3,225 research outputs found

### Hadronic B Decays to Charmless Final States and to J/psi K*

Preliminary results from the BABAR experiment on charmless B decays to
charged pions or kaons, and the measurement of the B -> J/psi K* decay
amplitudes are presented. The data sample, collected at the asymmetric-energy
B-factory PEP-II at SLAC, comprises a total number of 22.7 million Y(4S)
decays, corresponding to an integrated on-resonance luminosity of approximately
21 fb-1. We measure the following CP-averaged branching fractions:
BR(B -> pi+pi-) = (4.1 +- 1.0(stat) +- 0.7(sys))xE-6 BR(B -> K+pi-) = (16.7
+- 1.6(stat) +1.2-1.7(sys))xE-6
and an upper limit of BR(B -> K+K-) < 2.5xE-6, at 90% confidence limit. The
measurement of the J/psi K* decay amplitudes results in R_\perp = 0.160 +-
0.032(stat) +- 0.036(sys), and reveals a dominant longitudinal component. The
phase of the longitudinal amplitude shows evidence for non-vanishing final
state interaction.Comment: 13 pages, 8 postscript figures, submitted to Proceedings for BCP

### Evaluation of \alpha (M^2_Z) and (g-2)_\mu

This talk summarizes the recent development in the evaluation of the leading
order hadronic contributions to the running of the QED fine structure constant
\alpha(s), at $s=M_{\rm Z}^2$, and to the anomalous magnetic moments of the
muon $(g-2)_\mu$. The accuracy of the theoretical prediction of these
observables is limited by the uncertainties on the hadronic contributions.
Significant improvement has been achieved in a series of new analyses which is
presented historically in three steps: (I), use of $\tau$ spectral functions in
addition to e^+e^- cross sections, (II), extended use of perturbative QCD and
(III), application of QCD sum rule techniques. The most precise values obtained
are: $\Delta\alpha_{had} (M_2^Z) =(276.3\pm1.6)\times10^{-4}$, yielding
$\alpha^{-1}(M_{\rm Z}^2)=128.933\pm0.021$, and $a_\mu^{\rm
had}=(692.4\pm6.2)\times 10^{-10}$ with which one finds for the complete
Standard Model prediction $a_\mu^{\rm SM}=(11 659 159.6\pm6.7)\times10^{-10}$.
For the electron $(g-2)_e$, the hadronic contribution is $a_e^{\rm
had}=(187.5\pm1.8)\times 10^{-14}$.Comment: 9 pages, Talk given at the ICHEP'98 Conference, Vancouver, Canada,
July 23-29, 199

### Reply to: ''Improved Determination of the CKM Angle alpha from B -> pipi decays''

In reply to hep-ph/0701204 we demonstrate why the arguments made therein do
not address the criticism exposed in hep-ph/0607246 on the fundamental
shortcomings of the Bayesian approach when it comes to the extraction of
parameters of Nature from experimental data. As for the isospin analysis and
the CKM angle alpha it is shown that the use of uniform priors for the observed
quantities in the Explicit Solution parametrization is equivalent to a
frequentist construction resulting from a change of variables, and thus relies
neither on prior PDFs nor on Bayes' theorem. This procedure provides in this
particular case results that are similar to the Confidence Level approach, but
the treatment of mirror solutions remains incorrect and it is far from being
general. In a second part it is shown that important differences subsist
between the Bayesian and frequentist approaches, when following the proposal of
hep-ph/0701204 and inserting additional information on the hadronic amplitudes
beyond isospin invariance. In particular the frequentist result preserves the
exact degeneracy that is expected from the remaining symmetries of the problem
while the Bayesian procedure does not. Moreover, in the Bayesian approach
reducing inference to the 68% or 95% credible interval is a misconception of
the meaning of the posterior PDF, which in turn implies that the significant
dependence of the latter to the chosen parametrization cannot be viewed as a
minor effect, contrary to the claim in hep-ph/0701204.Comment: 5 pages, 1 figure. Fig. 1 corrected (wrong file

### Improved Determination of alpha_QED(M_Z^2) and the Anomalous Magnetic Moment of the Muon

We reevaluate the hadronic contribution to the running of the QED fine
structure constant alpha(s) at s = M_Z^2. We use data from e+e- annihilation
and tau decays at low energy and at the qq-bar thresholds, where resonances
occur. Using so-called spectral moments and the Operator Product Expansion
(OPE), it is shown that a reliable theoretical prediction of the hadronic
production rate R(s) is available at relatively low energies. Its application
improves significantly the precision on the hadronic vacuum polarization
contribution. We obtain delta_alpha^had = (277.8 +/- 2.6) x 10^-4 yielding
alpha^-1(M_Z^2) = 128.923 +/- 0.036$. Inserting this value in a global
electroweak fit using current experimental input, we constrain the mass of the
Standard Model Higgs boson to be M_Higgs = (129 +103 -62) GeV. Analogously, we
improve the precision of the hadronic contribution to the anomalous magnetic
moment of the muon for which we obtain a_mu^had = (695.1 +/- 7.5) x 10^-10.Comment: tex, 18 pages and 3 figure

### Stau as the Lightest Supersymmetric Particle in R-Parity Violating SUSY Models: Discovery Potential with Early LHC Data

We investigate the discovery potential of the LHC experiments for R-parity
violating supersymmetric models with a stau as the lightest supersymmetric
particle (LSP) in the framework of minimal supergravity. We classify the final
states according to their phenomenology for different R-parity violating decays
of the LSP. We then develop event selection cuts for a specific benchmark
scenario with promising signatures for the first beyond the Standard Model
discoveries at the LHC. For the first time in this model, we perform a detailed
signal over background analysis. We use fast detector simulations to estimate
the discovery significance taking the most important Standard Model backgrounds
into account. Assuming an integrated luminosity of 1 inverse femtobarn at a
center-of-mass energy of 7 TeV, we perform scans in the parameter space around
the benchmark scenario we consider. We then study the feasibility to estimate
the mass of the stau-LSP. We briefly discuss difficulties, which arise in the
identification of hadronic tau decays due to small tau momenta and large
particle multiplicities in our scenarios.Comment: 26 pages, 18 figures, LaTeX; minor changes, final version published
in PR

### Weak Phase gamma Using Isospin Analysis and Time Dependent Asymmetry in B_d -> K_s pi^+ pi^-

We present a method for measuring the weak phase gamma using isospin analysis
of three body B decays into K pi pi channels. Differential decay widths and
time dependent asymmetry in B_d -> K_s pi^+pi^- mode needs to be measured into
even isospin pi pi states. The method can be used to extract gamma, as well as,
the size of the electroweak penguin contributions. The technique is free from
assumptions like SU(3) or neglect of any contributions to the decay amplitudes.
By studying different regions of the Dalitz plot, it is possible to reduce the
ambiguity in the value of gamma.Comment: 11 pages, 1 figur

### The global electroweak fit at NNLO and prospects for the LHC and ILC

For a long time, global fits of the electroweak sector of the Standard Model
(SM) have been used to exploit measurements of electroweak precision
observables at lepton colliders (LEP, SLC), together with measurements at
hadron colliders (Tevatron, LHC), and accurate theoretical predictions at
multi-loop level, to constrain free parameters of the SM, such as the Higgs and
top masses. Today, all fundamental SM parameters entering these fits are
experimentally determined, including information on the Higgs couplings, and
the global fits are used as powerful tools to assess the validity of the theory
and to constrain scenarios for new physics. Future measurements at the Large
Hadron Collider (LHC) and the International Linear Collider (ILC) promise to
improve the experimental precision of key observables used in the fits. This
paper presents updated electroweak fit results using newest NNLO theoretical
predictions, and prospects for the LHC and ILC. The impact of experimental and
theoretical uncertainties is analysed in detail. We compare constraints from
the electroweak fit on the Higgs couplings with direct LHC measurements, and
examine present and future prospects of these constraints using a model with
modified couplings of the Higgs boson to fermions and bosons.Comment: 26 pages, 9 figure

### Can the Renormalization Group Improved Effective Potential be used to estimate the Higgs Mass in the Conformal Limit of the Standard Model?

We consider the effective potential $V$ in the standard model with a single
Higgs doublet in the limit that the only mass scale $\mu$ present is
radiatively generated. Using a technique that has been shown to determine $V$
completely in terms of the renormalization group (RG) functions when using the
Coleman-Weinberg (CW) renormalization scheme, we first sum leading-log (LL)
contributions to $V$ using the one loop RG functions, associated with five
couplings (the top quark Yukawa coupling $x$, the quartic coupling of the Higgs
field $y$, the SU(3) gauge coupling $z$, and the $SU(2) \times U(1)$ couplings
$r$ and $s$). We then employ the two loop RG functions with the three couplings
$x$, $y$, $z$ to sum the next-to-leading-log (NLL) contributions to $V$ and
then the three to five loop RG functions with one coupling $y$ to sum all the
$N^2LL...N^4LL$ contributions to $V$. In order to compute these sums, it is
necessary to convert those RG functions that have been originally computed
explicitly in the minimal subtraction (MS) scheme to their form in the CW
scheme. The Higgs mass can then be determined from the effective potential: the
$LL$ result is $m_{H}=219\;GeV/c^2$ decreases to $m_{H}=188\;GeV/c^2$ at
$N^{2}LL$ order and $m_{H}=163\;GeV/c^2$ at $N^{4}LL$ order. No reasonable
estimate of $m_H$ can be made at orders $V_{NLL}$ or $V_{N^3LL}$. This is taken
to be an indication that this mechanism for spontaneous symmetry breaking is in
fact viable, though one in which there is slow convergence towards the actual
value of $m_H$. The mass $163\;GeV/c^2$ is argued to be an upper bound on
$m_H$.Comment: 24 pages, 5 figures. Updated version contains new discussion,
references, figures, and corrects errors in reference

### A configuration system for the ATLAS trigger

The ATLAS detector at CERN's Large Hadron Collider will be exposed to
proton-proton collisions from beams crossing at 40 MHz that have to be reduced
to the few 100 Hz allowed by the storage systems. A three-level trigger system
has been designed to achieve this goal. We describe the configuration system
under construction for the ATLAS trigger chain. It provides the trigger system
with all the parameters required for decision taking and to record its history.
The same system configures the event reconstruction, Monte Carlo simulation and
data analysis, and provides tools for accessing and manipulating the
configuration data in all contexts.Comment: 4 pages, 2 figures, contribution to the Conference on Computing in
High Energy and Nuclear Physics (CHEP06), 13.-17. Feb 2006, Mumbai, Indi

### A population-based approach to background discrimination in particle physics

Background properties in experimental particle physics are typically
estimated using control samples corresponding to large numbers of events. This
can provide precise knowledge of average background distributions, but
typically does not consider the effect of fluctuations in a data set of
interest. A novel approach based on mixture model decomposition is presented as
a way to estimate the effect of fluctuations on the shapes of probability
distributions in a given data set, with a view to improving on the knowledge of
background distributions obtained from control samples. Events are treated as
heterogeneous populations comprising particles originating from different
processes, and individual particles are mapped to a process of interest on a
probabilistic basis. The proposed approach makes it possible to extract from
the data information about the effect of fluctuations that would otherwise be
lost using traditional methods based on high-statistics control samples. A
feasibility study on Monte Carlo is presented, together with a comparison with
existing techniques. Finally, the prospects for the development of tools for
intensive offline analysis of individual events at the Large Hadron Collider
are discussed.Comment: Updated according to the version published in J. Phys.: Conf. Ser.
Minor changes have been made to the text with respect to the published
article with a view to improving readabilit

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