Physics with n’s: first evidence of an NN resonant...

Physics with n’s:first evidence of

an NN resonant state

Hadron Physics induced with Hadron BeamsGeorge Washington University

July 23 - 25, 2001, Washington DC

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

Outlook NN system np vs. pp interactions np vs. pd interactions n’s beams the OBELIX n’s “factory” n’s physics

– meson spectroscopy np cross section– n-nucleus cross section

NN interaction: why?historical

60’s: description of ordinary meson spectrum(π, ρ, …)

QCD 80’s - 90’s: search for exotic configurations

• multiquark (q2q2)• glueball (gg or ggg)• hybrids (qqg)• other non qq mesons

unsuccessful

NN interaction: why?nuclear physics

understanding of nuclear forces:• clearing up the rôle of the G-parity rule

(pp ↔ pp and np ↔ np)• search for NN bound states or resonances

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

interaction (comparison of pp with pn or np data)• determination of the annihilation strength

dependence on some channels (fit to the scatteringand annihilation data)

nn’s interaction: why? scarce data on low energy np interaction complementary/alternative to pp interaction

technically difficult low n production rate

The pp systemp

0 100 200 300 400

The np systemn

0 100 200 300 400

np vs. pp (advantages) pure I = 1 state:

– reduced number of initial statesinvolved

the percentages of s- and p-waves can be selected bychoosing the n momentum

absence of Coulomb interaction:– no distortion of the σ trend in

the low momentum region(σ ∝ 1/v)

mixture of the I = 0 and I = 1amplitudes

σ ∝ 1/v2

βσ ann(pp)

np vs. pp (advantages) no energy loss in the target:

– possibility of preciselyreconstructing the energy atwhich the interaction occurs

– only one target– the target thickness can be

increased to obtain highercounting rates

target thickness is a “function”of p momentum

np vs. pp (advantages) at least one prong in the final

state:– good opportunity for trigger– easier detection– selection of

exclusive final states(e.g. np → π+π+π-

np → π+π+π+π-π-)

all neutral annihilations(see Crystal Barrel)

after a 4C kinematical fitafter a 4C kinematical fit

better energy and momentumresolution:– no kinematical corrections due to

the presence of the spectatorproton

pd → 2π-π+(p)p

np → 3π+2π-n

np vs. pd (advantages)n pnp → 2π+π-n

pd → 3π-2π+(p)p

np vs. pp (drawbacks) low intensity:

– 30 ÷ 50 × 10-6 n/p

poor energy definition

in flight annihilations:– beam divergence– annihilation vertex

delocalization

e.g. LEAR p beam:– ∆p/p ~ 10-4

– I ~ 107 p/s

1983 - 1996

n’s productionbeam dumping

AGS/BNL (1981):– 10 < pp < 30 GeV/c– I ~ 10-8 n/p GeV/c sr– 0.3 < p < 1 GeV/c– no physics output:

• poor beam quality• n high contamination level

pp → nn charge exchange

AGS/BNL (1987):– target: CH2– 2% of p → n– 100 < p < 500 MeV/c– physics output:

• σtot(np) and σann(np)

LEAR/CERN (1988):– target: LH2 (2-body kinematics!)– I ~ 10-4 n/p– 50 < p < 300 MeV/c– physics output:

• σtot(np) and σann(nFe)

n npnp

first n tagged beam

first n tagged beamn

The n tagging techniquen

En = En(ϑn)ϑn = ϑn(z) En = En(z) tmeas = tmeas(z)

−++=Q)(P,

)EP,(Qt(z)ttt 0meas

The OBELIX n “factory“ On

1990 - 1996

n’s momentum evaluationn

algorithm based on t.o.f. measurement:

STOPsignal

STOPsignalSTART

signalSTARTsignal

ππ ϑ v

z-dl)z(v

z(0)vs

ttt(0)tt cz

0measc +++

′′

+=+++= cosd

fixedfixed

knownknown measuredmeasured

measuredmeasured

iterative procedureto determine = p

p ddβdp

(0)pc zEpz by guessing ppp

n’s momentum spectrumncapability of reconstructingthe momentum of each interacting nn

systematic

errors

greatly

reduced

systematic

errors

greatly

reduced

data corresponding todifferent n momenta are collected:• at the same time• with the same experimental and beam setup

wide and continue

n‘s beam spot on the targetn

*untagged n beam no direct flux measurement!!!

target target

flux*: (13 ± 0.5) x 10-6 n/p (56 ± 1.5) x 10-6 n/p

n p pn

The OBELIX n physics output

50 ÷ 200 MeV/c 200 ÷ 300 MeV/c

50 ÷ 405 MeV/c300 ÷ 405 MeV/c

π+π- system invariant mass

meson spectroscopymeson spectroscopy

~ 35000 events

np → 2π+π-np → 2π+π-n

contributionto the discussion

about the Ax/f0 case

contributionto the discussion

about the Ax/f0 case

(1522 ± 25) MeV/c2

(108 ± 33) MeV/c2

(1575 ± 18) MeV/c2

(119 ± 24) MeV/c2

Phys. Rev. D 57 (1998) 55Phys. Rev. D 57 (1998) 55

The OBELIX n physics outputmeson spectroscopymeson spectroscopy

pure phase spacepure phase space

(1359 ± 17) MeV/c2

(425 ± 30) MeV/c2

np → 3π+2π-np → 3π+2π-n

difference spectrum

LEAP ‘94LEAP ‘94

(1664 ± 1) MeV/c2

(53 ± 2) MeV/c2

The OBELIX n physics outputannihilation dynamicsannihilation dynamics

nucleon ss content?s

np → φπ+

np → ωπ+np → φπ+

np → ωπ+nn str

ong devia

(of a f

~ 30!)

from OZI ru

ong devia

(of a f

~ 30!)

from OZI ru

Rs = 0.112 ± 0.007Rs = 0.112 ± 0.007

Nucl. Phys. A 655 (1999) 453Nucl. Phys. A 655 (1999) 453

The OBELIX n physics outputnuclear physicsnuclear physics

np annihilation cross section np total cross section

anomaly in the elastic np cross section n-nucleus annihilation cross sectionn

-30 -20 -10 0 10 20 30 40 50

n-nucleus annihilation cross section

0 50 100 150 200 250 300 350 400 450

nνA)(pσA)(pσ 0

annann ×= nn ,

n pa)(pσ 0ann +=

a = (66.5 ± 3.0) mbb = (19.87 ± 0.86) mb × GeV/cν = (0.6526 ± 0.0060)

0 50 100 150 200 250 300 350

A and p dependencies

0 50 100 150 200 250 300 350 400 450

Comparisons

0 100 200 300 400 500 600 700 800 9000

0 200 400 600 800 1000 1200 1400 1600

n-nucleus data N-nucleus dataNn

0 50 100 150 200 250 300 350 400

Annihilation products

0.0025

0.0075

0.0125

0.0175

0 50 100 150 200 250 300 350 400

• both p and d production increases with A (and saturate)• d/p ~ 3-4% (average values in literature ~ 10-15%: different reconstruction efficiency?)

• K/π ~ 3%: about the same than np• no enhancement of strangeness production with A• very slight decreasing with A (energy loss?)

100 200 300 400

np annihilation cross sectionn

σanni =

1ρNA∆z

1εε trig

Nanni (1− γ i)

[Nucl. Phys. B

56A (1997) 227]

[Nucl. Phys. B

56A (1997) 227]

Comparison

100 200 300 400

sensible reduction

of the statistical errors!

sensible reduction

of the statistical errors!

100 200 300 400

S wave

P wave

Effective range expansion

σann =4πk2 (2l +1)(Imfll − fl

2) fl =

1cotδ l − i

k cotδ0 =1a1

k 3 cotδ1 =1b1

100 200 300 400

S wave

P wave

D wave

k 5 cotδ2 =1c1

cotδ l − i

k cotδ0 =1a1

k 3 cotδ1 =1b1

The transmission technique

zN)z,p(I)p()z,p(N Annnannnann ∆=∆ ρσ

zNnnnn

Atote),p(I)z,p(I ρσ−= 0

zNAnannnnz

)z,p(N Atotnann eN)p(),p(I ρσρσ −∆

∆ = 0

0 100 200 300 400

-35% off

not to scale

60708090

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

-10 0 10

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

90 < p < 110 MeV/c

-10 0 10

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

np total cross section On

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

0 100 200 300 400

100 200 300 400

Slopes

70 < p < 90 MeV/cnp

Comparisons

0 100 200 300 400

no systematic errors (practically)

statistical errors significantly reduced

0 100 200 300 400

OBELIXMahalanabisPirnerITEP

σ tot =4πk2 (2l +1)Imfll

totaltotal annihilationannihilation

0 100 200 300 400

OBELIXMahalanabisPirnerITEP

100 200 300 400

S wave

P wave

cotδ l − i

k cotδ0 =1a1

k 3 cotδ1 =1b1

100 200 300 400

S wave

P wave D wave

k 5 cotδ2 =1c1

cotδ l − i

k cotδ0 =1a1

k 3 cotδ1 =1b1

Anomalous p-wave contributiontotaltotal annihilationannihilation

100 200 300 400

S wave

P wave D wave0

100 200 300 400

S waveP wave

D wave

Different binning

np elastic cross section O

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

0 100 200 300 400

4 totelk σπ

σ ≥

Isospin dependence

0 100 200 300 400

0 100 200 300 400Rtot = =

(σ totnp)

σ tot(pp)

2σ tot

σ tot

0+σ tot

−= tottot

tot Rσσ

The ρρρρ parameter

[W. Brückner et al., Phys. Lett. B

158 (1985) 180]

[W. Brückner et al., Phys. Lett. B

158 (1985) 180]

Which origin for such anomalies? threshold of the pp → nn channel

(plab = 98 MeV/c) s-wave dominance, in the frame of the coupled

channel approach quasi-nuclear bound states near threshold

measurement of σela(pp)at low momentum

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

essential to discriminate among different hypotheses

FENICE experiment F(a)

σ(e+ e- →

s (GeV2)

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

The threshold region

1.84 1.85 1.86 1.87 1.88 1.89 1.9 1.91 1.920

10001000

Photoproduction experiments

FNAL E687FNAL E687

e+e- → 3π+3π-e+e- → 3π+3π-

Mx = (1.911 ± 0.004) GeV Γx = (29 ± 11) MeV

DM2DM2

Summaryn’s validated as projectile for meson spectroscopyσann(np) and σtot(np) measured for the first time:

❶ down to 50 MeV/c❷ with high statistics

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

indication for a structure below 100 MeV/c

in the elastic channel???

indication for a structure below 100 MeV/c

in the elastic channel???

Physics with n’s: first evidence of an NN resonant...

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