Z bb measurement in ATLAS (with ATLFAST)

Sept 30th 2004 Iacopo Vivarelli – INFN Pisa

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ZZbb measurement in ATLASbb measurement in ATLAS(with ATLFAST)(with ATLFAST)

Iacopo Vivarelli, Alberto Annovi

Scuola Normale Superiore,University and INFN Pisa


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IntroductionThe possibility of measuring the Zbb peak would be extremely helpful in the

search for bb final state (ttHttbb, WHlνbb, A/Hbb)

The “golden” channel for the b calibration in ATLAS is PT balance in bZbμμ events at present (easy to trigger, high purity). The Zbb sample can be a very

important complementary tool to cross check b-jet calibration.

Due to the huge QCD background (tens of mb of cross section), the reconstruction of such a signal is difficult.

Work in this sense has been done looking for a trigger with a μ6 (but this introduces a bias in the invariant mass reconstruction)

The aim of the present work is to check the feasibility of the Zbb measurement with a jet trigger, i.e., no requests on leptons in the final state

Very hard to trigger.


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First trigger considerations Since we want to trigger on low-medium Pt range jets, the present LVL1 menus

are less than satisfactory.

In fact, although the ATLAS LVL1 trigger is designed to work at a maximum output rate of 75(100) KHz (the input is 40 MHz), the staging of the LVL2 allows

only a 25 KHz LVL1 output. Since the LVL2 rejection on jets is limited, the thresholds on the jets are very high.

How can FTK contribute?

- high quality tracks @ LVL2 means high rejections against multijet

background for final state with b-jets

- track fitting not done @ LVL2 less computing power required

Lower thresholds can be used - more LVL1 bandwith availableLower thresholds can be used - more LVL1 bandwith available

LVL1 HLT Signature Rate

foreseenSignature

1J200 200 Hz 1J400

3J90 200Hz 2J350

4J65 200Hz 4J110


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Signal and background generation

Both signal and background generated with PYTHIA 6.203

- Signal (Zbb): MSEL=11 ; σ = 9.1 nb

- Background (Generic QCD): MSEL=1 with in 6 QT bins (beginning with QT > 10 GeV) ; σ = 9.45 mb. Cross check on the accepted cross section made with Alpgen generator. Results are

consistent.

Underlying event in PYTHIA tuned on the CDF data (A. Moraes et. al.)

Simulation/reconstruction made using ATLFAST-OO in Athena version 7.0.2 with FastShower included.


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Few words on ATLFAST…. The ATLAS fast simulation program (ATLFAST) is a particle level simulation.

Calorimeters are simulated by a grid of cells for geometric acceptance. The EM and HAD resolution is parametrized using the TDR results with full simulation.

No detailed simulation of the shower profile.

All the predicted rates in the following are evaluated using ATLFAST. The extimated rates could be underestimated by a factor 2-8

The b-tagging efficiencies are parametrized as well. Those used are

εb = 50% εc=9.2% εj=0.5%

ASSUMPTIONS IN THE FOLLOWING:

-Offline b-tag quality available at LVL2

- Few KHz available at LVL1 output


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Quest for high PT thresholds… First attempt: try a 2 jet selection. Rate limitations call for high jet

thresholds.

Since the PT of the signal jets is expected harder than background, a hard cut on the PT of the jets would also improve

the S/B ratio.


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…quest for low PT thresholds The final S/B ratio is expected to be 0.1-1%. A MC prediction at

this level of precision is impossible. A background subctration a la UA2 is needed. This means we need to have a lower and a higher

side band to evaluate the background contribution.

In order not to destroy the low-mass side band, taking into account that the Z0 peak has long radiative tails at low masses, we need to avoid too high thresholds on the

Pt of the jets.

Signal subtraction requirements call

for low PT thresholds

σ(M)/M~13%

μ = 90.47 GeV

Mbb

dN/d

Mbb


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Minv distributionsA scan on the thresholds of the 2 leading jets has been made.

Even with a very loose selection, the background mass distribution peaks around 50 GeV.

Even in the (very) optimistic scenario of a full efficient LVL1 on very low Pt jets, the background subctraction is made difficult by

poor low-mass side band.

The 2σ window for the signal is also drawn for completeness

1j20-1j151j25-1j152j251j25-1j15 nr

Mbb

50 GeV


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2 jet selection results

LVL1 selection

LVL1 Prescale to get 10

KHz

LVL1 Efficiencies LVL2 rates (Hz)

Invariant mass selection (80 GeV < M < 100 GeV)

S/√B

(1y 1033 cm-2 s-1, prescale included)

Background Signal Background Signal

1j20-1j15 137 14% 51% 7 1.9x10-3 % 3.2% 5.7

1j25-1j15 72 7.6% 48% 13 1.6x10-3 % 3.2% 8.6

2j25 26 2.8% 34% 23 1.1x10-3 % 3.1% 16.7

1j25-1j15 nr

19 2% 34% 18 0.8x10-3 % 2.7% 22

All the channels need a strong prescale factor already for the ATLFAST estimate.

High significances can be reached with the two hardest selections, but the peak of the background invariant mass is too high.

The signal to background ratio is around 0.2%


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3 jet selection We look for different strategies.

We require a leading non-b jet. This decreases the LVL1 rate, and moves at low masses the trigger turn-on in the background

invariant mass distribution

The reason is the following: requiring the leading jet to be non-b, one

strongly reduces the contribution from direct bb production and selects

mainly gluon splitting events (mainly ggggbbg). They are

characterized by low invariant mass of the bb couple, because of the small

angle between the b-jets

2j selection

3j selection

Rbb


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3 jet selection (2)

The first selection tried is 1j40-2j25. The LVL1 ATLFAST rate estimate goes down to 25 KHz. The signal to background ratio is

0.4%, while the significance is 24 for a LVL1 prescale of 2.5.50 Gev

μ=90.23 GeV

σ=10.74 GeV

Mbb Mbb


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3 jet selection (3)To reduce further the LVL1 rate & the trigger turn-on mass, a hard selection on the leading jet has been tried. A leading jet of PT > 80

GeV is required. This reduces the value of the peak in the background invariant mass. The LVL1 rate is 2.6 KHz.

50 GevSignal

QCD background

Mbb Rbb


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3 jet selection (4)

The selection can be refined further by raising the thresholds on the b-jets to 30 GeV. While the background mass does not change much, the Z mass distribution becomes narrower.

LVL1 Sel.

LVL1 Accepted σ In mass window

(80 GeV < Mbb<100 GeV)

LVL1 Rate

S/B S/√B

Signal QCD Signal QCD

1j80 2j25 157 pb 2.6 ub 7.9 pb 1.2 nb 2.6 KHz 0.7% 23

1j80 2j30 120 pb 2.0 ub 6.1 pb 702 pb 2.0 KHz 0.9% 23.3


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3 jet selection (5)In general, the application of a hard cut on the leading jet of the events leads to

more reasonable trigger configurations, to better S/B, even if the statistical significance decreases over a certain threshold.

The Z mass shape get better as the selection gets harder, with less signal in the low tail, which should lead to a better background subtraction

Significance

S/B

LVL1 prescale for

2.6 KHz included

1j80-2j30

1j80-2j25

1j40-2j25

Mbb


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LVL1 trigger considerationsThe main caveats are about the LVL1 efficiencies and rates

- Can we trigger efficiently on the hard PT 3jet selections?

- Which is the rate for them?

I am currenly analysing the LVL1 performances with the full G3 detector simulations. Unfortunatly the software is not the final one

Going back to the TDR results

With a 5 GeV tower threshold, a 30 GeV jet can be identified (i.e., the

LVL1 will provide a Region Of Interest –ROI) with ~95% efficiency


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LVL1 trigger considerations (2)At present, the LVL1 multijet trigger rates are given in terms of

symmetric thresholds (i.e. 3 jets of the same energy)

The trigger menu for this channel should include a high PT LVL1 jet plus 2 very low PT jets.


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Background subtractionWe tried to address the following question: is it possible to extract a signal with

high significance when the signal to background ratio is below 1%?

The background distributions is regular down to (at least) 40 GeV in the 1j80-2j30 case. It is well fit by an exponential, but the statistics is limited.

Even using the fast simulation, a big amount of CPU time is needed to generate the statistics of the background a for some fb-1 of integrated luminosity

We made the following:

- Physicist A was generating histograms with parametrized (and unknown to the physicist B) distributions and “real” statistics and statistic fluctuations for the

background for 10 fb-1. Then the signal is added using a sample a factor 10 lower than the one that will be collected in 10 fb-1.

- Physicist B fitted the background using the sidebands, a la UA2

-The function that fits the sidebands was subctracted from the histogram and the residual is fitted with a gaussian


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Background subtractionFirst example: simple exponential (known physicist B). On the left the 1j40-2j25

case (low sideband is 56-68 GeV, well inside the signal window) leads to a symmetric peak for the signal. The effect of the low radiative tails is clearly

visible also in the 1j80-2j25 selection (low sideband is 40-50) on the right. On the right bottom plot the found peak is compared to the MC generated Z0

1j40-2j25: μ=88.7 GeV σ=10.7 GeV S/√B=20.5

1j80-2j25 selection


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Background subtractionIn the case of the best considered selection (1j80-2j30) and a low sideband

window 38-52 GeV (achievable in the analysis context), we obtain good results on the peak reconstruction. The radiative tails are well reproduced after the

background subtraction. Good reconstruction of the peak details.

The points reproducethe MC Z0 peak shown

in the right low plot.

The fit is done in the 80-110 GeV window


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Background subctractionFirst unknown (to the physicist B) distribution: 35 // MbMa

Fit in the sidebands made with ))((tan 11

edMcbaF

1j40-2j25 selection

The fit of the sidebands (54-68 GeV and 116-

160 GeV) gives a good Χ2 (1.3)

The fitted gaussian (black Line) is very

similar to the fit found with the exponential

background (red line)


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Background subctractionSame background as before but with a different fitting function:

cbxaF )(2 Because of the

presence of a non negligible amount of

signal in the low sideband, part ofthe signal is lost.

systematic uncertainties on the Z significance and

parameters


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Background subtractionUsing the same background distribution as before. Using the fit function F1.

Check the results with the 1j80-2j30 distribution.

Better low sideband and narrower signal Less signal in the low mass sideband the Z parameters and the significance are less dependent on the

fit function chosenF1 used F2 used


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Background subctractionNew unknown distribution: a/M4 NO SIGNAL INCLUDED

Fit made with both F1 and F2

F1 does not fit the low sideband. As a consequence, a fake bump is produced.

F2 does fit the low sideband. No signal is found.


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Conclusions

• Different trigger/selection strategies are available

• The signal can be triggered (more investigation with full simulation needed)

• The signal can be reconstructed

• The amount of found signal and the parameter accuracy depends on the sample selection and reconstruction algorithm

• A very simple procedure gives good results if signal-free low mass side band can be used. To evaluate the parameter stability

we have to fit the reconstructed signal with a Landau, not a gaussian shape, however the comparison with the MC generated

Z0 is significant.


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Some work done also in the Vector Boson Fusion H production with Hbb. It is one of the most promising channels for the measurement of the WWH coupling. A measurement with a

precision of 20% is expected after 600 fb-1 of integrated luminosity. Impossible to trigger in the ATLAS enviroment without

good b-tagging at LVL2.

Seehttp://agenda.cern.ch/askArchive.php?base=agenda&categ=a036321&id=a036321s1t29/transparencies

http://agenda.cern.ch/askArchive.php?base=agenda&categ=a04587&id=a04587s1t10/transparencies

There are preliminary results also in the bbA/H4b MSSM channel. This can be used as a complementary channel for the

MSSM heavy neutral Higgs discovery.

See http://agenda.cern.ch/askArchive.php?base=agenda&categ=a041955&id=a041955s1t1/transparencies


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BACKUP


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Calorimeters in ATLASTile Calorimeter

Forward Calorimeter

EM barrel and EndCap

Hadronic EndCap

EM LAr || < 3 :

Pb/LAr 24-26 X0

3 longitudinal sections1.2

= 0.025 0.025

Central Hadronic || < 1.7 :

Fe(82%)/scintillator(18%)

3 longitudinal sections 7.2

= 0.1 0.1

End Cap Hadronic 1.7 < < 3.2 :

Cu/LAr – 4 longitudinal sections

< 0.2 0.2

Forward calorimeter 3 < < 4.9 :

EM Cu/LAr – HAD W/Lar

3 longitudinal sections

EE

8.1%8.1

%9.41

E

EM LAr + TileCal resolution (obtained EM LAr + TileCal resolution (obtained at 1998 Combined TestBeam) at 1998 Combined TestBeam)

Linearity within Linearity within ±2% (10-300 GeV)±2% (10-300 GeV)


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Background subctraction


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FIT RESULTS


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Documents

Z bb measurement in ATLAS (with ATLFAST)