s. bolognesi & m.a. borgia for the cp-violation exam
DESCRIPTION
A measurement of the B 0 B 0 oscillation frequency and determination of flavor-tagging efficiency using semileptonic and hadronic B 0 decays. S. Bolognesi & M.A. Borgia for the CP-violation exam. Introduction. The strategy The experimental environment. S. Bolognesi & M.A. Borgia. - PowerPoint PPT PresentationTRANSCRIPT
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A measurement of the B0B0 oscillation frequency and
determination of flavor-tagging efficiency using semileptonic and
hadronic B0 decays
S. Bolognesi&
M.A. Borgia
for the CP-violation exam
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Introduction
The strategy The experimental environment
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The measurement
• top contribute is dominant sensible to Vtd element of CKM matrix
one B reconstructed in a flavour eigenstate (Brec)
one B only tagged as B0 or B0 from its decay products (Btag)
• mixed if same flavor / unmixed if opposite flavor
PDF for the two categories (mixed + / unmixed -)
“dilution factor” due to mistag rate
distance between Brec and Btag decay (≈ B = 1.548±0.032 ps)
if perfect flavour taggingť
•
•
• time resolution function with parameters
B0B0 mixing through NLO EW diagrams involving exchange of up-type quarks
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Likelihood and time independent analysis
Likelihood = sum over all events (mix. & unmix.) and over different tag types (with its own Di)
minimized to extract simultaneously md, Di (and some ai)
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Time-independent analysis = neglecting background and assuming Brec correct identification, the observed time-integrated fraction of mixed events χobs as a function of B0B0 mixing probability χd can be expressed as:
where ω is the mistag rate, and χ d = ½ xd2/(1+xd2) = 0.174 ± 0.009 and xd2 = Δmd/Γ
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BaBar detector DCH + SVTdetection and momentum measurement for charged particles
SVTvertex information
DIRC
z ≈ 50 m for Brec
z ≈ 100-150 m for Btag
particle identification (charged hadrons)
DCHparticle identification (dE/dx)
EMC photons, electrons and neutral hadrons
IFR (RPC) muons and neutral hadrons
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8.9 fb-1 @ Y(4s) +
0.8 fb-1 @ 40 MeV below Y(4s)
(10.1 ± 0.4) × 106 BB pairs
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Particle identification Electrons
• track + EMC (shower shape, E/p)
• dE/dx in DCH
• Cherenkov angle in DIRC
efficiency 92%
mistag () 0.3%
Muons
• interaction lenghts and # hits in IFR• MIP in EMC
efficiency 75%
mistag () 2.5%
Kaons
15combKcomb
L
L where
comb SVT DCH DIRCL L L L
dE/dx• Cherencov angle• # photonsefficiency 85%
mistag () 5%
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Time resolution(t) dominated by (z) (z) dominated by (zBtag)
pseudo-track extrapolated from the interaction point in the Btag direction
reconstruct Brec
compute the Btag direction from the energy conservation
Btag vertex = intersection of pseudo-track with all the other tracks
Brec
Beam Spotpseudo track (Btag)
z
z ≈ 260 mz) ≈ 180 m
Resolution function is the sum of three gaussians
(3 parameters from MC3 parameters from fit)
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Flavor tagging
4 strategies to define if Btag is B0 or B0 4 tagging cathegories
Lepton tag: presence of a prompt lepton (pCM>1.1 GeV against charm semileptonic decay)
Kaon tag: total kaons charge not 0
2 neural network cathegories:
5 neural network algorithm
4 based on tracks1 exploits the charge of high momentum particles
whose outputs are combained in a single full neural network tagger (xNT [-1,1])
NT1
NT2
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Analysis
Hadronic decay channels Leptonic decay channels
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Hadronic decaysB0
rec D* - + ( / + / a1+ )
D0 -
K+ -
K+ - 0
K+ + - -
K0s + -
D - + ( / + / a1+ )
K+ - -
K0s -
B0rec
B0rec J/ K*0
e+ e- / + -
(K0s → + -, 0 → )
Usual cuts on intermediate/final particles: resonances invariant mass (±2) vertex 2 threshold on momenta opening angle between decay products
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B0 candidates characterized by
beam energy sobstituted mass:5.270 < mES < 5.290 (GeV)
EB0 – Ebeam in Y(4s) CM:|E| < 3E where E = E resolution (19 -40 MeV)
Cuts against continuum (e+e- → qq) normalized second Fox-Wolfram moment
(R2=H2/H0) < 0.5
large angle between thrust axis of B0 and of the remaining tracks
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Backgrounds (HD*)
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Data extracted from fit to the mES distrbution
* H
D =
Had
roni
c de
cays
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Semileptonic decays
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*0 DB
0D
0
K
K
K
D0 candidates: combination with all charged tracks (pTmin 50 MeV/c and charge opposite to that of the candidate K) => D* candidates
Usual cuts on intermediate/final particles: invariant mass (±2) around nominal D0 mass vertex 2 > 1% threshold on momenta
Mass difference: m(D*-)-m(D0) (± 2.5σ) of the nominal value
D* candidates:
D*- , pl > 1.2 GeV back-to-back => cosθ(D*- ) < 0
Neutrino existence consistency:
lDB
lDBlDBlDB pp
EEMMlDBpppp
*
*2
*2
22* 2
2)*,(cos0)(
(solving in the Υ(4s) system frame)
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Sample composition
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After cuts, 7517±104 B→D*lν events3101±64 in the mode1986±51 in the mode2430±56 in the mode
KD0
00 KD KD
0
Mass difference distributions for each flavor tagging category
Backgrounds are larger for semileptonic modes than
for hadronic modes
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Background
Combinatorial
•Due to falsely reconstructed D* candidates•Estimated by fitting Δm(D*-D0) distributions•Gaussian + threshold function with a sharp rise followed by exponential tailoff•Signal region within ±2.5σ of the peak in Δm(D*-D0) •Combinatorial background control sample provided by the sidebands region
0.150 < Δm(D*-D0) < 0.160 GeV/c2
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Three types of background to B0→D*lν :
• Combinatorial background
• Wrong-lepton background
• B+ background
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Wrong-lepton Wrong-lepton:D* combination with wrong lepton
Four potential sources:• ”Fake lepton”
(estimated selecting events in which a track candidate has failed very loose lepton criteria is
substituted for the lepton candidate)
• Real D* from one B + real lepton from the other B (“uncorrelated lepton” bg) (estimated by parity-inversion of the lepton momentum in the Y(4s) frame => control sample)
• Events of the type B0→D*DX in which the D decays semi-leptonically produce
a non-primary lepton (estimated with Monte Carlo, less than 1% => neglected)
• cc events producing real D* and lepton in back-to-back configuration.
(estimated using combinatorial-subtracted off-resonance data)
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B+ background
Due to B-decays which involve additional final state particles (B→D*(nπ)lν)
• B0→D*(nπ)lν that pass selection criteria are considered as signal (they
contribute to the measurement of Δmd and the additional low momentum π
does not affect the tagging algorithm)
• B-→D*+(nπ)l-ν considered as bg: they do not oscillate and must be corrected
for in extracting Δmd and their mistag rate may differ from that of B0 decays
as well
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Results
Likelihood fit results Time integrated method results
Combined results
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Backgr. treatment (LM*) PDF must be extended with background contributions
b = background sourcesi = tagging cathegories)
f = fraction of signal or background events
B = empirical description of t distribution in background events
(where
Fit to the background control samples (mES sidebands) to determine time dependence, dilution factor, resolution function:
three components
for each background source
• zero lifetime:
• non zero lifetime, no mixing:
• non zero lifetime with mixing:
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* LM = Likelihood method
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t distribution (LM*)
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* LM = Likelihood method
HADRONIC SAMPLE LEPTONIC SAMPLE
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Time dependent asymmetry a(t) (LM*)
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* LM = Likelihood method
HADRONIC SAMPLE LEPTONIC SAMPLE
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Fit results (LM*)
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* LM = Likelihood method
Identical analysis procedure on MC data with detailed detector simulation:
fit results consistent with a priori insterted value and MC truth information
observed differences applied as a correction to the measured values
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Systematic errors (LM*)
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* LM = Likelihood method
HADRONIC SAMPLE
Statistical error dominant, followed by MC correction uncertainties (t for md)
Systematic error dominant due to big uncertanties in background characteristic (t for md)
LEPTONIC SAMPLE
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Time integrate (single bin) method
First aim: measurement of the mistag rate
Main feature: restriction of the sample to events in a single optimized Δt interval (| Δt | < 2.5 ps because of Babar vertex resolution)
Events with | Δt | > 2.5 ps have on average equal numbers of mixed and unmixed events => contribute nothing to the determination of the mistag rate
Considering the different background contribution:
ff ddsobs ))21((
fs, fβ = fraction of signal and background source
χβ = fraction of mixed events in each background source
χobs = Observed fraction of mixed events
χ d = ½ xd2/(1+xd2) and xd2 = Δmd/Γ, while χ’d takes into account the sample restriction
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=>
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Results
HadronicSignal region defined as events with mES > 5.27 GeV/c2
Fraction of mixed events in the background determined by tag category using the sideband control sample, mES < 5.27 GeV/c2
Semileptonic - bg evaluated for each tag category and for each D0 decay - mistag fractions calculated individually by tag category and decay
mode using the Eq. shown - combination of the different decay modes, using the statistical errors to weight the individual results
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Systematic errors
• Hadronic
• Semileptonic
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Sources of systematic error for the mistag measurement on the hadronic and semileptonic samples
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Comparison between the two methods
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Combining results for the hadronic and semileptonic B samples for the likelihood fit method and for the single-bin method and taking
into account the systematic errors
Preliminary mistag rate
Single bin fit uses a subset of the sample used for the other method
The two sets of results are uncorrelated
Good agreement between the two methods. Final result: Q ≈ 0.28
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Final result
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Hadronic sample:
Leptonic sample:
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Back-up slides
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Fox-Wolfram momentsThe Fox-Wolfram moments , , are defined by
is the opening angle between hadrons and
the total visible energy of the event
are the Legendre polynomials
To the extent that particle masses may be neglected, . It is customary to normalize the results to , i.e. to give .
2-jet events tend to give for even and for odd.
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