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Measurements of g at LHCb

This presentation discusses the methods used by LHCb to measure the parameter g, including the ADS+GLW strategy and the use of various decay channels. It also presents expected sensitivities and background estimates.

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Measurements of g at LHCb

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  1. Measurements of g at LHCb Mitesh Patel (CERN) (on behalf of the LHCb Collaboration) 14th December 2006

  2. Introduction • LHCb will use a number of methods to measure g: • “ADS + GLW” • B±→D0(Kp,KK,pp)K± • B0→D0(Kp,KK,pp)K*0 • “Dalitz” • B±→D0(KSpp,KSKK)K± • B0→D0(KSpp,KSKK)K± • Four body “Dalitz” • B±→D(Kppp,KKpp)K± • Bs→DsK • B→hh [Jacopo Nardulli, WG4, Friday, PM-1] • Will present : • Expected sensitivity as a function of the relevant parameters • Signal selection/background estimate taking B±→D0(Kp)K± as an example Mitesh Patel, CKM 06

  3. s D0 D0 b u u u u s u u u u u u d c p- K+ c b s D0 d c s c D0 p- K+ u u B±→DK± Decays – ADS Method K- • B- can decay into both D0 and D0, diagrams very different amplitudes colour favoured colour suppressed • Decays of D0, D0 to same final state gives access to interference (doubly) cabibbo suppressed cabibbo favoured • For ‘suppressed’ B-→(K+p-)DK- (+ c.c.) decays : reversed suppression of D decays cf. B decays → ~ equal amplitudes → big interference effects • Counting experiment – no need for flavour tagging or t determination [For ‘favoured’ B-→(K-p+)DK- (+ c.c.): higher rates, but smaller B± asymmetry] B- B- K- Mitesh Patel, CKM 06

  4. (1) (2) (3) (4) Interference parameters • Interference depends on a number of parameters : • From the B decays : g– because have b→u, b→c interference rB – the ratio in magnitude of two diagrams (≤0.1 for D0K±) δB – a CP conserving strong phase difference • From the D decays : rDK – the ratio in magnitude of two diagrams (0.060) δDK – a CP conserving strong phase difference • Have 4 B± →D(Kp)K± rates we can measure : • Two rates are favoured, (1) and (3) • Two rates are suppressed, (2) and (4) but these suppressed rates have order 1 interference effects as rB ~ rD [+CP eigenstates → GLW Method] [+another D decay → ADS Method] Mitesh Patel, CKM 06

  5. B±→DK± Strategy : ADS+GLW • D0→Kp: 3 observables from the relative rates of the 4 processes depends on the 4 unknowns: g, rB, dB, dDKp • rDKp is already well measured • CLEO-c constraining cos(dDKp) → Need an additional D0 decay channel to solve for all unknowns • D0→Kppp: provides an additional 3 observables which depend only on one additional unknown: dDK3p (rDK3p also well measured) • At present have ignored resonant structure in D0→Kppp and just used rate for illustration, exploitation of sub-structure is under investigation • The CP eigenstate decays D0→KK/pp provide one more observable with no new unknowns: • Use the modes D0→Kp, K3p, KK, pptogether to give best sensitivity Mitesh Patel, CKM 06

  6. Experimental Situation • B-factories have not yet observed the suppressed modes • Dalitz analyses give rB~0.2 (BELLE) and rB~0.1 (BABAR) → Suggest suppressed modes should soon be observed • We takerB=0.08 (UTFIT), δB = 130o (average B-factory results), rDKp=rDK3p=0.06 (PDG), -25<dDKp <25o, -180<dDK3p <180o UTFIT best fit : rB = 0.08 ± 0.03 Mitesh Patel, CKM 06

  7. What can LHCb add... ? • 2fb-1 in 107 sec → equivalent to 1012 bb, 0.4 of which expected to be B± In both sign combinations signal yields then : 1012 bb × 0.4 × 2 × eTOT × BR • Our total efficiency, eTOT, and resulting sensitivity depend entirely on our ability to control the background – in very different environment to the B factories • Full simulation indicates that acceptance × trigger efficiency× selection efficiency gives eTOT = 0.5% (more in a moment) : • Favoured B±→D0(Kp)K±→ ~56,000 events/2fb-1 • Suppressed B±→D0(Kp)K± → ~700 events/2fb-1 • Favoured: BR = 1.4×10-5 • Suppressed: BR = 1.8×10-7 Mitesh Patel, CKM 06

  8. Full MC Performance • LHCb uses full MC simulation to estimate the signal selection efficiency and the background : • PYTHIA - generation of p-p collisions at √s = 14TeV • GEANT - full detector response/spill-over and tracking through material • on/offline pattern recognition, full trigger chain, selections • Signal selection efficiency eTOT(B±→D0(Kp)K±)=0.5%: 8.2% (geom.) × 87.8% (rec.) × 28.4% (seln.) × 25.0% (trig.) • Mass resolutions • B± ~15 MeV • D0 ~6.5 MeV B± mass /MeV Si Sensors RF foils Interaction region • Vertex resolutions • Primary vertex sz ~ 50 mm • B decay vertex sz ~ 200 mm fsensor R sensor 100 cm Mitesh Patel, CKM 06

  9. Estimating the Background • From a large sample of minimum bias events find that no events are selected • To study background in more detail, focus on bb events where one b decays in 400mrad – after the application of the trigger most likely source of background • Background sample 20 million bb events generated with above condition ( → factor 0.434, sample equivalent to ~46M bb events) • Still equivalent to only a few minutes of LHCb running ! Mitesh Patel, CKM 06

  10. Background Studies Efficiency / % • Favoured Modes • Background from D0p decays dominates • Use RICH PID to separate D0p and D0K • Use dedicated sample of D0p to estimate B/S → Expect ~17k bkgrd events /2fb-1 from D0p • Use bb sample to assess combinatorial bkgrd → Expect ~0.7k bkgrd events /2fb-1 → ~28k B+ signal events/2fb-1 B/S~0.6 → ~28k B- signal events/2fb-1 B/S~0.6 • Suppressed Modes • bb sample → combinatorial bkgrd dominates → Expect ~0.7k bkgrd events /2fb-1 → ~530 B+ signal events/2fb-1 B/S~1.5 → ~180 B- signal events/2fb-1 B/S~4.3 e(KK ,p) = 93% e(pK ,p) = 4.7% Momentum / GeV MC B±→D0(Kp)p± events 687 / 580k pass all cuts except B mass 387 / 580k inside 3s B mass cut B± mass /MeV Mitesh Patel, CKM 06

  11. LHCb Sensitivity w/o bkgrd dDKp, dDK3p • Toy MC used to generate 2fb-1 data(*) • Combine Kp with : • K3p – similar yields and identical background level as Kp • KK and pp: • 4300 B+, 3350 B- decays with B/S ~ 2 → sg=5–15o with 2fb-1 data [degrees] Estimated bkgrd dDKp, dDK3p [degrees] (Non-Gaussian distribution of fit results highlighted …) (*) g = 60º, rB=0.08, dB= 130º, rDKp=rDK3p=0.06, -25º<δDK< 25º and -180º< δDK3<180º Mitesh Patel, CKM 06

  12. LHCb Sensitivity w/o bkgrd dDKp, dDK3p • Toy MC used to generate 2fb-1 data(*) • Combine Kp with : • K3p – similar yields and identical background level as Kp • KK and pp: • 4300 B+, 3350 B- decays with B/S ~ 2 → sg=5–15o with 2fb-1 data [degrees] Estimated bkgrd dDKp, dDK3p [degrees] (Non-Gaussian distribution of fit results highlighted …) (*) g = 60º, rB=0.08, dB= 130º, rDKp=rDK3p=0.06, -25º<δDK< 25º and -180º< δDK3<180º Mitesh Patel, CKM 06

  13. LHCb Sensitivity w/o bkgrd dDKp, dDK3p • Toy MC used to generate 2fb-1 data(*) • Combine Kp with : • K3p – similar yields and identical background level as Kp • KK and pp: • 4300 B+, 3350 B- decays with B/S ~ 2 → sg=5–15o with 2fb-1 data [degrees] Estimated bkgrd dDKp, dDK3p [degrees] (Non-Gaussian distribution of fit results highlighted …) (*) g = 60º, rB=0.08, dB= 130º, rDKp=rDK3p=0.06, -25º<δDK< 25º and -180º< δDK3<180º Mitesh Patel, CKM 06

  14. B±→D*(D0g)K± BB incl sample B± Mass /MeV B±→D*(D0p0)K± B±→D*(D0g)K± D0K mass / MeV B±→D*K± Decays Signal/bkgrd arbitary normaln One entry from BB sample corresponds to 5k bkgrd evts/ 2fb-1 • B → D*K has attractive feature : • D*→D00 – has CP cons. phase δB • D*→D0 – CP cons. phase δB+p • Potentially very powerful : • Adding D* decays (w/o bkgrd) to prev. study: sg=5-15o → sg=2-5o • No phases with non-Gaussian fit results • However, reconstruction efficiency for soft gis small → large B/S • Ignore neutrals and fit DK mass shape ? • Fav. modes – yields 17k (p0) and 9k (g)/ 2fb-1 • Sup. modes – yields similar to D0K case - but bkgrd problematic → investigating use of event topology to reconstruct p0,g momentum Mitesh Patel, CKM 06

  15. B0→DK*0 Decays • For B0→DK*0 decay rB ~ 0.4 (both diagrams colour suppressed) • Treat with the same (“ADS+GLW”) method, so far have used Kp, KK, pp modes • Removes g bias from DCS decays seen using traditional ‘GLW’ approach → sg=7-10o with 2fb-1 data (taking rB=0.4, -180<dB<180o, -180< dDKp <180o) Mitesh Patel, CKM 06

  16. B±→D(KSp+p-)K± Decays – Dalitz • Three body decay D0→Ksp+p- fully parameterized with parameters m+2=m2(Ksp+) and m-2=m2(Ksp-) • Use pre-determined model to describe D0 decay amplitudes as a function of (m+2, m-2) • Total B decay amplitude is sum of contributions via D0 and D0 : • Interference has sensitivity to g N: number of resonances aj,aj: amplitude and phase parameters from B factories Aj: model-dependent parameterization of matrix element + 2 rB Re [ f(m-2,m+2)f*(m+2,m-2)ei (-g+dB) ] G(m-2,m+2) = |f(m-2,m+2)|2 + rB2|f(m+2,m-2)|2 Mitesh Patel, CKM 06

  17. B±→D(KSp+p-)K± – Sensitivity • Signal selection gives : • 5k events/2fb-1 assuming “good” KS efficiency(*) • Combinatorial bkgrd B/S < 0.7 @ 95% C.L. • D(KSpp)p bkgrd B/S = 0.2±0.1 → 0.2<B/S<1.0 @ 90% C.L. • Model parameters from B and charm factories • With rB = 0.08,s(g) ~ 8º (signal only and w/o acceptance effect in fit) + model uncertainty • Similar method will be used for D0→KSK+K- decays: reduced BR but less bkgrd (PID from RICH) • B0→D(KSp+p-)K*0 decays also under investigation LHCb generator studies m-2(GeV2/c4) D0/D0 r(770) K* and DCS K* m+2(GeV2/c4) (*) Assuming all KS found offline can be reconstructed online Mitesh Patel, CKM 06

  18. Four-body “Dalitz” Analyses • Idea for 3-body D0 decays can be extended to 4-body D0 decays • Five parameters are then needed to describe the decays • Two modes are presently being investigated: • B±→D(KKpp)K± • For g=60º, dB=130º, rB=0.08, expect 1.7k events/2fb-1 • B/S=0.9±0.4 (Combinatorial, D0p) →s(g) ~ 15º (signal only) [hep-ph/0611272] • B±→D(Kppp)K± (as used in ADS+GLW analysis) • Take into account strong phase dependence across “Dalitz” space • Sensitivity under study Mitesh Patel, CKM 06

  19. Bs→DsK Decays • Interference between tree level decays via Bs mixing • Measures g + fs(fs from BsJ/yf decays) • Main background from Dsp: • Factor 10 higher branching ratio • Suppressed using kaon id from RICH detectors • B/S < 1 @ 90% CL • Expect 5.4k signal events/ 2fb-1 • Excellent proper-time resolution (st~40fs) allows to resolve Bs oscillations →s(g) ~ 13 from 2fb-1 data [ ms = 17.3 ps–1 ] • Parallel analysis possible with Bd→DŦp± (~790k events/2fb-1 with B/S ~ 0.3, g extraction requires rDp or combined Bs→ DsK U-spin analysis) Mitesh Patel, CKM 06

  20. Summary of Performance • Combining all modes, with a nominal year of data (~2 fb-1), LHCb will be able to extract g from combined analysis B→DK with the ~5º precision required to match the indirect determination • Comparison of g from B→DK and indirect determination will become a stringent test of the SM Signal only, no accep. effect Mitesh Patel, CKM 06

  21. Measuring g: B+ D0(K0π+π-)K+ Giri, Grossman, Soffer, Zupan (PRD 68, 054018 (2003)) • Use three body Cabibbo allowed decays of the D0/D0 • BR(D0 K0π+π-)=(5.97±0.35)% • Large strong phases between the intermediate resonances allow the extraction of rB , d and g by studying the Dalitz distribution of events LHCb generator studies m-2(GeV2/c4) D0/D0 r(770) where K* and DCS K* m+2(GeV2/c4) Mitesh Patel, CKM 06

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