ATLAS UK Physics meeting Invisible Higgs at ATLAS Brian Cox, Jeff Forshaw, Rohini Godbole, Irina Nasteva ATLAS UK Physics meeting May 12, 2004

Invisible Higgs, I. Nasteva, Manchester Motivation In some extensions to the Standard Model, the Higgs can decay into invisible final states: SUSY models H χ0χ0 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. 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% May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

Invisible Higgs signal jet Production via vector boson fusion: qq qqVV qqH (where V = W,Z) 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 May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

Invisible Higgs, I. Nasteva, Manchester Main backgrounds* Z + jets associated production (Zjj) where Z νν W + jets associated production (Wjj) where W lν and the lepton is undetected QCD multi-jet production: QCDjj, QCDjjj + fake missing PT due to particles escaping detection or to semileptonic decays. * from a study by L. Neukermans and B. Di Girolamo [ATL-PHYS-2003-006] May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

Invisible Higgs, I. Nasteva, Manchester [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) 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 May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

Invisible Higgs, I. Nasteva, Manchester [ATL-PHYS-2003-006] cut (1) – jet PT and |Δη| cut (2) – Mjj cut (3) – missing PT azimuthal angle cut – ΔΦjj May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

BFKL pomeron background rapidity gap 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 May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

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 May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

Invisible Higgs, I. Nasteva, Manchester BFKL QCDjj Missing PT distribution after cuts (1) - (4) BFKL background in plots is a factor of 2 – 5 smaller than the full LO calculation The BFKL and QCD backgrounds are eliminated by the azimuthal angle cut ΔΦjj < 1 rad (at leading order) May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

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 May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

Invisible Higgs, I. Nasteva, Manchester 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 May 12, 2004 Invisible Higgs, I. Nasteva, Manchester

Invisible Higgs, I. Nasteva, Manchester Summary and outlook a first estimate of leading order BFKL background to the invisible Higgs need further analysis to include NLO contributions May 12, 2004 Invisible Higgs, I. Nasteva, Manchester