Higgs10: The dramatic last year of CERN’s flagship LEP collider
By: Luciano Maiani& Roger Cashmore
31 May, 2022 · Voir en français
The year 2000 was set to be the last year of running for CERN’s Large Electron–Positron (LEP) collider, and it ended in dramatic fashion. Luciano Maiani was Director-General and Roger Cashmore Research Director as the new millennium dawned.
Roger Cashmore :
The final year of LEP operation, 2000, had been agreed on at CERN by all of the relevant committees. By this time, the LEP experiments – ALEPH, DELPHI, L3 and OPAL – had established the Standard Model of particle physics with great precision. LEP had achieved its mission, and the only thing missing from the Standard Model was the elusive Higgs particle. Nobody knew whether the Higgs was within LEP’s reach, but detailed analysis suggested that its mass might be not much more than 100 GeV and that it would be produced in electron–positron collisions in association with a Z particle. In other words, the LEP experiments might have a chance of crowning their achievements with a spectacular discovery to start the new millennium.
There was nothing to lose and, as the 2000 run got underway, the machine was pushed to its limits. A cut-off date of 1 September had been set, and a closing celebration planned for the following month. Throughout the year, regular reports were made to the LEP Experiments Committee (LEPC), but there was no sign of a Higgs up to a mass of about 110 GeV. The decision was taken to push the beam energy beyond the limits through July and August: at this stage, if something broke, it really didn’t matter. And that was when the situation became exciting. A small excess of events was observed by the ALEPH experiment at a mass of about 114 GeV, but with no supporting evidence from the other experiments. Nevertheless, I telephoned Luciano to keep him informed that we might have an exciting time on our hands, and potentially a very difficult one! As a result of the ALEPH candidates, LEP’s final run was extended through to the end of October.
Luciano Maiani :
I remember Roger’s call like it was yesterday. Whatever happened next was going to require some difficult decisions. In October, we celebrated the conclusion of the LEP programme in the presence of eminent representatives of the Member States, even though the machine was still running. ALEPH’s excess was still there so, after the speeches were done, we discreetly started to work out the cost of running LEP for another year, and the repercussions it would have on the construction of the LHC.
The problem was that LHC excavations would soon reach the LEP tunnel, so an extra year of running would mean that work would have to stop, contracts be terminated and penalties paid to the companies involved, not to mention the extra running costs that had not been budgeted for. In total, we worked out that it would cost some 120 MCHF, and deal a major psychological blow to the LHC community. We had no way of anticipating how the LHC experiments’ funding agencies would react to the news of a year’s delay.
As October progressed, the other LEP experiments did not see anything, and ALEPH did not find any more candidates. LEP’s illustrious career seemed to be coming to an uneventful end, but there was to be one final twist: towards the end of month, the L3 experiment announced an event that seemed to change everything. It was a two-jet event. Each jet contained a b quark, and there was missing energy corresponding to the mass of a Z particle. Significantly, the jets had the fateful energy of around 114 GeV.
L3’s event could be interpreted as the production of the same particle that ALEPH seemed to see decaying into a b-anti-b quark pair, with the accompanying Z decaying into two invisible neutrinos. In short, it could be another trace of the existence of the Higgs boson.
We discussed the L3 event thoroughly with LEPC Chair Michel Spiro and concluded that it was inconclusive. It could be a Higgs, but it could equally well be something much more mundane: there was no imbalance in transverse energy as there had been in the 1980s when Carlo Rubbia had announced the discovery of the Z boson. Without that, the missing energy could have been lost down the beam pipes and so gone undetected and, importantly, there were well-known electromagnetic processes that would produce just such an outcome.
The L3 event was not a smoking gun after all, and we were left at the end of the month with a very difficult decision to take. Whatever we decided, some part of the community would be disappointed. Events proceeded quickly. On 3 November, LEPC delivered its verdict: not conclusive. Similar verdicts were then delivered by the Research Board and the Scientific Policy Committee (SPC). The decision was left to us and, along with Roger and the whole Directorate, we made our decision. For us, LEP was over; the LHC was the best machine to tell us whether there was a Higgs at 114 GeV, or whether LEP had been chasing phantoms.
By 4 November I had already written to George Kalmus, the Chair of the SPC. “The idea that we may find ourselves in September 2001 with 3.5–4 sigma, CERN’s financial position aggravated, LHC delayed and LHC people disbanded is not very encouraging. I am not going to go this way.” On 17 November, we recommended no additional year of LEP running to the Committee of Council. Faced with the alternative of betting 120 MCHF on the roulette wheel of a few anomalous events, the Council wisely accepted our advice.
LEP’s final year had been an emotionally charged rollercoaster ride. The lights never went out at CERN as analyses were refined around the clock and, when our decision became known, it was greeted with relief, shock and disbelief in equal measure. At the end of 2000, the Council’s decision moved us firmly into the LHC era, ready to fully explore the Higgs and much more.