Antinucleon-nucleoncross section

near the threshold region

Antinucleon-nucleoncross section

near the threshold region

Third International Conference onPERSPECTIVES IN HADRONIC PHYSICS

7 - 11 May 2001Miramare-Trieste, Italy

Alessandro FelicielloI.N.F.N. - Sezione di Torino

abdus salaminternational centre for theoretical physics

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Physics motivations

Anomalies in the NNsystem, near threshold np total cross section

A possible measurementof pp elastic cross sectionat AD the ELAPP project

Outlook

N

p

n

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Physics motivations

historical 60’s: description of ordinary mesons

(π, ρ, …) spectrum (unsuccessful)

QCD 80’s - 90s’: search for multiquark

configurations (exotics, glueballs,hybrids, non qq meson, …)

nuclear physics understanding of nuclear forces:

• G-parity rule (pp ↔ pp and np ↔ np)• several NN bound states or resonances

(NN potential deeper than the NN one)• isospin dependence of the NN interaction

(comparison of pp with np or pn data)• dependence of annihilation strength on

some channels (fit to the scattering andannihilation data)

q

N

pN

p nN

n p

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Experimentalsituation

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np totalcross section

[Phys. Lett. B 475 (2000) 378][Phys. Lett. B 475 (2000) 378]

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The transmissiontechnique

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-35% off

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The transmissiontechnique

60708090

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300 49.27 / 38P1 75.11 1.380P2 0.2659E-01 0.2928E-02

49 < p < 59 MeV/c

50.25 / 38P1 83.30 1.447P2 0.1599E-01 0.2831E-02

59 < p < 70 MeV/c

49.68 / 38P1 130.6 1.812P2 0.1438E-01 0.2302E-02

70 < p < 90 MeV/c

60708090

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34.60 / 38P1 129.7 1.806P2 0.1774E-01 0.2257E-02

90 < p < 110 MeV/c

Cou

nts

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39.17 / 38P1 150.7 1.946P2 0.1511E-01 0.2079E-02

110 < p < 130 MeV/c

vertex z [cm]-10 0 10

46.53 / 38P1 197.0 2.223P2 0.1218E-01 0.1805E-02

130 < p < 150 MeV/c

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Comparisons

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Effective rangeexpansion

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OBELIXMahalanabisPirnerITEP

annihilationannihilation

totaltotal

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OBELIXMahalanabisPirnerITEP

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np elasticcross sectionn

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Isospindependence

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Rtot = =(σ totnp)

σ tot(pp)

2σ tot

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σ tot

0+σ tot

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FENICE experiment(ADONE/LNF)

σσσσ(e+e-) →hadrons

[Nuc

l. Ph

ys. B

517

(199

8) 3

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(a)

σ(e+ e- →

mh

) (n

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s (GeV2)

(b)

|GM(p

) |

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3 3.25 3.5 3.75 4 4.25 4.5 4.75 5

Mx = (1.87 ± 0.01) GeV Γx = (10 ± 5) MeV

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The threshold region

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1.84 1.85 1.86 1.87 1.88 1.89 1.9 1.91 1.920

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other anomalies:• ρ parameter• old hints from DM2 experiment

(still unpublished!)• new results from FOCUS experiment

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The ρρρρ parameter

[W. Brückner et al., Phys. Lett. B

158 (1985) 180]

[W. Brückner et al., Phys. Lett. B

158 (1985) 180]

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−+−+ → ππ 33ee

DM2 dataDM2 data

)(MeVs

−+ ππ 33 inv. mass distribution in high energy photoproduction

FNAL E687FNAL E687

Photoproductionexperiments

MeVnb 20/)(σ

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Which origin forsuch anomalies?

threshold of the p channel(p = 98 MeV/c)

s-wave dominance, in the frame ofcoupled channel approach

quasi-nuclear bound states nearthreshold

nnpp →labpp

measurement of σela(pp)at low momentum

Is it possible to perform such a measurementat a machine like AD???

(the unique source of p in world today)

measurement of dσ/dΩ(relative importance of s- and p-wave contributions)

essential to discriminate among different hypotheses

p

p

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❶ simultaneous detection of: p - large dE/dx

- secondary particle emission

p - very low energy (0 ÷ 5 MeV)

❷ no trigger possible,due to the AD beam structure:

intensity: 106 ÷ 107

burst duration: ≤ 1 µsfrequency: ~ 10-2 Hz

❸ the detector must be operated in vacuum

❹ very thin CH2 target needed (1 ÷ 10 µm)– difficult to produce– difficult to sustain in place

❺ fixed beam energy

Experimentalproblems

p

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The ELAPP project

x

y

z 1 cm

p beamp

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Measurementstrategy

8 bits FADC systemwith 10 ns sampling rate

up to 102 samplings ofscintillating fibers arrayup to 102 samplings of

scintillating fibers array

topological discrimination betweenscattering and annihilation events

provided that:• no more than 30 ÷ 40 interactions/spill• no (few) particles from spurious events

– p annihilation from beam halo– p annihilation in structures surrounding the apparatus

pp

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The typical event

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The "anti"typical event

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Expected event rates(rough estimation)

Ap NxNN ⋅⋅⋅+⋅⋅= −27int 10)2600

141500

142()( ρ

ρ)(15int xNN p ⋅⋅≈

rApela NxNN ερ ⋅∆Ω⋅⋅⋅⋅⋅= −2710)150142()(

ρ)(5.0 xNN pela ⋅⋅≈

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int103 −⋅≈elaN

40 int./burst →→→→ 1 elastic scatt./burst40 int./burst →→→→ 1 elastic scatt./burst

1.5 · 103 elastic scatt./day1.5 · 103 elastic scatt./day

σT(pp) = 500 mbσT(p12C) = 2600 mbρ = 1.1 g/cm3

pp

σE(pp) = 150 mb∆Ω ~ 0.7 · 4πεr ~ 50%

p

N expressed in units of 106

x expressed in µmpN

N expressed in units of 106

x expressed in µmpN

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Requirementsfor the beam

Variable energy: 50 ÷ 100 MeV/c…but also 100 ÷ 200 MeV/cno measurements in this regioneasier p detection

good focus: Ø ≤ 2 mmno halo at r ≈ 3 cm

all possible sources of background(degrader, collimator, beam dump)far away from the detector (≥ 4m)

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Conclusions

σtot(np) measured for the first time: down to 50 MeV/c with high statistics

evident anomalous behaviourof σtot (np) (→ σela (np)) near threshold

possibility of looking at this effect in theelastic (pp) channel, never measured

(but where???)

n

indication for a structure below 100 MeV/c

in the elastic channel???

indication for a structure below 100 MeV/c

in the elastic channel???

nn

p