Fisica ai collisionatori adronici C. Gemme (INFN Genova) Corso di Dottorato 2014
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Fisica ai collisionatori adroniciC. Gemme (INFN Genova)Corso di Dottorato 2014
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Outline del corso Collisionatori (adronici)
• Motivazioni• LHC
Rivelatori ai collisionatori• Anatomia del rivelatore
(ATLAS)• Trigger• Operazione• Performance
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Osservabili e Analisi dati Misure di standard model
• Introduzione• EW: fit, W/Z, top
Higgs• Richiami teorici• La scoperta• Higgs gg• Proprieta’ dell’Higgs• Aspettative dei prossimi
run Oltre il modello standard
• Supersimmetria• “Esotici”Slides: www.ge.infn.it/~gemmec/talks/Didattica/
LHCP 2014: https://indico.cern.ch/event/279518/ICHEP 2014: http://ichep2014.es/ (this week!)ATLAS: https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHomeCMS: https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults
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W/Z bosons
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W/Z bosons
Last time we have seen how to identify Z/W boson.
Next:• W mass• W/Z Production• W/Z + jets Production• Z4l• Diboson production• Measurement of aTGC
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~67% hadrons, ~33% l ~20% , ~10% ll, 70% hadrons
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W mass World measurement by Tevatron and LEP:
• Analysis being performed with full Tevatron dataset (~10 fb-1): • CDF ~10 MeV, D0 ~ 15 MeV total uncertainty projection;
• More LHC measurement to come
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With MW
Without MW
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First step is to measure the fiducial cross-section
• Where N are the candidate events in data, B background events computed using data and simulation, Lint the integrated luminosity corresponding to data and trigger used. CW/Z is correction factor measuring the efficiency in the fiducial region determined by the selection cuts, for instance pT> xx GeV, etc…
W/Z production W/Z production studied (mainly) in the
leptonic modes:• W boson signature (~33% lv, ~67% hadrons): one
high pT lepton and missing energy
• Z boson signature (~20% inv, ~10% ll, ~70% hh): two high pT opposite charge, same flavour leptons
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NMC,rec events after simulation, reconstruction and
selection
NMC,gen,cut events at the generation in the fiducial cuts
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W/Z production The fiducial cross-section is dependent on the analysis cuts,
therefore not very useful for comparisons with other results or theoretical expectations.
The total cross-section in the leptonic channel is instead given:
• where the acceptance AW/Z is used to extrapolate the cross-section measured in the fiducial volume, σfid, to the full kinematic region:
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NMC,gen,cut events at the generation in the fiducial cuts
NMC,gen,all events at the generation
√s = 7 TeV, L = 33 pb-1, muon channel√s = 7 TeV, L = 36 pb-1, electron channel
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W/Z production Once the cross-sections in the electron/muon channels are
measured, the ‘leptonic’ result is given, properly accounting for the common systematics errors:
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• Measurements already limited by sys and lumi uncertainties
• Good agreement with NNLO prediction
• Discriminating power against different PDF sets
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W/Z production
Measuring the ratio between e and muon channel is a measurement of the branching ratio between the two channels
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Total cross-section vs cms energy
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W/Z production PDF Cross-sections and mainly the
differential cross-sections provide discrimination power for the PDFs.
• Sensitive to gluon PDF 10 bins in pT(Z) and 8 bins in Y(Z):
CMS PAS SMP-13-013
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W/Z production with jets Production of jets in association with vector bosons allow to
test pQCD• Difficult to provide NLO predictions at high multiplicities • PDFs probe region of high Q2 and low x
Important background for many searches of new physics Sensitive to renormalization scales, PDFs and hard parton
radiation at large angles.
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W/Z production with jets Up to 7 jets measured (17
events)
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Rare processes, an example: Z4l
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Diboson production (WW,ZZ,WZ,W,Z,)
At the LHC dibosons measurements allow Precision tests of Standard Model and searches of new physics: The major background to many Higgs channels Background to many BSM searches Probes of anomalous gauge boson self-interaction
Sensitive to BSM physics Closely related to Higgs/EWSB mechanism
The electroweak Lagrangian of the SM allows for triple gauge couplings:
Leads to two bosons in the final state
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Diboson production (WW,ZZ,WZ,W,Z,)
In the boson production: Largest LO contributions :
s-channel with triple gauge vertex
qqbar t- and u-channel (without triple coupling!)
Gluon-gluon: typically a few % of the 7/8
TeV cross sections: from 10s of fb to 10s of pb
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Anomalous Triple Gauge couplings SM(+Higgs): Existence and strength of Triple and
Quartic gauge boson couplings fully constrained• Deviations = New Physics!
However, physics beyond the SM could allow for ”anomalous” Triple Gauge Couplings (aTGCs)• Search for an excess in high energy tails where SM
backgrounds are small
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Parametrization of aTGCs Non-SM processes can affect TGCs:
• aTGCs modify total production rate as well as event kinematics• Possible vertices using an effective Lagrangian with a
parametrization:
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For ZZZ and ZZ couplings (forbidden!):
For WWZ and WW couplings:
No deviation from SM expectation found!
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Diboson production
Example: measurement of ZZ production at 8 TeV• Reconstructed Z in both e+e- and +-• Small background contamination
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Diboson production
Example: measurement of ZZ production at 8 TeV• Reconstructed Z in both e+e- and +-• Small background contamination
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Measurements areslightly above theoryexpectations (but inagreement within 1)
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The big picture
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Top quark
https://indico.cern.ch/event/279518/session/27/contribution/37/material/slides/0.pdf
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By Grannis, Tevatron Legacy@ LHCP2014
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Top Quark is special!
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Latest results:ΔMtop/Mtop <1%
Best relative precision available for any of the quark masses
Its mass is much larger than any other fermion only quark that has weak decay in real W, rather than a virtual W.
The decay width is large: decay happens before any strong bound may be formed as happens for c and b measure quark properties before hadronization.
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Top production
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Sin
gle
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pTop-
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top
t-chan tW-chan s-chan
Top production at Tevatron vs LHC: complementary information!
Single top quarks produced by EW interaction has been observed at Tevatron in 2009
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Top decay
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Each top decays ~100% tWb
~67% hadrons, ~33% l
Top pair production decays
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Top-antitop cross-sections at LHC
Cross-sections are measured in all the channels, despite the S/B being different between them.
Similar for 8 TeV, results are coming…
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Top-antitop cross-sections: summary
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* Representative exp. uncertainty: ~8% @ 7 TeV;5% @ 8 TeV per experiment; 5% Tevatron* Prediction in agreement with observed data
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Single top production
Small cross sections: very small yields and overwhelming background
• ttbar• W+jets (t-channel) Z+jets (tW-channel) • QCD multijet, dibosons
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Single top production Small cross sections: very small yields and overwhelming
background• ttbar, W/Z+ jets, QCD multijet, dibosons
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Single top production: Summary
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No significant deviation from SM observed
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Top mass
Top mass world combination: mtop = 173.34 ± 0.76 GeV
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Top and calibration
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Top final states involve all types of particles...• light-quark and b jets, missing transverse energy (ν), electrons,
muons, tau decays products The whole detector is involved: used for detector
commissioning and calibration• Example: a method to perform a b-tagging calibration exploits
the very high purity of the tt pairs with both W’s decaying leptonically.
• The b-tagging efficiency can be computed either with tag-and-probe or applying b-tagging algorithm to both jets and comparison with simulation.