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Analisi di dati del Run I

Analisi di dati del Run I

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Analisi di dati del Run I

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  1. Analisi di dati del Run I Congregazioneper laDottrina dellaFede dal top al bottom con un pizzico di charm P. Giromini

  2. 5 analisi completate fra il 1999 ed il 2002 • Usano campioni di dati diversi, ma hanno un filo logico comune • 1: Heavy flavor properties of jets produced in p p interactions at s1/2=1.8 TeV (blessata, draft in preparazione) • 2: Studio di decadimenti semileptonici sequenziali (b l c X, c l s X) (blessing frozen dopo 3 mesi di presentazioni, non si trovano sbagli ma il risultato non piace, analogo dei superjet) • 3: Misura di c, time-integrated mixing parameter di adroni con beauty (presentata, blessing parte inizi ottobre) • 4: Studio dello spettro di massa invariante di dimuoni fra la J/y e la U(1S) (presentata, in coda) • 5: Studio di eventi con J/y +leptone (analogo del BC) (presentata, in coda)

  3. 1Heavy flavor properties of jets produced in p p interactions at s1/2=1.8 TeV A small step forward in resolving the long-standing discrepancy between the measured and predicted b-cross section at the Tevatron

  4. Jets with heavy flavor • Over several years CDF has been comparing the fraction of jets with heavy flavor (b and c quarks) to a simulation based upon the Herwig and CLEO (QQ) Monte Carlo generators • Heavy flavor-identification : Efficiency • SECondary VerTeX (SECVTX) 50% • Jet-ProBability (JPB)50% • Data sets : W+ jet events, generic-jet data (JET20, JET50, and JET 100), di-jet events with one jet containing a lepton (low-pT lepton sample)

  5. Jets with heavy flavor • We have used the low-pT lepton sample to calibrate the data-to-simulation scale factors for the SECVTX and JPB tagging algorithms • We have used generic-jet data to tune the parton-level cross sections evaluated in Herwig within experimental and theoretical uncertainties W+jet events P~50% • PRD 64, 032002 (2001)

  6. Jets with heavy flavor P~50% • We also identify heavy flavors by searching jets for semileptonic decays (SoftLeptonTagging ) efficiency 20% • PRD 65, 052007 (2002) P~0.4%

  7. Anomalous W+ 2,3 jet events with a supertag • The kinematics of these events has a 10-6 probability of being consistent with the SM simulation [PRD 64, 032004 (2002)] • Some of us took these events very seriously; hep-ph/0109020 shows that the superjets can be modeled by postulating the existence of a low mass, strong interacting object which decays with a semileptonic branching ratio of the order of 1 and a lifetime of the order of 1 ps • Since there are no limit to the existence of a charge –1/3 scalar quark with mass smaller than 7 GeV/c2 [PRL 86, 1963 (2001)], the supersymmetric partner of the bottom quark is a potential candidate

  8. Light sbottom (bs) • Lot of very recent buzz • hep-ph/0007318 uses it to resolve the long-standing discrepancy between the measured and predicted value of R for 5 < s1/2 < 10 GeV at e+ e- colliders • PRL 86, 4231 (2001) uses it in conjunction with a light gluino which decays to b bs to explain the difference of a factor of 2 between the measured b-quark production cross section and the NLO prediction • If light bs existed, Run 1 has produced 109 pairs; why we did’t see them ?

  9. Strategy • the NLO calculation of p p bsbs predicts s = 19.2 mb for a squark mass of 3.6 GeV/c2 (Prospino MC generator program). • The bb production cross section at the Tevatron is s = 48.1 mb (MNR) • The cc production cross section at the Tevatron is s = 2748.5 mb(MNR) • The NLO calculation has a >50% uncertainty because of the renormalization scale m

  10. Strategy • We have adjusted the heavy flavor production cross sections calculated by Herwig within the theoretical and experimental uncertainties to reproduce the rate of SECVTX and JPB tags observed in generic-jet data. • In that study we have used jets with with uncorrected ET>15 GeV and |h|<1.5; they correspond to partons with transverse energy approximately larger than 18 GeV • For partons with transverse energy larger than 18 GeV, s = 84 nb , s = 298 nb , and s = 487 nb (10% contamination) • we could have easily tuned the Herwig generator to explain in terms of SM processes an additional 10% pair production of scalar quarks: sf= 382 nb , and sf= 487 nb

  11. Strategy • What if there is a bsquark with a 100% semileptonic branching ratio • In b-quark decays, a lepton is produced in 37% of the cases • In c-quark decays, a lepton is produced in 21% of the cases • By requiring that at least one jet in the previous QCD data contains a lepton (inclusive low-pT lepton sample or generic-jet data with SLT tags), the production cross sections are s = 84 nb , s = 110 nb , and s = 102 nb (28% contamination) • By fitting a simulated pseudo-experiment which includes sbottom production with a conventional QCD simulation using rates of SECVTX and JPB tags, one might find sf= 194 nb , and sf= 102 nb

  12. Strategy • As a next step, we select events in which an additional jet contain a soft lepton (57% contamination) • If the data are due to bottom and charmed quark production only, we will find s = 93.4 nb (71.8 nb due to b-quarksand 21.6nb due to c-quarks) in the data and the simulation • If the data contain bs production, the cross section in the data will be s = 146.3 nb (40.7 nb due to b-quarks, 21.6nb due to c-quarks, and 84.0 nb due to scalar quarks) • In generic-jet data with SLT tags, one would finds = 244 nbwithout a scalar quark, or , with a scalar quark, s = 296 nb in the data ands = 244 nb in the simulation • Use generic-jet data to cancel out experimental uncertainties (efficiencies, fake removal,…)

  13. away jet lepton jet l Data sample • Events with 2 or more jets with ET > 15 GeV and at least two SVX tracks (taggable,|h|<1.5) • one electron with ET> 8 GeV or one muon with pT > 8 GeV/c contained in one of the jets • Require I > 0.1 • Reject conversions • Apply all lepton quality cuts used in the high-pT lepton sample • 68544 events with an electron jet and 14966 events with a muon jet

  14. Strategy • Perform a detailed comparison between data and simulation using SECVTX, and JPB tags on both the lepton- and away-jets • Differently from previous CDF analyses, this study checks at the same time the cross section for producing at least 1 b with |h|<1.5 (imperfect NLO calculation), 1 b +1 b with |h|<1.5 (apparently robust NLO calculation) • Then we check the semileptonic branching ratio of heavy flavor hadrons by counting the number of a-jets with SLT tags in the data and in the simulation

  15. Mistags and tagging efficiencies • PRD 64, 032002 (2001) • Mistags (tags in a jet without heavy flavor) are evaluated with parametrized probability functions derived in generic-jet data. We estimate a 10% uncertainty. • Since we use a parametrized simulation of the detector, we have measured the data-to simulation scale factor for the tagging efficiency of the SECVTX and JPB algorithms.These factors were determined with a 6% accuracy and implemented into the simulation. • The SLT simulation uses efficiencies for each selection cut measured using data; we estimate a 10% uncertainty, which includes the uncertainty on the semileptonic branching ratio • The SLT efficiency in supertags is corrected for the data-to-simulation scale factor measured in generic-jet data: (85±5)%

  16. Simulation • Use the Herwig generator program (option 1500, generic 2 2 hard scattering with pT > 13 GeV/c) • bb and cc production are generated through processes of order a2such as qq bb • Processes of order a3 are implemented through flavor excitation diagrams, such as g b g b, or gluon splitting, in which the process g g g g is followed by g bb • Use MRS (G) PDF’s • The bottom and charmed hadrons are decayed with QQ (version 9_1) • We select simulated events which contain hadrons with heavy flavor and at least one lepton with pT > 8 GeV/c • These events are passed through QFL, a parametrized simulation of the CDF detector and treated as real data • We have simulated 27156 electron events (98.9 pb-1) and 7267 muon events (55.1 pb-1) with heavy flavor

  17. away jet lepton jet l Evaluation of the heavy flavor content of the data • Before tagging, approximately 50% of the lepton jets do not contain heavy flavor; they are mostly due to fake leptons • Mistags in the lepton-jets and away jets are evaluated with a parametrized probability and removed • The fraction (1-hf) of events in which the l-jet does not contain heavy flavor is not simulated. In these events, away-jets can have tags due to heavy flavor. Their rates are estimated using a parametrized probability of finding a tag due to heavy flavor in generic-jet data. Using a sample of l-jets containing electrons due to identified conversions, we estimate a 10% accuracy. It is a slight overestimate.

  18. Fit of the simulation to the data • Use 6 fit parameters corresponding to the direct, flavor excitation and gluon splitting production cross sections evaluated by Herwig for b and c-quarks • Ke and Km account for the luminosity and b-direct production • The parameters bf, bg, c, cf, cg account for the remaining production cross sections, relative to the b-direct production • The ratio of b to c direct production constrained to the default value (about 1) within 14% • the ratio of b to c flavor excitation constrained to the default value (about 0.5) with a 28% uncertainty • bg constrained to (1.4±0.19) • cg constrained to (1.35±0.36) • The tagging efficiencies are also fit parameters, and are constrained to their measured values within their uncertainties (6% for b-quarks, 28% for c-quarks)

  19. Fit result • c2/DOF=4.6/9

  20. Fit result • Fhf = (45.3±1.9)% for electrons • Fhf = (59.7±3.6)% for muons

  21. Kinematics SECVTX tagged

  22. Kinematics SECVTX tagged

  23. Kinematics L -jet SECVTX tagged A-jet A-jet with SECVTX tags

  24. Kinematics L -jet SECVTX tagged

  25. Kinematics L -jet SECVTX tagged A-jet with SECVTX tags

  26. Comparison of a-jets with SLT tags in the data and the normalized simulation SEEN 1137±140.0 (±51.0 STAT.) EXPECTED 746.9±75.0 (SYST) SEEN 453±29.4 (±25 STAT.) EXPECTED 316.5±25.4 (SYST) (±15.8 SLT efficiency, ±20 fit)

  27. Systematics (away-jets with SLT tags) • In events due to heavy flavor, there is an excess of 391 a-jets with a SLT tag with respect to the simulation (1137.8 observed and 746.9 expected), having removed 619.3 fake tags [the events in which the l-jet does not have heavy flavor contain 901.9±91 a-jet with SLT tags (74% fake+ 26% heavy flavor): slight overestimate]. • If one could increase the fake rate in events with heavy flavor by 60%, the excess would disappear. However, in generic-jet data, the fake rate is already 74% of the SLT tagging rate. • Since fakes are approximately 74% of the SLT rate, the 10% uncertainty of the fake removal was evaluated by comparing observed rates of SLT tags to the parametrized prediction in all QCD samples. Most of the 10% comes from the fact that different QCD sample have slightly different heavy flavor purity

  28. Systematics (fake SLT tags) Data – simulated H.F. = 15783±423 fakes Parametrized SLT fakes 15570 • The heavy flavor content of generic-jet data has been evaluated using SECVTX and JPB tags • In generic-jet data the number of SLT tags due to heavy flavor is therefore known with a 13% error, mostly due to the 10% uncertainty of the SLT tagging efficiency • Therefore the real uncertainty on the fake rate is no larger than 2.6%

  29. Systematics (fake SLT tags) • Away-jets in the inclusive lepton have a higher heavy flavor content (26%) than generic-jet data (13%) . • Could the fake rate in jets with heavy flavor be anomalously large ? Could the SLT efficiency or the semileptonic branching ratio in the simulation be grossly wrong ? • Jets with SECVTX or JPB tags in generic-jet data have a heavy flavor content ranging from 86%(JET 20) to 71% (JET 100) . The rate of SLT tags in these jets is not higher than in the simulation • This does not support simulation deficiencies as an explanation for the discrepancy h.f . fakes low pT (SLT) 1138 619 SECVTX+SLT 944 507 JPB+SLT 1167 856

  30. Conclusions • We have measured the heavy flavor content of the low pT inclusive lepton sample by comparing rates of SECVTX and JPB tags in the data and the simulation • We find good agreement between the data and the simulation tuned within the experimental and theoretical uncertainties • We find a 50% excess of a-jets with SLT tags due to heavy flavor with respect to the simulation; the discrepancy is a 3.5 s systematic effect due to the uncertainty of the SLT efficiency and background subtraction. However, comparisons of analogous tagging rates in generic-jet data and their simulation do not support any increase of the efficiency or background subtraction beyond the quoted systematic uncertainties

  31. Conclusions • A discrepancy of this kind and size is expected, and was the motivation for this study, if pairs of light scalar quarks with a 100% semileptonic branching ratio were produced at the Tevatron • The data cannot exclude alternate explanations for this discrepancy • Previously published measurements support the possibility, born out of the present work, that approximately 50% of the presumed semileptonic decays of heavy flavor hadrons produced at the Tevatron are due to unconventional sources

  32. correlated m+b-jet cross section sbb•BR • PRD 53, 1051 (1996) • 90% of the cross section at j>2 • Data are 1.5 times larger than the NLO calculation • The NLO cross section is not very sensitive to the scale m • The NLO value is approximately equal to the Born value

  33. bb correlations (dimuons) sbb•BR2 • PRD 55, 2547 (1997) • Data are 2.2 times larger than the NLO calculation • D0 has a similar result • The dependence of the NLO prediction on the scale m is less than 20% • Born and NLO values are within a few percents

  34. 2Study of sequential semileptonic decays of b-hadrons produced at the Tevatron • Given the previous anomaly, it is of interest to study sequential semileptonic decays of hadrons with heavy flavor • We use the same data set and simulation of the previous analysis

  35. Comparison of data and simulation • Data: 1447±65 dileptons (±44 stat. and ±48 syst.) • Sim.: 1181±129 dileptons (±51 fit and sim. stat., ±118 SLT eff.) • 2 s effect

  36. Dilepton kinematics

  37. Dilepton kinematics

  38. Systematics (fake dileptons) • “Misidentified leptons are expected to be present in equal amount in OS and SS dileptons” [PRD 49].The technique of subtracting SS from OS dileptons to remove this background has been used many times in CDF [PRD 49 (1994), PRD 59 (1999)]. • In contrast with previous analyses which used this technique, we now find a discrepancy between the data and the simulation prediction. We want to investigate further. Use the electron sample only (larger statistics)

  39. Fake dileptons • The heavy flavor simulation of the inclusive electron sample, after normalization, contain 955 ± 108 OS and 63 ± 9 SS dileptons • In the data, there are 1450 OS and 399 SS dileptons • To get rid of this discrepancy, one would like to explain in terms of background 495 ± 114 OS and 276 ± 20 SS dileptons (R=1.79±0.43) • On average, a jet contain the same number of positive and negative lepton-candidate tracks. • When searching for a second lepton in a jet in which one track has been already identified with an electron, one expects the number of OS candidates to be larger than the number of SS candidates [Nc(OS)=54938 and Nc(SS)=34744] (R=1.58) • If this is correct, there is no dilepton excess, but a number of previous analyses are wrong

  40. Dilepton background (conclusions) • Applying the parametrized fake probability to the Nc(OS) candidates one gets OS=302 ± 30 [used in PRD 60, (1999)] • However, the standard fake parametrization derived from generic-jet data might not be adequate to describe the fake rate in jets which already contain a lepton with pT> 8 GeV/c • Use generic-jet data to derive a fake probability for jets which already contain a lepton

  41. Dileptons (generic-jet data)

  42. Dilepton background (conclusions) • When applying P(OS) and P(SS) of generic-jet data to the candidate tracks, one predicts 236 ± 44 OS and 229 ± 14 SS fake dileptons • Note that b-hadron mixing is not simulated; when using c =0.118, the simulation predicts 915 ± 105 OS and 103 ± 15 SS dileptons due to heavy flavor; the difference between the data and the heavy flavor expectation is 535 ± 111 OS and 236 ± 24 SS dileptons • P(SS) of generic-jet data provides the correct prediction of SS dileptons • A background of 276 events with a 10% accuracy seems to be a fair estimate of the fake OS dileptons

  43. Sequential b-decays • According to the simulation, most of the dileptons in the same jet are contributed by semileptonic cascade decays of b-hadrons. They are simulated with the QQ Monte Carlo program. How good is it ? • Help from DELPHI (2000-060 PHYS 861, 2000) • DELPHI compares 573474 hadronic Z-decays to a JETSET 7.3 simulation consisting of 992988 events normalized to the same luminosity of the data • Use events with |cos qT|<0.95, where T is the thrust axis • Select events with two leptons with pT > 3 GeV/c and |h |<1 in the same emisphere as defined by the thrust axis. Divide events in OS and SS • First show agreement between DELPHI data and simulation. Then, show agreement between the DELPHI and our simulations at the Z-pole

  44. DELPHI

  45. Sequential b-decays DELPHI simulation: comparison between OS-SS dileptons and sequential b-decays identified at generator level Comparison between our and DELPHI simulation at the Z-pole

  46. ~ b bb 3Ratio of like-sign to opposite-sign dileptons • Recent publications by Ed Berger et al. explore an explanation within the context of MSSM to the discrepancy between the measured cross section and the NLO prediction • Hep-ph/0103145 postulates the existence of a light gluino which decay to b and • The pair production of gluinos provides an inclusive b-cross section comparable to conventional QCD production • However, pair production of gluinos and subsequent decays to b quarks also leads to an increase of like-sign dileptons

  47. The like-sign dilepton increase could be confused with an enhanced B0-B0 mixing and in a value of c, the time-integrated mixing parameter, larger than the world average 0.118±0.005 • Hep-ph/0103145 predicts c  0.17 , and, after comparing to the CDF Run 1A result of 0.131±0.026, concludes that a better measurement is needed in Run II • Other results: UA1 measured c = 0.1570.038 and CDF (Run 0) c = 0.1760.043 • We repeat the Run 1A analysis using also Run 1B dimuons and e-m

  48. Method of analysis • Use the same method of RUN 1A: fit the impact parameter distributions with the expected distributions for various sources • The main sources of dileptons are semileptonic decays of charmed and bottom mesons, and prompt decays of onia and Drell-Yan production • Impact parameter distributions for b and c quarks are derived from a simulation which uses Herwig (option 1500), QQ version 9_1, and QFL’

  49. Prompt shape • The prompt shape is derived using muons from U(1S) decays (9.28< Mmm <9.6 GeV) • The background is removed using dimuons with 9.04< Mmm <9.2 and 9.54< Mmm <9.7 GeV

  50. Shapes • Ratio of numbers of leptons with d<0.008 to that with d>0.008 cm • b quarks 0.85 • c quarks 2.03 • prompt 32.3