Science - USA (2021-11-12)

PHOTO: CHANG HYON HA

SCIENCE science.org 12 NOVEMBER 2021 • VOL 374 ISSUE 6569 805

T

he drama of the world’s most con-

troversial dark matter claim may

have reached its last act, if not its

final scene. For 2 decades, physicists

with an experiment called DAMA

have claimed that particles of dark

matter—the unseen stuff whose gravity

appears to bind our galaxy—are bumping

into atomic nuclei in their subterranean

particle detector, even as other dark matter

hunts come up empty. Now, physicists with

a detector called COSINE-100, designed

to mimic DAMA, present the most direct

refutation yet of the findings.

And in 2020, theorists identified

a way in which the DAMA signal

could have arisen inadvertently

in the team’s analysis.

The DAMA team rejects both

claims. Rita Bernabei, a physi-

cist at the University of Rome

Tor Vergata and DAMA’s leader,

declined to be interviewed. But

she dismissed the new expla-

nation in an email: “We have

already demonstrated that the

assumptions there reported are

untenable and the conclusions

are worthless.”

The Milky Way is thought to

whirl within a vast cloud of dark

matter, which could consist of

hypothetical weakly interacting

massive particles (WIMPs). As

the Solar System orbits the galactic cen-

ter at 225 kilometers per second, Earth

presumably plows into a wind of WIMPs.

Because our planet also orbits the Sun at

30 kilometers per second, the wind should

strengthen slightly when Earth is moving

in the same direction as the Sun, in June,

and abate when it’s moving the opposite

way, in December.

DAMA physicists have long claimed

to see this yearly cycle in their detector,

which now contains 25 10-kilogram crys-

tals of thallium-doped sodium iodide. Each

crystal produces a flash of light when a

particle pings a nucleus. The DAMA team

says that, in a low-energy range that corre-

sponds to WIMPs, the number of collisions

has gone up and down each year since ob-

servations began in 1995 (Science, 3 March

2000, p. 1570).

Other detectors see no such thing. But

those experiments use different target ma-

terials. So groups have built sodium iodide

detectors that can test the DAMA result in

an apples-to-apples comparison. One is CO-

SINE , which comprises eight crystals total-

ing 100 kilograms and has been taking data

since 2016 in South Korea’s subterranean

Yangyang Laboratory. Since 2018, COSINE

has improved its sensitivity 100-fold, says

Hyun Su Lee, the team’s co-spokesperson and

a particle physicist at South Korea’s Institute

for Basic Science. But in 1.7 years of data,

they see no sign of WIMPs, the researchers

report this week in Science Advances.

There’s a caveat. The new COSINE analy-

sis does not look for the annual cycle in the

event rate, but simply for an excess number

of low-energy events. That tiny signal would

appear on top of a much larger background

of events caused by ordinary particle radia-

tion from sources both inside and outside

the crystals. So the analysis depends on re-

searchers’ ability to model the subtle details

of those backgrounds, Bernabei notes.

COSINE is not the only experiment testing

the DAMA result, however. The ANAIS-112

detector contains nine sodium iodide crys-

tals with a total mass of 112 kilograms and

has been taking data in Spain’s Canfranc

Underground Laboratory since 2017. Three

years of data show no annual cycle in low-

energy events, ANAIS researchers reported

on 27 May in Physical Review D. However,

the uncertainties were slightly too high to

rule out the DAMA signal, says María Luisa

Sarsa, a physicist at the University of Zara-

goza and ANAIS co-leader.

If the DAMA signal isn’t real, “The field of

particle physics owes itself to find out what

DAMA is seeing,” says Reina Maruyama, a

nuclear particle physicist at Yale University

and co-spokesperson for COSINE. And last

year, Dario Buttazzo, a theorist at the Pisa

section of Italy’s National Institute of Nuclear

Physics, and colleagues identified a way in

which the DAMA team might have inadver-

tently created the annual cycle.

DAMA researchers collect data in year-

long runs. To make the variation in each

run stand out, they subtract

the average rate over the year

from the event rate measured

each day. But if the event rate

steadily rises or falls year after

year, the subtraction can turn a

steady trend into an oscillation,

Buttazzo says. For example, if

the rate were increasing by 1%

every year, DAMA’s method

would produce a signal that be-

gins each run at –0.5% and ends

each run at +0.5%.

“If you do the analysis like

DAMA was doing it, and if the

background has a particular fea-

ture, then you could have such

an effect,” says Buttazzo, whose

group reported its analysis in

the Journal of High Energy

Physics on 21 April 2020. But he

cautions that because DAMA hasn’t pub-

lished its data, “we cannot know if this ef-

fect is actually there or not.”

Still, the COSINE and ANAIS papers

suggest the issue could be important. The

ANAIS detector’s total event rate is decreas-

ing steadily as short-lived radioactive nuclei

in the crystals decay away, says María Lucía

Martínez Pérez, a physicist at the University

of Zaragoza and co-leader of the team. CO-

SINE sees a similar steady decline, Lee says.

Closure could come soon. In weeks, CO-

SINE will release a 3-year annual cycle

analysis, Lee says. ANAIS’s Sarsa says

that by next summer, “We expect to have

a high-impact publication with the aver-

age rate measured over the year with all

5 years of data.” That may be enough data,

she says, to bring the curtain down on the

DAMA claim. j

Dark matter particles should generate flashes of light in the COSINE experiment’s

sodium iodide crystals. It aims to test a similar experiment’s dark matter claim.

PARTICLE PHYSICS

By Adrian Cho

Is the end in sight for famous dark matter claim?

Data cast more doubt on DAMA result as an alternative explanation emerges