Christoph Schwanda1 The Belle B Factory Past, present and future Christoph Schwanda, Innsbruck, Oct-18, 2006

Christoph Schwanda 1

The Belle B FactoryThe Belle B FactoryPast, present and futurePast, present and future

Christoph Schwanda, Innsbruck, Oct-18, 2006


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)


B mesons

• Bound state of a light d- or u-with a heavy b-quark

• M(B0) = ( 5279.4 +/- 0.5 ) MeV/c2

M(B+) = ( 5279.0 +/- 0.5 ) MeV/c2

(B0) = ( 1.530 +/- 0.009 ) ps(B+) = ( 1.638 +/- 0.011 ) ps

d

b

u

b

B0 B+


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


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


Common misunderstandings

• Did the B factories discoverCP 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 K0 - anti-K0 mixing ( “indirect” CP violation ) and described by the parameter

= (2.28 +/- 0.02 ) x 10-3


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 inK decays

' = (1.72 +/- 0.18 ) x 10-3


Take away message

“The B factories did not discoverCP violation(*) but they confirmed the Kobayashi-Maskawa mechanismof CP violation”

(*) They first observed CP violation in B meson decays and new CP-violating observables though.


• Charged current interaction in the SM

• VCKM 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!

The KM mechanism

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.



[Kobayashi, Maskawa, Prog. Theor. Phys. 49, 652 (1973)]


Wolfenstein parametrization



= |Vus| = 0.22


The unitarity triangle

QuickTime™ and a

TIFF (LZW) decompressorare needed to see this picture.



= 1

= 2

= 3

(1,0)(0,0)

(

QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.




The B factories can

• Measure the sides (mainly |Vcb|, |Vub|) and the angles 1, 2 and 3 in the unitarity triangle

• Thus overconstraining the UT, we can test the Kobayashi-Maskawa mechanism


KEKB and Belle

~1 km in diameter

Mt. Tsukuba

KEKBBelle

8 GeV e- x 3.5 GeV e+

• Ecm = 10.58 GeV ( Y(4S) resonance )

• Lpeak = 1.652 x 1034 cm-2s-1

(I will not discuss BaBar and PEP-II)


Luminosity history

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


The Belle detector

/ KL detection 14/15 lyr. RPC+Fe

CsI(Tl) 16X0

Si vtx. det. 3(4) lyr. DSSD

SC solenoid 1.5T

8 GeV e

3.5 GeV e

Aerogel Cherenkov cnt. n=1.015~1.030

Central Drift Chamber small cell +He/C2H5

TOF counter


The Belle collaboration

13 countries, 55 institutes, ~400 collaborators

IHEP, ViennaITEPKanagawa U.KEKKorea U.Krakow Inst. of Nucl. Phys.Kyoto U. Kyungpook Nat’l U. EPF Lausanne Jozef Stefan Inst. / U. of Ljubljana / U. of MariborU. of Melbourne

Aomori U.BINPChiba U.Chonnam Nat’l U.U. of CincinnatiEwha Womans U.Frankfurt U.Gyeongsang Nat’l U.U. of HawaiiHiroshima Tech.IHEP, BeijingIHEP, Moscow

Nagoya U.Nara Women’s U.National Central U.National Taiwan U.National United U.Nihon Dental CollegeNiigata U.Osaka U.Osaka City U.Panjab U.Peking U.U. of PittsburghPrinceton U.RikenSaga U.USTC


Extracting a B signal

Methods to extract B signal yield:1) Cut on MB and fit to E2) Cut on E and fit to MB

3) Double dimensional fit to MB and E distribution

4) If B->P1P2P3: cut E and MB box and look at resonant structures in M(P1P2) mass distribution.

Using special Y(4S) kinematics, two nearlyindependent variables MB and E can beused to select B meson signal:

MB = (Ebeam)2 – ( Pi)2

E = Ei - Ebeam

*

*


Continuum suppression

e+

e-

e+

e-

qq

Signal B

Other B

Dominant Background for rare Decays:

Continuum

Jet-like

e+e qq “continuum” (~4x BB)

To suppress: use event shape variables

continuum

Y (4S)

Fox-Wolfram momentsAngle between B meson and beam axis direction

B eventsSpherical


The decay B0 J/ Ks0

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

b c

d

csd

J/KS

b

d cJ/KS

bcsddt

t

+

tree diagram box diagram + tree diagramVtd

Vtd

“golden mode”


Time-dependent CP asymmetry

B0(t) = rate of B’s decaying to J/ Ks (at t2) when the B flavor has been B0 (at t1)

anti-B0(t) = rate of B’s decaying to J/ Ks (at t2) when the B flavor has been anti-B0 (at t1)

t = t2-t1 time difference between flavor measurement and decay

for J/ Ks:

S = CPsin21 = +sin21 A = 0(CP : CP eigenvalue)

Mixing-induced CPVMixing-induced CPV Direct CPVDirect CPV


Principle of the measurement

t = z/c, = 0.425 at KEKB

e+ e-

l+

J/

Ks

z

z1 z2

B0

anti-B0

tag-side

CP-side

z


B0 J/ Ks with 535M BB pairs

2*/

2*KsJbeambc PEM −=

Nsig = 7482Purity 97 %

CP odd

Nsig = 6512Purity 59 %

CP even

0 0B0 J/ KS B0 J/ KL

_535M BB


B0 J/ KS0 B0 J/ KL

B0 tag_B0 tag

0

Asym. = -CPsin21sinmt

sin21= +0.643 ±0.038 A = - 0.001 ±0.028

sin21= +0.641 ±0.057 A = +0.045 ±0.033

stat error stat error

backgroundsubtracted

B0 tag_B

0 tag


sin 21 from b c anti-c s trees

5.5% rel. err.


sin2 history(1998-2005)


The decay B0 Ks

• This decay proceeds through a penguin loop diagram

• In the SM:S(J/Ks)=S(Ks)

• New physics in loops (new CP violating phases) would lead to: S(J/Ks)S(Ks)


307 21 KS signal

307 21 KS signal

unbinned fitSM

“sin21” = 0.50 0.21(stat) 0.06(syst) A = 0.07 0.15(stat) 0.05(syst)

“sin21” = 0.50 0.21(stat) 0.06(syst) A = 0.07 0.15(stat) 0.05(syst)

_535M BB


Theory tends to predictpositive shifts(originating from phasein Vts)

sin 21 from b q anti-q s penguins

Smaller than bccs in all of 9 modes

Smaller than bccs in all of 9 modes

Naïve average of all b s modes

sin2eff = 0.52 ± 0.052.6 deviation betweenpenguin and tree (b s) (b c)

Naïve average of all b s modes

sin2eff = 0.52 ± 0.052.6 deviation betweenpenguin and tree (b s) (b c)


The decay B0 + -

B0

d

b–

d–

bt

–tB0

–V*

tb Vtd

V*tbVtd

Mixing diagram Decay diagram (tree)

B0

b–

d du–

d–u

/

/Vud

V*ub

With the tree diagram only

S = +sin22

A = 0

S = +sin22

A = 01

3

2VudV*ub

VtdV*tb

VcdV*cb


04.010.061.0

05.008.055.0

±±−=±±+=

SA

first error: stat., second: syst.

background subtracted

+−

yie

lds

+−

asy

mm

etr

y _535M BB 1464±65 signal

events

Large Direct CP violation (5.5)in disagreement with BaBar

Large mixing-induced CP violation (5.6)

02.014.053.0

03.011.016.0

±±−=±±+=

SA


2() BaBar(//) + Belle(/)

Global Fit = [ 98 ] º +5-19

/2 = [93 ]+119 consistent with a global fit w/o /2


r

3()

• Time-dependent analyses get sin(21+3)• Best contraint on 3 comes from Dalitz analysis of B-

D(*)K(*)- with DKs+-

r =|A2|

|A1|B+: r

2m−

2m+

0 D 2m−

2m+

0 D

B-:

/3 = [53 ]+1518


|Vub|



M.Morii diagram


Summary

• Many, many results… Belle published ~200 journal papers

• All these measurements beautifully consistent with KM mechanism


A look into future

• Now, have we learned everything about CP violation?

– No! There must be more sources ofCP 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


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)


SuperKEKB

• Asymmetric energy ee collider at ECM=m((4S)) to be realized by upgrading the existing KEKB collider.

• Super-high luminosity 81035/cm2/sec 110 10 BB per yr.

910 9 per yr.

Higher beam current,more RF, smaller y* andcrab crossing L = 41035/cm2/sec

Belle with improved rate immunity

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


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


Thank you very much for your invitation!

Documents

Christoph Schwanda1 The Belle B Factory Past, present and future Christoph Schwanda, Innsbruck, Oct-18, 2006