Comparison with the Results from the Other Experiments

As described in Chapter 1, the charged Higgs searches have been done at the ATLAS ex-periment and the other exex-periments. We focused on the charged Higgs searches with the mass range from 200 to 600 GeV. We discuss our result by comparing with the results from the other heavy charged Higgs searches. There are five candidates of the charged Higgs analysis for the comparison in this mass region, the result of theH⁺ → tbsearch at the D0 experiment [29], the pp → W_L,R^′ (W_L,R^′ → tb) search at the ATLAS experiment [36], the search forH^± → W^±Z through Vector Boson Fusion at the ATLAS experiment [35], theH⁺ → τ⁺ν search at the AT-LAS experiment [33], theH⁺ →tbsearch at the CMS experiment [34]. We compared the results from the ATLASH⁺ →τ⁺ν[33] and the CMSH⁺ →tb[34] analyses with ours because these charged Higgs searches had similar production and decay process to our charged Higgs search, and get good model-independent results. The result from the ATLASH⁺ → τ⁺νresult is shown in Figure 7.4. They employ theCLsmethod for the limit setting. use the It is the model-independent analysis result. They observed no excess of data and set the upper limit of the cross section for the charged Higgs production in association with a top quark like our analysis, in the charged Higgs mass region from 180 to 1000 GeV. They set the observed limit of approximately 0.5 pb atm_H⁺of 200 GeV and 0.012 pb at 600 GeV. On the other hand, we set the observed limit of approximately 6.28 pb atm_H⁺ of 200 GeV and 0.24 pb at 600 GeV, which is set higher limit thanH⁺ → τ⁺ν analysis result by one order. As we searched for the charged Higgs in different decay process, it has no inconsistency. In one of MSSM scenario likem_h mod- scenario, H⁺ → tbdecay mode is dominant in higher mass region than 200 GeV as shown in Figure 1.3. There is approximately

one order difference in the branching ratio between tb and τ ν atm_H⁺ of 600 GeV. Figure 7.5 shows theH⁺ → tbsearch result including the expected limit without systematic uncertainty by a gray curve. Due to the large systematic uncertainty, we can only set higher upper limit on the production cross section times BR(H⁺ → tb) than H⁺ → τ ν analysis. For Run 2 analysis of H⁺→tbchannel at ATLAS experiment, we will be able to improve the result of the cross section limit in high mass region. The result from CMSH⁺ → tbresult is shown in Figure 7.6. They also employ theCLsmethod for the limit setting. It is the model-independent result in semilepton channel from the CMS analysis in the same production process, decay mode, and final state as our charged Higgs analysis. They do not observe the excess of data. They set the upper limit of the production cross section to approximately 2.2 pb atm_H⁺ of 200 GeV and to 0.15 pb atm_H⁺ of 600 GeV. Our observed limits on the same mass points are approximately 6.28 pb atm_H⁺ of 200 GeV and 0.24 pb atm_H⁺ of 600 GeV. In the CMS analysis, ttis the main background, and they do not separatettbackground process into subprocesses,tt+Light Flavour jets,tt+c-jets, and tt+b-jets as we did. The expected limits of ATLAS and CMS are summarized in Table 7.1. ATLAS results have good agreement with CMS results within uncertainty of 30%in the higher mass re-gion than 400 GeV. On the other hand, there is significant discrepancy between our expected limit and their expected limit in the lower mass region, our limit at m_H⁺ of 200 GeV is about twice as large as CMS result. The difference in the expected limits may originate from the estimate of the systematic uncertainties. As shown in Figure 6.3, the systematic uncertainty of tt+b-jets cross section which is not estimated by CMS group is the main uncertainty at 300 GeV at ATLAS.

This uncertainty has lower influence on the fitting at 500 GeV as shown in Figure 6.4. When we consider that thett+b-jets cross section uncertainty has strong anticorrelation with signal normal-ization in the lower mass region as shown Figure 6.5, this uncertainty is the first candidate making the systematic uncertainties large at ATLAS. Therefore, we need improvement in the treatment of thistt+b-jets cross section uncertainty in run 2 analysis. CMS also searches for charged Higgs bosons in the other channels, dilepton channel andµτ_hadchannel. The dilepton channel considers that the both top quarks from the charged Higgs decay and the production in association withH⁺ decay leptonicaly, but not include tau lepton. This channel has two leptons likee⁻e⁺, orµ⁻µ⁺, or eµ. µτ_hadchannel considers that both top quarks also decay leptonicaly, but oneW boson decays intoτ νand the otherW boson decays intoµν, then the tau lepton decays hadronicaly. Figure 7.7 shows the combined result of theH⁺ → tbsearch in all channels: semileptonic, dilepton, and µτ_had channel. The result of semileptonic channel has the best sensitivity in the channels. As shown in Figures 7.6 and 7.7, the combined result slightly improves the result of semileptonic channel.

Figure 7.4: Upper limits on the production cross section, σ(gb → tH⁺) × BR(H⁺ → τ⁺ν), from ATLAS charged Higgs analysis at √

s = 8TeV.

[GeV]

H+

m 200 250 300 350 400 450 500 550 600 tb) [pb]→+)xBR(H+tH→(gbσ

10-1

1 10

Observed limit (CLs) Expected limit (CLs)

σ

± 1 2σ

±

xBR=1.65 pb σ

=300 GeV, H+

m

Exp. limit with injected signal

β=0.5

mod- tan xBR mh

σ

β=0.7

mod- tan xBR mh

σ

β=0.9

mod- tan xBR mh

σ without sys.

Expected limit (CLs)

ATLAS

=8 TeV, 20.3 fb-1

s

+(tb)

→tH gb

Figure 7.5: Upper limits on the production cross section,σ(gb→tH⁺)×BR(H⁺→tb), includ-ing the expected limit without systematic uncer-tainty.

Figure 7.6: Upper limits on the production cross section, σ(pp → t(b)H⁺) ×Br(H⁺ → tb), in semileptonic channel for charged Higgs bosons at√

s= 8TeV at the CMS experiment.

Figure 7.7: Upper limits on the production cross section, σ(pp → tH⁺) × BR(H⁺ → tb), in combined channels for charged Higgs bosons at

√s= 8TeV at the CMS experiment.

Table 7.1: Comparison of expected 95%CL limits onσ(gb→tH⁺)×BR(H⁺ →tb)between the ATLAS and CMS experiments.

m_H⁺ [GeV] Expected limit from ATLAS [pb] Approximately expected limit from CMS [pb]

200 3.78 2.00

250 1.98 1.20

300 1.44 0.80

350 0.96 0.60

400 0.64 0.50

450 0.45 0.40

500 0.40 0.30

550 0.31 0.25

600 0.25 0.20

Chapter 8