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

http://www.ge.infn.it/~gemmec/talks/Didattica/

https://indico.cern.ch/event/279518/

http://ichep2014.es/

https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults

Giugno 2014 Fisica ai Collisionatori Adronici 3

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


~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

Giugno 2014 5

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


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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√s = 7 TeV

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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


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


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.


W/Z production with jets Up to 7 jets measured (17

events)


Rare processes, an example: Z4l


arX

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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


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


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


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:


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


Diboson production

Example: measurement of ZZ production at 8 TeV• Reconstructed Z in both e+e- and +-• Small background contamination


Measurements areslightly above theoryexpectations (but inagreement within 1)

The big picture



Top quark

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

By Grannis, Tevatron Legacy@ LHCP2014


Top Quark is special!


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


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…


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


Single top production Small cross sections: very small yields and overwhelming

background• ttbar, W/Z+ jets, QCD multijet, dibosons


Single top production: Summary


No significant deviation from SM observed

Top mass

Top mass world combination: mtop = 173.34 ± 0.76 GeV


ATLA

S-C

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F-2014-0

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Top and calibration


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.

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

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