Transcript Slide 1

Evidence for Bs Mixing and
measurement of ms at CDF
S. Giagu and CDF Collaboration
University of Rome “La Sapienza”
INFN Sezione di Roma 1
XXXIII International Conference On High Energy Physics, 28 July 2006 - Moscow
Outline
•
•
•
•
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
2
3
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
s,
s,
s,
s,
Mixing involves CKM elements,
measuring Δmq constraints
the unitarity triangle
New exotic particles may
run in the loop
 mixing sensitive to NP
GF2 mW2 S ( xt2 )
2
*
2
mq 
m
f
B
|
V
V
|
Bq Bq Bq
tq tb
2
6
S.Giagu - ICHEP 2006, Moscow
Form factors and B-parameters
from Lattice calculations are
known at ~15% level
ms and the side of UT
4
md  f2BBB [(1-r)2+2]  circle centered in (r,)=(1,0)
f2BBB known at 15% from LQCD
ms mBs f BBs

md mBd f BBd Vtd
2
Bs
2
Bd
Vts
2
2

Vts
mBs 2

mBd
Vtd
many theoretical uncertainties cancel
in the ratio
0.047
(hep-lat/0510113)


1
.
210
0.035
•
• |Vts|/|Vtd| can be determined at ~4%
Experimental challenge:
|Vts| >> |Vtd|  ms >> md  needs to resolve > 2.3 THz oscillations
Status of ms measurements:
LEP/SLD/CDF-I: ms > 14.4 ps-1
@ 95% CL
D0 Run-II:
ms  [17,21] ps-1 @ 90% CL
HFAG Average for PDG 2006
Phys. Rev. Lett. 97, 021802 (2006)
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2
Road map to ms measurement
vertexing (same) side
e,,Jet
Pnomix  Pmix
A(t ) 
 D cosm  t 
Pmix  Pnomix
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4
2
4
1
1
“opposite” side
1.
•
2.
5
A

D S
2
2
e

ms t 2

2
3
Collect as many Bs as possible
Tevatron, Trigger (SVT)
Extract Signal
•
3.
•
4.
•
•
5.
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
S
SB
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
Decay mode
Events
BsDs ()
1600
Bs Ds (K* K)
800
Bs Ds  (3)
600
Bs Ds3 ( )
400
Bs Ds3 (K*K)
200
Total
3600
Cleanest decay mode:
BsDs[] [KK] 
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Event Selection: Semileptonic Bs
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Ds Mass
Ds    , K *0 K  ,    
• Missing momentum (n)
• Poorer decay time resolution
• Large BR  high statistic
• Good sensitivity at low values of ms
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
l+Ds Mass
Proper decay time reconstruction
D
decay
B
decay
Lxy
PV
RUN 304720 EVENT 109026
L
m( B)
ct 
 Lxy 

pT B 
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-+
1.508  0.017
1.530  0.009
B-  D0-
1.638  0.017
1.638  0.011
Bs  Ds()
1.538  0.040
1.466  0.059
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9
Decay time resolution
• Finite resolution dilutes the amplitude of mixing asymmetry:
D t  e
( m t ) 2

2
• 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
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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
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)
Same-side kaon tag increases effective statistics  ~4
Courtesy of J.Kroll 11
Likelihood
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:
k
k
k
k
k
k=sig,bg
sig =
D
St
pdg
pT [GeV/c]
isolation
Amplitude method(*): scan ms space: fix ms fit for A:
A consistent with 1  mixing detected at the given ms
K-factor
(*)
ct [cm]
H-G.Moser, A.Roussarie,
NIM A384 (1997)
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Results
Likelihood ratio:
A=1 VS A=0 hypothesis
hep-ex/0606027 – accepted by PRL
A/A (17.3 ps-1) = 3.7
1


ms  17.3100..33
stat
.

0
.
07
(
syst
.)
ps
18
V td
Vts
001
 0.20800..002
( stat.  syst .)  00..008
006 (theo.)
+0.047
Inputs from PDG 06 and ξ=1.210 -0.035 (hep-lat/0510113)
P-value = 0.2% (>3)
small systematic uncertainty
dominated by knowledge of the
absolute scale of the decay-time
measurement
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Impact on the overall UT Fit
SM fit
SM+NP fit
CDF
measurement
CBs = 0.97 ± 0.27
CKM fit (no Δms)
(21.5 ± 2.6) ps-1
full
Bs
A
no angles
angles only
r  0.197  0.034
  0.397  0.025
r  0.203  0.055
  0.316  0.025
 A CBs e
SM
Bs

2 i  BSM
 Bs
s
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
• Collecting new integrated luminosity
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BsDs+-+ (Ds +--)
CDF Run II Preliminary
L=1 fb-1
• 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 BsDs*/K and Dsr:
• large BR
• good momentum resolution
3. Improve Flavor taggers:
•
•
NBs = 220
OST: +15% D2
• NN to combine OS taggers
• OSKT
SSKT: ~+10% D2
• better use of combined PID and kinematics
BsDs+ (Ds-)
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Summary and Outlook
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•
•
•
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|
0.33
 0.18
ms  17.31
V td
Vts
stat.  0.07( syst .)
ps
1
001
 0.20800..002
( stat.  syst .)  00..008
006 (theo.)
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
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Random Slides
S.Giagu - ICHEP 2006, Moscow
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Data Sample
Bs candidates collected by SVT
trigger
used in this analysis
• TTT:
two displaced tracks
• L+SVT: lepton + displaced track(s)
PV
Decay
Vertex
d0 = impact parameter
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 ms
LEP, SLD, CDF-I
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Recent from D0 collaboration
1st direct single experiment upper bound
ms > 14.4 ps-1@ 95% CL
HFAG Average for PDG 2006
S.Giagu - ICHEP 2006, Moscow
ms  [17,21] ps-1 @ 90% CL
Null hypothesis probability: 5%
D0 Coll.: Phys. Rev. Lett. 97, 021802 (2006)
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
PV
Decay
Vertex
d0 = impact parameter
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Semileptonics:
Correction for Missing Momentum
Reconstructed quantity
S.Giagu - ICHEP 2006, Moscow
Correction Factor (MC)
oscillation period
@ ms=18 ps-1
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Decay Time
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PID
Separartion Power
Combined PID: TOF + dE/dx

S.Giagu - ICHEP 2006, Moscow
K
Systematic Uncertainties
Hadronic
Semileptonic
• 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
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Incertezze sistematiche su ms
• systematic uncertainties from fit
model evaluated on toy Monte
Carlo
• have negligible impact
• only relevant systematic:
knowledge of lifetime scale
Syst. Unc
All other syst.
< 0.01ps-1
SVX Alignment
0.04 ps-1
Track Fit Bias
0.05 ps-1
PV bias from
tagging
0.02 ps-1
Total
0.07 ps-1
All relevant systematic uncertainties are common
between hadronic and semileptonic samples
S.Giagu - ICHEP 2006, Moscow
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Amplitude Scan: Hadronic decays
data period 1
data period 2
data periodo 3
S.Giagu - ICHEP 2006, Moscow
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Amplitude Scan: Semileptonic decays
data period 1
data period 2
data period 3
S.Giagu - ICHEP 2006, Moscow
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Parameterization of the tagging decision
• Exploit peculiarity of each tagger to minimize mistag probability
• example: soft muon tag
 from b decay
jet axis

ptrel
 from c decay
S.Giagu - ICHEP 2006, Moscow
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
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Negative log likelihood ratio
S.Giagu - ICHEP 2006, Moscow
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