Lepton Tagging Research Articles

Outcome of the KLOE-2 experiment after the conclusion of the data-taking period

The KLOE-2 experiment at the INFN-LNF has concluded the data taking campaign at DA Φ NE collider with more than 5 fb −1 of integrated luminosity collected. The KLOE detector received several upgrades to improve the performance and enlarge the physics reach: two new low angle calorimeters , a lepton tagging system for γ γ physics, and near the interaction point a state-of-the-art cylindrical triple-GEM detector has been installed as Inner Tracker. Performances of the detectors, preliminary results obtained including the Inner Tracker in the reconstruction and data quality monitoring are here discussed. • KLOE-2 first HEP experiment that build/operated ultra-light/compact tracking systems. • The biggest transparent Drift Chamber in 5.2 kGauss field of a superconductor coil. • Carbon fiber walls, 55000 stereo wires, 2 m radius, 4 m long, He/iC 4 H 10 gas mixture. • First multi-layer cylindrical triple-GEM detector for fine vertex reconstruction. • Total material budget below 2% of the radiation length X 0 .

Measurement of the DirectCPAsymmetry inB¯→Xs+dγDecays with a Lepton Tag

We report the measurement of the direct CP asymmetry in the radiative B¯→X(s+dγ) decay using a data sample of (772 ± 11)×10(6) BB[over ¯] pairs collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. The CP asymmetry is measured as a function of the photon energy threshold. For Eγ*≥2.1 GeV, where Eγ* is the photon energy in the center-of-mass frame, we obtain A(CP)(B¯→X(s+dγ))=(2.2±3.9±0.9)%, consistent with the standard model prediction.

Erratum: Measurement of thett¯production cross section inpp¯collisions ats=1.8TeV[Phys. Rev. D64, 032002 (2001)

We update the measurement of the top production cross section using the CDF detector at the Fermilab Tevatron. This measurement uses $t\bar{t}$ decays to the final states $e+\nu$+jets and $\mu+\nu$+jets. We search for $b$ quarks from $t$ decays via secondary-vertex identification or the identification of semileptonic decays of the $b$ and cascade $c$ quarks. The background to the $t\bar{t}$ production is determined primarily through a Monte Carlo simulation. However, we calibrate the simulation and evaluate its uncertainty using several independent data samples. For a top mass of 175 $GeV/c^2$, we measure $\sigma_{t\bar{t}}=5.1 \pm 1.5$ pb and $\sigma_{t\bar{t}}=9.2 \pm 4.3$ pb using the secondary vertex and the lepton tagging algorithms, respectively. Finally, we combine these results with those from other $t\bar{t}$ decay channels and obtain $\sigma_{t\bar{t}} = 6.5^{+1.7}_{-1.4}$ pb.

Measurement of the inclusive semileptonic branching fraction of B mesons and |Vcb|

We present a measurement of the electron spectrum from inclusive semileptonic B decay, using 5.1 fb−1 of ϒ(4S) data collected with the Belle detector. A high-momentum lepton tag was used to separate the semileptonic B decay electrons from secondary decay electrons. We obtained the branching fraction, B(B→Xe+ν)=(10.90±0.12±0.49)%, with minimal model dependence. From this measurement, we derive a value for the Cabibbo–Kobayashi–Maskawa matrix element |Vcb|=0.0408±0.0010(exp)±0.0025(th).

Measurement of the b quark forward–backward asymmetry around the Z0 peak using an inclusive tag

The b quark forward–backward asymmetry has been measured using hadronic Z0 decays collected by the OPAL experiment at LEP. Z0→bb̄ decays were selected using a combination of secondary vertex and lepton tags, and the sign of the b quark charge was determined using an inclusive tag based on jet, vertex and kaon charges. The results, corrected to the quark level, are: AFBb=0.0582±0.0153±0.0012ats=89.50GeV,AFBb=0.0977±0.0036±0.0018ats=91.26GeV,AFBb=0.1221±0.0123±0.0025ats=92.91 GeV, where the first error is statistical and the second systematic in each case. Within the framework of the Standard Model, the result is interpreted as a measurement of the effective weak mixing angle for electrons of sin2θeff,eW=0.23205±0.00068.

Measurement of thett¯production cross section inpp¯collisions ats=1.8TeV

We update the measurement of the t{bar t} production cross section using the CDF detector at the Fermilab Tevatron. This measurement use t{bar t} decays to the final states e+v+jets and {mu}+v+jets. We search for b quarks from t decays via secondary-vertex identification or the identification of semileptonic decayse of the b and cascade c quarks. The background to the t{bar t} production is determined primarily through a Monte Carlo simulation. However, we calibrate the simulation and evaluate its uncertainty using several independent data samples. For a top quark mass of 175 GeV/c{sup 2}, we measure {sigma}{sub t{bar t}}=5.1{+-}1.5 pb and {sigma}{sub t{bar t}}=9.2{+-}4.3 pb using the secondary vertex and the lepton tagging algorithms, respectively. Finally, we combine these results with those from other t{bar t} decay channels and obtain {sigma}{sub t{bar t}}=6.5{sub -1.4}{sup +1.7} pb.

Measurement of $R_\mathrm{b}$ and $\mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X})$ at LEP using double-tag methods

We present a combined measurement of $\Rb = \Gamma(\mathrm{Z \rightarrow b\overline{b}}) / \Gamma(\mathrm{Z} \rightarrow\mbox{hadro ns})$ and the semileptonic branching ratio of b quarks in Z decays, $\Brbl$, using double-tag methods. Two analyses are performed on one million hadronic Z decays collected in 1994 and 1995. The first analysis exploits the capabilities of the silicon microvertex detector. The tagging of b-events is based on the large impact parameter of tracks from weak b-decays with respect to the $\mathrm{e^+e^-}$ collision point. In the second analysis, a high-$p_t$ lepton tag is used to enhance the b-component in the sample and its momentum spectrum is used to constrain the model dependent uncertainties in the semileptonic b-decay. The analyses are combined in order to provide precise determinations of $\Rb$ and $\Brbl$: Rb = 0.2174 \pm 0.0015\mbox{(stat.)} \pm 0.0028\mbox{(sys.)}; $\Brbl = (10.16 \pm 0.13\mbox{(stat.)} \pm 0.30\mbox{(sys.)})\%.

Determination of the HQET parameterλ1from an inclusive semileptonicBmeson decay spectrum

We estimate the heavy quark effective theory parameter ${\ensuremath{\lambda}}_{1}$ from the inclusive semileptonic B-meson decay spectrum. By using recent CLEO double lepton tagging data of $\stackrel{\ensuremath{\rightarrow}}{B}\mathrm{Xe}\ensuremath{\nu}$, which show the lepton momentum as low as 0.6 GeV, we extracted ${\ensuremath{\lambda}}_{1}\ensuremath{\sim}\ensuremath{-}0.58 {\mathrm{GeV}}^{2}$. We also derived $\overline{\ensuremath{\Lambda}}\ensuremath{\sim}0.46 \mathrm{GeV}$ and $|{V}_{\mathrm{cb}}|=0.041\ifmmode\pm\else\textpm\fi{}0.002$.

FastCPviolation

$B$ flavor tagging will be extensively studied at the asymmetric $B$ factories due to its importance in CP asymmetry measurements. The primary tagging modes are the semileptonic decays of the $b$ (lepton tag), or the hadronic $b \to c (\to s)$ decays (kaon tag). We suggest that looking for time dependent CP asymmetries in events where one $B$ is tagged leptonically and the other one is tagged with a kaon could result in an early detection of CP violation. Although in the Standard Model these asymmetries are expected to be small, $\sim 1%$, they could be measured with about the same amount of data as in the ``gold-plated'' decay $B_d \to \psi K_S$. In the presence of physics beyond the Standard Model, these asymmetries could be as large as $\sim 5%$, and the first CP violation signal in the $B$ system may show up in these events. We give explicit examples of new physics scenarios where this occurs.

Measurement of the charged multiplicity of bottom and light quark events in e+e− annihilation at [formula omitted] GeV

The charged-particle multiplicity is measured for b-quark and light-quark ( u, d, s) events in e + e − annihilation at s =58 GeV. Two independent methods, lepton tagging and decay-length tagging, are applied to enrich b-quark events. These methods provide event samples having different concentrations of b-quark events. The mean charged multiplicity is measured to be 19.38±0.27±0.84 for b-quark events, and 15.91±0.04±0.70 for light-quark events. The results are compared with predictions of phenomenological and theoretical models.

Semileptonic branching fraction and hadronic width of the B mesons in light of the new CLEO observation of b → D¯X

This talk reports the latest CLEO observation of D production in b decays, which provides new insite of the QCD interactions in heavy quark systems. The observed B semileptonic branching fraction has been significantly lower than QCD expectations, as was confirmed by recent lepton-lepton correclation studies at various experiments that removed most of the model dependence in previous generation measurements. One possible explanation to accommodate this lepton shortage or a wider hadronic width of B mesons is that the process b → cc¯s contributes at a higher rate than expected. That explanation, however, predicts more charm production in than is observed. Using lepton tags in B events this analysis measures the fraction of b → DX in addition to b → DX . The first observation of B → DDX has profound implications to both experiment and theory.

A review ofRb at LEP

In the Standard Model, the ratioR b of the Z partial width into b $$\bar b$$ to its total width into hadrons is sensitive to the mass of the top quark but largely independent of the Higgs mass and the strong-coupling constant. A measurement with a relative precision below one percent would be a landmark test of the predictive powers of the Standard Model. The four LEP Collaborations have presented a series of increasingly precise analyses, using lepton tagging, event shape tagging and lifetime tagging, improving the methods as the data sets have grown in size. The current world average of 0.2205±0.0016, dominated by results from lifetime tagging, is about three standard deviations above the Standard Model value, assuming a top quark mass around 175 GeV/c2. Further improvements in precision are expected.

Measurement of the B semileptonic branching fraction with lepton tags.

We have used the CLEO II detector and 2.06fb^(-1) of ϒ(4S) data to measure the B-meson semileptonic branching fraction. The B→Xeν momentum spectrum was obtained over nearly the full momentum range by using charge and kinematic correlations in events with a high-momentum lepton tag and an additional electron. We find B(B→Xeν) = (10.49±0.17±0.43)%, with overall systematic uncertainties less than those of untagged single-lepton measurements. We use this result to calculate the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element V_(cb) and to set an upper limit on the fraction of ϒ(4S) decays to final states other than BB .

Test of the flavour independence of αs

Using about 950000 hadronic events collected during 1991 and 1992 with the ALEPH detector, the ratios r b = α s b α s udsc and r uds = α s uds α s cb have been measured in order to test the flavour independence of the strong coupling constant α s . The analysis is based on event-shape variables using the full hadronic sample, two b-quark samples enriched by lepton tagging and lifetime tagging, and a light-quark sample enriched by lifetime antitagging. The combined results are r b = 1.002±0.023 and r uds = 0.971 ± 0.023.

A model-independent determination of the inclusive semileptonic decay fraction of B meson

Abstract With the ARGUS detector at the e+ e− storage ring DORIS II, we have determined decay fraction and electron momentum spectrum of the inclusive decay mode B → ev X. Using lepton tags from the second B meson in 209000 Γ(4S)→B decays, we could determine the momentum spectrum for all momenta Pe > 0.6 GeV/c. Including the small extrapolation to Pe > 0, we find the model-independent decay fraction Br(B→evX) = (9.6 ± 0.5 ± 0.4)%. Adding D meson tags, our result is (9.7 ± 0.5 ± 0.4)%.

A model-independent determination of the inclusive semileptonic decay fraction of B mesons

With the ARGUS detector at the e +e - storage ring DORIS II, we have determined decay fraction and electron momentum spectrum of the inclusive decay mode B → eνX. Usinng lepton tags from the second B meson. in 209 000 γ(4 S) → B B decays, we could determine the spectrum for all electron momenta p e > 0.6 GeV/ c . Including the small extrapolation to p e > 0, we find the model-independent decay fraction B(B → eνX) = (9.6 ± 0.5 ± 0.4)%. Adding D meson tags, our result is (9.7 ± 0.5 ± 0.4)%.

A direct observation of quark-gluon jet differences at LEP

Quark and gluon jets in e +e − three-jet events at LEP are identified using lepton tagging of quark jets, through observation of semi-leptonic charm and bottom quark decays. Events with a symmetry under transposition of the energies and directions of a quark and gluon jet are selected: these quark and gluon jets have essentially the same energy and event environment and as a consequence their properties can be compared directly. The energy of the jets which are studied is about 24.5 GeV. In the cores of the jets, gluon jets are found to yield a softer particle energy spectrum than quark jets. Gluon jets are observed to be broader than quark jets, as seen from the shape of their particle momentum spectra both in and out of the three-jet event plane. The greater width of gluon jets relative to quark jets is also visible from the shapes of their multiplicity distributions. Little difference is observed, however, between the mean value of particle multiplicity for the two jet types.

A study of [formula omitted] in Z 0 decays

In this paper an investigation of the production of D ∗ ± mesons produced in e +e − collisions at energies around the Z 0 pole is presented. Based on 115 D ∗ ± mesons with x D∗  2E D∗ /E cm > 0.2 the properties of D ∗ mesons produced in the reaction Z 0 → c c are studied. Fixing the yield and the fragmentation function of bottom quarks to the values obtained at LEP using lepton tags, and average energy fraction of the D ∗ ± mesons from primary charmed quarks of 〈x c → D ∗〉 = 0.52 ± 0.03 +- 0.01 is found and Γ z 0→c c = (323 ± 61 ± 35) MeV is determined. The first error is the combined statistical and systematic error from this experiment, and the second the total error from other sources.

CP violation at the Z o with polarized beams

The forward-backward asymmetry in e +e − annihilation induced by neutral currents allows an effective particle-antiparticle separation. This is used to define a simple CP violating observable. At the Z o resonance, where th production rates are high, polarized electrons may increase the forward-backward asymmetries by up to a factor six. The number of events necessary to establish a CP-violating effect with neutral B mesons is reduced by about an order of magnitude compared with the usual lepton tag method.