beautiful lessons from b physics - epfl

Beautiful Lessons from B physics

Thomas Schietinger

Laboratory for HighEnergy PhysicsEPF Lausanne

Colloquium, University of Neuchatel 6 June 2005

Thomas Schietinger 6 June 2005Beautiful lessons from B physics 2

Contents● Introduction: quarks and their interactions,

the bottom quark● CP violation – the KobayashiMaskawa Model● B mesons and their phenomena● B physics experiments: Belle, BABAR, LHCb● Current challenges:

1.CP violation in B oscillation (top down transition)

2.CP violation in B decay (bottom up transition)

3.CP violation beyond the Standard Model


The quarks

up charm

down strange bottom

top

+2/3

1/3


up~0.004 GeV

down~0.008 GeV

top175 GeV

strange0.15 GeV

charm1.25 GeV

bottom4.2 GeV

+2/3

1/3

The quarks


The mass hierarchy is explained by the substructure!


There is a message here!

up~0.004 GeV

down~0.008 GeV

top175 GeV

strange0.15 GeV

charm1.25 GeV

bottom4.2 GeV

+2/3

1/3


Quark interactions

q q

gravitational

q q

electromagnetic

q q

strong g weak Z0

q qW+

q q'


Flavor change

W+

q q'

...only possible via exchangeof charged W boson:


Flavor change

W+

c s

for instance:charm decays to strange


Flavor change

W+

c s

Coupling strength given bya 3x3 complex matrix:

Vcs


The CKM matrix (another message!)

down strange bottom

up

charm

top

CK

M =

Cab

ibbo

, Ko

bay

ash

i, M

aska

wa


The CKM matrix

Complex elements:some carry a phase⇒ the flavor and thephase of the quark are changed!

down strange bottom

up

charm

top

~25˚

~115˚


The CKM matrix

This phase change goes in the oppositedirection for antiquarks!

antidown antistrange antibottom

antiup

anticharm

antitop

~ –25˚

~ –115˚

⇒ CP violation!

(in some processesinvolving interferencewith top and bottom)


CP violation

“ CPmirror”

K0

+ –

K0

– +

The decay K0 +– is slightly moreprobable than the decay K0 +–!

Discovered in 1964 with K (strange) mesons

Christenson, Cronin, Fitch, Turlay, Phys. Rev. Lett. 13, 138 (1964)

K0 = (sd)

K0 = (sd)

+= (ud)

–= (ud)


PS

195

Since then measured with high precision: (for instance CPLEAR) Angelopoulos et al.,

Phys. Lett. B 458, 545 (1999)

K0 +–

K0 +–

decays

decay time [S]


The CP puzzle● CP violation came as a surprise and it was puzzling:

– A tiny effect!

– Only observed with neutral kaons.

● It took almost ten years until Kobayashi and Maskawa realized 1973 that complex couplings (i.e. nontrivial phases) are possible if there are at least three generations of quarks. (At the time only the first generation and the strange quark were known.)

➔ Prediction of the bottom and the top quark, experimentally confirmed:

✔ 1977 discovery of bottom (Lederman, Fermilab)

✔ 1995 discovery of top (CDF&D0 coll., Fermilab)

● Note the contrast to Dirac's “invention” of antimatter!


Dirac: “My equation has negative solutions⇒ there must be antiparticles!”

Kobayashi/Maskawa:

“There is this experimental nuisanceCP violation, let's try with six quarks!”

(purely aesthetical, no experimental evidence)

CP

(purely experimental evidence, no aesthetical merit)


The CP puzzle (cont'd)

● If the KobayashiMaskawa (KM) picture was correct then the tiny effects seen in the kaon system were only faint echoes of much larger violations of CP in decays of bottom and top!

– Strong interaction effects prohibit a decisive test of the KM model with neutral kaons (except for extremely rare ones).

● Top very heavy and shortlived ⇒ difficult to produce and analyze!

● Bottom quarks, however, can be produced in large quantities.

● They come shrinkwrapped in the form of B mesons (and baryons, but those are much less useful).

● B mesons exhibit many interesting phenomena that can be harnessed by the experimenters.

➔ Strong motivation to build socalled “ Bfactories” !


B mesons

bd ububbd

B0 B0 B+B–

bs cbcbbs

Bs0 Bs

0 Bc+Bc

–


B meson phenomena

direct decay

b

W–

q

q

s,d

u,c,t

loop decay

bW–

q, ℓ

q',

u,c

mixing / oscillation:

( = 489 GHz)

b

W

u,c,t

Wu,c,td

B0 B0

d

b


B meson phenomena

direct decay

b

W–

q

q

s,d

u,c,t

loop decay

bW–

q, ℓ

q',

u,c

mixing / oscillation:

“ tree diagram” “ penguin diagram”

“ box diagram” b

W

u,c,t

Wu,c,td

B0 B0

d

b


B meson phenomena

direct decay

b

W–

q

q

s,d

t

loop decay

bW–

q, ℓ

q',

u,c

mixing / oscillation:b

W Wd

B0 B0

d

bt

t


B meson phenomena

bW–

q, ℓ

q',

u,c

direct decay

b

W–

q

q

s,d

t

loop decay

mixing / oscillation:Vts

VtdVtd

Vub Vcb

b

W

t

Wtd

B0 B0

d

b


back to the CKM matrix:

down strange bottom

up

charm

top



Vud

Vcd Vcb

Vub

Vts

Vcs

Vus

Vtd Vtb



Vud

Vcd Vcb

Vub

Vts

Vcs

Vus

Vtd Vtb

Accessible via direct decays

Accessible via loop decays andoscillation

The B meson is theideal laboratory to study the “ interesting”quark couplings!


Testing the KM hypothesis● Unitarity (conservation of probability) imposes strong

constraints on the quark mixing matrix:

● It depends on only 4 real parameters, among them the phase responsible for CP violation

● This fact renders the KM hypothesis extremely predictive!

– All CPviolating phenomena depend on just one parameter!

– Whereas almost all “new physics” scenarios (supersymmetry etc.) introduce new sources of CP violation.

➔ Many relations among the 9 (complex) matrix elements that can be tested experimentally!

● In particular: the “unitarity triangle”


The unitarity triangle

Vud

Vcd Vcb

Vub

Vts

Vcs

Vus

Vtd Vtb

Unitarity conditionfor the two “ interesting”columns:

VudVub*+VcdVcb*+VtdVtb* = 0

⇒ Vub*+Vtd = Vcb* ( = 0.22)

In the complex plane:

Im

Re

Vub*

Vtd

Vcb*

= arg(Vtd)

= arg(Vub*)


A remarkable prediction!

|Vtd|

|Vcb|

The extent of CP violation (the CP violating phases and )is completely determined by the strengths of the quark couplings

upbottom and topdown!

|Vub|A simple highschool

geometry construction!

Is it true?...


The CP agenda

1)What is the phase of Vtd? ()

● Is it compatible with the prediction from |Vub| and |Vtd|?

2)What is the phase of Vub? ()


3)Are there additional sources of CP violation?

● We know there have to be: the KobayashiMaskawa mechanism is not sufficient to explain cosmological matterantimatter imbalance!

● |Vub| is known from the rate of semileptonic b u decays.

● |Vtd| is known from the frequency of B0–B0 oscillation.● The phases are very hard to measure!


e+e– colliders: clean production, but limited statisticsLEP at CERN: e+e– Z0 bb {B0, B±, Bs0, Bc±, bbaryons}

ALEPH, DELPHI, L3, OPAL (multipurpose experiments)have each collected ~700k bb pairs (1989–1993)

B factories: e+e– (4S) B0B0 or B±B∓

ARGUS at DORIS (DESY, Hamburg) 190k BB 1982–1992CLEO (and CUSB) at CESR (Cornell) 16 M BB 1979–2001Belle at KEKB (KEK, Tsukuba) 275 M BB 1999–2007(?)BABAR at PEPII (SLAC, Stanford) 227 M BB 1999–2006(?)

hadron colliders: huge statistics, but messy environmentTevatron at Fermilab: pp X+bb {B0, B±, Bs0, Bc±, bbaryons} (20 kHz!)

CDF, D0 (multipurpose experiments) 1985–2008(?)BTeV (dedicated B experiment) cancelled

LHC at CERN: pp X+bb {B0, B±, Bs0, Bc±, bbaryons} (100 kHz!)LHCb (dedicated B experiment) 2007– ?

The experimental players


Asymmetric B factories

● (4S) is the first bottomonium resonancethat can decay to B mesons.

● ~50% B0B0 , ~50% B±B∓mesons (no Bs or Bc!)● The B0B0 system is entangled! ⇒ Always one B0 and one B0!

Why asymmetric?

symmetric:asymmetric:

B0

B0B0

B0

⇒ Boost gives better time resolution and a reference “t0”!

~30

µm

~200 µm

e+e– (4S) B0B0 or B±B∓


BelleAn asymmetric Bfactory at the

KEKB e+e– collider in Tsukuba (Japan)

● Silicon vertex detector● Gas drift chamber● Aerogel Cherenkov counters (PID)● Timeofflight scintillator counters● CsI crystal calorimeter● Muon chambers (RPC)● 1.5 Tesla superconducting solenoid

8 GeV e–

3.5 GeV e+● Started in 1999● Collaboration of

~300 physicists from ~60 institutes in14 countries.

● Has collected ~275 million BB pairs


BABARAn asymmetric Bfactory at the

PEPII e+e– collider in Stanford (USA)

● Silicon vertex detector● Gas drift chamber● Ring image Cherenkov detector, based

on internal reflection (DIRC)● CsI crystal calorimeter● Muon chambers (RPC)● 1.5 Tesla superconducting solenoid

3 GeV e+

9 GeV e–

● Started in 1999● Collaboration of

~550 physicists from ~72 institutes in9 countries.

● Has collected ~227 million BB pairs


LHCbA dedicated B physics experiment

at the LHC pp collider at CERN (Geneva, Switzerland)

● Under construction, start planned for 2007

● Collaboration of ~560 physicists from ~45 institutes in13 countries.

Singlearm forward spectrometer: production of b hadrons is strongly peakedin the forward and backward direction.

14 TeV pp

b [rad]

b [rad]


The CP agenda










Measurement of arg(Vtd)

B0

bW–

d

c

c

s

d

Vcb*Vcs

J/

K0 KS

B0

bW+

d

c

c

s

d

Vcb

Vcs*

J/

K0 KS

● Both B0 and B0 can decayinto the CP eigenstate J/ KS.

● Striking experimental signature!● No phases are involved in

the decay (only Vcs and Vcb).

Bigi and Sanda, Nucl. Phys. B 193, 85 (1981)

Carter and Sanda, Phys. Rev. D 23, 1567 (1981)


B0

bW–

d

c

c

s

d

Vcb*Vcs

J/

K0 KS

B0

bW+

d

c

c

s

d

Vcb

Vcs*

J/

K0 KS

B0

d

b

t

t

Vtd*

Vtd

⇒ The B0 has two ways to decay into J/ KS:

“ unmixed” decay: no phase

“ mixed” decay: phase of Vtd!

⇒ Timedependent interference term proportional to:

Vtb* Vtd

Vtb Vtd*= e2i


1) Find events containing the final state (f )● Reconstruction of tracks, neutrals; combination to give B candidate

2) Determine whether at t = t0 it was a B0 or a B0

● “Flavour tagging”: exploit the correlation between the two B's produced in association.● t0 is the time of the production vertex (hadron colliders) or the time of the decay of the

other B (asymmetric B factories).

3) Measure the time t ● Distance from the production vertex (hadron colliders), or distance between the two B

decays (asymmetric B factories).

Timedependent CP asymmetries

ACP(t ) = R(B0 f )(t ) – R(B0 f )(t )

R(B0 f )(t ) + R(B0 f )(t ) = sin(2) sin(t )


Event displayReconstruction, tagging, time dependence


Observation of large CP asymmetry!

Belle: sin(2) = 0.728 ± 0.056 ± 0.023

BABAR: sin(2) = 0.722 ± 0.040 ± 0.023

World average: sin(2) = 0.733 ± 0.037

⇒ arg(Vtd) = = 23.6°± 1.6°∨ = 66.4°± 1.6°

BABAR



“CKM fitter”:http://ckmfitter.in2p3.fr/

A. Höcker et al.,Eur. Phys. J. C21, 225 (2001)


Beyond sin(2)● sin(2) was a triumph for the KobayashiMaskawa

model!

● But an important piece is still missing to complete the picture:– All CP violation observed so far arises from particle loops (K and

B oscillation)

– These loops could be affected by “ new physics” (heavy exotic particles can virtually participate in the loop)

● We must confirm CP violation in direct decays! – With our current experimental tools, Vub is the only possibility!

– Many strategies have been proposed to measure the phase of Vub, I will just focus on the most promising one.


The CP agenda










Measurement of = arg(Vub)

b

W–

cVcb*

Basic idea:measure interference betweenb cW– and b uW– decays!

b

W–

uVub*

For this, we need:1.Common final states to which both

b cW– and b uW– can contribute2.Calculable or measurable finalstate

interaction phases to measure CPphase against.

Only two possibilities:1. Exploit B mixing (similar to sin(2) ⇒ measure sin(2+)!) 2.Charged B decays to D0K±, where D0 and D0 decay to the same final state.

( )


B+ D0K+ vs B+ D0K+

D0b

W+

cVcb

u uB+

K+u

s

Vus*

B+ D0K+:external tree diagram⇒ colorallowed

B+ D0K+:internal tree diagram⇒ colorsuppressed

r = ≈ 0.1–0.2Aallowed

Asuppr.

ratio:

K+

D0

bW+

s

u uB+

c

u

Vub Vcs*

Unknown strong phase shift between the two diagrams.

The “DDalitz” method:Both D0 and D0 can decay, via resonances, to KS+– ⇒ interference

sensitive to angle !

KS+–

But:Requires a detailed analysis of all resonances! (Dalitz plot analysis)


Extraction of D0 KS+– Dalitz ampl.

~187k signal events (+ 6k background)

Binned comparison of fit and data yields 2 = 2543 for

1106 degrees of freedom (⇒ systematic error)

Maximum likelihood fit

m2(KS–)= m–

2

m2(+–) m2(KS+)= m+

2

from e+e– continuum data

(Belle collab.)


18 resonance + 1 nonresonant contribution!

...a major contribution to charm physics in itself!

D0 KS+– intermediate resonancesExtraction of D0 KS+– Dalitz ampl.

(Belle collab.)


B± D(*)( KS+–)K± Dalitz plots

B+DKK+

B+D*K+

B–DKK–

Any difference signals(direct!) CP violation!

B–D*K–

276 candidate events(209 ± 16 signal)

Can also use:

B± D*0 (D KS+–, 0) K±

69 candidate events(58 ± 8 signal)


Maximum Likelihood fit

B+ D ( KS+–) K+ decay:

A+ = f(m+2, m–

2) + r ei(+) f(m–2, m+

2)

B– D ( KS+–) K– decay:

A– = f(m–2, m+

2) + r ei(–) f(m+2, m–

2)

Amplitudes:

Knowing the D0 Dalitz amplitude from the continuum sample, can fit for

r ei±

separately to B+ and B– sample,

with + = + , – = –

: weak phase : strong phase

Result: (for B±DKK±)


Maximum Likelihood fit

B+ D ( KS+–) K+ decay:

A+ = f(m+2, m–

2) + r ei(+) f(m–2, m+

2)

B– D ( KS+–) K– decay:

A– = f(m–2, m+

2) + r ei(–) f(m+2, m–

2)

Amplitudes:

Knowing the D0 Dalitz amplitude from the continuum sample, can fit for

r ei±

separately to B+ and B– sample,

with + = + , – = –

: weak phase : strong phase r = 0.247±0.071

= 156.6°±15.6°

= 63.7°±15.2°

Result: (for B±DKK±)


Impact on the unitarity triangle

= 63.7° (stat.) ±13°(syst.) ±11°(model) +14° –15°

(with twofold ambiguity ↔ +)

Belle's final result:


Progress on In principle, can measure it with B0 +–:

B0b

W–

d

d

uVub*

Vud

+

–

d

u

sin (2+2) = –sin(2)!

B0

b

d

Vub

B0

d

b

t

t

Vtd*

Vtd

W+

u

Vud*

–

+

d

u

d


Progress on But there is a contamination from loop decays, which carry a different phase (penguin pollution):

Tree decay:

B0

b

d

d

u

u

d

–

+

W–

t

VtsVtb*

B0

b

d

Vub*W–

d

uVud

+

–

d

u

Penguin decay: no phase

Early B factory results showed that loop contribution is much larger thanexpected! Bad news for the measurement of sin(2)...

But there is a way out!


Progress on

● B , B are mediatedby the same diagrams, but suffer in different ways fromfrom “penguin pollution”

● Key observation: B is toalmost 100% longitudinallypolarized!

● Simplified isospin relations todetermine extent of “penguinpollution”

= 103°±10°


Unitarity triangle from , , alone

Now better than indirectmeasurements!

Complete agreement withindirect measurements

The CKM model haspassed its longawaited“ test” with flying

colours!


Beyond KobayashiMaskawa● The KobayashiMaskawa mechanism alone cannot explain the

observed matterantimatter asymmetry of the universe:

– If the KM mechanism were responsible for the matter dominance after the big bang, we would expect 1018 photons for every proton.

– The observed value is 109 photons per proton.

– ⇒ There must be additional sources of CP violation from a new kind of physics! (Supersymmetry? Extra dimensions?)

● Can we hope to observe this kind of CP violation in B decays?

– Yes: if we look at decays that are strongly suppressed in the Standard Model, new physics effects have a better chance to be observable.

– Loop decays are particularly suitable, as heavy exotic particles may appear virtually in the loop.

– Again, I will pick just one example out of many possibilities.


The CP agenda










B0 KS

Let's go back to B0 J/ KS:

B0

bW–

d

c

c

s

d

Vcb*Vcs

J/

K0 KS


B0 KS

B0

b

d

s

s

s

d

K0 KS

Replace the J/ (cc) by a (ss):


B0 KS

b s transition: must occur via a loop! (“gluonic penguin”)

B0

b

d

s

s

s

d

K0 KS

W–

t

VtsVtb*

The loop carries no phase in the Standard Model...


B0 KS

...but new physics could change this!

B0

b

d

s

s

s

d

K0 KS

squark

??

New physics contributions can alter the phase of the decay


B0 KS

B0

b

d

s

s

s

d

K0 KS

W–

t

VtsVtb*

B0

b

d

s

s

s

d

K0 KS

W+

t

Vts*Vtb

B0

d

b

Vtd

Vtd*

t

t

experimentally completely analogous to J/ KS:

Measure CP asymmetry, expect sin(2) in Standard Model


Belle surprise!

Belle has found 139±14 B0 KS decays and measured the CP asymmetry:

“ sin(2)” = 0.06 ± 0.33 ± 0.09

Expect the same asymmetry as in J/ KS, i.e. +0.74! ⇒ a 2 deviation!

● Errors are still large!● BABAR result also low, but

smaller discrepancy:“sin(2)” = +0.50 ± 0.25 ± 0.07

● What about other b s channels?

Nsig = 139±14purity = 63%


b s Penguin ranking(quantifying the b u pollution)

b

d

s

ss

d

W–

t

b

d

s

dd

d

W–

t

b

d

W–

s

u

d

u

b

d

u

u

s

d

W–

b u tree (colorsuppressed)

b u tree (colorallowed)

b ddd penguin

b sss penguin

P

P'

T

T'

KS ✔ ✘ ✘ ✘KSKSKS ✔ ✔ ✘ ✘

'KS ✔ ✔ ✔ ✘f0KS ✔ ✔ ✔ ✘

0KS ✘ ✔ ✔ ✘0KS ✘ ✔ ✔ ✘KS ✘ ✔ ✔ ✘K+K–KS ✔ ✔ ✔ ✔

B0 ... P P' T T'

“ Golden”

“ Silver”

“ Bronze”


B0 KSKSKSanother golden mode

Belle has found 88 ± 13 B0 KSKSKS decays and measured the CP asymmetry:

“sin(2)” = –1.26 ± 0.68 ± 0.18

Expect the same asymmetry as in J/ KS, i.e. +0.74! ⇒ a 2.8 deviation!

● Again, not confirmed byBABAR:“sin(2)” = +0.71 ± 0.38 ± 0.04

● Very interesting channel tofollow!

Nsig = 88±13purity = 53%


b s Penguin results(spring 2005 averages)

Still a fuzzy picture!

sin(2) = 0.726 ± 0.037

“sin(2)” = 0.43 ± 0.07

b c modes averaged:

b s modes averaged:

● b s modes appear to be systematically lower than b c!

● At face value (naïve) a 3.7 discrepancy.

● BUT: simple averaging is... problematic already within the

Standard Model (theor. uncert.) Not meaningful at all if these loops

are dominated by new physics!➔Need more precise measurements

of individual channels!


b s Penguin results(spring 2005 averages)

● b s modes appear to be systematically lower than b c!

● At face value (naïve) a 3.7 discrepancy.

● BUT: simple averaging is... problematic already within the

Standard Model (theor. uncert.) Not meaningful at all if these loops

are dominated by new physics!➔Need more precise measurements

of individual channels!

sin(2) = 0.726 ± 0.037

“sin(2)” = 0.43 ± 0.07

b c modes averaged:

b s modes averaged:

Belle/BABAR averaged


b s: a job for LHCb?

bsbs

Bs0 Bs

0

b

W Ws

Bs0 Bs

0

s

bt

t

The (bs) bound state Bs allows for a particularly close examination

of the b s transition!


Conclusion (1): the present● We are living in a golden age of B physics!

– Data pouring in from B factories! It's a gold rush!

● The KobayashiMaskawa Model has passed its long awaited “test” with flying colours:

– Direct measurement of arg(Vtd) = in excellent agreement with predictions from unitarity (2001). More recently:

– Direct measurement of arg(Vub) = by Belle: again excellent unitarity prediction!

– ...and constraints on = 180° – – .

● There are tantalizing hints for new physics in B decays

– Belle sees deviation in B0 KS CP asymmetry, not confirmed by BABAR.


Conclusion (2): the future● The B factories will continue their hunt for new physics

until at least 2008.– More data eagerly awaited on b s transitions.

– But also incredible harvest of general beauty and charm physics!

● LHCb at CERN will join in 2007 and measure the Bs system with exquisite accuracy (among other things...).

● Beyond that, the future of B physics depends entirely on what the new physics is, and how it manifests itself!– Either there are important effects to be measured in B decays (CP

violating phases) which will help to understand the phenomena seen directly by the LHC and a possible linear collider...

– ...or the new physics will turn out to manifest itself at scales far beyond what is accessible with B decays.


Further readingGeneral level:

Nature:Michael Peskin: “The matter with antimatter”, Nature 419, p. 24–27 (2002).John Ellis: “Antimatter matters”, Nature 424, p. 631–634 (2003).

Physics Today:Helen Quinn: “The asymmetry between matter and antimatter”, Physics Today, Feb. 2003 (Vol. 56, Nr. 2), p. 30–35.

Neue Zürcher Zeitung:Thomas Schietinger: “Vom Kuriosum zum Präzisionsinstrument der Teilchenphysik:Die Verletzung der CP Verletzung als Schlüssel zur Erforschung der Quarks”, Neue Zürcher Zeitung, 23. Januar 2003 (Nr. 23), p. 63.

European Physical Journal A:Klaus Schubert: “CP violation in Bmeson decays” , Eur. Phys. J. A 18, p. 147–153 (2003)

Slightly more advanced:

beautiful lessons from b physics - epfl

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