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Recent results on antiparticles in cosmic rays from PAMELA experiment

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Recent results on antiparticlesin cosmic raysfrom PAMELA experiment

Sergio Ricciarini

INFN – Florence, Italy

On behalf of the PAMELA collaboration

Summary

The PAMELA experiment: short introduction.

Discussion of recent results.

(1) Antiproton/proton ratio at high energies

(submitted to Phys. Rev. Lett.).

(2) Positron fraction at high energies

(submitted to Nature).

Conclusion and prospects.

S. Ricciarini GDR-SUSY 08

Bari

Frascati

Naples

Rome

Trieste

Italy

Florence

Russia

Moscow,

St. Petersburg

Germany

Sweden

Siegen

Stockholm

S. Ricciarini GDR-SUSY 08

PAMELA scientific objectives

Study antiparticles in cosmic rays.

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

Study cosmic-ray production and propagation.

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

Search for anti-He (primordial antimatter).

Study solar physics and solar modulation.

Study of terrestrial magnetosphere and radiation belts.

S. Ricciarini GDR-SUSY 08

PAMELA nominal capabilities

Energy range (with 3 years statistics)

Antiprotons80 MeV - 190 GeV

Positrons 50 MeV - 270 GeV

Protons up to 700 GeV

Electrons up to 400 GeV

Electrons+positrons up to 2 TeV (from calorimeter)

Light Nuclei up to 200 GeV/n (He/Be/C)

AntiNuclei search

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

S. Ricciarini GDR-SUSY 08

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

+ -

- Time-Of-Flight (TOF)
- plastic scintillators + PMT:
- Trigger
- Albedo rejection
- Mass identification up to 1 GeV
- - Charge value from dE/dL
- Electromagnetic calorimeter
- W/Si sampling (16.3 X0, 0.6 λI)
- Discrimination e+ / p, p-bar / e-
- (shower topology)
- Direct E measurement for e-/e+
- Neutron detector
- polyethylene + 3He counters:
- High-energy e/h discrimination

GF: 21.6 cm2 sr Mass: 470 kg

Size: 130x70x70 cm3

Power Budget: 360 W

Spectrometer

microstrip Si tracking system (TRK)+ permanent magnet

- Magnetic rigidity R = pc/Ze (GV); magnetic deflectionη=1/R (GV-1)

- Charge sign, momentum

- Charge value from dE/dL

S. Ricciarini GDR-SUSY 08

Satellite and orbit

350 km

70o

SAA

610 km

orbit period ~90 min

350 km

70°

610 km

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.

S. Ricciarini GDR-SUSY 08

S. Ricciarini GDR-SUSY 08

High-energy antiproton/proton analysis

Results discussed here have been submitted to Phys. Rev. Lett.

Analyzed data: July 2006 - February 2008.

Total acquisition time ~ 500 days.

Collected ~ 1x109 triggers (~ 8.8TB of data).

Identified ~10 x 106 protons and ~1 x 103 antiprotons with kinetic energy between 1.5 and 100 GeV.

- Collected 100 antiprotons above 20 GeV.

S. Ricciarini GDR-SUSY 08

- 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

CARD

CAT

S2

.

TOF

TRK

CAS

S3

CALO

S4

ND

S. Ricciarini GDR-SUSY 08

- 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.

hadron (R=19GV)

electron (R=17GV)

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

Total depth: 16.3 X0 or 0.6 λI

S. Ricciarini GDR-SUSY 08

- Minimal track requirements for good rigidity measurement:
- at least 4 X (bending view) + 3 Y hits;
- energy-dependent cut on track c2 (~95% total efficiency);
- consistent TRK+TOF+CALO spatial information.

Magnetic rigidity R = pc/Ze (GV)

Magnetic deflectionη = 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.

S. Ricciarini GDR-SUSY 08

Rejection of p “spillover” background

- 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).

- 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).

Protons

and spillover

R = - 10 GV

R = - 50 GV

Antiprotons

S. Ricciarini GDR-SUSY 08

Rejection of p “spillover” background

R= - 50 GV

Preliminary!!

- 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.

p-bar

subsample with

MDR > 850 GV

Protons and spillover

MDR > 10 ∙ |R|

R = - 10 GV

S. Ricciarini GDR-SUSY 08

Antiproton/proton ratio

- Excellent agreement with recent data from other experiments.
- One order of magnitude improvement in statistics.
- Most extended energy range ever achieved.
- Expected further improvements with new data.

- Correction factors are included and ~ one order of magnitude less than statistical error.
- CALO efficiency (different for p-bar and p);
- loss of particles for interactions.

S. Ricciarini GDR-SUSY 08

PAMELA p-bar/p ratio and theory

- Ratio increases smoothly with energy from 4 x 10-5 and levels off at ~ 1 x 10-4.
- Our results are enough precise to place tight constraints on parameters relevant for secondary production calculations.
- Our data above 10 GeV place limits on contributions from exotic sources, e.g. dark matter particle annihilations.

S. Ricciarini GDR-SUSY 08

Positron fractionat high energies(energy 1.5 - 100 GeV)

S. Ricciarini GDR-SUSY 08

High-energy positron fraction analysis

Results discussed here have been submitted to Nature.

Analyzed data: July 2006 - February 2008.

Total acquisition time ~ 500 days.

~ 1x109 triggers (~ 8.8TB of data).

Identified ~150 x 103 electrons and ~9 x 103 positrons with energy between 1.5 and 100 GeV.

- Collected 180 positrons above 20 GeV.

S. Ricciarini GDR-SUSY 08

Distribution of fraction F before CALO cuts

Preliminary!!

Rigidity: 20-30 GV

Fraction F of energy released in CALO along the track in a cylinder of radius 0.3 rMolière

(central + 2 lateral Si strips)

Z = -1

e-

p-bar (non-int)

p-bar (int)

after basic event cuts

Z = +1

p (non-int)

(e+)

p (int)

S. Ricciarini GDR-SUSY 08

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

e-

e+

int. p

total energy measured in CALO/

rigidity measured in TRK (MIP/GV)

‘electron cut’

non-int. + int. protons

non-int. p-bar

S. Ricciarini GDR-SUSY 08

Cut on “energy-rigidity match”

Preliminary!!

Rigidity: 20-30 GV

Fraction F of energy released in CALO along the track

Z = -1

e-

+

Constraints on:

p-bar

Energy-rigidity match

Z = +1

all non-interacting and most interacting protons are rejected

e+

p

S. Ricciarini GDR-SUSY 08

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

e-

e+

p

S. Ricciarini GDR-SUSY 08

Cut on shower starting point

Constraints on:

- Proton background was also characterized at beam tests.

Energy-rigidity match

Shower starting point

Flight data.

Rigidity: 20-30 GV

Beam-test data after same cuts are applied.

Rigidity: 50 GV

Z = -1

e-

e-

e-

e+

p

Z = +1

e+

p

p

S. Ricciarini GDR-SUSY 08

Cross-check with ND flight data

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

Fraction F

Rigidity: 20-30 GV

Neutrons detected by ND

Z = -1

e-

e-

Z = +1

e+

e+

p

p

Residual p background

S. Ricciarini GDR-SUSY 08

Cut on longitudinal profile

Flight data:

51 GV positron

S. Ricciarini GDR-SUSY 08

Cut on longitudinal profile

Preliminary!!

Fraction F of energy released in CALO along the track

Rigidity: 20-30 GV

+

Constraints on:

Z = -1

Energy-rigidity match

Shower starting point

Longitudinal profile

Z = +1

S. Ricciarini GDR-SUSY 08

Cross-check with energy loss in TRK

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

Rigidity: 10-15 GV

Rigidity: 15-20 GV

p

e-

p

e-

p

e+

p

e+

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

S. Ricciarini GDR-SUSY 08

Proton contamination

- 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.

e-

Rigidity: 28-42 GV

electron selection

reduced CALO (20 out of 22 modules)

p

proton selection

p

e+

positron + residual p background selection

S. Ricciarini GDR-SUSY 08

Positron fraction at high energies

- One order of magnitude improvement in statistics over previous measurements.
- Most extended energy range ever achieved.
- Expected further improvements with new data.

- At high energies our data show a significant increase with energy.
- This cannot be explained by standard models of secondary production of cosmic rays.
- Either a significant change in the acceleration or propagation models is needed;
- or a primary component is present.

- Among primary-component candidates:
- annihilation of dark matter in the vicinity of our galaxy;
- near-by astrophysical sources, like pulsars.

line: secondary production,

Moskalenko and Strong,

Astrophys. J. 493 (1998)

S. Ricciarini GDR-SUSY 08

Positron fraction at low energies

- At low energies our results are systematically lower than data collected in 1990’s.
- Clem 2007 (with much lower statistics) is consistent with PAMELA.

- This is interpreted as effect of charge-sign dependent solar modulation.
- our data are enough precise to allow tuning of models of the heliosphere.

- Ruled out as negligible a possible combined effect of:
- asymmetry of spectrometer magnetic field;
- East-West effect or reentrant albedo particles.

S. Ricciarini GDR-SUSY 08

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.

2000: inversion of solar magnetic field

Neutron intensity (Rome monitor)

PAMELA

Clem

now: solar minimum

low-energy p-bar/p ratio

(BESS)

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

year

A-

A+

A-

A+

S. Ricciarini GDR-SUSY 08

Charge-sign dependence of solar modulation

A > 0

Positive particles

A < 0

Preliminary!!

(Preliminary!)

A>0

A<0

A<0

“drift” component of solar modulation

is enhanced during solar minimum

for low-mass particles

[Potgieter et al., Space Sci. Rev. 97 (2001)]

A>0

S. Ricciarini GDR-SUSY 08

Conclusion and prospects

- Precise measurements of p-bar/p ratio and of positron fraction over a wide energy range have been presented and discussed.
- PAMELA is expected to collect data until at least December 2009.
- increase in statistics will allow to extend energy range for p-bar/p ratio and positron fraction to the design limits.

- Several other items are currently under analysis:
- p-bar/p ratio and positron fraction in the energy range 100 MeV - 1 GeV;
- absolute differential spectra of |Z|=1 cosmic rays;
- nuclei (up to Z = 8);
- spectra of high-energy Solar Energetic Particles (SEP);
- radiation belts: morphology and energy spectrum;
- search for anti-He;
- study of isotope composition (d, 3He).

S. Ricciarini GDR-SUSY 08