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)

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Backups

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

Other top Sleuth states from CDF

D0 Results in the Most Discrepant CDF States

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D0 example of collaboration scale factors- muon tracking efficiency

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D0 MIS Checking Histogram

Not used in fitCheck for consistency in bulk of distribution