# Presentation: 770092

## Measurements of Time Dependent CP Asymmetries in Charmless Penguin-Dominated B Meson Decays at BaBar

- Alfio Lazzaro
- Dipartimento di Fisica and INFN, Milano
-  Research Progress Meeting
- December 4th, 2007
- Lawrence Berkeley National Laboratory

## Overview

- Symmetries
*CP* violation timeline discoveries: from *K* to *B* mesons system
- Two states formalism
- CKM picture in Standard Model
- Penguin decays
*B* physics facilities
- Time Dependent measurements in *B*-factory detectors
- Time Dependent measurement in *b→s* modes
- Observation of CP violation in B0  ' K0
- Summary of the results
- Conclusion

- Alfio Lazzaro

- December 4, 2007

## Introduction

- Alfio Lazzaro

- December 4, 2007

## Symmetries help to describe the Nature: they are key factors for the development of the physics theories

Understanding the symmetries and finding eventual violations is a crucial test for our understanding of the Nature

- Understanding the symmetries and finding eventual violations is a crucial test for our understanding of the Nature

- Symmetries in Nature

- Mirror

- Original

- Alfio Lazzaro

- December 4, 2007

## Discrete Symmetries

- Parity (**P**): *x *→ −*x*
- Violated in weak interaction (C. S. Wu, 1956)
- Charge (**C**): particles ↔ antiparticles
- Violated in weak interaction
- Time (**T**): *t* → −*t*
- _Before 1964_:** C** and **P**
- 	violated separately in weak
- 	interaction, but **CP** should
- 	be conserved

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 47, 48, 49

## CP Violation timeline discoveries

- J.H. Christenson, J. Cronin, V. Fitch and R. Turlay at Brookhaven Lab: discovery of CP violation symmetry in *K* system (1964)
- Are there other systems where we can observe CP violation?
- Discovery of *b* quark at FNAL (1977)
- Discovery of “naked beauty” (mesons with *b* quark)   e+e−  (4S)  *B**B* (Cornell, 1982)
*B* lifetime measurement (SLAC, 1982)
*B-**B* mixing measurement (DESY, 1987)
*CP* established in *B* decays with charm quark in the final state (SLAC and KEK, 2001): 37 years later than *K* system!

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 49--55

## Formalism and theory

- Alfio Lazzaro

- December 4, 2007

## B mesons flavor states

- Charged and neutral pseudoscalar *B* mesons (flavor states):

- PDG 2006

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 56-57

## Mixing

- We can describe *B* meson in terms of two physical states, combination of flavor states:
- For each physical state (denoted by *H* and *L*) we define a mass term *m* and a decay term 
- Mixing diagram (mixing frequency: *m = m**H** – m**L*):
- In the Standard Model (SM) these diagrams are dominated by top quark
- *m* = (0.507 ± 0.005) ps–1 for *B*0 (*m*d) (PDG 2006), in agreement  with SM prediction

- Alfio Lazzaro

- December 4, 2007

## Classification of CP-violating effects

- Decay amplitudes for *B**** f* and CP-conjugated states:
- 	(H is the Hamiltonian)
- Three kinds of CP violation in *B* mesons:
-  In the decay (direct CP)
- In the mixing
-  Interference between decay and mixing

- Alfio Lazzaro

- December 4, 2007

## “Direct” CP asymmetry (1)

- Time-integrated “direct” CP asymmetry (where   |AT|2):

****

****** ******** ******

****** ****+ ******

**A****T**** ****=**** ****A****1****+**** ****A****2**

**A****T**** ****=**** ****A****1****+**** ****A****2 ******** ****A****T**

- Require at least two amplitudes and 

- Alfio Lazzaro

- December 4, 2007

## “Direct” CP asymmetry (2)

- Time-integrated “direct” CP asymmetry (“CP violation in decay”) is valid for neutral and charged mesons
- 	                                             

- Alfio Lazzaro

- December 4, 2007

## Time evolution of neutral B mesons

- We can derive the time evolution of neutral *B* mesons, starting from specific flavor physical states which evolve as function of *t*
- We define the time-dependent asymmetry
- Special case: neutral pseudoscalar mesons produced coherently in pairs, from the decay of a vector resonance: *V*→*P*0*P*0,  where *P*0 and *P*0 decay at times *t*1, *t*2: (4S)→*B*0*B*0; →*K*0*K*0. In this case we can redefine the time-dependent asymmetry as function of *t *= *t*2−*t*1, with *t *→ *t*

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 58--59

## CP Asymmetry in the Mixing

- We use self-tagging flavor events
- 					
- Expected very small for *B* mesons in the SM
- This was the effect observed in 1963 for *K* mesons. Not observed for *B* mesons yet.

- Alfio Lazzaro

- December 4, 2007

## CP asymmetry in Interference between decay and mixing (1)

- Final state in common to *B*0 and *B*0  CP eigenstate *f*CP
- Interference between mixing and decay to a *f*CP
- The important parameter here
- 	is the “CP parameter”:

*CP*
- eigenvalue

- Amplitude
- ratio

- from mixing

- Alfio Lazzaro

- December 4, 2007

## CP asymmetry in Interference between decay and mixing (2)

- CP asymmetry in Interference between decay and mixing (2)

- For one
- decay amplitude

- Asymmetry

- Direct CP

- Interference between decay and mixing

- Physical Observables

- Alfio Lazzaro

- December 4, 2007

## The CKM paradigm in the SM

- 1973: M.Kobayashi and T.Maskawa
- CP violation  third generation of quarks
- Cabibbo-Kobayashi-Maskawa matrix V
- couples quark charged currents to W
- mixes the left-handed (qj=d,s,b) quark  mass eigenstates to give weak eigenstates
- unitary, with _4 independent parameters_ (e.g., 3 angles and 1 phase)
- complex elements: phase changes sign under CP
- Interfering amplitudes can give observable CP-violating rate asymmetries

- CP

- Quark doublets

- Alfio Lazzaro

- December 4, 2007

## CKM matrix parameterization

***u***

***d***

***s***

***b***

- Wolfenstein parameterization:

***c***

***t***

**Complex phase**
**It changes sign under CP**

- Orders of magnitude for Wolfenstein parameters:

- Alfio Lazzaro

- December 4, 2007

## The Unitarity Triangle (UT)

**u**
**c**
**t**

**d     s      b**

- These three triangles (and the three triangles corresponding to the rows) all have the same area. A nonzero area is a measure of CP violation and is an invariant of the CKM matrix.

- apply unitarity constraint to V

- 6 triangles in the complex plane

- relative
- magnitudes

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Page 60

## The Unitarity Triangle (UT)

**d•b* **= 0

- (Bd system)

**(1,0)**

****

****

****

**(********,********)**

**(0,0)**

- α, β, γ angles related to *CP* violation measurements
- Many process available to measure these angles

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Page 61

## TD in b→ccs “tree” Amplitudes (sin2 measurement)

*b*

*d*

*c*

*c*

*s*

*d*

- W-

*B**0** decay*

*K**0** mixing*

*s*

*d*

*d*

*s*

*b*

*d*

*d*

*b*

*B**0** mixing*

- TD in *b→c**c**s* “tree” Amplitudes (sin2** measurement)

- “Large” branching fractions, i.e.: BF(*B*0→*J*/*K*0) = 8.7  10−4
- Low background (purity up to 97%)
- All decay amplitudes have the same weak phase  clean theoretical prediction
- Expected
- Measured (PDG 2006): sin(2**) = 0.725 ± 0.037, *C* consistent with zero
**First observation of CP violation outside *****K***** mesons, in agreement with the SM prediction**

- Golden Channel: *B*0*→J/**** K*0

**angle of the UT

- Tree diagram dominant

- Alfio Lazzaro

- December 4, 2007

## bs “Penguin” Decays

- Flavor-Changing Neutral Current *b****s* decays forbidden at tree level in SM, possible at loop (“penguin”) level
- In the loop we can have *u*, *c*, or *t* quarks (*t* quark diagram dominant)
- Several amplitudes for the final state  possible uncertainties in the theoretical prediction
- Many modes available: Κ0, η'K0, π0K0,  ρ0K0,  ωK0, etc
- Rare modes (BF ~ 10−6 ÷ 10−4) with low purity

- Tree diagram suppressed

- Loop (penguin) diagram dominant

- Alfio Lazzaro

- December 4, 2007

## Why “penguins”?

- “Mary K. [Gaillard], Dimitri [Nanopoulos], and I first got interested in what are now called penguin diagrams while we were studying CP violation in the Standard Model in 1976... The penguin name came in 1977, as follows.
- In the spring of 1977, Mike Chanowitz, Mary K. and I wrote a paper on GUTs [Grand Unified Theories] predicting the b quark mass before it was found. When it was found a few weeks later, Mary K., Dimitri, Serge Rudaz and I immediately started working on its phenomenology.
- That summer, there was a student at CERN, Melissa Franklin, who is now an experimentalist at Harvard. One evening, she, I, and Serge went to a pub, and she and I started a game of darts. We made a bet that if I lost I had to put the word penguin into my next paper. She actually left the darts game before the end, and was replaced by Serge, who beat me. Nevertheless, I felt obligated to carry out the conditions of the bet.
- For some time, it was not clear to me how to get the word into this b quark paper that we were writing at the time.... Later...I had a sudden flash that the famous diagrams look like penguins. So we put the name into our paper, and the rest, as they say, is history.”
- John Ellis in Mikhail Shifman’s “ITEP Lectures in Particle Physics and Field Theory”, hep-ph/9510397

- Alfio Lazzaro

- December 4, 2007

## TD in b→s “Penguin” Amplitudes

- Alternative measurements of sin 2**
- Contributions from suppressed diagrams with different weak phase produce deviation *S* = *S* − sin2**, which are mode dependent
- We measure *S* = sin2**eff
- Several approaches to estimate *S *inside the SM: QCDF (Beneke,Neubert), SCET (Zupan), SU(3) (Gronau, Chiang).  In SM Δ*S* expected small (0.01 ÷ 0.2)

- some of recent QCDF estimates
- sin2eff − sin2

- We present here the TD measurements of B0’K0

- KS, ’KS, 3KS smallest theoretical uncertainties

- Alfio Lazzaro

- December 4, 2007

## SU(3) Upper Bound on S for B0→'K0

- Constraints on *S* in η′K0 can be written in terms of rates of other SU(3) related processes in different superpositions, depending on the level of approximation chosen
- Several combinations, where we consider amplitudes of isoscalar pairs including π0, η and η′ mesons in final state:
- Using a frequentist approach we extract *S **≤* 0.15. Using simpler combinations we obtain *S* Upper bound in the range 0.08 ÷ 0.15

- Alfio Lazzaro

- December 4, 2007

## New Physics Contribution

- In the other models *S* expected of order of 0.02 ÷ 0.05. In particular QCDF gives positive *S*
- Penguins modes are sensitive to New Physics (NP) contribution: new heavy particles in the loop could give large value of *S*, not explained by the SM (Buchalla et al., JHEP 0509,074(2005) (hep/ph 0503151)
- Sensitive to NP: expected as deviation from sin2** (large *S*): Low energy probe for NP effects

- Loop (penguin) diagram dominated

**NP?**

- Possible contribution from New Physics (NP): new heavy particles in the loop

**SUSY**

- Alfio Lazzaro

- December 4, 2007

## CP Measurements

- Alfio Lazzaro

- December 4, 2007

## The B-factory approach

- ee  (4S)  B0B0, BB (~50% neutral, ~50% charged)
- CM energy = 10.58 GeV

- Favorable
- Signal / Background:

- BB threshold

- Alfio Lazzaro

- December 4, 2007

## Symmetric Collider

- CESR (CLEO, several upgrades) until 2001
- 17 M BB events
- Monochromatic *B* mesons, small boost:  0.06
- Vertex detector, particle identification

**In CM e****+****e****−**** rest frame**

***t*****1**

***t*****2**

**Not measurable!**

**30 ********m**

**Distance**

**Time**

**No Time Dependent measurements **
**are possible!**

*******t = t*****2****−*****t*****1**

*e**–*

- Alfio Lazzaro

- December 4, 2007

## Asymmetric energy beams: boost the B pair to measure t (P. Oddone, 1987)

- Asymmetric energy beams: boost the *B* pair to measure t (P. Oddone, 1987)

**

- 

*z*

*~*

*2*

*6*

*0*

*m*

- l

- 

- 

*K*

**Exclusive *****B***** Meson Reconstruction**
**+**
** ****Decay Vertex Reconstruction**
**CP eigenstate**

- 

- 

- 0

*S*

*K*

- 

- 

- 

- 

- 

- /

- J

- 

- 

******(4*****S*****) produces coherent *****B***** pair:**** **
***t***** → *********t***

**B-flavor tagging**
**+**
**Decay Vertex Reconstruction**

- Asymmetric Collider: B-factory

**

**

- 

- 

- (

- 4

- )

- 0

- .

- 5

- 6

*S*

**

**

**

**

- 

- 

- 

- ~

- 1

- p

- s

- 1

- 7

- 0

- m

- ~

- 1

- .

- 6

- p

- s

- 2

- 5

- 0

- m

*t*

*B*

- Alfio Lazzaro

- December 4, 2007

## Decay rate distributions

- Vertex Reconstruction: resolution effect
- Tag of the other *B*: wrong tag identification effect

- *m**d* = 0.507  0.005 ps–1
- B0 = 1.530  0.009 ps

**B****0**
**tag**

**Tag from**
**“****other B”**

- t/

**C=0, S********0**

**Time-integrated asymmetry = 0**
****** ****Need both ********t and tag!**

**PDG 2006**

- Alfio Lazzaro

- December 4, 2007

## Impact of tagging and resolution on CP

**sin2******

**D********sin2******

*Asymmetry*

**perfect tagging + perfect resolution**

**imperfect tagging + ****perfect resolution**

**dilution ****D**** = (1 – 2 ********)**
****** ****: ****probability of wrong assignment**

**imperfect tagging + ****imperfect resolution**

***R(************t) ~****** ******e******-|************t|/************B***
***  *****(1***** ± ************CP****** ******D****** sin2************sin ************m******d ************t***** )***** ********** R(************t)****** ***

- B0 tag

- B0 tag

***t*

******t resolution dominated by tag side: ********1 ps ******** ****170 ********m**

******B ** 1.6 ps  250 m

**B-flavor tagging efficiency** and ******t resolution function** are obtained from data (measurement of mixing, with exclusively reconstructed self-tagging flavor states)

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 67--68

## Asymmetric B-Factory PEP-II

- Asymmetric *B*-Factory PEP-II

**Stanford Linear Accelerator Center (SLAC)**

- 9 GeV e− × 3.1 GeV e+
- Boost (4S) : βγ  = 0.56

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 62--66

## PEP-II Integrated Luminosity

- PEP-II Project Design

- ~ 4 × Project Design

- ~460 Million *B**B* pairs collected

- 384 Million *B**B* pairs used in B0 K0 TD analysis

- Alfio Lazzaro

- December 4, 2007

## BaBar detector

**DIRC ********PID)**
- 144 quartz bars
- 11000 PMs

**1.5T**** solenoid**

**EMC**
- 6580 CsI(Tl) crystals

**D****rift ****C****hamber**
- 40  layers

**I****nstrumented ****F****lux ****R****eturn**
- Iron / Resistive Plate Chambers or Limited Streamer Tubes  (muon / neutral hadrons)

**S****ilicon ****V****ertex ****T****racker**
- 5 layers, double sided strips

- e (3.1GeV)

- e (9GeV)

**BaBar Collaboration**
** ****11 Countries**
**80 Institutions**

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 72--73

## 0→'K0

## Overview

- BF is the highest in the Charmless Quasi-Two body modes: (in 10–6)
- (first error statistical, second error systematics)
- Sub-decays:

- PRL **94**, 191802 (2005)

- In total: 6 neutral sub-decays (5 with K0S and 1 with K0L)

- Alfio Lazzaro

- December 4, 2007

## B 0→'K0: Reconstruction and Selection

- Two types of background (estimated using simulated samples)
- e+e−→*q**q*  (*q* = *u*,*d*,*s*,*c*) (continuum background) _dominant_
- e+e−→*b**b* (non-continuum background) ~ 8 - 10%
- Standard cuts for the reconstruction of 0, ρ0, η, η', K0S: reconstructed combining charged tracks and photons, applying kinematical cuts and constraints
- PID requirements for charged tracks
- K0L candidates reconstructed in EMC or IFR
- Special procedure to selected K0L candidates, since the detector doesn’t reconstruct the momentum
- Use a Neural Network for background suppression (mainly photons)
- Kinematical and topological cuts applied to reconstruct and select *B* mesons candidates

- Alfio Lazzaro

- December 4, 2007

**Notes:**
KL: pages 75--78
Bbbar bkg: 83--84

## Event Selection: Variables

- Topological variables: based on different event distribution for continuum VS *B**B* events
- Kinematical variables:
- Daughter resonances mass
- Two kinematical variables to describe *B* candidate: mES and E

- * = variable in CMS

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 79--81

## Results extraction

- We use an unbinned Maximum Likelihood fits, combining several variables: mES, E, t, shape variable
- Simultaneous extraction of signal-background yields, and *S* and *C* TD parameters
- Simultaneous fit of all sub-decays with as much as possible free parameters (93 free parameters)
- Very complex fit: used a general program, MiFit, based on ROOT and RooFit classes
- Several checks to validate results: MC generated experiments, fit on MC samples, Likelihood ratio, ...

- Alfio Lazzaro

- December 4, 2007

**Notes:**
MiFit: pages 85--89
Details: page 95
Checks: pages 92--95

## Slide 41

- Alfio Lazzaro

- December 4, 2007

## CP Violation in B0→'K0: observation in 2006 

- CP Violation in *B*0→'*K*0: observation in 2006

***B******A******B******AR***

***B******A******B******AR***

- 384 M *B**B* pairs
- 1252 ± 50 events

**First observation (5.5******** from *****S*****=0) of CP violation in a *****b*****→*****s***** penguin mode, in**** ****agreement with the SM prediction**

- Important Achievement: constraint to New Physics Contribution

- PRL 98, 031801 (2007)

- Background

- Main Systematics: Fit parameterization and background contribution

- Projections: cut on likelihood ratio to enhance the signal
- The curves are not fits to distributions, just the distributions rescaled

- Alfio Lazzaro

- December 4, 2007

## Penguin b→s Modes Summary

- World Averages for each mode
- Naïve average of all modes compatible with sin2 from b→ccs, but poor fit:
- If f0K0S from B→KS is excluded:
*C* consistent with zero (no evidence of direct CP violation):
- Extreme caution when interpreting this averages: neglecting theoretical uncertainties, assumption of Gaussian errors not justified for all measurements, correlated systematic errors

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 69--71

## Conclusions

- The flavor physics is crucial for the Standard Model
*B* physics will be producing results for many years from now (B-factories, LHCb, SuperB-factory(?)): several measurements are statistical limited
*B* Penguin decays give a powerful probe for low energy New Physics effects
- 0→'K0 gave the first observation of CP violation in Penguin *b→s* Modes
- Up to now the experimental scenario is consistent with the SM: needed more data to clarify the situation
- Precision experiments can reveal New Physics if it is out there around 1 TeV, jointly with LHC experiments

- Alfio Lazzaro

- December 4, 2007

**Notes:**
Pages 69--71

## Backup Slides

## Symmetries in Nature

- Symmetries help to describe the Nature
- Symmetries are key factors for the development of the physics theories:
- Understanding the symmetries and finding eventual violations is a crucial test for our understanding of the Nature

| Symmetry under transformation | Conservation law |
| --- | --- |
| Spatial translation | Linear momentum |
| Spatial Rotation | Angular momentum |
| Temporal translation | Energy |

- Alfio Lazzaro

- December 4, 2007

## Discrete Symmetries

- Parity (**P**): *x *→ −*x*
- Violated in weak
- 	interaction
- 	(C. S. Wu, 1956)
- Charge (**C**): particles ↔ antiparticles
- Violated in weak interaction
- Time (**T**): *t* → −*t*

- Alfio Lazzaro

- December 4, 2007

## CP symmetry

**C** and **P** violated separately in weak interaction, but **CP** should be conserved (before 1964)
- We are interested to find discrepancies between the two “images” of the “CP mirror” (CP violation)

- Alfio Lazzaro

- December 4, 2007

## The Cosmological issue

- Only matter in the Universe: where is the primordial antimatter?
- Ratio anti-protons/protons ~10-4 in cosmic rays; no evidence of photons from annihilations of intergalactic clouds
- No antimatter in our galaxy
- No evidence of antimatter in the primordial cosmic rays
- Cosmological Asymmetry Generation: Sakharov’s Conditions (1967):
- Baryonic Number Violation
- Deviation from thermal equilibrium
- Violations of Discrete Symmetries *C* and *CP*

- Alfio Lazzaro

- December 4, 2007

## K Mesons

- Discovered in cosmic rays: L. Leprince-Ringuet, M. L’Heritier (1944); G.D. Rochester, C.C. Butler (1947)
- “Strange” particles: produced in strong interaction (time 10−23 s), they decay in weak interaction (10−10 s)  Strangeness hypothesis: a quantum number conserved by strong interactions and not by weak interactions
- Two charged conjugate (C) states for neutral mesons (K0 and K0) with transition K0 ↔ K0  use the superimposition of the 2 physical states K1 and K2
- No transition between K1 and K2, well-defined masses (not a particle-antiparticle pair) and width (expected to be different because different final states available)
- Observation in 1956 of long-lived K mesons: K1=KS (short-lived (~10−10 s)) and K2=KL (long-lived (~10−8 s))

- Alfio Lazzaro

- December 4, 2007

## CP symmetry and K mesons

- If **CP** is conserved, for the two physical states of *K* meson we have KS →2 and KL→3, and the physical states = CP eigenstates
- In 1963 found events with KL →2 (CP violated) in a Brookhaven experiment (L.B. Leipuner *et al*, Phys. Rev. 132, (1963) 2285). Needed further confirmation

- Alfio Lazzaro

- December 4, 2007

## The discovery of CP violation

- J.H. Christenson, J. Cronin, V. Fitch and R. Turlay proposed an experiment in Brookhaven lab: April 1963
- Apparatus ready: June 2nd 1963
- 40 days of running
- Evidence of events from the decay          KL→2: **(45****±****9)/22700 ****≈ (2.0 ± 0.4)×10****−3**** (tiny effect!)** (PRL 13, 4 (1964))
- Evidence of *CP* Symmetry Violation

- Alfio Lazzaro

- December 4, 2007

## From the discovery of b quark...

- "Observation of a Dimuon
- Resonance at 9.5 GeV in
- 400 GeV Proton-Nucleus
- Collisions,"
*PRL* 39, 252, (1977)

**Summer 1977 at FNAL:**
**Discovery of  ********(9.46) ******** ******************** **
**interpreted as 1****3****S****1**** b****b**

- Alfio Lazzaro

- December 4, 2007

## ... to B mesons.

- CESR at Cornell:
- “naked beauty” (mesons with *b* quark)
- e+e−  (4S)  BB

- CLEO Collaboration, "Observation of Exclusive Decay Modes of b-Flavored Mesons”, *PRL* 50, 881 (1983)

- Alfio Lazzaro

- December 4, 2007

## B meson lifetime

- 1982 MAC-MARKII Discovery: Longevity of *B*
- (SLAC experiments)
-  ≈ 10−12 s
- (Theoretical prediction placed their lifetime at 10−15 s, i.e. beyond the reach of observation)
- Longevity of *B* meson is important to perform the TD CP measurements!!!

- Alfio Lazzaro

- December 4, 2007

## Discovery of B−B Mixing

- Argus 1987

- The Mixing is linked to the coefficients *p* and *q*

- Results consistent with the SM

- Alfio Lazzaro

- December 4, 2007

## K0, D0, B0d, B0s parameters (1)

- Alfio Lazzaro

- December 4, 2007

## K0, D0, B0d, B0s parameters (2)

- Alfio Lazzaro

- December 4, 2007

## Time evolution of neutral B mesons (1)

- Time evolution of mass eigenstates:

- Time evolution of initially (t = 0) pure flavor eigenstates:

- Alfio Lazzaro

- December 4, 2007

## Time evolution of neutral B mesons (2)

- Time-dependent decay rate for                 :

- “decay”

- “oscillation, then decay”

- “interference”

- Alfio Lazzaro

- December 4, 2007

## The “normalized” Unitarity Triangle (UT)

**u**
**c**
**t**

**d     s     b**

**(1,0)**

****

****

****

**(********,********)**

**(0,0)**

- α, β, γ angle related to CP violation measurements

- Alfio Lazzaro

- December 4, 2007

## UT Angles determination

- Mixing
- Beta in trees *b **** c**c**s*
- Beta in penguins *b **** s**s**s*
- Alpha in trees *b **** u**u**d*
- Gamma: tree B decays to open charm

- Alfio Lazzaro

- December 4, 2007

## Experimental facilities

- Past:     DORIS, LEP, SLC, CESR
- Present: Tevatron, PEP-II, KEK-B
- Future:  LHC, Super B-Factory(?)

**PEP-II**

**KEK-B**

- Symmetric Collider

- Asymmetric Colliders

- Alfio Lazzaro

- December 4, 2007

## Recent past: LEP, SLC and CLEO

- ee  Z0  bb
- LEP (Aleph, Delphi, L3, OPAL)
- 4 M Z0    1.4 M Bu,d ; Bs, b-baryons, ...; bbhad  
- plab   30GeV    boost c 3 mm
- Silicon vertex detectors, particle identification
- Lifetimes, mixing, b-tags
- SLC (linear collider: SLD)
- 400K Z0   140K Bu,,d ; Polarized beam (*A**FB  *measurement!)
- Smaller diameter vacuum pipe, pixel vertex detector
- ee  (4S)  B0B0, BB
- CESR (CLEO, several upgrades) until 2001
- 17 M events, Bu,d only;  bbhad  0.25
- Monochromatic B mesons, small boost:  0.06
- Vertex detector, particle identification; many important results!

- Alfio Lazzaro

- December 4, 2007

## Present: Tevatron, Run II

- Tevatron @ FNAL: pp collider (CDF, D0)
- Large cross section *and*  backgrounds, at Ecm = 1.96 TeV:
- bb  105 nb** ,      **bbhad  0.001
- All species of b-hadrons: B, B0, BS, BC, b, b ;
- c 3 mm
- Present *peak*  luminosity: 1032 cm-2 s-1 ; *integrated*  about 3 fb-1
- Physics reach on *B*-physics:
- Rare decays, CPV, CKM, *B*c
***B******s****** mixing: ********************s**********************m****************s***

- Alfio Lazzaro

- December 4, 2007

## e+e−  (4S) with asymmetric energy to boost the B mesons: PEP-II at SLAC and KEK-B at KEK
clean environment, low backgrounds, high efficiency
small cross section (≈1nb): luminosity is the key factor. KEK-B record: 15.8 events/(nb s)
Two experiments: BaBar (PEP-II) and Belle (KEK-B)
> 1000 M B-meson pairs recorded in total by Belle and BaBar! (CLEO recorded 17 M B-meson pairs)

- clean environment, low backgrounds, high efficiency
- small cross section (≈1nb): luminosity is the key factor. KEK-B record: 15.8 events/(nb s)
- Two experiments: **BaBar (PEP-II) and Belle (KEK-B)**
**> 1000 M *****B*****-meson pairs** recorded in total by Belle and BaBar! (CLEO recorded 17 M *B*-meson pairs)

*B*-Factories: PEP-II and KEK-B

- Alfio Lazzaro

- December 4, 2007

## Future: LHC & Super B-Factories(?)

- Alfio Lazzaro

- December 4, 2007

## t  resolution effect

- *t*  resolution effect

******t resolution dominated by tag side: ********1 ps ******** ****170 ********m**

- Convolution of Decay Rate distribution with Resolution Model: Triple Gaussian

******t resolution function** obtained from data with exclusively reconstructed self-tagging flavor states (BReco)

- Alfio Lazzaro

- December 4, 2007

## B Flavor Tagging

*B* Flavor Tagging

- We should consider the effect of wrong tag identification

- Neural Network for the tagging: events in 6 categories

**Parameter of wrong tag identification **obtained from BReco data

- Alfio Lazzaro

- December 4, 2007

## b→ccs “tree” Amplitudes Results

*b→c**c**s* “tree” Amplitudes Results

***B******A******B******AR***

***B******A******B******AR***

**227 M *****B******B***** pairs 7730 CP events**

**First observation of CP violation outside *****K***** mesons (large effect!), in agreement with the SM prediction.**

- PRL 94, 161803 (2005)

- Alfio Lazzaro

- December 4, 2007

## CPV in the B sector: CKM angles

*b*

*d*

*d*

*b*

*B**0** mixing*

*B**0** decay: tree*

*B**0** decay: penguin*

- Alfio Lazzaro

- December 4, 2007

## Penguin mode (tree strongly suppressed) with BF ≈ 19 × 10−6
BaBar result (significance: 4.3), soon confirmed by Belle

Observation of Direct CP in K meson in 1999 (35 years later the observation of CPV in mixing!): small effect in K meson

- BaBar result (significance: 4.3), soon confirmed by Belle
- Observation of Direct CP in K meson in 1999 (35 years later the observation of CPV in mixing!): small effect in K meson

- Direct CP Violation in *B*0→K+−: observation in 2004

- Large Effect

- PRL 93, 131801 (2004)

- Alfio Lazzaro

- December 4, 2007

## Tracking system: SVT

- Great precision of the measurement of position of the charged particle: possibility to reconstruct the vertex of the primary *B* with great accuracy!

- Alfio Lazzaro

- December 4, 2007

## Particle identification: DIRC

- For particle up to 4 GeV/c of momentum we are able to distinguish a charged kaon from a charged pion.

- Alfio Lazzaro

- December 4, 2007

## Previous Measurement: TD CP

- 232 million of *B**B* pairs (PRL 94, 191802 (2005)): only K0→ K0S
- Most precise measurements in penguin channels
*S* is 3 sigma from the charmonium value!!!
- To improve the measurement we need more statistics  we include mode with K0→K0L
- Opposite CP eigenvalue (***f* = +1) wrt K0S (***f* = −1)
- Results presented to EPS2005 (232 million of BB pairs): combined final results from η'*K*0S and η'*K*0L

- Alfio Lazzaro

- December 4, 2007

## K0L Reconstruction

- We cannot measure the magnitude of K0L meson momentum
- We measure the direction of K0L momentum as the direction from the η' vertex to the centroid of the EMC or IFR candidate
- Using the η' four-momentum and fixing the *B* mass and K0L mass to PDG values, we can determinate the magnitude of K0L meson momentum (the same procedure used in J/K0L analysis)  we obtain a K0L candidate with all information
- We combine the η' and K0L candidate to obtain a *B* candidate

- Alfio Lazzaro

- December 4, 2007

## K0L Selection

- No other selection for IFR K0L candidates
- EMC K0L candidates selected also using a Neural Network
- We use a NN using the cluster shapes variables:
- Number of crystals
- Second Moment
- Lateral Moment
- S1/S9
- S9/S25
- Zernike Moments:
- 	|Z20|, |Z42|
- 7 variables, 1 layer,
- 	10 hidden units,
- 	1 output

- Alfio Lazzaro

- December 4, 2007

## K0L Selection (EMC Neural Network) (1)

- Trained on side-band events and MC signal events
- 5000 cycles, 1000 events (for signal and background) as training, 400 events (for signal and background) as validation

- Alfio Lazzaro

- December 4, 2007

## K0L Selection (EMC Neural Network) (2)

- Cut optimization using statistical significance  cut at > 0.4 (−12% efficiency, −50% fake K0L)
- We have checked NN on B0J/ K0L control sample

- Alfio Lazzaro

- December 4, 2007

## Event Selection: Topological Variables (1)

- Cosine of angle of the thrust axis of *B* candidate and the thrust axis of the rest of event

- Thrust:

- Cut: |cos (T)|<0.9

*p**i* =  particle momentum

- signal

- bkg

- Alfio Lazzaro

- December 4, 2007

## Event Selection: Topological Variables (2)

- 1: absolute value of the cosine of the angle between the *B* direction and beam axis
- 2: absolute value of the cosine of the angle between the thrust axis of *B* and the beam axis
- 3, 4: *L*0 e *L*2:

- 4 variables

*p**i* =  particle momentum
***T**B,i*= angle between *p**i* direction and the thrust axis of the *B* candidate

- Alfio Lazzaro

- December 4, 2007

## Daughter resonances mass
Two kinematical variables to describe B candidate: mES (not used in K0L mode) and E
Cuts: 5.25<mES<5.29 GeV, |E|<0.2 GeV for K0S modes and −0.01<E<0.04 GeV for K0L mode

- Two kinematical variables to describe *B* candidate: mES (not used in K0L mode) and E
- Cuts: 5.25<mES<5.29 GeV, |E|<0.2 GeV for K0S modes and −0.01<E<0.04 GeV for K0L mode

- * = variable in CMS

- Event Selection: Kinematical Variables

- Alfio Lazzaro

- December 4, 2007

## Input to ML fits: “best” candidate selection

- In case of multiple candidates per event, we chose the “best” candidate using an algorithm based on the /' mass (in case of main modes) or *B* vertex probability (other modes).
- In mode with K0L we have 2 categories of candidates: EMC, IFR. If the direction of IFR candidate is compatible with a candidate of EMC, we consider the two candidates as the same candidate (category EMC+IFR) and we drop the IFR one
- In mode with K0L, if several candidates per event have the same B vertex probability, we chose the candidate with K0L coming from (in order): EMC+IFR, EMC, IFR.

- Alfio Lazzaro

- December 4, 2007

## BB studies (1)

*B**B* expected as input to ML
- ’   modes clean
- Most of background for ’ modes is due to charm decays  we take in account it floating the background PDFs parameters in our fits
- We study in detail contribution from charmless decays.
- We make PDFs using these charmless events
- We use these events in our MC generated experiment studies

- Alfio Lazzaro

- December 4, 2007

## BB studies (2)

- Alfio Lazzaro

- December 4, 2007

## MiFit: the ML fitter

- A general program for ML fit, developed during my PhD thesis
- simple framework where it is not necessary to compile the code  only simple configuration files to use
- Every feature must be integrated for general use  no particular custom for a specific analysis
- Standalone code using ROOT and RooFit classes
- It allows: TD fits, MC generation experiments (“Toy-Experiments”), projections and sPlots, Lihelihood ratio plots, etc.
- Used in all Milan analyses

- Alfio Lazzaro

- December 4, 2007

## MiFit: Starting Point

- The case: different analyses have the same procedure
- The idea: to have a simple framework where it is not necessary to compile the code  only simple configuration files to use
- Every feature must be integrated for general use  no particular custom for a specific analysis

- Alfio Lazzaro

- December 4, 2007

## MiFit: What we have

- ROOT
- very nice framework for data analysis developed in CERN (R. Brun and F. Rademakers)
- RooFit
- powerful collection of C++ classes, mainly developed for ML fits (W. Verkerke and D. Kirkby)
- Integration in ROOT (since version 5)
- User program
- Every user should know how works ROOT and RooFit to write his personal code for the analysis
- Many analysis have in common different procedure (BR, TD, projections, Toy-Exp, plots, ...)

- Alfio Lazzaro

- December 4, 2007

## MiFit solution

- You don’t need to know how ROOT and RooFit work.
- Standalone code using ROOT and RooFit libraries
- You can configure using simple text configuration files  you don’t need to recompile the code
- Requested only knowledge of how to write the configuration files

- Alfio Lazzaro

- December 4, 2007

## MiFit description

- 4 main classes
- MFConfiguration: parser to read the configuration files. It provides a simple spelling syntax check
- MFModels: it provides the PDF models (Gaussians, Polynomials, CP models, ...)
- MFFits: it performs fit for a PDF, Extended ML fits and Toy-Exp studies
- MFPlots: for plots of the PDFs (single plot or split for different category)

- Alfio Lazzaro

- December 4, 2007

## ML fits (1)

- Unbinned Maximum Likelihood fit with: mES (not used in K0L mode), E, Fisher discriminant, t variables
- Small correlations between the input variables
- Two components in the fit: signal and continuum background. In η'ρ0  modes we add a charmless BB component
- Main background parameters floated in the fits on on-peak samples
- Signal and BB PDFs from MC events; continuum background PDFs from off-peak and mES-E side bands (mES < 5.27 GeV, 0.1<|E|<0.2 GeV) events
- PDFs parameterization:

- Alfio Lazzaro

- December 4, 2007

## PDFs

- Alfio Lazzaro

- December 4, 2007

## Likelihood definition for i event of c tagging category:

fsig,c taken from BReco sample fits

Likelihood for all events belonging to category c:

Total likelihood (and likelihood when we put together different sub-decays):

Our fits minimize the quantity –ln L as function of free parameters

*f**sig,c* taken from BReco sample fits
- Likelihood for all events belonging to category *c*:
- Total likelihood (and likelihood when we put together different sub-decays):
- Our fits minimize the quantity –ln L as function of free parameters

- ML fits (2)

- Alfio Lazzaro

- December 4, 2007

## Checks

- Several checks performed:
- Check on vertex reconstruction for K0L modes
- Fit on signal MC samples to check the ability to extract correctly the events
- TD fits of MC signal events
- Simulated MC experiments (events generated from PDFs): Pure Toy-Experiments
- Simulated MC experiments with background events generated from PDFs and signal and charmless events taken from MC to extract eventual bias (due to background events and residual correlations between the input variables): MC Toy-Experiments  we correct the final results for these biases and we assign a systematic error
- Effect of misreconstructed events
- Fit removing one PDF each time
- TD CP Fit on charged modes
- LH ratio
- Comparison Old-New Results

- Alfio Lazzaro

- December 4, 2007

## MC Generated Experiments

- Composition of the samples as expected in data

- Alfio Lazzaro

- December 4, 2007

## Results: details

- Alfio Lazzaro

- December 4, 2007

## E & mES projections

- Done applying a cut to select the events on the ratio:
- The curve are not fits to distributions, just the PDFs rescaled

- η'KL

- η'KS

- η'KS

- Alfio Lazzaro

- December 4, 2007

## Systematics

- Alfio Lazzaro

- December 4, 2007

## Consistence Old-New CP TD results (1)

- Overlap events Old−New

- Alfio Lazzaro

- December 4, 2007

## Consistence Old-New CP TD results (2)

- We consider a Gaussian distribution of values for *S* with a mean of 0.273 and sigma of 0.153 for the 511 common events, and another Gaussian with mean of the “true value” of *S* with a sigma of 0.129 (rescaling 0.155 to the different number of events) for the 741 additional events.
- We add the two Gaussians, weighted for the numbers of events 511 and 741, respectively, to have the total distribution.
- We use this distribution to perform 100000 toy-experiments and then we calculate the fraction of *S* values greater than 0.58.
- Performing toys in two cases, with a true value of *S* = 0.7 and *S* = 0.61, where the latter is the current world average for S in η′K0, we obtain a probability of 49.0% and 35.5%, respectively.
- A probability of nearly 50% is clearly a reasonable result consistent with statistics.

- Alfio Lazzaro

- December 4, 2007

## BaBar and Belle results consistent
Combining all results of CP Violation (K and B system) in a global fit
All results are consistent each other and consistent with the SM prediction
No clear evidence of contribution coming from physics beyond SM

- Combining all results of CP Violation (K and B system) in a global fit
- All results are consistent each other and consistent with the SM prediction
- No clear evidence of contribution coming from physics beyond SM

- Summary of the Results: The Unitarity Triangle

- − plane with UT

- Only Angles

- Alfio Lazzaro

- December 4, 2007

## − Plane: Past to Present

- Results as Summer, 2007

- Results as May, 2001

- Alfio Lazzaro

- December 4, 2007

## From Jim Olsen’s talk at ARGUS Symposium

- November 9, 2007

## #B’s

- 109--

**B meson observed (1982)****;**** **“long” lifetime (~1.5ps)

**B****0**** mixing observed (1987)**; Vub measured; lower bound on mt > 42 GeV;  Vub ≠ 0 → KM in the game as source of CPV

**Radiative penguin** **b ****→ ****s******** observed (1993)**;  major constraint on NP models; asymmetric B Factory proposals

**CPV in B decays**** ****observed (2001)**; sin2 consistent with SM and constraints from existing CKM measurements

**The ab****-1**** era**: precision tests of the CKM paradigm; constraining new physics with flavor physics

**Direct CPV observed (2004)**; confirmation of KM mechanism; CKM parameters over-constrained

- 108--

- 107--

- 104--

- 105--

- 106--

- #B’s

**1999 - B Factories start operation**

- Alfio Lazzaro

- December 4, 2007

## Constraining NP with Flavor Physics

- Trees

- Loops

- Alfio Lazzaro

- December 4, 2007

## “New Physics Flavor Problem”

- If new physics lives at the TeV scale it cannot have a generic flavor structure → Minimal Flavor Violation!

- Model-independent constraints from Bd mixing

- Alfio Lazzaro

- December 4, 2007

## #B’s

- 109--

**B meson observed (1982)****;**** **“long” lifetime (~1.5ps)

**B****0**** mixing observed (1987)**; Vub measured; lower bound on mt > 42 GeV;  Vub ≠ 0 → KM in the game as source of CPV

**Radiative penguin** **b ****→ ****s******** observed (1993)**;  major constraint on NP models; asymmetric B Factory proposals

**CPV in B decays**** ****observed (2001)**; sin2 consistent with SM and constraints from existing CKM measurements

**The ab****-1**** era**: precision tests of the CKM paradigm; constraining new physics with flavor physics

**Direct CPV observed (2004)**; confirmation of KM mechanism; CKM parameters over-constrained

- 108--

- 107--

- 104--

- 105--

- 106--

- #B’s

**1999 - B Factories start operation**

- Alfio Lazzaro

- December 4, 2007