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Quark-gluon tagged jet quenching studies in PbPb collisions with the CMS detector at LHC

Final Report Summary - QGTAGCMSHI (Quark-gluon tagged jet quenching studies in PbPb collisions with the CMS detector at LHC)

The primary objective of the project is to investigate the color-charge related properties of jet quenching phenomena in heavy-ion collisions. The color structure of quarks and gluons are different, therefore the jets originating from quarks and gluons may differ in either the amount of energy-loss or the pattern of the radiated energy, or both. The project initially aimed to exploit the correlations between these properties in order to distinguish the different types of jets.
In practice, this can be achieved by finding jet-by-jet variables that are most sensitive to the radiation pattern, so that samples can be selected based on these variables. These variables can be related to the total mass of the jet constituents, or the width of the jet or the transverse momentum composition of the constituents.
Studies of simulated Monte-Carlo samples can determine the identification of the quarks and gluons based on these variables, however they must be validated by real data of pp and pPb collisions.
The main sample of interest is the inclusive dijets, in which both quarks and gluons are abundant. As control samples, several different types of collisions with a well-known initial state can be used, such as photon+jet events and Z+jet events. In such events, the ratios of the quarks and gluons are different, therefore one can validate the identification performance which was determined from simulations.

Before the beginning of the project, the researcher was elected as the convenor for the sub-group of high-pt physics under the heavy-ion group of CMS and took part in the organization of multiple analyses ongoing within the collaboration, of which several were in parallel with the needs of the quark-gluon tagging objective of the project. To name the three separate parts of the research programme:

1- Jet-track correlations in PbPb : A step towards quantifying properties of jets according to their radiation patterns
2- Z-jet correlations in pPb and PbPb : A step towards isolating samples of different quark and gluon jet mixtures
3- b-tagged jets in pPb and PbPb : A first direct comparison of jet quenching in a quark dominated sample and a gluons dominated sample.

As of 2015, the researcher handed over the convenorship responsibility and focused on a single analysis project, namely the correlations of b-tagged dijets and the measurement of momentum imbalance in tagged versus untagged dijets. This study explored the differences and similarities between light-quark and gluon dijets versus b-quark originated dijets, which is very important for understanding the radiation pattern of partons in dense medium of quark-gluon plasma.

The study involved two main components: the identification of b-jets, and the correlations between jet pairs. For the identification of b-quark originated jets, the researcher took part in the supervision of a PhD student, who performed an optimization and analysis of b-identification algorithms in PbPb collisions. Certain techniques of background subtraction and efficiency correction were implemented to minimize the systematic uncertainty of the measurement, and as a consequence, a good measurement of the momentum imbalance between the correlated b-jets was achieved.

On the other side, an important aspect of the study was the baseline constructed by inclusive dijets, which originate from light quarks and gluons. Although this was a common practice in the history of PbPb studies in CMS, it posed new challenges due to the high background levels in the new collisions energy of 5 TeV. New correction methods were invented in order to cope with the new effects, which are described in more detail in the analysis documentation. As a result, both b-dijets and inclusive dijets were measured with systematic uncertainties that are optimal for the available statistics of the b-jet sample.

The summary of the results are published in the CERN Document Server with open access:

The measurement found no significant difference between b-tagged dijets and other dijets. This may have various theoretical implications, which may be resolved with more detailed studies of the light dijets, in particular, the difference between the gluon dijets and light-quark dijets. Such measurements are expected to be performed with the data obtained from PbPb collisions in the 2016 LHC run.