Achievement of former postdocs & grad. students

INTRODUCTION

The Wisconsin group made a strong effort in preparation for Higgs searches for a number of years. Since the second half of 2011 it intensified its effort in the following Higgs analyses:
(1) Hγγ ,
(2) HZZ4 leptons,
(3) HWWlvlv,
(4) Hττlvvhv,
(5) WH (Wlv, Hbb)
(6) ATLAS Higgs combination.
This effort culminated in our strong contribution to the discovery of the Higgs boson in 2012
Sau Lan Wu is committed to the physics of Run II (2015-2018). For this period, our manpower will be evenly divided between the study of the Higgs sector and the exploration of the energy frontier of 14 TeV - searching for dark matter, SUSY and other exotic particles.
Below you can find more information on each of these efforts, and on our research plans for Run II of the LHC.

 

HIGGS ANALYSES


(1) Hγγ,
In this channel, the results presented in the 2012 ICHEP conference show a significance of 4.5σ at 126.5 GeV (Figure 1). With the full Run I dataset, a 7.4σ excess is found at mH=126.8GeV (Figure 5; result and plot were produced by Wisconsin graduate student H. Wang together with other physicists). We contributed to the observed rate in this channel being 1.65±0.34 times the Standard Model prediction; if this excess is confirmed, physics beyond the Standard Model is just around the corner. We also contributed to the spin 2 hypothes is being excluded in favor of spin 0 at the 99% C.L.
Our group has worked on most aspects of this analysis: selection studies, background modeling, Vector Boson Fusion (VBF) and associated production (VH) analyses, systematics estimation, significance and signal strength determination, mass measurement and spin determination.


(2) HZZ4 leptons,  
This ‘golden channel’ has also yielded exciting results for the discovery. With the full dataset a clear mass peak is seen at 124.3±0.8 GeV (Figure 6); the significance of the excess is 6.6σ (Figure 7, statistical result and plot were produced by Wisconsin Grad. Student X. Ju together with other physicists). We also contributed to the result of its spin-parity-- JP=2+ (0-) is excluded in favor of 0+ at 83%(97.8%) C.L.
Our group has worked on this channel including event selection, data-driven background extraction, systematic estimations, limit and significance evaluation, mass and rate measurements and spin-parity determination.


(3) HWWlvlv,
This is a challenging channel because it needs good control of leptons, jets and Missing Transverse Energy (MET). With the Run I dataset, the significance of the excess at mH=125 GeV is 3.8σ. A spin analysis yields a 95% C.L. exclusion of spin-2 in favor of spin-0.
Our group has contributed to this analysis for a number of years. We worked on the full analysis chain, including jet energy scale and MET-related systematics. Postdoc L. Kashif is responsible for combining results from the 0+1 jet, VBF and VH analyses. In addition, he is measuring fermionic and bosonic coupling strengths in this channel.

(4) Hττlvvhv,
Recently, ATLAS reported the first evidence of SM Higgs Boson decaying into a pair of tau-leptons, the first time a significant excess having been observed in the fermionic decay mode of the Higgs Boson. H→ττ decays are the most sensitive probe for studying direct Yukawa coupling of the SM Higgs Boson to fermions.

Our postdoc S.Banerjee and Wisconsin graduate student Y. Heng contributed to the data analysis of H→τlepτhad, decays 
using cut-based and multi-variate discrimination techniques for the data collected at 7 TeV and 8 TeV, respectively.
We employ data-driven background estimation, and perform signal extraction via simultaneous fit to the data in multiple control regions enriched with signal and individual background components.

Postdoc S.Banerjee pioneered the fit model for combination of signatures from SM Higgs Boson decaying to ττ in the three decay channels (H→τlepτlep, τlepτhad and τhadτhad). The observed and expected significances are 4.1σ and 3.2σ, respectively. The observed strength of combined H→ττ signal as well the signal strengths in gluon-gluon fusion and VBF production modes  are consistent with the predictions of the Standard Model with in roughly 1σ.


 (5) WH (Wlv, Hbb)
This channel is not yet sensitive to the SM Higgs; with improvements, by summer 2013 it can provide a Higgs-bottom quark coupling strength measurement.
Our group runs the full analysis chain in WH→lvbb, including object and event selection, data-driven background estimation, systematics evaluation and statistical treatment. Postdoc L. Ma and Wisconsin graduate student Y. Ming are among the physicists who are in charge of the statistical combination of the three sub-channels (WH→lvbb, ZH→llbb, ZH→vvbb) and of providing the input for the ATLAS Higgs combination. Results with the full 2011 and 20012 dataset is on going in the 2013.


(6) ATLAS Higgs combination.
We played a leading role in the ATLAS Higgs combination for the discovery from all Higgs decay channels.
Over several years, our group has made substantial contributions to the statistical techniques for the combination of Higgs search results. We have contributed intensively the statistical results shown in all seven major conferences and four publications between 2011 and early 2013. Also our group contriubuted the ATLAS inputs in the Higgs decay channels γγ  (H. Wang), 4 leptons (X. Ju), WWlvlv (L. Kashif), ττ (S. Banerjee) and bb (L. Ma). Grad. Student H. Ji provided most of the results for the ATLAS Higgs combination; he provided the ATLAS combination result shown on July 4, 2012 (5.1σ), in PLB 716 (2012) (5.9σ) and CERN council Dec. 2012 (7σ). He is actively producing results for the ATLAS combinations for rate and coupling strength measurements. Fig. 8 shows the signal significance with the full dataset, observing 10σ at mH~125.5 GeV.
Coupling and spin combinations: we also make very strong contributions to the coupling and spin combinations. For Moriond 2013, Grad. Student H. Ji produced the ratios of Higgs couplings to fermions and vector bosons. Postdoc L. Kashif is in charge of combining JP = 0+ vs 2+ discrimination analyses from H→γγ , H→ WWlvlv and H→ ZZ4 lepton channels. The full spin combination yields an exclusion of 99% C.L. in favor of JP = 0+.

 

CONTRIBUTION TO ATLAS SILICON READ OUT DRIVER (ROD)

The ATLAS Pixel and Semi-Conductor Tracker have 80 million and 6.2 million channels, respectively. The silicon Read-out Driver (ROD) is a large VME board which interfaces these detectors to the Level-2 Trigger and the data acquisition systems. Since 1994, our group has assumed major responsibilities for the design, production, installation and commissioning of the RODs, as well as the long-term maintenance of their hardware and firmware. All 260 RODs were successfully delivered to ATLAS on time. The recent focus has been adding speed and new functionality for calibrations and data-taking.


CONTRIBUTION TO HIGH LEVEL TRIGGER AND UPGRADE PLAN

This task is ongoing. Over the past 15 years our group has made important contributions to the definition of the High Level Trigger (HLT) selection strategy, its hardware and software architecture and its implementation, commissioning and operation. Our group has had important impact on the present and future architecture of the ATLAS High Level Trigger:

  1. 1) During 2000-2002, W. Wiedenmann, S. Gonzalez (former scientist, now Program Director at NSF), and H. Zobernig pioneered the development and proposed an architecture that allowed ATLAS to use all the tools and methods of the offline software inside the trigger framework, so that the physics algorithms for event selection could be developed by physicists not specialized on DAQ. This architecture was accepted by ATLAS in 2003 (TDAQ TDR), implemented and used with great success in all LHC data taking periods up to Feb. 2013.
  2. 2) In 2009 our group put forward a proposal to treat the Level-2 and Event Filter selection much more uniformly. In 2012 ATLAS accepted this proposal as new architecture, which is now the basis for the new HLT software. The new architecture is being implemented and will  be ready for data taking by end of 2014. It will be used in the 2015-2018 LHC running period.
  3. 3) This activity is of great relevance for the software changes ATLAS foresees for the run starting 2019. ATLAS TDAQ plans to decide on new software frameworks by the end of 2015, explore more parallel algorithm execution in both offline and online software until 2017, and decide on the PC hardware architecture for new HLT farm nodes also by 2017.
  4. 4) Our group plans to be involved in these developments in a leading position, both for the structure of the new HLT software as well as for the selection of the most appropriate hardware platform. Due to rapidly increasing parallelism in new computing hardware both aspects will be strongly interdependent.
  5. 5) Our continued involvement in the design of the HLT software architecture and evaluation of hardware platforms enable us to contribute to the HLT core software for 2019 and beyond.

New graduate students and postdocs will work on the new software architecture and hardware platform with engineers Wiedenmann and Zobernig.

 

STUDENT INVOLVEMENT IN TRIGGER PERFORMANCE

Our students and postdocs are very active in the ATLAS trigger performance group. Their activities are briefly listed below.

 

SELECTED TALKS AND SELECTED REVIEW ARTICLES


Higgs at LHC

+ Higgs Search at ATLAS. 40th SLAC Summer Institute, “The Electroweak Scale: Unraveling the Misteries at the LHC”: SLAC SSI 2012. Higgs search at ATLAS: July 25, 2012.
+ The Discovery of the Higgs – the God Particle. Invited talk at the Wisconsin Institutes for Discovery. Wisconsin institutes: September 20, 2012. Video
+ The Discovery of the Higgs – the God Particle. Physics colloquium, University of Wisconsin. October 19, 2012.
+ The Discovery of the Higgs – the God Particle. Colloquium at Vassar College. Poughkeepsie, New York. Vassar: October 22, 2012. Video
+ Historic Review of Higgs Searches, in Aspen 2013 - Higgs Quo Vadis, Aspen Center for Physics. Aspen: March 10, 2013.

+ Historic Review of Higgs Searches, in ISHP 2013 - The long journey to the Higgs discovery, International Symposium on Higgs Physics. IHEP, Beijing, August 15, 2013.
+ The Higgs at Last. Co-authored by Michael Riordan, Guido Tonelli and Sau Lan Wu. Scientific American, October 2012 (pages 66-73).


B Physics

+ Highlights of 10 years of LEP B physics. Invited plenary talk. Proceedings of the Third International Conference on B Physics and CP Violation; Taipei, Taiwan, December 3-7, 1999, World Scientific (pages 67-87).
+ Recent Results on B Meson Oscillations. Invited Plenary talk, Proceedings of the XVII International Symposium on Lepton-Photon Interactions, August 10-15, 1995, Beijing, China.


Higgs at LEP

+ Higgs Searches at LEP II. Invited plenary talk, Proceedings of the 5th  International Conference on Physics Potential and Development of μ+μ- Colliders, December 15-17, 1999, San Francisco, California.
+ Search for Neutral Higgs at LEP 200. Invited Talk, Proceedings of the ECFA Workshop - LEP 200, September 29-October 1, 1986, Aachen, West Germany (pages 312-367).
+ The Higgs Particle in the Standard Model: Experimental Results from LEP, co-authored by Sau Lan Wu and Peter McNamara. Report on Progress in Physics 65 (2002), pages 465-528.
+ Observation of Higgs candidates around 114 GeV/c2 at LEP's highest energies, SLAC Departmental Colloquium, LBL/FNAL Physics Seminars. October 2-5, 2000.


Gluon discovery

+ The First Evidence for three-jet events in e+e- collisions at PETRA--First direct observation of the gluon. Proceedings of the International Europhysics Conference on High Energy Physics. July 27-August 2, 1995, Brussels, Belgium (pages 3-10). This talk was given on the occasion of the award ceremony for the authors to accept the 1995 EPS High Energy and Particle Physics Prize.

+ e+e- Physics at PETRA - The First Five Years, Physics Reports 107 (1984) 59, 266 pages (pages 60-324).

+ Hadron Jets and Discovery of the Gluon. The 3rd International Symposium on the History of Particle Physics: The Rise of the Standard Model-Particle Physics in the 1960s and 1970s.  Edited by L. Hoddeson, L. Brown, M. Riordan, and M. Dreseden.  Cambridge University Press, 1997, (pages 600-621).

+ The Discovery of Gluons. Proceedings of the International Symposium on 30 Years of Neutral Currents from Weak Neutral Currents to the (W)/Z and Beyond, February 3-5, 1993, Santa Monica, California (pages 598-623).