XMASS continues dark matter debate
XMASS is the latest of multiple experiments to contradict a previous dark matter discovery claim, but the conversation isn't over yet.
Since 1998, scientists on the DAMA-LIBRA experiment at Gran Sasso National Laboratory in Italy have claimed the discovery of an increasingly statistically significant sign of dark matter.
This week, the XMASS experiment in Japan joined the LUX, Xenon100 and CDMS experiments in reporting results that seem to contradict that claim.
Scientists look for dark matter in many ways. Both this result from the XMASS experiment and the results from DAMA-LIBRA look for something called annual modulation, a sign that the Earth is constantly moving through a halo of dark matter particles.
As the sun rotates around the center of the Milky Way, the Earth moves around the sun, completing one revolution per year. During the first half of the year, the Earth moves in the same direction as the sun; during the second half, the Earth completes its circle, moving in the opposite direction.
When the sun and Earth are moving in the same direction, the Earth should move through slightly more dark matter than when the sun and Earth are moving in opposite directions. So scientists should see a few more dark matter particles hit their detectors during that part of the year.
Experiments other than DAMA-LIBRA have seen hints of an annual modulation, but only the CoGeNT experiment has ever provided support for DAMA-LIBRA’s claim that this modulation comes from dark matter.
The effect could be caused by other annual changes. Pressure and temperature could affect an experiment. Atmospheric changes with the seasons could affect the number of cosmic rays that reach the experiment. Background radiation from radon gas has been known to change seasonally for underground experiments because of its interaction with the water table in the rock, says Fermilab scientist Dan Bauer of the CDMS experiment.
“Nobody’s been able to put their finger on what’s causing the DAMA modulation,” he says. “We can’t find the smoking gun.”
The XMASS experiment in Kamioka, Japan, looks for signs that dark matter particles have bounced off the nuclei in their 832-kilogram container of liquid Xenon. The experiment has sensitivity to two types of possible dark matter interactions, says scientist Yoichiro Suzuki, principal investigator for XMASS at the Tokyo-based Kavli Institute for the Physics and Mathematics of the Universe, in an email.
After taking data for about 16 months, the XMASS experiment disagreed with the DAMA-LIBRA claim, if one assumes dark matter particles scatter like billiard balls when they collide with nuclei. XMASS did find a low level of annual modulation, though, and that could be a hint of dark matter interacting with normal matter in a different way.
However, XMASS scientists deduced from their signal some characteristics that the dark matter particles causing the modulation would need to have: their masses and their rates of interaction with normal matter. And experiments that search for dark matter directly have already ruled out those masses and interaction rates.
But scientists still don’t know for sure what dark matter particles are like. Until they do, or until they identify the source of the annual modulation signals, they might have a hard time dissuading scientists on DAMA-LIBRA.
The XMASS experiment continues to take data, Suzuki says. XMASS scientists hope eventually to build a 5-ton version of the experiment.
XMASS continues dark matter debate
XMASS is the latest of multiple experiments to contradict a previous dark matter discovery claim, but the conversation isn't over yet.
Since 1998, scientists on the DAMA-LIBRA experiment at Gran Sasso National Laboratory in Italy have claimed the discovery of an increasingly statistically significant sign of dark matter.
This week, the XMASS experiment in Japan joined the LUX, Xenon100 and CDMS experiments in reporting results that seem to contradict that claim.
Scientists look for dark matter in many ways. Both this result from the XMASS experiment and the results from DAMA-LIBRA look for something called annual modulation, a sign that the Earth is constantly moving through a halo of dark matter particles.
As the sun rotates around the center of the Milky Way, the Earth moves around the sun, completing one revolution per year. During the first half of the year, the Earth moves in the same direction as the sun; during the second half, the Earth completes its circle, moving in the opposite direction.
When the sun and Earth are moving in the same direction, the Earth should move through slightly more dark matter than when the sun and Earth are moving in opposite directions. So scientists should see a few more dark matter particles hit their detectors during that part of the year.
Experiments other than DAMA-LIBRA have seen hints of an annual modulation, but only the CoGeNT experiment has ever provided support for DAMA-LIBRA’s claim that this modulation comes from dark matter.
The effect could be caused by other annual changes. Pressure and temperature could affect an experiment. Atmospheric changes with the seasons could affect the number of cosmic rays that reach the experiment. Background radiation from radon gas has been known to change seasonally for underground experiments because of its interaction with the water table in the rock, says Fermilab scientist Dan Bauer of the CDMS experiment.
“Nobody’s been able to put their finger on what’s causing the DAMA modulation,” he says. “We can’t find the smoking gun.”
The XMASS experiment in Kamioka, Japan, looks for signs that dark matter particles have bounced off the nuclei in their 832-kilogram container of liquid Xenon. The experiment has sensitivity to two types of possible dark matter interactions, says scientist Yoichiro Suzuki, principal investigator for XMASS at the Tokyo-based Kavli Institute for the Physics and Mathematics of the Universe, in an email.
After taking data for about 16 months, the XMASS experiment disagreed with the DAMA-LIBRA claim, if one assumes dark matter particles scatter like billiard balls when they collide with nuclei. XMASS did find a low level of annual modulation, though, and that could be a hint of dark matter interacting with normal matter in a different way.
However, XMASS scientists deduced from their signal some characteristics that the dark matter particles causing the modulation would need to have: their masses and their rates of interaction with normal matter. And experiments that search for dark matter directly have already ruled out those masses and interaction rates.
But scientists still don’t know for sure what dark matter particles are like. Until they do, or until they identify the source of the annual modulation signals, they might have a hard time dissuading scientists on DAMA-LIBRA.
The XMASS experiment continues to take data, Suzuki says. XMASS scientists hope eventually to build a 5-ton version of the experiment.
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LA FISICA INCONTRA L’ARTE AL CASTEL DELL’OVO
Sculture, dipinti, collage fotografici, manufatti tessili, installazioni digitali e video ispirati alle sfide alla frontiera della conoscenza del grande acceleratore del CERN, LHC, alla passione dei migliaia di fisici che vi lavorano e alla recente scoperta del bosone di Higgs. Trenta lavori originali di artisti di tutto il mondo, che saranno esposti dal 15 al 21 Settembre nella Sala delle Carceri del Castel dell’Ovo di Napoli per la mostra Arte & Scienza, 30 opere di artisti internazionali illustrano la scoperta del bosone di Higgs, a cura del CERN e dell’Istituto Nazionale di Fisica Nucleare. Le opere sono state realizzate per il programma art@CMS dell’esperimento CMS, uno dei quattro grandi rivelatori, che come gigantesche macchine fotografiche catturano le collisioni tra i fasci protoni nell’anello di LHC. Art@CMS ha coinvolto a partire dal 2012 artisti, ricercatori, studenti ed educatori in progetti creativi mirati alla partecipazione del pubblico. “Quasi tutte le opere esposte – afferma Pierluigi Paolucci, ricercatore INFN che ha curato l’allestimento napoletano – sono il frutto di una collaborazione tra un’artista e uno scienziato, che si sono avventurati ognuno nel mondo dell’altro. 
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Eventi collaterali
Accompagneranno la mostra due eventi collaterali organizzati in collaborazione con la Città della Scienza di Napoli:
Disegnare coi ragazzi 16 Settembre ore 10.00, Castel dell’Ovo
Incontro con la musica elettronica G. Di Giugno e M. Fargnoli 17 Settembre ore 16.00, Castel dell’Ovo
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