b+Jet+MET topology-based physics signatures
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While most SUSY searches focus on particular points in the parameter space of SUSY models, our group is developing a broad research program based on physics signatures containing multi-b jets and missing ET in the final state. This includes understanding (and measuring) Standard Model processes and probing SUSY in this final state signature.
The requirement of b-jets in events with large missing ET and multi-jets is well motivated both theoretically and experimentally:
- QCD, and WZ+(light quark) jets backgrounds are significantly reduced.
- Top background is irreducible, but b-tagging provides additional handles to control and measure it.
- Reducing non-top SM backgrounds by requiring b-tags could be one of the keys to the early discovery of SUSY at the LHC.
- In many scenarios, where the third-generation of squarks are expected to be light, most new physics events will contain two or four b- or t-quarks.
- Even if SUSY is first discovered in inclusive jet + MET or lepton events, understanding the production of third-generation squarks will be critical to answer important physics questions.
- Is new physics related to Dark Matter? since b-tags reduce the QCD multi-jet background, looser MET cuts can open up the possibility to study the shape of the MET, to determine if the missing energy originates from a massive object.
- Quadratic divergences in the top sector: third-generation squarks (stops, sbottoms) are responsible for the largest quantum corrections that stabilize the weak scale. b-tagging can help determine the mass scale for third-generation partners.
- Origin of the electroweak symmetry breaking: if third-generation partners are light, higgs particles can be produced in decay chains starting with stops and sbottoms, as well as in gluino decays. A light Higgs could be first discovered in this channel.
The b+MET signature is almost unexplored in ATLAS: b-tag and missing ET are among the most challenging and complex techniques. Expertise in multiple areas is required, which matches well SLAC strengths.
There are several experimental challenges to b+MET analyses:
- Efficient b-tag trigger to afford lower missing ET and jet pT requirements (to increase the event selection efficiency)
- Jet energy scale and resolution, and the effect of multiple interactions.
- Missing ET clean up, calibration, and significance. In particular, in the presence of b-jets in the final state.
- b-tagging:
- b-tag background estimation at high tag multiplicities.
- Estimation of b production from gluon splitting.
- b/c flavor separation.
- high pT b-tagging
Our group is heavily involved in all these areas is making significant contributions to ATLAS reconstruction software and analysis.
An overview of the b+Jet+MET topology-based research can be found in the diagram and links below.
Top quark cross section measurement in the missing ET + b-jet channel
Verifying top x section and mass and probing for SUSY at the same time.
Inclusive missing ET + multi-b search
missing ET + 4b analysis
Top + b
Top + MET
ttH
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Jet reconstruction, calibration, and resolution
introduction...
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Track-jets
Jet energy resolution improvements using tracks
Jet energy resolution improvements using longitudinal calorimeter segmentation
Jet-Vertex association algorithm
Baseline correction for pile-up and noise
Semi-leptonic b-jet energy scale
Measurement of the jet energy resolution
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b-tagging
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Track-jet based b-tagging
High pT b-tagging
Event Tagging
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Hadronic flavor tagging
The aim of this research program is to develop techniques to identify the flavor of hadronic jets, in particular: quark/gluon separation, tagging b-jets from gluon splitting, and the identification of highly boosted (bb) jets.
Gluon bb tagging
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highly boosted (bb)-jets
Quark/gluon separation
High Level Trigger b-tagging
LVL2 impact parameter Chi2 algorithm
b+MET combined trigger
The SLAC group has proposed, for the ?rst time in an hadron collider experiment, a technique to identify b-tagged jets from gluon splitting, and distinguish them from single b-tagged jets. This is important because the main backgrounds containing b?jets to signals from new physics, as well as for single-top, and Higgs production, consist of the production of a W boson in association with b?quarks produced by gluon splitting.
Next-to-leading order calculations show that, at the LHC, the cross section for merged b?jets is a factor of two higher compared to the
case in which the two b?quarks are produced at larger angles and reconstructed as separate jets.
The method developed uses a Neural Network (NN) trained with tracking and calorimeter information based on the fact that merged (bb) jets are wider, and have larger calorimeter-cluster and track multiplicity than single b jets.
We plan to use this neural network to select control samples with different gluon splitting content to test data/Monte Carlo agreement, reject b-tag background events, and improve the estimation of b-tag backgrounds.
Highly boosted (bb)-jets
Very recently it has been proposed that (low mass) Higgs production in association with W boson could be efficiently identified at the LHC if the Higgs is produced with high transverse momentum. Highly boosted Higgs decaying to b-quarks produce a signature of merged (bb)-jets, which can be distinguished from generic QCD jets.
The original paper proposes a method based on jet sub-structure to separate (bb) jets from QCD jets. We would like to investigate the use of a neural network technique, similar to the gbb NN developed to separate gluon->(bb) jets from b-jets, and study the kinematic differences of merged (bb) jets from gluon splitting and merged (bb) jets from Higgs.
Quark/gluon separation
We are working in the development of a neural network based algorithm that assigns jets a probability that they originated from quarks or gluons, based on Monte Carlo simulations.
There are several analysis and problems that can benefit from this technique, in particular:
- Validation of jet energy scale on quark/gluon enhanced data samples.
- Improve the understanding of Standard Model backgrounds: ttbar+jets.
- Use flavor tagging to improve inclusive Super-symmetric jet+met searches: reduce the QCD background by rejecting quark-like jets.
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High Level Trigger b-tagging
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Level 2 impact parameter Chi2 algorithm
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