op quark physics at the of t evatron a n dconferences.fnal.gov/win03/talks/ewk/ciobanu.pdfn in 107.1...

C. C

iobanu

W

IN’03

Single Top Quark Physics at the Tevatron and

Beyond

Catalin

Cio

ban

u, U

niversity o

f Illino

is, CD

F

Fo

r the L

HC

part, valu

able h

elp received

from

:

Serg

ey Sab

losp

itsky, IH

EP

Pro

tvino

, CM

S

Du

gan

O’N

eil, Sim

on

Fraser U

niv., A

TL

AS

WIN

’03, Octo

ber 7, 2003

C. C

iobanu

W

IN’03

Single Top Productionl

Why look for single top:

‰C

ross section ~ | V

tb | 2: the only w

ay todirectly m

easure CK

M m

atrix el. Vtb

‰S

ingle top is background to othersignals: e.g. H

iggs searches.‰

Test non-S

M phenom

ena:ÿ

Heavy W

’ bosonÿ

Anom

alous Wtb

couplingsÿ

FC

NC

couplings tug or tcg

lA

t the Tevatron relevant channels are:

‰t-channel W

-gluon fusion:ÿ

1.98 pb at ÷s=

1.96 Te

Vÿ

Hard b-jet, W

decay products, soft b(usually lost), light quark jet

‰s-channel W

*:ÿ

0.88 pb at ÷s=1.96

TeV

ÿT

wo hard b-jets, W

decay products

‰B

.W. H

arris et al.: Phys. R

ev. D 66, 054024

(theoretical calculations)

C. C

iobanu

W

IN’03

From Run I to Run II

lR

esults at ÷s = 1.8 T

eV from

CD

F and D

∅ :

‰S

ingle top has not been observed; 95% C

L limits w

ere set:‰

t-channel: D∅

limit: 22

pb, C

DF

limit: 13

pb (theoretical x-section: 1.40 pb

)

‰s-channel: D

∅ lim

it: 17 pb

, CD

F lim

it: 18 pb (theoretical x-section: 0.76

pb)

‰com

bined s- and t- channels search: CD

F lim

its: 14 pb (H

T , Mlnb )

‰m

ost recent Run 1 C

DF

study finds a 2.2s excess and lim

it of 24 pb (7-inp N

N)

‰D

∅:P

hys. Lett. B 517, 282 (2001); C

DF

: PR

D65, 091102 (2002), C

. Ciobanu

’s Ph.D

. thesis

lF

or Run II:

‰H

igher rate: 32% increase in the com

bined s and t channel x-section:ÿ

still small: 2.9 pb versus 6.7 pb for top pair production

‰M

ore luminosity – right now

CD

F has 180 pb

-1 – that is 70% m

ore data!‰

Better detector acceptance

‰P

reliminary C

DF

analysis done for 107 pb

-1 of data:ÿ

Search for s- and t-channels com

bined productionÿ

Seaparate

search for t-channel

C. C

iobanu

W

IN’03

Combined Search

lLook in the W

+2 tight jets channel:

‰“tight” jet: E

T >

15 GeV

, |h| <

2.8‰

at least one SV

X B

-tag‰

exactly one lepton with E

T >20 G

eV, |h

|<2.0

‰m

issing energy: ET >

20 GeV

‰veto Z

’s, dilepton events‰

apply a top mass w

indow cut: 140 <

Mlnb <

210 GeV

/c2

lM

onte Carlo sam

ples:‰

single top signal:ÿ

Pythia

(1M events)

ÿM

adevent+P

ythia (~

200k)

‰H

ER

WIG

tt background‰

ALP

GE

N+

HE

RW

IG W

bb used to model the shapes of the

non-top backgrounds: Wbb, W

cc, Wc, m

istags, non-W events

C. C

iobanu

W

IN’03

Expected Yieldl

For L=

107.1 pb-1:

‰expect 2.4 signal events

‰expect 16.1 backgrd events

‰observe 19 (expect 18.5)

lT

he variable with the m

ostdiscrim

ination power is H

T ,the total transverse energy inthe event:‰

Will fit the H

T shape in data asthe sum

of weighted M

Cshapes.

‰U

se maxim

um likelihood

method w

ith Gaussian

background constraints

ProcessN

in 107.1 pb-1

W-gluon

1.61 ± 0.51

W*

0.80 ± 0.18

tt2.33 ± 0.70

Wbb

3.5 ± 1.5

Wcc

1.4 ± 0.8

Wc

3.1 ± 1.0

Mistags

2.9 ± 0.9

Non-W

2.2 ± 0.7

Diboson

0.7 ± 0.2

Z+bb

0.07 ± 0.03

Total non-top

13.8± 2.8

C. C

iobanu

W

IN’03

Fitting HT distributions

lH

T distributions for signal and backgrounds:‰

signal shapes similar for s- and t- channels

‰fit yields 2.9

±4.5 signal events (exp. 2.4 events).

C. C

iobanu

W

IN’03

95% C.L. lim

itl

Limit at 95%

C.L. is 17.5 pb

lT

his accounts for the systematics:

‰Jet E

T scale: 20.9%‰

ISR

/FS

R: 4.0%

‰T

op Mass: 6.2%

‰P

DF

, signal generator, backgroundm

odel 1-2%

lC

ompare to R

un I:‰

Sam

e luminosity 107 vs 106 (p

b-1)

‰14 pb lim

it for 2.16 pb x-section‰

Now

: 17.5 pb limit for 2.86 pb x-s.

marginally better lim

it!

‰S

everal ways to im

prove this will be

discussed later.

C. C

iobanu

W

IN’03

Separate Searchl

Looking for the two signal channels individually is desired:

‰D

ifferent sensitivities to new physics:

ÿs-channel: heavy charged vector bosons W

’, CP

-violation effects within

MS

SM

, Kaluza

-Klein excited W

-boson within M

SS

M

ÿt-channel: F

CN

C couplings, anom

alous V+

A contributions to the W

-t-bvertex, etc.

ÿT

o go back to extracting Vtb w

e will have to know

the individual rates

lW

e use the same selection as in the com

bined search.‰

How

ever, using HT is no longer appropriate

‰A

variable with good W

* - W-gluon separation potential is Q

x h, w

hereQ

is the lepton charge and h the pseudorapidity of the non-b jet.

‰N

ow treat W

* as a background, therefore perform a 4-com

ponent fit

C. C

iobanu

W

IN’03

Fitting Q¥h

lt-channel exhibits an asym

metry tow

ard Qxh

positive values (twice as

many events than in the negative Q

xh range)

l fit to data yields: 1.9± 3.7 t-channel signal events (exp. 1.6 events).

C. C

iobanu

W

IN’03

95% C.L. lim

itl

Limit at 95%

C.L. is 15.4 pb

lT

his accounts for the systematics:

‰Jet E

T scale: 21.5%

‰S

ignal generator: 21.1%

‰B

ackground model: 43.0%

‰IS

R/F

SR

: 6.9%

‰T

op Mass: 6.7%

‰P

DF

4.3%

lC

ompare to R

un I:‰

13 pb limit for 1.40 pb x-section

‰N

ow: 15.4 pb lim

it for 1.98 pb x-s.

again a better limit!

C. C

iobanu

W

IN’03

Future Prospectsl

Neural N

etwork w

ith 5 input variables: HT , E

T (j1), ET (j2), E

T , PT (j1-j2),

Looking in the W+

2j channel, no M

lnb cutA

ssuming the background uncertainties w

ill scale as L-1/2

lR

ight now: 3 sigm

a with 2.5 fb

-1

lIm

provements to com

e. We try:

‰S

oft lepton tagger, Jet Prob. tagger

‰U

se of forward electrons

‰B

etter discriminant variables:

ÿM

atrix elements (or ratios of these)

‰U

se the top mass hypothesis to

constrain the eventÿ

Will lead to better M

lnb resolution

‰B

etter understanding of thesystem

atic uncertainties is crucial.

C. C

iobanu

W

IN’03

Single Top at the LHC

lLH

C w

ill be a top quark factory:‰

8 million top pairs per experim

ent per year (10 fb-1 / year)

‰S

ome cross section values for ÷s =

14 T

eV:

ÿtop pair production: ~

800 pb (mostly via gluon-gluon fusion)

ÿt-channel single top: 153 (top) and 90 (antitop

) = 243 pb

ÿe.g

. per d

ay ~6000 even

ts, at 1033cm

-2s-1

ÿs-channel single top: 6.6 (top) and 4.8 (antitop

) = 11

pb

ÿassociated W

t production: 50-60 pb

ÿN

eglig

ible at th

e Tevatro

n

ÿW

ill discuss each of the three modes in w

hat follows

C. C

iobanu

W

IN’03

t-channel single top (CMS)

lC

MS

study (hep-ph/0003033):

‰F

ull calorimeter sim

ulation + b-tag efficiency param

etrization

‰P

ythia 5.72 signal, tt, W

Z; V

ecbos+H

erwig W

+jets. S

election:ÿ

One isolated lepton w

ith PT >

20 GeV

/c, |h|<

2.5

ÿM

issing ET >

20 GeV

, 50 < M

ln <100 G

eV/c

2

ÿT

wo jets w

ith ET

> 20

GeV

, |h| <

4.0

ÿO

ne jet: E

T >

20 G

eV, |h

| < 2.5, th

e oth

er ET

> 50 G

eV, 2.5<

|h| <

4.0

ÿL

eadin

g jet E

T <100 G

eV (to

redu

ce tt)

ÿE

xactly on

e b-tag

ged

jet – the cen

tral on

e

ÿR

eject WZ

candidates with: 80 <

Mjj <

100 GeV

/c2

‰S

ignal peak visible in reconstructed Mtop distribution

‰W

ith 10 fb-1, ÷(S

+B

)/S =

1.4%

lT

he quest continues – newer det. sim

ulation‰

use fancier generators: TopR

ex, Single T

op‰

Work-in-progress, unofficial results ds

/ s ~

10%

V.Abram

ov, CMS Week 9

/17/03

C. C

iobanu

W

IN’03

lA

TLA

S study of W

-gluon single top (Dugan O

’Neil’s P

h.D. thesis):

‰A

TLF

AS

T param

etrized detector simulation

‰O

netop+

Pythia

5.72 signal, tt; M.E

.+H

ER

WIG

Wbb

, Wjj

‰S

elections are rather similar to C

MS

study, plus:ÿ

150<M

top <200

GeV

/c2

ÿR

esult: ÷(S+

B)/S

= 0.9%

with 10 fb

-1.

lW

hat about Vtb ? E

xtracting |Vtb | 2 from

x-section picks up uncertainties:‰

x-section uncertainty:ÿ

Statistical: 5%

is a perhaps reasonable/optimistic guess for L=

10 fb-1

ÿS

ystematic <

20%. H

ere things can easily be off by a lot, especially in the 1st year…

‰Lum

inosity uncertainty: traditionally 4-5%

‰T

heoretical uncertainty:ÿ

From

factorization and renormalization scale dependence (3%

from N

LO calculation)

t-channel single top (ATLAS)

C. C

iobanu

W

IN’03

lC

MS

study (S. S

ablospitsky, CM

S w

eek 9/17/03)‰

disclaimer: w

ork-in-progress, not yet ‘official’ results

‰use T

opRex to generate single top, tt, W

bb. Pythia W

+jets. U

se CM

SJE

T 4.801

‰selection:ÿ

one lepton with E

T >10 G

eV, |h

|<2.5

ÿ2 B

-tags with E

T >20 G

eV, no other jets w

ith ET >

20GeV

ÿV

ector sum all final state P

T : |SP

T |<15 G

eV

ÿR

econst. top mass: 150 G

eV<

Mtop <

200 GeV

/c2

ÿR

esults: ÷(S+

B)/S

= 12%

, S/B

= 8%

for L = 30 fb

-1

ÿw

hich implies d|V

tb |=8.3%

s-channel single top (CMS)

C. C

iobanu

W

IN’03

s-channel single top (ATLAS)

lA

TLA

S study (hep

-ph/0003033)‰

Onetop

+P

ythia 5.72 signal, tt; M

.E.+

HE

RW

IG W

bb, W

jj

‰S

election:ÿ

One isolated lepton w

ith PT >

20 GeV

/c, |h|<

2.5

ÿ2 b-jets w

ith ET >

75 GeV

ÿS

calar sum all final state P

T : |SP

T |<175 G

eV

ÿR

econst. top mass: 150

GeV

<M

top <200 G

eV/c

2

‰R

esults: ÷(S+

B)/S

= 5.5%

for L = 30

fb-1, d|V

tb |~1%

lW

t single top production at AT

LAS

:‰

65< M

jj < 95 G

eV/c

2

‰E

vent invariant mass

< 300 G

eV/c

2

‰÷(S

+B

)/S=

4.4% for L =

30 fb-1

C. C

iobanu

W

IN’03

Top Polarization in Single TopEvents

lE

W-produced top quarks are highly polarized:

‰In the top rest fram

e, its spin points along the direction of the down (d) quark.

‰R

estrict to t-channel single top:80% (69%

) of the top (antitop) events have the

‘d’ quark in the final state. The angular distrib

. of the charged lepton:

‰used signal sam

ples with P

=+

1 and P=

-1. Consider only W

+jet background.

‰C

hisquare fit, letting the two P

contributions float.

‰T

he error on polarization measurem

ent is 1.6% for 10 fb

-1 (Dugan

’sPh

.D.thesis)

lW

boson helicity measurem

ent:‰

Again restrict to t-channel signal.

‰M

easure cos(yl ) distribution. y

l between the direction of the lepton in W

restfram

e and the direction of the W in the top rest fram

e.

‰T

hree component fit: fL , fR , fLong , w

ith fL +fF

R +fLong =

1

‰U

ncertainties of the order 2-3% for 30 fb

-1 of data.

()l

lq

qcos

12 1

)(cos

Pf

+=

C. C

iobanu

W

IN’03

Conclusionsl

CD

F and D

ø accum

ulated more events than in entire R

un I (factor of 2).l

Prelim

inary CD

F study from

107 pb-1 sets prom

ising single top limits

lH

owever, m

any improvem

ents needed if we are to observe single top in

the next 2-3 years.l

Precise m

easurements of |V

tb |, as well as top polarization are not w

ithin(im

mediate) sight

lLH

C w

ill see very exciting results in the first (few) years of running:

‰10 –30

fb-1 needed for observation in m

ost searches.‰

Higher collider energy =

considerable higher cross sections

lA

TLA

S and C

MS

single top studies are very mature

‰T

he Monte C

arlo efforts are particularly strong: Top R

ex, SingleT

op.

lC

DF

/Dø

can help with insight into system

atics. Search strategies are

already similar betw

een Tevatron and C

ER

N, but big im

provements are

just an idea away!

op quark physics at the of t evatron a n dconferences.fnal.gov/win03/talks/ewk/ciobanu.pdfn in 107.1...

Documents