High Precision Measurements of B s Parameters in B s J/ ˆ ï¦

download High Precision Measurements of B s  Parameters in B s J/ ˆ ï¦

of 18

  • date post

  • Category


  • view

  • download


Embed Size (px)


High Precision Measurements of B s Parameters in B s J/ ψ . Roger Jones University of Lancaster United Kingdom for the ATLAS B-physics Group. Beauty 2005, Assisi, Italy. Overview of ATLAS. Weight: 7000 tonnes. Length: 46m. Muon chambers. Barrel toroid. Radius: 11m. EM calorimeter. - PowerPoint PPT Presentation

Transcript of High Precision Measurements of B s Parameters in B s J/ ˆ ï¦

Slide 1Roger Jones
Beauty 2005, Assisi, Italy
Radius: 11m
Length: 46m
Muon chambers
Barrel toroid
Inner detector
EM calorimeter
Forward calorimeter
Hadronic calorimeter
End-cap toroid
ATLAS is a central detector whose primary aims are the discovery of new particles and the precise study of known particles.
The B-physics programme falls into this second category.
The detector contains two features in particular which enable it to fully exploit the enormous B-hadron production rate:
(i) The inner detector.
Provides fine-granularity measurements of particle loci (tracks) enabling momentum and vertex position measurements to be made with very high precision.
The inner detector uses a combination of silicon pixel detectors, silicon microstrips and straw tube trackers, arranged in cylindrical layers and wheels around the interaction point,
(ii) The muon spectrometers measure the transverse momentum of muons leaving the detector
provide a trigger for B-physics events. A number of non-silicon technologies are deployed in the muon system.
2010+: 1034 cm-2s-1 “High”
1 proton bunch crossing every 25ns
4.6/23 pp collisions/crossing @ low/high luminosity
~1% of pp collisions produce a bb pair
At luminosity 2 x 1033 cm-2s-1
bb events produced with rate of 106 Hz
10Hz output to permanent storage for B-physics
so highly selective and adaptable B-physics
trigger required
This slide contains all the words needed. The points for particular emphasis are
The luminosity of the LHC will increase over a period of months, but will decrease during individual runs as protons are consumed in collisions.
This changing environment will require a highly flexible triggering strategy in order to extract the most data from the beam
In general the trigger will need to be highly selective to reduce the data-rate down to financially acceptable levels
B-physics trigger strategies (see Natalia Panikashvili)
As luminosity drops during the fill, more triggers are turned on
2 muons pT > 6 GeV (barrel) 3 GeV (end-caps)
Confirm muons Refit tracks in ID Decay vertex reconstr. Select decays using mass/decay length cuts
Bd→ J/ψ(μμ)K0S Bs→ J/ψ(μμ)φ B→μμ B→K0*μμ B→fμμ Λb→Λ0 J/ψ(μμ) Λb→Λ0 μμ
EM+μ L < 2.1033
1 muon pT > 6 GeV 1 EM cluster ET > 2GeV
Confirm muons & EMC Decay vertex reconstr. Refit tracks Selections
Bd→ J/ψ(μμ)K0S+ b→ eX Bd→ J/ψ(ee)K0S + b→ μX Bd→K0*g Bs→fg + b→ μX
Hadronic L < 2.1033
1 muon pT > 6 GeV 1 jet cluster ET > 5GeV
Confirm muons & j.c. Decay vertex reconstr. Refit tracks Selections
Bs→Ds(φ(KK))π Bs→Ds (φ(KK)) a1(ρ0π+) B+ → K+K+p- Bd → p+p- (+ b→ μX)
The B-physics trigger must be able to handle luminosities of 2x10^33 cm-2s-1 and adapt to falling luminosities during a run.
The di-muon trigger requires two muons with the indicated pt al LVL1, with confirmation and ID scans taking place at LVL2, and mass/decay length selections at EF. This trigger will pick up the J/psi->mumu decays which are crucial to much of the B-physics programme. It will be used into the high luminosity period.
The other two trigger schemes will be turned on as the luminosity falls, and will extend the physics reach of the detector to encompass the channels shown. The essential message is that the B-physics reach of the detector improves with falling luminosity, and the trigger strategies are designed to capitalise on this. The di-muon trigger is sufficiently robust to allow B-physics measurements to be made in the higher luminosity environments.
S : mixing phase = 2 sin θc sin γ |Vub| / |Vcb|
Arises through the interference of mixing and decay
Highly sensitive to SUSY contributions
Parameter is small in the Standard Model (~0.02) so challenging measurement
General box diagrams for Bs-Bs mixing:
Γs = ½ ( ΓH + ΓL )
Extracting mixing parameters requires separation of CP eigenstate amplitudes
Scalar Vector + Vector decay: final state described by three helicity amplitudes
Determined by the angular distribution of the decay, and also proper times and tag
|A| |A| δ1 δ2
Γs ΔΓs ΔMs S
Distribution is model-independent – new physics enters through the modification of existing values
Accurately modelled by EvtGen
For Bd(s)→ J/ψ(μ6μ3)K0S
εtag(electron) = 0.012
εtag(muon) = 0.025
Wtag(electron) = 0.27
Wtag(muon) = 0.24
Signal B/B-meson
Many of the studies, especially those involving CP violation, require the flavour of neutral B-mesons to be known at the time of fragmentation of the bb quark pair produced by the pp collision.
Two tactics are deployed.
Same-side tags exploit correlations between the flavour of the b-quark producing the B-meson, and the “charge” of the accompanying jet (“charge” = the kinematically weighted average of the charge of all particles in the jet). This is sometimes referred to as a “jet-charge tag”. Such tagging algorithms look for jets in the event which are consistent with coming from the same quark as the B-meson under study. Such tags are highly efficient, in that most B-mesons have such a jet accompanying them and can hence be tagged, but have a high “wrong-tag” probability due to particles which are not from the jet being mistakenly assigned to it.
Opposite side techniques look at the products of the other quark to tag the flavour of the B-meson. They rely on the quark decaying semi-leptonically; the lepton charge can easily be identified, which tags the quark and hence the other quark and the B-meson. These tags have a low wrong-tag probability but a low efficiency (the SL branching ratio is low).
Details of Analysis
All studies based on fully simulated ATLAS events and using the current reconstruction software
1 000 000 Bs decays produced with PythiaB and EvtGen to generate the correct angular distribution and mixing
(model input: A,A, δ1,δ2,Γs,ΔΓs, s,ΔMs)
Cuts: 1 muon > 6 GeV; 1 muon > 3 GeV; kaons > 0.5 GeV
All computations performed and all results stored on the Grid (LCG)
pT(μ1) > 3GeV && pT(μ2) > 3GeV
σ = 38MeV
pT(K) > 0.5GeV
|η(K)| < 2.4
J/ψ invariant mass
must point at primary vertex
Bs proper decay time > 0.5ps
pT(Bs) > 10 GeV
Bs invariant mass
Analysis Results and projections
Total number of signal events within kinematic cuts after 30 fb-1: 810 000
LVL1/LVL2 trigger di-muon efficiency: 77%
Number of signal events after trigger: 623 700
Reconstruction Efficiency
S/B ~ 15.1
Background 2:
S/B ~ 6.8
bb→J/ψ(μμ)X (background 2)
(Anti B)-tag: εtag1 = w; εtag2 = 1 – w
No tag: εtag1 = εtag2 = 0.5
Convolution with Gaussian to account for proper decay time resolution
Reconstruction efficiency and acceptance corrections: determined from simulation
Background (level determined from simulation)
Theoretical PDF:
No sensitivity to Standard Model values
(nor have LHCb or CMS)
CDF recently made a unexpectedly large measurement of ΔΓs/Γs