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Radiative B Decays (an Experimental Overview)
E.H. ThorndikeUniversity of Rochester
CLEO Collaboration
FPCPMay 18, 2002
The Observables Rates for exclusive decays. eg, B K*(892) Rate for inclusive decay b s (actuallyB Xs ) CP asymmetry, inclusive decays
CP asymmetry, exclusive decays Photon energy spectrum in inclusive decays B Xs
Same observables for b d
)()(
)()(
sbsb
sbsbacp
What do you learn? Rate for exclusive decays
• Experimentally easiest.• B K*(892) first penguin seen(1993).• Form factors not known, so not good for “New Physics”.
Rate for inclusive decays• Loops, w &t, so sensitive to other heavy things in loop (i.e. “New
Physics”)• Reliably calculated with SM and with “New Physics” excellent for revealing or limiting “New Physics”.
CP asymmetry• Expected to be very small in SM.• 10-20% in some “New Physics”.• Inclusive more reliably calculated than exclusives,
but if big in either, New Physics.
What do you learn? – cont’d
Photon energy spectrum in b s • Insensitive to New Physics (b s is 2-body, a line)• Depends on quark mass and Fermi momentum
Can give B light cone shape function (useful for obtaining |Vub| from b u l inclusive).
Can help determine HQET OPE expansion parameters (needed for obtaining |Vcb| from b c l inclusive).
b d • Initial interest will be in determing |Vtd|
(but watch out for long distance effects, and for additional CKM factors from c - and u - quark loops).
The Experimental Problems
MUST suppress continuum.
MUST subtract continuum.
To push spectrum down below 2.2 GeV, must handle backgrounds from other B decay processes.
Outline for Rest of Talk
Branching Fractions for Exclusive Decays Branching Fraction for Inclusive Decays CP Asymmetries Photon Energy Spectrum b d
Discovery of PenguinsCLEO -1993
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B K* (BaBar)
Run I (22.7 M BB) H Tanaka Moriond 2002
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B K* (Belle)
BK* Branching Fractions
BK*o B-K*-
CLEO ’93 4.0+1.7+0.8 5.7+3.1+1.1
CLEO ’00 4.55+0.70+0.34 3.76+0.86+0.28
BaBar ’02 4.23+0.40+0.22 3.83+0.62+0.22
Belle (prelim) 4.08+0.34+0.26 4.92+0.57+0.38
average 4.21+0.25+0.26 4.32+0.38+0.30
(All numbers, X10 -5)
BK*2(1430) Branching
Fractions
CLEO ’00 1.66+0.56+0.13 x 10-
5
Belle (prelim) 1.50+0.56+0.12 x 10-
5
Other Exclusives (Belle)
B+ K+x
K*ox K+ox K+ (NR) < 0.9 x
Continuum Suppression forInclusives -CLEO
Leptons: If event has lepton (e or ), use l , El for additional continuum suppression.
Weight: For each event with a high energy , determine probability that it is b s , rather than continuum background. Weight each such event, according to probability.
Event shape variables: R2, S , R’2, cos’, cone energies within 20o, 30o of direction and - direction. Into neural net, 8 inputs, 1 output.
“Pseudoreconstruction”: Search events for combinations of particles that look like B->Xs. For Xs use K+ or Ko
s, and 1-4 (at most 1 o). Calculate
If event has 2B<20, use
2B , |costt| for additional
suppression.
)()(22
2
M
B
E
beam MMEEB
CLEO, PRL 87, 251807 (2001)Title:
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Photon energy spectra (weights per 100 MeV)
Top shows the On Y(4S) and the scaled Off-resonance spectra.
Bottom shows the difference and the spectrum estimated from B decay processes other than b s and b d.
B( b s )
CLEO ‘95CLEO ‘95
ALEPH ‘98ALEPH ‘98
Belle ‘01Belle ‘01
CLEO ‘01CLEO ‘012.0 GeV
2.2 GeV
?? GeV
2.2 GeV
TheoryBuras,Misiak, et alHep-ph/0203135
x10-4
CP Asymmetry NOTE sign convention FOLLOW sign convention (so far, everyone seems to have.)
BK*(892)
CLEO, ‘00 +0.08+0.13+0.03
BaBar, ‘02 -0.044+0.076+0.012
Belle, new +0.032+0.069+0.020
Average +0.009+0.048+0.018
CLEO, ‘01 Inclusive -0.079+0.108+0.022(0.965A(bs )+0.02A(bd ))
)()(
)()(
sbsb
sbsbacp
Photon Energy Spectrum- the B Backgrounds ’s from o , , that have escaped the o/ veto.
The big one (90% of total). Measure o, yields, treating o () as if it were a , all cuts as
for b s analysis. Use Monte Carlo to determine o/ veto efficiency.
’s from other sources o, ’ o, radiative decay, , a1 , final state
radiation. b u processes, b sg processes. They’re small, and with modest effort to have Monte Carlo event
generator ok, one can trust the Monte Carlo. Klong, interactions in calorimeter.
Determine contribution from lateral distribution in calorimeter (E9/E25).
n
CLEO (PRL 87, 251807 (2001))
Observed laboratory frame photon energy spectrum (weights/100 MeV) for ON minus scaled OFF minus B backgrounds, the putative b s plus b d signal.
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Moments of the Spectrum CLEO obtains moments in the B rest frame, for E(rest frame) > 2.0 GeV:
)]/1(482.741.10491.14.7482.7
153.6997.728.3298.0474.7185.3
946.0)87.01(648.1)(43.154.11[192
||)3689.0(
433
33
32
31
32
31
32
33
3
22
21
2
2
02
3
222
BBBBBB
BBB
l
BBB
B
s
B
ssBcbFs
MOMMMMM
MMMMMM
MM
MVG
GeV10.008.035.0
HQET plus OPE allows inclusive observables to be written as double expansions in powers of s and 1/MB to order o2
s and 1/M3B
C2 and C7 are Wilson coefficients and o is the one-loop QCD function. The 1/M3B parameters are
estimated from dimensional considerations to be (0.5GeV)3.
Using the first CLEO obtains
The expression for the second moment converges slowly in 1/MB, and so CLEO made no attempt to extract parameters from it.
011.0032.0346.2 E 0020.00066.00226.022 EE
)
1(
94
33
12
3313))(175.1954.01()(620.0385.01
2 47
222
34321
3212
02
0M
OCMM
C
MMM
ME
BBDBB
ss
B
ssB
)
1(
12
3
12
32))(05412.005083.0()(01024.000815.0
12 4321
3212
02
02
122
MO
MMMMMEE
BBB
ss
B
ss
BB
GeV GeV 2
Convolute with light cone shape function.
b s (parton level)
B Xs (hadron level)
B lightquark shape function, SAME (to lowest order in QCD/mb) for b s B Xs and b u l B Xu l.
b u l (parton level)
B Xu l (hadron level)
BaBar limit by far the best [(1-)2+2]1/2 < 1.6 (Tanaka, Moriond ’02)
Still, not an improvement in limit on |Vtd| over that from Bs- s mixing.
b d So far nothing on inclusive. Only upper limits on exclusives. Expect B(B + ) = 2 x B(B o ) = 2 x B(B )
B Pairs
(Million) B(B + ) 2 x B(B o ) 2 x B(B )
CLEO ‘00 9.7 13 34 18
Belle ‘01 11 10 21 ---
BaBar prelim
63 2.8 3.0 ---
Branching Fraction Upper Limits (10-6)
B
Summary and Conclusions I b s Exclusive branching fractions.
Not of great fundamental interest, but by identifying a larger
fraction of the makeup of B Xs decays, one will reduce some systematic errors on the branching fraction for the inclusive process b s . Belle progress on this front.
b s inclusive branching fraction. Experiment agrees with SM theory, places strong restrictions on
New Physics. But really only one good measurement. Babar and Belle should
get to work! They will need to:1. Accept photons down to 2.0 GeV, or lower. (2.2 GeV is no longer
good enough)2. Take a reasonable amount of data below the Y(4S) resonance.
(continuum subtraction MUST be done with DATA.)
Summary and Conclusions II CP asymmetry
No hint of a non-zero value. Limits place weak restrictions on New Physics. Plenty of room for improvement. Asymmetry for inclusive wanted (Babar, Belle??)
b s photon energy spectrum Has helped provide precise determination of |Vcb| from the inclusive
semileptonic decay branching fractions, and (more important) a good determination of |Vub| from the lepton endpoint yield in b u l, with DEFENSIBLE ERRORS. Will be key for future determinations of |Vub| from inclusive b u l.
Improvements in spectrum very desirable. b d
So far, nothing on inclusive, only upper limits on exclusives. Not yet an improvement in limit on |Vtd| over that from mixing. Stay tuned.
BB ss