A new proton spill from CERN to Gran Sasso
Since 21 October, CERN has been sending a new type of neutrino beam to Gran Sasso. The new configuration is intended to allow the experiments to define the departure time of the neutrinos more accurately and thus check the previous results obtained using the nominal beam configuration.
The CERN Neutrino to Gran Sasso (CNGS) beam no longer operates using the standard beam time structure. Instead, a new type of proton pulse is being produced by CERN’s accelerators and sent to the graphite target to generate neutrinos. “We are now producing extremely short beam pulses,” explains Edda Gschwendtner, the physicist in charge of the CNGS secondary beam. “During a CNGS cycle we now have a LHC type bunched beam with four bunches, each about 2 ns long. Each bunch contains more than 2.5 x 1011 protons; bunches are spaced by 500 ns. In total, this makes about 1012 protons on target for each extraction from the SPS.”
The CNGS beam was originally designed to maximize the number of muon neutrinos produced and sent to Gran Sasso. This was done to increase the probability of observing the “oscillations” as muon neutrinos turn into tau neutrinos. However, after the recent measurement of the neutrino’s speed, it has become important for experiments downstream to receive shorter beam pulses to allow for a more precise definition of the departure time. “The nominal CNGS beam cycle has two pulses each, 10.5 μs long and has a total of about 4x1013 protons on target per cycle. In the present configuration, we have a factor of 40 fewer protons per cycle, but in 4 much shorter pulses. Thanks to a huge effort from many experts in the accelerator sector and to experience acquired with the LHC beam, it was possible to make this special proton beam available and to send it to the CNGS target,” says Edda.
“This beam configuration allows experiments to connect each single neutrino event to the corresponding pulse of protons that generated it. In this way, one can obtain the statistical significance of the result, which is comparable to the one obtained by OPERA, using two orders of magnitude fewer events,” says Sergio Bertolucci, CERN Director for Research and Computing. “Moreover, several if not all the possible details of neutrino production during the spill – such as energy distribution, production rate, etc. – become irrelevant. This allows experiments to remove an important source of systematic error.”
The experimental teams at Gran Sasso are accumulating data using the beam with the short proton spills. This 2011 technical run will make it possible, in the short time available, to test this new operational mode. This will also help to assess the benefits of a dedicated run with such short proton spills in 2012, when all the four Gran Sasso experiments – BOREXINO, ICARUS, LVD, OPERA – will be in a position to perform independent measurements of the neutrino’s time of flight.
The CNGS beam will be kept in this mode of operation until 7 November. After that, there will be a technical stop until 11 November.
by CERN Bulletin