# Presentation: 861260

## The Mirror Crack’d*: History and Status of CP Violation Studies

*Eric Prebys **(UR ‘90*)**, Fermi National Accelerator Laboratory*
- Representing the
**BELLE Collaboration**

- *apologies to Agatha Christie

- University of Rochester

- September 26, 2001

## The BELLE Collaboration

- 300 people from 49 Institutions in 11 Countries:
- Australia, China, India, Korea, Japan, Philippines, Poland, Russia, Taiwan, Ukraine,  and USA

- University of Rochester

- September 26, 2001

## Just to set the tone....

- Dear Eric,
- I just returned to Rochester and I am happy to know that Tom has invited you for a colloquium on Sep 26. Can you send me a title of your talk at the earliest. I would like to tell you a few things that Tom may not have mentioned. First, you will be the first speaker of the semester and, therefore, you carry a great responsibility for presenting a very good colloquium. Second, since our colloquium attendance has thinned over the years (because of bad talks, specialized talks), I have assured the students that I will only invite extraordinary speakers who can give a very general talk to graduate students across all disciplines. So, I would like you to prepare your talk keeping this in mind. In particular, what this means is that please do not make it a talk on experimental physics, rather on physics. Remember the time when you were a student and the kinds of things you hated in colloquia, please avoid them. Not all the students will be from high energy physics. In fact, many are from optics, astronomy and so a talk with less display of detectors etc and with a greater balance of theoretical motivation and the explanation of results would be highly appreciated.
- Why am I telling you all this? Well, first of all, you were our former student and as such *I have a right to ask you for things*. Second, you will be the first speaker and if the students are not thrilled with your talk, the attendance may shrink in the subsequent talks. On the other hand, if your talk is superb, which I hope it will be, more people will show up for the later talks (people have a tendency to extrapolate). In any case, please keep in mind that you will be talking to a general audience and not to a group of experimentalists.
- Let me know when your itinerary is complete, but please send me a title in a couple of days.
- With very best regards,
- Ashok.

- University of Rochester

- September 26, 2001

## Outline

- Why do we care?
- History
- Parity Violation
- V-A Currents and CP (almost) Conservation
- CP Violation in the Neutral K System
- The Cabbibo-Kobayashi-Maskowa Mechanism
- “The” Unitarity Triangle
- The Present
- Direct CP Violation in the Neutral K System (’/)
- Indirect CP Violation in the B meson System (B-Factories)
- The Future?

- University of Rochester

- September 26, 2001

## Why do We Care?

- Dirac first predicted antimatter in 1930 as a consequence of the “extra” solutions to his relativistic formulation of quantum mechanics - and was widely ridiculed.
- The positron (anti-electron) was discovered by Anderson in 1932 and the anti-proton was discovered by Segre and Chamberlain in 1955.
- Now we are all quite comfortable with the idea of antimatter as “equal and opposite” to matter, e.g.
- ...but why does the universe seem to be made entirely of matter?
- Why do there seem to be *tiny* differences in the physics of matter and antimatter?
- These legitimately qualify as **“big questions”.**

- “Of course, there is only one correct mixing ratio of matter and  antimatter: **one to one**!” – Star Trek, The Next Generation

- University of Rochester

- September 26, 2001

## Parity Violation

- The “parity” operation transforms the universe into its mirror image (*goes from right-handed to left-handed*).
- Maxwell’s equations are totally parity invariant.
- BUT, in the 50’s huge parity violation was observed in weak decays...

- Example decay of polarized **Co...**

- electron preferentially emitted opposite spin direction

- University of Rochester

- September 26, 2001

## Weak Currents and Parity Violation

- Review: QED

- Transform like vectors

- For weak interactions,* try* (“four fermion interaction”)

- vector

- axial vector

- Manifestly Violates Parity!!

- University of Rochester

- September 26, 2001

## “V-A” Current

- Experimentally, it was found that data were best described by

- Maximum Parity Violation!!!!

- Recall that for Direct Spinors, the left handed projection operator is

- “Left-handed” current

- For massless particles, spinor state = helicity state

- Only Left-handed Neutrinos

- University of Rochester

- September 26, 2001

## CP Conservation (sort of)

- When we apply the usual Dirac gymnastics, we find that for *anti-particles*

- Right-handed current

- Only Right-handed anti-Neutrinos

- Overall symmetry restored under the combined operations of C(harge conjugation) and P(arity).

- CP Conservation!!!

- well, maybe not....

- University of Rochester

- September 26, 2001

## The Neutral Kaon System

-      In experiments in the 1950s, it was found that there were two types of neutral strange particles, of indistinguishable mass (498 MeV), but with different decay properties.

- CP = -1

- CP = +1

-     Because *3*m**** **** m**K* , the *K**L* lives about 600 times longer than the *K**S*, hence the names.

- Possible explanation:

- Strangeness eigenstates

- close, but not *quite* correct...

- University of Rochester

- September 26, 2001

## CP Violation in the Neutral K System

- In 1964, Fitch, Cronin, *etal*, showed that in fact *K**L****2****  *with a branching ratio on the order of* 10**-3**.*

- Interpretation:

- CP Eigenstates

- Mass Eigenstates

- University of Rochester

- September 26, 2001

## The Significance

- In other words...

- where

-     This generated great interest (not to mention a Nobel Prize), and has been studied in great detail ever since, but *until recently* had only been conclusively observed in the kaon system.

-     Unlike parity violation, it is *not* trivial to incorporate CP violation into the standard model.  To understand how it is done, we must now digress a bit into some details of fundamental particle interactions....

- University of Rochester

- September 26, 2001

## Weak Interactions in the Standard Model

- In the Standard Model, the fundamental particles are leptons and quarks

- In this model, weak interactions are analogous to QED.

- OR

- quarks combine as
- to form hadrons

- leptons exist
- independently

- University of Rochester

- September 26, 2001

## Quark Mixing

- In the Standard Model, leptons can only transition *within* a generation (NOTE: probably not true!)

- Although the rate is *suppressed*, quarks can transition *between* generations.

- University of Rochester

- September 26, 2001

## The CKM Matrix (1973)

- The weak quark eigenstates are related to the strong (or mass) eigenstates through a unitary transformation.

- Cabibbo-Kobayashi-Maskawa (CKM) Matrix

- The only straightforward way to *accommodate** *CP violation in the SM is by means of an irreducible phase in this matrix
- This requires at least three generations and  led to prediction of *t* and *b* quarks ... a year *before* the discovery of the c quark!

- University of Rochester

- September 26, 2001

## Wolfenstein Parameterization 

- The CKM matrix is an SU(3) transformation, which has four free parameters.  Because of the scale of the elements, this is often represented with the “Wolfenstein Parameterization”

- CP Violating phase

- First two generations *almost* unitary. = sine of “Cabbibo Angle”

- University of Rochester

- September 26, 2001

## “The” Unitarity Triangle

- Unitarity imposes several constraints on the matrix, but one (product first and third columns)...

- results in a triangle in the complex plane with sides of similar length                , and appears the most interesting for study

- University of Rochester

- September 26, 2001

## The Plane

- Remembering the Wolfenstein Parameterization

- we can divide through by the magnitude of the base (A3)....

- CP violation is generally discussed in terms of this plane

- University of Rochester

- September 26, 2001

##  Direct CP Violation

- CP Violation is manifests itself as a difference between the physics of matter and anti-matter

*Direct* CP Violation is the observation of a difference between two such decay rates; however, the amplitude for one process can in general be written

- Weak phase changes sign

- Strong phase does not

- Since the observed rate is only proportional to the _amplitude_, a difference would only be observed if there were an *interference* between two diagrams with different weak *and* strong phase*.*

*** *Rare and hard to interpret

- University of Rochester

- September 26, 2001

## Direct CP Violation in the Neutral Kaon System (’/ Measurement)

- Recall...

- If there is only *indirect* CP violation, then ALL 2 decays *really *come from *K**1* , and we expect (among other things)

- But the Standard Model allows

*Direct* CP Violation

- University of Rochester

- September 26, 2001

## Direct CP Violation in the Neutral Kaon System (cont’d)

- ’

- CP=+1

- CP=-1

- CP=+1

- Formalism:

- Theoretical estimates for ’range from 4-30 x 10-4

- University of Rochester

- September 26, 2001

## Easy to Measure....NOT!

- Detector

- Must take great steps to understand acceptances and systematic errors!!

- University of Rochester

- September 26, 2001

## KTeV Experiment (Fermilab)

- (Images from Jim Graham’s Fermilab “Wine and Cheese” Talk)

- University of Rochester

- September 26, 2001

## Current Status of ’

- At this point, the accuracy of this measurement is better than that of the theoretical prediction:
- (4-30 x 10-4)

- (ibid.)

- This bothered people

- University of Rochester

- September 26, 2001

## Indirect CP Violation in the B Meson System

- Let’s Look at B-mixing...

- Mixing phase

- University of Rochester

- September 26, 2001

## Indirect CP Violation (cont’d)

- If both                  can decay to the same *CP eigenstate** f,* there will be an *interference*

- And the time-dependent decay probability will be

- Decay phase

- CP state of* f*

- Mixing phase

- Difference between B mass eigenstates

- University of Rochester

- September 26, 2001

## The  Resonances

- 

- At the right energies, electrons and positrons can produce a spectrum of bound resonant states of *b* and *anti-b* quarks

- The 1- states are called the “ (‘Upsilon’)resonances”

- Starting with the (4S), they can decay strongly to pairs of B-mesons.

- The lighter states must decay through quark-antiquark annihilation

- University of Rochester

- September 26, 2001

## The Basic Idea

- We can create          pairs at the           resonance.
- Even though both *B’*s are mixing, if we tag the decay of one of them, the other must be the CP conjugate *at that time**.* We therefore measure the time dependent decay of one B relative to the time that the first one was tagged (EPR “paradox”).
- PROBLEM:  At the          resonance, B’s only go about 30 m in the center of mass, making it difficult to measure time-dependent mixing.

- University of Rochester

- September 26, 2001

## The Clever Trick (courtesy P. Oddone)

- If the collider is *asymmetric*, then the entire system is Lorentz boosted.
- In the Belle Experiment, 8 GeV e-’s are collided with 3.5 GeV e+’s  so

- 

- So now the time measurement becomes a *z* position measurement.

- University of Rochester

- September 26, 2001

## “Gold-Plated” Decay

- Total state CP

- University of Rochester

- September 26, 2001

## Predicted Signature

*t* = Time of tagged decays

- University of Rochester

- September 26, 2001

## “Tin-Plated” Decay

- Complicated by “penguin pollution”, but still promising

- University of Rochester

- September 26, 2001

## Make LOTS of       pairs at the (4S) resonance in an asymmetric collider.
Detect the decay of one B to a CP eigenstate.
Tag the flavor of the other B.
Reconstruct the position of the two vertices.
Measure the z separation between them and calculate proper time separation as
Fit to the functional form

Write papers.
Over the last ~8 years, there have been two dedicated experiments under way to do this – BaBar (SLAC) and Belle (KEK)

- Detect the decay of one *B* to a CP eigenstate.
- Tag the flavor of the other *B.*
- Reconstruct the position of the two vertices.
- Measure the *z* separation between them and calculate proper time separation as
- Fit to the functional form
- Write papers.
- Over the last ~8 years, there have been two dedicated experiments under way to do this – BaBar (SLAC) and Belle (KEK)

- Review - What B-Factories Do...

- University of Rochester

- September 26, 2001

## Motivations for Accelerator Parameters

- Must be asymmetric to take advantage of Lorentz boost.
- The decays of interest all have branching ratios on the order of 10-5 or lower.
- Need lots and lots of data!
- Physics projections assume 100 fb-1 = 1yr @ 1034 cm-2s-1
- Would have been pointless if less than 1033 cm-2s-1

- University of Rochester

- September 26, 2001

## The KEKB Collider (KEK)

- Asymmetric Rings
- 8.0GeV(HER)
- 3.5GeV(LER)
- Ecm=10.58GeV= M((4S))
- Target Luminosity: 1034s-1cm-2
- Circumference: 3016m
- Crossing angle: 11mr
- RF Buckets: 5120
-  2ns crossing time

- University of Rochester

- September 26, 2001

## The PEP-II Collider (SLAC)

- Asymmetric Rings
- 9.0GeV(HER)
- 3.1GeV(LER)
- Ecm=10.58GeV= M((4S))
- Target Luminosity: 3x1033s-1cm-2
- Crossing angle: 0 mr
- 4ns crossing time

- University of Rochester

- September 26, 2001

## Motivation for Detector Parameters

- Vertex Measurement
- Need to measure decay vertices to <100m to get proper time distribution.
- Tracking...
- Would like p/p.5-1% to help distinguish B decays from BK  and BKK decays.
- Provide dE/dx for particle ID.
- EM calorimetry
- Detect ’s from slow, asymmetric ’s   need efficiency down to 20 MeV.
- Hadronic Calorimetry
- Tag muons.
- Tag direction of KL’s from decay BKL .
- Particle ID
- Tag strangeness to distinguish B decays from Bbar decays (low p).
- Tag ’s to distinguish B decays from BK  and BKK decays (high p).

*Rely on mature, robust technologies whenever possible!!!*

- University of Rochester

- September 26, 2001

## The Belle Detector

- University of Rochester

- September 26, 2001

## BaBar Detector (SLAC)

- University of Rochester

- September 26, 2001

## The Accelerator is Key!!!

**   ****STOP Run**** +HV Down **** +Fill HER**** +Fill LER**** +HV Up**** +START Run**
** ****= 8 Minutes!**

- University of Rochester

- September 26, 2001

## Luminosity

- Daily integrated luminosity

- Total integrated luminosity

- Total for first CP Results (Osaka):

- Total for *these* Results:

- Our Records:
- Instantaneous:
- Per (0-24h) day:
- Per (24 hr) day:
- Per week:
- To date:

- (on peak)

- Note: integrated numbers are accumulated!

- World Records!!

- University of Rochester

- September 26, 2001

## The Pieces of the Analysis

- Event reconstruction and selection
- Flavor Tagging
- Vertex reconstruction
- CP fitting

- University of Rochester

- September 26, 2001

## J/ and KS Reconstruction

- J/ and KS Reconstruction

- Mev
- Require mass
-  within 4of PDG

- University of Rochester

- September 26, 2001

## BKS Reconstruction

*B****K**S** *Reconstruction

- In the CM, both *energy* and *momentum* of a real B0 are constrained.
- Use “Beam-constrained Mass”:

- Signal

*123 Events*
*3.7 Background*

- University of Rochester

- September 26, 2001

## All Fully Reconstructed Modes (i.e. all but L)

| Mode | Events | Background |
| --- | --- | --- |
| BS | 457 | 12 |
| All Others | 290 | 46 |
| Total | 747 | 58 |

- University of Rochester

- September 26, 2001

## BKL Reconstruction

- Measure direction (only) of *K**L** *in lab frame
- Scale momentum so that M(*K**L**+***)=M(*B**0*)
- Transform to CM frame and look at *p(B**0**)*.

- KLM Cluster

*J/**** *daughter particles

*K**L*

- University of Rochester

- September 26, 2001

## BKL Signal

*0<p**B*****<2 *GeV/c

- Biases spectrum!

*346 Events*
*223 Background*

- University of Rochester

- September 26, 2001

## Flavor Tagging           

- X

- University of Rochester

- September 26, 2001

## Flavor Tagging (Slow Pion)

- Very *slow* pion

- Combined effective efficiency eff = t(1-2w)2 = 27.0.2%

- University of Rochester

- September 26, 2001

## Vertex Reconstruction (SVD)

- Overall efficiency = ~85%.    In total 1137 events for the CP fit.

- University of Rochester

- September 26, 2001

## CP Fit (Probability Density Function)

*f**BG* = background fraction.  Determined from a 2D fit of *E* vs *M.*
*R(**** t) = *resolution function.  Determined from *D*****’s* and MC.
*PDF**BG**(**** t) = *probability density function of background. Determined from  sideband.

- University of Rochester

- September 26, 2001

## Resolution Function

- Fit with a double-Gaussian...

- University of Rochester

- September 26, 2001

## Test of Vertexing – B Lifetime

- University of Rochester

- September 26, 2001

## The Combined Fit (All Charmonium States)

- University of Rochester

- September 26, 2001

## Sources of Systematic Error

- Bottom Line

- Published in **Phys.Rev.Lett. 87, 091802 (2001)**

- University of Rochester

- September 26, 2001

## The BaBar Measurement

**Phys.Rev.Lett. 87 (2001)**

- Based on 32 million B-Bbar pairs

- University of Rochester

- September 26, 2001

## Summary of 21 Measurements

- University of Rochester

- September 26, 2001

## How About That  Plane?

- World Average Sin21  (1)

- Constraints of Everything *but* Sin21

**Looks good for the Standard Model, but a little *****dull***** for experimenters !**

- University of Rochester

- September 26, 2001

## Current Status

- The study of CP Violation has been going on for *almost 40 years! *
- A number of experiments are currently taking data which seem to be confirming the Standard Model (CKM) explanation of CP Violation, and thereby constraining that model
- Direct CP violation is observed in the neutral K system!
- CP *is* violated in the B-Meson system!
- Over the next several years, the existing B-Factories will continue to take data, providing tighter and tighter constraints.
- New players are also coming on the scene:
- Fermilab Run II (CDF and D0) - now
- BTeV (dedicated B Experiment at Fermilab) - ~2005
- LHC (Atlas and CMS) - 2006
- LHC-B (dedicated B Experiment at LHC) - ?

- University of Rochester

- September 26, 2001

## More “Out There”

- CP Violation in the  sector? (probably there, hard to study)
- CPT Violation?
- CPT Conservation is a direct consequence of the Lorentz invariance of the Lagrangian.
- Evidence of its violation would be observation (direct or indirect) of
-     and would be big news.
- We still can’t answer why the unverse is all matter. Maybe it *isn’t*!
- The AMS experiment, set to fly on the ISS, will look for massive anti-nuclei to test the hypothesis that distant parts of the universe *might* be antimatter (!!)

- University of Rochester

- September 26, 2001

## Are Two B-Factories Too Many?

- These are not discovery machines!
- Any interesting physics would manifest itself as small deviations from SM predictions.
- People would be very skeptical about such claims without independent confirmation.
- Therefore, the answer is NO (two is not *one* too many, anyway).

- University of Rochester

- September 26, 2001

## Differences Between PEP-II (BaBar) and KEKB (Belle)

- PEP-II has complex IR optics to force beams to collide head-on.	Pros: 	Interaction of head-on beams well understood.	Cons: 	Complicates IR design.		More synchrotron radiation.		Can’t populate every RF bucket.
-  In KEK-B, the beams cross at ±11 mr.	Pros: 	Simple IR design.		Can populate every RF bucket.		Lower (but not zero!!!) synchrotron radiation.	Cons: 	Crossing can potentially couple longitudinal 		and transverse instabilities.

*At present, both designs seem to be working.*

- University of Rochester

- September 26, 2001

## Differences (cont’d)

- Readout:
-  BaBar uses an SLD-inspired system, based on a continuous digitization. The entire detector is pipelined into a software-based trigger.	Pros: 	Extremely versatile trigger.		Less worry about hardware-based trigger systematics.		Can go to very high luminosities.		Cons: 	Required development of lots of custom hardware.
-  Belle’s readout is based on converting signals to time-pulses. The trigger is an “old-fashioned” hardware-based level one. Events satisfying level one are read out after a 2 μs latency.	Pros: 	Simple.		Readout relies largely on “off-the-shelf” electronics.	Cons: 	Potential for hardware-based trigger systematics.		Possible problems with high luminosity.

- University of Rochester

- September 26, 2001

## Particle ID needs

- University of Rochester

- September 26, 2001