Neutrino Physics at Short Baseline

Transcription

1 Neutrino Physics at Short Baseline E. D. Zimmerman University of Colorado Lepton-Photon 2005 Uppsala, Sweden 2 July 2005

2 Neutrino Physics at Short Baseline Parameter regions accessible with short baselines History and status of oscillation measurements Non-oscillation physics from these experiments (new results) Future opportunities

3 Parameter regions accessible at short baseline Confine discussion to accelerator-based experiments Short baseline means < 2 km. Neutrino energies range from stopped muon decay ( 40 MeV) to 100 GeV The m 2 sensitivity range is higher than for astrophysical or long-baseline accelerator experiments: Region of interest is m 2 >10 2 ev 2. ( ) ( ) ( ) Primarily e or disappearance in current searches

4 History and current searches Current status of searches Higher-energy accelerators, reactor limits LSND and KARMEN Joint analysis Physics implications Current searches MiniBooNE

5 The Experimental Picture Highest m 2 limits: Dominated by high energy experiments: CCFR, CDHS, NuTeV, BNL, NOMAD/CHORUS Lower m 2 limits: Reactor experiments (Bugey) Moderate m 2 and sin 2 2 experiments: LSND (positive signal) and KARMEN2 (no signal) Atmospheric Solar

6 Separating and : a) NOMAD, CHORUS (approx.) NuTeV (ν) LSND, KARMEN only antineutrinos BNL, NOMAD, CHORUS only neutrinos b) ν µ ν e 90% Conf. Limits BNL-E734 (ν) BNL-E776 (ν) Band is LSND (ν ) Allowed Region 90% Conf. 99% Conf. NuTeV does them separately NuTeV (ν ) Combined results only: CCFR (appearance) CDHS (disappearance) ν µ ν e 90% Conf. Limits KARMEN (ν )

7 LSND concept (Liquid Scintillator Neutrino Detector) Stopped + beam at Los Alamos LAMPF produces e,, but no e (due to capture). Search for e appearance via reaction: ν e + p e + + n 167 ton mineral oil detector doped with scintillator Positron produces prompt Cherenkov + scintillation light signal Neutron thermalizes, captures in 200 s to form deuteron and 2.2 MeV gamma ray scintillation signal Look for the delayed coincidence.

8 LSND data Major background non-beam (measured off-spill, subtracted); 4 standard dev. excess above background. beam excess events data! Bgd. Small "m 2 +! Bgd. Large "m 2 +! Bgd. Oscillation probability: P ( ν µ ν e ) = (2.5 ± 0.6 stat ± 0.4 syst ) 10 3 positron energy (MeV)

9 LSND and KARMEN2 compatibility KARMEN2: similar experiment at RAL (UK), shows no excess above background. KARMEN2 had shorter baseline, so sensitive to higher m 2. Joint LSND+KARMEN confidence regions Joint LSND+KARMEN confidence region E. Church et al., Phys. Rev. D66, (2002) Combined analysis: Consistency at 64% confidence level Restricted parameter region

10 The Overall Picture LSND m 2 > 0.1eV 2 ν µ ν e Atmos. m ev 2 ν µ ν? Solar m ev 2 ν e ν? With only 3 masses, can t construct 3 m 2 values of different orders of magnitude! Is there a fourth neutrino? If so, it can t interact weakly at all because of Z 0 boson resonance width measurements consistent with only three neutrinos. We need one of the following: A sterile neutrino sector New ideas: CPT violation (excluded?), neutrino decay, mass varying neutrinos,... Discovery that one of the observed effects is not oscillations

11 MiniBooNE at Fermilab Purpose is to test LSND with: Higher energy Different beam x10 statistics Different oscillation signature Different systematics Antineutrino-capable beam 8 GeV primary proton beam from FNAL Booster L=500 meters, E=0.5 1 GeV: same L/E as LSND.

12 MiniBooNE Beamline BooNE Target and Horn LMC Booster 8 GeV protons 451 meters undisturbed earth Collimator Decay pipe 91 cm radius, 50 m long MiniB BooNE will test the LSND result with: Cartoon not to scale! Booster provides about 5 pulses per second, protons per 1.6 s pulse under optimum conditions

13 MiniBooNE horn Welding the inner conductor Horns provide pulsed toroidal magnetic field to focus pions Horn welding into the decay region. and assembly Assembled horn Beryllium target inside! This horn survived 96 million pulses -- a world record! -before failing in July New horn works well.

14 The BooNE neutrino detector Pure mineral oil 800 tons; 40 ft diameter Inner volume: 1280 PMTs Outer veto volume: 240 PMTs

15 Oscillation Signature at MiniBooNE Oscillation signature is charged-current quasielastic scattering: ν e + n e + p Backgrounds to oscillation: Intrinsic e in the beam π µ ν e in beam K + π 0 e ν e, KL 0 π0 e ± ν e in beam Particle µ decaysmisidentification to e, µ unobserved in detector µ mis-id as e, decay unseen π 0 produced in NC, decays to γγ, mis-id as e nγ branching ratio ~1%

16 Oscillation Analysis Steps to an oscillation result: Understand the flux Understand the detector: optical issues Particle Identification Expected statistics and sensitivity Progress

17 Understanding the neutrino flux Primary p-be interactions: from fit to external data including BNL E910. Soon, will include result from HARP at BooNE beam energy. K + from external data fit too, agrees well with MARS. Will incorporate internal measurements from dedicated beam monitor, studies of highest-energy neutrinos in detector. HARP data will be coming too. K 0 scaled from K + using GFLUKA Fraction of Flux / 0.1 GeV Flux e Flux E (GeV)

18 MiniBooNE optical issues Cherenkov light production Occurs when n >1 (n 1.47) Emitted promptly, in cone 1/ 2 wavelength distribution Scintillation light Emission from molecular excitations from ionization Emitted isotropically Several lifetimes, emission modes = nm Particles below Cherenkov threshold still scintillate Optical properties of oil, detectors must be well understood, including: Absorption Rayleigh and Raman scattering Fluorescence PMT response

19 MiniBooNE Particle ID Use ring shape, topology to identify particles: e Early PID efficiency studies based on neural nets; boosted decision trees appear to give better results: B. Roe et al., Nucl. Inst. Meth. A (2005)

20 Expected oscillation candidates at MiniBooNE Process All Events After Selection! " CC quasi-elastic 553,000 8! " NC! 0 110, If LSND correct Radiative # decay 1, Intrinsic! e 2, Oscillation Signal 1, Signal/Background 300/780=0.38 For protons-on-target NC 0 is dominant reducible background

21 MiniBooNE Sensitivity Expected limit curves assuming protons on target Sensitive to most of LSND at 5 sigma

22 MiniBooNE data collection progress Running since August 2002 Collected protons on target as of June 30. Scheduled to run through 2006 (next year may be in antineutrino mode)

23 MiniBooNE oscillation analysis progress Progress is being made on all aspects, including understanding of: Neutrino Flux Particle ID understanding and performance Optical model of detector is improving rapidly e appearance analysis is still blind Potential e candidates are hidden from detailed analysis Anticipate first oscillation result late this year

24 Non-Oscillation Physics Interesting non-oscillation results are coming out of these experiments MiniBooNE, K2K N scattering cross-section studies (new results!) Proton interaction measurements for neutrino flux predictions Deep Inelastic Scattering results are still coming out

25 Cross-section studies: necessary to understand oscillation analyses Event rates at 1 GeV: (Numbers are from MiniBooNE MC, but similar for K2K, and off-axis beams) n p 39% CCQE 25% CC + Used for oscillation analysis K2K paper; BooNE preliminary result N=nucleon 16% NC Elastic 7% NC 0 13% other Background to oscillation analysis Coherent, resonant combined

26 Pion production This represents a third of neutrino interactions at 1 GeV! Must understand it to characterize total event rates, backgrounds, etc. Two production modes for nuclear targets: Nucleon resonance: N N Coherent nuclear: A A Analogous neutral-current 0 modes too. D. Rein and L. M. Sehgal, Nucl. Phys. B (1983) is standard for describing cross-section, kinematics MiniBooNE, K2K making first measurements in this energy range on nuclear targets

27 Charged-current production N π + p+ N' p, n p, n Pre-2005 data set is small: no nuclear targets below 3 GeV proton target neutron target

28 K2K pion production K2K has measured the q 2 distribution of CC + production using the SciBar fine-grained scintillation detector at the near detector site. The q 2 distribution distinguishes nucleon resonance (high) from coherent nuclear pion production (low) Paper is available as hep-ex/

29 K2K fit to data All CC events (a) 1 Track Data CC coherent! CC 1!, DIS, NC CC QE q 2 (GeV/c) 2 rec (c) non-qe Proton q 2 (GeV/c) 2 rec (b) QE q 2 (GeV/c) 2 rec (d) non-qe Pion q 2 (GeV/c) 2 rec Remove events with evidence of extra proton Data CC Coherent pion CC1!,DIS,NC CC QE q 2 rec (GeV/c) 2 Final coherent candidates (q 2 rec<0.1 GeV 2 ): Data 113; Background 111 No evidence for coherent pion production! Rein-Seghal prediction for coherent pion production 2.67% of all CC K2K limit <0.60% at 90% CL

30 MiniBooNE Charged-current analysis Final state has a nucleon, muon, and pion. Generally, pion is sub-cherenkov threshold, so MiniBooNE fitter reconstructs only the muon. Both muon and leave stopped- decay electron signatures in detector This is an unusual enough signature that these events can be isolated well without particle ID. Primary event and decay electrons separated in time

31 MiniBooNE charged-current analysis N π + p+ N' p, n p, n Measure visible energy, lepton direction from fit to Cherenkov ring only (avoid scintillation light from pion, nucleon)

32 MiniBooNE charged-current analysis N π + p+ N' p, n p, n Use lepton energy, momentum and direction (from known neutrino beam) to reconstruct a quasielastic energy assuming a recoil mass: In this case, recoil mass is 1232 MeV ( mass), vs. proton mass for true quasielastics This method gives 20% resolution on neutrino energy (compared to 10% for CCQE)

33 MiniBooNE Chargedcurrent analysis Without cut efficiency corrections: measured N(CC )/N(CCQE) vs. E QE CCQE cut efficiency degrades at high E due to exiting CC cut off by kinematic threshold at low E Motivation for measuring (CC /CCQE) ratio: possibility of disappearance oscillations like branching ratio measurements, normalize to golden mode in our own data CCQE is the golden mode of low E n ss Efficiency corrected ratio measurement: estimate efficiency correction in MC systematic errors due to: cross sections (~15%), extinction and scattering length in oil (~20%), energy scale (~10%) Syst. err %, stat. err. 5-6%

34 MiniBooNE charged-current analysis Muon direction: Note deficit in small-angle (low q 2 ) region vs. Rein- Sehgal model. Coherent production is clearly lower than expected. (This is consistent with K2K). beam : high cos low q 2

35 Neutral current 0 production Major background for all e appearance searches including MiniBooNE, T2K. Measurements from MiniBooNE: Invariant mass reconstruction of candidates in neutral current data π 0 2γ Angular distribution of 0 with respect to the beam: note deficit at small angles (low q 2 ). Once again here, coherent production lower than expected.

36 Hadron Production Studies of pion, kaon production cross-sections by protons on a variety of targets Not actually neutrino experiments, but the results are critical for understanding neutrino experiments Recently, neutrino physicists have led these experimental and analysis efforts: BNL E910: MiniBooNE collaborators have analyzed 6, 12 GeV protons on Be; current flux fits depend heavily on this FNAL E907 (MIPP) current effort geared toward NuMI energies CERN PS214 (HARP): Major effort involving K2K, MiniBooNE

37 General purpose, wide-acceptance particle production detector Kinematics, PID by TPC, drift chambers, Cherenkov counters, and TOF HARP Took data at CERN PS in 2002, 2003 Targets included 5%, 100% interaction length Al and Be, as well as replica targets for MiniBooNE and K2K Pion production cross-sections for BooNE and K2K will be released imminently!

38 Neutrino Deep Inelastic Scattering NuTeV electroweak anomaly is still unresolved Deep Inelastic Scattering structure functions from NuTeV (see M. Tzanov, DIS2005 proceedings -- coming soon). No more beams available or projected for this kind of study, unfortunately. Atomic Parity Violation ν DIS Moller Scattering High Energy

39 Non-oscillation physics The Future MiniBooNE continuation, SciBar MINER A: Recently approved by FNAL for NuMI Possible reactor-based studies of electroweak anomaly (Braidwood) -- PRD 71: Oscillation physics MiniBooNE antineutrinos? Full BooNE -- if MiniBooNE has a signal SNS T2K/NuMI off-axis near detectors?

40 Summary Short baseline oscillation experiments are the main source of our knowledge of possible oscillations in the m 2 >10 2 ev 2 range. Rich recent history of these experiments Near future has many interesting results coming out: Looking forward to a conclusive resolution of LSND Many cross-section measurements are emerging, including some that are important for understanding long baseline experiments (Lots of opportunity for theoretical work on neutrino interactions!) Exciting (if a little uncertain) medium-term future for this sub-field