1. Measuring the B+→J/ψ (μμ) K+ Channel with the first LHC data in Atlas
Anastopoulos Christos
Petridou Chariclia
Sampsonidis Dimitrios
Bachas Konstantinos
Aristotle University Of Thessaloniki

2. Why we study B+→J/ψ (μμ) K+
Important reference channel. Rare decay channels like B→μμ will be measured relative to this.
Our task is to determine with first ATLAS data the B+
Mass
Lifetime
Differential Cross Section

3. ATLAS DETECTOR Inner Detector (ID) Calorimeter
Si pixel and strip detector
Transition Radiation Tracker: e/π separation
Solenoid magnet of 2T
Calorimeter
High granularity LAr EM calorimeter: |η|< 3.2
Hadron calorimeter: |η|< 4.9 (scintilator-tile in barrel and LAr in end-caps and forward)
Muon spectrometer
Air-core toroid system average ~ 0.5 T
MDTs & CSCs; RPCs & TGCs

4. Muon rates and samples’ composition in ATLAS
single-muon
di-muon
all h b c
J/y nb
@1031
100 pb-1
pp  mu6 X
60 Hz
bb  mu6 X
25 Hz
cc  mu6 X
10 Hz
pp  mu6 mu3 X
1.6 Hz
bb  mu6 mu3 X
1.2 Hz
pp  J/ψ(mu6mu3)
0.1 Hz
bb  J/ψ(mu6mu3) X
0.04 Hz
Y(mu6mu3)
0.01 Hz
At LEVEL-1 :
single muon or di-muon.
At HLT :
Reconstruct exclusive decays. Making Selection on mass, decay distance. Using a LVL-1-seeded Region Of InterestI for fast rejection

5. An Example of the ROI Mechanism in the HLT
Look for a J/ decay where only one  has been triggered at LVL1
Adpating the ROI mechanism to the B-physics challenges
MDT
RPC/TGC
m RoI ( φ, η )
LVL1 pT(m) > 6GeV μ μ
Invariant mass constraint

6. B+→J/ψ(μμ)K+ channel Branching ratio ~ 10-3
17000 events expected in 100 pb-1 (1 year of running at cm-2s-1)
Important reference channel. New channels (B→μμ) measured relative to this.
Our task in the view of DC3 (Data Challenge 3) is to determine with first ATLAS data the B+
Mass
Lifetime
Differential Cross Section

7. Latest ATLAS Software Latest Athena Release 11.0.41
Fully simulated Geant4 data
Use of Grid technology
Analysis Object Data (AOD) files for storing and manipulating reconstruction data.

8. B+→J/ψ(μμ)K+ in ATHENA
ATHENA release
167 Fully reconstructed events (more in the pipeline)
Events Generated with PYTHIA (Grid)
Events Simulated and Digitized with (Grid)
Events Reconstructed with
/castor/cern.ch/grid/atlas/bphys/fordc3/reco/18603.B_plus_Jpsi _mumuK_plus/data
Dataset contains one Low Pt LVL1 muon (Pt> 6GeV) and a second HLT muon :one track segment in the inner station of the muon Spectrometer matched with one inner detector track.

9. B-Physics Analysis Code in Athena
The ATLAS offline analysis objects allow to write analysis code in Athena using AOD (Analysis Object Data) files
B-physics group provides additional tools that supplement the standard ones.
Vertex fitters
Track manipulation utilities ,etc..
The analysis code is Object Oriented (C++ language)
Does extensive use of C++ STL (Standard Template Library) containers and iterators in order to store and manipulate the data .

10. J/ψ selection
Form pairs with one Low Pt muon (Pt>6GeV) and a MUID or MUTAG muon .
The muon pairs are fitted to a common vertex using CDF fitter.
A chi2/NDF <10 and a Lxy>0.1 mm cut is applied to the fitted vertices.
From the vertices surviving the previous cuts we keep only these that have an invariant mass inside a window around mJ/ψ .

11. J/ψ invariant mass PDG values Selection Efficiency Mean 3096.9
88 % mainly from muon Pt cuts

12. K+ Selection
We select from the collection of reconstructed tracks the ones that have Pt >1.5GeV and |η| < 2.7.
From these we exclude the ones that are previously identified as muons coming from the J/ψ.
Finally we keep only the ones that have positive charge . These are our K+ candidates

13. B+ Selection
Combining the muon-pairs and the K+ candidates we form triplets of particles
We fit these triplets to a common vertex with a mJ/ψ constrain applied to the muon – pairs
We apply a chi2/NDF <6 a Pt > 5GeV and a Lxy>0.1 mm cut to the fitted vertices.
From the vertices surviving the previous cuts we keep only these that have an invariant mass inside a window around m B+ .
These are considered to be B+

14. B+ Selection additional cutsΚ+
Two more cuts to be considered for handling combinatorals
Angle Between B+ vertex direction and B+ Pt
Isolation cone with radius R<0.7 (CDF).
Made no difference in our case because only signal events were generated
Pt Β+ μ- Β+ μ+
Beam Spot
P Β+ R
R2 = Δφ2+Δη2 Pt pt

15. B+ invariant mass PDG values Selection Efficiency Mean 5279 MeV
63.5 % from J/ψ efficiency and K+ Pt cuts

16. Conclusions
We gained experience running on Grid which is vital for future ATLAS Data Challenges.
We used state of the Art OO analysis tools in order to access and manipulate Analysis Object Data (AOD).
Our Algorithm is to be included in the Bphys analysis package in the future Athena Releases
We created and tested a first analysis algorithm for the B+→J/ψ(μμ) K+ channel
With a small data sample we were able to determine the J/ψ and B+ mass.
Our selection efficiency was 88% for J/ψ and 63.5 % for B+

17. Future Plans
Our code will be tested in a large J/ψ inclusive sample (250K) which is under preparation by Atlas Bphys Group.
Two more cuts to reduce combinatorial background .
Detailed studies of the cuts and efficiency
Use a large data sample to conclude our studies on
Mass
Lifetime
Differential Cross Section
Of B+→J/ψ(μμ) K+ channel