OPERA Neutrino Experiment

Tija Sīle

http://arxiv.org/abs/1109.4897This presentation is based on:

Doktorantūras skolas “Atomāro un nepārtrauktās vides fizikālo procesu pētīšanas, modelēšanas un matemātisko

metožu pilnveidošanas skola” seminārs

28. 09. 2011

Outline• Introduction• Experimental setup• Experimental data and analysis• Previous experiments concerning

neutrinos

Introduction•In September 2011 the OPERA collaboration announced that their experimental results show that neutrinos move with a velocity that exceeds the speed of light.• A neutrino beam produced by a particle accelerator at CERN moves through Earth’s crust and is detected at Gran Sasso Laboratory• The beam was measured at its creation and detection using synchronized clocks• Comparison of those events allows one to calculate the Time of Flight (TOF)

IntroductionFrom the preprint: “Despite the large significance of the measurement

reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results. “

The paper has ~ 150 authors that represent ~ 40 institutions

The measured deviation from the speed of light is:60 ns ± 14 ns for the total time of flight = 243 µs(v-c)/c = (2.5 ±0.6) ·10-5

What is neutrino?• Neutrino is a lepton (like electrons)• Neutrinos have a property of “flavour” – there are electron,

muon and tau neutrinos • Neutrinos can change their flavour – so called “neutrino

oscillations”• Neutrinos does not participate in strong or EM interaction• From the neutrino oscillations follows that neutrinos have

mass• The precise mass of neutrino is still unknown

How to create a neutrino beam?

1. High energy protons are produced using Super Proton Synchrotron2. Protons collide with a graphite target producing pions and kaons

3. Magnetic horns focus particles with an energy of 35 GeV4. Pions and kaons decay into muons and neutrino

5. A stopper absorbs everything that is not muons or neutrinos (protons and pions/kaons that have not decayed)

6. Muons are measured after the hadron stop and afterwards are absorbed by Earth’s crust within a kilometre

How to create a neutrino beam?

How to detect a neutrino beam?

• Neutrinos can be detected by their interaction with other particles

• Muon neutrino interacts with an electron producing muon and electron neutrino

• Muons are detected by the products of their decay• The experimental system is optimized to detect neutrino

oscillations

How to measure velocity?

The measurement of distance is straightforward: GPS + geodesyAccounts for ~ 10 % of the total uncertainty in measurements

The measurement of time accounts for the most of the uncertainty. At this experiment it is not possible to calculate the time of flight for a single neutrino, because a neutrino can be produced anytime in the 10.5 μs extraction

How to measure velocity?Distance

• Very precise measurements using GPS

• The distance is ~ 730 km with an uncertainty of 20 cm

• Movement of the Earth’s crust can be seen in the measurements

How to measure velocity?Time

• There are two atomic clocks – at the beam generator and at the detector. Both clocks are synchronized using GPS (“common view”)

• The structure of the proton beam is measured using coaxial transformator 700 m before the proton beam hits the target

• The neutrino beam is measured at the detector

• The time of flight is calculated comparing the time distribution of proton beam with the time distribution of the neutrino beam

Atomic clockThe proton beam is measured

Atomic clockNeutrinos are detected

The analysis of experimental data• The protons are ejected from SPS in two extractions. Each extraction is

10 μs long. The extractions are separated by 50 ms.• The structure of the proton beam is different in each extraction

therefore they are analyzed separately.

Summed proton waveforms for each extraction

1 2 1 2

6s

10 μs 50 ms

The analysis of experimental data

• There are many factors that can influence the measurement of the time of flight – delays in the experimental devices

• Many of these factors were not known at the beginning of the experiments in 2006

• In order to eliminate bias, first a blind analysis was carried out – using the information about the experimental conditions in 2006

• After the blind analysis corrections were introduced

The analysis of experimental results

• Maximal likelihood function provides the probability of observing given experimental data as a function of a parameter

• There is a single parameter to be determined – time difference δt between the time of flight of for neutrinos TOFν and for photons TOFc

Likelihood distributions for both extractions, fitted with parabolic shape

The analysis of experimental resultsFinal analysis

After the blind analysis is done, the result is corrected taking into account more precise calibrations of experimental devices

Experimental resultsThe maximal likelihood analysis gives the value of δt (blind)

The correction δtcor

That gives the final result

Experimental results

The comparison betweeen proton and electron beam distributions. Zoom of the leading and trailing edges.

Experimental resultsStability of the results

In order to find parameters that could influence the result, the data was divided into subsets • over time• day and night Δ = 17.1 ± 15.5 ns• spring and fall Δ = 11.3 ± 14.5 ns

No significant difference was found.

Experimental resultsEnergy dependence

• In charged current interactions it is possible to calculate the energy of the neutrino• The data from the CC interactions were separated and divided into two bins – with mean energies of 13.9 and 42.9 GeV• No significant difference in neutrino velocity was observed

Previous experiments concerning neutrino velocity

Experiment Year Energy |v-c|/c

OPERA 2009-2011 17 GeV (2.48 ± 0.28 ±0.30 ) 10∙ -5

Fermilab 1976 > 30 GeV <4 10∙ -5

MINOS 2007 ~3 GeV (tail above 100 GeV)

(5.1 ± 2.9) 10∙ -5

Supernova 1987a 1987 10 MeV < 2 10∙ -9

Supernova 1987a The neutrinos arrived to Earth three hours before photons