Latest Results from ATLAS Higgs Search
4 July 2012
On 4 July, 2012, the ATLAS experiment presented a preview of its updated results on the search for the Higgs Boson. The results were shown at a seminar held jointly at CERN and via video link at ICHEP, the International Conference for High Energy Physics in Melbourne, Australia, where detailed analyses will be presented later this week. At CERN, preliminary results were presented to scientists on site and via webcast to their colleagues located in hundreds of institutions around the world.
"The search is more advanced today than we imagined possible," said ATLAS spokesperson Fabiola Gianotti. "We observe in our data clear signs of a new particle, at the level of 5 sigma, in the mass region around 126 GeV. The outstanding performance of the LHC and ATLAS and the huge efforts of many people have brought us to this exciting stage. A little more time is needed to finalize these results, and more data and more study will be needed to determine the new particle’s properties." The Higgs Boson is an unstable particle, living for only the tiniest fraction of a second before decaying into other particles, so experiments can observe it only by measuring the products of its decay. In the Standard Model, a highly successful physics theory that provides a very accurate description of matter, the Higgs Boson is expected to decay to several distinct combinations of particles, or channels, with the distribution among the channels depending on its mass. ATLAS concentrated its efforts on two complementary channels: Higgs decays to either two photons or to four leptons. Both of these channels have excellent mass resolution; however, the two-photon channel has a modest signal over a large but measured background, and the four-lepton channel has a smaller signal but a very low background. Both channels show a statistically significant excess at about the same place: a mass of around 126 GeV. A statistical combination of these channels and others puts the significance of the signal at 5 sigma, meaning that only one experiment in three million would see an apparent signal this strong in a universe without a Higgs. The current results are an update on previous analyses shown at a CERN seminar last December and published at the beginning of this year. The December results, based on 7 TeV proton collision data collected in 2011, limited the mass of the Higgs Boson to two narrow windows in the range between about 117 GeV and 129 GeV. A small excess of events above the expected background was seen by both ATLAS and CMS at around 126 GeV, about the mass of an iodine atom. The next steps for ATLAS, the LHC and the high-energy physics community are to measure the properties of this particle and compare these measurements with the predicted properties of the Higgs Boson. Already some of these properties match the predictions: the fact that it is seen in the predicted channels and at a mass favoured by other, indirect measurements. In the weeks and months ahead, ATLAS will better measure these properties, enabling a clearer picture to emerge about whether this particle is the Higgs Boson, or the first of a larger family of such particles, or something else entirely. The 2012 data set comes from proton collisions with an increased centre of mass energy of 8 TeV and includes more data (collected in only three months) than was collected in all of 2011. This rapid accumulation of data was possible thanks to the outstanding efforts of the LHC accelerator group. The data set presented at the seminar comes from approximately one quadrillion (million billion) proton collisions. The ATLAS detector has performed remarkably well, even under the more difficult beam conditions of 2012, and has, with nearly full efficiency, collected high quality data for this search. Powerful computing provided by the worldwide LHC Computing Grid was essential for the reconstruction and analysis of the data. The LHC is expected to provide ATLAS with double the data again by the end of the 2012, before the beginning of a long shutdown to upgrade the accelerator. When the machine starts up again toward the end of 2014, it will operate at nearly twice its current energy. The new 2012 data and the data generated by the improved accelerator will allow scientists to address the questions about the Higgs prompted by today’s announcement as well as other questions fundamental to our knowledge of nature. About ATLAS ATLAS is a particle physics experiment at the Large Hadron Collider (LHC) at CERN. The ATLAS detector is searching for new phenomena in the head-on collisions of hadrons of extraordinarily high energy. ATLAS is studying the basic forces that have shaped our Universe since the beginning of time and that will determine its fate. Among the possible unknowns are the origin of mass, extra dimensions of space, the unification of fundamental forces, and evidence for dark matter candidates in the Universe. At the time of writing, the ATLAS Collaboration comprises 3000 physicists from 176 institutions located in 38 different countries around the world. More than 1000 PhD students are involved in the operation of ATLAS and in the analysis of its data. |
2 July 2012
Every other year, particle physicists gather together to share their latest results at the ICHEP (International Conference on High Energy Physics) conference. This year, more than 700 are attending the conference in Melbourne, Australia, July 4-11.
ATLAS will be sending the results of several dozen new analyses to ICHEP, covering the full scientific programme of the experiment, both precision measurements of Standard Model processes and searches for new physical phenomena - including the Higgs boson. A preview of the Higgs search results will be presented at a special joint video-linked seminar at CERN on the opening day of the conference. ATLAS physics results to be presented are based upon the full 2011 data set and, in several cases, data from the first part of the 2012 run. The great performance of the LHC this year at 8 TeV collision energy has already allowed ATLAS to more than double the total data collected. The scope of results extends well beyond the Higgs. For instance, there are searches for new forces between particles; ATLAS is sensitive to particles with ranges only one twentieth of the range of the so-called "weak force", which itself has a range of only a tiny fraction of the radius of the proton. Several searches for new “supersymmetric” particles will be reported; these hypothesized particles may be related to the mysterious dark matter that apparently makes up four fifths of the matter in the universe. Rounding out the searches are many new results on direct searches for signatures of “exotic” particles, for instance particles that could be candidates for the above mentioned dark matter or evidence for the existence of “extra dimensions” - spatial dimensions beyond the familiar three. In addition, new precision studies of the Standard Model will be presented, including studies that probe either rare processes or the properties of the heavy quarks, bottom and top; these studies could provide indirect hints of new physics. In addition, ATLAS will be combining its measurements of the mass of the top quark - the heaviest elementary particle known - with similar measurements from CMS for an accuracy better than either experiment has reached on its own. More than physicists are benefiting from the excitement of ICHEP; an unprecedented effort has been made this year to include the public and the media. Starting with a public webcast of the Higgs Seminar on the 4th, activities include an ATLAS exhibit at the Planetarium in Scienceworks, Masterclasses for high school students, teacher development day, a public panel discussion, and a public talk by ATLAS spokesperson Fabiola Gianotti. Conference attendees will have the chance to take part in an Outreach & Education parallel session on Saturday morning. The session will feature presentations by scientific communication experts, including CERN’s Director General Rolf-Dieter Heuer and Australia’s famous radio personality Dr. Karl Kruszelnicki. An interactive session includes a panel discussion with Social Media experts and a Public Q&A session in the form of a Google Hangout. More information and a schedule can be found on the conference web site and links to various events and venues are included below. |
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Public Webcast [web portal]
ATLAS Event Display Images
Candidate Higgs to 4-electron (2012) [img] [img] [img] [img] [img] [img] [img]
Candidate Higgs to 4-muon (2012) [img] [img] [img]
Candidate Higgs to 2-muon & 2-electron (2012) [img] [img] [img]
More ATLAS Event Images [web portal] [even more]
More ATLAS Multimedia Material [web portal]
ATLAS Event Display Animations
Candidate four-muon event (2011) [animation]
Candidate two-electron, two-muon event (2011) [animation]
Candidate two-photon event (2011) [animation]
Media Support Material
Photos of ATLAS Physicists Meeting [img, img]
B-roll of ATLAS Physicists at a Meeting [video]
Background Material on the Higgs Boson
The Origin of Mass [text]
The Higgs Mechanism [animation]
Explanation of Higgs Mass Limit Plot (Brazil Plot) [text, image]
LHC Backgrounder on Higgs: General Information: [text]
LHC Backgrounder on Higgs: Glossary of Terms: [text]
LHC Backgrounder on Higgs: Evolution or Revolution? [text]
Physics World Interview with Peter Higgs [text, audio]
Explanation of Look-Elsewhere Effect by CMS [text]
CERN Interview with ATLAS and CERN Experts [video]
Other Sources of Information from ATLAS
ATLAS Home Page [web portal]
ATLAS Live Webcast Streams [web portal]
ATLAS on Social Media [Twitter] [Google +] [Facebook] [YouTube]
ATLAS Blog [web portal]
Some Fun
Jon Butterworth on Why Does the Higgs Decay? [Guardian Article]
What would happen if you put your hand in the LHC? [60 Symbols Video]
The ATLAS Boogie (How we really find the Higgs!) [YouTube Video]
New LHSee Android App with Real Higgs Candidate Events [Google Store]