recent results on antiparticles in cosmic rays from pamela experiment

GDR Supersymétrie, Orsay, 3-4 December 2008

Recent results on antiparticles

in cosmic raysfrom PAMELA experiment

Sergio Ricciarini INFN – Florence, Italy

On behalf of the PAMELA collaboration

S. Ricciarini GDR-SUSY 08

• The PAMELA experiment: short introduction.The PAMELA experiment: short introduction.

• Discussion of recent results.Discussion of recent results.

(1) (1) Antiproton/proton ratioAntiproton/proton ratio at high energies at high energies

(submitted to (submitted to Phys. Rev. Lett.Phys. Rev. Lett.).).

(2) (2) Positron fractionPositron fraction at high energies at high energies

(submitted to (submitted to NatureNature).).

• Conclusion and prospects.Conclusion and prospects.

Summary


Moscow, St. Petersburg

Russia

Sweden

Stockholm

Germany

Siegen

Italy

Bari Florence FrascatiTriesteNaples Rome

The PAMELA collaboration


• Study Study antiparticles in cosmic raysantiparticles in cosmic rays..

• Search for dark matter annihilationSearch for dark matter annihilation (e(e++ and p-bar spectra). and p-bar spectra).

• Study cosmic-ray production and propagation.Study cosmic-ray production and propagation.

• Study Study composition and spectracomposition and spectra of cosmic rays (including of cosmic rays (including light nuclei).light nuclei).

• Search for Search for anti-Heanti-He (primordial antimatter). (primordial antimatter).

• Study Study solar physicssolar physics and and solar modulationsolar modulation..

• Study of Study of terrestrial magnetosphereterrestrial magnetosphere and and radiation beltsradiation belts..

PAMELA scientific objectives


Energy range (with 3 years statistics)Energy range (with 3 years statistics)• AntiprotonsAntiprotons 80 MeV - 190 GeV80 MeV - 190 GeV• Positrons Positrons 50 MeV - 270 GeV50 MeV - 270 GeV

• Protons Protons up to 700 GeVup to 700 GeV• Electrons Electrons up to 400 GeV up to 400 GeV

• Electrons+positrons Electrons+positrons up to 2 TeV (from calorimeter)up to 2 TeV (from calorimeter)• Light Nuclei Light Nuclei up to 200 GeV/n (He/Be/C)up to 200 GeV/n (He/Be/C)• AntiNuclei search AntiNuclei search

– Simultaneous measurement of many cosmic-ray species.Simultaneous measurement of many cosmic-ray species.– New energy range.New energy range.– Unprecedented statistics.Unprecedented statistics.

PAMELA nominal capabilities


PAMELA detectors

GF: 21.6 cm2 sr Mass: 470 kgSize: 130x70x70 cm3

Power Budget: 360 W Spectrometer microstrip Si tracking system (TRK) + permanent magnet- Magnetic rigidity R = pc/Ze (GV); magnetic deflection η=1/R (GV-1)- Charge sign, momentum- Charge value from dE/dL

Time-Of-Flight (TOF)plastic scintillators + PMT:- Trigger- Albedo rejection- Mass identification up to 1 GeV- Charge value from dE/dL

Electromagnetic calorimeterW/Si sampling (16.3 X0, 0.6 λI) - Discrimination e+ / p, p-bar / e- (shower topology)- Direct E measurement for e-/e+

Neutron detectorpolyethylene + 3He counters:- High-energy e/h discrimination

Main requirements: high-sensitivity antiparticle identification and precise momentum measurement+ -


PAMELA

• PAMELA mounted on Russian satellite Resurs-DK1, inside a pressurized container.

• Minimum lifetime 3 years starting from June 2006.

• Quasi-polar low-earth elliptical orbit (70.0°, 350 - 610 km).

• Traverses and operates in the South Atlantic Anomaly.• Crosses the outer (electron) Van Allen belt at south

pole.350 km

610 km

70o

SAA

orbit period ~90 min

°

km

km

Satellite and orbit


Antiproton/proton ratioat high energies

(kinetic energy 1.5 - 100 GeV)


• Results discussed here have been submitted to Results discussed here have been submitted to Phys. Rev. Phys. Rev. LettLett..

• Analyzed data: July 2006 - February 2008.Analyzed data: July 2006 - February 2008.

• Total acquisition time ~ 500 days.Total acquisition time ~ 500 days.• Collected ~Collected ~ 1x10 1x109 9 triggers (~ 8.8TB of data).triggers (~ 8.8TB of data).

• Identified Identified ~~10 x 1010 x 1066 protons protons and and ~1 x 10~1 x 1033 antiprotons antiprotons with with kinetic energy between 1.5 and 100 GeV.kinetic energy between 1.5 and 100 GeV.– Collected 100 antiprotons above 20 GeV.Collected 100 antiprotons above 20 GeV.

High-energy antiproton/proton analysis


Antiproton and proton: basic cuts

• All requirements in the p-bar/p analysis are applied for both charge signs.

• Clean event pattern (reject spurious events):– single track in TRK;– no activity in CARD+CAT;– no multiple hits in S1+S2 (segmented).

• MIP |Z|=1 particle.– TRK+S1+S2 dE/dL < 3 MIP.

• Galactic particle (reject albedo, reentrant, East-West effect):– downward-going particle (300 ps TOF

resolution over 3 ns flight time);– measured rigidity R > 1.3 vertical

geomagnetic cutoff.

S1

S2

CALO

S4

CA

RD

CA

S

CAT

TOF

ND

TRK

.

S3


• Contamination from e- on p-bar sample is reduced to a negligible amount.– e- are easily identified in CALO from interaction topology

(rejection factor >104): they interact in the first CALO layers and give well contained and compact EM showers;

– on the other hand, most hadrons interact well deep in the CALO or do not interact at all.

Electron/hadron separation with CALO

5%aE

ba

E

σE

electron (R=17GV) hadron (R=19GV)

22 modules (Y Si-strip + W layer + X Si-strip)

Total depth: 16.3 X0 or 0.6 λI


Momentum and charge sign with TRK

• Minimal track requirements for good rigidity measurement:– at least 4 X (bending view) + 3 Y hits;– energy-dependent cut on track 2 (~95% total efficiency);– consistent TRK+TOF+CALO spatial information.

Magnetic rigidity R = pc/Ze (GV)Magnetic deflection η = 1/R (GV-1)

MDR (Maximum Detectable Rigidity):Def.: |R|=MDR σR=|R|

MDR=1/ση (ση spectrometer deflection resolution)

MDR depends on event characteristics and is evaluated event-by-event with the fitting routine:

- number and distribution of fitted points along the track;

- spatial resolution of the single position measurements (varies with track inclination and strip noise);

- magnetic field intensity along the track.


Rejection of p “spillover” background

• Defined additional optimized track requirements to improve MDR:- stronger constraints on χ2 at high energies (~75% efficiency);- rejected tracks with low-spatial-resolution clusters along the trajectory:

- faulty strips (high noise);- δ-rays (high signal and multiplicity).

Protonsand spillover

Antiprotons R = - 50 GVR = - 10 GV

• Main difficulty here is the background from “spillover” protons in the p-bar sample at high energies (protons with wrong charge sign):– finite MDR limits the precision of η (R) measurement;– high (~ 104) p/p-bar ratio in cosmic rays.

Minimal track requirements plus: MDR > 850 GV (high-precision subsample).


p-bar

R = - 10 GV

R= - 50 GV

Rejection of p “spillover” background

Protons and spillover

• Rigidity-dependent cut to reject residual spillover: MDR > 10 ∙ |R|This cut is equivalent to: |η| > 10 ∙ ση

This conservative rejection cut reduces residual spillover contamination to a negligible amount.

subsample withMDR > 850 GV

MDR > 10 ∙ |R|


Antiproton/proton ratio

• Excellent agreement with Excellent agreement with recent data from other recent data from other experiments.experiments.– One order of magnitude One order of magnitude

improvement in statistics.improvement in statistics.– Most extended energy Most extended energy

range ever achieved.range ever achieved.– Expected further Expected further

improvements with new improvements with new data.data.

• Correction factors are Correction factors are included and ~ one order included and ~ one order of magnitude less than of magnitude less than statistical error.statistical error.– CALO efficiency (different CALO efficiency (different

for p-bar and p);for p-bar and p);– loss of particles for loss of particles for

interactions.interactions.


PAMELA p-bar/p ratio and theory

• Ratio increases smoothly Ratio increases smoothly with energy from 4 x 10with energy from 4 x 10-5-5 and levels off at ~ 1 x 10and levels off at ~ 1 x 10--

44..

• Our results are Our results are enough enough precise to place tight precise to place tight constraintsconstraints on on parameters relevant for parameters relevant for secondary production secondary production calculations.calculations.

• Our data above 10 GeV Our data above 10 GeV place limits on place limits on contributions from exotic contributions from exotic sourcessources, e.g. dark matter , e.g. dark matter particle annihilations.particle annihilations.


Positron fractionat high energies

(energy 1.5 - 100 GeV)


• Results discussed here have been submitted to Results discussed here have been submitted to NatureNature..

• Analyzed data: July 2006 - February 2008.Analyzed data: July 2006 - February 2008.

• Total acquisition time ~ 500 days.Total acquisition time ~ 500 days.• ~~ 1x10 1x109 9 triggers (~ 8.8TB of data).triggers (~ 8.8TB of data).

• Identified Identified ~150 ~150 x x 101033 electrons electrons and and ~9 x 10~9 x 1033 positrons positrons with energy between 1.5 and 100 GeV.with energy between 1.5 and 100 GeV.– Collected 180 positrons above 20 GeV.Collected 180 positrons above 20 GeV.

High-energy positron fraction analysis


(e+)

p (non-int)

p (int)

p-bar (int)

e-

p-bar (non-int)

Z = -1

Z = +1

Fraction F of energy released in CALO along the track in a cylinder of radius 0.3 rMolière(central + 2 lateral Si strips)

Rigidity: 20-30 GV

Distribution of fraction F before CALO cuts

after basic event cuts


Cut on “energy-rigidity match”

e- e+

int. p

non-int. + int. protonsnon-int. p-bar

‘electron cut’

tota

l en

erg

y m

easu

red

in

CA

LO

/ri

gid

ity m

easu

red

in

TR

K (

MIP

/GV

)

• Consider the ratio between total energy measured by CALO and rigidity measured by TRK.– For electrons (positrons) ratio is constant over rigidity.


e+

p

p-bar

e-

Z = -1

Z = +1

Fraction F of energy released in CALO along the track

+Constraints on:

Energy-rigidity match

Rigidity: 20-30 GV

Cut on “energy-rigidity match”

all non-interacting and most interacting protons are rejected


e- e+

p

• Less than 1 proton out of 105 survives the complete set of CALO cuts, with e+ efficiency 80%.

e+ and e- identification in calorimeter


• Proton background was also characterized at beam tests.

pp

ee--

ee++

pp

Flight data.Rigidity: 20-30 GV

Beam-test data after same cuts are applied.Rigidity: 50 GV

ee--

ee--

ee++ pp

Cut on shower starting point


Shower starting point

Z = -1

Z = +1

Constraints on:


• Cross-check with flight data from neutron detector to validate the selection procedure.

ee--

pp

ee--

ee++

pp

Neutrons detected by NDRigidity: 20-30 GV

ee++

Cross-check with ND flight data

Z = -1

Z = +1

Residual p background

Fraction F


Flight data: 51 GV positron

Cut on longitudinal profile


Rigidity: 20-30 GVFraction F of energy released in CALO along the track

+Constraints on:


Shower starting point

Longitudinal profile

Z = -1

Z = +1

Cut on longitudinal profile


Rigidity: 10-15 GV Rigidity: 15-20 GV

ee--ee--

ee++ee++pp

pp

pp

pp

Cross-check with energy loss in TRK

• Top: proton and electron samples, identified with TRK only (charge sign).

• Bottom: proton and positron (+ residual p background) samples, identified with present CALO requirements.


Proton contamination

e-

p

e+

Rigidity: 28-42 GVelectron selection

positron + residual p background selection

proton selection

p

reduced CALO (20 out of 22 modules)

• Proton contamination obtained directly from flight data (no simulation involved) and subtracted with statistical “bootstrap” analysis.

Considered three F distributions in a reduced calorimeter after applying all CALO cuts:

(a) electrons and (c) e+ with residual p background: selected in upper CALO.(b) protons: “pre-sampled” in first 2 modules and then selected in lower CALO.


Positron fraction at high energies

line: secondary production,Moskalenko and Strong,Astrophys. J. 493 (1998)

• One order of magnitude One order of magnitude improvement in statistics improvement in statistics over previous measurements.over previous measurements.

• Most extended energy range Most extended energy range ever achieved.ever achieved.

• Expected further Expected further improvements with new data.improvements with new data.

• At high energies our data At high energies our data show a significant increase show a significant increase with energy.with energy.

• This cannot be explained by This cannot be explained by standard models of standard models of secondary production of secondary production of cosmic rays.cosmic rays.– Either a significant change Either a significant change

in the acceleration or in the acceleration or propagation models is propagation models is needed;needed;

– or a primary component is or a primary component is present.present.

• Among primary-component Among primary-component candidates:candidates:– annihilation of dark matter annihilation of dark matter

in the vicinity of our galaxy;in the vicinity of our galaxy;– near-by astrophysical near-by astrophysical

sources, like pulsars.sources, like pulsars.


Positron fraction at low energies

• At low energies our At low energies our results are results are systematically lower than systematically lower than data collected in 1990’s.data collected in 1990’s.

– Clem 2007 (with much Clem 2007 (with much lower statistics) is lower statistics) is consistent with PAMELA.consistent with PAMELA.

• This is interpreted as This is interpreted as effect of charge-sign effect of charge-sign dependent solar dependent solar modulation.modulation.

– our data are enough our data are enough precise to allow tuning precise to allow tuning of models of the of models of the heliosphere.heliosphere.

• Ruled out as negligible Ruled out as negligible a possible combined a possible combined effect of:effect of:

– asymmetry of asymmetry of spectrometer magnetic spectrometer magnetic field;field;– East-West effect or East-West effect or reentrant albedo reentrant albedo particles.particles.


Low-energy e+ fraction and solar modulation

• Solar modulation (through solar wind) of cosmic ray fluxes depends on:– amount of solar activity;– polarity of solar magnetic field;– cosmic-ray energy and mass;– charge sign of cosmic ray.

low-energy p-bar/p ratio(BESS)

low-energy e+ fraction (Caprice, MASS, HEAT, AMS98...)

PAMELANeutron intensity (Rome monitor)

year

2000: inversion of solar magnetic field

now: solar minimum

A+

A+ A- A-

Clem


A>0

A > 0

Positive particles

A < 0

A<0

Charge-sign dependence of solar modulation

A<0

A>0(Preliminary!)

“drift” component of solar modulationis enhanced during solar minimumfor low-mass particles[Potgieter et al., Space Sci. Rev. 97 (2001)]


• Precise measurements of p-bar/p ratio and of positron Precise measurements of p-bar/p ratio and of positron fraction over a wide energy range have been presented and fraction over a wide energy range have been presented and discussed.discussed.

• PAMELA is expected to collect data until at least December PAMELA is expected to collect data until at least December 2009.2009.– increase in statistics will allow to extend energy range for p-increase in statistics will allow to extend energy range for p-

bar/p ratio and positron fraction to the design limits.bar/p ratio and positron fraction to the design limits.

• Several other items are currently under analysis:Several other items are currently under analysis:– p-bar/p ratio and positron fraction in the energy range 100 MeV - p-bar/p ratio and positron fraction in the energy range 100 MeV -

1 GeV;1 GeV;– absolute differential spectra of |Z|=1 cosmic rays;absolute differential spectra of |Z|=1 cosmic rays;– nuclei (up to Z = 8);nuclei (up to Z = 8);– spectra of high-energy Solar Energetic Particles (SEP);spectra of high-energy Solar Energetic Particles (SEP);– radiation belts: morphology and energy spectrum;radiation belts: morphology and energy spectrum;– search for anti-He;search for anti-He;– study of isotope composition (d, study of isotope composition (d, 33He).He).

Conclusion and prospects

recent results on antiparticles in cosmic rays from pamela experiment

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