LEPSUSYWG/04-10.1

LEP2 SUSY Working Group

Single photons

130-208 GeV


S. Arcelli (OPAL), S. Ask (DELPHI), A. Favara (L3), M. Gataullin (L3), V. Hedberg (DELPHI),
P. Krieger (OPAL), C. Matteuzzi (DELPHI), G. Taylor (ALEPH), G. Wilson (OPAL).

Introduction:
In the context of the Minimal Supersymmetric Standard Models (MSSM) single photon events are expected from the processes:
1) e+e- -> grav chi_1 (with chi_1 -> grav gamma)
2) e+e- -> chi_2 chi_1 (with chi_2 -> chi_1 gamma)
3) e+e- -> grav grav gamma
where `grav' is the gravitino and chi_1 (chi_2) is the lightest (next-to-lightest) neutralino. Reactions 1) and 3) assume a very light gravitino (with mass much smaller than 1 eV). No anomalous production of single-photon events has been observed by any experiment individually, so combined exclusion limits have been produced from the results. The standard model background is largely dominated by neutrino pair production with photon radiation (e+e- -> nu nubar gamma(s)). The Koralz generator was adopted for the simulation of the standard model expectation up to sqrt{s}=183 GeV. For higher energies ALEPH and L3 adopted the KK and OPAL the NUNUGPV generators. 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. When data samples taken at different centre-of-mass energies are combined, the combined cross-section limit is evaluated at the highest centre-of-mass energy. All limits are given at 95% C.L.

In addition to the SUSY scenarios the single photon results have been used to search for large extra dimensions through the process,
4) e+e- -> graviton gamma
and the individual limits from these searches have been combined as well.
References:
The theoretical framework is described in:
[1] J.L. Lopez, D.V. Nanopoulos, A. Zichichi, Phys. Rev. D55 (1997) 5813 and references therein.
[2] A. Bartl, H. Fraas and W. Majerotto, Nucl. Phys. B278 (1986) 1.
[3] A. Brignole, F. Feruglio and F. Zwirner, Nucl. Phys. B516 (1998) 13;
     A. Brignole, F. Feruglio and F. Zwirner, Erratum-ibid. B555 (1999) 653.
[4] G.F. Giudice, R. Rattazzi and J.D. Wells, Nucl. Phys. B544 (1999) 3;
     E.A. Mirabelli, M. Perelstein and M.E. Peskin, Phys. Rev. Lett. 82 (1999) 2236.
References for the experimental results:
ALEPH:
[5] " 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:
[6] " 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:
[7] "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:
[8] ``Photonic Events with Missing Energy in e+e- Collisions at sqrt(s) = 189 GeV'',
OPAL Collab., G. Abbiendi et. al., CERN-EP/2000-050, April 6 2000,
Eur. Phys. J. C18 (2000) 253.

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: 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 (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
103 100.6 3272 3408.9 1350 1378.7 1557 1535.8 6282 6424.1


Exclusion Limits:
Gravitino - Neutralino production:

Cross section upper limit for the process e+e- -> chi_1 grav -> grav grav gamma. The chi_1 particle is assumed to be heavier than the Z, since, for smaller chi_1 masses, LEP1 data should be considered. A \beta^8 scaling of the signal cross-section is assumed for the limit computation combining data samples taken at different centre-of-mass energies.
For these limits, the following data samples were provided by the LEP experiments,
ALEPH: 189 192 196 200 202 204-208 (GeV)
DELPHI: 161 172 183 189 192 196 200 202 204-208 (GeV)
L3: 161 172 189 192 196 200 202 204-208 (GeV)
OPAL: 130 136 161 172 183 189 (GeV)
130 to 208 GeV
Plot A Plot B

Plot A shows the cross section limit assuming 100% branching ratio for the neutralino decay into a photon and a gravitino. 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.
The cross section limit in ASCII format can be found HERE.
Plot B shows the excluded region in the neutralino - gravitino mass plane. The solid and dashed lines corresponds to the obtained and expected limits when the left and right handed selectron masses are equal to 150 GeV/c^2 and under the assumption that the neutralino is a pure bino. The dotted line correspond to the obtained limit when the SUSY parameters are determined from the so-called LNZ model, where only the neutralino and gravitino masses are free parameters. To obtain the limits in plot B the branching ratio BR(chi_1 -> grav gamma) is taken into account.

Neutralino pair production:

Cross section limit for the process e+e- -> chi_2 chi_1 -> chi_1 chi_1 gamma under the assumption BR(chi_2 -> chi_1 gamma) = 1. A (p/sqrt(s))/s scaling of the signal cross-section is assumed for the limit computation combining data samples taken at different centre-of-mass energies and the limit is computed at $\sqrt{s} =$ 208 GeV. 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.
For these limits, the following data samples were provided by the LEP experiments,
ALEPH: 189 192 196 200 202 204-208 (GeV)
DELPHI: 200 202 204-208 (GeV)
L3: 189 192 196 200 202 204-208 (GeV)
OPAL: 189 (GeV)
189 to 208 GeV
Obtained Expected

The cross section limit in ASCII format can be found HERE.
Gravitino pair production:

The results from the individual DELPHI and L3 searches [6,7] for Gravitino pair production have been combined. The combination have been based on the (Bayesian) likelihood functions of the parameter (1/sqrt(F))**8 from the two experiments, where the 95% CL have been obtained from integration over the physically allowed region ((1/sqrt(F))**8.GE.0). The sqrt(F) limit presented here corresponds to the SUSY breaking scale in [3].
For these limits, the following data samples were provided by the LEP experiments,
DELPHI: 183 189 192 196 200 202 204-208 (GeV)
L3: 189 192 196 200 202 204-208 (GeV)

SQRT(F) (GeV/c^2) M_Grav (eV/c^2)
240 1.37E-5

Large Extra Dimensions:

Originating from the work of combining SUSY searches using single photon events, the results from the ALEPH, DELPHI and L3 searches [6,7] for large extra dimensions, by photon + graviton emission [4], have been combined. The search, combination procedure and results will be presented within the LEP Exotica working group.