Experimentelle Flavourphysik

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Studies of B->*D+D*- decays at the LHCb experiment
(2024) Langer, Jan; Albrecht, Johannes; Elsässer, Dominik
In this thesis, two measurements probing the Standard Model of particle physics in B->D*+D*- decays are presented. Both measurements are performed using proton-proton collision data recorded by the LHCb experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 9fb-1. The first analysis is a branching fraction measurement of the Bs->D*+D*- decay. The decay is observed with a significance of more than $10$ standard deviations. The absolute time-integrated Bs->D*+D*- branching fraction is measured to be B(Bs->D*+D*-) = (2.15 +- 0.26 +- 0.09 +- 0.06 +- 0.16) * 10^(-4) where the first uncertainty is statistical, the second systematic, the third the uncertainty of the fragmentation fraction ratio fs/fd and the fourth is due to the limited precision of the BdToDstDst branching fraction used in the normalisation. The second analysis presented is the measurement of CP violation in BdToDstDst decays. As the final state is a mixture of CP-even and CP-odd states, the CP observable S(D*+D*-) is diluted. Therefore, an angular analysis is performed to determine the dilution factor. The current state of the analysis and its sensitivity are presented and discussed. With the current precision, the uncertainty of the measurement is expected to be statistically dominated.
MIBAD - an FPGA-based readout system for the LHCb Beam Conditions Monitor
(2024) Bieker, Martin Stefan; Albrecht, Johannes; Elsässer, Dominik
Experiments at particle accelerators, such as the Large Hadron Collider (LHC), are at risk of damage due to the high-energy beams. The Beam Conditions Monitor protects the LHCb detector at the LHC. It measures the particle flux with diamond sensors at two positions near the interaction point. If the measured signals exceed safe levels, the BCM automatically triggers a dump of the LHC beams. The LHCb detector was upgraded from 2018 to 2022. Consequently, the BCM needed to be adapted to a five-fold increase in instantaneous luminosity and a new detector geometry. Moreover, the pre-existing BCM hardware card is no longer supported due to an experiment-wide replacement of the readout electronics. This thesis presents the Machine Interface Beam Abort (MIBAD) system as the BCM readout for the post-upgrade era. The development of the system, encompassing hardware, firmware, and software, is detailed. The main emphasis of this work is the implementation of the beam abort decision logic. As a vital part of the BCM safety system, the MIBAD continuously evaluates the beam conditions while meeting strict requirements regarding reliability and availability. A review of the testing and commissioning phase, as well as the experiences in the first months of operation, indicate that these goals have been met.The upgraded BCM, equipped with the MIBAD system, has safeguarded the LHCb detector since June 2023.
Precision measurements of 𝐬𝐢𝐧 (𝟐𝜷) with the LHCb experiment
(2023) Meier, Gerwin; Albrecht, Johannes; Delitzsch, Chris
High-precision measurements of observables of the Standard Model of particle physics, like the CKM matrix parameters, are key to answering the open questions of particle physics. The most precise CKM angle sin(2β) can be optimally determined by a decay-time dependent measurement of CP violation in B→ψKS0 decays. In this thesis data from B→J/ψ(→μμ)KS0, B→ψ(2S)(→μμ)KS0 and B→J/ψ(→ee)KS0 decays is analysed with KS0→π+π- collected at a centre-of-mass energy of √s = 13TeV with pp collisions by the LHCb experiment between 2015 and 2018. The data corresponds to an integrated luminosity of 6/fb and results in the combination of all three decays for the CP violation parameters in S(ψKS0) = 0.717 +/- 0.013 (stat.) +/- 0.008 (syst.), C(ψKS0) = 0.008 +/- 0.012 (stat.) +/- 0.003 (syst.), where S corresponds to sin(2β). This is consistent with the Standard Model predictions and with previous measurements. Furthermore, this is the most precise single measurement and more precise than the current world average.
Decay-time-dependent studies of strange beauty mesons
(2023) Führing, Marc Quentin; Albrecht, Johannes; Kröninger, Kevin
In this thesis, two decay-time-dependent analyses of Bs0-meson decays are presented. Precise measurements are performed using proton-proton collision data recorded by the LHCb experiment between 2015 and 2018. Based on the reconstruction and selection of 378 700 +/- 700 Bs0 to Ds- pi+ decays, the most precise measurement of the Bs0-Bsbar0 oscillation frequency DeltaMs = (17.7683 +/- 0.0051 +/- 0.0032) ps−1 in the world to date is achieved. This measurement alone is an essential test of the Standard Model of particle physics. In addition, it is a crucial ingredient to increase the precision of decay-time-dependent measurements of other Bs0-decay modes. The presented analysis of Bs0 to Ds K decays benefits from this updated measurement of DeltaMs and from further improvements of algorithms and methods. The CKM angle gamma = (74 +/- 11)°, an important Standard Model parameter, is measured based on 20 950 +/- 180 selected decays. Compared to the analysis using data from 2011 and 2012, the precision is improved beyond what would be expected from the increase in data sample sizes.
Brachytherapy meets X-rays
(2023) Manke, Henning; Albrecht, Johannes; Lühr, Armin
Intraocular tumours are a life-threatening risk. Smaller tumours can be treated with 106Ru applicators. Meanwhile, more prominent tumours get treated with 125I applicators, which results in a higher risk of side effects, or proton therapy, which is only available at a limited number of specialised centres and may lead to toxic tumour syndrome. Therefore, a novel combined therapy concept is presented and analysed in this thesis. The idea combines the use of 106Ru applicators and external beam radiotherapy performed with soft X-rays. With the confocal irradiation technique and the utilisation of the 106Ru applicator as a beam-stop for the X-ray beams, healthy tissue is spared from high dosages to reduce the risk of side effects. The combined therapy concept can be implemented in hospitals with relatively low costs and resembles a novel bulb-preserving modality for large intraocular tumours. As the first steps to this therapy concept, measurements are performed with a novel solid phantom with three different detector types to assess the absorption of the X-rays in the 106Ru applicator. Additionally, the clinical benefit is investigated with a novel software workflow, in which 3D models are created using patient data from the University Hospital Essen in Germany. The modalities are simulated with these 3D models. Dose-volume histograms are presented to evaluate the combined therapy concept and to compare it to stand-alone brachytherapy.
Measurement of rare leptonic and semi-baryonic B meson decays with the LHCb experiment
(2023) Becker, Maik; Albrecht, Johannes; Kröninger, Kevin
This thesis presents two measurements probing the Standard Model of particle physics with data recorded by the LHCb experiment in Run 1 and Run 2 of the Large Hadron Collider. The first measurement is a precise branching fraction determination of the purely leptonic Bs→μ⁺μ⁻ and Bd→μ⁺μ⁻ decays. The Bs→μ⁺μ⁻ branching fraction is determined with a relative uncertainty of 15%, making it the most precise single-experiment result available at publication time. The Bd→μ⁺μ⁻ decay is not observed. An upper limit on its branching fraction is calculated to be of O(1e-10) at a confidence level of 95%. These published results strongly constrain theoretical models of new physics effects in the flavour sector. The second analysis is the first search for the semi-baryonic Bs→ppμ⁺μ⁻ and Bd→ppμ⁺μ⁻ decays. It is the first time that decays of Bs mesons via flavour-changing neutral b→sl+l- transitions into a final state with baryons are studied. The current state of the measurement and preliminary results are presented. Expected upper limits on the branching fractions are determined to be of O(1e-9) at a confidence level of 95%.
Standalone track reconstruction on GPUs in the first stage of the upgraded LHCb trigger system & Preparations for measurements with strange hadrons in Run 3
(2022) Calefice, Lukas; Albrecht, Johannes; Kröninger, Kevin
The LHCb experiment is undergoing its first major detector upgrade to operate at a five times higher instantaneous luminosity during the Run 3 data taking period. It is equipped with a new set of tracking detectors (VELO, UT, SciFi) to match the conditions of an increased track multiplicity and radiation damage. The hardware trigger stage is removed. The first stage of the software trigger system is implemented to run on about 200 GPU cards with a throughput of 30MHz. An alternative tracking algorithm called Seeding & Matching for the first trigger stage is developed and presented in this thesis. Other than the formerly used forward tracking, the presented algorithm performs the tracking without making use of the UT which allows to run in the early data taking of Run 3 before the UT will be installed. The Seeding is a standalone reconstruction of track segments in the SciFi, which is followed by a Matching step where the SciFi seeds are matched to VELO track segments reconstructed beforehand. The physics and computing performance of the Seeding & Matching is evaluated and found to be compatible with the forward tracking. The Seeding & Matching is now used as the new baseline algorithm and currently being commissioned on the first Run 3 data. Furthermore, preparations for an early Run 3 measurement of the ratios of the production cross-section of Λ and KS hadrons are presented in this thesis.
Search for new physics in rare decays of 𝑩 mesons at the LHCb experiment
(2022) Battig, Alexander; Albrecht, Johannes; Kröninger, Kevin
Diese Arbeit präsentiert Suchen nach den rein leptonischen Zerfällen Bs0 → e+e- und B0 → e+e-, ebenso wie den leptonfamilienzahlverletzenden Zerfällen B+ → K+ e+ μ-. Da diese Zerfälle im Standardmodell selten oder verboten sind ermöglichen sie Nulltests des Standardmodells. Außerdem ermöglichen Messungen dieser Kanäle es Modellen von Physik jenseits des Standardmodells Einschränkungen aufzuerlegen. Die Suche nach den Zerfällen Bs0 → e+e- und B0 → e+e- nutzt Daten, welche vom LHCb Experiment in den Jahren 2011, 2012, 2015 und 2016 aufgezeichnet wurden und stellt die erste Suche am LHC nach diesen Zerfällen dar. Die bestimmten oberen Ausschlussgrenzen auf die Verzweigungsverhältnisse von Bs0 → e+e- und B0 → e+e- liegen in der Größenordnung O(10-8), was die vorhergegangenen Ausschlussgrenzen um einen Faktor 30 verbessert. Die Suche nach den leptonfamilienzahlverletzenden Zerfällen B+ → K+ e+ μ- nutzt den kompletten LHCb Datensatz, was obigen Datensätzen die Daten aus den Jahren 2017 und 2018 hinzufügt und so die Menge der B Mesonen im analysierten Datensatz effektiv verdoppelt. Da diese Analyse noch nicht veröffentlicht wurde, wird der Status der Analyse zum Zeitpunkt der Abfassung dieses Textes präsentiert.
Measurements of decays in the B->DD family at the LHCb experiment
(2022) Mödden, Antje; Albrecht, Johannes; Kröninger, Kevin
Precision measurements of CKM-matrix elements are used to test the Standard Model of particle physics. Decays of Bs0 mesons that proceed via b→ ccs transitions are used to measure the CP -violating weak mixing phase φs that is directly related to the CKM angle βs and has a precise theoretical prediction. The mixing phase φs can be measured using Bs0 → Ds+Ds− decays, where higher-order Standard Model effects can be assessed by exploiting results from analyses in additional decays of the B → DD family. These analyses comprise CP -violation and branching-ratio measurements. Two different analyses of B → DD decays are presented, which utilise data collected by the LHCb experiment. A search for the Bs0 → D∗±D∓ decay is presented using data corresponding to an integrated luminosity of 9 fb−1 collected at centre-of-mass energies of 7, 8 and 13 TeV. The Bs0 → D∗±D∓ decay is observed and its branching fraction is measured for the first time. The ongoing measurement of CP violation in Bs0 → Ds+Ds− decays uses data corresponding to 6 fb−1 collected at 13 TeV. The measurement of the CP parameters is still kept blind, but the sensitivities to the CP parameters are evaluated and indicate that this will be the most precise measurement in this decay channel.
Measurements of CP violation in B0→ DD decays at the LHCb experiment
(2022) Ibis, Philipp; Albrecht, Johannes; Kröninger, Kevin
Precise measurements of CKM matrix parameters to probe the Standard Model of particle physics are a main focus of current flavour-physics experiments. Analyses of B → DD decays allow to assess higher-order Standard Model corrections in these measurements. Two decay-time-dependent measurements of CP violation in different B0 → DD decay modes are presented using data collected by the LHCb experiment. The analysis of B0 → D∗±D∓ decays utilises data corresponding to an integrated luminosity of 9 fb−1 collected at centre-of-mass energies of 7, 8 and 13 TeV. The results of the CP parameters correspond to the most precise measurement in this channel. They are consistent with the previous world average. The analysis of B0 → D+D− decays uses data corresponding to 6 fb−1 collected at 13 TeV. The analysis is ongoing and currently still blind. The evaluated sensitivities show that this will be the most precise measurement of CP violation in this channel.
Test of lepton flavour universality with rare beauty-quark decays at the LHCb experiment
(2022) Seuthe, Alex; Albrecht, Johannes; Kröninger, Kevin
Recent measurements of the lepton flavour universality show evidence of a violation of the Standard Model of particle physics. However, further investigation with higher sensitivity is needed to either falsify or confirm the anomalies. This thesis describes the first simultaneous measurement of the lepton flavour universality ratios 𝑅𝐾 with 𝐵+ → 𝐾 +ℓ+ℓ− decays and 𝑅𝐾∗0 with 𝐵0 → 𝐾∗0ℓ+ℓ− decays using the entire current available data set of the LHCb experiment recorded during the years 2011 to 2018. Both ratios are evaluated in two regions of 𝑞2, which is the squared invariant mass of the dilepton system. Even though the central values of the results are kept blind and will only be published once the analysis has passed the collaboration-internal review process, the presented sensitivities on 𝑅𝐾 and 𝑅𝐾∗0 already show that this will be the most precise test of lepton flavour universality with these decays to date. The analysis work described in this thesis comprises the first measurement of 𝑅𝐾 at low 𝑞2 at LHCb and an updated measurement of 𝑅𝐾∗0 with an enlarged data set. Statistical and systematic uncertainties as well as correlations between the four measurements are thus determined coherently for the first time and allow for improved interpretations. The calibration of the simulation used to determine the selection efficiency is a crucial component of the analysis to ensure that deviations of the measurements from the Standard Model expectation are not an effect due to remaining imperfections of the simulation. Therefore, a complex multi-stage calibration procedure has been developed together with an optimised selection strategy. Simulation of the signal decays is calibrated with the help of high-yield control samples chosen to minimise correlations between calibration and signal data sets. The experimental procedure of the analysis is validated by multiple stringent cross-checks.
Strange oscillating beauty-mesons
(2021) Heinicke, Kevin; Spaan, Bernhard; Rhode, Wolfgang
In dieser Arbeit wird eine Messung der Bs-BsBar Oszillationsfrequenz DeltaMs vorgestellt. Diese Frequenz ermöglicht die Bestimmung einiger Parameter der CKM-Matrix und dient als wichtiger externer Parameter für Messungen von CP-Verletzung mit Bs-Mesonen. Die Analyse wird mit einem Datensatz von 378 700 Zerfällen von Bs → DsPi durchgeführt, was einer integrierten Luminosität von 6/fb von Proton-Proton Kollisionen entspricht. Der Datensatz wurde zwischen 2015 und 2018 bei einer Schwerpunktsenergie von 13 TeV mit dem LHCb-Experiment aufgenommen. Die Oszillationsfrequenz wird mit Hilfe eines ungebinnten und gewichteten Maximum-Likelihood-Fits zu DeltaMs = (17.7683 ± 0.0051 ± 0.0032)/ps bestimmt. Das Ergebnis stellt eine der bisher präzisesten Messungen der LHCb-Kollaboration dar. Es ist im Einklang mit aktuellen Voraussagen des Standard Modells.
SciFi meets GPU
(2021) Stevens, Holger; Spaan, Bernhard; Rhode, Wolfgang
This thesis presents studies towards the LHCb detector upgrade for the LHC Run III. Mostly the entire detector is renewed to profit from the changed beam conditions. Two of the main changes are a fivefold instantaneous luminosity and a pure software trigger. Because of the higher luminosity, the track density is increased. Therefore, components with higher granularity are used, and one of them is the SciFi tracker. It is made of 250 μm thick scintillating fibres. The light output of this fibres decreases through radiation damages and natural material ageing. These effects are added to the detector simulation. Thereby the tracking efficiency of the SciFi is simulated for the entire planned lifetime. The first stage of the LHCb software trigger reduces the incoming data-rate of 40 Tbit s−1 by a factor of 40. The necessary computations are paralellisable, and for this reason, a GPU based implementation is suitable. A GPU version of the SciFi hit decoder was implemented during the baseline architecture change process from CPU to GPU. The LHCb detector has a diamond-sensor based safety system, which monitors the LHC beam. This system is renewed in parallel to the detector upgrade. Measurements towards the diamond sensor efficiency characterisation with a strontium source are presented.
Measurements of beauty hadron to charmonium decays
(2021) Mackowiak, Patrick; Spaan, Bernhard; Kröninger, Kevin
This thesis presents two measurements involving a beauty hadron decaying to a charmonium meson and a strange hadron. First, a measurement of the ratio of branching fractions for Λb→ψ(2S)Λ and Λb→J/ψΛ decays is presented. This analysis uses a data set corresponding to 3 fb-1, collected by the LHCb experiment at a centre-of-mass energy of 7 and 8 TeV. The ratio of branching fractions is determined to be BR(Λb→ψ(2S)Λ)/BR(Λb→J/ψΛ)= 0.513 ± 0.023 (stat) ± 0.016 (syst) ± 0.011 (BR). The second analysis is a on-going measurement of CP violation in g in B0→J/ψKS, B0→ψ(2S)KS and Bs0→J/ψKS decays. The analysed LHCb data sets correspond to 6 fb-1, at a centre-of-mass energy of 13 TeV. The measured sensitivities of the CP parameters S and C in the analysed Bd0 decays are σ(S) = 0.0133 (stat) ± 0.0063 (syst), σ(C) = 0.0125 (stat) ± 0.0042 (syst). This single measurement will have a better sensitivity than the current world average.
Beautiful leptons - setting limits to New Physics with the LHCb experiment
(2020) Mombächer, Titus; Albrecht, Johannes; Kröninger, Kevin
In this thesis, searches for physics effects beyond the Standard Model are performed by studying purely leptonic and lepton-flavour violating B-meson decays with the LHCb experiment. Advanced statistical tools to interpret nulltests are implemented and used in the analysis of the very rare decays discussed in this thesis. A search for lepton-flavour violating B+→K+µ±e∓ decays is performed based on the LHCb Run 1 sample as the first analysis of these transitions with LHCb data. Upper limits on their branching fractions are set below 10^{-8}, which improves previous limits by more than an order of magnitude. Similarly, B0(s)→e+e- decays have never before been studied with the LHCb experiment. The upper limits on their branching fractions determined in this thesis with Run 1 and part of Run 2 data improve previous limits by a factor of 30, reaching a sensitivity of O(10^{-8}). With this improvement the limits become sensitive to scenarios of new physics effects. In a third analysis, the B0(s)→µ+µ- decays are studied with the full LHCb Run 1 and Run 2 data sample. The analysis is currently being reviewed by the collaboration. Its precision is expected to be similar to that of the combination of previous results of the LHC experiments obtained on a partial data set. The results obtained in this thesis impose strong constraints on popular scenarios of new physics effects.
Alternative approaches in the event reconstruction of LHCb
(2019) Hasse, Christoph; Albrecht, Johannes; Kröninger, Kevin
Starting in 2021, the LHCb experiment will switch to a fully software-based trigger system. It is an ongoing challenge to ensure that this system will be able to process events at the required rate of 30 MHz. Two alternative approaches to improve and speed up the particle track reconstruction performed in the first stage of LHCb’s software trigger are presented. An alternative method to the Kalman filter track fit inside the VELO reconstruction is presented. This method uses neural networks to perform the fit and additionally provides an estimation of a track’s momentum based on the track’s scattering. The momentum information enables improved uncertainty estimates for a trajectory’s impact parameter, an important quantity to select tracks from secondary vertices. It is shown that the neural network’s uncertainty prediction enables an equally efficient selection of secondary tracks while reducing the amount of falsely selected prompt tracks by 30%. Additionally, an alternative procedure to reconstruct tracks that traverse the entire LHCb detector is presented. These tracks are LHCb’s main physics objects and their reconstruction previously required a third of the overall processing time of the first trigger stage’s reconstruction sequence. A new algorithm design is proposed, which is shown to yield similar reconstruction efficiencies while providing an over six-fold speedup over the current algorithm. A comparison of trigger configurations, which are able to process data at the rate of 30 MHz, is presented. It shows that a trigger based on the new algorithm yields significantly higher selection efficiencies.
Measurement of 𝘾𝙋 violation in the decay 𝘽0→𝘿∗±𝘿∓ with data from the LHCb experiment
(2019) Schellenberg, Margarete; Spaan, Bernhard; Kröninger, Kevin
This thesis presents the first LHCb measurement of 𝐶𝑃 violation in 𝐵0 → 𝐷∗±𝐷∓ decays. Due to the 𝑏 → 𝑐𝑐𝑑 transition, 𝐵0 → 𝐷∗±𝐷∓ decays give access to higher-order Standard Model contributions. The decay-time-dependent measurement is performed on a dataset corresponding to an integrated luminosity of 9 fb−1 recorded by the LHCb detector in proton-proton collisions at centre-of-mass energies of 7, 8 and 13 TeV. The 𝐶𝑃 observables are measured with an unbinned maximum likelihood fit as 𝑆𝐷∗𝐷 = −0.861 ± 0.077 (stat) ± 0.019 (syst), Δ𝑆𝐷∗𝐷 = 0.019 ± 0.075 (stat) ± 0.012 (syst), 𝐶𝐷∗𝐷 = −0.059 ± 0.092 (stat) ± 0.020 (syst), Δ𝐶𝐷∗𝐷 = −0.031 ± 0.092 (stat) ± 0.016 (syst), 𝐴𝐷∗𝐷 = 0.008 ± 0.014 (stat) ± 0.006 (syst). To date, the analysis is the most precise single measurement of 𝐶𝑃 violation in this decay channel. All parameters are consistent with their current world average values.
Search for high-confidence blazar candidates and their MWL counterparts in the Fermi-LAT catalog using machine learning
(2016-08-26) Einecke, Sabrina
A large fraction of the gamma-ray sources presented in the Third Fermi-LAT source catalog (3FGL) is affiliated with counterparts and source types, but 1010 sources remain unassociated and 573 sources are associated with active galaxies of uncertain type. The purpose of this study is to assign blazar classes to these unassociated and uncertain sources, and to link counterparts to the unassociated. A machine learning algorithm is used for the classification, based on properties extracted from the 3FGL, an infrared and an X-ray catalog. To estimate the reliability of the classification, performance measures are considered through validation techniques. The classification yielded purity values around 90% with efficiency values of roughly 50%. The prediction of high-confidence blazar candidates has been conducted successfully, and the possibility to link counterparts in the same procedure has been proven. These findings confirm the relevance of this novel multiwavelength approach.
Multiwavelength picture of the blazar S5 0716+714 during its brightest outburst
(2016-11-24) Manganaro, Marina; Pedaletti, Giovanna; Doert, Marlene; Bastieri, Denis; Ramazani, Vandad Fallah; Gasparrini, Dario; Lindfors, Elina; Lott, Benoit; Nievas, Mireia; Rani, Bindu; Thompson, David J.; Angelakis, Emmanouil; Borman, George; Gurwell, Mark; Hovatta, Talvikki; Itoh, Ryosuke; Jorstad, Svetlana; Kraus, Alex; Krichbaum, Thomas P.; Kuin, Paul; Lähteenmäki, Anne; Larionov, Valeri; Lien, Amy Yarleen; Myserlis, Ioannis; Tornikoski, Merja; Troitsky, Ivan; Zensus, J. Anton
S5 0716+714 is a well known BL Lac object, and one of the brightest and most active blazars. The discovery in the Very High Energy band (VHE, E > 100 GeV) by MAGIC happened in 2008. In January 2015, the source went through the brightest optical state ever observed, triggering MAGIC follow-up and a VHE detection with ∼ 13σ significance (ATel ♯6999 ). Rich multiwavelength coverage of the flare allowed us to construct the broad-band spectral energy distribution of S5 0716+714 during its brightest outburst. In this work, we will present the preliminary analysis of MAGIC and Fermi-LAT data of the flaring activity in January and February 2015 for the HE (0.1 < HE < 300 GeV) and VHE band, together with radio (Metsähovi, OVRO, VLBA, Effelsberg), sub-millimeter (SMA), optical (Tuorla, Perkins, Steward, AZT-8+ST7, LX-200, Kanata), X-ray and UV (Swift-XRT and UVOT), in the same time-window and discuss the time variability of the multiwavelength light curves during this impressive outburst.
Long-term monitoring of bright blazars in the multi-GeV to TeV range with FACT
(2017-02-20) Temme, Fabian; Adam, Jan; Ahnen, Max L.; Baack, Dominik; Balbo, Matteo; Bergmann, Matthias; Biland, Adrian; Blank, Michael; Bretz, Thomas; Brügge, Kai A.; Buss, Jens; Dmytriiev, Anton; Dorner, Daniela; Einecke, Sabrina; Hempfling, Christina; Hildebrand, Dorothee; Hughes, Gareth; Linhoff, Lena; Mannheim, Karl; Müller, Sebastian; Neise, Dominik; Neronov, Andrii; Nöthe, Max; Paravac, Aleksander; Pauss, Felicitas; Rhode, Wolfgang; Shukla, Amit; Thaele, Julia; Walter, Roland
Blazars like Markarian 421 or Markarian 501 are active galactic nuclei (AGN), with their jets orientated towards the observer. They are among the brightest objects in the very high energy (VHE) gamma ray regime (>100 GeV). Their emitted gamma-ray fluxes are extremely variable, with changing activity levels on timescales between minutes, months, and even years. Several questions are part of the current research, such as the question of the emission regions or the engine of the AGN and the particle acceleration. A dedicated longterm monitoring program is necessary to investigate the properties of blazars in detail. A densely sampled and unbiased light curve allows for observation of both high and low states of the sources, and the combination with multi-wavelength observation could contribute to the answer of several questions mentioned above. FACT (First G-APD Cherenkov Telescope) is the first operational telescope using silicon photomultiplier (SiPM, also known as Geigermode—Avalanche Photo Diode, G-APD) as photon detectors. SiPM have a very homogenous and stable longterm performance, and allow operation even during full moon without any filter, leading to a maximal duty cycle for an Imaging Air Cherenkov Telescope (IACT). Hence, FACT is an ideal device for such a longterm monitoring of bright blazars. A small set of sources (e.g., Markarian 421, Markarian 501, 1ES 1959+650, and 1ES 2344+51.4) is currently being monitored. In this contribution, the FACT telescope and the concept of longterm monitoring of bright blazars will be introduced. The results of the monitoring program will be shown, and the advantages of densely sampled and unbiased light curves will be discussed.

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