strong phase in d →kπ decay and ycp measurement at besiii...
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Strong phase in D0→Kπ decay
and yCP measurement at BESIII
Xiao-Rui Lu (E-mail: [email protected])
(on behalf of the BESIII collaboration)
• Introduction
• BEPCII/BESIII experiment
• Strong phase in D0→Kπ decay
• yCP measurement
• Summary
List of Contents:
University of Chinese Academy of Sciences (UCAS)
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Xiao-Rui Lu @ Charm 2013 2
Introduction
DDbar mixing is highly suppressed by the GIM mechanism
and by the CKM matrix elements within the Standard Model
Observation of DDbar mixing by LHCb
Improving the constraints on the charm mixing parameters is
important for testing the SM, such as long-distance effect
In addition, strong phase is an important ingredient for (over-
)constraining the CKM unitary triangle, which is an crucial
for searching for new physics
The mixing parameters describes the magnitude of DDbar mixing
where M1,2 and Γ1,2 are the masses and widths of the neutral D
meson mass eigenstates.
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Production at threshold
Xiao-Rui Lu @ Charm 2013 3 3
Threshold production at 3.773 GeV
Double Tag techniques: (partial-)reconstruct both D mesons
Charm events at threshold are very clean and unique in studying D decays
Quantum correlation of two D mesons
Very clean environment with little to no non-DDbar background
Lots of systematic uncertainties uncertainties cancel when applying double tag method
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Xiao-Rui Lu @ Charm 2013 4
The decay rate of a correlated state
Taking advantage the quantum coherence of DDbar pairs, BESIII
can study the charm physics in an unique way
• strong phase
• mixing parameters
• direct CP violation
• ...
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Xiao-Rui Lu @ Charm 2013 5
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The BESIII detector
Xiao-Rui Lu @ Charm 2013 6 6
The new BESIII detector is hermetic for neutral and charged particle with excellent resolution, PID, and large coverage.
NIM A614, 345 (2010)
Magnet: 1 T Super conducting MDC: small cell & He gas xy=130 m sp/p = 0.5% @1GeV dE/dx=6%
TOF:
T = 90 ps Barrel
110 ps Endcap
Muon ID: 8~9 layer RPC RΦ=1.4 cm~1.7 cm
EMCAL: CsI crystal E/E = 2.5% @1 GeV φ,z = 0.5~0.7 cm/E
Trigger: Tracks & Showers
Pipelined; Latency = 6.4 s
Data Acquisition:
Event rate = 3 kHz
Throughput ~ 50 MB/s
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Collected data samples at BESIII
Xiao-Rui Lu @ Charm 2013 7 7
• world’s largest samples of on-threshold ψ(3770) data and keep increasing in the future
• the aim is to have 20 /fb data
Previous data BESIII now Goal
J/ψ BESII: 58 M 1.2 B 20*BESII 10 B
ψ(3686) CLEO: 28 M 0.5 B 20*CLEO 3 B
ψ(3770) CLEO: 0.8 /fb 2.9 /fb 3.5*CLEO 20 /fb
Above open
charm
threshold
CLEO: 0.6/fb
@4160MeV
2011: 0.5 /fb @ 4.009 GeV
2013: 1.9 /fb @ 4.26 GeV,
0.5 /fb @ 4.36 GeV
and data for lineshape
5-10 /fb
R scan BESII 2012: R @2.23,2.4,2.8,3.4 GeV
25 /pb tau mass
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Xiao-Rui Lu @ Charm 2013 8
Implications of strong phase difference
Time-dependent D0Kπ analysis:
phase difference δ to relate (x', y') with (x, y).
CKM unitarity triangle γ/ɸ3 extraction from B- D0 K-
Atwood, Dunietz, Soni (ADS): Use doubly Cabibbo-suppressed decays, e.g. D0
K+π -
PRL 110, 101802 (2013)
)(
0
0
3
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Bi
BerKDBA
KDBA
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Strong phase in D0→Kπ decay: formalism
Xiao-Rui Lu @ Charm 2013 9 9
The strong phase difference δKπ between the doubly Cabibbo-suppressed (DCS) decay D0→ K−π+ and the corresponding Cabibbo-favored (CF) D0→ K −π+ is denoted as
Omitting the higher orders of the mixing parameters, and assuming CP conservation, we have
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Accessing strong phase δKπ at threshold
Xiao-Rui Lu @ Charm 2013 10 10
We measure the strong phase difference using quantum correlated production of D-Dbar at the production threshold
based on 2.9 fb-1 ψ(3770) data
When we neglect CPV, CP of the two D mesons are anti-symmetric.
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Xiao-Rui Lu @ Charm 2013 11
For the CP-eigenstates, yields of D → CP ST events will be
The DT yields with D → CP and D → Kπ will be
Therefore, the branching fraction is
Here, εCP±/εKπ,CP± cancels most systematic effects within the D → CP± decay mode. Therefore, ACP→Kπ can be obtained. With external inputs of
the other parameters, we can obtain δKπ.
To determine δKπ in experiment
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Single tags of CP modes
Xiao-Rui Lu @ Charm 2013 12 12
preliminary preliminary
preliminary preliminary preliminary
preliminary preliminary preliminary
22
DbeamBC pEM
unique variable in threshold-production
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Xiao-Rui Lu @ Charm 2013 13
CP purity check of CP-tag modes
preliminary preliminary
preliminary preliminary preliminary
preliminary
events with same-CP decays are consistent with 0
consider as systematic uncertainty
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Xiao-Rui Lu @ Charm 2013 14
Double tags of (CP, Kπ) modes
preliminary preliminary
preliminary preliminary preliminary
preliminary preliminary preliminary
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Xiao-Rui Lu @ Charm 2013 15
Preliminary numerical results
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Preliminary results of δKπ
Xiao-Rui Lu @ Charm 2013 16 16
With external inputs of the parameters in HFAG2013 and PDG,
we obtain
We measure
We have
CLEO measurements of strong phase differences and coherence factors done with 0.8 fb−1 at ψ(3770). [CLEO, PRD 86 (2012) 112001]
without external inputs:
with external inputs:
BESIII result: the most precise measurement of δKπ and compatible with the world average
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Determination of the mixing parameter yCP
Xiao-Rui Lu @ Charm 2013 17 17
For any final states of CP eigenstates, the decay rate is:
where
Considering the process in which one D decays into CP eigenstates and the other D decays semileptonically, the decay rate is:
Neglecting terms to order y2 or higher, we can derive
In the limit of no CPV,
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Xiao-Rui Lu @ Charm 2013 18
Measurement of yCP: formalism
On experiments, we have
where the efficiency-corrected yields are denoted to be
then
is the average ratio over different CP modes by minimizing
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CP- tag
flavor tag
Measurement of yCP: CP tag and flavor tag
Xiao-Rui Lu @ Charm 2013 19 19
We measure the yCP using CP-tagged semi-leptonic D decays
based on 2.9 fb-1 ψ(3770) data
missmissmiss pcEU
Semi-leptonic signal peaks at zero!
The observable can be :
lKDDbeammiss pppmEpCP
ˆ22
lKbeammiss EEEE
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Single tags of CP modes
Xiao-Rui Lu @ Charm 2013 20 20
preliminary
preliminary preliminary
preliminary preliminary
preliminary
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Xiao-Rui Lu @ Charm 2013 21
Double tags of Keν modes
• signal: MC shape convoluted with an asymmetric Gaussian
• background: a 1st-order polynomial function
preliminary
preliminary preliminary
preliminary preliminary
preliminary
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Xiao-Rui Lu @ Charm 2013 22
Double tags of Kμν modes
• signal: MC shape convoluted with an asymmetric Gaussian
• backgrounds:
Kππ0: use control sample of D→Kππ0 in data
Kev: fixed to MC shape and size
others: a 1st-order polynomial function
preliminary
preliminary preliminary
preliminary preliminary
preliminary
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Xiao-Rui Lu @ Charm 2013 23
To evaluate Kππ0 backgrounds in Kμν modes
take Eextra>0.5GeV as control sample to estimate the shape and size of Kππ0 backgrounds
shape : the smearing Gaussian is fixed to the parameters obtained from fit in the control sample
Eextra>0.5GeV
size: scale the MC size in the signal region with the ratio of the number of Kππ0 events in data to that in MC in the control sample
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Preliminary numerical results
Xiao-Rui Lu @ Charm 2013 24 24
Signal yields of the full data set
preliminary result:
• result is statistically limited
• systematic uncertainty is relatively small
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Comparison with world measurement
Xiao-Rui Lu @ Charm 2013 25 25
compatible with world average results
BESIII (pre.) -1.6 ±1.3 ±0.6 %
best precision in Charm factory
World average directly from HFAG2013 (BESIII (pre.) not included)
CLEOc 2012: [PRD 86 (2012) 112001]
yCP=(4.2±2.0 ±1.0)%
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26
arXiv:1304.6170
Toward global fit at BESIII
least squares fitter: used for extracting expected physics
parameters from the correlated experimental data
Monte Carlo validation of the fitter
• seven external inputs in
the test: RWS, r2, δKπ, xD,
yD, x′2 and y′
• their uncertainties are
assumed to be
uncorrelated
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MC study corresponds to 3.0 / fb data
input of the central values of the world average in 2012:
with the external constrains of :
output:
27
arXiv:1304.6170
Sensitivity of the global fit at BESIII
CLEO 2013:
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Xiao-Rui Lu @ Charm 2013 28
Summary
• Quantum-correlated D0-D0 production on threshold provide an unique way to measure the charm mixing parameters
• BESIII collected 2.9 /fb e+e- collision data at 3.773 GeV the world-largest on-threshold data in charm factory
• Strong phase difference in D0→Kπ decays is measured with the best accuracy help to improve the world measurement of the mixing parameters x and y
• The mixing parameter yCP is determined, which is compatible with the world average still statistically limited
• More charm data will be collected at BESIII; work on global fit is ongoing
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Xiao-Rui Lu @ Charm 2013 29
Thank you!
谢谢大家!