Discovery and precision studies of the Bc meson family are the analysis focus of Sally, Konstantin, and their students. Because of their high masses and doubly heavy quark content, the Bc family provides a set of unique benchmarks for predictions by potential models and lattice calculations. Precision studies of their properties also have the potential to cast light on new physics processes. We discovered the Bc(2S) state the same year in the 7 TeV data, confirmed it the next year in the 8 TeV data, and announced the discovery at ICHEP2014, for which it was singled out on the atlas.ch web page as a "conference highlight." Our paper on the excited state (the primary subject of Rui's dissertation) has been accepted by PRL.
Supervised by Sally and Konstantin, Simon produced the first measurement at 8 TeV of the differential cross section for production of the ground state Bc relative to that of the B+, where the decays are observed through Bc+→J/ψπ+ and B+→J/ψK+ respectively. This is the first measurement of this relative production cross section by ATLAS at any energy, and the world's first measurement at 8 TeV. It accesses a new kinematic regime and provides valuable input to theoretical models of heavy quark production and hadronization. This work has been accepted as Simon's Master's thesis and is now under ATLAS collaboration review for publication in PRL. Simon reported on his work at five meetings of the ATLAS BtoJpsi Subgroup in the past 12 months. Conversion of an M.S. thesis to an ATLAS publication is fairly rare; Simon's work is excellent. Simon left ATLAS in September 2014, and Sally, Konstantin, and Aidan will shepherd the analysis through the review process in 2015. This will naturally involve responding to internal and external requests for cross checks and related physics studies and so will constitute a significant analysis project for Aidan. Aidan has taken over as primary editor of the paper.
Aaron (supervised by Sally and Konstantin) is writing his dissertation on a measurement of the absolute production cross section of the Bc at 8 TeV. He achieves this by combining Simon's relative production cross section measurement with a first measurement of the absolute B+ production cross section at 8 TeV, which Aaron is making now. The absolute Bc cross section is difficult in part because of the requirement of detailed knowledge of ATLAS acceptances. It is especially sensitive to our map of the magnetic field. This work will be important input to models of heavy quark production and hadronization. Aaron presented a talk on "Studies of the Bc Meson" in the UNM Nuclear, Particle, and Astro-particle Physics Seminar Series of 2013 as well as 3 status reports to the ATLAS BtoJpsi Subgroup in 2014. Aaron expects to publish his work and graduate in 2016.
Aidan's dissertation research (also supervised by Sally and Konstantin) investigates the properties of the Bc(2S), including its spin and unexpectedly high production rate relative to the ground state. This research, which will include 7, 8, and 14 TeV data, will provide valuable input to NRQCD and lattice models of heavy bound states and could turn up something interesting in the source of the high production rate. He presented a talk on the Bc(2S) discovery at the 2014 APS Four Corners Section Meeting. We anticipate that Aidan will complete his research in 2019.
Igor convenes the ATLAS vector boson + heavy flavor analysis group, which includes teams at Berkeley (Marjorie Shapiro) and Tokyo (Kunihiro Nagano). Their effort is focused on the measurement of cross sections for W/Z-associated production of heavy quarks Q. Igor is the principal advisor on the analyses being conducted by Neil at UNM as well as the Tokyo students Toyonobu Okuyama and Yoichi Ninomiya. Their studies simultaneously connect with searches for new physics and investigate fundamental QCD. Furthermore the W/Z+Q are a significant background to Higgs produced in W/Z0 + H(→bƃ). The process gb→Zb is an irreducible background to gb→Hb, where the Z and the H decay to the same final state bƃ, t+t-, or μμ. On the QCD front, W/Z+Q probes parton distribution functions (pdfs) at high momentum transfer, Q2~mW/Z.
During 2013-14, Igor and his students have been analyzing events in which the charm is reconstructed in an exclusive D(*)-meson state. The W-associated production is directly sensitive to the s-quark pdf at momentum fractions x in the range 0.001 to 0.1. This expanded x range is important, since prior to the LHC era, the s-quark pdf had to be taken primarily from neutrino-nucleon DIS experiments. For those DIS data, (x, Q2) ~ (0.1, 10 GeV2), and interpretation is sensitive to the modeling of the c-quark fragmentation and nuclear corrections. First results from the ATLAS vector boson plus heavy flavor analysis are available at. Our study of Z+D(*) production probes directly the c- or b-quark pdfs, again in the x regime 0.001 to 0.1. This leads to a new measurement of the b-quark pdf, which is needed for the analyses of gb→Hb, bb→H, and the BSM process gb→H-t. The precision of the Z-associated measurement will substantially improve upon that of a recent publication.
Igor reported at the May 2014 ATLAS W/Z plenary meeting on the status of their two paper drafts (one for W, one for Z) using the full 8 TeV dataset. Neil is working now on comparing the measurements to predictions by aMC@NLO. The two papers, including Neil's predictions, are planned for release to the ATLAS Collaboration in December 2014. Igor and Neil are responsible for guiding those papers to publication in 2015, including carrying out cross checks and all analyses required by internal and external referees.
Igor spoke on "Open c and b States Production" at the Workshop on Heavy Flavor Production at Hadron Colliders (2013) and on "Heavy Flavor Physics at ATLAS and CMS" at the 2013 Hadron Structure conference. He presented the subject in a Joint Experimental-Theoretical ("Wine and Cheese") seminar at Fermilab in 2013 as well. Neil is presenting a talk on "Measurement of the Production of a W Boson in Association with a Charm Quark in pp Collisions at √s=7 TeV with the ATLAS Detector" at the APS Four Corners Section Meeting in October 2014.
Igor Gorelov is collaborating with Marge Shapiro (UC Berkeley) on a suite of Standard Model measurements with implications for the Higgs. The first is a measurement of the cross section for production of W in association with D0/D±/Ds/Λc+, relative to the cross section for production of inclusive W. This measures the strange quark pdf in a new regime. Differences between the strange and anti-strange seas can be inferred. The measurement depends critically on vertexing and is thus nicely linked to our 18-year record of contributions to the ATLAS Pixel system. It relies on tracking information in the 8 TeV data (and beyond) and is thus relatively immune to pile-up contamination. An extension of this study will replace the W+charm with Z(→dileptons)+bottom and will contribute additionally to the description of the background to Higgs→bb. Neil is training on ATLAS Offline now with the goal of contributing to this analysis full-time by summer 2014.
Aaron, Konstantin, Simon, and Sally are measuring the differential cross section for central production of the Bc in ATLAS data, first relative to the cross section for B+ (this will be a publication led by Simon) and later absolutely (a second paper, led by Aaron). This measurement is complementary to what can be achieved by LHCb. It is expected to resolve the confusion surrounding the immense range of theoretical predictions for the process by pointing to optimized QCD choices.
Sally, Konstantin, and Rui are interested in the production of the Bc meson at the LHC. Results from this program will provide critical tests of the QCD production models describing heavy quark production at LHC. We observed the Bc state at CERN in March 2012 through its decay to J/Ψ +π. The foundation of this analysis was prepared by Sally, Konstantin, and Sally's Lund University doctoral student, Weina Ji, in "A Study of the ATLAS Detector Sensitivity for the Event Yield in the Bc Meson Mass Region,". Konstantin's implementation of Bc production in the Pythia+ EvtGen Monte Carlo generator for LHC kinematic conditions was intrinsic to this project, as generation of the full spectrum was not possible with default Pythia. Rui reported on heavy meson production in ATLAS in a plenary at the BEACH 2012 conference. Konstantin reported on "Measurement of Mass and Lifetime of B-Hadrons at ATLAS" at ICHEP 2012. Sally gave the plenary "Highlights from the ATLAS Experiment" at the First International Conference on the Frontiers of Physics in 2012