ATLAS experiment’s winter round-up

The ATLAS experiment sent the results of more than 40 new analyses to “La Thuille”, “Moriond” and other winter conferences. These results covered the full scientific programme of the experiment, from precision measurements of Standard Model processes, through searches for the Higgs boson and new physical phenomena to the study of hot and dense matter probed in heavy-ion collisions.

The collaboration has made a great deal of progress since the seminar on 13 December where preliminary results on the Higgs search in both the ATLAS and CMS experiments were presented. ATLAS has since used the full 2011 data set to search for a Standard Model Higgs boson in 12 different decay channels. Combining all of these results has left only three remaining windows for the Higgs: 115.5–131 GeV, 237–251 GeV and above 520 GeV. The upcoming data at the new centre-of-mass energy of 8 TeV will increase the sensitivity of both CMS and ATLAS, and allow the collaborations to make more definitive statements on the existence and mass of a Standard Model Higgs boson by the end of the year.

At moderate to high masses, the most sensitive searches for the Higgs boson involve its decays to two heavy electroweak bosons, either WW or ZZ. Because this search looks for an excess of events over the background from other diboson production it is important to measure the diboson background as accurately as possible. This measurement is also important in its own right: it depends on the strength of the interaction between the W boson, the Z boson and the photon. This strength is a fundamental parameter of the Standard Model and distinguishes it from other theories. These new measurements use the full 2011 data set and are twice as precise as previous ATLAS measurements.

In the Standard Model, the B_s meson is predicted to decay to a μ⁺μ^– pair rarely: only 3 or 4 times in a billion. However, in various extensions of the Standard Model this rate can be increased by a factor of 10 or even a 100; it is one of the more sensitive indirect tests for new physics and complements some of the direct searches for new phenomena at the LHC. Other experiments both at Fermilab’s Tevatron and at the LHC have set upper limits on this decay in the range of 4.5–51 decays per billion. The ATLAS collaboration has now joined this search and has reported an upper limit of 22 decays per billion.

At the International Conference on High-Energy Physics (ICHEP) in Paris, ATLAS showed the first LHC results with sensitivity beyond that of previous experiments: limits on quark substructure obtained by studying events containing two jets. ATLAS has continued to investigate this category of events using the full 2011 data set. The additional data have provided extremely energetic events for ATLAS to study: in some cases the two jets have a combined mass above 4 TeV. There is still no evidence that quarks are made of smaller objects – but if they existed and were at least as large as 3 × 10^–20 m, ATLAS would have detected them. This lack of observation allows the experiment to set a limit on the size of any quark substructure. In a complementary measurement looking for excited electrons or muons, which would also be an indication of substructure, ATLAS has set limits around 10^–19 m.

Based on the new results previewed at these conferences, the ATLAS collaboration has sent more than 30 articles to scientific journals, with more in preparation. The next major round of conferences will be in summer and will include ICHEP 2012 in Melbourne. In addition to new results at a total collision energy of 7 TeV, the collaboration intends to show early results from 8 TeV collisions.