Analyses

We are currently preparing seven analyses for you in python and C++. The analyses range from measuring Standard Model particles including a Higgs analysis to a search for a Beyond the Standard Model particle.

$W$ Analysis and $Z$ Analysis

The $W$ and $Z$ bosons are together known as the weak or intermediate vector bosons. These Standard Model elementary particles mediate the weak interaction.

It is important to measure well known Standard Model particles. This way we can confirm that we understand the detector and software properly. We are then ready to search for new physics.

Candidate event with a $Z$ decay to muon pairs, recoiling against missing transverse energy (MET). The muon candidates have transverse momenta of 50 and 126 GeV. The invariant mass of the two muons attributed to the Z candidate is 94 GeV; the MET is measured to be 161 GeV.

Top pair Analysis

The top pair analysis is interesting because it is a good test of the Standard Model.

It is important for discrimination between different data simulation generators, QCD models and parton distribution functions.

In addition, top pair production is an important background in various Higgs boson analyses as well as beyond the Standard Model searches. It is therefore crucial to understand this process in detail.

Display of a top pair candidate event recorded by ATLAS with LHC stable beams at a collision energy of 13 TeV. The red line shows the path of a muon with transverse momentum around 140 GeV through the detector. The green line shows the path of an electron with transverse momentum around 170 GeV through the detector. The green and yellow bars indicate energy deposits in the electromagnetic and hadronic calorimeters. From these deposits 3 jets are reconstructed with transverse momenta between 30 and 80 GeV. Two of the jets are identified as having originated from b-quarks. Tracks reconstructed from hits in the inner tracking detector are shown as arcs curving in the solenoidal magnetic field.

$WZ$ Analysis

This analysis looks for both a $W$ boson and a $Z$ boson. It is a relatively clean signature due to three leptons in the final state. It is interesting for physics since it is a probe for triple gauge couplings.

Candidate for a $WZ \rightarrow \mu\nu\mu\mu$ decay, collected on 25 October 2010. The invariant mass of the two muons attributed to the $Z$ is 92 GeV. The transverse mass of the potential $W$ boson is 88 GeV.

Further event properties:

$Z$ candidate:

• $p_\text{T}(\mu^-)$ = 60 GeV
• $p_\text{T}(\mu^+)$ = 35 GeV

$W$ candidate:

• $p_\text{T}(\mu^-)$ = 40 GeV
• $E^\text{T}_{\text{miss}}$ = 55 GeV (indicated by a blue line in the display)

$ZZ$ Analysis

This analysis looks for two $Z$ bosons where both $Z$ bosons decay to leptons.

The production of $Z$ boson pairs with subsequent decay to leptons is one of the very few Standard Model process with four prompt leptons in the final state. Its low production cross section results in a very low yield for the ATLAS open data dataset and highlights the limitations of the dataset. Although some events can be selected the low event yield prohibits detailed analysis and conclusions drawn are of a more qualitative nature.

The Higgs boson decaying to two $Z$ bosons which in turn decay to 4 leptons is known as the golden channel. The importance of this channel was shown by its major role in the discovery of a Higgs-like boson, by the ATLAS and CMS collaborations, with mass near 125 GeV.

H → ZZ → 4 leptons. Event display of a 4 muon candidate. EventNumber: 71902630 RunNumber: 204769

Further event properties:

Higgs candidate=125.1 GeV.

• $\mu_1$: $p_\text{T}$=36.1 GeV, $\eta$=1.29, $\phi$=1.33.
• $\mu_2$: $p_\text{T}$=47.5 GeV, $\eta$=0.69, $\phi$=-1.65.
• $\mu_3$: $p_\text{T}$=26.4 GeV, $\eta$=0.47, $\phi$=-2.51.
• $\mu_4$: $p_\text{T}$=7.17 GeV, $\eta$=1.85, $\phi$=1.65

$H\rightarrow W^+W^-$ Analysis

This analysis searches for the standard model Higgs boson. Here, the Higgs boson decays into 2 $W$ bosons which subsequently decay into leptons and neutrinos.

The Higgs boson appears as a consequence of the breaking of electroweak symmetry which describes the origin of mass for all massive particles. The central role played by the Higgs boson in the SM explains why the understanding of this particle is a centrepiece of the LHC physics programme.

The $H\rightarrow W^+W^- \rightarrow l^+\nu l^-\bar\nu ($$ℓ$=electron or muon) channel is particularly sensitive in the intermediate mass range 120 < $m_\text{Higgs}$ < 240 GeV

It has been shown at the LHC that the major background contributions to the search in this decay mode are:

• top pair production,
• $WW$, and
• $Z$+jets events.

All of these processes contain two isolated high-$p_\text{T}$ leptons from $W$ boson decays which is also the signature of the signal.

$Z^\prime$ Analysis

The $Z^\prime$ boson is a hypothetical gauge boson that arises from extensions of the electroweak symmetry of the Standard Model. The $Z^\prime$ boson is named in analogy with the Standard Model $Z$ boson.

Further Information

The purpose of these example analyses is to showcase the abilities and limitations of the measured and simulated data included in this data release.

Three high statistics Standard Model analyses:

• a selection of events with one leptonically decaying $W$ boson,
• a selection of leptonically decaying $Z$ boson, and
• a selection of semileptonic top pair production.

These analyses are intended to show that the general description of the data for these important background processes is sound. They also pose the possibility to study Standard Model observables like the mass of the $Z$ boson. Observable discrepancies are due to the simplified nature of the used ntuples.

Three low statistics Standard Model analyses: These analyses show the limitations of this dataset with respect to rare processes. They are:

• a $WZ$ analysis,
• a $ZZ$ analysis, and a $H\rightarrow W^+W^-$ analysis.

It is possible to obtain results in these analyses and achieve educational objectives.
However, the statistical limitations prohibit more meaningful analyses. This point is particularly important as it demonstrates that the proposed datasets are intended for educational purposes only.

A beyond the Standard Model analysis: Multiple samples of simulated data containing $Z^\prime$ signal events are provided to implement a simplified analysis. The goal is to set exclusion limits on new physics.

This list should not be seen as an exhaustive list of all possible analyses. Further processes that may be explored include $WW$ production, dileptonic top pair production, single top production and many others.

Suggested activity

In theoretical physics, Feynman diagrams are pictorial representations of the mathematical expressions describing the behaviour of subatomic particles. Feynman diagrams are a valuable tool for understanding physics processes.

• Have a go at drawing / finding the Feynman diagram for each of the above seven analyses.