Stefano Bianco

1
New results on charm mixing and CP violation
Stefano Bianco Laboratori Nazionali di Frascati
CPConf2000, Ferrara, September 18-22 2000
2
Outline

• Basic Mixing Phenomenology
• FOCUS Detector
• Part I Charm Mixing and yCP
• Part II Search for CP violation
• Part III Measurement of D? p (Kp-)
• Conclusions and Outlook

J.Wiss, Fermilab Seminar Apr 2000 FOCUS Coll.
(J.M. Link et al.) Phys.Lett.B48562-70,2000
FOCUS Coll. (J.M. Link et al.) hep-ex/0005037
Preliminary J.M. Link, DPF2000 Columbus (USA)
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Formalism of P0-P0 mixing
_
The time evolution of flavour eigenstates
is given by the Schrödinger equation

mixing can be described by the ratio r
Processes which allow mixing appear
in H12 and H21.
If
are not mass eigenstates. Mass eigenstates
are (assuming CP conservation)
xgt0 mixing is caused by genuine
transitions(Df2) x splits masses
ygt0 mixing is caused by the short-lived
component disappearing rapidly, leaving behind
the long-lived component, made of and
y splits lifetimes
and mixing amplitude is
Mass and width differences are parametrized by
4
K0, D0, B0 _ Wheres The Difference ?
Also recent OPE-based limits I.Bigi,
N.G.Uraltsev hep-ph/0005089
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Theoretical guidance
From compilation of H.N.Nelson hep-ex/9908021
Triangles are SM x Squares are SM y Circles
are NSM x
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TECHNIQUES FOR STUDYING D0D0 MIXING (I)
_
Measure yCP, the y parameter for CP eigenstates
Measure r
?2
These Results
Also E791, BELLE
HADRONIC DECAYS
SEMILEPTONIC DECAYS
(?1 ?2)/2
E791
E791, CLEO, ALEPH
7
TECHNIQUES FOR STUDYING D0D0 MIXING (II)
_
via
HADRONIC DECAYS
Measure r
Cabibbo-Favoured Decay
mixing
Strong phase d between CFD and DCSD mixes x
and y
D tags D0 flavour
Double Cabibbo-Suppressed Decay
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TECHNIQUES FOR STUDYING D0D0 MIXING (III)
_
SEMILEPTONIC DECAYS
via
Measure r
Cabibbo-Favoured Decay
mixing
D tags D0 flavour
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TECHNIQUES FOR STUDYING D0D0 MIXING (Concl.)
_

• Direct measurement of yCP requires good
  lifetime resolution
• Measuring rws in hadronic decays requires
• to disuntangle rDCS from r
• and the knowledge of d
• Measuring rws in semileptonic decays provides
  the r parameter, but it is experimentally more
  difficult

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Spectrometer at FNAL Wide Band
Photon Beam
Vertexing
Cerenkov
Over 1 million reconstructed!
PWC
Successor to E687. Designed to study charm
particles produced by 200 GeV photons using a
fixed target spectrometer with updated
Vertexing, Cerenkov, EM Calorimeters, and Muon
id capabilities. Member groups from USA, Italy,
Brazil, Mexico, Korea.
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The FOCUS Collaboration
Univ. of California-Davis, CBPF-Rio de Janeiro,
CINVESTAV-Mexico City, Univ. Colorado-Boulder,
FERMILAB, Laboratori Nazionali di Frascati,
Univ. of Illinois-Urbana-Champaign, Indiana
Univ.-Bloomington, Korea Univ.-Seoul, INFN and
Univ.-Milano, Univ. of North Carolina-Asheville,
INFN and Univ.-Pavia, Univ. of Puerto
Rico-Mayaguez, Univ. of South Carolina-Columbia,
Univ. of Tennessee-Knoxville, Vanderbilt
Univ.-Nashville, Univ. of Wisconsin-Madison,
Yonsei Univ.-Seoul
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Detector Performance related to lifetime study

• Vertex algorithm is driven by D candidates
• The Fit variable ( reduced proper time
  ) acceptance is
  flat.
• Excellent ( and flexible) Cerenkov
  identification
• Minimized systematics on particle ID
  misidentification.

• Segmented target
• 62 of charm decay in air.
• small absorption correction.
• Extremely good proper time resolution
  ?(?)/?(D0) 8 (30fs)
• No resolution convolution systematics/error
  inflation ? Binned likelihood method.

Detachment cut
Z (cm)
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Selection of D?? K?, KK

• Common base cuts detachment (l /s gt 5), Kaon ID
  (Wp-WK gt 4)
• Tagged sample
• Or inclusive sample
• More Cerenkov cuts pion ID (W-W? gt -2), Kaon
  ID 2 ( WP-WK gt -2 )
• Primary vertex in target (PIT)

• Important Get a sample with a flat efficiency
  over t

D?? K? 119738 selected D ? KK 10331 selected
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D?KK signal for several cleanups
l /? gt 5 Wp-WKgt1
l /? gt 5 Wp-WKgt4
Y16532 S/N2.3
Kp reflection
l /? gt 9 Wp-WKgt1
l /? gt 9 Wp-WKgt4
Y11528 S/N4.3
Y7151 S/N5.7
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Fit technique non-parametric background

• Binned likelihood 20 ? 200 fs
• Acceptance/absorption f(t) correction by MC
  nearly 1
• mi signal in each t bin
• bi background from sidebands
• Fit t (Kp), B (Kp) (but see next page)

• option w or w/o B-tie term.

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Fitting technique

• The KK sample has some K? reflection at its
  side B2 Kp nominal ? one more player in
  fitting.
• Subtract K? reflection by a mass fit.
• The reflection mass shape from MC.
• The subtraction level by the mass fit.
• Time evolution of the reflection from t (Kp)

• Background under KK signal ? B1 (B2 - K?
  reflection)
• Simultaneous time evolution fit of both K? and
  KK histos.
• 4 variables in the time fit
• t (Kp), yCP, B (Kp), B (KK)

D?KK
Region B1
Region B2
Kp reflection
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Fitted time evolutions

• Background subtracted and f(t) corrected time
  evolution of Kp and KK events in the final fit.

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Ycp Results and Systematics
3 K ID ? 3 l/? ? B-tie or not ? 15/20 bin 36
fit variants shown.
yCP 3.42?1.39?0.74
Sample standard deviation of fit variants is
?0.63.
?(K?) 409.2?1.3??? fs
Sample standard deviation of fit variants
?0.3. Absolute lifetime systematics not ready
until we analyze K3?, etc.
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Additional systematic checks
Could curvature in reflection line- shapes cause
the sideband background to fail? Lets reduce
the sideband size by half and see...
Results are nearly identical to standard fits
How important is the f(t) correction? Set
f(t)1 and rerun all the fits.
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Consistency of D and full sample
Further suppress non-charm and non-Do reflection
by looking at D-only sample.
21
Comparisons to CLEO, E791 and BELLE
The comparison to CLEO is valid only if one
assumes a small strong phase difference ?. About
the same sensitivity to the CLEO CP constrained
fit, but the opposite sign!
FOCUS
FOCUS
CLEOII.V
E791
FOCUS yCP 3.42 ? 1.39 ? 0.74
Recent Measurements E791 yCP 0.8 ? 2.9 ? 1
CLEO -5.8 lt y lt 1 ( 95 CL) BELLE prelim.
yCP
E791
BELLE
95 CL
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Search for CP asymmetry in charm decay
CP asymmetries may show via the interference of
two distinct amplitudes with phases d1 and d2
which contain both a weak (CKM) and a strong
(FSI) contribution.
The weak contribution in the phases changes sign
under CP, while the strong one does not. The most
accessible decays which contain two weak phases
are the SCSD (tree and penguin diagrams). The CP
asymmetry will then be
Buccella et al predict state specific asymmetries
in the range of 0.002 ? 0.14 The Do asymmetry
is complicated by a direct as well mixed
contribution but expectation are that the D
might be smaller than D
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Search for CP asymmetry in charm decay (D?KK)
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Search for CP asymmetry in charm decay (D?KK?)

• Cabibbo suppressed mode.

D
D
DS?
DS
D ? K?Kp
D? ? K?Kp?

• Cabibbo favored mode.

D ? K? p p
D? ? Kp? p?
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CP asymmetry results

• No evidence for CP violation.
• Our limits on
• Need to use tagged Do sample which cuts our
  sample by 80.
• 23 times better limit than the previous
  published measurements.
• also, New Unpublished CLEO limits on KK,
  pp (CIPANP, Quebec City 2000)

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Measurement of D? p (Kp-)
Event Selection

• Very loose Cerenkov based particle id cuts on K
  and p.
• The D0 candidate is used as a seed to find the
  production vertex.
• The production vertex has at least 2 tracks in
  addition to the D0.
• The production vertex is required to be within
  1s of target material.
• Production and decay vertices are required to be
  well formed (CLgt1).
• D0 daughter tracks inconsistent with coming from
  the production vertex.
• The vertex separation L/sLgt5.
• Cut Kp pairs with high momentum asymmetry and
  low D0 momentum.

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Measurement of D? p (Kp-)
First You Have to Tag the D0 Flavor

• The decay DgD0p is used to identify the D0
  flavor.
• So we study the D-D0 mass difference.

D-D0
(CF-like Tags)
pp-
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Measurement of D? p (Kp-)
The Worst BG is CF Kp Double Mis-id
- Standard cuts.
- Double mis-id cut.
The double mis-id Dm is indistinguishable from
the correctly identified signal.
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Measurement of D? p (Kp-)
A New Method

• Divide the data into 1 MeV wide bins in Dm, and
  fit the D0 in each bin.
• Fit the KK and pp reflections with Monte Carlo
  events.

• Fit BG to a polynomial.
• Fit D0 to a gaussian.

.
DCS-like tags 146ltDmlt147 MeV
.
.
A total of 80 fits!
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Measurement of D? p (Kp-)
Fit the Dm Distributions

• Fitted D0 yields are plotted in the appropriate
  Dm bins.
• Background is fit to .
• DCS signal is fit directly to the CF histogram
  signal region.

CLEO (0.332 ?0.064 ?0.040)
rWS (0.482 ? 0.093) Preliminary! Stat err only
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Conclusions

• Measurement of yCP, asymmetry between the
  KK(CP1) and K?(CPmixed) lifetimes
• 
  
• New limits on CPV asymmetries
• Preliminary Measurement of D? p (Kp-)
  Assuming No Mixing
• rDCS (0.482-0.093) (stat only)

yCP 3.42 ? 1.39 ? 0.74 ?(K?)
409.4 ? 1.34 ??? fs
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Outlook

• FOCUS yCP result becomes more compatible to CLEO
  r measurement if a very large strong phase d is
  allowed
• Fairly intense theory production to explain the
  CLEO and FOCUS mixing results
• Expect new FOCUS results on r soon
• Expect new CLEO and FOCUS results on r from
  semilep analysis soon
• The FOCUS 2.2s evidence for charm mixing should
  be verified soon by B-factories

• I.I.Bigi, ICHEP2K Osaka Plenary talk
  hep-ph/0009021
• A.Golutvin, ICHEP2k Osaka Plenary talk
• A.Petrov, hep-ph/0009160
• Yosef Nir, hep-ph/0008226
• D.Atwood et al., hep-ph/0008090
• J. P. Silva, hep-ph/0007214
• J. L. Rosner, hep-ph/0007194
• J.L. Rosner, hep-ph/0005258
• S.Bergmann et al., Phys.Lett.B486418-425,2000
• I.I. Bigi, hep-ph/0005089

Citations to date