Soft QCD physics
Bose-Einstein correlations
As one could guess, these are correlations of bosons (particles with an integer spin). Wave function describing a pair of identical bosons needs to remain the same when we exchange them. This property of the wave function results in an increased number of pairs with a small four-momentum difference (a combination of energy and momentum, which appears in the Special theory of relativity). This effect is called the Bose-Einstein correlations. We search for it in pions (lightest bosons) frequently produced in proton-proton collisions at the LHC accelerator and recorded by the ATLAS detector.
In order to study the Bose-Einstein correlations experimentally, we need to create pairs with no correlations present which are used as a reference sample. For this purpose we are using pion pairs with each pion created in a different proton-proton collision which cannot be correlated in any way, or pairs of pions from the same collision with momentum vector of one of the pions inverted which destroys the correlations, or opposite charge pion pairs which are non-identical and therefore not subject to Bose-Einstein correlations. Measuring the Bose-Einstein correlations, we are able to determine a characteristic size of the volume, where the particles are created in proton-proton collisions at LHC. It is of great importance, because we hope that it could help us to better understand the quark confinement in protons and other hadrons. This characteristic size is comparable with the size of an atomic nucleus.
Our team contributed to the following publications:
- Analysis of 7 TeV data: Eur. Phys. J. C 75 (2015) 466 (arXiv:1502.07947),
- Analysis of 13 TeV data: Eur. Phys. J. C 82 (2022) 608 (arXiv:2202.02218).
Search for Intrinsic Charm
One way to look at the composition of the proton is, that it consists of the three valence quarks (up, up and down - abbreviated |uud>) and a quark-gluon sea with composition that depends on the energy of the probe. Within the Standard model it is possible that there are more states which do not depend on the probe energy. In fact there should be a whole series of states |uudg>, |uudqq>, …. The quarks and gluons originating from those states are called intrinsic.
In search for intrinsic quarks it is necessary to focus on heavy quarks (c and b-quarks), originating from states |uudcc> and |uudbb>, because only these are possibly distinguishable from sea quarks. Typically heavy intrinsic quarks tend to carry larger portion of proton momentum. We search events in which c-quark originates from one of the protons and in the output there is a particle which can give us information about how big portion of the energy the c-quark carried. In our case it is a photon but it can be also Z boson. In the end we expect than number of photon + c-quark events will be higher compared to the predictions which only take three valence quark model in consideration.
The results of this analysis are published in