Search for nucleon short-range correlations
in neutrino-argon scattering
Kajetan Niewczas, Jan T. Sobczyk
Institute for Theoretical Physics, University of Wrocław
ArgoNeuT
ArgoNeuT conditions
Two proton events
Nucleon-nucleon short-range corelations
Short-range correlations (SRC) are baryonic configurations in nuclei which are defined by local properties of
nucleon-nucleon (NN) interactions rather than by mean
field [1]. At low momentum, one expects their momentum
distribution to be identical to the deuteron distribution. NN
SRC is a pair two high-momentum nucleons in a nucleus
with large relative momentum and a small total momentum,
i.e. back-to-back configuration ~k1 ' −~k2.
Experiment
The most appropriate venue to probe NN correlations are
two-proton knockout processes. For further references on
the experiment see [2].
νµ
µ
W
n
νµ
−
p
CC RES
p
+
µ−
+
W
n
p
MEC
DIS
RES
CCQE
3000
2500
p
2000
1500
p
+
p
1000
+
500
νµ
µ−
0
W
n
MEC
DIS
RES
CCQE without SRC
CCQE with SRC
3000
2500
p
p
+
2000
1500
p
n
+
1000
500
FIG. 2. (Top left) nucleon-nucleon correlation and one-body interaction resulting in a
two proton emission, (top right) nucleon-nucleon correlation and two-body interaction
resulting in a two proton emission, (bottom) final state interactions that result in a
similar multi-nucleon knockout.
3.5
3
p
We indentify the higher in momentum proton to be the
one struck by neutrino. Then we try to reconstruct the
neutrino energy.
Eν = Eµ + Tp1 + Tp2 + TA−2 + Emiss,
(3)
where Emiss is an averaged 2 proton knock-out energy
for argon and
T
(~pmiss
)2
.
(4)
TA−2 '
2MA−2
ArgoNeuT Coll. observed an enahncement in the number of events for cos(γ i) < −0.90 area [2].
1.5
3
1
2.5
0.5
MEC
DIS
RES
CCQE without SRC
CCQE with SRC
ArgoNeuT
4
3.5
3
2.5
1.5
1
0.5
0
2
1.5
3
1
2.5
0.5
ν-mode
-1
-0.5
0
0.5
1
1.5
cos(γ)
1
FIG. 3. Distribution of the cosine of the angle between two protons in the final state.
The NuWro results are normalized to the same area as the experimental data.
(Top) LFG model, (bottom) SP approach.
If we treat the NuWro results as the probability distribution
and use Poisson statistics to calculate the probability of
obtaining 4 or more events P (4+) in the first bin of Fig. 3:
0.5
0
(% of investigated 2p events)
% of all 2p events
in total sample
cos γ i ≤ −0.9
Extra condition: pp1, pp2 > 250MeV
NuWro: SF
4
3.5
3
cos(~qrec, p~2) is peaked at ∼ 0.6
~q ≈ p~1 + p~2
~q ≈ ~qrec
0.5
0
3.5
3
2.5
52th WINTER SCHOOL OF THEORETICAL PHYSICS , 14-21 FEBRUARY 2016, LADEK
˛
ZDRÓJ, POLAND
Back-to-back configuration is kinematically preferred.
Conclusions
2
1.5
1
0.5
-1
-0.5
0
0.5
1
cos(γ)
FIG. 4. Distribution of the cosine of the angle between two protons in the final state.
Subsample with both protons momenta above argon Fermi momentum. The NuWro
results are normalized to the same area as the experimental data. (Top) LFG model,
(bottom) SP approach.
The probability of having four or more events P (4+) out of 19
in the first bin of Fig. 4 using NuWro results as the probability
distribution:
4.6%
TABLE I. Two-proton sample statistics from both ArgoNeuT and NuWro. The last
column shows the percentage of the two-proton events without detector effects. The
previous two columns show the contribution of two-proton events from both neutrino
beams to the investigated subsample with detector effects.
p~1 rec = p~1 − ~qrec ≈ −~p2
MEC
DIS
RES
CCQE without SRC
CCQE with SRC
ArgoNeuT
4
0
P (3+) = 70.9% P (6+) = 50.6%
cos(~qrec, p~1) is peaked at ∼ 0.85
NuWro: SF
61910 (22.1%) 217982 (77.9%)
cos γ i ≤ −0.8
Configuration origin:
2.5
P (4+) = 0.9% for the SF approach.
(63%)
1
TABLE II. Probabilities of detecting three (or more) and six (or more) events with
protons in the reconstructed initial back-to-back configuration according to NuWro.
MEC
DIS
RES
CCQE
ArgoNeuT
4.4%
19
0.5
NuWro: LFG P (3+) = 65.0% P (6+) = 46.5%
NuWro: LFG 57979 (21.9%) 206955 (78.1%)
(37%)
0
FIG. 6. Distribution of the cosine of the reconstructed angle between two protons in
the final state. The NuWro results are normalized to the same area as the
experimental data. (Top) LFG model, (bottom) SP approach.
P (4+) = 2.6% for the SF approach.
P (4+) = 1.1% for the LFG model,
11
-0.5
P (4+) = 2.9% for the LFG model,
3.4%
ArgoNeuT
-1
cos(γi)
1
ν̄-mode
MEC
DIS
RES
CCQE without SRC
CCQE with SRC
ArgoNeuT
2
1.5
We performed a simulation using NuWro Monte Carlo event
generator [3]. We have:
νµ interactions from two beams (νµ and ν̄µ dominated).
Normalization according to numbers of efficiency corrected CC events (729 + 3759).
Two NuWro modes; one with spectral function (SF)
for CCQE (some events occur on back-to-back
neutron-proton SRC pairs).
Proton tracks fully contained in the detector (47x40x90
cm3). We use here ArgoNeuT formula connecting proton
kinetic energy (T) and the track length (R)
A b+1
T (R) =
R ,
(5)
b+1
where A = 17 in the units of MeV/cm1+b, b = -0.42.
Only events with two protons and no pion in the final
state.
MEC
DIS
RES
CCQE
ArgoNeuT
2
0
0
2000
pTmiss (MeV/c)
2
NuWro simulation
1500
2
miss
n
1000
CC QE
2.5
ArgoNeuT Coll. observed an enahncement in the number of events in this cos(γ) < −0.95 area [2].
CC QE one-body on a neutron from a SRC np pair. Both
of the protons found in the final state should exceed the
Fermi momentum.
p~p1 = p~i + ~q >> kF , p~p2 = p~i >> kF
(2)
500
We can see in Fig. 5 that for hammer events with
pTmiss ≥ 300MeV/c, the RES contribution starts to dominate.
MEC
DIS
RES
CCQE
ArgoNeuT
4
SRC Identification
To specify the role of SRC, one can consider two interactions:
CC RES pionless mechanisms involving a preexisting
SRC np pair in the nucleus. We identify them in events
with the proton pair in a back-to-back configuration in
the lab frame (cos(γ) < −0.95) and a rather large missing
transverse momentum, pTmiss > 300 MeV/c, where
p~ T = −(~kµ + p~p1 + p~p2)T .
(1)
0
FIG. 5. Missing transverse momentum distribution for 2 proton hammer events. (Top)
LFG model, (bottom) SP approach.
Angle between two protons
FIG. 1. Two-dimensional views of one of the "hammer events," with a forward going
muon and a back-to-back proton pair (pp1 = 552MeV/c, pp2 = 500MeV/c). Source: [2].
Transformations from the TPC wireplanes coordinates (w, t "collection plane" [top], v, t
"induction plane" [bottom]) into lab coordinates are given in the further references
of [2].
0
NuWro predicts too few hammer events.
According to NuWro, the most interesting is the
excess in the ArgoNeuT hammer events in the
LAB frame.
The excess in the reconstructed back-to-back
nucleon pairs is kinematical in origin and is not
directly related to existence of SRC nucleon pairs.
References
[1]
J. Arrington, D. W. Higinbotham, G. Rosner, and M. Sargsian,
Prog. Part. Nucl. Phys. 67, 898 (2012)
[2]
ArgoNeuT Coll. (R. Acciarri et al.),
Phys. Rev. D 90, 012008 (2014)
[3]
K. Niewczas, J. T. Sobczyk, [arXiv:1511.02502]
Mail:
[email protected]