Search for η-meson production in γγ interactions with e+e- colliding beams at

√s = 1 GeV

23-09-2009

2

ee++ e e-- ηη e e++ e e-- process process

N. expected events = L σ (e+ e- η e+ e-) BR (η π + π – π 0) ε

ηη ππ ++ ππ –– ππ 00

Integrated luminosity L= 240 pb-1

BR(+-

0)=22.73%

γ split or plus 1 acc. γ50π+π-γ

KS2π0, KL π± l± v2.0KSKL

μ± ν π± π0, π+ π- 2π08.6K+K-

30π+π-π0

5.72ω(π+π-π0)π0

0.23η(π+π-π0)γ

σ (nb)process

γ split or plus 1 acc. γ50π+π-γ

KS2π0, KL π± l± v2.0KSKL

μ± ν π± π0, π+ π- 2π08.6K+K-

30π+π-π0

5.72ω(π+π-π0)π0

0.23η(π+π-π0)γ

σ (nb)process

3

SND

e+e- →

SND

e+e- (4.6%)

e+e- , ee(0.68%)

e+e- 30, with N e+e-

N

e+e-, with N e+e-N

don’t survive the analysis cuts

4

)()(3

0 LSKKeeKKee

The measured cross section (K+K-) has a very large error.

We prefer to evaluate it using the formula

5

Phase space distribution

EPA

The MC generator used accounts for non negligible high pT values for e+e- and so for the , that is different from EPA predictions

6

signal

e+e- 0

kinematics

Mmiss2 vs π+ π- π0 longitudinal momentum from MC distributions

Signal

Unreducible background Φ ηγ

7

kinematics

Φ ηγ

Signal

Mmiss2 and π+ π- π0 longitudinal momentum from MC distributions

(MeV)(GeV2)

8

• TRIGGER, FILFO

• γγ filter (see KLOE Memo n.346), in detail:

• 2 tracks with opposite charge from a cylinder with ρPCA < 8 cm, |zPCA|< 8 cm, ρfirst-hit < 50 cm

Analysis criteriaAnalysis criteria

• at least 2 neutral prompt clusters with Eclu> 15 MeV

• 100 MeV < Σ Eγ < 900 MeV

• Eγ1 > 50 MeV

• Only 2 neutral prompt clusters

• Xγγ pairing < 8

• Cut on Xη

• “Electron likelihood” cut• Cuts to reduce

“pathological” background:– “Split track” cut

– Eγ1< 225 MeV

– sinθγ1> 0.462

> 30°• Kinematic cut

Event selectionEvent selection

9

Kinematic fit using Lagrange multipliers

method

m2 = m2

m2 = m2

tγ - |r γ|/c = 0 for

2

variablvariablesesconstraintsconstraints

Xγγ pairing:

Xη:

)cos1(2

2

1)(

)(

2

2

2 0

ijji

j

E

i

E

pair

EEM

EEM

M

M

MM

ji

Energy resolution

M

j

kN

kj

kj

N

ii

measii PPCPP

1 11 2

22 )...(

)(

10

2 signal

2

2

data

X2pair distribution for data and MC signal

X2η distribution for data, MC signal and MC ηγ

X2η distribution

for low values of X2η: MC

signal (red) and MC ηγ

(blue)

Very good agreement between official ALLRAD and private e+e- e+e- eta MC productions

2

signal

2

data

11

Mω ≈ 780 MeVMη ≈ 550 MeV 0

e+e

peak not visible

2< 400

0

11

• π+π- γγ invariant mass distribution (MeV): A) data events with npro=2 (black); B) npro=2 and X2pair < 8 (red)

• π+π- γγ invariant mass distribution (MeV) after the cut Xη < 400

12

Energy of the most energetic photon (MeV) vs X2η for MC ηγ before and after the requirement npro=2

Pairing with the monochromatic !

E1< 225 MeV and 2<~20 to further reduce this contribution

13

QED background

θπ + vs θπ – (deg), data e+e e+e

e+e

Likelihood cut

14

“Pathological” background

SPLIT TRACKS

After the electron likelihood cut

15

Split tracks

|p(π +)|-|p(π -)| (MeV) vs θπ+π- (deg)

Event display

• Large values of the angle between the two tracks

• Opposite values of the momenta

16

Split tracks

Correlation between the tracks opening angle and the distance between the first hit of the positive track and the first hit of the negative track for data (top) and for signal simulated events (bottom)

The cut applied is indicated by the straight line

+ > 30° to further reduce background due to e+e-N

17

Eγ1 (MeV) vs θγ1 (deg)

data

MC signal

Cut out with the cuts on Eγ1 and θγ1

QED background (2)

• Eγ1< 225 MeV

• 27.5° < θγ1<

152.5°

18

Kinematic cut

Mmiss2 vs π+ π- π0 longitudinal momentum for data (top) and MC signal (bottom)

19

Efficiencies and reduction factors

20

Trigger, filfo, data filter and track requirements efficiencies

(GeV2

)

(MeV)

A. Squared missing mass

B. Longitudinal momentum of π+π-

π0

A

B

(MeV)

(GeV2

)

A

BAnalysis cuts efficiencies

21

pL distributions

21

signal

22

signal

Mmiss2 distributions

23

FractionsFractions

N(data) after cuts = 1576 Integrated luminosity L= 240 pb-1

fD

B

N

Lf

Intervals for the fractions

24

Fite+e- e+e-

e+e-

signal

2 < 16

fit /d.o.f.= 75.9/83 fit /d.o.f.= 59.8/43

25

Fite+e- e+e-

e+e-

signal

2 < 8

fit /d.o.f.= 91.4/83 fit /d.o.f.= 38.4/43

26

Results

BR(+-

0)=22.73%

27

Systematics Output () value always 15% lower than expected

value Spread among results from different cuts ~ 11% For a fixed cut, the difference between pL and

Mmiss2 fits ~ 2%

With or without pT<100 MeV cut, related to the generated pT distribution for MC signal ~ 3%1 (no pT cut) = 0.209 ± 0.003

2 (pT cut) = 0.196 ± 0.003

(e+e e+es=1 GeV) = (50±2stat±9syst) pb

28

Conclusions

events have been observed in the same mass range of interest for other studies (see )

The cross section e+e-e+e-e+e- has been measured and found consistent with the EPA prediction even at small √s value compared with existing literatures