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Evidence for B s Mixing and measurement of m s at CDF. S. Giagu and CDF Collaboration University of Rome “La Sapienza” INFN Sezione di Roma 1. Outline. Introduction Search for B s -B s oscillations in CDF Impact on the overall UT fit Work in progress and Outlook. CDF Collaboration,

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evidence for b s mixing and measurement of m s at cdf

Evidence for Bs Mixing and measurement of msat CDF

S. Giagu and CDF Collaboration

University of Rome “La Sapienza”

INFN Sezione di Roma 1

  • Introduction
  • Search for Bs-Bs oscillations in CDF
  • Impact on the overall UT fit
  • Work in progress and Outlook

CDF Collaboration,

“Measurement of the Bs-Bs Oscillation Frequency”

hep-ex/0606027 – accepted by Physical Review Letters

S.Giagu - ICHEP 2006, Moscow

b meson flavor oscillations





B Meson Flavor Oscillations

Neutral B mesons can spontaneously transform in the corresponding antiparticle

In the SM generated via F=2 2nd order weak interactions, dominated by the exchange of a top quark

Mixing involves CKM elements,

measuring Δmq constraints

the unitarity triangle

New exotic particles may run in the loop

 mixing sensitive to NP

Form factors and B-parameters from Lattice calculations are known at ~15% level

S.Giagu - ICHEP 2006, Moscow

m s and the side of ut
ms and the side of UT

md  f2BBB [(1-r)2+h2] circle centered in (r,h)=(1,0)

f2BBB known at 15% from LQCD

  • many theoretical uncertainties cancel in the ratio
    • |Vts|/|Vtd| can be determined at ~4%


Experimental challenge:

|Vts| >> |Vtd|  ms >> md  needs to resolve > 2.3 THz oscillations

Status of Dms measurements:

LEP/SLD/CDF-I: ms > 14.4 ps-1 @ 95% CL HFAG Average for PDG 2006

D0 Run-II: ms  [17,21] ps-1 @ 90% CL Phys. Rev. Lett. 97, 021802 (2006)

road map to m s measurement

vertexing (same) side






“opposite” side


Road map to ms measurement


  • Collect as many Bs as possible
    • Tevatron, Trigger (SVT)
  • Extract Signal
    • Bs flavor at decay inferred from decay products
  • Measure proper decay time of the Bs meson
    • L00, per event primary vertex, candidate specific decay time resolution
  • Determine Bs flavor at production (flavor tagging)
    • PID (TOF, dE/dx)
    • Flavor tag quantified by Dilution: D=1-2w, w = mistag probability
  • Measure asymmetry between unmixed and mixed events
    • In practice: perform likelihood fit to expected unmixed and mixed distributions
event selection fully hadronic b s
Event Selection: Fully Hadronic Bs

used in this analysis

  • Bs momentum completely reconstructed
  • Excellent decay time resolution, good S/N
  • Small BR  low statistic
  • Good sensitivity at high values of ms

Cleanest decay mode:

BsDs[] [KK] 

event selection semileptonic b s
Event Selection: Semileptonic Bs

Ds Mass

  • Missing momentum ()
  • Poorer decay time resolution
  • Large BR  high statistic
  • Good sensitivity at low values of ms

l+Ds Mass

48000 l+Ds candidates, 75% are from Bs decay

  • Minv(l+Ds) helps reject BG
  • BG Sources:
    • Ds + fake lepton from PV
    • Bs,dDsDX (DslnX)
    • cc
proper decay time reconstruction








RUN 304720 EVENT 109026

Proper decay time reconstruction


Detector length scale and proper treatment of detector/selection biases controlled by performing lifetime measurements

decay time resolution
Decay time resolution
  • Finite resolution dilutes the amplitude of mixing asymmetry:
  • Sensitivity maximized by making full use of all available information:
    • layer-00, candidate specific primary vertex and decay time resolution
  • Resolution measured in data in large samples of prompt D meson decays
    • D+ combined with prompt tracks to mimic B0-like topologies

oscillation period

@ ms=18 ps-1

M(lDs) > 3.3 GeV/c

first bin of ct

4 sampling per cycle

Hadronic decays gives CDF sensitivity at much

larger values of ms than previous experiments

flavor tagging performances
Flavor Tagging Performances

Two 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











ct [cm]

pT [GeV/c]

Courtesy of J.Kroll


Data fitted with an unbinned likelihood function to the expected unmixed and mixed distributions

Procedure checked on B0 by fitting for md

for each event:





(*) H-G.Moser, A.Roussarie,

NIM A384 (1997)

Amplitude method(*): scan ms space: fix msfit for A:

A consistent with 1  mixing detected at the given ms


A/A (17.3 ps-1) = 3.7



Inputs from PDG 06 andξ=1.210 (hep-lat/0510113)


Likelihood ratio:

A=1 VS A=0 hypothesis

hep-ex/0606027 – accepted by PRL

P-value = 0.2% (>3)

small systematic uncertainty

dominated by knowledge of the absolute scale of the decay-time measurement

impact on the overall ut fit
Impact on the overall UT Fit

SM fit

SM+NP fit



CBs = 0.97 ± 0.27

CKM fit (no Δms)

(21.5 ± 2.6) ps-1

no angles

angles only

UTfit Coll.: hep-ph/0605213

and Vincenzo’s talk

Similar results from CKMfitter group: http://ckmfitter.in2p3.fr and Stephane T’Jampens talk

work in progress

CDF Run II Preliminary L=1 fb-1

Work in progress

BsDs+-+ (Ds +--)

  • Collecting new integrated luminosity
  • Squeezing maximum information from the data
  • we already have:
  • Systematic use of Neural Networks in signal extraction:
    • use decays modes previously discarded cause high BG
    • more signal in already used modes
  • Use partially reconstructed BsDs*/K and Ds:
    • large BR
    • good momentum resolution
  • Improve Flavor taggers:
    • OST: +15% D2
      • NN to combine OS taggers
      • OSKT
    • SSKT: ~+10% D2
      • better use of combined PID and kinematics

NBs = 220

BsDs+ (Ds-)

summary and outlook
Summary and Outlook
  • CDF finds evidence for flavor oscillations in the Bs sector
  • Probability of a random fluctuation 0.2%
  • Measurement of the mixing frequency with <2% precision
  • Most precise measurement of |Vtd/Vts|

An important and precise experimental input for the overall test of the SM and the end of a very long effort to measure ms

… but not the end of the CDF B-physics programme


Random Slides

S.Giagu - ICHEP 2006, Moscow

data sample




d0 = impact parameter

Data Sample
  • Bs candidates collected by SVT trigger
  • TTT: two displaced tracks
  • L+SVT: lepton + displaced track(s)

used in this analysis

Typical inst. Luminosity 1032 cm-2 s-1

~1.4 fb-1 collected by CDF

~1 fb-1 (good runs) used in this analysis

S.Giagu - ICHEP 2006, Moscow

other results on m s
Other results on ms


Recent from D0 collaboration

1st direct single experiment upper bound

ms  [17,21] ps-1 @ 90% CL

Null hypothesis probability: 5%

ms > 14.4 ps-1@ 95% CL

D0 Coll.: Phys. Rev. Lett. 97, 021802 (2006)

HFAG Average for PDG 2006

S.Giagu - ICHEP 2006, Moscow

example of specific trigger for b physics




d0 = impact parameter

Example of Specific Trigger for B Physics

Level 1

- 2 XFT tracks with pT > 1.5 GeV

- opposite charge

-  < 135o

- |pT1| + |pT2| > 5.5 GeV

Level 2

- confirm L1 requirements

- both XFT tracks

- SVT 2<15

- 120 m< |d0| <1mm

- 2o <  < 90o

- Decay length Lxy > 200m

Level 3

- confirm L2 with COT & SVX

“offline” quality track reco.

S.Giagu - ICHEP 2006, Moscow

semileptonics correction for missing momentum

Correction Factor (MC)

Decay Time

Reconstructed quantity

Semileptonics: Correction for Missing Momentum

oscillation period

@ ms=18 ps-1

S.Giagu - ICHEP 2006, Moscow


Separartion Power

Combined PID: TOF + dE/dx


S.Giagu - ICHEP 2006, Moscow

systematic uncertainties
Systematic Uncertainties



  • related to absolute value of amplitude, relevant only when setting limits
    • cancel in A/A, folded in in confidence calculation for observation
    • systematic uncertainties are very small compared to statistical

S.Giagu - ICHEP 2006, Moscow

incertezze sistematiche su m s
systematic uncertainties from fit model evaluated on toy Monte Carlo

have negligible impact

only relevant systematic: knowledge of lifetime scale

Incertezze sistematiche su ms

All relevant systematic uncertainties are common

between hadronic and semileptonic samples

S.Giagu - ICHEP 2006, Moscow

amplitude scan hadronic decays
Amplitude Scan: Hadronic decays

data period 1

data period 2

data periodo 3

S.Giagu - ICHEP 2006, Moscow

amplitude scan semileptonic decays
Amplitude Scan: Semileptonic decays

data period 1

data period 2

data period 3

S.Giagu - ICHEP 2006, Moscow

parameterization of the tagging decision


Parameterization of the tagging decision
  • Exploit peculiarity of each tagger to minimize mistag probability
  • example: soft muon tag

 from b decay

jet axis

 from c decay

S.Giagu - ICHEP 2006, Moscow

sskt calibration
SSKT Calibration
  • Dilution measured in high statistic samples of light B meson decays and compared with the results of simulation

Dominant source of systematic uncertainty: Data/MC agreement ~O(14%)

S.Giagu - ICHEP 2006, Moscow

negative log likelihood ratio
Negative log likelihood ratio

S.Giagu - ICHEP 2006, Moscow