Observation of the W b -

Brad Abbott 1

Observation of the b-

Brad AbbottUniversity of Oklahoma

On behalf of the DØ Collaboration

SLAC January 6, 2009

Brad Abbott 2

B physics at the Tevatron

• Upsilon discovered in 1977 at Fermilab by Lederman et. al.

• Since then other observations of other B mesons – B+, B0, Bs, Bc

+ (pre Tevatron RunII)

– B* (pre Tevatron RunII), – Bd**(Tevatron RunII)– Bs** (Tevatron RunII)

• Little experimentally known about B baryons

• Only one B baryon known for many years

Summer 1977

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B baryonsCounting only quark content 15 b baryons are expected

Charmless b baryons multiplet

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B baryons more recently

• Prior to Run II only 1 b baryon, b(udb), was considered observed (b J/ )

• However in last 2 years 4 new b baryons have been discovered b

+(uub) b-(ddb) (CDF, 2007)

b-(dsb) (D0/CDF 2007)

b-(ssb) (D0 2008)

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(*)-b in October 2006

CDF announces the observation of the b’s with 1.1 fb-1

PRL 99, 202001 (2007)

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Last year: --bb observation observation

(syst) (stat) 0.09 0.28 0.09 0.08 -

R

5.5ln2ln2

BS

B

L

LL

Signal Significance:

)/()(

) /()(

JBR

JBRR

bb

bb

PRL 99, 052001 (2007)

M(b-)

5792.9 2.5 (stat) 1.7 (syst) MeV/c2

PRL 99, 052002 (2007)

Signal significance = 7.8

CDF

M(b)=5.774 ± 0.019 GeV

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B baryonsVarious predictions for b baryon masses

“Know” approximately where to look and what decay modes are expected

B quark section large at Tevatron(pp bb) = 150 ub @ √s=2 TeV

(e+e- bb) = 7 nb @ √s=Mz

(e+e- bb) = 1 nb @ √s=M(4S)

However large backgrounds make detection challenging

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Tough environment

BaBar

~ 5 tracks/event

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DataTevatron is running well: ~ 5 fb delivered to D0/CDF

D0 Efficiency high: ~ 4.5 fb recorded by each experiment

This analysis

1.3 fb-1

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DØ detector

Large coverage

No particle ID(limited /K separation)

Unprescaled dimuontriggers at all luminosities

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Triggers

• Entire b physics program based on muon triggers (single and dimuon triggers)

• Do not have bandwidth available to trigger on displaced vertices

• Many b physics analyses use dimuon triggers (unprescaled) and search for J/ decaying to .

• Dimuon triggers have several Hz final rate

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B physics program at DØ

Over 34 papers published/accepted for publicationCP violation in Bs decaysFlavor oscillations in Bs decaysBs Branching ratio of Bs Ds*Ds*Bc meson mass/lifetimeDirect CP violation….For full list seehttp://www-d0.fnal.gov/Run2Physics/WWW/results/b.htm

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B baryons at DØb J/

Aids in understanding momentum scale

Possible biases in reconstruction

Lambda long lived so gives practicein reconstructing long lived particles

Discovered that official processing of data inefficient for tracks with large impactparameterTakes a significant time to reconstruct each event andDØ cannot afford to increase reconstruction time forall events

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Data reprocessing

p

When tracks are reconstructed, a maximum impact parameter is required to increase the reconstruction speed and lower the rate of fake tracks.

But for particles like the b-,

this requirement could result in missing the and proton

tracks from the and - decays

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Reprocessing J/ sample

• Since interesting decays all have a J/ only reconstruct subset of data

• Much smaller sample and can be reconstructed in a reasonable time (few months)

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Increase of reconstruction efficiency

0

SK p

Opening up the IP cut: (Before) ( After )GeV GeV GeV

D0 D0 D0

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Similarities of decays

b J/ b J/

Techniques learned in measuring b led to observation of b in 2007

Apply same idea to searching for b

Use techniques learned from b to help in searching for b

“extra pion”

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Search for the -b(bss)

bss quarks combination Mass is predicted to be

5.94 - 6.12 GeV

0.83<(-b)<1.67 ps

(prediction)

M(-b) > M(b)

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--

b

+

-

-

p

K-

~3 cm

~5 cm

Similar decay topologiesb J/ b J/

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Slight differences

• c() = 7.89 cm• c() = 2.46 cm• Lifetime(b) = 1.42 ps• Lifetime(b)= .84-1.69 ps (predicted)• Mass(b) = 5.792 GeV• Mass(b) = 5.94-6.12 GeV (predicted)

• Much more difficult to reconstruct than

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vs reconstruction →p decays:

– pT(p)>0.7 GeV

– pT()>0.3 GeV

- → decays:– pT()>0.2 GeV

– Transverse decay length>0.5 cm

– Collinearity>0.99

)( p

D0

D0

reconstruction

Black symbols: right sign combinationsGreen/red symbols: wrong sign combinations

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reconstruction

• Need to clean up mass peak

• First clean up peak

• Apply Boosted Decision Trees to further clean up sample

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optimization

Apply a cut on proper decay length significance

( decay length significance > 10)

D0 Before cut

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reconstruction

• Minimum selection cuts: +K vertex

reconstructed– Transverse decay

length significance>4– Proper decay length

uncertainty<0.5 cm

Wrong-sign

events+K+

Right-sign events(+K-)

D0

PDG mass value

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Boosted Decision Trees (BDT)

• 20 input variables and - vertex

quality, decay lengths and decay kinematics

• For training we use MC signal and background from wrong-sign events (J/(K+)).

• Most important variables: pT(K)

pT(p)

pT()

decay length

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- after BDT selection

- signal much

cleaner

D0

Signal yield approximately the sameBackgrounds greatly reduced

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Contamination due to --

• There is a reflection due to assigning a pion as a kaon.

• Remove by requiring M()>1.34.

DØ

DØ

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- after BDT selection and removing decays

Wrong-sign combination events

DØ

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Final b optimization

• Optimized for the efficiency

• Further reduce background (based on level we observe in the wrong-sign combinations.)

• Uncertainty on b proper decay length < .03 cm

b transverse momentum greater than 6 GeV

J/ and in same hemispere

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Mass ResolutionPDGPDG

JJJ MMMMMM ///

= 80 MeV = 34 MeV

“poor man’s” way to reduce mass resolution

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Look where we don’t expect any signal

• After “optimize” J/+ decays by using wrong-sign combination events: <0.03 cm

J/ and in the same hemisphere

pT(J/+)>6 GeV

30 events remain

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Side band control samples

DØ DØ

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Summary of all control samples

No excess is observed in any MC sample after selection criteria applied.

)))((())((/

)))((())((/

))((/0*

pJ

KKJB

pJ

b

S

b

Study known decays using MC samples with statistics larger than data set

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“Open Box” and look at right-sign combinations

Excess of events

79 events selected

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Mass measurement

• Fit:Unbinned log-

likelihood fitGaussian signal, flat

backgroundNumber of

background/signal events are floating parametersNumber of signal events: 17.8 ±

4.9

Mean of the Gaussian: 6.165 ± 0.010(stat) GeV

Width of the Gaussian fixed (MC): 0.034 GeV

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Signal Significance

• Two likelihood fits are performed to estimate the significance:1.Signal + background hypothesis (LS+B)

2.Only background hypothesis (LB)

• We evaluate the significance:

• Significance of the observed signal: 5.4

BS

B

L

LL ln2ln2

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Consistency check: Increase pT(B)

Significance > 6

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Look back plots

D0

D0

See consistent yields

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Another consistency check

We can compare proper decay length of sample with MC sample with a lifetime of 1.54 ps

We do not know the lifetime of the b

We do not have enough events to measure the lifetime of the b

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Alternative Cuts Based Analysis (CBA)

Variable BDT CBApT() (GeV) >0.2 and input to BDT >0.2

pT(p) (GeV) >0.2 and input to BDT >0.7

pT(K) (GeV) input to BDT >0.3

collinearity input to BDT >0.99

Transverse decay length (cm)

input to BDT >0.5

Proper decay length uncertainty (cm)

<0.3 <0.3

Variables selected based on relative importance in BDT

performance

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Cut Based Analysis

Number of signal events: 15.7 ± 5.3

Mean of the Gaussian: 6.177 ± 0.015(stat) GeV

Width of the Gaussian fixed (MC): 0.034 GeV

Signal significance reduced to 3.9 due to increased background

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BDT or Cut Base Analysis

After removing duplicate events and combining analyses we observe 25.5 ± 6.5 events. Significance: 5.4

Cut based and BDT can select different events. Overlap is ~ 50%

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Signal confirmed without BDT.

• BDT vs CBA– Consistent number of

observed signal candidates

– Consistent mass– Consistent

reconstruction efficiencies

– BDT has better background rejection power.

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Systematic uncertainties on the mass

• Fitting models– Linear background instead of flat background gives

negligible change.– Varying the signal Gaussian width between 28 – 40

MeV resulted in a 3 MeV uncertainty• Momentum scale correction:

– Fit to the b mass peak in data gives a 4 MeV uncertainty.

• Event selection:– Varying selection criteria and from the mass shift observed

between the cut-based and BDT analysis gives a 12 MeV uncertainty.

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b mass

M(b)=6.165 ± 0.010(stat) ± .013(stat) GeV

A little on the high side of the predictions1-2

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Production rate

)()(32.080.0)/()(

)/()( 14.022.0 syststat

JBrbf

JBrbf

bb

bb

To determine production rate we normalize to b

)(

)(

)(

)(

)/()(

)/()(

b

b

b

b

bb

bb

N

N

JBrbf

JBrbf

2.06.1)(

)(

b

b

The systematic uncertainty includes contributions from the signal yields as well as selection efficiencies

Determined from MC

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Production rate

8.9)/(

)/(

: (1997) 2799 56, D Rev. Phys. From

J

J

b

b

ps 42.1)( 28.024.0

b

theory

ps 67.1)(83.0 b

ps) 67.1)(( 062.0

ps) 83.0)(( 126.0

)(

)(

b

b

b

b

bf

bf

14.007.0)(

)(

b

b

bf

bf

Spin of b = ½ Spin of b = 3/2

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Summary of b

Number of signal events: 17.8 ± 4.9 (stat) ± 0.8(syst)

Mass: 6.165 ± 0.010(stat) ± 0.013(syst) GeV

Significance= 5.4

arXiv:0808.4142(2008) PRL 101, 232002 (2008)

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Summary of b

14.007.0)(

)(

b

b

bf

bf

Consistent with expectations

)()(32.080.0)/()(

)/()( 14.022.0 syststat

JBrbf

JBrbf

bb

bb

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Future plans

• Reprocess Run IIb data– Lots more data (1.3 pb-1Run IIa vs 3.2 pb-1

Run IIb) – Much higher instantaneous luminosities so

much larger combinatorial background– Layer 0 silicon

• Begin searches for other B baryons

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Other possible B baryons(need a J/ to trigger)

0cb(dcb) 0

c J/ +

cb(ucb) +c J/

0bb(ubb) 0

b J/low rate

-bb(dbb) -

b J/low rate

0cb(scb) 0

c J/

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Backup

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Event display of b candidate

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Boosted Decision Trees (BDT)

• All variables are on - or its decay products.

• For training we use MC signal and background from wrong-sign events (J/(K+)).