CERN Accelerating science

LHC experiments present latest results in ICHEP16

by Panos Charitos

New results from the LHC experiments were presented during the 38th International Conference on High Energy Physics (ICHEP 2016) that took place in Chicago, USA. The collaborations of the LHC experiments can now dive in and explore at the new energy frontier of 13 TeV, following last year’s first glimpse of physics at this unprecedented energy level. LHC collaborations are presenting more than 100 different new results, including many analyses based on newly taken 2016 data.

Thanks to the outstanding performances of the LHC, experiments have already recorded about 5 times more data in 2016 than in 2015, in just a few months of operations. The LHC surpassed its design luminosity in June reaching a peak luminosity of 1 billion collisions per second so that even the rarest processes at the highest effective energy could occur. The LHC is thus running beyond expectations and the objective of 25 inverse femtobarn3 of proton–proton collisions delivered to experiments for the whole of 2016 is within sight. “The LHC really entered a new regime by reaching its nominal luminosity, now exceeded by 20%,” said CERN Director for Accelerators and Technology, Frédérick Bordry. “It’s a major achievement and we can be confident that we will go beyond our goals for the full second run of the LHC.”

Physicists have been hard at work in the past months dealing with the huge amount of data recorded by the LHC experiments. With a larger data set now analysed, more precise measurements of the Standard Model processes and more sensitive searches for the direct production of new particles at the highest energy are possible. As an example, the 125 GeV Higgs boson, discovered in 2012, has now also been observed at the new energy of 13 TeV with higher statistical significance. In addition, both ATLAS and CMS experiments have made new precise measurements of Standard Model processes, especially looking for anomalous particle interactions at high mass, a very sensitive but indirect test for physics beyond the Standard Model.

CMS is significantly increasing its physics reach with respect to Run 1, especially in studies of the Higgs sector and in searches for new physics.One of the highlights from CMS for ICHEP is the full re-discovery programme around the Higgs boson with a mass of 125 GeV, providing independent observations in the H→γγ and H→ZZ*→4l channels well above 5 standard deviations. The analyses are completed with dedicated measurements of the cross-section in the fiducial volume, mass and spin-parity studies, and searches for additional Higgs bosons at higher masses. All results were found to be consistent with Standard Model expectations and with previous CMS results from Run 1.

Mass spectra obtained in the 2016 CMS Higgs search using the di-photon (left) and four-lepton (right) decays channels. The significance of the observed signals around 125 GeV is larger than 5 standard deviations in both channels. The analysed data correspond to an integrated luminosity of 13 fb−1, collected with the CMS detector at a centre-of-mass energy of 13 TeV.

SUSY searches are also taking full advantage of the increased statistics. While no significant excesses were observed in the new data, mass limits increased by a few hundreds of GeV on almost all fronts. That is the case of gluino and top-squark (or stop) searches in high-multiplicity states associated to missing transverse energy. Updated analyses in photonic or leptonic final state also led to improved limits. Finally, a new generation of SUSY analyses is targeting complicated scenarios like compressed spectra or particles exclusively produced via electroweak interactions. 

CMS keeps an intensive, comprehensive dark-matter search programme in the so-called “mono-X” final states, where high-mass mediators decay into invisible particles (dark matter) and recoil against high-pT X visible objects. The most sensitive “mono-jet” analysis is now excluding mediator masses up to 2 TeV in several standard scenarios for low masses of the dark-matter candidate.

CMS continues to perform precise measurements of Standard Model processes, with a special focus on high-mass final states, where Run 2 is particularly advantageous. CMS is also presenting new heavy-ion results from the most recent pp, pPb and PbPb runs taken at 5 TeV centre-of-mass per nucleon. Last but not least, data from Run 1 keep providing extremely interesting physics results, like a first high-statistics study of Z+charm production at the LHC. An investigation of the B0sπ spectrum did not show any evidence for resonant structures around 5568 MeV, leading to a conservative limit of about half the rate claimed by the D0 experiment at the 95% CL. More information about the CMS results can be found at http://cern.ch/cms-results/public-results/preliminary-results/ICHEP-2016.

ATLAS released results with 12 inverse femtobarns of data recorded at 13 TeV in 2016. This is nearly four times larger than the 2015 dataset – and the year is not yet over.

Preparations for ICHEP have kept everyone at ATLAS on their toes. From detector operations and trigger to computing and analysis, ATLAS teams have worked tirelessly to collect and analyse this new wealth of data. Thanks to their efforts, 50 new conference notes have been prepared especially for ICHEP. 

Along with detailed studies of Standard Model processes and searches for new physics, new studies of the Higgs boson will be shown, including searches for SUSY particles. ATLAS physicists have been eagerly searching the collected data for evidence of the production of the supersymmetric top quark (squark). Theory predicts that the top squark would decay to ordinary quarks, a dark matter particle and possibly leptons (electrons or muons). Recent ATLAS results feature five separate searches for this elusive particle. Each search differs in the number of leptons in the final state, covering all the possible decay modes. Four searches require respectively zero, one, two, or three light leptons (electrons or muons) and the fifth targets decays to tau leptons. 

Overall, the data gathered have been found to be consistent with the production of only known particles. You can read more about the key results presented during ICHEP16 in ATLAS Physics Briefings,

For both experiments, the analysis of the di-photon spectrum near 750 GeV with the new 2016 data became one of the priorities, The new results including more data from the 2016 run, show no significant excess in the relevant region and do not confirm the previously observed evidence of an excess. Additional searches in related channels (like X→Zγ) have not shown any significant excess around 750 GeV mass either.

Searches for any signs of the direct production of new particles predicted by Supersymmetry and other exotic theories of physics beyond the Standard Model, but no compelling evidence of new physics has appeared yet. “This is one of the most exciting times in recent years for physicists, as we dig into the unknown in earnest: the particle physics at an energy never explored before,” said CERN Director for Research and Computing, Eckhard Elsen.

LHCb are presenting many interesting new results as well, in the domain of flavour physics. A particular highlight is the discovery of the decay mode B0->K+K-, the rarest B-meson decay into a hadronic final state ever observed, as well as studies of unprecedented sensitivity of CP violation, a very subtle phenomenon explaining nature’s “preference” for matter over antimatter. LHCb have also conducted measurements that could help to reveal some new phenomena such as the first measurement of the photon polarisation in radiative decays of Bsmesons and determinations of the production cross-sections of several key processes at a collision energy of 13 TeV – some of which, at first sight, are at variance with current predictions.

Results from heavy-ion collisions were also presented from ALICE - the dedicated heavy-ion LHC experiment- and the other LHC experiments. The wealth of results marks a bright future for the rich research programme in heavy-ions. The new energy regime reached at the LHC allows both pursuing discovery of new phenomena and improving our knowledge about the quark-gluon plasma..

“We're just at the beginning of the journey,” said CERN Director-General, Fabiola Gianotti. “The superb performance of the LHC accelerator, experiments and computing bodes extremely well for a detailed and comprehensive exploration of the several TeV energy scale, and significant progress in our understanding of fundamental physics.”

 
 
 
 
 
 
 
 
 
 
 
 
 
 

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