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Register a new userTau lepton reconstruction at collider experiments using impact parameters
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Daniel Jeans
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We present a novel method for the reconstruction of events containing pairs of hadronically decaying tau leptons at collider experiments. This method relies on accurate knowledge of the tau production vertex and precise measurement of its charged decay products. The method makes no assumptions about the centre-of-mass or invariant mass of the tau pair, and is insensitive to momentum loss along the beam direction. We demonstrate the method using e+e- -> mu+ mu- tau+ tau- events fully simulated in the ILD detector.
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This report is to provide a novel method for the lepton energy calibration at Hadron Collider Experiments. The method improves the classic lepton energy calibration procedure widely used at hadron collider experiments. The classic method parameterizes the potential bias in the lepton en- ergy calibration, and determines the value of the parameter by the invariant mass of $Z/gamma^*rightarrow ell^+ell^-$ events. The precision of the calibration is dominated by the number of parameters or terms consid- ered in the parameterization, for example, a polynomial extension. With one physics constraint of the reconstructed Z boson mass, the classic procedure can use and determine one parameter. The novel method improves the precision of lepton calibration by introducing more terms in the parameterization. To precisely determine the values of multiple parameters, the method first ac- quires more constraints by separating the $Z/gamma^*rightarrow ell^+ell^-$ samples according to the decay kinematics, and then reduces the correlation between multiple parameters. Since the new method is still using the reconstructed Z boson masses as the only constraints, it is much faster and easier than detailed study of detector simulations.
Through the last three decades, accurate simulation of the interactions of particles with matter and modeling of detector geometries has proven to be of critical importance to the success of the international high-energy physics (HEP) experimental programs. For example, the detailed detector modeling and accurate physics of the Geant4-based simulation software of the CMS and ATLAS particle physics experiments at the European Center of Nuclear Research (CERN) Large Hadron Collider (LHC) was a determinant factor for these collaborations to deliver physics results of outstanding quality faster than any hadron collider experiment ever before. This review article highlights the impact of detector simulation on particle physics collider experiments. It presents numerous examples of the use of simulation, from detector design and optimization, through software and computing development and testing, to cases where the use of simulation samples made a difference in the precision of the physics results and publication turnaround, from data-taking to submission. It also presents estimates of the cost and economic impact of simulation in the CMS experiment. Future experiments will collect orders of magnitude more data with increasingly complex detectors, taxing heavily the performance of simulation and reconstruction software. Consequently, exploring solutions to speed up simulation and reconstruction software to satisfy the growing demand of computing resources in a time of flat budgets is a matter that deserves immediate attention. The article ends with a short discussion on the potential solutions that are being considered, based on leveraging core count growth in multicore machines, using new generation coprocessors, and re-engineering HEP code for concurrency and parallel computing.
The reconstruction of tau-pair production, $e^{+}e^{-} to tau^{+}tau^{-}$, from the subsequent 3-prong ($tau^{+} rightarrow pi^{+} pi^{-} pi^{+} bar{ u}_{tau}$) and 1-prong ($tau^{-} to ell^{-} bar{ u}_{ell} u_{tau}$, $tau^{-} to h^{-} u_{tau}$ or $tau^{-} to pi^{-} pi^0 u_{tau}$) decays, is presented using 8.8 fb$^{-1}$ of $e^{+}e^{-}$ collision data of Belle II at the center-of-mass energy $sqrt{s} = m_{Upsilon(4S)}$. The pseudomass technique developed by the ARGUS experiment is used to measure the $tau$-lepton mass $m_{tau}$ in the 3-prong $tau^{+} to pi^{+} pi^{-} pi^{+} bar{ u}_{tau} $ decay, resulting in $m_{tau} = 1777.28 pm 0.75~{rm (stat.)} pm 0.33 ~{rm (sys.)}~{rm{MeV}/rm{c}^2}$.
The lifetime of the $tau$-lepton is measured using the process $e^+e^-rightarrowtau^+tau^-$, where both $tau$-leptons decay to $3pi u_tau$. The result for the mean lifetime, based on $711,mathrm{fb}^{-1}$ of data collected with the Belle detector at the $Upsilon(4S)$ resonance and $60,mathrm{MeV}$ below, is $tau = (290.17 pm 0.53(mathrm{stat.}) pm 0.33(mathrm{syst.}))cdot10^{-15},mathrm{s}$. The first measurement of the lifetime difference between $tau^+$ and $tau^-$ is performed. The upper limit on the relative lifetime difference between positive and negative $tau$-leptons is $|Deltatau| / tau < 7.0 times 10^{-3}$ at 90% CL.
Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays $tau^{pm}rightarrowell^{pm}gamma$ ($ell=e,mu$) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via $tau^{pm}rightarrowell^{pm}gamma$ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb$^{-1}$, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, $mathcal{B}(tau^{pm}rightarrow mu^{pm}gamma)$ $leq$ $4.2times10^{-8}$ and $mathcal{B}(tau^{pm}rightarrow e^{pm}gamma)$ $leq$ $5.6times10^{-8}$, are set at 90% confidence level.
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