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RWTH Aachen Particle Physics Theory

The Higgs Boson in 2015

October 23rd, 2015 | by

The Higgs boson has been discovered in 2012 at CERN’s Large Hadron Collider (LHC), or more precisely at the LHC experiments ATLAS and CMS. The discovery of the Higgs resonance is definitely a milestone in particle physics and two of the fathers of the Higgs mechanism, Peter Higgs and Francois Englert, were awarded the nobel prize in physics in 2013.

After the discovery three years ago, there was immediately a decisive question to be answered: Has the discovered particle all the properties which are predicted by the Standard Model (SM) of particle physics. Or turning the question around in more scientific terms: Are there any statistically significant deviations from the SM predictions which can be identified with the recorded proton-proton collisions. Any such deviation would of course call for physics beyond the SM. Until the end of LHC run 1 at the beginning auf 2013, there have been great efforts to collect as many Higgs collisions as possible. And similar efforts have been invested in recent years to extract as much information as possible from the recorded data.

Higgs coupling strength measurements

Signal strength measurements for different Higgs-production channels, where a signal strenath of 1 is the SM expectation (taken from ATLAS CONF-2015-044)

Only recently the Higgs legacy of run 1 has been finalized performing the combination of the Atlas and the CMS data. The so-called signal strength for different production channels and the global signal strength is shown in the diagram on the right relative to the SM prediction. So far, the discovered particle does not give any hints for new physics beyond the SM. It simply looks more or less as predicted decades ago.

For these and similar measurements, the interplay between theory predictions and the experimental analysis is most crucial. Mid of October, experimentalists and theorist working on Higgs physics have met at the conference “Higgs Couplings 2015” which was hosted by the IPPP in Durham and took place in the beautiful medieval Lumley Castle close to Durham. The latest run 1 measurements have been presented and discussed. But run 1 is already part of the past. Everybody is looking forward to seeing the measurements from run 2 and gearing up for the upcoming analyses.

Run 2 has already started this year with the record breaking proton-proton energy of 13 TeV (run 1 has provided 7 and 8 TeV collisions). In 2015, a year to learn how to operate at the record-breaking energy and with collisions every 25 nano-secons, there will be not enough data recorded to make a major step forward in the precision of Higgs measurements (this is very different for other new physics searches, e.g. for multi-TeV resonances). However, the coming years of run 2 will be exciting for Higgs physics for sure.

So far, measurements are still statistically limited, i.e. by the relatively small number of recorded Higgs-bosons. However, residual uncertainties within the theoretical predictions will soon enter as a major player in the quest for ultimate measurements of Higgs properties, and therefore also in the quest for physics beyond the SM in the Higgs sector. Hence, improving theory predictions and making them available for the analysis of the data is more important than ever in the field of Higgs physics, and one of the research topics at our institute. Conferences like the “Higgs Couplings 2015” are providing an important forum for discussions on these topics between experimentalists and theorists.

So, let’s see what nature will teach us about the Higgs in the coming years.

Exploring dark matter with IceCube and the LHC

October 2nd, 2015 | by

Various astrophysical and cosmological observations point towards the existence of dark matter, possibly a novel kind of fundamental particle, which does not emit or reflect light, and which only interacts weakly with ordinary matter.

DM_detection

If such a dark matter particle exists, it can be searched for in different ways: direct detection looks for the elastic scattering of dark matter with nuclei in highly sensitive underground experiments, as Earth passes through our galaxy’s dark matter halo. Indirect detection experiments on Earth or in space look for cosmic rays (neutrinos, photons, or antiparticles) from the annihilation of dark matter particles in the centre of the Galaxy or in the Sun. And last but not least, if dark matter interacts with ordinary matter, it may be produced in high-energy proton collisions at the LHC.

To explore the nature of dark matter, and to be able to combine results from direct, indirect and collider searches, one can follow a more model-independent approach and describe dark matter and its experimental signatures with a minimal amount of new particles, interactions and model parameters. Such simplified or minimal models allow to explore the landscape of dark matter theories, and serve as a mediator between the experimental searches and more complete theories of dark matter, like supersymmetry.

 

ATLAS_monojet

About 1027 dark matter particles per second may pass through the Sun. They can loose some energy through scattering off protons and eventually be captured in the core of the Sun by the Sun’s gravitational pull. Dark matter particles in the Sun would annihilate with each other and produce ordinary particles, some of which decay into neutrinos. Neutrinos interact weakly with matter, can thus escape the Sun and could be observed by the neutrino telescope IceCube near the South Pole. Neutrinos therefore provide a way to search for dark matter in the core of the Sun.

At the LHC, dark matter may be produced in high-energy proton collisions. As dark matter particles interact at most weakly with ordinary matter, they would leave no trace in the LHC detectors. However, dark matter (and other novel weakly interacting particles) can be detected by looking at exotic signatures, where a single spray of ordinary particles is seen, without the momentum and energy balance characteristic for standard particle collisions (so-called mono-jet events, see right figure).

 

DM
We have recently joined forces with members of the RWTH IceCube team to explore dark matter searches from neutrinos in the Sun and through dark matter production at the LHC, see http://arxiv.org/abs/1411.5917 and http://arxiv.org/abs/1509.07867.  We have considered a minimal dark matter model where we only add two new particles to the ordinary matter: a new dark matter fermion, and a new force particle, which mediates the interaction between the dark matter fermion and the ordinary matter. As no signal for dark matter has been observed, we can place limits on the masses of the dark matter particle and the new force particle, see figure to the left. We find a strinking complementarity of the different experimental approaches, which probe particular and often distinct regions of the model parameter space.

Thus only the combination of future collider, indirect and direct searches for dark matter will allow a comprehensive test of minimal dark matter models.

Anticipating Discoveries: LHC14 and Beyond

July 17th, 2015 | by

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.

 

screen_shot_2015-07-14_at_08.13.25In 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.

 

ATLAS_jets

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

Axions, WIMPs or WISPs? Searching for dark matter

July 3rd, 2015 | by

PhD student Mathieu Pellen reports from a dark matter workshop in Zaragoza.

The quest for the understanding of dark matter is certainly one of the greatest challenges of the 21st century. It is thus an extremely hot topic in the particle physics community. 

conference_photo

The 11th Patras Workshop on Axions, WIMPs and WISPs has been held in the beautiful and hot city of Zaragoza (Spain) (21-26 June 2015). As the title indicates, the focus was on dark matter and more particularly on axions.

Axions have been originally proposed to solve the strong CP problem. They are light particles (of the order of an electron-Volt or even lighter). These can be detected in light-shinning-through-wall experiments or in low background underground laboratories like the one of Canfranc (which has been one the highlights of the conference). During the conference, several innovative experiments looking for axions, axion-like particles or dark photons have been presented. New mechanisms predicting the existence of light particles have been also proposed.

In addition to light particles, Weakly Interacting Massive Particles (WIMPs) are the best motivated solution to account for the dark matter observed in our Universe. WIMPs are studied in three different ways: the first is their production at collider experiments such as the Large Hadron Collider (LHC, Geneva). The second is the detection of nuclear recoils produced by dark matter particles scattering on heavy nuclei in underground facilities such as the Grand Sasso laboratory in Italy. Finally, when two dark matter particles annihilate in the galaxy, they produce cosmic rays of standard model particles. These can be detected in satellite-based experiments such as the Alpha Magnetic Spectrometer (AMS-02, partly built at the RWTH Aachen University) on the International Space Station (ISS).

My contribution to the conference focuses on the last possibility. I have reported exciting results on a project carried out with Leila Ali Cavasonza and Michael Krämer.CCnew2_09_14 Indeed, AMS-02 has reported an excess in the measurement of the positron flux (red date points, left figure) compared to standard expectations from astrophysical sources (green curve, left figure). This has triggered a lot of interest recently. The reason is that anti-particles are an extremely interesting observable when searching for dark matter. Indeed they are rarely produced from standard astrophysical sources. Thus the discovery of excesses in anti-particles fluxes could be already a smoking gun for the existence of dark matter. Nowadays, the dark matter contribution is believed to be sub-dominant in the AMS-02 observations. However, the absence of a “bump” – as expected from a from a dark matter signal – in the very smooth AMS-02 spectrum is a great opportunity to set limits on dark matter annihilation cross sections.

MathieuWe have derived new upper limits on the annihilation cross section using a new method that allows us to study dark matter with masses ranging from several TeV down to 1 GeV. In particular we have focused on the impact of massive electro-weak gauge bosons on these limits. Even if their contributions are limited, they are of prime importance as they produce all standard model particles when decaying. I have thus shown that there is a promising complementarity between different fluxes of anti-particles. This opens up new ways to exclude or find dark matter in the next few years using indirect detection.

Why the Large Hadron Collider is good for Philosophy

June 14th, 2015 | by

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 philosob6d92a04-2231-42ce-814c-c82d86b80ebd-2060x1236phy 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

May 1st, 2015 | by

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.

AMS_on_ISS_700k

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.

CCnew2_09_14

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

February 13th, 2015 | by

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.

squark_decayIt 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.

missinge

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. T2_CMS t_channel_gluinoIn 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. kk_quark_decay 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

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.

pictureout

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.

Read the rest of this entry »

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 »