Schlagwort: ‘supersymmetry’
Anticipating Discoveries: LHC14 and Beyond
PhD students Leila Ali Cavasonza and Mathieu Pellen report from the workshop “Anticipating Discoveries: LHC14 and Beyond”
Few months ago, the Large Hadron Collider (LHC) in Geneva woke up from a long shut-down phase. It is now operating at a centre of mass energy of 13 TeV (it might reach 14 TeV in the upcoming phases). This is the first time in the history of humanity that particles are collided at such high energy in a machine built by humans.
Thus the Run II of the LHC is just starting and is lifting once again the excitement in the particle physics community. It is thus the right time to discuss what particles or theories could be discovered by the ATLAS and CMS detectors. In this spirit, a topical workshop organised by the Munich Institute for Astro- and Particle Physics (MIAPP) has been held in Munich: “Anticipating Discoveries: LHC14 and Beyond” from the 13th to the 15th of July.
In the last few days, the so-called pentaquark has been claimed to be discovered by the LHCb collaboration. This is an extraordinary discovery but the particle physics theorists are after another kind of particles. Indeed this pentaquark (a composite object made of 5 quarks, see picture to the right) has been predicted many years ago by quantum-chromo dynamics (QCD) but has never been observed so far. What theorists are looking for are theories beyond the standard model. These are introduced to explain experimental and theoretical problems. In general, these predict new resonances or effects that can be traced by experimental collaborations.
During this workshop many theories or extensions of previous ones have been proposed. In particular since the discovery of the Higgs boson, extensions of the Higgs sector are under high scrutiny. The beautiful theory of supersymmetry which predicts a special relation between bosons and fermions is still greatly discussed.
In particular extension of its minimal version have been proposed. Finally, as we know there is a huge amount of unexplained, invisible matter in our Universe, the so-called Dark Matter, it is justified to propose myriads of models that could explain various anomalies. In particular during these three days, several theories involving a non-abelian structure of the dark sector have been presented. These have a particular phenomenology at very different scales and are currently being tested against observations.
During this workshop many theories have been discussed and all theorists are craving to find signs of their favourite theory at the next LHC run. The kind of signs they are looking for is similar to the one reported by the ATLAS and CMS collaboration. The experimental collaborations have made public an excess in the Z/W channels (see picture on the left) and especially in the one where the gauge bosons are decaying into two jets. Future will tell us whether this is a sign of hope and the beginning of a new exciting hera.
Leila and Mathieu
Simplified models for new physics
Jan Heisig and Jory Sonneveld report on their recent work on simplified models
With about a petabyte of data processed in Switzerland everyday, the Large Hadron Collider (LHC) provides an enormous amount of information on high energy physics processes. This information is used in order to test theories beyond the Standard Model of Particles Physics — theories that are motivated either by outstanding theoretical problems or experimental evidence, like in the case of dark matter. While experimentalists work their way through the data, theorists line up to convince them to search for their favorite a model in the currently collected 20 fb-1 of data.
It is impossible for experimentalists to search for each possible model theorists came up with. This is why they often try to search for simple characteristics that represent a larger class of possible new models of physics. One new model of physics, supersymmetry, for example, predicts new spin-0 (scalar) quarks, or squarks (supersymmetric quarks) among many other particles. These new squarks decay to a quark and so-called neutralino (see Feynman diagram on the left), which in many models of supersymmetry is assumed to be the lightest supersymmetric particle.
From FNAL.gov
What would be seen at the LHC if supersymmetry were realized in nature? As the neutralino is a neutral, stable particle it is invisible for the detector. But as it carries away energy and momentum it could be reconstructed with the missing energy in an event. However, in order to recognize that energy is missing we have to measure visible particles the neutralino recoils against. If at the LHC a pair of squarks is produced in the collision of two protons and both squarks decay in a quarks and a neutralino, we would see events with two quarks (recognized as “jets” in the detector) and missing energy from the invisible neutralinos. This is an important signature that is looked for at the LHC.
How could we interpret the presence or absence of a signal in the search for jets and missing energy in a specific model? This is not trivial, since the significance of the search in general depends on details of the model. For instance, as supersymmetry has a huge number of free parameters, the significance of the search can in principle depend on the masses of supersymmetric particles other than the squark and neutralino. 
In this article we investigated this question studying to what extent the “simplified squark model” (left figure) introduced by the experimental collaborations can be used to draw conclusions about more general supersymmetric models where the production is also mediated by a gluino, the supersymmetric partner of the gluon (as shown in the Feynman diagram on the right).
In addition to supersymmetry, there are many other possible models of physics beyond the Standard Model. 
One such model postulates extra spatial dimensions (see also notes by various speakers at the TASI Lectures). It also predicts new quarks (see Feynman diagram on the right), but this time particles with spin 1/2: this means they have the same spin as the Standard Model quarks. We can call this model a same-spin model. Could we also use the results for a supersymmetric simplified squark model to say something about excluded masses of quarks and the lightest particles of a same-spin model? It turns out that one can.
We theorists then continue to use results from searches for simplified models and apply these to our favorite models of physics beyond the Standard Model. Many tools exist for exactly this purpose; one example is SModelS. We look forward to the fresh start of the LHC this year!
Super-precision for the Large Hadron Collider
PhD student Mathieu Pellen describes his research on precision calculations for supersymmetry as published in two recent scientific articles (open access versions can be found here and here).
Despite the discovery of the Higgs boson, numerous theoretical issues in particles physics remain unexplained. This is the reason why new theories are required. These theories can be tested in experiments such as the LHC (Large Hadron Collider, CERN, Geneva) and supersymmetry is one of the best motivated theories beyond the standard model. It is thus a major task of the experimental collaborations to search for supersymmetric particles. So far no sign of the existence of supersymmetry in collider experiments has been seen. Nonetheless, there is still lots of room for supersymmetry to be discovered and the next run of the LHC might unravel its nature.
In order to match the unprecedented accuracy of experimental measurements, precise and appropriate theoretical predictions are required. This is achieved by calculating supersymmetric processes with high accuracy. This means calculating it at next-to-leading order (NLO), i.e. the second order in perturbation theory. In addition to this, in order to have more realistic predictions, these calculations have to be matched with so-called parton showers that account for further radiations of quarks and gluons. The aim of this article has thus been to perform a calculation of squark-antisquark (superpartners of the quark) production supplemented by their decay at NLO in perturbation theory and matched with parton showers. The conclusion of this study is that precise predictions in supersymmetric theories are important for LHC phenomenology.
Exemplary diagram of NLO calculation matched with parton shower in supersymmetry. The particles with a tilde are supersymmetric particles.
Supersymmetry or just a bunch of logarithms?
In its first phase from 2010 to 2013, CERN’s Large Hadron Collider has delivered an impressive amount of new results. The LHC experiments ATLAS and CMS have measured a myriad of particle scattering cross sections with unprecedented accuracy. These cross sections reflect the probability of producing certain particles in the collisions of protons, smashed at each other by the LHC. The so-called “Stairway to Heaven” plot below shows the remarkable agreement between experimental cross section measurements (points) and the theoretical predictions (lines) within the Standard Model of particle physics. Read the rest of this entry »
Susy edges
Two weeks ago, the SUSY conference took place in Manchester, and we already had a nice report from Mathieu and Jory here in our blog. However, I also want to draw your attention to this talk about an overview of CMS searches for supersymmetric particles at the LHC. Generic searches for supersymmetric particles depend mainly on two possible observations: in most supersymmetric scenarios, one has a lightest stable supersymmetric particle (that can play the role of the dark matter candidate). This particle, if produced at a collider like the LHC, does not decay any more (stable!) and does not leave any trace in the detectors. No trace? No! Read the rest of this entry »