Cosmic ray consists of high energy particles that mostly originate from outer space, with some very high energy particles even thought to have an extragalactic origin. Primary cosmic rays consist of 90% protons, 9% alpha particles and of other heavier nuclei [1] . This primary cosmic rays interact with the gas molecules in the atmosphere and produce secondary cosmic rays. These secondary particles consist mostly of pions and some kaons. Neutral pions (π 0 ) decay into gamma rays that generate electromagnetic showers (e + , e − , γ), which posses low penetration power. Charged pions (π + , π − ) decay into muons and neutrinos. Neutrinos have a very small cross-section for interaction and typically pass through the earth without any further interactions. On the other hand, muons are heavy particles and thus loss of energy through bremsstrahlung is negligible for them. This makes the muon a very penetrating particle, unlike electron. The muon has a lifetime of 2.2 µs yet it still makes it down to detectors at the surface of the earth traversing through the atmosphere. This is because the muon travels at a speed that is close to that of light and thus experiences relativistic time dilation and therefore can be detected by our detectors. Since the secondary cosmic rays are mostly muons and they can travel large distance through the atmosphere before they are detected, it will be really interesting if any correlation of this cosmic ray muon flux with the change in atmosphere in terms of the concentration of air pollutants is found [2, 3, 4] .
For this study, cosmic ray flux has been measured in our laboratory using plastic scintillator detectors before and after the imposition of lockdown due to the COVID19 pandemic. The effects of atmospheric pressure and temperature on the muon flux has also been studied here. A brief description of the change of atmospheric parameters due to lockdown is discussed in the next section.
The succeeding sections consist the details of the experimental setup, results followed by summary and discussions.
India is at a critical stage in its fight against COVID19 with positive cases crossing 71,73,565 and death toll at 1,09,894 until October 12, 2020 [5]. The entire country was under complete lockdown from March 25 to April 14, 2020, for 21 days, which was further extended by the Government of India until May 3, 2020, followed by the third phase of lockdown till May 17, 2020, and the fourth phase till May 31, 2020, to tackle the spread of COVID19. Restrictions on social gathering and travelling resulted in the shutdown of all the businesses which include industries, transport (air, water, and surface), markets, shops, tourism, construction and demolition, hotels and restaurants, mining and quarrying, etc. except the essential services like groceries, milk, medicines and emergency services like hospital, fire service and administration. In June 2020 Governments, both state and central, declared restricted unlocking phase. While Unlock phases were started from 1 st June 2020, there were complete lockdown in West Bengal on some selected dates to combat against COVID19. The dates of lockdown are mentioned in Fig. 1 increased by ∼17% due to the clearer atmosphere as compared to that before lockdown period [2] . We also looked into the last year data for the same period (March-April, 2019) and found that, during the lockdown, the concentration of air pollutants are decreased by ∼27% (PM 2.5 ), ∼32% (PM 10 ), ∼66% (NO 2 ), ∼16% (CO) and O 3 concentration is increased by ∼87% [2] .
We have used the live day to day data from Ref [6] of the concentrations of the seven major air pollutants and studied the effects of them on cosmic ray flux. In our work, we have reported the measured muon fluxes before and after the lockdown at Kolkata and tried to correlate the same with the change in the concentrations not only of the individual components of air pollutants but also the total amount of pollutants.
The schematic of the muon flux measurement setup is shown in Fig. 2 . Three plastic scintillators tagged as SC1, SC2, and SC3, made by using BC400 material are used in this setup [7, 8] . The The width of each discriminator output is kept at 50 ns. The coincidence of these three signals is achieved using a logic unit. The three-fold coincidence signal is then counted using a scaler and then divided by the product of the area of coincidence window (20 cm 2 ), muon detection efficiency of the system (∼72%) [8] and the measurement time to get the muon flux. Each data point represents a 30 minutes long measurement. To check the health of the individual detectors, the singles count rate of all the modules is measured several times. It is found that the singles count rate of scintillators SC1, SC2, and SC3 are found to be ∼77, ∼28, and ∼171 Hz respectively.
The cosmic ray flux is measured in Kolkata before and after the lockdown due to the COVID19 outbreak. The average muon flux before and after the lockdown is shown in Fig 3 as a function of time. We had a very small amount of data before lockdown. The cosmic ray flux has a dependence on atmospheric parameters like temperature and pressure [9, 10] . In this work, the temperature and pressure data are collected from Ref [11] . The ratio of temperature and pressure as a function of day is also shown in Fig 3. In order to normalise the temperature (T = t + 273 ) and pressure (p) A positive correlation is observed between the muon flux and T /p. Using the parameters, the muon flux measured before and after the lockdown is normalised and shown in Fig 5. In Fig. 6 , the distribution of the normalised muon flux is shown before and after the lockdown.
It is found that the mean normalised muon flux before the lockdown period is 0.996 with a standard In Fig. 7 , we have reported the individual air pollutants (average air quality index (AQI)) before and after the lockdown (on the dates of cosmic ray data recording) and a clear decrement in the concentrations of the air pollutants have been observed after lockdown. One striking thing we observe during this study, is that the concentration of O 3 also decreases after lockdown in our case unlike at Ref [2] where the concentration of O 3 was reported to be increased during lockdown (for a short time period though). Table 1 below.
T /p normalised muon flux is also plotted as a function of the average air quality index for the Figure 9 : (Colour online) Normalised muon flux as a function of the air quality index (AQI) of 7 most abundant air pollutants. seven air pollutants individually and with the gross pollutants present in Fig. 9 . Here also we observed that the normalised flux increases with decrease in the concentrations of the air pollutants.
Air pollutant type p0 p1 PM 2.5
1.23 ± 0.003328 -0.0002962 ± 5.312e-05 PM 10 1.235 ± 0.003955 -0.0003795 ± 6.645e-05 NO 2 1.232 ± 0.003666 -0.00139 ± 0.0002634 NH 3 1.243 ± 0.006047 -0.007628 ± 0.001701 SO 2 1.236 ± 0.004197 -0.001255 ± 0.0002339 CO 1.243 ± 0.006191 -0.001622 ± 0.0003632 O 3 1.226 ± 0.004799 -0.0001535 ± 7.864e-05 Gross 1.24 ± 0.004969 -0.0001563 ± 3.044e-05 Table 1 : Value of the fit parameters of the muon flux vs air quality index (AQI) curve with 7 most abundant air pollutants.
Cosmic ray muon flux is measured using the coincidence technique with plastic scintillation detectors.
To restrict the outbreak of COVID19, Government of India imposed 67 days of nationwide complete lockdown in three phases. After that, the unlocking was declared in phases in different parts of India.
Before the lockdown, we collected some cosmic ray flux data. After lockdown the measurement is continued to compare with the flux as measured before lockdown. In our measurement, it is found that the cosmic ray flux remained more or less unchanged before and after the lockdown. However it is well known that there is an effect of atmospheric temperature and pressure on the cosmic ray flux and we also looked for any such possible correlation. A positive correlation is indeed observed between the muon flux and the ratio of atmospheric temperature and pressure. This correlation is fitted well by a function of the form p0+p1(T/p), and the fit parameters p0 and p1 are used to normalise the T/p effect on the cosmic muon flux. It is found that the mean normalised muon flux before and after the lockdown period are 0.996 with a standard deviation of 0.029 and 1.001 with a standard deviation of 0.012 respectively.
A lockdown such as the one implemented due to the COVID19 typically has significant influence on the atmospheric condition in terms of presence of pollutants. We wanted to study any possible correlation of measured cosmic ray muon flux with this. To realise this, we considered the seven most abundant air pollutants (PM 2.5 , PM 10 , NO 2 , NH 3 , SO 2 , CO, O 3 ) and investigated the change in their concentrations with time (before and after lockdown). We found significant declination in the concentrations of the pollutants and we tried to look for any correlation with the measured muon flux within the stipulated time window. The result shows a clear correlation as with decreasing concentrations of the air pollutants we observed an increasing trend of the normalised muon flux.
From our observation, one can comment that the increase in cosmic ray flux can also be considered as one of the secondary indicators of less polluted air.
However, there are a few limitations of our measurement. First, the detector coverage area is very small, resulting in low statistics. Second, the statistics of muon data before lockdown is small.
It will be very interesting if any other research laboratory having a large facility of cosmic ray flux measurement can try to study such correlation.