I describe two methods for studying hadronic backgrounds to prompt photon
production with L3, and compare the observed background rates with Monte Carlo
predictions. I find that the Monte Carlo models JETSET and HERWIG underestimate
the production of isolated neutral hadrons in hadronic Z decays at LEP. By
extrapolating results obtained with L3, I estimate that the rate of
prompt-photon + jet background to a H -> gamma gamma search at the LHC will be
larger than Monte Carlo predictions by a factor of 1.5-2.5.Comment: 4 page

Kirkby, David

English

arXiv

I describe two methods for studying hadronic backgrounds to prompt photon production with L3, and compare the observed background rates with Monte Carlo predictions. I find that the Monte Carlo models JETSET and HERWIG underestimate the production of isolated neutral hadrons in hadronic Z decays at LEP. By extrapolating results obtained with L3, I estimate that the rate of prompt-photon + jet background to a H -> gamma gamma search at the LHC will be larger than Monte Carlo predictions by a factor of 1.5-2.5

Kirkby, David P.

CERN Document Server

hep-ex/9505012   24 May 1995CALT-68-1992A STUDY OF HADRONIC BACKGROUNDSTO ISOLATED HARD PHOTONPRODUCTION WITH L31David KirkbyCalifornia Institute of TechnologyPasadena, CA 91125, U. S. A.The L3 CollaborationAbstractI describe two methods for studying hadronic backgrounds to prompt photon production withL3, and compare the observed background rates with Monte Carlo predictions. I nd that theMonte Carlo models JETSET and HERWIG underestimate the production of isolated neutralhadrons in hadronic Z decays at LEP. By extrapolating results obtained with L3, I estimatethat the rate of prompt-photon + jet background to a H!  search at the LHC will be largerthan Monte Carlo predictions by a factor of 1.5{2.5.(Talk given at the XXXth Rencontres de Moriond, QCD and High-EnergyHadronic Interactions, Les Arcs, Savoie, France, March 19{26, 1995)1Work supported in part by a grant from the United States Department of Energy.IntroductionEnergetic and isolated (\prompt") photons produced in hadronic nal states originate pri-marily from hard scattering processes. At LEP, cuts on energy and isolation are used to studynal-state radiation in hadronic Z decays[1]. At the LHC, similar cuts will be applied to selectpairs of photons from the decay H! [2].Energetic and isolated neutral hadrons present a potentially large background to any studyof prompt photons, since they are typically recorded in the detector as two or more overlappingphotons and thus can be dicult to distinguish from single photons. The description of thisbackground in a hadronization model is sensitive to the tails of the fragmentation function.Since these tails are poorly constrained, both theoretically and experimentally, these modelsare not expected to be reliable.In this paper I use data collected with the L3 detector at LEP[3] to investigate the productionof hadronic backgrounds to isolated hard photon production in hadronic Z decays. I comparethe measured production rates with the predictions of the JETSET[4] and HERWIG[5] MonteCarlo models. Since present design studies for LHC detectors rely heavily on Monte Carlopredictions, I evaluate the reliability of these models for estimating the expected hadronicbackgrounds to an LHC search for the decay H! , based on measurements at LEP.Resonance ReconstructionThe rst method I have used to study isolated neutral hadronic backgrounds is directreconstruction of 0and  resonances via their decay into two resolvable photons. I selectphoton candidates in hadronic Z decays by requiringE> 500 MeV ; 10< < 170; jtrkj > 17 mrad ;where trkis the dierence in azimuthal angle between the photon candidate and the nearestcharged track. I then reconstruct candidate pairs and require that the reconstructed objectsatisesE> 3 GeV ; 45< < 135;and that it is also isolated from other particles fpig in the event by an angle  = 10{25cos 1(p pi) >  :Figure 1 shows the invariant mass distributions of reconstructed pairs in L3 data collectedduring 1991{94, with isolation requirements of  = 10(a) and  = 25(b). Narrow peaksdue to 0and  resonances are clearly identiable in both cases. The JETSET Monte Carloreproduces the observed rate of isolated 0and  with  = 10, but signicantly underestimatesthe observed background with  = 25.Shower Shape AnalysisThe second method I have used to study isolated neutral hadronic backgrounds exploits thedierent patterns of energy deposit (shower shapes) in the L3 electromagnetic calorimeter dueto a single photon or to multiple overlapping photons. To quantify this eect I use a neuralnetwork discriminator that classies calorimeter clusters based on the energies measured in a55 matrix of BGO crystals. Figure 2 shows the discriminator output distributions for clustersselected in hadronic Z decays withE > 3 GeV ; 45<  < 135;2025005000750010000125001500017500200000 0.2 0.4 0.6 0.8JETSET MCL3 DATAReconstructed mgg (GeV)Events / 10 MeV ISOL > 10o020040060080010000 0.2 0.4 0.6 0.8L3 DATAJETSET MCReconstructed mgg (GeV)ISOL > 25oFigure 1: Invariant mass distributions for photon pairs reconstructed in hadronic Zdecays, for which the reconstructed pair is isolated from other particles in the eventby at least 10(a) or 25(b).010000200003000040000500000 0.2 0.4 0.6 0.8 1L3 DATAZ → qq_ gIsolated p 0Other Hadr BGNN Disc OutputL3 EM Clusters / 0.05ISOL > 10o050010001500200025000 0.2 0.4 0.6 0.8 1L3 DATAZ → qq_ gIsolated p 0Other Hadr BGNN Disc OutputISOL > 25oFigure 2: Neural network discriminator output distributions for electromagneticcalorimeter clusters selected in hadronic Z decays, and isolated by at least 10(a)or 25(b).and isolated from other particles by  = 10(a) and  = 25(b). These plots show clearly thatan isolation cut improves the purity of the prompt photon signal. The Monte Carlo predictionof isolated hadronic background, which is concentrated in the lowest bin, is slightly low with = 10and signicantly too low with  = 25.Comparison of MethodsThe two methods described above are complementary and select independent samples ofhadronic background to isolated hard photon production, based on decay kinematics. At lowenergies, neutral hadrons usually decay into well separated photons that are selected by therst method; at intermediate energies, decay photons typically overlap in the detector and areselected by the second method; at the highest energies decays are almost collinear and thusindistinguishable from single photons.Figure 3 gives a compilation of results obtained with the two methods described above,for an isolation requirement of  = 15. The two methods are found to be consistent. Thedisagreement between the observed and predicted background rates has the following qualitativefeatures:3xp = 2 Ep / √sIsolated p0 Rate (DATA/MC)0.811.21.41.61.822.22.40.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Figure 3: Compilation of measurements of the rate of isolated 0's in hadronic Zdecays, expressed in terms of the ratio (DATA/MC) as a function of the scaled 0energy, x. Results were obtained with the JETSET Monte Carlo. the observed background rate is larger than the predicted rate, and this discrepancyincreases with a tighter isolation cut; the discrepancy is largest for intermediate energies (0:3 < x < 0:7); and the Monte Carlo model HERWIG is in slightly better agreement with data thanJETSET (by 10{20%).Implications for Higgs Searches at the LHCThe dominant background to a search for photon pairs from Higgs decay at the LHC isexpected to be from events containing a single genuine prompt photon, together with a hardisolated 0that is produced from a jet and then misidentied as a prompt photon. In order toperform a crude extrapolation of the backgrounds observed with L3 to the likely backgroundsto an LHC H!  search, I have used PYTHIA[4] to estimate the average scaled energyx= E=Ejetof neutral pions that combine with a prompt photon to pass CMS selection cuts[2]and which also give an invariant mass in the range 90 GeV < m< 130 GeV, where the twophoton Higgs decay mode is expected to be competitive; I nd hxi ' 0:4. The proposed CMSisolation scheme in azimuthal angle and pseudorapidity corresponds approximately to  = 15at LEP. By combining these results, I estimate that the prompt photon + jets background toan LHC H!  search will be larger than Monte Carlo predictions by a factor of 1.5{2.5.References[1] See for example, L3 Collab., O. Adriani et al., Phys. Lett. B 292 (1992) 472.[2] See for example, C. Seez and T. S. Virdee, CMS Technical Note CMS TN/94-289.[3] L3 Collab., B. Adeva et al., Nucl. Instrum. Methods A 289 (1990) 35.[4] T. Sjostrand, Computer Physics Commun. 82 (1994) 74.T. Sjostrand, CERN Preprint CERN-TH.7112/93.[5] G. Marchesini et al., Computer Physics Commun. 67 (1992) 465.4

A Study of Hadronic Backgrounds to Isolated Hard Photon Production with L3

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