Physics with lepton pairs at the CDF experiment

1
Physics with lepton pairs at the CDF experiment

• Julia Thom, Fermilab
• HEP Seminar
• University of Washington, 12/7/04

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Particle Physics Today

• Impressive progress made over last decades
• Precision test of the Standard Model (SM)
• Closing last gaps
• Key questions Can we find the Higgs? Physics
  beyond the SM? Connection with cosmology?
• Hadron Collider experiments are a tool to address
  these questions today
• CDF and D0 are taking data
• LHC in preparation discover Higgs, New Physics?

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Addressing key questionsat the Tevatron

• Is there Physics beyond the Standard Model?
• top quark properties
• Bs physics
• SUSY searches
• Mechanism of EWK symmetry breaking
• Searches for the Higgs
• Understanding important backgrounds to Higgs
  searches
• I will cover physics analyses that I have done,
  and that address some of these key questions

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Outline of the talk

• Introduction to the Tevatron and CDF
• Physics with Lepton Pairs, or Searching for
  New Physics at CDF
• Top production with two leptons in the final
  state
• WW production an important background to
  Higgs-searches
• search for rare Bs?mm- decays and implication
  for SUSY parameters
• Conclusions

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Tevatron Run II
Tevatron Peak Luminosity
For this talk results with 200pb-1 (2x Run I
data set) Now analyzing 400pb-1 Will have 2 fb-1
by 2007
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pp Collisions at the Tevatron
W, Z, photon, etc
p
Initial state radiation
Underlying event
final state radiation
p
Very complex event structure, need a complicated
detector to study the physics
JET
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The CDF Detector
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Muon
Calorimeter
tracking
installing silicon tracker, before detector
roll-in
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Event observables
Top candidate

• Leptons
• tracking and cal.
• High-PT lepton id
• eff. of 90

• jets
• Collimated flow of hadrons
• Calibrate calorimeter response
• (3 systematic)

• Missing ET (ET)
• From ET balance
• correct for undermeasured tracks and
  mis-measured jets

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Why lepton pairs?

• Simple signature, low background (not QCD!)
• e.g. electron identification cuts
• Small ratio of HAD/EM energy
• Calorimeter isolation
• Matching track
• Shower shape
• Many interesting processes accessible at CDF and
  LHC have dilepton signatures

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TOP production with leptons in the final state
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Why measure TOP properties?

• Only quark at weak scale
• ?Window into EWSB?
• New physics may appear
• in production (e.g. topcolor)
• or in decay (e.g. charged Higgs)
• Start by establishing a top sample and measuring
  the top production cross-section!

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Top Decay into Lepton Pairs

• Signature
• Two high-PT leptons
• ET from ns
• 2 or more jets (from b)
• BR only 11, good S/N
• (other final states
• Leptonjets BR44
• All hadronic BR45)

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Selection of top dilepton sample

• Identify leading lepton, ETgt20 GeV
• Instead of 2nd lepton
  identify isolated track
  (new technique)
• Require ETgt25 GeV
• Require 2 jets,
  ETgt20 GeV
  
• Require opposite charge

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Experimental challenge background

• Drell-Yan (DY)
• Major background source!
• Must have fake ET
• Not modeled well in MC, use data as much as
  possible
• Wjets
• Major background source!
• Jet fakes a lepton
• determine fake rate from data (0.3)
• Diboson (WW,WZ,..)
• Small cross-section
• estimate from MC
  

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DY background
Missing energy

• fake missing energy
• Incomplete detector coverage
• Mis-reconstructed jets
• Undermeasured MIPs
• reduce this background
• ETgt25 GeV
• if dileptons in Z mass window
• ETgt40 GeV

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DY background

• How many DY background events in signal sample?
• Find bad MC modeling of tails! But poor data
  statistics
• Number of events in Z mass window from data
• Ratio of events inside/outside Z mass window from
  MC

Z?mm data
Number of events
MC
Missing Energy
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DY background

• Main systematic due to data statistics (30)
• Checked technique by
• varying ET cut
• Veto all events inside Z mass window
• results in lt2 variation on top cross-section

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Fake Background
Whats the rate for jet to look like isolated
track? In a generic jet data sample determine
Fake Rate Fake
rate
Fake rate
EtGeV
Fake rate
h
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Fake Background

• Predict number of fakes
• (fake rate from Jet50 sample)
• Compare with number of fakes in Jet20, Jet70
  sample

Jet20(50, 70) trigger at least one jet with
Etgt20(50,70) GeV
Jet20
observed predicted
Jet70
EtGeV
EtGeV
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Signal and Background Table
200pb-1
signal region!
Control samples Check 0, 1 jet first
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Dilepton Kinematics
ET
HTSum of all ET (jets, leptons, ET)
Lepton transverse momentum
good agreement with SM
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Run I a SUSY explanation of dilepton event
distribution
SM ttbar 300 GeV
Barnett and Hall, Phys. Rev. Lett. 77 3506 (1996)
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Top production cross-section

• Systematics
• Backgrounds 30
• Selection efficiency 6 from acceptance, trk
  ID
• Luminosity 6

First Run II high-pt paper at Tevatron PRL
93142001, 2004 hep-e/0404036
hep-ph/0303085 (Mangano et al)
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Other top decay channels

• comparison with other
• cross-section measurement
• Working on combination

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TOP with dileptons summary

• Established higher statistics Run II sample, new
  techniques!
• looks consistent with SM
• Cleared up Run I kinematic inconsistencies
• Many analyses ongoing that make use of this
  sample..

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• Top Mass measurements with dileptons, e.g.
• Top decay
• BR(t-gtWb) (identify jets from b-quarks)
• Limits on t?Hb (charged Higgs)
• Leptont final state
• Angular correlations between leptons due to top
  spin

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Physics with high-PT lepton pairs WW production
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Why measure WW production?

• Background for top measurement
• Interesting in its own right
• Run I only 5 events observed
• calculate cross-section with higher statistics
• Interesting test of SM ? 2 TeV!
• Critical channel at LHC

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WW irreducible background for Higgs search at
CDF/LHC. Need to know production rates. At CDF
Dilepton invariant mass
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WW signal and background table
Same signature as top, except look at zero (or
one) jet sample!
Problem
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Reducing DY background
Reject events consistent with ET being a
fluctuation of the total ET flow again
estimate DY using data-based methods.
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WW cross-section

• 200pb-1
• (to be submitted to PRL)
• In good agreement
• with NLO calculation

RunII measurements from CDF, D0
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WW kinematic distributions
..good agreement with SM prediction
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WW production summary

• Interesting test of SM
• checked WW cross-section at 2 TeV consistent
  with NLO calculations
• Can use similar methods at LHC important
  background in H?WW discovery channel

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Physics with low-PT lepton pairs search for
Bs?mm- decays
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Why search for Bs?mm- decays?

• Highly suppressed in SM (FCNC to leptons)
• Not yet experimentally observed
• Good indirect search channel for New Physics!

Buchalla, Buras, Nucl.Phys.B398,285
CDF, PRD 57(1998)3811R
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Bs?mm- decays in SUSY

• 2 Higgs doublets Hd, Hu
  Md3x3Yd3x3ltHdogt C ltHuogt
• M,Y NOT simultaneously diagonalizable
• ?off-diagonal quark-neutral scalar couplings
  
• large tanb enhances BR(Bs?mm) by 1-2 orders
  of magnitude, observable in Run II!

b
m
Raby, Hamzoui, Babu, Kolda
H,A
tanb
m-
s
tanb2
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B-physics at CDF

• Large production rates
• s(pp?bX)30mb (central, PTgt6 GeV)
• Heavy states produced (Bs, Bc, Lb,..)
• Backgrounds 3 orders of magnitude higher
• Inelastic cross-section 100mb
• Using a di-muon trigger (PTgt1.5 GeV, htriglt1)

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CDF Di-Muon Triggers
Bs?mm

• Pink used in this analysis
• Bs?mm expect at most 5 events
• ? beat down 104 background events !

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Bs?mm- topology
Signal - M(mm) M(Bs) - long
lifetime (ct 438 mm) - few addl tracks
- LT and PT(mm) co-linear Backgrounds
- semileptonic b decays - continuum mm,
mfake, fakefake
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Silicon Vertex Detector
8 layers 704 ladders 722,432 channels
Improve IP resolution
Forward tracking
3D vertexing, Tracking Trigger (L2)!
Use SVX information to reconstruct lifetime of
the Bs cand. Require gt4 hits on each muon leg,
require good vertex
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Bs?mm- signal vs. background

• Discriminating variables
• - invariant mass M(mm)
• - Lifetime ctLxyM/PTB
• - Pointing angle FBs-Fvtx
• isolation ratio PTB/(StrkPTB)

Signal MC bkg dominated data
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Optimization
Blind analysis Optimize selection cuts
excluding search region

• Search Region
• 4s search window
• s(Mmm) 27 MeV/c2
• Sideband Regions
• additional 0.5 GeV/c2 on either side

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Background estimate

• Usually apply all cuts, count events in side-band
  
• ?leaves few surviving events
• Alternative factorize analysis cuts
• Only if correlations are small!
• Background prediction cross-checked in control
  samples (e.g. same sign leptons)

ltBgdgt in 171 pb-1 1.1 /- 0.3 events
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Bs?mm results
171 pb-1
Observe 1 event in search window At 90 CL
Phys.Rev.Lett.93032001,2004 hep-ex/0403032

• 3x better than previously published limit
• Competitive with Belle Bd result
• improve limit by factor of 2 by adding data on
  tape

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Combining with results from D0
240 pb-1
D0 BR(Bs?mm-)lt3.8 x 10-7 _at_ 90 C.L. Combining
CDFD0 limits BR(Bs?mm-) lt 2.7 x 10-7 _at_ 90 C.L
.
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Title
Example for mSUGRA constraints
Tighter constraints on Bs?mm forces heavier M0 or
smaller tan b
(Dedes, Dreiner, Nierste, PRL (2001) 251804)
P.Ko
BR(Bs?mm) lt1x10-7
am(g-2)/2
Higgs mass
Light Higgs mass
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Bs?mm- summary

• Extremely interesting search channel for SUSY
  (large tanb)
• Improved previous limit significantly
• Starting to cut into parameter space of many
  mSUGRA, SO(10),..models
• Future
• Use more data (gt2x on tape)
• Use L00 for primary vertex finding
• Include forward triggered muons

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Outlook whats next?

• Working on now
• Same sign lepton pairs or trileptons very low SM
  background
• Predicted in many SUSY scenarios
• Search for medium PT leptons to gain acceptance
  for SUSY signatures
• Intermediate decays to charginos and neutralinos
  at least one lepton PTgt5GeV

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Talk Summary

• Tevatron Run II Wealth of physics topics, have
  looked for New Physics in different processes
  using lepton signatures
• Most precise top dilepton cross-section
  measurement using new techniques
• WW cross-section measurement at hadron collider
• Improved upper bound on BR(Bs?mm) ?important
  constraint on SUSY parameter space
• Will take at least 10 times more data before
  LHC-era
• Exciting times ahead! Gaining experience for
  head-start on LHC physics

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Bs-gtmm and Dark Matter
(P. Ko, SUSY 04)

• Probe mSUGRA parameter space consistent with WMAP
  data
• Upper limit on BR(Bs?mm) could strongly affect DM
  cross-section
• Bs-gtmm complementary to direct DM detection
  experiments

CDFD0 Limit
Consistent with WMAP data
M0300 GeV, A00