SM + Top physics Summary

1

SM + Top physics Summary

Marina Cobal,ATLAS Overview WeekFreiburg, October 2004

Marina Cobal - Atlas Week, Oct 2004

P 2

Outline Lot of activity…I had to make some selection!

SM results in 2004 TGCs and AQGCs

PDFs

Underlying Events and Minimum bias

Top results in 2004 New spin correlation studies

New study on resonances

Commissioning with the top

Plans

3

TGCs and QGCs TGCs and QGCs


P 4

Triple Gauge-boson Couplings

Non-abelian SU(2)L×U(1) Y gauge group (foundation of SM!)

WWγ WWZ couplings

most-general C & P conserving WWZ,WWγ vertices are specified by just 5 parameters:

model independent parameterization Probe tool: sensitive to low energy remnants of new physics operating at a higher scale complement to direct searches

big advantage for LHC

S.M in the ZERO , Δκ ,Δκ ,g

s like grow

Z

s like grow

Z1Z

L. Simic


P 5

Events selection

New study covers the purely leptonic channel:

Selection

PT>25 GeV, |η|<2.5 ET

miss >50 GeV Z mass constraint |MZ-Mll|<15 GeV Veto Jets with PT(jet)>10 GeV and |η(jet)|<3

The expected number of events at 30 fb-1 is 7800, with 11% background.

Background:

μ)e,(lν,νllWW

ZZZ,Z,W,tt

New!!


P 6

TGC’s in WW processesInclusion of anomalous couplings at WWZ and WWγ vertices yields enhancements in: (WW) at large values in and k PT(W) or PT(e) distributions PT(WW) or PT(e+e-) distributions

NLO corrections are large in the same regions.

Theoretical arguments suggest that anomalous TGC’s are at most of

O (m2W/Λ2)

Λ is scale of new physics.For Λ~1 TeV TGC’s are O(10-2).


P 7

Confidence intervals for TGC’s

In WW processes 95% confidence intervals for TGC parameters

for 30 fb -1 and ΛFF= 2 TeV are:

WW process WZ and Wγ process

-0.028 < ΔκZ < 0.057 -0.11 < ΔκZ < 0.12

-0.035 < λZ < 0.026 -0.0073 < λZ < 0.0073

-0.077 < Δκγ < 0.15 -0.075 < Δκγ < 0.076

-0.061 < λγ < 0.063 -0.0035 < λγ < 0.0035

-0.13 < Δg1Z < 0.42 -0.0086 < Δg1

Z < 0.011

for HIZ (equal coupling) scheme:

ΔκZ= Δκγ=Δκ λZ= λγ=λ -0.024 < Δκ < 0.025-0.017 < λ < 0.017 WW channel will be competitive

(1d fit, not systematic included) with WZ and Wγ in determining

limits for ΔκV

M. Dobbs,M. Lefebvre: ATL-PHYS-2002 023/022


P 8

W : Quartic Gauge Couplings LHC able to probe AQGCs through vector boson fusion and triple

vector boson production

Wgood place to start looking for: low partonic centre of mass required suppressed by only one branching ratio

W production is sensitive to possible AQGC of the form WW – this is SM?

Effects of AQGC on the qq to l with l = e or at LHC have been studied by Eboli et al who have a MC with AQGCs implemented

Effects of AQGC can be seen on distributions of PT() and invariant mass of the photon pair, M.

P. Bell


P 9

Consider only the exclusive channel qq to e Process events using ATLFAST Cross section falls rapidly with the cut on PT

Two photons, PT>15 geV, |n|<2.4

One e with PT > 25, |n|<2.4

Missing ET > 20 GeV R > 0.4, Re > 0.8 MT (l)> 65 GeV

Simulation chain Weighted events from W2GRAD Implemented as external process in PYTHIA within ATHENA-ATLFAST PYTHIA added beam remnants, QCD showers and underlying events and do the

frag/decay etc as usual Accept/reject alg then passes events of unit weight to ATLFAST

W in ATLAS

Eboli et al

PT


P 10

Results

Background: W +1 jet and

W+2 jet events with one or both

jets are misidentified as a , with

probability 1/Rjet (ATL-PHYS-99-016

gives Rjet=1300 @ low L, Pj = 20GeV)

106 W + jet and W + 2jet events

going through PYTHIA + ATLFAST

If ~ 80% for e and , S=14 evts

in 30fb-1, so about 60 for l+-.

Background ~ 13 evts

PT and M dist. show that the SM background and mis id can still be removed by cutting at PT = 200 GeV, so that Eboli’s conclusions hold even with these backgrounds

Feature of W production In SM. Amplitude for qq W vanishes for cos* = -1/3 ( angle between the q and the W in parton CMS)

This radiation is preserved in the limit of 2 collinear ’s

Vanishes as increases separation criteria for detection

Independent from PT() and same in lab and parton frames

Dip at 0.1 for cos > 0 after suppressing radiative W events

Previous selection + opening angle requirement

Chance in 3 y, clear with 100fb-1

The Radiation Zeroe-e expectation in 30fb-1

[1] x 4

[1]

[2]

[3]

e-e expectation in 100fb-1 x 4

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PDFs PDFs


P 13Parton parameterizations

Uncertainty on the PDF’s: Propagation of uncertainties on

experimental data to the fitted PDF’s Statistical uncertainties and

(correlated) systematic effects Uncertainties in the theoretical

description of the fit procedure Flavour thresholds, s

Scales uncertainties Nuclear effects Higher twist, …

A number of groups have published the PDF fits with propagated experimental uncertainties: Botje (Eur Phys J C14 (dec 1999)) CTEQ (J. Pumplin et al, hep-ph/0201195) MRST (A. Martin et al, hep-ph/0211080)

Alekhin (S. Alekhin, hep-ph/0011002) Fermi2001 (Giele et al, hep-ph/0104052)

Theoretical uncertainties not treated here

PDF’s obtained from QCD DGLAP evolution fits to data. DIS data from fixed target and

HERA Jet cross sections pp colliders Drell-Yan processes

Z rapidity ZEUS-S W+ rapidity ZEUS-S

Z rapidity MRST02 W+ rapidity MRST02

For LHC:

fits of ZEUS and MRST02in agreement within PDF uncertainties

Both fits use conventional NLOQCD evolution in the DGLAP formalism including data taken at very low-x (down to x=6 10 -5)

Whereas this formalism still fits the data very well, there are theoretical reasons to doubt its validity at low-x

(Devenish and Cooper-Sarkar, ‘Deep Inelastic Scattering’, OUP 2004, Section 6.6.6 and Chapter 9)

A.M. Cooper-Sarkar

Z rapidity MRST02 W+ rapidity MRST02

W+ rapidity MRST03Z rapidity MRST03

PDFs derived from a fit without low-x data : MRST03 conservative partons’.

Compare predictions for W/Z production to those of the ‘standard’ PDFs

The two predictions are very different for LHC(not so different for tevatron)

W and Z rapidity distributions for │η│< 2.4 are potentially sensitive to the treatment of low-x QCD evolution.

R = W- / W+ MRST02

(W)/(W+)=0.74

R = W- / W+ MRST03

(W-)/(W+)=0.79

R = W- / W+

ZEUS-S

(W-)/(W+)=0.75±0.02

The shape of the ratio of W rapidities seems well suited to NOT seeing these differences between different PDFs – minimizing PDF errors-

The magnitude of the ratio differs by ~5%. Is the luminosity error small enough to see this? Is this a good luminosity monitor?

Suppose we WANT to see the differences ? Differences evident for the Z rapidity distributions, once one has moved away from central rapidity. How well can we measure the Z rapidity distribution?

Need to know the efficiency of Z reconstruction for │y│> 1.2 accurately because the difference between MRST02 and MRST03 at central rapidity is mostly normalisation.

The shape differences become most evident for │y│~2

Z reconstruction efficiency vs y

MRST02

MRST03

Similar exercise done with the W’s

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UE and Min Bias UE and Min Bias


P 19

Min bias events

Definition depends on the experiment trigger! Usually is associated to non-single-diffractive events (NSD) – (see ISR<UA5,E735,CDF…)

Dominated by soft interactions, although there is some contributions from hard scattering

difndifddifselastot ...

σNSD ~ 65 - 73mbσtot ~ 102 - 118 mb

(PYTHIA) (PHOJET)

(PYTHIA)


P 20

UE in charged jet evolution

Everything except the two outgoing hard scattered jets

In a hard scattering process, the underlying event has a hard

component (ISR+FSR and particles from the outgoing hard

scattered partons) and a soft component (beam-beam

remnants)

Many published data:

Durham HEP database

JetWeb

ljet

CDF analysis:• charged particles: pt>0.5 GeV and |η|<1

• cone jet finder:

7.022 R


P 21

Comparing different PYTHIA tunings to data

F(z

) =

<n ch

g > P

(nch

g)

z = nchg /<nchg >

High-multiplicity events are described differently by each tuning

√s (GeV)

dN

chg/

dη

at

η=

0

LHC


P 22

• CDF tuning: tuned particularly to UE data; doesn’t correct for particle decays which affect minimum bias distributions.

Tra

nsv

erse

< N

chg >

Pt (leading jet in GeV)

Rat

io (

MC

/Dat

a)

Comments:

• ATLAS – TDR: not tuned to UE data; doesn’t include double diffraction for minimum bias events; uses a model with a small hadronic core size; doesn’t correct for particle decays which affect minimum bias distributions and doesn’t includea pTmin energy dependence.

dNchg/dη ~ 10

dNchg/dη ~ 15

Central Region(data dNchg/dη ~ 4)


P 23


P 24

Data-MC Multiplicity information: ‹nch›, dN/dη, KNO

Transverse region (UE): ‹nch› and ‹pTsum›


P 25

LHC predictions: PYTHIA6.214 – tuned vs. CDF Tuning

Tra

nsv

erse

< N

chg >

Pt (leading jet in GeV)

LHC

Tevatron

x 2

x 3

dNchg/dη ~ 20

dNchg/dη ~ 30

Central Region(min-bias dNchg/dη ~ 7)


P 26

LHC predictions


P 27Summary on UE and min bias

Current min bias and UE data can be described with appropriate

tuning for PYTHIA and PHOJET

PYTHIA6.214-tuned and PHOJET1.12 with its default settings give

the best global agreement to the data. They generate LHC

predictions with ~30% difference for min bias, and ~ a factor of 2 for

UE distributions

More activity in the UE than predicted for an average min bias event!

PYTHIA6.214-tuned predicts an increase (~ a factor of 2) in this

activity when extrapolating from Tevatron to LHC, whereas

PHOJET1.12 suggests the ratio UE/min bias will remain the same

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Top: Resonances Top: Resonances


P 29

Many theoretical models include resonances decaying to ttbar SM Higgs (but BR smaller with respect to the WW and ZZ decays) MSSM Higgs (H/A, if mH,mA>2mt, BR(H/A→tt)≈1 for tanβ≈1) Technicolor Models, strong ElectroWeak Symmetry Breaking, Topcolor, “colorons” production,

[…]

Generic resonance with 350 GeV<MX<5 TeV ; X < det and X > det

Signal observation above continuum BKG within a 2det mass window

=0.9545 fraction of signal within window Signal must have

a stat significance > 5

must contains ≥ 10 evts

Analysis : determination of X , tt and (fraction of BKG in window)

decaytX BrBr L10).(

New study on Resonances

)()5(. decaytXttBrBr

ttL

E. Cogneras, D. Pallin

Need to look for an alternative method for high MX As for high pt jet method used in Top mass measurement No isolated muon in the preselection (and trigger menu)

Efficiency Detector resolution

Low efficiency above MX ~2 TeV Jet overlap Muon less isolated


P 31

Analysis repeated for standard model (the BKG!)

Effect of jet calibration in the

continuum:

Resonance mass shift (mtt)

all jets mis-cal. (mtt)= 0.61 10-2 mtt - 0.48 per % of mis-calib.

b jets mis-cal (mtt)= 0.36 10-2 mtt - 0.21 per % of mis-calib.

Typically for a 1TeV resonnance (mtt)= 5.6 GeV (all jets)

(mtt)= 3.4 GeV (b jets)

tt

ttbar continuum


P 32

Resonance width x < det

Discovery potential Resonance width x = 2 det

Results close to TDR. For 300 fb-1

MX = 5OO GeV/c2 discovery if (.Br)> 1500 fb Needs to investigate newMX = 1 TeV/c2 discovery if (.Br)> 650 fb methods to improve atMX > 3 TeV/c2 discovery if (.Br)> 11 fb high mass

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Top: Spin Correlation Top: Spin Correlation


P 34

Since no hadronisation: daughter keep spin info Study in semi-leptonic and di-leptonic

Spin analyser:

Leptonic: lepton

Hadronic: (W, b) or least energetic jet (lej)

Interesting angles:

• Θ1 (Θ2) : angle between chosen spin axis and spin

analyzer direction in the t(t) rest frame.

Spin axis is t(t) direction in the parton c.m.s. (helicity basis)

• φ : angle between spin analyzers direction in the t(t) rest

frame

Top Spin CorrelationE. Monnier, P. Pralavorio,

F. Hubaut


P 35

)coscos1(4

1

)(cos)(cos

121

21

2

Cdd

Nd

N

TopReX 4.05 (SM): LO spin correlation simulation Pythia 6.221 (NC): CTEQ5L and ISR-FSR AlpGen: used for W+jets background Tauola+Photos 2.6: t decay and radiative corrections Atlfast 2.60: ATLAS fast simulation and reconstruction

Unbiased estimator of C : -9 < cos 1 cos 2 > = 0.16

Unbiased estimator of D : -3 < cos φ > = -0.11

)cos1(2

1

cos

1

Dd

dN

N

Variables

C = degree of spin correlation in the helicity base


P 36

Results for S + B ( stat. syst errors) : 80500 S, S/B=15 C(lej) = 0.21 0.015 0.04 = ~ 5 σ from 0 D(lej) = -0.12 0.01 0.02 = ~ 5 σ from 0 Study with full simulation just started scientific note

C extraction

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Top: Commissioning(see talk of S. Bentvelsen)

Top: Commissioning(see talk of S. Bentvelsen)


P 38

Scenarios under study pp collisions

What variations in predictions of t-tbar – which generator to use? Underlying event parameterization Background estimation from MC

Try to be as independent from MC as possible.

Detector pessimistic scenarios Partly or non-working b-tagging at startup Dead regions in the LArg Jet energy scale

Use data to check data

Software tools Many studies (not all!) only in fast simulation

It is clear we need to redo most important studies with full simulation

Estimate top physics potential during first few months of running

S. Bentvelsen, M. Cobal


P 39 Status of top event generators

‘Old’ Leading Order MC: Pythia: full standalone MC Herwig: full standalone MC TopRex (include spin correlations – interfaced to Pythia)

‘New’ NLO QCD calculations implemented in MC MC@NLO – interfaced to Herwig shower and fragmentation

This is relevant theoretical improvement Superseeds the old Pythia and Herwig MC’s. Validation done for this generator

Currently DC2 processes 106 MC@NLO t-tbar events Crucial for us to analyse these Waiting for Tier0 exercise to obtain reconstructed objects


P 40

Background events Top physics background

Mistags or fake tags Non-W (QCD) W+jets, Wbbar, Wccbar Wc WW,WZ,ZZ Z tt Single top

AlpGen W+4 jets samples produced Very CPU intense (NIKHEF grid)

Un-weighting to W lepton (e,,) decay Production:

Effective : 2430 pb 380740 unweighted events generated

(2.6 10-5 efficiency) 3.41% (13002) events pass first selection

~ 150 pb-1 W+4jet background available

Largest background is W+4 jet.

This background cannot be simulated by Pythia or Herwig shower process. Dedicated generator needed: e.g. AlpGen. Large uncertainties in rate

Ultimately, get this rate from data itself. For example, measure Z+4 jets rate in data, and determine ratio (Z+4 jets)/(W+4 jets) from MC

W+4 extra light jets

Jet: Pt>10, ||<2.5, R>0.4

No lepton cuts

Initial grid: 200000*3

Events: 150·106

Jobs: 98~1.5 1010

events!


P 41

Non b-tag tops

Selection: Isolated lepton with PT>20 GeV

Exactly 4 jets (R=0.4) with PT>40 GeV

Reconstruction: Select 3 jets with maximal resulting PT

t bjj

M (bjj)V. Kostiouchine


P 42

Extraction of top signal

Fit to signal and background Gaussian signal 4th order polynomal Chebechev background In this fit the width of top is fixed at 12 GeV

Extract

cross section

and Mtop?

150 pb-1

Need full simulation!!


P 43

Lower luminosity?

Go down to 30 pb-1 Both W and T peaks already

observable See something!

30 pb-1 mean σ(stat)

in peak 0.8% 17%

Mtop 170.0 3.2

Mw 78.3 1.030 pb-1


P 44

More efforts on..

Mtt reconstruction Top mass reconstruction using full simulation (“In situ”) W-calibration studies Better evaluation of the FSR systematics in the Mtop

reconstruction Single top studies… Involvement in the DC2 validation

(our group has a link-person dedicated to this since 1 year)


P 45 SM Group: Priorities for November & Rome

LHC Physics Environment: Parton Density Functions

studies just beginning. Expect results for Rome. understand how PDF knowledge will evolve in first years of data (i.e. PDF is largest syst. uncertainty in many studies, but will this be true

after LHC further constrains them?)

Underlying event need uniform tunings for Pythia / Jimmy timescale: DC2. Presentation in November. important e.g. for studies that use MC@NLO for signal, and Pythia for

backgrounds

Fundamental SM Measurements W-mass, Drell-Yan, and AFB in Z0e+e-

fast simulation only thus far specific performance issues need full sim. are performance assumptions realistic?

focus on lepton energy scale (dominates W-mass) forward electron tagging (for AFB)


P 46 Top Group: Priorities for November & Rome

Understand the interplay between using the top signal as tool to improve the understanding of the detector (b-tagging, jet E scale, ID, etc..) and top precision measurements

More studies on the QCD W+jet events

Perform a preliminary cross section study for November. Finalize it for Rome

Present a “final” Mtt analysis for Rome

Draft note on commissioning with top events ready before Rome

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SM + Top physics Summary

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