Vector Boson Scattering at high energy at the LHC
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picture produced with actual CSC-WZ(ChL) full simulated events. CSC-01-00-00 Layout
John Idárraga Georges Azuelos P.-A. Delsart
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Outline
2- ChL model 1- Motivation
3- Phenomenology 4- Results
John Idárraga
John Idárraga
3 Problems with the SM
Despite the fact that it provides a unified description of the weak an electromagnetic interactions:
● The Higgs boson has not been found yet
● No dynamical explanation of EWSB
● Hierarchy problem
● Fine tuning
● The three families, why ?
● Not a unification for electromagnetic and weak interactions
● Yukawa couplings
● Neutrino mass
● Gravitation is not included
● No dark matter candidate
motivation
John Idárraga
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Supersymmetry:
• Divergences are canceled in a natural way by the presence of scalars and fermions in supersymmetric fields.
• It is a theory where the electroweak and strong forces find their common origin at very high energy.
• A light Higgs is present. Five higgses in MSSM.
motivation
SUSY
Little Higgs
Solves the hierarchy problem, still a light Higgs boson is present.
Any other options including a Higgs field ?
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motivation
Extra dimensions Technicolor
It introduces new massless fermions whose chiral symmetry is spontaneously broken by a mechanism that at the same time is responsible for EWSB. Three composite Goldstone bosons (called technipions) produced at the breaking of the symmetry provide with longitudinal components for vector boson masses.
What about higgsless models ?
In the Higgsless models with extra dimensions, Kaluza-Klein excitations of gauge bosons yield the vector boson masses.
?!
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It is important to consider the scenario where no light Higgs boson is found at the LHC
How can we understand ewsb ? Two approaches:
1)Effective theory (Chiral Lagrangian model)
2)Dynamical model
(technicolor, higgsless models in theories with extra dimensions, ...)
motivation
John Idárraga
7 Limits on the Higgs mass:
In the SM the diagrams with a light Higgs boson are essential in vector boson scattering. Without the Higgs exchange process unitarity is violated for energies > ~1 TeV.
Non violating unitarity give the conditions
s
c 1.7 TeV
M
h 870 GeV
motivation
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Either we will discover a light higgs Boson with a mass smaller that
~870 GeV, in which case SM or MSSM would be a good description, or we will discover new phenomena in the region > 1 TeV. (MSSM light higgs must have mass < 150 GeV)
As a matter of fact, the only way of avoiding a light Higgs is to presume new physics at High
Energy (LHC scale)
motivation
The big
picture !
Copyrighted
Jorge Cham phdcomics.com
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Chiral Lagrangian Model
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ChL model
The Chiral Lagrangian model
The “bottom-up” approach
The lagrangian includes mass terms
Also kinetic terms
where
At low energies, interactions of quarks are dominated by QED and QCD
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ChL model
The Chiral Lagrangian model
The “bottom-up” approach
We include vector bosons and their kinetic terms
We also have terms violating the symmetry.
The mass terms
At energies close to the mass of the vector bosons
e.w. quark interactions go exactly as in the SM with SU(2) x U(1) symmetry
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ChL model
The Chiral Lagrangian model
The “bottom-up” approach
To solve the problem we introduce the sigma field
Fermion mass terms respect the symmetry now.
VB mass terms arise from kinetic terms of the sigma field
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ChL model
The Chiral Lagrangian model
The “bottom-up” approach
Anomalous couplings:
Assuming CP invariance, additional dim-4 operators are possible. Here we find the VV interaction terms
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ChL model
If the symmetry breaking mechanism, present in models with Higgs, is not valid, we could expect that there will not exist a light higgs boson. In such scenario the interaction between vector bosons become strong at high energies.
This a good probe of alternative models of EWSB is Vector Boson Scattering (VBS) in the high energy range. The ChL model is an effective theory that reproduces well the phenomenology of VBS at low energy and includes terms that allow extrapolation to higher energies.
The Chiral Lagrangian model
The “bottom-up” approach
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Scattering pion-pion Unitarization of ChL model
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Must assume some unitarization procedure at high mass since we don’t have a full expansion.
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Pade (or inverse amplitude) method. We get very good description of pion resonances.
ChL model
W-M Yao et al 2006 J. Phys. G: Nucl. Part. Phys. 33 1
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Phenomenology
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The parameter space of VV scattering can be reduced to 2 parameters, and on this space we can find resonances in the range 500 GeV to 3 TeV, hopefully visible at LHC.
phenomenology
V: vector resonances S: scalar resonances
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Vector boson fusion – WZ case
Main characteristics of the signal
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Forward jets
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Central jets that might be merged
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Dileptons in the central region
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MissingET for W reconstruction
phenomenology
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Signals
ChL and CSC
* cross section in pb
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phenomenology
Pseudorapidity of the initial quarks in signal events.
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phenomenology Before continuing with the signal characteristics let us list
The backgrounds:
Meant for high energy samples have special
preselection cuts
Includes
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phenomenology
Z+4 jets
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phenomenology
Z+3 jets
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phenomenology
More signal characteristics and background production preselection cuts
F o rw a rd J e ts a p p lie d o n Z + 4 a n d Z+ 3 j e ts
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phenomenology
C e n tr a l p a rt o n s a p p li e d o n Z+ 4 a n d Z+ 3 j e ts
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phenomenology
C e n tr a l p a rt o n s o n ly a p p lie d t o Z+ 3 j e ts
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Discovery potential - Results
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W,Z reconstruction from jet pair
Z reconstruction:
dilepton and
Forward jet
Forward jet
W reconstruction:
single lepton + MET
Produced with v-atlas CSC-01-00-00 thanks to M. Gallas and V. Tsulia
Vector boson fusion – WZ case – 1.15 TeV resonance
ChL and CSC
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results
Leptonic vector boson
identification
After lepton quality
selection, we apply the following cuts
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results
Forward Jet
tagging
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results
Jet Inner structure
Reconstruction with 2 cases:
● Single jet
● Double jet
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results
Hadronic vector boson
identification
Single and double jet cases.
Jets with
inner structure
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results
Event Selection
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results
Transverse momentum cut (first and second jet)
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results
W mass cut
Single and double jet cases
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results
Resonance reconstruction ! Single jet case
Inner W jet decay structure not considered yet
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results
Resonance reconstruction ! Single jet case
Inner W jet decay structure applied
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results
Resonance reconstruction ! Double jet case
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results
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results
Resonance reconstruction !
+ forward jets + W reco with missing Et
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results
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results
Resonance transverse mass reconstruction !
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results
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results
Conclusions
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results
