Expected results from top physics with early data

14 July 2008

  Left: Expected distribution of the three-jet invariant mass in the muon channel for the right combination (white) or the wrong jet pairing (blue) after selection and imposing the W-boson mass constraint. The plot is obtained with 100 pb-1 of integrated luminosity.
  Right: Distribution of the expected statistical significance of the top signal in the peak as a function of the integrated luminosity for two background scenarios.

After a long preparation phase, we are finally approaching the exciting moment of getting first data. Depending on how well the machine behaves, the integrated luminosity could reach between ten to a few tens of inverse picobarns before the end of the year. This data set is likely to already give some hints on how the ATLAS detector will behave and, consequently, on the quality of the physics objects that all of us will use in their analysis in the next years.

How many real leptons are reconstructed with the calorimeter and the spectrometer; what is the response of the calorimeters when measuring the jet energy? Quite a few of these problems will need to be addressed in this new energy regime and with a new detector before one can claim to observe something unexpected. A range of processes will provide samples which will allow us to test and tune the detector. At the LHC, top physics will be in this group even with modest amounts of data.

Reconstruction of top candidates will allow us to test object identification. One of the first measurements of interest that we expect to perform with ATLAS is the production cross-section of ttbar pairs in pp collisions, coming mainly from gluon-gluon strong interactions. At 14 TeV, the Standard Model prediction is about 830 pb. This implies that at this center-of-mass energy, almost 107 ttbar pairs will be produced each year at L = 1032 cm-2s-1. At 10 TeV, the cross-section is expected to be about half of the above and probably just some initial detector studies will be possible at this stage by reconstructing top candidates. Nevertheless, preliminary studies by the Top group found that both the kinematics and the efficiencies should not change much with respect to what we observed at 14 TeV.

Each of the two tops decays, according to the Standard Model, into a W boson and a b quark. Semileptonic channels, in which one of the W’s decays into an electron or a muon and the other one into quarks, are a good compromise between high statistics and an affordable level of background (the wealth of fully hadronic decays is overwhelmed by the QCD background accompanying the hard gluon-gluon interaction). To select semileptonic top decays, one will rely on the lepton triggers, looking for a high transverse momentum lepton in the event (an electron reconstructed in the calorimeter and matched with a track in the Inner Detector; or a muon formed in the spectrometer and then combined with an Inner Detector  track). In addition, kinematic constraints such as the number of high transverse momentum jets and large missing transverse energy from the neutrino from the W decay (to reject the QCD background) will be applied.

The main source of background is expected to come from W’s decaying semileptonically, produced with a varying number of jets, making them look like top events. Both a counting method and a fit for signal and background in the top mass distribution have been studied by the Top Working Group to determine the ttbar cross-section for the CSC notes exercise. The uncertainty on this measurement will mostly come from the jet energy scale (JES) in the calorimeters and the estimation of the background and the fitting model. Finally, radiation of quarks or gluons from the initial or final partons in the hard process, will lead to additional jets, which will also contribute to the systematic uncertainty.

From these studies, we are confident we can already get quite a large and clean sample of ttbar events with early data. One can now see the importance of top event reconstruction: it involves using various physics objects (leptons, jets, missing energy). For instance, constraining the mass of the hadronic top W to the world average will be a valuable tool to calibrate the JES. The leptonic W provides the expected value of the missing transverse energy, showing initial miscalibrations (and/or new physics contributions!).

Valuable information coming from algorithms (such as b-tagging) will need more time to be calibrated and will likely not be used at the beginning. Evaluating the efficiency of the various b-taggers from the presence of the b’s from top quark decays is going to be done directly from data. Last but not least, studies are being carried out to evaluate the lepton trigger efficiencies from the overlap with the jet triggers when selecting the hadronic part of the decay.

We have already had a chance to look at the cross-section measurement in a way closer to real data-taking mode with samples from the Full Dress Rehearsal. This exercise, mainly meant to test our technical ability to access and analyse data with our code, has also proven to give results on the cross-section in good agreement with the expectations using an equivalent statistics of just a few pb-1: a nice result in view of the very early data!

A cross-section measurement with fully leptonic ttbar decays (about 5% of the total) has also been studied by requiring two isolated leptons of opposite charge. A veto is applied on the Z → ll decay by cutting away lepton pairs consistent with the Z mass. In this case, one can also use a cut-and-count approach as for semileptonic decays, or disentangle the signal from other contributions by looking at kinematic and angular distributions and maximizing a likelihood.

For the CSC exercise, the Top Group has also looked at how well ATLAS can measure the top mass with the first inverse fb of data in the semileptonic channel. For the mass measurement, the selection is similar to the cross-section analysis but additional constraints on the masses of combined objects and on energy differences are needed. This reduces combinatorial background in the final invariant mass distribution of the hadronically decaying top from random jets association. Depending on whether the b-tag information will be available at that time, requirements on only the W mass or W and b combined mass will be necessary before the final fit.

I have tried to give a general scenario of what can be achieved with early data in terms of top physics. Given the good knowledge of top quark properties inherited from the great work of our LEP and Tevatron colleagues, top physics will be a primary tool to understand our detector “at the physics object level”, making it one of the most interesting topics in the months to come. Later on, measuring the single top production cross-section, the top quark properties and looking for high PTtop and ttbar resonances will provide a very challenging field of study in itself, and might open the door to new physics.



Giuseppe Salamanna