1 / 88

Analisi di dati del Run I

Analisi di dati del Run I. C ongregazione per la D ottrina della F ede. dal top al bottom con un pizzico di charm. P. Giromini. 5 analisi completate fra il 1999 ed il 2002 Usano campioni di dati diversi, ma hanno un filo logico comune

joshua-mercer
Download Presentation

Analisi di dati del Run I

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. 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

More Related