rpv stop paper seminar - Texas A&M...

16
Search for pair production of scalar top quarks decaying to a τ τ τ lepton and a b quark in___ collisions at TeV 1.96 s = Autors: M. Chertok, T. Kamon, V. Khotilovich, R. Lander, A. Safonov CDF Paper Seminar, February 21, 2008 pp Godparents: T. Liss (chair), C. Neu, K. Osterberg CDF Note 9038

Transcript of rpv stop paper seminar - Texas A&M...

Page 1: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

Search for pair productionof scalar top quarks

decaying to a ττττ leptonand a b quark in___

collisions at TeV1.96s =

Autors:M. Chertok, T. Kamon, V. Khotilovich, R. Lander, A. Safonov

CDF Paper Seminar, February 21, 2008

pp

Godparents:T. Liss (chair), C. Neu, K. Osterberg

CDF Note 9038

Page 2: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 2

� Process, motivation and existing limits� Data and MC samples� Event selection� Background estimation� Fitting procedure� Cross-section and mass limits� Summary

Outline

Page 3: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 3

RPV Stop (and LQ3) Pair Production

� Supersymmetric top quark can be pair-produced at the Tevatron � R-parity separates SM and SUSY particles:

� if violated, stop can decay into tau and a b-jet� for wide range of parameters high is expected

� Set of leading order Feynman diagrams:

q g

g q

g 1 t ~

1 t ~

b

τ

τ

b

g

g

1 t ~

1 t ~

1 t ~

b

τ

τ

b

g

g

1 t ~

1 t ~

b

τ

τ

b

SLBpR 23)1( ++−=

� Identical to the case of 3rd generation scalar leptoquark (LQ3 )� Existing Limits:

� LQ3: m > 99 GeV/c2 (LEP / CDF Run I)� RPV stop: m >122 GeV/c2 (CDF Run I)

( )Br t bτ→ɶ

Page 4: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 4

Signature, Run II Improvements

Signature to look for:� e or µµµµ :

from leptonically decaying tau� ττττh : hadronically decaying tau� 2 jets : from b-quarks

Improvements in Run II:� Increased cross-section (~35%)� Increased muon coverage (CMX)� Dedicated tau triggers� More data� Better particle ID� Improved analysis technique

pp

t~

t~

b

b

τ

τ

jet

jet

Page 5: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 5

Data and MC Samples

� Dataset: L=322 pb-1

� Lepton+Track Triggers� 8 GeV lepton – electrons & muons in |η| < 1� 5 GeV/c Isolated Track

� Signal: m(stop) = 100 - 180 GeV/c2

� AcceptancePYTHIA (CTEQ5L PDF) + CDF detector simulation

� NLO cross-sectioncalculated with PROSPINO v2 (CTEQ6M)

Page 6: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 6

Backgrounds

� Z (� τlτh ) + ≥ 2jets (dominant; use MC corrected with Data)

� tt-bar, WW/WZ/ZZ (small; use MC)

� Z (�ee/µµ) + ≥ 2jets (dominant; use MC corrected with Data)

� QCD (≥ 4jets) (use Data)

� W (�l / �τ � l ) + ≥ 3jets (small, but hard to estimate; use Data)

Jet, e, µµµµ: misidentified as ττττh

Page 7: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 7

Event Selection

� Central e or µµµµ : pT(l)>10 GeV/c� Hadronic tau : pT(τ)>15 GeV/c� ID cuts� Background removal:

� DY, cosmics rays, conversions� ST = pT(l) + pT(ττττ) + MET > 110 GeV

� strong suppression of QCD and Z→ττ� cut value is optimized

� Sort remaining events by N jet & mT(l,MET) � Signal dominates in Region A2

( “the blind box” )� mT(l, MET) < 35 GeV/c2

separates signal and W, top and di-boson backgrounds

� Njet≥≥≥≥2 (ET >20 GeV is optimized)separates signal and most of backgrounds (including Z →ττ)

B0A0

B2A2

MT

N “

extr

a”je

ts0

≥≥≥≥2

35 GeV

1

S

Page 8: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 8

Validation: Njet=0 Regions A0 and B0

� Use ST >80(better statistics for validation)

� A0 is dominated by Z→ττ� clear tau signal

A0 B0

A0

B0 is mostly W+jets

mT(l

, ME

T)

< 35

GeV

/c2

Page 9: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 9

Signal Event Selection: Summary

Total signal event selectionacceptance (in region A2 asa function of m(stop) for:� Electron channel� Muon channel:

CMUP (|η|<0.6)CMX (0.6<|η|<1.0) separately and combined

� Muons (combined) and Electrons give similar acceptance

Page 10: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 10

� Use fitting to the Data to find:� Probability Density Function

as a function of signal cross-section

� estimate W+jets background

� “4-region” method:� Assume for W+jets

R≡≡≡≡[N(A2)/N(B2)]/[N(B0)/N(A0)]~1� unreliable rates but reliable ratios� verified with MC

� Estimate all non-W+jets BG� Estimate NB2/NA2, NA0/NA2, NB0/NA2

for signal events� Include systematic uncertainties

(are in range of 7.0%~11.2%)� Fit to Data in all 4 regions

Fit Strategy

B0A0

B2A2

MT

N “

extr

a”je

ts

0

≥≥≥≥2

35 GeV

1

Result of fit:can set limits or discover stop

A1 B1

Page 11: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 11

Signal Region

Njet Distribution for mT<35 GeV/c2

� Signal region (Njet ≥ 2):� was looked after

the validation in control regions was completed

� Nobs = 2 (1e+1µµµµ)� NBG = 0.5

-0.43.0+

used in paper

A0 A1 A2

Page 12: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 12

Number of events in Different Regions

used in paper� Note: W+jet and other BG’s are not combined

W+jet are estimated in the fit and not independent from number of data evt., other BG’s and expected signal fractions

Page 13: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 13

Stop Cross-Section Upper Limits

� No excess observed � set limits

� Theoretical uncertainties on cross-section:� CTEQ6.1M

� ± ½ of QCD renorm./factor. scale

� 95% C.L. upper limit on whereas function of stop mass, calculated for the cases when theoretical uncertainties were or were not considered:

used in paper

21 1( )t tσ β×ɶ ɶ

1( )Br t bβ τ= →ɶ

Page 14: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 14

Stop Mass Limits

� Stop mass 95% C.L. limits for the case :� m > 153 GeV/c2 (theoretical uncertainties considered)� m > 156 GeV/c2 (not considering theor. uncertainties)

used in paper

1( ) 1Br t bτ→ =ɶ

Page 15: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 15

Summary and Plans� No RPV stop discovery with L=322 pb -1

� Expected BG events in primary signal region� Observed 2 events� Set 95% C.L. upper limit on

as function of stop mass� Stop mass 95% C.L. limit for case:

� m > 153 GeV/c2 (theoretical uncertainties considered)

� Results are fully applicable to Scalar LQ3

� Possible Prospects:� Bigger dataset will need completely new analysis with b-tagging� Combined search of modes

has more potential

� Thanks� Godparents: Tony Liss, Christopher Neu, Kenneth Osterberg� Readers: SPRG, Cantabria, Pisa, IPP (Canada), Fermilab

0.5-0.43.0+

21 1 1( ) ( )t t Br t bσ τ× →ɶ ɶ ɶ

1 1 1& &t eb t b t bµ τ→ → →ɶ ɶ ɶ

1( ) 1Br t bτ→ =ɶ

Page 16: rpv stop paper seminar - Texas A&M Universitypeople.physics.tamu.edu/.../080221_rpv_stop_paper_seminar.pdf · 02/21/2008 CDF Paper Seminar 7 Event Selection Central e or µµµ: pT(l)>10

02/21/2008 CDF Paper Seminar 16

Backup