# Presentation: 549288

## UIUC paper review

**UIUC paper review**

** ****Introduction to Bc**
** ****Event selection**
** ****The blind analysis**
** ****The final result**
** ****The systematic error**
** ****A further check**

- -**a paper written simply and neatly with full physics concepts associated to the analysis **

## Short introduction to Bc

** ****Meson(bc): The highest combination of two different quark flavors**
** ****Reco. event   : **_**has not been observed**_** in any fully reconstructed decay modes.**
**         ****(CDF RunI, D0 RunII measurement of mass : using semileptonic decay.)**
** ****Mass   : 6.40********.39(stat) ******** 0.13(syst) GeV/c****2**** (PDG-2004)**
_**The measurement in this analysis results:**_
** ****~19 fully reconstructed B****c********J/******** events, J/******** ******** ********.**
** ****Mass : ****6287.********.8(stat) ******** 1.2(syst) MeV/c****2**

## Online event selection

**360 pb****-1**** data sample**
**Di-muon triggered sample**
**Level 1:**** CMU, CMX stubs are matched to COT axial tracks. A muon pair (CMU-CMU, CMU-CMX) is required with pT > 1.5  and 2.0  GeV/c for the CMU and CMX muons respectively. **
**Level 2:**** require the opposite charge and impose requirement on the opening angle. **
**Level 3:**** three dimensional tracking is done for the muon pair; require the invariant mass from 2.7 to 4.0 GeV/c****2**** .**

## Reconstruct BcJ/ events offline

**Ensure good vertex resolution**
**Three axial SVX hits.**
**Good matching between the fitted muon tracks and the muon stubs.**
**Di-muon invariant mass window ****[3.042, 3.152]**** GeV/c****2**** .**
**pion candidate : each charged particle track that is not part of the reconstructed J/********.**
**The J/******** candidate and pion candidate are constrained to a common 3-D vertex with a vertex fit cut.**
**B****c ****search mass window is defined ****from 5.6 to 7.2 GeV/c****2**** .**

## The blind analysis

**The search mass window [5.6, 7.2] GeV/c****2**** ****. **
**~ 2 standard deviation region around the CDF Run I measurement of the B****c**** mass.**
**The mass values of the events in the search window are temporarily hidden**
**Binning the invariant mass in two intervals above or below 6.4 GeV/c****2****.**
****** ****function: **
**Optimize the ******** function with MC samples:**
-  A quality requirement on the J/ three-track 3-D vertex fit.
- A requirement on the pion track contribution to the 3-D 2 vertex fit.
- The impact parameter of the J/ pair with respect to the beam line.
- The maximum proper decay time of the J/ pair.
- The transverse momentum of the pion.
- The 3-D angle between the momentum and decay direction of the J/ pair.
- The significance of the transverse decay length of the J/ pair
**Find 390 candidate events in the search window in data sample.**

- 

- J/

- 

- 

## Control sample: BuJ/k 

**B****u********J/********k vs B****c********J/******** **
**Same decay topology**
**Different masses**
**Check the understanding of the reconstruction of the relevant variables in the simulation.**
**Mass resolution**
**Estimate the expected B****c ****yield. **
**10 – 50 B****c**J/** events in data sample**

**Three Gaussians and **
**a linear function**
**2378 signal events**
**11.5********0.3MeV/c****2**

**Agree with the mass resolution from MC simulation.**

## A sliding fit window

**The sliding search window**
**extends from –100 to +200 MeV/c****2**** in mass **
**around each nominal peak position, m. **
**seeach mass region [5.7,7.0]**
**In each search window, do a binned likelihood fit**
**Signal : Gaussian with fixed width (13-19 GeV/c****2****) from MC.**
**Background: linear function including two components**
**Partially reconstructed B****c****.**
**Combinatorial background.**
**Calculate ******** in each search window**

## A sliding fit window to search for maximum 

**Establish a signal or set an upper limit for the production?**
_**Toy MC**_** determines the expected distribution of ******** for pure bdg samples.**
**The overall distribution is consistent with the two mass bins of the search window**
**Two types of backgrounds are statistically varied in each toy MC experiment. **
****** ****>3.5 establishes a signal.**
******  ****distribution from the sliding fit window scans**
**Maximum value 3.6 at a mass of 6.29 GeV/c****2**
**The background is consistent to that in the toy MC scans. **
**The chance for a pure bgd distribution to yield ******** >3.6 is 0.27%.**

## The final fit

**Use unbinned likelihood fit to the mass region containing the peak.**
**Gaussuan + linear function**
**Gaussian width is fixed to 15.5 MeV/c****2****, mean is allowed to float.**
**Results: ****(within a region of ******** 2 standard deviation of the central value)**
**Mass:****	****	****6287.0 ******** 4.8 MeV/c****2**
**Signal events: ****	****18.9********5.7**
**Background:****	**** 10.0********1.4**

**B****c********J/********  exclusive decay **
**signal Established.**

## Systematic error

**The systematic errors are evaluated from the corresponding uncertainties on the B****u**** mass analysis. **
**The uncertainties on the track parameters. **
**The uncertainty on the momentum scale.**
**Differences in the pT spectra of the B****u**** and B****c**** mesons. **
**Limited knowledge of the background shape used in the final mass fit. **
**The total systematic uncertainty: 1.1 ****MeV/c****2**

## A further check to this Bc enriched sample

**For data sample, lower mass window include both partially reconstructed B****c****  and the linear combinatorial bgd. The upper mass window contains mostly the linear combinatorial bgd. **
**Lose cuts to get more events in the data sample**
**The lower mass window should be enriched with events having small impact parameter of the pion wrt the jpsi vertex.**

**Data sample**