Physics at the B Factories: Progress and Prospects

Caltech Colloquium Patricia Burchat November 13, 2003 Stanford University Two new “Asymmetric-energy B Factories” started accumulating data ~June 1999 with the goal of studying CP violation in B decays as a tool to search for new physics.

at the Stanford Linear Accelerator Center

at the KEK Laboratory in Japan

A B meson is a particle made up of a heavy quark called the “bottom” quark and an ordinary light quark (“up” or “down”).

Asymmetric-energy e+e- storage rings ⇒ B mesons are moving in the laboratory frame of reference.

Caltech Colloquium Patricia Burchat, Stanford 2 11/13/03 John Seeman (SLAC) and Katsunobu Oide (KEK) are the recipients of the 2004 Wilson Pier Oddone (LBNL) and Jonathan Dorfan Prize for outstanding (SLAC) beside PEP-II rings. achievement in the physics of particle accelerators.

Caltech Colloquium Patricia Burchat, Stanford 3 11/13/03 The B Factory Experiments

Peak luminosity: ~7 BB / second ~12 BB / second

Total recorded integrated ~140 fb-1 ~160 fb-1 luminosity:

c.f. integrated luminosity for pioneers in B physics:

•Argus (1983-1987): ~ 0.1 fb-1

•CLEO (1981-2000): ~ 16 fb-1 Caltech Colloquium Patricia Burchat, Stanford 4 11/13/03 What is CP violation and why are we trying to study it? Start with the “mirror” transformations C, P, and T:

C = charge conjugation C2 = 1 P = parity inversion P2 = 1 T = time reversal T2 = 1

Any symmetry has an associated nonobservable quantity:

C⇒ no absolute sign of electric charge P ⇒ no absolute right-handed coordinate system T ⇒ no absolute direction of time

C, P, and T were assumed to be symmetries of nature until…

Caltech Colloquium Patricia Burchat, Stanford 5 11/13/03 Parity Violation In 1956, Lee and Yang proposed, and in 1957, Wu and others showed experimentally, that nature is not invariant under the PARITY transformation.

In the Standard Model, C and P are maximally violated in charged weak interactions… C

+ − e R e R NO YES W+ W−

νL νL P + − e L e L W+ W− NO YES ν νR R

…but CP appears to be OK. Caltech Colloquium Patricia Burchat, Stanford 6 11/13/03 CP Violation In 1964, Cronin, Fitch and others found that even CP symmetry is violated in the weak decays of neutral Kaons.

In the Standard Model, CP violation can be accommodated in (again) the charged weak interaction, in the quark sector. NO YES C q L q L W+ W− Vqq´

q´R q´R P

q R q R W+ W− V*qq´ NO q´ YES L q´L Caltech Colloquium Patricia Burchat, Stanford 7 11/13/03 NO YES Significance of CP Violation

• If C, P and CP are violated, does nature respect any mirror symmetries? • CPT: combined action of C, P, and T is a symmetry of any local relativistic field theory. • If CPT is a good symmetry, CP violation ⇒ T violation.

• Sakharov’s three conditions for a net excess of matter over antimatter in the universe include CP violation. However, the Standard Model mechanism can accommodate no more than 10-10 of the observed amount of matter.

• CP-violating processes provide an absolute distinction between matter and antimatter.

• Example: CP-violating asymmetry in KL →πeν.

The KL decays slightly more often (0.33%) into the final state containing an e+ than into that containing an e-.

Caltech Colloquium Patricia Burchat, Stanford 8 11/13/03 How does the fundamental CP violation shown below show up as a difference in the behavior of particles and antiparticles?

NO YES C q L q L W+ W− Vqq´

q´R q´R P

q R q R W+ W− V*qq´ NO q´ YES L q´L

Caltech Colloquium Patricia Burchat, Stanford 9 11/13/03 δi = a phase that does not change sign under CP φi = a phase that does change sign under CP

A1 = |A1| exp(iδ1) exp(iφ1) |A1| exp(iδ1) exp( - iφ1) i f CP i f

A2 = |A2| exp(iδ2) exp(iφ2) |A2| exp(iδ2) exp( - iφ2)

Choose a phase convention such that δ = φ = 0. P(i→f) - P(i→f) α 2 |A1 A2| sin(1δ1 - 1δ2) sin(φ1 - φ2)

A1 + A2

φ2

−φ CP 2

δ2 δ2

A1 A1 + A2 exp(-2iφ2)

⇒ Need to know δ1 - δ2 in order to extract φ1 - φ2. CP-Violating Phases in the Standard Model:

Elements of the Cabibbo-Kobayashi-Maskawa (or CKM) “quark mixing matrix”

d s b Vqq’ q q’ u c W t

Important V V * Example: tb t td b d B0 B0 mixing B W W B d t b * Vtd Vtb Caltech Colloquium Patricia Burchat, Stanford 11 11/13/03 The Quark Mixing Matrix and the Unitarity Triangle d s b

u * V *V c Vub Vud α tb td

t γ β

* Vcb Vcd

apply unitarity constraint to these two columns

Caltech Colloquium Patricia Burchat, Stanford 12 11/13/03 So, the phase in the CKM quark mixing matrix can provide a relative “CP-violating” phase.

What about the relative “non-CP-violating” phase?

A. Relative phase due to the non-CP-violating strong interaction.

B. Relative phase due to “mixing”.

Caltech Colloquium Patricia Burchat, Stanford 13 11/13/03

DK A. Strong interactions can lead to relative “non-CP- violating” phase. ⇒

Interference between two decay diagrams (e.g., tree and loop diagrams with different CKM phases) can lead to CP-violating asymmetries …

W u b t d π- π- W d u 0 b u B u 0 + B d d π + d d π

… but relative strong phase cannot be calculated!

Caltech Colloquium Patricia Burchat, Stanford 14 11/13/03

DK B. Meson mixing provides a source of error-free “non-CP- violating” phase.

b t d m Δm/2 0 0 B W W B H = d t b Δm/2 m

|B0 (t)〉 ∝ cos(Δm t/2) |B0〉 – i sin(Δm t/2) |B0〉 exp(2iφ)

Meson mixing provides a source of error-free non-CKM phase shift by -90o (from the factor of - i ).

The weak phase φ is associated with the mixing diagram.

Caltech Colloquium Patricia Burchat, Stanford 15 11/13/03

DK No mixing: cos(Δm t/2)

0 B fCP B0 Mixing followed by decay: – i sin(Δm t/2) exp(2iφ) relative relative non-CP-violating CP-violating phase phase

Caltech Colloquium Patricia Burchat, Stanford 16 11/13/03 CP violation in decays of B mesons is expected to exhibit itself as oscillations in the decay rate.

Ratio of oscillation frequency to decay B0 rate:

very large

B0

Decay Rate ~ few

In B decays, the oscillation frequency is small ~ 0.1 compared to the decay rate!

time (ps) Graphics provided by David Kirkby (UCI)

Decay Time (picoseconds) The Asymmetric-Energy B Factories

π+

ϒ(4S) B0 / B0 e - e + B0 / B0 π—

e ±, µ ±, K± tag Δz

〈 | Δz | 〉 ~ 250 µm

18 The Asymmetric-Energy B Factory at the Stanford Linear Accelerator Center

Caltech Colloquium Patricia Burchat, Stanford 19 11/13/03 Silicon Vertex Tracker (SVT)

Caltech Colloquium Patricia Burchat, Stanford 20 11/13/03 From the ideal world to “reality”… Now add effect of imperfect measurement of Δt. B = B0 B = B0 tag tag 0 0 Btag= B Btag= B

Δt (ps) Δt (ps) First add effect of imperfect tagging.

Time-dependent CP asymmetry is diluted. Finally add background contribution. 0 B = B0 0 B = B0 Btag= B tag Btag= B tag

Δt (ps) Δt (ps) Graphics provided by David Kirkby (UCI) 21 Since we have a “Factory”, we must have a lot of signal events, right? Wrong…

~200 million B pairs have been recorded and analysed by BABAR and Belle.

~100 million of these are neutral B pairs.

~one B in a thousand decays to the CP final states we need.

Of these, ~10% decay into final states we can reconstruct.

Of these, ~50% pass all the selection criteria.

We are left with about 5000 signal events (for the most copious useful decay mode). Caltech Colloquium Patricia Burchat, Stanford 22 11/13/03 sin2β

* Vtb Vtd

β

* Vcb Vcd

Caltech Colloquium Patricia Burchat, Stanford 23 11/13/03 Charmonium modes used for measuring sin2β “The Golden Modes”

b c One dominant decay c J/ψ, χc, ηc W B0 s amplitude ⇒ d d KS,L theoretically clean!

“Large” branching fraction: ~4 x 10-4

ccKs modes “Distinctive” final state ⇒ experimentally clean!

24 Blind Analysis Techniques

BABAR uses “blind” analysis strategies for the extraction of the time-dependent and time- integrated asymmetries in order to minimize possible experimenters bias.

In time-dependent asymmetries, we use a technique that hides not only the result of the fit, but also the visual CP asymmetry in the time distribution.

The statistical error on the asymmetry is not hidden.

Caltech Colloquium Patricia Burchat, Stanford 25 11/13/03 K modes K modes BABAR s L

0 0 Red Curve (B ) minus Blue Curve (B ) sin2β is the amplitude of Red Curve (B0) plus Blue Curve (B0) this asymmetry.

sin2β = 0.74 ± 0.07 ± 0.03 (BABAR)

= 0.73 ± 0.06 ± 0.03 (Belle) 26 sin2β measurement history

a) “Osaka 2000” measurement (compiled by Owen Long) (hep-ex/0008048)

• Only J/ψ Ks and ψ(2s) Ks.

d e b) 1st Paper (PRL 86 2515, 2001)

c • Added J/ψ KL. • Simultaneous sin2β and mixing b fit. a c) 2nd Paper (PRL 87 201803, 2001) *0 • Added J/ψ K and χc Ks. • Better vertex reconstruction. • Better SVT alignment and

higher Ks efficiency for new data.

d) Winter 2002 (hep-ex/0203007) • Improved event selection. • Reprocessed 1st 20 fb-1.

e) Current measurement (hep- ex/0207042, PRL) • Improved flavor tagging.

• One more CP mode: ηcKs. 27 Decrease in Statistical Uncertainty

Curves represent 1/√∫Ldt.

Improvements in statistical uncertainty due to

• adding new B decay modes,

• improved vertex reconstruction,

• improved SVT alignment,

• improved tagging performance.

Caltech Colloquium Patricia Burchat, Stanford 28 11/13/03 Constraints on upper vertex of Unitarity Triangle from all measurements EXCEPT sin2β

With BABAR and Belle, we are directly measuring the angle β shown in the triangle with unit base. β

Imaginary axis

95% CL allowed regions.

Real axis 29 World Average sin2β = 0.74 ± 0.05

The Standard Model wins again … at least at the current level of experimental precision,

Imaginary axis in this decay mode.

Caltech Colloquium PatriciaReal Burchat, axis Stanford 30 11/13/03 0 Other studies of sin2β: B → φKs

W One dominant decay b t s amplitude ⇒ s φ B0 s theoretically clean ! 0 d d K (And sensitive to NEW PHYSICS)

“Distinctive” final state Small branching fraction: ~few x 10-6 ⇒ experimentally difficult !

Caltech Colloquium Patricia Burchat, Stanford 31 11/13/03 BABAR, 2003: time-dependent asymmetry in

0 B → φKs

Caltech Colloquium Patricia Burchat, Stanford 32 11/13/03 0 Measurements of “sin2β” from B → φKs

Winter Summer 2003 2003 s

BABAR K +1 φ sin2β

BABAR

Average Average 2.6

Belle σ from 2.7σ

” 0 β

Belle sin2 “ -1

Caltech Colloquium Patricia Burchat, Stanford 33 11/13/03 Heavy Flavor Averaging Group

loop diagrams

Caltech Colloquium Patricia Burchat, Stanford 34 11/13/03 sin2α

* * Vub Vud α Vtb Vtd

Lots of experimental and theoretical activity!

Caltech Colloquium Patricia Burchat, Stanford 35 11/13/03 CP Violation in B0 → π+π-

u b t d π- π- d u 0 b u B u + B0 d d π + d d π

Two decay amplitudes contribute (BAD) ⇒

Relative size of two amplitues and relative strong phase δ are unknown but can, in principle, be determined from an isospin analysis that requires measuring BF for B0→π+π-, B0→π+π-, B±→π±π0, B0→π0π0, and B0→π0π0.

Modes like B →ρ ρ look more hopeful now.

Caltech Colloquium Patricia Burchat, Stanford 36 11/13/03 sin2γ

* Vub Vud

γ

* Vcb Vcd

No single “clean” measurement.

Measurement of γ will rely on a lot of ideas in a lot of modes …

Caltech Colloquium Patricia Burchat, Stanford 37 11/13/03 Timeline for B-Factory Physics Program

Discovery of Discovery of new narrow new narrow

Ds states state X(3872)

1999 2001 2003 2005 2007 2009

? t0 Observation ~70 journal of CP violation articles sub’d ~500 fb-1 ~1000 fb-1 in B system by each expt Precision CP Precision CP tests for tests for CP violation New Physics New Physics in rare modes ~100 fb-1

Caltech Colloquium Patricia Burchat, Stanford 38 11/13/03 The Study of CP in “Flavor Mixing”

Quarks Leptons

d s b ν1 ν2 ν3 u e

c µ

t τ

Caltech Colloquium Patricia Burchat, Stanford 39 11/13/03 This is just the beginning…

Caltech Colloquium Patricia Burchat, Stanford 40 11/13/03 Backup Slides CP asymmetries due to interference between direct decay and mixing followed by decay:

dN ∝ exp(–t/τB) [ 1 ± S sin(Δm t)] S is directly related to elements of quark mixing matrix if decay proceeds through single diagram.

interference interference between between mixing 2 direct decays, such More generally, and decay as Penguin and Tree

dN ∝ exp(–t/τB) [ 1 ± S sin(Δm t) -+ C cos(Δm t)]

S and C difficult to interpret in terms of elements of quark mixing matrix.

Caltech Colloquium Patricia Burchat, Stanford 42 11/13/03 Detector of Internally Reflected Cherenkov Light (DIRC)

Quartz bar θ c Active Detector Measure angle of Cherenkov Cone in quartz Surface – Transmitted by internal reflection

– Detected by~10,000 PMTs Particle Cherenkov light Caltech Colloquium Patricia Burchat, Stanford 43 11/13/03