Minimum bias pp collisions: The very first data from the LHC can be used to study the typical inelastic pp collision at high energy.

Why bother?

As seen on the plot (from the ATLAS CSC book, 2009, Expected Performance of the ATLAS Experiment.) different QCD-based models differ in their predictions of the final state particle density.

Minimum bias

Studies of particle production in such "minimum bias" events are crucial for distinguishing QCD effects in heavy ion collision from simple scaling of the number of nucleons. It is also crucial in high luminosity pp collisions to understand the background from many overlapping minimum bias events in each bunch-crossing.

First collisions

On 23 November 2009, during the very first commissioning of the accelerator with beams circulating at the injection energy of 450 GeV, the beams were for short while brought into collision and each of the four LHC experiments recorded about 200 collision events. The ALICE experiment immediately published these observations into a publication on minimum bias events that were equally immediately accepted for publication, see this first paper on LHC data.

Precise measurements

Many millions of inelastic pp collisions of have now been recorded at injection energy, in December 2009 data were briefly taken at 2.3 TeV and on 30 March 2010, the energy was raised to 7 TeV which will be the working point for the next two years. The results are interesting. There are more charged particles produced than expected, as shown by the precise results from ATLAS.

Preliminary results from higher energies indicate that the high multiplicity tail of the multiplicity distribution also grows faster than expected with energy. The experiments are working hard to have official results out for the summer conferences (see ATLAS Public Results and ALICE presentations).