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Observation of the W b -

Observation of the W b -. Brad Abbott University of Oklahoma On behalf of the D Ø Collaboration. SLAC January 6, 2009. B physics at the Tevatron. Upsilon discovered in 1977 at Fermilab by Lederman et. al. Since then other observations of other B mesons

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Observation of the W b -

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  1. Observation of the Wb- Brad Abbott University of Oklahoma On behalf of the DØ Collaboration SLAC January 6, 2009

  2. B physics at the Tevatron • Upsilon discovered in 1977 at Fermilab by Lederman et. al. • Since then other observations of other B mesons • B+, B0, Bs, Bc+ (pre Tevatron RunII) • B* (pre Tevatron RunII), • Bd**(Tevatron RunII) • Bs** (Tevatron RunII) • Little experimentally known about B baryons • Only one B baryon known for many years Summer 1977

  3. B baryons Counting only quark content 15 b baryons are expected Charmless b baryons multiplet

  4. B baryons more recently • Prior to Run II only 1 b baryon, Lb(udb), was considered observed (Lb J/yL) • However in last 2 years 4 new b baryons have been discovered • Sb+(uub) Sb-(ddb) (CDF, 2007) • Xb-(dsb) (D0/CDF 2007) • Wb-(ssb) (D0 2008)

  5. (*)-b in October 2006 CDF announces the observation of the b’s with 1.1 fb-1 PRL 99, 202001 (2007)

  6. Last year:-b observation CDF Signal Significance: M(b-) 5792.9  2.5 (stat)  1.7 (syst) MeV/c2 Signal significance = 7.8 PRL 99, 052002 (2007) PRL 99, 052001 (2007) M(Xb)=5.774 ± 0.019 GeV

  7. B baryons Various predictions for b baryon masses “Know” approximately where to look and what decay modes are expected B quark section large at Tevatron s(pp  bb) = 150 ub @ √s=2 TeV s(e+e- bb) = 7 nb @ √s=Mz s(e+e- bb) = 1 nb @ √s=M(4S) However large backgrounds make detection challenging

  8. Tough environment BaBar ~ 5 tracks/event

  9. Data Tevatron is running well: ~ 5 fb delivered to D0/CDF D0 Efficiency high: ~ 4.5 fb recorded by each experiment This analysis 1.3 fb-1

  10. DØ detector Large h coverage No particle ID (limited p/K separation) Unprescaled dimuon triggers at all luminosities

  11. Triggers • Entire b physics program based on muon triggers (single and dimuon triggers) • Do not have bandwidth available to trigger on displaced vertices • Many b physics analyses use dimuon triggers (unprescaled) and search for J/y decaying to mm. • Dimuon triggers have several Hz final rate

  12. B physics program at DØ Over 34 papers published/accepted for publication • CP violation in Bs decays • Flavor oscillations in Bs decays • Bsmm • Branching ratio of Bs  Ds*Ds* • Bc meson mass/lifetime • Direct CP violation …. For full list see http://www-d0.fnal.gov/Run2Physics/WWW/results/b.htm

  13. B baryons at DØ Lb J/YL Aids in understanding momentum scale Possible biases in reconstruction Lambda long lived so gives practice in reconstructing long lived particles Discovered that official processing of data inefficient for tracks with large impact parameter Takes a significant time to reconstruct each event and DØ cannot afford to increase reconstruction time for all events

  14. Data reprocessing When tracks are reconstructed, a maximum impact parameter is required to increase the reconstruction speed and lower the rate of fake tracks.   p   But for particles like the b-, this requirement could result in missing the  and proton tracks from the  and - decays  

  15. Since interesting decays all have a J/y  mm only reconstruct subset of data Much smaller sample and can be reconstructed in a reasonable time (few months) Reprocessing J/y sample

  16. Increase of reconstruction efficiency D0 D0 D0 GeV GeV GeV Opening up the IP cut: (Before) ( After )

  17. Similarities of decays “extra pion” Lb J/yLXb J/yX Lp Techniques learned in measuring Lb led to observation of Xb in 2007 Apply same idea to searching for Wb Use techniques learned from Xb to help in searching for Wb

  18. bss quarks combination Mass is predicted to be 5.94 - 6.12 GeV 0.83<(-b)<1.67 ps (prediction) M(-b) > M(b) Search for the -b(bss)

  19. Similar decay topologies Xb J/yX Wb J/yW + - p  - -b - ~5 cm ~3 cm p K-

  20. Slight differences • ct(L) = 7.89 cm • ct(W) = 2.46 cm • Lifetime(Xb) = 1.42 ps • Lifetime(Wb)= .84-1.69 ps (predicted) • Mass(Xb) = 5.792 GeV • Mass(Wb) = 5.94-6.12 GeV (predicted) • Much more difficult to reconstruct W than L

  21. X vs  reconstruction • →p decays: • pT(p)>0.7 GeV • pT()>0.3 GeV • - → decays: • pT()>0.2 GeV • Transverse decay length>0.5 cm • Collinearity>0.99 Black symbols: right sign combinations Green/red symbols: wrong sign combinations W reconstruction D0 D0

  22. W reconstruction • Need to clean up W mass peak • First clean up L peak • Apply Boosted Decision Trees to further clean up sample

  23.  optimization Apply a cut on proper decay length significance ( decay length significance > 10) D0 Before cut

  24. Minimum selection cuts: +K vertex reconstructed Transverse decay length significance>4 Proper decay length uncertainty<0.5 cm  reconstruction D0 PDG mass value Wrong-sign events +K+ Right-sign events (+K-)

  25. 20 input variables L and - vertex quality, decay lengths and decay kinematics For training we use MC signal and background from wrong-sign events (J/(K+)). Most important variables: pT(K) pT(p) pT() W decay length Boosted Decision Trees (BDT)

  26. - after BDT selection D0 - signal much cleaner Signal yield approximately the same Backgrounds greatly reduced

  27. There is a reflection due to assigning a pion as a kaon. Remove by requiring M()>1.34. Contamination due to -- DØ DØ

  28. - after BDT selection and removing X-  Lp- decays DØ Wrong-sign combination events

  29. Final Wb optimization • Optimized for the efficiency • Further reduce background (based on level we observe in the wrong-sign combinations.) • Uncertainty on Wb proper decay length < .03 cm • Wb transverse momentum greater than 6 GeV • J/y and W in same hemispere

  30. Mass Resolution s = 80 MeV s = 34 MeV “poor man’s” way to reduce mass resolution

  31. Look where we don’t expect any signal • After “optimize” J/+ decays by using wrong-sign combination events: • <0.03 cm • J/ and  in the same hemisphere • pT(J/+)>6 GeV 30 events remain

  32. Side band control samples DØ DØ

  33. Summary of all control samples Study known decays using MC samples with statistics larger than data set No excess is observed in any MC sample after selection criteria applied.

  34. “Open Box” and look at right-sign combinations 79 events selected Excess of events

  35. Mass measurement • Fit: • Unbinned log-likelihood fit • Gaussian signal, flat background • Number of background/signal events are floating parameters Number of signal events: 17.8 ± 4.9 Mean of the Gaussian: 6.165 ± 0.010(stat) GeV Width of the Gaussian fixed (MC): 0.034 GeV

  36. Signal Significance • Two likelihood fits are performed to estimate the significance: • Signal + background hypothesis (LS+B) • Only background hypothesis (LB) • We evaluate the significance: • Significance of the observed signal: 5.4

  37. Consistency check: Increase pT(B) Significance > 6

  38. Look back plots See consistent yields D0 D0

  39. Another consistency check We do not know the lifetime of the Wb We do not have enough events to measure the lifetime of the Wb We can compare proper decay length of sample with MC sample with a lifetime of 1.54 ps

  40. Alternative Cuts Based Analysis (CBA) Variables selected based on relative importance in BDT performance

  41. Cut Based Analysis Number of signal events: 15.7 ± 5.3 Mean of the Gaussian: 6.177 ± 0.015(stat) GeV Width of the Gaussian fixed (MC): 0.034 GeV Signal significance reduced to 3.9 s due to increased background

  42. BDT or Cut Base Analysis Cut based and BDT can select different events. Overlap is ~ 50% After removing duplicate events and combining analyses we observe 25.5 ± 6.5 events. Significance: 5.4

  43. BDT vs CBA Consistent number of observed signal candidates Consistent mass Consistent reconstruction efficiencies BDT has better background rejection power. Signal confirmed without BDT.

  44. Systematic uncertainties on the mass • Fitting models • Linear background instead of flat background gives negligible change. • Varying the signal Gaussian width between 28 – 40 MeV resulted in a 3 MeV uncertainty • Momentum scale correction: • Fit to the b mass peak in data gives a 4 MeV uncertainty. • Event selection: • Varying selection criteria and from the mass shift observed between the cut-based and BDT analysis gives a 12 MeV uncertainty.

  45. Wb mass M(Wb)=6.165 ± 0.010(stat) ± .013(stat) GeV A little on the high side of the predictions 1-2 s

  46. Production rate To determine production rate we normalize to Xb Determined from MC The systematic uncertainty includes contributions from the signal yields as well as selection efficiencies

  47. Production rate Spin of Xb = ½ Spin of Wb = 3/2

  48. Summary of Wb Number of signal events: 17.8 ± 4.9 (stat) ± 0.8(syst) Mass: 6.165 ± 0.010(stat) ± 0.013(syst)GeV Significance= 5.4 arXiv:0808.4142(2008) PRL 101, 232002 (2008)

  49. Summary of Wb Consistent with expectations

  50. Future plans • Reprocess Run IIb data • Lots more data (1.3 pb-1Run IIa vs 3.2 pb-1 Run IIb) • Much higher instantaneous luminosities so much larger combinatorial background • Layer 0 silicon • Begin searches for other B baryons

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