Of mesons and bosons
24 July 2015 | By
ATLAS is ready for detailed physics studies. The experiment used early data collected from the LHC’s Run 2 to calibrate its detectors. Measurements of the production and leptonic decay of certain particle resonances have shown that the detectors and software are working as expected.
Three of the high-mass resonances measured are: J/psi meson, W boson and Z boson. On 23 July, new measurements of the three resonances were presented, with focus on their identification and reconstruction.
The discovery of the J/psi meson in 1974 and its subsequent identification as a bound state of the new “charm” quark and its anti-matter partner (a system now known as charmonium) led to a revolution in our understanding of particle physics. However, over 40 years later, many aspects of how J/psi mesons are produced in hadron colliders are still not understood, and the characteristics of J/psi production at the LHC remains under scrutiny.
When protons collide at the LHC, J/psi mesons with a mass of 3.1 GeV can be produced in two distinct ways. First, the hard scattering of the constituents of each proton can directly produce J/psi mesons (occasionally via excited charmonium states). Second, J/psi mesons can result from B hadron decays. LHC collisions produce a copious amount of beauty quarks, which subsequently become bound up into B hadrons. About one in every hundred B hadrons decays into a system of particles including a J/psi.
The ATLAS experiment can efficiently identify J/psi mesons decaying to a muon and an anti-muon. The ATLAS pixel detector measures the J/psi decay vertex position and the initial proton-proton interaction precisely vertex. As the J/psi meson decays far too quickly to travel a measurable distance, one can very reliably approximate the J/psi production position by its decay position. The decay position (or time) distribution of a sample of reconstructed J/psi mesons originating from B hadron decays can then be used to determine the fraction that was displaced before they decayed.
The distributions observed in the 13 TeV data have been compared to the expected contributions from W and Z boson production, including associated jet production and other Standard Model processes with similar final states. These distributions agree, within the estimated uncertainties, with the Standard Model expectations.
In 13 TeV proton collisions, ATLAS has observed that the majority of J/psi mesons produced with low momentum originate from the initial proton-proton interaction with no measurable displacement. At higher momentum, decays from B hadrons begin to dominate the production rate. This early analysis of the initial 13 TeV data marks the first step towards more detailed studies of J/psi production, which can elucidate the nature of their production in high-energy proton collisions.
W and Z bosons
The production and decay of W and Z bosons offer opportunities to study high-mass resonances, especially in leptonic decay signatures. When a W boson decays to an electron or muon plus a neutrino, the lepton’s transverse momentum distribution takes a specific shape, easily identified as the hallmark of W production. Similarly, the invariant mass of oppositely charged electrons or muons from Z boson decays is a known quantity that can be used for calibration.
ATLAS’ high rate of W and Z boson production makes it possible to collect large samples of candidate events. In fact, inclusive W and Z boson production is expected to increase by a factor of 1.7 in 13 TeV proton collisions compared to 8 TeV. Selecting and identifying the W and Z decays depend on charged lepton identification, missing energy reconstruction and efficient online triggering algorithms.
The distributions observed in the 13 TeV data have been compared to the expected contributions from W and Z boson production, including associated jet production and other Standard Model processes with similar final states. These distributions agree, within the estimated uncertainties, with the Standard Model expectations. Ongoing studies are proceeding to allow the extraction of precise cross section measurements for W and Z production in the 13 TeV data.