Transcript Document

Physics with radiative B decays at Babar
CERN EP Seminar, June 19th 2006
Wouter Hulsbergen (CERN)
 (* )
o B physics in a nutshell
o radiative B decays
o PEP-II and BaBar
o b->sγ branching fraction and CP asymmetry
o probing the photon polarization with B0->Ksπ0 γ

Physics with B decays
o the aim of the heavy flavour physics program is to understand the flavour
structure of the quark sector of the standard model
u c t
d s b
~
0.01 1.2 173
0.01 0.1 4.5
GeV
 why are there 6 quarks? why are there masses so different?
 is the standard model ‘CKM’ picture of quark mixing correct?
 is it the only source of CP violation at low energies?
o decays of B hadrons are our richest source of flavour phenomenology
 B’s are heavy: many different decay modes; decays to both other
families
 long lifetime (cτ=0.5mm), large mixing probability (for B0: τΔm=0.8)
 large CP violation effects
 mB>>ΛQCD  perturbative QCD works, at least sometimes
o B decays also improve our understanding of hadronization (long distance
effects), important to extract the short distance physics
2

The CKM matrix
o for quarks weak eigenstates are different from mass eigenstates
νe
leptons
e+
d
quarks
u
g
g Vud
W+
W+
o V is unitary and called the Cabibbo-Kobayashi-Maskawa (CKM) matrix
o it has 4 physical parameters, one of which is a complex phase
 this phase is the “origin of CP violation in the SM”
o VCKM is almost diagonal:
1
λ
λ
λ3
1
λ3
λ2
λ2
1
λ ≈ 0.22
3
The unitarity triangle

o the condition that Vckm is unitary can be visualized with triangles in the
complex plane, for example
o there are 6 such triangles, but this one is most relevant for B0 decays
o the angles of this triangle are the famous ‘CKM angles’
*) since the ‘phase’ of a quark field itself is arbitrary, Vckm depends on a
phase convention. however, as you can see, these angles do not.
4
The unitarity triangle

o branching fractions and CP asymmetries provide experimental constraints
on different sides and angles of this triangle, for example
o we can test the SM quark flavour mixing picture by
 measuring all sides and angles: overconstraining the triangle
 measuring a single side or angle in more than one way
5
Where do we stand?

this graph shows the constraints used
for a ‘global fit’ of all 4 CKM parameters
one way to express the consistency is to
compare the measured WA for sin(2β)
from CP-violation in B0->ψ Ks
sin(2β)exp = 0.69 ± 0.03
with the value of a fit that does not use
this measurement
sin(2β)fit = 0.74 (+0.07 -0.03)
o the success of the SM shows that new physics effects in B decays are small !
o therefore, we now concentrate on searches for NP in processes dominated by
loop diagrams in the SM, in particular b->s transitions
6
bs and bd transitions

o b -> s and b -> d transitions are a Flavour Changing Neutral Current
 absent in the standard model at tree-level
 exist only at loop level, for example via a W-top loop
Vtb*
Vts
o SM contribution receives even additional suppression
 CKM suppression: t- and c-quark loop ~λ2, u-quark loop ~λ4
 for radiative decays there is also a helicity suppression
o new physics enters ‘at leading order’, for example SUSY
o physics at the virtual high-energy frontier!
7

sin(2β) in penguins: a smoking gun for NP?
The decay B0->ψKs is dominated by
a tree diagram
Vcb*
b
c
c ψ
B0
Vcs
d
s
d
K0
The rare decays to the left all proceed
through a b->s gluonic penguin:
Vtb*
Vts
For all these decays, time-dependent
CP-violation probes sin(2β) in the SM …
Is there new physics in b->s penguins?
‘naïve’ penguin average is
~2.5σ away from SM value
Let’s look at radiative b->s decays …
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Radiative B decays

o radiative B decays are b ->s,d transitions with a high energy photon or
lepton pair in the final state
`radiative penguin’
`WW box’
o in contrast with other b → s decays
 QCD plays a relatively minor role
 one can actually calculate something (even formfactors)
 one can measure inclusive BFs and asymmetries
o last point is important because predictions for inclusive decays are more
accurate than those for exclusive decays
9

How do we calculate things?
The theoretical framework is the operator product expansion (OPE)
VtbVtd* x C7γt(mB) x
Wilson coefficient: physics above mB
 calculated pertubatively in SM and
beyond
 this where new physics enters
Local operator: physics below mB
 use Heavy Quark Expansion:
systematic expansion in Λ /mB
 exclusive decays need formfactors
o calculations become a ‘double expansion’ in αs and ΛQCD /mB
o precise predictions for (a.o.)
 inclusive b->sγ and b->sl+l- branching fractions
 CP asymmetries, both inclusive and exclusive
 polarization of the photon
 mass and angular distribution of the lepton pair in b->sll
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Inclusive b->sγ branching fraction
o

in the Heavy Quark Expansion:
BF(B->Xs γ) ≈ BF(b->s γ) + O[ ( ΛQCD / mB )2 ]
o
NLO SM prediction
BF(B→Xs) = (3.57 ± 0.30) x 10-4
(Buras et al 2002)
(for recent update, see eg Hurth, Lunghi, Porod 2005)
o
expected theoretical accuracy 5% within a few years
 most importantly: scale dependence reduced by going to NNLO
o
there is a subtlety here connected to the minimum photon energy:
 convention: ‘inclusive’ means Eγ > 1.6 GeV
 experiments cannot measure that low: needs extrapolation
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Direct CP Violating Asymmetries

o direct CP violation or ‘charge asymmetry’:
o occurs if >= 2 amplitudes with different weak and strong phases
PRL93:131801,2004
o well established in B0->K+π- decays
WA (HFAG): Acp(K+ π-) =-0.108 +/- 0.017
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Predictions for direct CP Violation in b->Xγ

o b->sγ
 loops always dominated by top-quark contribution, VtbVts*
 small asymmetry: Acp = 0.004 ± ~0.002
(Nucl.Phys.B704,2005)
o b->dγ
 ‘up’ contributions VubVud* about as important as ‘top’ VtbVtd*
 large asymmetry: Acp = -0.10 ± ~0.04
(Nucl.Phys.B704,2005)
o in the limit ms=md (‘U-spin’), CP violation vanishes in the SM
 inclusive asymmetry Acp(B->Xdγ+B->Xsγ) ≈ 0
 corrections are of order (ms/mb)2 times small CKM factors
o for exclusive decays results are more model dependent, but uncertainties
not much larger
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
The photon polarization
o SM: W couples to left-handed quarks:
bL
tL
sL
o photon has spin 1: to conserve helicity in a two body decay, one of the
quarks needs to ‘flip helicity’
bRsL γL: spin-flip on the b-quark
bLsR γR: spin-flip on the s-quark
o
the probability for spin-flip is proportional to the quark mass
 two important consequences:
1. SM: b -> γL and anti-b -> γR
 opposite helicity suppressed by ms/mb.
 measurement of polarization is excellent probe for NP
2. anomalous WbRtR coupling would strongly affect the b->sγ
branching fraction, because it is enhanced by mt/mb
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
B factories: e+e-  Y(4S)  BB
o B factories operate at the Y(4S)
resonance (10.58 GeV)
o hadronic cross-sections:
uds/cc/ bb = 2.1 / 1.3 / 1.1 nb
o in the Y(4S) frame the B mesons
are practically at rest
 need boost to measure decay
lengths with high accuracy
 PEP-II is an asymmetric collider
9.0 GeV electrons vs
3.1 GeV positrons
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PEP-II and BaBar at SLAC

linac
PEP-II
storage ring
SLD
BaBar
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
Integrated luminosity
when performing well,
off-resonance data
on-resonance data
PEP-II produces about 10
BB event per second
since 2000 BaBar has
recorded over 300M BB
events
about 8% of data is taken
below the Y(4S)
resonance
results presented here
are based on
80 fb-1 and 210 fb-1
on-resonance data
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
The BaBar detector
Electromagnetic Calorimeter
6580 CsI crystals
e+ ID, π0 and γ reco
Instrumented Flux Return
19 layers of RPCs
μ and KL ID
Cherenkov Detector (DIRC)
144 quartz bars
K, π, p separation
3 GeV
positrons
Drift Chamber
9 GeV
electrons
1.5 T magnet
40 layers,
tracking + dE/dx
Silicon Vertex Tracker
5 layers of double-sided
silicon strips
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BaBar (artist’s perspective)

19
Examples of samples of exclusive B decays
very many:
B-> D(*) + N π
qqbar continuum
very clean:
B->charmonium Ks
about 0.4% of
all produced B

very rare: B->Kll
BF≈0.4x10-6
bump from other B decays:
‘peaking background’
Distributions show
‘beam-energy substituted mass’:
One of main ingredients in multi-dimensional ML fits used for the
analysis of exclusive decays
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B->Xγ at the B-factories

Three types of probes
o various exclusive final states, such as K*γ, Kππγ, Kφγ, Kη’γ
 branching fractions, CP asymmetries, isospin asymmetries
 small systematic uncertainties
 theory accuracy limited by FF calculations, but improving
o semi-inclusive: as a sum-of-exclusive-modes
 reconstruct B->Kγ + up to 3 or 4 pions, at most 1 pi0
 cross-feed between difference B->Xγ modes not negligible
 larger uncertainty from backgrounds than exclusive analyses
 extrapolation to fully inclusive BF not trivial
o fully inclusive
 use only the photon as a tag
 measures really b-> (s+d) γ
 dominating systematic: background from other B decays
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
What do BXsγ events look like?
o one high energy photon and an s-quark ‘jet’
Eγ≈M/2

B
M = 5.28 GeV
Xs
K+
hadrons
o not exactly a 2-body decay, because the b quark is bound in a meson and
because of higher order corrections (e.g. bsgγ)
K*γ
o the simplest final state is
B->K*(890)γ (about 12%)
o the photon spectrum itself
is important physics as well:
it can be used to extract
HQET parameters
photon spectrum measured
by Babar (PRD 72, 052004)
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
Radiative penguin portrait
B+→ K*+γ (K*+->Ksπ+) candidate
Muon from
other B decay
Detached vertex
from Ks → ππ
High energy
photon in EMC
π+ from K*+
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
Measuring BF(B->Xsγ)
Simplest technique: count events with one high energy photon
Very large backgrounds from
o continuum qq-bar (q=u,d,s,c) and ττ-bar
qq + ττ
BB
o other B decays
most photons come from π0 and η decays
Backgrounds are suppressed by
B->Xsγ
o explicit veto of π0 and η candidates
o ‘tagging’ the other B
o using event-shape
o analysis does not separate b->dγ (about 4% of total rate)
 subtracted using theory prediction
o B not exactly at rest in CMS: measure ‘Eγ*’ rather than ‘Eγ’
24
Continuum background suppression

Tagging the second B
o leptons common in B decays: B->Xlν ~ 20%
o leptons are not common in qqbar background
 require high pT muon or electron
Exploiting event shape:
o qqbar events are ‘jet-like’
o B events are spherical
 combine ‘event-shape’ variables in
multivariate discriminant
25

Background subtraction
Remaining background is subtracted
o use ‘off-resonance’ data to subtract continuum.
 reliability tested in high Eγ sideband
o use Monte Carlo to subtract BB background.
 mostly π0 and η, but also electrons,
anti-neutrons, ω, …
 MC carefully tuned on control samples
(for example by reversing π0/η veto)
tested in low Eγ sideband
BB
continuum
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
Result
Remaining events in 81/fb, for Eγ = 1.9-2.7 GeV
Nsig = 1042 ± 84 (stat) ± 62(syst)
Preliminary result (hep-ex/0507001)
BF(b->sγ, Eγ>1.9 GeV) =
stat
o main systematic: BB background subtraction
 improves with larger control sample size
syst
model-dependence
of efficiency
o efficiency depends on Eγ  correction model-dependent
 improves with better measurement of photon energy spectrum
o result still requires extrapolation to Eγ = 1.6 GeV
 leads to additional uncertainty
 also improves with better measurement of photon energy spectrum
27
Other techniques for measuring BF(b->sγ)

1. as a sum-of-exclusive-modes (BaBar, PRD 72,052004 (2005) )

reconstruct as many final states as possible, about 55% of total

kinematic constraint on Eγ: detailed information on photon spectrum

dominant systematic: fraction of missing final states (fragmentation)
 not competitive with fully inclusive measurement in the long run
2. in the recoil of fully reconstructed B decays (no results yet)

exploit large sample of about 200,000 fully reconstructed B decays

may provide interesting information on fragmentation
28
Measurements of the bsγ branching fraction

10% measurements agree with
10% NLO SM predictions
improvements of both to 5%
seems feasible:
 theory: NNLO
 experiments: towards 10x
more statistics
agreement already highly
constrains new physics!
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
An example: bsγ and the charged Higgs
b->sγ receives contribution from
charged Higgs in many SM extensions
(eg supersymmetry)
assuming that only 2 doublet Higgs sector
contributes at low energy (2HDM), we
rule out a considerable part of the
parameter space!
m(H+,type-II 2HDM) > ~450 GeV
(using technique from Gambino and Misiak)
limit is much better than that from
direct searches at LEP and Tevatron!
of course, exclusion power model
dependent …
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
Another example: b→ sγ and top quark couplings
new Wtb couplings will also affect BF(bsγ)
from hep-ph/9906329,
(uses slightly smaller
value for SM contribution)
new right-handed
coupling, enhanced by mt/mb
new left-handed
coupling
10% error on BF constrains right-handed
coupling to be few % of SM coupling
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Direct CP violation in the inclusive sample

o event selection uses lepton tag  events are already tagged!
 B0 contribution ‘mixes’: asymmetry diluted by factor 0.816+/-0.004
 cannot subtract b->dγ contribution: measure really Acp[ b -> (s+d)γ ]
o statistical precision optimized by reducing Eγ* window to [2.2,2.7] GeV
 ‘extrapolation’ error for theory predictions better under control than
for the branching fraction
o events observed: N(l+) = 349 +/- 48 versus N(l-) = 409 +/- 45
o stat. uncertainty much larger than for ‘exclusive’ analyses (next slide)
o systematic uncertainty again dominated by B background
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
Summary of Acp measurements
inclusive
semi-inclusive
exclusive
o current B-factories cannot reach inclusive Acp better than ~0.03
o semi-inclusive and exclusive below 0.01 seems feasible
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
Intermezzo: time-dependent CP-violation in a nutshell
o suppose ‘f’ is a final state that is accessible to both B0 and anti-B0
o the total amplitude contains at least two contributions, one appearing
through B0-anti-B0 mixing
decay
B0
In simpl(istic) words:
f
anti-B0
o the contribution from ‘mixing’ depends on how long it takes the B to decay
 relative contribution is ‘time-dependent’
o furthermore, if the contributions have a relative phase, we get CP-violation
o the sum of these 2 effects is “time-dependent CP violation” (TDCPV)
coefficients between -1 and 1
Mixing
frequency
B
factory
specialty:
BB
decays
in ‘entangled state’
flavour of B
Δm=0.51/ps
o att t=0
 Δt
Lifetime: τ=1.5ps
o flavour at (t=0)  anti-flavour of other B at time of decay
*) expression becomes more complicate if f is not a CP eigenstate.
34
Example: B->ψKs (the ‘golden’ mode)

In the Standard Model:
S(ψKs) = sin 2β ≈ 0.7
C(ψKs) = 0
35
Probing the photon polarization through TDCPV

o now, consider the decay BXγ with X a CP-eigenstate
o just like before, but interference is suppressed (Atwood,Gronou,Soni)
XsγR
B0
anti-B0
Xs γL
o the suppression is proportional to fraction of opposite helicity photons
S ≈ - sin 2β x 2ms/mb ≈ -0.04
o NP contributions with different photon polarization enhance interference
 TDCPV becomes a probe of the size/chirality of NP in bsγ
o this works for several final states, but B0->K*0 γ with K*0->Ksπ0 is
currently the only experimentally accessible mode
o main theoretical uncertainty: contributions from b->sγg
(Grinstein,Grossman,Ligetti,Pirjol 2004)
36

Mixing induced CP violation in B→Ks0
o main experimental complications:
 measure a B vertex with one trajectory (well established by now!)
 large continuum background: exploit event shape, 0/η vetos
 background from other B decays (such as other B->Xsγ and B->Ksπ0π0)
Ksπ0 mass distribution
o in principle, all B-> Ksπ0 γ final states can
be used (Atwood, Gershon,Hazumi,Soni)
o however above the K*(890)
 poorly known background from other B
decays becomes much larger
 theoretical uncertainties are larger
 only small number of events
(background subtracted)
K*(890)
K*(1430)
(Belle seems to gain more from including
the ‘high-mass’ region than Babar does)
37
Mixing induced CP violation in B→Ks0
signal+background
background
data

(beamenergy-constrained) mass
for B->K*(890)γ candidates
Δt distributions and asymmetry
Result from fit in 210/fb:
Nsig = 156  16
S = −0.21  0.40  0.05
C = −0.40  0.23  0.04
(PRD72,051103,2005)
Errors still very large:
consistent with both SM and ‘no polarization’
38
Future prospects for this technique
non-SM physics is not going to be very large:
this method only becomes interesting with errors
of about σ(S)≈0.05

super-B-factory
statistics
also accessible at LHC-b
39
Not today: other important radiative B decays

o the b->dγ transition
 estimated branching fraction few parts in 10-6
 experimentally hard because of large background from b->sγ
 searched for in exclusive decays B0->ρ0γ, B+->ρ+γ and B->ωγ.
 together with B->K*γ these modes provide a constraint on |Vtd/Vts|
o the b->sl+l- transition
 branching fraction also few parts in 10-6
 studied both in exclusive B->K(*) l+l- and ‘semi-inclusive’ B->(Kll + (1-4)π)
 3 competing SM amplitudes at leading order:
 very rich phenomenology: SM provides clean predictions for l+l- mass
and angular distribution, the ratio of e+e- to μ+μ-, etc
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Summary

o b->sγ decays are an excellent prove for new physics
 beyond-the-standard-model physics enters at leading order
 serious predictions exists for several measurable observables
this is the ‘virtual high energy frontier’
o most important measurement: b->sγ branching fraction
 agreement between data and theory highly constrains new FCNC
 either NP is very heavy (not very likely)
 or it is flavour blind (the SUSY flavour problem)
 or there are accidental cancellations (bad luck)
 expect considerable improvements in both experiment and theory
within next few years
o polarization of the photon
 experimental uncertainties from TDCPV still large
 other methods exist (angular distribution in Kππγ), but do not look
very promising yet
o B factories have still much more data coming … stay tuned!
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