Activities
of the
Top quark physics
group
ATLAS Physics Workshop
Lund, 12-16 September 2001
Marina Cobal (University of Udine)
Marina Cobal
Lund, 12-16 Sept 2001
Outline
tt reconstruction
t-quark mass measurement
Rare decays
Single top production
tt spin correlations
CP violation in tt production
Summary and outlook
Marina Cobal
Lund, 12-16 Sept 2001
tt reconstruction and mt
In Physics TDR
Inclusive single lepton + jets
High Pt single lepton plus jets
Di-lepton events
First look at all jets mode
s(mt) < 2 GeV possible (systematic errors)
Updates performed for all of these
topologies, and new analyses started
Marina Cobal
Lund, 12-16 Sept 2001
After Physics TDR
Kinematic fit of entire event
(P. Roy, see presentation at this meeting)
Systematics of using different MC generators

ATLAS COM-PHYS-2001-004
(M. David, M. Cobal, A. De Angelis, A. Maio)

Mt measurement with Herwig 6.1, Pythia 6.115, Isajet 7.49

Herwig, Pythia very similar. More gluon rad in Isajet

DMjjb ~ 0.1 ± 0.1 GeV (Herwig- Pythia)
~ -2.2 ± 0.1 GeV (Isajet-Pythia)
High PT subsample
(F. Fassi, see presentation at this meeting)
Marina Cobal
Lund, 12-16 Sept 2001
After Physics TDR (II)
All jets final state
(Lj. Simic,G. Skoro, I. Mendas, D. Popovic)

Extract signal above the large QCD bkgnd
QCD with Pythia and s(Pt > 100 GeV) = 1.7 mb
Njets >= 6, Pt(j) > 40 GeV, Nbjet>=2, S/B ~1/19
 Subsample
with Pt(top) > 200 GeV, has 3300 evts
per 10 fb-1, with S/B ~ 18/1
 Kinematic
and event shapes does not help to
improve S/B
 Instead, perform
kinematic fit to tt
hypothesis after cut
on c2, S/B ~ 5/1
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Next:
- Use Herwig
- Trigger studies
Marina Cobal
Lund, 12-16 Sept 2001
After Physics TDR (III)
 Dilepton events
(V. Simak, P. Homola, J. Valenta, R. Leitner)
 Solving
event kinematics leads, typically,
to several solutions

Examine mean prob. of solution with heighest weight,
according to angle of top in tt CMS system and En
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 tt events with J/Y -> mm
(P. Grenier, see next transparencies)
“Continuous” jet cone algorithm
(V. Kostiukhine, see next transparencies)
Marina Cobal
Lund, 12-16 Sept 2001
Mt measurement from t  l + J/Y + X
Correlation between M(l-J/Y) and Mt
W
m
nl
W
t
t
b
b
About 2700 evts/year at high L expected
J/y
After analysis cuts: 430 evts/year
- 1 iso lepton Pt > 30 GeV and |h| < 2.4
- 3 non-iso muons Pt > 3 GeV and |h| < 2.4
(mass of two of them compatible with MJ/Y)
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Marina Cobal
D M(l-J/Y) (stat)  0.5 GeV

D M(top) (stat)  0.9 GeV
for 5 years of running
Lund, 12-16 Sept 2001
s(pp  tt  bX lnb)  350 pb
Backgrounds:
 W+jets,
Z+jets, WW, WZ, ZZ,bb negligible
 Wbb
(complete analysis)
s( W  ln ) ~ 85 pb -> at production level S/B ~ 4
Rec. e ~ 1.2% compared to 16% for signal  S/B ~ 55
 Systematic uncertainties:
look at ISR, structure function, B-quark
fragmentation < 1 GeV might be possible

 Pt
top dependence  needs good tuning MC to data
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Marina Cobal
Lund, 12-16 Sept 2001
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Marina Cobal
Lund, 12-16 Sept 2001
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Marina Cobal
Lund, 12-16 Sept 2001
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Marina Cobal
Lund, 12-16 Sept 2001
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Marina Cobal
Lund, 12-16 Sept 2001
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Marina Cobal
Lund, 12-16 Sept 2001
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Marina Cobal
Lund, 12-16 Sept 2001
Rare decays
In Physics TDR
 FCNC top decays
- according to SM, would be impossible to be detected
even at the LHC (Br < O(10-10))
- however, enhanced in many Beyond SM theories
- results for t  Zc, gc,gc presented
sensitivity to Br ~ O(10-5) (7.10-3 for gc)
After Physics TDR
 t  Hc
(C. Chikovani, T. Diobava, see next tranparencies)
Marina Cobal
Lund, 12-16 Sept 2001
t -> Hc
Strongly suppressed in SM ( Br (t  Hq) ~ 10-8-10-7)
Analysis on final states: tt  HqWb  bbj,lnb (l=e,m)
Sensitivity of ATLAS requiring >5 signal events,
assuming no bckgnd at high L, 100% signal efficiency
Hypothesis: bckgnd for WH channel can be reduced
by additional cuts and reconstruction of t  Hq mass
Processes
MH (GeV)
Sensitivity
tt -> HqWb -> ggj,lnb
150
0.7.10-3
tt -> HqWb -> bbj,lnb
160-170
4.7.10-6
tt -> HqWb -> ZZj,lnb
tt -> HqWb -> bbj,lnb
Marina Cobal
120-130
100-120
0.7.10-4
2.4.10-7
Lund, 12-16 Sept 2001
Background
The most dangerous backgnd is due to the tt process.
After all the analysis cuts :
~ 12000 tt evts are left, with 56 signal events
Cuts
t  Hq
tt
ttH
Nevt
(eff %)
Nevt
(eff %)
Nevt
(eff %)
W+jets
Nevt
(eff %)
Generated
~2.104
4.106
2.105
12.106
evts
Expected evts
~ 25.107
1.9.104
122.106
1 lep.,
3811(18.5) 3.106(12.8) 7910 (42) ~ 1.2.105 (0.1)
|h|<2.5, Pt >
20 GeV
4 Jets, Pt > 40 1610 (7.8) 2.106 (8.1) 5466 (29) ~ 4.4.104 (0.04)
GeV
3 b-tags
268 (1.3) ~106 (0.40) 1384(7.3) 146 (1.2.10-4)
. -5
MH  20 GeV 173 (0.8) 46833(0.19) 823 (4.4) 55 (5.0 10 )
3 (2.2.10-6)
Mt  12 GeV 34 (0.17) 7410(0.03) 107(0.57)
9326(0.04) 132(0.70)
4 (2.9.10-6)
Mt  16 GeV 47 (0.2)
5 (3.7.10-6)
Mt  32 GeV 56 (0.3) 12350(0.05) 185(0.98)
Marina Cobal
Lund, 12-16 Sept 2001
Cuts
Mbbj
Cuts
3 b-tag
MH  20 GeV
Mt  12 GeV
Mt  16 GeV
Mt  32 GeV
- 1 lepton, Pt > 20 GeV
- 4 jets, Pt > 40 GeV
- 3 b-tags
- MH ± 20 GeV
- Mt ± (12,16,32) GeV
Sensitivity to Br( t -> Hq)
3.0.10-3
3.1.10-3
6.0.10-3
5.0.10-3
4.5.10-3
A Br(t  Hq) as low as 4.5.10-3 could be discovered at
the 5s level with an integrated luminosity of 100 fb-1
Marina Cobal
Lund, 12-16 Sept 2001
RPV SUSY decays of top
If RPV exists can have significant top decay widths
for certain 3-body final states
(hep-ph/0002220, Belyaev, Ellis, Lola).
(i.e: t  c0ql)
F. Martin used COMPHEP to calculate decay
width for t  c0bm as a fcn of RPV coupling l’233
Title:
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Marina Cobal
Lund, 12-16 Sept 2001
Isajet used to generate tt evts with t  c0bm

If c0 escape see excess of events at low Pt (m)

Analysis under way
Other possibilities can be considered:
- t  c0ql, followed by c0  qql
(can result in same-sign lepton signal)
- t  c+qn, followed by c+  c0 W
Marina Cobal
Lund, 12-16 Sept 2001
EW single top
In Physics TDR
Possible to:
see signals
measure Vtb in each prod. mode
see predicted top and W polarization in Wg fusion
(at parton level) showed
After Physics TDR
ATLAS PHYS-2000-017
(B. Gonzalez Pineiro, D. O’ Neil, R. Brock, M. Lefebvre)
top, W polarization with detector smearing
with 10 fb-1 can measure W and top pol.,
with errors < 2%
varying bkgnd by +-20% 
<1% syst. error in polarization.
Marina Cobal
Lund, 12-16 Sept 2001
tt spin correlation
In Physics TDR
studied lab-frame angles between charged leptons
in dilepton events
(V. Simak)
After Physics TDR
extended study to single lepton + jet events
(L. Hinz)
Dileptonic channel :
 low statistics  k = -0.33
Semileptonic channel :
 high statistics  k = 0.11
dilepton events analysed using event reconstruction
(K. Smolek, V. Simak, P. Homola)
see presentation of V. Simak at this meeting
Marina Cobal
Lund, 12-16 Sept 2001
W+f, W+g, bb, g*/Z0+f, g*/Z0+g, g*/Z0,Wbb, Z0Z0, Z0W, WW
Background:
Selection >= 4 jets with pTj > 20 GeV/c and |h| < 2.5, 2 b
| Mjj - MW | < 20 GeV/c2
1 iso lepton with pTl > 20 GeV/c and |h| < 2.5
pTmiss > 20 GeV/c
240000 tt with S/B=5.5
•
•
•
•
Observables from lepton angles
Comparison SM - SM without spin effects (NC)
Maximum in angular distribution : 1.5%
For 1 year at low L with BCK, ISR and FSR :
Marina Cobal
Lund, 12-16 Sept 2001
 Models distinguishable with 3s after 10 fb¯ 1 for flj
After 3 years at low luminosity sensitivity to a
max effect of 0.7% with 2s
New physics
 necessary to be sensitive to a max effect
little than 1%
 possible after 3 years at low luminosity
Semileptonic channel

easy to reconstruct neutrino

direct measurement of k in top rest frame
Dileptonic channel

greater effect but lower statistics compared to
semileptonic
Semileptonic is complementary to
dileptonic channel

 Have to completely simulate new physics models to
better quantify ATLAS sensitivity to each one
Marina Cobal
Lund, 12-16 Sept 2001
CP violation
Due to CKM, extremely small in top sector
 any observation would be a sign of new physics
Different observables can be considered
- DNLR = (Nll-NRR)/(all tt evts)
- A = <Elep+> - <Elep-> / [< Elep+> - <Elep->]
In Physics TDR
Dedicated studies not yet started
After Physics TDR
Studies underway
(see presentation of J. Sjolin)
Marina Cobal
Lund, 12-16 Sept 2001
Summary and conclusions
Understanding of how to reconstruct tt event
topologies and precisely determine mt improved
Several new studies recently started
CP violation in tt production
FCNC decay t-> Hc
novel top decays in RPV SUSY model
Some ideas still to be exploited
tt
 HqWb  WW*j,lnb  lnlnj,lnb
Determination
of the top charge
Searches (i.e: W’ study in EW single top)
Exploit
extra-dimensions in top sector
Marina Cobal
Lund, 12-16 Sept 2001