BS Mixing Measurementsat the Tevatron
Michael KirbyRadboud University, Nijmegen
NIKHEF
New Trends in High Energy Physics, CRIMEA 2006
B Meson Flavor Oscillations
2*22
222
||6
)(tbtqBBB
tWFq VVBfm
xSmGm
qqqπη
=Δ
Form factors and B-parameters from Lattice calculations are
known at ~15% level
Mixing involves CKM elements,measuring Δmq constraints
the unitarity triangle New exotic particles may
run in the loop mixing sensitive to NP
Neutral B mesons can spontaneously transform in the corresponding antiparticle
3
many theoretical uncertainties cancel in the ratio
• • |Vts|/|Vtd| can be determined at ~4%
Δms and the side of UTΔmd f2
BBB [(1-)2+η2] circle centered in (η)=(1,0)
f2BBB known at 15% from LQCD
2
2
22
2
2
2
td
ts
Bd
Bs
td
ts
BdBd
BsBs
Bd
Bs
d
s
V
V
m
m
V
V
Bf
Bf
m
m
m
m ξ==ΔΔ
047.0035.0210.1 +
−=ξ (hep-lat/0510113)
Experimental challenge:|Vts| >> |Vtd| Δms >> Δmd needs to resolve > 2.3 THz oscillations
Previous Δms measurements: LEP/SLD/CDF-I: Δms > 14.4 ps-1 @ 95% CL HFAG Average for PDG 2006
Unitarity Triangle Fit• just for illustration, other fits exist
• CKM Fit :Δms: 18.3+6.5 (1) : +11.4 (2) ps-1
from Δmd
from Δmd/ΔmsLower limit on Δms
-1.5 -2.7
B Mixing• Neutral B Meson system
mixture of two mass eigenstates (No CP violation case):
• BH and BL may have different mass
and decay width– Δm = MH – ML
(>0 by definition)– Δ = H - L
• The case of Δ = 0
€
BH =1
2B + B( )
BL =1
2B − B( )
Measurement .. In a Perfect World
what about detector effects?
“Rig
ht
Sig
n”
“Wro
ng
Sig
n”
Realistic Effects
flavor tagging power,background
displacementresolution
momentumresolution
mis-tag rate 40% L) ~ 50 m p)/p = 5%
Real Measurement Layout
momentum resolutiondisplacement resolutionflavor tagging power
UnbinnedLikelihood
Fitter
Data
A(Δms=15 ps-1)= ?
Δ ms = ?
scan for signal:
measure frequency:
p ~ e-t/[1±AD cosΔmt] R(t)
9
1. Collect as many Bs as possible• Tevatron, Trigger, Reconstruction
2. Extract Signal • Bs flavor at decay inferred from decay products
3. Measure proper decay time of the Bs meson• Si trackers, per event primary vertex, candidate specific decay time resolution
4. Determine Bs flavor at production (flavor tagging) • PID (TOF, dE/dx) • Flavor tag quantified by Dilution: D=1-2w, w = mistag probability
5. Measure asymmetry between unmixed and mixed events• In practice: perform likelihood fit to expected unmixed and mixed distributions
Road map to Δms measurementvertexing (same) side
“opposite” side
e,Jet
14
4
3
2( )
BS
Se
SD tsm
A +=
Δ−
22
2
2
1 σε
σ
( )tmDPP
PPtA
nomixmix
mixnomix ⋅Δ=+
−= cos)(
5
This analysis: Feb 2002 – Jan 2006 1 fb-1
Tevatron Luminosity
DZero Detector• Spectrometer : Fiber and Silicon Trackers in 2 T Solenoid • Energy Flow : Fine segmentation liquid Ar Calorimeter and Preshower• Muons : 3 layer system & absorber in Toroidal field• Hermetic : Excellent coverage of Tracking, Calorimeter and Muon Systems
SMT H-disks SMT F-disks SMT barrels
Signal Selection (e)+
π -
K+K-
D-
S
(e) B
X
00ss BB ↔
Muons were selected by triggers without lifetime bias
= no online/offline Impact Parameter cuts
Trigger muon can be used as tag muon : gives access to eDs sample with enhanced
tagging purity
Signal Selection
+
π -
K+K-
D-S
(e) B
X
PV
LT(DS)
00ss BB ↔
Ds lifetime is used to have non-zero selection efficiency at Interaction Point
Bs can decay at IP and be reconstructed
Eff=30%
Flavor Tagging and dilution calibration
• Identify flavor of reconstructed BS candidate using information from B decay in opposite hemisphere.
a) Lepton Tag : Use semileptonic b decay :Charge of electron/muon identifies b flavor
Ds
cos (l, Bs) < 0.8
Bs
e /
b) Secondary Vertex Tag : Search for secondary vertex on oppositeSide and loop over tracks assoc. to SV.
c) Event charge Tag:All tracks opposide to rec. B Secondary Vertex
Dilution in Δmd measurement• Combine all tagging variables using likelihood ratios
• Bd oscillation measurement with combined tagger
Δmd= 0.5010.030±0.016ps-1 Combined dilution: D2=2.48±0.21±0.08 % Input for Bs
measurement
Cross-check on BdXD±(π)
• EXACTLY the same sample & tagger• Amplitude Scan shows Bd oscillations
– at correct place no lifetime bias– with correct amplitude correct dilution calibration
• Same results for two other modes
Amplitude ScanDØ Run II Preliminary
Measure Resolution Using Data• Ultimately Δms sensitivity is limited by decay length
resolution – very important issue • Use J/ sample
– Fit pull distribution for J/ Proper Decay Length with 2 Gaussians– Resolution Scale Factor is 1.0 for 72% of the events and 1.8 for the rest
• Cross-checked by several other methods+
PVJ/ vertex
-
L±σL
DØ Run II Preliminary
Results of the Lifetime Fit( )( )cKxÄme
c
Kxp s
c
Kx
B
oscnoss
sB
s
/15.0)(/ ⋅±⋅⋅=−
cos Dτ
τ• From a fit to signal and background region:
Decay Mode cBs (m) cbkg (m)
BsDs X, Ds π 4049 6276
BsDs e X, Ds π 44429 64518
BsDs X, Ds K*K 40722 54910
BsDs e X
Ds πDs K*K
BsDs X
Bs decay samples after flavor tagging
• NBs( π ) = 5601 102
• NBs(π + e) = 1012 62 (Muon tagged)
• NBs(K*K + ) = 2997 146
BsDs e X
Ds π
Ds K*K
BsDs X
BsDs XDs π
Amplitude Scan of BsXDs(π)
• Deviation of the amplitude at 19 ps-1– 2.5σ from 0 1% probability– 1.6σ from 1 10% probability
Log Likelihood Scan
Δms < 21 ps-1 @ 90% CL assuming Gaussian errorsMost probable value of Δms = 19 ps-1
Systematic
Resolution K-factor variation BR (BsDsX) VPDL model BR (BsDsDs)
Have no sensitivity above 22 ps-1
“Direct Limits on the Bs Oscillation Frequency”hep-ex/0603029 – published by Physical Review Letters
More Amplitude Scans• New results : Amplitude scans from two additional
modes
Ds K*K
BsDs (π e XBsDs X
Ds π
Combined D0 Result
• Amplitude is centred at 1 now, smaller errors• Likelihood scan confirms 90% CL Δms limits: 17-21 ps-1
• Data with randomized tagger : 8% probability to have a fluctuation (5% before for π mode)
• Detailed ensemble tests in progress
CKM Fit Before D0
CKM Fit With D0 Limit
The CDFII Detector• multi-purpose detector• excellent momentum
resolution (p)/p<0.1%
• Yield:– SVT based triggers
• Tagging power:– TOF, dE/dX in COT
• Proper time resolution:– SVXII, L00
27
Event Selection: Fully Hadronic Bs
used in this analysis
Decay mode Events
BsDsπ (π) 1600
Bs Dsπ (K* K) 800
Bs Ds π (3π) 600
Bs Ds3π ( π) 400
Bs Ds3π (K*K) 200
Total 3600
Cleanest decay mode:BsDsπ[π]π [KK] ππ
• Bs momentum completely reconstructed• Excellent decay time resolution, good S/N• Small BR low statistic• Good sensitivity at high values of Δms
28
• Minv(l+Ds) helps reject BG• BG Sources:
• Ds + fake lepton from PV• Bs,dDsDX (DslX)• cc
Event Selection: Semileptonic Bs
• Missing momentum ()• Poorer decay time resolution• Large BR high statistic• Good sensitivity at low values of Δms
48000 l+Ds candidates, 75% are from Bs decay
Ds Mass
l+Ds Mass
−−+−−−→ πππφπ ,, 0* KKDs
29
Flavor Tagging PerformancesTwo types of flavor tags used in CDF
– OST: produce bb pairs: find 2nd b, determine flavor, infer flavor of 1st b• calibrated on large samples of B0 ad B+ decays
– SST: use charge correlation between the b flavor and the leading product of b hadronization
• performances (D) evaluated in MC, after extensive comparison data VS MC
Same-side kaon tag increases effective statistics ~4
εD2 Hadronic (%)εD2 Semileptonic
(%)
Muon 0.48 0.06 (stat) 0.62 0.03 (stat)
Electron 0.09 0.03 (stat) 0.10 0.01 (stat)
JQ/SecVtx 0.30 0.04 (stat) 0.27 0.02 (stat)
JQ/Displ’d trk 0.46 0.05 (stat) 0.34 0.02 (stat)
JQ/High pT 0.14 0.03 (stat) 0.11 0.01 (stat)
Total OST 1.47 0.10 (stat) 1.44 0.04 (stat)
SSKT 3.42 0.96 (syst) 4.00 1.12 (syst)
Amplitude Scans• example: B0 Mixing signal in hadronic decays• points: A§(A) from likelihood fit for different Δm • yellow band: A § 1.645 (A) Δm values where A+1.645 (A) < 1 are excluded at 95% C.L.• dashed line: 1.645 (A) as function of Δm• measurement sensitivity: 1.645 (A) = 1
narrow Δms range wide Δms range
Calibrating the Proper Time Resolution
• utilize large prompt charm cross section• construct “B0-like” topologies of prompt D- + prompt track• calibrate ct resolution by fitting for “lifetime” of “B0-like” objects
trigger tracksprompt track
Ds- vertex
P.V.
“Bs” vertex
period 3
+
32
Proper decay time reconstruction
( )Bp
BmL
Lct
Txy
)(×==
βγ
Detector length scale and proper treatment of
detector/selection biases controlled by performing lifetime measurements
Decay CDF [ps]
(stat. only)
PDG 06 [ps]
B0 D-π+ 1.508 0.017 1.530 0.009
B- D0π- 1.638 0.017 1.638 0.011
Bs Dsπ(ππ) 1.538 0.040 1.466 0.059
RUN 304720 EVENT 109026PV
D decay B
decayLxy
π π
l+Ds ct* Projections
Bs lifetime in 355 pb-1: 1.48 ± 0.03 (stat) psWorld Average value: 1.469 ± 0.059 ps
Lepton
Ds- vertex
P.V.
Bs vertex
Hadronic Scan: CombinedPreliminary
Bs ! Dsπ / Dsπππ
Semileptonic Scan: Combined
Combined Amplitude Scan
A/A (17.25 ps-1) = 3.5
How significant is this result?
Preliminary
25.3 ps-1
37
ct [cm]K-factorisolation
St
D
pT [GeV/c]
Likelihood
Amplitude method(*): scan Δms space: fix Δms fit for A:A consistent with 1 mixing detected at the given Δms
=
k k k k k k=sig,bg
sig
for each event:
pdg
Procedure checked on B0 by fitting for Δmd
Courtesy of J.Kroll
(*) H-G.Moser, A.Roussarie,NIM A384 (1997)
Data fitted with an unbinned likelihood function to the expected unmixed and mixed distributions
Δms = 17.33 +0.42 (stat) ± 0.07 (syst) ps-1
Measurement of Δms
-0.21
limit Δms in [16.94, 17.97] ps-1 at 95% CL-0.21
“Measurement of the Bs-Bs Oscillation Frequency”hep-ex/0606027 – published by Physical Review Letters
|Vtd| / |Vts|
• compare to Belle bd (hep-ex/0506079):
|Vtd| / |Vts| = 0.199 +0.026 (stat) +0.018 (syst)
inputs: m(Bd)/m(Bs) = 0.9830 (PDG 2006) ξ = 1.21 +0.047 (M. Okamoto, hep-lat/0510113) Δ md = 0.507 ± 0.005 (PDG 2006)
-0.035
|Vtd| / |Vts| = 0.208 +0.008 (stat + syst)-0.007
-0.025 -0.015
CKM Fit including CDF Δms measurement
• Add Same Side Tagging
• Add hadronic modes triggering on tag muon
• Add more data (4-8 fb-1 in next 3 years) with improved detector – additional layer of silicon between beampipe and Silicon Tracker (Layer0) – better impact parameter resolution
Layer0 has been successfully installed in April 2006- S/N = 18:1 & no pickup noise - First 50 pb-1 of data on tape, first tracks have been reconstructed, and commissioning advancing quickly
D0 Outlook
42
CDF Run II Preliminary L=1 fb-1
CDF Outlook• Collecting new integrated luminosity• Squeezing maximum information from the data we already have:
1. Systematic use of Neural Networks in signal extraction:
• use decays modes previously discarded cause high BG
• more signal in already used modes
2. Use partially reconstructed BsDs*π/K and Ds:• large BR• good momentum resolution
3. Improve Flavor taggers:• OST: +15% D2
• NN to combine OS taggers• OSKT
• SSKT: ~+10% D2
• better use of combined PID and kinematics
BsDsπ+π-π+ (Ds π+π-π-)
NBs = 220
BsDsπ+ (Dsπ-)
Conclusions• Frequency of Bs mixing successfully measured at
the Tevatron!
• D0 reported the first two-sided limit on Δms
Δms [17,21] ps-1 @ 90% C.L.
• CDF confirmed result with measurement
Δms = 17.33 +0.42 (stat) ± 0.07 (syst) ps-1
|Vtd / Vts| = 0.208 +0.008 (stat + syst)
-0.21
-0.007