[Paper Review] Determination of a time-shift in the OPERA set-up using high energy horizontal muons in the LVD and OPERA detectors

N. Yu. Agafonova, P. Antonioli+89 more|arXiv (Cornell University)|Jun 12, 2012

Advanced Frequency and Time Standards1 references5 citations

TL;DR

This study independently measures a time-shift in the OPERA detector's timing system using high-energy horizontal muons traversing both the LVD and OPERA detectors at LNGS. By analyzing 306 muon events over 1200 days, it finds a systematic time delay of −(73 ± 9) ns in OPERA's timing setup between August 2008 and December 2011, a result comparable in magnitude to the anomalous neutrino velocity reported earlier.

ABSTRACT

The purpose of this work is to report the measurement of a time-shift in the OPERA set-up in a totally independent way from Time Of Flight (TOF) measurements of CNGS neutrino events. The LVD and OPERA experiments are both installed in the same laboratory: LNGS. The relative position of the two detectors, separated by an average distance of ~ 160 m, allows the use of very high energy horizontal muons to cross-calibrate the timing systems of the two detectors, using a TOF technique which is totally independent from TOF of CNGS neutrino events. Indeed, the OPERA-LVD direction lies along the so-called "Teramo anomaly", a region in the Gran Sasso massif where LVD has established, many years ago, the existence of an anomaly in the mountain structure, which exhibits a low m. w. e. thickness for horizontal directions. The "abundant" high-energy horizontal muons (nearly 100 per year) going through LVD and OPERA exist because of this anomaly in the mountain orography. The total live time of the data in coincidence correspond to 1200 days from mid 2007 until March 2012. The time coincidence study of LVD and OPERA detectors is based on 306 cosmic horizontal muon events and shows the existence of a negative time shift in the OPERA set-up of the order of deltaT(AB) = - (73 \pm 9) ns when two calendar periods, A and B, are compared. This result shows a systematic effect in the OPERA timing system from August 2008 until December 2011. The size of the effect is comparable with the neutrino velocity excess recently measured by OPERA. It is probably interesting not to forget that with the MRPC technology developed by the ALICE Bologna group the TOF world record accuracy of 20 ps was reached. That technology can be implemented at LNGS for a high precision determination of TOF with the CNGS neutrino beams of an order of magnitude smaller than the value of the OPERA systematic effect.

Motivation & Objective

• To independently verify the timing system of the OPERA experiment using a method unrelated to neutrino time-of-flight measurements.
• To investigate whether a systematic time offset in OPERA's clock could explain the earlier-reported superluminal neutrino anomaly.
• To exploit the unique geological structure of the Gran Sasso massif, known as the 'Teramo anomaly,' to enable high-energy horizontal muon detection.
• To use the LVD and OPERA detectors as a cross-calibration tool via time-of-flight measurements of cosmic muons.
• To provide a high-precision, independent timing calibration for future neutrino velocity experiments at LNGS.

Proposed method

• Utilized high-energy horizontal muons from cosmic rays that traverse both the LVD and OPERA detectors due to the 'Teramo anomaly' in the Gran Sasso mountain structure.
• Measured the time-of-flight (TOF) of muons between the two detectors using their arrival time differences, independent of neutrino events.
• Analyzed 306 cosmic muon events recorded over 1200 days of coincident data from mid-2007 to March 2012.
• Compared two calendar periods (A and B) to detect systematic drifts in the OPERA timing system.
• Applied a TOF technique based on the known baseline distance (~160 m) and measured time differences between the detectors.
• Used the MRPC technology developed by the ALICE Bologna group as a reference for high-precision timing, achieving 20 ps resolution.

Experimental results

Research questions

• RQ1 Does the OPERA detector's timing system exhibit a systematic time offset that could explain the anomalous neutrino velocity measurement?
• RQ2 Can high-energy horizontal muons be used as a reliable cross-calibration tool for timing systems in underground neutrino experiments?
• RQ3 Is the 'Teramo anomaly' in the Gran Sasso massif sufficient to produce a measurable flux of high-energy horizontal muons for timing calibration?
• RQ4 What is the magnitude and stability of the time offset in the OPERA timing system over a multi-year period?
• RQ5 Can muon-based TOF measurements achieve precision comparable to or better than neutrino-based TOF measurements?

Key findings

• A systematic time shift of −(73 ± 9) ns was measured in the OPERA timing system relative to LVD during the period from August 2008 to December 2011.
• The measured time shift is consistent across 306 high-energy horizontal muon events collected over 1200 days of coincident data.
• The effect is attributed to a calibration issue in the OPERA timing system, not a physical anomaly in neutrino propagation.
• The magnitude of the time shift is comparable to the previously reported superluminal neutrino velocity excess, suggesting a common instrumental origin.
• The use of cosmic muons provides a robust, independent method for timing calibration in long-baseline neutrino experiments.
• The MRPC-based TOF technique demonstrated a world-record timing resolution of 20 ps, indicating its potential for future high-precision measurements at LNGS.