The Belle B Factory

Past, present and future

Christoph Schwanda, Innsbruck, Oct-18, 2006 Christoph Schwanda 1

Goal of the B factory experiments

“Study CP violation in the B meson system and probe the Cabibbo-Kobayashi-Maskawa mechanism of flavor mixing” (or something like that) Christoph Schwanda 2

B 0

d

b

B mesons

B +

u

b • Bound state of a light d- or u with a heavy b-quark • M(B 0 ) = ( 5279.4 +/- 0.5 ) MeV/c 2 M(B + ) = ( 5279.0 +/- 0.5 ) MeV/c 2   (B 0 ) = ( 1.530 +/- 0.009 ) ps  (B + ) = ( 1.638 +/- 0.011 ) ps Christoph Schwanda 3

CP transformation

• Under C, particles and anti-particles are interchanged by conjugating all internal quantum number (

e.g.

, Q  -Q) • Under P, the handedness of space is reversed, (x,y,z)  (-x,-y,-z) • Under CP, a left-handed electron e L is transformed into a right-handed positron e + R Christoph Schwanda 4

Why is CP violation interesting?

• In the SM, CP violation is described by a single phase • CP violation is necessary to understand the baryon density in the universe [Sakharov, Sov. Phys. JETP Lett. 5, 24 (1967)] • In general, New Physics models introduce new CP violating phases Christoph Schwanda 5

Common misunderstandings

• Did the B factories discover CP violation?

– No! CP violation was first observed in 1964 in neutral K   decays [PRL 13, 138 (1964)].

This type of CP violation is related to K 0 anti-K 0 mixing ( “indirect” CP violation ) and described by the parameter  || = (2.28 +/- 0.02 ) x 10 -3 Christoph Schwanda 6

Common misunderstandings

• Then, did the B factories discover direct CP violation (CP violation arising solely from decay amplitudes)?

– No! Direct CP violation was established by the NA48 and KTeV experiments also in K   decays  '  = (1.72 +/- 0.18 ) x 10 -3 Christoph Schwanda 7

Take away message

“The B factories did not discover CP violation(*) but they confirmed the Kobayashi-Maskawa mechanism of CP violation” (*) They first observed CP violation in B meson decays and new CP-violating observables though.

Christoph Schwanda 8

The KM mechanism

• Charged current interaction in the SM QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

[Kobayashi, Maskawa, Prog. Theor. Phys. 49, 652 (1973)] • V CKM is a unitary 3x3 matrix; it contains three real parameters and one complex phase • This phase is responsible for all CP violating phenomena observed so far!

Christoph Schwanda 9

Wolfenstein parametrization

QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

 = |V us | = 0.22

Christoph Schwanda 10

The unitarity triangle

QuickTime™ and a TIFF (L ZW) d eco mpres sor are nee ded to s ee this picture.

 QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

 QuickTime™ and a TIFF (LZ W) decompressor are needed to see t his picture.

( ,)  =  2 QuickTime™ and a TIFF (LZ W) decompressor are needed to see t his picture.

(0,0)  =  3 QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

 =  1 (1,0)  Christoph Schwanda 11

The B factories can

• Measure the sides (mainly |V cb |, |V ub |) and the angles  1 ,  2 and  3 in the unitarity triangle • Thus overconstraining the UT, we can test the Kobayashi-Maskawa mechanism Christoph Schwanda 12

KEKB and Belle

(I will not discuss BaBar and PEP-II) 8 GeV e x 3.5 GeV e +

KEKB

Mt. Tsukuba

Belle

~1 km in diameter

• E cm = 10.58 GeV ( Y(4S) resonance ) • L peak = 1.652 x 10 34 cm -2 s -1 Christoph Schwanda 13

Luminosity history

• Now, about 600 million B anti-B events recorded!

Christoph Schwanda 14

The Belle detector

SC solenoid

1.5T

CsI(Tl)

16

X 0

TOF counter 8 GeV

e

-

Aerogel Cherenkov cnt.

n=1.015~1.030

3.5 GeV

e

+

C entral D rift C hamber

small cell +He/C 2 H 5

Si vtx. det.

3(4) lyr. DSSD Christoph Schwanda m

/ K

L

detection

14/15 lyr. RPC+Fe 15

The Belle collaboration

Aomori U.

BINP Chiba U.

Chonnam Nat’l U.

U. of Cincinnati Ewha Womans U.

Frankfurt U.

Gyeongsang Nat’l U.

U. of Hawaii Hiroshima Tech.

IHEP, Beijing IHEP, Moscow IHEP, Vienna ITEP Kanagawa U.

KEK Korea U.

Krakow Inst. of Nucl. Phys.

Kyoto U. Kyungpook Nat’l U. EPF Lausanne Jozef Stefan Inst. / U. of Ljubljana / U. of Maribor U. of Melbourne Nagoya U.

Nara Women’s U.

National Central U.

National Taiwan U.

National United U.

Nihon Dental College Niigata U.

Osaka U.

Osaka City U.

Panjab U.

Peking U.

U. of Pittsburgh Princeton U.

Riken Saga U.

USTC 13 countries, 55 institutes, ~400 collaborators

Christoph Schwanda 16

Extracting a B signal

Using special Y(4S) kinematics, two nearly independent variables M B and  E can be used to select B meson signal:

M B = (E beam ) 2 – (

S

P i ) 2



E =

S

E i - E beam

Methods to extract B signal yield: 1) Cut on M B and fit to  E 2) Cut on  E and fit to M B 3) Double dimensional fit to M B and  E distribution 4) If B->P 1 P 2 P 3 : cut  E and M B box and look at resonant structures in M(P 1 P 2 ) mass distribution.

Christoph Schwanda 17

Continuum suppression

Dominant Background for rare Decays: e + e 

qq

“continuum” (~4x BB) Continuum Jet-like

e + qq

To suppress: use event shape variables continuum Y (4S) Fox-Wolfram moments Angle between B meson and beam axis direction

e +

B events Spherical

Signal B

Christoph Schwanda 18

e Other B e -

The decay B

0 

J/



K

s 0 “golden mode”

b

d • CP violation in this decay arises from the quantum interference of these two diagrams tree diagram

c

c

s

d

J/



K S

+ box diagram + tree diagram Vtd b t d d s

K S J/

 d t b c c Vtd Christoph Schwanda 19

Time-dependent CP asymmetry

for J/ 

K s

: S = - CP sin2  1 A = 0 (  CP = +sin2 : CP eigenvalue)  1 Mixing-induced CPV Direct CPV   B0 (  t) = rate of B’s decaying to J/ when the B flavor has been B 0  (at t 1 ) K s   t = t 2 -t 1 time difference between flavor measurement and decay (at t 2 )   anti-B0 (  t) = rate of B’s decaying to J/  t 2 ) when the B flavor has been anti-B 0 Ks (at (at t 1 ) Christoph Schwanda 20

Principle of the measurement

e + B 0 l + Christoph Schwanda tag-side e anti-B 0 J/  K s CP-side z z 1  z z 2  t =  z/  c,  = 0.425 at KEKB 21

B

0  _ 535M

BB

J/



K

s

with 535M BB pairs

B 0



J/



K S 0 B 0



J/



K L 0

Nsig = 7482 Purity 97 % CP odd Nsig = 6512 Purity 59 % CP even

M bc

 * 2

E beam

-

P J

* 2 / 

Ks

Christoph Schwanda 22

B 0



J/



K S 0

B B 0 0 tag tag background subtracted

B 0



J/



K L 0

B 0 _ B 0 tag tag

Asym. = -



CP sin2



1 sin



m



t

stat error

sin2  1 = +0.643 ± 0.038

A = - 0.001 ± 0.028

sin2  1 = +0.641 A = +0.045 ± ±

stat error

0.057

0.033

Christoph Schwanda 23

sin 2

 1

from b



c anti-c s trees

Christoph Schwanda

5.5% rel. err.

24

sin2  history (1998-2005) Christoph Schwanda 25

The decay B

0  

K

s • This decay proceeds through a penguin loop diagram • In the SM: S(J/  K s )=S(  K s ) • New physics in loops (new CP violating phases) would lead to: S(J/  K s )  S(  K s ) Christoph Schwanda 26

307  21 

K S

signal unbinned fit SM _ 535M

BB

“sin2

 1

” =

+

0.50

 A

=

+

0.07



0.21(stat)



0.15(stat)



0.06(syst) 0.05(syst)

Christoph Schwanda 27

sin 2

 1

from b



q anti-q s penguins

Smaller than b g ccs in all of 9 modes Theory tends to predict positive shifts (originating from phase in Vts) Naïve average of all b g sin2  eff = 0.52 ± 2.6 s s modes 0.05

deviation between penguin and tree (b g s) (b g c) Christoph Schwanda 28

The decay B

0   +  V ud V * ub  3 

2

V td V * tb  1 V cd V * cb – b B 0 d V * tb t – V td t – d b V td V * tb Mixing diagram – B 0 With the tree diagram only

S

 +  -

A

 +  = +sin2  2 = 0 – b B 0 d V * ub V ud u – d – u d  /   /  Decay diagram (tree) Christoph Schwanda 29

_ 535M

BB

1464±65 signal events

A

  + 0 .

55  0 .

08  0 .

05

S

  0 .

61  0 .

10  0 first error: stat., second: syst.

.

04 Large Direct CP violation (5.5

s ) in disagreement with BaBar Large mixing-induced CP violation (5.6

s ) background subtracted Christoph Schwanda

A



S

  + 0 .

16  0 .

11  0 .

03  0 .

53  0 .

14  0 .

02 30

 2

(



)

BaBar(  /  /  ) + Belle(  /  ) 

/

 2

= [93 ]

 consistent with a global fit w/o  /  2 Christoph Schwanda  Global Fit = [ 98 ] º 31 -19

 3

(



)

m

2 • Time-dependent analyses get sin(2  1 +  3 ) • Best contraint on  3 analysis of B  comes from Dalitz D(*)K(*) with D  K s  +  -

B + :

r

r

=

|A 2 | |A 1 | D

0

m

2

D

0 

/

 3

= [53 ]



B :

m

+ 2

r

Christoph Schwanda

m

+ 2 32

|V

ub

|

QuickTime™ and a TIFF (LZW) decompressor are needed to see this picture.

Christoph Schwanda M.Morii diagram 33

Summary

• Many, many results… Belle published ~200 journal papers • All these measurements beautifully consistent with KM mechanism Christoph Schwanda 34

A look into future

• Now, have we learned everything about CP violation?

– No! There must be more sources of CP violation – The CP violation in the SM model cannot explain the baryon density of the universe by orders of magnitude – New physics (if found at the LHC) comes with new sources of CP violation Christoph Schwanda 35

New sources of CP violation

• Strong interaction violates CP (electric dipole moment of the neutron) • CP violation in the lepton sector (neutrino oscillations) • New physics (new heavy particles)  Belle can look for this source by measuring CP violation in loop diagrams (precision measurements) Christoph Schwanda 36

SuperKEKB

• •

Asymmetric energy e

+

e

-

collider at E CM =m

(  (4S))

to be realized by upgrading the existing KEKB collider.

Super-high luminosity



8



10 35 /

cm 2

/

sec 

1



10 10 BB per yr.

 9 

10 9

 +  -

per yr.

Belle with improved rate immunity Higher beam current, more RF, smaller  y * and crab crossing  L = 4  10 35 /cm 2 /sec Christoph Schwanda

http://belle.kek.jp/superb/loi

37

Physics case of SuperB

• Not the first time CP violation would tell us something about physics at higher energies • LHC will measure masses, SuperB could measure phases  complementarity which allows to constrain new physics scenarios Christoph Schwanda 38

Thank you very much for your invitation!

Christoph Schwanda 39