Particle Physics at Colliders

Simulation of the decay of a Higgs boson into two photons in the ATLAS detector at the LHC
Figure 1: Simulation of the decay of a Higgs boson into two photons in the ATLAS detector at the LHC.

Our goal are improved theoretical predictions for processes at high energy particle colliders, in particular at the Large Hadron Collider (LHC). Heavy new particles produced at colliders decay almost immediately back into lighter, known particles. The effects of heavy particles are therefore in general only indirectly detectable, as deviations of observations from theoretical predictions within the Standard Model. Precise theoretical predictions for collider processes, both in the standard model and in specific extensions thereof are therefore crucial to interpret the measurements from the LHC.

Collider physics and flavor physics

There are two ways to search for the effects of new physics. One can either try to produce new particles using high-energy collisions, or one can study their virtual effects from quantum fluctuations in low-energy processes. Our group is working on predictions for both approaches. Christoph Greub specializes on rare decays of heavy B-mesons, in particular on radiative decays. An example is the process b → sγ, the decay of a heavy b quark into a strange quark s and a photon γ, which is depicted in Figure 2. Since such quark-flavor changing decays are suppressed in the Standard Model, they are sensitive probes of virtual effects of new physics. Thomas Becher mainly works on high-energy collision processes, in particular on the production of hadronic jets and electroweak bosons (Higgs, Z, W±). Admir Greljo focuses on the interplay between low-energy flavour and high-energy collider physics.

<i>b &rarr; s&gamma;</i>
Figure 2: Contributions to the decay b → sγ. The right diagram shows the contribution of a hypothetical charged Higgs particle

Effective field theory

Collider processes typically involve many disparate scales. An important tool to analyze multi-scale processes are effective field theories such as Heavy Quark Effective Theory (HQET) and Soft-Collinear Effective Theory (SCET). We use these effective field theories to separate the physics associated with different scales and to improve predictions by resumming logarithmically enhanced terms to all order in perturbation theory. In the effective field theory, this resummation is achieved using renormalization group methods. This technique yields accurate predictions also in regions of phase space where standard perturbation theory breaks down, e.g. in the region of low transverse momentum qT depicted in the plot in Figure 3.

Comparison of the theoretical prediction for <i>Z</i>-production with the measurement from ATLAS
Figure 3: Comparison of the theoretical prediction for Z-production with the measurement from ATLAS (from JHEP 1202 (2012) 124).