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Physics potential of Physics potential of ATLAS at LHC ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1 M. Aharrouche Physics with ATLAS
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Page 1: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Physics potential of Physics potential of ATLAS at LHCATLAS at LHC

Mohamed Aharrouchefor the ATLAS Collaboration

1M. Aharrouche Physics with ATLAS

Page 2: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

OutlineOutline

IntroductionQCD measurements Electroweak measurementsHiggs SearchSupersymmetry

2

part-1

part-2

part-3

M. Aharrouche Physics with ATLAS

Page 3: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

IntroductionIntroduction

Our universe Which theory can describe its behavior, quantitatively!?

Particle Physics Study of fundamental constituents of our

universe the interactions between the constituents

Standard Model is the only particle theory that has been verified experimentally which ´partially´ answers the question above

M. Aharrouche Physics with ATLAS 3

Page 4: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Standard ModelStandard Model

M. Aharrouche Physics with ATLAS 4

Standard ModelStandard Model

TheoryTheory(QFT, symmetries…)(QFT, symmetries…)

ExperimentExperiment(spectrum of particles…)(spectrum of particles…)

PredictionsPredictions

Test with dataTest with data Consistent with all Consistent with all current experimental current experimental data!data!

Page 5: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Standard ModelStandard Model

Relativistic quantum field theory + local gauge symmetries-> particle-antiparticle symmetry required by QFT

Quarks and leptons in doublets

Forces between particles due to exchange of particles (bosons)

Unify the electromagnetic and weak interactions

M. Aharrouche Physics with ATLAS 5

Page 6: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

EWSBEWSB

The construction of the electroweak sector of the Standard Model from its gauge symmetry results in four massless bosons W+, W-, Wo and Bo!

Three of the physical bosons are not massless in natureEWSB

The Higgs boson introduced by hand into Standard Model by the mechanism for EWSB, but not seen yet

Does the Higgs exist ?

M. Aharrouche Physics with ATLAS 6

Page 7: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

7M. Aharrouche Physics with ATLAS

Large Hadron ColliderLarge Hadron Collider

Page 8: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

pp collisionspp collisions

Protons are composite interaction unknown at parton level interaction energy ≪ proton energy proton remnants disappear in the

beampipe kinematics must be reconstructed from

the decay products

Constituents of the protons are described by structure functions

Protons have strong interactions cross sections for production of

strongly interacting particles are large huge QCD backgrounds

8

pp

M. Aharrouche Physics with ATLAS

Page 9: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Structure of an eventStructure of an event

M. Aharrouche page 9 Physics with ATLAS

from T. Sjoestrand

Page 10: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Structure of an eventStructure of an event

The hard sub-process

M. Aharrouche page 10 Physics with ATLAS

Page 11: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Structure of an eventStructure of an event

Resonance decays

M. Aharrouche page 11 Physics with ATLAS

Page 12: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Structure of an eventStructure of an event

Initial-state radiation

M. Aharrouche page 12 Physics with ATLAS

Page 13: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Structure of an eventStructure of an event

Final-state radiation

M. Aharrouche page 13 Physics with ATLAS

Page 14: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Structure of an eventStructure of an event

Multiple parton–parton interactions

M. Aharrouche page 14 Physics with ATLAS

Page 15: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

LHC vs previous hadron collidersLHC vs previous hadron colliders

Cross sections at the LHC are essentially one order of magnitude larger than at the Tevatron new (x, Q2) regime Gluons play a more dominant role at higher energies

M. Aharrouche Physics with ATLAS 15

Page 16: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Road Road

16

10 TeV

14 TeV

10 pb-1

100 pb-1

1 fb-1

time

Initial detector & trigger synchronisation, commissioning, calibration & alignment, materialRediscover SM processes

Understand SUSY and Higgs background from SMMore accurate alignment & EM/Jet/ETmiss calibrationEarly discoveries

Higgs discovery sensitivity (MH=130~500 GeV)

Explore SUSY to m ~ TeV SM Precision measurements

M. Aharrouche Physics with ATLAS

Page 17: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

ATLASATLAS

17

see Andree‘s talk for more details

Multi-purpose detectorcoverage up to |η| = 5;design to operate at L= 1034cm-2s-1

Inner Detector (tracker)Si pixel & strip detectors + TRT;2 T magnetic field;coverage up to |η|< 2.5.

Calorimetryhighly granular LAr EM calorimeter( | η |< 3.2);hadron calorimeter – scintillator tile- LAr for endcap&forward ( | η |< 4.9).

Muon Spectrometerair-core toroid system(| η | < 2.7).

M. Aharrouche Physics with ATLAS

Page 18: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Particle detectionParticle detection

1818

No signals at all ; only missing energy

Track; energy deposit in ECAL

Track; tracks/deposits in muon chambers

No track; only energy deposit in ECAL

Hadronic jets ; signals in all devices

Displaced vertices; signals in all devices

e

/ /q g b

Everything must be reconstructed only from:

M. Aharrouche Physics with ATLAS

Page 19: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Common variables used in the Common variables used in the analysis of pp collisionsanalysis of pp collisions

Transverse momentum

Rapidity

Pseudo-rapidity

Angular separation

19

2 2T x yp p p

1log

2z

z

E py

E p

log tan2

2 2R

x

y

z

M. Aharrouche Physics with ATLAS

Page 20: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

QCD MEASUREMENTS

20M. Aharrouche Physics with ATLAS

Page 21: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

IntroductionIntroduction

At high energy hadron collider most of the bunch-crossings involves a mixture of interactions with high and low transverse momentum transfer from initial to final state.

Hard interactions Emission of at least one particle with high pT

Interactions at high scale can be well described using perturbative QCD

Soft interactions Very few particles are produced with significant pT (pT > 2 GeV) Interactions with soft components, not easily calculable within QCD,

require non-perturbative phenomenological models

21M. Aharrouche Physics with ATLAS

Page 22: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Minimum Bias EventsMinimum Bias Events

Any activities in the detector above certain pT threshold (e.g. 100 MeV)

Event type non-diffractive event (low pT) single-difractive event double diffractive event

Characteristics very rare high pT objects uniform energy deposits in calorimeter along η low pT tracks distributed at all azimuthal positions

How we measure them? <nchg>, <pT>, dN/dη

22M. Aharrouche Physics with ATLAS

Page 23: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Minimum BiasMinimum Bias MC modeling

Remember for the soft interactions we need to use phenomenological models

Predictions for the LHC• Use existing data from other experiments to predict for the

LHC• Different Models tuned to agree to SppS and Tevatron • Different predictions at LHC energies

Model dependence (Pythia vs PHOJET at 14TeV)• σTOT : 102-119 mb

• <Nchg>: 70-91

• dNchg/dη at η=0: 5.1-6.8

• <pT> at η=0: 550-640 MeV

Large uncertainties on MB!

Measuring MB special scintillator trigger Reconstruct charged tracks Limitations

• Limited rapidity coverage

• Standard tracking only down to 500 MeV in pT

23

A. Moraes

M. Aharrouche Physics with ATLAS

Page 24: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Underlying EventsUnderlying Events

All particles from a single p-p collision not to do with a hard sub-process (ISR/FSR) B e a m R e m n a n t s M u l t i p l e I n t e r a c t i o n s

Ho we measure them? Look at tracks in transverse

region w.r.t. jet activity

24M. Aharrouche Physics with ATLAS

Page 25: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Underlying EventsUnderlying Events

UE are measured in different colliding energies

Modelling of UE necessary tool to predict at LHC energies

MC models extrapolates (from SppS) to the LHC energies Challenge to describe entire energy

range from SppS to LHC

Underlying event uncertain at LHC

TevatronPythia Phojet

25M. Aharrouche Physics with ATLAS

Δ%

Page 26: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Inclusive Inclusive jetsjets

Inclusive jet cross section measurement Tests perturbative QCD in new energy

regime Sensitive to new physics (e.g. quark

compositeness). One of the first physics measurements

for ATLAS (after calibration!)

Difficulty Jet energy scale uncertainty

26

Effect due to 1% jet energy scale uncertainty Relative change in the inclusive jet cross-section as calculated with error PDFs w.r.t to best fitM. Aharrouche Physics with ATLAS

Page 27: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Dijet studiesDijet studies

The measurement of di-jets and their properties can be used to constrain PDF’s.

Dijet production in hadron-hadron collisions results in Δφjj = | φj1 - φj2| = π in the absence of

radiative effects. Δφjj small deviations from π → additional soft

radiation outside the jets Δφjj as small as 2π/3 → one additional high-pT

jet small Δφjj– no limit → multiple additional hard

jets in the event

27M. Aharrouche Physics with ATLAS

Page 28: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

W/Z+jetsW/Z+jets

W/Z + jet production at LHC proceeds via quark-antiquark or quark-gluon interactions

Measure W/Z + jet(s) cross-section Probing pQCD important background to new physics, Higgs and top Physics benchmark

• In-situ calibration of lepton efficiencies• Jet energy balancing• Missing transverse energy resolution

Z+jets (e channel) Isolated di-electron OR single electron trigger Electron ET>25 GeV

Cone 0.4 jets (ET>40 GeV) Fit Z mass sidebands

28M. Aharrouche Physics with ATLAS

Page 29: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

ELECTROWEAK MEASUREMENTS

29M. Aharrouche Physics with ATLAS

Page 30: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Z and W Cross Z and W Cross SectionSection

30M. Aharrouche Physics with ATLAS

Page 31: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Z and W productionZ and W production Z production

at 14TeV ~1.5x107 events/year at

low luminosity (1033cm-2s-1) From qq annihilation xqxqbar ~ 4x10-5

Longitudinal momentum• PL = 0.5*√s*(xq - xqbar)

Z decays To two energetic fermions with

opposite charge 70% to quarks pair: dominated

by the background leptonic decay channels: cleaner

31

W production ~10x Z production There are more W+ than

W- W+ is peaked at high

rapidity & W- is peaked at mid-rapidity

W decays ~32% to one energetic

lepton and one neutrino ~68% to hadrons

W l

ν

M. Aharrouche Physics with ATLAS

Page 32: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Z Event SelectionZ Event Selection Z->ll event selection:

An electron trigger with a 10 GeV threshold

Two isolated electrons, pT > 15 GeV

An invariant mass in a window of 20 GeV around the Z mass

Background In the electron channel signal and

background fraction are simultaneously estimated via a fit that leads to (8.5±1.5)% of background rate, with the uncertainty coming from modeling the shape

In the muon channel the dominant background is t]tbar and the total uncertainty on this background is 20%

Ze+e-

50 pb-1

32M. Aharrouche Physics with ATLAS

Page 33: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

W Event SelectionW Event Selection

W->lν selection: An electron trigger with a 20 GeV

threshold one isolated electron in the

geometrical acceptance with more than 25 GeV of transverse momentum

missing transverse energy > 25 GeV transverse mass > 40 GeV

Background: In the electron channel the jet fraction

estimated with a data driven method to be (0±4)%, and the W->τν with an uncertainty of 3%

In the muon channel a theoretical uncertainty of 15% is assumed on t]tbar background, plus a 10% one on the rejection of the isolation cut that is a total 20% on this background rate

33M. Aharrouche Physics with ATLAS

50pb-1

Page 34: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Z and W Cross SectionZ and W Cross Section

Signal AcceptancexSign

al selection efficiency

expected number of background events (from

data)

number of observed events

Luminosity:Large uncertainty in earliest data (up to 10%)

Expectations for 50 pb-1

Expectations for 1 fb-1

34M. Aharrouche Physics with ATLAS

Page 35: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

W MassW Mass

35M. Aharrouche Physics with ATLAS

Page 36: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

W Mass MeasurementW Mass Measurement

Select W candidate events ( previous slides). 2 observables sensitive to the W mass

n pT (lepton)

Build templates distributions pT(mW) and MT(mW);

Fit the templates to data -> find mW .

36M. Aharrouche Physics with ATLAS

15 pb-1

Page 37: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

W Mass MeasurementW Mass Measurement

Results at 15 pb-1

37

With pT lepton

δMW = 110 (stat) 114 (exp.) 25 (PDF) MeV⊕ ⊕

With mT

δMW = 60 (stat) 230 (exp.) 25 (PDF) MeV⊕ ⊕

M. Aharrouche Physics with ATLAS

Page 38: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Diboson Diboson

38M. Aharrouche Physics with ATLAS

Page 39: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Diboson Physics StudiesDiboson Physics Studies

DibosonProduction cross-sections WW / WZ / ZZ / Z / W typically 10 times higher than Tevatron tens to hundreds events in the first fb-1

Anomalous charged triple-gauge-boson coupling Self interaction among 3 gauge bosons deviations from SM counterparts:

• Δg1Z= g1

Z - 1, ΔκZ = κZ - 1, Δκγ = Δκγ - 1, λγ 0, λZ 0

Anomalous neutral triple-gauge-boson coupling Neutral TGC: ZZZ, ZZ & Z: forbidden in SM tree level

• Study ZZ processes

f4 0,f5

0,f4Z 0,f5

Z 0

New physics control samples background to Higgs and beyond SM physics

39M. Aharrouche Physics with ATLAS

Page 40: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

WW SelectionWW Selection

Electron selection: Require 2 isolated leptons of opposite charge. Tight cuts associating calorimeter

information with tracks. Pseudorapidity cuts: || < 2.5, excluding inter-calorimeter gap regions 1.35 < || <

1.57. Isolation: ET < 8 GeV in cone R = 0.45. Helps discriminate against WW, ttbar and

Drell-Yan processes. Muon selection

Muon pT > 5 GeV. Isolation: ET < 5 GeV in cone R = 0.45.

Jet veto: Seeded cone jets with R = 0.7 Jet Veto: ET > 20 GeV. Jet || < 3.

Missing ET, with correction for energy loss in cryostat. MET > 50 GeV.

MZ veto: |Ml+l−- MZ| > 15 GeV

(pT(l+l−), miss_pT) > 175º.

40M. Aharrouche Physics with ATLAS

Page 41: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

WW Detection Sensitivity with 1 WW Detection Sensitivity with 1 fb-1 fb-1

Cut based analysis Drell-Yan and ttbar are

dominant backgrounds. After cuts at 1 fb-1:

Significance = S/√B = 15.5

Multivariate Analysis Boosted Decision Trees 15 inputs to BDTs were used,

e.g. pT and isolation of muon, E/p for electron, jet multiplicity

At 1 fb-1 after cut at 200: signal – 469 events, background - 92. Significance = S/√B = 23 (in gaussian standard deviation)

41M. Aharrouche Physics with ATLAS

Page 42: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Signal Rates at 1 fbSignal Rates at 1 fb-1 -1 : All Process: All Process

42M. Aharrouche Physics with ATLAS

Page 43: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Anomalous Triple Gauge CouplingAnomalous Triple Gauge Coupling

The signature of anomalous couplings in diboson production is an increase in the cross-section at high values of gauge boson transverse momentum (pT ) and diboson transverse mass (MT ).

To probe sensitivity we compare the ’measured’ diboson production cross sections and the vector boson pT or diboson MT distributions to models with anomalous TGC’s

A binned likelihood using the MT or pT spectrum for each channel ->to extract the 95% C.L intervals of anomalous coupling parameters

43

ATLAS

M. Aharrouche Physics with ATLAS

Page 44: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Limits on Anomalous TGCLimits on Anomalous TGC

WW ProductionWW Production

Charged TGC Charged TGC 95% CL limits, = 2 TeV: All processes

Neutral TGC Neutral TGC 95% CL limits, = 2 TeV: All processes

44

ATLAS

M. Aharrouche Physics with ATLAS

Page 45: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Weak mixing angleWeak mixing angle

45M. Aharrouche Physics with ATLAS

Page 46: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Forward backward asymmetryForward backward asymmetry

Parity violation in the neutral current

Consequence: Asymmetry in the angular distribution of leptons from Z decay

The probabilities to produce a lepton with a polar angle and with π- are different

Theta dependence of the cross section

At Z-pole Determination of the Weinberg effective angle and the precision on this value

AFB = b(a – sin²(eff))

46

q q

e-

e+

M. Aharrouche Physics with ATLAS

Page 47: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

cos(cos(*)*)

Defined in the Collins-Soper frame to take into account the non zero transverse momentum of the incoming quark

In pp collisions we suppose that the quark direction is the same as the Z boost

47

pp

*

e-

ATLAS

M. Aharrouche Physics with ATLAS

Page 48: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Weak mixing angleWeak mixing angle

Forward electron identification important sensitivity increases with forward electrons

48M. Aharrouche Physics with ATLAS

World average value: 1.5x10-4

Events with correct quark direction

Page 49: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Top QuarkTop Quark

49M. Aharrouche Physics with ATLAS

Page 50: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Top ProductionTop Production

Top pair production (σtt ~ 830pb)

Single top production

50

t-channel Wt-channel W* (s-channel)

~ 250 pb ~70pb ~ 10 pb

Vtb

Vtb Vtb

Vtb

gg->tT ~90% qq->tT~10%

M. Aharrouche Physics with ATLAS

Page 51: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Top DecaysTop Decays

51

Lepton side

Hadron side

In the Standard Model, the decay of top quarks takes place almost exclusively through the t->Wb. W-boson decays in about 1/3 of the cases into a charged lepton and a neutrino and in 2/3 of the cases, decays into a quark-antiquark pair

M. Aharrouche Physics with ATLAS

Page 52: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Top Physics StudiesTop Physics Studies

52

Productions mechanisms Production X-sections Vtb Spin correlations ttbar production by new resonances

Properties Top mass Charge Decay properties

• Electroweak (V-A) vertex: W helicity• Rare Top decays

Search for New physics using heavy flavour (>1fb-1)

M. Aharrouche Physics with ATLAS

Page 53: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Top Pair SelectionTop Pair Selection

Event selection Only semileptonic channel will be shown here dilepton channel can also be used lepton trigger pT lepton>20 GeV

4 jets pT >20 GeV and 3 jets pT >40 Gev

ET miss>20 GeV

Top = 3 jets giving Highest PT sum No b tag (W constraint Mw+- 10 GeV) for 1 jj comb.

Main backgrounds W+jets (dominant) single top Z → l+l− + jets. QCD with fake leptons and MET diboson WW,WZ,ZZ

53

100pb-1

M. Aharrouche Physics with ATLAS

Page 54: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Top Pair Cross Section Top Pair Cross Section Measurement Measurement

Two methods maximum likelihood fit on the

three-jet invariant mass distribution

Counting the number of top candidate events that pass the selection, and subtracting all backgrounds

Systematic uncertainties

54

100pb-1

M. Aharrouche Physics with ATLAS

Page 55: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Top Quark MassTop Quark Mass

With same cuts as before, but require all jets to have pT > 40GeV, since below that jets not very well calibrated.

Require exactly two b-tags. Use χ2 method to reconstruct

hadronic W, by minimising:

over all light jet pairs.

55

mt = 175.0 ± 0.2GeV(stat)

M. Aharrouche Physics with ATLAS

Page 56: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

VVtb tb from Single Top (t channel) from Single Top (t channel)

Event Pre-Selection 1 high PT isolated lepton in the central

region At least 2 jets with PT > 30 GeV

≤ 4 jets with PT > 15 GeV Among those jets, at least one has to

be b-tagged Missing energy > 20 GeV

Final selection (Multivariate analysis) Δσ/σ= 5.7%(stat)±22%(sys) (BDT) Δ|Vtb|/|Vtb| = 12%

56

1 fb-1

M. Aharrouche Physics with ATLAS

Page 57: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

HIGGS SEARCH

57M. Aharrouche Physics with ATLAS

Page 58: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Higgs Production at LHCHiggs Production at LHC

mainly via gluon fusion

ttbar-fusion

W,Z fusion: increasinglyimportant at high masses

associated productionin the low mass region

58M. Aharrouche Physics with ATLAS

Page 59: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

Higgs DecaysHiggs Decays

Low Higgs mass (m(H) < 2mZ) H , H bb H , via VBF H ZZ* 4 H WW* or jj, via VBF

m(H) > 2mZ

H ZZ 4

qqH ZZ

qqH ZZ jj

qqH WW jj

59M. Aharrouche Physics with ATLAS

Page 60: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

H->H->

60M. Aharrouche Physics with ATLAS

Page 61: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

H->H-> Signal:

2 high-pt photons from a narrow resonance

Irreducible background: Di-photon events from QCD Quark-photon events with extra photon from

fragmentation

Huge reducible background: QCD photon-jet QCD di-jets

Excellent photon identification to reject the large QCD background. Rejection larger than 8000 per single jet with photon efficiency larger than 80%.

Mass resolution Photon calibration : energy scale and resolution, separation of converted

and unconverted photons Photon direction: from calorimeter pointing and tracking based vertices

61M. Aharrouche Physics with ATLAS

Page 62: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

H->H->: Signal/Background : Signal/Background

62

Inclusive analysis(gluon fusion)

Higgs plus 1 jet analysis(gluon fusion + VBF)

Higgs plus 2 jet analysis(80% VBF, 20% gluon)

M. Aharrouche Physics with ATLAS

Page 63: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

H->H->: : SignificanceSignificance

Expected signal significance for 10 fb‐1 of integrated luminosity as a function of the mass.

Various analyses (see previous slide) including events counting and combined fits

63M. Aharrouche Physics with ATLAS

Page 64: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

HH

64M. Aharrouche Physics with ATLAS

Page 65: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

HH Due to poor Higgs mass resolution for H,

inclusive analysis not possible

Exclusive (VBF) searches: Reduce QCD backgrounds by using distinct topology of jets in association with Higgs

Signal: 2 high-pT jets from quarks, at large η (“forward

tag jets”), no jets in between -pair from a resonance leptonic and hadronic decays are considered

(ℓℓ, ℓh, hh topologies) Missing ET

Background: Irreducible: Z+jets (Z→) Reducible: W+jets, tt+jets

65

Leading PT jet

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HH

Mass Reconstruction Mass reconstruction via collinear

approximation:Tau decay products collinear to tau direction

Approximation breaks down when the two taus are back-to-back

Mass resolution limited by missing ET (8-10 GeV) and tau reconstruction (≈10-13 GeV

Significance based on fitting mττ spectrum, background

uncertainties incorporated by using profile likelihood ratio. Pile-up not included

Only lep-had and lep-lep channels used for combination due to challenge in predicting QCD bgd for had-had final state

66

MH=120 GeV

Z jj

tt, WW EW

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H->ZZH->ZZ(*)(*)->4->4ℓℓ

67M. Aharrouche Physics with ATLAS

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H->ZZH->ZZ(*)(*)->4->4ℓℓ

Golden channel in the mass range mH > 130 GeV

Signature Two opposite sign pairs of leptons coming from the primary

vertex compatible with Z mass (at least 1 couple)

Irreducible Background: continuum ZZ(*) →4 leptons

Reducible Backgrounds: Zbb 4 leptons tt 4 leptons suppressed by:

• impact parameter • lepton isolation in the tracker and in the calorimeter

Issues: lepton efficiencies: Reconstruction and Identification lepton energy resolution

68M. Aharrouche Physics with ATLAS

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H->ZZH->ZZ(*)(*)->4->4ℓℓ

Reconstructed 4‐lepton mass for signal and backgrounds, in the case of a 130 and 180 GeV Higgs boson

Sensitivity in channels with different lepton flavours calculated with Poisson statistics and without systematic errors

69

MH=130GeV

MH=180GeV

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H->WWH->WW(*)(*)->-> ℓν ℓν ℓν ℓν

70M. Aharrouche Physics with ATLAS

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H->WWH->WW(*)(*)->-> ℓν ℓν ℓν ℓν

Large H → WW BR for mH ~ 160 GeV/c2

No mass peak use transverse mass

Signal gg and VBF production 2 leptons (e or μ) + MET

Backgrounds: tt, tWb : rejected by jet-veto WW,WZ, ZZ: rejected by kinematical cuts Two main discriminants:

• Lepton angular correlation• 0 jet channel

– jet veto• 2 jet channel

– 2 forward jets and no jets in between

Need exact knowledge of background shape

71M. Aharrouche Physics with ATLAS

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H->WWH->WW(*)(*)->-> ℓν ℓν ℓν ℓν

The expected significance at 10fb‐1 for gluon‐gluon process, and the VBF process are shown. For combined channels sensitivity 5σ for mH larger than 140 GeV

The ee and μμ channels are under study72M. Aharrouche Physics with ATLAS

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Combination Combination

Needed, to cover the full mass range and increase the sensitivity General combination method, based shapes as well as taking

systematics into account by use of the profile likelihood ratio, has been prepared.

Four important search channels (shown here) used in the combination. The median p-value obtained for excluding SM Higgs boson for various

channels as well as combinations. Value below p=0.05 indicates an exclusion at 95%. Value below p=2.87×10-7 claims for a discovery at 5σ significance

Confidence level need good knowledge of the background shapes data-driven methods have been studied, based on control (signal-free)

regions

73M. Aharrouche Physics with ATLAS

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Discovery PotentialDiscovery Potential

74M. Aharrouche Physics with ATLAS

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SUPERSYMMETRY

75M. Aharrouche Physics with ATLAS

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The actual problem of Standard The actual problem of Standard Model Model

Does not unify the electroweak and strong force Does not include gravitational forces Many free parameters

26 parameters (for mν>0)

Requires inputs from experiment not completely predictive

No candidate for Dark Matter No explanation for the Dark energy 3 generations? Hierarchy problem

Corrections to the Higgs boson mass diverge quadratically

V-A structure of the weak interaction? …

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Alternative?Alternative?

Need a more fundamental theory of which SM is low-E approximation

M. Aharrouche Physics with ATLAS 77

Fundamental theoryFundamental theory

Standard Standard ModelModel

Page 78: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

ExtensionsExtensions

Technicolor Super-strong interaction

Grand Unification Theories Larger symmetry group including SM symmetry groups

Preons Composite quarks and leptons

String theories Particles are strings

Supersymmetry

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SupersymmetrySupersymmetry Postulate symmetry between fermions-bosons

fermions ↔ bosons

SUSY modfies running of SM gauge couplings to give grand unificcation at single scale

SUSY can be a new source of CP-violation• may explain the matter/anti-matter asymmetry in the universe

SUSY partners to have same masses as SM states Not observed! SUSY must be a broken symmetry at low energy Various possible SB mechanisms exist

79M. Aharrouche Physics with ATLAS

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R-ParityR-Parity New quantum number in order to warrant the conservation of

baryonic and leptonic R-parity

R = (−1)(2S+L+3B)

R=1 for SM particles and R=-1 for superpartners R is a multiplicative quantum number

R-parity conservation Not required by proton stability (protects proton decays) SUSY-particles are always produced in pairs and each decays to the lightest SUSY

particle (LSP) Lightest SUSY-particle (LSP) is stable

• should be colorless and neutral• weakly interacting → escapes the detector undetectable• large missing energy• dark matter candidate

R-parity violation LSP decays (into leptons, jets) No missing energy! Sparticles may be produced singly

80M. Aharrouche Physics with ATLAS

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SUSY ModelsSUSY Models

not possible to explore in full the 100-dimensional parameter space of the MSSM adopt some specific assumptions for the SUSY breaking

minimal SuperGravity (mSUGRA): simple boundary conditions at GUT scale reduce the number of parameters to ~5 SUSY breaking is mediated by gravitational interactions LSP: neutralino Parameters

• Common scalar mass m0

• Common gaugino mass m1/2

• Common trilinear scalar interaction A• Ratio of vevs of two Higgs fields tan• Sign of Higgs mass parameter

GMSB: gauge messengers; light gravitino LSP SUSY breaking is mediated by a gauge interaction through messenger

gauge fields

81M. Aharrouche Physics with ATLAS

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SUSY Studies at ATLASSUSY Studies at ATLAS

mSUGRA and GMSB models with R-parity conservation

Set of benchmark points has been chosen in the mSUGRA and GMSB frameworks

Evaluate SM background to SUSY searchesEstimate background sources using real data

wherever possible

SUSY searches and measurements Inclusive searchesSearches for specific signaturesParameters determination

82M. Aharrouche Physics with ATLAS

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SUSY SignaturesSUSY Signatures

Strongly interacting sparticles (squarks, gluinos) dominate production

Long cascade decay into the LSP: e.g. lightest neutralino Cascades produce also leptons : easier background

rejection Channels

multi jets + missing ET + (leptons) photons, tau leptons, b-jets

Main Background W boson, Z boson and top quark production each in association with

jets constitute major backgrounds

83M. Aharrouche Physics with ATLAS

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Global Event Variables Global Event Variables

Effective mass measure of the total activity

in the event distribution peaks at a value

which is strongly correlated with the mass of the pair of SUSY particles produced

Transverse sphericity ST

λ1, λ2 eigenvalues of the 2×2 sphericity tensor calculated over all jets (pT

jet > 20 GeV) and leptons

SUSY events are more `spherical’ (ST~1)

Transverse mass Stransverse mass

84M. Aharrouche Physics with ATLAS

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SUSY SearchSUSY Search

85M. Aharrouche Physics with ATLAS

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Background DeterminationBackground Determination

QCD background is difficult to predict

Estimate it in a `control’ sample and propagate this measurement to the `signal’ sample

Background estimation from data

Example: Z+jets Select Z+jet events with Z → ℓ+ℓ− Calculate MET removing leptons Use MC to get corrections

86M. Aharrouche Physics with ATLAS

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No Lepton ModeNo Lepton Mode Signature

MET + 4 jets

Selection At least four jets with pT > 50 GeV at least

one of which must have pT > 100 GeV MET > max(100GeV,0.2Meff) transverse sphericity ST > 0.2 Δφ(MET, jet1-3) > 0.2 Reject events with an e or a µ Meff>800GeV

Clear excess of events is visible with 1 fb-1

For the SU2 point to be found in the 0-lepton channel, one would have to select larger values of Meff and a greater integrated luminosity would be required.

87

1fb-1

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One Lepton ModeOne Lepton Mode

Has smaller cross section, but with better control over background

Selection Exactly one isolated lepton with pT > 20

GeV At least four jets with pT > 50 GeV at

least one of which must have pT > 100 GeV

MET > max(100GeV,0.2Meff) transverse sphericity ST > 0.2 Δφ(MET, jet1-3) > 0.2 Transverse mass, MT > 100 GeV

Meff>800GeV

ttbar+jets dominant, QCD negligible

88

1fb-1

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Other ModesOther Modes

tau mode ≥1 τ + 4 jets + MET τ reconstruction efficiency estimated

from real data by replacing e or μ Tt+jets dominated background Significance (assuming 20% syt.

uncertaint for the background)

b-jet mode ≥1 b-jet + 4 jets + MET b-tagging performance important

89M. Aharrouche Physics with ATLAS

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Discovery Reach @ 1fbDiscovery Reach @ 1fb-1-1

90M. Aharrouche Physics with ATLAS

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SUSY MeasurementsSUSY Measurements

91M. Aharrouche Physics with ATLAS

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Mass MeasurementMass Measurement

Two missing LSPs with unknown mass No mass peak!

Strategy apply kinematics on long decay

chains to link endpoints with combinations of masses

measure endpoints (edges,thresholds) in invariant mass distributions. mll , mllq , mlq , etc.

Endpoint kinematic

92

Allanach et al., JHEP 09 (2000) 004

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Dilepton EndpointDilepton Endpoint

invariant mass of dileptons after flavor subtraction and efficiency correction When at least one of the sleptons is lighter than the

the two-body decay channel is the dominant. The distribution of the invariant mass of the two leptons is triangular

When the sleptons are heavier than the the decay proceeds through the three body

channel. the distribution of the invariant mass of the two leptons has a non-triangular shape

93

SU3 1fb-1 SU4 0.5fb-1

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Leptons + squark endpoointLeptons + squark endpooint

Invariant mass combinations of leptons and jets

94

SU4 0.5fb-1SU4 0.5fb-1

edge thresholdM. Aharrouche Physics with ATLAS

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mSUGRA MassesmSUGRA Masses

Several mass edges can be reconstructed

SUSY masses can then be obtained from a fit to all edges

Use only the endpoints involving leptons and jets: five measurements

SU3: 4 masses and 5 meaurements

Endpoints most sensitive to mass differences

95

SU3 1fb-1

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MSSM HiggsMSSM Higgs

96M. Aharrouche Physics with ATLAS

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MSSM HiggsMSSM Higgs

MSSM provides two complex Higgs doublets Each doublet has charged and neutral components Each doublet contributes to the up-type or down-type fermions

5 physical Higgs boson states remain after EWSB Two like SM-Higgs: h and H One pseudoscalar: A Two charged higgs: H±

All SUSY Higgs masses are given, at tree level, by the two parameters: MA and tan

97M. Aharrouche Physics with ATLAS

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Neutral Higgs Neutral Higgs

Produtcion Dominantly produced in gg fusion of low and intermediate tanβ. bbH

dominant for high tanβ

Decays Φ->bb: dominant decay but large QCD background Φ->ττ: large BR Φ->μμ: low BR, but good mass resolution (3% vs ττ’s 20%.)

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Neutral Higgs: h/A/HNeutral Higgs: h/A/H->2->2 ℓ ℓ44νν

Signal Only channels with associated b studied 2 isolated leptons 1 b-jet at least Not more than two jets (including the b-

tagged jets) Higgs mass reconstruction

• Collinear approximation: decay products emitted in τ-direction

Background Z->ll, ttbar, W+jets, QCD multijets need to be estimated from control data

• QCD multijets: Selecting same-sign τ-pairs• Z->ττ: Z->μμ as controle sample

99M. Aharrouche Physics with ATLAS

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Neutral Higgs: h/A/HNeutral Higgs: h/A/Hµµµµ

Signal 2 isolated muons of opposite charge MET < 40 GeV Separate handling of two event types

• 0 b-jets : suppresses ttbar background• >= 1 b-jets

– suppresses Z background– Jet veto against ttbar

background Z+jets, ttbar, ZZ Background estimated from data

using:• Background enriched control sample.

Primarily useful for ttbar• e+e− and e±μ control samples with same shape

as μ+μ− background.

100M. Aharrouche Physics with ATLAS

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Neutral Higgs: discovery potentialNeutral Higgs: discovery potential

Experimental systematic uncertainties primarily from jet resolution, jet energy scale, and b-tagging.

M. Aharrouche Physics with ATLAS 101

h/A/H->2l4ν h/A/Hμμ

Less coverage than ττ but combination of μμ and ττ channels can improve the discovery reach

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Charged HiggsCharged Higgs

For the light charged Higgs boson (mH± < mtop) dominant production mode is from top decay t ->bH+

dominant decay is H+->τν.

For the heavy charged Higgs boson (mH+ > mtop) dominant production modes are gg->tbH+ and gb-

>tH+

dominant decays are H+tb and H+-->τν.

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Charged Higgs Charged Higgs DiscoveryDiscovery

103M. Aharrouche Physics with ATLAS

MH+ <= mtop : Will be Covered at LHC

MH+ > mtop : Sensitivity Only for High tanβ

Page 104: Physics potential of ATLAS at LHC Mohamed Aharrouche for the ATLAS Collaboration 1M. AharrouchePhysics with ATLAS.

ConclusionConclusion

Atlas will perform new tests of the Standard Model predictions Predictions of Quantum Chromodynamics can be tested in

High pT jet production W/Z production

Precise measurements of Standard Model parameters competitive results to LEP/Tevatron

ATLAS have a huge discovery potential If Higgs exists

full mass range, already at low luminosity;

If SUSY exists: discovery of TeV-scale SUSY should be easy, Despite missing LSP, precision measurements of masses will be also

possible determination of model parameters is difficult

104M. Aharrouche Physics with ATLAS


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