Apresyan, A.

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arXiv:0809.0347v1  [hep-ex]  2 Sep 2008Hadron Collider Physics Symposium (HCP2008), Galena, Illinois, USASearch for SM Higgs Boson Produced in Association with a Z or a WBoson in events with /ET and b-jetsA. ApresyanPurdue University, West Lafayette, IN 47907, USAWe present a search for the Standard Model Higgs boson produced in association with a Z or a W boson, using datacollected with the CDF II detector at the Tevatron accelerator. A scenario where the Z decays into neutrinos or chargedleptons originating from the W-decay escape detection and the Higgs decays into a bb pair is considered. Thereforethe expected signature is large missing transverse energy (/ET ), no isolated leptons, and two b-jets. We present thepreliminary results in this search using 1.7fb−1 of data collected by CDF and the work on future improvements toincrease the sensitivity of the analysis.1. INTRODUCTIONIn the Higgs mechanism of the Standard Model, the fermions and weak gauge bosons acquire mass via interactionwith the Higgs field which is described by a complex doublet. Three of the four real fields of the doublet couple tothe SU(2) gauge bosons. The observable quantum of the fourth Higgs field is called the Higgs boson. The existenceof this undiscovered particle is the cornerstone of the Standard Model[1].Direct searches performed with the LEP experiments have constrained the Higgs mass to be larger than 114.4 GeVat 95% C.L. [2]. In pp collisions at the Tevatron, the most probable production mode of the Higgs is by gluon fusionthrough a virtual top loop. Around 70% of the Higgs would decay into two b-quarks yielding two b-jets in the finalstate. Since the QCD b-quark production is an irreducible background, this analysis would have a low sensitivity.The second most frequent production mode is when a virtual W or Z decays into a W or Z and a Higgs. In this case,it is possible to trigger on the decay products of the W/Z boson and significantly reduce the QCD background.We are analyzing Z-Higgs and W-Higgs associated productions when the Z decays into two neutrinos, or the Wdecays leptonically but the charged lepton escapes the detection. Because the neutrinos will not be detected in thecalorimeter, either, they lead to an unbalanced transverse energy sum in the transverse plain (/ET ).2. DATA SAMPLE AND EVENT SELECTIONWe use data collected through March 2007, which corresponds to 1.7 fb−1 integrated luminosity. The events arecollected by CDF II detector with a trigger that selects events with/ET > 25 GeV at Level 1 at least two Level 2clusters with ET > 10 GeV and/ET > 35 GeV at Level 3 .In the first step of the analysis, both the Monte Carlo and real data events are to pass a set of quality cuts toensure that the possible beam and detector effects are removed from the data sample making it compatible with thesimulation. The standard CDF jet clustering algorithm is used [3] with a jet cone of radius 0.4. Jet energies arecorrected for calorimeter non-uniformity, non-linearity and energy loss in the un-instrumented regions of calorimeterand energy coming from different pp interactions during the same bunch crossing. The/ET of the event is thencorrected with new jet energies.The trigger efficiency is obtained from data and is used to scale the signal and Monte-Carlo backgrounds to correctfor event loss during data taking. The efficiency of the two-jet requirement is 100% if the oﬄine transverse energy ofthe most energetic jet is above 35 GeV, the second most energetic jet is above 25 GeV, and at least one of the jetsFERMILAB-CONF-08-341-Ehas |η| < 1.0. The overall efficiency of the online event selection is then parameterized by the oﬄine corrected/ETand applied on the Monte Carlo samples providing a proper scaling for the simulated events.The final requirement imposed on the data and simulation before comparing them is the b-tag requirement. We usetwo categories of SECVTX b-tagging algorithm [4], tight and loose. The main difference between the loose and tighttagging algorithms is that the loose tagger has more efficient track selection. The b-tagging efficiency for tight (loose)tagger is ∼ 40% (50%) and mistag rate is ∼ 2% (4%). In this analysis events are split into two exclusive categories:events with 2 jets tagged by SECVTX loose algorithm or exactly one jet tagged by SECVTX tight algorithm.2.1. BACKGROUND ESTIMATIONThe backgrounds in this data sample have contributions from the following processes: QCD multi-jet production,top quark pair and single production, W or Z boson production with jets and diboson production (WW,WZ,ZZ).We simulate processes which yield real taggable objects, that is, when a b- or a c-quark pair is created. Events withlight flavor jets with a positive tag are considered to be mistags and are estimated from the data[4]. The remainingbackground processes were generated with pythia[5] Monte Carlo event generator passed through CDF II detectorsimulation.After defining two control regions in the events passing the basic selection criteria, the Standard Model backgroundis compared to the data. In the first control region (CR1) all events with identified leptons are vetoed, and theazimuthal angular separation between the second leading jet and the/ET is less than 0.4. This control region isdominated by QCD multi-jet events. The second control region (CR2) contains events with at least one lepton orisolated track and ϕ(2nd jet,/ET ) > 0.4. This region is sensitive to Electroweak processes, and is used to check theoverall shapes and normalizations of the Monte Carlo simulated processes.Control Region 1 Control Region 2Process 1 Tight tag 2 Loose Tags 1 Tight tag 2 Loose TagsQCD h.f. 24337.1 ± 111.4 ± 5445.4 3768.5 ± 45.8± 688.2 50.7 ± 5.1± 12.6 7.0± 2.0± 1.9Top 7.1± 0.4± 0.8 2.3± 0.2± 0.4 134.8 ± 1.6± 16.4 55.9± 1.0± 9.0Di-boson 1.1± 0.2± 0.2 0.1± 0.1± 0.1 14.7 ± 0.8± 2.5 1.9± 0.2± 0.4W + h.f. 26.2± 2.7± 11.1 1.1± 0.5± 0.4 80.5 ± 4.1± 34.9 8.2± 1.3± 3.6Z + h.f. 8.7± 1.2± 3.6 0.9± 0.4± 0.4 17.5 ± 1.8± 7.9 1.3± 0.5± 0.7Mistag 6181.0 ± 63.6 ± 498.5 415.2 ± 10.0± 71.1 86.5 ± 4.3± 6.3 3.7± 1.2± 0.8Expected 30561.2 ± 5469.7 4188.1 ± 693.4 384.7 ± 42.6 77.9 ± 10.3Observed 29431 4190 373 79Table I: Number of expected background and observed events in the control regionsAfter achieving a good agreement between the simulation and the data in the control regions (Table I, Figure 1), aset of cuts are selected optimizing the signal Monte Carlo against the background prediction. The optimization yieldsthe following selection requirements for signal region (SR): ϕ(1stjet,/ET ) > 0.8, ϕ(2ndjet,/ET ) > 0.4,/HT /HT > 0.451,1stjet ET > 60 GeV,/ET > 70 GeV and no isolated leptons in the event. Table II shows the comparison betweenexpected and observed event yields in the signal region. The dijet invariant mass distributions in the signal regionare shown in Fig.2. The excess of observed events in double-loose tagged sample has been extensively studied. Nosystematic source for disagreement has been found. The probability of having such an excess as a result of backgroundonly fluctuation was estimated to be ∼3%.1HT is the scalar sum and/HT is the magnitude of the vectorial sum of the pT ’s of the two leading jetsProcess Single Tag Double TagsQCD h.f. 157.4 ± 9.0± 49.1 10.6± 2.4± 3.9Top 48.2± 1.0± 4.1 14.0± 0.5± 2.1Di-boson 11.5± 0.6± 2.4 1.9± 0.2± 0.4W + h.f. 59.9 ± 4.1± 26.6 4.6± 1.1± 2.0Z + h.f. 28.3 ± 1.9± 12.5 4.1± 0.7± 1.9Mistag 98.2 ± 7.3± 12.8 4.7± 1.0± 1.1Expected 403.5 ± 60.1 39.9± 6.1Observed 443 51(V H mH = 115GeV/c2) (1.9) (1.2)Table II: Number of expected background and signal events in the signal region after applying the final cuts.2.2. CROSS-SECTION LIMITSSince there is no significant excess in the data compared to the predicted backgrounds, we set 95% C.L. upper limitson Higgs boson production cross-section times the branching fraction. The systematic uncertainties are classifiedas correlated and uncorrelated errors considering the relations between the signal and the background processes.The correlated errors are taken into account separately for each processes in the limit calculation. The uncorrelatedsystematic uncertainties are: statistical error in negative tag estimate, negative-positive tag rate asymmetry factor,QCD multi-jet Monte Carlo normalization (14% in single tagged, 6.3% in double tagged sample), MC statisticalfluctuations. The correlated systematics are: luminosity (6.0%), b-tagging efficiency scale factor between data andMonte Carlo (4.3% for single and 10.2% for double tags), trigger efficiency (3%), lepton veto efficiency (2%), PDFuncertainty (2%) and Jet Energy Scale. ISR/FSR systematic uncertainties (between 1% and 5%) are applied on thesignal.Considering the systematic uncertainties listed above, we computed the expected limit for the Higgs cross-sectionwhen the Higgs is produced with a Z/W boson and decays to two b-quarks where Z decays to neutrinos and W toleptons. We use Bayesian method for deriving the limits[6]. Table III shows the final result. All the cross-sectionsare ratios with respect to the Standard Model cross-section. (GeV)jjM0 100 200 300 400 500 600 700 800 900 10000100200300400500 Z+H.F.W+H.F.DibosonTopQCD H.F.MistagData-1CDF Run II Preliminary, 1.7 fbDijet Mass, CR1, L+L (GeV)jjM0 50 100 150 200 250 300 350 400 4500102030405060 Z+H.F.W+H.F.DibosonTopQCD H.F.MistagData-1CDF Run II Preliminary, 1.7 fbDijet Mass, CR2, Single TightFigure 1: Dijet invariant mass distributions in control regions for: (left) double-tagged events in CR1, (right) single-taggedevents in CR2)2 (GeV/cjjM0 50 100 150 200 250 300 350 400020406080100 Z+H.F.W+H.F.DibosonTopQCD H.F.MistagDataZH115 X 10-1CDF Run II Preliminary, 1.7 fbDijet Mass, SR, Single Tight)2 (GeV/cjjM0 50 100 150 200 250 300 350 400024681012141618 Z+H.F.W+H.F.DibosonTopQCD H.F.MistagDataZH115 X 10-1CDF Run II Preliminary, 1.7 fbDijet Mass, SR, L+LFigure 2: Dijet invariant mass in the Signal Region, single- and double-tagged eventsHiggs mass VH limit, 1 Tight Tag VH limit, 2 Loose Tags VH limit, Combined(GeV) Predicted Observed Predicted Observed Predicted Observed110 19.7+9.7−6.0 36.6 10.4+4.4−2.9 18.7 9.3+4.4−2.9 18.5115 22.7+9.5−7.2 37.2 11.1+4.4−3.3 20.8 9.7+5.0−2.8 19.7120 27.5+11.4−7.7 40.8 13.0+6.5−3.9 25.2 11.5+5.5−3.7 22.6125 31.2+14.8−9.3 46.6 15.9+6.6−4.8 30.1 13.4+6.1−4.1 26.6130 40.6+16.7−12.6 58.7 19.5+10.6−5.5 39.3 16.6+7.3−5.3 33.4135 52.0+22.4−16.7 74.6 24.7+10.7−7.5 48.3 21.0+9.7−6.3 43.0140 71.6+31.5−23.7 110.0 35.3+17.5−10.9 64.3 31.5+16.4−7.2 61.5150 172.3+71.7−61.4 238.6 77.1+37.1−22.4 133.6 72.1+30.9−23.4 127.0Table III: The predicted and observed cross-section limits of the ZH/WH processes combined when H → bb¯ divided by theSM cross-section3. FUTURE PROGRESSImprovements of the analysis technique are being developed to further increase the sensitivity of the analysis.Below we summarize the progress in the main directions that are pursued.One of the main challenges in this search channel is the modeling of the large QCD multi-jet background. A modelto estimate this background directly from data has been developed. In order to estimate the multi-jet background inthe single-tagged sample we measure the probability to tag one jet from the “pretag”2 sample. Similarly, to estimatethe multi-jet background in the double-tagged sample we measure the probability to tag a jet in a sample that alreadyhas one jet tagged. This method allows us to estimate the shapes and normalizations for multi-jet production insingle-tagged and double-tagged categories, Fig.3(a).In order to increase the acceptance to the Higgs signal we accept events with three jets, for the first time at CDFin this channel. The main motivation is to accept events where one of the b quarks coming from the Higgs radiates agluon. In addition to that, we also accept WH events where the charged lepton coming from the W is reconstructedas a jet. The latter case happens when the W decays to an electron and it fails the CDF electron identification2i.e. the events passing the basic selection criteria before the tagging requirement(a)Dijet invariant mass, single-taggedevents, CR2(b)Number of jets, single-taggedevents, CR2(c)∆φ(/ET ,/PT ), double-tagged events,CR1Figure 3: Kinematic distributions in control regionsalgorithm, but is reconstructed as a jet; or when the W decays to τν and τ →hadrons. As can be seen from Fig.3(b)our background prediction for jet multiplicity agrees very well with the observed data.In order to effectively reduce the large QCD background, we need to get a good estimate of the event true missingenergy. We do that by calculating the/PT , which is defined as the negative vectorial sum of charged particle pT ’s.For events with true/ET the/PT is highly correlated and parallel with calorimeter/ET , while for QCD events withmismeasured jets it is not. As shown in Fig.3(c) the ∆φ(/ET ,/PT ) can serve as an excellent kinematic variable todiscern real from fake/ET .For searches of H → bb decays it is crucial to precisely meausure the energies of the jets coming from the b-quarks.An algorithm similar to the one used by H1 collaboration [7] has been implemented, which combines tracks andcalorimeter towers to improve the reconstruction of the jets energies. Initial studies show that an improvementof ∼ 10% in dijet mass resolution can be achieved. To further increase the final discriminant sensitivity, we aredeveloping an artificial neural network to maximally separate Higgs boson signal from backgrounds.In summary, we have performed a direct search for the Standard Model Higgs boson decaying into b-jet pairsin 1.7 fb−1 data accumulated in Run II of the CDF detector. We do not observe any significant excess over thebackground predicted by the Standard Model, thus we set a 95 % C.L. upper limit for the Higgs boson at variousmasses. Multiple improvements in the analysis technique are being developed and will be incorporated in the nextiterations of the analysis.References[1] J. Gunion et al., The Higgs Hunter’s Guide (Addison-Wesley; New York, 1990).[2] A. Heister et al., (The LEP Higgs Working Group), Phys. Lett. B 565, 61 (2003).[3] F. Abe et al., Phys. Rev. D 45, 1448 (1992).[4] D. Acosta et al., Phys. Rev. D. 71, 052003 (2005)[5] T. Sjostrand et al., Comput. Phys. Commun. 135, 238 (2001)[6] T. Junk, Sensitivity, Exclusion and Discovery with Small Signals, Large Backgrounds, and Large SystematicUncertainties, CDF8128[7] H1 Collaboration, C.Adloff et al, Z.Phys. C74 (1997) 221.

Search for SM Higgs Boson Produced in Association with a Z or a W Boson in events with E(T) and b-jets

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Search for SM Higgs Boson Produced in Association with a Z or a W Boson in events with E(T) and b-jets

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