Various Work for Belle Detector FGIP-student Forum TIT, 2005-06-17
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Transcript of Various Work for Belle Detector FGIP-student Forum TIT, 2005-06-17
Various Work for Belle Detector
FGIP-student ForumTIT, 2005-06-17
Saša Fratina,
Jožef Stefan Institute, Ljubljana, Slovenia
Outline of the talk
A little bit about Slovenia.
Ring Imaging Cherenkov Counter (RICH)
Silicon Vertex Detector (SVD) CP-asymmetry measurement
in B meson system
Where do I come from?
Slovenia
Croatia
HungaryAustria
Italy
Few facts : 2M inhabitants, 20.000 km2, capital Ljubljana, EU members (www.slovenia.info)
Education system: public schools
Primary school (9 years, children 6-15 years old) Secondary school – high school (4 years,
students 15-19 years old), at the end of which you have to pass national exams from 5 subjects (slovene language, math, english and two by student`s choice)
University (4-5 years for graduation, usually takes longer)
Graduate studies: masters or PhD course depending on the field, in physics mostly PhD.
Universities in Slovenia
University of Ljubljana (largest, 22 faculties, 50.000 students)
University of Maribor Nova Gorica Polytechnic
My education
Primary school: Ljubljana High school: Gimnazija Bezigrad,
Ljubljana (finished with International Baccalaureate)
graduated at the University of Ljubljana, Faculty of Mathematics and Physics
started PhD study in 2002 at the University of Ljubljana (and working at Jozef Stefan Institute, Ljubljana)
Belle Control room
About work…
Main goal of Belle experiment: study of CP violation in B-meson system
Mt. Tsukuba
KEKBBelle
(4s)e+ e-
p(e+)=3.5 GeV/c
p(e-)=8.0 GeV/c
(4s)
B
B z ~ 200 m
Silicon vertex detector
Ring Imaging
Cherenkov Counter
Data analysis:
study of CP violation
RICH for super-Belle (2008?)
Requirements
Compact detector
Good /K separation in the forward (high momentum) region
for few-body decays of B's (B , B K)
for b -> d g , b -> s g ( , K )
Low momentum (<1GeV/c) e/μ/ separation (B ->Kll)
High efficiency for tagging kaons
Basic principle
Ring Imaging Cherenkov counter, RICH
cos Ch = 1/n
= v/c
Position sensitive photon detector
particle
Cherenkov photons
aerogel
Aerogel
n = 1.05 Ch () ~ 308 mrad
Ch () - Ch () ~ 23 mrad few cm thickness transmission length
2.5 - 4.5 cm approx. 10 emitted
photons / cm
100x100x20mm3 n=1.050
No cracks
Photon detector photo multiplier tube
(PMT) single photon sensitive position sensitive:
position resolution few mm quantum efficiency 20%
Photon detector: array of 16 H8500 PMTs
Beam test measurements Confirm feasibility of
such detector Study /K separation
capability
Clear rings, little background
Beam test: Cherenkov angle resolution and number of photons
Beam test results with 2cm thick aerogel tiles: >4K/separation
-> Number of photons has to be increased.
Typically around 13 mrad (for 2cm thick aerogel)
How to increase the number of photons?
What is the optimal radiator thickness?
Use beam test data on 0 and Npe
Minimize the error per track: (Npe) Optimum is close to 2
cm
0 Npe
(Npe)
● measure overlaping rings“focusing” configuration
● measure two separate rings “defocusing” configuration
How to increase the number of photons without degrading the resolution?
normal
Radiator with multiple refractive indices
4cm aerogel single index
2+2cm aerogel
FOCUSING CONFIGURATION - data
Increase the number of photons without degrading the resolution!
● number of detected hits: dual radiator has a clear advantage
● single photon resolution: dual radiator ~same as single (of half the thickness) for the full momentum range
FOCUSING CONFIGURATION – momentum scan
Development and testing of photon detectors for 1.5 T Baseline: large area HPD of the proximity focusing type Backup: MCP-PMT (micro channel plate)
-10kV15~25mm
e-
Multialkali photocathode
Pixel PD or APD
R&D project in collaboration with HPK
RICH - conclusions
Feasibility of RICH detector was confirmed. More photons: employ radiators with multiple
refractive indices. Idea successfully tested in beam tests.
Aerogel production: transmission length improved, new cutting methods tested, multiple layer samples.
R&D issues: development and testing of a multichannel photon detector for high mag. fields
Silicon vertex detector
50 cm
20 cm
basic SVD unit: Double Sided Strip Detector (DSSD)
separate measurement of r and z coordinate
strip pitch: 50 and 75 m for r and z coordinate, respectively
pitch
Evaluate the performance of SVD – measure its intrinsic resolution:Incident angle dependence, occupancy study,alignment cross-check
SVD intrinsic resolution
Error on the track position measurement
Track position
is determined
from the SVD
hits on other layers
DSSD
track
residual
SVD hit
Typical residual distributions
Intrinsic resolution is determined from the width of the residual distribution: i = 10 m, RMS = 12-15 m for r and i = 25 m, RMS = 30 m for z coordinate
residual [cm] residual [cm]
r coordinate z coordinate
Incident angle dependence Simple estimate for the perpendicular tracks:
signal collected by single strip → resolution ≈ strip pitch / 12
Small incident angle: signal collected by few strips → resolution improved
Large incident angle: signal
collected by many strips
→ resolution gets
worse due to smaller
signal to noise ratio
track
x strips
x
y
angl
e
Incident angle dependence: result
RMS [m] RMS [m]60
40
20
0-20 0 20 40 Incident angle [degrees]
-20 0 20 40 60Incident angle [degrees]
r coordinate z coordinate
Innermost layer
30
20
10
0
Incident angle dependence: result
RMS [m] RMS [m]
Incident angle [degrees]-20 0 20 40 60
Incident angle [degrees]
r coordinate z coordinate
Different colors show the result for all four layers: black, red, green and blue for the innermost, second, third and outermost layer.
-20 0 20 40
60
40
20
0
30
20
10
0
Magnetic Field Effect Intrinsic resolution is not symmetric with respect to
perpendicular
incident angle Reason:
magnetic field,
confirmed
by the plot of
hit cluster size next SVD:
Distribution of hits
with cluster size
1 strip, 2 strips, 3 strips, …
-20 0 20 40
incident angle [o] -12o
Magnetic field effect
n side
p side
E
B .Fe
Fm
e- direction
x
y
track
n side
p side
+
++
track
E
B .
angl
e
Fm
+
++
Fm
x
y
Negative angle:
smaller cluster size
Positive angle:
bigger cluster size
Degradation of SVD intrinsic resolution due to higher detector occupancy (background): cluster distortion or cluster mis-association?
residual
occupancy < 0.04
residual
occupancy 0.3
Number of clusters with MC hit (correctly associated)
occupancy, x coordinate
Rel
ativ
e fr
actio
n of
SV
D h
itswrong cluster association is negligible
Number of clusters with rel. change in E > 0.2
Number of clusters with MC hit (correctly associated)
Alignment cross-check
Check if residual distributions for individual SVD units are shifted (mis-aligned)
Try to correct
the shift
residual [cm]
r coordinate
Conclusions on the SVD intrinsic resolution study Best resolution (RMS) at small track incident
angle is found to be 12 and 30 m for r and z coordinate, respectively.
Results of this study provided important information for cross-check of the SVD alignment different geometry design to take into account
magnetic field effect improvement of clustering algorithm in the case of
higher detector occupancy
Analysis of data collected at the Belle detector:
Measurement of Time Dependent CP Violation in B0 → D+D- Decays
B 0B 0 fCPfCP
B 0B 0
fCPfCPB 0B 0
B 0B 0
B0 → D+D- CP eigenstate
tree
penguin
B0
B0
c
c
d
c
c
d
D-
D+
• B 0 and B 0 mixes with each other via “box-diagram”.
• The box-diagram includes CKM complex phase.• A path from B 0 to fCP via mixing has different weak
phase from one from B 0 to fCP directly due to the CKM
phase.
W - W +
t
t
d
b
b
dVtb
*
Vtb*Vtd
Vtd
Time-dependent decay amplitude
PCP±(t) = exp(-|t| / ) / (4) ·
(1±(1-2w)(S sin(mt)- C cos(mt))) S = - sin(2) (if only tree diagrams are present)
CKM triangle
Analysisfinal step: fit S and C to t distribution Reconstruct the events Measure the time of B meson decay from the
reconstructed B vertices. Determine B0 flavor
Properly fit the t distribution (taking into account detector resolution, mis-tagging of B mesons,…)
5 steps toward the CP asymmetry measurement
• Reconstruct B KS decays• Measure proper-time difference: t • Determine flavor of Basc
• Evaluate asymmetry from the obtained t distributions
• Discuss observed CP asymmetry
e- e+e: 8.0 GeVe: 3.5 GeV
Brec
z
Basc
Y(4S)~ 0.425
fCP (fKS)fCP (fKS)
z c tB ~ 200 mm
Flavor tagFlavor tag
Event reconstruction
Br ~ 1.7 10-4
D mesons are reconstructed only in decays to charged particles (10%)
Detector efficiency for reconstructing such an event ~ 10 – 20 % (at S/B ~ 1)
Expect only few (2-3) events per 10 M BB events!
High luminosity B-factory is needed – KEK!
Approx. 350 M recorded BB events – about 100 of them will be correctly reconstructed as B0 → D+D-
B0 decay time
Average distance between the two decay vertices is 200 μm
Need to measure the vertex position with better accuracy
With SVD we are able to measure the vertex z coordinate with accuracy about 100 μm
Flavor tagging
Use flavor-specific decays ( slow, K, leptons)
t =trec
BrecBrec
(4S)(4S)BascBasc
Brec = B 0 decay(flavor-specific)
Brec = B 0
?????? D+D- decay
t =tasc
B 0-B 0 mixing
t
Current status
Event selection is optimised according to best signal to background ratio.
Vertex position and tagging are determined using standard Belle software
Fitting procedure is under development
(I can not show any preliminary results)
Conclusion…
It is great to work for
SVD
RICH
Study of CP violation