Minimum bias physics at the LHC with the ATLAS detector

16 June 2009

 

We are preparing for first collision data at the end of 2009 and minimum bias data may well yield our first results. Minimum bias data will help us to understand soft QCD processes at the LHC center-of-mass energy. Physics processes at the LHC involve the interactions of quarks and gluons at small and large transferred momentum. Hard processes (high-pT) should be well described by perturbative QCD. But soft interactions (low-pT) require non-perturbative phenomenological models, and experimental data is needed to validate these models. Minimum bias events are dominated by “soft” partonic interactions with low transverse energy and low charged track multiplicity.

Studies of minimum bias events are vital to understand QCD in the LHC energy regime; such events are also needed to model additional proton-proton interactions (“pileup”), which will be abundant at higher instantaneous luminosities. Measuring minimum bias event characteristics will help give us a better understanding of the structure of the proton at low-x (low energy transfer).

The total p-p cross-section at the LHC includes both elastic and inelastic processes. Furthermore, inelastic processes contain different kinds of soft interactions known as non-diffractive, single-diffractive and double-diffractive processes. (Central diffractive processes are very small, and are not considered here). Minimum bias events are here taken to include the non-single diffractive inelastic events. Studies have been carried out at different center-of-mass energies by earlier experiments, such as UA5 at CERN and CDF at Fermilab to understand this “soft” part.


 

Central charged particle density for non-single diffractive inelastic p-pbar collisions.


 

The figure illustrates the measured and predicted central charged particle density for non-single diffractive events as measured by these experiments at different centre-of-mass energies. Such results have been used to tune PYTHIA and PHOJET (two typical Monte Carlo event generators). There are clear differences in the predicted multiplicities of PYTHIA and PHOJET when extrapolating to LHC centre-of-mass energies of 10 TeV and 14 TeV. Making such a measurement in ATLAS, in order to constrain the models at LHC energies, is thus very important. The very first results from the ATLAS detector may well be  measurements, in minimum bias events, of the charged track multiplicity as a function of pseudo-rapidity and transverse momentum.

The LHC is expected to run with a range of different operating parameters, providing different mean numbers of interactions per p-p bunch crossing. During the initial running it is expected that the mean number of inelastic interactions per p-p bunch crossing will be much less than one. Under these conditions it will be important to select the events resulting from real inelastic interactions rather than events where the beams do not produce an interaction and only detector noise is recorded. Once the average number of interactions approaches or exceeds unity, the majority of inelastic interactions will be selected by simply requiring the presence of two crossing proton bunches. The production cross-section for these processes is very large at LHC and all of the events cannot be recorded. For the nominal LHC luminosity, we will record one such event in every 100K produced.

The key detector components for early minimum bias physics measurements are the Inner Detector (ID) and special parts of the trigger system dedicated to the selection of inelastic interactions with minimal bias, namely the Minimum Bias Trigger Scintillators (MBTS) for initial data taking.

The ATLAS detector has a three stage trigger for event selection: Level 1, Level 2 and the Event Filter (EF). A Level 1 MBTS trigger is formed by requiring two out of the 32 MBTS counters above threshold or by simply requiring the presence of two proton bunches and a random clock cycle. During the initial running period, the luminosity is expected to be too low for the second type of trigger to be useful; and events will be collected without filtering at Level 2. Later on, the Level 2 and the Event Filter will only retain events containing a number of reconstructed tracks with a minimum pT and originating from the interaction point. This requirement will reject background tracks related to beam-gas interactions and empty events. These triggers should efficiently retain non-diffractive and doubly-diffractive events while rejecting about half the single-diffractive and beam-gas events.

The minimum bias trigger is now ready for data from the first collisions expected at the end of this year. Already with as few as 10 to 100 thousand minimum bias events, statistics will be enough to make the track multiplicity measurements, once the tracking is sufficiently understood. This makes these studies quite likely to be one of ATLAS' first results!

 

Prafulla Behera

University of Iowa