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Summary of the 2005 Rome ATLAS Physics Workshop. M. Cobal, University of Udine Physics Plenary, ATLAS Week, June05. Rome physics workshop. 91 entries (out of about 100 talks), 21 F plus 70 M. Some numbers : ~450 participants 100 talks ~ 35 hours of presentations and discussions.

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Summary of the 2005 Rome ATLAS Physics Workshop

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Summary of the 2005 Rome ATLAS Physics Workshop

M. Cobal, University of Udine

Physics Plenary, ATLAS Week, June05

Rome physics workshop

91 entries (out of

about 100 talks),

21 F plus 70 M

Some numbers:

  • ~450 participants

  • 100 talks

  • ~ 35 hours of presentations and discussions

June 05 ATLAS Week - M. Cobal

Sessions for physics groups

  • B-physics

  • Top

  • Higgs

  • Standard Model

  • SUSY

  • Exotics

  • Heavy Ions

  • HUGE amount of work/results

  • Cannot do justice to everything presented!!

  • Give general flavour of the workshop highlights

  • Focus as requested in talk title on early physics

For all groups bulk of analyses performed on fully simulated “Rome” samples

Concentrate on analyses possible with few fb-1

Displace center of interest from exploration of ATLAS parametrised potential to:

  • Control of detector systematics affecting measurements and discovery

  • Study of dependency of discovery potential from achieved level of alignment calibration

  • Development of strategies for estimate of systematics on background evaluation

June 05 ATLAS Week - M. Cobal

A new point of view: Commissioning!

The game to play:

Understand detector /Minimize MC dependency

  • Knowing the detector

    • Redundancy between detectors

    • Straight tracks, etc.

  • Physics: available ‘candle’ signals in physics

    • Presence and mass of the W±, Z0, top-quark

    • Presence of b-jets

    • Balance in transverse plane, PT

  • Prepair with detector pessimistic scenarios

    • Non-perfect alignment at startup, e.g. in b-tagging

    • Dead regions in the calorimeter / noise

    • Unknown precise jet energy scale

    • Assess trigger dependencies

Only after full understanding of these the road to discovery starts…

June 05 ATLAS Week - M. Cobal

Top physics

Standard Model

  • Minimum bias/Underlying event

    • Before Rome: comparing existing models with SPS/Tevatron

      and extrapolating to LHC

    • Now: based on the full ATLAS software chain, explore how well

      we can measure typical quantities:

  • Studies on W

    • Large statistics

    • Basic benchmark process

    • Aim at constraining proton PDFs

    • Emphasis on understanding systematic detector effects

Minimum bias events (~20/beam cross)

Example of “very early” physics: only need a few thousands interactions

“Soft” part of pp interactions not described by PQCD

Constitutes unavoidable background

for all physics

Measure typical quantities using

full ATLAS chain:



Large uncertainty track densities!

Charged particle density at  = 0


Multiple interaction model in PHOJET predicts a ln(s) rise in energy dependence. PYTHIA suggests a rise dominated by the ln2(s) term.

June 05 ATLAS Week - M. Cobal

Generated vs reconstructed tracks

Explore special runs without solenoid magnetic field?

Charged particle densities


1000 events




Black = Generated charged tracks

Blue = Reconstructed: NO TRT, NO solenoid

Red = Reconstructed: NO TRT, WITH solenoid

limited rapidity coverage

Can only reconstruct track down to ~500 MeV PT


June 05 ATLAS Week - M. Cobal

Uncertainty in pdf transferred to sizeable variation in rapidity distribution electrons

Limited by systematic uncertainties

To discriminate between conventional PDF sets we need to achieve an accuracy ~3% on rapidity distributions.

Error boxes:

The full PDF







MRST03 (Herwig+k-Factors)

Stat ~6 hours

at low Lumi.



Pdf determination using W bosons

W+ and W- Rapidity

June 05 ATLAS Week - M. Cobal

Full simulation

Pdf determination using W bosons

  • Generator level for Ws

e+ e- Pseudo-Rapidity

W+ and W- Rapidity




Selection Cuts applied


W- /W+


e- /e+ Ratio

Selection Cuts applied

June 05 ATLAS Week - M. Cobal

Charge Misidentification dilutes Asymmetry


Charge Asimmetry

W Asymmetry

e+e- Asymmetry

Selection Cuts applied

ARAW = Measured Asymmetry

ATRUE = Corrected Asymmetry

F-= rate of true e-

misidentified as e+

F+ = rate of true e+

misidentified as e-

June 05 ATLAS Week - M. Cobal

Systematics using Full Simulation

Charge misidentification


ARAW = Measured Asymmetry

ATRUE = Corrected Asymmetry

F-= rate of true e-

misidentified as e+

F+ = rate of true e+

misidentified as e-


Detector Level


  • Use Z -> e+e- sample from

  • Full Simulation Rome production

  • ~98K events, Herwig+CTEQ5L

  • data-like analysis (No MC-Truth)

    • Mis-ID rate negligible?


June 05 ATLAS Week - M. Cobal

Ratio <NTrackReco>/<NTrackMC>

Underlying event

UE is defined as the Transverse Region

Transverse <nch>

Df = f - fljet

  • Soft component in hard scattering event

  • On fully simulated jet sample compare

    reconstructed and generated multiplicity.

Njets > 1,

|ηjet| < 2.5,

ETjet >10 GeV,

|ηtrack | < 2.5,

pTtrack > 1.0 GeV/c

Good agreement reconstructed/generated

Can use to tune MonteCarlo

Pt leading jet (GeV)

June 05 ATLAS Week - M. Cobal

Aim to determine M(W) with precision of 15 MeV

Highest precision expected in Wν


Transverse mass MT

Missing PTmiss

PT lepton


June 05 ATLAS Week - M. Cobal

Top physics

Top physics

  • Top production: basic calibration tool for early physics 1500 tt->bW(ln)bW(jj) requiring 4 jets above 40 GeV/day at low L.

  • Need to select clean top sample from the beginning

    • Past work: show in fast simulation that top signal observable with no b-tagging

    • Rome work: perform signal and background analysis in full simulation

stt(tot) = 759 pb

stt(semi-lept: e,m)~ 30%

Nevents ~ 700 per hour


Reconstruct top w/o b-tag

Observe top quarks after ~1 week?

When no b-tag is yet present?

Hadronic top:

Three jets with highest vector-sum pT as the decay products of the top

W boson:

Two jets with highest momentum in reconstructed jjj C.M. frame.

Selection cuts:

Missing ET > 20 GeV

1 lepton Pt > 20 GeV

Selection efficiency = 5.3%

4 jets PT > 40 GeV

Trigger efficiency not taken into account yet

June 05 ATLAS Week - M. Cobal

Analysis including W+4jets background

Observe both top and hadronic W peaks!

W+jets bckg is large (and has large uncertainty)



S/B = 0.45

S/B = 0.27


300 pb-1

Number of events / 5.1 GeV

Number of events / 5.1 GeV


W+jets and MC@NLO signal

W+jets and MC@NLO signal

W mass (GeV)

Top mass (GeV)

Use peak position M(W) for light jet energy calibration

June 05 ATLAS Week - M. Cobal

Various cuts to improve purity

Ask for: 70 < M(jj) < 90 GeV


Top peak clearly visible after 1 week of LHC data


Top mass (GeV)


Ask for: b signal probability> 0.90 on 4th jet

Top mass (GeV)

June 05 ATLAS Week - M. Cobal


EPart / E



Use W in top events for jet calibration

Effect of a mis-calibration of jet energy dominant systematics

Several methods to calibrate. Simplest one:

  • compute R for k bins in E

  • apply kfactors on R and recompute R n times =>

June 05 ATLAS Week - M. Cobal

Results after recalibration

  • Use Top sample to correct jet energies of Z+jet sample

  • TOP 12000 jets, Z+jet 8000 jets

  • Apply same cuts on jets energies

  • => Top light jet scale seems to work for all light jets

  • In progress: repeat exercise with backgrounds



EPart / E

EPart / E

After calib ‘Top’



June 05 ATLAS Week - M. Cobal

Single top production

  • Three production mechanism

    • Some could be seen at Tevatron

    • At LHC ‘precise’ determination of all of them

  • Main backgrounds

    • Non top events

      • Z+jets, W+jets

    • Top-pair production

  • B-tagging essential in this case!

Detailed simulation of single top only just started. No realistic backgrounds yet.

NLO generator MC@NLO expected!

June 05 ATLAS Week - M. Cobal

Finding the Higgs particle

  • We have two options:

  • We find the Higgs at the LHC

    • Gain deep knowledge on the Standard Model

  • We do not find the Higgs at the LHC

    • Something serious wrong with our understanding of the Standard Model and it is observable at LHC

    • In the absence of Higgs, the WW scattering amplitude violates unitarity

H is very sensitive to detector performance

Study impact of new layout (initial/Rome) is underway

Energy reconstruction of converted photons is critical issue

Inclusive H to NLO

  • Energy reconstruction of converted and non-converted photons






June 05 ATLAS Week - M. Cobal

NLO QCD corrections

Higgs production via MC@NLO generator

Higgs decay via HDecay program

Used QCD NLO corrections to background pp+X

Signal significance possibly further enhanced by 40%.

H may be a discovery channel on its own for 10 fb-1

Inclusive H to NLO


TDR-like analysis with NLO σ


June 05 ATLAS Week - M. Cobal

H 4 leptons

  • The HZZ*4leptons channel is the golden channel for SM Higgs search in the mass range 120 GeV < MH<~800 GeV

  • TDR studies on both e and m channels

  • Main backgrounds are:

    • ZZ*/g* (irreducible)

    • Zbb, tt (reducible)

  • Background rejection based on cuts on leptons pT, reconstructed Z and Higgs masses, lepton isolation based on calorimeter energies, impact parameter significance

  • Current studies aim mainly at assessing the reconstruction and selection performance

    • 4-muons channel

    • 4-leptons channel

June 05 ATLAS Week - M. Cobal

Preselection cuts as in TDR

First two leptons pT>20 and |h|<2.5, second pair pT>7 and |h|<2.5

Likelihood for reducible background (Zbb and ttbar) rejection

2 largest IP, 2 largest pT, 2 largest transverse energies in a DR=0.2 cone

Likelihood for irreducible background (ZZ) rejection

Z invariant masses, angles between two Z’s decay planes, m angles in Z’s frame

Normalized to 30 fb-1

H 4 muons

June 05 ATLAS Week - M. Cobal

H4m - different group





NLO, Normalized to 30fb-1

June 05 ATLAS Week - M. Cobal


Significances using LO cross sections, 10 fb-1:

Significances using LO and NLO for 10 fb-1:


◦ LO

  • For large range of Higgs masses discovery after 10 fb-1 (one year?)

  • Combining electron and muon channels essential

June 05 ATLAS Week - M. Cobal

Counting experiment

No Higgs mass peak!

Discriminant variable is e.g. angle φbetween leptons

Background top-pair productionand di-boson production:

Event topology

A nasty one: HW+W-l+νl-ν

Require forward jets

Two opposite leptons

Reject central jets

Missing energy

  • This decay mode significant in region 150 < MH < 180 GeV

    • At MH=170 BR 100 times HZZ

  • Understanding of bckgr’s critical!

    • Develop clever methods to assess backgrounds from data

    • Statistically can claim discovery with ~5fb-1 of data

June 05 ATLAS Week - M. Cobal


Top-pair production with extra jets

Rely heavily on ID tracking and b-tagging capabilities

Very interesting alternative to Higgs discovery using photons

Determination largest Yukawa coupling from production cross section: (ttHttbb,tttt,ttWW) g2ttHBR(Hbb,Htt,HWW)

Challenging channel:

4 b-jets

2 light jets

Missing energy

Isolated lepton

Another one: ttH signal

  • Detailed knowledge detector needed

  • Not done with realistic simulation and backgrouond treatment yet…

Low Luminosity: 30/fb

June 05 ATLAS Week - M. Cobal

Search for SUperSYmmetry

Search for SuperSymmetry

Elegant extension to the ‘Standard Model’ that…

stabilizes the Higgs mass; predict light Higgs mass.

unifies the coupling constants of the three interaction

provides a candidate for dark matter

is consistent with all electroweak precision data

Complex signatures: e, µ, t, jets, b-jets, Etmiss

Good test for detector performance and reconstruction.

Analyses divided by signature

Various ways to create some order in the chaos of multi-parameter space

Unified boson and fermion masses at GUT scale as in mSUGRA models:

Only 4 free parameters remain: m0, m½, tanβ, A0, sign =±

Select several mSUGRA points

Consistent with WMAP data for cold dark matter

Don’t believe mSUGRA, but use it to suggest interesting possible particle spectra

Typically σ>1 pb, so early discovery physics

Analyze each of these points

E.g. point SU1:

SuSy parameter space





June 05 ATLAS Week - M. Cobal

Susy characterized by decays:

Decay to jets, perhaps leptons, and escaping LSP (missing ET)

Events characterized by large Meff = ETmiss+Σ|pT, jet|

All hadronic decay

Backgrounds given by SM processes: Z and W-production, top production, multi QCD jets

At TDR this background was estimated

Convincing SuSy signal obtained using parton shower MC’s

Hadronic SuSy topologies


June 05 ATLAS Week - M. Cobal

However, it is well known that parton showers underestimate the high PT region

So complete background estimation is redone

Using ME approach where possible

Susy signal effectively disappeared in this channel

Use the right MC generators!

Hadronic SuSy

June 05 ATLAS Week - M. Cobal


Main difference from PT jet

For ETmiss> 700 GeV :

clear excess

ETmiss vital for SUSY searches

High PT jets are emitted by

background as well:

not clear separation

June 05 ATLAS Week - M. Cobal

SUSY: s-transverse mass for SU1

In all possible ways and compute:

June 05 ATLAS Week - M. Cobal

Signal reduced by factor 5

Background reduced by factor 20-30

Dominant background are semi-leptonic top-quark pairs

Largest uncertainty in Meff originates from estimation of ETmiss

ETmiss distribution sensitive to detector imperfections

One-lepton SuSy

Simulation of 3-4% calo dead channels

June 05 ATLAS Week - M. Cobal

Add SuSy

Repeat procedure with SuSy signal included

ETmiss distribution from data

Clear excess from SuSy at high ETmiss observed: method works!

Obtain the ETmiss distribution from data using top events

By fixing the top mass in the leptonic channel, predict ETmiss

Select top without b-tagging

ETmiss for top signal minus sideband

Reduce combinatorical background

Normalise at low ETmiss, where SuSy signals are small

One-lepton SuSy: ETmiss estimate

Estimate background from data

Example of reducing MC dependency on ETmiss distribution

June 05 ATLAS Week - M. Cobal

In most scenarios the first SUSY decay reconstructed is leptonic decay of neutralinos.

“Smoking gun”: excess of opposite-sign lepton pairs with an edge structure in invariant mass

No mass peak themselves can be reconstructed

Muon reconstruction efficiency is essential

Example at point SU3:

















Di-lepton SuSy

4.37 fb-1

No cuts


opposite sign

same sign

June 05 ATLAS Week - M. Cobal

SUSY: SU1 Leptonic Signatures

D. Costanzo, F.Paige

20.6 fb-1, No cuts

Coannihilation point

MC Truth, lL

Soft lepton

MC Truth, lR

Hard lepton

MC Data

Two edges from:

Each s-lepton close in mass to one of the neutralinos – one of the

leptons is soft

June 05 ATLAS Week - M. Cobal

SUSY: SU-2 Dileptons


Heavy scalars:

no scalar lepton in  decay

1° edge


6.9 fb-1

No cuts

Direct 3-body decays:

The two edges measure

the two mass differences

Δm = m(n0) -m(10)

2° edge


6.9 fb-1

No cuts

Two edges expected at 57.0 and 76.4 GeV

June 05 ATLAS Week - M. Cobal

SUSY: SU-2 Dileptons

  • SU2 SUSY production is:

  •  (direct) (4.5 pb)

  • Do not pass cuts to reject SM

  • (little jets & ETmiss)

  • gg →+jets(0.5 pb)

  • This can be separated

  • efficiently from SM

  • After cuts (from fast sim),

  • only few events remain.

  • Edge reconstruction in SU2 needs higher integrated luminosity.

6.9 fb-1


2.6s excess

SU2 dilepton invariant mass,

after cuts to reject SM

June 05 ATLAS Week - M. Cobal

Tau signatures in SuSy

  • Tau signatures (mostly hadronic decays) are important in much of the mSUGRA parameter space, particularly at high tan

  • At some points in the parameter space (e.g. funnel) can only observe kinematic endpoints in  invariant mass distributions

  • Can often see endpoints in m, mq, etc, but:

    •  triangular shape distorted due to ETmiss from ν

    •  statistics much lower due to t-reconstruction efficiency (expecially for soft-taus, coannihilation point)

  • typically achieve /jet  100 for a t-reconstruction ε of 50%

June 05 ATLAS Week - M. Cobal

Typical distortion due to escaping neutrino’s in tau decay

However, can still fit this distorted distribution to obtain edge point

Black points: MC truth

 note the triangular shape

Red line: distribution from non-leptonic decay products

 (distorted shape)

SuSy: Tau signatures

(98.3 GeV)

4.9 fb-1

4.9 fb-1

a strong di-t edge has been identified in the bulk region and it looks

Possible to extract a useful measurement in the coannihilation region

June 05 ATLAS Week - M. Cobal

SUSY: b-tagging

June 05 ATLAS Week - M. Cobal


Among the most popular:

- Alternatives to EW symmetry breaking

- Extended gauge symmetries

- Extra dimensions besides our 4D space time


High pT bosons

Few/no jets in central region

(no colour exchange)

Forward tag jets

No light Higgs at the LHC?

  • Scenario without ‘light Higgs’ particle: VL VL → VL VL violates unitarity at scales ~TeV, reachable by LHC!

  • Increase in cross section damped e.g. by strong symmetry breaking mechanism

    • VL VL → VL VL described at low energy by an effective theory

    • General parameterisation of the “new physics”.Can lead to resonances in WW / WZ scattering

Important backgrounds :

  • W+jets, Z+jets

  • ttbar

  • qq→WZqq , WWqq

June 05 ATLAS Week - M. Cobal

Separation of signal from background difficult

Again ttbar background is essential ; need better undertanding

Example resonances in Wlν, Wjj

no resonance



30 fb-1 of data

- Signal

- ttbar

June 05 ATLAS Week - M. Cobal

Exotics: H++

L-R symmetric model would be a natural extension of the SM

  • SU(2)L x SU(2)R x U(1)B-L

  • predicts new fermions: heavy Majorana neutrino

  • predicts new gauge bosons: WR

  • predicts new Higgs sector

June 05 ATLAS Week - M. Cobal







150 GeV

Exotics: H++








- Signal:150, 200, 500 GeV(~5K)

- Backgrounds: WW+jets (~5K)

- Fake rate: jet/e

Mass Mean Sigma Expected Selected

(GeV) GeV GeV

150149.2 ± 0.089.9 ± 0.06

148.9 ± 0.09 11.9 ± 0.06


240 20 ± 1.

WW+jets 0

June 05 ATLAS Week - M. Cobal

Exotics: Narrow Resonance Z’ ee

June 05 ATLAS Week - M. Cobal

Exotics: Narrow resonance Z’ tt


June 05 ATLAS Week - M. Cobal

Exotics: Narrow resonance G*ee

June 05 ATLAS Week - M. Cobal













1 TeV




Exotics: Little Higgs

New approach to the hierarchy problem  many new particles:

  • T, heavy top

  • New gauge bosons WH, ZH, AH

  • Higgs tripletf0, f+, f++

cannot distinguish  treated together

1, 2 jets

1 ou 2 jets

1, 2 jets


Fully simulated evts

with Z/W  qq, primary vertex can be determined

Z vertex used to correct  of the photons

June 05 ATLAS Week - M. Cobal

Exotics: Little Higgs

  • Signal at M(H)=120 GeV

  • ZH/WH Z/W H  qq 

1 TeV

Full reco 10.0.1


Estimation of the significance

arbitrary units

arbitrary units




M(ZH/WH) (GeV)

M(ZH/WH) (GeV)


x =


June 05 ATLAS Week - M. Cobal

Not only science fiction!

  • First cosmic event in UX15!!

  • Barrel TileCal is completein the cavern.

  • Single tower trigger

  • First cosmic eventsobserved last Tuesday!

June 05 ATLAS Week - M. Cobal

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