radiative penguin decays at babar - university of edinburgh · pepii asymmetric bfactory: 9gev eˉ,...
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Radiative Penguin decays at BaBar
Debbie BardUniversity of Edinburgh
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PEPII
Asymmetric Bfactory: 9GeV eˉ, 3.1GeV e+
Record instantaneous luminosity just over 1x1034cmˉ²sˉ²
Total recorded BaBar luminosity runs 14: ~240 fbˉ¹
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CKM parameters
VudVub* + VcdVcb* + VtdVtb* = 0
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Radiative PenguinsFlavour Changing Neutral Current (FCNC) transitions
Forbidden at tree level in SM
Permitted at loop level
Photon emitted in loop ∴ radiative
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This is why it's a 'penguin' diagram
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Physics Interest bs
NonSM particles may enter the virtual loop and alter expected Branching Fraction (BF)
Good agreement between SM theory/experiment in bs constrains new physics at electroweak scale
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Physics Interest bdVery rare decay
Conflicting experimental results for BF
SM interest – ratio of Vtd/V
ts not well known
SU(3) breaking of form factors in ratio
R nonpenguin contribution to loop
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Physics Interest Acp
Direct probe for new physics Acp = (B) (antiB) (B) + (antiB)
SM predicts:
Acp(bs) ~ 0.006
Acp(bd) ~ 0.160
NonSM loop contributions could alter Acp~10%
Acp(b(s+d)) = 0
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What's a SemiInclusive analysis?Exclusive analysis:
exclusive hadronic modes eg. BK*
cleaner BF measurement both experimentally and theoretically
Inclusive analysis
measure photon spectrum, inclusive BF
SemiInclusive analysis
sum of exclusive hadronic modes
can measure BF of individual modes and inclusive BF
can measure photon spectrum
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bs Semiinclusive BF
Sum of 38 exclusive modes
Hadronic mass range M
Xs
0.62.8GeV
89 million BBbar pairs
Single bin fit to all events
Signal
Continuum BG (fixed from MC)
Peaking BG (fixed from MC)
BB BG and crossfeed
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bs Semiinclusive BF
BF = (3.310.19 +0.64 0.42)x104
Consistent with Standard Model prediction of BF(bs) = 3.600.30 x104 (Gambino & Misiak '01)
World average: 3.47 (HFAG)
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bs Semiinclusive AcpUsed 12 exclusive final states
Hadronic mass range MXs
0.62.3GeV
Acp = 0.025 0.050 0.015Recent Belle result: Acp = 0.004 0.051 0.038 Consistent with standard model prediction of 1%
antiB mesonsB mesons
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bs– Photon Spectrum
HQET expands around limit of static b quark
motion of b is universal – information from bscan be used for extraction of Vub/Vcb in Bxl
Spectral shape independent of new physics, expressed in terms of HQET parameters, dependent on scheme
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bs– Photon Spectrum II
Fit to photon spectrum Fit to HQET parameters in Shape Function scheme
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Exclusive B()Simplest and most common bd exclusive decays
Reconstruct
B with and ,
B with
Theory predicts BF[Bx106
Use full run 14 dataset: 211 million Y(4S)BBbar pairs
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Exclusive B()BaBar results: no evidence for decays found
Improved upper limits (at 90% CL)B+ BF(<1.8x106)B0 BF(<0.4x106)
B0 BF(<1.0x106)BBF(<1.2x106)
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CKM constraints from BBF ratio limit: <0.029 (90% CL)
Neglecting theoretical errors <0.19|Vtd| |Vts|
BF(B) BF(B)
(ζ2,∆R) = (0.75,0.00) (ζ2,∆R) = (0.85,0.10) – with theory error no theory error
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Semiinclusive bdSum of 7 Exclusive modes
Also reconstruct 7 equivalent bsmodes
Measure ratio of branching fractions gives
Theoretically clean
Experimentally clean (systematic errors cancel)
B0(K)
(K)
K
K
K
K
K
BF(bd) BF(bs)
|Vtd| |Vts|
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Semiinclusive bd background suppression
Continuum
Dominant background
From ISR, decays
Use eventshape variables in neural net
Tagging variables
Peaking backgrounds
Crossfeed From misidentified bs decays
Particle ID kaon/pion
Generic B decays
vetos
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Expected YieldsScaled n events Scaled in
Expected Signal box
Signal 31 24
Uds 15067 488
Ccbar 1480 50
Generic B 4220 38
Xs gamma 19 11
All cuts incl. best cand selectionScaled to 211 million BB pairsAssume signal BF = 1e5
Signal efficiency over whole hadronic mass region M
Xd=1.01.8 GeV
Signal box: |∆E|<0.1, 5.274<mES<5.286
∆E = E*BE*
beam
mES2 = E*beam
2 p*B
2
simple cut and count significance: ~1including region: ~22D LH fit should take us to > 3significance
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More interesting hints of New Physics...bsl+l
SM Predictions for many observables
BF ~ 106, negligible direct Acp
Ratio of e/ sensitive to h0 contribution in loop
q2 distribution and Afb sensitive to relative contribution of , Z penguins and box diagram
q2(Afb=0) well predicted in SM
sign of Afb sensitive to relative signs of Wilson coefficients C9/C10 and C7
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Interesting Experimental Results
Belle finds in 350 fb1 (prelim., hepex/0508009)
SM: Afb(Kll)=0
SM: integrated Afb(K*ll) =0.160.20
excludes charmonium resonances
BF Discrepancy!
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Conclusion
Radiative penguin decays starting to provide stringent (and interesting!) constraints of SM parameters
Close to more exciting results
First signs of nonSM behaviour could be seen here!
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Backup slides
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BK
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bd Neural Net performance
NN cut @ 0.85: 94% continuum rejection, lose 55% signal.
NN output: signal in red, uds in blue, scaled to equal area
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Afb: