Post on 08-Jul-2020
XLIVth Recontres de MoriondQCD and High Energy Interactions
March 14-21, 2009
Global Searches at the Tevatron
Joel Piper Michigan State University
on behalf of the DØ and CDF Collaborations
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The Standard Model is incomplete. With shrinking resources at Fermilab, we must address a central question:
Do we see what we expect?
Outline
Search Strategies at DØ and CDF
General Searches using Vista
Targeted Searches using Bump Hunter (CDF) and Sleuth ?
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A global search is an involved process with complicated results
Where do we look?Event counts in final states
Shape agreement over many distributionsEvent p
T distributions
Mass peaks
What if we see something we do not expect?
Is MC simulation accurate here?
Could detector modeling or event reconstruction cause this?
Could this be new physics?
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Divide data/MC for each *exclusive* final stateBased on high- p
T objects: , e,,, jets, b-jets, MET
Check numbers of eventsSignificance diluted by number of final states
Compare shapesUsing Kolmogorov-Smirnov probabilities
Significance diluted by number of histograms
Sample variables: (pT,M(all combinations),M
T(w/MET),R, ,)
Report significant discrepancies> 3 after trials factor
Vista checks number of events and shapes in exclusive channels
μ+μ-
e+ e-
jet jet
jet
jet
metμ±
jet
jet
jetμ±
jet
metμ±
met
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CDF uses only Monte Carlo events in SM simulationPrimarily PYTHIA and MadEvent
Fit for 43 correction parametersDetermined by a constrained global fit
Used as few parameters as possible for agreement
CDF develops a SM implementation using common MC and a global fit
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DØ uses common collaboration scale factors to search leptonic final state
DØ also uses common MCPYTHIA and ALPGEN
Multijets background from data
Apply common collaboration-wide scale factors
Can be bin-by-bin or several parameter functions
Use phase space dominated by SM processes
Then fit for normalization factorsTrigger efficiencies, k-factors, etc
7 inclusive final states
Exclude high-pT tails
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Vista Results
Large majority (>97%) of states and histograms in each experiment agree, but focus is on disagreement
CDF sees 0/399 number and 555/19650 shape discrepanciesMost shape disagreements QCD related
DØ sees 4/180 number and 24/9335 shape discrepanciesSearch limited to final states with leptons
3 basic modeling issues
-dependent trigger efficiency in + jets + MET
Muon resolution
in + MET
Jets misidentified as
photons in states
2 fb-1
1 fb-1
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Distributions of Final States' Data/MC Count Agreement
Left sidedata deficit
Right side data excess
DØ Curve: GaussianCDF Curve: pseudoexperiments
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Distributions of KS Probabilities in Histogram Shapes
Agreement Disagreement
555 discrepancies mostly in multijets dominated states
24 histograms show discrepancies after trials
Curve to the right is centered at mean rather than 0
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CDF shape discrepancies primarily in all jet final states
Soft jet emission in PYTHIA
R between jets
mass of jets
No new physics claim Generators are not in
agreement here
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At DØ, focus on leptons
-dependent trigger efficiencysingle muon
muon + jets
Track resolution modeling high p
T
muon and missing transverse energy are back-to-back
appears again in Sleuth
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CDF Bump Hunter
Search for narrow resonances in invariant masses
Define a search window of 2DM(DM=expected detector mass resolution)
Compare data to SM background
Define a possible bumpat least 5 data events
verify that 'side-bands'
agree better than center
Estimate significance of
bumps by pseudoexperiments
CDF Run II (2 fb-1)
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Bump Hunter Results
Agreement Disagreement
CDF Run II (2 fb-1)
One bump found beyond threshold
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Sleuth performs a quasi-model independent search
Focuses on tails of Σp
T for each final
state
Searches for excesses
Find cut that has most significant data excess
1-sided region
check each data point
Quantify significancepseudoexperiments
ttcross-check
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CDF Sleuth Results
The most discrepant states all SS eAfter trials
~2 effect
Probability before final state trials factor
Low probability-modeling issues, new physics?
High probability- overestimate of SM background?
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DØ Sleuth Results
OS e MET
Events in tailslarge p
T
back-to-back with MET
same problem as + MET
Other highly populated final states w/ MET
similar excess w/high-pT
Vista number excesses also trigger Sleuth “discoveries”
+ 2 jet + MET
bulk of distribution
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Conclusions
Vast majority of states and distributions are in agreement at DØ and CDF
Vista considers final state populations and the shapes of 1-D histograms
CDF- 0 population, 555 shape discrepant distributions in 2 fb-1
DØ- four population, 24 shape discrepant distributions in 1 fb-1
All of the given discrepancies point to modeling difficulties
Sleuth and Bump Hunter used to examine specific distributions for high-p
T excesses and narrow mass resonances
CDF- no Sleuth discrepancies
DØ- one Sleuth discrepancy probably related to resolution
CDF Bump Hunter finds one discrepant mass distribution (light jet emission)
No hints yet... but with 5.0 fb-1 of data already collected at each experiment, much more data are available for searches
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CDF Publications
Global Search on 1.0 fb-1:
T. Aaltonen et al., Phys Rev D 78 012002 (2008)
Global Search on 2.0 fb-1:
T. Aaltonen et al., Phys Rev D 79 0111101 (2009)
7 inclusive final states
D0 Run II Preliminary
D0 Run II Preliminary
Additional objects X
D0: Inclusive final states and object selection
's have additional 300 GeV maximum p
T cut
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Additional CDF Sleuth Results
Low probability-modeling issues, new physics?
High probability- overestimate ofStandard Model background?
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Additional D0 Sleuth Results
Most significant states all show issues with resolution
Search concentrates on a single most significant state
Just leading state crosses threshold after trials... but systematic problem
Few states on far left as
would be expected for
a background
overestimate
Cluster of states with high ; many related to resolution issue
D0 Run II Preliminary
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D0 example of collaboration scale factors- muon tracking efficiency