May 12, 2004 1

Invisible Higgs at ATLAS

Brian Cox, Jeff Forshaw, Rohini Godbole, Irina Nasteva

ATLAS UK Physics meeting

May 12, 2004 Invisible Higgs, I. Nasteva, Manchester 2

MotivationIn 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%

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Invisible Higgs signalProduction 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

jet

jet

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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]

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Analysis[ATL-PHYS-2003-006]

Selection cuts:1) Two tag jets with PT > 40 GeV and |η| < 5.0, separated in

rapidity: |η1 – η2 | > 4.4 , η1.η2 < 02) Invariant mass of the two jets Mjj > 1200 GeV3) Missing PT > 100 GeV4) 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

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[ATL-PHYS-2003-006]

cut (1) – jet PT and |Δη|

cut (2) – Mjj

cut (3) – missing PT

azimuthal angle cut – ΔΦjj

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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

yedt

d ~

t

sy

ˆln

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BFKL pomeron measurementsHard 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

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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 HiggsBFKLQCDjj

BFKL background in plots is a factor of 2 – 5 smaller than the full LO calculation

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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

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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 progressNo results yet

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Summary and outlook

• a first estimate of leading order BFKL background to the invisible Higgs

• need further analysis to include NLO contributions