LHC Physics News and Progress
Geneva, 22 August 2011. Results from the ATLAS and CMS collaborations, presented at the biannual Lepton-Photon conference in Mumbai, India today, show that the elusive Higgs particle, if it exists, is running out of places to hide. Proving or disproving the existence the Higgs boson, which was postulated in the 1960s as part of a mechanism that would confer mass on fundamental particles, is among the main goals of the LHC scientific programme. ATLAS and CMS have excluded the existence of a Higgs over most of the mass region 145 to 466 GeV with 95 percent certainty.
As well as the Higgs search results, the LHC experiments will be presenting new results across a wide range of physics. Thanks to the outstanding performance of the LHC, the experiments and the Worldwide LHC Computing Grid, some of the current analyses are based on roughly twice the data sample presented at the last major particle physics conference in July.
“These are exciting times for particle physics,” said CERN’s research director, Sergio Bertolucci. “Discoveries are almost assured within the next twelve months. If the Higgs exists, the LHC experiments will soon find it. If it does not, its absence will point the way to new physics.”
The Standard Model Higgs mechanism is one of a range of ways that fundamental particles could acquire their masses. According to the Higgs mechanism, space is filled with a so-called Higgs field with which particles interact. Those that interact strongly with the field have more mass than those that interact weakly, rather like a streamlined racing car cuts through air more easily than a bus.
At the first major particle physics conference of 2011, the European Physical Society’s High Energy Physics conference held in Grenoble, France, in July, both ATLAS and CMS were careful to stress that possible hints of a Higgs signal in their data could be explained by statistical fluctuations. Now, with additional data analysed, the significance of those fluctuations has slightly decreased.
“Thanks to the superb performance of the LHC, we have recorded a huge amount of new data over the last month, ” said ATLAS spokesperson Fabiola Gianotti. “This has allowed us to make great strides in our understanding of the Standard Model and in the search for the Higgs boson and new physics.”
CMS Spokesperson Guido Tonelli concurred, saying: “It’s great that the LHC’s fantastic performance this year has brought us this close to a region of possible discovery. Whatever the final verdict on Higgs, we’re now living in very exciting times for all involved in the quest for new physics.”
The Lepton-Photon conference runs until 27 August. There will be a press conference on 25 August at which CERN Director General, Rolf Heuer, will be one of the speakers. CERN’s LHCb experiment will present its latest measurements on the Standard Model on Saturday 27 August. Following the Lepton Photon conference, the results from the LHC experiments will be available through the CERN website.
The LHC is on track to at least double the amount of data delivered so far to the experiments by the end of the year.
Physicists at the University of Iowa are busily analyzing the data recently recorded at the LHC. Professors Yasar Onel and Jane Nachtman lead a group of researchers on the Compact Muon Solenoid (CMS) experiment. Professor Onel has been a part of CMS since the design stages, and was responsible for the construction and installation of part of the detector (the calorimeter) which measures the energy from the collision byproducts, the spray of energetic subatomic particles into the detector. Now they are able to reap the benefits of their work, by searching through the LHC collisions to find rare and interesting events. There is a second UI group led by Professor Usha Mallik which is working on the ATLAS experiment.
Members of the Iowa team contributed to the latest physics results being shown by the CMS collaboration in international conferences, including the Lepton-Photon conference in Mumbai, India. Iowa team members search for evidence for new physics in several possible channels.
Only a tiny fraction of the matter in the universe is understood; the “Dark Matter” is mysterious and can’t be seen. However, analysis of LHC collisions may yield clues as to the origin of Dark Matter. U of Iowa graduate student Elif Albayrak and post-doctoral researcher Dr. Taylan Yetkin are searching for these events, relying on the excellent performance of the Iowa-built forward calorimeter to distinguish these Supersymmetric events from “normal” events.
Iowa post-doctoral researcher Dr. Kai Yi leads a search for exotic physics decaying to jets, sprays of particles originating from a quark or gluon. His analysis takes advantage of the CMS calorimeter to precisely measure their energy and position, allowing him to isolate anomalous events.
Iowa graduate student Anthony Moeller contributes to the search for the Higgs boson, in a channel that relies on the Iowa-built forward calorimeter to separate the very small Higgs signal from the overwhelming background from “normal” events.
The LHC (Large Hadron Collider) in Geneva, Switzerland, will accelerate lead, Pb, ions again in November 2011.The University of Iowa is a partner in this work through the efforts of the high energy nuclear physics group in the Department of Physics and Astronomy. The group includes Edwin Norbeck, Yasar Onel, Duane Ingram, and Paul Debbins, with help from several graduate students. This group has played a major role in developing the detectors for measuring particles that are produced at small angles from the beam, (less than 6 degrees). Cosmic ray studies have seen a variety of strange objects that are thought to be the product of collisions of cosmic-ray iron nuclei hitting nitrogen and oxygen at the top of the atmosphere.
It is expected that these strange objects will be seen in the laboratory by the high energy lead-lead collisions. They are expected to appear only in the small angle detectors. In such high energy nuclear collisions, the individual protons and neutrons are “melted” into quarks and gluons to form a quark-gluon plasma. This large, hot (two trillion degrees Celsius) ball of material has strange properties; it behaves like a fluid of almost zero viscosity. It is studied observing the particles that come off as it cools and evaporates into a variety of mostly known nuclear particles.