Invisible Higgs at ATLAS
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Transcript of Invisible Higgs at ATLAS
May 12, 2004 1
Invisible Higgs at ATLAS
Brian Cox, Jeff Forshaw, Rohini Godbole, Irina Nasteva
ATLAS UK Physics meeting
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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