ALEPH Status Report
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Transcript ALEPH Status Report
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
T itle:
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Marina Cobal
Lund, 12-16 Sept 2001
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Marina Cobal
Lund, 12-16 Sept 2001
T itle:
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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
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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