Neutrino interactions with carbon

PROCEEDINGS SUPPLEMENTS

~1 H.51:VIH~

Neutrino KARMEN

Nuclear Physics B (Proc. Suppl.) 38 (1995) 198~03

Interactions with Carbon Collaboration t

B. A r m b r u s t e r a, B.E. B o d m a n n b, I. Blair ~, N.E. B o o t h d, A.C. Dodd ~, G. Drexlin a, V. Eberhard a, J.A. E d g i n g t o n ¢, K. Eitel ~, M. Ferstl b, E. Finckh b, H. G e m m e k e a, W. Grandegger ~, J. HSfll b, T. Jannakos o, M. Kleifges o, j. Kleinfeller ~, W. Kretschmer b, R. Maschuw ], J. Rapp ~, P. Plischke a, F. Schilling b, B. Seligmann c, O. S t u m m b, J. Wolf ~, S. W61fle ~ and B. Zeitnitz ~ a Institut ffir Kernphysik I, Kernforschungszentrum Karlsruhe, and Institut ffir Experimentelle Kernphysik, Universit£t Karlsruhe, Postfach 3640, 76021 Karlsruhe, G e r m a n y b Physikalisches Institut, Universit£t Erlangen-Nfirnberg, 91058 Erlangen, G e r m a n y Physics D e p a r t m e n t , Queen Mary and Westfield College, Mile E n d Road, London E1 4NS, UK d D e p a r t m e n t of Nuclear Physics, Oxford University, Keble Road, Oxford OX 1 3RH, UK Rutherford Appleton Laboratory, Chilton, Didcot, OX 11 0QX, UK ] Institut ffir Strahlen- und Kernphysik, Universit~it Bonn, 53115 Bonn, G e r m a n y

presented by B. Zeitnitz The K A R M E N experiment at the pulsed spallation n e u t r o n facility ISIS studies the charged and neutral current reactions 12C ( v~, e- ) ~2N and 12C ( u , u' ) ]2C* ( 1 + 1 ) in the astrophysical i m p o r t a n t energy range up to 50 MeV. Neutrinos are detected by a 56 ton high resolution liquid scintillation calorimeter with spectroscopic quality. Efficient background rejection results in clear neutrino signatures and allows reliable cross section m e a s u r e m e n t s down to 10 -42 cm 2. We present cross section results for u - i n d u c e d reactions on carbon with special emphasis on their implications for neutrino astrophysics and weak nuclear formfactors and report a new flux-independent test of the equality of the couplings of u¢ and ~ to the weak neutral current at low energies.

1

Introduction

N e u t r i n o - i n d u c e d reactions in nuclei at low and intermediate energies play an i m p o r t a n t role in the study of f u n d a m e n t a l neutrino properties and their interaction with matter. One of the most interesting examples in this context is given by recent studies of the dynamic processes in core-collapse supernovae, where n e u t r i n o - n u c l e u s interactions play an essential role. New theoretical

concepts like u - i n d u c e d nucleosynthesis [1] or new experimental techniques like the flayour independent detection of SN neutrinos by the neutral current excitation of nuclei [2] underline the i m p o r t a n c e of neutrino interactions with nuclei for a variety of astrophysical scenarios. However, as in solar neutrino physics, the almost total lack of experimental cross section d a t a for neutrino nucleus reactions at low and intermediate energies introduces great uncertainties.

t Supported by BMFT (Germany) and SERC ( UK ) 0920-5632/95/$09.50 © 1995 Elsevier Scicncc B.V. All rights reserved. SSDI 0920-5632(94)00747-0

KARMEN Collaboration~Nuclear Physics B (Proc. Suppl.) 38 (1995) 198 203

Apart from the more recent awareness of the importance of neutrino-nucleus interactions in astrophysics, there is also a l o n g standing recognition of the significance of n e u t r i n o - n u c l e u s interactions as probes of the weak nuclear current [3]. Selecting transitions between discrete nuclear states with well defined q u a n t u m numbers Y T (spin, parity and isospin) allows to analyse specific parts of the weak hadronic current and to use the nucleus as a microscopic ' s p i n - i s o s p i n ' filter. The K A R M E N experiment is investigating charged current (CC) and neutral current (NC) neutrino interactions on carbon in the astrophysical important energy range up to 50 MeV, while simultaneously looking for u - oscillations in the appearance channels u~, --+ ue and ~ ~ Pc (see [4]). Main emphasis is put on clear neutrino signatures and efficient background rejection. In the following we report cross sections for neutrino induced reactions on 12C from analysis of data taken between July 1990 and February 1994 corresponding to 5 181 Coulombs (3.23 x 1022 protons ) of b e a m on target.

As can be seen in fig. 1, neutrinos from a b e a m stop source are the closest terrestrial analogue of neutrinos from core-collapse supernovae, and thus can be used for precise calibration measurements in the field of neutrino astrophysics.

T=5

10

The

KARMEN

Experiment

The K A R M E N experiment [5t is performed at the pulsed spallation neutron facility ISIS of R u t h e r f o r d - Appleton L a b o r a t o r y in Chilton, Didcot (UK). The b e a m stop of the 800 MeV 200 #A rapid cycling proton synchrotron delivers equal fluxes of three different neutrino species (u~, Ue and ~ ) from the ~r+ --* #+ --* e + decay cascade at rest. The time structure of the proton b e a m (two 100 ns bunches 330 ns apart, recurring at 50 Hz) gives a p r o m p t burst of monoenergetic 30 MeV v, from ~r+- decay at rest within the first 500 ns after b e a m - o n - t a r g e t . The subsequent decay of/x + provides u¢ and ~, with continuous spectra of energies extending up to 52.8 MeV at much later times from 0.5 #s up to 9 #s. The resulting duty factors of 10 -5 for v~ and about 2.5 x 10 -4 for u¢ and 17~ allow effective suppression of cosmic ray induced background.

20

30

MeV

40

,

50

60

Electron Neutrino Energy [ M e V ] rW

T = 10 M e V

6

2

199

10

::

20

30

..- . . . . . . . . "~

40

50

60

Muon Neutrino Energy [ M e V ]

Fig. 1: Comparison of neutrino energy spectra from/z + and 7r+ decay at rest (broken lines) with Fermi-Dirac type neutrino spectra of temperature T from core-collapse supernovae (solid lines) for a) electron-type neutrinos b) muontype neutrinos. Neutrinos are detected by a high resolution 56 ton liquid scintillation calorimeter located at a m e a n distance of 17.5 m from the spallation target, and housed in a 6 000 ton shielding blockhouse [6]. Consisting entirely of hydroearbons, the detector is an all active target of 12C and 1H nuclei optimised for the investigation of low energy neutrino nucleus interactions. Recent improvements of the experiment include further shielding around the spallation target and proton b e a m line, completion of a boron-loaded thermal neutron shield, and a refined trigger logic flagging cosmic ray induced events (stopped muons) over longer periods of time.

200 3

KARMEN Collaboration~Nuclear Physics B (P1vc. Suppl.) 38 (1995) 198-203

The

reaction

i2C ( b,e , e - ) 12Ng.s

The signature that uniquely identifies the exclusive charged current reaction

Ve + 12C

....,

12Ng.s + e 12Cg.s. @ e + + ue

is a spatially correlated delayed coincidence between an electron from the inverse /3- decay on 12C during the re- time window and a positron from the subsequent decay of the ground state of 12N. P r o m p t electrons from the initial reaction on 12C were looked for in a 9 # s long time window starting 0.635 #s after b e a m o n - t a r g e t . A visible electron energy in the range from 10 to 36 MeV was required, with the upper energy b o u n d given by the kinematic limit, the lower eliminating the majority of background events induced by beamassociated and cosmic ray produced neutrons. Positrons from the subsequent 12N- decay of visible energies in the interval from 3.5 to 16.5 MeV were searched for in a 36 ms long time range starting 0.5 ms after the p r o m p t event. Spatial correlation between the reaction steps was enforced by requiring b o t h to occur within a volume of 0.3 m 3 defined by a m a x i m u m measured separation of 0.5 m along the module axis, in the same (or adjacent ) detector modules. Finally, the sequence was vetoed by cosmic ray activity in the 20 its interval preceding the p r o m p t or delayed part of the coincidence or by a muon stopping nearby in the previous 60 ms. Applying these cuts to our b e a m on data set a total of 291 coincident events survived all cuts. Background, mainly due to capture reactions of stopped muons that evade the cosmic ray veto, comprises 4.0 true and 4.3 accidentiM coincidences measured with high statistical precision in the pre-beam period 200 #s before beam-on-target. This leaves 2 8 2 . 7 + 17.1 neutrino induced events from t h e 12C ( Ve, e - ) 12Ng.s' reaction. The energy distributions of these events are shown in fig. 2 in comparison with G E A N T 3 Monte Carlo simulations, the corresponding time

distributions are compared to the lifetimes of tt + (T = 2.2 #s) and 12N ( T = 15.9ms). There is excellent agreement between our d a t a and the simulations, in particular the decay constant of (2.21 + 0.16) tts fitted to the time spectrum of fig. 2 c) clearly proofs the neutrino induced nature of the observed events, as well the positionM correlation of coincident events which is fitted by a Gaussian with ~ = 13.4 cm, consistent with the measured resolution. The detection efficiency of K A R M E N for the 12C (v~ ,e-)12Ng.s. reaction was calculated with the G E A N T 3 Monte Carlo code using as input the kinematic and angular distributions of inverse/3- decay electrons provided by a calculation of Donnelly [3]. The cross section for the exclusive CC reaction averaged over the entire v~ energy distribution ( 0 - 52.8 M e V ) is a c c (v~) = [9.1 + 0.5(stat.) + 0.8 (syst.) ] x 10 -42 cm 2 in good agreement with recent calculations by Dolmelly [3], using a multipole expansion of the weak hadronic current, and by calculations based on the elementary particle t r e a t m e n t ( E P T ) by Fukugita et al. [2] and Mintz and Pourkaviani [9] giving values of 9.4, (9.2 i 1.1 ) and ( 8.0 :t: 0.3 ) × 10 -42 cm 2, respectively, and also with a new result of 9.3 × 10 - 4 2 c m 2 by Kolbe et al. [8] using the Continuum R a n d o m Phase Approximation. Since the recoil kinetic energy of 12N in this reaction is negligible, the primary neutrino energy E (v~) is related to the electron kinetic energy E ( e - ) by E ( u~ ) = E ( e - ) + 17.3 MeV. Thus a precise measurement of E ( e - ) , which is possible due to the good calorimetric properties of the K A R M E N detector, determines E (re). As the spectral shape of u¢ from #+ - decay at rest is given by the Michel p a r a m e t e r 0, one can deduce the energy dependence of the 12C ( re, e - ) 12Ng.s. cross section from the number of events within a given interval of E(v¢), corresponding to a known fraction of neutrino flux. A measurement of the excitation function ~r (E~,) is of particular interest for the

201

KARMEN Collaboration~Nuclear Physics B (Proc. Suppl.) 38 (1995) 198 203

Ve + 12C __>12Ng.s"+ @ ;>

12Ng.s"~ 12C + @ + Ve

35-

50-

~: 3oi ,-: 25i 20 15 10i

t

li

.,' i-

b)

40 30"

_i.l l-"

":

20

°'-][

lo-

5i

0

. . . .

,

10

15

20

70 60i "-'~ 50i ~-v=40! 30i 20i 10i 0:

':~4,..÷_. ~5~;~.,.~

.

. . . . . . .

[

25 30 35 Eleclron Energy (MeV)

5

. . .

.

.

.

.

. [

- -_ '

~

'

'

I

10 15 Positron Energy (MeV)

9O C)

75 6o ~ 45

"t = 2.2 gs

~

- t5.9

d) ins

30

0

1

2

3

4

5

6

7

8

9

10

Electron Time (Its)

0

'

'

'

'-i--'

10

. . . . . . .

~

'

'

20 30 Positron Time (ms)

Fig. 2: Energy (a),(b) and time (c),(d) distributions of electrons and positrons from the a2C ( ~,~, e- ) 12Ng.s. reaction. Energy spectra are compared to GEANT 3 Monte Carlo simulations, time distributions to decay curves of/z + ( r = 2.2 #s ) and 12N ( r = 15.9 ms). Background is shown shaded. theory of weak interactions in nuclei, as the energy dependence of the 12C ( ue, e - ) 12Ng.s. reaction is contained only in the kinematic phase space factor ( E , - Q ) 2 m o d u l a t e d by the q2 _ dependence of the weak n u c l e a r formfactor F A ( q 2) (using the E P T formalism of refs. [2, 9]). The dominant i s o v e c t o r a z i a l v e c t o r formfactor F a of 12C has so far only been determined at fixed values of mom e n t u m transfer q2 from f l - d e c a y (q2=0) and m u o n capture (q2=0.424 m2). Neutrino absorption on carbon allows the first determination of FA in the intervening range of m o m e n t u m transfer q2. Following the expression for a (E.) given in ref. [2] we first select a parametrization for FA ( q2 ). We use the theoretically preferred dipole form, suggested also by the behaviour of the electromagnetic formfactor /~ ( q2 ). The only unknown parameter remai-

ning in the parametrization is RA, the rms radius of the w e a k axial charge of the 12Cnucleus. To determine I~A we carry out a m a x i m u m likelihood fit of the resulting distributions to our measured cross section (for details see [10]), finding for the dipole form R.,x = ( 3.8 +- 1.8 1.4 ) fro, agreeing, within the errors, with the rms charge radius of 12C of Rm = 2.478 fro, determined in electron scattering. To compare our result with the theoretical formalism of [2, 9] we derive from RA a dipole mass MA = ( 180+~°°~-45/ MeV/c2 in agreement with the mass p a r a m e t e r used in the E P T calculations. This is the first experimental observation of the decrease of FA with increasing q2 which also confirms for the first time the identical q2 scaling behaviour of weak magnetic and weak axial formfactors of nuclei.

KARMEN Collaboration~Nuclear Physics B (Proc. Suppl.) 38 (1995) 198-203

202

4

The reaction

'2C (v,v')

12C*

The neutral current reaction 12C ( v , v I ) 12C* is of particular interest for neutrino astrophysics, as it allows the measurement of the bolometric flux of neutrinos from collapsing stars in large volume underground scintillation detectors like LVD or M A C R O . The K A R M E N experiment, which made the first observation of this neutral current process, can, apart from providing reliable cross section d a t a for this reaction, also test the underlying postulate of flavour universality of the neutrino coupling to the weak neutral current. The neutral current excitation of carbon leads to p r o m p t decay of the T = 1 level at 15.1 MeV with a branching ratio of ( 0.92 + 0.02 ) for single g a m m a ray emission back to the ground state, the signature therefore being a clear peak structure in visible energy around 15 MeV [7]. The analysis focuses on single prong events in the ( ue, vu ) time window, using the same d a t a set as for the exclusive CC reaction and similar pre- trigger requirements: no activity anywhere in the detector or its vetocounter system for 20 #s prior to the event, and no stopped muon locally in the previous 100 ms~ providing effective suppression of low-energy background from capture reactions of stopped cosmic ray muons. A fiducial volume cut excluding the outer layer of modules eliminates most events due to 7 - rays from Fe ( n, 3' ) capture reactions and bremsstrahlung from energtic electrons following # - decay in the inner passive shield. D a t a were accumulated in an extended ( ue, ~'u ) time window from 0.635-15.635 #s for analysis by a m a x i m u m likelihood method, assuming a signal with a 2.2 #s time constant superimposed on a time-independent cosmic ray induced background. The validity of this approach is proven by the time distribution shown in fig. 3 b ) : events distributed with the characteristic 2.2/~s decay time appear on a background that is accurately constant b o t h before and after b e a m o n - t a r g e t . Fig. 3 a) shows the resulting energy spectrum of v - i n d u c e d events from

12o

i

a)

100 1 80 ©

'°i

tf

4O

2°i 0

0

, i . . . . . . . . . . . . . . . .

10

2o

30

40

5o

Energy [MeV] 300 :::k tr] 250

b)

11MeV_
20n 1qo 100

,~

5O 0-10 . . . .

; ....

1; . . . .

2; . . . .

30

Time [gs] Fig. 3: Visible energies (a) and distribution in time after beam- on - target (b) of v- induced single prong events in the (re, ~'u ) time window. 10 MeV to 50 MeV after subtraction of all exclusive CC reactions which could be identified by their delayed coincidence signature. In the interval from 11 to 16 MeV the spect r u m clearly shows a peak which is due to the NC reaction 12C (/),/21 ) 12C*( 1 + 1 ; 15.1 MeV ) and corresponds to a total of 267 -t- 26 events. The broader underlying distribution has contributions from four reactions: neutrino- electron scattering, and the inclusive CC reactions 13C ( v ~ , e - ) 1aN , 12C ( r e , e - ) 12N and 12C ( v ~ , e - ) 12N*. There is excellent agreement between d a t a and corresponding G E A N T 3 simulations. After allowing for the fiducial volume and

KARMEN Collaboration/Nuclear Physics B (Proc. Supp£) 38 (1995) 198-203

correcting for time and energy cuts and pretrigger efficiency we derive the cross section for the neutral current excitation of 12C averaged over the energy spectra of ~'e and 5u : qNC(~'e + P u ) = [10"4± 1.0(stat.) ± 0.9 (syst.) ] X 10 -42 cm 2 For the ( ue, uu )- induced reaction studied here, Fukugita et al. [2] and Pourkaviani and Mintz [9] find (9.9± 1.2) and (9.8 ~= 0.4) × 10 -42 cm 2, while Kolbe et al. [8] calculate 10.5 x 10 -42 cm 2, almost independent of the residual interaction assumed. All calculations are in excellent agreement with our result. This clearly proofs that there are indeed two neutrino fiavours participating in the neutral current channel. As the exclusive CC reaction and the NC reaction link the ground strate of 12C to ( 1 + , 1 ) analogue states of the A=12 isospin triplet, the m a t r i x elements for both reactions are related by the Wigner-Eckart theorem, the isospin Clebsch- Gordon factor 1 / V~ implies that cross sections for CC and NC transitions in 12C are in the ratio 2 : 1. As the NC reaction is also induced by an equal flux of ~u, the ratio R = o'NC ( v e ~- vp ) / o'CC (//e ) is about 1, provided the ~ couples in the same way to the Z ° as re. Measurement of R is thus a flux-independent test of universality at low engies. Because of the slightly different energy spectra of ve and uu and also accounting for the small v - ~ difference in the NC cross section, the theoretical expectations for this ratio are 1.08 ± 0.02 [2], 1.21 ± 0.09 [9] and 1.13 [8] to be compared with our experimental result R = 1.15 -i- 0.13 (stat.) ± 0.06 (syst.). At the present level of precision this completely flux - independent result is confirming the universality postulate of the neutrino neutral current coupling for low energies up to 50 MeV, which is of considerable practical importance for future experiments intending to measure the bolometric fluxes of astrophyscial neutrinos via the NC excitation of nuclei [2].

5

203

Conclusion

Our measurements of CC and NC neutrino reactions on 12C are in excellent agreement with the most recent theoretical calculations, confirming the underlying theory of neutrino nucleus interactions and indicating that the the rates of allowed transitions can be calculated with confidence. We report the first m e a s u r e m e n t of a weak nuclear axial charge radius being in agreement with the corresponding electromagnetic radius. We also could report the first flux-independent test of universality in the neutral current sector confirming the universality postulate at low energies. The K A R M E N experiment is currently taking further d a t a to improve the precision of this test, clarify the violation of isospin s y m m e t r y in the A -- 12 system, and study first and second order forbidden transitions to excited states of 12N, whilst simultaneously testing f u n d a m e n t a l u - p r o p e r t i e s in the context of the Standard Model [4, 5]. REFERENCES

[1] S.E. Woosley et al., Astrophys. J. 356

(1990) 272. [2] M. Fukugita, Y. K o h y a m a and K. Kubodera, Phys. Lett. B 212 (1988) 139. [3] T.W. Donnelly, R.D. Peccei, Phys. Rep. 50 (1979) 1. [4] G. Drexlin et al., these proceeedings. [5] G. Drexlin et al., Progr. Part. Nucl., Vol. 32 (1994), pp. 375-396; B. Zeitnitz et al., Progr. Part. Nucl., Vol. 32 (1994), pp. 351-373. [6] G. Drexlin et al. ( K A R M E N Collab.), Nucl. Instr. Meth. A 289 (1990) 490. [7] B. B o d m a n n et al. ( K A R M E N Coll.), Phys. Lett. B 332 (1994) 251. [8] E. Kolbe et al., Phys. Rev. C 49 (1994) 1122. [9] S.L. Mintz et al., Phys. Rev. C 40 (1989) 2458; M. Pourkaviani et al., J. Phys. G 16 (1990) 569. [10] B. B o d m a n ~ et el. ( K A R M E N Collab.), acc. for publ. in Phys. Lett. B.