The Hunt for the Higgs Boson in the Standard Model

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The Hunt for the Higgs Boson in the Standard
Model
Produced by Andy Haasfrom theUniversity of
Washington for the DØ Experiment at Fermilab
HIGGSBOSON
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Starring WH ? leptons, neutrinos, and
bottom-quarks ZH ? leptons, neutrinos, and
bottom-quarks H ? WW ? electrons, muons, and
neutrinos The Plot Suggested over 35 years
ago by theorists to explain how particles get
their mass in the Standard Model, the Higgs boson
has been on high-energy experimentalists'
most-wanted list every since. The DØ
experiment is searching for the Higgs boson, and
is already placing new limits on Higgs its
production.
The final piece of the Standard Model is still
missing...
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Wjets with b-tagging
A major background to Higgs signals is events
with W bosons and bottom-quark jets (b-jets). DØ
has now studied and isolated events of this type!
Number of jets in W 2 jet events
First, we comparethe jets in W eventsto
simulations of theStandard Model.Good agreement!
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Wjets with b-tagging
Number of jets which are b-tagged in W 2 jet
events
B
b-jets are identified by finding tracks
fromsecondary verticesinside jets
We observe roughly the number of b-tagged jets
that we'd expect. Most of the 8 double b-tagged
events are from top-quark events.
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Wjets with b-tagging
Invariant mass of double b-tagged events
Invariant mass of double b-tagged events with
exactly 2 jets, to reduce tt background
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Wjets with b-tagging
A double b-tagged W 2 jets event
Two b-tagged jets!
electron and neutrino come from the W
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Search for Higgs ?WW
Invariant mass of two electrons in data compared
to Monte Carlo
If the Higgs is heavy (gt140 GeV), it will
decay to pairs of W's. DØ is placing new limits
on the excess production of W boson pairs.
Step 1 Understandthe data which has two
leptons and missing ET
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Search for Higgs ?WW
Angle between the two electronsin data
vs.MonteCarlo
Step 2 Make moreselective cuts to reducethe
backgrounds
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Search for Higgs ?WW
An event passing all criteria for WW
Two electrons (one from each W)Small angle
betweenthem!
Large missing ET from two neutrinos(one from
each W)
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Search for Higgs ?WW
New limits have been placed on the production of
Higgs bosons decaying to WW The limits are
compared to predictions from the Standard Model
and other theories
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Zjets with b-tagging
A large background to Higgs signals is Z events
with b-jets. DØ has now studied this process and
made a first measurement of its cross-section.
Jets in Z events compared to Monte Carlo
Events / 10 GeV
First, we comparethe jets in Z eventsto
simulations of theStandard Model.Good agreement!
Jet ET (GeV)
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Zjets with b-tagging
pT spectrum of b-tagged jets
Z b-jet event
Jet with secondary vertex
We have measured the ratio of cross-sections for
Zb / Zj0.0270.007
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Starring Doubly-charged Higgs bosons
Supersymmetric Higgs bosons Fermio-phobic Higgs
bosons The Plot More extended theories of
Electro-weak Symmetry Breaking predict many kinds
of Higgs bosons. DØ is looking hard for these
new Higgs bosons, and has now excluded regions of
parameter space in these models.
... and it could point the way to physics beyond
Standard Model.
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The Search for Higgs BosonsBeyond the Standard
Model
Produced by Andy Haasfrom theUniversity of
Washington for the DØ Experiment at Fermilab
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Search for bb(h,H,A)SUSY Higgs at high tanß
Feynman diagrams for bb(h,H,A) production
If Supersymmetry (SUSY) is correct, then there
are at least five Higgs bosons h, H, A, H, H-
DØ has completed a search for the neutral Higgs
bosons in the context of the Minimal
Supersymmetric extension of the Standard Model
(MSSM). We search for a signal from the Higgs in
triple b-tagged multi-jet events.
This coupling is proportional to tanß.The
cross-section is proportional to tan2ß!
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Search for bb(h,H,A)
Comparison of double b-tagged events with 4
jets to Monte Carlo
First, we comparedistributions of the double
b-tagged events to the Standard Model
The ALPGEN Monte Carlo describes the multi-jet
heavy-flavor background very well. We have
measured the normalization factor of data / MC
1.140.02 (stat.)
4 jets 2 b-tags
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Search for bb(h,H,A)
Comparison of triple b-tagged events to expected
background
The background shape was extrapolated from double
b-tagged data using a tag-rate-function derived
on the full multi-jet data sample. Limits were
set on the signal production, using simulated
signal shapes, the normalized background, and the
observed data.
Excluded Higgs signal
3 jets 3 b-tags
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Search for bb(h,H,A)
We have constrained the MSSM parameter space by
searching for Higgs bosons!
The limitswill continueto improve as new
dataand b-taggingalgorithms are used.
131 pb-1 of data
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Search for doubly-charged Higgs

• Many models predict the existence of Higgs bosons
  with twice the electric charge of other
  particles
• Left/Right Symmetric
• Higgs Triplet
• Little Higgs
• We search for decays of the doubly-charged Higgs
  into two like-signed muons.
• Assume a 100 branching ratio to muons when
  setting limits.

Coupling depends on L/Rhandedness
Feynman diagram for the production of
doubly-charged Higgs
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Search for doubly-charged Higgs
First, we comparedistributions before requiring
like-signed muons. Good agreement!
Comparisons of mass and ?f distributions of the
di-muon data to Monte Carlo
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Search for doubly-charged Higgs
No sign of an excessin events with isolated and
like-signed muons.
Comparisons of mass and ?f distributions of the
like-signed and isolated di-muon data to Monte
Carlo
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Search for doubly-charged Higgs
Confidence Limit set as a function of
doubly-charged (left-handed) Higgs mass
We have placed new lower limits on the masses of
doubly-charged Higgs bosons mHL gt 118.4
GeV mHR gt 98.2 GeV
113 pb-1
These limitswill improve when the full data set
is included!
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Search for Higgs ? ??
Invariant mass of photon pairs in the central and
end-cap Calorimeters

• Some models predict an enhenced branching
  fraction of Higgs bosons to photons
• Top-color Higgs
• Fermio-phobic Higgs
• We search for decays of the Higgs into two
  photons.

Error on background
Total bkgd. QCD Other
We compare thedistributions to expected
backgrounds are see... decent agreement.
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Search for Higgs ? ??
Limits set on the branching ratio of Higgs to
photons as a function of the Higgs mass
We are able to exclude a light Higgs boson which
only decays to photons.
The limitshave surpassed the Run I results!