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Invisible Higgs at ATLAS

Invisible Higgs at ATLAS. Brian Cox, Jeff Forshaw, Rohini Godbole, Irina Nasteva. ATLAS UK Physics meeting. In some extensions to the Standard Model, the Higgs can decay into invisible final states: SUSY models H χ 0 χ 0

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Invisible Higgs at ATLAS

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  1. Invisible Higgs at ATLAS Brian Cox, Jeff Forshaw, Rohini Godbole, Irina Nasteva ATLAS UK Physics meeting

  2. In some extensions to the Standard Model, the Higgs can decay into invisible final states: • SUSY models Hχ0χ0 • models with enlarged symmetry breaking sector (Majoron models) H JJ • Extra dimension models- H mixes with scalar fields arising from gravity propagating in the extra dimensions. Motivation It is possible that H is produced at SM rates, but decays predominantly in its invisible modes: in some regions of parameter space BR(invisible) ~ 100% Invisible Higgs, I. Nasteva, Manchester

  3. jet Production via vector boson fusion: qq qqVV qqH (where V = W,Z) jet Invisible Higgs signal • the vector bosons have PT~ mW/2 => H is • produced with transverse momentum ~ mW • jets from quarks have a small scattering • angle and are emitted in the high rapidity regions • W,Z have an energy of ~ mH/2 => the tag jets energy is ~ O(TeV) • no colour connection between the quarks – lack of hadronic activity in the central region (rapidity gaps) • The signatures of this process are: • Two far forward and backward tagging jets of moderate PT • Considerable missing PT in the central region • Rapidity gaps Invisible Higgs, I. Nasteva, Manchester

  4. Z + jets associated production (Zjj) where Zνν • W + jets associated production (Wjj) • where Wlν and the lepton is undetected • QCD multi-jet production: • QCDjj, QCDjjj + fake missing PT due to particles escaping detection or to semileptonic decays. Main backgrounds* * from a study by L. Neukermans and B. Di Girolamo [ATL-PHYS-2003-006] Invisible Higgs, I. Nasteva, Manchester

  5. The discriminating variable is the azimuthal angle separation of the tag jets ΔΦjj: • signal – flat azimuthal dependence • background – jets are back-to-back • azimuthal angle cut ΔΦjj < 1 rad [ATL-PHYS-2003-006] Analysis • Selection cuts: • Two tag jets with PT > 40 GeV and |η| < 5.0, separated in rapidity: |η1 – η2| > 4.4 , η1.η2 < 0 • Invariant mass of the two jets Mjj > 1200 GeV • Missing PT > 100 GeV • Lepton veto and jet veto (no jets with PT > 20 GeV between the tag jets) Invisible Higgs, I. Nasteva, Manchester

  6. [ATL-PHYS-2003-006] cut (1) – jet PT and |Δη| cut (2) – Mjj cut (3) – missing PT azimuthal angle cut – ΔΦjj Invisible Higgs, I. Nasteva, Manchester

  7. rapidity gap BFKL pomeron background • colour singlet exchange – gluon • radiation is suppressed (rapidity gaps) • mimics the invisible Higgs signal • when there is large missing PT: • from jets lost down the beam pipe, • when only some radiation is detected • BFKL pomeron background is potentially larger than QCD background (single gluon exchange): • where y is the rapidity separation • and ω is the pomeron intercept ω ~ 1.4 • Two jets, back-to-back in Φ, with rapidity gaps Invisible Higgs, I. Nasteva, Manchester

  8. BFKL pomeron measurements • Hard colour singlet exchange was measured at the TeVatron and found to agree with BFKL theory: • B. Cox, J. Forshaw, L. Lönnblad [hep-ph/9908464] • Gap fraction compared to D0 data: • gap fractions were calculated • from BFKL pomeron exchange • leading logarithmic calculation • of BFKL at fixed αs = 0.17 • using HERWIG 6.4 Invisible Higgs, I. Nasteva, Manchester

  9. Invisible Higgs BFKL QCDjj BFKL background in plots is a factor of 2 – 5 smaller than the full LO calculation Missing PT distribution after cuts (1) - (4) The BFKL and QCD backgrounds are eliminated by the azimuthal angle cut ΔΦjj < 1 rad (at leading order) Invisible Higgs, I. Nasteva, Manchester

  10. NLO contributions to BFKL • Monte Carlos can’t simulate reliably the high-PT and large-angle gluon radiation • this is important for both BFKL and QCD backgrounds – large-angle radiation is detected while the quark jet is lost down the beam pipe • need the next-to-leading order (NLO) • contribution to the 2 3 parton scattering • process. • NLO will increase backgrounds because of: • higher cross-sections for hard gluon emission • de-correlated azimuthal angle of the jets (if one jet is lost) => ΔΦjj cut becomes less effective Invisible Higgs, I. Nasteva, Manchester

  11. NLO calculations • gluon radiation in BFKL pomeron exchange is not calculated at next-to-leading order • it is expected to be similar to the NLO contribution to QCD three-jet production (2 3 scattering) • we can look at the NLO contribution to QCD three-jet production and estimate the BFKL background by this • use NLOJET++ QCD event generator to calculate three-jet cross sections at next-to-leading order with the KT algorithm Work in progress No results yet Invisible Higgs, I. Nasteva, Manchester

  12. Summary and outlook • a first estimate of leading order BFKL background to the invisible Higgs • need further analysis to include NLO contributions Invisible Higgs, I. Nasteva, Manchester

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