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Perspectives for indirect dark matter search with AMS-2 using cosmic-ray electrons and positrons
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2009 New J. Phys. 11 105021
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New Journal of Physics
T h e o p e n–a c c e s s j o u r n a l f o r p h y s i c s
Perspectives for indirect dark matter search with AMS-2using cosmic-ray electrons and positrons
B Beischer,P von Doetinchem,H Gast,T Kirn and S Schael
I.Physikalisches Institut,RWTH Aachen University,52074Aachen,Germany
E-mail:Stefan.Schael@physik.rwth-aachen.de
New Journal of Physics11(2009)105021(15pp)
Received17March2009
Published16October 2009
Online at https://www.sodocs.net/doc/506969864.html,/
doi:10.1088/1367-2630/11/10/105021
Abstract.The AMS-2experiment will be launched with the Space Shuttle
Discovery and installed on the International Space Station in2010.It is
designed to perform precision spectroscopy of many different cosmic-ray species
including electrons and positrons.While the nature of dark matter is as yet
unknown,dark matter annihilating in the Galactic halo is a well-motivated source
of cosmic-ray electrons and positrons.The cosmic-ray positron fraction data
available so far show signi?cant deviations between different measurements
and from the expectation for purely secondary production.The differences
between the measurements up to particle energies of6GeV can be understood
in a framework of charge-sign-dependent solar modulation and the spectra
show excellent agreement if corrected for these time-dependent effects.Recent
observations of an excess in the high-energy electron spectrum by ATIC might
be connected to the excess in the positron fraction.A possible source of both
signatures could be dark matter annihilation or a nearby pulsar.A measurement
of the anisotropy of high-energy electrons could distinguish between both
scenarios.Therefore the sky coverage of AMS-2will be discussed in addition
to possible dark matter scenarios and the sensitivity of the AMS-2experiment to
these effects.
New Journal of Physics11(2009)105021
2
Contents
1.Introduction2
2.Charge-sign-dependent solar modulation4
3.Sources of high-energy electrons and the sky coverage of AMS-27
4.Dark matter annihilation as a local fresh source for cosmic-ray electrons and
positrons?8
5.Alpha magnetic spectrometer(AMS)10
6.Summary13 References13
1.Introduction
Cosmic rays mainly consist of nuclei.Roughly90%of them are protons,9%areα-particles and heavier nuclei make up the rest.Electrons,positrons and antiprotons are found in small quantities.Starting in the GeV range,the number density of primary cosmic rays as a function of energy follows a power law(?gure1left),d N/d E∝E?γwith a spectral index for protons ofγp=2.7up to roughly1015eV.At energies below a few GeV,the spectra of cosmic rays are affected by solar modulation.
As cosmic rays travel through the Galaxy,their spectra are altered and their composition is changed by a variety of physical processes.Hadronic interactions of protons and nuclei with interstellar matter create secondary charged particles,as well asγ-rays byπ0production. Electrons lose energy by bremsstrahlung processes due to the interstellar matter,synchrotron radiation in the Galactic magnetic?eld,and inverse Compton scattering on photons of the cosmic microwave background and of starlight.Radioactive isotopes decay in?ight.To simulate these effects we have chosen to use the numerical model described by the Galprop code[8,9], version50p,which incorporates as much current information as possible,for example,on galactic structure and source distributions.There are two sets of propagation parameters for Galprop that have been found to give a good description of a wide range of available cosmic-ray data,the so-called conventional and the plain diffusion model[8].
Recent measurements of cosmic-ray electrons and positrons show interesting new features(?gure2)not expected within the Galprop model.At low energies,the new PAMELA measurement[10]of the positron fraction deviates signi?cantly from previous measurements[4],[11]–[14].We will show in this paper that this can be well understood in the framework of a charge-sign-dependent solar modulation(section2).At energies above10GeV, the positron fraction measured by PAMELA deviates signi?cantly from the expectation for purely secondary production as calculated in the framework of the Galprop program,con?rming earlier measurements but with much better statistics.The origin of this feature in the positron fraction has been discussed since the?rst HEAT measurements taken in1994[3,21].Various explanations have been suggested,varying from new effects in cosmic-ray propagation,to a nearby source for electrons and positrons like a pulsar or dark matter annihilation(sections3 and4).We will discuss the prospects to solve these questions with new measurements from AMS-2[22].At energies around500GeV,the ATIC experiment[18]and PPB-BETS[19]have reported an excess in the combined?ux of electrons and positrons.This could well be connected to the feature in the positron fraction if a local source produced an equal amount of electrons
3
/n ) / GeV
kin
(E
10
1
10
10F l u
x / (c m 2 s s r G e V /n )
–1
10101010
1010
101010
/ GeV
kin E 10110
10e +
Φ/p Φ
10101010Figure 1.Left:measured ?uxes of protons [1],helium [2],electrons and
positrons [3,4],antiprotons [5,6],and diffuse γ-rays from the Galactic center region [7].The solar-modulated predictions obtained in the conventional Galprop [8,9]model for each species are included.Right:energy dependence of the proton-to-positron ratio in cosmic rays as predicted by Galprop [8,9].
E / GeV
10
110
10 )
– + e + / ( e +e 10
10 E (GeV)
110
s –1 s r –1)
–2 c m 2 f l u x (G e V × 3E 101010Figure 2.Left:the positron fraction measured by PAMELA [10]and by
AMS01[4,11],HEAT [12],CAPRICE [13],TS93[14]and the weighted mean of these data [15]together with the secondary background as predicted by Galprop’s conventional and plain diffusion models.Right:the cosmic-ray electron spectrum measured by AMS01[4],CAPRICE [13],HEAT [16],SANRIKU [17],ATIC [18],PPB-BETS [19]and HESS [20],together with the prediction for a nearby pulsar.
and positrons at these energies as for example a pulsar or dark matter annihilation could do.We will discuss up to which energies a direct measurement with a magnetic spectrometer like AMS-2could contribute and help to understand the origin of this bump.
The new PAMELA data show that we have entered a new era of astroparticle physics where precision data are available to be compared with advanced theoretical models like Galprop for cosmic-ray propagation.The previous data sets for the cosmic-ray electron spectra as shown
4
in?gure2(right)show signi?cant variations in the normalization and in the spectral index. In order to ensure that this is not due to time variations in the cosmic-ray?ux an overlap in the measurement time between experiments is needed,as will be the case in the coming years between PAMELA,GLAST/FERMI[23]and AMS-2.
The other crucial issue is the understanding of the hadronic background,which is induced mainly by protons(?gure1right).Even though it seems unlikely that the rise in the positron fraction is a detector artifact,this option cannot completely be excluded today.The ratio of protons to positrons as predicted by Galprop(?gure1right)increases very fast with energy due to the different spectral indices of protons and positrons.With an electromagnetic calorimeter alone,as in PAMELA,GLAST/FERMI and ATIC,it is experimentally very challenging to achieve the required hadron rejection.The nonzero cross section forπ0production in hadron—nucleon interactions generates an irreducible background for all such experiments.In an event where a high-energyπ0is created at the top of the calorimeter,the shower in the calorimeter looks exactly like an electromagnetic shower.For better separation of electrons and positrons from hadrons,more than one subdetector is needed in the energy range of interest here.This will be achieved by AMS-2by the combination of an electromagnetic calorimeter and a transition radiation detector,which will allow us to reduce the hadronic background basically to zero. AMS-2will be launched by the Space Shuttle Discovery in2010and it will measure undisturbed by the Earth’s atmosphere on the International Space Station for three years.With an exposure for electrons and positrons of90sr day m2,its capabilities for particle identi?cation,and its excellent energy resolution due to the combination of a superconducting magnet and a silicon tracker,AMS-2will improve all existing measurements signi?cantly as will be discussed in section5.
2.Charge-sign-dependent solar modulation
The pre-PAMELA measurements of the cosmic-ray positron fraction are consistent[15]within the error bars(?gure2left)but they differ signi?cantly from the PAMELA data in the energy range below5GeV.In this regime,the?uxes of cosmic-ray particles are modulated due to interactions with the solar wind when they arrive at the outskirts of the solar system.The?rst hints at the effect of solar modulation came from observations of an anticorrelation between neutron monitor counts and the sunspot number,the latter being an indicator of the level of solar activity[24].The solar wind originates from the corona of the Sun.A magnetic?eld, rooted in the Sun,is frozen into the solar wind plasma,and the Sun’s rotation leads to the creation of the large-scale structure known as the Archimedes spiral.Cosmic-ray particles are scattered on the magnetic?elds.Gleeson and Axford[25]have modeled solar modulation by taking into account cosmic-ray diffusion through this magnetic?eld,convection by the outward motion of the solar wind,and adiabatic deceleration of the cosmic rays in this?ow.In the force-?eld approximation,the effect of solar modulation can be described by a single parameterφthat depends on the solar wind speed V and the diffusion coef?cientκ.The interstellar cosmic-ray ?ux J IS is then modulated to yield the locally observed one J as
J(E)=
E2?m2
(E+|z|φ)2?m2·J IS
(E+|z|φ),(1)
where E and m are the total and rest energy of a cosmic-ray particle,respectively,and z is the particle charge.The modulation parameterφhas the dimension of a rigidity and is typically of
5
/ GeV
kin E
10110
10 s s r G e V )–1
2F l u x / (c m 10101010
1010
10
Year
A v e r a g e S S N
(M V )
ΦM o d u l a t i o n p o t e n t i a l A +
A –
A –
Figure 3.Left:cosmic-ray proton spectrum as measured by BESS [28]–[30],
AMS01[1]and PAMELA [31],together with the prediction by the Galprop conventional model.The unmodulated model ?ux is plotted (full line),as well as the modulated ones,using the best-?t modulation parameter φfor each data set.Right:monthly average of the observed sunspot numbers [33]since 1985,
compared to the values of the modulation parameters φ+
p (black dots)determined
from the proton spectra and φ?
ˉp determined from anti-proton spectra (green
boxes)and φ?
e ?electron spectra (blue cross and blue circle).The cycle o
f the solar magnetic polarity is indicated by the bars at the top of the ?gure,with the approximate start and end dates taken from [34]and [35].
the order of 500MV but it changes with time in accordance with the solar cycle (?gure 3right).The modulation parameter is not a model-independent quantity.The values of φquoted in this paper have all been determined in the framework of the Galprop model.For example,using power spectra for protons and electrons instead,one would observe the same correlation with the sunspot numbers,but with a different scale factor.
The pre-PAMELA experiments all took place at a similar solar activity and at the same orientation of the solar magnetic ?eld.In 2000,the polarity of the Sun’s magnetic ?eld ?ipped and PAMELA data were taken under quiet solar conditions (?gure 3right).Drift models have been proposed [26,27]to account for charge sign-dependent effects in the solar modulation.In the framework of the force-?eld approximation the charge-sign-dependent solar modulation can be introduced by assuming that the spectra of positively and negatively charged particles can be described by two different parameters φ+and φ?.BESS [28]–[30]has measured continuously the proton and antiproton ?uxes,while only few electron and positron spectra are available.No special spectral feature is observed in the antiproton ?ux and therefore one can assume that the antiproton spectrum is purely secondaries as described in the Galprop model (?gure 4left).φ+is then determined for each measurement from a ?t to the proton spectrum (?gure 3left),while antiproton [28]–[30]and electron [3,4]measurements are used for the calculation of φ?.The obtained values for φ+and φ?are summarized in table 1.For several years we obtain a value for φ?ˉp =0which,if taken literally,implies that the negative cosmic-ray ?ux is not modulated by the solar magnetic ?eld at all.To our understanding this points to problems with the simpli?ed ansatz used in the force-?eld approximation presented here and one would expect that much better motivated descriptions like in drift models [26,27]would solve this problem.
6
/ GeV
kin E
10
110
10/p
p 1010101010
E / GeV
10
110
10
)– + e + / ( e +e –2
10
10Figure 4.Left:antiproton-to-proton ratio measured by PAMELA [32]compared
to the prediction obtained by assuming charge-sign-dependent solar modulation
with φ+
p =390MV and φ?ˉp =0MV.Right:positron fraction data corrected for
solar modulation effects according to the Galprop conventional model.Table 1.
Values obtained for φ+from the proton spectra and for φ?from
antiproton and electron spectra.
Experiment Year φ+p (MV )
φ?
ˉp (MV )
φ?
e ?(MV )
BESS 1993537
BESS 19973850
AMS11998454442
BESS 19984870BESS 1999571345BESS 20001238632BESS 20021037568BESS 2004689461PAMELA
2007
390
Fitting the low-energy part of the positron fraction as measured by PAMELA with the prediction from the Galprop model,leaving φ+and φ?as free parameters,leads to the same values as obtained from the ?t of the proton and antiproton PAMELA data (?gure 4left).However,the same exercise for the antiproton data from 1998from BESS and the electron data from AMS-1leads to signi?cantly different values of φ?,which is dif?cult to understand.The AMS-1electron spectrum has been measured in space and does not have any corrections due to atmospheric effects.We hence assume that it has signi?cantly smaller systematic uncertainties and use the electron φ?from 1998in the following.
With these data for φ+and φ?,the positron fraction measurements from AMS-1,HEAT,TS-93,CAPRICE and PAMELA can be corrected for solar modulation effects in the framework of the force-?eld approximation to obtain the local interstellar spectrum (J IS )as shown in ?gure 4(right).Now the agreement between the data sets from the various experiments and with the expectation from Galprop is intriguing.At the very least,it increases our con?dence in the measurements above 5GeV signi?cantly.
7
Figure5.Left:AMS-2on board the ISS together with the de?nition of the
angles,which de?ne the orientation of the ISS and hence of AMS-2.Right:sky
coverage of AMS-2on board the ISS in the Galactic coordinate system.
Corresponding to estimated changes is the ISS orientation yaw,pitch and roll
have been varied in the ranges[?180,+180]degrees,[?4,+22]degrees and
[?5,0]degrees,respectively.
3.Sources of high-energy electrons and the sky coverage of AMS-2
The direct detection of nearby electron sources by observing the energy spectrum in the TeV region is a well known,challenging goal of cosmic-ray physics.The energy loss of high-energy electrons(see e.g.[36]and references therein)per unit time is proportional to E2 and is caused by synchrotron radiation in the galactic magnetic?eld and inverse-Compton scattering on background photons.Therefore,in the TeV region,only electrons from sources at a distance within1kpc and with an age less than105years can reach the Earth.Since the number of such possible sources should be very limited,the energy spectrum of electrons might have a characteristic structure[37],and the arrival directions are expected to show a detectable anisotropy[38,39]modulated by the diffusion process in the Galaxy.The electron energy spectrum could,therefore,give direct knowledge of the nearby sources and the diffusion characteristics.
As several authors have pointed out already(e.g.[36],[40]–[42]),both a pulsar and dark matter annihilations could explain the features observed in the electron spectrum and in the positron fraction and both observations might be connected,i.e.for high energies there might be an equal amount of electrons and positrons in cosmic rays.If the dominant source was a nearby pulsar,for which Vela and Geminga are promising candidates,it could lead to an observable anisotropy of TeV electrons,while dark matter annihilation would lead to a homogeneous distribution of electrons on the sky[43].In order to experimentally test these hypotheses,a full sky map in the TeV electron and positron light is needed.The GLAST/FERMI experiment will be able to do this only for the sum of electrons and positrons and it will be challenging for GLAST/FERMI to control the hadronic background to the required precision.The AMS-2 instrument has an opening angle of50degrees and covers the sky as shown in?gure5taking changes in the orientation of the ISS into account[44].Anisotropies in this map could only be produced by local,nearby sources because of the diffusion process in the galactic magnetic ?eld.But to our knowledge,the idea that the TeV cosmic-ray electron?ux is isotropic has never been tested experimentally and as illustrated in?gure5this can be done by AMS-2with good statistics.
8
/n ) / GeV
kin (E –2
10–1
101
10
210B /C
E / GeV
–1
10
110
2
10 )– + e + / ( e +e –
1010Figure 6.Left:cosmic-ray B /C-ratio as a function of kinetic energy per nucleon,
compared to Galprop (red line).Data are from ACE-CRIS [45],Caldwell and Meyer [46],Chapell and Webber [47],Dwyer and Meyer [48],IMP-8[49],Gupta and Webber [50],HEAO-3[51],ISEE-3[52],Júliusson [53],Lezniak and Webber [54],Maehl et al [55],Orth et al [56],Simon et al [57],ULYSSES [58],and V oyager [59].The gray band corresponds to a variation of the Galprop model parameters as described in the text.Right:measurements of the cosmic-ray positron fraction compared to the predictions from Galprop (red line).The gray-shaded band corresponds to the variation for the Galprop parameters compatible with the measured B /C-ratio as described in the text.Models additionally falling within the 99%con?dence level interval with respect to the positron fraction data points below 3GeV,i.e.outside the expected signal region for dark matter annihilation,yield the green uncertainty band.
4.Dark matter annihilation as a local fresh source for cosmic-ray electrons and positrons?
Among the most intriguing open questions in modern physics is the nature of the dark matter,that has been shown to contribute around 23%to the total energy density of the universe [60,61].While the nature of dark matter is as yet unknown,dark matter annihilating (see e.g.[61])in the Galactic halo is a well-motivated source of cosmic-ray electrons and positrons.The cosmic-ray positron fraction data available so far indicate an excess over the expectation for purely secondary production (see ?gure 6right),a trend recently con?rmed and intensi?ed by measurements of the PAMELA experiment [10].Certain extensions to the standard model of particle physics like supersymmetry [62,63]or universal extra dimensions [64,65]predict a new particle,which would have all the properties required of a dark matter candidate.It will form halos around galaxies and annihilate pair-wise producing a variety of indirect signals in cosmic rays such as γ-rays,neutrinos and antiparticles.Its annihilation could be enhanced by the Sommerfeld effect [66]–[68]and by a possible clumpiness of dark matter [43,69],which
9
Table2.Range of parameter variation for random scans of the parameter space
of the conventional Galprop model.Slashes separate values below and above the
respective break rigidities quoted in the text.
Parameter Nominal value Lower bound Upper bound
γs1.82/2.361.77/2.311.87/2.41
γe1.6/2.51.4/2.31.8/2.7
D0(cm2s?1)5.75×10284.4×10287.2×1028
v A(km s?1)362646
z h(kpc)4 3.2 5.5
we account for by an additional boost factor in the modeling of the cosmic ray spectra.Rare cosmic antiparticles like positrons,antiprotons and anti deuterons are sensitive probes for new phenomena as there are no known primary sources of antiparticles in the Galaxy[70]–[73].
Before one can search for any signal of dark matter annihilation in charged cosmic rays one has to understand the uncertainties in the propagation within our galaxy.Charged cosmic rays in the energy range considered here do not contain any directional information due to the galactic magnetic?eld as opposed to gamma or neutrino rays.This makes the experimental signal discrimination much more dif?cult.In this context,it would therefore be rather dif?cult to establish a dark matter signal only by observing a slight deviation in the spectrum normalization as it has been discussed for the antiproton spectrum.A change in the spectral index as observed in the positron fraction is much more dif?cult to account for and since the?rst HEAT measurements in1994[3,21]this is a pending problem in particle astrophysics.
In the following,the key parameters of the conventional Galprop model have been varied to study their statistical uncertainties using the measured B/C ratio and the positron fraction outside the expected signal region from dark matter annihilation as constraints.The parameters considered are the spectral indicesγs for nuclei andγe for electrons at injection,the diffusion coef?cient D0,the Alfven velocity v A,and the halo size z h[8,9].The solar modulation parameters are kept?xed in the process.A random scan of the parameter space within in the limits as given in table2has been performed.Models giving a description of the B/C data with
equal or betterχ2
HEAO?3than the conventional Galprop model fall within the gray band in?gure6
(left and right).Models additionally falling within the99%con?dence level interval with respect to the positron fraction data points below3GeV i.e.outside the expected signal region for dark matter annihilation,yield the green uncertainty band in?gure6(right).It should be noted that
none of the individual curves in the gray band in?gure6(right)gives aχ2
ndof <360/29for the
measured positron fraction.The minimum found corresponds to the nominal Galprop parameter values in table2which are therefore used in the following.
As an example for a quantitative analysis of the sensitivity of the positron fraction to dark matter models we studied neutralino dark matter in the minimal supergravity grand uni?cation (mSUGRA)model.Aχ2minimization was performed with respect to the positron fraction data in the m1/2-m0-plane.The correction for solar modulation according to section2was applied to the data and the local interstellar spectra of the Galprop conventional model were used.The remaining statistical uncertainty of the background model corresponding to the green band as shown in?gure6(right)was taken into account in the calculation of theχ2.Only models ful?lling constraints on relic density,the B R(b→sγ)at the3σ-level and giving a value of
10
E / GeV
–1
10
110
2
10 )
– + e +
/ ( e +e
1010 / GeV
1/2m 240250
260270280290300
310320 / G e V
0m 1450
150015501600
1650
17001750
sr)2AMS-2 (3 years, 0.0875 m sr)
2PAMELA (3 years, 0.002 m 99% CL
Figure 7.Left:best-?t positron fraction with respect to the data of AMS-1,
HEAT,CAPRICE,TS93and PAMELA,for a representative mSUGRA model.Right:99%con?dence level areas derived from the χ2contours for ?ts to the projected data from PAMELA and AMS-2,for a ?xed mSUGRA parameter point in a small part of the m 1/2-m 0-plane,for tan β=40,m t =172.76GeV and m χ01=93GeV.The PAMELA contour refers to the projected data based on acceptance and mission duration,for comparison to AMS-2,not to the actual data.
the anomalous magnetic moment of the muon,a μ,falling within the preferred region at the 3σ-level,as well as existing mass limits and direct detection limits and having a best-?t boost factor of less than 104are included.As shown in ?gure 7(left)a moderately good ?t to the
high-energy PAMELA data can be obtained in the mSUGRA model.The χ2
ndof improves from 227/29for the background only ?t to 49/28for the model including a possible dark matter signal from neutralino annihilation.In this example,PAMELA would not be able to observe the return to the background curve due to its limitations in acceptance.This has a signi?cant impact on the capabilities to substantially constrain the mSUGRA parameter space as shown in ?gure 7(right).Details concerning the analysis are found in [74].
As has been pointed out in [75]radiative corrections could enhance the annihilation signal in the positron fraction without leaving any observable effect in the antiproton spectrum.Taking this effect into account improves the agreement between the measured positron fraction and the predictions within the mSUGRA model signi?cantly [75].
5.Alpha magnetic spectrometer (AMS)
AMS-2[22]will be launched on board the Space Shuttle Discovery in 2010.The detector design is optimized for precision particle spectroscopy in space and is based on the experience gained in the successful 10-day precursor ?ight of AMS-1in 1998[76].The AMS-2spectrometer design includes a superconducting magnet,a time-of-?ight (TOF)system,a silicon tracker,an anticoincidence counter (ACC)system,a transition radiation detector (TRD),an
11
Figure8.Left:full view of the AMS-2experiment mounted in the structural
interface(USS)which connects the detector to the shuttle and to the ISS.
Right:the AMS-2experiment at CERN in Spring2009after the completion of
the pre-integration.
electromagnetic calorimeter(ECAL)and a ring imaging Cherenkov detector(RICH).The total
weight is limited to6850kg and the total power consumption is2.7kW.A full view of the
AMS-2detector with its main components is shown in?gure8(left).A preintegration of
all subdetectors but the superconducting magnet has been performed at CERN in2007/2008
(?gure8right)including extensive and successful tests of the trigger and readout system with
cosmics.
The superconducting coils of the magnet system are situated inside a vacuum case
and operated at1.8K with super?uid helium.The superconducting magnet[77]generates a
magnetic?eld of0.86T in the center.Inside the cylindrical volume of the vacuum tank a double-
sided silicon strip detector measures the trajectories of charged particles at eight planes.The
single point resolution of the silicon tracker is0.0085mm in the bending plane and0.030mm in
the nonbending plane.The combination of the large lever arm,BL2=0.86Tm2,together with the high accuracy of the silicon microstrip detector gives a measurement of particle rigidity with
an accuracy ofσp/p=4.0×10?4p/GeV⊕0.018.This corresponds to a maximal detectable rigidity of2500GV and allows charge separation with three standard deviations up to800GV. The mechanical stability of the silicon tracker is monitored via an infrared laser system with a position accuracy of better than0.005mm[78].
The TOF system is made out of four scintillator planes(two on top and two below the
silicon tracker)read out by?ne-mesh phototubes due to the operation in regions with high
magnetic?eld[79].The TOF provides a fast trigger within200ns for the read out of the AMS-2
detector and it measures the particles charge and the time a particle needs to traverse the tracker
with a resolution ofσt=125ps.
The silicon tracker is surrounded by the ACC system,which consists of16scintillation
panels of8mm thickness read out by?ne-mesh phototubes(same type as for the TOF
system)[80].The ACC system detects and vetos particles,which enter the tracking volume
from the side,outside of the main acceptance,in coincidence with a particle going through the
TOF system and the silicon tracker.
A conical-shaped octagon structure is placed on top of the magnet vacuum case which
houses a20layer TRD for particle identi?cation.The combination of TRD and lead/?ber
ECAL,which is located at the bottom of AMS-2provides a proton rejection at the106level up
12
Figure 9.Left:the AMS-2TRD after the completion of the construction in the
clean room at the RWTH Aachen.Right:the construction principle of a TRD straw module.
to particle energies of 300GeV .The particle identi?cation capabilities of AMS-2are completed by the RICH system [81],which is located below the silicon tracker.The geometric acceptance of the AMS-2silicon tracker is 0.4m 2sr.The combination of TRD and ECAL reduces the geometric acceptance to 0.09m 2sr.
The AMS-2ECAL [82]–[84]is a 3D-sampling device made out of a 16.7X 0(radiation lengths)lead /scintillating ?ber structure,which measures gamma-rays,electrons and positrons and discriminates leptons from hadrons with a rejection of 103–104in the energy range from 1GeV up to 1TeV .The energy resolution is well parameterized by σ(E )/E =(10.2±0.3)%/√E /GeV ⊕(2.3±0.1)%.
Besides the enormous increase in acceptance the main difference in electron and positron identi?cation between PAMELA and AMS-2is the AMS-2TRD.This is the key feature of AMS-2,which allows the reduction of the proton background in the positron measurement basically to zero.The AMS-2TRD [85]–[88]is shown after its completion at the RWTH Aachen in ?gure 9.The transition radiation photons are produced by charged particles passing through the 20mm thick ?eece,which is used as radiator.The TR photons are detected in straw tubes,?lled with a Xe /CO2(80%/20%)gas mixture and operated at a voltage of 1400V .The TRD consists of 20layers of straw modules interleaved with a ?ber ?eece radiator and arranged in a conical octagon structure (?gure 9).The top and the bottom 4layers are oriented parallel to the AMS-2magnetic ?eld while the middle 12layers run perpendicular to provide three-dimensional (3D)tracking.Each straw module consists of 16straws as shown in ?gure 9.The straws have an inner diameter of 6mm,the wall material is a multilayer aluminum–kapton foil with a total thickness of 0.072mm.A gold-plated 0.030mm thick tungsten wire,?xed in a polycarbonate end-piece,is used as sense wire.The straw modules are mechanically stabilized by longitudinal and vertical carbon ?ber stiffeners.The AMS-2TRD has a proton rejection between 1000and 100in the energy range from 10to 250GeV at an electron ef?ciency of 90%[87].
AMS-2will not only measure electrons and positrons in cosmic rays.The combination of TRD,TOF,silicon tracker,RICH and ECAL will allow precision measurements of the B /C and the 3He /4He ratio [89].These measurements will allow us to constrain and test cosmic-ray
13
P (GeV)
110
1010
s r s )–1)
2 (c m 2 F l u x (G e V 3
E 10
10
10
10
P (GeV)
110
1010
)
–+e +
/(e +e 10
1010Figure 10.Left:projected electron and positron spectra as measured by AMS-2
compared to experimental data from AMS-1and ATIC-2.Right:projected positron fraction as measured by AMS-2compared to experimental data from PAMELA and the weighted mean of HEAT,AMS-1,TS-93and CAPRICE.
propagation models like Galprop to much greater precision than today.This will reduce the systematic errors in the background expectation for the positron fraction and the electron spectrum signi?cantly.In the example of the statistical uncertainties of the conventional Galprop model discussed in section 4the gray-shaded bands in ?gure 6(left and right)would be reduced to narrow lines,indistinguishable from the lines shown for the Galprop background expectation itself.
The AMS-2instrument as described above is expected to measure the electron and positron spectra in cosmic rays as shown in ?gure 10.Within the energy range considered here of up to 800GeV where AMS-2can separate positive and negative particle charges with three standard deviations the statistical error bars are smaller than the symbols displayed.
6.Summary
Earth’s atmosphere prohibits measurements of GeV-range cosmic rays from the ground.AMS-2[22]will be launched by the Space Shuttle Discovery to the International Space Station in 2010.It will measure charged cosmic rays with unprecedented precision and statistics for three years until the superconducting magnet runs out of helium.AMS-2covers a broad physics program and is an excellent detector for cosmic-ray electron and positron measurements.
In combination with the results on dark matter particles expected from the LHC collider in Geneva,indirect dark matter search in cosmic rays could become a major cornerstone of particle physics in the coming years.
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