1 / 34

B s physics at LHCb

B s physics at LHCb. Roger Forty, CERN. The LHCb experiment B s –B s oscillation CP violation Rare decays. WHEPP8, IIT Bombay, January 2004. LHCb is a dedicated B physics experiment at the LHC. LHC environment. LHC: pp collisions at  s = 14 TeV

shassell
Download Presentation

B s physics at LHCb

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Bs physics at LHCb Roger Forty, CERN The LHCb experiment Bs–Bs oscillation CP violation Rare decays WHEPP8, IIT Bombay, January 2004

  2. LHCb is a dedicated B physics experiment at the LHC

  3. LHC environment • LHC: pp collisions at s = 14 TeV • Bunches cross at 40 MHz frequency separated by 25 ns • sinelastic = 80 mb at high L >> 1 pp collision/crossing • Choose to run at ~ 2 × 1032 cm-2s-1 dominated by single interactions • Makes it simpler to identify B decays from their vertex structure(and reduces radiation dose) • Beams are defocussed locally  maintain optimal luminosity even when ATLAS and CMS run at 1034 Inelastic pp collisions/crossing LHCb

  4. B production • B hadrons are mostly produced in the forward direction (along the beam) • Choose a forward spectrometer10–300 mrad • Both b and b in the acceptance: important for tagging the production state of the B hadron b–b correlation (PYTHIA)

  5. Typical B event • Need to measure proper time of B decay: t=mL / pchence decay length L (typically ~ 1 cm in LHCb)and momentum p from decay products (which have ~ 1–100 GeV) • Also need to tag production state of B: whether it was B or BUse charge of lepton or kaon from decay of the other b hadron

  6. LHCb detector ~ 300 mrad p p 10 mrad  Forward spectrometer (running in pp collider mode) Inner acceptance 10 mrad from conical beryllium beam pipe

  7. LHCb detector  Vertex locator around the interaction region Silicon strip detector with ~ 30 mm impact-parameter resolution

  8. LHCb detector  Tracking system and dipole magnet to measure angles and momenta Dp/p ~ 0.4 %, mass resolution ~ 14 MeV (for Bs DsK)Magnetic field regularly reversed to reduce experimental systematics

  9. LHCb detector  Two RICH detectors for charged hadron identification Provide > 3s p–K separation for 3 < p < 80 GeV

  10. LHCb detector e h  Calorimeter system to identify electrons, hadrons and neutrals Important for the first level of the trigger

  11. LHCb detector m  Muon system to identify muons, also used in first level of trigger Efficiency ~ 95% for pion misidentification rate < 1%

  12. Design of experiment in the underground cavern Typical event (full GEANT simulation) Current status of magnet construction: Subdetectors also under constructionExperiment will be completed ready for first LHC beam in 2007

  13. Vertex locator Constructed from silicon discs perpendicularto the beam axis, with r–f geometry Made in two halves so can be retractedduring beam injection Interaction region ~1m Vacuumvessel

  14. RICH detectors RICH1 detector Vertex locator RICH2 uses a CF4 gas radiator for ID of high momentum tracks

  15. 80 mm 1000 pixels RICH photon detector: HPD Performance of particle ID Test beam: e–p separation Typical event in the RICH1 photon detectors

  16. sbb ~ 500 mb, < 1% of inelastic cross-section Use multi-level trigger to select interesting events: high pTelectrons, muons or hadrons vertex structure and pT of tracks full reconstruction Trigger ~ 200 Hz to tape    30–60%efficiency

  17. Comparison to other experiments • Enormous production rate at LHCb: ~ 1012 bb pairs per year much higher statistics than the current B factoriesBut more background from non-b events  challenging triggerand high energy  more primary tracks, tagging more difficult • Expect ~ 200,000 reconstructed B0 J/y KS events/yearcf current B-factory samples of ~ 2000 events precision on sin 2b ~ 0.02 in one year for LHCb (similar to expected world average precision in 2007) • Expect ~ 26,000 reconstructed B0 p+p- events/yearcf ~ 1000 from B-factory by 2007 • But in addition, all b-hadron species are produced: B0, B+, Bs, Bc , Lb … • Concentrate here on physics of the Bs meson (not produced at B factories)Only competition before LHC is from CDF+D0 (lower statistics, poorer ID) • ATLAS and CMS will only have lepton trigger, poor hadron identificationDirect competition will come from BTeV, expected at the Tevatron ≥ 2009

  18. B–B oscillation • Neutral meson with flavour eigenstates B0 (bq), B0 (bq) has CP eigenstates:B1 = (B0 + B0) /2 CP(B1) = +B1 B2 = (B0- B0) /2 CP(B2) = -B2 • Neglecting CP violation, mass eigenstates = CP eigenstatesTime development: id B0 = M – iG B0dt B0 2B0  initially pure B0 state decays as B0 (or B0) at time t with probability:P(t) = e-Gt e-DGt + e+DGt± cos Dmt22Dm = mass difference of B1, B2DG = width difference of B1, B2 • Hence oscillatory behaviour with frequency Dm:

  19. In Standard Model, oscillationproceeds via box diagram: • Oscillation frequency Dmvery precisely known for the B0, Dmd = 0.502  0.006 ps-1(= 3.3 × 10-4 eV !) but extraction of CKM element Vtdlimited by knowledge of hadronic terms in the expression • If the oscillation frequency Dms could be measured for the Bs toothen some uncertainty cancels in the ratio: • DG< 1% expected for the B0, but could be as high as ~ 10% for the BsG ~ 30  fast oscillations SU(3) breaking term

  20. Despite heroic effort at LEP + SLD, Dms has not yet been measured(although there is an interesting feature in the amplitude plot at ~ 18 ps-1) • Current world combined limits: Dms > 14.4 ps-1DGs < 0.29 Gs • If CDF+D0 don’t get there first…best channel for LHCb: Bs  Ds-p+ Expect 80,000 reconstructed signal/yearwith signal/background ~ 3 • Fully reconstructed decay  excellent momentum resolutionDecay length resolution ~ 200 mm Proper time resolution ~ 40 fs at 95% CL Ds-p+

  21. Tagging of production state:efficiency = 55% mistag rate = 30% Reconstructed proper-timeshows clear oscillations: (for two values of Dms, withacceptance, resolution, mistag) Error on the amplitude vsDms can make a 5s measurement in one year for Dms up to 68 ps-1 (far beyond Standard Model expectation) Once a Bs–Bs oscillation signal is seen, the frequency is precisely determined: s (Dms ) ~ 0.01 ps-1

  22. CP violation • At the level of precision that will be probed by LHCb, there are two unitarity relations of the CKM matrix that are of interest: • Possible situation of the measurements when LHCb starts to take data: Differ at the percent levelphase of Vts  measurement of the angle g will be crucial

  23. CP asymmetry: Bs J/yf • Bs counterpart of the golden mode B0 J/y KS • CP asymmetry arises from interference of Bs J/y fand Bs Bs J/y fmeasures the phase of Bs mixing • In Standard Model expected asymmetry  sin 2c = very small ~ 0.04 sensitive probe for new physics • Reconstruct J/y m+m- or e+e-, f  K+K- 120,000 signal events/year • Final state is admixture of CP-even and odd contributions angular analysis of decay products required • Define transversity angle qtr :Likelihood is sum of CP-odd and even termsL(t) = R-L-(t) (1+cos2qtr)/2 + (1-R-) L+(t) (1-cos2qtr) • Fit for sin 2c, R- and DGs/Gs s(sin 2c) ~ 0.06, s(DGs/Gs) ~ 0.02 in one year

  24. CP asymmetry: Bs Ds K+ • Arises from interference between two tree diagrams via Bs mixing:Bs Ds+K- and Bs Ds-K+ B ~ 20 × 10-5 B ~ 3 × 10-5 • CP asymmetry measures g - 2c (g from phase of Vub)c will be determined using Bs J/yf decays  extract g • Very little theoretical uncertaintyInsensitive to new physics, which is expected to appear in loops • Reconstruct using Ds- K-K+p-5400 signal events/year

  25. Bs Dsp is background for Ds KBranching ratio ~ 12  higher • Suppress it by cutting on difference in log-likelihood between K and phypotheses in RICH: Remaining contamination only ~ 10% Dsp should not have CP asymmetry use it as a control channeleg to measure any possible productionasymmetry of Bs and Bs

  26. Allow for possible strong phase difference D between the two diagrams • Fit two time-dependent asymmetries:Phase of Ds+K- asymmetry is D - (g - 2c)Phase ofDs-K+ asymmetry is D + (g - 2c)can extract both D and(g - 2c) Asymmetries for 5 years of simulated data s(g) ~ 14 in one year

  27. CP asymmetry: B(s) h+h- • B0 p+p- originally proposed for measurement of angle a = p - b - gBut clean extraction of a is compromised by influence of penguin diagrams • Measure time-dependent CP asymmetries for B0 p+p- and Bs K+K-ACP(t) = Adir cos(Dmt) + Amix sin(Dmt) • Extract four asymmetries:Adir(B0 p+p-) = f1(d, q, g) deiq = ratio of penguin and tree Amix(B0 p+p-) = f2(d, q, g, b) amplitudes in B0 p+p-Adir(Bs K+K-) = f3(d’, q’, g) d’eiq’ = ratio of penguin and tree Amix(Bs K+K-) = f4(d’, q’, g, c) amplitudes in Bs K+K-

  28. Assume U-spin flavour symmetry (under interchange of d and s quarks)d=d’ and q=q’ [R. Fleischer, PLB 459 (1999) 306] • Four measurements, three unknowns (taking b and c from other channels)  can solve for g • Plot dvsg • 37,000 reconstructed Bs K+K- events s(g) ~ 5 in one year • Theoretical uncertainty from U-spin assumption (can be tested)Sensitive to new physics in the penguin loops “fake” solution blue bands from Bs  KK (95% CL) red bands from B   (95% CL) ellipses are 68% and 95% CL regions(input = 65)

  29. Rare decays: Bs m+m- • Flavour-changing neutral currentstrongly suppressed in Standard ModelB (Bs m+m-) ~ 4 × 10-9 • New physics contributions could increase this significantly  excellent place to look for them • eg in SUSY: [G. Kane et al, hep-ph/0310042] • Expect ~ 16 signal events/year for the Standard Model branching ratio ~ 40 background events/year (mostly from b  m-, b  m+)~ 4 s significance after three years • Here ATLAS and CMS are competitive, due to their higher luminosityLHCb will also study many other rare decays: B0 K*g, K*m+m-...

  30. b  s penguin decays • One of the most interesting results from B factories is for B0f KS • Standard Model asymmetry = sin 2b(within ~10% theoretical uncertainty)= 0.736 ± 0.049 from B0 J/y KS • Measured values:+0.45 ± 0.43 ± 0.07 (BaBar)consistent with the Standard Model-0.96 ± 0.50 ± 0.10 (Belle) inconsistent with the Standard Model • Expect to reconstruct ~ 1000 B0f KS signal events/year in LHCb • However, if new physics does show up in B0f KS it is important to also examine other b  s penguin decays:Bs ff, KK, fg, … LHCb will also reconstruct large samples of all of these modes

  31. Conclusions • LHCb is dedicated to the study of B physics, with a devoted trigger,excellent vertex and momentum resolution, and particle identification • Construction of the experiment is progressing wellIt will be ready for first LHC collisions in 2007 • LHCb will give unprecedented statistics for B decays, including access to the Bs meson, unavailable to the B factories • Bs–Bs oscillations will be measured precisely> 5s for Dms up to 68 ps-1s (Dms ) ~ 0.01 ps-1 • Many measurements of rare decays and CP asymmetries will be performeds (sin 2b) ~ 0.02s (sin 2c) ~ 0.06s (g) ≤ 10 • CP angles determined via channels with different sensitivity to new physics  detailed test of the CKM description of the quark sector in one year in one year

  32. Outlook • A possible scenario before the LHCb measurement of g:

  33. Outlook • A possible scenario after the LHCb measurement of g:

  34. Possible topics Questions that might be of interest to discuss in the workshop: • How will different New Physics scenarios affect the quantities that LHCb will measure?eg MSSM, non-minimal SUSY, or other models… effect on Dmd, Dms, DGs, sin 2b, sin 2c, g in different channels… • What other channels would be of interest for LHCb to study? • If a large value is measured for B (Bs m+m-), how can we be sure that it is a sign of SUSY? • Is the precise measurement of Dms and DGs useful?How can the theoretical uncertainties in calculating them be reduced? • How might we improve the production state tagging?in addition to the standard “opposite-side” lepton and kaon tags: same-side tagging, vertex charge, jet charge… • What can be learnt from high statistics of Bc mesons and b-baryons?

More Related