Ballestrero, A

Mariotti, C

Treille, D

CERN Document Server

DELPHI Collaboration DELPHI 2001-045 PHYS 8958 May, 2001A study on collinear events in the framework of theHiggs analysisA. Ballestreroa, C. Mariottib,a and D. Treilleba: INFN Torino, b: CERNAbstractIn the search for the Higgs boson in the missing energy channels, collinear jets caneasily fake a signal. In this note we report a study on collinear jets in missing energyevents.1 IntroductionTwo different diagrams contribute to the Higgs production in the missing energy channel:the Higgstrahlung process and the WW fusion process as shown in figure 1. The twoprocesses interfere positively. The Higgstrahlung mechanism dominates for m(H)≤ √s-m(Z) (the so-called Higgstrahlung wall) while the WW-fusion mechanism is our onlypossibility to go beyond. Figure 2 shows the various contributions to the cross section ofthe process e+e−→H νν¯ as a function of the Higgs mass for a centre of mass energy of206 GeV. At Ecm =206 GeV and for a Higgs of 115 GeV, about 40% of the cross sectionis due to the fusion process and interference. This means that the kinematic distributionsare quite different from the ones expected from the Higgstrahlung process alone.Figure 1: The two Feynmann diagrams describing the Higgs production: the Hig-gstrahlung process and the WW-fusion process.In figure 3 the acollinearity of the two jets from the Higgs decay is shown at generatorlevel using the WPHACT generator [2] for the total signal (bottom), and for Higgstrahlung(top) and WW-fusion (center) only. The different behaviour in acollinearity of these twocontributions is evident and the net result on the signal distribution (Higgstrahlung +WW-fusion + interference) is that the jets tend to have an angle in space quite differentfrom 180 degrees. The fraction of events with acollinearity lower than 5 degrees is 5%and it is 15% for an angle smaller than 10 degrees. One may also notice that, even if oneconsiders the Higgstrahlung contribution alone, the two jets do not tend to be collinear.This corresponds to the fact that, even if we are at the kinematic limit for production ofan Higgs and a Z, they are not produced at rest. One can see from figure 4 (top) thatthe highest differential cross section dσ/dm(νν¯ ) is about 1 GeV lower than that of the Zmass. This may be understood as the result of two competing mechanisms: the Z Breit-Wigner which tends to put m(νν¯ ) as close as possible to m(Z) and the steep decreasenear the kinematic limit of the production cross section e+e−→HZ as a function of m(Z),which on the contrary tends to produce a much lower m(νν¯ ). With a mass m(νν¯ ) of,say, 90 GeV, the Higgs of 115 GeV gets at this energy a boost of 10 GeV. This behaviouris confirmed by the exact differential cross section dσ/d(boost) of figure 4 (bottom) whichhas a peak around 10 GeV for the Higgstrahlung and around 15 GeV for the total signal.In this note we study the characteristics of these events and the background compo-sition.1.1 The event selectionThe events are selected as in the DELPHI official Hνν¯ analysis, (see [1]) except that fewof the cuts were not applied in order not to suppress collinear event. As already stated in1The Higgsstrahlung           WallE      =  206 GeVCMTotalHiggsstrahlungFusionInterferenceFigure 2: The cross section for Higgs production at Ecm =206 GeV as function of theHiggs mass for the different production processes.[1] we expect very few events integrated over all years since the cross section for a highmass Higgs is very small. The only chance to see them is to work in an environment freeof background.The following cuts (applied in the standard Hνν¯ analysis) are not used in the presentanalysis:• the acoplanarity (of the event forced to 2 jets) should be larger than 6◦ and themissing transverse momentum should be larger than 6 GeV (the missing transversemomentum is defined as the total momentum in the plane perpendicular to thebeam direction).• two coplanar jets (acoplanarity ≤ 10◦) should not go both in the region of polarangle θ = 40◦ ± 5◦ and should be above 20◦ in θ.• √s′ ≤ 0.96× Ecm• Events defined by this two-dimensional cut in the θ(pmis) (degree) vs.√s′(GeV)plane are excluded:θpmis < 40◦:√s′< (−0.6× θpmis + 115) GeVθpmis > 140◦:√s′< (+0.6× θpmis + 7) GeVThese last two cuts suppress collinear events as it is shown in figure 5 for the selectedevents with a cut on the btag larger than 0 (the btag is defined as in [1]).Finally the collinearity of the 2 jets is required to be less than 10 degrees for this study.Two kinds of background dominate: the full energy qq¯ events and the double radiativereturn to the Z, Zγγ . In the first case the 2 quarks are the only energetic particles in2M(Higgs)=115 GeV, Ecm=206 GeV00.20.40.60.8x 10-40 10 20 30 40 50 60acollinearityσ(pb)00.20.4x 10-40 10 20 30 40 50 60acollinearityσ(pb)00.050.10.15x 10-30 10 20 30 40 50 60acollinearityσ(pb)Figure 3: Acollinearity distribution for the two b-quarks coming from a Higgs of 115GeV/c2 at Ecm =206 GeV, produced with WPHACT.3M(Higgs)=115 GeV, Ecm=206 GeV00.050.10.150.20.250.30.350.4x 10-388 90 92M(νν) (GeV/c2)σ(pb)00.10.20.30.40.50.6x 10-460 80M(νν) (GeV/c2)σ(pb)00.10.20.30.40.5x 10-380 85 90M(νν) (GeV/c2)σ(pb)00.10.20.30.40.50.60.70.8x 10-40 20 40Higgs Boost (GeV/c)σ(pb)00.10.20.30.40.5x 10-40 25 50Higgs Boost (GeV/c)σ(pb)00.020.040.060.080.10.12x 10-30 25 50Higgs Boost (GeV/c)σ(pb)Figure 4: Distribution of the invariant neutrino mass and the Higgs boost for a Higgs of115 GeV/c2 at Ecm =206 GeV, produced with WPHACT4Figure 5: The angle (in degrees) between the 2 jets (top) when not applying the cuts on√s′and on the polar angle of the missing momentum, (bottom) applying them.5the final state; of these events the “dangerous” ones are those where only about half ofthe energy is reconstructed due to detector effects and/or the presence of neutrinos insemileptonic decays.After these cuts, the efficiency for a Higgs of 115 GeV of mass is reduced to 16%, andit drops to 5% if one requires an acollinearity cut at 5 degrees. In the standard Higgsanalysis the efficiency after event selection is 75%.2 The resultsThe visible mass (Mvis) spectra for the selected events is shown in figure 6: with a btaglarger than 0 (top), 1 (bottom left) and 3 (bottom right). The Higgs efficiency after the 3btag cuts is respectively 10%, 8% and 4%. A 1 σ excess of data with respect to simulationis observed. The dark grey (purple) histogram is the contribution in the simulation comingfrom Zγγ events [3], while the light grey (yellow) part is the one due to non resonant qq¯(γ).If one forces the missing mass to be MZ , the events will have a recoiling mass to the Zin the highest kinematically accessible bins i.e.√s -MZ , as shown in figure 7 for btag≥0.We focus on the next on the events with a visible mass in the range 95-120 GeV/c2 whichare compatible with a high mass Higgs boson.The θ distribution of the jets and the acoplanarity (i.e. the angle between the jets in theRφ projection) are shown in figure 8 for the events in the region 95≤ Mvis ≤120 GeV/c2and with a btag≥ 0. From these plots there is no evidence that the events are concentratedin detector areas of poor energy resolution or lacking hermeticity (i.e. 40◦,90◦.) Thebackground from double radiative return events is reduced almost completely with thecut on the visible di-jet mass. The only background remaining is from the qq¯ (γ) events.Because of the cuts on the btag, 90% of the simulated events have b-quarks in the finalstate. About 80% of the events originate from a M(qq¯ ) ∼ Ecm , about 20% originatefrom M(qq¯ ) ∼ 120-180 GeV/c2, and less than 1% are still Z with two or many photonemitted, not recognised as a double radiative return, or with photons inside one of thejets thus increasing the visible mass.In figure 9 the visible mass spectra and the recoiling mass to the Z are shown forthe selected collinear events (with acollinearity ≤ 10◦), asking a btag≥1 (correspondingto a Higgs efficiency of 10%) for the different centre of mass energies: going from thetop to the bottom of the page respectively for 189 GeV, 196 GeV, 200+202 GeV and forthe 2000 data and 206.7 GeV simulation. It is obvious that the recoiling mass shifts withcentre-of-mass energy and that a discrepancy between data and simulation, or a statisticalfluctuation, could fake a Higgs signal at the kinematic limit. The number of events indata and simulation are the following: at Ecm =189 GeV 33 in data and 28.6 in MC, atEcm =196 GeV 18 in data and 13.2 in MC, at Ecm =200+202 GeV 18 in data and 21.1 inMC and for the year 2000 38 in data and 33 in MC.3 Energy carried away by neutrinosIn the full energy 2 quarks final state events, where only half of the energy is reconstructedin the detector, the loss is largely due to detector effects. However, the fraction of events6Figure 6: The visible mass distribution for the selected events for three different btagcuts, ≥ 0 (top left plots for data and simulation; top right plot for the Higgs signal) andbtag≥ 1 and 3 on the bottom. The simulation is at Ecm =206.7 GeV. The spectra endat about Mvis=160 GeV because of the cut on the visible energy ≤ 0.75× Ecm . The fullline (blue) on the top-right plot is for a Higgs of 115 GeV of mass; the dashed line (red)is for a Higgs of 114 GeV of mass.7Figure 7: The recoiling mass to the Z→νν¯ , for btag ≥1. The simulation is at Ecm =206.7GeV. In the bottom plot the full line (blue) is for a Higgs of 115 GeV of mass; the dashedline (red) is for a Higgs of 114 GeV of mass.8Figure 8: The polar angle distribution of the jets and the acoplanarity of the selectedevents and with 95≥Mvis ≥120 GeV/c2 and btag ≥0.9Figure 9: The visible mass and the recoiling mass spectra for collinear selected eventsand for a btag ≥ 1 for 189 GeV, 196 GeV, 200+202 GeV and 2000 year data respectivelygoing from the top to the bottom.10in which the main loss is due to neutrinos from semileptonic decays is not negligible andit is detector independent.The percentage of events that loose more than 30% of their energy in neutrinos fromsemileptonic decays has been estimated using generated MC events. 371000 e+e−→qq¯events have been generated at Ecm =206 GeV; of these 91000 have E(qq¯ )≥ 200 GeV. For217 of them, more than 60 GeV of energy has been carried away by the neutrinos: i.e.6×10−4 of the total qq¯ events. If b-quark events are selected, 172 have lost more than60 GeV of energy in neutrinos: i.e. 5×10−4 of the total qq¯ events.This means that in each experiment we expect, in the 2000 run only (i.e. for anintegrated luminosity of 220 pb−1 at Ecm =206 GeV), that about 10 events have lost atleast 60 GeV of their energy in neutrinos (i.e. neglecting detector effects). In figure 10the energy carried away by neutrinos for e+e−→qq¯ events and e+e−→bb¯ events is shownfor events generated at Ecm =206 GeV and with E(ff¯ )≥ 200 GeV. Similar results areobtained at the Z [4].Figure 11 shows the acollinearity of events that have lost at least 60 GeV of energyin neutrinos for e+e−→qq¯ events and e+e−→bb¯ events (events generated at Ecm =206GeV and with E(ff¯ )≥ 200 GeV): 70% have acollinearity ≤ 5◦, 90% acollinearity ≤ 10◦.Of the 1˜0 events expected in each detector during the 2000 run, 55% will remain atthe end of the analysis described in this note, while only 14% would survive the Hνν¯DELPHI standard analysis [1] (after event selection, i.e. for an Higgs efficiency of 75%).Trying to characterise the event categories where more than 60 GeV of energy is lostin neutrinos, one can see that:• for 85% of the events, there are neutrinos in both hemispheres as both the B (or D)hadrons decay semileptonically.• The total number of events with at least one τ lepton is 72 (of which 5 with twoτ). Looking more in details; the sample is enriched in B→τντ as one gets twice asmany as one would expect from the relative branching ratios.• In half of the events, the B decays semileptonically into a D hadron that decayssemileptonically as well.• The leptons in these events are quite soft and with a small transverse momentumwith respect to the thrust axis. In total, 70% of these events do not have at leastone lepton of high momentum (i.e. ≥ 5 GeV/c) and high pT (i.e. ≥ 2 GeV/c).4 ConclusionHiggs at the kinematic limits are always produced with a sufficient boost that allows tocut away collinear events without a serious loss in the number of expected signal events.We have shown in this note that collinear events can easily fake a high mass Higgs signalin case of a discrepancy between data and simulation. The most dangerous irreduciblebackground in this case is that of 2 fermions events produced at full energy that loosealmost half of their energy in neutrinos.1111010 210 310 40 20 40 60 80 100 E of neutrinos (GeV)events11010 210 30 20 40 60 80 100 E of neutrinos in bb events (GeV)eventsFigure 10: The energy carried away from neutrinos in (top) qq¯ generated events atEcm =206 GeV and in (bottom) bb¯ . The only request is that the energy of the 2 quarkshould be larger than 200 GeV.1201020304050600 5 10 15 20 25 30 35 40 45 50acollinearity (degrees)events0510152025303540450 5 10 15 20 25 30 35 40 45 50acollinearity in bb events (degrees)eventsFigure 11: The acollinearity of the events in which more than 60 GeV of energy has beencarried away by neutrinos. Upper plot: qq¯ generated events at Ecm =206 GeV askingE(qq¯ )≥200 GeV; lower plot: bb¯ events asking E(bb¯ )≥200 GeV.13References[1] P. Abreu et al. [DELPHI Collaboration], “Search for the standard model Higgsboson at LEP in the year 2000,” Phys. Lett. B 499 (2001) 23.[2] E. Accomando and A. Ballestrero, Comp. Phys. Comm. 99 (1997) 270.[3] C. Mariotti, E. Piotto,“Symmetric double radiative return with Z→qq¯ final state”,2000-132 OSAKA CONF 431.[4] A.DeAngelis (DELPHI), private comunications.14

A study on collinear events in the framework of the Higgs analysis

DELPHI Collaboration DELPHI 2002-085 CONF 61914 June, 2002A study on collinear events in the framework of theHiggs analysisA. Ballestreroa, C. Mariottib,a and D. Treilleba: INFN Torino, b: CERNAbstractIn the search for the Higgs boson in the missing energy channels, collinear jets caneasily fake a signal. In this note we report a study on collinear jets in missing energyevents.Contributed Paper for ICHEP 2002 (Amsterdam)1 IntroductionTwo different diagrams contribute to the Higgs production in the missing energy channel:the Higgstrahlung process and the WW fusion process as shown in figure 1. The twoprocesses interfere positively. The Higgstrahlung mechanism dominates for m(H)≤ √s-m(Z) (the so-called Higgstrahlung wall) while the WW-fusion mechanism is our onlypossibility to go beyond. Figure 2 shows the various contributions to the cross section ofthe process e+e−→H νν¯ as a function of the Higgs mass for a centre of mass energy of206 GeV. At Ecm =206 GeV and for a Higgs of 115 GeV, about 40% of the cross sectionis due to the fusion process and interference. This means that the kinematic distributionsare quite different from the ones expected from the Higgstrahlung process alone.Figure 1: The two Feynmann diagrams describing the Higgs production: the Hig-gstrahlung process and the WW-fusion process.In figure 3 the acollinearity of the two jets from the Higgs decay is shown at generatorlevel using the WPHACT generator [2] for the total signal (bottom), and for Higgstrahlung(top) and WW-fusion (center) only. The different behaviour in acollinearity of these twocontributions is evident and the net result on the signal distribution (Higgstrahlung +WW-fusion + interference) is that the jets tend to have an angle in space quite differentfrom 180 degrees. The fraction of events with acollinearity lower than 5 degrees is 5%and it is 15% for an angle smaller than 10 degrees. One may also notice that, even if oneconsiders the Higgstrahlung contribution alone, the two jets do not tend to be collinear.This corresponds to the fact that, even if we are at the kinematic limit for production ofan Higgs and a Z, they are not produced at rest. One can see from figure 4 (top) thatthe highest differential cross section dσ/dm(νν¯ ) is about 1 GeV lower than that of the Zmass. This may be understood as the result of two competing mechanisms: the Z Breit-Wigner which tends to put m(νν¯ ) as close as possible to m(Z) and the steep decreasenear the kinematic limit of the production cross section e+e−→HZ as a function of m(Z),which on the contrary tends to produce a much lower m(νν¯ ). With a mass m(νν¯ ) of,say, 90 GeV, the Higgs of 115 GeV gets at this energy a boost of 10 GeV. This behaviouris confirmed by the exact differential cross section dσ/d(boost) of figure 4 (bottom) whichhas a peak around 10 GeV for the Higgstrahlung and around 15 GeV for the total signal.In this note we study the characteristics of these events and the background compo-sition.1.1 The event selectionThe events are selected as in the DELPHI official Hνν¯ analysis, (see [1]) except that fewof the cuts were not applied in order not to suppress collinear event. As already stated in1The Higgsstrahlung           WallE      =  206 GeVCMTotalHiggsstrahlungFusionInterferenceFigure 2: The cross section for Higgs production at Ecm =206 GeV as function of theHiggs mass for the different production processes.[1] we expect very few events integrated over all years since the cross section for a highmass Higgs is very small. The only chance to see them is to work in an environment freeof background.The following cuts (applied in the standard Hνν¯ analysis) are not used in the presentanalysis:• the acoplanarity (of the event forced to 2 jets) should be larger than 6◦ and themissing transverse momentum should be larger than 6 GeV (the missing transversemomentum is defined as the total momentum in the plane perpendicular to thebeam direction).• two coplanar jets (acoplanarity ≤ 10◦) should not go both in the region of polarangle θ = 40◦ ± 5◦ and should be above 20◦ in θ.• √s′ ≤ 0.96× Ecm• Events defined by this two-dimensional cut in the θ(pmis) (degree) vs.√s′(GeV)plane are excluded:θpmis < 40◦:√s′< (−0.6× θpmis + 115) GeVθpmis > 140◦:√s′< (+0.6× θpmis + 7) GeVThese last two cuts suppress collinear events as it is shown in figure 5 for the selectedevents with a cut on the btag larger than 0 (the btag is defined as in [1]).Finally the collinearity of the 2 jets is required to be less than 10 degrees for this study.Two kinds of background dominate: the full energy qq¯ events and the double radiativereturn to the Z, Zγγ . In the first case the 2 quarks are the only energetic particles in2M(Higgs)=115 GeV, Ecm=206 GeV00.20.40.60.8x 10-40 10 20 30 40 50 60acollinearityσ(pb)00.20.4x 10-40 10 20 30 40 50 60acollinearityσ(pb)00.050.10.15x 10-30 10 20 30 40 50 60acollinearityσ(pb)Figure 3: Acollinearity distribution for the two b-quarks coming from a Higgs of 115GeV/c2 at Ecm =206 GeV, produced with WPHACT.3M(Higgs)=115 GeV, Ecm=206 GeV00.050.10.150.20.250.30.350.4x 10-388 90 92M(νν) (GeV/c2)σ(pb)00.10.20.30.40.50.6x 10-460 80M(νν) (GeV/c2)σ(pb)00.10.20.30.40.5x 10-380 85 90M(νν) (GeV/c2)σ(pb)00.10.20.30.40.50.60.70.8x 10-40 20 40Higgs Boost (GeV/c)σ(pb)00.10.20.30.40.5x 10-40 25 50Higgs Boost (GeV/c)σ(pb)00.020.040.060.080.10.12x 10-30 25 50Higgs Boost (GeV/c)σ(pb)Figure 4: Distribution of the invariant neutrino mass and the Higgs boost for a Higgs of115 GeV/c2 at Ecm =206 GeV, produced with WPHACTFigure 5: The angle (in degrees) between the 2 jets (top) when not applying the cuts on√s′and on the polar angle of the missing momentum, (bottom) applying them.5the final state; of these events the “dangerous” ones are those where only about half ofthe energy is reconstructed due to detector effects and/or the presence of neutrinos insemileptonic decays.After these cuts, the efficiency for a Higgs of 115 GeV of mass is reduced to 16%, andit drops to 5% if one requires an acollinearity cut at 5 degrees. In the standard Higgsanalysis the efficiency after event selection is 75%.2 The resultsThe visible mass (Mvis) spectra for the selected events is shown in figure 6: with a btaglarger than 0 (top), 1 (bottom left) and 3 (bottom right). The Higgs efficiency after the 3btag cuts is respectively 10%, 8% and 4%. A 1 σ excess of data with respect to simulationis observed. The dark grey (purple) histogram is the contribution in the simulation comingfrom Zγγ events [3], while the light grey (yellow) part is the one due to non resonant qq¯(γ).If one forces the missing mass to be MZ , the events will have a recoiling mass to the Zin the highest kinematically accessible bins i.e.√s -MZ , as shown in figure 7 for btag≥0.We focus on the next on the events with a visible mass in the range 95-120 GeV/c2 whichare compatible with a high mass Higgs boson.The θ distribution of the jets and the acoplanarity (i.e. the angle between the jets in theRφ projection) are shown in figure 8 for the events in the region 95≤ Mvis ≤120 GeV/c2and with a btag≥ 0. From these plots there is no evidence that the events are concentratedin detector areas of poor energy resolution or lacking hermeticity (i.e. 40◦,90◦.) Thebackground from double radiative return events is reduced almost completely with thecut on the visible di-jet mass. The only background remaining is from the qq¯ (γ) events.Because of the cuts on the btag, 90% of the simulated events have b-quarks in the finalstate. About 80% of the events originate from a M(qq¯ ) ∼ Ecm , about 20% originatefrom M(qq¯ ) ∼ 120-180 GeV/c2, and less than 1% are still Z with two or many photonemitted, not recognised as a double radiative return, or with photons inside one of thejets thus increasing the visible mass.In figure 9 the visible mass spectra and the recoiling mass to the Z are shown forthe selected collinear events (with acollinearity ≤ 10◦), asking a btag≥1 (correspondingto a Higgs efficiency of 10%) for the different centre of mass energies: going from thetop to the bottom of the page respectively for 189 GeV, 196 GeV, 200+202 GeV and forthe 2000 data and 206.7 GeV simulation. It is obvious that the recoiling mass shifts withcentre-of-mass energy and that a discrepancy between data and simulation, or a statisticalfluctuation, could fake a Higgs signal at the kinematic limit. The number of events indata and simulation are the following: at Ecm =189 GeV 33 in data and 28.6 in MC, atEcm =196 GeV 18 in data and 13.2 in MC, at Ecm =200+202 GeV 18 in data and 21.1 inMC and for the year 2000 38 in data and 33 in MC.3 Energy carried away by neutrinosIn the full energy 2 quarks final state events, where only half of the energy is reconstructedin the detector, the loss is largely due to detector effects. However, the fraction of events6Figure 6: The visible mass distribution for the selected events for three different btagcuts, ≥ 0 (top left plots for data and simulation; top right plot for the Higgs signal) andbtag≥ 1 and 3 on the bottom. The simulation is at Ecm =206.7 GeV. The spectra endat about Mvis=160 GeV because of the cut on the visible energy ≤ 0.75× Ecm . The fullline (blue) on the top-right plot is for a Higgs of 115 GeV of mass; the dashed line (red)is for a Higgs of 114 GeV of mass.7Figure 7: The recoiling mass to the Z→νν¯ , for btag ≥1. The simulation is at Ecm =206.7GeV. In the bottom plot the full line (blue) is for a Higgs of 115 GeV of mass; the dashedline (red) is for a Higgs of 114 GeV of mass.8Figure 8: The polar angle distribution of the jets and the acoplanarity of the selectedevents and with 95≥Mvis ≥120 GeV/c2 and btag ≥0.9Figure 9: The visible mass and the recoiling mass spectra for collinear selected eventsand for a btag ≥ 1 for 189 GeV, 196 GeV, 200+202 GeV and 2000 year data respectivelygoing from the top to the bottom.10in which the main loss is due to neutrinos from semileptonic decays is not negligible andit is detector independent.The percentage of events that loose more than 30% of their energy in neutrinos fromsemileptonic decays has been estimated using generated MC events. 371000 e+e−→qq¯events have been generated at Ecm =206 GeV; of these 91000 have E(qq¯ )≥ 200 GeV. For217 of them, more than 60 GeV of energy has been carried away by the neutrinos: i.e.6×10−4 of the total qq¯ events. If b-quark events are selected, 172 have lost more than60 GeV of energy in neutrinos: i.e. 5×10−4 of the total qq¯ events.This means that in each experiment we expect, in the 2000 run only (i.e. for anintegrated luminosity of 220 pb−1 at Ecm =206 GeV), that about 10 events have lost atleast 60 GeV of their energy in neutrinos (i.e. neglecting detector effects). In figure 10the energy carried away by neutrinos for e+e−→qq¯ events and e+e−→bb¯ events is shownfor events generated at Ecm =206 GeV and with E(ff¯ )≥ 200 GeV. Similar results areobtained at the Z [4].Figure 11 shows the acollinearity of events that have lost at least 60 GeV of energyin neutrinos for e+e−→qq¯ events and e+e−→bb¯ events (events generated at Ecm =206GeV and with E(ff¯ )≥ 200 GeV): 70% have acollinearity ≤ 5◦, 90% acollinearity ≤ 10◦.Of the 1˜0 events expected in each detector during the 2000 run, 55% will remain atthe end of the analysis described in this note, while only 14% would survive the Hνν¯DELPHI standard analysis [1] (after event selection, i.e. for an Higgs efficiency of 75%).Trying to characterise the event categories where more than 60 GeV of energy is lostin neutrinos, one can see that:• for 85% of the events, there are neutrinos in both hemispheres as both the B (or D)hadrons decay semileptonically.• The total number of events with at least one τ lepton is 72 (of which 5 with twoτ). Looking more in details; the sample is enriched in B→τντ as one gets twice asmany as one would expect from the relative branching ratios.• In half of the events, the B decays semileptonically into a D hadron that decayssemileptonically as well.• The leptons in these events are quite soft and with a small transverse momentumwith respect to the thrust axis. In total, 70% of these events do not have at leastone lepton of high momentum (i.e. ≥ 5 GeV/c) and high pT (i.e. ≥ 2 GeV/c).4 ConclusionHiggs at the kinematic limits are always produced with a sufficient boost that allows tocut away collinear events without a serious loss in the number of expected signal events.We have shown in this note that collinear events can easily fake a high mass Higgs signalin case of a discrepancy between data and simulation. The most dangerous irreduciblebackground in this case is that of 2 fermions events produced at full energy that loosealmost half of their energy in neutrinos.1111010 210 310 40 20 40 60 80 100 E of neutrinos (GeV)events11010 210 30 20 40 60 80 100 E of neutrinos in bb events (GeV)eventsFigure 10: The energy carried away from neutrinos in (top) qq¯ generated events atEcm =206 GeV and in (bottom) bb¯ . The only request is that the energy of the 2 quarkshould be larger than 200 GeV.1201020304050600 5 10 15 20 25 30 35 40 45 50acollinearity (degrees)events0510152025303540450 5 10 15 20 25 30 35 40 45 50acollinearity in bb events (degrees)eventsFigure 11: The acollinearity of the events in which more than 60 GeV of energy has beencarried away by neutrinos. Upper plot: qq¯ generated events at Ecm =206 GeV askingE(qq¯ )≥200 GeV; lower plot: bb¯ events asking E(bb¯ )≥200 GeV.13References[1] P. Abreu et al. [DELPHI Collaboration], “Search for the standard model Higgsboson at LEP in the year 2000,” Phys. Lett. B 499 (2001) 23.[2] E. Accomando and A. Ballestrero, Comp. Phys. Comm. 99 (1997) 270.[3] C. Mariotti, E. Piotto,“Symmetric double radiative return with Z→qq¯ final state”,2000-132 OSAKA CONF 431.[4] A.DeAngelis (DELPHI), private comunications.14

A study on collinear events in the framework of the higgs analysis

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