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

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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

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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

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1(

94

33

12

3313))(175.1954.01()(620.0385.01

2 47

222

34321

3212

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OCMM

C

MMM

ME

BBDBB

ss

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1(

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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.

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