5.1 Condence belt, at 90% condence level, for a Poissonian variable. . . 82 A.1 Calibration constant for each CCD. . . 123
Chapter 1 Introduction
The electric charge quantization is a fundamental open problem in particle physics arised from the historical Milikan's oil drop experiment in 1909 [1]. This rst evidence that electric charges in Nature come in discrete units e inspired numerous experiments not only to rene Milikan's original determination of the unit charge but also to search for free particles violating this quantization. Within the development of particle physics, the quark model proposed by Gell-Mann and Zweig [2, 3] modied the original charge quantization statement to its modern meaning: all elementary particles observed have an electric charge as an integer multiple of the d-quark. As quarks are conned in color-neutral baryons and mesons that carry integer electric charges, free charge in nature comes in discrete unitse.
No mechanism included in the Standard Model (SM) framework ensures electric charge quantization. In fact, it can be demonstrated that charge may be dequantized in the SM with massless neutrinos and three lepton generations [4], as the SM Lagrangian contains anomaly free global symmetries independent of the standard hypercharge Y. However, as described by Dirac in his seminal work [5], the existence of magnetic monopoles would explain the observed electric charge quantization in a way consistent with the quantum electrodynamics. This solution to the problem motivated an extensive research eld ded-icated to the observation of this new particle. The lack of experimental evidence for monopoles opened the possibility of encountering exotic particles with fractional electro-magnetic charge.
These hypothetical particles which vilolate the charge quantization principle are often referred in the literature to as milli- or minicharged particles (mCPs), fractionally charged particles (FCPs) and lightly ionizing particles (LIPs). The direct observation of a mCP is an undisputed signature of a beyond the Standard Model phenomenon. Unbound quarks, bound states of quarks with noninteger charge or even new leptons are only a few possible mCPs predicted by a great variety of theories. For example, a SU(2)L singlet can always be added to the SM Lagrangian, postulating in this way a new particle with an arbitrary small charge [6]. A hidden sector scenario (new interactions decoupled from the SM particles) with a local unbroken gauge group U(1)h added to the SM groups naturally, includes mCPs [7]. In these theories, the SM photon and the hidden photon has a kinect mixing so that a particle charged under the U(1)h appears to have a small coupling to the photon. Also, as there are four anomaly-free U(1) symmetries in the SM, the hypercharge operator can be redened to give a fractional electric charge i to the neutrinos without making the theory anomalous [8].
This great diversity of extensions to the Standard Model have been exploited experi-mentally over the years. First, the addition of new exotic particles to the Standard Model can drastically alter well studied physical process depending on the particle's properties.
As a consequence, possible mCP mass and charge should be constrained by the known physics. From the astrophysical point of view, limits on the mCP mass/charge parameter space have been set from red giants (RG) [812], horizontal-branch (HB) [814] stars, white dwarf (WD) [8, 1518], and supernova (SN) 1987A [8, 19, 20] data. For the sun in particular, sound speed proles and neutrino ux data are used to constraint mCP param-eter space [6]. Cosmologically, the existence of a mCP would have an impact at the Early Universe modifying the element abundance predicted by the Big Bang Nucleosinthesis (BBN) theory and the observed Cosmic Microwave Background (CMB) [6,10]. To probe the mass/charge parameter space outside these restrictions, dierent experimental tech-niques were designed. For example, direct production and observation at xed target experiments or colliders [8, 2124], detection of some relic abundance in bulk matter [25]
or in meteoritic materials [26] using modern Milikan oil drop experiments, search for excess of ν−e scattering events at reactor neutrino experiments [27], observation of in-visible ortho-positronium decay [28] and measurement of deviation on the expected Lamb shift [21,29,30] or magnetic moment of the electron [29]. The non observation of a mCP canditate provides more constraints on this particle characteristics. The summary of the observed constraints on fermionic mCPs revised in [6] is presented in gure 1.1.
Figure 1.1: Summary of constraints on fermionic MCPs in the mass/charge plane. The results are divided in models including a massless hidden photon (left) and models without (right). The bounds were derived using CMB, BBN and astrophysical objects (HB, WD, RG stars and supernova 1987A) data, collider search at SLAC, LHC and LEP, observation of the positronium decay, Milikan oil drop experiments in bulk matter and meteoritic materials, production at reactor neutrinos experiments. Figure taken from [6].
As depicted in gure 1.1, there is a signicant part of parameter space allowed for particles characterized by low charge fraction and mass larger than the electron mass. An important channel to directly exploit this region is the cosmogenic search. The interaction between high energy cosmic rays and nuclei in the atmosphere could produce mCPs in the same way they would be produced in accelerators. These particles are expected to have a low energy deposition in matter, proportional to the square of their charges, and a straight line trajectory. With this behavior, it is possible for the mCP to reach
a very specic class of detectors already built underground for science programs that require low natural radiation levels. Using this approach, stringent limits were placed on the mCP ux at the detectors site, including MACRO (Monopole, Astrophysics, Cosmic Ray Observatory) [31], Kamiokande-II [32], and LSD (Liquid Scintillation Detector) [33]
results for particles with charge fraction0.4<1/f <6, the Cryogenic Dark Matter Search (CDMS) experiment [34] in the range with 1/f < 200 and the Majorana Demonstrator for1/f <1000 [35]. The combination of these results found in [35] is shown in gure 1.2.
Figure 1.2: Limits at 90%of condence level on the mCP ux at the detector site derived from Majorana Demonstrator MACRO, Kamiokande-II, LSD, and CDMS data. Figure taken from [35].
The aim of this work is to search for cosmogenic mCPs with data obtained by the Dark Matter in CCDs (DAMIC) experiment [3639]. The existence of dark matter is well established by a variety of observational evidence [4042]. From astrophysical to cosmological scales, there is a discrepancy between the observed universe and the one expected when considering it contains only SM particles. Beside the regular barionic matter, the universe must have a non-luminous cold component interacting gravitationally.
This component, whose nature is still unknown and the subject of many investigations, is called cold dark matter. Hypothesized to be a new particle, there is a great eort to directly detect a dark matter particle. These direct detection experiments search for the scattering signal of a dark matter particle naturally present in our galaxy. They are installed deep underground to reduce the high level of radioactivity produced by cosmic rays and shielded against the natural radioactivity from the surrounding rock. In this context, the DAMIC experiment employs the bulk silicon of scientic-grade charge-coupled devices (CCDs) to detect coherent elastic scattering of weakly interacting massive particles (WIMPs). WIMPs are highly motivated candidates for being the dark matter in the Universe. As silicon is a relatively light element, DAMIC is optimized to search for low mass WIMPs, operating with low noise and at eV scale energy threshold.
This PhD thesis presents the limits on the ux of cosmogenic mCPs derived from DAMIC data. It shows that not only the DAMIC apparatus is adequate to search for free mCPs, being competitive with previous limits (see gure 1.2), but also it is capable of excluding new parameter space due to its unique energy resolution. In parallel to this main analysis, it also presents two results in the context of the dark matter search. First, the monitoring of the detector operation parameters for the experiment during the last science data acquisition run. This service work for the experiment is essencial to ensure good quality data. Second, the simulation of the expected radioactive background on the upgraded version of the experiment, DAMIC-M, exploring possible designs for the construction of the experiment according to our background level requirement. This work serves as guideline to decide the detector design which will be implemented.
The text of the thesis is organized as following: chapter 2 is dedicated to review in details the search for mCPs. Two theoretical models, one with a hidden photon and an-other without, are discussed as examples of the huge variety of models predicting mCPs.
After giving the details of the main techniques to search for these particles and their results, the method to search for cosmogenic mCPs is derived. Chapter 3, presents a brief introduction to the dark matter problem and extensively discusses the CCD operation as
a particle detector. DAMIC scientic goals and results are highlighted. The nal section of this chapter describes the DAMIC-M, an upgraded version of the DAMIC experiment, presenting the rst work of this thesis. Still under development, it is shown the simu-lation of the expected radioactive background of DAMIC-M, exploring possible designs for the construction of the experiment according to the experiment's background level re-quirement. Chapter 4 is dedicated to the application of event reconstruction chain of the DAMIC experiment and the monitoring of the detector operation parameters. The search for cosmogenic mCPs with the DAMIC data is presented in chapter 5. First, a simulation of the interaction between a mCP in the CCDs, including the event reconstruction, is explained. Then, based on this Monte Carlo, selection variables used to characterize a mCP were chosen. The nal analysis presents the limits on the ux of cosmogenic mCPs at the DAMIC site derived after analyzing the data acquired by the experiment. Finally, chapter 6 summarizes all the results, analyzing the future of the experiment and this analysis.