LEPSUSYWG/04-09.1

LEP2 SUSY Working Group

Acoplanar photon pairs

130-208 GeV

S. Arcelli (OPAL), S. Ask (DELPHI), A. De Min (DELPHI), A. Favara (L3), M. Gataullin (L3),
P. Krieger (OPAL), G. Taylor (ALEPH), G. Wilson (OPAL).

Introduction:

Acoplanar photon pairs are expected from neutralino pair-production in the context of the Minimal Supersymmetric Standard Models (MSSM). The exact process involved depends on the model assumptions about the LSP particle:

In models with a light gravitino such as the Gauge-Mediated SUSY Breaking (GMSB) models and the One-parameter No-Scale Supergravity Model (LNZ), two acoplanar photons can originate from the production of a pair of lightest neutralinos (chi_1), each decaying to a photon and a (invisible) gravitino.
In more conventional SUSY models with gravity-mediated supersymmetry breaking, acoplanar photon pairs can be a signature for the pair-production of next-to-lightest neutralinos, decaying to a photon and a lightest-neutralino.

In each experiment, a first set of loose cuts is used to preselect events with acoplanar photon pairs. The events passing the preselection are used in order to study the recoil mass distribution and to monitor the modeling standard model backgrounds. After preselection, the signal-to-background ratio is improved by tightening the selection criteria, which depend on the specific scenario considered.
In the GMSB and LNZ scenarios the signal is a pair of lightest neutralinos, each decaying into a photon and a gravitino. The neutralino decay is assumed to take place at the main vertex and the neutralino mass to be between 45 GeV and the beam energy. In the supergravity-MSSM scenario the signal is a pair of next-to-lightest neutralinos, each decaying into a photon and a lightest neutralino. The next-to-lightest neutralino mass is assumed to be between 45 GeV and the beam energy.
No anomalous production of acoplanar photon pairs has been observed by any experiment individually for any considered scenario. The standard model background is largely dominated by neutrino production with photon radiation (e+e- -> nu nubar gammas). The Koralz generator was adopted by all experiments for the simulation of the standard model expectation up to sqrt{s}=183 GeV. For higher energies ALEPH and L3 used the KK and OPAL the NUNUGPV generators respectively. The three generators have been found to agree at the level of ~1% for what concerns the total accepted single-photon cross-section, although differences have been observed in the shape of the recoil mass distribution.
Combined limits have been derived on the neutralino production cross-sections assuming 100% branching ratio for the neutralino decay into a photon and an invisible particle. When data samples taken at different centre-of-mass energies are combined, a beta/s scaling of the signal cross-section is assumed and the combined cross-section limit is evaluated at the highest of the centre-of-mass energies considered in the combination. All limits are given at 95% C.L. and the confidence level is computed using the multichannel likelihood ratio method (A.L. Read) using the FFT method (H. Hu and J. Nielsen) presented in CERN Yellow Report 2000-005.

References:

The theoretical framework is described in:
[1] A. Bartl, H. Fraas and W. Majerotto, Nucl. Phys. B278 (1986) 1.
References for the experimental results:
ALEPH:
[2] " Single- and multi-photon production in $e^+ e^-$ collisions at $\sqrt{s}$ up to 209 GeV",
ALEPH Collab., R. Barate et. al., CERN-EP/2002-033, May 22 2002,
Eur. Phys. J. C28 (2003) 1.
DELPHI:
[3] " Photon Events with Missing Energy in $e^+ e^-$ collisions at $\sqrt{s} =$ 130 to 209 GeV",
DELPHI Collab., J. Abdallah et. al., Submitted to Eur. Phys. J.C.
CERN-EP/2003-093, December 18 2003.
L3:
[4] "Single- and Multi-Photon events with Missing Energy in e+e- Collisions at LEP",
L3 Collab., P. Achard et. al., CERN-EP/2003-068, October 24 2003,
Phys. Lett. B587 (2004) 16.
OPAL:
[5] "Search for acoplanar photon events in e+e- collisions",
OPAL Collab., G. Abbiendi et. al., OPAL PR400, 12 July 2004.

Preselection:

Combined recoil mass distributions:

Combined recoil mass distribution based on all available data. For this histogram, the following data samples were provided by the LEP experiments,
ALEPH: 161 172 183 189 192 196 200 202 204-208 (GeV)
DELPHI: 130 136 161 172 183 189 192 196 200 202 204-208 (GeV)
L3: 161 172 183 189 192 196 200 202 204-208 (GeV)
OPAL: 130 136 161 172 183 189 192 196 200 202 204-208 (GeV)


All data (2 GeV bin)	All data (4 GeV bin)

M<70		70<M<110		110<M<160		M>160		Total
Data	Bkgd	Data	Bkgd	Data	Bkgd	Data	Bkgd	Data	Bkgd
6	5.1	241	251.0	87	91.0	58	59.3	392	406.3

Exclusion Limits:

Gravitino-LSP scenario:

The curves showing the GMSB expectations assume chi_1 to be pure Bino and the selectron_left to be mass-degenerate with the selectron_right (the excluded region in the /\-tan(beta) plane, from the acoplanar photon search, can be seen in the GMSB section).
For these limits, the following data samples were provided by the LEP experiments:
ALEPH: 161 172 183 189 192 196 200 202 204-208 (GeV)
DELPHI: 130 136 161 172 183 189 192 196 200 202 204-208 (GeV)
L3: 161 172 183 189 189 192 196 200 202 204-208 (GeV)
OPAL: 130 136 161 172 183 189 192 196 200 202 204-208 (GeV)


Comparison with GMSB models	Comparison with CDF event

The cross section limit in ASCII format can be found HERE.

Neutralino-LSP scenario:

For these limits, the following data samples were provided by the LEP experiments:
ALEPH: 161 172 183 189 192 196 200 202 204-208 (GeV)
DELPHI: 130 136 161 172 183 189 192 196 200 202 204-208 (GeV)
L3: 183 189 192 196 200 202 204-208 (GeV)
OPAL: 130 136 161 172 183 189 192 196 200 202 204-208 (GeV)


Observed	Expected

The cross section limit in ASCII format can be found HERE.

Updated April 7, 2004
Stefan Ask (E-Mail: stefan DOT ask AT cern DOT ch)