Heavy flavour physics - University of Warwick · 2013. 3. 19. · 9 8/Sept/2011 Michal Kreps -...

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Physics Department

Heavy flavour physics

Plan

Kaon physics and SM construction (bit of history)

Establishing SM experimentally

Looking for breakdown of SM

Hard to cover everything in details in four lectures

Michal Kreps

Michal Kreps - Quark flavour physics 22 6/Sept/2011

Outline

Discuss angles and unitarity triangle

Why we need new physics

What are places to look for new physics


Angle β

Sf is in fact sin(2β)

Two-fold ambiguity exists in measurement

One way to resolve it is to use decays B0→J/ψKπ with interference between K* and s-wave Kπ

Can exclude one of the two solutions at reasonable confidence


Angle α

α is phase between Vtb

*Vtd and

Vub

*Vud

Extracted from b→uud transitions using mixing induced CPV

B0→ππ, ρρ, πρ

As those decays are suppressed, penguin contributions play also role

Isospin analysis used to correct for penguin contributions

Current (2010) value is 89±4.4°


Angle γ

Interference of B-→D0K- (Vcb

) and B-

→D0K- (Vub

) when using common final state for D0 and D0

Plays special role as it is extracted from tree level decays

Provides CKM phase without being significantly affected by new physics

Three different D final states used

GLW: Cabbibo suppressed CP-eigenstates (KK, ππ)

ADS: Doubly Cabbibo suppressed D0→K+π-

Dalitz plot: Ksππ


Angle γ

B- decays are challenging for γ measurement as

CP asymmetry is large, but rate is very small (ADS)

Rate is reasonable, but CP asymmetry is small (GLW)

Only in about last year experiments start to see significant signals

Other promising decay is Bs→D

sK

But this requires to resolve fast Bs oscillation

First measurement from LHCb

Will discuss this bit later


Angle γ


Angle γ

Dalitz plot measurements are still most sensitive

Still not very precise

Some disagreement in uncertainty between different combinations of measurements


Why new physics?

What we saw up to now is extreme success of SM

So why we are so excited about new physics if we can describe all measurements without it?

SM does not tell why we have three generations (do we?)

SM does not tell anything about masses of fermions

Way SM works needs quite some fine tuning (final result is often difference between two huge numbers)

Higgs mass in SM is not really stable

Cosmology argument:

No candidate for Dark matter

Not possible to generate current matter-antimatter asymmetry in Universe


Sakharov

In 1967 Andrei Sakharov formulated three necessary conditions to produce baryon-antibaryon asymmetry

Baryon number violation

CP violation

Interactions out of equilibrium

Surprisingly SM can best cope with baryon number violation – but non-perturbative QCD effect

CP violation exists in SM, but definitely not large enough

In SM there is nothing which could drive system out of equilibrium

Those are in my view strongest needs for new physics


Typical NP scenarios

SUSY – most popular

Each SM particle gets its SUSY partner

Squarks can also mix (similar to quarks) and provide additional phases

Fourth generation

Puts in additional fermion generation

Not as appealing as SUSY and cannot answer all questions, but can make huge difference on CPV

Technicolor

Gets more popular again as it can dynamically create Higgs like effect without need of Higgs


NP DNA


Bs→μ+μ-

SM prediction Br=(3.2±0.2)10-9

Extremely sensitive to new physics

Already with limits 10 times above SM one could severely restrict NP models


Experiments


LHCb is not small one


Main experimental issues

As decay is rare, main issue is to control background

Special care is needed for B→hh' decays which peak in same place as signal

Selection ranges from cut based (CMS) to BDT (LHCb) to NN(CDF, D0)


Tevatron Bs→μ+μ-

Tevatron was for long time only place to really study this decay

With final data could come close to SM

D0: <15*10-9 (exp 23*10-9)

CDF: two-sided bound (0.8—3.4)10-8 (exp 13*10-9)

Small excess, but does not look like signal

Which experiment is better?


Bs→μ+μ- with GPDs @ LHC

LHC experiments can finally probed SM values

CMS: <7.7*10-9 (8.4*10-9)

ATLAS: <22*10-9 (16*10-9)

In long run, GPD should give us best branching fraction measurement

This comes from higher luminosity compared to LHCb


Bs→μ+μ- @ LHCb First evidence for signal

(3.5σ)

Branching fraction (3.2+1.4

-1.2+0.5

-0.3)10-9

Spot on Standard model prediction


CPV induced by Bs mixing

CPV stems from interference of decays with and w/o mixing

In SM it is expected to me tine – Effect of almost real Vts

Needs some care in interpretation of what is really measured

Up to now measured with Bs→J/ψφ and Bs→J/ψf

0(980)

Difficult due to large Δms~17.8 ps-1

Needs very good time resolution


Master formulas of time evolutionA

rXiv:0 90

4.1 869


Decay Bs→J/ψf

0

This final state is CP-eigenstate

In master formula this (with some additional assumptions) means |λ

f|=1

For Bs the a is at most ~ 1% and ΔΓ is in principle non-zero


Decay Bs→J/ψf

0 - Lifetime


Decay Bs→J/ψf

0 - CPV

Takes input of Γ and ΔΓ from LHCb B

s→J/ψφ result

Consistent with SM


Decay Bs→J/ψφ

This decay was first to study as it has higher signal yield

Complication arises from the fact that both J/ψ and φ are have spin 1 => mixture of CP-even and CP-odd

Need additional work (angular analysis) to statistically distinguish two

On the other hand it also helps to gain additional sensitivity from interference

There are many details to care about, much beyond scope of this lecture

If you are interested, check CDF paper arXiv:1112.1726, it has more details than you want to know (LHCb long paper on its way)

Update from LHCb very soon


Decay Bs→J/ψφ


Quantum interference in action

On previous page I showed only one solution while for CDF and D0 two, which are equally good


Going back to Bs→D

sK

ArX

iv:0 904.1 869


Going back to Bs→D

sK

Place where all terms in time evolution play role

First analysis ever by LHCb

Not yet constrain on g angle

Heavy flavour physics - University of Warwick · 2013. 3. 19. · 9 8/Sept/2011 Michal Kreps -...

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