## Selection of the most important publications

### Publications Research Area A

#### Proposal for a Cosmic Axion Spin Precession Experiment (CASPEr)

Dmitry Budker, Peter W. Graham, Micah Ledbetter, Surjeet Rajendran, Alex Sushkov

We propose an experiment to search for QCD axion and axionlike-particle dark matter. Nuclei that are interacting with the background axion dark matter acquire time-varying CP-odd nuclear moments such as an electric dipole moment. In analogy with nuclear magnetic resonance, these moments cause precession of nuclear spins in a material sample in the presence of an electric field. Precision magnetometry can be used to search for such precession. An initial phase of this experiment could cover many orders of magnitude in axionlike-particle parameter space beyond the current astrophysical and laboratory limits. And with established techniques, the proposed experimental scheme has sensitivity to QCD axion masses ma≲10-9 eV, corresponding to theoretically well-motivated axion decay constants fa≳1016 GeV. With further improvements, this experiment could ultimately cover the entire range of masses ma≲μ eV, complementary to cavity searches.

Journal reference: DOI: Cite as: |
Phys. Rev. X 4, 021030 (2014) 10.1103/PhysRevX.4.021030 arXiv:1306.6089 [hep-ph] |

#### New Exclusion Limits for the Search of Scalar and Pseudoscalar Axion-Like Particles from "Light Shining Through a Wall"

OSQAR Collaboration

Physics beyond the Standard Model predicts the possible existence of new particles that can be searched at the low energy frontier in the sub-eV range. The OSQAR photon regeneration experiment looks for "Light Shining through a Wall" from the quantum oscillation of optical photons into "Weakly Interacting Sub-eV Particles", such as axion or Axion-Like Particles (ALPs), in a 9 T transverse magnetic field over the unprecedented length of 2×14.3 m. In 2014, this experiment has been run with an outstanding sensitivity, using an 18.5 W continuous wave laser emitting in the green at the single wavelength of 532 nm. No regenerated photons have been detected after the wall, pushing the limits for the existence of axions and ALPs down to an unprecedented level for such a type of laboratory experiment. The di-photon couplings of possible pseudo-scalar and scalar ALPs can be constrained in the nearly massless limit to be less than 3.5⋅10−8 GeV−1 and 3.2⋅10−8 GeV−1, respectively, at 95% Confidence Level.

Journal reference: DOI: Cite as: |
Phys. Rev. D 92, 092002 (2015) 10.1103/PhysRevD.92.092002 arXiv:1506.08082 [hep-ex] |

#### Neutrinos from the primary proton–proton fusion process in the Sun

BOREXINO Collaboration (G. Bellini et al.)

In the core of the Sun, energy is released through sequences of nuclear reactions that convert hydrogen into helium. The primary reaction is thought to be the fusion of two protons with the emission of a low-energy neutrino. These so-called pp neutrinos constitute nearly the entirety of the solar neutrino flux, vastly outnumbering those emitted in the reactions that follow. Although solar neutrinos from secondary processes have been observed, proving the nuclear origin of the Sun’s energy and contributing to the discovery of neutrino oscillations, those from proton–proton fusion have hitherto eluded direct detection. Here we report spectral observations of pp neutrinos, demonstrating that about 99 per cent of the power of the Sun, 3.84 × 1033 ergs per second, is generated by the proton–proton fusion process.

Journal reference: DOI: |
Nature 512 (2014) no.7515, 383-386 10.1038/nature13702 |

#### The MuPix System-on-Chip for the Mu3e Experiment

Heiko Augustin, Niklaus Berger, Sebastian Dittmeier, et al.

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ+→e+e−e+. Decay vertex position, decay time and particle momenta have to be precisely measured in order to reject both accidental and physics background. A silicon pixel tracker based on 50μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a 1 T solenoidal magnetic field provides precise vertex and momentum information. The MuPix chip combines pixel sensor cells with integrated analog electronics and a periphery with a complete digital readout. The MuPix7 is the first HV-MAPS prototype implementing all functionalities of the final sensor including a readout state machine and high speed serialization with 1.25 Gbit/s data output, allowing for a streaming readout in parallel to the data taking. The observed efficiency of the MuPix7 chip including the full readout system is ≥99% in a high rate test beam.

DOI: Cite as: |
10.1016/j.nima.2016.06.095 arXiv:1603.08751 [physics.ins-det] |

#### Dark Photon Search in the Mass Range Between 1.5 and 3.4 GeV/c2

BESIII Collaboration M. Ablikim et al.

Using a data set of 2.93 fb−1 taken at a center-of-mass energy s√ = 3.773 GeV with the BESIII detector at the BEPCII collider, we perform a search for an extra U(1) gauge boson, also denoted as a dark photon. We examine the initial state radiation reactions e+e−→e+e−γISR and e+e−→μ+μ−γISR for this search, where the dark photon would appear as an enhancement in the invariant mass distribution of the leptonic pairs. We observe no obvious enhancement in the mass range between 1.5 and 3.4 GeV/c2 and set a 90% confidence level upper limit on the mixing strength of the dark photon and the Standard Model photon. We obtain a competitive limit in the tested mass range.

Journal reference: DOI: Cite as: |
Physics Letters B 774, 252 (2017) 10.1016/j.physletb.2017.09.067 arXiv:1705.04265 [hep-ex] |

#### Search for light massive gauge bosons as an explanation of the (g−2)μ anomaly at MAMI

H. Merkel, P. Achenbach, et al.

A massive, but light abelian U(1) gauge boson is a well motivated possible signature of physics beyond the Standard Model of particle physics. In this paper, the search for the signal of such a U(1) gauge boson in electron-positron pair-production at the spectrometer setup of the A1 Collaboration at the Mainz Microtron (MAMI) is described. Exclusion limits in the mass range of 40 MeV up to 300 MeV with a sensitivity in the mixing parameter of down to ϵ2=8×10−7 are presented. A large fraction of the parameter space has been excluded where the discrepancy of the measured anomalous magnetic moment of the muon with theory might be explained by an additional U(1) gauge boson.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 112 (2014), 221802 10.1103/PhysRevLett.112.221802 arXiv:1404.5502 [hep-ex] |

#### Search for a dark photon in e+e- collisions at BABAR

The BABAR Collaboration

Dark sectors charged under a new Abelian force have recently received much attention in the context of dark matter models. These models introduce a light new mediator, the so-called dark photon (A'), connecting the dark sector to the Standard Model. We present a search for a dark photon in the reaction e+e- -> gamma A', A'-> e+e-, mu+mu- using 514 fb-1 of data collected with the BABAR detector. We do not observe a significant signal and we set 90% confidence level upper limits on the mixing strength between the photon and dark photon at the level of 10^-4 - 10^-3 for dark photon masses in the range 0.02 - 10.2 GeV. We further constrain the range of the parameter space favored by interpretations of the discrepancy between the calculated and measured anomalous magnetic moment of the muon.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 113, 201801 (2014) 10.1103/PhysRevLett.113.201801 arXiv:1406.2980 [hep-ex] |

#### Constraints on Spin-Dependent Short-Range Interaction between Nucleons

Kathlynne Tullney, Fabian Allmendinger, et al.

We report on the search for a new spin-dependent P- and T-violating interaction between nucleons mediated by light, pseudoscalar bosons such as the axion which was invented to solve the strong CP problem. Our experimental approach is to use an ultra-sensitive low-field magnetometer based on the detection of free precession of co-located 3He and 129Xe nuclear spins using SQUIDs as low-noise magnetic flux detectors. In the presence of an unpolarized mass the precession frequency shif

DOI: Cite as: |
10.1103/PhysRevLett.111.100801 arXiv:1303.6612 [hep-ex] |

#### Laser spectroscopy of muonic deuterium

CREMA Collaboration, Randolf Pohl et al.

The deuteron is the simplest compound nucleus, composed of one proton and one neutron. Deuteron properties such as the root-mean-square charge radius rd and the polarizability serve as important benchmarks for understanding the nuclear forces and structure. Muonic deuterium μd is the exotic atom formed by a deuteron and a negative muon μ–. We measured three 2S-2P transitions in μd and obtain rd = Embedded Image fm, which is 2.7 times more accurate but 7.5σ smaller than the CODATA-2010 value rd = Embedded Image fm. The μd value is also 3.5σ smaller than the rd value from electronic deuterium spectroscopy. The smaller rd, when combined with the electronic isotope shift, yields a “small” proton radius rp, similar to the one from muonic hydrogen, amplifying the proton radius puzzle.

Journal reference: DOI: |
Science 353 (2016) no.6300, 669-673 10.1126/science.aaf2468 |

#### Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector

IceCube Collaboration

We report on results of an all-sky search for high-energy neutrino events interacting within the IceCube neutrino detector conducted between May 2010 and May 2012. The search follows up on the previous detection of two PeV neutrino events, with improved sensitivity and extended energy coverage down to approximately 30 TeV. Twenty-six additional events were observed, substantially more than expected from atmospheric backgrounds. Combined, both searches reject a purely atmospheric origin for the twenty-eight events at the 4σ level. These twenty-eight events, which include the highest energy neutrinos ever observed, have flavors, directions, and energies inconsistent with those expected from the atmospheric muon and neutrino backgrounds. These properties are, however, consistent with generic predictions for an additional component of extraterrestrial origin.

Journal reference: DOI: Cite as: |
Science 342, 1242856 (2013) 10.1126/science.1242856 arXiv:1311.5238 [astro-ph.HE] |

## Research Area D

#### The hadronic vacuum polarization contribution to the muon g−2 from lattice QCD

M. Della Morte, A. Francis, V. Gülpers, G. Herdoíza, G. von Hippel, H. Horch, B. Jäger, H.B. Meyer, A. Nyffeler, H. Wittig

We present a calculation of the hadronic vacuum polarization contribution to the muon anomalous magnetic moment, a^{hv}^{p}_{μ}, in lattice QCD employing dynamical up and down quarks. We focus on controlling the infrared regime of the vacuum polarization function. To this end we employ several complementary approaches, including Padé fits, time moments and the time-momentum representation. We correct our results for finite-volume effects by combining the Gounaris-Sakurai parameterization of the timelike pion form factor with the Lüscher formalism. On a subset of our ensembles we have derived an upper bound on the magnitude of quark-disconnected diagrams and found that they decrease the estimate for a^{hvp}_{μ} by at most 2%. Our final result is a^{hvp}_{μ}=(654±32+21−23)⋅10−10, where the first error is statistical, and the second denotes the combined systematic uncertainty. Based on our findings we discuss the prospects for determining a^{hvp}_{μ} with sub-percent precision.

Journal reference: DOI: Cite as: |
JHEP 1710 (2017) 020 10.1007/JHEP10(2017)020 arXiv:1705.01775 [hep-lat] |

#### Simplifying differential equations for multi-scale Feynman integrals beyond multiple polylogarithms

Luise Adams, Ekta Chaubey, Stefan Weinzierl

In this paper we exploit factorisation properties of Picard-Fuchs operators to decouple differential equations for multi-scale Feynman integrals. The algorithm reduces the differential equations to blocks of the size of the order of the irreducible factors of the Picard-Fuchs operator. As a side product, our method can be used to easily convert the differential equations for Feynman integrals which evaluate to multiple polylogarithms to ε-form.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 118, 141602 (2017) 10.1103/PhysRevLett.118.141602 arXiv:1702.04279 [hep-ph] |

#### Dark Matter Decay between Weak Scale Phase Transitions

Michael J. Baker, Joachim Kopp

We propose a new alternative to the Weakly Interacting Massive Particle (WIMP) paradigm for dark matter. Rather than being determined by thermal freeze-out, the dark matter abundance in this scenario is set by dark matter decay, which is allowed for a limited amount of time just before the electroweak phase transition. More specifically, we consider fermionic singlet dark matter particles coupled weakly to a scalar mediator S3 and to auxiliary dark sector fields, charged under the Standard Model gauge groups. Dark matter freezes out while still relativistic, so its abundance is initially very large. As the Universe cools down, the scalar mediator develops a vacuum expectation value (vev), which breaks the symmetry that stabilises dark matter. This allows dark matter to mix with charged fermions and decay. During this epoch, the dark matter abundance is reduced to give the value observed today. Later, the SM Higgs field also develops a vev, which feeds back into the S3 potential and restores the dark sector symmetry. In a concrete model we show that this "vev flip-flop" scenario is phenomenologically successful in the most interesting regions of its parameter space. We also comment on detection prospects at the LHC and elsewhere.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 119, 061801 (2017) 10.1103/PhysRevLett.119.061801 arXiv:1608.07578 [hep-ph] |

#### Four-gluon scattering at three loops, infrared structure and Regge limit

Johannes M. Henn, Bernhard Mistlberger

We compute the three-loop four-gluon scattering amplitude in maximally supersymmetric Yang-Mills theory, including its full color dependence. Our result is the first complete computation of a non-planar four-particle scattering amplitude to three loops in four-dimensional gauge theory and consequently provides highly non-trivial data for the study of non-planar scattering amplitudes. We present the amplitude as a Laurent expansion in the dimensional regulator to finite order, with coefficients composed of harmonic poly-logarithms of uniform transcendental weight, and simple rational prefactors. Our computation provides an independent check of a recent result for three-loop corrections to the soft anomalous dimension matrix that predicts the general infrared singularity structure of massless gauge theory scattering amplitudes. Taking the Regge limit of our result, we determine the three-loop gluon Regge trajectory. We also find agreement with very recent predictions for sub-leading logarithms.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 117, 171601 (2016) 10.1103/PhysRevLett.117.171601 arXiv:1608.00850 [hep-th] |

#### One Leptoquark to Rule Them All: A Minimal Explanation for R_{D}(∗), R_{K} and (g−2)_{μ}

Martin Bauer (U. Heidelberg), Matthias Neubert (JGU Mainz and MITP)

We show that by adding a single new scalar particle to the Standard Model, a TeV-scale leptoquark with the quantum numbers of a right-handed down quark, one can explain in a natural way three of the most striking anomalies of particle physics: the violation of lepton universality in B¯→K¯ℓ+ℓ− decays, the enhanced B¯→D(∗)τν¯ decay rates, and the anomalous magnetic moment of the muon. Constraints from other precision measurements in the flavor sector can be satisfied without fine-tuning. Our model predicts enhanced B¯→K¯(∗)νν¯ decay rates and a new-physics contribution to Bs−B¯s mixing close to the current central fit value.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 116, 141802 (2016) 10.1103/PhysRevLett.116.141802 arXiv:1511.01900 [hep-ph] |

#### Lattice QCD calculation of hadronic light-by-light scattering

Jeremy Green, Oleksii Gryniuk, Georg von Hippel, Harvey B. Meyer, Vladimir Pascalutsa

We perform a lattice QCD calculation of the hadronic light-by-light scattering amplitude in a broad kinematical range. At forward kinematics, the results are compared to a phenomenological analysis based on dispersive sum rules for light-by-light scattering. The size of the pion pole contribution is investigated for momenta of typical hadronic size. The presented numerical methods can be used to compute the hadronic light-by-light contribution to the anomalous magnetic moment of the muon. Our calculations are carried out in two-flavor QCD with the pion mass in the range of 270 to 450MeV, and contain so far only the diagrams with fully connected quark lines.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 115, 222003 (2015) 10.1103/PhysRevLett.115.222003 arXiv:1507.01577 [hep-lat] |

#### Gravitational Waves From a Dark (Twin) Phase Transition

Pedro Schwaller

In this work, we show that a large class of models with a composite dark sector undergo a strong first order phase transition in the early universe, which could lead to a detectable gravitational wave signal. We summarise the basic conditions for a strong first order phase transition for SU(N) dark sectors with n_f flavours, calculate the gravitational wave spectrum and show that, depending on the dark confinement scale, it can be detected at eLISA or in pulsar timing array experiments. The gravitational wave signal provides a unique test of the gravitational interactions of a dark sector, and we discuss the complementarity with conventional searches for new dark sectors. The discussion includes Twin Higgs and SIMP models as well as symmetric and asymmetric composite dark matter scenarios.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 115, 181101 (2015) 10.1103/PhysRevLett.115.181101 arXiv:1504.07263 [hep-ph] |

#### Lepton universality test in the photoproduction of e^{−}e^{+} versus μ^{−}μ^{+} pairs on a proton target

Vladyslav Pauk, Marc Vanderhaeghen

In view of the significantly different proton charge radius extracted from muonic hydrogen Lamb shift measurements as compared to electronic hydrogen spectroscopy or electron scattering experiments, we study in this work the photoproduction of a lepton pair on a proton target in the limit of very small momentum transfer as a way to provide a test of the lepton universality when extracting the proton charge form factor. By detecting the recoiling proton in the γp→l^{−}l^{+}p reaction, we show that a measurement of a ratio of e^{−}e^{+}+μ^{−}μ^{+} over e^{−}e^{+} cross sections with a relative precision of around 2%, would allow for a test to distinguish between the two different proton charge radii currently extracted from muonic and electronic observables.

Journal reference: DOI: Cite as: |
Phys. Rev. Lett. 115, 221804 (2015) 10.1103/PhysRevLett.115.221804 arXiv:1503.01362 [hep-ph] |

#### On axionic dark matter in Type IIA string theory

Gabriele Honecker, Wieland Staessens

We investigate viable scenarios with various axions in the context of supersymmetric field theory and in globally consistent D-brane models. The Peccei-Quinn symmetry is associated with an anomalous U(1) symmetry, which acquires mass at the string scale but remains as a perturbative global symmetry at low energies. The origin of the scalar Higgs-axion potential from F-, D- and soft breaking terms is derived, and two Standard Model examples of global intersecting D6-brane models in Type II orientifolds are presented, which differ in the realisation of the Higgs sector and in the hidden sector, the latter of which is of particluar importance for the soft supersymmetry breaking terms.

Journal reference: DOI: Cite as: |
Fortsch.Phys.62(2014)115-151 10.1002/prop.201300036 arXiv:1312.4517 [hep-th] |

#### Optimizing the basis of B->K*ll observables in the full kinematic range

Sebastien Descotes-Genon, Tobias Hurth, Joaquim Matias, Javier Virto

We discuss the observables for the B->K*(->K pi)ll decay, focusing on both CP-averaged and CP-violating observables at large and low hadronic recoil with special emphasis on their low sensitivity to form-factor uncertainties. We identify an optimal basis of observables that balances theoretical and experimental advantages, which will guide the New Physics searches in the short term. We discuss some advantages of the observables in the basis, and in particular their improved sensitivity to New Physics compared to other observables. We present predictions within the Standard Model for the observables of interest, integrated over the appropriate bins including lepton mass corrections. Finally, we present bounds on the S-wave contribution to the distribution coming from the B->K_0*ll decay, which will help to establish the systematic error associated to this pollution.

Journal reference: DOI: Cite as: |
JHEP, 1305:137, 2013 10.1007/JHEP05(2013)137 arXiv:1303.5794 [hep-ph] |