This result is not a sprint, it is a marathon. How the teams behind the measurement of the W-boson mass at the CMS experiment found working on this result, almost a decade in the making.
Precision measurements like that of the mass of the W boson are some of the hardest things to do at the LHC. The teams behind the W boson mass result at the CMS experiment worked for years – some almost a decade – to come to this result.
“We’ve put a lot of time into this. And it’s an incredible investment emotionally, scientifically, personally,” Kenneth Long tells us, a postdoctoral researcher for MIT who has been working at CERN for around 8 years now.
Even more unique is the fact that for most of the time the team worked on the measurement, they did not know what result they were going to get. The whole analysis was ‘blinded’, meaning that they worked on the inputs without seeing the real outputs until the very end.
The reason for keeping everything blinded is to remove any bias an individual might have: if the result seems just slightly off what they might expect, they could unintentionally shift some of the inputs or complex parameters to skew the result. Even small modifications could drastically change the output.
“First and foremost, you want to get the right answer, you want a result that will stand the test of time”, emphasises Kenneth.
But working on a blinded study comes with challenges: true, the result may not be biased, but some potential issues may slip through the net and only be visible at the very end.
“This was not the case,” explains Lorenzo Bianchini, who leads a group of researchers on the study in Italy. “Not just for the number, the final result, which came out consistent with the Standard Model, but also all the rest. That everything else came out very consistent with our expectations – I think this is what gives robustness to our result.”
We spoke to some of the researchers before the ‘unblinding’, when the final result is revealed after all the tests and checks, and asked them what they thought the outcome would be:
“You can’t really bet against the Standard Model,” laughs Kenneth. “It would be pretty exciting for the result to be in agreement with the Standard Model because in that case, we would have pretty good confidence that we did it – that we didn’t screw up.”
“If the result is consistent with the Standard Model then ... this is a spectacular validation of it,” says Josh Bendavid, a research scientist with MIT who has been working on the measurement while based at CERN, mainly focusing on the precise calibrations.
But there was also the chance that the result was not consistent with the Standard Model. Two years ago, another experiment, CDF, published such a result – something that no other experiment had observed before or since. CMS’ result has been much anticipated to see which way it would go.
“My dream is that we get the same result as CDF because it means that we finally found the culprit of our partial understanding of the universe,” says Elisabetta Manca, a postdoctoral researcher at UCLA.
Our current understanding of the universe doesn’t explain many phenomena such as dark matter, dark energy, and gravity to name a few. Probing the fundamental parameters of our best theories is one way of trying to see if there are any differences between what we calculate in theory and what we actually see experimentally. Every result that confirms the theory makes us more confident we are heading in the right direction. But if we do find those differences, then it could indicate that a gap has been identified where new physics might be lurking.
However, to do these kinds of measurements means working to such incredible precision that it presents all sorts of challenges for the teams.
“You can imagine what pressure we feel, both from the community and inside, from CMS, to deliver something... We have to resist many temptations to cut corners, do things quickly. And we’ve been very careful in scrutinising and understanding every single detail of the analysis,” explains Lorenzo.
The delicacy of working on something with such high precision is in itself a draw for the teams.
“To work on the measurement of the W mass is quite exciting to me because here we can reach a precision that is beyond what was achieved in other lab experiments [at hadron colliders]. This is one of the cases where, well, precision is the best! We can currently measure the parameters of the Standard Model with the best precision in the world. And that’s what makes it exciting for me!” explains David Walter, who joined the team as a CERN research fellow two years ago.
But for some people, the journey has been a lot longer
“Some of us have invested literally years of our lives into this. So yes, it will be a huge sense of accomplishment as well,” says Josh. “Many, many years of many, many late nights!”
"The really difficult part is to keep a group of people motivated enough to pursue this goal for many, many years,” says Elisabetta.
The group is made up of people collaborating from different universities. In fact, Elisabetta herself started while she was a student in Italy during her undergraduate degree, and is now with UCLA.
With a team spanning half the globe, most of the work is done from different locations, with the exception of some retreats in order to bring people together and to work on their code.
“It really helps in terms of group bonding. It has been a real challenge sociologically and psychologically: learning how to work together without even having a common boss because we all work together and everybody counts as one,” explains Elisabetta.
“We don’t have a management structure comparable to industry. ... It’s very collaborative, which has some big benefits and also some challenges when you’re trying to hit deadlines etc.” adds Kenneth.
The flat management structure is important to the way the teams work, especially considering the spread of people across different time zones. “We work with teams from Italy and from the US, from California and Fermilab and MIT for example,” explains David, who is based in Switzerland at CERN, “the timezones are quite different, but we sometimes overlap. Our weekly meeting is at 5pm (Central European Time) because of California – for them it is 9 in the morning – and after that we go home but they start their routine day.”
“I felt extremely relieved,” says Lorenzo after the final result was revealed, “happy and relieved! We should definitely celebrate and have a toast!”
Links to more information:
- For all resources on the W boson and this CMS measurement
- For other CMS physics highlights