ALEPH Status Report

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Activities of the Top quark physics group
ATLAS Physics Workshop Lund,
12-16 September 2001
Marina Cobal (University of Udine)
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Outline

• tt reconstruction
• t-quark mass measurement
• Rare decays
• Single top production
• tt spin correlations
• CP violation in tt production
• Summary and outlook

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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) lt 2 GeV possible (systematic errors)
• Updates performed for all of these
• topologies, and new analyses started

4

• 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)

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• 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 gt 100 GeV) 1.7 mb
• Njets gt 6, Pt(j) gt 40 GeV, Nbjetgt2, S/B
  1/19
• ? Subsample with Pt(top) gt 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
• Next
• - Use Herwig
• - Trigger studies

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• 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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Mt measurement from t ? l J/? X

• Correlation between M(l-J/Y) and Mt
• About 2700 evts/year at high L expected
• After analysis cuts 430 evts/year
• - 1 iso lepton Pt gt 30 GeV and h lt 2.4
• - 3 non-iso muons Pt gt 3 GeV and h lt 2.4
• (mass of two of them compatible with MJ/Y)

n
l
W
W
t
t
m
b
b
J/y
? M(l-J/?) (stat) ? 0.5 GeV
?
? M(top) (stat) ? 0.9 GeV for 5 years of running
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• s(pp ? tt ? bX l?b) ? 350 pb
• Backgrounds
• ? Wjets, Zjets, WW, WZ, ZZ,bb
  negligible
• ? Wbb (complete analysis)
• s( W ? ln ) 85 pb -gt 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 lt 1 GeV might be possible
• ? Pt top dependence ? needs good tuning
  MC to data

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Rare decays

• In Physics TDR
• FCNC top decays
• - according to SM, would be impossible to be
  detected
• even at the LHC (Br lt 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)

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t -gt Hc

• Strongly suppressed in SM ( Br (t ? Hq)
  10-8-10-7)
• Analysis on final states tt ? HqWb ? bbj,lnb
  (le,m)
• Sensitivity of ATLAS requiring gt5 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 -gt HqWb -gt ggj,lnb 150
0.7.10-3 tt -gt HqWb -gt bbj,lnb 160-170
4.7.10-6 tt -gt HqWb -gt ZZj,lnb
120-130 0.7.10-4 tt -gt HqWb -gt
bbj,lnb 100-120 2.4.10-7
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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
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Cuts - 1 lepton, Pt gt 20 GeV - 4 jets, Pt gt 40
GeV - 3 b-tags - MH 20 GeV - Mt (12,16,32)
GeV
Mbbj
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
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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
  l233

20
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 ? cqn, followed by c ? c0
W
21
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 lt 2
• varying bkgnd by -20 ?

22
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 Å k -0.33
• Semileptonic channel
• high statistics Å k 0.11
• dilepton events analysed using event
  reconstruction
• (K. Smolek, V. Simak, P. Homola)
• see presentation of V. Simak at this meeting

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• Background Wf, Wg, bb, g/Z0f, g/Z0g,
  g/Z0,Wbb, Z0Z0, Z0W, WW
• Selection gt 4 jets with pTj gt 20 GeV/c and h
  lt 2.5, 2 b
• Mjj - MW lt 20 GeV/c2
• 1 iso lepton with pTl gt 20
  GeV/c and h lt 2.5
• pTmiss gt 20 GeV/c
• 240000 tt with S/B5.5
• 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

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• 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

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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
ltElepgt - ltElep-gt / lt Elepgt - ltElep-gt In
Physics TDR Dedicated studies not yet
started After Physics TDR Studies underway
(see presentation of J. Sjolin)
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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-gt Hc
• novel top decays in RPV SUSY model
• Some ideas still to be exploited
• ?tt ? HqWb ? WWj,lnb ? lnlnj,lnb
• ?Determination of the top charge
• ?Searches (i.e W study in EW single top)