Physics with n’s: first evidence of an NN resonant...
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A. Feliciello - HPHB01 Washington July 23 - 25, 2001 1 Physics with n’s: first evidence of an NN resonant state Hadron Physics induced with Hadron Beams George Washington University July 23 - 25, 2001, Washington DC Alessandro Feliciello I.N.F.N. - Sezione di Torino n N
Transcript of Physics with n’s: first evidence of an NN resonant...
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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
n
N
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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
Nppn
n
n
nn
n
n
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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
N
unsuccessful
q
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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)
N
p n
NNnpp
N
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nn’s interaction: why? scarce data on low energy np interaction complementary/alternative to pp interaction
technically difficult low n production rate
but
n
pn
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The pp systemp
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The np systemn
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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
n p
βσ ann(pp)
n
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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
n p
p
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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)
n p
nn
after a 4C kinematical fitafter a 4C kinematical fit
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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
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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
n pp
np
p
1983 - 1996
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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)
np n
n npnp
np
np
pn
n
n n
n
first n tagged beam
first n tagged beamn
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The n tagging techniquen
En = En(ϑn)ϑn = ϑn(z) En = En(z) tmeas = tmeas(z)
=
−++=Q)(P,
)EP,(Qt(z)ttt 0meas
nn
nnp
ϑϑ
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The OBELIX n “factory“ On
1990 - 1996
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n’s momentum evaluationn
algorithm based on t.o.f. measurement:
STOPsignal
STOPsignalSTART
signalSTARTsignal
π
ππ ϑ v
sv
z-dl)z(v
z(0)vs
ttt(0)tt cz
0measc +++
′′
+=+++= cosd
npp
pnpp
fixedfixed
knownknown measuredmeasured
measuredmeasured
iterative procedureto determine = p
p ddβdp
(0)pc zEpz by guessing ppp
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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
n
wide and continue
wide and continue
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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
n p pn
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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
n
~ 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
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The OBELIX n physics outputmeson spectroscopymeson spectroscopy
n
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
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The OBELIX n physics outputannihilation dynamicsannihilation dynamics
n
nucleon ss content?s
np → φπ+
np → ωπ+np → φπ+
np → ωπ+nn str
ong devia
tion
(of a f
actor
~ 30!)
from OZI ru
lestr
ong devia
tion
(of a f
actor
~ 30!)
from OZI ru
le
Rs = 0.112 ± 0.007Rs = 0.112 ± 0.007
Nucl. Phys. A 655 (1999) 453Nucl. Phys. A 655 (1999) 453
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The OBELIX n physics outputnuclear physicsnuclear physics
n
np annihilation cross section np total cross section
anomaly in the elastic np cross section n-nucleus annihilation cross sectionn
nn
n
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-30 -20 -10 0 10 20 30 40 50
n-nucleus annihilation cross section
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nνA)(pσA)(pσ 0
annann ×= nn ,
bn
n pa)(pσ 0ann +=
a = (66.5 ± 3.0) mbb = (19.87 ± 0.86) mb × GeV/cν = (0.6526 ± 0.0060)
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A and p dependencies
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Comparisons
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n-nucleus data N-nucleus dataNn
! !
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0
0.005
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0.015
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0.025
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Annihilation products
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0.0025
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0.0075
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0.0125
0.015
0.0175
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0
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• 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?)
n
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np annihilation cross sectionn
σanni =
1ρNA∆z
1εε trig
Nanni (1− γ i)
Nni
[Nucl. Phys. B
56A (1997) 227]
[Nucl. Phys. B
56A (1997) 227]
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Comparison
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sensible reduction
of the statistical errors!
sensible reduction
of the statistical errors!
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0
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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
+12
r1k2
k 3 cotδ1 =1b1
−32
1R1
k 2
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S wave
P wave
D wave
Effective range expansion
k 5 cotδ2 =1c1
+52
1ρ1
3 k 2
fl =1
cotδ l − i
k cotδ0 =1a1
+12
r1k2
k 3 cotδ1 =1b1
−32
1R1
k 2
σann =4πk2 (2l +1)(Imfll − fl
2)
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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
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The transmission technique
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-35% off
not to scale
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The transmission technique
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
100
200
300
-10 0 10
34.60 / 38P1 129.7 1.806P2 0.1774E-01 0.2257E-02
90 < p < 110 MeV/c
Cou
nts
-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
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np total cross section On
[Phys. Lett. B 475 (2000) 378][Phys. Lett. B 475 (2000) 378]
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Slopes
70 < p < 90 MeV/cnp
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Comparisons
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no systematic errors (practically)
statistical errors significantly reduced
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Effective range expansion
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OBELIXMahalanabisPirnerITEP
σ tot =4πk2 (2l +1)Imfll
totaltotal annihilationannihilation
σann =4πk2 (2l +1)(Imfll − fl
2)
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OBELIXMahalanabisPirnerITEP
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S wave
P wave
Effective range expansion
fl =1
cotδ l − i
k cotδ0 =1a1
+12
r1k2
k 3 cotδ1 =1b1
−32
1R1
k 2
σ tot =4πk2 (2l +1)Imfll
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S wave
P wave D wave
Effective range expansion
k 5 cotδ2 =1c1
+52
1ρ1
3 k 2
fl =1
cotδ l − i
k cotδ0 =1a1
+12
r1k2
k 3 cotδ1 =1b1
−32
1R1
k 2
σ tot =4πk2 (2l +1)Imfll
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Anomalous p-wave contributiontotaltotal annihilationannihilation
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S waveP wave
D wave
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Different binning
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np elastic cross section O
[Phys. Lett. B 475 (2000) 378][Phys. Lett. B 475 (2000) 378]
n
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4 totelk σπ
σ ≥
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Isospin dependence
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(σ totnp)
σ tot(pp)
2σ tot
1
σ tot
0+σ tot
1
121
0
−= tottot
tot Rσσ
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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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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
nppp
p
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FENICE experiment F(a)
σ(e+ e- →
mh
) (n
b)
s (GeV2)
(b)
|GM(p
) |
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3 3.25 3.5 3.75 4 4.25 4.5 4.75 5
[Nuc
l. Ph
ys. B
517
(199
8) 3
][N
ucl.
Phys
. B 5
17 (1
998)
3]
Mx = (1.87 ± 0.01) GeV Γx = (10 ± 5) MeV
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The threshold region
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Photoproduction experiments
FNAL E687FNAL E687
e+e- → 3π+3π-e+e- → 3π+3π-
Mx = (1.911 ± 0.004) GeV Γx = (29 ± 11) MeV
DM2DM2
!
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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???
n
nn
nn
