The NOMAD experiment: a status report

UCLEARPHYSICS PROCEEDINGS SUPPLEMENTS ELSEVIER

N u c l e a r P h y s i c s B (Proc. Suppl.) 55C (1997) 4 2 5 - 4 3 2

The NOMAD

experiment:

a status report

Alessandro Cardini* University of California, D e p a r t m e n t of Physics and Astronomy, 405 Hiigard Avenue, Los Angeles, CA 90025-1547, USA. NOMAD is an experiment searching for v~(,) --* v, oscillations at the CERN SPS Wide-Band neutrino Beam. This search is performed via the appearance of vT in a predominantly v~ beam of 24 GeV average energy. The expez4.ment has been designed to allow a kinematical selection of u. charged current interactions from the background of v~ and v~ charged and neutral current interactions and has maximum sensitivity in sin 2 20 for A m 2 greater than 40 eV 2, and some sensitivity remains for Am 2 down to 1 eV2 in the case of full mixing. NOMAD has taken data with the full detector since August 1995, and will pursue the oscillations search until the end of 1997. Charged and neutral current interactions of v~, Y~,, v, and ~, have been identified and comparisons between data and Monte-Carlo are presented. The status of the v~ --+ v~ oscillation search using various tan decay channels (r --* ~, r --+ e, ~" --+ hadrons) is also reported.

i. INTRODUCTION

~

N O M A D [1] - N e u t r i n o Oscillation MAgnetic Detector - is a short baseline experiment searching for v~ appearance from v~(~) ---* v~ oscillations. Several high energy physics and cosmological models suggest m~. in the range from 1 to 100 eV ~. Searching for oscillations m a y be the only way to probe m~, in this range. The N O M A D experiment is performed at the C E R N SPS Wide Band Neutrino Beam, mainly composed of ~ , ' s and with a few percent contamination from ~u and v~. The v¢ search is performed by looking at the charged current interaction

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collaboration.

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length, which depends on g~ and on the eigenstates squared mass difference Am2: [Km] =

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ent techniques and looking for v~ --~ vr oscillations. At large A m 2 values the current experimental limit is sin 2 20 < 5 x 10 - s [2]. NOMAD (located at an average distance of 600 In from the neutrino source and with an average neutrino energy of 24 GeV) should be able to improve, in the case of a null result, the existing limit by an order of magnitude teaching sin 2 20 < 3.8 x 10 -4 for A m ~ > 40 eV 2. The CHORUS experiment [6] which uses the same b e a m line but an entirely different technique has a similar sensitivity.

2. T H E

NEUTRINO

BEAM

The SPS Wide-Band neutrino Beam is produced by 450 GeV protons hitting a Be target. The secondary particles produced in the interactions are focused by two magnetic lenses, the horn and the reflector, and are allowed to decay in a 300 m long vacuum tunnel. Surviving hadrons and muons are stopped by iron and earth shields. The b e a m composition and average energies of the various components are shown in Table 1. The contamination from prompt v~ was recently re-evaluated [7,8] and was found to be 50 times larger that previous estimates, still at

a negligeable level compared to the sensitivity of the experiment.

3. T H E

DETECTOR

The N O M A D detector [9-111 provides excellent electron and muon identification, a precise measurement of the event kinematics and at the same time a considerable target mass for neutrino interactions. All subdetectors (see Figure 2) are housed inside the recycled UA1 magnet which provides a 0.4 tesla bipolar horizontal magnetic field over a volume of 3.6 x 3.5 × 7.0 ms. The active target, weighting 2.7 tons in the 2.6 × 2.6 × 4.05 m s fiducial volume, consists of 44 drift chamber modules. Each module has three planes of sense wires with stereo angles of +5 °, 0 ° and - 5 0 with respect to the magnetic field direction, with a hit resolution of 180 p m . Charged particles are reconstructed with little degradation from multiple scattering given the low Z of the chamber material. (The whole target is 1 X0 and 0.5 R~t.) The transition radiation detector ( T R D ) is composed of nine identical modules made of a polypropylene foil radiator and straw tubes, and is used to tag electrons with a rejection factor for pions of

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Table 1 CERN Wide Band neutrino Beam composition and average energies. Neutrino v~ P~ v, Pe vr

Mean energy (GeV) 23.6 22.7 37.0 33.2 -

103 for 90% electron efficiency. The preshower (PRS) is composed of a 1.6 X0 long lead converter followed by two successive arrays of proportional tubes. The Cerenkov light electromagnetic calorimeter (ECAL) is placed downstream of the PRS, and it is made of 35 x 25 19 Xo thick lead-glass counters. ECAL has an energy resolution o ' ( E ) / E : 3.2%/v/E[GeV] + 1% [12]. The hadronic calorimeter (HCAL) is an ironscintillator calorimeter with an energy resolution of o ' ( E ) / E = 1 0 0 % / ~ . Two stations of muon detectors, separated by 80 cm of iron, follow HCAL. They consist of 10 drift chambers previously used in UA1 and arranged in pairs to provide track segment reconstruction. The scintillating trigger planes T1 and T2, together with the veto plane V, form the main trigger for the experiment, the logical V. T1- T2. The front calorimeter (FCAL) sitting upstream of the active target provides more data for neutrino flux measurements and acts also as an independent source of dimuon events. 4. T H E

1995 DATA

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SET

The NOMAD detector was completed during the 1995 data taking period, and has been operating smoothly since then. The 1995 data taking, corresponding to 9 x 1018 protons on target recorded on tape, resulted in approximately 2 x 10 s u~, charged current interactions analysed.

4.1. M u o n i d e n t i f i c a t i o n In order to select a clean sample of muon (anti)neutrino charged current interactions the following criteria were applied: (i) at least two primary tracks should be reconstructed; (ii) the

interaction vertex should lie inside the fiducial region defined s by IX, YI < 130 cm and 5 < [Z[ <

370 cm; (iii) a muon identified in the muon chambers should be matched in the drift chambers to a primary track; (iv) the muon momentum should be larger than 2.5 GeV/c. The momentum distribution for both positive and negative muons is shown in Figure 3. The dots are data and the histogram is Monte-Carlo. Monte-Carlo events are 2The Z axis coincide with the nominalbcam line,X is the directionof the N O M A D magnetic fieldand Y is pointing in the verticaldirection.

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generated with L E P T O 6.1 [13] and detector effects are included with a G E A N T / F L U K A [14] based program. The beam is simulated using a parametrisation developed by the CHARM-II collaboration and reweighted by the prediction of a G E A N T simulation of the actual neutrino production line. The ratio between the numbers of identified v~, and ~ , charged current interactions is in agreement with the fluxes estimated by the beam simulation. Charged hadrons are tracks found in the drift chambers and not identified as muons or electrons. The inclusive hadron spectrum is shown in Figure 4. The hadronic jets produced in neutrino interactions are reconstructed with a combination of tracking and calorimetric information. Charged hadrons are individually identified as tracks and the neutral component (photons and neutral hadrons) as neutral clusters in the calorimetric devices. In particular, for v~, charged current interactions, the total visible energy is measured by summing the muon energy and the whole hadronie jet energy. The hadronic jet energy

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also includes contributions from secondary interactions and decays of neutral particles. The distribution of the reconstructed visible energy in v i, charged current interactions is shown in Figure 5. In all the above distributions a good agreement between data and Monte-Carlo can be observed. 4.2. E l e c t r o n i d e n t i f i c a t i o n Electron identification relies strongly on the energy deposition in the transition radiation detector, the preshower and the electromagnetic calorimeter. The selection of events containing an electron requires that: (i) a track reconstructed in the drift chambers should be matched to a TRD track and to an ECAL cluster but not to a segment found in the muon chambers; (ii) the track should pass the T R D electron selection criteria, plus additional criteria in the PRS and a shower profile cut in the ECAL. The distribution of ( E , , , l - P ~ ) / ( E , , , , ~ + P ~ ) is shown in Figure 6 at different stages of electron identification. E , ~ l is the energy measured in ECAL and Pi is the reconstructed initial momentum of the track measured in the drift chambers. A clear electron signal is visible.

A. Cardini/Nuclear Physics B (Proc. Suppl.) 55C (1997) 425-432

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The sample of p r o m p t electrons is selected by taking events for which (E**~: - Pi)/(E**~l + Pi) is between - 0 . 2 and 0.3. This selection includes also a large fraction of soft electrons coming from photon conversions in the drift chamber volume. On average one photon conversion occurs per neutrino interaction. Photons are coming mostly from decays of ~r° mesons produced in neutrino interactions (on average 1.7 ~r° per interaction). These softer electrons are rejected by applying invariant mass cuts between the candidate electron and opposite sign tracks to which the electron mass is assigned, and also between the candidate electron and identified photons. Neutral current neutrino interactions are rejected by applying an isolation criterium using the variable QT defined as:

cuts (for electron energies larger than 10 GeV), while only 4 × 10 - s v# charged current and 3 x 10 -4 v# neutral current events survive the same selection. The final electron and positron spectra are shown in Figure 7. There are 607 (88) events with an electron (positron).

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Pe (Pto*) being the the electron (total visible) m o m e n t u m in the event. QT represents the electron transverse m o m e n t u m with respect to the total reconstructed m o m e n t u m in the event. The efficiency of this selection has been evaluated on Monte-Carlo v~ charged current events. 40% of the ve charged current events pass these

5. T A U N E U T R I N O

SEARCH

5.1. K i n e m a t i c s e l e c t i o n In NOMAD vr charged current interactions cannot be identified by looking at the r - track itself. With an average p a t h of 1 m m the r - decay vertex cannot be separated from the primary. The r - can however be identified using kinematic criteria in the transverse plane, making use of the fact that one or more neutrinos escaping detection produce a non-zero missing transverse mom e n t u m in the event. NOMAD intends to search for r - decaying to e - ~ e v r , # - ~ , v r , ~r-vr, p--vr and 7r-a'-Tr+n~r°vr, which sums up to 86% of the total branching ratio. The r - identification criteria are discussed in the following for the muonic channel, but they are similar in the electron case. Figure 8 shows the kinematic selection trite-

A. Cardini/Nuclear Physics B (Proc. Suppl.) 55C (1997) 425-432

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ria used to distinguish u~ charged current events where the ~'- decays to a negative m u o n from the background due to u s charged current interactions. The missing transverse m o m e n t u m in the event (~t) is built using the reconstructed m o m e n t a in the event. In the transverse plane ~o~,h (~o,,,h) is defined to be the angle between the hadronic jet direction and the muon (missing) momentum. The angle ~ogh is peaked strongly at 180 ° for u s charged current events, while a taft extending to smaller angles is present in the case of v¢ charged current events. In the case of P,~l, the distribution is flat for v~, charged current events because the direction of F t is determined mainly by resolution in momenta reconstruction, and it is peaked at 1800 for v~ charged current events, reflecting the fact that the r - and the hadronic jet are produced back-to-back. In practice this selection will be enhanced by combining these vari-

ables in the (~i,a, ~,~h) plane and performing a two dimensional selection, as shown in the two lower histograms in Figure 8. 5.2. Electron channel Figure 9 shows the plot of the variables ~eh and ~mh for the selected electron sample. While the ~,~h distribution agrees reasonably well with Monte-Carlo predictions, the q0~h data distribution is wider than the Monte-Carlo. These systematic differences are currently being studied by looking at m u o n events. These have the same kinematics but the statistics is m u c h larger. 5.11. Hadronic channels Also the analysis of the tau hadronic decays relies on kinematical cuts. Particle identification is used to reject events containing primary electrons or muons. In the following we will consider the

A. Cardini/Nuclear Physics B (Proc. Suppl.) 55C (1997) 425-432

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r - ---* Ir-v~ search. The largest backgrounds are neutral current interactions and charged current interactions where the leading lepton is not identiffed. The QT variable defined above is used to reject these backgrounds from the tau signal. In Figure 10 the QT distributions for ur charged current Monte-Carlo events and for v i, neutral current Monte-Carlo events are shown. There is a region populated by signal with little background. In order to cross check the v , neutral current Monte-Carlo QT distribution with the data a fake v~ neutral current data sample has been generated by removing the muon in vu reconstructed charged current events. These data, shown as dots in Figure 10, are in very good agreement with v;, neutral current Monte-Carlo. 6. C O N C L U S I O N S NOMAD has performed well during the 1995 and 1996 data taking period. A sample of neutrino interactions corresponding to 1.2 x 105 vu charged current events has been identified in the 1995 data. Preliminary data analysis shows that the detector is performing as expected. The neu-

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trino oscillation search is in progress, and first results are expected soon. With the inclusion of the 1997 data the experiment will have a statistics close to 10 s charged current vv interactions. REFERENCES

1. NOMAD Collaboration, P. Astier et al., CERN-SPSLC/91-21 (1991), C E R N SPSLC/91-48 (1991), SPSLC/P261 Addendum 1 (1991), CERN-SPSLC/93-31 (1993). 2. N. Ushida et al., Phys. Rev. Lett. 57 (1986) 2897. 3. K. McFarland et al., Phys. Rev. Lett 75 (1995) 3993. 4. M. Gruw6 et al., Phys. Lett B309 (1993) 463. 5. F. Dydak et al., Phys. Left. B134 (1984) 281. 6. CHORUS Collaboration, C E R N - S P S C / 9 0 42 (1990), C E R N - P P E / 9 3 - 1 3 1 (1993). 7. B. Van De Vyver, P. Zucchelli, "Prompt ur background in Wide Band u~ Beams", CERN-PPE/96-113.

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8. M.C. Gonzales-Garcia, J.J. Gomez-Cadenas. "Prompt v~ Fluxes in Present and Future Tau Neutrino Experiments", CERN-PPE/96-114. 9. A. Rubbia, Nucl. Phys. B (Proc. Suppl.) 40 (1995) 93. 10. NOMAD Collaboration, NOMAD memo 95027, contribution to the EPS HEP '95, 1995 International Conference on High Energy Physics, August 1995, Brussels, Belgium. 11. NOMAD Collaboration, contribution to the TAUP95 International Conference, September 95, Toledo, Spain. 12. D. Autiero et al., "The electromagnetic calorimeter for the NOMAD experiment", Nucl. Instr. and Meth., A373 (1996) 358. 13. G. Ingelman, "The LUND Monte-Carlo for Deep Inelastic Lepton-Nucleon Scattering, LEPTO 6.1", Physics at Hera, October 1991; T. SjSstrand, Comp. Phys. Commun. 39 (1986) 347; H.U.Bengtsson and T. SjSstrand, Comp. Phys. Commun. 43 (1987) 367. 14. GEANT 3.21, CERN Program Library, long write-up W5013.