# Presentation: 843127

## Eduard De La Cruz Burelo
CINVESTAV IPN Mexico
On behalf of the D0 collaboration

**August 28****th****, 2008**

**W&C Seminar at Fermilab**

**Eduard De La Cruz Burelo**
*CINVESTAV IPN Mexico*
***On behalf of the D0 collaboration***

- Observation of the doubly strange *b-*baryon -b

- _Outline:_
-  Introduction
-  b observation
-  b search
-  Summary

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## The -(sss) discovery (1964)

- The Eightfold way
- SU3

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## 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?

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**Notes:**
First, let clarify for of what energies we will be talking about in this example.

We can divide our energy reachable spectrum in very low energy, probably below 100 MeV, where Quantum Chromodinamics is practically non-preturbative and predictions are almost imposible, but where ineteresting effects like confinement take place.

Then we have the sector where I will concentrate in this example. It is what I call  the B Physics regime. Let’s say between 1 GeV to around 11 GeV. Most of the B hadrons have a mass within 5 and 7 GeV. 

From D0:
When confronted with its earlier known siblings, the bottom quark is considered heavy, with a mass about four times that of its next heaviest colleague, the charm quark. Such relatively high mass grants the bottom quark special status in the studies of QCD. Bottom quarks are produced in proton-antiproton collisions dominantly by the strong QCD interactions of gluons and light quarks that reside within the colliding beam particles.The large value of as and the non-abelian nature of QCD are responsible for the difficulty of making quantitative predictions. However, the higher the mass of the involved quark, the more reliable are the calculations. The mass of the bottom quark is high enough for obtaining reliable QCD calculations, but still low enough to have copious production at the Tevatron. This balance is one of the aspects that single out bottom quarks as an excellent source of data for confrontation with theory, a true "laboratory" for QCD studies. Consequently, one of the ways we test the reliability of QCD in DØ is by measuring the rate at which bottom quarks are produced. An added bonus of heavy quark production is that the dependence of the production rates has a direct correlation to the internal gluon distributions within the colliding protons, which are not well measured, and can be extracted from such data.
-----------------

## What energies we will look at?

- TeV

- MeV

- few GeV

- GeV

- B Physics

- Bs Mixing, CKM, lifetimes, etc.

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## 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

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## When Tevatron Run II begun: 

| Notation | Quark content | JP | SU(3) | (I,I3) | S | Mass |
| --- | --- | --- | --- | --- | --- | --- |
| b0 | b[ud] | 1/2+ | 3* | (0,0) | 0 | 5619.71.21.2 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

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## (*)-b in October 2006 

**CDF announced the observation of the ********b****’s with 1.1 fb****-1**
**PRL 99, 202001 (2007)**

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## Last year: -b observation

**Number of events:  15.2 ± 4.4**
**Mass: 5.774 ± 0.011(stat) GeV**
**Width: 0.037 ± 0.008 GeV**

- Signal Significance:

- We also measured:

**PRL 99, 052001 (2007)**

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## Last year: -b observation

- Signal significance = 7.8

- Also searched for in b-c0-

**M(********b****-****) = 5792*****.*****9 ******** 2*****.*****5 (stat) ******** 1*****.*****7 (syst) MeV*****/c*****2**

**PRL 99, 052002 (2007)**

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## During Tevatron Run II

| Notation | 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 |

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## 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)

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## 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

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## How did we look for the -b?

- +

- -

- p

- -

- -

- -

- 

- -b

- ~5 cm

- ~5 cm

- -b→J/+-

- ~0.7 mm

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**Notes:**
Talk about natural constraints

## 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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## Increase of reconstruction efficiency

- Opening up the IP cut: (**Before**)  ( **After** )

- GeV

- GeV

- GeV

**D0**

**D0**

**D0**

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## 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:**
** ********b****→J/******** decays in data**
** ****J/******** + ********(fake from ********(p********-****)********+**** )**
** ****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:

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## Search for the -b(bss)

*bss* quarks combination
- Mass is predicted to be 5.94 - 6.12 GeV
- M(-b) > M(b)
- Lifetime is predicted to be 0.83<(-b)<1.67 ps

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## How do we look for it?

- -b

- +

- -

- -

- 

- p

- -

- K-

**Similar**

- ~3 cm

- ~5 cm

**~****?**

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## -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

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##  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

- In this analysis for the  reconstruction:

**D0**

**D0**

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## 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**

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##  optimization

- First we select a proper decay length significance cut to clean  signal
- ( decay length significance > 10)

- +K will have huge combinatory background

**D0**

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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)

- 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

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## - after BDT selection

- Clean - signal

**D0**

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## -- 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**

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## - after BDT selection

- Wrong-sign combination events

-  decays
- removed

**D0**

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## 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.

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## Final optimization: pT(B)>6 GeV

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

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## Final optimization: ()<0.03 cm

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

- Uncertainty on 

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## 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

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## Other control sample

- We analyze candidates in the sidebands of - signal

**D0**

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## Other control sample

- We analyze candidates in the sidebands of   signal

**D0**

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## Nothing where nothing should be

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

- We check also high statistics MC samples

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## Looking at right-sign combinations

**Clear excess of events near 6.2 GeV**

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## Mass measurement

- Fit:
- Unbinned extended log-likelihood fit
- Gaussian signal, flat background
- Number of background/signal events are floating parameters

**Number 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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## Significance of the peak

- Two likelihood fits are performed:
- Signal + background hypothesis (LS+B)
- Only background hypothesis (LB)
- We evaluate the significance:
- Significance of the observed signal: 5.4

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## Consistency check: Increase pT(B)

- Significance >6

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## Consistency check: Look back plots

**D0**

**D0**

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## Consistency check: lifetime 

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

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## Alternative Cuts Based Analysis (CBA)

| Variable | BDT | CBA |
| --- | --- | --- |
| pT() (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 fit

- Fit:
- Unbinned extended log-likelihood fit
- Gaussian signal, flat background
- Number of background/signal events are floating parameters

**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: 3.9******

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## BDT or Cut Base Analysis

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

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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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## One example: Event display

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## One example: Event display

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## 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 .

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## Production rate

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

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## Production rate

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## 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******

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## Summary

- Consistent with expectations

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## Submitted to PRL

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## 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

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## Fermilab’s latest discovery

- b-baryon -b(bss)

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## Backup slides

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## Example: Decision Trees

- S/B

- S1,B1

- S2,B2

- S12

- S11,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

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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+)).

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## 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.

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