Studying CP violation in Charm Decays

1. Based on AB, Inguglia, Meadows: PRD and related work in progress with same authors Studying CP violation in Charm Decays Adrian Bevan Lancaster, 22nd February 2013 email: a.j.bevan@qmul.ac.uk

2. Overview • CP violation in the SM • Why study charm? • Methodology • Measurements to make • Something completely different • Summary Adrian Bevan: QMUL

3. CP Violation in the SM K and B decay results Adrian Bevan: QMUL

4. A brief history • 1964 • 1999 • 2001 • 2004 Christensen, Cronin, Fitch, and Turlay discover CP violation in K0 decay. NA48 and KTeV establish direct CP violation in K0 decay. BaBar establishes evidence for CP violation in B decay. BaBar establishes direct CP violation in B decay. Adrian Bevan: QMUL

5. 1980 The (other) 1964 revolution • CP violation was observed via the decay: • This wasn't supposed to happen J.H. Christenson et al., PRL 13 p138 (1964) Cronin & Fitch Adrian Bevan: QMUL

6. (1964) • The 1964 discovery of CP violation was a landmark. • A single experimental result required the invention of 3 additional quarks, and corresponding additional leptons. • This was for the sole purpose of explaining a tiny effect in a rather esoteric physical system. • CP violation led theorists to postulate half of the fundamental quarks and leptons of our Standard Model. • Why is there CP violation? • How does this relate to the Big Bang? • Given that quarks mix and exhibit CP violation, and neutrinos mix (so it is safe to assume that its worth searching for CP violation), what about charged leptons? Are they coming to the party as well? Adrian Bevan: QMUL

7. a few decades later... (1999) • ε'/ε had to be measured: but why? • Any non-zero value would mean CP violation in direct decay (ΔF=1). • Otherwise there would be possible super-weak transitions. • The theoretical picture was (and still is) hard to interpret – experiment on the other hand tells us all we needed to know! The NA48 result: [Phys. Lett B 465 (1999) 335-348]. KTeV was able to produce a similar result. Firmly established direct CP violation, and confirmed the NA31 evidence for direct CPV. Adrian Bevan: QMUL

8. (2001) • The B Factory revolution of our understanding of CP violation is quite profound. • Kaons dictated what we had known prior to these experiments. • The conjecture put forward to explain CP violation needed to be tested (i.e. the KM mechanism and CKM matrix). • We learned that • CP violation in B decays is large. • The KM mechanism, so the CKM matrix works (at the 10% level) • There are some tension between the value of S=sin(2β) and the SM. • Still room for higher order NP contributions. • Doesn't explain the Universal matter-antimatter asymmetry. ✓ ✓ ✓ ✓ ✗ Adrian Bevan: QMUL

9. The angle revolution • B Factories now dominate our knowledge of CP violation. B Factory Averages (See the Physics of the B Factories, to be submitted to EPJC this year for details) LHCb have measurement that make some contributions to β&γ, but B Factories dominate. Adrian Bevan: QMUL

10. (2004) • This measurement is equivalent to ε' for the kaon system. • Binary test of an effect, not useful to interpret in terms of the SM. Direct CP violation seen as the asymmetry between B+ and B− decays to Kπ final states. Massive effect (compared with K decay); ~ -13%. Clear confirmation of direct CPV in a second system. As with ε' it is not easy to interpret the result in terms of SM parameters. Phys.Rev.Lett.93:131801,2004 Adrian Bevan: QMUL

11. The CKM matrix • CP violation is encoded in a single complex phase in VCKM: • Generally we look at this in terms of an expansion in the sine of the Cabibbo angle: λ: • This has been tested for = Adrian Bevan: QMUL

12. Why study charm? (i) "Because we can" (ii) because we've never studied an up-type quark like this before Adrian Bevan: QMUL

13. Observables to measure • As with B and K decay there are several types of observables: • CP violation in mixing • Direct CP violation (CPV in decay) • CP violation in the interference between mixing and decay Elements of the effective Hamiltonian are related to the dynamics of the underlying mixing, and symmetry conservation: we want CPT to be conserved in the SM. Adrian Bevan: QMUL

14. Observables to measure • As with B and K decay there are several types of observables: • CP violation in mixing • Direct CP violation (CPV in decay) • CP violation in the interference between mixing and decay q and p are integral to our understanding of CPV in mixing and CPV in the interference between mixing and decay. Adrian Bevan: QMUL

15. Observables to measure • As with B and K decay there are several types of observables: • CP violation in mixing • Direct CP violation (CPV in decay) • CP violation in the interference between mixing and decay Adrian Bevan: QMUL

16. Observables to measure • As with B and K decay there are several types of observables: • CP violation in mixing • Direct CP violation (CPV in decay) • CP violation in the interference between mixing and decay CPV in mixing involves measuring |q/p| in a number of different decay modes. The problem is that mixing is slow in charm... Adrian Bevan: QMUL

17. Observables to measure • As with B and K decay there are several types of observables: • CP violation in mixing • Direct CP violation (CPV in decay) • CP violation in the interference between mixing and decay CPV in decay relies on interference between two or more amplitudes with different weak and different strong phases. Hadronic uncertainties will limit interpretation in the context of the SM. This is essentially a binary test of CP violation. c.f. ε' and B decays. Adrian Bevan: QMUL

18. Observables to measure • As with B and K decay there are several types of observables: • CP violation in mixing • Direct CP violation (CPV in decay) • CP violation in the interference between mixing and decay Indirect CP violation: can be theoretically clean – but the level of contamination depends on mode. c.f. the measurement of unitarity triangle angles by BaBar and Belle. Adrian Bevan: QMUL

19. Observables • There is no point in measuring a theoretically unclean observable: • We know direct CPV exists and the super-weak theory is now dead (twice over). • There will be direct CP violation in charm – the question is at what level will it occur. • ΔA should be measured, but won't teach us anything significant about the SM. • So CPV in mixing and indirect CPV are left as the real goal if we want to test the SM in terms of Unitarity of the CKM matrix. This requires an experimental and theoretical game change, along the lines we proposed. Adrian Bevan: QMUL

20. Haven't we done this twice before? • Yes .... but • K and B mesons are made from down type quarks. • D mesons are made of up type ones. • Actually D0 is the only all up type meson we can study. • The universe is matter dominated and we still don't know why. • The SM is incomplete: our understanding of quark mixing and CPV is ad hoc. • Charm physics has a potential to reach higher energy than B (but not quite as high as K) • "Because it's there!" A quote from Captain Kirk Adrian Bevan: QMUL

21. Haven't we done this twice before? • Yes .... but • K and B mesons are made from down type quarks. • D mesons are made of up type ones. • Actually D0 is the only all up type meson we can study. • The universe is matter dominated and we still don't know why. • The SM is incomplete: our understanding of quark mixing and CPV is ad hoc. • Charm physics has a potential to reach higher energy than B (but not quite as high as K) • And most importantly "Because it's there!" Captain J. T. Kirk Adrian Bevan: QMUL

22. Methodology Existing studies are good enough for current data samples, but soon they will be obsolete. Adrian Bevan: QMUL

23. CKM • Unitarity only holds for a theory with three generations: e.g. • We still need to over-constrain CKM in as model independent a way to see if it really works ... ? The Unitarity Triangle (Bd) Triangle to add Adrian Bevan: QMUL

24. CKM • Unitarity only holds for a theory with three generations: e.g. • We still need to over-constrain CKM in as model independent a way to see if it really works ... ? The Bs triangle Triangle to add Adrian Bevan: QMUL

25. CKM • Unitarity only holds for a theory with three generations: e.g. • We still need to over-constrain CKM in as model independent a way to see if it really works ... ? The charm triangle Triangle to add Adrian Bevan: QMUL

26. Production environments • Quantum correlated pairs of D mesons • Require one D to decay to an or hadronic tag final state. • The other D can decay into a CP eigen-state to study double tagged events. • Correlated production at the ψ(3770) • Clean signature • Semi-leptonic tags have no dilution • Require a boosted CM system • Require a vertex tracker • Ideal use case for an asymmetric tau-charm factory Adrian Bevan: QMUL

27. Production environments • D* tagged events: • Require slow pion reconstruction to tag the flavour of the D at the point of production. • Reconstruct the CP decay at some later point in time. • Complementary systematic errors to the threshold case. • Un-correlated production at the Y(4S) or in pp collisions at the LHC. • Higher multiplicity than at charm threshold. • low tag efficiency. • Slow pion reconstruction to deal with. • cc-bar cross section comparable to bb-bar at this energy. • Plenty of data to offset issues with tag efficiency and lower reconstruction efficiency. Adrian Bevan: QMUL

28. Time-evolution (e.g. uncorrelated production) • Formalism required for BaBar/Belle/LHCb and Belle II • No Taylor expansion approximation as this introduces bias at large t • If you prefer x and y, it's straightforward to re-phrase the physics. Uncorrelated: +ve sign only Adrian Bevan: QMUL

29. Time-evolution (e.g. uncorrelated production) • Formalism required for BaBar/Belle/LHCb and Belle II • No Taylor expansion approximation as this introduces bias at large t • If you prefer x and y, it's straightforward to re-phrase the physics. Additional terms (Bs and D decays only) Familiar from Bd decays q/p is determined from mixing A/A depends on the decay mode Adrian Bevan: QMUL

30. Measurements to make A shopping list of things to measure: to test the SM Adrian Bevan: QMUL

31. Before showing you a long list of modes... • There are a few things we can learn from time-dependent analyses: • The phase of mixing (generally q/p is best studied in semi-leptonic decays, so lets igore that here...) • Weak phases from the triangles Mixing related parameters (supposed to be) common for all neutral D mesons. Decay related (mode dependent) Adrian Bevan: QMUL

32. Before showing you a long list of modes... • There are a few things we can learn from time-dependent analyses: • The phase of mixing (generally q/p is best studied in semi-leptonic decays, so lets igore that here...) • Weak phases from the triangles • The mode dependent part can be understood interms of the contributions (+ can merge factors with the same weak phase): Adrian Bevan: QMUL

33. Before showing you a long list of modes... Tree Color Suppressed Tree Penguin W-exchange Adrian Bevan: QMUL

34. Now for the list + more in the paper: see PRD 84 114009 (2011). Adrian Bevan: QMUL

35. Now for the list Measures the phase of mixing (~real CKM facors) + more in the paper: see PRD 84 114009 (2011). Adrian Bevan: QMUL

36. Now for the list Measures a combination of mixing phase and βc + more in the paper: see PRD 84 114009 (2011). Adrian Bevan: QMUL

37. Expected precision • The estimated precision on ΦMIX and βC are Adrian Bevan: QMUL

38. And now for something completely different Adrian Bevan: QMUL

39. T violation • BaBar recently observed T violation in the decay of EPR correlated T-conjugate final states, i.e. using: • We want to point out the potential for a second CKM revolution: • Over-constrain the CKM matrix in terms of T violation tests in all possible combinations of quark transitions. • A second B Factory revolution is on the way... |i> vs |f> Adrian Bevan: QMUL AB, Inguglia, Zoccali

40. T violation • What needs to be measured? • Time-dependent T asymmetries in: • [c-cbar-s has been done] • The observed level of CP violation can be directly used to predict the expected level of T violation. • Deviations from expectation could point to quantum gravity scale violations of CPT. • Loop dominated, tree dominated and W exchange topologies all give the potential for different types of NP to manifest themselves. • The data is in hand. • Pity that the UK expertise isn't funded to exploit this new idea ... Adrian Bevan: QMUL AB, Inguglia, Zoccali

41. T violation • What needs to be measured? • Time-dependent T asymmetries in: • The observed level of CP violation can be directly used to predict the expected level of T violation. • Deviations from expectation could point to quantum gravity scale violations of CPT. • Loop dominated, tree dominated and W exchange topologies all give the potential for different types of NP to manifest themselves. • The data is in hand. • Pity that the UK expertise isn't funded to exploit this new idea ... Adrian Bevan: QMUL AB, Inguglia, Zoccali

42. Summary Adrian Bevan: QMUL

43. Summary • CP violation is not understood. • But remains a vital ingredient to understand the nature of the Universe. • Measurements from B decays confirmed the KM conjecture as the leading order source of CP violation. • This is a billion times to small to explain the universal matter-antimatter asymmetry. • While it seems to work – there is no fundamental explanation of the origin of CP violation. • Why study CP violation in charm. • The only up-type quark system accessible for study. • Small effects expected – but these could have a reach to higher energy scales than B meson decays. • Existing facilities are not good enough to do precision tests: • LHCb, Belle II and future τ-C facilities can perform measurements. • Need e+e− experiments to control hadronic uncertainties required for interpretation. Adrian Bevan: QMUL

44. Summary • The next decade of time-dependent measurements from LHCb and Belle II will be interesting. • A number of processes will shed light on the nature of charm mixing. • A number of modes with weak phase sensitivity can also be measured. • Systematic study of these is imperative to test the SM. • Hopefully LHCb can start to study these states. • Belle II is required to do the full set of measurements. • Important to control hadronic uncertainties: here Belle II/BES III will provide important inputs. • To measure a non-zero asymmetry at the SM rate one would need to record ~1000 times the data of Belle II (and control systematic uncertainties). Adrian Bevan: QMUL

45. Summary • If the indication that is non-zero has two possible implications: • Driven by final state interactions. • Given that strong phase differences are generally small – that would imply a large (non-SM) weak phase difference in ππ. • Systematic study of time-dependent asymmetries in charm may yield an indication of physics beyond the SM. Adrian Bevan: QMUL

46. Summary • We also note that there is a programme of T and CPT tests that can be performed at charm threshold. • Require EPR correlated decays to ensure that one can exchange initial and final states when constructing an asymmetry. • Formally need to test the behaviour of discrete symmetries for different physical systems to verify if CKM holds. • Can only be done at e+e− experiments: BaBar and Belle (now) • In the future: Belle II (asymmetric tau-charm) for B (D) decays. • Not done before: BaBar started off a new sub-field of measurements 5 years after data taking stopped! • Let's do it! T Adrian Bevan: QMUL