Cracking the CKM Triangle
– BABAR’s Next Step –
Masahiro MoriiHarvard UniversityBABAR Collaboration
October 2003
Oct 3, 2003 Masahiro Morii 2
Outline Very brief introduction
CKM triangle and CP violation in the B system BABAR and PEP-II
What we can do today, and in 3 years Measurements
Angle from B KS/L , KS , ’KS Angle from B
Angle from B D, DK |Vub| from B Xul decays
Summary
Oct 3, 2003 Masahiro Morii 3
CKM Matrix CKM matrix appears in the weak Lagrangian as
Unitary matrix translates mass and weak basis 3 real parameters + 1 complex phase
CPV in the Standard Model is uniquely predictive Attractive place to look for New Physics
. .2
ud us ub
cd cs cb
td ts tb
L
L L L L
L
V V VV V VV V V
dg u c t s W h c
b
L
The only source of CPV in the Minimal SM
2 3
2 2
3
12
12
2
1 ( )1
(1 ) 12
L
L L L L
L
i
i
dg u c t s
A
A AWA
b
L
Wolfenstein
parameters
Oct 3, 2003 Masahiro Morii 4
Why CP Violation? Standard Model is unreasonably successful
It predicts everything we measure, while We all know it’s wrong
BIG failing: Baryogenesis Matter-dominant universe is created through:
CP violation Baryon number violation Non thermal equilibrium
SM prediction falls way short of reality CKM mechanism was a postdiction of CPV
Never tested (before BABAR) its predictive power
All three availablein the Standard Model
Oct 3, 2003 Masahiro Morii 5
Unitarity Triangle V is unitary Consider
Dividing by gives the familiar triangle
Non-zero angles CP violation All sides are O(1) Can test closure with realistic
experimental precision
0ud ub cd cb td tbV V V V V V 3
cd cbV V A
1
*
*
*
*
*
*
arg
arg
arg
td tb
ud ub
cd cb
td tb
ud ub
cd cb
V VV V
V VV V
V VV V
td tb
cd cb
V VV V
ud ub
cd cb
V VV V
Oct 3, 2003 Masahiro Morii 6
Anatomy of B0 System B0-B0 system very similar to K0 system
Mixing through box diagrams Coupling constants appear as
Mass eigenstates BH and BL are linear combinations
Lifetime H and L close Ignore Follow the time evolution…
W+
W-
s/b
d
d
s/b* * * 2 5( ) ( ) ( )us ud cs cd ts tdV V V V V V O O O A * * * 3 3 3( ) ( ) ( )ub ud cb cd tb tdV V V V V V O A O A O A
0 0
0 0
L
H
B p B q B
B p B q B
2 2 1p q Mass difference m
causes mixing
Oct 3, 2003 Masahiro Morii 7
B0 Evolution and Decay Starting from pure B0(B0) state, after time t
Now, consider decays into a CP eigenstate fCP
It’s trivial (just tedious) to calculate the decay rates
0 0 0
0 0 0
( ) cos sin2 2
( ) sin cos2 2
B
B
im t t
im t t
mt q mtB t e e B i Bp
p mt mtB t e e i B Bq
0 0f CP f CPA f B A f B H H
20 0
20 0
( ( ) ) ( )
( ( ) ) ( )
CP CP
CP CP
B t f f B t
B t f f B t
H
HNeat problem for anundergrad. QM exam
Oct 3, 2003 Masahiro Morii 8
CP Asymmetry
2
2 2
1 2 Im
1 1
f f ff f f f
ff f
AqC S λ ηp A
0B
0B
CPfm
ixingfA
fA
Scenario 1:There is only one diagram
For B0 J/KS , -Im(f) = sin2
Scenario 2: More than one diagram contribute both sin and cos terms will survive Sf depends on Im UT angles Cf depends on || direct CP
)sin()cos())(())(())(())(()( 00
00
mtSmtCftBNftBNftBNftBNtA
ff
CPCP
CPCPCP
f = CP eigenvalue of fCP
1f fA A 0, Im( )f f fC S
( ) Im( )sin( )CP fA t mt
Oct 3, 2003 Masahiro Morii 9
Measuring CP Violation
Experiments must do 3 things: Produce and detect B fCP events
Typical BR: 10-4 – 10-5
Need a lot of lot of lot of lot of B’s Separate B0 from B0 = “Flavor tagging”
Use 4S B0B0 and tag one B Measure the decay time
Measure the flight length ct But B’s are almost at rest in 4S decays
0 0
0 0
( ( ) ) ( ( ) )( ) cos( ) sin( )
( ( ) ) ( ( ) )CP CP
CP f fCP CP
N B t f N B t fA t C mt S mt
N B t f N B t f
Solution:AsymmetricB Factory
Oct 3, 2003 Masahiro Morii 10
Three Ingredients
Ingredient #1:Exclusivereconstruction
Ingredient #2:Flavor tagging
Ingredient #3: t determination
e- (4S)
B0
B0
e-
+
-
Breco
Btag
e+
z~ c t
+
-
0 0
0 0
( ( ) ) ( ( ) )( ) cos( ) sin( )
( ( ) ) ( ( ) )CP CP
CP f fCP CP
N B t f N B t fA t C mt S mt
N B t f N B t f
Oct 3, 2003 Masahiro Morii 11
Asymmetric B Factory Collides e+e- at ECM = m(Y4S) but with E(e+) ≠ E(e-)
PEP-II: 9 GeV e- vs. 3.1 GeV e+ = 0.56 The boost allows measurement of t
Collides lots of them: Ibeam = 1 – 3A PEP-II luminosity 6.6 x 1033/cm2/s = 6.6 Hz/nb
That’s >2x the design KEKB has hit 1 x 1034/cm2/s
( ) 1nbe e bb
Oct 3, 2003 Masahiro Morii 12
Integrated Luminosity
>100 fb-1/expt. accumulated Physics results used 90-130 fb-1 so far
Oct 3, 2003 Masahiro Morii 13
Luminosity Projection – PEP-II
That’ll give BABAR a billion B mesons to play with
Inte
grat
ed lu
min
osity
[fb-1
]
Peak
lum
inos
ity [1
033 ]
5
10
15
200
400
600
PEP-II plans to
deliver 500 fb-1
by end 2006
Oct 3, 2003 Masahiro Morii 14
Luminosity Projection – KEKB
KEKB also shoot for 500
fb-1 by end 2006
Oct 3, 2003 Masahiro Morii 15
Detector: BABAR (or Belle)
Precise vertex with a silicon strip
detector
Charged particle momentum with a
drift chamber in a 1.5 T field
Photon energy with a CsI(Tl)
crystal calorimeter
Particle ID with a Cerenkov detector
(DIRC in BABAR,aerogel in Belle)
Muons detected after penetrating
iron yoke
Oct 3, 2003 Masahiro Morii 16
Harvard Group At WorkNew 3D track trigger systemfor better backgd. rejectionat higher luminosity
“ZPD”
Oct 3, 2003 Masahiro Morii 17
B0 Charmonium + K0
The “golden mode” Theoretically clean
Only the tree diagram matters Experimentally clean “Large” BF (~10-4)
CP sample in 89 M BB pairs
0Kb
c
sc
d0B
/J
d
ACP(t) = sin2 sinmt
Mode CP Nevents
J/KS , (2S)KS , cKS , cKS -1 1506J/KL +1 988J/K*0(KS0) mixe
d147
Full CP sample 2641
Oct 3, 2003 Masahiro Morii 18
CP Fit sin2CP=+1
CP=-1
sin2 = 0.741 ± 0.067stat ± 0.034systBABAR 81 fb-1 PRL 89, 201802
(2002)
Oct 3, 2003 Masahiro Morii 19
Unitarity Triangle World average:sin2 = 0.736 ± 0.049
Excellent agreement with SM constraint from indirect (non-CPV) data
Does that mean no New Physics?
Observed CP asymmetry consistent with
CKM mechanism being the dominant source of
CPV
sin2 vs. indirect constraints
Oct 3, 2003 Masahiro Morii 20
Next Step We’ve achieved(sin2
Already more precise than the indirect constraints Use it as the reference!
B0 K0 is a tree decay New Physics may be hiding
in more suppressed diagrams Strategy: Measure “other aspects” of the CKM
triangle Modes with different Feynman diagrams and Clean theoretical interpretations Look for inconsistencies = New Physics
0Kb
c
sc
d0B
/J
d
Oct 3, 2003 Masahiro Morii 21
Cracking the CKM Triangle Let’s measure everything we can!
ud ub
cd cb
V VV V
td tb
cd cb
V VV V
Bs mixing
at Tevatron
Vub from charmless semileptonic B
decays
sin2 from penguin
decays, e.g. B K
sin2 from B decays
from B D and
B DK decays
Oct 3, 2003 Masahiro Morii 22
sin2 Measurements “Redundant” measurements using other decay
modes Different diagrams Different sensitivity to New
Physics Loop diagrams are particularly interesting
Theoretically clean modes are more useful Single-diagram decays preferred
Modes under study
Final states Dominantdiagram
Otherdiagrams
KS Penguin Negligible’ KS Penguin TreeD* D*, D* D Tree Penguin
Clean
Less clean
Oct 3, 2003 Masahiro Morii 23
B0 /’ KS
Dominated by b sss penguin ACP
SM = sin2 = ACP(J/ KS) New Physics may enter the loop
KS is pure-penguin As clean as J/KS Small BR: 7.6 x 10-6
’KS has tree diagrams too Cabbibo- and color-suppressed |A/Ā| ~ 1 within a few % Larger BR: 5.5 x 10-5
’ decays harder to reconstruct
b
d
uu
sd
W
0K
sd
0K
ss
,
Oct 3, 2003 Masahiro Morii 24
Penguin SignalsBABAR KS
BABAR ’KS
Belle
Preliminary LP’03
Oct 3, 2003 Masahiro Morii 25
Penguin CPV Results Preliminary LP’03
Something very strange is happening here…
Oct 3, 2003 Masahiro Morii 26
B0 KS CP Fits Preliminary LP’03
BelleBABAR
B0 tags
B0 tags
ACP
Low-purity tags High-purity tags
Oct 3, 2003 Masahiro Morii 27
B0 ’ KS CP Fits Preliminary LP’03
BelleBABAR
B0 tags
B0 tags
ACP
Low-purity tags
High-purity tags
Oct 3, 2003 Masahiro Morii 28
Status of Penguins B0 KS
Belle 3.3 from J/KS Prob. < 0.1%
BABAR – Belle = 2.1 Prob. = 3.6%
Average = –0.14 ± 0.33 2.6 from J/KS
B0 ’KS Average = 0.27 ± 0.21 2.2 from J/KS
Do we have a hint of New Physics? Theorists “told you so” for this very decay mode Too early to tell 4x more data will settle the case
Oct 3, 2003 Masahiro Morii 29
Cracking the CKM Triangle
from B D and
B DK decayssin2 from penguin
decays, e.g. B K
from B decays
ud ub
cd cb
V VV V
td tb
cd cb
V VV V
|Vub| from charmless semileptonic B
decays
Oct 3, 2003 Masahiro Morii 30
Measuring Angle Tree diagram of B0 +- should give us sin2
But there are penguin diagrams
b cc
s
W
cbVb u
u
d
W
ubV
0SB J K 0B
sin 2 sin 2
b uu
d
W
ubV
effsin 2b d
u
u
W
T = Tree P = Penguin
gtbV
*tdV
Oct 3, 2003 Masahiro Morii 31
Taming Penguins To estimate eff – , we need:
P/T ratio – about 0.3 from (B K)/(B ) = strong phase difference between P and T
Gronau & London (1990) suggested using isospin relations0
0
0 0 0
( ) 2 2
( )
( )
B T P
B T C
B C P
AAA
b uu
d
ddb d
u
u
dd
b u
u
d
dd
TC
P
Measure BF for all modes and combine Extract eff – from data
Oct 3, 2003 Masahiro Morii 32
Isospin Analysis Isospin analysis requires BF(B0 00) separately
for B0 and B0
Too hard for BABAR/Belle Only average measured Use BF(B0 00) to put upper bound on eff –
Grossman and Quinn, 1998; Charles, 1998Gronau, London, Sinha, Sinha
PLB 514:315-320, 2001
0 0( ) ( )BR BR
eff2( )
Allowed assumed
0( ) 1.3( )
BRBR
Oct 3, 2003 Masahiro Morii 33
Observation of B0 00
BABAR has observed B0 00
at 4.2 significance
Weak limit on eff –
hep-ex/0308012
0 0 0 6( ) (2.1 0.6 0.3) 10BF B 0 0 0
0
( ) 0.38 0.13( )BB
Allowed
2(
eff –
)
Oct 3, 2003 Masahiro Morii 34
CPV in B0 +-
B-Physics News of 2002:Belle “discovery” of large CPVin B0 +-
PRD 68 (2003) 012001 CPV ≠ 0 at >99.9% CL Not seen by BABAR
2.6 discrepancy Did more data help?
BABAR
Belle
Oct 3, 2003 Masahiro Morii 35
Brief HistoryBABAR 33×106 BB
BABAR 60×106 BB
BABAR 88×106 BB
Belle 45×106 BB
Belle 85×106 BB
BABAR 123x106 BB
Preliminary LP’03
New world average:
S = -0.58±0.20C = -0.38±0.16
Oct 3, 2003 Masahiro Morii 36
CP Asymmetries
BABAR preliminary Belle PRD 68 (2003) 012001
B0 tags
B0 tags
B0 tags
B0 tags
Oct 3, 2003 Masahiro Morii 37
Status of eff
BABAR-Belle compatibility improved from 2.6 to 2.0 Waiting for new result from Belle
Interpretation of eff remains murky BF(B0 00) too large for useful limit on |eff – | Full isospin analysis beyond reach of existing B
factories Several model-dependent analysis proposed
BABAR (preliminary) Belle (PRD 68, 012001) Average
S 0.40 ± 0.22 ± 0.03 0.58 ± 0.20C 0.19 ± 0.19 ± 0.05 0.77 ± 0.27 ± 0.08 0.38 ± 0.16
0.080.071.23 0.41
Oct 3, 2003 Masahiro Morii 38
Status of Different interpretations
with various assumptions“agree” with the indirectconstraint of Theoretical error???
Experimental efforts areshifting towardB0 , final states Interference between
resonances give extra information for |P/T| and Broad on top of non-resonant makes analyses
incredibly complicated
indirect
Oct 3, 2003 Masahiro Morii 39
Cracking the CKM Triangle
from B D and
B DK decayssin2 from penguin
decays, e.g. B K
from B decays
ud ub
cd cb
V VV V
td tb
cd cb
V VV V
|Vub| from charmless semileptonic B
decays
Oct 3, 2003 Masahiro Morii 40
Why |Vub| Measurement of sin2 has become more accurate
than the indirect constraint Width of the indirect
ellipse is determined by|Vub/Vcb|
Better measurement of |Vub|
More stringent test of the Unitarity
Triangle
Vub
Oct 3, 2003 Masahiro Morii 41
Measuring |Vub| Measure the rate of charmless semileptonic
decays
Catch: charm background
There are many techniques Exclusive:
Better S/B Inclusive: Lepton endpoint spectrum, etc.
Better efficiency
2( ) ubb ul V
b u
l
W
ubV
2
2
( ) 1( ) 50
ub
cb
Vb ulb cl V
or B l l That’s not a good sign…
Vub
Oct 3, 2003 Masahiro Morii 42
Why |Vub| Is Hard
InclusiveExclusive
Poor S/B ratio
S/B bette
r
Error in extrapolation to full acceptance
Model-dependent calculation of FF
PDG 2002, p. 706
Vub
Oct 3, 2003 Masahiro Morii 43
How To Improve |Vub| Measure inclusive (B Xul)
Exclusive needs better FF calculation Lattice QCD Goal: better S/B ratio + larger kinematical acceptance
Minimize charm background Reduce extrapolation
Three kinematical variables in the Xul final state
We need a large sample of clean, isolated, and unbiasedB decays “B beam”
El lepton energy easy to measure
q2 (l- mass)2 needs momentum
mXhadron system mass needs all hadrons
Want to use all of them for optimal efficiency and S/B
Vub
Oct 3, 2003 Masahiro Morii 44
Tagged-B Events We have a large sample of (4S) BB events
with one B fully reconstructed ~1000 decay channels Efficiency ~0.2%/B
Look at the other B inthese events (“recoil” B) Almost-pure B0, B± with
known momentum Purity known from mES fit Subtract background using sideband
0 ( )
( )0
B D Y
B D Y
Y± is any combinationof ±, K±, KS and 0
Ideal sample for branching fraction measurements
Vub
Oct 3, 2003 Masahiro Morii 45
BF(B Xul) Start from the recoil-B sample
Find a lepton with p > 1 GeV Calculate mass of the remaining system “X”
Know pB, missing () mass = 0 2-C fit improves (mX) from 500 350 MeV
Normalization from fitting tag-B mass
All events with a leptonMostly B Xcl mX > 1.55 GeV Xul enriched
to ~60%
Vub
Oct 3, 2003 Masahiro Morii 46
BF(B Xul) |Vub| Fit the mX distribution to extract BR
Use OPE calculation
1
( )( )0.0206 0.0025( )
0.0023( ) 0.0036( , )
uB X lB Xl
statsyst
hep-ex/0307062submitted to PRL
1 2
31
( ) 1.55ps0.00445 (1 0.020 0.052)0.002
4.62 0.28( ) 0.27( ) 0.40( , ) 0.26 10
ubb
b ulV
stat syst
B
mbOPE
Vub
Oct 3, 2003 Masahiro Morii 47
Theoretical Errors Signal efficiency depends on (El, q2, mX) distribution
Differential rate predicted at parton level to O(s) Depends on the b quark mass and its Fermi motion
Leading systematics for |Vub| Parameterized in
Same parameters determine (El, q2, mX) for B Xcl Above values come from CLEO’s electron spectrum We should be able to improve!
Measure both El and mX spectra in B Xcl mX requires the recoil-B technique
210.480 0.120GeV, 0.300 0.105GeV
Vub
Oct 3, 2003 Masahiro Morii 48
Hadron Mass Moment Start from recoil-B with lepton
Calculate mX as in the |Vub| analysis Subtract (small) B Xul contribution Calculate moments <mX> <mX
2> <mX3> <mX
4> Vary the lepton p cut from 0.9 to 1.6 GeV
Vub
BABAR preliminary
Coming Soon: Combine with Ee spectrum and fit determine and 1
Oct 3, 2003 Masahiro Morii 49
Status of |Vub| BABAR/Belle/CLEO are
working hard on |Vub| BABAR is leading with
the recoil-B technique Improvements continue
El and mX spectra Better and 1
Cut on q2 Reduced theoretical error
Tag B with semileptonic decays higher efficiency
Vub
Oct 3, 2003 Masahiro Morii 50
Cracking the CKM Triangle
from B D and
B DK decayssin2 from penguin
decays, e.g. B K
from B decays
ud ub
cd cb
V VV V
td tb
cd cb
V VV V
|Vub| from charmless semileptonic B
decays
Oct 3, 2003 Masahiro Morii 51
Measuring Angle No magic bullet for Many difficult ideas
Most use interference between two competing diagrams to measure sin(2 + )
Modes under study include: first results from BABAR, Belle first result from Belle no results yet And a whole lot more…
0 (*)B D
0B D K 0B D K
Oct 3, 2003 Masahiro Morii 52
B0 D(*) Analysis Cabbibo-favored and suppressed amplitudes
Amplitude ratio r ~ 0.02 Weak phase from Vub Strong phase unknown
*cb udA V V
2*
212
* 1i i i i iub cd
cb ud
V VA e e e e eA V V
r
Time-dependent CPV with
ACP = r sin(2 + )
*ub cdA V V
Oct 3, 2003 Masahiro Morii 53
CPV in B0 D(*)
Measuring 4 ratesshould give useverything, but…
r is small (~0.02) cannot be extracted from C We need r from elsewhere to determine sin(2 + )
0 (*)
0 (*)
0 (*)
0 (*)
( ( ) ) 1 cos( ) ( ) sin( )
( ( ) ) 1 cos( ) ( ) sin( )
( ( ) ) 1 cos( ) ( ) sin( )
( ( ) ) 1 cos( ) ( ) sin( )
t L
t L
t L
t L
B t D e C mt S mt
B t D e C mt S mt
B t D e C mt S mt
B t D e C mt S mt
2
(*)2
(*)
11
1r
Cr
(*)
(*)2(*)
2sin(2 )
1r
Sr
Oct 3, 2003 Masahiro Morii 54
Estimating r Theoretical models
Estimates are around r = 0.02 with ~20% spread Measure B+ D+0
Isospin symmetry: O(10-7) We can’t measure this for a while
Use B0 Ds+- assuming SU(3)
0 (*) (*) 0( ) 2 ( )A B D A B D
* 0.0050.007( ) 0.019 0.004, ( ) 0.017r D r D
plus ±30% theoretical error
Oct 3, 2003 Masahiro Morii 55
B0 D(*) Signal BABAR 88x106 BB
Fully reconstructed signal
B0 D(*)-+ B0 D(*)+-
B0 D(*)-+ B0 D(*)+-
0 (*)
0 (*)
0 (*)
0 (*)
( ( ) ) 1 cos( ) ( ) sin( )
( ( ) ) 1 cos( ) ( ) sin( )
( ( ) ) 1 cos( ) ( ) sin( )
( ( ) ) 1 cos( ) ( ) sin( )
t L
t L
t L
t L
B t D e C mt S mt
B t D e C mt S mt
B t D e C mt S mt
B t D e C mt S mt
Submitted to PRL
Oct 3, 2003 Masahiro Morii 56
B0 D(*) Results
Measurements with ~100% statistical errors are arriving Small CPV CPV of the tag side dominates
systematics
2r sin(2+)cos 2r sin(2+)sinBABAR (82/fb)fully-reconstructedhep-ex/0309017
D–
0.038±0.038±0.021
0.025±0.068±0.035
D*–
0.068±0.038±0.021
0.031±0.070±0.035
BABAR (82/fb)partially-reconstructedhep-ex/0307036
D*–
0.063±0.024±0.017
–0.004±0.037±0.020
Belle (140/fb)fully-reconstructedhep-ex/0308048
D 0.058±0.038±0.013 0.036±0.038±0.038
D* 0.063±0.041±0.021 0.030±0.041±0.034
Prel
imin
ary
Oct 3, 2003 Masahiro Morii 57
B± D0K± Analysis
Phase between the two diagrams = Interference if D0/D0 decay into a common final
state,e.g., KS, KSKK
CPV:
B+ ucu
b
su
D0
K+
B+
usu
b uc D0
K+
*cb usA V V *
ub csA V V
( )0( ( ) ) 1 B DiiS B DA B D K K r r e e
|Vub/Vcb| D0 KS ratio
strongphase
s
Oct 3, 2003 Masahiro Morii 58
B± D0K± Analysis Strong phase D in the D0 decay comes from FSI
Varies across the Dalitz plot Select a region where
single resonance dominates, or
Fit D as a function over theDalitz plane
Things get tricky
Belle hep-ex/0308043preliminary
Oct 3, 2003 Masahiro Morii 59
B± D0K± SignalBelle 140/fb (hep-ex/0308043) preliminary
B- vs. B+
Oct 3, 2003 Masahiro Morii 60
B± D0K± Result Belle result: = (95±23±13±10)°
Last error due to D0 decay model, i.e., how to fit the Dalitz plot
Encouraging first step BABAR analysis in
progress Summer ’04?
Stat. only
Stat. + syst.90% CL
Belle 140/fb(hep-ex/0308043) preliminary
Oct 3, 2003 Masahiro Morii 61
Status of Serious efforts started to measure in several
channels First results from B0 D and B± D0K± show
promise B0 D – Cleaner analysis. How we get r? B± D0K± – Dalitz analysis powerful but messy
No “golden mode” Many measurements must be combined – How?
Remember: we are trying to discover non-SM effect Careless averaging may wipe out New Physics Area of intense debate
Oct 3, 2003 Masahiro Morii 62
Cracking the CKM Triangle
from B D and
B DK decayssin2 from penguin
decays, e.g. B K
from B decays
ud ub
cd cb
V VV V
td tb
cd cb
V VV V
|Vub| from charmless semileptonic B
decays
Oct 3, 2003 Masahiro Morii 63
Summary (1) BABAR/Belle working hard to crack the Unitarity Triangle
The Standard Model is holding remarkably well Tantalizing shifts observed in sin2 from penguin
decays Too early to declare death of the SM 4x data in 3 years will tell
from B seems harder than we hoped Dalitz analysis of B , pursued
New and improved |Vub| measurements with recoil-B Coming soon: better and 1 to reduce theory error
Early measurements of started to appear Huge effort going into the last angle
Oct 3, 2003 Masahiro Morii 64
Summary (2) With 109
B’s/experiment by 2006, we will learna lot more about the Unitarity Triangle Is it really closed, or
will we see a sign of New Physics?