search for supersymmetry with early lhc data david stuart, uc, santa barbara. may 12, 2010
TRANSCRIPT
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Search for Supersymmetry with early LHC dataDavid Stuart, UC, Santa Barbara. May 12, 2010
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Outline
What is Supersymmetry & why are we looking for it?
How do we go about searching for it?
When might we find it?
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
W, Z, fermion masses could be generated by Higgs
mechanism.
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
W, Z, fermion masses could be generated by Higgs
mechanism.
=> Search for Higgs at CMS
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
W, Z, fermion masses could be generated by Higgs
mechanism.
![Page 7: Search for Supersymmetry with early LHC data David Stuart, UC, Santa Barbara. May 12, 2010](https://reader035.vdocuments.site/reader035/viewer/2022062515/56649cd75503460f9499e779/html5/thumbnails/7.jpg)
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
W, Z, fermion masses could be generated by Higgs
mechanism. But mass values still not explained.
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
W, Z, fermion masses could be generated by Higgs
mechanism. But mass values still not explained.
And mH should diverge due to corrections:
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
W, Z, fermion masses could be generated by Higgs
mechanism. Though values not explained.
And mH should diverge due to corrections.
Would cancel if fermionboson “super symmetry”.
~
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
W, Z, fermion masses could be generated by Higgs
mechanism. Though values not explained.
And mH should diverge due to corrections.
Would cancel if fermionboson “super symmetry”.
Cancel the debt by printing more particles.~
~
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
Unification would be nice.
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The Standard Model, an incomplete success
Parameters unexplained: masses, mixings, couplings.
Unification would be nice, and corrections from the extra “SUSY” particles seem to provide it.
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The Standard Model, an incomplete success
The standard model does not explain dark matter.
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The Standard Model, an incomplete success
The standard model does not explain dark matter.
A conserved SUSY quantum number would. The LSP.
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Supersymmetry, an experimentalists dream
Many parameters to measure. Doubled mass spectrum, more mixings.
New puzzles to solve. Whence the masses and mixings?
Why is the symmetry broken?
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Supersymmetry, an experimentalists dream
Many parameters to measure. Doubled mass spectrum, more mixings.
New puzzles to solve. Whence the masses and mixings?
Why is the symmetry broken?
Supersymmetry’s variety of signatures makes it
a target representative of many new models.
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How to search for SUSY?
Produce super-partners, and
detect that we have done so.
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How to search for SUSY?
Produce super-partners, and
detect that we have done so.
Massive, so need energy => LHC
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How to search for SUSY?
Produce super-partners, and
detect that we have done so.
Massive, so need energy => LHC
Production via SM like diagrams.
For example...
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How to search for SUSY?
The same diagram that produces WZ in SM…
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How to search for SUSY?
…could produce Winos and Zinos.
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How to search for SUSY?
The cross section for weak production is small.
But squarks & gluinos would be strongly produced…
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How to search for SUSY?
In pairs if RP conserving.
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How to search for SUSY?
Cross-section is calculable given masses.
Example: 400 GeV squark and gluino
gives ≈40 pb at 7 TeV.
≈ 1/4 of top quark cross-section.
Could produce thousands this year.
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Varied signatures
Higgssector
LSP
squarks
gluino
sleptons
charginos/neutralinos
A CMS mSUGRA benchmark, LM0
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Higgssector
LSP
squarks
gluino
sleptons
charginos/neutralinos
A CMS mSUGRA benchmark, LM0
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Varied signatures
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Higgssector
LSP
squarks
gluino
sleptons
charginos/neutralinos
A CMS mSUGRA benchmark, LM0
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Varied signatures
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Many different search modes
Multijets + MET
Lepton + jets + MET
Dileptons + jets + MET
Same sign dileptons + jets + MET
Trileptons + jets + MET
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Many different search modes
Each mode has different backgrounds.
Search amounts to predicting these bkgdsand looking for an excess above prediction.
Multijets + MET Lepton + jets + MET
Dileptons + jets + MET Same sign dileptons + jets + MET
Trileptons + jets + MET
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Background cross-sections
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Background cross-sections
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Need to predict the Njet and MET tails?
But kinematic tails are hard to calculate:higher orders, pdfs, detector effects…
e.g., Z+jets
qZ
+
-
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How to predict the Njet and MET tails?
Monte Carlo predictions?
Sophisticated, higher order modeling,
e.g., ALPGEN.
Elaborate simulation of detector response.
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How to predict the Njet and MET tails?
Monte Carlo predictions?
Sophisticated, higher order modeling,
e.g., ALPGEN.
Elaborate simulation of detector response.
Both are software…Only trust in so far as validated with data.
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How to predict the Njet and MET tails?
Data validation challenges:
Slow.
Fit away signal?
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How to predict the Njet and MET tails?
Data validation challenges:
Slow.
Fit away signal?
Don’t want to wait…
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How to predict the Njet and MET tails?
Would like a quick, data-driven method to predict MET in Z+jet events.
In this case it is fake MET,
from mis-measurement.
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Missing ET in Z+jets
The Z is well measured. The MET comes from the detector’s response to the jet system.
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Missing ET in Z+jets
For each Z+jet event, find an event w/ a comparable jet system and use its MET as a prediction.
Huge QCD x-section makes such events SUSY free.
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Missing ET in Z+jets
For each Z+jet event, use a MET template measured from events with a comparable jet system in O(1) pb-1.
Templates measured in bins of NJ and JT = j ET.
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Missing ET in Z+jetsExample of template parameterization
Background predictionData distribution
For each data event...
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Missing ET in Z+jetsExample of template parameterization
Background predictionData distribution
For each data event,look up the appropriate template.Sum these, each withunit normalization, to get the fullbackground prediction
N JETSpT>50 GeV
sumETBin 1
sumET Bin 2
sumETBin 3
sumET Bin 4…
2
3
4…
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Missing ET in Z+jets, MC closure test
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Missing ET in Z+jets, MC closure test
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Missing ET in Z+jets, MC closure test
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Missing ET in Z+jets, MC closure tests
“Scaled” includes a low MET normalization, which is important for low NJ.
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Missing ET in +jets, MC closure test
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Missing ET in +jets, MC closure test
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Missing ET in +jets, MC closure test
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Missing ET in +jets, MC closure tests
“Scaled” includes a low MET normalization, which is important for low NJ.
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Missing ET in Z/+jets, robustness tests
Various detectoreffects could addMET tails.
Check robustnesswith MC tests,applied equally toall samples.
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Missing ET in Z/+jets, robustness tests
• R=0.8 hole at (h,f)=(0,0)• Double gaussian smearing• Randomly add 50-100 GeV “noise jets”• Vary nJ slope by ±50%.• Jet energy scale sensitivity.
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Missing ET in Z/+jets, robustness tests
• R=0.8 hole at (h,f)=(0,0)• Double gaussian smearing• Randomly add 50-100 GeV “noise jets”• Vary nJ slope by ±50%.• Jet energy scale sensitivity.
Now working to validate this with low ET +jets from early data.
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Missing ET in W()+jetsW
Can we predict W+jets, and ttW+jets?
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Missing ET in W()+jets
Templates can predict the fake MET in W+jet events, but we also need to predict the real MET, i.e., the pT.
Can we predict W+jets, and ttW+jets?
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Missing ET in W(+jets
Templates can predict the fake MET in W+jet events, but we also need to predict the real MET, i.e., the pT.
But, the pT spectrum is ≈ same as the pT spectrum,
if we ignore V-A or randomize W polarization.
Can we predict W+jets, and ttW+jets?
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Missing ET in +jets
Pretend we could detect and apply templates.
Mismatch due to b-jet dominance.
But, neutrino pT dominates MET.
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Missing ET in +jets
Combining template prediction with pT spectrum
gives a prediction for the full MET distribution.
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Missing ET in +jets
The same approach predicts W shape,
if polarization is not extreme.
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Comparison with signal
Benchmark points (LM4 and LM1) stand out with 200/pb at 14 TeV.LM4=(m0=210,m1/2=285); LM1=(60,250). tan()=10.
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But we don’t have 14 TeV.
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But we don’t have 14 TeV. So it will be a bit harder.
When will we discover SUSY with 7 TeV?
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But we don’t have 14 TeV. So it will be a bit harder.
When will we constrain SUSY with 7 TeV?
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When?
Systematic uncertainty assumed to be 50% overall.Separate tight selections for 100 pb-1 and 1 fb-1.
Expected limits in multijets + MET search
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When?Similar limits from same-sign dilepton search
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When?So far, we only have a few inverse nanobarns. But, that is a few hundred million events…
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Detector performance encouraging with first data.
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Tracking calibrated with Ks, , ,J/, D resonances.
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When?Expect 100 pb-1 by end of this year and 1 fb-1 by end of next year. Tuning up…
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Summary
Potential for quick sensitivity to Supersymmetry
If we can obtain robust, data-driven background predictions
QCD based templating gives an in situ prediction of MET distribution.
Charged lepton pT predicts neutrino pT.
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Acknowledgements
MET prediction method by Victor Pavlunin. PRD81:035005 arXiv:0906.5016
Several slides borrowed from Jeff Richman
Other material from
…and references therein.
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Extra slides
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Dileptons
m(e±m) m(l +l −)
%χ20 → l +%l −; %l − → l − %χ 1
0
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How to search for SUSY?
Leading order cross section, tan = 3, A=0, >0
LM0
LM1
Rough limit from Tevatron
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Typical decay patterns in LM0
%b1 → %t1W
−(43%)
%t1 → %χ1
+W−(100%
%b1 → %χ20b(29%)
→ %χ1−t(24%)
%χ20 → %χ 1
0e+e− (3.3%), %χ 10μ +μ − (3.3%)
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Typical decay patterns in LM0
%g → %b1b + c.c.(54%)
%b1 → %t1W
−(43%)
%t1 → %χ1
+W−(100%
%g → %t1t + c.c.(46%)
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Standard Model backgrounds to Z+jets
+ +
+q Z
e+
e-