ATLAS and CMS have just presented the most precise measurement of the Higgs boson mass yet – mH = 125.09 ± 0.24 GeV (0.21 stat. ± 0.11 syst.) - it’s one of the most precise measurements ever performed at the LHC, and the first time that the two experiments have combined their results on the mass of the Higgs.
In the LHC’s proton-proton collisions, the Higgs boson decays into various different particles. For this measurement, results on the two decay channels that best reveal the mass of the Higgs boson (H->γγ and H->ZZ*->llll) have been examined. The ATLAS and CMS teams have sifted through data from approximately 4000 trillion proton-proton collisions collected at the LHC in 2011 and 2012. Each experiment has found a few hundred events in the H->γγ channel and a few tens in H->ZZ*->llll. The two collaborations worked together and reviewed the analyses and their combination.
"The discovery of the Higgs boson in 2012 was a fundamental breakthrough,” says Dave Newbold (CMS and Bristol), “and we've followed up on that by combining the ATLAS and CMS capabilities in this measurement. The upgraded LHC will allow us to go even further together later this year, and to explore the properties of the new particle in detail"
“ATLAS and CMS use different detector technologies and different detailed analyses to determine the Higgs mass,” adds ATLAS spokesperson Dave Charlton (Birmingham). “The measurements made by the experiments are quite consistent, and we have learnt a lot by working together, which stands us in good stead for further combinations.”
The Standard Model does not predict the mass of the Higgs boson itself and therefore it must be measured experimentally. However, once supplied with a Higgs mass, the Standard Model does make predictions for all the other properties of the Higgs boson, and these can then be tested by the experiments. This mass combination represents the first step towards a combination of other measurements of Higgs boson properties, which will involve all other decays.
Up to now, increasingly precise measurements from the two experiments have established that all observed properties of the Higgs boson, including its spin, parity and interactions with other particles are consistent with the Standard Model Higgs boson. With the upcoming combination of other Run 1 Higgs results from the two experiments, and with higher energy and more collisions to come during LHC Run 2, physicists expect to improve the precision of the Higgs boson mass and explore in more detail the particle’s properties.
There’s much more to being a CERN Member state than simply turning up at Council meetings. We meet the UK chair of the CERN Finance Committee to find out more.
Charlotte Jamieson is STFC’s Programme Manager, looking after the UK’s strategic relationship with CERN and representing the views of experiment groups. But like her opposite numbers in the other Member States, Charlotte also contributes to the committees that feed into CERN Council; she is currently chairing the CERN Finance Committee and audit committee.
With an annual CERN budget of approximately 1 billion CHF, Charlotte and her committee colleagues have to balance the ambitious ideas of an enthusiastic community of engineers and researchers with the economic pressures facing the Member States. That’s a situation that was thrown sharply into focus when Switzerland ‘unpegged’ it’s currency from the euro.
To Chair the Finance Committee, “you don’t need to be an accountant,” says Charlotte, “but you do need experience of looking at financial information, and be able to ask a good question. It’s important to see the finances from every angle – not just CERN but also the Member States - and recognise the pressure that different Member States might be under at different times.”
Diplomacy is an essential skill. As Chair of Finance, Charlotte has earned considerable respect, and her social skills are in constant play to win around the views of the other member states.
“Chairing committees helps the UK influence strategic decision-making,” she explains. “You need to be objective and at the same time do the seemingly impossible by pleasing everyone. It’s important for the UK to volunteer to do their bit and take on additional responsibilities in the running of CERN.”
The job is definitely varied, “one day I can be talking about superconducting magnets, and pension funds the next!”
Chairing the Finance Committee contrasts with Charlotte’s previous role as deputy chair of TREF, CERN’s Tripartite Employment Conditions Forum. As well as reviewing changes to the pay and conditions for CERN staff, she was particularly interested in the committee’s discussion in improving gender diversity at CERN.
“Women are represented at every level in CERN, and it’s very exciting that the next DG at CERN will be a woman, but there’s still room for improvement.”
It’s clear that Charlotte has an ongoing personal as well as professional interest in CERN’s future.
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