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August 28 th , 2008 W&C Seminar at Fermilab 1 Eduard De La Cruz Eduard De La Cruz Burelo Burelo CINVESTAV IPN Mexico On behalf of the D0 collaboration Observation of the Observation of the doubly strange doubly strange b- b- baryon baryon - - b b Outline: Outline: Introduction Introduction b observation observation b search search Summary Summary

Observation of the doubly strange b- baryon - b

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Observation of the doubly strange b- baryon  - b. Eduard De La Cruz Burelo CINVESTAV IPN Mexico On behalf of the D0 collaboration. Outline: Introduction  b observation  b search Summary. August 28 th , 2008. W&C Seminar at Fermilab. 1. The  - (sss) discovery (1964). - PowerPoint PPT Presentation

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Page 1: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 1

Introduction b observation b search Summary

August 28th, 2008 W&C Seminar at Fermilab 1

Eduard De La Cruz BureloEduard De La Cruz BureloCINVESTAV IPN Mexico

On behalf of the D0 collaboration

Eduard De La Cruz BureloEduard De La Cruz BureloCINVESTAV IPN Mexico

On behalf of the D0 collaboration

Observation of the doubly Observation of the doubly strangestrange b-b-baryonbaryon --

bb

Observation of the doubly Observation of the doubly strangestrange b-b-baryonbaryon --

bb

Outline:Outline: IntroductionIntroduction

bb observation observation

bb search search

SummarySummary

Page 2: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 2

Introduction b observation b search Summary

The -(sss) discovery (1964)

The Eightfold way

SU3

Page 3: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 3

Introduction b observation b search Summary

What energies we will looking at?

TeV

MeV few GeV

GeV

Z’,W’,KK modes, etc.

Z, W, top, Higgs?

Do we miss something here?

Page 4: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 4

Introduction b observation b search Summary

What energies we will look at?

TeV

MeV few GeV

GeV

B Physics Bs Mixing, CKM, lifetimes, etc.

Page 5: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 5

Introduction b observation b search Summary

B baryons at the Tevatron

• Unique to Tevatron (not produced in B factories)

• B baryons expected to be produced copiously at the Tevatron

• Only b was considered as observed back in 2001 ~20 events.

• Interesting mass predictions using different models.

• However, very challenging.

b(bud)

b0(bud)

b+(buu)

b-(bdd)

b0(bus)

b-(bds)

b-(bss)

J=1/2, 1 b

Not yet observed

Page 6: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 6

Introduction b observation b search Summary

When Tevatron Run II begun: Notation Quark content JP SU(3) (I,I3) S Mass

b0 b[ud] 1/2+ 3* (0,0) 0 5619.71.21.2 MeV

b0 b[su] 1/2+ 3* (1/2,1/2) -1 5.80 GeV

b

-b[sd] 1/2+ 3* (1/2,-1/2) -1 5.80 GeV

b+ buu 1/2+ 6 (1,1) 0 5.82 GeV

b0 b{ud} 1/2+ 6 (1,0) 0 5.82 GeV

b

-bdd 1/2+ 6 (1,-1) 0 5.82 GeV

b0’ b{su} 1/2+ 6 (1/2,1/2) -1 5.94 GeV

b-’ b{sd} 1/2+ 6 (1/2,-1/2) -1 5.94 GeV

b- bss 1/2+ 6 (0,0) -2 6.04 GeV

b*+ buu 3/2+ 6 (1,1) 0 5.84 GeV

b*0 bud 3/2+ 6 (1,0) 0 5.84 GeV

b*- bdd 3/2+ 6 (1,-1) 0 5.84 GeV

b*0 bus 3/2+ 6 (1/2,1/2) -1 5.94 GeV

b*- bds 3/2+ 6 (1/2,-1/2) -1 5.94 GeV

b*- bss 3/2+ 6 (0,0) -2 6.06 GeV

from hep-ph/9406359

Page 7: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 7

Introduction b observation b search Summary

(*)-b in October 2006

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

PRL 99, 202001 (2007)

Page 8: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 8

Introduction b observation b search Summary

Last year: --bb observation observation

Number of events: 15.2 ± 4.4

Mass: 5.774 ± 0.011(stat) GeV

Width: 0.037 ± 0.008 GeV

(syst) (stat) 0.09 0.28 0.09 0.08 -R

5.5ln2ln2

BS

B

L

LL

Signal Significance:

)/()(

) /()(

JBR

JBRR

bb

bb

We also measured:

PRL 99, 052001 (2007)

Page 9: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 9

Introduction b observation b search Summary

Last year: --bb observation observation

Signal significance = 7.8

Also searched for in b

-c0-

M(b-) = 5792.9 2.5 (stat) 1.7 (syst)

MeV/c2

PRL 99, 052002 (2007)

Page 10: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 10

Introduction b observation b search Summary

During Tevatron Run IINotation Quark content JP SU(3) (I,I3) S Mass

b0 b[ud] 1/2+ 3* (0,0) 0 5620.2 1.6 MeV

b0 b[su] 1/2+ 3* (1/2,1/2) -1 5.80 GeV

b

-b[sd] 1/2+ 3* (1/2,-1/2) -1 5792.4 3.0 MeV

b+ buu 1/2+ 6 (1,1) 0 5807.8 2.7 MeV

b0 b{ud} 1/2+ 6 (1,0) 0 5.82 GeV

b

-bdd 1/2+ 6 (1,-1) 0 5815.2 2.0 MeV

b0’ b{su} 1/2+ 6 (1/2,1/2) -1 5.94 GeV

b-’ b{sd} 1/2+ 6 (1/2,-1/2) -1 5.94 GeV

b- bss 1/2+ 6 (0,0) -2 6.04 GeV

b*+ buu 3/2+ 6 (1,1) 0 5829.0 3.4 MeV

b*0 bud 3/2+ 6 (1,0) 0 5.84 GeV

b*- bdd 3/2+ 6 (1,-1) 0 5836.4 2.8 MeV

b*0 bus 3/2+ 6 (1/2,1/2) -1 5.94 GeV

b*- bds 3/2+ 6 (1/2,-1/2) -1 5.94 GeV

b*- bss 3/2+ 6 (0,0) -2 6.06 GeV

Page 11: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 11

Introduction b observation b search Summary

B hadron observation status

• Mesons: – B+, B0, Bs, Bc

+ (before Tevatron RunII)– B* (before Tevatron RunII) – Bd**(Tevatron RunII)– Bs** (Tevatron RunII)

• Baryons: b (before Tevatron RunII) b

+, and b*+(Tevatron RunII)

-b (Tevatron RunII)

Page 12: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 12

Introduction b observation b search Summary

Data

In this analysis we use 1.3 fb-1 of data collected by DØ detector (RunIIa data).

Thanks to the Fermilab Accelerator division for doing wonderful work.

Muon and central tracker subdetectors are particularly important in this analysis

Page 13: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 13

Introduction b observation b search Summary

How did we look for the -b?

+

-p

-

-

--

b ~5 cm

~5 cm

-b→J/+-

~0.7 mm

Page 14: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 14

Introduction b observation b search Summary

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

Page 15: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 15

Introduction b observation b search Summary

Increase of reconstruction efficiency 0

SK p

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

D0 D0 D0

Page 16: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 16

Introduction b observation b search Summary

b Reconstruction procedure Reconstruction procedure: Reconstruct J/→+-

Reconstruct →p

Reconstruct → +

Combine J/+

Improve mass resolution by using an event-by-event mass difference correction

The optimization:

1. b→J/ decays in data

2. J/ + (fake from (p-)+ )

3. Monte Carlo simulation of

b-→J/+-

→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

-b particle:

– Lifetime significance>2. (Lifetime divided by its error)

Final b selection cuts:

Page 17: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 17

Introduction b observation b search Summary

Search for the -b(bss)

bss quarks combinationMass is predicted to be

5.94 - 6.12 GeV

M(-b) > M(b)

Lifetime is predicted to

be 0.83<(-b)<1.67 ps

Page 18: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 18

Introduction b observation b search Summary

How do we look for it?

-b

+

-

-

p

-

K- Similar

~3 cm

~5 cm~?

Page 19: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 19

Introduction b observation b search Summary

-b vs -

b: differences

-b(bds) -

b(bss)Decay b J/(+-) () b J/(+-) (K)

Mass 5.792 0.003 GeV 5.94 - 6.12 GeV [1]

Lifetime 1.42 0.28 ps 0.84 – 1.69 ps. [2]

Daughters: ():

Mass: 1321.71 0.07 MeV

c =4.91 cm

(K):

Mass: 1672.45 0.29 MeV

c =2.461 cm

[1] Phys. Rev. D 77, 014031 (2008); arXiv:0708.4027 [hep-ph] (2007).

[2] arXiv:hep-ph/9705402

Page 20: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 20

Introduction b observation b search Summary

reconstruction a challenge →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

In this analysis for the reconstruction:

D0

D0

Page 21: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 21

Introduction b observation b search Summary

Analysis strategy

Fix selection criteria and then apply them to J/ +

Event per event mass correction

Combine J/ + (K+)

Reconstruction of → + K

Select p

Events are reprocessed to increase reconstruction efficiency of long-lived particles.

Select J/ candidates

Yield is optimized by using proper decay length significance cuts.

Optimize yield by using multivariate techniques

Keep blinded J/ + combinations and

optimize on J/ + (K+)

Improve mass resolution from 80 MeV to 34 MeV

Perform as many test as possible in different background samples

Page 22: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 22

Introduction b observation b search Summary

optimizationFirst we select a proper decay length significance cut to clean signal

( decay length significance > 10)

+K will have huge combinatory background

D0

Page 23: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 23

Introduction b observation b search Summary

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

Page 24: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 24

Introduction b observation b search Summary

Boosted Decision Trees (BDT)

• All variables are related to - or its decay products.

• We use a total of 20 variables.

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

• Most important variables:– pT(K)– pT(p) – pT() - transverse decay

length

Page 25: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 25

Introduction b observation b search Summary

- after BDT selection

Clean - signal

D0

Page 26: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 26

Introduction b observation b search Summary

-- contamination

• This is a reflection contamination due to mistaken a pion as a kaon.

• It is easy to eliminate by requiring M()>1.34.

D0

D0

Page 27: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 27

Introduction b observation b search Summary

- after BDT selection

Wrong-sign combination events

decays

removedD0

Page 28: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 28

Introduction b observation b search Summary

Final optimization

• We want to further reduce background (based on level we observe in the wrong-sign combinations.)

• We use - yield in MC signal verify that we maintain the highest possible signal efficiency.

Page 29: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 29

Introduction b observation b search Summary

Final optimization: pT(B)>6 GeV

• We compare MC signal vs wrong-sign background events pT distribution.

Page 30: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 30

Introduction b observation b search Summary

Final optimization: ()<0.03 cm

• Similarly, MC signal is compare with uncertainty from wrong-sign events.

Uncertainty on

xy

xyL

)(

Lxy

Primary vertex

B decay Vertex

Page 31: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 31

Introduction b observation b search Summary

Wrong-sign combinations

• After optimization: <0.03 cm

J/ and in the same hemisphere

pT(J/+)>6 GeV

• We define mass as:

• Mass window for the search: 5.6 - 7 GeV

After optimization, we look at wrong-sign combination first

PDGPDGJJJ MMMMMM ///

Page 32: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 32

Introduction b observation b search Summary

Other control sampleWe analyze candidates in the sidebands of - signal

D0

Page 33: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 33

Introduction b observation b search Summary

Other control sampleWe analyze candidates in the sidebands of signal

D0

Page 34: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 34

Introduction b observation b search Summary

Nothing where nothing should be

No excess is observed in any control samples after selection criteria is applied to them.

)))((())((/

)))((())((/

))((/0*

pJ

KKJB

pJ

b

S

b

We check also high statistics MC samples

Page 35: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 35

Introduction b observation b search Summary

Looking at right-sign combinations

Clear excess of events near 6.2 GeV

Page 36: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 36

Introduction b observation b search Summary

Mass measurement

• Fit:Unbinned extended

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

Page 37: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 37

Introduction b observation b search Summary

Significance of the peak

• Two likelihood fits are performed: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

Page 38: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 38

Introduction b observation b search Summary

Consistency check: Increase pT(B)

Significance >6

Page 39: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 39

Introduction b observation b search Summary

Consistency check: Look back plots

D0

D0

Page 40: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 40

Introduction b observation b search Summary

Consistency check: lifetime

We compare to a MC sample with a lifetime of 1.54 ps (~460 microns).

Page 41: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 41

Introduction b observation b search Summary

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

Page 42: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 42

Introduction b observation b search Summary

Cut Based Analysis fit

• Fit:Unbinned extended

log-likelihood fitGaussian signal, flat

backgroundNumber of

background/signal events are floating parametersNumber 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: 3.9

Page 43: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 43

Introduction b observation b search Summary

BDT or Cut Base Analysis

After we remove duplicate events, we observe 25.5 ± 6.5 events. Significance: 5.4

Page 44: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 44

Introduction b observation b search Summary

Signal confirmed without BDT

• BDT vs CBA– Consistent number of observed signal

candidates– Consistent mass– Consistent reconstruction efficiencies – BDT has better background rejection power.

Page 45: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 45

Introduction b observation b search Summary

One example: Event display

Page 46: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 46

Introduction b observation b search Summary

One example: Event display

Page 47: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 47

Introduction b observation b search Summary

Systematic uncertainties on the mass

• Fitting models– Linear background instead of flat. Negligible.– Varying Gaussian width between 28 – 40 MeV,

3 MeV• Momentum scale correction:

– Fit to the b mass peak in data, 4 MeV.• Event selection:

– Varying selection criteria and from the mass shift observed between the cut-based and BDT analysis, a 12 MeV variation is estimated .

Page 48: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 48

Introduction b observation b search Summary

Production rate

)()(32.080.0) /()(

) /()( 14.022.0 syststat

JBrbf

JBrbf

bb

bb

)(

)(

)(

)(

) /()(

) /()(

b

b

b

b

bb

bb

N

N

JBrbf

JBrbf

(stat) 2.05.1)(

)(

b

b

The systematic uncertainty includes contributions fromthe signal yields as well as selection efficiencies

Page 49: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 49

Introduction b observation b search Summary

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

Page 50: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 50

Introduction b observation b search Summary

Summary

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

Page 51: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 51

Introduction b observation b search Summary

Summary

14.007.0)(

)(

b

b

bf

bf

Consistent with expectations

)()(32.080.0) /()(

) /()( 14.022.0 syststat

JBrbf

JBrbf

bb

bb

Page 52: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 52

Introduction b observation b search Summary

Submitted to PRL

Page 53: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 53

Introduction b observation b search Summary

B baryons at the Tevatron

• B baryons: If it decays to J/ it the Tevatron produces at reasonable rate, we will find it.

• Precision measurements will come with statistics.

• A legacy from the Tevatron

b(bud)

b0(bud)

b+(buu)

b-(bdd)

b0(bus)

b-(bds)

b-(bss)

J=1/2, 1 b

Page 54: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 54

Introduction b observation b search Summary

Fermilab’s latest discovery

b-baryon -b(bss)

GeV )0.013(syst )0.010(stat 6.165)( bM

Page 55: Observation of the doubly strange b- baryon  - b

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Introduction b observation b search Summary

Backup slides

Page 56: Observation of the doubly strange b- baryon  - b

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Introduction b observation b search Summary

Example: Decision Trees

S/B

S1,B1 S2,B2

S12S11,B11 S21,B21 B22

Splitting continue …

• A decision tree is a binary decision.

• Starting from the first node, between the variables the best split is selected.

• In the next nodes from the first split the operation is repeated.

• Splitting stops until you reach a given proportion of signal or background, or until no more splits can be done.

• We combine 100 DT by using the bagger technique

Page 57: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 57

Introduction b observation b search Summary

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+)).

Page 58: Observation of the doubly strange b- baryon  - b

August 28th, 2008 W&C Seminar 58

Introduction b observation b search Summary

BDT output

• After training the BDT, it is applied to a validation sample:– Signal MC– Background from

wrong-sign events

• We select BDT>0.– BDT +1: Signal-like– BDT -1: background-

like.