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

\title[SENSEI]{Searching for Sub-GeV Dark Matter with SENSEI}

\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[February 22, 2021]

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

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

\begin{frame}{New results from SENSEI}
    \begin{columns}
        \column{0.7\textwidth}
        \begin{itemize}
            \item SENSEI has delivered world-leading results in low-threshold DM direct detection
                \begin{itemize}
                    \item 2017: Demonstration of 0.068 $e^-$ noise in prototype
                    \item 2018: DM search, surface run of prototype
                    \item 2019: DM search, underground run of prototype
                \end{itemize}
        \end{itemize}
        \column{0.29\textwidth}
        \includegraphics[width=\textwidth]{breakthrough}
    \end{columns}
    \begin{itemize}
        \item Today we present results from the 2020 SENSEI run
            \begin{itemize}
                \item First DM search with a science-grade SENSEI CCD
                \item Paper in PRL --- see arXiV 2004.11378
                \item Next step: full-scale experiment deep underground
            \end{itemize}
    \end{itemize}
    \begin{center}
        \includegraphics[width=\textwidth]{CCD_package_crop}
    \end{center}
\end{frame}

\begin{frame}
    \includegraphics[width=\textwidth, page=1]{SENSEI-Collaboration-List2}
\end{frame}

%\begin{frame}
%    {\Huge GIANT 12MB COLLABORATION PHOTO}
%    %\includegraphics[width=\textwidth, page=2]{SENSEI-Collaboration-List2}
%\end{frame}

\begin{frame}{Electron recoils for sub-GeV dark matter}
    \begin{itemize}
        \item We look for DM interactions with the electrons in a CCD
            \begin{itemize}
                \item Benchmark models: DM-electron scattering, absorption
                \item Mass range and sensitivity strongly depend on the energy threshold
            \end{itemize}
        \item Silicon bandgap gives us sensitivity to 1.2 eV excitations --- if we can capture and resolve a single electron
            %\begin{itemize}
            %\end{itemize}
        %\item DM-electron scattering mediated by 
        %\item Absorption: 
        %\item Skipper CCDs have sub-electron charge resolution
    \end{itemize}
    \begin{center}
        \includegraphics[width=\textwidth]{dmscatter_picture}
    \end{center}
\end{frame}

\begin{frame}{CCDs}
    \begin{columns}
        \column{0.4\textwidth}
        \begin{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{0.6\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}
    \includegraphics[width=\textwidth]{ccd_output}
\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}
    \includegraphics[width=\textwidth]{ccd_frequency}
\end{frame}

\begin{frame}{Sub-electron readout noise}
    \begin{columns}
        \column{0.55\textwidth}
        \begin{itemize}
            \item Skipper noise scales as $1/\sqrt{N}$
                \begin{itemize}
                    \item For the dark matter search we operate at $N=300$, 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=\textwidth]{rms_vs_n}
        \end{center}
        \column{0.45\textwidth}
        \includegraphics[width=\textwidth]{electron_by_electron}

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

\begin{frame}{Our CCDs}
    \begin{columns}
        \column{0.5\textwidth}
        \begin{itemize}
            \item $6144\times 886$ pixels (divided in quadrants), 15 $\mu$m pitch
            \item High-resistivity silicon $675$ $\mu$m thick, $1.59\times 9.42$ cm$^2$
            \item Designed by LBNL MSL, fabricated by DALSA
            \item The first dedicated production of Skipper CCDs for dark matter
                \begin{itemize}
                    \item 1.925 grams of active mass, up from 0.0947 in protoSENSEI
                    \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}{CCD package}
    \begin{columns}
        \column{0.45\textwidth}
        \begin{itemize}
            \item Densely packable and minimizes radioactive contamination
            \item Silicon pitch adapter serves multiple functions:
                \begin{itemize}
                    \item Electrical interface to flex cable
                    \item Mechanical support, with perfectly matched thermal expansion
                    \item Thermal connection to copper tray through machined leaf spring
                \end{itemize}
        \end{itemize}
        %\begin{center}
        %\includegraphics[width=\textwidth]{emcal_location}
        %\end{center}
        \column{0.55\textwidth}
        \includegraphics[width=\textwidth]{CCD_package}

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

%\begin{frame}{LTA readout board}
%    \begin{columns}
%        \column{0.55\textwidth}
%        \begin{itemize}
%            \item ``Low Threshold Acquisition'' --- single-board readout system for Skipper CCDs
%            \item Compared to previous solutions: compact, flexible, scalable, reliable
%            \item One LTA board reads one CCD; multiple LTAs can be run synchronously for multi-CCD systems
%        \end{itemize}
%        \begin{center}
%            \includegraphics[width=0.5\textwidth]{monsoon}
%        \end{center}
%        \column{0.45\textwidth}
%        \includegraphics[height=\textwidth,angle=270]{MINOS_LTA}
%
%    \end{columns}
%\end{frame}

\begin{frame}{MINOS setup}
    \begin{columns}
        \column{0.65\textwidth}
        \begin{itemize}
            \item Shallow underground site (MINOS cavern at Fermilab) reduces muon rate from cosmic rays; lead shielding reduces gamma rate from ambient radioactivity
            \item Cryocooler and insulating vacuum keep the CCD cold to minimize dark current
        \end{itemize}
        \begin{center}
            \includegraphics[width=\textwidth]{minos_location}
        \end{center}
        \column{0.35\textwidth}

        \includegraphics[width=\textwidth]{MINOS_setup}

    \end{columns}
\end{frame}

\begin{frame}{Inside the cryostat}
    \begin{columns}
        \column{0.65\textwidth}
        \begin{itemize}
            \item Shielding design adapted from DAMIC: cylindrical vacuum vessel with lead ``plugs'' above and below the CCD
            \item CCD at 135 K, biased at 70 V
        \end{itemize}
        \column{0.35\textwidth}
        %\includegraphics[width=\textwidth]{minos_lead}
        \includegraphics[width=\textwidth]{MINOS_inside}
    \end{columns}
\end{frame}

\begin{frame}{The dataset}
    \begin{columns}
        \column{0.4\textwidth}
        {\small
            \begin{itemize}
                \item 20 hours exposure, 6 hours readout
                \item Analysis developed using 7 commissioning images
                \item Blinded dataset of 22 images, Feb. 25 --- Mar. 20
                \item One quadrant is damaged, one has a light leak: we use the first two quadrants (19.93 gram-days) for the 1/2$e^-$ searches, and add part of the third quadrant for 3/4$e^-$ (total 27.82 gram-days)
            \end{itemize}
        }
        \column{0.6\textwidth}
        \includegraphics[width=\textwidth]{ds9_allquads}
    \end{columns}
\end{frame}

\begin{frame}{Images!}
    \begin{columns}
        \column{0.5\textwidth}
        \begin{itemize}
            \item This is 1/5th of one quadrant
            \item Muons: straight tracks
            \item Electrons: curly tracks
            \item X-rays: round clusters
        \end{itemize}
        \column{0.5\textwidth}
        \includegraphics[width=\textwidth]{ds9_skp_72000secs_exp_run10_NSAMP_300_36_hdu2}
    \end{columns}
\end{frame}

\begin{frame}{Searches and backgrounds}
    \begin{columns}[T]
        \column{0.5\textwidth}
        \begin{itemize}
            \item Single-pixel searches (we exclude any pixel with a nonempty neighbor):
                \begin{itemize}
                    \item Single-electron: background-dominated
                    \item Two-electron: low-background
                \end{itemize}
            \item Three-, four-electron clusters (we combine adjacent pixels): zero-background
        \end{itemize}
        \column{0.5\textwidth}
        \begin{itemize}
            \item Local sources of charge: high-energy clusters (ionizing radiation), CCD defects 
            \item Spatially uniform sources of charge
                \begin{itemize}
                    \item Spurious charge: charge generated during readout
                    \item Dark current: charge generated during exposure by thermal excitation
                    \item Others?
                \end{itemize}
        \end{itemize}
    \end{columns}
\end{frame}

\begin{frame}{Cuts: bad pixels/dark spikes}
    \begin{columns}
        \column{0.55\textwidth}
        \begin{itemize}
            \item Surface defects on the CCD can create pixels with high dark current
            \item We identify these with special high-temperature runs and by stacking images, and mask them out
        \end{itemize}
        \column{0.45\textwidth}
        \includegraphics[width=\textwidth]{darkspikes}
    \end{columns}
\end{frame}

\begin{frame}{Cuts: serial register hits}
    \begin{columns}
        \column{0.55\textwidth}
        \begin{itemize}
            \item Tracks that cross the serial register during readout can produce lines of charge in the image
            \item We mask out isolated horizontal lines of charge
        \end{itemize}
        \column{0.45\textwidth}
        \includegraphics[width=\textwidth]{sr_hit}
    \end{columns}
\end{frame}

\begin{frame}{Cuts: bleeding}
    \begin{columns}
        \column{0.73\textwidth}
        \begin{itemize}
            \item Some charge may be left behind when we transfer charge from one cell to the next
                \begin{itemize}
                    \item Surface defects can create traps that increase the bleeding tails in specific columns
                \end{itemize}
            \item We mask out bleed regions above and to the right of high-charge pixels
            \item We identify high-bleed columns by looking for excess charge above high-charge pixels
        \end{itemize}
        \column{0.27\textwidth}
        \includegraphics[width=\textwidth]{bleed}
    \end{columns}
\end{frame}

\begin{frame}{Cuts: halo}
    \begin{columns}
        \column{0.45\textwidth}
        \begin{itemize}
            \item We see an excess of charge near high-charge pixels, even after masking out bleed regions
                \begin{itemize}
                    \item Likely near-bandgap photons from Cherenkov radiation and electron-hole recombination (see arXiV:2011.13939)
                \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
        \end{itemize}
        \column{0.55\textwidth}
        \includegraphics[width=\textwidth]{halo1e}
        %\includegraphics[width=\textwidth]{ds9_skp_72000secs_exp_run10_NSAMP_300_36_hdu2}
    \end{columns}
\end{frame}

\begin{frame}{1$e^-$ rate}
    \begin{columns}
        \column{0.55\textwidth}
        \begin{itemize}
            \item We see $3.188(90)\times 10^{-4}$ $e^-$/pixel in our images, from a total exposure of 1.380 gram-days
                \begin{itemize}
                    \item If all exposure-dependent, this is a 1$e^-$ rate of $3.363(94)\times 10^{-4}$ $e^-$/pixel/day
                \end{itemize}
            \item Is this all dark current? Unlikely!
                \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}
        \end{itemize}
        \column{0.45\textwidth}
        \includegraphics[width=\textwidth]{DC_vs_T}
    \end{columns}
\end{frame}

\begin{frame}{Spurious charge measurement}
    \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$e^-$ rate we see is 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{0.45\textwidth}
        \includegraphics[width=\textwidth]{SC}

        \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 Ionizing radiation generates charge both locally (halo) and uniformly in our CCD
                    \item Better shielding will very likely further reduce our 1$e^-$ rate
                \end{itemize}
        \end{itemize}
        \column{0.25\textwidth}
        \includegraphics[width=\textwidth]{minos_lead}
    \end{columns}
    \begin{center}
        \includegraphics[width=0.45\textwidth]{Background_wineNcheese}
        \includegraphics[width=0.48\textwidth]{DCvsBackground}
    \end{center}
\end{frame}

%\begin{frame}{Cuts: loose clusters}
%    \begin{columns}
%        \column{0.6\textwidth}
%        \begin{itemize}
%            \item For the 2/3/4$e^-$ searches, we use a smaller halo cut (to preserve exposure) but apply additional cuts to remove regions with higher charge density
%            \item If two 1$e^-$ pixels are within 20 pixels of each other, we remove a radius-20 circle around both
%                \begin{itemize}
%                    \item This cut kills half of the 2$e^-$ events with a 11\% loss of exposure
%                \end{itemize}
%        \end{itemize}
%        \column{0.4\textwidth}
%        \includegraphics[width=\textwidth]{loosecluster}
%    \end{columns}
%\end{frame}

\begin{frame}{2/3/4$e^-$ rate}
    \begin{itemize}
        \item For these searches, we use a smaller halo radius (to preserve exposure) but apply an additional ``loose cluster'' cut to remove regions with higher charge density
        \item After all cuts, we see 5 pixels with 2$e^-$
        \item We see no 3$e^-$ or 4$e^-$ clusters
    \end{itemize}
    \begin{center}
        \includegraphics[width=\textwidth,page=2]{DM_skim_72000secs}
    \end{center}
\end{frame}

\begin{frame}{Charge diffusion}
    \begin{columns}
        \column{0.4\textwidth}
        \begin{itemize}
            \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
            \item 22.8\% for 2$e^-$ to stay in one pixel
            \item 76.1\%, 77.8\% for 3, 4$e^-$ to form a contiguous cluster
        \end{itemize}
        \begin{center}
            \includegraphics[width=\textwidth]{diffusion}
        \end{center}
        \column{0.6\textwidth}
        %\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}
    \begin{columns}[T]
        \column{0.6\textwidth}
        \begin{itemize}
            \item New record lows for semiconductor detectors and DM searches at these thresholds
        \end{itemize}
        \column{0.6\textwidth}
        {\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{columns}

    \begin{itemize}
        \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=0.33\textwidth]{SENSEI_MINOS2020_FDM1-Paper}
        \includegraphics[width=0.33\textwidth]{SENSEI_MINOS2020_FDMq2-Paper}
        \includegraphics[width=0.32\textwidth]{absorption_rho03_3pt8_v4}
    \end{center}
\end{frame}

\begin{frame}{The future of Skippers}
    \begin{columns}
        \column{0.65\textwidth}
        \begin{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{0.35\textwidth}
        \includegraphics[width=\textwidth]{Oscura-projection-2020-FDM1}

        \includegraphics[width=\textwidth]{Oscura-projection-2020-FDMq2}
    \end{columns}
\end{frame}

\begin{frame}{SENSEI@SNOLAB}
    \begin{columns}
        \column{0.6\textwidth}
        \begin{itemize}
            \item We are building the full-scale SENSEI experiment, deep underground at SNOLAB with a low-background shield
            \item ``Phase 1'' system is operating at SNOLAB, full experiment is ready for installation
        \end{itemize}
        \begin{columns}
            \column{0.6\textwidth}
            \includegraphics[width=\textwidth]{SNOLAB}

        %\includegraphics[width=\textwidth]{C_package}
            \column{0.4\textwidth}
            \includegraphics[width=\textwidth]{sensei100_inside}
        \end{columns}
        \column{0.4\textwidth}
        \includegraphics[width=\textwidth]{IMG_9521}

    \end{columns}
\end{frame}

\end{document}
