University of Science Technology of China USTC Search
- Slides: 23
University of Science & Technology of China (USTC) Search for heavy neutral resonances in vector boson fusion qq VVqq with the ATLAS detector Liang Han, Minghui Liu, Guangyi Zhang University of Science and Technology of China 第 12届全国粒子物理学术会议 1
Vector boson fusion An interesting process • A Promising mode of Higgs production • 2 vector bosons with decay products as signature • 2 forward jets with high invariant mass The first search for neutral resonances above the Higgs mass through VBF at ATLAS 第 12届全国粒子物理学术会议 2
Motivation nb nb u A direct test to SM EWSB A s Unitarity violation The unitarity can be restored by adding Higgs 第 12届全国粒子物理学术会议 3
Motivation u Sensitive to new physics VBF is the best channel if Ø Higgs: take full or partial responsibility for the R has no/weak coupling to fermion unitarity ? Ø g(H VV) =? SM Ø New resonance is needed to help restore the unitarity Full leptonic channel is favored due to low backgrounds rate ! 第 12届全国粒子物理学术会议 4
Signal model Model: 5 types of resonances Ø A benchmark model combines new resonances with an effective chiral Lagrangian. Ø Resonances couple with longitudinal vector boson only ! VBF is the only production process ! 第 12届全国粒子物理学术会议 0=g 2 m 3/64 v 2 Free parameters: coupling & Mass 5
Signal model • Higgs still exsits • New resonances R interfered with SM EW • Coupling fixed: g = 2. 5 interference SM EW R Signal = interf + R Whizard: Signal mass points: 200 -500 Ge. V Signal Xsec vs. resonance mass 第 12届全国粒子物理学术会议 6
Data and objects Data sample : 25 ns data in 2015, Luminosity = 3. 2 fb− 1 MC sample : ATLAS full simulated samples with all necessary corrections Objects Ø Electron: • Kinematic: p. T > 25 Ge. V, |η| < 2. 47 • Track and calo quality • isolated Ø Muon: • Kinematic: p. T > 25 Ge. V, |η| < 2. 5 • Track quality • isolated 第 12届全国粒子物理学术会议 Ø Jet: • Algorithm: Anti. Kt with cone 0. 4 • Kinematic: p. T > 30(50) Ge. V |η| < 4. 5 (2. 5<|η|<4. 5 ) • b-jets: BDT alg with 85% efficiency 7
Event selection • Exactly 2 high quality leptons, opposite charge • Mll>40 Ge. V • NOT inside Z mass window for ee and • At least 2 jets with p. T>30 (50)Ge. V, no b-jet • MET>35 Ge. V ϛ in VBS topology tends to be positive • Mjj>500 Ge. V • j 1 x j 2 < 0, | jj|>2. 4 • Lepton centrality > -0. 5 • frecoil < 2 the strength of the recoil to dilepton system 第 12届全国粒子物理学术会议 8
Backgrounds Physics backgrounds Backgrounds jets fake backgrounds • • • Z+jets Top related Diboson SM Higgs … • W+jets • QCD 第 12届全国粒子物理学术会议 MC prediction Data driven (Matrix method) 9
Control regions Ø Dominant background sources : Ø Definitions of validation regions : 第 12届全国粒子物理学术会议 10
Data vs. MC in Z+jets CR Mjj MTWW 第 12届全国粒子物理学术会议 11
Data vs. MC in ttbar CR Mjj MTWW 第 12届全国粒子物理学术会议 12
Data vs. MC in low-mjj VR Mjj MTWW 第 12届全国粒子物理学术会议 13
Data vs. MC in SR ee eµ µµ Reasonable agreement 第 12届全国粒子物理学术会议 14
Event yield in SR No significant excess above the SM background expectation is observed. 第 12届全国粒子物理学术会议 15
Systematics • Experimental • Theoretical : production Xsec of the backgrounds PDF, s , renormalization/factorization scale • Shape uncertainties For dominant backgrounds Z+jet & ttbar 第 12届全国粒子物理学术会议 16
95% CL upper limits ØNo significant excess above the SM background expectation is observed. Ø 95% CL upper limits are derived on the x Br for five new resonances: σ, φ, ρ, f and t Ø The frequentist method (CLs), is used to compute 95% CL upper limits 第 12届全国粒子物理学术会议 17
Summary l A first search for a heavy neutral resonance in VBF process using 3. 2 fb-1 13 Te. V ATLAS data was presented: ü Reasonable agreement of data and SM prediction in kinds of CRs ü No excess above the SM expectation is observed in signal region ü First sets of limits are obtained on the x Br of five types of new resonances ü CONF note: ATLAS-CONF-2016 -053 l Full 2016 datasets aiming at a paper 第 12届全国粒子物理学术会议 20
More plots in SR Met Njets 第 12届全国粒子物理学术会议 22
More plots in SR Mjj | jj| 第 12届全国粒子物理学术会议 23
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