Feasibility Studies in Feasibility Studies in bb!! d d Decays at Decays at BelleBelle

Clement Ng, EPP groupClement Ng, EPP group

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Overview of the ExperimentOverview of the Experiment

Studied a process that is sensitive to the Studied a process that is sensitive to the presence of new physics involving the presence of new physics involving the b b !! d d processprocessTuned analysis on Tuned analysis on simulated data setssimulated data sets to to model signal and background event model signal and background event distributionsdistributionsOptimised Optimised background cutsbackground cuts and studied and studied effect on the effect on the fraction of fraction of b b !! d d events events which passwhich pass

Motivations:Motivations:The The nneed for eed for New PhysicsNew Physics

Our Our Standard ModelStandard Model as has been able to as has been able to make precise predictions for interactions up make precise predictions for interactions up to the to the electroweakelectroweak energy scale ~100 GeV energy scale ~100 GeVIt is an effective theory; a It is an effective theory; a Hierarchy ProblemHierarchy Problem exists from the “energy desert” between exists from the “energy desert” between electroweak and electroweak and PlanckPlanck scales ~10 scales ~101818 GeV GeVCurrent experiments aim to test proposed Current experiments aim to test proposed new models – new physics expected at the 1 new models – new physics expected at the 1 TeV scaleTeV scale

Searches Searches for New for New PhysicsPhysics

Direct searches for the Higgs mass and Direct searches for the Higgs mass and new particles, cosmological observationsnew particles, cosmological observations

Also, amplitudes of Also, amplitudes of 22ndnd order order electroweak electroweak decay processes decay processes affected by proposed affected by proposed new new modelsmodels

bb!! s s studied extensively studied extensively

bb!! d d radiative decays recently measured radiative decays recently measured at at BelleBelle; no indication of new physics at the ; no indication of new physics at the current errors current errors BrBr((BB!!((,,))) = 1.32 ) = 1.32 §§ 0.44 0.44££1010-6-6

Radiative Penguin DecaysRadiative Penguin DecaysWhat makes What makes bb!! d d decays of interest? decays of interest?

Flavour-changing neutral currentsFlavour-changing neutral currents Forbidden in first-order (tree level)Forbidden in first-order (tree level)

Can proceed at Can proceed at looploop level level highly-suppressedhighly-suppressed, dependent on the CKM element , dependent on the CKM element VVtd td ;;

occurs 1 in 1 million decaysoccurs 1 in 1 million decays Because SM effects minimal, channel is sensitive to contributions from new physicsBecause SM effects minimal, channel is sensitive to contributions from new physics

Might be seen in: Branching ratio,Might be seen in: Branching ratio,CPCP violation, isospin violation violation, isospin violation

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Extinction?

bb!! d d in Meson Decaysin Meson Decays

How can this process be observed?How can this process be observed?BB mesonsmesons

Looked atLooked at neutral neutral BB’s; ’s; comprised of comprised of ddbb

bb!!dd found in found inBB00!!BB00!!

andand mesonsmesons Both comprise of same quark Both comprise of same quark

content, different bound state:content, different bound state:

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0

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1p2(u¹u+d¹d)

The Belle Experiment at the KEKThe Belle Experiment at the KEK B B FactoryFactory

ee++ee-- collider – world’s highest luminosity collider – world’s highest luminosity¼ of collisions form ¼ of collisions form BBBB meson pairs, other times meson pairs, other times qqqq pairs ( pairs (continuumcontinuum events)events)Produced over 600 million Produced over 600 million BBBB events events

Measurement ProceMeasurement Proceduredure

Monte Carlo simulated data generation Monte Carlo simulated data generation We used 200,000 for each signal mode, 140,000,000 continuumWe used 200,000 for each signal mode, 140,000,000 continuum

Event reconstructionEvent reconstruction Involves particle identification data taken from the detector Involves particle identification data taken from the detector

components, searches for invariant mass of candidate particlescomponents, searches for invariant mass of candidate particles

Background suppressionBackground suppression Add extra “cuts” to filter out random combinations which pass Add extra “cuts” to filter out random combinations which pass

reconstructionreconstruction

Signal fittingSignal fitting Based on signal models for each decay and background, try and Based on signal models for each decay and background, try and

fit them to your datafit them to your data

Event ReconstructionEvent ReconstructionFinal state particlesFinal state particles

BB!! !! ++--BB!! !! ++--00PionsPions selected from: selected from:

PID likelihood, mass, PID likelihood, mass, charge, track topologycharge, track topology

Expect high energy Expect high energy photonphoton

Intermediate state particlesIntermediate state particles ,, formed from pions formed from pions Invariant mass, helicity Invariant mass, helicity

requirementsrequirements

GeV

GeV

Events

Events

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Event ReconstructionEvent ReconstructionBB reconstruction reconstruction

Invariant mass with added information from the beam collisionInvariant mass with added information from the beam collision Modified Modified Beam Constrained MassBeam Constrained Mass

Energy shiftEnergy shift

AfterAfter this stage we have this stage we have ~~20% of initial signal events remaining20% of initial signal events remaining

Mbc =p(E ¤

beam=c2)2 ¡ jp¤B =cj2

¢E = E ¤B ¡ E ¤

beam

(jp° j = E ¤beam ¡ E½;! )

Background SuppressionBackground Suppression

2 types of backgrounds:2 types of backgrounds:BB backgrounds backgrounds

Uses same techniques as reconstruction, tuned towards vetoing Uses same techniques as reconstruction, tuned towards vetoing troublemaking decays, namely combinations of mass, helicitytroublemaking decays, namely combinations of mass, helicity

BB!! !! ++--– dominant background – dominant background ((bb!!ss process) process)

occurs 100 times more often than occurs 100 times more often than ,,due to ratio |due to ratio |VVtd td //VVtsts||22

End up with about End up with about 2 to 12 to 1 (for (for BB!!)) Was not investigated in depthWas not investigated in depth

ContinuumContinuum backgrounds backgrounds qqqq pairs created from off-resonance e pairs created from off-resonance e++ee-- beam collisions beam collisions Ratio of Ratio of qqqq events to events to BB events is 3 to 1 events is 3 to 1 qq qq events outweigh events outweigh , , more than a million to one more than a million to one Even after event reconstruction, still seeing Even after event reconstruction, still seeing 300 to 1300 to 1

Continuum Suppression: Event TopologyContinuum Suppression: Event Topology

Super Fox-WolframSuper Fox-Wolfram moments moments Describes spherical or jet-like Describes spherical or jet-like

events from momenta and events from momenta and angular distributionangular distribution

Reduced into a Reduced into a Fisher Fisher discriminant discriminant valuevalue

Based on finding optimal Based on finding optimal plane of discriminationplane of discrimination

Resonant e+e- ! Y(4S) ! BB events

Non-resonant e+e- ! qq (q = udsc) Continuum events

BB flight direction flight direction BB Angle between Angle between B B and decay particle and decay particle

direction in CM framedirection in CM frame True True BB events (blue) follow cos events (blue) follow cos22((BB) )

Combine Fisher discriminant with Combine Fisher discriminant with coscos22((BB) to form ) to form likelihood ratiolikelihood ratio

Continuum Suppression: Event TopologyContinuum Suppression: Event Topology

L = L F £ L cosµB

LR = L sigL sig+L q ¹q

Continuum Suppression: Continuum Suppression: Flavour TaggingFlavour Tagging

Insert more information about the event, by Insert more information about the event, by looking at the looking at the otherother BB meson meson fromfrom the the BB BB decay – called the decay – called the tag-sidetag-side BB Certain Certain BB decays are able to be decays are able to be well taggedwell tagged – flavour can be inferred from decay product – flavour can be inferred from decay product charge, momentum, polar angle etccharge, momentum, polar angle etcQuality of the Quality of the BB00 tag described by tag described by ||qq¢¢rr||((qq is the flavour of the is the flavour of the B B, , rr is the confidence is the confidence factor)factor)Partially correlated with Super Fox-Wolfram Partially correlated with Super Fox-Wolfram moments; must investigate a two-moments; must investigate a two-dimensional space of the two variables – dimensional space of the two variables – divide into divide into 6 |6 |qq¢¢rr| bins| bins

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tag-side signal-side

Continuum Suppression: Continuum Suppression: Selection Selection OptimisationOptimisation

Divide intoDivide into 6 | 6 |qq¢¢ r r| bins, find the | bins, find the LRLR cut cut which optimises which optimises for each one for each one

Finally have Finally have 140,000,000 140,000,000 continuum events continuum events suppressed tosuppressed to 4141 events events

Events

Min. LR requirement

Continuum

Signal

N SpN S +NB

N SpN S +N B

Results: Signal EfficiencyResults: Signal Efficiency

MMbcbc and and EE fits fits

Results: Signal EfficiencyResults: Signal Efficiency

600 fb600 fb-1-1 (current Belle integrated luminosity) (current Belle integrated luminosity)

Signficance for projected luminositiesSignficance for projected luminosities

MC valueMC value MC valueMC value KK*0*0 MC valueMC value

EventsEvents 30.130.1§§1717 2121 9.39.3§§1919 1212 1234.21234.2§§2020 12571257

EfficiencyEfficiency 0.0740.074

§§0.040.04

0.510.51 0.0230.023

§§0.050.05

0.380.38 0.0640.064

§§0.0010.001

0.6560.656

Luminosity (fbLuminosity (fb-1-1)) SignificanceSignificance

600600 30.1330.13§§17.117.1 11

10001000 43.8543.85§§21.821.8 22

22002200 100.13100.13§§33.133.1 33

33003300 117.7117.7§§35.535.5 33

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WConclusionConclusion

Supports the previous Belle Supports the previous Belle bb!! d d analysis, continued analysis, continued optimisation feasibleoptimisation feasible

Continuum suppression the key to measurements of rare Continuum suppression the key to measurements of rare penguin decayspenguin decays

Flavour tagging variables effective in adding new Flavour tagging variables effective in adding new information about eventsinformation about events

MC optimisation highly dependent on available statisticsMC optimisation highly dependent on available statistics Still additional tagging variables to be modelled in Still additional tagging variables to be modelled in

multidimensional likelihood analyses – currently in researchmultidimensional likelihood analyses – currently in research