CP violation measurements: status and prospects

Transcription

1 CP violation measurements: status and prospects Historical context CPV in the b-system CPV in the c-system The further future Conclusions Guy Wilkinson University of Oxford and CERN Guy Wilkinson 1

2 Guy Wilkinson 2

3 50 years ago events of differing significance Martin Luther King Jnr. wins Nobel Peace Prize Cassius Clay becomes heavyweight champion of the world Nelson Mandela sentenced to life imprisonment Guy Wilkinson 3

4 50 years ago events of differing significance Beatles take America by storm release of Dr Strangelove and of Mary Poppins Michelle Obama born as was Sarah Palin Guy Wilkinson 4

5 50 years ago events of differing significance Beatles take America by storm release of Dr Strangelove and of Mary Poppins Michelle Obama born as was Sarah Palin Guy Wilkinson 5

6 50 years ago events of differing significance Beatles take America by storm release of Dr Strangelove and of Mary Poppins Michelle Obama born as was Sarah Palin Guy Wilkinson 6

7 CP violation studies 50 years on Kaons still play a vital role in flavour physics studies, but in CP violation measurements the current focus of attention is in b- and c-hadrons. Executive summary of what is to follow: No new smoking guns those areas where we had discrepancies seem now to have fallen into line, or are heading in that direction. In some cases this is disappointing the nature of the observable and the precision of the measurement meant we were hoping for signs of tension (e.g. φ s ). This is of course the flavour problem reiterated. Generally, its no big deal as we re only getting going with many measurements (e.g. γ, charm indirect CPV). In all cases the prospects of experimental improvement are excellent. Very few measurements are systematically limited and we expect an order of magnitude improvement in precision over coming ~ decade. Guy Wilkinson 7

8 CPV in the beauty sector Setting the scene LHCb a reminder Selected topics: CKM metrology towards a precise measurement of γ Indirect CPV in the B s system Puzzles in charmless B decays Guy Wilkinson 8

9 B physics 25+ years of steady progress, e.g. time-dependent CPV Observation of mixing in (B 0 +B s ) and B 0 system UA1, PLB 186 (1987) 247 Argus, PLB 192 (1987) 245 Resolution of B 0 oscillations ALEPH, PLB 313 (1993) 498 Resolution of B s oscillations CDF, PRL 97 (2006) BELLE, PRL 87 (2001) Many facilities, many experiments... Observation of CPV in interference between B 0 mixing and decay BaBar, PRL 87 (2001)

10 The B-factory legacy validating the CKM mechanism BaBar & Belle made a host of very significant measurements, & flavour physics in the last decade has also received critical inputs from the Tevatron, but most important achievement remains the measurement of sin2β, primarily though B 0 J/ψK S decays This measurement validates the CKM mechanism as being the leading order agent of CP-violation in nature (and gave the cue for K & M s 2008 Nobel prize) Last decade we had 3 B-physics colliders in operation, now we have 1 - the LHC Guy Wilkinson 10

11 LHCb the essentials LHCb a forward spectrometer optimised for heavy-flavour physics at the LHC forward acceptance (2 < η < 5) high bandwidth trigger acceptance down to low p t precise vertexing (VELO) hadron identification (RICHes) LHCb operation proceeds in harmony with higher luminosity operation of ATLAS/CMS thanks to luminosity leveling. 37 pb -1 collected in fb -1 in 2011 and 2 fb -1 in 2012 aim for ~8 fb -1 before shutdown ATLAS/CMS lumi falls exponentially LHCb lumi continually leveled >225 papers & counting on CP violation, rare decays (see Diego s talk), spectroscopy, & other physics (EW, QCD, exotics ) Guy Wilkinson 11

12 Precision CKM-metrology: the next challenge B-factories (& others) have done a great job in mapping out unitarity triangle. But further progress needs, in particular, improved knowledge of angle γ Look in B ± DK ± decays using common mode for D 0 & D 0 γ sensitive interference different rates for B + & B - (CPV!) Many possibilities: Kπ, KK, Kπππ... Favoured Suppressed Tree-level decays: strategy very clean & yields result unpolluted by New Physics This is a good thing! Provides SM benchmark against which other loop-driven NP sensitive observables can be compared (e.g. Δm d /Δm s, sin2β, γ measured in B hh) Guy Wilkinson 12

13 γ measurement the last ~10 years The story so far... Derived from combination of observables in many B DK decay channels Consistent with the indirect prediction......but not nearly as precise...factor 3 improvement in 10 years. Guy Wilkinson 13

14 γ measurement: true precision needs statistical muscle of LHCb Rare, important decays just beyond the reach of the B-factories (e.g. the suppressed ADS B ± - (K + π ± ) D K ± mode (BR ~ 10-7 ) was soon seen at LHCb 1 fb -1 [PLB 712 (2012) 203] This CP asymmetry carries ultra-clean, easy to interpret, information on γ! Guy Wilkinson 14

15 [JHEP 10 (2014) 097] B [K S hh] D K at LHCb B DK method has been applied by LHCb to D K S ππ and K S KK. CPV leads to difference in D Dalitz plots for B + and B - decays Cleaner than B-factories! B + B - Data analysed in bins which have similar D decay strong-phase. To retain model independence these phases are taken from measurements of quantumcorrelated DDbar pairs at CLEO-c [PRD 82 (2010) ] - will be improved by BES-III. Cleanliness of measurement preserved exploiting synergy of facilities! Guy Wilkinson 15

16 LHCb: current precision on γ and future prospects Combination of LHCb B DK results obtained so far Uncertainty of <10 o - better than that obtained with combined B-factory samples Will improve steadily: LHCb-CONF still many important Run 1 modes to be published (e.g. 3 fb -1 B D(KK,ππ,Kπ,Kπππ,Kππ 0,πππ 0, KKπ 0,ππππ)K, B s D s K still to come) Repeat with much larger data set anticipated in Run 2. Aim for ~ 3 o uncertainty after first stage LHCb (matches current indirect precision) Guy Wilkinson 16

17 Mixing induced CPV in B s system CPV phase, φ s, in B s mixing-decay interference, e.g. measured in B s J/ΨФ, very small & precisely predicted in SM. Box diagram offers tempting entry point for NP! Tevatron results were tantalising with early data and remain intriguing with final sample: [PRD 85 (2012) ] [PRD 85 (2012) ] Results are consistent, & both are ~1σ away from SM. What about the LHC? Guy Wilkinson 17

18 Precision studies of Φ s The LHC (firstly LHCb & now ATLAS/CMS) has brought clarity to the φ s picture. LHCb attributes/contributions: Statistics and time resolution! B s J/Ψφ analysis with ~20x precision of Tevatron [arxiv: ] B s oscillation seen in D s π New J. Phys. 15 (2013) Augment this with novel analysis in complementary channel B s J/Ψππ [PLB 736 (2014) 186] Finally, perform study looking at strong-phase change w.r.t. KK invariant mass in J/ΨKK which resolves 2-fold ambiguity [PRL 108 (2012) ] Final LHCb run-1 results [arxiv: ] now out, together with ATLAS [PRD 90 (2014) ] and CMS [CMS-PAS-BPH ] studies - overall picture now clear. 18

19 Precision studies of Φ s Earlier hints of large NP effects emphatically not confirmed... zoom...but observable remains a priori very sensitive to non-sm contributions and essential to improve precision over coming decade. Guy Wilkinson 19

20 The elephant in the room: a sl Flavour-specific CP asymmetry in B decays, most easily measured in semileptonics (hence a sl ) accesses CP-violation in mixing. Extremely small in SM, especially in B s system. [D0, PRD 89 (2014) ] D0 measurement, made with dileptons, measures a superposition of a s sl and a d sl Result lies ~3 σ from SM (exact degree of tension depends on how comparison is made). Most usually interpreted as a B s driven effect. Challenging, however, to reconcile with other measurements, e.g. B s J/Ψφ, J/Ψππ Guy Wilkinson 20

21 Attempting to resolve the D0 dimuon anomaly Systematics associated with being a pp collider makes it very difficult to repeat D0 measurement at LHC. However it is possible to measure a sl s and a sld. LHCb has performed 1 fb -1 a sl s [PLB (728) (2014) 607] and 3 fb -1 a sl s [arxiv: ] studies. with LHCb input These recent measurements agree with SM, but do not exclude dimuon result. New, much more precise a sl s result on way. Improved ΔΓ d /Γ d would also be welcome. Guy Wilkinson 21

22 Attempting to resolve the D0 dimuon anomaly Systematics associated with being a pp collider makes it very difficult to repeat D0 measurement at LHC. However it is possible to measure a sl s and a sld. LHCb has performed 1 fb -1 a sl s [PLB (728) (2014) 607] and 3 fb -1 a sl s [arxiv: ] studies. and now there is also a very recent new a sl d from BaBar [arxiv: ] These recent measurements agree with SM, but do not exclude dimuon result. New, much more precise a sl s result on way. Improved ΔΓ d /Γ d would also be welcome. Guy Wilkinson 22

23 Charmless B decays Studies of B decays not involving charm are of great interest, as the CPV is usually generated through interference with a Penguin loop-diagram what enters the loop? Many studies have been performed, and some have thrown up surprises. e.g. naively it is expected that the CPV in B 0 K + π - and B - K - π 0 should be the same, but this is emphatically not so [from BaBar, Belle, CDF & LHCb] K - π + K + π - K - π 0 K + π 0 Belle, Nature 452 (2008) 332 This puzzle has been around for several years, and should be taken seriously. Improved measurements are needed in B - K - π 0 and K 0 π - and B 0 K 0 π 0 Guy Wilkinson 23

24 Charmless B decays Other surprises are emerging from the large samples now available at the LHC - not all necessarily with New Physics consequences, but still of great interest. e.g. large CPV seen in low mass non-resonance region of 3-body decays Big asymmetry In Kππ Same story in πππ seemingly mirrored by one of opposite sign in KKK when compared to πkk LHCb, arxiv: Perhaps due to long-distance ππ KK rescattering? Guy Wilkinson 24

25 CPV in the charm sector Setting the scene Selected topics: Direct CPV Indirect CPV Guy Wilkinson 25

26 The charm renaissance For many years charm was the Cinderella of flavour physics studies tiny CPV and mixing effects expected in the SM and no evidence of either despite intensive searches long-distance effects complicate predictions Excluded regions 2006 No mixing 2007 No mixing 2014 All results are null. Ian Shipsey, Charm Measurement contours; no-mixing excluded at 5σ No-mixing excluded at lots and lots Then combination of B-factory analyses finally saw mixing. New outlook! mixing parameters not tiny (~1%); good news for (indirect) CPV observables smallness of SM pollution not a bad thing in looking for New Physics signal internal down-type quarks in loops complementary to b-physics huge potential of LHC for improving sensitivity 26

27 LHCb, PRL 110 (2013) Rise of the hadron machines Power of hadron colliders is now clear. Last year LHCb and CDF published first individual (>)>5σ measurements, in WS Kπ analyses. 36k WS signal 1 fb σ away from no-mixing hyp. 33k WS signal 9.6 fb -1 CDF, PRL 111 (2013) σ away from no-mixing hyp. Although e + e - machines retain advantages for many modes with neutrals, LHC has huge advantages for charged modes (e.g. # WS Kπ in Run 1 at LHCb = 230 x 10 3 ; at Belle in 0.9 ab -1 = 12 x 10 3 ) and also time resolution. 27

28 ΔA CP : first sighting of direct CPV in charm? The observable ΔA CP A CP (KK) - A CP (ππ) is robust against detector systematics & production asymmetries, and is sensitive to any direct CPV in SCS charm decays Majority opinion in literature before 2012: direct CPV at or above a few per-mille in SCS decays is very unlikely in SM Hence LHCb result [PRL 108 (2012) ] with 0.6 fb -1 of D* decays of great interest: KK ππ Soon after, measurements from CDF 9.7 fb -1 [PRL 109 (2012) ] and Belle 976 fb -1 [arxiv: ] increased the excitement: These (together with other measurements) only consistent with no-cpv at ~2%. A story to get the theorists interested? Yes! And surely very unlikely to be SM??? CDF Belle Guy Wilkinson 28

29 ΔA CP : informing the theory community LHCb, CDF and Belle collaborations Flavour physics theorist Please, sir. We ve done what you told us. We ve brought you the broomstick of the Wicked Witch of the West. We melted her. Oh... You liquidated her, eh? Very resourceful! Yes, sir. So we d like you to keep your promise to us, if you please sir. Not so fast! I ll have to give the matter a little thought. Go away and come back tomorrow. CP conservation in charm Experimentalist reaction Guy Wilkinson 29

30 No CPV ΔA CP : too early to celebrate So it seemed a consistent picture had emerged of ~0.5% direct CPV in SCS charm decays (thanks to LHCb, CDF, Belle) This caused the theory community to re-evaluate their position... but later LHCb results (1 fb -1 D* update [LHCb-CONF ] and 3 fb -1 B D 0 μx [JHEP 07 (2014) 041]]) indicate reduced / ~null effect Next steps required for progress: Final LHCb D* results from 2012 data set (& beyond) More precise results in other SCS modes 30

31 Recent progress in SCS direct CPV searches Other two-body (& pseudo-body) SCS searches now have 0.1% precision, e.g. D + Φπ + LHCb [JHEP 06 (2013) 112] Belle [PRL 108 (2012) ] BaBar [PRD 87 (2012) ] Moreover a developing area, of great promise, is to look for regions of local CPV in multi-body modes, where interference between neighbouring resonances may allow effects to be seen. e.g. LHCb analysis of 0.7 M D 0 π + π - π 0 decays with model independent energy test [arxiv: ] Prospects of direct CPV revealing clear sign of NP appears to be receding. But still of great interest to find a non-0 signal and LHCb statistics hold promise. Guy Wilkinson 31

32 CPV searches in mixing More important is to search for CPV in mixing related phenomena. Observables are pre-multiplied by x,y, so large (~1%) value of mixing is encouraging in this quest. Already plenty of progress in last few years SM SM any non-zero signal with current and near-future precision would indicate NP. Guy Wilkinson 32

33 Searching for indirect CPV Hunt for indirect CPV can be performed with: Dedicated observables, such as A Γ A ( D ( D 0 0 KK) ( D KK) ( D 0 0 KK) KK) (similarly for any other CP eigenstate) A 1 2 q / p p / q y cos q / p p / q xsin (neglecting direct CPV) Current precision 0.05% and is LHCb driven. Significant improvements already expected with full Run 1 data set. Generalising WS Kπ fit to D 0 and D 0 bar e.g. LHCb PRL 111 (2013) Time-dependent Dalitz studies of multi-body decays, e.g. K S ππ Very promising for Belle-II as well as LHCb upgrade We re at the start of a long journey let s travel hopefully! e.g. BaBar PRL 105 (2010)

34 The further future Guy Wilkinson 34

35 Dreaming about ultra-high statistics Big improvements foreseen before long shutdown (e.g. ~8 fb -1 at LHCb, ~ doubling in x-sec from E CM w.r.t. 2012, improved analysis methods) but we can dream of what could be achieved with a very large increase in sample sizes e.g. CKM metrology Determine γ with sub-degree precision to match anticipated improvements in indirect precision coming from lattice QCD. Improve β down to ~0.02 o. CPV in B s mixing Measurement of φ s with precision much better than SM central value, to probe for sub-leading contributions from NP. B 0 (s) μμ True precision measurement of BR down to theory uncertainty and first measurement of ultra-suppressed B 0 μμ BR. B 0 K*μμ Precision studies of all observables of interest through full angular analysis Charm Extensive study of direct CPV across wide range of modes. Sensitivity to indirect CPV down to SM expectation. Plus great improvements in precision, & new measurements, in many other topics! 35

36 Unwise to assume ~10% (or even 0.1%) is good enough Courtesy Browder and Soni Many of the arguments for increasing the statistical precision are motivated by clear numerical facts, e.g. matching theoretical precision or opening up new decay modes. But history tells us that whatever the argument, improved precision is always welcome. We should exploit existing facilities to the utmost. BR (K 0 L ππ) ~ 2 x 10-3 Cronin, Fitch et al., 1964 Guy Wilkinson 36

37 Realising the dream the LHCb upgrade LHCb collaboration plans an upgrade, to be installed in shutdown Essential features: Full software trigger: will readout into DAQ all subdetectors at 40 MHz (c.f. 1 MHz at present). This will improve efficiency compared with current hardware trigger, giving factor of two improvement for hadronic final states Increase operational luminosity to 2 x10 33 cm -2 s -1 Annual yields in muonic final states will increase 10x w.r.t. most published analyses, and 20x for hadronic decays. Aim to collect ~50 fb -1 in total. All aspects of Upgrade have been approved by CERN research board. Final R&D on sub-detector technologies underway, with construction to begin very soon. 37

38 The e + e - frontier: Belle-II Looking forward, the LHC is not the only show in town. The e + e - programme will resume with SuperKEKb (accelerator) & Belle-II (detector) aiming for x50 Belle yield. Capabilities highly complementary to LHCb, with particular strengths in: final states with soft neutrals and neutrinos inclusive studies tau physics Guy Wilkinson 38

39 Conclusions Impressive level of precision achieved by B-factories in sin2β is now being matched in other CPV observables at the LHC. So far all measurements are compatible with the CKM paradigm alone. In the charm sector the initial indications of direct CPV in SCS decays have receded. But the precision is now becoming truly impressive, and prospects for improvement are excellent, for this and for indirect CPV. Many results to come, and full impact of LHC Run 1 data is still to be seen (in particular watch the γ measurement and indirect CPV in charm). Big improvements will come in Run 2. And much more significant leaps in sensitivity are expected with the LHCb Upgrade. Belle II will provide complementary measurements on the same timescale. Guy Wilkinson 39

40 Backups Guy Wilkinson 40