Title: Search for magnetic monopoles

In 1931, Dirac realised that the existence of one magnetic monopole in the Universe would be sufficient to explain the quantisation of electric charge. It follows that monopoles should carry a magnetic charge equivalent to many electron charges in terms of ionisation energy loss. Thus, monopoles would appear as extremely highly-ionising particles in a detector. Searches for direct monopole production have been performed each time an accelerator of a new type or surpassing power was operated. ATLAS performed the first monopole search at the LHC, using part of the data taken in 2011 [1]. The search exploits the characteristic signature of a high fraction of high-threshold hits in the transition radiation tracker (TRT) and a narrow energy deposition in the electromagnetic (EM) calorimeter (see figure). However, this search had one severe limitation: the triggers used were designed for electrons or photons and required the particle to reach the second layer of the EM calorimeter, while highly-ionising particles such as monopoles would tend to range out before they penetrate deep into the calorimeter. In 2012, the Geneva group developed and validated a dedicated high-level trigger for monopoles. This trigger is based on TRT hits rather than calorimeter energy, which reduces the energy threshold and removes the condition on the second EM calorimeter layer. The search being performed in Geneva with the data taken in 2012 with this trigger is therefore expected to vastly surpass the previous results in sensitivity.   

In addition to direct detection in ATLAS, there are several other techniques which can be used to search for monopoles. Our group investigated such options [2], creating new experimental opportunities at the LHC [3] and beyond the LHC [4]. 


[1] ATLAS Collaboration, Search for magnetic monopoles in sqrt(s) = 7 TeV pp collisions with the ATLAS detector, arXiv:1207.6411, Phys. Rev. Lett. 109, 261803 (2012). 

[2] A. De Roeck, A. Katre, P. Mermod, D. Milstead, and T. Sloan, Sensitivity of LHC experiments to exotic highly ionising particles, arXiv:1112.2999, Eur. Phys. J. C 72, 1985 (2012).

[3] A. De Roeck, H.P. Hächler, A.M. Hirt, M. Dam Joergensen, A. Katre, P. Mermod, D. Milstead, and T. Sloan, Development of a magnetometer-based search strategy for stopped monopoles at the Large Hadron Collider, arXiv:1206.6793, Eur. Phys. J. C 72, 2212 (2012).

[4] K. Bendtz, H.-P. Hächler, A.M. Hirt, P. Mermod, P. Michael, D. Milstead, C. Tegner, T. Sloan, and S.B. Thorarinsson, Search for magnetic monopoles in polar volcanic rocks, arXiv:1301.6530, Phys. Rev. Lett. 110, 121803 (2013).


Figure caption: High-threshold TRT hit fraction versus electromagnetic energy dispersion. The circles represent 1000 simulated single monopoles with mass 800 GeV. The crosses represent ATLAS data.