search for anomalous production of multi-lepton events at cdf

Search for Anomalous Production of Multi-

lepton Events at CDF

Alon Attal UCLASept. 5, 2006

Sept. 5, 2006 Alon Attal 2

Outline

Standard Model

SUSY Theory & Motivation

CDF Detector

Signal & Background

Analysis

Results


Standard Model

The Standard Model (SM): SU(3) x SU(2) x U(1)Y

The SM agrees extraordinarily well with detector measurements

SM requires a Higgs boson to explain massive particles

Is there physics beyond the SM?


New Physics Clues

SM does not contain gravity New physics at Planck Scale (~1018 GeV)

Higgs has not been discovered yet Could be an indication of non-SM Higgs

Hierarchy problem Unnatural for Higgs mass corrections to depend on Planck Scale

Dark Matter None of the particles within SM can account for it

Particle – antiparticle asymmetry Can’t be accounted for with SM CP violation

Particle masses, quantized electric charge, 3 families SM doesn’t answer why it is the way it is


What is SuperSymmetry?SuperSymmetry (SUSY) is a proposed symmetry between fermions

and bosons:Fermions Bosons

SM particles

SM particles/Higgs

SUSY particles

SUSY particles

SUSY must be broken particles gain mass

SUSY contains particles exactly the same as Standard Model particles but with different spin:


SUSY Motivation

SUSY cancels quadratic divergences in the Higgs sector:

Works best when particle, SM masses are similar

particles should have masses on the order of Mtop

-

f

f

H

f~

f~

H H H≈ 0


SM

(GeV)

Grand Unification

Addition of particles leads to unification at GUT Scale

SUSYi-1

()


Minimal Supergravity (mSUGRA) SUSY introduces > 100 free parameters

The mSUGRA framework constrains the theory down to 5: M0,

M1/2, tan,A0, sign(). In my analysis M( ) is the most important and depends on M1/2

0~1


R-Parity (Rp)

Rp = (-1)[3(B-L) + 2S]

+1 for SM particles

-1 for particles

B = baryon #, L = lepton #, S = spin

If Rp is conserved, the lightest supersymmetric particle (LSP) is stable.

LSP = Dark Matter candidate


R-Parity Violation (RpV)

“RpV SUSY is no WIMP”

Violate lepton # Violates baryon #

3 additional couplings:

• Only consider Li Lj Ek term, protecting proton lifetime• |λ| < 0.1, assume only LSP decays via RpV coupling

WRPV ijk LiL jE k ijk LiQ jD k ijkU iD jD k


4-Lepton SignatureStep 1: Dominant Sparticle Production

2 particles

p p


Step 2: Chargino/Neutralino Decay

2 particles

2 LSPs

cascade decay

p p Leptons may be produced in cascade decays.


Step 3: RpV Decays

121 diagrams:

2 particles

2 LSPs

≥ 4 Charged Leptons

cascade decay

RpV decay

p p Most sensitive to 121 and 122.


Restrictions on There are theoretical and experimental limits on :

Upper limit set by muon lifetime

Analysis is not sensitive to the specific value of since

Lower limit set by impact parameter cut on identified leptons (d0 ≤ 0.02 cm)


Tevatron & Luminosity

Currently the highest energy accelerator in the world

346 pb-1 used


CDF Detector & Lepton ID

Lepton ID important to analysis Studied in data and MC Efficient (~90%) Probability that jets are misidentified

as leptons is small (< 0.02%)Muon Detectors

Drift ChamberCalorimeter

Electrons Track + Calorimeter Cluster 95% of energy in EM calorimeter |η| < 2.0

Muons Track plus “stub” in muon detector Minimum ionizing |η| < 1.1

η = 1.0

η = 2.0

η = 0

~


Signal Distributions

muon coverage

electron coverage

181.4

M

(GeV)

0.13

(pb)

0

A0

+

sign

182.299.45260250

M

(GeV)

M

(GeV)

tanβM1/2

(GeV)

M0

(GeV)

±~1

0~2

0~1

Analysis Reference Point:


Needle in a Haystack

SUSY σ much smaller than backgrounds

Key: understand and reduce backgrounds


Data Collection

All Events

High-pT lepton triggers:Electron or muon with pT > 18 GeV/c

Triggered Events

Filtered Events

Dilepton Filter:“Tight” lepton w/ pT > 18 GeV/c and

“loose” lepton w/ pT > 5 GeV/c


Backgrounds (Before Final Cuts)

Luminosity = 346 pb-1

Individual processes t-tbar / heavy flavor Diboson

Composite backgrounds W / Z/* + Misidentified

Jets – Determined from data

W / Z/* + Photon Conversions (+ material → e+e-)– Determined from MC


Analysis Strategy

Counting experiment with 346 pb-1

No Jet or missing energy cuts to reduce model dependency

Also sensitive to H±± and non-minimal SUSY models Two signal regions “boxes” to optimize result:

4 or more leptons Exactly 3 leptons (to increase acceptance)

Choose selection criteria to optimize S/B Validate with control regions

Goal: Find new physics in multi-lepton channel!


Selection Criteria & Targeted Backgrounds

Di-Boson

Heavy Flavor

Drell-Yan

Z/ + Jets

Trilepton Signal Regions: At least one of the two leading leptons must be an e± (±) for 121 (122)

ET (pT) > 20, 8, 5 GeV/c2 (GeV/c)

for 1st, 2nd, additional leptons, respectively

Electron track + opp. sign partner track:

S < 0.1 cm, cot < 0.02

Low Mass Cut: Mll < 15 GeV/c2

Isolated Leptons

Z Veto Cut: ≠ (76 < Ml+l- < 106 GeV/c2)

Cut: ≠ ( 160 < < 200°)

, J/Selection Requirement


Systematic Uncertainties

Lepton ID Per lepton (0.7 – 11% depending on lepton type, pT)

Mis-ID Jets 50%

Conversion Removal 28.8%

Luminosity 6%

PDFs 0.3% (signal), 2% (background)

ISR 1.5% (signal), 4% (background)

Cross Section 8% (DY), 5% (Diboson), 10% (t-tbar), 7% (signal)

Understanding uncertainties important for reporting the accuracy of the result

Largest systematic uncertainties: Mis-ID Jets for background expectation Lepton ID for signal expectation


Control Region (Example) Control regions are crucial to understanding our procedure

Validate lepton ID efficiencies Validate selection cuts

Trilepton events that fail 1 or more cuts


Control Region Overview

y = x

26 total control regions (summary on left) By lepton type Inside & outside Z window Number of leptons Pass/Fail cut

Analysis procedure is validated through agreement between data and MC prediction

Each point corresponds to a singlecontrol region w/ error bars = ±1σ


Opening the Box

?


Signal Regions

Signal events: 4 eee, 1ee, 1 e Probability of observing ≥ 5 events with 3.1 expected = 17% Signal regions consistent with background and no signal

Trilepton Signal Regions

Dataset λ121 λ122

Z/ + 2.1 ± 0.8 1.2 ± 1.0

Z/ + W 0.2 ± 0.1 0.1 ± 0.1

Fakes 0.7 ± 0.4 0.5 ± 0.3

Total Background 3.1 ± 0.9 1.9 ± 1.0

RpV SUSY (121) 3.8 ± 0.4 -----

RpV SUSY (122) ----- 4.0 ± 0.4

Data 5 1

≥ 4 Lepton Signal Region

Dataset Signal

Z/ + 0.001 ± 0.001

Z/+Z/ 0.004 ± 0.002

Fakes 0.004 ± 0.003

Total Background 0.008 ± 0.004

RpV SUSY (121) 1.5 ± 0.2

RpV SUSY (122) 1.5 ± 0.3

Data 0


Setting Limits

Use Bayesian method to find σobs, combining 3 and ≥4 lepton signal regions

Set limits for multiple SUSY scenarios


Results


Trilepton Event

Jet 16 GeV

Leading Mee = 70 GeV/c2

e- 50 GeV, e+ 30 GeV, e- 13 GeV

e-50 GeV

e+30 GeV

e-13 GeV

e+30 GeV

e-50 GeV

e-13 GeV

Jet 16 GeV

Jet 16 GeV

ET = 1.5 GeV


Trilepton Invariant Mass

TrileptonSignal Events


DØ RpV SUSY Search

DØ searched for same process but in 3l + ET channel

Disadvantages: Sensitive to less areas

of new physics Overlap with Rp

conserving search

Advantages: Missing energy cut

reduces Z/* background

Better limits


Rp Conserving SUSY Searches CDF and DØ both have conducted searches for associated

production of chargino-neutralino events Results combine 3l + ET and like l±l± ET searches


Comparing Limits

RpV mass limits > RpC mass limits Small branching ratio into leptons El from LSP decay > El from cascade decays

DØ RpC mass limits > CDF mass limits Expected limits are similar DØ assumes no slepton mixing

103450ViolatedLEP

229-2343603l + ETViolatedDØ

186-203350≥ 3lViolatedCDF

140300-1,1003l + ETConservedDØ

127310-7503l + ETConservedCDF

M( ) Limit (GeV/c2)

Luminosity (pb-1)

SignatureRpExperiment ±~1


Conclusions

We completed a search for new physics in the multilepton channel

No significant evidence of physics beyond the SM was detected

We set mass limits on the lightest neutralino and chargino using an RpV SUSY framework

1st draft approved by internal review committee and sent to collaboration for review on 8/2/06


Backup


particle Masses


mSUGRA Parameter Dependence


Electron ID CutsVariable Tight Cuts (TCE) Loose Cuts (LCE)

Fiducial fidEle == 1 fidEle == 1

ET ≥ 5 GeV ≥ 5 GeV

Track pT ≥ 4 GeV/c ≥ 4 GeV/c

Track |Z0| < 60 cm < 60 cm

# COT Ax. Seg. ≥ 3 (5 hits) ≥ 3 (5 hits)

# COT St. Seg. ≥ 2 (5 hits) ≥ 2 (5 hits)

Had/Em < 0.055 + 0.00045*E

< 0.055 + 0.00045*E

Track |d0| < 0.02cm or < 0.2 cm (No SVX hits)

< 0.02 cm or < 0.2 cm (No SVX hits)

Iso/ET < 0.1 < 0.1

E/p < 2 or pT>50 GeV -----

Lshr < 0.2 -----

Q*X -3 – 1.5 cm -----

|Z| < 3 cm -----

2strip < 10 -----

Variable Phoenix Plug

Stand-Alone

1.2 – 2.0 1.2 – 2.0

ET > 15 GeV 5-15 GeV

Track Phoenix Pad Track

Had/Em < 0.05 < 0.05

PES 5/9 (U &V) > 0.65 > 0.65

2PEM < 10 < 10

Iso/ET < 0.1 < 0.1

E/p ----- < 3

Track |d0| < 0.02cm or < 0.2 cm (No SVX hits)

< 0.02 cm or < 0.2 cm (No SVX hits)


Muon ID CutsVariable CMUP CMX CMIO

Stub CMU & CMP CMX -----

Track Fiducial CMU & CMP CMX ≠ (CMU & CMP) and ≠ CMX

CES fiducial (2nd lepton only)

pT > 5 GeV > 5 GeV > 10 GeV

Track |Z0| < 60 cm < 60 cm < 60 cm

Em + Had Energy > 0.1 GeV > 0.1 GeV > 0.1 GeV

Em Energy < max[2 , 2 + (pT -100) * 0.0115] GeV < 2 GeV

Had Energy < 3.5 + pT / 8 GeV (pT < 20 GeV)

< max[6 , 6 + (pT -100) * 0.028] GeV (pT > 20 GeV)

< 3.5 + pT / 8 GeV (pT < 20 GeV)

< 6 GeV (pT > 20 GeV)

Track |d0| < 0.02 cm or < 0.2 cm (No SVX hits or pT < 20 GeV)

# COT Ax. Seg. ≥ 3 (5 hits) ≥ 3 (5 hits) ≥ 3 (5 hits)

# COT St. Seg. ≥ 2 (5 hits) ≥ 2 (5 hits) ≥ 3 (5 hits)

Iso/pT < 0.1 < 0.1 < 0.1

CMU Stub Cuts |X| < 5 cm or

(2 < 9 and pT < 20 GeV)

CMX Stub Cuts |X| < 6 cm or

(2 < 9 and pT < 20 GeV)

Exit Radius > 140 cm (1st lepton only)


Additional Control RegionsExactly 2 opp. Sign electrons

Z MassWindow

Trilepton events that fail 1 or more cuts


Control Region Overview

26 total control regions (summary on left) By lepton type Inside & outside Z window Number of leptons Pass/Fail cut

Analysis procedure is validated through agreement between data and MC prediction

StMu = Stubbed Muon, X = electron or muon


Limits

Limits

SUSY

Scenario

M

(GeV/c2)

M

(GeV/c2)

expected observed expected observed

> 0 105.0 101.5 191.9 185.3

< 0 101.1 97.7 192.2 185.6

> 0 107.7 110.4 197.5 202.7

< 0 102.7 106.3 195.3 201.9

0~1

±~1


DØ Limits

Limits

SUSY

Scenario

M

(GeV/c2)

M

(GeV/c2)

> 0 118 229

< 0 115 230

> 0 119 231

< 0 117 234

> 0 86 166

0~1

±~1


Trilepton Event 2

e-

e+

Jet #1

Jet #1

Jet #3

Jet #2

e-e+

-

-ET = 2.2 GeV

search for anomalous production of multi-lepton events at cdf

Documents

sm masses

lepton signaturestep

wimpviolate lepton

tight lepton w pt

broken particles

massive particles

standard model particles

gevc andloose lepton