summary of the 2005 rome atlas physics workshop

Summary of the 2005 Rome ATLAS Physics Workshop

M. Cobal, University of UdineM. Cobal, University of Udine

Physics Plenary, ATLAS Week, Physics Plenary, ATLAS Week, June05 June05

June 05 ATLAS Week - M. Cobal

Rome physics workshop

Speakers age distribution

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91 entries (out of about 100 talks),21 F plus 70 M

Some numbers:

— ~450 participants

— 100 talks

— ~ 35 hours of presentations and discussions


Sessions for physics groups

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

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


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…

Top physicsStandard 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


Charged particle density at Charged particle density at = 0 = 0

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: dNch/d dNch/dpT

Large uncertainty track densities!

LHC?

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


Charged particle densities

Generated vs reconstructed tracks

limited rapidity coverage

Can only reconstruct track down to ~500 MeV PT

1000 events1000 events

Explore special runs without solenoid magnetic field?

dNdNchch/d/ddNdNchch/d/d

dNdNchch/dPT/dPT

B=0

Black = Generated charged tracks

Blue = Reconstructed: NO TRT, NO solenoid

Red = Reconstructed: NO TRT, WITH solenoid

MeVMeV


Pdf determination using W bosons

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.

CTEQ61 (MC@NLO)

MRST02 (MC@NLO)

ZEUS02 (MC@NLO)

MRST03 (Herwig+k-Factors)e-

Error boxes: The full PDF Uncertainties

e+

Stat ~6 hours at low Lumi.

W+ and W- Rapidity

Wud

Wdu


Pdf determination using W bosons Full simulation

W+ and W- Rapidity

e- /e+ Ratio

e- e+

e+ e- Pseudo-Rapidity

W-

W+

W- /W+ Ratio

)(/

)(/)(

Wdyd

WdydyR

W

WW

Selection Cuts applied


Generator level for Ws


Charge Asimmetry

W Asymmetry

)(/)(/

)(/)(/)(

WdydWdyd

WdydWdydyA

WW

WWW

e+e- Asymmetry


Charge Misidentification dilutes Asymmetry

Correction:

FF

FFAA

RAWTRUE

1

ARAW = Measured AsymmetryATRUE = Corrected AsymmetryF- = rate of true e- misidentified as e+

F+ = rate of true e+ misidentified as e-


Systematics using Full Simulation

FF

FFAA

RAWTRUE

1

ARAW = Measured AsymmetryATRUE = Corrected AsymmetryF- = rate of true e- misidentified as e+

F+ = rate of true e+ misidentified as e-

F-

F+

Detector Level

Charge misidentification

Use Z -> e+e- sample from Full Simulation Rome production~98K events, Herwig+CTEQ5L

data-like analysis (No MC-Truth)

Mis-ID rate negligible?


Pt leading jet (GeV)

—Soft component in hard scattering event— On fully simulated jet sample compare

reconstructed and generated multiplicity.

Ra

tio

<N

Tra

ckR

eco>

/<N

Tra

ckM

C>

ljet

UE is defined as the UE is defined as the Transverse RegionTransverse Region

Njets > 1, |ηjet| < 2.5, ET

jet >10 GeV,,

|ηtrack | < 2.5, pT

track > 1.0 GeV/c

Underlying event

Good agreement reconstructed/generated

Can use to tune MonteCarlo

Tra

nsv

erse

<n

ch>


W-mass

Aim to determine M(W) with precision of 15 MeV Highest precision expected in Wν

Observables Transverse mass MT

Missing PTmiss

PT lepton

Top physicsTop physics

tttot = 759 pb

tt(semi-lept: e,) ~ 30%

Nevents ~ 700 per hour

•Top production: basic calibration tool for early physics 1500 tt->bW(l)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


Reconstruct top w/o b-tag

1 lepton Pt > 20 GeV

Missing ET > 20 GeV

4 jets PT > 40 GeV

Selection cuts:

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.

TOP CANDIDATE

Selection efficiency = 5.3%

Trigger efficiency not taken into account yet

Observe top quarks after ~1 week?

When no b-tag is yet present?


Analysis including W+4jets background

W+jets and MC@NLO signal

W+jets and MC@NLO signal

m(t) m(W)

Top mass (GeV) W mass (GeV)

Observe both top and hadronic W peaks! W+jets bckg is large (and has large uncertainty)

Nu

mb

er

of

even

ts /

5.1

GeV

Nu

mb

er

of

even

ts /

5.1

GeV

S/B = 0.45 S/B = 0.27

B

S

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

300 pb-1


Various cuts to improve purity

Top mass (GeV)

m(t) Top peak clearly visible after 1 week of LHC data

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

B-JET CANDIDATE

m(t)

Top mass (GeV)Ask for: b signal probability> 0.90 on 4th jet


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 k factors on R and recompute R n times =>

jeti

parti

iWPDGW

E

EwithMMR 21/

1 2k j j True nk k

n

R

E

E

Pa

rt /

E

E


Results after recalibration E

Pa

rt /

E

E 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

After calib ‘Top’

E E

Pa

rt /

E

Top

Z+jets


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!

Finding the Higgs particle

We have two options:

We find the Higgs at the LHCWe find the Higgs at the LHCGain deep knowledge on the Standard Model

We do not find the Higgs at the LHCWe do not find the Higgs at the LHCSomething serious wrong with our understanding of the Standard Model and it is observable at LHCIn the absence of Higgs, the WW scattering amplitude violates unitarity


Inclusive H to NLO

H is very sensitive to detector performance Study impact of new layout

(initial/Rome) is underway Energy reconstruction of

converted photons is critical issue

Energy reconstruction of converted and non-converted photons

Non-converted

converted

E(

)/E

(tru

e)E

()/

E(t

rue)

E()


Inclusive H to NLO

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

H+1j

TDR-like analysis with NLO σ

H+0j


H 4 leptons

The HZZ*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 channels Main backgrounds are:

ZZ*/* (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


H 4 muons

Preselection cuts as in TDR First two leptons pT>20 and ||<2.5, second pair pT>7

and ||<2.5 Likelihood for reducible background (Zbb and ttbar) rejection

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

Likelihood for irreducible background (ZZ) rejection Z invariant masses, angles between two Z’s decay

planes, angles in Z’s frame

Normalized to 30 fb-1

Higgs

Mass(Gev)

DC1Muid Comb

Mass res(GeV)

DC2Muid Comb

Mass res (GeV)

TDR

Mass res.(GeV)

130 1.68±0.02 1.9±0.1 1.42±0.06

150 1.88±0.03 2.0±0.1 1.62±0.06

180 2.50±0.02 2.9±0.2 2.20±0.06


H4- different group

SignalQCD ZZZbbttbar

NLO, Normalized to 30fb-1


Significances

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

-Combining electron and muon channels essential

Significances using LO cross sections, 10 fb-1:

MH [GeV] 4e 2e2 4 combined

130 Rome 1.4 2.3 1.7 3.2

DC1 1.0 1.9 1.6 2.8

150 Rome 2.9 4.2 3.2 5.8

DC1 2.4 4.0 3.1 5.6

180 Rome 1.5 2.3 1.5 3.1

DC1 1.2 2.0 1.5 2.8

300 Rome 2.5 3.8 2.7 5.2

DC1 2.1 3.2 2.4 4.5

● NLO◦ LO

Significances using LO and NLO for 10 fb-1:


A nasty one: HW+W-l+νl-ν

Counting experiment No Higgs mass peak!

Discriminant variable is e.g. angle φ between leptons Background top-pair production

and di-boson production:

Event topology

Reject central jets

Require forward jets Two opposite leptons

Missing energy

This decay mode significant in region 150 < MH < 180 GeV At MH=170 BR 100 times HZZ

Understanding of bckgr’s critical! Develop clever methods to

assess backgrounds from data Statistically can claim discovery

with ~5fb-1 of data


Another one: ttH signal

Backgrounds: 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: (ttHttbb,tttt,ttWW) g2

ttHBR(Hbb,Htt,HWW)

Challenging channel: 4 b-jets 2 light jets Missing energy Isolated lepton

Low Luminosity: 30/fb

Detailed knowledge detector needed

Not done with realistic simulation and backgrouond treatment yet…

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

Search for SuperSymmetry


SuSy parameter space

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:

SU1

SU2SU3

SU6


Hadronic SuSy topologies

Susy characterized by decays: Decay to jets, perhaps leptons, and

escaping LSP (missing ET)

Events characterized by large Meff = ET

miss+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

SU2


Hadronic SuSy

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!


SUSY:Background

Main difference from PT jetFor ET

miss> 700 GeV :

clear excess

ETmiss vital for SUSY

searches

High PT jets are emitted by background as well: not clear separation


SUSY: s-transverse mass for SU1

In all possible ways and compute:


One-lepton SuSy

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 ET

miss

ETmiss distribution sensitive to

detector imperfections

Simulation of 3-4% calo dead channels


One-lepton SuSy: ETmiss estimate

Add SuSy Repeat procedure with SuSy

signal included ET

miss 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 ET

miss, where SuSy signals are small

Example of reducing MC dependency on ET

miss distribution

Estimate background

from data


Di-lepton SuSy

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:

lqq

l

g~ q~ l~~ ~p p

Muonsopposite signsame sign

4.37 fb-1

No cuts


SUSY: SU1 Leptonic Signatures

Two edges from:

01

02

~ llll R

01

02

~ llll L Each s-lepton close in mass to one of the neutralinos – one of the leptons is soft

D. Costanzo, F.Paige20.6 fb-1, No cuts

MC Truth, lL

MC Truth, lR

MC Data

Hard lepton

Soft lepton

Coannihilation point


SUSY: SU-2 Dileptons

Direct 3-body decays:

The two edges measure

the two mass differences

Δm = m(n0) -m(1

0)

01

02 ll

01

03 ll

Focus-Point Heavy scalars: no scalar lepton in decayT.L.

6.9 fb-1

No cuts

2° edge

1° edge

Z

Two edges expected at 57.0 and 76.4 GeV

6.9 fb-1

No cuts


SUSY: SU-2 Dileptons

6.9 fb-1

2j100+4j50+xE100

SU2 dilepton invariant mass,after cuts to reject SM

2.6 excess

SU2 SUSY production is: (direct) (4.5 pb)Do not pass cuts to reject

SM(little jets & ET

miss) 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.


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, mq, etc, but: triangular shape distorted due to ET

miss from ν statistics much lower due to -reconstruction efficiency

(expecially for soft-taus, coannihilation point)

typically achieve /jet 100 for a -reconstruction ε of 50%


SuSy: Tau signatures

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)

4.9 fb-1

4.9 fb-1

(98.3 GeV)

a strong di-edge has been identified in the bulk region and it looks Possible to extract a useful measurement in the coannihilation region


SUSY: b-tagging

Among the most popular: - Alternatives to EW symmetry breaking

- Extended gauge symmetries- Extra dimensions besides our 4D space

time

Exotics


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

High pT bosons

Few/no jets in central region (no colour exchange)

Forward tag jets

Signal: Important backgrounds :

• W+jets, Z+jets

• ttbar

• qq→WZqq , WWqq


Example resonances in Wlν, Wjj

Separation of signal from background difficult Again ttbar background is essential ; need better undertanding

- Signal- ttbar

Scalar Vector no resonance

30 fb-1 of data


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

(if Lagrangian is invariant under symme) try)

0

0

01,2 1,2

( , , )

( , ,

,

R R R R

L L L L L R


Exotics: H++150 GeV

WW+jets

H++

Mass Mean Sigma Expected Selected(GeV) GeV GeV150 149.2 ± 0.08 9.9 ± 0.06

148.9 ± 0.09 11.9 ± 0.06 240 20 ± 1.

ee+

WW+jets 0

- Signal: 150, 200, 500 GeV (~5K)- Backgrounds: WW+jets (~5K) - Fake rate: jet/e

ee

ee

Exampleq q

q

l

lW

W L

q W+

W-


Exotics: Narrow Resonance Z’ ee


Exotics: Narrow resonance Z’ tt

e


Exotics: Narrow resonance G*ee


New approach to the hierarchy problem many new particles:

•T, heavy top

•New gauge bosons WH, ZH, AH

•Higgs triplet 0, ,

Exotics: Little Higgs

cannot distinguish treated together

1, 2 jets

WWHH

WW

HH

qq

qq

1 ou 2 jets1, 2 jets

ZZHH

ZZ

HH

qq

qq

+

1 TeV

Fully simulated evts

with Z/W qq, primary vertex can be determined

Z vertex used to correct of the photons

Example…


Exotics: Little Higgs Signal at M(H)=120 GeV

ATLFAST Full reco 10.0.1

ZH/WH Z/W H qq

1 TeV

Efficiency 31.6 %

Resolution in mass (GeV)

48

Significance 16.6

Efficiency 22.6 %

Resolution in mass (GeV)

35

Significance 13.9

Estimation of the

significance

x =FullSim

ATLFAST

xefficiency

xresolution

Signif. ATLFAST

arb

itra

ry

un

its

arb

itra

ry

un

its

M(ZH/WH) (GeV) M(ZH/WH) (GeV)


Not only science fiction!

First cosmic event in UX15!! Barrel TileCal is complete

in the cavern. Single tower trigger First cosmic events

observed last Tuesday!

summary of the 2005 rome atlas physics workshop

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