M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China _ A Search for D 0 D 0 mixing in...
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Transcript of M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China _ A Search for D 0 D 0 mixing in...
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
__ A Search for DA Search for D00DD0 0 mixing in mixing in
Semileptonic decays, A search for CP Semileptonic decays, A search for CP Violation in DViolation in D++ K K++KK-- decays decays
and a Measurement of and a Measurement of the Branching ratiothe Branching ratio
Milind V. Purohit
Univ. of South Carolina
(for the BaBar collaboration)
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
The BaBar Collaboration
China [1/5]Inst. of High Energy Physics, Beijing
Germany [4/28]Ruhr U BochumTU DresdenPhys. Inst., HeidelbergU Rostock
France [5/60]LAPP, AnnecyEcole PolytechniqueLAL OrsayLPNHE des Universités Paris 6/7CEA, DAPNIA, CE-Saclay
United Kingdom [10/72]U of BirminghamU of BristolBrunel UniversityU of EdinburghU of LiverpoolImperial CollegeQueen Mary & Westfield CollegeRoyal Holloway, University of LondonU of ManchesterRutherford Appleton Laboratory
Italy [13/105]INFN BariINFN FerraraINFN Frascati INFN GenovaINFN MilanoINFN NapoliINFN PadovaINFN PaviaINFN PerugiaINFN PisaINFN RomaINFN TorinoINFN Trieste
Canada [4/19]U of British ColumbiaMcGill UU de MontréalU of Victoria
Norway [1/3]U of Bergen
Russia [1/11]Budker Inst., Novosibirsk
Netherlands [1/5]NIKHEF
USA [38/301]Caltech, PasadenaUC, IrvineUC, Los AngelesUC, San DiegoUC, RiversideUC, Santa BarbaraUC, Santa CruzU of CincinnatiU of ColoradoColorado StateFlorida A&MHarvardU of IowaIowa State ULBNLLLNLU of LouisvilleU of MarylandU of MassachusetsMITU of MississippiMount Holyoke CollegeU of Notre DameOhio StateU of OregonU of PennsylvaniaPrairie View A&MPrincetonSLACUniv. of South CarolinaStanford USUNY, AlbanyU of TennesseeU of Texas at DallasU of Texas, AustinVanderbiltU of WisconsinYale U
(78 institutions,
609 Collaborators)
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
The BaBar Detector
•Silicon Vertex Tracker z hit resolution 15m•Drift CHamber (pT)/pT = 0.14% pT + 0.45%
•Detector of Internally Reflected Cherenkov light K- separation 4.2 @ 3.0GeV•ElectroMagnetic Calorimeter E/E = 2.3%E-1/4 1.4%
•RPC based Instrumented magnetic field Flux Return 18/19 layers of RPC in 60/65 cm of iron
~250 fb-1 of data collected so far.
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Searching for New Physics in Charm CP violation with BaBar data
CP violation in charm is expected to first manifest itself in Singly Cabibbo-suppressed (SCS) decays.
Within the Standard Model, one expects CP violation asymmetries in SCS Decays ~10-3, while New Physics can give CP violation asymmetries ~10-2.
[G. Burdman & I. Shipsey, Ann. Rev. Nucl. Part. Sci., 2003, hep-ph/0310076. See also S. Bianco, F. L. Fabbri, D. Benson & I. Bigi, hep-ex/0309021.]
Current experimental limits are ~ (2 – 5) x 10-2 leaving a considerable window for new physics discovery.
The analysis reported here is based on ~43,000 D+ KK decays from ~80 fb-1. [BaBar has ~250 fb-1 of data.]
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Standard Model interfering amplitudes:
A ~Vcs*¢ Vus A ~ Vcs
*¢ Vus, Vcd*¢ Vud, Vcb
*¢ Vub
•In the Wolfenstein parameterization, the CKM matrix (below) clearly gives only a small CP violating asymmetry.
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Data Sample and Analysis• Our final sample contains ~43,000 D+ KK decays
from ~80 fb-1.• We measure the asymmetry
where
• We also measure the asymmetry in the & K* regions.
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
KK Yields
21632 ± 228 D+ events 20940 ± 226 D- events
23066 ± 217 Ds+ events 22928 ± 214 Ds
- events
•Likelihood ratio cut uses p* and beam-spot constrained 2
•Kaons have kaon ID, pions must not have kaon ID
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
ACP in , Regions of KK Dalitz Plot
5452 ± 87 D+ events 5327 ± 86 D- events
• mass is required to lie within 10 MeV/c2 of nominal mass
• |cos(H)| is required to be ≥ 0.2 ; H is helicity angle in rest frame
1. The region:
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
5247 ± 96 D+ events 5113 ± 96 D- events
• K* mass is required to lie within 50 MeV/c2 of nominal K* mass
• |cos(H)| is required to be ≥ 0.3 ; H is helicity angle in K* rest frame
2. The region:
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Systematic Errors on ACP
• Systematic errors on ACP estimated as:– Largest difference using other normalizations (0.8%)– Largest uncorrected asym. in control samples (1.1%)– From table (in units of 10-2):
Source KK K*0K
MC simulation 0.06 0.06 0.06
Background estimate 0.63 0.32 0.49
Event Selection 0.51 0.56 0.54
Total 0.81 0.65 0.73
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
ACP Results
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
The Branching Ratio (D+ K-K+) / (D+ K-)
• Using measured yields and efficiencies in bins of the Dalitz plots, the total branching ratio is determined: 0.107 ± 0.001 ± 0.002
• Sources of systematic errors:Source Error (10-2)
PID, tracking 0.21
Background estimate 0.05
Event Selection 0.02
Total 0.22
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
The Branching Ratio Compared
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Searching for New Physics in mixing with BaBar data
For decays of neutral Ds Rmix – The standard model predicts a low rate (~10-7) of for the
box diagram which goes up to ~10-3 when long distance effects are included; nevertheless we would like to observe mixing as a first step.
– New physics effects can easily produce > 10-7 rates of mixing, and are the only way we can get CP violation in mixing.
[G. Burdman & I. Shipsey, Ann. Rev. Nucl. Part. Sci., 2003, hep-ph/0310076. See also S. Bianco, F. L. Fabbri, D. Benson & I. Bigi, hep-ex/0309021.]
The analysis reported here is based on ~50,000 semi-electronic decays of neutral D mesons from ~87 fb-1 with mixing rate sensitivity down to ~10-3.
DD M ixing
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
(A. Petrov, hep/ph 0311371)
• Possible enhancements due to new particles and interactions in new physics models:
•gluinos, squarks•fourth generation quarks
•lepto-quarks, etc.• No CP-violating effects
expected in SM -- CP violation in mixing would be unambiguous signal of new physics
x=M/y=/2
x=M/
mix
ing
ra
te =
|am
pli
tud
e|2
mix
ing
ra
te =
|am
pli
tud
e|2
New Physics Mixing Predictions
current experimental sensitivity
Charm Mixing, continuedSM Mixing Predictions
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
cc continuum event topology
K
e,
slow
other charmed hadron(will hadronically reconstruct in next iteration of analysis)
interaction point
beamspot D0
D*+ D0 +tag
D0 K+e-
D*- D0 -tag
D0 K-e+
Wrong-sign mixed decays
D*+ D0 +tag
K-e+
D*- D0 -tag
K+e-
Right-sign unmixed decays
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
The Analysis Strategy
4exp)(
222
00
yxtttDD
WS
exponentialenvelope
Quadratic time dependence
mixing rate
• We use neutral D mesons from D*+ decays: D*+ D0
• Flavor at birth is tagged by pion from D* decay• Flavor at decay is tagged by electron: D0 (K-/K*-) e+e
Clearly, an e+ signifies a RS (Right Sign, or unmixed) D0
while an e- would signify a WS (Wrong Sign or mixed) D0
• The mixing rate is given by
• where x M / , y / 2 • No DCS decay background exists
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Semileptonic Mixing Analysis Technique
• A neural network event selector is used.• Another neural network is used for p*(D0) reconstruction.
(p* is the c.m. momentum)• An unbinned extended maximum likelihood fit
using M & transverse lifetime is then done, where M m(D0) – m(D0)• First, a fit to high-statistics RS sample gets
– signal M shape and – unmixed D0 lifetime
for use in WS pdf and to get N(unmixed), the normalization for Rmix.
• Subsequent fit to WS sample for N(mix)• Rmix ≈ N(mix) / N(unmixed)• Data sample: 80 fb-1 on resonance, 7.1 fb-1 off-resonance
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
• Unbinned extended maximum likelihood fit to transverse lifetime and M = M(D*)-M(D0) with 15 floated parameters
RS Unmixed Fit to 87 fb-1 of Data
D+D0 bkgdzero life
M signal region
M sideband
D+
D0 bkgdzero life
lifetime tails
D0 sgnlD0 sgnl
Results of fit:
• Unmixed D0 yield: 49620 ± 324 evts (stat)
• M and lifetime pdfs
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
WS Mixed D0 Fit to 87 fb-1 of Data
•Mixed signal pdf parameters taken from high-statistics unmixed data fit– Ni and zero lifetime triple Gaussian
parameters floated (11 parameters)
Random D+
Random D0
Zero Life
M projection D0 signal
Peaking D+
•Distribution of N(mix) from fits to an ensemble of 170 WS generic MC datasets with zero embedded mixed signal events
(~5% probability of getting a larger result for Rmix=0)
•Unblinded N(mix): 114 ± 61 evts
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
WS Mixed D0 Fit to 87 fb-1 of Data
zero life
Lifetime projection showing mixed signal
random D+
random D0
D0 signal
pkng D+
Full lifetime projection
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
stat stat + syst
90% Confidence limiton number of mixingsignal.
Final result for semileptonic mixing• The fit gives a yield of N
mix=114 ± 61 wrong sign signal events.
• Systematics evaluated as fraction of statistical error:
• Normalising to number of right-sign events gives result on mixing.
Rmix 0.0023 0.0012 stat 0.0004 syst
Rmix 0.0042 90% C.L.
Systematic ErrorMixed m PDF 0.27Mixed decay time PDF 0.06Combinatoric m PDF 0.13Bkg D0 decay model 0.13Bkg D+ decay model 0.10Total systematic 0.35Total stat. + syst. 1.06
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Comparison to other results
Summary of all BaBar D mixing measurements:
•Hadronic (K) analysis:
Rmix < 1.3 x 10-3
•Lifetime difference for D0 KK
and D0
yCP = 0.8 ± 0.4 +0.5 -0.4 %
•This (semileptonic) analysis: Rmix < 4.2 x 10-3 (90% CL)
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Conclusions
• We have demonstrated that
ACP is consistent with zero. The measured values are:
D+ KK+0.0136 ± 0.0103 ± 0.0110
D+ +0.0024 ± 0.0152 ± 0.0080
D+ K*K+0.0088 ± 0.0177 ± 0.0080
• We find that the Branching Ratio for D+ KKis:
0.1070 ± 0.0009 ± 0.0022
• We find, using semi-electronic neutral D decays, that
Rmix < 4.2 x 10-3 (90% CL)
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Acknowledgements
• Thanks to all BaBar collaborators including, particularly:
• Francisco Yumiceva, South Carolina graduate student (D+ K+K- decays)
• Kevin Flood, graduate student at the
Univ. of Massachusetts, Amherst
(semileptonic mixing analysis)
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Backup Slides
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
The Status of CP Violation
CP violation is necessary if the universe is indeed matter-antimatter asymmetric but the Big Bang is not
Standard Model CP violation cannot explain CP violation in the universe
New physics is needed, SM is not enough
Charm may be a good place to look
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Raw CP asymmetries in control samples
• Ds+ K+K- decays: +3.0 x 10-3
• D+ K-+ decays: -2.9 x 10-3
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Event Sample
• Nearly all Run 1, 2 on/off-resonance (R10 processing)
–80.0 fb-1 on-resonance
–7.1 fb-1 off-resonance
mode N events (x106) equiv lumi (fb-1)b0 154 294b+ 156 298cc 185 142uds 293 140
• SP4 generics
• Wrong-sign mixed D0 lifetime: 100k events
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Track Selection• Pion candidates
– GoodTracksVeryLoose– .45 < lab < 2.5– p* < 0.45 GeV/c– track fit prob. > 0.001– beamspot refit prob. > 0.01– ≥ 2 SVT r-phi (z) hits with at
least 1 hit on inner 3 r-phi (z) layers
– ≥ 6 total SVT hits
• Multiple D0 candidates– Veto events with more than
one RS or WS D0 candidate passing all selection criteria (~11% of RS signal lost after other cuts)
• Kaon candidates– GoodTracksVeryLoose– KLHVeryTight
• plus KMicroVeryTight (plab > 2.1 GeV/c)
– .45 < lab < 2.5
• Electron candidates– GoodTracksVeryLoose– PidLHElectrons (default)– .45 < lab < 2.409– 0.8 < E/p < 1.05– conversion veto
• K/e vertex– GeoKin vertex prob. > 0.01– M(KeVtx) < 1.82 GeV/c2
– Lifetime error < 2 D0 lifetimes
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Neural Network Event Selection
• Events are selected using a neural network with the following inputs taken from signal and generic MC:
– p*(K/e vertex)– p*()– thrust magnitude (w/o K, e)– Opening angle between p*(K/e) and
thrust (w/o K, e)– opening angle between p*(K) and p*(e)
• RS/WS signal selection efficiencies are identical
• RS/WS signal/bkgd NN event selector output
signal
bkgd
Lumi-scaled NN output
signal
bkgd
Normalized NN output
final cut optimized for best statistical sensitivity
final cut optimized for best statistical sensitivity
final cut
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
•Novel use of two hidden-layer neural network to map NN input vector to signal p*(D0)
• Momentum resolution fit to double gssn– phi core sigma/fraction: 82 mrad / 0.81– theta core sigma/fraction: 80 mrad / 0.94– total magnitude RMS: 371 MeV/c– transverse magnitude RMS: 350 MeV/c
•Trained with inputs from signal MC:– p*(K/e vertex)– p*()– thrust vector (w/o K, e)– Opening angle between p*(K/e) and thrust (w/o
K,e)– Opening angle between p*() and thrust (w/o
K,e)– opening angle between p*(K) and p*(e)– opening angle between p*(K/e) and p*()
Neural Network D0 Reconstructionp*(D0) Neural Network Residuals
GeV/c
GeV/c
rad
rad
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
zero lifetime
D+
WS mixed D0
RS unmixed D0 mis-reco’d D0
1-d lifetime pdfs
M, Lifetime PDFs
signal
D+ (RS only)
1-d M pdfs
random comb.
signal (zoom)
off-resdatafit
fit
fit
shapefromMC
shapefromMC
fitfromRS fit
M = M(D*) - M(D0)
MC/fit
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Testing the Mixed Fit with Generic MC and Embedded Mixed Events
• WS mixed fit tested with full fit on ~540 lumi-scaled generic MC datasets with different levels of embedded mixed events
• N(mix) pull plots (right) show no evidence of bias or improperly scaled errors in the fit number of N(mix) with or w/o the presence of mixed events
N(mix) R(mix) fit mean err stat mean0 0 0.008 0.089 -0.06650 0.001 0.05 0.097 0.091
100 0.002 0.012 0.07 0.012
N(mix) R(mix) fit sigma err stat rms0 0 0.965 0.079 0.99450 0.001 0.995 0.093 0.999
100 0.002 0.994 0.052 0.994
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Goodness-of-Fit to Run 1,2 Data
•Toy MC datasets generated from RS/WS data fit pdfs and fit with unmixed/mixed fit models to determine goodness-of-fit
•Both RS/WS NLL values from data fits lie well within range predicted by the fits to toy MC
NLL Distribution for WS Toy Datasets
NLL distribution for RS toy datasets
WS data fit NLL value
RS data fit NLL value
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
• No significant variations in the mixing rate were found when making the following changes:– TwoTrksVtx → Hadronic beamspot– GeoKin K,e vertex → FastVtx– KLHVeryTight → KMicroVeryTight– E/p <1.05 → 1.10– Lifetime error cut +/- 10%– Separate fitting of initial D0 and D0 – Different NN event selector cuts
Evaluation of Systematics
• Systematic checks fall into two categories:– reasonableness or “sanity”
checks– systematic variations which
encode lack of knowledge/ understanding and biases in the fit model
• First class demonstrates robustness of result
• Latter class determines quantitative estimation of the systematic error
Reasonableness Checks
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
RS Unmixed M, Lifetime PDF Fit Classes
RS random M D0 lifetime
RS D+
RS random M zero lifetime
RS peaking M D0 unmixed lifetime
full M range
full M range
full M range
M M M
M M
M M
c
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
WS Mixed M, Lifetime PDF Fit Classes
Random comb. M D0 lifetime
WS random M D+ lifetime
Random comb. M zero lifetimePeaking M D+ lifetime
WS peaking M D0 mixed lifetime
full M range
full M range
full M range
MM
M M
M M
M M
c
c
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
stat
•Floated unmixed fit RS signal M shape parameters correlated, so N(mix) systematic error from this source calculated using RS fit correlation matrix
Quantitative Systematic Error
• Total systematic error
Delta M shape 0.00032Unmixed D0 lifetime 0.00008Random comb. shape 0.00015Lifetime resolution model 0.00000Bkgd D0 lifetime model 0.00016Bkgd D+ lifetime model 0.00012
sys
•List of systematic errors:
M. V. Purohit, Univ. of S. Carolina, ICHEP 2004, Beijing, China
Semileptonic Mixing Summary
Rmix = 0.0023 ± 0.0012 (stat) ± 0.0004 (sys)• Rmix consistent with no mixing
• CP fit was performed and no CP-violating effects were found
• Rmix < 0.0047 (95% C.L.) from NLL scan
• E791: rmix < 0.50% @ 90% CL
• FOCUS: rmix < 0.05% @ 90% CL
rmix < 0.10% @ 90% CL(using Feldman-Cousins)
Stat error is ~0.1%FOCUS result is UNPUBLISHED
(Plot courtesy of G. Burdman & I. Shipsey, hep/ph 0310076)
N(mix) NLL Scan