Fisica ai collisionatori adronici C. Gemme (INFN Genova) Corso di Dottorato 2014

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

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

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

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

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

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

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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arX

iv:1

40

3.5

65

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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!

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)

The big picture

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Top quark

https://indico.cern.ch/event/279518/session/27/contribution/37/material/slides/0.pdf

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.

Top production

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Sin

gle

to

pTop-

anti

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

Top decay

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Each top decays ~100% tWb

~67% hadrons, ~33% l

Top pair production decays

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

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

Top mass

Top mass world combination: mtop = 173.34 ± 0.76 GeV

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ATLA

S-C

ON

F-2014-0

08

/ C

DF

note

11071

C

MS P

AS T

OP-1

3-0

14 /

D0 n

ote

64

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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.