SLD Experiment Research Articles

Precision polarimetry at the ILC: Concepts, simulations and experiments

The precision physics program of the ILC requires precise knowledge of the state of beam polarisation. In fact the Compton polarimeters intended for the ILC will have to measure the polarisation with error a factor of 2 smaller than the previous best measurement at the SLAC SLD experiment. In order to further reduce measurement error, spin tracking simulations in the ILC Beam Delivery System subject to ground motion induced misalignment have been performed and the expected variation in polarisation has been quantified. A prototype of a high precision spectrometer to record Compton scattered electrons from the interaction of a longitudinal laser and the charged beams has been developed. The Compton electrons interact with a gas in the polarimeter channels to produce Cherenkov radiation measured by photodetectors. The calibration of the photodetectors is crucial and exhaustive bench tests of the photodetector linearity have been performed. The polarimeter prototype itself will be tested at the ELSA testbeam in Bonn in Spring 2009.

CCD-based vertex detectors

Over the past 20 years, CCD-based vertex detectors have been used to construct some of the most precise ‘tracking microscopes’ in particle physics. They were initially used by the ACCMOR collaboration for fixed target experiments in CERN, where they enabled the lifetimes of some of the shortest-lived charm particles to be measured precisely. The migration to collider experiments was accomplished in the SLD experiment, where the original 120 Mpixel detector was later upgraded to one with 307 Mpixels. This detector was used in a range of physics studies which exceeded the capability of the LEP detectors, including the most precise limit to date on the B s mixing parameter. This success, and the high background hit densities that will inevitably be encountered at the future TeV-scale linear collider, have established the need for a silicon pixel-based vertex detector at this machine. The technical options have now been broadened to include a wide range of possible silicon imaging technologies as well as CCDs (monolithic and hybrid silicon pixel devices, DEPFET-based and SOI-based devices). However, there is a good chance that CCD-based detectors, or an architecture derived from CCDs, will still prove to be superior for this application. Groups in Europe, Asia and the USA are working semi-independently on various aspects of this development, with the goal of evaluating prototype detector elements within the next 5 years. If the CCD option is selected for one of the LC detector systems, it is hoped that these groups will join forces to construct the new detector. If the design goals can be achieved, this vertex detector will provide a tool not only for b and c tagging, but also for the measurement of ‘vertex charge’, allowing discrimination between b and b ¯ jets, and between c and c ¯ jets. Given the complex topological nature of much of the potential new physics in the TeV regime (multiple hadronic jets), such a tool could provide the key to unravel novel processes which may be unintelligible at the LHC.

Fragmentation in [formula omitted] processes

I study bottom quark fragmentation in the Standard Model Higgs decay H → b b ¯ , within the framework of perturbative fragmentation functions. I resum large collinear logarithms ln ( m H 2 / m b 2 ) in the next-to-leading logarithmic (NLL) approximation, using the DGLAP evolution equations. Soft contributions to the MS ¯ coefficient function and to the initial condition of the perturbative fragmentation function are resummed to NLL accuracy as well. The implementation of collinear and soft resummation has a relevant impact on the b energy spectrum, which exhibits a milder dependence on factorization and renormalization scales and on the Higgs mass. I present some predictions on the energy distribution of b-flavoured hadrons in Higgs decay, making use of data from LEP and SLD experiments to fit a few hadronization models. I also compare the phenomenological results yielded by a few processes recently provided with NLL collinear and soft resummations.

Identified hadron production at SLD and BABAR

We present measurements of identified hadron spectra in e{sup +}e{sup -} hadrons from the BABAR and SLD experiments at respective c.m. energies of 10.54 GeV and 91.2 GeV. The scaling violations between the two energies are found to be large and described by hadronization models for charged pions, but not for heavier hadrons. SLD measurements show large differences between b{bar b}, c{bar c} and light-flavor events that are qualitatively consistent with expectations. The light flavor events are somewhat more consistent with the predictions of MLLA QCD than are events of all flavors, but a discrepancy in the mass dependence remains. SLD have made several new and/or improved measurements of light leading particle production, paving the way for future tagging of u, d and s jets. They have also made a unique study of signed rapidity correlations, showing direct evidence for charge ordering of protons along the quark-antiquark axis.

First Measurement of the Double-inclusive $B/\overline{B}$ Hadron Energy Distribution in e+e– Annihilations, and of Angle-Dependent Moments of the B and $\overline{B}$

We have made the first measurement of the double-inclusive B/ $\overline{B}$ energy distribution in e+e– annihilations, using a sample of 400,000 hadronic Z0-decay events recorded in the SLD experiment at SLAC between 1996 and 1998. The small and stable SLC beam spot and the CCD-based vertex detector were used to reconstruct B/ $\overline{B}$ -decay vertices with high efficiency and purity, and to provide precise measurements of the kinematic quantities used to calculate the B energies in this novel technique. We measured the B/ $\overline{B}$ energies with good efficiency and resolution over the full kinematic range. We measured moments of the scaled energies of the B and $\overline{B}$ hadrons vs. the opening angle between them. By comparing these results with perturbative QCD predictions we tested the ansatz of factorisation in heavy-quark production. A recent next-to-leading order calculation reproduces the data.

First measurement of the double-inclusive [formula omitted] hadron energy distribution in e+e− annihilations, and of angle-dependent moments of the B and [formula omitted] energies

We have made the first measurement of the double-inclusive B/B energy distribution in e+e− annihilations, using a sample of 400000 hadronic Z0-decay events recorded in the SLD experiment at SLAC between 1996 and 1998. The small and stable SLC beam spot and the CCD-based vertex detector were used to reconstruct B/B-decay vertices with high efficiency and purity, and to provide precise measurements of the kinematic quantities used to calculate the B energies in this novel technique. We measured the B/B energies with good efficiency and resolution over the full kinematic range. We measured moments of the scaled energies of the B and B hadrons vs. the opening angle between them. By comparing these results with perturbative QCD predictions we tested the ansatz of factorisation in heavy-quark production. A recent next-to-leading order calculation reproduces the data.

Search for [formula omitted] oscillations at SLD

We present preliminary results on the time dependence of B s 0− B s 0 and B d 0− B d 0 mixing using a sample of 400, 000 hadronic Z 0 decay collected by the SLD experiment at the SLC. The analyses take advantage of the excellent decay length resolution of the CCD vertex detector. The B or B production flavor is determined by exploiting the large forward-backward asymmetry of polarized Z 0 → b b decays. This flavor tag is enhanced by incorporating additional information from the hemisphere opposite that of the reconstructed B decay. The B d 0- B d 0 mixing analysis uses the charge of high momentum kaons to identify the B or B decay flavor. The result is Δm d = 0.503 ± 0.028(stat) ± 0.020(syst) ps −1. The results of three separate B s 0- B s 0 mixing analyses are presented. These analyses determine the B decay flavor using the lepton charge in B semileptonic decays, the charge of fully reconstructed D s decays, and the charge difference between topologically reconstructed B and D decays. The three analyses are combined to set a lower limit on the B s 0 − B s 0 oscillation frequency of 12.0 ps −1 at 95% CL.

Search for time-dependentBs0−Bs0¯oscillations using a vertex charge dipole technique

We report a search for B{sub s}{sup 0} - {bar B}{sub s}{sup 0} oscillations using a sample of 400,000 hadronic Z{sup 0} decays collected by the SLD experiment. The analysis takes advantage of the electron beam polarization as well as information from the hemisphere opposite that of the reconstructed B decay to tag the B production flavor. The excellent resolution provided by the pixel CCD vertex detector is exploited to cleanly reconstruct both B and cascade D decay vertices, and tag the B decay flavor from the charge difference between them. We exclude the following values of the B{sub s}{sup 0} - {bar B}{sub s}{sup 0} oscillation frequency: {Delta}m{sub s} < 4.9 ps{sup -1} and 7.8 < {Delta}m{sub s} < 10.3 ps{sup -1} at the 95% confidence level.

Erratum: Measurement of theb-quark fragmentation function inZ0decays [Phys. Rev. D65, 092006 (2002)

We present a measurement of the b-quark inclusive fragmentation function in Z0 decays using a novel kinematic B-hadron energy reconstruction technique. The measurement was performed using 350,000 hadronic Z0 events recorded in the SLD experiment at SLAC between 1997 and 1998. We compared the sacled B-hadron energy distribution with models of b-quark fragmentation and with several ad hoc functional forms. A number of models and functions are excluded by the data. The average scaled energy of weakly-decaying B hadrons was measured to be <x_B>= 0.709 +-0.003 (stat) +-0.003 (syst) +-0.002 (model).

Improved study of the structure ofe+e−→bb¯gevents and limits on the anomalous chromomagnetic coupling of thebquark

The structure of three-jet e{sup +}e{sup -} {yields} b{bar b}g events has been studied using hadronic Z{sup 0} decays recorded in the SLD experiment at SLAC. Three-jet final states were selected and the CCD-based vertex detector was used to identify two of the jets as b or {bar b}; the remaining jet in each event was tagged as the gluon jet. Distributions of the gluon energy and polar angle with respect to the electron beam were measured over the full kinematic range, and used to test the predictions of perturbative QCD. We find that beyond-leading-order QCD calculations are needed to reproduce the features seen in the data. The energy distribution is sensitive to an anomalous b chromomagnetic moment {kappa} at the b{bar b}g vertex. We measured {kappa} to be consistent with zero and set 95% C.L. limits on its value, -0.06 < {kappa} < 0.04.

Search for time-dependentBs0−Bs0¯oscillations using exclusively reconstructedDs±mesons

A search for Bs0-Bs0bar oscillations is performed using a sample of 400,000 hadronic Z0 decays collected by the SLD experiment. The Bs0 candidates are reconstructed in the Bs0 to Ds-,X channel with Ds- to phi,pi- or K*0K-. The Bs0 production flavor is determined using the large forward-backward asymmetry of polarized Z0 to b,bbar decays and charge information in the hemisphere opposite that of the Bs0 candidate. The decay flavor is tagged by the charge of the Ds+/-. From a sample of 361 candidates with an average Bs0 purity of 40%, we exclude the following values of the oscillation frequency: Delta m_s < 1.4 ps^{-1} and 2.4 < Delta m_s < 5.3 ps^{-1} at the 95% confidence level.

Internal alignment of the SLD vertex detector using a matrix singular value decomposition technique

The tracking resolution and vertex finding capabilities of the SLD experiment depend upon a precise knowledge of the location and orientation of the 96 elements of the SLD pixel vertex detector (VXD3) in 3D space. At the heart of the deterministic procedure described here to align the 96 CCDs is the matrix inversion technique of singular value decomposition (SVD). This tool is employed to unfold the detector geometry corrections from the track hit residual data in the VXD3. The algorithm is adapted to perform an optimal χ 2 minimization by careful treatment of the errors and correlations in the residual measurements. The general form of the problems that might be solved with this technique is discussed. The tracking resolution obtained with the aligned geometry is compared with the starting point, based on an optical survey of the CCDs, and is shown to achieve the design performance.

Electroweak asymmetries from SLD

Abstract We present a summary of the results on electroweak asymmetries performed by the SLD experiment at the Stanford Linear Collider (SLC). Most of these results are final and are based, unless otherwise stated, on the full 1993–1998 data set of approximately 550,000 hadronic decays of Z 0 bosons, produced with an average electron beam polarization of 73%.

Electroweak asymmetries from SLD

We present a summary of the results on electroweak asymmetries performed by the SLD experiment at the Stanford Linear Collider (SLC). Most of these results are final and are based, unless otherwise stated, on the full 1993–1998 data set of approximately 550,000 hadronic decays of Z0 bosons, produced with an average electron beam polarization of 73%.

Measurement of theb-quark fragmentation function inZ0decays

We present a measurement of the b-quark inclusive fragmentation function in ${Z}^{0}$ decays using a novel kinematic B-hadron energy reconstruction technique. The measurement was performed using 350 000 hadronic ${Z}^{0}$ events recorded in the SLD experiment at SLAC between 1997 and 1998. The small and stable SLC beam spot and the charge-coupled-device--based vertex detector were used to reconstruct B-decay vertices with high efficiency and purity, and to provide precise measurements of the kinematic quantities used in this technique. We measured the B energy with good efficiency and resolution over the full kinematic range. We compared the scaled B-hadron energy distribution with models of b-quark fragmentation and with several ad hoc functional forms. A number of models and functions are excluded by the data. The average scaled energy of weakly decaying B hadrons was measured to be $〈{x}_{b}〉=0.709\ifmmode\pm\else\textpm\fi{}0.003(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.003(\mathrm{syst})\ifmmode\pm\else\textpm\fi{}0.002(\mathrm{model}).$

Beautiful mirrors and precision electroweak data

The standard model (SM) with a light Higgs boson provides a very good description of the precision electroweak observable data coming from the CERN LEP, SLD and Fermilab Tevatron experiments. Most of the observables, with the notable exception of the forward-backward asymmetry of the bottom quark, point towards a Higgs boson mass far below its current experimental bound. The disagreement, within the SM, between the values for the weak mixing angle as obtained from the measurement of the leptonic and hadronic asymmetries at lepton colliders, may be taken to indicate new physics contributions to the precision electroweak observables. In this article we investigate the possibility that the inclusion of additional bottomlike quarks could help resolve this discrepancy. Two inequivalent assignments for these new quarks are analyzed. The resultant fits to the electroweak data show a significant improvement when compared to that obtained in the SM. While in one of the examples analyzed the exotic quarks are predicted to be light, with masses below 300 GeV, and the Higgs boson tends to be heavy, in the second one the Higgs boson is predicted to be light, with a mass below 250 GeV, while the quarks tend to be heavy, with masses of about 800 GeV. The collider signatures associated with the new exotic quarks, as well as the question of unification of couplings within these models and a possible cosmological implication of the new physical degrees of freedom at the weak scale are also discussed.

A fast CCD vertex detector for the future linear collider: some recent developments

Experiments at a future e +e − linear collider will require a high-performance vertex detector to identify heavy quarks in multi-jet final states. The SLD experiment at SLAC has successfully demonstrated that charge-coupled devices (CCDs) are ideal candidates for this role, offering excellent two-dimensional spatial resolution with extremely low mass.This paper discusses recent R&D studies aimed at further reducing the vertex detector material budget and significantly increasing the CCD readout speed, in order to extend the physics reach of the future linear collider. A radical new CCD architecture is described, which would enable a CCD-based vertex detector to be used in the extremely challenging environment of the superconducting TESLA collider.

A Preliminary Study of the Structure of $b\bar bg$ Events using Z0 Decays

The structure of three-jet [Formula: see text] events has been studied using hadronic Z0 decays recorded in the SLD experiment at SLAC. Three-jet final states were selected and the CCD-based vertex detector was used to identify two of the jets as [Formula: see text]. The remaining jet in each event was tagged as a gluon jet, and distributions of the gluon energy and polar angle with respect to the electron beam were examined and were compared with perturbative QCD predictions. These distributions are potentially sensitive to an anomalous b chromomagnetic moment κ. We compare our data with theoretical predictions including such a moment and discuss limits on the value of κ.

Z-QUARK COUPLING MEASUREMENTS AT SLD

The SLD experiment at SLAC has performed several precise tests of the electroweak Standard Model. This paper summarizes the measurements of Z couplings to quarks. Most of these measurements are preliminary and many incorporate the full SLD dataset of 550,000 polarized Z decays.

SLD's Final Leptonic Asymmetry Parameter Measurements

We report the SLD experiment's final measurements of leptonic asymmetry parameters from which [Formula: see text] at the Z0 pole is accurately determined. Based on these measurements, the Higgs boson mass is estimated to be less than 147 GeV/c 2 at 95% confidence level within the context of the standard model.