Antimatter dark matter and cosmic rays with AMS

J.P. Vialle

To cite this version:

J.P. Vialle. Antimatter dark matter and cosmic rays with AMS. Cosmion Conference 5, May2001, Moscow St-Petersburg, Russia. 2001. <in2p3-00011231>

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LAPP�EXP �������

September ����

Antimatter� Dark Matter and Cosmic rays with AMS

J�P� VIALLE

LAPP� IN�P��CNRS� Chemin de Bellevue� BP����

F������ Annecy�le�Vieux

abstract

The AMS experiment aims at searching for primordial antimatter� non�baryonic darkmatter� and at measuring with high statistics and high accuracy the electrically chargedcosmic ray particles and light nuclei in the extraterrestrial space beyond the atmosphere�AMS is the �rst magnetic spectrometer which will be �own in space� It will be installedfor � to � years on the international space station �ISS in ���� covering a range in energyfrom GeV to a few TeV� with an increase in sensitivity of � to � order of magnitudescompared to existing experiments� A test �ight with the space shuttle DISCOVERY tookplace in June ��� with a dedicated instrument and gave many results on the �uxes ofparticules above and below the geomagnetic threshold� These results are described herewith their interpretation� The physics goal and perspectives for AMS on the space stationare described as well

Talk given at the �th COSMION Conference� ���� May �� � Moscow�St�Petersburg�Russia

Gravitation � Cosmology� Vol� � ������� Supplement� pp� ���c� ���� Russian Gravitational Society

ANTIMATTER� DARK MATTER AND COSMIC

RAYS WITH AMS

J�P�Vialle�

LAPP�IN�P��CNRS and University of SavoieAMS Collaboration

�Laboratoire d�Annecy�le�Vieux de Physique des Particules� Chemin de Bellevue� B�P� ���F����� Annecy�le�Vieux Cedex� France

Abstract

The AMS experiment aims at searching for primordial antimatter� non�baryonic darkmatter� and at measuring with high statistics and high accuracy the electrically chargedcosmic ray particles and light nuclei in the extraterrestrial space beyond the atmosphere�AMS is the rst magnetic spectrometer which will be �own in space� It will be installedfor � to � years on the international space station �ISS� in ��� covering a range in energyfrom � GeV to a few TeV� with an increase in sensitivity of � to � order of magnitudescompared to existing experiments� A test �ight with the space shuttle DISCOVERYtook place in June ���� with a dedicated instrument and gave many results on the �uxesof particules above and below the geomagnetic threshold� These results are describedhere with their interpretation� The physics goal and perspectives for AMS on the spacestation are described as well

� Introduction

The apparent absence of primordial antimatter �antihelium� anticarbon� ���� in the universe isone of the great puzzles in physics� The absence of annihilation of gamma rays peaks excludesthe presence of large quantity of antimatter within a distance of the order of � Mpc fromthe earth� and balloon borne experiments have searched for antinuclei of helium or carbonfor more than � years with negative results��� On the other hand� was demonstrated byA� Zacharov�� that to generate an asymmetric universe made of matter only� starting froma symmetric universe� i�e� an universe in which there is the same amount of matter and ofantimatter� as it is in the big�bang theory at the very beginning of the universe� � conditionsmust be ful lled � i� Violation of the baryonic number ii� Violation of the charge conjugationC iii� large CP violation iv� Baryogenesis out of the thermal equilibrium� These conditions arenot supported by particle physics experimental data so far� On the other hand it is advocated�from the absence of observation of gamma ray peaks from matter�antimatter annihilation� andfrom the spectrum of Cosmic Microwave Background� that large amounts of antimatter if itexists are beyond several hundreds Mpc of the earth��� As a matter of fact� there is no rmexperimental foundation to the theories predicting the absence of antimatter� Universe can stillhave islands of antimatter���

Antimatter� Dark Matter and Cosmic rays

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Figure �� Fraction of positrons in the summed spectra of electrons and positrons with the e�ectof a neutralino of mass �� GeV�c� according to Diehl et al�

From the motion of the spiral arms of galaxies and of the cluster of galaxies it can be deducedthat about �� of the mass of the universe is non�radiating� Experimental data�� show that

most of the dark matter is of non baryonic origin� the so�called WIMP�s� In supersymmetrictheories� a good candidate for WIMP�s is the neutralino �� the lightest supersymmetric particle�It could be detected in cosmic rays through its annihilation into antiprotons� positrons� andgammas�� resulting in speci c structures in the falling energy spectra of such particles at highenergy�

The search for antimatter and for dark matter necessitates measurements at high accu�racy and high statistics of electrically charged cosmic rays� with excellent identi cation of thenature of particles and of the sign of their electric charge� The AMS �Alpha Magnetic Spec�trometer�experiment aims at such a measurement� It is a large acceptance high resolutionspectrometer which will be installed in �� for � to � years on the International Space Sta�tion� taking pro t of the background free environment of the extraterrestrial space beyond theatmosphere�

AMS will be able to improve our knowledge of the �uxes of charged cosmic rays and nuclei ina rigidity range typically from � GeV to a few TeV� where little or no precise experimental dataexist� For light nuclei �uxes� the isotopic content� gives information on the con nement in thegalaxy� The unstable isotopes act like a clock which gives the time spent in the galaxy by thenuclei between their production and the measurement� This is for instance the case for the Be�

and Be�� for which the ratio of abundance gives a direct measurement of the con nement time�AMS will be able to measure accurately the �uxes of light nuclei with their isotopic contentup to the oxygen on a kinetic energy range from few tens of MeV�Nucleon up to around ��GeV�N� AMS will also be able to measure the nuclei spectra for masses up to the aluminiumin an energy range up to about � TeV� providing invaluable informations on the mechanismsof galaxies�

� AMS on the international space station ISS� AMS��

AMS is scheduled to �y to the international space station ISS in march ��� and to take datafor � to � years�

J�P� Vialle AMS Collaboration

Figure �� The AMS� detector to be installed on the International Space Station

The AMS detector is designed around a superconducting magnet with a cylindrical innervolume of ����� meters diameter and � cm height giving a bending power of BL� � ���Tm��Eight layers ����� m�� of double�sided silicon tracker placed inside the magnet provide a co�ordinate resolution of �� in the bending plane and �� in the non�bending plane� This willallow to measure particle momenta with a resolution better than �� up to � GeV� �� at �TeV� and to have a excellent determination of the sign of electric charge up to several TeV�

Four layers of Time of Flight �TOF� hodoscopes in plastic scintillator� � above and � belowthe tracker�provide precise time of �ight measurement �about �� picoseconds� and dE�dxmeasurements� A layer of Veto counters made of plastic scintillator lines the inner face of themagnet to ensure that only particles passing through the magnet aperture will be accepted�

A Transition Radiation Detector placed above the TOF detector� made of � planes of strawtubes and foam� will identify electrons and positrons with a rejection factor against hadrons of����� from ��� GeV to � GeV� and will give additional measurements of the trajectory ofparticles�

Below the TOF� a RICH cerenkov counter with an aerogel radiator measures the velocity �to

���� and the charge jQj of particles and nuclei� Combined with the momentum measurementin the tracker� it will enable AMS to measure directly the mass of objects� Nuclei up toAluminium will be identi ed on the whole energy range of AMS �up to � TeV�n�� and isotopiccontent will be measured up to Oxygen for nuclei with an energy per nucleon below �� GeV�

Underneath the RICH� an imaging electromagnetic calorimeter made of a sandwich of leadand scintillating bers of depth ���� X will measure the electromagnetic shower energy withan accuracy of a few percent and the direction of electromagnetic particles to about �� Thanksto its very ne granularity ��� sampling in depth� and a transverse granularity of �� Moliereradius in both direction� the calorimeter will allow to achieve a rejection power of electron�positron and gammas against hadrons close to ��� which combined with the TRD will achievethe identi cation power required for the precise measurement of charged cosmic rays �uxes upto the TeV range�

The detector was designed with two main principles �

� Antimatter� Dark Matter and Cosmic rays

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i� a minimal amount of material on the particle trajectory to avoid possible sources ofbackground andto minimize large angle nuclear scattering along the trajectory�

ii� many repeated measurements of momentum� velocity� charge� energy loss� in order to getexcellent identi cation power and background rejection capability�

The acceptance of the detector for antihelium search is �� m�Sr�

� The precursor �ight STS�� � AMS��

In order to test the technology and to have a rst measurement of the environmental conditionson the space station� it was decided to �y a rst version of the detector on board of thespace shuttle DISCOVERY� This so�called AMS� precursor �ight took place in june ����and lasted � days� AMS consisted mainly of a permanent magnet� a silicon tracker� time of�ight hodoscopes� anticoincidence counters� and an aerogel threshold Cerenkov counter� Theacceptance was about �� m�sr�

The permanent magnet had the same inner volume and the same eld structure as thesuperconducting magnet of AMS�� with a bending power of ��� Tm�� There was only sixplanes of double sided silicon microstrip detectors� The four layers TOF hodoscope was thesame as in AMS�� The typical accuracy on the time measurement was �� psec for particlesof unit charge� and �� psec for charge jZj � �� allowing to identify electrons and positronsagainst protons up to ��� GeV�c� An other velocity measurement for identifying positrons andelectrons was made by a threshold Cerenkov counter� with an aerogel radiator of refractiveindex n������ giving a rejection factor of about � up to �� GeV�c� Finally� the detectorwas also shielded from low energy �a few MeV� particles by thin carbon ber walls� The basictrigger was made of the coincidence of signals in � out of � of the TOF planes� with a veto ona signal in the anticoincidence counters�

After the �ight� the detector was extensevely checked at two accelerators� At GSI Darmstadtwith Helium and Carbon beams from �� to ��� GV in rigidity at � incident angles andlocations for a total of � events� and at CERN�PS with protons of � to �� GeV at ��incident angles and locations for a total of �� events� This ensured that the behaviour ofthe detector and its performances on momentum measurement� mass separation and chargeseparation were thoroughly understood�

J�P� Vialle AMS Collaboration �

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During the �ight STS��� the orbit was the same as the orbit of the space station� It wasinclined by ����o on the geographic equator� at an altitude varying between �� kms and ��kms� More than � millions triggers in total were gathered�

The �uxes of cosmic rays from outer space is a�ected by the earth magnetic eld whichbends their trajectories back to space when the energy is below a threshold that depends onthe impinging angle and the geomagnetic latitude of the particle� Near the magnetic pole thekinetic energy threshold is as low as � MeV while at the magnetic equator it raises beyond �GeV� In the South Atlantic the magnetic eld is much weaker �i�e� the so�called SAA or SouthAtlantic Anomaly� which allows a high �ux of low energy particles to reach the Earth� Datataken while passing through or near the SAA were excluded from all the analysis�

Search for antihelium

For this search��� the sample was restricted to data taken with the shuttle attitude such thatthe z�axis of AMS was pointing toward the zenith within an angle of �� degrees� Events wereselected after full reconstruction �momentum� sign� velocity� direction upward or downward�energy loss in the tracker and in the TOF estimated by truncated mean� and were requiredto give hits in at least � tracker planes out of �� and to have an energy loss in tracker andin TOF compatible with Z�� charge� The latter criteria rejected the background of electronswith wrongly measured charge� since the combined charge confusion probability was found tobe less than �� � g ��� The possibility of misidentifying the direction upward or downwardof the cosmic ray was found to be negligible from a control sample� To get rid of large anglenuclear scattering on one of the tracker plane� it was also requested that the parameters of thetrack reconstructed with the rst � hits� the last � hits� and using all the hits in the tracker becompatible� To remove events with colinear delta rays which could disturb the reconstruction alast cut was applied on events with an excess of energy deposited within � � mm of the track�Finally a probability function was constructed from the measurement of velocity� rigidity� andenergy loss to to check the compatibility of the track with the passage of an helium or antiheliumnuclei with A�� or �� While this last cut removed the � last remaining antihelium candidates�the Helium sample was very little a�ected� A total of ��� � �� He was obtained in a rigidityrange of up to �� GV while there was no antihelium at any rigidity� g ��� The limit onantihelium is� N�antihelium��N�helium�� ��� ���� over the range � to �� GV in rigidity� The

� Antimatter� Dark Matter and Cosmic rays

same study applied to nuclei with jZj � � gave ���� � �� events in the same rigidity range andno antinucleus candidate�

� Measurement of spectra at high energy

Spectra of cosmic rays are generally expected to fall o� like a power law at high energy� A mea�surement of the slope for protons and helium nuclei has been done with the AMS�� data�� ���

Protons and Helium are easy to study since they are the most abundant charged cosmicrays� All the other ones are orders of magnitude lower in �ux�The data used for this study wererestricted to the periods when the axis of AMS was pointing toward the Zenith within ��o� Itwas required to have at least � hits in the tracker in the bending plane and � in the non�bendingplane in order to t the particle rigidity R�pc�jZj � e� The charge was measured from energyloss in tracker and TOF� leaving less than ��� proton contamination in the Helium eventswhose �ux is typically � times lower� The remaining background events in the proton samplefrom pions produced in the top part of AMS was found to be less than ��� below � GeV andvanishing rapidly with energy� while the contamination of deuterons was reduced to a negligiblelevel by requiring the measured mass to be within � standard deviations of the proton mass�

The distributions obtained were corrected for the di�erential acceptance of the detector as afunction of the latitude and of the pointing angle� giving a contribution to the total systematicerror of ��� They were then unfolded for the e�ect of the rigidity resolution�

After applying all these corrections the spectrum was parametrized by a power law in rigidityas� � �R

��

Fitting the parameters on the rigidity range � to � GV for the protons yields �

� � ���� ���fit�� ����sys�

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Fitting the parameters on the rigidity range � to � GV for the Helium yields�

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� Particles spectra below the geomagnetic threshold

��� Protons �uxes

Due to the di�culty to launch and operate a magnetic spectrometer in space� the previousmeasurements of charged particles at high energy were done with balloon�borne experiment��ying at a maximum altitude of � kms�where there is still � g�cm� of atmosphere above� Ithad been demonstrated by Stoermer����� that if there was under cuto� particles they could notcome from the outer space� Under cuto� particles were detected and classi ed in � categories �i� Atmospheric Secondaries� for the particles produced in the air above the detector and goingdownward ii� Splash Albedo for upward going particles produced by interaction in the air iii�Return Albedo for the fraction of splash albedo going back to the earth on the opposite atmo�sphere� Particle produced and then detected in these experiments were immediately absorbedin the atmosphere� At higher altitude like the � km orbit of AMS� no measurement existedabove � MeV�

J�P� Vialle AMS Collaboration

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Figure �� Fluxes of proton upward and downward in bins of geomagnetic latitude

It was a surprise in AMS to observe a large �ux of under cuto� particles up to few GeV inenergy� and that around the magnetic equator particles were trapped for a long duration����

For the study of the proton �ux� the sample was restricted to the period in which the z�axiswas pointing within �o of the zenith �downward going particles� or of the Nadir �upward goingparticles�� The background was found negligible like at higher energy and the corrections forthe di�erential acceptance and the detector resolution were the same�

The e�ect of geomagnetic cuto� shows up very neatly as a function of the geomagnetic lat�itude� At high energy� say above � GeV� all the spectra have the same behavior� Surprisingly�a second �secondary� spectrum starts below the geomagnetic cuto� with a rate about an orderof magnitude below� Near the equator the secondary spectrum has a distinct structure� In theregion �M � ��� the spectrum extend from the lowest measured energy� �� GeV to about �GeV� with a rate of m���sec���sr��� In all the bins in latitude up to �M � � the secondaryspectrum of downward particle has a �ux equal to the one of upward particles measured withthe detector pointing toward the earth � g ��� By tracing back �� protons of the secondary�ux in the earth magnetic eld and stopping the extrapolation either when the particle reachthe top of the atmosphere de ned at � kms� or when the �ight time reached � seconds� itwas found that all the protons from the secondany spectrum originate in the atmosphere� ��of the protons �ew for less than �� seconds �so�called �short lived� particles� and their originin the atmosphere is uniformly distributed around the globe re�ecting the shuttle orbits� Theremaining � � so�called long�lived particles� �ew for more than �� sec and originate from arestricted geographic region �Fig���� This global behaviour with a complicated path of particlesin the geomagnetic eld is di�erent from what is observed either with balloon�borne experi�ments in which particles are all �short lived�� or with satellites in the radiation belts where theprotons bounce across the equator for a much longer time�

� Antimatter� Dark Matter and Cosmic rays

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Figure �� Geographical origin of a� short�lived and b� long�lived protons with p � �GeV�c �left�compared with geographical origin of long�lived electrons and positrons �right�� The dashedlines indicate geomagnetic eld contour at �� km�

��� Electron and Positron measurements

AMS was also able to measure electrons and positrons���� The electrons thanks to their negativesign have clear identi cation and very small background� hence AMS was able to measuretheir spectra with limited statistics up to energies of � GeV� For a geomagnetic latitude of�M � ��� it was found that the electron spectrum has the same behaviour as the proton one�with a secondary �ux below the geomagnetic cuto� extending up to � GeV � g � and thesame amount of electrons moving downward and upward� uniformly spread around the globe�For positrons� the high �ux of protons with a rate about thousand time higher gives a lotof background at high energy� hence the study was done only below � GeV� an energy rangewhere the time of �ight of the particle and the signal in the aerogel counter allows to reducethe proton misidenti cation to a negligible level� As a consistency check� the positron fractionover the sum of positron and electron rate was compared in the polar region ��M � ��� to theprevious measurement by balloon borne experiments and was found in excellent agreement�

The electron and positron spectra exhibits the same behaviour as protons spectra� with asecondary �ux below the geomagnetic threshold which contains an equal number of downwardand upward going particles� In the geomagnetic equator region �M � �� radian the energyrange �� to ��� GeV� the sample contains also the so�called short�lived and long�lived particles�Short�lived electrons and positrons origin re�ects the orbits of the shuttle while long�livedpositrons come from the same limited geographical region than protons� Long�lived electronsoriginate from a restricted geographical region which is complementary of the one of protonsand electrons� Eventually� the �ux of positrons becomes � times higher than the electron onein the equator region�

��� Measurement of Helium �uxes and mass spectrum

Below the energy threshold after the fall of the primary spectrum a secondary spectrumshows up� � to � order of magnitude smaller in �ux� in the equatorial region j�M j � ��rad� Fitting the mass of helium nuclei above the threshold one gets M � ���� � �� GeV�c�

J�P� Vialle AMS Collaboration �

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Figure �� The left part of gure shows the spectra of Helium nuclei in bins of geomagnetic lati�tude� which exhibits also a secondary spectrum� The central gure shows the mass distributionof Helium candidates above threshold� compatible with a strong content of Helium �� while theright gure shows the mass distribution of particles below threshold which is compatible withHelium ��

in the polar region j�M j � �� rad� and M � ���� � �� GeV�c� in the equatorial regionj�M j � �� rad� Comparing with the He� mass of ��� GeV�c�� it shows that the content ismostly He�� However� the same mass tting applied to Helium nuclei below the threshold inthe equatorial region gives a mass of ���� � �� GeV � g ��� close to the He� mass of ����GeV�c� which implies that these nuclei are mostly He� which could be due to spallation of He�

on the atmosphere�

�� Antimatter� Dark Matter and Cosmic rays

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Figure �� Comparison of Simulation and AMS data� The left part shows the energy spectrumof protons near the equator� while the right gure shows the �ux of electrons and positrons forupward and downward particle�

Interpretation of the physics results of AMS���

To understand the data under geomagnetic cuto��a simulation has been made in LAPP���

with a detailed description of the earth atmosphere and of the earth magnetic eld� Protons�electrons� and heliums were generated isotropically at � earth radii with energy spectra takenfrom the measurement of AMS� Particles were traced along the magnetic eld and for thoseinteracting with the atmosphere the secondary particles from the interaction were also traced tocheck if they cross the orbit of the detector� This simulation reproduced quite well the observedbehaviour below the geomagnetic cuto�� As an example� the gure � shows the comparisonof experimental data and Monte�Carlo simulation for the energy spectrum of protons� and therelative behaviour of electrons and positrons� This result was con rmed by simulations of othergroups����

The e�ect of trapping arould the geomagnetic equator� and the concentration of the originof long�lived particle in a restricted geographic area is also well explained by the structure ofthe earth magnetic eld� The earth magnetic eld is in rst approximation a dipole inclined ofabout ��o with respect to the earth rotation axis� and the south magnetic pole near the northgeographic pole� The dipole axis is shifted by about � kms from the center of the earth� Dueto this con guration� particles are trapped in the earth magnetic eld and they bounce northand south around the equator and oscillate east�west� but the altitude of the trajedctory variesas a function of the longitude� which impose that only particle generated in a restricted range oflongitude can be trapped with their trajectory staying most of the time out of the atmosphere�

� Conclusion

Though the �ight STS��� was only a technological test� AMS obtained numerous� precise� andsometimes unexpected results on charged cosmic rays���� which demonstrates the quality ofthe instrument and its capability to ful ll its physics program in the future� A very detailedstudy of particle �uxes under the geomagnetic cuto� was achieved� The AMS� experiment onthe International Space Station will now concentrate on primary �uxes� and higher energy� Itsdesign is specially suited to the range of energy from � GeV to a few TeV� a domain which is

J�P� Vialle AMS Collaboration ��

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Figure �� Measurement of electrons�positrons with AMS�

still largely unexplored� The long duration of data taking on the space station� � to � years�will enable AMS to collect a huge amount of data � � order of magnitude more than in AMS��a much larger domain in energy� and a particle identi cation capability very powerful� The gure � shows an example of the capability of the new detector compared to AMS��

All the �uxes of charged particles and nuclei will be measured at the same time on a wideenergy range� This is especially important for the models of the dynamics of galaxies� since itallows to get rid of systematic errors on the relative �uxes� Furthermore� AMS is also capableof measuring high energy gamma rays� either by electron�positron pair conversion in the upperpart of the detector� or by conversion in the electromagnetic calorimeter� AMS will be able toobserve cataclysmic phenomena like AGN or GRB�s in the domain of a few GeV to typically� GeV where no data exist yet� This should improve dramatically our knowledge of these

phenomena�

With AMS on the space station� a new domain of energy for cosmic rays becomes accessible�

in which many surprises could happen�

Acknowledgements�

I would like to thank the Professors Samuel C�C� Ting and Maxim Y� Khlopov for giving methe opportunity to present AMS at the Fifth Cosmion Conference� I would like also to thank allmy colleagues of the AMS experiment and specially the AMS group at LAPP for many fruitfuldiscussions� The nancial support of the IN�P��CNRS� the �Conseil Regional Rhone�Alpes��and the �Conseil General de Haute�Savoie� to the AMS experiment is gratefully acknowledged�

References

� G�F�Smoot et al� Phys� Rev� Lett��� ����� ���� G� Steiman et al� Ann� Rev� Astr� Ap�������� ���� G� Badhwar et al� Nature �� ����� ��� A� Bu�ngton et al� ApJ ��� ���������� R�L� Golden et al� ApJ �� ����� ���� J�F�Ormes et al�� ApJ Letters ��� �������� T� Saeki et al�� Phys� Lett� B��� ������ ����

� A� D� Sacharov� JETP Lett� �� ��� ������

� Cohen� De Rujula� and Glashow ApJ ��� no����� pp��������� �������

� M� Yu� Khlopov et al�� Phys� Rev� D�� ���

� Antimatter� Dark Matter and Cosmic rays

� V�S� Berezinsky et al�� in TAUP �� Conference� Nucl� Phys� B� Proc� Suppl� � �� ������� E� Diehl et al�� Phys�Rev� D ��� ������� F� Barancourt et al� The AMS��� Aerogel Threshold Cerenkov counter Nucl� Instrum� Meth�

ods A� V������� ������ J� Alcaraz et al�� Search for antihelium in cosmic rays� AMS Collaboration� Phys� Lett�

B�������������� J� Alcaraz et al��Cosmic Protons� AMS Collaboration�Phys� Lett� B��������� J� Alcaraz et al��Helium in near earth orbit� AMS Collaboration�Phys� Lett� B��� ���

������ C� St ormer� The Polar Aurora� �Clarendon Press� Cambridge ������� J� Alcaraz et al�� Protons in near earth orbit� AMS Collaboration�Phys� Lett� B��� ���

������ J� Alcaraz et al��Leptons in near earth orbit� AMS Collaboration�Phys� Lett� B���� � ������ J� Favier� R� Kossakowski and J�P�Vialle� LAPP�Note�� L� Derome et al��Phys� Lett� B��������� V� Plyaskin� HEP�PH�������� J� Alcaraz et al�� AMS Collaboration� Physics Report in preparation�