CCQE xsec: from Minerva to T2K
based on arXiv:1305.2243 (and 1305.2234)
S.Bolognesi (IRFU, CEA Saclay)
T2K-Saclay internal meeting
29 October 2013
Charged-current quasi-elastic xsec
µ−

νµ
u
u
d
p
d
u
d
n
Xsec parametrized by 6 form factors to describe hadronic
structure (not an elementary process → cannot be calculated
in perturbative EWK theory)
• vector form factors same as EM interaction → measured
with e- scattering
●
axial and pseudoscalar form factors
✗
depend on Q2=-(pν-pµ)2
✗
parametrized with axial mass (MA)
(proper description for free nucleon, eg deuterium or high energy)

Nu oscillation experiments use heavy nuclei and Eν ~ 1 GeV →
modification from nuclear environment:
usual approx: Relativistic Fermi Gas (independent nucleons in uniform
binding potential)
→ interaction between nucleons neglected (needed eg for e- scattering!)
(this affect the kinematic of outgoing muon and hadrons)
S.Bolognesi (IRFU, CEA Saclay)
2
T2K Saclay internal meeting
CC inclusive – CCQE exclusive
µ−
νµ
µ−
νµ
p

For normalization of νµ disappearance
results CC-inclusive is sufficient ...
u
u
d
d
u
d
extrapolation from ν µ to ν µ and from a
nucleus to another (near/far detector)
need full modeling of different
contributions to CC
n
●
●
RES: single pion production through
baryon resonances (CC1π)
DIS: deep inelastic scattering
CCQE
(RES-DIS boundary arbitrary: cut on
hadronic invariant mass W)
●

NOMAD 2009,
arXiv:0812.4543
CC incl includes other processes
(backgrounds to CCQE)
DIS
(COH: coherent interaction with whole
nucleus: νµ N → µ- p+ N)
RES
Final state interactions: big uncertainty
→ interplay with experimental capability of
distinguishing the final state
S.Bolognesi (IRFU, CEA Saclay)
3
Minerva analysis strategy (1)

Experimental setup:
NuMi beam: >95% νµ, 1.5-10 GeV, 9.42 * 1019 POT
Minerva (2m upstream MINOS for µ): tracker of planes scintillator strips (95% CH)
surrounded by calorimeters

Reconstruction and selection:
• µ- used to reco Q2 in QE hypothesis
(Eb = 30 MeV binding energy, formula in RFG approximation, Q2 is model dependent)
• clusters (>1MeV)
vertex energy region (Ep ~225MeV): large theoretical
uncertainty (correlation among nucleons, FSI) → NOT USED
recoil energy region: sensitive to high E proton and hadrons
(cut vs Q2 → 95% signal eff. for each Q2 bin)
→ 29620 events

Unfolding and efficiency corrections from MC (without pµ or pp fiducial cuts)
S.Bolognesi (IRFU, CEA Saclay)
4
T2K Saclay internal meeting
Minerva analysis strategy (2)
purity 0.49
efficiency 0.47
(detector level)
Signal/backgrounds normalization in
each Q2 bin from fit to recoil energy
•
•
recoil energy shape from MC (!)
very similar shapes (!)
Systematics:
I = muon scale
II = recoil reco
III + IV = theor. modeling
V = flux (norm. only)
VI = others
S.Bolognesi (IRFU, CEA Saclay)
5
T2K Saclay internal meeting
Minerva results (1)
σ = 0.93 ± 0.01(stat) ± 0.11(syst) 10-38 cm2/n
(same results in νµ analysis)
●
●
●
Best agreement with TEM (Transverse Enhancement Model) which describe
underlying dinamical multinucleons processes (as in e- scattering)
Shape prefers large axial mass in RFG model
Normalization prefers Spectral Function over RFG (more sophisticated energymomentum spectrum of bound nucleons)
S.Bolognesi (IRFU, CEA Saclay)
6
T2K Saclay internal meeting
Contextualizing...
Wascko, 2011
(arXiv:1107.3400)

NOMAD high energy (!), in agreement with free nucleons MA ~1GeV (= world average
measured from deuterium (!))

All low energy exp. on heavier nuclei measure reduced xsec at low Q2 and enhanced
at high Q2 (→ larger MA~1.35GeV) and larger xsec normalization
Possible explications:
●
●
problem in modeling CC1π background (low Q2 deficit mitigated when CC1π from data)
too simple RFG model (impulse approximation) not valid at low energy: missing
coherent interaction of initial state nucleon with other nucleons
Minerva results tend to favor TEM → second hypothesis (but with low sensitivity)
S.Bolognesi (IRFU, CEA Saclay)
7
T2K Saclay internal meeting
Minerva results (2)

First order initial state model improvement: coherent interaction with 2 nucleons
νµ n p → µ- p p

(eg, Martini et al. 2009,
arXiv 0910.2622)
; νµ n p → µ+ n n
Minerva results in vertex region in agreement with this hypothesis
ν µ data suggest additional proton with E<225MeV
in 25 ± 1(stat) ± 9(syst) % of events
ν µ Q2<0.2 GeV2
ν µ data: no additional proton (low sensitivity of
Minerva to low E neutrons)
ν µ Q2>0.2 GeV2
ν µ Q2>0.2 GeV2
ν µ Q2<0.2 GeV2
Unlikely to be due to systematics (eg, FSI):
highly correlated (0.7) btw νµ and νµ
S.Bolognesi (IRFU, CEA Saclay)
8
T2K Saclay internal meeting
Conclusions
Minerva measurement CCQE dσ/dQ2 in ν µ and ν µ on Carbon

●
●
analysis inclusive in vtx region (mostly affected by theor. syst. on FSI and initial
state multinucleons processes)
● cut to improve S/B
analysis based on recoil region
● fit S/B norm. with shape from MC
➔ results in agreement with other experiments on C at low energy
(different than NOMAD)
➔
➔

larger MA in RFG needed or
TEM model with nucleons-nucleons dynamics
(slightly preferred by Minerva νµ and νµ data)
Minerva measurement of energy in vtx region
➔ need for coherent interaction with 2 nucleons
(comparison of νµ vs νµ results makes this result convincing)
S.Bolognesi (IRFU, CEA Saclay)
9
T2K Saclay internal meeting
Going further... (T2K)

More model-independent results but complete info → to allow theoreticians to test
our data against different model
●
dσ/dQ2 → dσ/dpµ dθ µ
signal extraction:
Minerva looks only to recoil energy →
T2K fit together 1track and 2tracks samples + with fiducial
cut on proton momentum to separate the 2 samples (!)
●
●

Minerva uses MC for shape of recoil energy → better to use control
region in data (T2K: common fit with CC1π in control region)
T2K plan for CCQE xsec measurement:
θ μ , pμ bins topol react Etrue
ν
θμ , pμ bins
∑
i
ni =
∑i ∑
∑
m
k
i, m, k , j
scale
eff
i, m, j
f
⋅w(
M
,
FSI
,
...)
⋅r
(μ
,
p
,...)
⋅T i ,m ,k , j
∑ j
A
j
Topology: 1tr, 2tr (µ+p TPC, µ TPC + p FGD, µ FGD + p TPC)
Reaction: CCQE, CC1π (in control region), DIS
Tijkm = MC template of number of events in each pµ−θµ bin for each reaction,
topology and true Eν
→ for k=CCQE: Tijm * σi ← xsec weight we want to measure (same for all topo, Eνtrue)
S.Bolognesi (IRFU, CEA Saclay)
10
T2K Saclay internal meeting
T2K CCQE
θ μ , pμ bins topol react Etrue
ν
θμ , pμ bins
∑
i
ni =
i, m, k , j
scale
eff
i , m, j
f j⋅w( M A , FSI , ...)
⋅r (μ
, p ,...) ⋅T i ,m ,k , j
∑i ∑
∑
∑
m
k
j
✗ fj = flux normalization, known as a function of Eνtrue bins
✗ w = response function: how theory parameters affect number of events in each
react, topol, Eνtrue – θµ,pµ bins
systematics change the kinematics → move events from one θµ,pµ bin to another
and change distribution of Eνtrue in each θµ,pµ bin
systematics move events from one topol to another
(eg p detection affected by FSI uncertainty)
systematics move events from one reaction to another
(eg definition of DIS-CC1p modeling)
→ covariance matrix
✗ r = response function: how detector systematics affect the number of events in each
topol, θµ,pµ bin (given Eνtrue)
eg, systematics on µ scale affect pµ bin
systematics on p reco move events from one topol to another
→ covariance matrix
S.Bolognesi (IRFU, CEA Saclay)
11
T2K Saclay internal meeting
CCQE xsec: from Minerva to T2K
BACK-UP
S.Bolognesi (IRFU, CEA Saclay)
T2K-Saclay internal meeting
29 October 2013
Results review
νµ
(… all hypotheses
still compatible with
data at <2σ level ...)
νµ