Rare B decay searches with BABAR using hadronic tag ...steven/StevenWebFiles/Talks/cap...Jun 10,...
Transcript of Rare B decay searches with BABAR using hadronic tag ...steven/StevenWebFiles/Talks/cap...Jun 10,...
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Rare B decay searches with Rare B decay searches with BABAR using hadronic tag BABAR using hadronic tag
reconstructionreconstruction
Steven RobertsonInstitute of Particle Physics
CAP Congress 2009Moncton, NB
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Jun 10, 2009 2Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Physics with BABARPhysics with BABAR
PEP-II Asymmetric B Factory produced e+e- interactions at the Υ(4S) resonance (~10.5 GeV)
σ(BB) ~ 1.1nb ⇒ 1.1M/fb-1 Total of 430 fb-1 collected by BABAR
between 1999 – 2008:
~500 million BB pairs
BB
thre
shol
d
B0B0 pair is produced in a coherent L=1 state
See talk by Tom Mattison on Weds afternoon
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Jun 10, 2009 3Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Rare decay searches with BABARRare decay searches with BABAR
Substantial potential for New Physics searches in rare B decays
e- (9.0 GeV)
e+ (3.1 GeV)
EM calorimeter6580 CsI(Tl) crystals
1.5 T solenoid
5-layer silicon vertex detector
DIRC – RICH utilizing total internal reflection
Magnet flux return instrumented with RPCs ( LSTs)→
40-layer drift chamber
u l+
νb
W+, H+B+
rH
● non-SM contributions can be comparable to or larger than (suppressed) SM contribution:
● Experimental challenge is to identify rare signal decays with missing energy or limited kinematic information
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Jun 10, 2009 4Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Rare decay searches with BABARRare decay searches with BABAR
How can we distinguish particles associated with the “interesting” signal event from those arising from the decay of the second B meson in a Υ(4S)→ BB event?
● Reconstruct the other B instead, then check if whatever is left looks like a signal decay
B+→l+νγ signal simulation
Y(4S)B- B+
ν
l+
γ
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Jun 10, 2009 5Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Hadronic “tag” reconstructionHadronic “tag” reconstructionReconstruct a large sample of hadronic (or semileptonic) B decays in
specific “clean” decay modes
● method based on a D(*) “seed”, to which individual charged and neutral pions and kaons are added until a B candidate is identified
● exploit knowledge of CM energy and tag B kinematics
mES (GeV/c2)
BB-- →→ D D(*)0(*)0 X X-- ΚΚ π π ΚΚ π π π π00 ΚΚ π π ππ π π ΚΚ00ss π π
ππ
Advantages:
● clean separation of signal and tag decay products
● improved determination of missing energy and signal B 4-vector
● strong suppression of (and precise determination of) continuum backgrounds
Disadvantage:
● efficiency ~0.3%
correctly reconstructed
mis-reconstructed
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Jun 10, 2009 6Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Leptonic B decays Leptonic B decays
Helicity suppressed SM process with potential sensitivity to tree-level New Physics contributions
● SM BFs scale with final state lepton mass squared, hence τ mode is most prominent (BF ~10-4), but mode is within μ statistical reach of B current factories (BF ~10-7),
u l+
ν
W+,H+
bB+
fBVub
In MSSM (or generic 2HDM), charged Higgs boson gives equal relative enhancement/suppression in all modes
● At one-loop level, large (Higgs mediated) lepton flavour violating effects can break hierarchy imposed by helicity suppression
i.e. NP constrained by measuring ratios of branching fractions of leptonic modes
Standard Model Rates B(B+ → τ+ν) 1x10-4
B(B+ → μ+ν) 4x10-7 B(B+ →e+ν) 10-12
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Jun 10, 2009 7Steven Robertson Institute of Particle PhysicsCAP Congress 2009
LHC vs Flavour ConstraintsLHC vs Flavour Constraints
Results of hadronic and semileptonic tag searches for B+ → τ+ ν have been reported by both BABAR and Belle
● Resulting charged Higgs bound is fairly model-independent and substantially constrains region of parameter space accessible to LHC experiments
arXiv:0901.0512
U. Haisch arXiv:0805.2141 [hep-ph](presented at FPCP 2008)
ATLAS
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Jun 10, 2009 8Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Tagged Tagged BB++ →→ll++ ν ν ((ll = e, = e, μμ) )
CM frame B rest frame
μν
B+
B-
e+
e-
Can use the hadronic B reconstruction method to search for other leptonic modes (e, μ) as well as τ mode● only 1 neutrino, so reconstruction of tag B
completely constrains event kinematics
● Signal B rest frame estimated from tag B 4-vector, permitting 2-body signal kinematics to be exploited
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Jun 10, 2009 9Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Tagged Tagged BB++ →→ll++ ν ν ((ll = e, = e, μμ) ) and LFVand LFVHadronic tag reconstruction method used
for first time for this search by BABAR (thesis work of M. Klemetti, McGill)
● also placed limits on LFV neutral B modes
PRD-RC 77, 091104 (2008)
Mode Data Sample 90% CL limit
● Sensitivity limited by statistics with BABAR data sample but excellent control of kinematics and background suppression would make this a slam-dunk for SuperB
μν
B+
B-
e+
e-
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Jun 10, 2009 10Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ll++ ν ν γγ ((ll = e, = e, μμ)) Presence of (hard) photon removes helicity suppression in purely
leptonic decay modes, but adds factor of and form factor αuncertainties:
G. Korchemsky, D. Pirjol, T.-M. Yan Phys. Rev. D61:114510, 2000. (hep-ph/9911427)
B(B+→l+νγ) ~(1-5)x10-6 for all three modes
● In practice, can extract λB which
is related to B light cone wave function
Model uncertainties make it desirable to provide model-independent measurement...
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Jun 10, 2009 11Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ll++νν γ γTagging provides precise measurement of
tag B (and hence signal B) 4-vector
● in combination with lepton and photon, neutrino 4-vector is fully determined
● strong suppression of (dominant) b→ ulν decay background
● no additional particles should be present in the event
μν
B+
B-
γ
Work by Dana Lindemann (McGill)
μν
B+
B-
γγB+ →π0 l+ν
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Jun 10, 2009 12Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ll++νν γ γ backgroundsbackgroundsNon-B backgrounds are strongly suppressed by tag reconstruction
procedure, and remaining background can be estimated directly from data
reconstructed invariant mass of tag B candidate
● Only B “peaking” backgrounds rely on MC, and this can be explicitly verified against data
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Jun 10, 2009 13Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ll++νν γ γ resultsresultsUnblinded data revealed
slight excess over expectation, but consistent with no signal
BABAR preliminary
To be submitted to PRL
Model independent branching fraction results:
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Jun 10, 2009 14Steven Robertson Institute of Particle PhysicsCAP Congress 2009
SummarySummary
Tag reconstruction techniques provide an extremely clean analysis environment for studies of rare B decay processes which would otherwise lack sufficient kinematic constraints
● tag efficiency is low, but large B factory datasets still make these methods viable
● very interesting sensitivity for high luminosity SuperB factory!
Recent BABAR results on rare leptonic B decays B+ → l+ ( )ν γ provide interesting constraints on New Physics which are fully competitive with expected direct LHC sensitivity
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Jun 10, 2009 15Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB+/-+/- tag reconstruction tag reconstruction
Realistic tagging efficiency (per B+/-) of ~0.24% in events containing a low-multiplicity “signal” event
● typically “signal-side” selection is fairly efficient (~10% - 70%)
Assuming ~30% efficiency gives “single-event sensitivity” at
Br(B → rare) ~2x10-8 with 75 ab-1
mES (GeV/c2) mES (GeV/c
2)
After continuum background suppression
No combinatorial background in signal events!
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Jun 10, 2009 16Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ll++νν γ γ at SuperBat SuperBBABAR analysis (optimized for ~0.5ab-1 ) can be naively extrapolated
to SuperB luminosities
● Simple cut-and-count with no kinematic or angular constraints on the signal lepton or photon
Sensitive to entire SM range with 75ab-1
● Ultimate limitation will likely be b→ ulν form factors needed for “peaking” background estimate
Dedicated SuperB study currently in progress
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Jun 10, 2009 17Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Neutral B tagsNeutral B tags
Realistic tagging efficiency (per B0) of ~0.16% in events containing a low-multiplicity “signal” event
● Assuming 30% efficiency gives “single-event sensitivity” at
Br(B → rare) ~3 x 10-8 with 75 ab-1 Obviously, actual sensitivity depends on the level of background...
mES (GeV/c2) mES (GeV/c
2)
After continuum background suppression
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Jun 10, 2009 18Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Run 7Run 7
● Data taken between December 16, 2007 and April 7, 2008
● 30.2 fb-1 at ϒ(3S)● 14.5 fb-1 at ϒ(2S)● ~5 fb-1 above ϒ(4S) scan● Several ϒ(3S) analyses and
above ϒ(4S) scan presented at ICHEP 2008
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Jun 10, 2009 19Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Tagged Tagged BB++ →→ll++ ν ν ((ll = e, = e, μμ)) ● Signal lies at kinematic endpoint in lepton p* for B decays, hence
essentially no B background
● Continuum background can produce high p* leptons, but this background can be directly determined from data using the tag B mES sideband
● Narrow signal peak would lead to a very compelling signal with a very small number of events
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Jun 10, 2009 20Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ττ++ ν ν Experimentally challenging due to presence of multiple neutrinos:
● Search for 1-prong τ decays: e , , and τ→ νν τ→μνν τ→πν τ→ππ0 ν● “Topological” signal candidate selection, then require residual
calorimeter energy to be consistent with “noise”
Phys. Rev. D76, 052002 (2007)
Phys. Rev. D77, 011107 (2008)
BABAR searches based on 383 x 106 B meson pairs
Combined result: (2.6 significance) σ
(SL tag)
(Had tag)
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Jun 10, 2009 21Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ll++ ν ν γγ ((ll = e, = e, μμ)) In limit of zero photon energy
(actually, below a few 100 MeV) “signal” is helicity suppressed leptonic modes
- KPY model only valid down to ~few hundred MeV
● Difference in models mainly impacts angular distributions
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Jun 10, 2009 22Steven Robertson Institute of Particle PhysicsCAP Congress 2009
BB++ →→ll++ ν ν γγ ((ll = e, = e, μμ))
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Jun 10, 2009 23Steven Robertson Institute of Particle PhysicsCAP Congress 2009
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Jun 10, 2009 24Steven Robertson Institute of Particle PhysicsCAP Congress 2009
NP @ tree level and beyondNP @ tree level and beyond
● Uncertainties from Vub and fB cancel in ratio of modes!
⇛ NEED DETERMINATION OF BOTH τ AND μ MODES
Tree-level charged Higgs (Type-II 2HDM or MSSM) contribution has the same relative effect on all leptonic modes:
B(B+→l+ν)MSSM = B(B+→l+ν)SM x [ 1 – (m2B/m2H+) tan2 /(1+β ϵ0 tan2 )β ]2
● “Universality” preserved at tree level
At one-loop level, potentially large Lepton Flavour Violation (LFV) effects entering from e.g. SUSY in grand unification scenarios:
i.e. B+ → l+ νl' where l' ≠ l via effective l H+ ν coupling● Observable effects in ratios, Rb
l/τ, of B leptonic branching ratios:
G. Isidori and P. Paradisi hep-ph/0605012A. Masiero, P. Paradisi and R. Petronzio hep-ph/0511289
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Jun 10, 2009 25Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Lepton Flavour ViolationLepton Flavour Violation
● In Standard Model, lepton flavour conservation is not associated with any underlying symmetry principle
● LFV generally permitted in New Physics models containing more than one Higgs doublet
● In SUSY seesaw models, flavour changing insertions arise from Yukawa couplings in the slepton mass RGEs:
● ⇛ New Physics effects in τ→ l (γ l = e, )μ can saturate experimental bounds in natural and well-motivated models
● MSSM with heavy right-handed neutrinos and seesaw mechanism: Br~10-7
● heavy Dirac neutrinos, RPV SUSY models, flavour changing Z' models...
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Jun 10, 2009 26Steven Robertson Institute of Particle PhysicsCAP Congress 2009
Higgs mediated LFVHiggs mediated LFV
● Higgs mediated LFV present in MSSM at loop level given that there is a source of LFV among sleptons:
From A. Dedes, Super B Factory proceedings: hep-ph/0503261
Effective SUSY
● Predicts LFV effects in a variety of τ and B decay modes (with preference for 3rd generation couplings):