Central Exclusive Production and Spectroscopy Studies at LHCb

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


At the LHCb experiment, on the Large Hadron Collider at CERN, the counter-circulating beams of protons pass through each other many millions of times per second. In most of these passings, collisions occur in which the quarks or gluons inside the protons interact, splitting apart the parents, and produce new states of interest, together with many other particles. LHCb uses such 'inelastic' interactions to study the phenomenon of CP-violation, and the other experiments at the LHC have benefitted from these classes of interaction to discover the Higgs boson. Other categories of interaction occur however; one example of great interest is so-called Central Exclusive Production.

In Central Exclusive Production the protons interact through both emitting a system of gluons termed the pomeron. The protons themselves remain intact, and continue their journey with minimal deflection, much as if they had only 'rubbed shoulders'. The pomerons however fuse together and produce a new state which then decays in the detector. Rather than the usual picture of a multitude of particles being seen, a beautifully clean 'event' is produced with only a handful of particles observed. Measuring the rate and characteristics of this process is an important and necessary test of our understanding of the strong force. Even more importantly, the process may produce states which are hard to isolate and study in inelastic collisions. There has been much interest in recent years in exotic mesons - a class of strongly interacting particles that are hard to explain by the conventional picture of a bound system of quark and anti-quark, but rather requires a more complex arrangement of constituents. Central Exclusive Production provides an excellent laboratory to study these exotic mesons.

During the forthcoming LHC run 2, due to begin in Spring 2015, Central Exclusive Production interactions will be selected from the LHCb data. These will then be studied and their characteristics compared with expectation. Searches will be performed for the presence of exotic mesons in this data set, and the properties of these mesons studied. An important challenge in this work will be to ensure that the interactions are truly exclusive, which means ensuring no additional particles have been produced close to the proton beams that evade detection in the experiment. For this purpose, new detectors have been installed in the tunnel at distances up to 100 m away from the experiment, in order to be sensitive to such additional particles. Operating these detectors, ensuring that they perform well, and making best use of the information they provide in the analysis, will be a critical aspect of the research programme.

Planned Impact

It is important for the public to understand that the scope of physics studies at the LHC go far beyond the Higgs. CEP is a suitable topic for getting this message across, given the very stark different in event characteristics between CEP and inelastic interactions (see Fig. 1 in 'case for support').

Any CEP results on exotic spectroscopy could be presented in a manner which would be stimulating and informative for the public. This is without question an area where there is significant interest and thirst for new information, as has been borne out by the high level of activity on social media that followed the posting on LHCb recent findings on the Z(4430), which for the first time unambiguously showed results that cannot be interpreted within the classical quark model (see April 24 posting on http://lhcb-public.web.cern.ch/lhcb-public/ ).

As Spokesperson, I shall be in an ideal position to draw attention to these results during outreach events and interviews. I shall collaborate with the postdoc in contributing one or more articles to the popular science press about CEP and exotic spectroscopy.


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Description The award has enabled the successful construction and operation of the HeRSCheL detector, which enhances the capabilities of the LHCb experiment for the type of science targetted in this grant. HeRSCheL detects particles produced at low angles in LHC collisions and allows background processes to the physics channels of interest to be rejected. During the past year a publication (JINST 13 (2018) P04017) has appeared which records the great success of this project. Furthermore an important physics measurement has appeared (JHEP 10 (2018) 167) that benefits from the HeRSHeL system.
Exploitation Route HeRSCheL is now an important tool for all the analysts in the LHCb collaboration, and is being used in many ongoing analyses. Many future publications will benefit from this system.
Sectors Other