Roadmap Chapters: Proper Time Resolution and B →p p

1. Roadmap Chapters: Proper Time Resolution and B→p p Laurence Carson γwith Loops Meeting 19.02.09

2. Overview • I will review studies described in two public LHCb notes. • Each is the basis for a chapter in the roadmap. • “Measurement of the Proper Time Resolution for B→h+h’- Channels from Data”, author L. Carson (LHCb-2008-060) • “Sensitivity of LHCb to Observe Bd→p p”, authors L. Carson and E. Rodrigues (LHCb-2009-006, in preparation)

3. Proper Time Studies: Status • Work on this was completed last year; public note LHCb-2008-060 released in November. • Roadmap chapter: First draft exists; revised draft will be circulated in the next draft of the roadmap.

4. Motivation for Proper Time Study • Time-dependent CP asymmetries in Bd→π+π- and Bs→K+K- together give sensitivity to γ. • Fast Bs oscillations mean that the reconstructed proper time (τrec) distribution for Bs→K+K- is dependent on detector proper time resolution. • So an incorrect proper time resolution model may bias the extraction of γ. • Hence a method is needed which extracts the resolution model for Bs→K+K- from data. • Method in LHCb-2007-059 uses Bs→K-π+ as a control channel. This works because Bs→K-π+ is flavour-specific and so cannot have time-dependent CPV.

5. Validation of Resolution Model • First step: produce a model that accurately describes the proper time residual (Δτ≡ τrec- τtrue) distributions for B→hh channels. • Model used: Secondary Gaussian (width Sfixed = 10ps, mean 0) which describes the wide tails in the distribution. Main Gaussian (width S, mean M) which describes the distribution for almost all of the events. • Parameters M and S are calculated event-by-event, using M ≡ GM. στrec and S ≡ GS.στrec (στrec is per-event error). Goal is to find the values of GS, GM and F2 that allow R(Δτ) to accurately describe the Δτ distribution in each B→hh channel.

6. Validation of Resolution Model • The four channels considered are Bd→π+π-, Bs→K+K-, Bs→K-π+and Bd→K+π-. • The four Δτdistributions were all found to be well described by R(Δτ), and the values of GS, GM and F2 are compatible across the distributions.

7. Model Extraction on Data • Toy study (using flavour-tagged τrecdistributions created in RooFit) was done to estimate the sensitivity to GM, GS and F2 from data. • Distribution for Bs→K-π+ made using R(Δτ) as resolution model. Other parameters such as ω, τBsetc. assumed known from other studies. • Distributions for main specific background (Bd→K+π-) and b-inclusive background made with Gaussian of 40fs fixed width as resolution model. This is because these backgrounds aren’t sensitive to the resolution model.

8. Model Extraction on Data • Five parameters are fitted for: GM, GS, F2, B/Sspecific, and B/Sb-inclusive. • 300 toy jobs were ran, with each dataset having 6,000 signal events, corresponding to 2fb-1 of data. • The oscillations are just visible above the “noise” coming from mistag, proper time resolution and background. toy datapoints PDF used to make toy data

9. Results • Input values are recovered without biases (see next slide). • Pull widths are all compatible with 1. • A few % of fits fail to converge correctly. GS GM

10. Results • The input values are correctly recovered within the errors. • The sensitivities are such that from 2fb-1 onwards the fit can start to give non-trivial input into the analysis of Bs→K+K- , and hence the γ measurement.

11. Conclusions on Proper Time • A model has been developed which accurately describes the proper time residuals for B→hh. • The parameters of this model can be extracted on data via a fit to the flavour-tagged τrecdistribution for Bs→K-π+. • Once 2fb-1 of data has been collected, the fit to the model parameters can provide useful input into the γ measurement using Bd→π+π- and Bs→K+K-.

12. B→p p Study: Status • Work completed recently; first draft of public note LHCb-2009-006 is nearing completion. Should be circulated around the WG soon. • Roadmap chapter: Included in the currently circulating draft of the roadmap.

13. Motivation for B→p p • Despite searches by B-factories, no charmless two-body baryonic B-decay has been observed. • Particular case of Bd→p p ideal for LHCb as it differs from standard B→hh modes only in PID. Current best upper limit for its branching ratio (BR) is 1.1*10-7 (from Belle). • Theoretical predictions for BRs of such decays in the SM is challenging; results vary in order of magnitude depending on model used (e.g QCD sum rule vs. diquark model). • Most of the theoretical predictions for Bd→p p are excluded by experiment; lowest prediction is 1.1*10-7. Not a copy-paste error: it really is the same value as the experimental limit!

14. Backgrounds to B→p p • The backgrounds considered are: • L0-passed minimum bias • b-inclusive • B→hhh specific backgrounds • B→hh specific backgrounds • As selection already exists for the “standard” B→hh (where h = π or K) which keeps the B/S for min. bias and b-inclusive low, this was used as the starting point for the Bd→p p selection. • After studying the few background events which survived the standard selection, a few extra cuts were added to lower the B/S further:

15. Selection for Bd→p p • Added PID cuts: DLL(p-π)>5 and DLL(p-K)>0. • Very effective against both inclusive bkgs (few h in typical event are true p), and specific bkgs (most of B→hh and B→hhh don’t involve p). Bd→ π+π-π0 DLL(p- π) Signal DLL(p- π) • Added track quality cut: track χ2/ndf < 3. • Reduces the number of ghosts from min. bias and b-inclusive events. b-inclusive ghost track χ2/ndf Signal track χ2/ndf

16. Specific Bkgs Before PID • Plots show expected mass distributions after 2fb-1, applying full selection except the PID cuts. Bd→ p p Bd→ p p Bd→ K+π- Bd→ π+π-π0 Bs→ K+ K- Λb→ p K- Bd→ p p Bd→ p p Bu→ π+π-π+ Bu→ K+K-K+ Bu→ π+π-K+ Bd→ Ks π-π+ Bu→ ppK+ Bu→ π+K-K+ Bu→ ppπ+

17. Specific Bkgs After PID • Thankfully we have two RICHes!  Recall: DLL(p-π)>5 and DLL(p-K)>0 Bd→ p p Λb→ p K- Bs→ K+ K- Bu→ ppK+ Bd→ K+π- Bu→ ppπ+ Bd→ π+π-π0

18. Selection Efficiency • Final step: apply mass window of mBd±50MeV. • The resulting selection has an efficiency of 9.8% on signal. Assuming a BR of 1.1*10-7, this gives 1882 events in 2fb-1. • No background events of any type survive the selection, allowing upper limits on the yield to be set using the Feldman-Cousins method. • The B/S for min. bias is clearly saturated by lack of available simulated MB events. • From now on we assume that no MB will pass the selection.

19. Signal Significance • This can be plotted against lumi for different BR values. • If B.R. = 1.1*10-7: • The total B/S (ignoring min. bias) is <2.24 at 90% confidence level. • 5σ observation is possible with only 0.09fb-1, i.e. in the 2009/10 running period! • Even if the true B.R. is 5 times lower, an observation can be made with 2.0fb-1. 1.1*10-7 8*10-8 4*10-8 2*10-8 Signal significance = S/sqrt(S+B)

20. Trigger Issues: Mass Window • So far trigger has been ignored, and pp daughter hypothesis used. Trigger uses π+π-hypothesis, shifting mass peak far below mBd. • A mBd±600MeV window loses ~13% of signal events. Could live with this. • The [4GeV,6GeV] window of the Topological Trigger would be even better. • BUT the roadmap proposes a mass window of [5.0GeV, 5.8GeV]. • This fails to include the peak for Bd→ p p! Bd→ p p • The trigger mass window needs to be widened to include this discovery channel. • A wider window also improves sideband knowledge. End of roadmap window Bs→ p p End of 600MeV window mBd

21. Other Similar Rare Decays • Other unobserved decays suitable for LHCb include Bs→p p and Bd→ ΛΛ. • Former comes “for free” with the search for Bd→p p. • Latter will be more challenging due to the need to reconstruct two hyperons. Also invariant mass will be lower than Bd→p p. • Also still to be observed are Bs→π+π- and Bd→K+K-. • Their BRs can help constrain U-spin breaking effects; important in extracting γfrom Bd→π+π- and Bs→K+K-. • Selection should be kept the same as for main B→hh modes to help control systematics. • BR upper limits in 2008 PDG are 5.9*10-5 (Bs→p p), 3.2*10-7 (Bd→ ΛΛ), 1.7*10-6 (Bs→π+π) and 4.1*10-7 (Bd→K+K-).

22. Conclusions on Bd→p p • Offline selection has been developed. • ηsel= 9.8% • For high signal BR: • Total B/S < 2.24 at 90% C.L • 2fb-1 yield≈1900 events • 5σ observation with 0.09fb-1

23. Backups

24. F2 Fitted Values • For 2fb-1, the value of F2 is often fitted to 0 as the sensitivity is comparable to its magnitude. But for 10fb-1 the sensitivity is better, and a Gaussian is obtained for the fitted values distribution. • In any case this is not a problem as the fit result is still correct within its error, and the pulls are okay. 2fb-1 10fb-1

25. Yield Calculation Details • The following does not take the trigger efficiency into account. • The Feldman-Cousins method gives 2.44 as 90% upper limit when 0 events are selected; ηsel<2.44/(# of events in bkg sample). • For the signal and specific bkgs, the yield in 2fb-1 is given by From previous slide Events with both daughters in LHCb acceptance Since have both b and bbar in an event Hadronisation probability Number of bbbar in 2fb-1 Branching ratio • For stripped b-inclusive the yield in 2fb-1 is Probability for bb inclusive event to pass at least one of the preselections in the stripping Scaling mass window from 600MeV to 50MeV • The L0-yes min. bias yield is calculated by assuming that the output rate of L0 is 1MHz.