TeVPA 2017 - Antiproton Flux and Antiproton-to-Proton Flux Ratio … · 2017-08-12 · 07.08.2017 |...

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Antiproton Flux and Antiproton-to-Proton Flux Ratio in Primary Cosmic Rays Measured with AMS on the Space Station Andreas Bachlechner on behalf of the AMS collaboration 07.08.2017 | TeVPA 2017, Columbus

Antiproton Flux and Antiproton-to-Proton Flux Ratio in Primary Cosmic Rays Measured with AMS on the Space Station Andreas Bachlechner, I. Physikalisches Institut, RWTH Aachen 07.08.2017 | TeVPA 2017, Columbus

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2

Antiprotons in cosmic rays

AMS

p, He + ISM è p + …

p, He

ISM

p

The collision of cosmic rays with the interstellar medium (ISM) produces antiprotons



3

~85% of the matter in the Universe is dark

AMS

p, He + ISM è p + …

p, He

ISM

p

χ + χ è p + …

p

χ

χ

The collision of cosmic rays with the interstellar medium (ISM) produces antiprotons

Annihilation of dark matter particles can produce additional antiprotons



4

•  Antiproton excess can be measured by AMS

Antiproton to proton ratio in the cosmic rays

Ant

ipro

ton

/ pro

ton

ratio

Donato et al., PRL 102, 071301 (2009)

10-2

10-3

10-4

10-5

10-6

10-7 0.1 1 10 100 1000

Kinetic energy [GeV]

Dark matter + collision of cosmic rays with ISM

Ratio of ~10-4 requires great background rejection

Boost factor=40 ︎



5

AMS: A TeV precision, multipurpose spectrometer

TRD

TOF

Tracker

TOFRICH

ECAL

1

2

7-8

3-4

9

5-6

Electromagnetic Calorimeter Separate p,p from e±

Time of Flight Z, β

Magnet ± Z Silicon Tracker

Z, R

Ring Imaging Cherenkov Z, β

Transition Radiation Detector Separate p,p from e±

The charge and energy or rigidity are measured independently by several sub-detectors



6

Antiproton signal

Λ ×sign(R)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

RICH

β

0.96

0.98

1.0

1

10

210

310

410

510

p p

-e

+π+e-π

TRD

Λ TRD Λ CC Λ CC Λ TRD 0.5−0 0.5 1

0.51

Eve

nts

0

5

10

15

Fit p

0.5−0 0.50.5

1

Eve

nts

0

10

15

Data

(a)

(b) (c)

5

-e

p

Events

0

10

15

5

5.4<|R|<6.5 GV

Well separated antiproton signal



7

Event selection for the p/p analysis

TRD

ECAL

RICH M

AG

NE

T

AC

C

2

3-4

5-6

7-8

R = −363 GV antiproton

9

1

Tracker

§  ISS data from May 19, 2011 to May 26, 2015 M. Aguilar et al., Phys. Rev. Lett. 117, 091103 (2016)

•  The number of antiprotons is determined from template fits.

•  To maximize the measurement accuracy, different approaches are used depending on the backgrounds to challenge.



8

Antiproton identification (low rigidity region)

|Rigidity| / GV1 10 210

-1 s

sr)

2Fl

ux /

(GV

m

6−10

5−10

4−10

3−10

2−10

1−10PAMELA(2006/07-2009/12)

1.00 - 4.02 GV

!

•  Electron, pion background

•  Mass calculated from TOF β and Tracker rigidity

•  Cut on TRD estimator to suppress electrons

•  Template events selected from ISS data via the RICH p

e-+π-



9

Antiproton identification (intermediate rigidity region)


-1 s

sr)

2Fl

ux /

(GV

m

6−10

5−10

4−10

3−10

2−10

1−10PAMELA(2006/07-2009/12)

1.00 - 4.02 GV 3.67 - 18.0 GV

TRD estimator0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

# E

vent

s

100

200

300 5.4<|R|<5.9 GV

p e-

TRD estimator ∧TRD

•  Electron and small amount of pion background

•  Utilize RICH information to remove residual Pion contamination

•  Templates from ISS data



10

Antiproton identification at high rigidities


-1 s

sr)

2Fl

ux /

(GV

m

6−10

5−10

4−10

3−10

2−10

1−10PAMELA(2006/07-2009/12)

1.00 - 4.02 GV 3.67 - 18.0 GV 16.6 - 450 GV

Charge confusion estimator1− 0.5− 0 0.5 1

Prob

abilit

y3−10

2−10

1−10

p →p p

100<|R|<450GV

Charge confusion estimator ∧CC

•  Events may be reconstructed with wrong charge sign, due to: -  Finite tracker resolution -  Particle interactions with detector material -  Leads to charge confused events

•  Electron and charge confused proton background

•  Charge confusion estimator ∧CC: -  MVA BDT 10 variables based on e.g.:

§  Track fit quality §  Rigidity algorithms and track pattern §  Charge measurements



11

Antiproton identification at high rigidities


-1 s

sr)

2Fl

ux /

(GV

m

6−10

5−10

4−10

3−10

2−10

1−10PAMELA(2006/07-2009/12)

1.00 - 4.02 GV 3.67 - 18.0 GV 16.6 - 450 GV

Λ ×sign(R)

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

RICH

β0.96

0.98

1.0

1

10

210

310

410

510

p p

-e

+π+e-π

TRD

Λ TRD Λ CC Λ CC Λ TRD 0.5−0 0.5 1

0.51

Eve

nts

0

5

10

15

Fit p

0.5−0 0.50.5

1

Eve

nts

0

10

15

Data

(a)

(b) (c)

5

-e

p

Events

0

10

15

5

χ2/d.f. = 138/154

175 < |R| < 211 GV

Charge confused



12

Antiproton identification


-1 s

sr)

2Fl

ux /

(GV

m

6−10

5−10

4−10

3−10

2−10

1−10PAMELA(2006/07-2009/12)

1.00 - 4.02 GV 3.67 - 18.0 GV 16.6 - 450 GV

1. Low rigidity region: Electron, pion background 1.00 - 4.02 GV The mass calculated from TOF and Tracker

2. Intermediate region: Electron and small amount of pion background

3.67 - 18.0 GV RICH and TRD estimator 3. High rigidity region: Electron and charge confusion proton background

16.6 - 450 GV 2D template in (∧TRD - ∧CC ) plane •  The regions overlap, the analysis with the smallest error is taken

3.49 x 105 antiprotons and 2.42 x 109 protons are selected in the rigidity range 1<|R|<450 GV



13

Antiproton uncertainties

|Rigidity| [GV]1 2 3 4 5 10 20 30 100 200

Rel

ativ

e er

ror [

%]

5

10

15

20

25

30

RσAσ

Nσsystσ

statσ

•  Statistical error dominants below 2 GV and above 100 GV

•  In the intermediate region the systematic uncertainty from the acceptance correction dominates

Rigidity scale

Counting

Acceptance

Total Syst.

Statistical 24%

13% 12%

2% 2%



14

Antiproton uncertainties

|Rigidity| [GV]1 2 3 4 5 10 20 30 100 200

Rel

ativ

e er

ror [

%]

5

10

15

20

25

30

RσAσ

Nσsystσ

statσ

Rigidity scale

Counting

Acceptance

Total Syst.

Statistical 24%

13% 12%

2% 2%

•  Affecting the antiproton counting σCounting •  Geomagnetic cutoff •  Selection •  Charge confusion templates

•  Affecting the acceptance, σAcceptance •  Cross sections •  MC statistic fluctuations •  Migration matrix



15

Systematic uncertainty from cross section uncertainty

The antiproton-to-proton flux ratio is defined as

Number of observed antiprotons and protons

Acceptance ratio of protons to antiprotons

P(GeV/c)1 10 210 310

[mb]

abs

σ+C

p

200

300

400

500Abrams(1971)Denisov(1973)Carroll(1979)Nakamura(1984)Cork(1957)Aihara(1981)Allaby(1970)

+C absorptionp

proton momentum [GeV/c]-110 1 10 210 310

p+C

inel

astic

cro

ss s

ectio

n [m

b]200

300

400

500NA61 (2012)Denisov (1973)Bellettini (1965)Carroll (1979)Bowen (1958)Dubna-exp db dataIHEP-exp db dataLetaw (1983)GEANT 4-09-06

Exp. Data compiled by J. Hoffman

p p

The typical uncertainty is 20% at 1 GV, 15% at 10 GV, and 5% at 100 GV

Uncertainty is still significant.



16

Systematic error from charge confusion templates

]-1 [ GV4001 - Rigidity

1

-0.02 -0.01 0 0.01 0.02

Even

ts

10

210

310

410

510

610400 GeV/c Test Beam Data400 GeV/c Monte Carlo Simulation

Good agreement between data and MC in 5 orders of magnitude

•  The minor difference in charge confusion between MC simulation and data is taken as associated error

Rigidity resolution function



17

Systematic error from the rigidity scale uncertainty

RigidityEnergy

0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

Bin

Events

10

210

E>30 GeV+e E>30 GeV, Scaled-e

1/Δ∼0andσ(1/Δ)=1/26TV-1

Nor

mal

ized

ent

ries



18

Rigidity [GV]210 310

410

1

10]

1.7GV ⋅-1 s ⋅-1 sr ⋅

⋅-2

[m2.7 RΦ

ˇ

p

p

The Spectra of Protons and Antiprotons

•  If p are secondaries, their spectrum should be different than p. •  Unexpectedly p and p have identical spectra.

M. Aguilar et al., Phys. Rev. Lett. 117, 091103 (2016)

104

3×103

2×104

8×103



19

Antiproton-to-Proton Flux Ratio

•  The antiproton-to-proton flux ratio reaches its maximum at 20 GV •  The antiproton-to-proton flux ratio shows no rigidity dependence above 60 GV

Nikolas Zimmermann - RWTH Aachen - EPS HEP 2017 - 07.07.2017 Slide 11

Antiproton/proton ratio - resultsM. Aguilar et al., Phys. Rev. Lett. 117 (2016) 091103.

The fitted value of the slopek = (-0.7 ± 0.9) x 10-7 GV-1

is consistent with zero.Above 60.3 GV:

The fitted value of the slope Above 60.3 GV: k = (-0.7 ± 0.9)x10-7 GV-1 is consistent with zero.



20

Antiproton-to-proton ratio

37

• AMS

Darkmatter

Momentum [GeV] DarkMatterModelexample: Donatoetal.,Phys.Rev.Lett.102,071301(2009).AstrophysicsModelexamples:P.Mertsch and S.Sarkar,Phys.Rev.D90,061301(2014);K.Kohri,K.Ioka,Y.Fujita,and R.Yamazaki,Prog.Theor.Exp.Phys.2016,021E01(2016).

The excess of antiprotons observed by AMS cannot come from pulsars.

It can be explained by Dark Matter collisions or by new astrophysics phenomena

Antiproton-to-Proton Flux Ratio

•  Different then the positron fraction excess the antiproton excess cannot be explained by Pulsars but could be explained by Dark Matter collisions or by new astrophysics phenomena



21

Conclusions

•  Antiproton measurement up to 450 GV measured based on 3.49 x 105 antiprotons and 2.42 x 109 protons extending existing measurements with high precision

•  The antiproton-to-proton flux ratio shows no rigidity

dependence above 60 GV - Hint for dark matter or new astrophysical phenomena

•  To test these signals a very precise theoretical prediction for the ISM background is needed

•  AMS will continue to collect more data till the end of the ISS lifetime to further explore the high rigidity region, increase the precision and investigate the time dependent effects

TeVPA 2017 - Antiproton Flux and Antiproton-to-Proton Flux Ratio … · 2017-08-12 · 07.08.2017 |...

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