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Search for η -meson production in γγ interactions with e + e - colliding beams at √s = 1 GeV

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23-09-2009. Search for η -meson production in γγ interactions with e + e - colliding beams at √s = 1 GeV. e + e -  η e + e - process. Integrated luminosity L= 240 pb -1. η  π + π – π 0. BR( h  p + p - p 0 )=22.73%.

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slide2

e+ e- η e+ e- process

Integrated luminosity L= 240 pb-1

η  π+π–π0

BR(hp+p-p0)=22.73%

N. expected events = L σ (e+ e- η e+ e-) BR (η  π+π–π0) ε

slide3

SND

e+e- → p+p-p0

SND

e+e- hg, h p+p- g (4.6%)

e+e- hg, h  e+e- g (0.68%)

e+e- hg,h 3p0, with gN  e+e-N

e+e- gg, with gN  e+e-N

don’t survive the analysis cuts

slide4

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

We prefer to evaluate it using the formula

slide5

Phase space distribution

EPA

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

kinematics

signal

e+e- hg p+p-p0g

kinematics

Signal

Unreducible background Φ ηγ

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

slide7

Mmiss2and π+ π- π0 longitudinal momentum from MC distributions

Φ ηγ

Signal

(GeV2)

(MeV)

kinematics

analysis criteria
Analysis criteria

Event selection

  • 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
    • a > 30°
  • Kinematic cut
  • 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
slide9

Energy resolution

Xγγ pairing:

Xη:

Kinematic fit using Lagrange multipliers method

  • 10 g variables
  • constraints
slide10

c2h signal

c2h hg

c2h data

c2gg signal

X2pair distribution for data and MC signal

c2gg data

X2η distribution for low values of X2η:MC signal (red) and MC ηγ (blue)

X2η distribution for data, MC signal and MC ηγ

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

slide11

Mη ≈ 550 MeV

Mω ≈ 780 MeV

+ c2h< 400

e+e-g

p+p-p0

wp0

h peak not visible

  • π+π- γγinvariant mass distribution (MeV): A) data events with npro=2 (black); B) npro=2 and X2pair < 8 (red)
  • π+π- γγinvariant mass distribution (MeV) after the cut Xη < 400

11

slide12

Pairing with the monochromatic g!

Eg1< 225 MeV and ch2<~20 to further reduce this contribution

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

slide13

QED background

gN  e+e-N

θπ+ vs θπ– (deg), data

e+e- e+e-g

Likelihood cut

slide14

“Pathological” background

After the electron likelihood cut

SPLIT TRACKS

slide15

Split tracks

Event display

|p(π+)|-|p(π-)| (MeV) vs θπ+π- (deg)

  • Large values of the angle between the two tracks
  • Opposite values of the momenta
slide16

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

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

slide17

QED background (2)

data

  • Eγ1< 225 MeV
  • 27.5° < θγ1< 152.5°

MC signal

Cut out with the cuts on Eγ1 and θγ1

Eγ1 (MeV) vs θγ1 (deg)

slide18

Kinematic cut

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

slide20

A

Trigger, filfo, data filter and track requirements efficiencies

(GeV2)

B

(MeV)

  • Squared missing mass
  • Longitudinal momentum of π+π- π0

A

(GeV2)

Analysis cuts efficiencies

B

(MeV)

slide21

signal

pL distributions

21

slide22

signal

Mmiss2 distributions

slide23

N(data) after cuts = 1576

Integrated luminosity L= 240 pb-1

Fractions

Intervals for the fractions

slide24

e+e- e+e-g

Fit

e+e-+-

c2fit /d.o.f.= 75.9/83

c2fit /d.o.f.= 59.8/43

signal

ch2 < 16

slide25

e+e- e+e-g

Fit

e+e-+-

c2fit /d.o.f.= 91.4/83

c2fit /d.o.f.= 38.4/43

signal

ch2 < 8

slide26

Results

BR(hp+p-p0)=22.73%

slide27

e1 (no pT cut) = 0.209 ± 0.003

e2 (pT cut) = 0.196 ± 0.003

Systematics

  • Output s(hg) value always 15% lower than expected value
  • Spread among results from different ch2 cuts ~ 11%
  • For a fixed ch2 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%

s(e+e- e+e-h, s=1 GeV) = (50±2stat±9syst) pb

slide28

Conclusions

  • ggh events have been observed in the same mass range of interest for other studies (see ggp0p0)
  • The cross section e+e-e+e-hp+p-p0e+e- has been measured and found consistent with the EPA prediction even at small √s value compared with existing literatures
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