Observation of the decay Z→ψ ℓ+ℓ−→μ+μ−ℓ+ℓ−\mathrm{Z}\to\psi\,\ell^+\ell^-\to\mu^+\mu^-\ell^+\ell^- with the CMS detector

The observation of the $\mathrm{Z}$ boson rare decay to a $\psi$ meson and two oppositely charged same-flavour leptons, $\ell^+ \ell^-$, where $\psi$ represents the sum of $\mathrm{J}/\psi$ and $\psi(\mathrm{2S})\to\mathrm{J}/\psi\, X$, and $\ell=\mu,\mathrm{e}$, is presented. The data sample used corresponds to an integrated luminosity of $35.9\,\mathrm{fb}^{-1}$ of proton-proton collisions at a center-of-mass energy of $13\,\mathrm{TeV}$ accumulated by the CMS experiment at the LHC. The signal is observed with a significance in excess of 5 standard deviations. Removing contributions from $\psi(\mathrm{2S})$ decays to $\mathrm{J}/\psi$, the signal is interpreted as being entirely from $\mathrm{Z}\to\mathrm{J}/\psi\,\ell^+\ell^-$, with its fiducial branching fraction relative to that of the decay $\mathrm{Z}\to\mu^+\mu^-\mu^+\mu^-$ measured to be $$\frac{\mathcal{B}(\mathrm{Z}\to\mathrm{J}/\psi\,\ell^+\ell^-)}{\mathcal{B}(\mathrm{Z}\to\mu^+\mu^-\mu^+\mu^-)}=0.70\pm 0.18\, \mathrm{(stat)} \pm 0.05\, \mathrm{(syst)}.$$ This result is obtained with the assumption of no $\mathrm{J}/\psi$ polarisation, where extreme polarisation scenarios can create $-24\%$ to $+22\%$ variations

Identification of circles from datapoints using Gaussian sums

We present a pattern recognition method which use datapoints on a plane and estimates the parameters of a circle. MC data are generated in order to test the method's efficiency over noise hits, uncertainty in the hits positions and number of datapoints. The scenario were the hits from a quadrant of the circle are missing is also considered. The method proposed is proven to be robust, accurate and very efficient.Comment: 4 pages, 5 figure

Performance Studies of a Micromegas Chamber in the ATLAS Environment

Five small prototype micromegas detectors were positioned in the ATLAS detector during Large Hadron Collider running at $\sqrt{s} = 7$ and $8\, \mathrm{TeV}$. A $9\times 4.5\, \mathrm{cm^2}$ double drift gap detector was placed in front of the electromagnetic calorimeter and four $9\times 10\, \mathrm{cm^2}$ detectors on the ATLAS Small Wheel, the first station of the forward muon spectrometer. The one attached to the calorimeter was exposed to interaction rates of about $70\,\mathrm{kHz}/\mathrm{cm^2}$ at $\mathcal{L}=5\times 10^{33}\,\mathrm{cm^{-2}s^{-1}}$ two orders of magnitude higher than the rates in the Small Wheel. We present the results from performance studies carried out using data collected with these detectors and we also compare the currents drawn by the detector installed in front of the electromagnetic calorimeter with the luminosity measurement in ATLAS.Comment: 9 pages, 11 figure

Performance Studies of Micromegas Chambers for the New Small Wheel Upgrade Project

Micromegas, an abbreviation for Micro MEsh Gaseous Structure (MM), is a robust detector with excellent spatial resolution and high rate capability. An $R\&D$ activity, called Muon ATLAS MicroMegas Activity (MAMMA), was initiated in 2007 in order to explore the potential of the MM technology for use in the ATLAS experiment. After several years of prototyping and testing, the ATLAS collaboration has chosen the MM technology along with the small-strip Thin Gap Chambers (sTGC) for the upgrade of the inner muon station in the high-rapidity region, the so called New Small Wheel (NSW) upgrade project. It will employ eight layers of MM and eight layers of sTGC detectors per wheel. The NSW project requires fully efficient MM chambers, able to cope with the maximum expected rate of $15\,\mathrm{kHz/cm^2}$ featuring single plane spatial resolution better than $100\,\mu\mathrm{m}$. The MM detectors will cover a total active area of $\sim1200\,\mathrm{m^2}$ and will be operated in a moderate magnetic field with intensity up to $0.4\,\mathrm{T}$. Moreover, together with the precise tracking capability the NSW MM chambers will contribute to the ATLAS Level-1 trigger system. An extensive $R\&D$ program is ongoing to determine the best configuration that satisfies these requirements. Several tests have been performed on small ($10\times10\,\mathrm{cm^2}$) and medium ($1\times0.5\,\mathrm{m^2}$) size prototypes using medium ($1-5\,\mathrm{GeV/c}$) and high momentum ($120-150\,\mathrm{GeV/c}$) hadron beams at CERN. A brief overview of the results obtained from recent performance tests concerning the aspects discussed above is presented