Commissioning StudiesTop Physics Group

M. Cobal – University of Udine

ATLAS Week, Prague, Sep 2003

Under study

In Athens:

Effects of a non-optimized jet energy scale on the Mtop reconstruction (M. Cobal)

First evaluation of (statistical) error on the cross section and on Mtop (M. Cobal and J. Parsons)

Here:

First evaluation of Mtop, assuming no b-tagging at the startup (V. Kostiouchine)

Investigation of differences found in the combinatorial backgnd between TDR and DC1 (V. Kostiouchine)

Top measurement during detector/physics commissioning

Gold-plated channel : single lepton

• pT (lep) > 20 GeV• pT

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

~ 5%S/B ~ 65

Period evts dMtop(stat)

1 year 3x105 0.1 GeV

1 month 7.5x104 0.2 GeV

1 week 1.9x103 0.4 GeV

L = 1033 cm-2s-1

/ (stat)

0.2 %

0.4%

2.5%

To reconstruct Mtop:

Reconstruction of the hadronic part W from jet pair with the closest invariant mass to MW cut on |mjj-mW| < 20 GeV Association of W with a b-tagged-jet

Cut on |mjjb-<mjjb>| < 20 GeV

Kinematic fit

Using remaining l+b-jet, the leptonic part is reconstructed |mlb-<mjjb>| < 35 GeV Kinematic fit to the ttbar hypothesis, with Mtop and MW mass constraints

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

Results presented at the Athens Workshop

An initial uncertainty of 5% on the b-jet energy scale, gives a top massuncertainty of 3.5 for the mass reconstuction.If we go to 10% , the uncertainty on the top mass is of ~7 GeV

An initial uncertainty of 10% on the light jet energy scale, gives a top mass uncertainty of 3 GeV for the mass reconstuction. Kinematic fit less sensitive to light jet energy scale. But can have very large combinatorial background in case of b-tagging not working

After 1 week of data taking we should be able to measure the cross-section with a 2% statistical error

Even without b-tagging, with two days of data taking, can measure at < 10% (stat. error)

Mtop reconstruction in ATLAS at startup

Work done by V. Kostioukhine

Assumptions:

• No jet energy calibration, no b-tagging.• Uniform calorimeter response • Good lepton identification.

TDR signal+backgrounds estimation

In case of no b-tag:

tt signal: ~500k evt ( 4 times reduction due to b-tag)W+jets: ~85k evt (50 times reduction due to b-

tag)

Signal selection without b-tag

Lepton+4jets exactly (R=0.4): signal ~76% with respect to

4jet W+jets ~83% with respect to

4jets

Select the 3-jet combination with maximal

Select among them 2 jets with maximal

3

1iiPP

2

1iiPP

jjb

jj

Having 3 jets from t-quark decay,there are 3 possible jet assignments for W(jj)b.

• A kinematical constraint fit can be used for a further selection: MW

1=MW

2 and Mt

1=Mt

2.

An approximate calibration is obtained with the W peak

• Select the combination with lowest 2 out of the 3 available. Event is accepted is this minimal 2 is less than a fixed value.

Big 2 events

Reconstructed Mtop

Signal selection: ( 4jets exactly+2 cut) ~40% (~200k evt)

W+jets selection: with the same cuts ~9% (~8k evt)

2 signal 2 W+jets3-jet mass W+jets

Preliminary results with full simulation

TDR top sample(same cuts as fast sim.)

Top mass

W mass

DC1 sample (same cuts as fast sim.)

Top mass

W mass

Conclusions on Mtop

1. A tt signal can be selected without b-tagging and precise jet energy calibration

2. Signal / backgnd ratio is ~20 in this case (~70 in the region Mjjb<200 GeV) . Here only W+jets events are considered as background.

3. Such a clean sample could be also used for jet energy calibration.

4. Results confirmed by full simulation

Combinatorial background in DC1 data

Work done by V. Kostioukhine

• Increase of the combinatorial background in DC1 samples with respect to the TDR ones

• Vadim checked better and.....

W(TDR) W (DC1)

TDR +jets sample

Selection: 1 lep with Pt>20 GeV, Pt miss >20 GeV, at least 4 jets with

Pt>40GeV, 2 b-jets (parton level). 2 non-b jets with min|Mjet-jet – MW|

taken as W decay products. b jet is selected so that Pt jet-jet-b -> max

t-quark peak after application of constraint fit

jj mass jjb mass

top

DC1 +jets sample

Same selection

DC1 sample

t-quark peak after application of constraint fit

DC1 sample with application of“TDR-like” generation level cuts

jj mass jjb mass

top top

jj mass jjb mass

DC1 e+jets sample

Selection: the same

DC1 sample

t-quark peak after application of constraint fit

DC1 sample with application of“TDR-like” generation level cuts

jj mass jjb mass jjb massjj mass

toptop

DC1 summary e,+jets sample

Same selection DC1 sample with application of“TDR-like” generation level cuts DC1 sample

t-quark peak after application of constraint fit agreement with TDR !!

toptop

jj mass jj massjjb mass jjb mass

Next Steps

More detailed MC study: W + jets background.

Study of background level dependence on b-tagging .

Measure the cross-section and top mass assuming different efficiency for the b-tagging (and no b-tagging at all) and looking at various channels. What is the minimal b-tagging needed?

……………

First look at data in 2007

Study of high pT isolated electrons and muons

Select a “standard” top sample, and a “golden” top sample with tighter cuts.

Try to reconstruct the two top masses (in single lepton events, one top decays hadronically, the other one leptonically)

Take top events: try a first measurement of the cross section, and of the mass in various channels (as a cross check, since systematic errors are different)

(tt) : initial measurement dominated by L and detector uncertainties 10-20%?

In addition, very pessimistic scenario considered : b-tag not yet available S increases by ~ 4 S/B decreases from 65 to 6 large combinatorial background

W jj t bjj

M (jj) M (bjj)

Still a top peak is visible Statistical error from fit: from 2.5% (perfect b-tag) to 7% (no b-tag) for ~ one weekWhat about B systematics ?

M (jj)

W jj

difference of distributionsfor events in the top peak andfor events in the side-bands

Feedback on detector performance:-- m (top) wrong jet scale ? -- golden-plated sample to commission b-tag