&Bullet; physics 14, 96

A neutrino background that could confuse the dark matter search now becomes an opportunity to explore new physics.

The “neutrino soil” has been emerging for years in the search for dark matter. This neutrino background is still below the sensitivity of dark matter detectors, but as such detectors become more sensitive it is only a matter of time before neutrino events begin to dominate the signal. Reaching this level may sound like bad news, but some researchers see it as an opportunity to uncover new information about neutrinos and potentially uncover particles and interactions beyond the Standard Model of particle physics.

The neutrino bottom is the result of a particular neutrino interaction called coherent elastic neutrino nucleus scattering, or

$\text{CE}\nu \text{NS}$

(pronounced “seven”). “If you’re looking for dark matter,

$\text{CE}\nu \text{NS}$

is a background. If you’re looking for neutrinos, that’s a signal, ”says Louis Strigari of Texas A&M University. The first observation of

$\text{CE}\nu \text{NS}$

happened just four years ago in an accelerator-based experiment. Now a dozen other experiments aim to: a

$\text{CE}\nu \text{NS}$

Signal. At the Weak Interactions and Neutrinos 2021 conference earlier this month, Strigari gave an overview of what

$\text{CE}\nu \text{NS}$

First postulated in the 1970s,

$\text{CE}\nu \text{NS}$

occurs when a neutrino “hits” a nucleus and gives it a kick. Compared to other neutrino-core interactions, where the neutrino interacts with a single neutron or proton,

$\text{CE}\nu \text{NS}$

is a coherent interaction between the neutrino and all neutrons and protons in the nucleus. Makes this bigger goal (or “cross section”)

$\text{CE}\nu \text{NS}$

much more likely than other neutrino interactions with single nucleons or electrons. On the other hand, the kick or recoil that the core receives is tiny, which results in that

$\text{CE}\nu \text{NS}$

very difficult to watch.

Despite this difficulty, there are suggestions for the detection of

$\text{CE}\nu \text{NS}$

Go back 40 years. The early designs for

$\text{CE}\nu \text{NS}$

Detectors were based on scintillators, in which the nuclear recoil produced an observable flash of light. Researchers later adopted this scintillator technology to look for dark matter particles called WIMPs. “It’s actually funny that we should come full circle again,” says Strigari as the researchers try to use dark matter detection technology for studies

$\text{CE}\nu \text{NS}$

.

The XENON collaboration, which is running a major dark matter experiment at the Gran Sasso National Laboratory in Italy, recently did a special search for

$\text{CE}\nu \text{NS}$

. The team focused on a specific source of neutrinos – those resulting from boron-8 nuclear reactions in the sun. In the XENON detector, these neutrinos should a

$\text{CE}\nu \text{NS}$

Signal indistinguishable from a WIMP with a ground of

$6th\phantom{\rule{2.77695pt}{0ex}}{\text{GeV / c}}^{2}$

. This indistinguishability makes

$\text{CE}\nu \text{NS}$

Neutrinos anathema to dark matter scientists. “It’s a background you can’t tell from your signal,” says Joseph Howlett, a member of the XENON team, a graduate student at Columbia University in New York. He and his colleagues appreciated that

$\text{CE}\nu \text{NS}$

Neutrinos could already be identifiable in XENON observations. To test this possibility, they re-analyzed some archived data – relaxing certain selection criteria and eliminating some other background – but in the end they found none

$\text{CE}\nu \text{NS}$

Signal. The situation could change with improved detectors at XENON and other dark matter facilities. “There’s a good chance we’ll see these events in the next few years,” Howlett says.

The only experiment that captured a

$\text{CE}\nu \text{NS}$

Signal is so far COHERENT – a dedicated one

$\text{CE}\nu \text{NS}$

Project at the Spallation Neutron Source in Tennessee. The experiment uses a high-energy neutrino beam that is generated as a by-product of an accelerator experiment. In 2017 the COHERENT Collaboration reported the first observations of

$\text{CE}\nu \text{NS}$

Events in a cesium iodide scintillator. And earlier this year the team released evidence for

$\text{CE}\nu \text{NS}$

in another type of detector based on argon. The reason for changing the detector type is that the

$\text{CE}\nu \text{NS}$

Interaction should depend on the size of the target core, explains Kate Scholberg of Duke University in North Carolina, spokeswoman for COHERENT. especially the

$\text{CE}\nu \text{NS}$

Rate is predicted as proportional to

${\mathrm{No}}^{2}$

, Where

$\mathrm{No}$

is the neutron number. “We want different values ​​of

$\mathrm{No}$

to test this dependency, ”says Scholberg.

COHERENT plans to measure

$\text{CE}\nu \text{NS}$

with other types of detectors, such as those based on germanium and sodium iodide. At the same time, several other experiments, such as TEXONO in Taiwan and CONNIE in Brazil, have installed detectors next to nuclear reactors that allow them to study

$\text{CE}\nu \text{NS}$

with reactor neutrinos. Some of these projects, Strigari says, arose spontaneously in physics departments that had additional dark matter detector equipment and a nearby neutrino source. “It’s a really experimental field that doesn’t require huge detectors or huge collaborations,” says Strigari.

One of the reasons to study

$\text{CE}\nu \text{NS}$

is looking for new physics. “Because the nucleus looks like a single, structureless particle for a neutrino that knocks it over a step

$\text{CE}\nu \text{NS}$

, there are few uncertainties in the process due to the internal core structure, ”says Scholberg. For example, if neutrinos have non-standard interactions with neutrons, then the coherent aspect is that

$\text{CE}\nu \text{NS}$

Interaction could increase this effect. “In a way, you amplify a new physical signal,” says Strigari.

There are other questions that

$\text{CE}\nu \text{NS}$

Research could address, for example, whether the so-called sterile neutrino exists or not (see position: Sterile Neutrino Down but Not Completely Out). In some neutrino experiments a deficit in the neutrino count was observed, which could be explained by the conversion of the three standard flavor neutrinos into undetectable sterile neutrinos. How

$\text{CE}\nu \text{NS}$

is a taste-independent interaction, it could offer a new way to test this sterile neutrino hypothesis.

The more sensitive detectors naturally become

$\text{CE}\nu \text{NS}$

Neutrinos, the more difficult it will be for them to discover other particles. “Mitigating this background is one of the biggest challenges for dark matter experiments in the next decade,” says Howlett. Some mitigation strategies are proposed, such as the production of detectors with direction-sensitive or spin-dependent effects (see Synopsis: Discriminating Dark Matter from Neutrinos). But the meeting

$\text{CE}\nu \text{NS}$

-Recognition challenge could be an opportunity to learn new and perhaps unexpected things about neutrinos. “This is an opportunity that dark matter detectors can take advantage of,” Howlett says.

–Michael Schirber

Michael Schirber is the corresponding editor for physics based in Lyon, France.

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