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B Physics prospects at LHCb

B Physics prospects at LHCb. Emanuele Santovetti on behalf of the LHCb Collaboration Università di Roma Tor Vergata e INFN. Physics motivations Detector requirements Physics program. Physics motivation.

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B Physics prospects at LHCb

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  1. B Physics prospects at LHCb Emanuele Santovetti on behalf of the LHCb Collaboration Università di Roma Tor Vergata e INFN • Physics motivations • Detector requirements • Physics program DIS2007 Munich 16-20 April

  2. Physics motivation • LHCb is a dedicated B physics precision experiment at LHC to study CP violation and rare decays • Standard Model describes CP violation by a single complex phase in the unitary CKM matrix • We will over constrain the unitarity triangles and search for new physics LHCb precision enough to distinguish unitarity triangles (equal up to the l3 order) • New Physics can manifest through the exchange of a new intermediate particle in box and penguin diagram • compare measurementswhere NP effects are expected with treelevel ones (no loop or penguins) DIS2007 Munich 16-20 April

  3. Pythia production cross section 100 μb 230 μb bb correlation Detector design: B acceptance • Designed to maximize B acceptance (within cost and space constraints) • Forward spectrometer, 1.9 <  < 4.9 • more b hadrons produced at low angles • single arm OK since b-b pairs produced correlated in space • Luminosity tuned (2×1032 cm-2s-1) to maximize the probability of single interaction per x-ing • pT trigger can be lowered up to 2 GeV/c, efficient also for purely hadronic B • 1 year of running = ~2 fb-1 and 1012 b-b events @nominal luminosity DIS2007 Munich 16-20 April

  4. Detector requirements • Time dependent measurements • Reconstruction in the harsh LHC environment • B events ~ few % of the total cross section • Need of a selective trigger • Mass and pointing constraint to reduce background p+,K+ Good vertex spatial resolution to determine proper time K- ~1 cm Good K/p separation K+ Bs SV p+ PV p p Btag B flavor tagging DIS2007 Munich 16-20 April

  5. LHCb detector Muon stations Tracking stations beam VELO RICH system ECAL and HCAL DIS2007 Munich 16-20 April

  6. Momentum resolution dp/p = 0.5% p0 invariant mass spectrum Detector performances + 12C → 0 + X → 2 + X B vertex (0) =4.7 MeV/c2 PV IP Particle Identification Eff. DIS2007 Munich 16-20 April

  7. Flavor tagging Tagging power in ε(1-2ω)2 • Opposite side • Charge of the kaon in the b→ c→ s chain • Charge of the lepton in semi-leptonic decays • Charge of accompanying b jet • Same side • Charge of the K accompanying Bs • Charge of the p from B** → B*p± DIS2007 Munich 16-20 April

  8. LHCb Physics program • Bs mixing parameters: DGs, Dms,fs • a with Bd→p0p+p- • b with Bd → J/Y Ks • g with different methods • Rare decays • Bs → mm to the level of the SM prediction • Radiative penguin Bd → K*g, Bs → fg, Bd → wg • Electroweak penguin Bd → K*mm • and much more, e.g. Bc, charm physics, D0 mixing and CP violation) • .... not exhaustive list ! DIS2007 Munich 16-20 April

  9. ACP(t) - background subtracted sin2b with B0J/KS • One of the first CP measurements • golden mode, very well measured by b-factories • will be an important check of CP analyses and of tagging performance • can search for direct CP violating term  cosDmdt • Expect 240k reconstructed*B0 J/y KS events/2fb-1 • Precision sstat(sin 2b) ~ 0.02 in 2fb-1 of collected data [currently s(sin 2b) ~ 0.03] *after trigger and reconstruction DIS2007 Munich 16-20 April

  10. Measure g • g is the least well measured CKM angle and LHCb has several ways to measure it (independent) • Most promising method is the ADS+GLW applied to B → DK • B+→D(Kp)K+, D(K3p)K+, D(pp,KK)K+ • B+→D*(Kp)K+ • B0→D(Kp)K*0, D(KK)K*0, D(pp)K*0 • BS→DSK • Dalitz analysis for the neutral and charged B → DK decays • B0 → p+p- and Bs → K+K-, sensitive to new physics DIS2007 Munich 16-20 April

  11. Cabibbo favored Cabibbo suppressed The B → DK decays (GLW and ADS method) Consider the following decays (tree level) Color favored Color suppressed • Measuring the three different decay rates (relative) and the c.c. will allow to extract the gamma angle in a clean way, but… • because the color suppression, thetwo amplitudes, A1 and A2, very different → large error • need the D0 tag Decays of D0, D0 to same final state allows the two tree diagrams (theoretically clean!) to interfere. Consider the decay D0 → K-p+ For these decays the reversed suppression of the D decays relative to the B decays results in much more equal amplitudes → big interference effects ~ O(1) Counting experiment: no need for flavor tagging or proper time determination → measure of BR ~ O(10-7) or smaller DIS2007 Munich 16-20 April

  12. (1) 56000* (2) 700 (3) 56000 (4) 700 The B → D(Kp)K decays 4 B± → (Kp)DK± decays *both charges • Interference parameters • From the B decays: g– because have b→u , b→c interference rB – the ratio in magnitude of two diagrams (≤0.1 for DK±) δB – a CP conserving strong phase difference • The D decays introduce: rDK – the ratio in magnitude of two diagrams (0.060) δDK – a CP conserving strong phase difference rates (1) and (3) are favored + CP eigenstate = GLW method rates (2) and (4) are suppressed + another D decay = ADS method but these suppressed rates have order 1 interference term, as rB~rD DIS2007 Munich 16-20 April

  13. B →DK strategy: ADS+GLW • D0→Kp 3 observables from the relative rates of the 4 processes, depend on 4 unknowns g, rB, dB, dDKp • rDKpis already well measured • may benefit from cos(dD) measurements in CLEO-c and/or BES III • Need another D0 decay channel to solve for all unknowns • D0→Kppp (BR ~ 8%): provides 3 observables which depends on 4 unknowns g, rB, dB, dDK3p only dDK3pnew, rDK3pis already well measured • D0→KK/pp each CP mode provides one more observables with no new unknowns DIS2007 Munich 16-20 April

  14. LHCb sensitivity w/o bkgrd δDKp, δDK3p • Toy MC to simulate 2 fb-1 signal data: → fit results return input values • Combine Kp with: • K3p similar yields and identical background level • KK pp4300 B+ and 3300 B- with B/S ~ 2 estimated bkgrd δDKp, δDK3p (HighlightedareRMS quoted from non-Gaussian distribution of fit results due to close ambiguous solutions, will disappear as statistics increase. Global analysis using all modes will also help.) s(g) ~ 5º-15º in 2 fb-1depending on rB, δDK and δDK3 DIS2007 Munich 16-20 April

  15. D0K mass ADS with B →D*K decays • Attractive feature • D*→D0p0 (BR~2/3) – has strong (CP con.) phase δB • D*→D0g (BR~1/3) – strong phase δB+p → if can distinguish the two decays, powerful additional constraint ! • Preliminary studies (without background) show that, including D*K improves precision of previous analyses to s(g) = 2º - 5º (favored mode: 17k/2 fb-1) • However… • Reconstruction efficiency is small for soft g while background is enormous • Non trivial to separate D0p0 and D0g • Fit DK mass shape to get p0 and g components ignoring neutrals Signal/bkgrd Arbitrary norm. B±→D*(D0g)K± 1 bb event here represents 5k/2fb-1 bb sample B± mass Charged reconstruction B±→D*(D0p0)K± B±→D*(D0g)K± DIS2007 Munich 16-20 April

  16. A1 = A(B0 D0K*0): bc transition, phase 0 A2 = A(B0 D0K*0): bu transition, phase + A3 = 2 A(B0 DCPK*0) = A1+A2, because DCP=(D0+D0)/2 GLW method - g from B0 →D0K*0 • Dunietz variant of Gronau-Wyler method makes use of interference between two color-suppressed diagrams interfering via D0 common final states (pp, pK, KK) • Measuring the 6 decay rates, B0D0(Kp,pp,KK)K*0 + CP conjugates, allows g to be extracted without flavor tagging or proper time determination s(g) ~ 7º - 10º (2 fb-1) depending on , δDK and δDK3 *in 2 fb-1, both charges DIS2007 Munich 16-20 April

  17. Bs→ DsK • Interference between tree level decays via BS mixing (s-version of Bd→D*p): time dependent analysis (clean, no penguins) • Measuresg+fS (fS from BS→J/ΨΦ) • Expect 5400 events/2fb-1 • Excellent proper-time resolution (st ~ 40 fs) allows to resolve BS oscillations • s(g) ~ 13°from 2fb-1 data [Dms=17.3 ps–1] • Main background from DSp (BR×10 ) • Suppressed using kaon ID from RICH detector • B/S <1 @ 90% CL • Parallel analysis possible with Bd→Dp± (~790k events/2fb-1 with B/S ~ 0.3, g extraction requires rDp or combined Bs→ DsK U-spin analysis) DIS2007 Munich 16-20 April

  18. g from B0 hh • B0→ pp originally proposed to measure a but the influence of penguin diagrams makes the task difficult From the time dependent CP asymmetry deiq = ratio of penguin and tree amplitude in B→pp d’eiq’ = ratio of penguin and tree amplitude in B→KK Extract four asymmetries Assuming U-spin flavor symmetry (u↔s) d=d’ and q=q’ and taking b from Bd → J/y Ks and c from Bs → J/yf we can solve for g DIS2007 Munich 16-20 April

  19. g from B0 hh • 26k Bd → pp events with 2fb-1, B/S < 0.7 • 37k Bs → KK events, B/S = 0.3 • s(g) ~ 5º + uncertainty from U-spin symmetry breaking • Sensitive to new physics DIS2007 Munich 16-20 April

  20. Summary of performances on g Signal only, no accept. effect Combining all modes, with a nominal year of data (2 fb-1), LHCb will be able to extract g with ~ 4° resolution, and compare the B→DK direct measurement with the indirect determination (B0 +–,Bs  K+K–) to make a stringent test of the SM DIS2007 Munich 16-20 April

  21. ? Bs mixing phase, fs, from Bs→J/y(m+m-) f In the SM fsSM = -2c = -2l2h (small) and from UT fits we get fs= -0.037± 0.002 Direct measurements not very precise: recent D0: -0.79+0.47-0.39 CP violating decay can proceed directly or through mixing. The “mixing box” can have contributions from NP particles, resulting infs = fsSM + fsNP≠ -2l2h Tagged Bs Tagged Bs Measure proper time dependence Measure time-dependent CP asymmetry hf = ± 1 CP DIS2007 Munich 16-20 April

  22. Bs mixing phase, fs, from Bs→J/y(m+m-) f • Because the final state contains two vector particles, it is a mixture of CP odd and CP even • Use qtr angle between m+andnormal to f decay plane to do an angular analysis to identify the states. Total With:2 fb-1 of data Precision on fss(fs) = 0.023 CP even bkgnd From pure CP states Bs → J/yh,s(fs)= 0.059 Combining s(fs)= 0.021 (UT fit value: -0.037) CP odd Simultaneous fit to time and angular distributions DIS2007 Munich 16-20 April

  23. Very small branching ratio in SM: (3.4 ± 0.5) × 10-9 Present limit from Tevatron at 95% CL(1 fb-1): < 7 × 10-8 (expected final limit at 95% CL (8 fb-1): < 2 × 10-8) Sensitive to New Physics through loops Could be strongly enhanced by SUSY. ? ? MSSM Bs→m+m- DIS2007 Munich 16-20 April

  24. LHCb Sensitivity (signal+bkg is observed) BR (×10-9) 5 SM prediction 3 Integrated Luminosity (fb-1) Bs→m+m- • LHCb should have good prospect for significant measurement: 17 SM events/2fb-1 • Difficult to get reliable estimate of expected background • No background events selected in sample of 33M events but estimation limited by statistic • Combinatorial: B to m+X, B to m-X known as the main source of background, addressed by very good mass resolution 18 MeV/c2 • Bd, Bs to pp, pK, KK and mis-id. addressed by particle identification and mass resolution. with L=2fb-1 3s observation if at SM value DIS2007 Munich 16-20 April

  25. Conclusions • LHCb will be ready to collect data with its full detector as LHC turns on. Physics at 14 TeV starting in 2008 • It will make precision measurements that will severely constrain the unitarity triangle fits and probe rare decays • These measurements could either limit New Physics contributions to B decays or, more optimistically, uncover them DIS2007 Munich 16-20 April

  26. The B → DK decays (GLW method) Consider the following diagrams and the c.c. colour suppressed colour favoured • Measuring the three different decay rates (relative) and the c.c. will allow to extract the gamma angle in a clean way, but… • two amplitudes (A1 and A2) very different → large error • need the D0 tag DIS2007 Munich 16-20 April

  27. The ADS method Decays of D0, D0 to same final state allows the two tree diagrams (theoretically clean!) to interfere. Consider the decay D → K-p+ B+→D0K+ (colour suppressed) B+→D0K+ (colour favoured) Cabibbo favoured Cabibbo suppressed For these decays the reversed suppression of the D decays relative to the B decays results in much more equal amplitudes → big interference effects ~ O(1) Counting experiment: no need for flavor tagging or proper time determination → measure of BR ~ O(10-7) or smaller DIS2007 Munich 16-20 April

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