The hunt for dark matter 4,850 ft below

A team of physicists in rural South Dakota search an abandoned gold mine for dark matter, a theoretical particle which has never been seen or directly detected.

TRANSCRIPT

IN AN ABANDONED GOLD MINE IN RURAL SOUTH DAKOTA, A TEAM OF PHYSICISTS ARE HUNTING FOR ASTROPHYSICAL TREASURE.

THEIR RARE AND ELUSIVE QUARRY IS DARK MATTER -- A THEORETICAL PARTICLE WHICH HAS NEVER BEEN SEEN OR DIRECTLY DETECTED.

PRODUCERS FOR 'SCIENCE FRIDAY' MADE THE JOURNEY DEEP UNDERGROUND TO DOCUMENT THE TEAM'S EXPERIMENT IN THEIR FILM '4,850 FEET BELOW.'

WE'LL SPEAK WITH THE FILMMAKER ABOUT THE RESEARCHER'S LARGE UNDERGROUND XENON EXPERIMENT KNOWN AS LUX AND THE INCREDIBLE CHALLENGES THEY FACED.

BUT FIRST, HERE'S A CLIP.

THE FIRST TIME I CAME HERE, IT WAS VERY MUCH A 'WOW' MOMENT FOR ME.

I NEVER THOUGHT I WOULD END UP IN A MINE SHAFT A MILE UNDER THE EARTH.

WE ARE 4,850 FEET BELOW THE SURFACE, SO ALMOST A MILE.

WE MAKE THAT JOURNEY BECAUSE, ON THE SURFACE, MORE THAN ONE COSMIC RAY IS GOING THROUGH MY HAND EVERY SECOND.

HERE ON THE 4,850 LEVEL, LESS THAN ONE COSMIC RAY IS GOING THROUGH MY HAND EVERY THREE MONTHS.

SO LUX HAS BEEN DESIGNED TO BE THE QUIETEST PLACE IN THE WORLD.

AND THROUGH SUCH AN INSTRUMENT, WE HOPE TO BE ABLE TO STUDY DARK MATTER.

YOU KNOW, THERE'S A PRETTY SMALL SLICE OF REALITY THAT WE CAN ACTUALLY SEE OR TOUCH.

IN PHYSICS, YOU CAN GO WELL BEYOND THESE THINGS.

WE CAN'T FEEL OR TOUCH RADIO WAVES OR INFRARED OR ULTRAVIOLET OR X-RAYS OR GAMMA RAYS, NEUTRONS, BUT THOSE THINGS ARE ALL AROUND US ALL THE TIME.

AND WITH THE RIGHT INSTRUMENTS, YOU CAN DETECT THEM.

SO THIS IS LUX RIGHT HERE.

THIS IS THE WATER TANK.

LUX IS DEEP INSIDE OF THIS GUY RIGHT HERE.

THE WATER ACTS AS OUR NEUTRON AND GAMMA-RAY SHIELD FROM ALL THE RADIATION IN THE LAB SPACE.

THE LUX DETECTOR IN ITS MOST BASIC FORM IS SIMPLY A BUCKET OF XENON.

LIQUID XENON -- WELL, XENON, THE NOBLE ELEMENT, IS VERY PURE.

IT HAS VERY FEW RADIOACTIVE ISOTOPES IN IT, WHICH MEANS, IN ITS NATURAL FORM, IT'S INCREDIBLY QUIET.

WE'RE ABLE, THEN, TO USE THE FEATURE OF XENON, WHICH IS WHEN PARTICLES INTERACT IN IT, IT WILL GENERATE A SMALL AMOUNT OF LIGHT.

IN ORDER TO DETECT DARK MATTER EVENTS, WE NEED TO STUDY, OR WATCH, THE XENON INCREDIBLY CLOSELY.

WE'VE DESIGNED A SYSTEM OF PHOTOMULTIPLIER TUBES THAT ARE WATCHING THE XENON FROM BELOW AND ABOVE.

IN OUR EXPERIMENT, WE HAVE 122 PHOTOMULTIPLIER TUBES.

THEY'RE SINGLE PHOTON-SENSITIVE DEVICES.

THE AVERAGE 60-WATT LIGHT BULB PRODUCES 1 BILLION BILLION PHOTONS PER SECOND, AND WE SEE ONE PHOTON.

WE EXPECT A DARK-MATTER PARTICLE IS GONNA FLY INTO OUR DETECTOR.

IT WILL, ALMOST BILLIARD-BALL STYLE, INTERACT DIRECTLY WITH A XENON NUCLEUS.

ASSOCIATED WITH THAT, THERE IS THE EMISSION OF LIGHT.

AND WE CAN PICK UP THE INDIVIDUAL PHOTONS THAT ARE BEING EMITTED FROM THE XENON.

WHEN THAT HAPPENS, YOU HAVE OUR S1 SIGNAL.

THE ELECTRONS THEMSELVES SOMETIMES ACTUALLY GET REMOVED FROM THE ATOM.

WHEN THAT HAPPENS, YOU NOW HAVE FREE ELECTRONS, INDIVIDUAL ELECTRONS COMING OUT OF THE XENON LIQUID, GENERATING THESE INCREDIBLE BURSTS OF LIGHT THAT LIGHT UP LIKE CHRISTMAS TREES.

SO WE GET A SECOND SCINTILLATION SIGNAL.

WE CALL IT S2.

JOINING ME NOW IS 'SCIENCE FRIDAY' VIDEO PRODUCER LUKE GROSKIN.

WHAT'S IT LIKE BEING IN THIS PLACE, THIS 4,850 FEET BELOW HEAVY ROCK?

IT'S... IT'S A LITTLE BIT TERRIFYING.

I'M NOT GONNA LIE.

TAKING A 12-MINUTE ELEVATOR RIDE DOWN TO THE --

IN THE WRONG DIRECTION.

YEAH, IN THE WRONG DIRECTION, IT'S A LITTLE BIT TERRIFYING.

IT'S A RICKETY KIND OF CAGE -- THEY CALL IT A CAGE -- ELEVATOR RIDE.

YOUR EARS ARE POPPING THE ENTIRE TIME.

THEY GIVE YOU THIS EXTENSIVE SAFETY LESSON BEFORE YOU GO DOWN THERE, BASICALLY SAYING LIKE, 'IN CASE OF A COLLAPSE, YOU'RE GOING TO HAVE SURVIVE USING THESE THINGS ON YOUR BELT.

JUST LISTEN TO THE INSTRUCTIONS AND YOU'LL BE OKAY.'

SO IT'S A LITTLE INTIMIDATING, BUT WHEN YOU GET DOWN THERE AND YOU ROUND THIS CURVE AND YOU GO INTO THE SCIENCE CAMPUS INSIDE THIS MINE, IT'S LIKE BEING IN A REALLY, REALLY HIGH-TECH SCIENCE LAB.

REALLY HIGH-TECH SCIENCE LAB.

YOU HAVE PEOPLE IN TYVEK SUITS.

YOU HAVE TO WIPE DOWN ALL OF YOUR GEAR TO MAKE SURE NONE OF THE DUST COMES IN, AND YOU'RE PRESENTED WITH THESE EXPERIMENTS THAT INVOLVE INCREDIBLY ELABORATE SETUPS OF FILTERS, PUMPS, XENON, THIS REALLY RARE NOBLE GAS, AND ALL OF IT JUST IS -- IT'S VERY SURREAL.

THE WHOLE EXPERIENCE IS REALLY, REALLY SURREAL.

SO WHY DO THEY THINK THAT THEY CAN -- THAT THEY MIGHT HAVE A CHANCE AT FIGURING OUT WHAT DARK MATTER IS DOWN THERE VERSUS ANYWHERE ELSE?

WELL, WHY DOWN THERE?

SO, IF YOU GO THAT DEEP IN THE EARTH, YOU'RE BEING PROTECTED FROM ALL THE COSMOGENIC RAYS THAT ARE BOMBARDING US.

SO RIGHT NOW, IN THIS STUDIO, WE ARE BEING BOMBARDED WITH RAYS, COSMOGENIC RAYS.

AND ALL OF THAT CREATES NOISE INSIDE OF A DETECTOR.

THEY CALL IT NOISE.

IT'S JUST BASICALLY PARTICLE INTERACTIONS.

NOW, YOU WANT TO GET AWAY FROM THAT.

IF YOU WANT TO DETECT SOMETHING THAT'S VERY, VERY SUBTLE, THIS WEAK INTERACTION, YOU GOT TO GET AWAY FROM ALL THAT, SO THEY GO DEEP-DOWN UNDERGROUND.

THEN THEY PUT LEVELS UPON LEVELS OF SHIELDING AROUND THE DETECTOR TO MAKE SURE THAT ANY SORT OF BETA DECAYS OR OTHER SORT OF PARTICLES THAT MIGHT BE ABLE TO ZIP DOWN THROUGH THE EARTH INTO THERE GET SHIELDED OUT, AND THEN, EVEN THEN, THEY RULE OUT ALL OF THIS OTHER SORT OF INTERACTIONS INSIDE THE PARTICLE DETECTOR, AND THEY'RE LOOKING FOR ONE SPECIFIC TYPE OF INTERACTION.

AND THEY BELIEVE, BASED ON CURRENT ASTROPHYSICAL EVIDENCE, THAT THIS PARTICLE WILL BEHAVE IN A CERTAIN WAY AND HAVE A CERTAIN MASS.

THERE'S LOTS OF EVIDENCE, IF YOU LOOK AT THE ROTATION OF GALAXIES AND THE WAY THAT GALAXIES FORM CLUSTERS, THAT THIS PARTICLE, DARK MATTER, HAS CERTAIN PROPERTIES.

MM-HMM.

AND SO IF IT HAS THESE CERTAIN PROPERTIES, IT WILL INTERACT INSIDE XENON IN A VERY SPECIFIC MANNER.

THIS COULD BE A WHILE.

OH, YEAH.

AND ARE THEY WILLING TO WAIT THAT LONG?

I MEAN, IS THE FACILITY ITSELF -- DOES IT HAVE A 100-YEAR PLAN, A 500-YEAR PLAN, YOU KNOW WHAT I MEAN?

SO RIGHT THERE, THEY'RE IN THE MIDST OF THEIR 300-DAY RUN OF LUX.

AND AFTER THAT, THEY'RE GONNA DISMANTLE LUX AND THEY'RE GONNA BUILD AN EVEN BIGGER VERSION OF IT CALLED LZ -- LUX ZEPLIN.

THEY'RE NOT VERY GOOD WITH THEIR ACRONYMS.

WHERE'S THAT GONNA BE?

THAT'S GONNA BE RIGHT WHERE LUX IS.

SO THEY'RE GONNA PUT IT RIGHT IN THE SAME SPOT.

NOW, THAT'S A FIVE-TIMES-BIGGER DETECTOR.

AND SIZE REALLY MATTERS WHEN YOU'RE TRYING TO DETECT PARTICLE INTERACTIONS BECAUSE YOU'RE NOT ACTUALLY LOOKING AT THE ENTIRE BUCKET OF XENON.

YOU'RE NOT LOOKING FOR ANY INTERACTIONS THERE.

YOU'RE ONLY LOOKING AT A VERY, VERY, VERY TINY SPOT IN THE CENTER.

SO IF YOU HAVE A BIGGER DETECTOR, YOU CAN HAVE A BIGGER CENTER...

SURE.

...AT THE CENTER OF YOUR DETECTOR.

OKAY, SO LET'S SAY BEST-CASE SCENARIO, THEY GET A HIT.

WHAT DOES THAT MEAN?

SO, IT MEANS TWO THINGS.

IF, BY CHANCE, THEY GET A WEAK INTERACTION BETWEEN A DARK-MATTER PARTICLE AND XENON, IT MEANS THAT THEY CAN IDENTIFY THE MASS OF THAT PARTICLE.

IT MEANS THEY CAN UNDERSTAND ITS ENERGY.

THEY CAN UNDERSTAND THAT, 'OH, IT DOES, INDEED, INTERACT WITH NORMAL MATTER IN THIS WAY.'

SO THAT'S REALLY IMPORTANT BECAUSE IT'S A BIG MYSTERY AS TO HOW THIS DARK MATTER ACTUALLY INTERACTS WITH NORMAL MATTER.

THE NEXT STEP IS, YOU KNOW, IT'S A LOT LIKE HUNTING FOR THE RIGHT RABBIT HOLE AND THEN JUMPING DOWN THAT NEXT RABBIT HOLE, ONLY TO FIND ANOTHER RABBIT HOLE THAT YOU HAVE TO JUMP INTO.

SO, YOU KNOW, IT'S GONNA OPEN UP A WHOLE NEW FIELD OF RESEARCH WHERE THEY'RE GONNA TRY TO HAVE TO FIGURE OUT, YOU KNOW, 'OKAY, IF IT'S THIS SIZE, AND IT INTERACTS IN THIS WAY, YOU KNOW, HOW DOES IT ACTUALLY SHAPE GALAXIES?

HOW IS IT ACTUALLY INTERACTING ON A GALAXY SCALE?

HOW IS IT INTERACTING ON A UNIVERSAL SCALE?

WHAT DOES THAT MEAN ABOUT THE BIG BANG AND ITS CREATION -- THE CREATION OF THIS PARTICLE?'

SO IF THIS MATTER, DARK MATTER, IS ALL AROUND US, WHY IS IT SO HARD TO JUST CATCH A LITTLE SLIVER OF IT IN THIS TINY...

SO IT DOESN'T INTERACT WITH THINGS THE WAY THAT WE MIGHT -- I MIGHT INTERACT WITH THIS TABLE.

SO THIS IS -- IF I'M TOUCHING THIS TABLE, IT'S THE ELECTROMAGNETIC FORCE.

IT DOES INTERACT WITH THINGS THROUGH GRAVITATIONAL FORCE.

SO THAT'S WHAT CAUSES IT TO ACTUALLY HELP SHAPE AND SPEED UP GALAXIES.

BUT THE SCIENTISTS THINK THAT DOES INTERACT USING THE WEAK FORCE.

SO THE WEAK FORCE -- THERE'S A STRONG --

THIS IS NOT A 'STAR WARS' REFERENCE?

NO. NO, NO, NO.

THE WEAK FORCE IS THIS VERY -- YEAH, IT'S KIND OF MYSTICAL, ACTUALLY, FORCE THAT IT ONLY -- TWO PARTICLES CAN ONLY INTERACT WHEN THEY GET REALLY CLOSE TOGETHER.

AND THAT'S WHERE THE WEAK FORCE CAN COME IN, AND THEY TRADE A SUBATOMIC PARTICLE WHEN THEY COME CLOSE TOGETHER.

AND THEY THINK THAT THAT'S HOW IT INTERACTS WITH NORMAL MATTER, BUT THEY DON'T KNOW FOR SURE.

IT COULD BE A STRONGLY INTERACTING FORCE.

AND JUST TO BE CLEAR, THE STRONG FORCE IS A WHOLE OTHER TYPE OF FORCE.

[ LAUGHS ]

SO THERE'S THE STRONG FORCE, THE WEAK FORCE, AND THEY'RE NOT THE SAME.

ALL RIGHT, LUKE GROSKIN, THANKS FOR GOING DOWN IN THAT RICKETY CAGE FOR US.

MY PLEASURE.

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