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
August 28th, 2008 W&C Seminar 2
Introduction b observation b search Summary
The -(sss) discovery (1964)
The Eightfold way
SU3
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?
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.
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
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
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)
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)
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)
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
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)
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
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
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
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
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:
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
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~?
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
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
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
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
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
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
August 28th, 2008 W&C Seminar 25
Introduction b observation b search Summary
- after BDT selection
Clean - signal
D0
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
August 28th, 2008 W&C Seminar 27
Introduction b observation b search Summary
- after BDT selection
Wrong-sign combination events
decays
removedD0
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.
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.
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
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 ///
August 28th, 2008 W&C Seminar 32
Introduction b observation b search Summary
Other control sampleWe analyze candidates in the sidebands of - signal
D0
August 28th, 2008 W&C Seminar 33
Introduction b observation b search Summary
Other control sampleWe analyze candidates in the sidebands of signal
D0
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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
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
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
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
August 28th, 2008 W&C Seminar 38
Introduction b observation b search Summary
Consistency check: Increase pT(B)
Significance >6
August 28th, 2008 W&C Seminar 39
Introduction b observation b search Summary
Consistency check: Look back plots
D0
D0
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).
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
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
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
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.
August 28th, 2008 W&C Seminar 45
Introduction b observation b search Summary
One example: Event display
August 28th, 2008 W&C Seminar 46
Introduction b observation b search Summary
One example: Event display
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 .
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
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
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
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
August 28th, 2008 W&C Seminar 52
Introduction b observation b search Summary
Submitted to PRL
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
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
August 28th, 2008 W&C Seminar 55
Introduction b observation b search Summary
Backup slides
August 28th, 2008 W&C Seminar 56
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
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+)).
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.