TIN KH NUaCJJGQAl
NGHIEIV Ctfq TIA VU TRU D J TIM, DAU mfiu TOIV TAI VAT CHAT T 6 I
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ebsite Livescience.com ngay 19/9 dua tin: Phd ke tu tinh Alpha (AMS) dat tren Tram vu try qudc td ISS dang bay each Trai ddt 400 km da tien hdnh phan tich 41 ty hat tia vii try, nhd dd giiip chiing ta hieu biet ban ve vat chat tdi - loai vdt chdt than bi khdng nhin thay ehiem 27% todn bd vu try'.Hdi nam ngodi AMS da thu dugc chung cd vfi sy tdn tai eiia vdt chat tdi. Ket qud phdn tich mdi day Id su do ludng chinh xac nhat ddi vdi hat tia vii try,
Ngdy 18/9, phat bidu trong budi truyfin hinh tryc tuydn tir phdng thi nghiem cita Td chtic Nghien cihi hat nhan chau Au CERN dat d Thyy ST, Giao su Hgc vien Cdng nghe Massachusetts - dng Samuel Ting (ttic Ding Zhaozhong hay Dinh Trieu Trung), ngudi phat ngdn ciia Dy an AMS cho bifit khdng nhiing nhdm nghifin ciiu AMS hien cd sd lieu phan tich nhieu hon trudc day khoang 50% ma hg edn nghien ctiu sau ve nguon gdc ctia cac hgt tia vu try.
GS Samuel Ting di^g trudc AMS khi tiiiet bj nay (6n dura diroi phong len Ai ghep vao tram ISS. (Anh: CBS News)
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Vat chdt tdi - loai vat chdt khdng nhin thdy va cho tdi nay chua phat hifin thdy - Id khdi nifim dugc su dung dfi giai thich tai sao cdc thien ha va thifin thfi khdng bi vo tung va bay tan tdc. Tdt ca mpi vat chdt deu sinh ra trgng lyc, thfi nhung can cir theo Iinh todn thi trgng lyc ciia cac vat thfi trdng thay dugc lgi khdng dii stic dfi Idm cho vii try giii duge Irang thai ty hop. Vi thd cac nha vat ly suy ludn rdng vat chat tdi tdt phdi nhieu gdp nam lan vat chdt khd kifin thi mdi cd thfi bao dam vu try tdn tgi dugc.
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Nhtmg khoa hgc hien nay con chua ro vdt chdi i6i cau tao bdi cdc thdnh phdn gi, va lam each nao dfi true tiep dd tim thay vat chat tdi. Can cii theo mdt ly thuyfil dang phd bien thi vdt chdt tdi la do nhiing hai tirong tac yeu cd khdi Iugng ldn (cdn ggi la hat nan|; tirong lac yeu -WIMPs) hgp thdnh. Cac nha vdt ly suy doan la khi hai hat WIMPs va chgm nhau thi chiing se huy diet nhau, qua dd tgo ra mgt difin tti (electron) vd mdt d i ^ tir duong (positron) - ddi tdc phan vat ehdt ciia dipn tit. Po- sitron cd khoi Iugng bdng electrgn nhirag mang dipn tich duong, edn electron mang dien tich dm.
Vi the d day chiing la cdn tdi AMS vd sd lifiu tia vii try.
Phdn ldn tia vu try dugc hlnh thanh bdi cac thdnh phan Iam nen nguyfin tu, gdm proton v^
hat nhdn nguyfin tir da hi ldy mat cac electron, cung nhu cac positron va electron nang lugng cao. Song thyc ra posi- tron thiia qud nhieu so vdi electron, bdi vdy cdc nha vgt Iy cho rang tia vii try khong phai Id ngudn duy nhdt ctia positron. Hg ngh! rdng sy va cham giiia cdc hat kho tim thdy ctia vat chdt tdi cd the la mgt ngudn khac sinh ra posi- tron.
HI^U B l ^ Mdi Vfe VAT C H A T T 6 I AMS cd the do dugc mijc nang lugng ciia timg hgt vdi don vi la GeV (gigaelectron volt), tiic 1 ty vdn-di?n tii. Can Cli theo md hinh vat ly hgc thien thfi vfi sy va cham ctia cdc hat khdng gian, so lugng positron tuong iing vdi electron phai gidm di theo sy ndng cao mirc nang Iugng ciia tia vii try. Thfi nhimg cac s6 lieu mdi do AMS thu lugm dugc lai cho thdy tren thyc tfi, trudc khi mtic nang lugng cua tia vii try dgt tdi 275 GeV thi so lugng positron Iai tang theo su nang muc nang lupng dd. Chi khi miic nang lugng cao hon 275 GeV thi so
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TIN KH HUCyC NGOAl
AMS ngay s w Uilien tdi Tram vii b u quoc t e ISS (Nguon: NASA)
lugng posifron mdi bat ddu gidm.
Sam Ting ndi: ro rang la tdc do gidm positron ehgm hon rat nhieu so vdi electron. Khde biet ndy dugc gidi thich Id su va cham ciia vat chdt tdi se bd sung them positron. Theo dng, hifin nay khoa hgc chua chiing minh dugc su ton tai cua vdt chdt tdi. De chimg minh dugc dieu do, cac nhd vgt ly phdi tryc tiep tdm bat dugc loai Imt nay.
P H A I C H d K ^ QUA C U 6 l C U N G
H i ^ nay dya vao ket qua quan trdc thien van, gidi khoa hgc suy ra rang cd the tdn tai vdt chdt tdi, nhimg trong cdc nghien cim thi nghiem ve tia vii try sudt nira thfi ky qua, hg van chua phat hifin thdy chiing cd tryc tifip vfi sy tdn tai ciia nd. Can cii theo cac ly thuyfit hipn cd, khi v^t chdt tdi va chgm nhau se sinh ra mgt lugng positron du thvra, vi the vipc do ludng chinh xdc cac dac diem ciia positron se cd y nghia rdt quan trgng giiip lim hieu vat chdt tdi.
Theo ket qua nghien ciiu cdng bd tren tap chi Physical Review Letters (My), cdc nhd khoa hgc da phat hipn thdy cdc positron du thiia trong tia vii try cd nam dgc diem la: - sy tdng len ciia ty lfi positron bdt dau tir mirc ndng lugng 8 GeV; - vd tdc dg tang, ty le ciia positron trong tong sd electron va positron tdng nhanh; - d miic 275 GeV thi positron ngung tang; - qua trinh tang ty Ip positron tucmg ddi deu, khdng cd cac eye difim; - ngudn positron dudng nhu dfin tir cdc Iiudng trong khdng gian vii try chii khdng cd dinh tti mgt hudng ndo.
GS Ting, ngudi timg nhan giai Nobel Vat ly nam 1976 do phat hipn h^t J, ndi phat hipn mdi ve ddu hipu vgt chat tdi la kfit qua nghien cuu quan trgng nhdt ciia ong kfi tir ndm 1963 tdi nay. "Can cir vao kfit qua hipn nay, cai ma chiing tdi tim kiem nhat djnh phai Id cai mdi, cai chua limg thdy; nhung nd cd phdi
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la vat chat ldi hay khdng, dieu dd phai chd xem ket qua cudi cung," dng nhdn manh.
Nhdm nghien cuu AMS cho rang mudn chiing minh positron du thira la do vat chat tdi va chgm sinh ra thi phdi kham pbd dii sdu dac tnmg cua positron du thira. Dac trung cudi cimg Id ty le tang cua positron sau khi toi diem cao nhdt ed sut gdp hay khdng, neu quan sat thdy cd syt gdp, dd tiic la positron du thira den tir su va chgm cua vat chdt tdi;
ndu sut giam tir tir thi cd the Id positron du thiia den tir cdc thien thfi nhu pulsar.
Do vgt chdt toi khdng tdc dung vdi sdng difin tir nfin edc nhd khoa hgc chi cd thfi thdng qua tuang tac ciia nd vdi cdc vdt chat khdc dd gidn tiep "quan sdt"
vat chdt tdi. GS Ting gidi thich, chu yeu cd ba phuang phdp tim kiem vdt chat tdi: - Thi nghiem sti dyng sy va chgm hat sinh ra vdt chdt tdi de quan sdt sy suy bifin ciia vat chdt tdi; - Thi nghiem tan xg do sy tan xg vdt chdt tdi den thfi ldng hodc the rdn; - Thi nghiem hiiy dipt tifin hdnh trong khdng gian vii try, dya tren nguyen ly vat chdt tdi va chgm nhau sinh ra positron vd phan proton.
AMS Id dgi dien ciia phuang phap Thi nghipm hiiy diet, nd cd the tien hanh nhifiu do ludng dgc lap ddi vdi electron, positron vd tdng hda electron-positron.
Thdng 5/2011, AMS dugc dua len ISS, sau dd nd da do va phdn tich 41 ty hat tia vu try, trong dd cd 10,9 trieu electron vd positron.
Trd ldi cdu hdi lieu phuang phdp AMS cd gidnh vi tri ddu trong cudc canh tranh vdi cdc thi nghipm khac ve phat hifin sy tdn tai ciia vat chit tdi hay khdng, GS Ting ndi, viec phan tich sd Heu vd ciing khd khan, chira the dy kifin se mdt bao lau, nhung dng "hy vgng khdng d vi tri thii hai".
Vdt chat tdi Id mgt trong nhung bi dn khoa hgc ldn nhdt cho tdi nay chua cd gidi dap. Vdt chdt tdi khdng phat sang, tuc khdng phdt sdng dien tir, cho nen ta khdng nhin thdy. Nhung ciing nhu mgi vgt chat thdng thudng khdc, chiing cd tac dung lyc hdp dan. Nhd dd ma cac nha khoa hgc cd can cii de cho rang vdt chat tdi chiem tdi 23% vii try, ndng lugng Idi chifim 73%, nghia Id todn bg vai chai thdng thudng Ioai ngudi da quan sat thdy chi chiem 4% vii try. n
NGUYIN HAI HOANH tdng hgp
' Theo nha v^t ly Stephen Hawking thi vat chat itha kii-n chi chicm 5 % vu try. con I91 2 7 % la v^l chai l6i va 6 8 % la nang luyng (61
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