Feasibility Studies in b ! d  Decays at Belle Clement Ng, EPP group

1. Feasibility Studies in b! dDecays at BelleClement Ng, EPP group Vtb Vtd W

2. Overview of the Experiment • Studied a process that is sensitive to the presence of new physics involving the b ! dprocess • Tuned analysis on simulated data sets to model signal and background event distributions • Optimised background cuts and studied effect on the fraction of b ! d  events which pass

3. Motivations:The need for New Physics • Our Standard Model as has been able to make precise predictions for interactions up to the electroweak energy scale ~100 GeV • It is an effective theory; a Hierarchy Problem exists from the “energy desert” between electroweak and Planck scales ~1018 GeV • Current experiments aim to test proposed new models – new physics expected at the 1 TeV scale • Direct searches for the Higgs mass and new particles, cosmological observations • Also, amplitudes of 2nd orderelectroweak decay processes affected by proposed new models • b! s studied extensively b! d radiative decays recently measured at Belle; no indication of new physics at the current errors Br(B!(,)) = 1.32 § 0.44£10-6 Searches for New Physics

4. Extinction? Radiative Penguin Decays What makes b! d decays of interest? • Flavour-changing neutral currents • Forbidden in first-order (tree level) • Can proceed at loop level • highly-suppressed, dependent on the CKM element Vtd ; occurs 1 in 1 million decays • Because SM effects minimal, channel is sensitive to contributions from new physics • Might be seen in: Branching ratio, CP violation, isospin violation Vtb Vtd W

5. 1 ¹ ( ) d d + ¹ u u p 2 b! din Meson Decays How can this process be observed? • Bmesons • Looked at neutral B’s; comprised of db • b!d found in B0!0B0!w • 0andmesons • Both comprise of same quark content, different bound state: g B0 0 g B0 

6. The Belle Experiment at the KEK B Factory • e+e- collider – world’s highest luminosity • ¼ of collisions form BB meson pairs, other times qq pairs (continuum events) • Produced over 600 million BB events

7. Measurement Procedure • Monte Carlo simulated data generation • We used 200,000 for each signal mode, 140,000,000 continuum • Event reconstruction • Involves particle identification data taken from the detector components, searches for invariant mass of candidate particles • Background suppression • Add extra “cuts” to filter out random combinations which pass reconstruction • Signal fitting • Based on signal models for each decay and background, try and fit them to your data

8. Event Reconstruction g B0 p+ • Final state particles • B!!+- B!w!+-p0g Pions selected from: PID likelihood, mass, charge, track topology • Expect high energy photon • Intermediate state particles • , formed from pions • Invariant mass, helicity requirements 0 p- g p+ B0 p0  p- Events GeV Events GeV

9. ( j j ) ¤ E E ¡ p = b ° ½ ! e a m ; p ( = ) j = j 2 2 2 ¤ ¤ M ¢ E E E E ¤ ¤ ¡ ¡ c p c = = b B b b B c e a m e a m Event Reconstruction • B reconstruction • Invariant mass with added information from the beam collision • Modified Beam Constrained Mass • Energy shift After this stage we have ~20% of initial signal events remaining

10. Background Suppression 2 types of backgrounds: • B backgrounds • Uses same techniques as reconstruction, tuned towards vetoing troublemaking decays, namely combinations of mass, helicity • B!K*0!K+K-g – dominant background (b!s process) • occurs 100 times more often than , due to ratio |Vtd /Vts|2 • End up with about 2 to 1 (for B!g) • Was not investigated in depth • Continuum backgrounds • qq pairs created from off-resonance e+e- beam collisions • Ratio of qq events to B events is 3 to 1 • qq events outweigh g, g more than a million to one • Even after event reconstruction, still seeing 300 to 1

11. Continuum Suppression: Event Topology • Super Fox-Wolfram moments • Describes spherical or jet-like events from momenta and angular distribution • Reduced into a Fisher discriminant value • Based on finding optimal plane of discrimination Resonant e+e- !Y(4S)! BB events Non-resonant e+e-!qq (q = udsc) Continuum events

12. L µ F i L L R L L c o s s g B £ = = L L + i ¹ s g q q Continuum Suppression: Event Topology • B flight directionB • Angle between B and decay particle direction in CM frame • True B events (blue) follow cos2(B) • Combine Fisher discriminant with cos2(B) to form likelihood ratio

13. Continuum Suppression: Flavour Tagging V g l+ B0 B0 p+ • Insert more information about the event, by looking at the otherB meson from the BB decay – called the tag-sideB • Certain B decays are able to be well tagged – flavour can be inferred from decay product charge, momentum, polar angle etc • Quality of the B0 tag described by |q¢r| (q is the flavour of the B, r is the confidence factor) • Partially correlated with Super Fox-Wolfram moments; must investigate a two-dimensional space of the two variables – divide into 6 |q¢r| bins 0 p- n tag-side signal-side

14. N S p N N + S B N S p N N + S B Continuum Suppression: Selection Optimisation Divide into 6 |q¢ r| bins, find the LR cut which optimises for each one Finally have 140,000,000 continuum events suppressed to41 events Events Continuum Min. LR requirement Signal

15. Results: Signal Efficiency • Mbc and E fits

16. Results: Signal Efficiency g B0 p+ • 600 fb-1 (current Belle integrated luminosity) • Signficance for projected luminosities 0 p-

17. Vtb Vtd W Conclusion • Supports the previous Belle b! d analysis, continued optimisation feasible • Continuum suppression the key to measurements of rare penguin decays • Flavour tagging variables effective in adding new information about events • MC optimisation highly dependent on available statistics • Still additional tagging variables to be modelled in multidimensional likelihood analyses – currently in research