Dark matter results from 100 live days of XENON100 data

The Xenon Road to Direct Detection of Dark Matter at LNGS: The XENON Project

Dark matter is a milestone in the understanding of the Universe and a portal to the discovery of new physics beyond the Standard Model of particles. The direct search for dark matter has become one of the most active fields of experimental physics in the last few decades. Liquid Xenon (LXe) detectors demonstrated the highest sensitivities to the main dark matter candidates (Weakly Interactive Massive Particles, WIMP). The experiments of the XENON project, located in the underground INFN Laboratori Nazionali del Gran Sasso (LNGS) in Italy, are leading the field thanks to the dual-phase LXe time projection chamber (TPC) technology. Since the first prototype XENON10 built in 2005, each detector of the XENON project achieved the highest sensitivity to WIMP dark matter. XENON increased the LXe target mass by nearly a factor 400, up to the 5.9 t of the current XENONnT detector installed at LNGS in 2020. Thanks to an unprecedentedly low background level, XENON1T (predecessor of XENONnT) set the world best limits on WIMP dark matter to date, for an overall boost of more than 3 orders of magnitude to the experimental sensitivity since the XENON project started. In this work, we review the principles of direct dark matter detection with LXe TPCs, the detectors of the XENON project, the challenges posed by background mitigation to ultra-low levels, and the main results achieved by the XENON project in the search for dark matter.

Dark Matters on the Scale of Galaxies

The cold dark-matter model successfully explains both the emergence and evolution of cosmic structures on large scales and, when we include a cosmological constant, the properties of the homogeneous and isotropic Universe. However, the cold dark-matter model faces persistent challenges on the scales of galaxies. Indeed, N-body simulations predict some galaxy properties that are at odds with the observations. These discrepancies are primarily related to the dark-matter distribution in the innermost regions of the halos of galaxies and to the dynamical properties of dwarf galaxies. They may have three different origins: (1) the baryonic physics affecting galaxy formation is still poorly understood and it is thus not properly included in the model; (2) the actual properties of dark matter differs from those of the conventional cold dark matter; (3) the theory of gravity departs from General Relativity. Solving these discrepancies is a rapidly evolving research field. We illustrate some of the solutions proposed within the cold dark-matter model, and solutions when including warm dark matter, self-interacting dark matter, axion-like particles, or fuzzy dark matter. We also illustrate some modifications of the theory of gravity: Modified Newtonian Dynamics (MOND), MOdified Gravity (MOG), and f(R) gravity.

The waning of the WIMP? A review of models, searches, and constraints

Weakly Interacting Massive Particles (WIMPs) are among the best-motivated dark matter candidates. No conclusive signal, despite an extensive search program that combines, often in a complementary way, direct, indirect, and collider probes, has been detected so far. This situation might change in near future due to the advent of one/multi-TON Direct Detection experiments. We thus, find it timely to provide a review of the WIMP paradigm with focus on a few models which can be probed at best by these facilities. Collider and Indirect Detection, nevertheless, will not be neglected when they represent a complementary probe.

Search for Electronic Recoil Event Rate Modulation with 4 Years of XENON100 Data

We report on a search for electronic recoil event rate modulation signatures in the XENON100 data accumulated over a period of 4 yr, from January 2010 to January 2014. A profile likelihood method, which incorporates the stability of the XENON100 detector and the known electronic recoil background model, is used to quantify the significance of periodicity in the time distribution of events. There is a weak modulation signature at a period of 431_{-14}^{+16} day in the low energy region of (2.0-5.8) keV in the single scatter event sample, with a global significance of 1.9σ; however, no other more significant modulation is observed. The significance of an annual modulation signature drops from 2.8σ, from a previous analysis of a subset of this data, to 1.8σ with all data combined. Single scatter events in the low energy region are thus used to exclude the DAMA/LIBRA annual modulation as being due to dark matter electron interactions via axial vector coupling at 5.7σ.

The pMSSM10 after LHC run 1

We present a frequentist analysis of the parameter space of the pMSSM10, in which the following ten soft SUSY-breaking parameters are specified independently at the mean scalar top mass scale [Formula: see text]: the gaugino masses [Formula: see text], the first-and second-generation squark masses [Formula: see text], the third-generation squark mass [Formula: see text], a common slepton mass [Formula: see text] and a common trilinear mixing parameter A, as well as the Higgs mixing parameter [Formula: see text], the pseudoscalar Higgs mass [Formula: see text] and [Formula: see text], the ratio of the two Higgs vacuum expectation values. We use the MultiNest sampling algorithm with [Formula: see text]1.2 [Formula: see text] points to sample the pMSSM10 parameter space. A dedicated study shows that the sensitivities to strongly interacting sparticle masses of ATLAS and CMS searches for jets, leptons [Formula: see text][Formula: see text] signals depend only weakly on many of the other pMSSM10 parameters. With the aid of the Atom and Scorpion codes, we also implement the LHC searches for electroweakly interacting sparticles and light stops, so as to confront the pMSSM10 parameter space with all relevant SUSY searches. In addition, our analysis includes Higgs mass and rate measurements using the HiggsSignals code, SUSY Higgs exclusion bounds, the measurements of [Formula: see text] by LHCb and CMS, other B-physics observables, electroweak precision observables, the cold dark matter density and the XENON100 and LUX searches for spin-independent dark matter scattering, assuming that the cold dark matter is mainly provided by the lightest neutralino [Formula: see text]. We show that the pMSSM10 is able to provide a supersymmetric interpretation of [Formula: see text], unlike the CMSSM, NUHM1 and NUHM2. As a result, we find (omitting Higgs rates) that the minimum [Formula: see text] with 18 degrees of freedom (d.o.f.) in the pMSSM10, corresponding to a [Formula: see text] probability of 30.8 %, to be compared with [Formula: see text] in the CMSSM (NUHM1) (NUHM2). We display the one-dimensional likelihood functions for sparticle masses, and we show that they may be significantly lighter in the pMSSM10 than in the other models, e.g., the gluino may be as light as [Formula: see text]1250 [Formula: see text] at the 68 % CL, and squarks, stops, electroweak gauginos and sleptons may be much lighter than in the CMSSM, NUHM1 and NUHM2. We discuss the discovery potential of future LHC runs, [Formula: see text] colliders and direct detection experiments.

Search for Event Rate Modulation in XENON100 Electronic Recoil Data

We have searched for periodic variations of the electronic recoil event rate in the (2-6) keV energy range recorded between February 2011 and March 2012 with the XENON100 detector, adding up to 224.6 live days in total. Following a detailed study to establish the stability of the detector and its background contributions during this run, we performed an unbinned profile likelihood analysis to identify any periodicity up to 500 days. We find a global significance of less than 1σ for all periods, suggesting no statistically significant modulation in the data. While the local significance for an annual modulation is 2.8σ, the analysis of a multiple-scatter control sample and the phase of the modulation disfavor a dark matter interpretation. The DAMA/LIBRA annual modulation interpreted as a dark matter signature with axial-vector coupling of weakly interacting massive particles to electrons is excluded at 4.8σ.

Physics at the [Formula: see text] linear collider

A comprehensive review of physics at an [Formula: see text] linear collider in the energy range of [Formula: see text] GeV-3 TeV is presented in view of recent and expected LHC results, experiments from low-energy as well as astroparticle physics. The report focusses in particular on Higgs-boson, top-quark and electroweak precision physics, but also discusses several models of beyond the standard model physics such as supersymmetry, little Higgs models and extra gauge bosons. The connection to cosmology has been analysed as well.

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Search for dark matter, extra dimensions, and unparticles in monojet events in proton-proton collisions at [Formula: see text][Formula: see text]

Results are presented from a search for particle dark matter (DM), extra dimensions, and unparticles using events containing a jet and an imbalance in transverse momentum. The data were collected by the CMS detector in proton-proton collisions at the LHC and correspond to an integrated luminosity of 19.7[Formula: see text]at a centre-of-mass energy of 8[Formula: see text]. The number of observed events is found to be consistent with the standard model prediction. Limits are placed on the DM-nucleon scattering cross section as a function of the DM particle mass for spin-dependent and spin-independent interactions. Limits are also placed on the scale parameter [Formula: see text] in the Arkani-Hamed, Dimopoulos, and Dvali (ADD) model of large extra dimensions, and on the unparticle model parameter [Formula: see text]. The constraints on ADD models and unparticles are the most stringent limits in this channel and those on the DM-nucleon scattering cross section are an improvement over previous collider results.

Joining forces of Bayesian and frequentist methodology: a study for inference in the presence of non-identifiability

Increasingly complex applications involve large datasets in combination with nonlinear and high-dimensional mathematical models. In this context, statistical inference is a challenging issue that calls for pragmatic approaches that take advantage of both Bayesian and frequentist methods. The elegance of Bayesian methodology is founded in the propagation of information content provided by experimental data and prior assumptions to the posterior probability distribution of model predictions. However, for complex applications, experimental data and prior assumptions potentially constrain the posterior probability distribution insufficiently. In these situations, Bayesian Markov chain Monte Carlo sampling can be infeasible. From a frequentist point of view, insufficient experimental data and prior assumptions can be interpreted as non-identifiability. The profile-likelihood approach offers to detect and to resolve non-identifiability by experimental design iteratively. Therefore, it allows one to better constrain the posterior probability distribution until Markov chain Monte Carlo sampling can be used securely. Using an application from cell biology, we compare both methods and show that a successive application of the two methods facilitates a realistic assessment of uncertainty in model predictions.

Fourth generation parity

We present a very simple fourth generation (4G) model with an abelian gauge interaction under which only the 4G fermions have nonzero charge. The U(1) gauge symmetry can have a Z2 residual discrete symmetry (4G parity), which can stabilize the lightest 4G particle (L4P). When the 4G neutrino is the L4P, it would be a neutral and stable particle and the other 4G fermions would decay into the L4P, leaving the trace of missing energy plus the standard model fermions. Because of the new symmetry, the 4G particle creation and decay modes are different from those of the sequential 4G model, and the 4G particles can be appreciably lighter than typical experimental bounds.

Search for dark matter candidates and large extra dimensions in events with a photon and missing transverse momentum in pp collision data at sqrt[s]=7 TeV with the ATLAS detector

Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum in proton-proton collisions at sqrt[s] = 7 TeV are reported. Data collected by the ATLAS experiment at the LHC corresponding to an integrated luminosity of 4.6 fb(-1) are used. Good agreement is observed between the data and the standard model predictions. The results are translated into exclusion limits on models with large extra spatial dimensions and on pair production of weakly interacting dark matter candidates.

Dark matter results from 225 live days of XENON100 data

We report on a search for particle dark matter with the XENON100 experiment, operated at the Laboratori Nazionali del Gran Sasso for 13 months during 2011 and 2012. XENON100 features an ultralow electromagnetic background of (5.3 ± 0.6) × 10(-3) events/(keV(ee) × kg × day) in the energy region of interest. A blind analysis of 224.6 live days × 34 kg exposure has yielded no evidence for dark matter interactions. The two candidate events observed in the predefined nuclear recoil energy range of 6.6-30.5 keV(nr) are consistent with the background expectation of (1.0 ± 0.2) events. A profile likelihood analysis using a 6.6-43.3 keV(nr) energy range sets the most stringent limit on the spin-independent elastic weakly interacting massive particle-nucleon scattering cross section for weakly interacting massive particle masses above 8 GeV/c(2), with a minimum of 2 × 10(-45) cm(2) at 55 GeV/c(2) and 90% confidence level.

Astrophysics-independent bounds on the annual modulation of dark matter signals

We show how constraints on the time integrated event rate from a given dark matter (DM) direct detection experiment can be used to bound the amplitude of the annual modulation signal in another experiment. The method requires only mild assumptions about the properties of the local DM distribution: that it is temporally stable on the scale of months and spatially homogeneous on the ecliptic. We apply the method to the annual modulation signal in DAMA/LIBRA, which we compare to the bounds derived from XENON10, XENON100, cryogenic DM search, and SIMPLE data. Assuming a DM mass of 10 GeV, we show that under the above assumptions about the DM halo, a DM interpretation of the DAMA/LIBRA signal is excluded for several classes of models: at 6.3σ (4.6σ) for elastic isospin conserving (violating) spin-independent interactions, and at 4.9σ for elastic spin-dependent interactions on protons.

Modified Newtonian Dynamics (MOND): Observational Phenomenology and Relativistic Extensions

A wealth of astronomical data indicate the presence of mass discrepancies in the Universe. The motions observed in a variety of classes of extragalactic systems exceed what can be explained by the mass visible in stars and gas. Either (i) there is a vast amount of unseen mass in some novel form - dark matter - or (ii) the data indicate a breakdown of our understanding of dynamics on the relevant scales, or (iii) both. Here, we first review a few outstanding challenges for the dark matter interpretation of mass discrepancies in galaxies, purely based on observations and independently of any alternative theoretical framework. We then show that many of these puzzling observations are predicted by one single relation - Milgrom's law - involving an acceleration constant a₀ (or a characteristic surface density Σ_† = a₀/G) on the order of the square-root of the cosmological constant in natural units. This relation can at present most easily be interpreted as the effect of a single universal force law resulting from a modification of Newtonian dynamics (MOND) on galactic scales. We exhaustively review the current observational successes and problems of this alternative paradigm at all astrophysical scales, and summarize the various theoretical attempts (TeVeS, GEA, BIMOND, and others) made to effectively embed this modification of Newtonian dynamics within a relativistic theory of gravity.

Enhancement of dark matter capture by neutron stars in binary systems

We study the capture of dark matter particles by neutron stars in close binary systems. By performing a direct numerical simulation, we find that there is a sizable amplification of the rate of dark matter capture by each of the companions. In the case of the binary pulsar PSR J1906+0746 with the orbital period of 4 hours the amplification factor is approximately equal to 3.5. This amplification can be attributed to the energy loss by dark matter particles resulting from their gravitational scattering off moving companions.

Search for dark matter and large extra dimensions in pp collisions yielding a photon and missing transverse energy

Results are presented from a search for new physics in the final state containing a photon (γ) and missing transverse energy (E[combininb /](T)). The data correspond to an integrated luminosity of 5.0 fb(-1) collected in pp collisions at √[s]=7 TeV by the CMS experiment. The observed event yield agrees with standard-model expectations for the γ+E[combininb /](T) events. Using models for the production of dark-matter particles (χ), we set 90% confidence level (C.L.) upper limits of 13.6-15.4 fb on χ production in the γ+E[combininb /](T) state. These provide the most sensitive upper limits for spin-dependent χ-nucleon scattering for χ masses (M(χ)) between 1 and 100 GeV. For spin-independent contributions, the present limits are extended to M(χ)<3.5 GeV. For models with 3-6 large extra dimensions, our data exclude extra-dimensional Planck scales between 1.64 and 1.73 TeV at 95% C.L.

Final analysis and results of the Phase II SIMPLE dark matter search

We report the final results of the Phase II SIMPLE measurements, comprising two run stages of 15 superheated droplet detectors each, with the second stage including an improved neutron shielding. The analyses include a refined signal analysis, and revised nucleation efficiency based on a reanalysis of previously reported monochromatic neutron irradiations. The combined results yield a contour minimum of σp=5.7×10(-3)  pb at 35  GeV/c2 in the spin-dependent sector of weakly interacting massive particle (WIMP) proton interactions, the most restrictive to date for MW}≤60  GeV/c2 from a direct search experiment and overlapping, for the first time, with results previously obtained only indirectly. In the spin-independent sector, a minimum of 4.7×10(-6)  pb at 35  GeV/c2 is achieved, with the exclusion contour challenging a significant part of the light mass WIMP region of current interest.

Outstanding questions: physics beyond the Standard Model

The Standard Model of particle physics agrees very well with experiment, but many important questions remain unanswered, among them are the following. What is the origin of particle masses and are they due to a Higgs boson? How does one understand the number of species of matter particles and how do they mix? What is the origin of the difference between matter and antimatter, and is it related to the origin of the matter in the Universe? What is the nature of the astrophysical dark matter? How does one unify the fundamental interactions? How does one quantize gravity? In this article, I introduce these questions and discuss how they may be addressed by experiments at the Large Hadron Collider, with particular attention to the search for the Higgs boson and supersymmetry.

Sneutrino dark matter in gauged inverse seesaw models for neutrinos

Extending the minimal supersymmetric standard model to explain small neutrino masses via the inverse seesaw mechanism can lead to a new light supersymmetric scalar partner which can play the role of inelastic dark matter (IDM). It is a linear combination of the superpartners of the neutral fermions in the theory (the light left-handed neutrino and two heavy standard model singlet neutrinos) which can be very light with mass in ~5-20 GeV range, as suggested by some current direct detection experiments. The IDM in this class of models has keV-scale mass splitting, which is intimately connected to the small Majorana masses of neutrinos. We predict the differential scattering rate and annual modulation of the IDM signal which can be testable at future germanium- and xenon-based detectors.