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