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Title: Multilepton Signatures at Tevatron


1
Multilepton Signatures at Tevatron
Maxim Titov UNIVERSITY OF FREIBURG (ON BEHALF OF
THE CDF AND DØ COLLABORATIONS)
Luminosity vs Projections
2 fb-1
2007
Aspen Winter Conference New Physics at the
Electroweak Scale and New Signals at Hadron
Colliders, January 8-14, 2007
2
SUperSYmmetry (SUSY)
  • Supersymmetric extensions of the SM provide a
    consistent framework
  • for gauge unification and stabilization of EWK
    scale
  • Each SM particle gets a SUSY partner (spin
    differ by DS ½)
  • Superpartners are heavy ? SUSY must be broken
  • Several SUSY breaking scenario under
    consideration
  • mSUGRA, Gauge-Mediated SB, Anomaly-Mediated SB ?
    determines SUSY structure

Typical mass spectrum of SUSY particles (importa
nt decays are shown
Strong limits on SUSY searches from LEP M(c) gt
103.5 GeV Mslepton gt 95 GeV
3
Phenomenology R-Parity Violating (RPV) SUSY
SUSY / Gauge invariance do not require R-parity
conservation Define multiplicative quantum
number Rp (-1)3B L 2S (1 for SM -1 for
SUSY partners)
Superpotential of general MSSM with R-parity
violation term (W WMSSM WRPV)
WRPV lijk LiLjEk lijk LiLjDk lijk UiUjDk
45 NEW YUKAWA COUPLINGS, ASSUME ONLY ONE COUPLING
DOMINATE AT A TIME !
  • Experimental signatures
  • _at_ Tevatron
  • Pair production and RPV decays of LSP
  • -- l and l couplings
  • Resonant sparticle production
  • -- l and l couplings
  • R-parity is violated (RPV)
  • SUSY particles can be singly produced
  • The LSP is unstable and decays to SM
  • particles ? no dark matter candidate
  • SUSY particles decay into quarks,
  • leptons, neutrinos
  • ? multi-jet, multi-leptons, small ETmiss
  • R-parity is conserved (RPC)
  • SUSY particles are pair produced
  • Lightest SUSY particle (LSP) is
  • stable ? dark matter candidate
  • The LSP (neutral, colourless) interacts
  • only weakly with matter
  • ? Large ETmiss from LSP (SUSY signature)

4
Minimal SuperGravity (mSUGRA)
EW scale
  • THE SYMMETRY BREAKING TAKES PLACE IN A
  • HIDDEN SECTOR AND IS TRANSMITTED TO
  • THE VISIBLE SECTOR BY GRAVITATION
  • Only FIVE parameters
  • m0 common scalar (Higgs,
  • sleptons, squarks) mass at the GUT scale
  • m1/2 common gaugino (bino,
  • wino, gluino) mass at the GUT scale
  • A0 common trilinear scalar
  • couplings at the GUT scale
  • (sfermion mixing)
  • tan b ratio of Higgs vacuum
  • expectation values

GUT scale
Tevatron
Assume mSUGRA mass relation m (c1) m (c20)
0.8 m1/2 m (c10) 0.4 m1/2
5
SUSY in Multilepton Signatures
Many SUSY models give rise to multi-lepton
signatures ? cascade decays from charginos,
neutralinos, sleptons, squarks (topics presented
in this talk are shown in red)
6
Tevatron Experiments CDF D0
Excellent particle ID, coverage, tracking and
powerful trigger system Mature understanding of
detectors Data taking efficiency 85 - 90
General Purpose Detectors CDF
DØ Electron ID acceptance ?lt2.0
?lt3.0 Muon ID acceptance ?lt1.5
?lt2.0 trigger
?lt1.0 ?lt2.0 Precision tracking (Si)
?lt2.0 ?lt3.0 Jet ID
?lt3.6 ?lt4.2
7
Searches for SUSY (R-parity Conserved)
Lepton signatures indicate presence of top, W, Z,
weak gauginos and sleptons
8
Chargino / Neutralino Production and Decays
  • ( ) DEPEND ON GAUGINO-
  • HIGGSINO MIXING, SQUARK MASSES

Destructive interference between s- and
t-channels
t - channel
s - channel
  • Clean signature 3 charged leptons ETmiss (low
    SM bkg)
  • Electroweak production ? small event rates (s x
    Br (3l) 0.1- 0.5 pb)
  • Large cross section ? low gaugino masses (m1/2),
    large squark masses
  • Large leptonic branching fraction ? low slepton
    masses ? low m0
  • Large e(m) branching fraction ? low degree of
    stau mixing ? low tan b

9
miss
Charginos and Neutralinos in 3 leptons E
T
  • Experimental challenge low-pT leptons
  • Need multilepton triggers w/ low pT-thresholds
  • Need efficient lepton ID _at_ low pT
  • ANALYSIS STRATEGY / SIGNATURE
  • 3 leptons ETmiss
  • -- 2 leading PT leptons and tight ID criteria
  • -- Isolated 3rd track PT 3-5 GeV (no ID)
  • (3rd track e, m, t, including hadronic thad)
  • 2 leptons ETmiss
  • -- 2 Like-sign leptons
  • -- No 3rd track requirement
  • (important when 3rd lepton PT is very soft)
  • Major SM Backgrounds
  • WZ/ZZ ? 3 l (irreducible)
  • Fake Leptons
  • (g-conversion or fake leptons p0)
  • W g/jet ? 1 l 2 fakes
  • Z/g g/jet ? 2 l fake
  • WW g/jet ? 2 l fake
  • ttbar ? 2 l fake
  • QCD multijet ? no isolated
  • leptons (determined from data)

10
miss
Charginos and Neutralinos in 3 leptons E
T
6 D0 ANALYSIS 1 ? LS (mm), 3? ll (e,m)
track 2 ?(e/m thad track)
ANALYSIS CHANNEL (ee track) PTe1 gt 12 GeV
PTe2 gt 8 GeV Anti Z/g ? ee Cuts 18 GeV lt Mee lt
60 GeV Dj (e,e) lt 2.9 Anti - ttbar Cut HT
SPTjet lt 80 GeV ETmiss-based Cuts ETmiss gt 22
GeV ET significance gt 8 MT (e, ETmiss) gt 20
GeV Third Isolated Track PT gt 4 GeV,
calorimeter tracker isolation Anti W
Cut pT3rd trackgt 7 GeV, if MT(e,ETmiss)gt 65
GeV Product of ETmiss and 3rd track PT ETmiss
PT3rd track gt 220 GeV2
? L dt 1.1 fb-1
Mee
ETmiss PT3rd track
11

Trilepton Analysis Selection
m m
_ _
Increase acceptance by requiring 2 out of 3
leptons, while reducing SM bkg
? L dt 0.9 fb-1
M (m,m)
Preselection (2 isolated LS-muons) PTm1 gt 13
GeV PTm2 gt 8 GeV Anti Z/g ? mm Anti-QCD
Cuts 25 GeV lt Mmm-lt 65 GeV Dj (m,m) lt
2.9 ETmiss-based Cuts ETmiss gt 10 GeV ET
significance gt 12 15 GeV lt MT (ETmiss, pTm2) lt 65
GeV LS Invariant Mass Cut 12 GeV lt M (m,m) lt
110 GeV Product of ETmiss and 3rd track
PT ETmiss PT3rd track gt 160 GeV2
MT
LS dilepton channel is also sensitive to squark
/ gluino production
12
RPC Chargino / Neutralino Limits in mSUGRA
D0 ANALYSIS LUMI (PB-1) BACKGROUND EXPECTED DATA
e e track 1100 0.82 ? 0.66 0
LS m m 900 1.1 ? 0.4 1
e m track 300 0.31 ? 0.13 0
m m track 300 1.75 ? 0.57 2
e t track 300 0.58 ? 0.14 0
m t track 300 0.36 ? 0.13 1
  • mSUGRA - inspired Model
  • s x Br (3 l) is mainly a function
  • of m( ) and m ( )
  • Degenerate slepton masses
  • (no slepton mixing)
  • ms? mse m s?

Combine analysis eetrack (1.1 fb-1), emtrack
(0.3 fb-1), mmtrack (0.3 fb-1), LS m m (0.9
fb-1)
DM lt 0 2-body decay into real sleptons DM lt - 6
GeV High efficiency, Br(c20c1 ?e,m) dominant -6
GeV lt DM lt 0 GeV Soft 3rd lepton PT ? limit set
by LS m m DM gt 0 3-body decay via virtual
or W/Z DM ? 0 -exchange dominates Large
Br(c20c1 ?e,m) ? 3l-max scenario DM gtgt 0
W/Z-exchange dominates Small Br(c20c1 ?e,m)
large-m0 scenario
DM M(slepton) M (c02) (GeV)
13
RPC Chargino / Neutralino Limits in mSUGRA
Combination eetrack (1.1 fb-1), emtrack (0.3
fb-1), mmtrack (0.3 fb-1), m m (0.9 fb-1)
3 l MAX SCENARIO M ( ) ? M (
) (sfermion decay dominates) M ( ) gt
140 GeV s Br lt 0.07 pb
HEAVY SQUARKS M ( ) gtgt M ( ) (no
scalar mass unification) M ( ) gt 155
GeV s Br lt 0.06 pb
  • LARGE - m0 SCENARIO
  • M ( ) gtgt M ( , )
  • (large masses, small Br (3l)
  • ? W, Z decays dominate)
  • No Sensitivity

t channel suppressed

14
miss
Charginos and Neutralinos in 3 leptons E
T
14 CDF ANALYSIS 6 ? LS (ll), 7 ? trilepton
(lll), 1 ? eetrack
?Ldt 1 fb-1
  • Analysis Channels ell, mel,
  • mml (high-PT rrigger), mml (low-pT trigger)
  • 2 leading pT leptons (l e,m)
  • 20 GeV lt Mll lt76 GeV Mll gt106 GeV
  • ETmiss gt 15 GeV
  • Njets (PTjet gt 20 GeV) lt 2
  • 3rd lepton PT gt 5 GeV

ee/me/m
?Ldt 0.7 fb-1
m e m/e
?Ldt 0.7 fb-1
m m m/e
15

l l
Trilepton Analysis and eetrack
_ _
  • Analysis ll (ee, em, mm)
  • 2 leading pT LS leptons, Mllgt25 GeV
  • ETmiss gt 15 GeV Z-veto
  • Challenge modelling SM bkg (conversions)
  • Analysis Channel e e track
  • 2 leading pT electrons
  • 20 GeV lt Mll lt 60 GeV M ll gt106 GeV
  • ETmiss gt 20 GeV Min MT gt 10 GeV
  • Dj(e1,e2) lt 2.8 HT SPT jet lt 80 GeV
  • PT (3rd track) gt 4 GeV

?Ldt 1 fb-1
l l
?Ldt 1 fb-1
?Ldt 1 fb-1
ee track
ee track
16
RPC Chargino / Neutralino Limits in mSUGRA
mSUGRA inspired tan b 3 A 0 m gt 0 M0
60 GeV M1/2 162 - 230 GeV
CDF ANALYSIS LUMI (PB-1) BACKGROUND EXPECTED DATA
e?e?, e???, ???? 1000 7.9 ? 0.3 13
?? e/? (low-pT) 1000 0.4 ? 0.1 1
ee track 1000 0.97 ? 0.28 3
e e/m e/? 1000 0.73 ? 0.09 0
? ? e/? 750 0.64 ? 0.18 1
? e e/? 750 0.78 ? 0.11 0
Slepton mixing is on
No slepton mixing
s Br lt 0.25 pb
W/Z decays dominate
No sensitivity
M(c1) expected limit is 160 GeV _at_ 95CL ? s
Br lt 0.1 pb
17
RPC Chargino / Neutralino Observed Limits
  • Degenerate slepton masses
  • Stau mixing is on
  • m0 60 GeV, m ( ) lt m ( ) ?
  • 2-body decays via real enhanced
  • M ( ) gt 130 GeV s Br lt 0.25 pb

DIFFERENT LUMINOSITIES, ANALYSIS CHANNELS
AND SLIGHTLY DIFFERENT SCENARIOS
  • Degenerate slepton masses
  • Stau mixing is off
  • m0 100 GeV, m ( ) ? m ( ) ?
  • only 3-body decays
  • via off-shell enhanced
  • M ( ) gt 140 GeV s Br lt 0.07 pb

18
Search for Direct Production of Scalar Top Quarks
Squark mixing mq ? stop might be the lightest
squark
  • Heavy stop
  • t ? tc10
  • Medium stop
  • t?bc1?bln
  • Light stop
  • t ? cc10


FINAL STATE Pair production of lightest stop
quark with subsequent decays via virtual
charginos t1t1 ? bb ll nnc10c10 (n ?nc10)





SIGNATURE 2l (ee, em) 2 jets ETmiss

Stop exclusion up to m top
2 different analysis to improve sensitivity
em,mm em optimized for different Dm (Mstop
Msneutrino) Cuts 2 l (opposite sign) at least
1 b-jet ETmiss gt 15 GeV (n ?nc10) topological
cuts Background Z/g ? tt ttbar
ANALYSIS BACKGROUND EXPECTED DATA
em (Dm low) 23.0 ? 3.1 21
em (Dm high) 40.7 ? 4.4 42
mm 2.9 ? 0.4 1
19
Searches for SUSY (R-parity Violation)
20
RPV in Production and Decay of SUSY Particles
RPV DECAY OF LSP ( ) INTO SM FERMIONS
RESONANT SPARTICLE PRODUCTION
(For non-zero LiLjEk - coupling)
(For non-zero LiLjDk -coupling)
Experimental Signatures at Hadron Colliders
l121 ? eeee, eeem or eemm nn l122 ? mmmm, mmme
or mmee nn l133 ? tttt, ttte or ttee nn
(l211 coupling)
Single sparticle production rate depends on lijk
coupling strength s (lijk)2 Decay of LSP
The lijk coupling strength influences only
lifetime (decay length) a) lijk gt O (10-2) ?
prompt decay b) lijk lt O (10-2) ? long lived
particle, decay outside detector
21
l
l
R-Parity Violation Couplings
112
122
  • FINAL STATE
  • RPC SUSY Production
  • Sparticles cascade decay into c10
  • Prompt RPV decay of c10 via l121 or l122

SIGNATURE At least 4 l 2 n
SIMILAR TO TRILEPTON ANALYSIS Cuts optimized
for ? 3 leptons to improve acceptance (eel,
mml) pTl1 ? 20 GeV pTl2 ? 8 GeV pTl3 gt 5 GeV
channel dependent cuts (anti-Z/g) Background
Drell Yan Z/g
Sensitive to all new physics with gt 4 leptons in
the final state
m0 250GeV m gt 0 m0 250GeV m gt 0 m0250GeV m lt 0 m0250GeV m lt 0
COUPLING l 121 l122 l121 l122
M ( ) GeV 101 110 98 106
M ( ) GeV 185 203 186 202
mSUGRA limits l121 from eel (l e,m) l122
from mml (l e,m)
22
l
l
R-Parity Violation Couplings
R-Parity Violation LLE Couplings
1jk
1jk
Analysis require 3 charged leptons eel (l
e,m), mml (l m,e), eethad channels loose
ETmiss cut
m0 1 TeV conservative choice (heavy
sleptons) ? limits should be valid for any m0
mSUGRA m0 1TeV tan b 5 mSUGRA m0 1TeV tan b 5 mSUGRA m0 1TeV tan b 5 mSUGRA m0 1TeV tan b 5 m0100 GeV tanb 20
COUPLING ?121 l122 l133 l133
M ( ) GeV 119 118 86 115
M ( ) GeV 231 229 166 217
eel, mml, eethad analysis are combined to set
limits for each coupling l121, l122, l133
No-GUT MSSM
mSUGRA
Phys. Lett. B 638, 441-449 (2006)
23
l
'
Resonant Smuon Production Coupling
211
FINAL STATE RPV in resonant production and
decay via l211 d u ? ? mc10 ? mm du
l211 exclusion contours within mSUGRA
_
_
SIGNATURE ? 2 ? 2 jets (no ETmiss)
Cuts pT?1 ? 15 GeV pT?2 ? 8 GeV pTjets(1,2)
? 15 GeV ?R (?,jets) ? 0.5 Background Z/g2
jets, QCD
Final Cuts depend on the reconstructed mass M (
) M (mmjj) M(c10) M (mjj)
M (mmjj)
Excluded slepton mass range lijk strength
M ( ) lt 210 GeV for l211 gt 0.04
M ( ) lt 340 GeV for l211 gt 0.06
M ( ) lt 363 GeV for l211 gt 0.10
Phys. Rev. Lett. 97, 111801 (2006)
24
ll
Signature-based Searches in Channel
g
Signature-based Approach Investigate events by
the final state products ? Quasi-model
independent technique
25
Signature-based Searches in Dilepton X
Look for excess above SM prediction in eeg, mmg
and emg X final states
  • Motivation
  • CDF Run I eegg ETmiss event ? rare in SM,
    expected in GMSB or ll ? llgg
  • Excess in the l g ETmiss X events above the SM
    predictions

No excess over the SM predictions in 1
fb-1 of dataset
mmg
eeg
llg
ETmiss in eeg events
ETmiss in mmg events
26
Summary and Outlook
  • CDF and DØ searches are exploring new territory
    beyond LEP limits
  • No evidence of any SUSY signal yet
  • Start counting on gt 2 fb-1 data sample
  • ? We hope that chargino and neutralino are light
    enough to find them at Tevatron

The results of ?2 fits based on the current
experimental results for the precision
observables MW, sin2?eff, (g-2)?, BR(b?s?)
Expected sensitivity in the search for SUSY via
trilepton decay signature (D0 CDF)
Fit to Electroweak precision data JHEP02
(2005)013 (2005)
M(c20), M (c1) (GeV)
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