% our theorists, on DM-e scattering in Si: https://arxiv.org/pdf/1509.01598.pdf
% DOE BRN: https://science.osti.gov/-/media/hep/pdf/Reports/Dark_Matter_New_Initiatives_rpt.pdf
% PDG DM: https://pdg.lbl.gov/2020/reviews/rpp2020-rev-dark-matter.pdf
% last SENSEI: https://arxiv.org/pdf/2004.11378.pdf


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%\title[GEANT4/EGS5]{GEANT4/EGS5}

\title[SENSEI]{Skipper-CCDs and the SENSEI Search for Sub-GeV Dark Matter}

\author[Sho Uemura]{Sho Uemura}
\institute{Tel Aviv University\\for the SENSEI Collaboration\\\vspace{1cm}{\tiny SU was supported in part by the Zuckerman STEM Leadership Program}}
\date[May 28, 2021]

%\titlegraphic{
%\includegraphics[height=0.1\textheight]{SLAC_Logo}\hspace*{4.75cm}~
%\includegraphics[height=0.1\textheight]{partner_logo_v2}
%}

\begin{document}

%\setcounter{framenumber}{2} 
\begin{frame}
    \titlepage
\end{frame}

\begin{frame}{The search for sub-GeV dark matter}
    \begin{itemize}
        \item Direct-detection experiments have focused on WIMP DM down to $O(\mathrm{GeV})$
            \begin{itemize}
                \item Well motivated (``WIMP miracle''), experimentally accessible (nuclear recoils)
            \end{itemize}
        \item New complementary searches for lower masses
            \begin{itemize}
                \item New theoretical interest in dark sectors
                \item New technologies with lower thresholds
            \end{itemize}
    \end{itemize}
    \begin{columns}
        \column{0.6\textwidth}
        \begin{itemize}
            \item One promising direction: Skipper-CCDs for electron recoil, with thresholds near the silicon bandgap
                \begin{itemize}
                    \item Many others, both for NR and ER --- refer back to M-C. Piro Monday talk
                \end{itemize}
            \item The DM results I show are for the 2020 test run of SENSEI (arXiV:2004.11378), but the program is just getting started
        \end{itemize}
        \column{0.4\textwidth}
        \includegraphics[width=\textwidth]{dm_landscape}
    \end{columns}
\end{frame}

\begin{frame}{Electron recoils in silicon}
    \begin{columns}
        \column{0.6\textwidth}
        \begin{itemize}
            \item We look for DM interactions with electrons in silicon
                \begin{itemize}
                    \item DM-electron scattering: $m_\chi$ in the MeV-GeV range
                        \begin{itemize}
                            \item Energy transfer is a few eV, good match to semiconductor band gap
                        \end{itemize}
                    \item DM absorption: bosonic DM at the eV scale
                        \begin{itemize}
                            \item Energy transfer equals $m_\chi$
                        \end{itemize}
                \end{itemize}
            \item Energy transfer creates ionization, and we measure the number of electron-hole pairs
                %\item Typical electron recoil energy is few eV
                %\item Silicon bandgap gives us sensitivity to 1.2 eV excitations --- if we can capture and resolve a single electron
        \end{itemize}
        \begin{center}
            \includegraphics[width=0.6\textwidth]{dmscatter_picture}
        \end{center}
        \column{0.4\textwidth}
        %\includegraphics[width=\textwidth]{dmscatter_picture}
        \includegraphics[width=\textwidth]{SiSpectrum}

    \end{columns}
\end{frame}

\begin{frame}{Strategy}
    \begin{itemize}
        \item Challenges:
            \begin{enumerate}
                \item Unambiguously ID 1$e^-$, 2$e^-$ events
                \item Minimize background sources of charge
                \item Minimize coincidence background
            \end{enumerate}
        \item Our starting point is the CCD
            \begin{itemize}
                \item Mature technology, used for DM in DAMIC
                \item Achieves \#3 through pixel segmentation
            \end{itemize}
        \item Same physics as EDELWEISS, CDMS HV --- very different detector
    \end{itemize}
    \begin{center}
        \includegraphics[width=0.6\textwidth]{ccd}
    %    \includegraphics[width=\textwidth]{dmscatter_picture}
    \end{center}
\end{frame}

\begin{frame}{CCD searches for dark matter}
    \begin{columns}
        \column{0.75\textwidth}
        \begin{itemize}
                %\item Overlapping collaborations, sharing LBNL's CCD technology
            \item DAMIC (40 g, complete): first CCD DM experiment
            \item SENSEI ($O(100)$ g starting 2021): first DM experiment with Skippers
                \begin{itemize}
                    \item Several physics results (most recent 2020) from prototypes and test runs; full-scale experiment under construction
                    \item Active mass and radiopurity similar to DAMIC
                \end{itemize}
            \item DAMIC-M ($O(1)$ kg starting 2023): next-generation Skipper experiment
                \begin{itemize}
                    \item Scaling up in mass, with corresponding improvement in radiopurity
                \end{itemize}
            \item Oscura ($O(10)$ kg, in development): large-scale
                \begin{itemize}
                    \item Major R\&D for readout, cooling, integration
                \end{itemize}
                %\item We expect significant improvements in all measurement channels:
                %    \begin{itemize}
                %        \item 1$e^-$: better shielding $\to$ lower 1$e^-$ rate
                %        \item 2$e^-$: reduced spurious charge $\to$ shorter exposures $\to$ lower coincidence rate
                %        \item 3, 4$e^-$: increased detector mass
                %    \end{itemize}
                %\item SENSEI@MINOS demonstrates that Skipper CCDs have the performance we need to reach theory targets
                %    \begin{itemize}
                %        \item SENSEI@SNOLAB: 100 grams
                %        \item DAMIC-M: 1 kg
                %        \item Oscura: 10 kg
                %    \end{itemize}
        \end{itemize}
        \column{0.25\textwidth}
        \includegraphics[width=\textwidth]{Oscura-projection-2021-FDM1-DoE-review}

        \includegraphics[width=\textwidth]{Oscura-projection-2021-FDMq2-DoE-review}

        %\includegraphics[width=\textwidth]{Oscura-projection-2021-Absorption-DoE-review}
    \end{columns}
\end{frame}

\begin{frame}
    \begin{center}
        \includegraphics[height=\textheight, page=1]{SENSEI-Collaboration-List2}
    \end{center}
\end{frame}

\begin{frame}{CCDs}
    \begin{columns}
        \column{0.5\textwidth}
        \begin{itemize}
            \item Holes drift through the substrate and collect in pixels near the surface
            \item Charge packets are shifted to a shared amplifier (1 per quadrant) for readout
            \item All CCDs for DM are designed by LBNL MSL, based on fully-depleted CCD designs proven in astronomy
                \begin{itemize}
                    \item High-efficiency charge collection and transport, low dark current
                    \item Thickness limited only by commercial foundry capabilities
                \end{itemize}
                %\item CCDs can read millions of charge packets with minimal loss
                %    \begin{itemize}
                %        \item The result of decades of R\&D in imaging CCDs
                %    \end{itemize}
            \item Conventional CCDs are limited to noise of $\sim 2e^-$
        \end{itemize}
        %\includegraphics[height=\textwidth,angle=270]{protoccd}
        \column{0.25\textwidth}
        \includegraphics[width=\textwidth]{ccd_schematic}
        \column{0.25\textwidth}
        \includegraphics[width=\textwidth]{ccd-multiple-con-flechas}
    \end{columns}
\end{frame}

\begin{frame}[t]{Skipper readout}
    \begin{itemize}
        \item In a conventional CCD, charge moved to the sense node must be drained
            \begin{itemize}
                \item You can integrate longer, but you cannot beat the $1/f$ noise
            \end{itemize}
        \item The Skipper amplifier lets you make multiple measurements!
    \end{itemize}
    \begin{center}
        \includegraphics[width=0.75\textwidth]{ccd_output}
    \end{center}
\end{frame}

\begin{frame}[t]{Skipper readout}
    \begin{itemize}
        \item In a conventional CCD, charge moved to the sense node must be drained
            \begin{itemize}
                \item You can integrate longer, but you cannot beat the $1/f$ noise
            \end{itemize}
        \item The Skipper amplifier lets you make multiple measurements!
    \end{itemize}
    \begin{center}
        \includegraphics[width=0.9\textwidth]{ccd_frequency}
    \end{center}
\end{frame}

\begin{frame}{Sub-electron readout noise}
    \begin{columns}
        \column{0.55\textwidth}
        \begin{itemize}
            \item Skipper noise scales as $1/\sqrt{N}$: trade resolution for speed
                \begin{itemize}
                    \item For the 2020 DM search we operated at $N=300$ (13 ms/pixel), noise of $\sim 0.14e^-$
                \end{itemize}
            \item We can count single electrons: self-calibrating charge measurement with zero noise
                %\begin{itemize}
                %    \item Other applications, such as a very clean measurement of the Fano factor in silicon
                %\end{itemize}
        \end{itemize}
        \begin{center}
            \includegraphics[width=0.9\textwidth]{rms_vs_n}
        \end{center}
        \column{0.45\textwidth}
        \begin{center}
            \includegraphics[width=\textwidth]{electron_by_electron}

            \includegraphics[width=0.9\textwidth]{peaks-crop}
        \end{center}
    \end{columns}
\end{frame}

\begin{frame}{The SENSEI CCDs}
    \begin{columns}
        \column{0.5\textwidth}
        \begin{itemize}
            \item High-resistivity silicon $675$ $\mu$m thick, $1.59\times 9.42$ cm$^2$
            \item $6144\times 886$ pixels, 15 $\mu$m pitch (1.925 grams active)
                %\item Designed by LBNL MSL, fabricated by Teledyne DALSA
            \item The first dedicated production of Skipper CCDs for dark matter
                %\begin{itemize}
                %    \item Orders of magnitude improvement in dark current and amplifier luminescence
                %\end{itemize}
        \end{itemize}
        \column{0.5\textwidth}
        \includegraphics[width=\textwidth]{ccds}
    \end{columns}
\end{frame}

\begin{frame}{MINOS setup}
    \begin{columns}
        \column{0.5\textwidth}
        \begin{itemize}
            \item Shallow underground site (MINOS cavern at Fermilab), simple lead shield
            \item CCD at 135 K, biased at 70 V
                %\item Shielding design adapted from DAMIC: cylindrical vacuum vessel with lead ``plugs'' above and below the CCD
        \end{itemize}
        \begin{center}
            \includegraphics[width=\textwidth]{minos_location}
        \end{center}
        \column{0.25\textwidth}

        \includegraphics[width=\textwidth]{MINOS_setup}

        \column{0.25\textwidth}
        %\includegraphics[width=\textwidth]{minos_lead}
        \includegraphics[width=\textwidth]{MINOS_inside}
    \end{columns}
\end{frame}

\begin{frame}{Images!}
    \begin{columns}
        \column{0.5\textwidth}
        \begin{itemize}
            \item This is 1/6th of one quadrant
            \item 20 hours exposure, 5.15 hours readout
                \begin{itemize}
                    \item During readout, the image continues to accumulate hits and charge
                \end{itemize}
            \item 22 images in the blinded dataset
        \end{itemize}
        \begin{center}
            {\small
            \begin{tabular}{l|r}
                Search & Exposure post-cuts  \\
                \hline
                1$e^-$ & 1.38 gram-days  \\
                2$e^-$ & 9.17 gram-days  \\
                3$e^-$ & 11.87 gram-days  \\
                4$e^-$ & 11.70 gram-days  \\
                %\hline
            \end{tabular}
            }
        \end{center}

        \column{0.5\textwidth}
        \includegraphics[width=\textwidth]{ds9_skp_72000secs_exp_run10_NSAMP_300_36_hdu2_annotated}
    \end{columns}
\end{frame}

\begin{frame}{Searches and backgrounds}
    \begin{columns}[T]
        \column{0.5\textwidth}
        \begin{itemize}
            \item Single-electron: background-dominated
            \item Multiple electrons (single-pixel or cluster): low-background
                \begin{itemize}
                    \item Coincidence of $1e^-$ processes
                    \item True multi-$e^-$ processes
                \end{itemize}
        \end{itemize}
        \includegraphics[width=\textwidth]{spectrum}
        \column{0.5\textwidth}
        \begin{itemize}
            \item $1e^-$ processes
                \begin{itemize}
                    \item Local sources of charge: high-energy clusters (ionizing radiation), amplifier luminescence, CCD defects 
                    \item Spatially uniform sources of charge
                        \begin{itemize}
                            \item Spurious charge: charge generated during readout
                            \item Intrinsic dark current: charge generated during exposure by thermal excitation
                        \end{itemize}
                    \item Skippers give us new insight into these processes: paper in prep
                \end{itemize}
            \item Multi-$e^-$ processes: rare but irreducible
                \begin{itemize}
                    \item Compton scattering
                    \item Partial charge collection
                \end{itemize}
        \end{itemize}
    \end{columns}
\end{frame}

\begin{frame}{Localized charge from high-energy events}
    \begin{columns}
        \column{0.8\textwidth}
        \begin{itemize}
            \item Bleeding: charge ``left behind'' by charge transfer
            \item Halo: excess of charge near high-charge pixels
                \begin{itemize}
                    \item Probably near-bandgap photons from Cherenkov radiation and electron-hole recombination (see arXiV:2011.13939)
                \end{itemize}
            \item Loose clusters: regions with high charge density
                \begin{itemize}
                    \item May be Cherenkov photons (reflected, or generated outside of CCD)
                \end{itemize}
                %\item We apply a tight cut (>60 pixels from any high-charge pixel) for the 1$e^-$ analysis, looser (>20 pixels) for the 2/3/4$e^-$ analyses
            \item After cuts, charge density of $3.188(90)\times 10^{-4}$ $e^-$/pixel
        \end{itemize}
        \includegraphics[width=0.4\textwidth]{halo1e}
        \column{0.2\textwidth}
        \includegraphics[width=\textwidth]{bleed}

        %\includegraphics[width=\textwidth]{ds9_skp_72000secs_exp_run10_NSAMP_300_36_hdu2}
    \end{columns}
\end{frame}

\begin{frame}{Spurious charge}
    \begin{columns}
        \column{0.55\textwidth}
        \begin{itemize}
            \item Measurements with shorter exposures show a limiting value for the CCD charge: $1.66(12)\times 10^{-4}$ $e^-$/pixel
                \begin{itemize}
                    \item Half of the 1$e^-$ events we see are due to spurious charge!
                    \item Optimization of the CCD voltage waveforms will reduce this background in future runs
                \end{itemize}
            \item Subtracting the exposure-independent charge, our 1e- rate is $1.59(16)\times 10^{-4}$ $e^-$/pixel/day
        \end{itemize}
        \column{0.4\textwidth}
        \includegraphics[width=\textwidth]{SC}

        %\includegraphics[width=\textwidth]{DC_vs_T_subtracted}

        \includegraphics[width=\textwidth]{sc_vs_v}
    \end{columns}
\end{frame}

\begin{frame}{Intrinsic dark current}
    \begin{columns}
        \column{0.55\textwidth}
        \begin{itemize}
            \item Subtracting the exposure-independent charge, our 1e- rate is $1.59(16)\times 10^{-4}$ $e^-$/pixel/day
            \item Intrinsic dark current is the usual suspect
                \begin{itemize}
                    \item Thermal generation of electron-hole pairs, mediated by lattice defects 
                \end{itemize}
            \item However:
                \begin{itemize}
                    \item Extrapolation from higher temperatures (dashed black line) predicts $\ll 1\times 10^{-5}$ $e^-$/pixel/day at our operating temperature of 135 K
                        \begin{itemize}
                            \item Suppressing surface dark current gets us from red data points to blue
                        \end{itemize}
                \end{itemize}
            \item High-quality silicon has made this a subdominant background
        \end{itemize}
        \column{0.45\textwidth}
        %\includegraphics[width=\textwidth]{DC_vs_T}
        \includegraphics[width=\textwidth]{DC_vs_T_subtracted}
    \end{columns}
\end{frame}

\begin{frame}{1$e^-$ rate vs. shielding}
    \begin{columns}
        \column{0.75\textwidth}
        \begin{itemize}
            \item We have data with and without the outer ring of lead bricks
            \item Factor of 3 reduction in the rate of high-energy tracks $\to$ similar reduction in the 1$e^-$ rate
                \begin{itemize}
                    \item Radiation generates charge in halos, in loose clusters, and uniformly
                    \item Better shielding will very likely further reduce our 1$e^-$ rate
                \end{itemize}
        \end{itemize}
        \begin{center}
            \includegraphics[width=0.45\textwidth]{Background_wineNcheese}
            \includegraphics[width=0.48\textwidth]{DCvsBackground}
        \end{center}
        \column{0.25\textwidth}
        \includegraphics[width=\textwidth]{minos_lead}
    \end{columns}
\end{frame}

\begin{frame}{Multi-$e^-$ backgrounds}
    \begin{columns}
        \column{0.5\textwidth}
        \begin{itemize}
            \item At the low-energy end of the Compton spectrum, a gamma ray can create arbitrarily small-energy electron recoils
                \begin{itemize}
                    \item DAMIC and SENSEI efforts to measure this spectrum and compare to theory
                \end{itemize}
            \item Depending on details of processing, some CCDs have a highly-doped backside layer with partial charge collection: this can create low-charge events
                \begin{itemize}
                    \item DAMIC and SENSEI are affected, but future CCDs will not have this layer
                \end{itemize}
        \end{itemize}
        \begin{center}
        \end{center}
        \column{0.4\textwidth}
        \includegraphics[width=\textwidth]{compton}

        \includegraphics[width=\textwidth]{pcc}
    \end{columns}
\end{frame}

\begin{frame}{Multi-$e^-$ searches}
    \begin{columns}
        \column{0.6\textwidth}
        \begin{itemize}
            \item We observe 5 2$e^-$ pixels and no 3, 4$e^-$ clusters (adjacent nonempty pixels)
            \item Some multi-$e^-$ events will be lost when they diffuse
                \begin{itemize}
                    \item We calibrate the diffusion width using muon tracks and simulate the geometric efficiencies: 22.8\% for 2$e^-$ to stay in one pixel, 76.1\%, 77.8\% for 3, 4$e^-$ to form a contiguous cluster
                \end{itemize}
            \item Now we can put limits on the rate of events
                %\item What is the probability for both electrons from a DM interaction to end up in the same pixel?
                %\item We use muon tracks to measure diffusion as a function of depth
        \end{itemize}
        \begin{center}
            \includegraphics[width=0.9\textwidth]{diffusion}
        \end{center}
        \column{0.4\textwidth}
        \includegraphics[width=\textwidth]{spectrum}

        \begin{center}
            {\small
            \begin{tabular}{l|r}
                & 90\% CL  \\
                \hline
                1$e^-$ & 525.2 events/g-day  \\
                2$e^-$ & 4.449 events/g-day  \\
                3$e^-$ & 0.255 events/g-day  \\
                4$e^-$ & 0.253 events/g-day  \\
                %\hline
            \end{tabular}
            }
        \end{center}
        %\includegraphics[width=\textwidth,angle=270]{simulations_in_4umPixelCCD}
        %\includegraphics[width=\textwidth,page=1]{SENSEI_size_calibration}

        %\includegraphics[width=0.4\textwidth,page=12]{selected_muons}
        %\includegraphics[width=0.55\textwidth]{diffusion_sim15}

    \end{columns}
\end{frame}

\begin{frame}{Limits from SENSEI@MINOS}
    \begin{itemize}
        \item New record lows for semiconductor detectors and DM searches at these thresholds
        \item The derived DM constraints are also world-leading
            \begin{itemize}
                \item Left to right: $F_{DM}=1$ scattering (heavy mediator), $F_{DM}=(\alpha m_e/q)^2$ scattering (light mediator), absorption
            \end{itemize}
    \end{itemize}
    \begin{center}
        \includegraphics[width=0.3\textwidth]{SENSEI_MINOS2020_FDM1-Paper}
        \includegraphics[width=0.3\textwidth]{SENSEI_MINOS2020_FDMq2-Paper}
        \includegraphics[width=0.3\textwidth]{absorption_rho03_3pt8_v4}
    \end{center}
\end{frame}

%%%%
% PCC
% Compton
% other applications
%%%%

\begin{frame}{SENSEI@SNOLAB}
    \begin{columns}
        \column{0.3\textwidth}
        \begin{itemize}
            \item We are building the full-scale SENSEI experiment, deep underground at SNOLAB with a low-background shield
            \item Installation in progress by SNOLAB team
        \end{itemize}
        \column{0.25\textwidth}
        \includegraphics[width=\textwidth]{sensei100_inside}
        \column{0.45\textwidth}
        \includegraphics[width=\textwidth]{IMG_7493}
    \end{columns}
\end{frame}

\appendix
\backupbegin

\begin{frame}[t]{Backup: CCD amplifiers}
    \begin{itemize}
        \item Floating diffusion: standard CCD amplifier
        \item Floating gate: used in our Skippers
            \begin{itemize}
                \item Enables nondestructive readout at the cost of S/N
            \end{itemize}
        \item SiSeRO: under development by MIT-LL
            \begin{itemize}
                \item Best of both worlds: nondestructive, and excellent S/N
                \item Possible option for Oscura
            \end{itemize}
    \end{itemize}
    \begin{center}
        \includegraphics[width=0.3\textwidth]{floating_diff}
        \includegraphics[width=0.3\textwidth]{floating_gate}
        \includegraphics[width=0.38\textwidth]{sisero}
    \end{center}
\end{frame}

%\begin{frame}[t]{Backup: Other applications}
%    \begin{itemize}
%        \item Fano factor measurement
%        \item Quantum imaging
%        \item Spectroscopy
%    \end{itemize}
%\end{frame}

\backupend

\end{document}
