Why the Large Hadron Collider is good for Philosophy
Guest post in Jon Butterworth’s Guardian blog: The philosophy of the Large Hadron Collider
There have been many tedious and futile discussions about the value of philosophy for modern science. I find it much more interesting and fruitful to ask if and in what way modern science can advance philosophy. The complexity, the new challenges and the new methods that arise in modern science in general – and at the LHC in particular – raise a number of questions that concern core issues of philoso
phy of science: what are the methods of acquiring knowledge, what is the role of models, and how does the intricate relationship between theory, computer simulations and experimental data work? The LHC has been built for fundamental physics, but it will also challenge and advance the philosophy, sociology and history of science!
See the full text at: The philosophy of the Large Hadron Collider
Exploring dark matter through electroweak radiation
PhD student Leila Ali Cavasonza reports on a her recent work on indirect dark matter searches.
Investigating the nature of Dark Matter is certainly one of the most compelling and exciting goals of particle and astroparticle physics nowadays, both on the experimental and on the theoretical side.
It is now almost universally assumed that the Dark Matter consists of one or more new particles. According to the observations, this new particle is neutral, non relativistic, massive, weakly interacting and with a small self-interaction cross section. One of the most prominent candidate are the so-called Weakly Interactive Massive Particles (WIMPS).
These particles could interact with ordinary matter and be detected in the so-called direct detection experiments. Or they could be produced via annihilation of standard particles and discovered at colliders. Or they could annihilate into standard particles like photons, electrons and positrons or neutrinos and produce an excess in the fluxes of standard model particles observed in cosmic rays.
Indirect detection experiments, like the Alpha Magnetic Spectrometer (AMS, see left figure) measure the composition and the fluxes of the cosmic rays with very high precision in order to detect such possible excesses.
The AMS experiment has actually found a significant excess in the positron fluxes, that is at the moment not explained by standard astrophysics (figure below). On the other hand no excess has been observed for example in the antiproton fluxes.
The positron fraction measured by AMS (red circles) compared with the expectation from ordinary cosmic-ray collisions.
To explain this situation, the so called leptophilic models for Dark Matter have been introduced: According to these models the Dark Matter particles can annihilate only into leptons, like electrons and positrons, but not in hadrons, like the antiprotons.
However, the situation is not so simple. A very energetic electron produced via WIMPS annihilation can actually radiate a Z or W boson and produce at the end all stable standard particles, including antiprotons. To have a consistent picture and accurate predictions for the AMS experiment in the frame of leptophilic models it is then necessary to take into account the electroweak radiation off the standard model final state.
The so called fragmentation functions approximation has been developed in order to include these contributions in a simple and model independent way. In our paper arXiv: 1409.8226 we analyse the quality of this approximation. In particular we produce predictions for the AMS experiment within a simple leptophilic model including the electroweak radiation in the complete theory and with the approximation respectively. We then compare the predicted fluxes to understand how reliable the approximation is. It turns out that for some models the approximation is not valid. On the other hand, when valid, the approximation is actually very reliable and it is possible to obtain accurate predictions in a faster and simpler way.
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.
Bouncing robots and deformed planets
On a long hiking trip we were bored on our way back. So what do you do if you are a physicist and if you are outside and have no data and nothing to look up? You bring up some Fermi-Problem to solve as a pastime. Our up-to-date problem chosen was concerning the marvelous landing of Philae on the comet 67P/Churyumov–Gerasimenko. After the first “landing” the little robot bounced back and it took it around 2 hours to touch the comet again (resulting in at least one more bounce). The question at hand is: How far up did the robot bounce? We tried to answer this question, but this lead us to some more questions with quite unintuitive results.
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 »
Autumn school in Maria Laach
(c) by Andreas Künsken
One of our PhD students, Lennart, participated at the recent autumn school for High Energy Physics in Maria Laach. This school is aimed at PHD students in theoretical and experimental high energy physics and besides particle physics, it also gives you an insight into the live in a monastery…
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Species
The origin and size of the neutrinos masses is one of the unsolved mysteries in High energy physics. Their masses must be very small (< 1eV) in comparison to the other elementary particles: if the neutrino had the weight of a house mite, the top quark would have the mass of a sperm whale! For theorists, this hierarchy in the masses is really unsatisfying, and the question is if there is a mechanism that makes the neutrino mass automatically that small.
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 »