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RWTH Aachen Particle Physics Theory

Super-precision for the Large Hadron Collider

February 5th, 2015 | by

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.

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Exemplary diagram of NLO calculation matched with parton shower in supersymmetry. The particles with a tilde are supersymmetric particles.

Bouncing robots and deformed planets

November 22nd, 2014 | by

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.

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Supersymmetry or just a bunch of logarithms?

November 20th, 2014 | by

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

October 3rd, 2014 | by
The monastery of Maria Laach, foto by Andreas Künsken

(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

August 9th, 2014 | by

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.

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Susy edges

August 2nd, 2014 | by

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 »

Back from the SUSY conference

July 29th, 2014 | by

Jory and Mathieu are PHD students working on beyond the standard model physics. They presented their current projects (click and click) at the SUSY 2014 conference in Manchester last week. Read on about their experiences at the conference…

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Schüleruni Physik

July 21st, 2014 | by

logo4_hell_merge

This week we have the “Schüleruni Physik “. 25 very motivated high school students aged 16-18 visit the institute to get an impression of life at the university and get some insight in whether they like to study physics at the RWTH. This year, we have planned great talks about particle physics at the collider and in space. In the afternoon, the students have the possibility to experiment and visit labs.

If you have missed to apply for this year, go here to find out about other RWTH events for high school students.

Dark matter mysteries

July 20th, 2014 | by

The bullet cluster are two galaxy clusters roughly 3.8 billion light years away in the Carina constellation in the southern sky.  Galaxy clusters are gravitationally bound accumulations of galaxies. The bullet cluster is an object of particular interest: Since its discovery in 1995, it has been an object of study with different observation methods. In the optical light, there seem to be two separate galaxy clusters with a distance of roughly 0.7 Mpc. The X-ray observation reveals, that these two galaxy cluster collided in the past and are now separating again. The bullet cluster is a textbook example for such two objects interacting, leading to a bow shock which can be nicely studied in the X-ray image of the object. However, there is something else which is very interesting about this object: The collision separates two components of the galaxy clusters, namely the luminous mass of the cluster and the main mass components of the cluster, that can not be seen in the optical or X-ray region. This hints towards a large amount of dark matter taking part in the collision. And this makes it very interesting for particle physicists as well! Read the rest of this entry »

Are there leptoquarks?

July 16th, 2014 | by

leptoquarks
Leptoquarks are interesting objects: These (so far) hypothetical particles are able to connect a single Lepton and a single quark in one interaction. For triggering such an interaction, leptoquarks have to carry both lepton number (such as leptons) and baryon quantum number (such as quarks). Hence the name: Lept-o-quark.

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