higgs - page 5

The Physics Teacher
Vol. 50, S
sentations focused on specific analyses and many presenta-
tions occurred simultaneously), followed by a couple of days
of plenary meetings (which were overview talks with no other
simultaneous presentations). This format guaranteed that the
Higgs boson plenary session would be anticlimactic because
the measurements would have been already announced.
In addition, important experimental particle physics
results are generally announced in a seminar at the hosting
laboratory before being released more generally. In order to
accommodate these various considerations, the CERNman-
agement decided to have a seminar at 9:00 in the morning on
July 4th. This would kick off the ICHEP conference but had
the unfortunate consequence of occurring on a major holiday
at 3:00 a.m. in the Eastern U.S. The CMS and ATLAS Higgs
results were described first in a joint seminar, followed by a
press conference that included the CERN director and leaders
400-500 physicists, while each of the LHC collaborations in-
volve on the order of 3000 physicists. With those numbers of
personnel, every conceivable effect was checked and double
checked. Peer review has an entirely different meaning under
these conditions.
When the LHC resumed operations in March of 2012 (af-
ter the scheduled winter shutdown), the energy of the accel-
erator was raised to 8 TeV. This increase resulted in a 20-30%
increase in the Higgs production cross section. Further, the
LHC is running superbly. By late June of 2012, the LHC had
delivered as much integrated luminosity to the experiments
as it had in all of 2011. With June came the moment when the
LHC’s accumulated luminosity surpassed the Tevatron’s inte-
grated luminosity that took a decade to record.
In order to make the best measurement possible, the LHC
experimenters deliberately blinded themselves to data in
those mass regions in which the Higgs boson could still exist.
Because there were those earlier hints of an excess at
125 GeV, we didn’t want to bias our event selection criteria
to either amplify or suppress a similar excess in the 2012 col-
lision data. It was equally important that both the CMS and
ATLAS experiments work independently. If the data taken in
both experiments tell the same story, we are much more likely
to believe that something has been discovered.
The initial unblinding for the data in both experiments
was done in mid-June. For this first look at the new data, all
of the 2011 data and about half of the 2012 data taken to that
point were included. It was at this time that the blogosphere
started heating up. To understand precisely what was be-
ing observed, we must define the term Standard Model Lite
(SML), which is the Standard Model but with the Higgs boson
removed from the theory. According to the blogs, both exper-
iments were seeing more events in which two photons or two
Z bosons were produced than could be accounted for by SML.
In addition, the blogs reported that the 2011 and 2012 data
were telling a common story. Neither experiment confirmed
the rumors, as there were cross-checks that remained to be
done. Further, both experiments needed to include the entire
set of data collected in 2012, a step that was not performed
until the last days of June.
On July 2, the Tevatron experimenters (DZero and CDF)
announced in a seminar held at Fermilab our nearly final
results in our searches for the Higgs boson. By combining
dozens of analyses and both experiments’ data sets, we found
an excess of 2.9 sigma, indicating the existence of a particle
not predicted by the Standard Model Lite. The mass range for
this possible particle was in the range of 115–135 GeV. This
announcement was a delightful aperitif for the LHC results
released but two days later.
Both LHC experimental teams (ATLAS and CMS) expect-
ed to present their measurements at the International Confer-
ence on High Energy Physics (ICHEP) held in Melbourne
this year. This particular conference was organized so that the
first few days were parallel meetings (meaning that the pre-
Fig. 8. This data distribution shows the ATLAS collaboration’s
study of diphoton production. The small bump at a mass of
near 125 GeV indicates the observation of a new boson that is
consistent with being the Higgs boson. (Figure courtesy of CERN
and the ATLAS collaboration.)
Fig. 7. The announcement of evidence of a new boson was fol-
lowed by a press conference, attended by international media.
L to r: Fabiola Gianotti (leader of ATLAS), Rolf-Dieter Heuer
(CERN director), and Joe Incandela (leader of CMS). (Figure
courtesy of CERN.)
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