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

2. 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

3. 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

4. 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 ]
May 12, 2004
Invisible Higgs, I. Nasteva, Manchester

5. Invisible Higgs, I. Nasteva, Manchester
[ATL-PHYS ]
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

6. Invisible Higgs, I. Nasteva, Manchester
[ATL-PHYS ]
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

7. 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

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/ ]
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

9. 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

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 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

11. 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 ( 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

12. 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