top at startup of lhc

Top at startup of LHC

Stan Bentvelsen (NIKHEF)October 2nd, 2004

Stan Bentvelsen Commissioning Meeting Freiburg

P 2

Top commissioning studies

Top physics at LHC Top one of ‘easiest’ bread and butter

Cross section 830±100 pb

1. Used as calibration tool

2. Rich in ‘precision and new physics’ Top mass Mt, cross section σt

Resonances decaying into top

Commissioning for the top group: Summarize studies already performed Tasks to do before startup What do we need as input from others? What can/should we provide to collaboration? Goals

Semi-leptonic top channel

detector tools involved:

Lepton reconstruction

Missing ET

Jets + calibration

B-tagging

Work done together with Marina Cobal


P 3

Initial commissioning studies

First evaluation of statistical uncertainty on σtop and Mtop

Gold-plated channel : single lepton

• pT (lep) > 20 GeV• pT

miss > 20 GeV• ≥ 4 jets with pT > 40 GeV• ≥ 2 b-tagged jets• | mjjb-<mjjb>|< 20 GeV

For initial run at LHC:(L = 1033 cm-2s-1)

Pretty small uncer-tainties after very short time of LHC running!

Period evts dMtop(stat)

1 year 3x105 0.1 GeV

1 month

7x104 0.2 GeV

1 week 2x103 0.4 GeV

/ -st

0.2 %

0.4%

2.5%

P. Grenier


P 4 Systematic error on Mtop (TDR performance, 10 fb-

1)

Initial performance : uncertainty on b-jet scale expected to dominate

b-jet scale uncertainty Mtop

1% 0.7 GeV 5% 3.5 GeV 10% 7 GeV

Cfr: 10% on q-jet scale 3 GeV on Mtop


P 5 Various scenarios currently 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

Get good ‘feel’ for important systematic uncertainties – 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 realistic potential for top physics during the first few months of running


P 6

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 7

Generators: MC@NLO, Herwig, Pythia

PT(tt system) Herwig & MCatNLO agree at low PT,

At large PT MCatNLO ‘harder’ PYTHIA completely off

Many more comparisons: see talks in top meeting

Example: distributions on top-anti-top characteristic – PT of the whole system

PT of t-tbar system is balanced by ISR & FSR


P 8

Underlying event (UE) / minimum bias

Extremely difficult to predict the magnitude of the UEat LHC

Will have to learn much more from Tevatron before startup

Various models exists Herwig’s UE and minimum bias shows much less activity compared

to Pythia. This has always been a problem in Herwig.

Jimmy is developed as alternative model for UE at ep collisions Various ‘tunings’ exist – leading to wildly different results

More work is mandatory here Wish list to generate fully simulated events with Jimmy during DC2

post-production


P 9

Tune of Butterworth

Standard Jimmy

Running at LHC energies

Standard Pythia

Standard Herwig


P 10

Jimmy UE: Cells & Jets in Atlfast

Herwig vs Jimmy LO t-tbar

At jet-level effectreduced

10 GeV

Cell multiplicity

Cluster multiplicity

Jet multiplicity


P 11

Reconstruct the top

Top peak for various reconstruction methods

Difference in mass can be as large as 5 GeV

Really need data to check data on UE Study effect better (as said)


P 12

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 13

Non-W (QCD-multijet) background

Not possible to realistically generate this background Crucially depends on Atlas’ capabilities to minimize

mis-identification and increase e/ separation This background has to be obtained from data itself

E.g. method developed by CDF during run-1:

Rely now on e/ separation of 10-5

Use missing ET vs lepton isolation to define 4 regions:

A. Low lepton quality and small missing ET

Mostly non-W events (i.e. QCD background)

B. High lepton quality and small missing ET

Observation reduction QCD background by lepton quality cuts

C. Low lepton quality and high missing ET

W enriched sample with a fraction of QCD background

D. High lepton quality and high missing ET

W enriched sample, fraction of QCD estimated by (B·C)/(A·D)


P 14

Detector scenarios: HV problems

Effects of dead HV regions om Mtop

Argon gap (width ~ 4 mm) is split in two half gaps by the electrode HV by Dη x Dφ = 0.2 x 0.2 (or 0.1 x 0.2) sectors, separately in each

half gap ~ 33 / 1024 sectors where we may be unable to set the HV on one

half gap multiply energy by 2 to recover

A.I. Etienvre, J.P. Meyer, J.Schwindling

particle


P 15

100 000 tt events (~ 1.5 days at LHC at low L)

o Simulated using PYTHIA + ATLSIM

/ G3 (initial detector, h < 3.2)

o Reconstructed using ATHENA 7.0.0

Preselection of events:

o At least one recontructed e or μ

with PT > 20 GeV and η < 2.5

o ETmiss > 20 GeV

o At least 4 jets with PT > 20 GeV and h < 2.5

EM clusters

Jets

Analysis


P 16

Results

If the 33 weak HV sectors die (very pessimistic), the effects on the top mass measurement, after a crude recalibration, are:

o Loss of signal: < 8 %

o Increase in background: not studied

o Displacement of the peak of the mass distribution: -0.2 GeV

This effect on the Top mass is (much) better known than other systematic uncertainties

Mtop(without ) – mtop(with dead regions)


P 17

Detector scenarios: b-tagging

Precise alignment of ID can be reached only after few months of data taking.

The impact of misalignment can be much larger than having 2 instead of 3 layers

Top events to evaluate b-tagging efficiencies from data Select a pure t-tbar sample with tight kinematical cuts

Count the number of events with at least 1 tagged jet Compare 0 vs 1 vs 2 b-tagged jets in top events Can expect the b-tagging efficiency different in data from MC

In most pessimistic scenario b-tagging is absent at start

Can we observe the top without b-tagging?


P 18

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 19

Non btag: top sample

Signal plus background at initial phase of LHC

Most important background for top: W+4 jets Leptonic decay of W, with 4 extra ‘light’ jets

With extreme simple selection and reconstruction the top-peak should be visible at LHC

L = 150 pb-1

(2/3 days low lumi)


P 20

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


P 21 Can we see the W? (4 jets sample)

Select the 2 jets with highest resulting PT

W peak visible in signal No peak in background Better ideas well possible!

E.g. utilizing 2 body decay in top rest frame.

Select 2 jets with invariant mass closest to Mw (80.4 GeV)

Large peak in background Enormous bias Not useable!

150 pb-1


P 22

Fit to W mass

Fit signal and background also possible for W-mass

Not easy to converge fit Fix W width to 6 GeV

150 pb-1 mean σ(stat)

in peak 3.0% 5%

Mtop 167.0 0.8

Mw 77.8 0.7

These numbers for statistical uncertainties are consistent to the earlier study


P 23

Jet Energy scale / MC dependence

Variation of the jet energy scale to infer systematics Bjet scale: 0.92 – 0.96 – 1.00 –

1.04 – 1.08 Light scale:0.94 – 0.98 – 1.00 – 1.02 – 1.04

(1) (2) (3) (4) (5)

1) Analysis with jet energyscaled

2) All with MC@NLO, Herwigand Pythia;

3) Redo analysis with doubled W+4jet background (stat indep)

Top mass

155

160

165

170

175

180

0 5 10 15 20 25

Scale variations

To

p m

ass

Raw Top Mass

Scaled Top Mass

Cross section

0

200

400

600

800

1000

1200

0 5 10 15 20 25

Cross section

Determine Mtop and σ(top)

‘Raw’, i.e. no correction for jet scale

‘Corrected’, i.e. apply percentage difference of W-peak to the reconstructed top

Dependence on top mass reduced by scaling with W:

Rms Raw: 6.2 GeV

Rms Scaled: 1.2 GeV

Large dependence σ(top) on jet energy scale Via event selection.


P 24

Some results… (still no b-tag)

Using 150 pb-1 of data:

Statistic uncertainty already smaller than these systematic variations

Note these numbers are very preliminary –

Luminosity uncertainty (15-20%) to be added!

How to judge these values? Systematics overestimated:

since all generators are used, with all energy scales; double counting

W+4jets rate can be measured from data

Systematics underestimated: No real FSR variation No other backgrounds

(e.g. WW, QCD) Trigger Non-uniformities

Need further detailed studies!

Please don’t thrust any of this

without Full simulation

mean Stat std percent

Mtop raw 168,1 0.8 6,2 3,7%

correcte

d 171,9 0.8 1,2 0,7%

σ(top) raw 817,2 5% 94,8 11,6%


P 25

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 26

Fit the lepton+hadronic top

Full kinematic fit to t-tbar system – no b-tagging Fit the neutrino Px and Py - and get Pz via W constraint Use W-mass constraints Require equal top masses for lepton and hadron side Repeat fit over all possible combinations of jets

Better suited for mass than for cross section Looks promising but further study needed…

E. Bos

Background shows structure

Cut on quality of fit (X2)


P 27

What with b-tagging on?

Now assume full b-tagging Efficiency 60%, mistag 1%

Background is rapidly decreasing See for example the W-mass peaks for 1 and 2 b-tags Same selection: 4 jet events W reconstructed as dijet mass; |Mjj-80.4| minimal for light-jets j

150 pb-1


P 28

Reconstruct top mass

Sharp top mass peaks with little background Only use events for which |Mjj-80.4| < 20 GeV

Standard kinematic top reconstruction for 1 and 2 b-tags Background from W+4jets removed by b-tag requirement These results are very sensitive to b mis-tag rate

150 pb-1


P 29

Mtt at startup?

Can we determine dσ(tt)/dmtt without b-tagging? Interesting to SuSy models that modify this cross section

Mtt is invariant mass over all final state to tt products In principle no

assignment of b-jet to top is needed

Suffer a lotfrom background

No reliable measurement of mtt without b-tagging

W+4jet background


P 30

Resonances decaying to t-tbar

Observe heavy resonances Xt-tbar during commissioning? Plot invariant mass of 4 jets + lepton + neutrino No intelligence in determining Pz neutrino here

Insert resonance at 2000 GeV Cross section * BR(tt) = 350 pb

‘True’ mtt distributions of resonance and ttbar events

Heavy resonances with large cross sections visible


P 31

Need checks with full simulation

We are eagerly waiting for reconstructed DC2 events Repeat with full simulation for Rome next year


P 32

Top group in Atlas:

Inputs to the top group

Estimate of the single electron trigger efficiency

Can be done by using the Z triggered as single electron

How much time is needed to arrive to a reasonable evaluation of this efficiency?

Estimate of the initial lepton identification efficiency

Estimate of the integrated luminosity

At the beginning the precision on L should be around 10-20%.

The ultimate precision should be < 5%

Eventually: B-tagging efficiency Jet scales

What we have to provide

Top candidates enriched samples A “pure” one, obtained with quite

tight selection criteria A “loose” one: a more “background

enriched” sample, to be used as control sample for background calculations etc…

Estimate of a light jet energy scale correction

Assume 10% for light and b-quark jets, look at effect on Mtop and stop

Assume that at the very beginning only the EM scale is known (means: do not put any weight on the hadronic scale)

Output: provide the MW peak to rescale the light jets

Estimate of the b-tagging efficiency


P 33

Summary / Conclusion

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

At LHC Top more easily found than W’s in 4 jet channel Using extreme simple selection, no b-tagging Need more work on background estimation, both from W’s and QCD, e/

ratio, trigger, lepton ID, etc … Remove dependence of results on MC as much as possible

Using few days of data taking (150 pb-1) Current estimate on cross section accuracy of (ball park) 10%

plus luminosity uncertainty Interestingly mass of top via fits to mass peak looks promising

(use W-mass as constraint to jet-scale) Need better ideas to isolate very pure top sample without b-tagging

Its clear we need full simulation Eagerly awaiting reconstructed DC2 events to repeat/extend these studies

top at startup of lhc

Documents

low pt

herwig shower

herwig mcs

characteristic pt

nlo ttbar eventscrucial

pt mcatnlo harderpythia

bjet scale

gev ptmiss