Search For Rare Processes Research Articles

Techniques for Background Identification in the Search for Rare Processes with Crystal Scintillators

In astroparticle, nuclear and subnuclear physics, low-counting experiments play an increasingly important role in the investigation of rare processes such as dark matter, double beta decay, some neutrino processes and low-background spectrometry. Extremely low-background features are more and more required to produce detectors and apparata of suitable sensitivity. Over time, a great deal of interest and attention in developing experimental techniques suitable to improve, verify and maintain the radiopurity of these detectors has arisen. In this paper, the characterization of inorganic crystal scintillators (such as, e.g., NaI(Tl), ZnWO4 and CdWO4) using α, β and γ radioactive sources and the main experimental techniques applied in the field to quantitatively identify the radioactive contaminants are highlighted; in particular, we focus on inorganic crystal scintillators, widely used in rare processes investigation, considering their applications at noncryogenic temperatures in the framework of the DAMA experiment activities at the Gran Sasso National Laboratory of the INFN (National Institute for Nuclear Physics, INFN).

Search for rare processes with DAMA experimental set-ups

Profiting of the favourable conditions offered by the Gran Sasso underground laboratory and of the several low-background DAMA set-ups, many and competitive results have been obtained for rare processes. Here the main results will be briefly resumed and some more details about the latest activities will be given. In particular, the searches on ββdecays of 106Cd and 116Cd and the perspectives of a complementary investigation on those Dark Matter (DM) candidates inducing just nuclear recoils, by exploiting the directionality approach with the anisotropic ZnWO4 scintillators, will be underlined.

The Matrix Element Method in the LHC era

The Matrix Element Method (MEM) is a powerful multivariate method allowing to maximally exploit the experimental and theoretical information available to an analysis. The method is reviewed in depth, and several recent applications of the MEM at LHC experiments are discussed, such as searches for rare processes and measurements of Standard Model observables in Higgs and Top physics. Finally, a new implementation of the MEM is presented. This project builds on established phase-space parametrisations known to greatly improve the speed of the calculations, and aims at a much improved modularity and maintainability compared to previous software, easing the use of the MEM for high-statistics data analyses.

The Matrix Element Method at the LHC: status and prospects for Run II

The Matrix Element Method (MEM) is a powerful multivariate method allowing to maximally exploit the experimental and theoretical information available to an analysis. Applications of the MEM at LHC experiments are discussed, such as searches for rare processes and measurements of properties of the Standard Model Higgs boson. The MadWeight software, allowing for a fast and automated computation of MEM weights for any user- specified process, is briefly reviewed. A new implementation of the MEM in the C++ language, MoMEMta, is presented. Building on MadWeight's tricks to accelerate the calculations, it aims at a much improved modularity and maintainability. Examples of this modularity are discussed: the possibility to compute several weights in parallel (propagation of systematic uncertainties), the Differential MEM (DMEM), and a novel way to search for lion-resonant. New Physics.

Crystal scintillators for low background measurements

Some general arguments in the light of the application of the inorganic crystal scintillators in the search for rare processes are addressed. The continuous innovation provided by physics, chemistry and technology in their development allows the improvement of their performances. Also the possibility of detectors exploiting new materials is open and the further application to this field of research is promising.

Search for dark matter with CRESST

The search for direct interactions of dark matter particles remains one of the most pressing challenges of contemporary experimental physics. A variety of different approaches is required to probe the available parameter space and to meet the technological challenges. Here, we review the experimental efforts towards the detection of direct dark matter interactions using scintillating crystals at cryogenic temperatures. We outline the ideas behind these detectors and describe the principles of their operation. Recent developments are summarized and various results from the search for rare processes are presented. In the search for direct dark matter interactions, the CRESST-II experiment delivers competitive limits, with a sensitivity below 5×10-7 pb on the coherent WIMP-nucleon cross section.

New approaches to dark matter and neutrino detection

Two new initiatives in astroparticle and neutrino physics are briefly reviewed. COUPP employs ultra-stable heavy liquid bubble chambers to search for WIMP dark matter. First results leading to improved limits on spin-dependent WIMP couplings are presented, together with the most recent progress and prospects. CoGeNT aims at the detection of very faint (∼100 eV) signals in detectors massive enough (∼1 kg) to allow searches for rare processes, using recently developed p-type point contact (PPC) germanium detectors. The broad range of applications (coherent neutrino scattering, light WIMP searches, double-beta decay) for these semiconductor devices is succinctly described. The implications from their first underground operation are discussed, as well as the status of an ongoing deployment at the San Onofre nuclear generation plant.

Coherent neutrino detection in the year 2008

CoGeNT, a recent collaboration between Argonne National Laboratory, CANBERRA Industries, Lawrence Livermore National Laboratory, Pacific Northwest National Laboratory, Sandia National Laboratory, the University of Chicago and the University of Washington, aims at the detection of very faint (∼100 eV) signals in devices massive enough (∼1 kg) to allow searches for rare processes, using recently developed p-type point contact (PPC) germanium detectors. The broad range of applications for these detectors is succinctly described. Implications from their first underground operation are discussed. Finally, the status of an ongoing deployment at the San Onofre nuclear generation station, aiming at obtaining a first evidence for coherent (anti)neutrino interactions with nuclei, is briefly described.

Associated charm production in neutrino–nucleus interactions

In this paper a search for associated charm production both in neutral and charged current $\nu$-nucleus interactions is presented. The improvement of automatic scanning systems in the {CHORUS} experiment allows an efficient search to be performed in emulsion for short-lived particles. Hence a search for rare processes, like the associated charm production, becomes possible through the observation of the double charm-decay topology with a very low background. About 130,000 $\nu$ interactions located in the emulsion target have been analysed. Three events with two charm decays have been observed in the neutral-current sample with an estimated background of 0.18$\pm$0.05. The relative rate of the associated charm cross-section in deep inelastic $\nu$ interactions, $\sigma(c\bar{c}\nu)/\sigma_\mathrm{NC}^\mathrm{DIS}= (3.62^{+2.95}_{-2.42}({stat})\pm 0.54({syst}))\times 10^{-3}$ has been measured. One event with two charm decays has been observed in charged-current $\nu_\mu$ interactions with an estimated background of 0.18$\pm$0.06 and the upper limit on associated charm production in charged-current interactions at 90% C.L. has been found to be $\sigma (c\bar{c} \mu^-)/\sigma_\mathrm{CC} < 9.69 \times 10^{-4}$.

Search for rare processes with DAMA/LXe experiment at Gran Sasso

After a short introduction on the low background liquid xenon DAMA set-up (DAMA/LXe) and its main previous results, we discuss the search for the nucleon, di-nucleon and tri-nucleon decays into invisible channels (disappearance or decay to neutrinos, Majorons, etc.) in the 136Xe isotope. The obtained limits (90% C.L.) on the lifetimes are: τ n > 3.3 · 1023 yr, τ p > 4.5 · 1023 yr, τ np > 3.2 · 1023 yr, τ pp > 1.9 · 1024 yr, τ nnp > 1.4 · 1022 yr, τ npp > 2.7 · 1022 yr and τ ppp > 3.6 · 1022 yr. In particular, the tri-nucleon decay into invisible channels is investigated here for the first time.

Liquid rare gas detectors: recent developments and applications

In past years, liquid rare gas detectors (LRGD) have made a considerable progress and found application in various fields of research, such as high energy experiments, astrophysics, search for rare processes and medical imaging. This review paper starts by a short summary and discussion of the properties of liquid rare gases (LRG) that make them attractive as radiation detector media. Next, the two best-established technologies based on liquid rare gases, electromagnetic calorimeters and time projection chambers, are briefly reviewed along with their main applications. The bulk of the paper is focused on the challenging development of LRGD for the search of rare processes in low background experiments with an emphasis on those dealing with the direct detection of dark matter. The challenges posed by those experiments, the detectors proposed and their present status of development are described. A brief review of the proposals of using LRGD for medical imaging is outlined and the present status of some of the more significant developments under way is presented.

Lepton flavor violation at linear collider experiments in supersymmetric grand unified theories

Lepton flavor violation at linear collider experiments is discussed. We show that detectable lepton flavor violation could occur in scalar lepton pair production and decay in the supersymmetric SU(5) grand unified theory in spite of the stringent present experimental constraints by searches for rare processes. Possible cross sections of $\ensuremath{\sim}40$ fb for an ${e}^{+}{e}^{\ensuremath{-}}$ collider and $\ensuremath{\sim}280$ fb for an ${e}^{\ensuremath{-}}{e}^{\ensuremath{-}}$ collider are illustrated.

A method which eliminates the discreteness in Poisson confidence limits and lessens the effect of moving cuts specifically to eliminate candidate events

In setting upper limits in counting experiments, a method exists which removes the discreteness in limits normally present for the Poisson probability distribution. The method, while essentially the same mathematically as a controversial statistical technique proposed around 1950, is now given more physical meaning: event quality information supplements the pure number of candidate events in calculating an upper limit. The method ameliorates the problem in searches for rare processes whereby small changes in data selection criteria (cuts) can have large, discontinuous effects on the limit quoted. It may thus have the potential to ease the reconciliation of different choices of cuts, while reducing the potential for bias in cut selection when one has knowledge of potential candidate events. Paradoxically, the method generally gives more restrictive upper limits than the usual method (assuming no background subtraction in either method), while strictly adhering to the classical definition of confidence intervals. Background subtraction is not yet incorporated.