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Model-independent approaches to missing energy at the...
Transcript of Model-independent approaches to missing energy at the...
Model-independent approaches to missing energy at the LHC
Johan Alwall, SLAC
NTU, Taipei, Taiwan, 16 Sep 2009
Based on:arXiv:0810.3921 (PRD 79, 075020)arXiv:0809.3264 (PRD 79, 015005)
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● The Standard Model – one of the most successful theories in the history of physics
Introduction: The Standard Model
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Introduction: The Standard Model
● The Standard Model – one of the most successful theories in the history of physics
● Describes the particle content and interactions of the microscopical world with no major discrepancies with experiment
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Introduction: The Standard Model
● The Standard Model – one of the most successful theories in the history of physics
● Describes the particle content and interactions of the microscopical world with no major discrepancies with experiment
● Yet undiscovered: The Higgs particle, breaks the Electroweak symmetry and gives mass to all particles
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Introduction: The Standard Model
Problems with the Standard Model● Quadratic quantum corrections to the mass of
the Higgs particle → hierarchy problem
t
t
H H ΔMH
2 ~ M2new physics
~100 GeV MPl
~1019 GeV
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Introduction: The Standard Model
Problems with the Standard Model● Quadratic quantum corrections to the mass of
the Higgs particle → hierarchy problem ● Dark matter observations in the sky
Luminous (X-ray-emitting) gas(stopped by collision)
Dark matter (deduced by graviational lensing)(unaffected by collision)
MACS J0025.4-1222 cluster collision (Bradac et al)
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Introduction: The Standard Model
Problems with the Standard Model● Quadratic quantum corrections to the mass of
the Higgs particle → hierarchy problem ● Dark matter observations in the sky
● Grand unification
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Introduction: The Standard Model
Solutions to hierarchy problem● New weakly interacting particles and
symmetries that cancel the “bad” loops
● Composite Higgs (new strong interactions, Technicolor)
● Removing the hierarchy by strengthening gravity (extra spacial dimensions)
t
t
H H ~ M2t - M2
t
t
H H
~
+ ~
~1 TeV 175 GeV
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Introduction: The Standard Model
Solutions to dark matter● New particles + symmetry that ensures stability
of lightest new particle (WIMP)– Simplest version: Internal parity under which SM
particles are even, new particles odd
Solutions to unification● New particles between
weak scale and GUT scale – modifies coupling evolution
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New Physics at the LHC
“Favourite” model: Supersymmetry– Symmetry between fermions and bosons –
automatically stabilizes the Higgs mass to all orders
– SM particle ↔ “superpartner” – same quantum numbers and SM couplings, different spin and massquark ↔ squark gluon ↔ gluino top ↔ stop (etc.)
– Extra quantum number carried by partners (R-parity)● Stable lightest superpartner (LSP) → Dark matter candidate● SUSY particles always pair produced
– Allows to solve all three problems within one framework!
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New Physics at the LHC
Supersymmetry not yet found (and has problems) – Need to consider alternatives
– Little Higgs
– Randall-Sundrum
– ADD (large extra dimensions)
– …
➔ To solve Hierarchy problem and dark matter, need similar features as SUSY– But can have very different mass spectra and
coupling structures!
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So why the Large Hadron Collider?
New Physics at the LHC
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So why the Large Hadron Collider?
● Particle masses around 100 GeV- 1 TeV – within reach for the 14 TeV LHC
● Some particles charged under QCD – easy to produce at a hadron collider
● We are (quite) confident that something new will be discovered at the LHC!
New Physics at the LHC
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“SUSY-like” MET signatures● At the LHC, mainly particles charged under
QCD (quark and gluon partners) will be directly produced
● Decay through “cascades” emitting quarks and leptons until reach dark matter particle
● Signature: High-E hadron jets, leptons and missingtransverse momentum (MET)
Typical process in
SUSY!
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Difficulties with MET signatures
● No visible resonances – no simple features above Standard Model backgrounds
● Event reconstruction not possible
● Complicated to measure jets and missing transverse momentum
● Difficult determining overall mass scale – only mass differences readily observable
● Extra hard jets from QCD radiation, besides the decay products – difficulties in combinatorics
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Model-independent approaches to MET+jets/leptons signals
Many models/ideas for physics beyond the Standard Model at the LHC giving signatures with
leptons, jets and missing energy● Supersymmetry: over 120 free parameters /
many mediation scenarios (SUGRA, GMSB, AMSB, Gaugino mediation, Mirage mediation, non-standard scenarios, ...)
● Little Higgs: dozens of models● Extra-dimensional scenarios (RS, UED)
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Model-independent approaches to MET+jets/leptons signals
Many models/ideas for physics beyond the Standard Model at the LHC giving signatures with
leptons, jets and missing energy● Differences between models often subtle –
difficult to distinguish in early data● Many models (esp. SUSY) have many
parameters – most unmeasurable at LHC● Constrained versions of models introduce
relations between observables not necessarily seen in data
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Model-independent approaches to MET+jets/leptons signals
Many models/ideas for physics beyond the Standard Model at the LHC giving signatures with
leptons, jets and missing energy
How to analyze and present excesses – Without model bias– Possible to compare to any model
?
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Present approaches
● Most common: Exclusions/analysis in model space of minimal model (mSUGRA/mGMSB/mAMSB) with few (~4) parameters
● Benchmark analyses (e.g. Snowmass points)● Signature-based exclusions (cross section
limits given some “standard” set of cuts)● In case of excesses: Scans of SUSY space
(~20 param) using high-level kinematical information
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Present approaches
● Most common: Exclusions/analysis in model space of minimal model (mSUGRA/mGMSB/mAMSB) with few (~4) parameters
● Problems:– Fixed relations between parameters, e.g.
mg:m
W:m
B ~ 6:2:1
– Fixed decays and branching ratios
➔ Not all parameter space covered by analysis
~LSP
~ ~
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Present approaches
mSUGRA mass ratiog → qqB
g → qqW+B
~ _~
~ _ ~mSUGRA exclusion (D0)
Example: mSUGRA/mGMSB assumes fixed ratio between gluon partner and photon partner➔ No Tevatron study allowing free mass ratio
J.A., Le, Lisanti, Wacker [arXiv:0803.0019]
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Present approaches
● Benchmark analyses (e.g. Snowmass points)– Only relevant to assess power of methods
● Signature-based exclusions (cross section limits given some “standard” set of cuts)– Restricted in scope, reduced power
● In case of excesses: Scans of SUSY space (~20 param) using high-level kinematical information and rate information– Assumes SUSY, need high-statistics data
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Presenting experimental data
Twofold problem:● How to analyze/report limits on cross sections
in a way that can be compared (by theorists) with any model?
● How to analyze/report/characterize stable excesses over background in a way that can be compared to any model and give relevant information on the underlying physics without bias to a certain model?
J.A., Le, Lisanti, Wacker [arXiv:0809.3264]
J.A., Schuster, Toro [arXiv:0810.3921]
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Presenting experimental data
Twofold problem:● How to analyze/report limits on cross sections
in a way that can be compared (by theorists) with any model?
● How to analyze/report/characterize stable excesses over background in a way that can be compared to any model and give relevant information on the underlying physics without bias to a certain model?
J.A., Le, Lisanti, Wacker [arXiv:0809.3264]
J.A., Schuster, Toro [arXiv:0810.3921]
This talk
In backup
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Model-independent characterization of excesses
If excesses in MET+jets/leptons are found at the LHC (signatures in leptons, jets and missing E
T),
the first questions* we want answered are:● Consistent with pair-production of particles with
cascade decays to massive dark matter?● Which colored particles dominate production?● Which decay chains are present?● Cross sections, mass scales?
Precision physics will come later!
*First ~500 pb-1(?)
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Model-independent characterization of excesses
How do this in a well-defined matter without introducing model bias in analysis?
● Small set of “simplified models” addressing the specific questions we want answered
● Well-defined method to compare any theoretical model to the simplified models
➔ The simplified models can act both as a detector-independent representation of data andas a basis for information about possible model space consistent with the data
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Simplified models
● Each simplified model: 3-4 mass parameters and 3-4 cross sections/branching ratios– All parameters directly measured in data
● One set of models to determine decay chains involving leptons
● One set of models to determine b quark content● For each case, one model with quark partners as
start of decay chain, one with gluon partners➔ Surprisingly good description of “data” generated
also with quite complicated models
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Simplified models for lepton content
Lep(Q)
*on- or off-shell
Lep(G)
g
g
g
g*on- or off-shell
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Simplified models for b quark content
Btag(Q)
Btag(G)
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Fitting examples
Two examples presented in our paper:● One “simple” example, where structure of
underlying model mirrors the simplified models● One “complicated” example, where underlying
model has complicated structure, including multi-level decays, squeezed spectra, competing decays
➔ Simplified models give good fit also to data from complicated model!
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Fitting examples
Simple example
Q
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Fitting examples
Dilepton invariant massHT (effective mass)
Examples of plots used for mass determination:
Lep(G) (MG=600 GeV)Lep(Q) (MQ=500 GeV)
Lep(G) (MG=600 GeV)Lep(Q) (MQ=500 GeV)
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Fitting examples
Number of leptons in lepton-incl.signal region
OSO
F
OSSF
Z cand
SSOF
SSSF
Examples of plots used for lepton BR fitting:
Lep(G) (MG=600 GeV)Lep(Q) (MQ=500 GeV)
Lep(G) (MG=600 GeV)Lep(Q) (MQ=500 GeV)
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Number of b jets above 75 GeV
Fitting examples
In this example: Lep(G)/Btag(G) “perfect fit” – can draw strong conclusions about underlying physics
Number of jets above 75 GeV
Examples of plots used for b content fitting and jet structure
Lep(G) (MG=600 GeV)Lep(Q) (MQ=500 GeV)Btag(G) (MG=600 GeV)
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qR
gq
Lb
t1
lL, ν
x4
0,x2+
x3
0
x2
0,x1+
x1
0
840
680
518
380
312
137
116
G
NI, CI
Ninv
28% qq l ν
650
440
100
5% qq ll
60% qq
G
Ninv
700
100
Lep(G) Btag(G)
t
q
qt bqb
W,(Z) l,ν
l,νW
5% qq Z
qq,ll,νν
63% tt34% qq3% bb
~
~~ ~
~~ ~
Complicated exampleFitting examples
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Fitting examples
OSO
F
OSSF
Z cand
SSOF
SSSF
Most distributions/counts still well describedDeviations hint at what might be missing in description
One-lepton pT
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How to use the simplified models?
● Fit parameters give direct information– Mass scales and mass differences
– Presence of weak bosons or lepton partners
– Over- or underrepresentation of heavy flavor
● Inspiration / starting point for model building– Deviations from data indicates missing features of
the simplified models – for second iteration
● Allows direct comparison with theory using tools available to theorists
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How to use the simplified models?
Experimental fit of Simplified model
over SM background
Theoretical comparison Simplified model
vs. three SUSY models
From experimental note Comparison by theorist
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Conclusions
● How present/analyze limits and excesses?● Model-independent approaches:
– Simplified models for analysis, characterization and presentation of early excesses
– (Not discussed: Cross-section grids for limits)
● Avoids model-bias – can fit any model● Allows detector-independent comparison with
any theoretical model● Attempt to maximize usefulness of experimental
analyses for the HEP community!
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Backup slides
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Cross section limits
Example: Non-unified/non-standard SUSY scenarios can have m
g:m
B ~ 1
● Then, gluino decays to 2 soft jets and LSP– No hard jets, no large missing transverse energy
– Need initial state QCD radiation
● A priori unclear whether Tevatron is sensitive– Need combination of MET+1-jet, 2-jet, 3-jet and
multijet searches to cover whole g-B mass plane
● Very difficult to find limits outside collaboration
~ ~
~~
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Cross section limits
Our suggestion:● Provide differential cross section limits for phase
space bins (in relevant variables) for mutually exclusive searches (1j+MET, 2j+MET, ...)
● Provide detector simulation and event generation chain verified to allow comparison with presented experimental cuts
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Cross section limits
Our suggestion:● Provide differential cross section limits for phase
space bins (in relevant variables) for mutually exclusive searches (1j+MET, 2j+MET, ...)
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Cross section limits
● Then, easy for theorists to generate the corresponding model cross sections and compare, point-by-point in parameter space, to get exclusion region.
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Cross section limits
mSUGRA mass ratiog → qqB
g → qqW+B
~ _~
~ _ ~mSUGRA exclusion
● Then, easy for theorists to generate the corresponding model cross sections and compare, point-by-point in parameter space, to get exclusion region.