Study of Standard Model Scalar Production in Bosonic Decay Channels in CMS
SStudy of Standard Model Scalar Production in Bosonic Decay Channels in CMS
Guillelmo G´omez-Ceballos
Massachusetts Institute of Technology, Cambridge, USA
The status of the Standard Model Scalar Boson search in the bosonic decay channels at theCMS experiment at the LHC is presented. The results are based on proton-proton collisionsdata corresponding to integrated luminosities of up to 5.1 fb − at √ s = 7 TeV and 19.6 fb − at √ s = 8 TeV. The observation of a new boson at a mass near 126 GeV is confirmed by theanalysis of the new data and first measurements of the boson properties are shown. One of the open questions in the standard model (SM) of particle physics , , is the origin of themasses of fundamental particles. Within the SM, vector boson masses arise from the spontaneousbreaking of electroweak symmetry by the Higgs field , , , , , . In 2012, the LHC experiments,ATLAS and CMS, reported the discovery of a new boson at approximately 125 GeVwith 5 ormore standard deviations each , . Both observations are consistent with expectations for theSM Higgs boson within the large statistical uncertainties.The H → V V modes have the largest sensitivity among all Higgs decays. In particular, theH → ZZ → (cid:96) and H → γγ modes have very good mass resolution, while the H → WW → (cid:96) ν mode has very large signal yield. In these proceedings the updated analyses with the full availabledataset by the time of the conference are summarized. The data sample corresponds up to 5.1 ± − (19.5 ± − ) of integrated luminosity collected in 2011 (2012) at a center-of-massenergy of 7 (8) TeV collected by the CMS experiment at the LHC.These proceedings are organized as follows. Section 2 briefly describes the main componentsof the CMS detector used in the analyses, while Section 3 describes the general selection usedto define the objects. The following Sections 4 to 7 explain each updated final state analysis:H → ZZ → (cid:96) , H → WW → (cid:96) ν , WH → WWW → (cid:96) ν and H → Z γ . It is worth notingthe spin and parity studies on H → ZZ → (cid:96) and H → WW → (cid:96) ν are summarized in theCMS Standard Model Scalar properties proceeding. By the time of the conference, there wasno update on H → γγ , and therefore the quoted results in Reference were still valid.All Higgs production mechanisms are considered: the gluon fusion process, the associatedproduction of the Higgs boson with a W or Z boson (VH), the t¯tH process, and the vectorboson fusion (VBF) process. The SM Higgs boson production cross sections are taken fromReference . The CMS detector is described in detail elsewhere . The key components used for this analysisare summarized here. A superconducting solenoid occupies the central region of the CMS a r X i v : . [ h e p - e x ] A p r etector, providing an axial magnetic field of 3.8 Tesla parallel to the beam direction. Chargedparticle trajectories are measured by the silicon pixel and strip tracker, which cover the pseudo-rapidity region | η | < .
5. Here, η is defined as η = − ln tan θ/
2, where θ is the polar angleof the trajectory of the particle with respect to the direction of the counterclockwise beam. Acrystal electromagnetic calorimeter (ECAL) and a brass/scintillator hadron calorimeter (HCAL)surround the tracking volume and cover | η | <
3. A quartz-fiber Cherenkov calorimeter (HF)extends the coverage to | η | <
5. The muon system consists of gas detectors embedded in the ironreturn yoke outside the solenoid, with a coverage to | η | < .
4. The first level of the CMS triggersystem, composed of custom hardware processors, is designed to select the most interestingevents in less than 3 µs , using information from the calorimeters and muon detectors. The HighLevel Trigger processor farm further reduces the event rate to a few hundred Hz before datastorage.Several Monte Carlo event generators are used to simulate the signal and background pro-cesses. For all of them, the detector response is simulated using a detailed description of theCMS detector, based on the geant4 package . Minimum bias events are superimposed on thesimulated events to emulate the additional pp interactions per bunch crossing (pile-up). Thesesamples are re-weighted to represent the pile-up distribution as measured in the data. Theaverage number of pile-up events per beam crossing in the 2011 data is about 10, and in the2012 data it is about 20. The selection requirements to define the objects in the different final states depend on theirspecific characteristics, both in terms of the signal topology and the background processes.Nevertheless, the general strategy to select the different objects is common for all of them, andit is described below.Signal candidates are selected online by trigger paths requiring the presence of one or severalelectrons or muons. The use of a combination of them make the efficiencies for events satisfyingthe analysis selection above 95% for the final states under study.Muon candidates are reconstructed combining two algorithms, one in which tracks in thesilicon detector are matched to hits in the muon system, and another in which a global fitis performed on hits in both the silicon tracker and the muon system. Muons are requiredto be isolated to distinguish between muons from W / Z boson decays and those from QCDbackground processes, which are usually in or near jets. For each muon candidate, the scalarsum of the transverse energy of all particles compatible with originating from the primaryvertex is reconstructed in cones of several widths around the muon direction, excluding thecontribution from the muon itself. This information is combined using a multivariate algorithmwhich exploits the differences in the differential energy deposition between prompt muons andmuons from hadron decays inside a jet, to discriminate between signal and background.Electron candidates are identified using a multivariate approach based on variables whichexploit information from the tracker, the ECAL, and the combination of these two detectors.Electron isolation is characterized by the ratio of the sum of the transverse energy of the particlesreconstructed in a cone around the electron, excluding the contribution from the electron itself,and the transverse energy of the electron. Isolated electrons are selected by requiring this ratioto be below a threshold.For both electrons and muons corrections are applied to account for the contribution to theenergy in the isolation cone from the pile-up. A median energy density ( ρ ) is determined eventby event and the pile-up contribution is estimated as the product of ρ and an effective isolationcone area. This contribution is subtracted from the transverse energy in the isolation cone.Hadronically decaying τ leptons are reconstructed and identified using an algorithm whichargets the main decay modes by selecting candidates with one charged hadron and up to twoneutral pions, or with three charged hadrons.The lepton candidates are required to originate from the primary vertex of the event, whichis chosen as the vertex with the highest (cid:80) p , where the sum runs over all tracks associatedwith the vertex.Photon candidates are reconstructed from clusters of channels in the ECAL around channelswith significant energy deposits, which are merged into superclusters. The clustering algorithmsresult in almost complete recovery of the energy of photons in spite of the large fraction ofBremsstrahlung and converted photons. In the endcaps, the preshower energy is added wherethe preshower is present ( | η | > . , the ratio of hadronic energy in the hadroncalorimeter towers behind the supercluster to the electromagnetic energy in the supercluster,the transverse width of the electromagnetic shower, and an electron veto to avoid misidentifyingan electron as a photon.Jets are reconstructed using the anti-k T clustering algorithm with distance parameter∆R = 0 .
5, as implemented in the fastjet package . A similar correction as for the leptonisolation is applied to account for the contribution to the jet energy from pile-up events. Jetenergy corrections are applied as a function of the jet E T and η . Events are classified accordingto the number of selected jets with E T >
30 GeV and | η | < . E missT , definedas the modulus of the negative vector sum of the transverse momenta of all reconstructedparticles (charged or neutral) in the event . Since the E missT resolution is degraded by pile-up,the minimum of two different observables is used. The first includes all particle candidates inthe event . The second uses only the charged particle candidates associated with the primaryvertex. The use of both variables exploits the presence of a correlation between the two variablesin signal events with genuine E missT , and its absence otherwise, as in Drell-Yan events.To suppress the top-quark background, a top tagging technique based on soft-muon and b-jettagging is applied. The first method rejects events with soft muons which likely come fromsemileptonic b-decays coming from top-quark decays. The second method uses a b-jet taggingalgorithm which looks for tracks with large impact parameter within jets. For the second methodjets with E T >
15 GeV are considered. The rejection factor for the top-quark background isabout 50% in the 0-jet category and above 80% for events with at least one jet passing theselection criteria. H → ZZ → (cid:96) analysis The H → ZZ → (cid:96) analysis presented here relies critically on the reconstruction, identification,and isolation of leptons. The high lepton reconstruction efficiencies are achieved for a ZZ systemcomposed of two pairs of same-flavour and opposite-charge isolated leptons, in the measurementrange m (cid:96) , m (cid:96) τ >
100 GeV. One or both of the Z bosons can be off-shell. The backgroundsources include an irreducible four-lepton contribution from direct ZZ (or Z γ ∗ ) production via qq annihilation and gg fusion. Reducible contributions arise from Z + b ¯ b and t¯t, where thefinal states contain two isolated leptons and two jets producing secondary leptons. Additionalbackground of instrumental nature arises from Z + jets , Z + γ + jets and WZ + jets events,where jets are misidentified as leptons.A matrix element likelihood approach , , is used to construct a kinematic discriminant( K D ) based on the probability ratio of the signal and background hypotheses, K D = P sig / ( P sig + P bkg ), where the likelihood ratio is defined for each value of m (cid:96) , being P sig and P bkg the signaland background probabilities, respectively.To improve the sensitivity to the production mechanisms, the event sample is split into twoategories based on the jet multiplicity: events with fewer than two jets, and events with at leasttwo jets. In the first category the transverse momentum divided by the mass of the four leptonsystem ( p T /m (cid:96) ) is used to discriminate VBF and VH from gluon fusion. In the second categorya linear discriminant ( V D ) is formed combining two VBF sensitive variables, the difference inpseudo-rapidity and the invariant mass of the two leading jets. The discriminant is tuned toseparate vector boson from gluon fusion processes.In summary, m (cid:96) , K D , and the two distributions after splitting into two categories based onthe jet multiplicity are used to discriminate between signal and background. The reconstructedfour-lepton invariant-mass distributions for the 4 (cid:96) and 2 (cid:96) τ final states are shown in Figure 1and compared with the expectation from SM background processes. The observed distributionis in good agreement with the expectation. The Z → (cid:96) resonance peak is observed withnormalization and shape as expected. The measured distribution at higher mass is dominatedby the irreducible ZZ background. A clear peak around m (cid:96) = 126 GeV is seen. [GeV] m E v en t s / G e V Data = 126 GeV H m *, ZZ g Z Z+X
CMS preliminary -1 = 8 TeV: L = 19.6 fbs -1 = 7 TeV: L = 5.1 fbs (GeV) t m
100 200 300 400 500 600 700 800 900 1000 E v en t s / G e V CMS preliminary -1 = 8 TeV, L = 19.3 fbs -1 = 7 TeV, L = 5.1 fbs DataZ+XZZ=350 GeV H m Figure 1: Distribution of the 4 (cid:96) (left) and 2 (cid:96) τ (right) reconstructed mass in the full mass range. Points representthe data, shaded histograms represent the background and unshaded histogram the signal expectation. The distributions of the kinematic discriminant K D versus m (cid:96) are shown for the selectedevents and compared to SM background expectation in Figure 2. The distributions of p T /m (cid:96) and the VBF discriminant V D are presented in Figure 3.The local p -values, representing the significance of local excesses relative to the backgroundexpectation, are shown as a function of m H in Figure 4. The minimum of the local p -value isreached around m (cid:96) = 125 . . σ (for an expec-tation of 7.2 σ ). This constitutes an observation of the new boson in the four-leptons channelalone. As a cross-check, the 1D ( m (cid:96) ) and 2D ( m (cid:96) , K D ) models are also studied, and observeda local significance of 4.7 and 6.6 σ , for an expectation of 5.6 and 6.9 σ , respectively. The upper95% confidence level (CL) limits obtained from the combination of the 4 (cid:96) and 2 (cid:96) τ channelsusing the modified frequentist construction CL s method , , are also shown in Figure 4. TheSM-like Higgs boson is excluded by the four-lepton channels at 95% CL in the range 130–827 GeV(for an expectation of 114–778 GeV). The signal strength µ , relative to the expectation for theSM Higgs boson, is measured to be µ = 0 . +0 . − . at 125.8 GeV.The mass measurement of the new resonance is performed with a three-dimensional fit usingfor each event the four-lepton invariant mass, the associated per-event mass uncertainty, andthe kinematic discriminant. Per-event uncertainties on the four-lepton invariant mass are calcu-lated from the individual lepton momentum uncertainties. Figure 5 shows the one-dimensionallikelihood scan versus SM Higgs boson mass performed under the assumption that its width is (GeV) l m
100 110 120 130 140 150 160 170 180 D K m m CMS preliminary -1 = 8 TeV, L = 19.6 fbs -1 = 7 TeV, L = 5.1 fbs (GeV) l m
100 110 120 130 140 150 160 170 180 D K m m CMS preliminary -1 = 8 TeV, L = 19.6 fbs -1 = 7 TeV, L = 5.1 fbs Figure 2: Distribution of the kinematic discriminant K D versus m (cid:96) in the low-mass region. The contoursrepresent the expected relative density of signal events for m H = 126 GeV (left) and for background events(right). The points show data and measured invariant mass uncertainties as horizontal bars. /m T p E v en t s Data * g ZZ , ZZ+XggH+ttH (126 GeV)qqH+VH (126 GeV)CMS preliminary -1 = 8 TeV, L = 19.6 fbs -1 = 7 TeV, L = 5.1 fbs D V E v en t s Data * g ZZ , ZZ+XggH+ttH (126 GeV)qqH+VH (126 GeV)
CMS preliminary -1 = 8 TeV, L = 19.6 fbs -1 = 7 TeV, L = 5.1 fbs Figure 3: Distributions for the p T /m (cid:96) in the first category for the VBF discriminant in second category. Onlyevents in the mass region 121 . < m (cid:96) < . much smaller than the detector resolution. The resulting fit gives m H = 125 . ± . ± . µ F , µ V ) are introduced as scalefactors to the SM expected cross section. A two dimensional fit is performed for the two signalstrength modifiers assuming a mass hypothesis of m H = 125 . µ V , µ F ) fit, leading to the measurements µ V = 1 . +2 . − . and µ F = 0 . +0 . − . . [GeV] H m
110 120 130 140 150 160 170 180 l o c a l p - v a l ue -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 CMS Preliminary -1 = 7 TeV, L = 5.05 fbs -1 = 8 TeV, L = 19.63 fbs → ZZ → H
3D Fit 7+8TeVExpected
Figure 4: Observed and expected 95% CL upper limit (left) on the ratio of the production cross section to theSM expectation in the H → ZZ → (cid:96) analysis. The 68% and 95% ranges of expectation for the background-onlymodel are also shown with green and yellow bands, respectively. Significance of the local excess (right) withrespect to the SM background expectation as a function of the Higgs boson mass. F m -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 V m -8-6-4-2024681012
68% CL95% CLbest fitSM
CMS Preliminary -1 = 8 TeV, L = 19.6 fbs -1 = 7 TeV, L = 5.1 fbs Figure 5: 1D test statistics q( m H )= −
2∆ ln L scan vs tested Higgs boson mass m H , obtained from the 3D teststatistics profiling the minimum of the signal strengths, with and without systematics (left). Likelihood contourson the signal strength modifiers associated with fermions ( µ F ) and vector bosons ( µ V ) shown at 68% and 95%CL (right). H → WW → (cid:96) ν analysis The search strategy for H → W + W − is based on the final state in which both W bosons decayleptonically , resulting in a signature with two isolated, oppositely charged, high p T leptons(electrons or muons) and large missing transverse momentum, E missT , due to the undetectedneutrinos.To improve the signal sensitivity, the events are separated according to lepton flavor intoe + e − , µ + µ − , and e ± µ ∓ samples and according to jet multiplicity into 0-jet and 1-jet samples.To reduce the background from WZ production, any event that has a third lepton passingthe identification and isolation requirements is rejected. The contribution from W γ production,when the photon is misidentified as an electron, is reduced by about 90% in the dielectron finalstate by γ conversion rejection requirements. The background from low mass resonances isrejected by requiring a dilepton mass ( m (cid:96)(cid:96) ) greater than 12 GeV. A minimum requirement onhe dilepton transverse momentum ( p (cid:96)(cid:96) T ) is applied to reduce the W + jets background.The Drell-Yan process produces same-flavor lepton pairs (e + e − and µ + µ − ). In order tosuppress this background, a few additional cuts are applied in the same-flavor final states. First,the resonant component of the Drell-Yan production is rejected by requiring a dilepton massoutside a 30 GeV window centered on the Z pole. Then, the remaining off-peak contribution issuppressed by exploiting different E missT -based approaches.To enhance the sensitivity to a Higgs boson signal, a cut-based approach is chosen for thefinal “Higgs” selection in all categories. Because the kinematics of signal events change as afunction of the Higgs mass, separate optimizations are performed for different m H hypothesesin a cut–based analysis. In addition, a two-dimensional shape analysis technique is also pursuedfor the different-flavor final state in the 0-jet and 1-jet categories. This second analysis is moresensitive to the presence of a Higgs boson and is used as a baseline for the final results.In the cut-based approach extra requirements, designed to optimize the sensitivity for a SMHiggs boson, are placed on p (cid:96), maxT , p (cid:96), minT , m (cid:96)(cid:96) , ∆ φ (cid:96)(cid:96) and the transverse mass m T , defined as (cid:113) p (cid:96)(cid:96) T E missT (1 − cos ∆ φ E missT (cid:96)(cid:96) ), where ∆ φ E missT (cid:96)(cid:96) is the difference in azimuth between E missT andthe transverse momentum of the dilepton system. The m (cid:96)(cid:96) and m T distributions in the 0-jet inthe different-flavor final state are shown in Figure 6 for a SM Higgs boson with m H = 125 GeV. ] [GeV/c ll m e v en t s / G e V / c data =125 GeV H m H125 W+jets VV Top* g Z/ WWsyst. ¯ stat. CMS Preliminary -1 = 8 TeV, L = 19.5 fbs -1 = 7 TeV, L = 4.9 fbs ] [GeV/c ll m da t a / M C ] [GeV/c ll m e v en t s / G e V / c data =125 GeV H m H125 W+jets VV Top* g Z/ WWsyst. ¯ stat. CMS Preliminary -1 = 8 TeV, L = 19.5 fbs -1 = 7 TeV, L = 4.9 fbs ] [GeV/c ll m da t a / M C Figure 6: Distributions of dilepton mass (left) and the transverse mass (right) in the 0-jet category, in thedifferent-flavor final state for a m H = 125 GeV SM Higgs boson and for the main backgrounds. The cut-basedH → W + W − selection, except for the requirement on the variable itself, is applied. The two-dimensional shape analysis for the different-flavor final state uses two independentvariables, m T and m (cid:96)(cid:96) . It allows for a simpler physical interpretation of the observed data witha sensitivity comparable to other more complex techniques. The two-dimensional distributionsfor the m H = 125 GeV Higgs signal hypothesis and background processes are shown in Figure 7for the 0-jet bin.After applying the Higgs selection, upper limits are derived for the ratio of the product of theHiggs boson production cross section and the H → W + W − branching fraction, σ H × BR(H → W + W − ), and the SM Higgs expectation, σ/σ SM . For the shape approach, the analysis in thedifferent-flavor final state in the 0-jet and 1-jet categories is combined with the cut-based analysisin all other categories.The 95% observed and median expected CL upper limits for the shape analysis are shownin Figure 8, which excludes a Higgs boson in the mass in the range 128–600 GeV at 95% CL.The expected exclusion range for the background only hypothesis is 115–575 GeV. An excessof events is observed for hypothetical low Higgs boson masses, which makes the observed limitsweaker than the expected ones. Due to the poor mass resolution of this channel the excess (GeV) T M60 70 80 90 100 110 120 ( G e V ) ll M = 125 GeV H M (8TeV) -1 CMS preliminary L = 19.5 fb (GeV) T M60 70 80 90 100 110 120 ( G e V ) ll M Background (8TeV) -1 CMS preliminary L = 19.5 fb
Figure 7: Two-dimensional m T − m (cid:96)(cid:96) distributions in the 0-jet bin for the m H = 125 GeV SM Higgs signalhypothesis (left) and the background processes (right). extends over a large mass range.The observed (expected) significance for a SM Higgs with a mass of 125 GeV is 4.0 (5.1)standard deviations for the shape-based analysis. The observed and expected significances andfor each Higgs mass hypothesis is shown in Figure 8. The observed µ value for m H = 125 GeVusing the shape-based analysis is 0.76 ± ± ± [GeV] H m S M (cid:109) / (cid:109) % C . L . L i m i t on -1 ObservedMedian Expected (cid:109) ± Expected (cid:109) ± Expected (cid:109) ± =125 GeV H Injection m
CMS Preliminary -1 =7 TeV, L = 4.9 fbs -1 =8 TeV, L = 19.5 fbs 0/1-jet (cid:105) (cid:65) WW (cid:65) H [GeV] H m s i gn i f i c an c e ExpectedObserved =125 GeV H Injection m (cid:109) ± Injection (cid:109) ± Injection
CMS Preliminary -1 =7 TeV, L = 4.9 fbs -1 =8 TeV, L = 19.5 fbs 0/1-jet (cid:105) (cid:65) WW (cid:65) H
100 200 300 400 500 600
Figure 8: Expected and observed 95% CL upper limits on the cross section times branching fraction, σ H × BR(H → W + W − ), relative to the SM Higgs expectation for the shape-based analysis (left). The expected limits in thepresence of the Higgs with m H = 125GeV and its associated uncertainty are also shown. The observed andexpected significances and for each Higgs mass hypothesis (right) for the shape-based analysis. The expectedsignificance under the presence of a m H = 125GeV Higgs is also shown. WH → WWW → (cid:96) ν analysis In the WH → WWW → (cid:96) ν channel , events are selected by requiring three charged leptoncandidates, electrons or muons, with total charge equal to ± p T >
20 GeVfor the leading lepton and p T >
10 GeV for the other leptons. The data analysis is performedy using a shape-based approach, with a cross check from a single bin counting experiment. Tofurther improve the sensitivity the events are split into two categories: all events that have anopposite-sign same-flavor lepton pair are classified in one category (OSSF), everything else isclassified as in the same-sign same-flavor category (SSSF). While 1/4 of the events are selectedin the second category, the expected background is rather small since physics processes leadingto this final state have small cross section.Events are required to have E missT above 40 (30) GeV in the OSSF (SSSF) category. Toreduce the background from top decays, events are rejected if there is at least one jet with E T above 40 GeV. The WZ → (cid:96)ν background is largely reduced by requiring that all the OSSFlepton pairs have a dilepton mass at least 25 GeV away from m Z . To reject the Z / W + γ ∗ background, the dilepton mass of all opposite-charge lepton pairs are required to be greaterthan 12 GeV. Finally, the signal region is defined by requiring in addition to all the above cutsthat the smallest dilepton mass m (cid:96)(cid:96) is less than 100 GeV and that the smallest distance betweenopposite-charge leptons ∆ R (cid:96) + (cid:96) − is less than 2.A shape-based analysis is carried out as a main analysis due to its superior performancewith respect to a simple counting experiment. In this analysis a cut on ∆ R (cid:96) + (cid:96) − is not applied,and instead it is used as the final discriminant. Tests have shown this variable to provide thebest discrimination between signal and background events, in terms of both expected limits andsignificance.No significant excess of events is observed with respect to the background prediction, and95% CL upper limits are calculated for the Higgs boson cross section with respect to σ/σ SM . Theexpected and observed upper limits are shown in Figure 9. Since the analysis is independent ofHiggs mass, and the shape of the ∆ R (cid:96) + (cid:96) − distribution changes just slightly for the Higgs signal,only small fluctuations are expected between different Higgs mass hypotheses. For the cut-basedanalysis, the observed (expected) upper limit at the 95% CL is 3.7 (3.6) times larger than theSM expectation for m H = 125 GeV. For the shape-based analysis, the observed (expected)upper limit at the 95% CL is 3.3 (3.0) times larger than the SM expectation for m H = 125 GeV. Higgs mass [GeV]
110 120 130 140 150 160 170 180 190 200 S M s / s % C L li m i t on
10 CMS preliminary (cut-based) n fi VH (8 TeV) -1 (7 TeV) + 19.5 fb -1 L = 4.9 fb observed median expected s – expected s – expected Higgs mass [GeV]
110 120 130 140 150 160 170 180 190 200 S M s / s % C L li m i t on
10 CMS preliminary (shape-based) n fi VH (8 TeV) -1 (7 TeV) + 19.5 fb -1 L = 4.9 fb observed median expected s – expected s – expected Figure 9: Upper limits at 95% CL in the WH → (cid:96) ν final state for the cut-based analysis (left) and shape-basedanalysis (right) H → Z γ analysis The H → Z γ decay channel is a clean final state, with the Z boson decaying into an electronor a muon pair, plus an isolated photon.The invariant mass of at least one (cid:96) + (cid:96) − pair is required to be greater than 50 GeV. If twoilepton pairs are present, the one closest to the Z mass is taken. The invariant mass of the (cid:96) + (cid:96) − γ system, m (cid:96)(cid:96)γ , is required to be between 100 and 180 GeV. Other conditions that combinethe information from the photon and the leptons are: (1) the ratio of the photon transverseenergy to m (cid:96)(cid:96)γ must be greater than 15/110, this requirement allows us to reject backgroundswithout significant loss in signal sensitivity and without introducing a bias in the m (cid:96)(cid:96)γ spectrum;(2) the ∆ R separation between each lepton and the photon must be greater than 0.4 in orderto reject events with initial-state radiation avoiding photon influence lepton isolation; and (3)final-state radiation events are rejected by requiring a minimum of 185 GeV on the sum of m (cid:96)(cid:96)γ and m (cid:96)(cid:96) .The sensitivity of the search is enhanced by subdividing the selected events into classesaccording to indicators of the expected mass resolution and the signal-to-background ratio, andthen combine the results in each class. For this purpose, four mutually exclusive event classesare defined: in terms of the pseudo-rapidity of the leptons and the photon and on the showershape of the photon for one of the topologies. The background model fit to the m µµγ distributionfor two event classes is shown in Figure 10.No excess over the background is observed, and therefore the data are used to derive upperlimits on the proton-proton Higgs boson production cross section times the H → Z γ branchingfraction, σ H × BR(H → Z γ ). The expected and observed limits are both shown in Figure 10.The expected exclusion limits at 95% confidence level are between 6 and 19 times the standardmodel cross section and the observed limit fluctuates between about 3 and 31 times the SMcross section. (GeV) H m
120 125 130 135 140 145 150 S M BR ] ·s / [ % C L BR ] ·s [ ObservedMedian Expected s – Expected s – Expected
CMS Preliminary -1 = 7 TeV L = 5.0 fbs -1 = 8 TeV L = 19.6 fbsElectron + muon channels Figure 10: Background model fit to the m µµγ distribution for two event classes (left). The statistical uncertaintybands shown are computed from the data fit. Exclusion limit on the cross section of a SM Higgs boson decayinginto Z-boson and a photon as a function of m H (right). The status of the SM Scalar Boson search in the bosonic decay channels at the CMS experimentat the LHC has been presented. The results are based on proton-proton collisions data corre-sponding to integrated luminosities of up to 5.1 fb − at √ s = 7 TeV and 19.6 fb − at √ s = 8TeV. The observation of a new boson at a mass near 126 GeV is confirmed by the analysis ofthe new data and first measurements of the boson properties have been shown. eferences
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