A new LHC Run 2 search for diboson (WW, WZ and ZZ) resonances has been carried out with 37 inverse femtobarns of pp collisions collected by the ATLAS detector in 2015 and 2016. The search exploits jet substructure techniques to tag hadronic jets originating from the decay of W or Z bosons. The paper has been submitted to Phys. Lett. B, see http://arxiv.org/abs/1708.04445. Prof. Willocq, postdoctoral Research Associate Picazio and graduate student Meadows had responsibility for multiple aspects of the analysis (e.g. evaluating the boson-tagging performance, studying the modeling of the background, determining large-radius jet systematic uncertainties, etc.) and editing of the paper.

See the mass distribution for pairs of large-radius jets (figure on the right) with dashed curves showing the expected deviations in the presence of Kaluza-Klein excitations of the graviton, as predicted by the Randall-Sundrum model with one warped extra spatial dimension. Interpretations were also made in the context of a heavy scalar model and the heavy vector triplet model.

For more information, see https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2016-19/.

Run 2 of the LHC at the higher center of mass energy of 13 TeV has begun. It provides a unique opportunity for further discovery in the next few years thanks to the energy increase. For some additional information see the UMass press release at http://www.umass.edu/newsoffice/article/umass-amherst-physicists-eager-begin.

A new LHC Run 1 search for di-Higgs resonances in the 4b decay channel has now been submitted to Eur. Phys. J. C, see http://arxiv.org/abs/1506.00285. The data sample corresponds to about 20 inverse femtobarns of proton-proton collisions collected by the ATLAS detector in 2012 at a collision energy of 8 TeV. Prof. Willocq and postdoctoral Research Associate Bellomo had responsibility for the statistical analysis and editing of the paper for the so-called boosted analysis. This boosted analysis features the first application of b-tagging to track-jets associated with large-radius jets in ATLAS. Such a novel approach is required for tagging of high-momentum Higgs bosons decaying in the dominant bbbar mode and is expected to have broad application in searches involving Higgs bosons in Run 2 of the LHC.

See the mass distribution for pairs of large-radius jets (figure on the right) with dashed curves showing the expected deviations in the presence of Kaluza-Klein excitations of the graviton, as predicted by the Randall-Sundrum model with one warped extra spatial dimension. Interpretations were also made in the context of Two-Higgs Doublet models.

For more information, see https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2014-11/.

The latest LHC Run 1 search for contact interactions and large extra dimensions with dielectrons and dimuons is now published in Eur. Phys. J. C, see EPJC 74 (2014) 3134 or http://arxiv.org/abs/1407.2410. The data sample corresponds to about 20 inverse femtobarns of proton-proton collisions collected by the ATLAS detector in 2012 at a collision energy of 8 TeV. Prof. Willocq and Graduate Student Varol have led the analysis of the dimuon channel. Varol completed her dissertation on this topic and earned her Ph.D. degree in 2014.

See the dimuon mass distribution (first figure on the right) with colored curves showing the expected distributions in the presence of new phenomena from contact interactions or large extra dimensions.

A novel aspect of this search is the inclusion of the muon angular distribution (second figure on the right) for the first time at the LHC, in addition to the dimuon mass distribution, as an independent observable sensitive to new phenomena in the dimuon final state.

For more information, see https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/EXOT-2013-19/.

On the evening of October 1st. 2014, the UMass ATLAS group presented the acclaimed documentary film “Particle Fever” at the local cinema, Amherst Cinema. A packed audience (sold out 24 hours prior to the showing) followed a group of theorists and experimentalists on the ATLAS experiment as they passed from the early days of the start-up of the LHC to the climactic announcement, on 4 July 2012, of the discovery of the long-sought Higgs boson. Profs. Brau, Dallapiccola and Willocq, joined by special guest Nobel Laureate Prof. Sheldon Glashow, led a 45-minute Q&A session after the film. The audience asked a number of great questions! Details of the Q&A, here

Video clip highlights from the Q&A:

• What do you think of the multiverse concept? Glashow responds unequivocally that, being untestable, non-falsifiable and motivated by anthropic arguments, it is not good science. Other prominent physicists would disagree (a panel of physicists hotly debate the topic here). What do you think?
• What are your thoughts on the Nobel Prize awarded to Higgs and Englert, for discovery of the mechanism responsible for electroweak symmetry breaking? Should the experimental collaborations at CERN, ATLAS and CMS, have been awarded the prize for having made the discovery of the Higgs boson? Video here.

An update of the search for production of micro black hole states at the LHC, the focus of research efforts of Prof. Dallapiccola, postdoc Dan Ventura and graduate students German Colon and Nathan Bernard, has been published in the Journal of High Energy Physics: arxiv.org/abs/1405.4254. The 2012 data were scrutinized for telltale signs of black hole evaporation via Hawking radiation.  No evidence of a signal was present, and thus lower limits were placed, in the context of models of extra spatial dimensions, on the fundamental energy scale of gravity, M_D, and the energy threshold above which semi-classical black hole states could be produced, M_th.  Based on 20.3 fb^-1 of integrated luminosity collected in proton-proton collisions with the ATLAS detector, values of M_th < 4.8-6.2 TeV have been ruled out at 95% confidence level.  These new results place the most stringent constraints to date on models proposing large, compactified extra dimensions.

The latest search for contact interactions and large extra dimensions with dielectrons and dimuons is now published in Phys. Rev. D, see http://prd.aps.org/abstract/PRD/v87/i1/e015010. The data sample corresponds to about 5 inverse femtobarns of proton-proton collisions collected by the ATLAS detector in 2011. Prof. Willocq and Graduate Student Varol have led the analysis of the dimuon channel.

See the dimuon mass distribution with colored curves showing the expected distributions in the presence of new phenomena from contact interactions or large extra dimensions.

The UMass ATLAS group led by faculty members Willocq, Dallapiccola, and Brau played a key role in this week’s preliminary observation of a new particle, possibly the long-sought Higgs boson, announced by scientists at the European Organization for Nuclear Research (CERN) particle physics laboratory. It is located at the Large Hadron Collider (LHC) deep below Switzerland and France.

The Standard Model of particle physics can correctly explain the elementary particles and forces of nature after more than four decades of experiments. But it cannot, without the Higgs boson, explain how most of these particles acquire their mass, which is a key ingredient in the formation of the universe. Though scientists are not certain this week that the new particle is indeed a Higgs, Willocq and colleagues at UMass Amherst are elated by the possibility and excited that their work since joining the international team eight years ago is paying off.

The UMass Amherst physicists work on the ATLAS project, one of two general-purpose detectors at the LHC used to detect the debris from the collisions of the LHC’s high-energy beams of protons. The UMass group has significant contributions to the identification and reconstruction of muons, a kind of particle produced in some of these collisions and a key component of the Higgs search. They have written large parts of the code now used to reconstruct the trajectories of muons detected in the ATLAS Muon Spectrometer.

Willocq says, “The ability to find muons among the debris of the proton collisions from the collider is critical to the research done with ATLAS. The long-sought Higgs boson is being hunted by hundreds of particle physicists eager to discover this hypothetical particle or disprove its existence. Higgs bosons may manifest themselves by decaying into muons as well as other particles. By identifying and measuring those muons and accompanying Higgs decay particles we hope to find evidence for the Higgs. Muons thus provide telltale signs of the ephemeral presence of a Higgs boson produced by the collider.”

A muon is a negatively charged, unstable elementary particle similar to the electron, but about 200 times heavier. The ATLAS detector includes a huge magnet that bends the paths of charged particles to allow their momenta to be measured. The device records and measures particles created in collisions, along with their paths, energies and identities.

Willocq adds, “We are responsible for the maintenance and further development of the muon event data classes, as well as substantial parts of the reconstruction software. Since joining ATLAS in 2004, we have held coordinator positions for muon reconstruction, muon software, muon combined data quality and muon combined performance. Graduate students have also played a significant role in the commissioning and continued operation of the endcap Muon Spectrometer detector system, as well as the muon data acquisition.”

Scientists at the LHC announced their latest results at a seminar at the LHC in Switzerland this week on July 4. Physicists from across the United States had gathered at laboratories and universities in the middle of the night to watch a live video stream of the seminar online. Most U.S. scientists participate in experiments from their home institutions, remotely accessing and analyzing their data through high-capacity networks and grid computing.

It was in 1964 that Peter Higgs proposed the existence of a new particle, now known as the Higgs boson, whose coupling with other particles would determine their mass, but it has eluded discovery. Now scientists are approaching the moment of knowing whether the Higgs is the right solution to this problem.

More than 1,700 people from U.S. institutions, including 89 American universities and seven U.S. Department of Energy national laboratories, helped design, build and operate the LHC accelerator and its four particle detectors. The U.S. Department of Energy’s Office of Science and the National Science Foundation provide support for research and detector operations at the LHC along with computing for the ATLAS experiment.

Preliminary findings announced this week are based on data collected in 2011 and 2012, while 2012 data are still under analysis. More presentations about the results from the CERN particle physics lab can be found at the International Conference on High Energy Physics Melbourne, Australia.

A search for production of mini black hole states at the LHC, the focus of research efforts of Prof. Dallapiccola and graduate student German Colon, has been submitted to Physics Letters B: arxiv.org/abs/1204.4646. The data were scrutinized for telltale signs of black holes decaying via Hawking radiation.  No evidence of a signal was present, and thus lower limits were placed on the fundamental energy scale of gravity, M_D, and the energy threshold above which classical black states could be produced, M_thresh.  Based on 1 fb^-1 of integrated luminosity collected in proton-proton collisions with the ATLAS detector, values of M_D < 1.5 TeV and M_thresh < 4.5 TeV have been ruled out at 95% confidence level.  These new results place stringent constraints on models proposing large, compactified extra dimensions.

These results, along with other, related searches for signatures of extra dimensions, were presented by Prof. Dallapiccola at a Fermilab JETP seminar, 11 May 2012, and can be viewed here.