# Presentation: 919707

## First observation of a New b-baryon b at D0: Celebrating 30 Years of Beauty @ Fermilab 

**First observation of a New b-baryon ********b**** at D0: *****Celebrating 30 Years of Beauty @ Fermilab***** **

- Eduard De La Cruz Burelo
*University of Michigan*
- for the DØ Collaboration

## The quest ... a long journey

- Everything begins at ... Fermilab
- The quest for B hadrons
- The quest for ... the b
- b signal
- Mass measurement
- Relative production ratio
- Summary

## The quest begins 30 years ago at...

**Fermilab's giant accelerator reveals another new sub-nuclear particle**

**Summer 1977**

- !! EXTRA!! Fermilab Experiment Discovers New Particle "UPSILON"

**B Physics, a whole field, was born on June 30 , 1977, here at Fermilab.**

## Since then ...

**Exclusive *****B***** decays**
- CLEO (1983)

**Bc by CDF (1998)**

- Main assumption to look for these particles: 1⁄2 of the mass of the upsilon!.

- Last B meson in the ground  0- state to be observed

## Recently ... October 2006

**CDF announced a preliminary result using 1.1 fb****-1**** of the ********b****’s almost 8 months ago.**

## Status:

- Mesons:
- B+, B0, Bs, Bc+ (established)
- B* (established),
- Bd**(submitted to PRL DØ, Preliminary CDF)
- Bs** (Preliminary DØ & CDF)
- Baryons:
- b (established)
- b+, and b*+(preliminary CDF)

## The quest for b baryons

**Plus there is a J = 3/2 baryon multiplet **

## Data we use ...

**Data we use ...**

**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.** **Keep delivering, and we will keep collecting data and analyzing** **.**

- The entire D0 detector is important, but the muon and central tracker subdetectors are particularly important in this analysis

## Motivation

- Spectroscopy:
- One of the best ways to test our understanding of QCD and potential models
- Production and Fragmentation:
- Major source of uncertainty in many measurements
- Discovery:
- Practice techniques for BSM searches by finding  undiscovered SM particles.

## Understanding these:

- Helps us understand these:

*D**s*****

*B**d*****

***b*

*X(3872)*

*D**sJ*

****

- BELLE

- BABAR

- JLAB

## Theoretical prediction of the masses

- E. Jenkins, PRD 55 ,
- R10-R12, (1997).

**Predicted mass hierarchy: **
**M(****Λ****b****)< M(********b****) < M(********b****) **

**Just today, first citation ...**
**Karliner et al., arXiv:0706.2163**

## LEP measurements:

- LEP experiments only deduce the presence of the b- indirectly; they look for an excess of events in the right-sign  combinations of -(-) *l **-*

******-****   ****mass**

- Wrong sign combination events

- They measure the lifetime of this excess of events.
- 1*.*45 +0*.*55/*−*0*.*43(stat*.*) *± *0*.*13(syst*.*) ps*.*
- Eur. Phys. J. C 44, 299–309 (2005)

## What do we know about the b-? 

- Predicted mass: 5805.7 ± 8.1 MeV
- Predicted to follow the mass hierarchy
- M(Λb)< M(b-) < M(b)
- By using preliminary b mass measurement from CDF and predicted mass hierarchy:
- 5.624 GeV < M(b-) < 5.808 GeV
- b- lifetime by LEP: 1.42 +0.28/-0.24 ps.*

- * This is the world average (ALEPH+DELPHI). HFAG: arXiv:0704.3575 [hep-ex]

## Our knowledge about b-baryons

- D0 has experience with the b: 3 results on the b  lifetime.

****

****

**Λ**

**π**

**p**

**Beam line**

## Searching for b in b-→J/+-

****

****

**π**

**p**

**Beam line**

**π**

****

****

**Searching for ********b**** in ********b****-****→J/********+********-**

## Impact parameter cut ... a killer

- 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

- 

- 

## What did we do to solve this problem?

- We need to open up the IP at reconstruction
- To reprocesses all DØ data with a wider IP for track reconstruction is a very difficult task. But ...
- Thanks to our muon detector (and the guys from the muon team), J/→+- is  a golden channel.. Although B -> J/X is fairly rare, it is very clean channel for us and easy to trigger on.
- We therefore reprocessed DØ RunIIa data for events containing a J/, which is ~35 million events.

## Mass distribution for K0,Λ0 and - signals for the “standard”  (bottom histograms) and “extended” (opening up IP) tracks reconstruction.

**K****0****S****→********+********-**

******→****p********-**

## Mass distribution for - signal for the “standard”  (bottom histograms) and “extended” (opening up IP) tracks reconstruction.

## Reconstruction strategy for b 

- Reconstruct J/→+-
- Reconstruct →p
- Reconstruct →+
- Combine J/+ 
- Improve mass resolution by using an event-by-event mass difference correction .
- The guides:
- The sister: b→J/ decays in data
- The impostor: J/+ (fake from (p-)+ )
- The clone: Monte Carlo simulation of b-→J/+-

##  Natural constraints inb-→J/+-

- Three daughter signal particles need to be reconstructed:
- →p+
- →+
- J/→+-
- The final state particles (p, -, -) have significant impact parameter with respect to the interaction point.
- Charge correlation: both pions must have the same charge

- ~5 cm
- c=7.89 cm

- ~5 cm
- c=4.91 cm

## More features in b-→J/+-

- b has a decay length of few hundred microns, PV separation

- - has a decay length of few centimeters.

-  has a decay length of few centimeters

- ~5 cm

- ~5 cm

- ~0.1 cm

## Reconstructing the daughters

- Background events from wrong-sign combinations  ( Λ(p-) + )

- J/→+-

- →

## What background do we expect?

- Prompt background:
- ~80% of the J/ are directly produced at the collision.
- Real B’s:
- The remaining ~20% of J/  come from B decays
- Combinatoric background:
- Real J/ plus fake -
- Fake J/ plus fake -
- Fake J/ plus real -
- Real J/ plus real - , but not from b-

- Our wrong-sign combination events have these.

## Determination of Selection Criteria 

- To retain efficiency, try to keep cuts loose
- We use independent samples:
- b→J/ decays from data
- Background from wrong-sign combination
- Background from J/ sideband events
- Background from - sideband events
- Use b- signal MC events only when no choice (e.g., pion from Xi)

## Example 1: pT(-) from  

- Λb→J/(μ+μ-)(p)

## Example 2: pT(-) from -

**Monte Carlo of**
******b****-****→J/********+********-**** **

- Background events from wrong sign combination (Λ(p-) + )

## Example 3: topology cut

**Collinearity in XY:**
**Cosine(********)**

- pT()

**Monte Carlo of**
******b****-****→J/********+********-**** **

- Background events from wrong sign combination (Λ(p-) + )

**Cos(********)>0.99**
**100% efficiency**

## Finally we have:b Selection

- →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)

## So now ... let’s first look at the background control samples after all cuts

- We have three independent background samples:
- Wrong sign combination (fake -’s from (p-)+ )
- J/ sideband events
- - sideband events.

## Background: Wrong sign combinations

**Background: Wrong sign combinations**

- No peaking structure observed in this background control sample

## Background: J/ sideband events

**J/****** (p-)+

- No peaking structure observed in this background control sample

## Background: - sideband events

- J/ (p-)-

- No peaking structure observed in this background control sample

## Now let’s look at background MC

- We investigated with high MC statistics, B decay channels such as:

- No peaking structure observed any these B decays MC samples

## What we expect: signal MC

**Mean of the Gaussian: ****5.839 ± 0.003 GeV**
**Width of the Gaussian: ****0.035 ± 0.003 GeV**

**MC events of**
******b-→J/********+********-**
**Input mass at generation level =****5.840 GeV**

## Looking at data

**Clear excess of events just below 5.8 GeV**

## Y

**Y**

**Z**

**Event scan of event in the signal peak**

## Y

**Y**

**Z**

**Event scan of event in the signal peak**

## Mass measurement

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

**Number of signal events:  15.2 ± 4.4**
**Mean of the Gaussian: 5.774 ± 0.011(stat) GeV**
**Width of the Gaussian: 0.037 ± 0.008 GeV**

- Compare to width measured in MC:
- 0.035 ± 0.003 GeV

## Nothing in the background samples:

- Bkg from  sidebands

- Bkg from J/ sidebands

- Bkg from wrong-sign

## Significance of the peak

- Two likelihood fits are perform:
- Signal + background hypothesis (LS+B)
- Only background hypothesis (LB)
- We evaluate the significance:
- Significance of the observed signal: 5.5

## Alternative significance

- In the mass region of 2.5 times the fitted width centered on the fitted mass, 19 candidate events are observed while 14.8 ± 4.3 (stat.)+1.9/−0.4 (syst.) signal and 3.6 ± 0.6 (stat.)+0.4/−1.9 (syst.) background events are estimated from the fit. The probability of backgrounds fluctuating to 19 or more events is **2.2 ×10****−7**, equivalent to a Gaussian significance of **5.2σ**

## Consistency checks

- Decay length distribution

## Intermediate Resonances

- Signal visible in all intermediate resonances

***J/*********

************

************

***m(************)***

***m(p************)***

***m(************)***

## A second analysis approach would be ...

- A different approach is to rely heavily on Monte Carlo simulation.
- There are many multivariate techniques in the market:
- Artificial Neural Networks,
- Boosted Decision Trees,
- Likelihood ratio,
- etc.
- As a cross check we used Decision Trees

## Example: Decision Trees (BDT)

- In order to apply the same BDT to b in data, we use only J/ and  variables as input to the BDT.

**Minimum overlap between BDT and cut-based variables**

## Decision Trees 

- We observe a signal consistent with that observed with cut-based analysis.
- Only ~50% overlap between selected events and the cut-based analysis.
- Width consistent with MC
- Background shape consistent with wrong-sign combination shape.

******b****-**** using BDT**

- A multivariate technique with a simple set of input variables, not including - variables, also results in a b- signal.

## Combining: cuts+BDT

- After we remove duplicate events, we observe 22.8 ± 5.8 events.
- Significance:
- Sqrt(-2L) = 5.9

## Systematic Uncertainties on Mass 

- Fitting models
- Two Gaussians instead of one for the peak. Negligible.
- First order polynomial background instead of flat. Negligible.
- Momentum scale correction:
- Fit to the b mass peak in data, < 1 MeV.
- Fit to B0 signal peak. Negligible effect < 1 MeV
- Study of dE/dx corrections to the momentum of tracks finds a maximum deviation of 2 MeV from the measured mass .
- Event selection:
- From the mass shift observed between the cut-based and BDT analysis, once removing the statistical correlation, a 15 MeV variation is estimated .

## Discovery!

## Production ratio

- In addition to the observation, we also measure:

- This provides a measurement to allow other experiments to compare their production rate with this result.

**f(b****→X) : fraction of times b quark hadronizes to X**

## Production ratio

- We use Monte Carlo samples of:
- b-→J/+-
- b→J/+
- MC passed through D0 detector simulation
- Same reconstruction and selection criteria as used on data is applied to Monte Carlo.
- Monte Carlo distributions need to be reweighted due to the Data/MC pT spectrum differences and to account for trigger effects.
- From comparison of b kinematic distributions in data and MC, determine further weighting factor, then apply to b-

## Systematic uncertainties in the relative production ratio

| Source | Uncertainty (%) |
| --- | --- |
| b/b hadronization models | Negligible |
| MC stat. on b /b | 10 |
| pT() reconstruction | 7 |
| Effect of mass difference between data and MC | 5 |
| b/b MC reweighting | 27  |
| Syst. uncertainties on the number of b in data | +13, -3 |

- Conservatively take difference between reweighting result and no reweighting .

## Production ratio

**Ignoring the ratio of Br’s, from ratio of hadronization fractions of B****s**** to B****d****, expect ~1/4 or less**

## Last Tuesday June 12, DØ submitted a PRL article announcing the discovery a new b baryon: b-

**Last Tuesday June 12, DØ submitted a PRL article announcing the discovery a new b baryon: ********b****-**

## Production ratio

- We measure the relative production ratio to be:

-        Allows comparison between experiments

## Submitted to PRL 6/12/07

- arXiv:0706.1690, Fermilab-Pub-07/196-E

## www.fnal.gov 6/13/07

**www.fnal.gov 6/13/07**

## The quest begins ... and continues @ Fermilab

**The quest begins ... and continues @ Fermilab**

**Celebrating 30 years of the b quark discovery @ Fermilab**

**A New b baryon: *****an anniversary gift***