Hedberg, V

Matteuzzi, C

Perrotta, A

CERN Document Server

DELPHI Collaboration DELPHI 2000-023 CONF 3445 March, 2000Update at 192-202 GeV of theanalysis of single photon events withmissing energy.PreliminaryV. Hedberg 1, C. Matteuzzi 2, A. Perrotta 3.AbstractAn update of a study of the production of single photon events at√s = 192 − 202GeV has been made with the data collected with the DELPHI detector. The analysisuses an integrated luminosity of 229 pb−1. No excess of events beyond that expectedfrom the Standard Model was observed and limits are set on new physics. A newlimit on the gravitational scale is also determined.1 Lund University, Lund, Sweden.2 Universita` e Sezione INFN, Milano, Italy.3 Universita` e Sezione INFN, Bologna, Italy.1 IntroductionThis paper describes the update of a study of the production of single photon events [1]with 229 pb−1 of data collected by DELPHI during 1999 at√s = 192−202 GeV (average√s =197.6 GeV).The physics motivations and the procedure to select the events have been discussedin detail in [1]. Several other results have also been published on this topic by DELPHIand other LEP collaborations [2].2 The single photon selectionThe general criteria for the selection of events are based mainly on the three electro-magnetic calorimeters (STIC, FEMC and HPC) and the tracking system of the DELPHIdetector [3]. The basic selection criteria of events are: no charged tracks detected and noelectromagnetic showers apart from the shower from the single photon candidate. Scin-tillation counters and silicon microvertex detectors are used to veto on charged particles.The details of the selection were described in [1].The selection of events in the FEMC is not identical to the previous selection since ithas been extended to lower photon energies. A sufficient rejection of QED background atthese photon energies was obtained by using the scintillator counters [4] mounted on theLEP beampipe and by a new theta-dependent energy cut.3 Single photon cross sectionThe energy spectrum (xγ = Eγ/Ebeam) of the 540 selected single photon events is shownin Figure 1 together with the expected contributions from known sources. The νν¯γprocess was simulated by the KORALZ [5] program and then passed through the extensivedetector simulation package of DELPHI [6].The number of events and cross sections obtained from these event samples after cor-recting for background and efficiencies are given in Tables 2 and 1. The main sourceof background is the QED process e+e− → e+e−γ [7] where the two electrons escapeundetected along the beampipe but also the contribution from other processes such asγγ collisions, e+e− → γγγ, off-energy electrons [8], cosmic events, e+e− → µ+µ−γ ande+e− → τ+τ−γ have been estimated. The contribution from various sources to the sys-tematic error amounts to ±8%, ±8% and ±9% for the HPC, FEMC and STIC analysisrespectively. The dominant uncertainty comes from the estimation of trigger and detec-tion efficiencies.Figure 2 shows the expected behaviour of the cross section, for three neutrino families,compared with the values measured with the HPC detector at different LEP energies. Thenew points at√s = 192− 202 GeV are in a good agreement with the expectation of theStandard Model.Averaging the three independent measurements done with the three different calorime-ters at√s = 192− 202 GeV, Nν = 2.78± 0.13(stat)± 0.14(syst), with the measurementat 183 and 189 GeV, the number of light neutrino generations becomes:Nν = 2.82± 0.09(stat)± 0.14(syst)14 Limits on the production of an unknown neutralstateThe observed single photon events have been used to set a limit on the production crosssection of a new hypothetical particle, X, produced in association with a photon and beingstable or decaying to invisible decay products. Limits are calculated from the recoil massdistribution (Figure 3) at√s = 192− 202 GeV of the 428 single γ events in the angularregion 12◦ − 168◦ and the 173 events in the angular region 45◦ − 135◦ while taking intoaccount the expected background. The limit is valid when the intrinsic width of the Xparticle is negligible compared to the detector resolution (the recoil mass resolution variesbetween 10 GeV at the Z0 peak to 1 GeV at high masses). The upper limit at the 95%confidence level of the cross section for e+e− → γ+X is given in Figure 3 for photonsin the HPC region and in the FEMC+HPC region. In the latter case an assumption ofan ISR-like photon angular distribution has been made to correct for losses between thecalorimeters.5 Limits on the production of gravitonsIf there are extra compact dimensions of space in which only gravity can propagate,gravitational interactions could be unified with gauge interactions already at the weakscale [9, 10]. The consequence of this model is that at LEP gravity could manifest it-self by the production of gravitons (G), which themselves would be undetectable by theexperiments. Instead single photons from the e+e− → γG reaction are observable.In these gravitational models, a fundamental mass scale, MD, is introduced, which isrelated to the gravitational constant, the size of the compactified space and the number ofdimensions, n, in addition to the usual 4 dimensional space. The differential cross-sectionfor the e+e− → γG process has been calculated by [10].After the sensitivity had been optimised for each calorimeter, the single photon sampleat√s = 196− 202 GeV consisted of 51 events with a photon in the HPC with 6 < Eγ <50 GeV and 73 events with a photon in the FEMC with 10 < Eγ < 50 GeV. The numbersof events expected in the Standard Model were 63.1 and 72.7 for the two calorimetersrespectively. A cross-section limit ofσ < 0.17 pb at 95% C.L. (1)results in limits on the fundamental mass scale of MD > 1.25 TeV and MD > 0.79 TeVfor 2 and 4 extra dimensions (Figure 4).6 Limits on compositenessThe model used in this analysis [11] considers leptons, quarks and weak bosons as com-posite particles. Some of the predicted new particles contribute to the cross section of theprocess e+e− → γ + invisible particles. Calculating the cross sections with the hypoth-esis that a composite boson exists with mass between mD = 5 ·mZ0 and mD = 7 · mZ0and summing the contributions to the cross sections coming from direct production ofU0U¯0 pairs and the exchange of U±, a limit can be obtained on mU from the measured2σ(e+e− → γ + inv.) after subtracting the contribution expected from neutrino productionin the Standard Model.The cross section limit obtained at√s = 196 − 202 GeV from the FEMC and theHPC data is the same as in the graviton analysis (σ < 0.17 pb) and this limit translatesinto a limit on the mass of the U boson which ranges betweenmU > 78− 87 GeV/c2 at 95% C.L.varying mD in the range indicated above.7 SUSY particles7.1 Limits on the gravitino massThe cross section for the process e+e− → G˜G˜γ has been computed under the assumptionthat all other supersymmetric particles are too heavy to be produced [12] and lower limitson the mass of such a light gravitino has been extracted previously at LEP [1, 13]. Thelargest sensitivity is obtained with photons at low energy and/or low polar angle. Sincethe signal cross section grows as the sixth power of the center-of-mass energy, the highestsensitivity is also found at the highest available beam energy.Combining the data at√s = 196 − 202 GeV from the FEMC and the HPC, thesame limit of σ < 0.17 pb at 95% C.L. was obtained as in the graviton analysis. Thiscorresponds to a lower limit on the gravitino mass which ismG˜ > 10.0 · 10−6 eV/c2 at 95% C.L.Since the supersymmetry-breaking scale |F | 12 is related to the gravitino mass by |F | =√38pi/GN ·mG˜, the limit on this scale is |F |12 > 205 GeV.8 ConclusionsWith the 229 pb−1 of data collected by DELPHI in 1999 at a center-of-mass energy of192-202 GeV, an update of the analysis of single and photon events with missing energyhas been made.The measured single photon cross section is in agreement with the expectations fromthe Standard Model process e+e− → νν¯γ.The absence of an excess of events has been used to set limits on the production ofa new unknown model-independent neutral state, a W-type U -boson as described by acompositeness model, a light gravitino and neutralinos. A new limit on the gravitationalscale is also determined.References[1] DELPHI Collaboration, P. Abreu et al., ’Photon events with missing energy at 183and 189 GeV’, to be published in Eur. Phys. J.3[2] DELPHI Collaboration, P. Abreu et al., Phys. Lett. B380 (1996) 471;DELPHI Collaboration, P. Abreu et al., Eur. Phys. J. C1 (1998) 1;DELPHI Collaboration, P. Abreu et al., Eur. Phys. J. C6 (1999) 371;ALEPH Collaboration, R. Barate et al., Phys. Lett. B420 (1998) 127;ALEPH Collaboration, R. Barate et al., Phys. Lett. B429 (1998) 201;L3 Collaboration, M. Acciarri et al., Phys. Lett. B411 (1997) 373;L3 Collaboration, M. Acciarri et al., Phys. Lett. B444 (1998) 503;OPAL Collaboration, K. Ackerstaff et al., Eur. Phys. J. C2 (1998) 607;OPAL Collaboration, K. Ackerstaff et al., Eur. Phys. J. C8 (1999) 23.[3] DELPHI Collaboration, P. Aarnio et al., Nucl. Inst. and Meth. A303 (1991) 233;DELPHI Collaboration, P. Abreu et al., Nucl. Inst. and Meth. A378 (1996) 57.[4] S.J. Alvsvaag et al., Nucl. Inst. and Meth. A425 (1999) 106.[5] S. Jadach et al., Comp. Phys. Comm. 66 (1991) 276;S. Jadach et al., Comp. Phys. Comm. 79 (1994) 503.[6] DELPHI Collaboration, DELPHI 89-67 PROG 142;DELPHI Collaboration, DELPHI 89-68 PROG 143.[7] D. Karlen, Nucl. Phys. B289 (1987) 23.[8] E. Falk, V. Hedberg and G. von Holtey, CERN SL/97-04(EA).[9] N. Arkani-Hamed, S. Dimopoulos and G. Dvali, Phys. Lett. B429 (1998) 263.E.A. Mirabelli, M. Perelstein and M.E. Peskin, Phys. Rev. Lett. 82 (1999) 2236.[10] G.F. Giudice, R. Rattazzi and J.D. Wells, Nucl. Phys. B544 (1999) 3.[11] H. Senju, Prog. Theor. Phys. 95 (1996) 455 and references therein.[12] A. Brignole, F. Feruglio and F. Zwirner, Nucl. Phys. B516 (1998) 13.[13] ALEPH Collaboration, R. Barate et al., Phys. Lett. B420 (1998) 127;ALEPH Collaboration, R. Barate et al., Phys. Lett. B429 (1998) 201;L3 Collaboration, M. Acciarri et al., Phys. Lett. B411 (1997) 373;L3 Collaboration, M. Acciarri et al., Phys. Lett. B444 (1998) 503;OPAL Collaboration, K. Ackerstaff et al., Eur. Phys. J. C2 (1998) 607;OPAL Collaboration, K. Ackerstaff et al., Eur. Phys. J. C8 (1999) 23.4HPC FEMC STIC√s: 192-202 GeV 192-202 GeV 192-202 GeVθγ : 45◦ − 135◦ 12◦ − 32◦ , 148◦ − 168◦ 3.8◦ − 8.0◦ , 172◦ − 176.2◦xγ : > 0.06 > 0.1 > 0.3Nobserved: 173 255 112Nbackground: 0.5 23.8 3.6Ne+e−→νν¯γ : 172 229 120Table 1: Number of selected and expected single photon events in the 1999 DELPHI data.191.6 GeV 195.5 GeV 199.5 GeV 201.5 GeV26.3 pb−1 77.5 pb−1 84.6 pb−1 40.8 pb−1STIC σmeas (pb) 1.32±0.37 1.33±0.22 1.00±0.19 0.75±0.253.8◦ − 8◦ σνν¯γ(γ) (pb) 1.32 1.29 1.22 1.21172◦ − 176.2◦ Nν 3.01±0.37 3.09±0.51 2.45±0.48 1.87±0.63FEMC σmeas (pb) 2.12±0.42 1.73±0.22 2.03±0.23 2.11±0.3312◦ − 32◦ σνν¯γ(γ) (pb) 2.14 2.09 1.96 1.97148◦ − 168◦ Nν 2.97±0.58 2.49±0.32 3.11±0.35 3.22±0.50HPC σmeas (pb) 2.13±0.44 1.42±0.21 1.86±0.22 1.61±0.3045◦ − 135◦ σνν¯γ(γ) (pb) 1.93 1.83 1.78 1.73Nν 3.31±0.68 2.33±0.34 3.14±0.37 2.79±0.52Table 2: Measured and calculated cross-section with the three calorimeters for e+e− →νν¯γ(γ) (KORALZ with three neutrino generations) and the number of neutrino genera-tions calculated from the cross-sections.. The errors are statistical only.502040600 0.2 0.4 0.6 0.8 101020300 0.2 0.4 0.6 0.8 101020300 0.2 0.4 0.6 0.8 1050100150 0.2 0.4 0.6 0.8 1FEMCSTICHPCBackgroundγνν_DataxγEvents/0.025Events/0.05Events/0.05Events/0.05Figure 1: xγ of selected single photons at 192-202 GeV in the three calorimeters STIC,FEMC and HPC. The bottom plot shows the combined spectrum. The light shaded areais the expected distribution from e+e− → νν¯γ and the dark shaded area is the totalbackground from other sources.6110100 120 140 160 180 200E c.m (GeV)σ(ννγ) (pb)DELPHI 192 - 202 GeV preliminaryFigure 2: The measured cross-sections in the HPC for Eγ >6 GeV at different√s com-pared to the expected σ(νν¯γ) (for three neutrino generations).7051015202540 60 80 100 120 140 160 18001020304050640 60 80 100 120 140 160 180HPCFEMC+HPCBackgroundγνν_DataMissing mass (GeV)Events/6 GeVEvents/6 GeV00.050.10.150.20.250.340 60 80 100 120 140 160 180Missing Mass (GeV)σLIMIT (pb)DELPHI192+196+200+202 GeV12o < Θγ < 168ο45o < Θγ < 135οFigure 3: Left: The distributions of the missing mass for the events at 192-202 GeV in theHPC and in the FEMC+HPC. The light shaded area is the expected distribution frome+e− → νν¯γ and the dark shaded area is the total background from other sources. Right:upper limit at 95% C.L. (within the solid angles described) for the production of a newunknown stable neutral object .811010 210 310 4400 600 800 1000 1200 1400 1600 1800 2000M D > 1250 GeV   for  n=2M D >  792 GeV   for   n=4n=2n=4M D (GeV)σ (fb)196+200+202 GeV(202 pb-1)HPC+FEMC acceptanceDELPHI95% limitFigure 4: The cross-section limit at 95% C.L. for e+e− → γG production at<√s >=198.4 GeV and the expected cross-section for 2 and 4 extra dimensions.9

Update at 192-202 GeV of the analysis of single photon events with missing energy

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