Haichen Wang
Assistant Professor
Department of Physics, University of California, Berkeley
423 Physics South Hall, Berkeley CA
Faculty Scientist
Physics Division, Lawrence Berkeley National Laboratory
1 Cyclotron Rd
Berkeley, CA 94720
(Last updated: March 2021)


PhD, University of Wisconsin-Madison, 2013

BS, Peking University, 2007


Research Interests:

I am an experimental particle physicist, and I use particle collider to study elementary particles and their interactions. My current research program is focused on analyzing data delivered by CERN's Large Hadron Collider (LHC) to discover new particles and new interactions, and on the construction of the new tracking detector for the High Luminosity LHC program. Specifically, my research efforts are carried out in the context of the ATLAS experiment at the LHC, and they include

  • Probe new physics through precision measurements of the Higgs boson properties
  • Search for physics beyond the Standard Model
  • Upgrade the ATLAS detector for the High Luminosity LHC program

In 2021, I received a CAREER award from the National Science Foundation to discover new physics at the LHC through the development and deployment of novel machine learning applications in the data analysis and the construction of silicon trackers for the High Luminosity LHC.

See the description below for more information -

Meausring the Properties of the Higgs boson

At the beginning of my scientific career, I was fortunate enough to be in a position to make important and direct contributions to the discovery of the Higgs boson at the Large Hadron Collider. The picture on the right was taken at CERN Building 32 RA-14, shortly before the midnight of June 24, 2012, when it became clear we discovered the Higgs boson. On my monitor, you can see the p-value, diphoton mass distribution, the signal strength, and 95% confidence limit from the Higgs → γγ channel.

Since then, a main component of my research program has ben using the Higgs boson as a tool to explore the unknown in particle physics. Between April 2018 and March 2020, I was one of the co-conveners to lead the ATLAS HGamma working group, consisting of over 100 physicists from more than 20 international research and educational institutions. The primary activity of this group is to study the Higgs boson property and search for physics beyond the Standard Model using Higgs boson decays that involve photons.

Recently, my group made significant progress in understanding how the Higgs boson interacts with the heaviest elementary particle, the top quark, through a series of measurements of the associated production of the Higgs boson with top quarks (ttH). This process is important to study because it provides a direct access to the top-Higgs Yukawa coupling. As shown in the left figure below, the Feynman diagram of the ttH process involves the interaction between the Higgs boson and the top quarks. This interaction is referred to as the t-H Yukawa coupling, and its strength is proportional to the mass of the top quark. The top quark, having a mass of about 173 GeV, is the heaviest elementary particle in the Standard Model. Its Yukawa coupling is at the order one and is significantly larger than the Yukawa couplings of other elementary fermions. This gives reasons to particle physicists to suspect that careful measurements of the ttH process may uncover physics beyond the Standard Model.

In 2018, both the ATLAS and CMS experiments observed the ttH production, using a combination of several Higgs boson decay channels. In 2019, we further established the observation of the ttH process in a single decay channel of the Higgs boson, and the result was highlighted at the Moriond conference in La Thuile. The right figure above shows the distribution of the invariant mass of two photon system in collision events selected as the ttH candidates. The excess at around 125 GeV corresponds to the decay of the Higgs boson to photons.

In 2020, my group completed a very first direct test of the CP property of the top-Higgs interaction using the ttH process, which probes mechanism that may break the symmetry between matter and anti-matter in our Universe. I wrote this article for the ATLAS Physics Briefing, and you can find more about this study there.

The investigation of the top-Higgs Yukawa interaction using the ttH process with the ATLAS data taken during the LHC Run-2 was written up in Berkeley graduate student Jennet Dickinson's Ph.D. thesis, which was recognized with a 2021 ATLAS Thesis Award.

Currently, we are investigating an even more rare SM process in which four top quarks are produced in a single proton-proton collision. This is an extremely complex final state, and yet the rate of this process is senstive to new physics contributions, particularly those that modify the top-Higgs interaction. Part of our NSF supported effort is to develop and deploy advanced machine learning applications to improve the sensitivity of the ATLAS experiment to such kind of processes.

Search for Physics beyond the Standard Model

The search for Physics Beyond the Standard Model is one of the primary goals of the LHC physics program. At Berkeley, I have led a number of searches for new physics using the data collected from the LHC Run-2. These included a search for the production of TeV-scale black hole in the high energy collisions of protons at the LHC, and a search for massive supersymmetric particles decaying to jets via R-parity violating couplings. While these searches did not lead to discovery of new physics, they yielded the most stringent constraints on the respective theoretical models at the time.

My current BSM effort has been focused on the search for exotic decays of the Higgs boson, in particular, one in which the Higgs boson decays to long-lived particles that result in delayed and non-pointing photons in the collision event. Processes like this is not well constrained and there is still room for BSM particles to hide. In order to detect delayed and non-pointing photons, we need to explore the unique capabilities of the ATLAS Liquid Argon Calorimeter, namely, the timing measurement and pointing measurement. Depending on the locality of the incident photon, the detector can measure the resolution of the photon arrival time with a resolution down to about 200 pico-seconds, and the displacement between the z position of the photon origin and the interaction point with a resolution down to a few centimeters, enabling the detection of the "long-lived"-ness of the new particle that decays to photons. We are investigating novel ways to represent detector data, in order to apply machine leanring techniques to significantly improve the detecter's capability of measuring photon pointing.

At this point, the total width of the Higgs boson is not particularly well constrained, and therefore, the branching ratio of the Higgs boson decaying to non-SM final states can be up to 20%. This ongoing search, once completed, will close a large chunk of sensitivity gap in the current LHC search program.

Recently, I have initiated a new search for exotic production of the 125 GeV Higgs boson. This search will systematically examine the Higgs boson data collected by the ATLAS experiment and probe possible unknown ways in which the Higgs boson could be produced.

Detector R&D for future colliders

The Large Hadron Collider was initially turned on in 2009. Its first extended data taking period was between 2010 and 2012, a.k.a., the Run-1, during which the Higgs boson was discovered. From 2015 to 2018, the LHC completed its Run-2, delivering a data set approximately 14 times the data set used for the Higgs boson discovery. The energy of the LHC Run-2 was also much higher than its Run-1 energy. This large data set is yet to be fully explored. The LHC is expected to enter its Run-3 in 2021 and delivers data until 2024. In between extended data taking periods of the LHC, there are long shutdowns, in which both the LHC accelerator complex and the collider detectors typically undergo maintenance and upgrade. Significant upgrade activities are expected to take place between 2025 and 2026, so that the LHC will be operated under the "high luminosity" configuration afterwards. The High Luminosity LHC (HL-LHC) physics program will last until approximately 3000 fb-1 data is delivered. This data set is approximately 300 times the data set used for the Higgs boson discovery, and is considered to be sufficient to significantly improve the Higgs boson property measurements and extend the sensitivity of the BSM searches.

The “Phase-2” upgrade of the ATLAS detector, to enable physics at the HL-LHC, is a large international program with significant participation from US institutions, funded by the Department of Energy (DOE) and the National Science Foundation (NSF). Technical and engineering effort, and procurements are supported by construction projects which are making their way through the DOE Critical Decision, and NSF review processes. Complementary to the construction projects are significant contributions by faculty, laboratory scientific staff, postdoctoral researchers, and graduate students, supported by the respective research programs. In the language of the construction projects, these are the “sceintific efforts” and are a vital element of the upgrade.

The Strip Tracker upgrade is part of a new Inner Tracker (ITk) for the ATLAS experiment at the HL-LHC and is the largest in scale among all the ATLAS upgrade projects. The Strip Tracker upgrade project is at a stage where the Final Design Reviews (FDRs) will take place in late summer and early fall of 2019. The production of tracker components will soon ramp up. Given the unprecedented instantaneous and integrated luminosities at the HL-LHC, it is critical to verify that the Strip Tracker components used for production, in particular, the front end Application Specific Integrated Circuits (ASICs), meet the specification of radiation tolerance. Irradiation studies need to be performed for quality assurance; their findings will also inform future detector operation. In the production phase, approximately 6000 modules, corresponding to half of the modules in the barrel part of the Strip Tracker, will be built in the US. It is imperative to develop and implement a high throughput electrical testing program to efficiently and effectively scrutinize the production modules.

My research group, including myself, postdocs, graduate students and ocasionally undergraduate students, have been leading some of the above activities, and will continue to play an important role in the upgrade of the ATLAS detector for the High Luminosity LHC.

Group members and local collaborators

Graduate students - current graduate students working with me and/or supported as GSRA by me include Ryan Roberts, and Sai Neha Santpur. Past: Brian Amadio, Jennet Dickinson

Undergraduate students - current: Earl Russell Almazan, Allison Xu, Shikai Qiu, Digvijay Roy Varier. Past: Samuel Bright-Thonney (now graduate at Cornell), Sicong Lu (now graduate at Penn), Alex Wang (now graduate at Wisconsin), Paul Kottering, Donald Aurum, Manuel Silva Jr. (now graduate at Wisconsin), Charles Cooper (visiting from Chicago), Chenji Han (BIPE student visiting from UCAS, now graduate student at Caltech), Miela Gross, Ximeng Yu, Nikita Shalimov, Rohith Karur

SULI and BLUR Interns - current: Pamela Pajarillo

Postdoc - Dr. Shuo Han

Local collaborators at UC Berkeley and LBNL - an incomplete list of UC faculty and LBL scientists I collaborate with closely: Professors Marjorie Shapiro and Heather Gray, Dr. Ian Hinchliffe, Dr. Carl Haber, Dr. Alessandra Ciocio, Dr. Timon Heim, Dr. Fabio Cerutti, Dr. Simone Pagan-Griso, Dr. Zachary Marshall, Dr. Xiangyang Ju, Dr. Weiming Yao, Dr. Hongtao Yang, Dr. Maurice Garcia Sciveres, and Dr. Kevin Einsweiler

also see the homepage of the LBL ATLAS group here.

Write to me by email if you are
  1. UC Berkeley graduate students or incoming students who are interested in joining the ATLAS experiment
  2. Prospective students who are submitting a graduate schoool application for Berkeley Physics
  3. Graduate students at other institutions who are interested in visiting LBNL or applying for postdoc position at UC Berkeley or LBNL

UC Berkeley undergraduate students who want to conduct research at the ATLAS group should apply for the URAP at the beginning of a semester. Visiting students through UC Berkeley Extension should also submit application through URAP so that we can vet you.

To get a sense what we do with the ATLAS Experiment at Berkeley, you can look at Physics 251 presentations I gave to first-year Berkeley graduate students in recent years here: 2018, 2019, 2020

Selected Publications
  1. CP Properties of Higgs Boson Interactions with Top Quarks in the ttH and tH Processes Using H to Diphoton with the ATLAS Detector, Phys. Rev. Lett. 125, 061802 (2020)
  2. Combined measurements of Higgs boson production and decay using up to 80 fb−1 of proton-proton collision data at s√= 13 TeV collected with the ATLAS experiment, Phys. Rev. D 101 (2020) 012002
  3. Observation of Higgs boson production in association with a top quark pair at the LHC with the ATLAS detector, Phys. Lett. B 784 (2018)
  4. Measurements of Higgs boson properties in the diphoton decay channel with 36.1 fb−1 pp collision data at the center-of-mass energy of 13 TeV with the ATLAS detector, Phys. Rev. D 98 (2018), p. 052005
  5. Search for massive supersymmetric particles in multi-jet final states produced in pp collisions at 13 TeV using the ATLAS detector at the LHC, Phys. Lett. B 785 (2018)
  6. Search for strong gravity in multijet nal states produced in pp collisions at s = 13 TeV using the ATLAS detector at the LHC, JHEP 1603 (2016) 026
  7. Real Time Tracker Based Upon Local Hit Correlation Circuit for Silicon Strip Sensors, Nuclear Inst. and Methods in Physics Research, A (2016), pp. 21-29
  8. Expected Performance of the ATLAS Inner Tracker at the High-Luminosity LHC, ATL-PHYS-PUB-2016-025 (2016)
  9. Technical Design Report for the ATLAS Inner Tracker Strip Detector,CERN-LHCC-2017-005 ; ATLAS-TDR-025 (2017)
  10. Evidence for the spin-0 nature of the Higgs boson using ATLAS data, Phys. Lett. B 726 (2013), pp. 120-144
  11. Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC, Phys. Lett. B 716 (2012) 1-29
  12. Search for the Standard Model Higgs boson in the diphoton decay channel with 4.9 fb-1 of pp collisions at sqrt{s} = 7 TeV with ATLAS, Phys. Rev. Lett. 108, 111803 (2012)
  13. Search for Diphoton Events with Large Missing Transverse Energy in 7 TeV Proton-Proton Collisions with the ATLAS Detector Phys.Rev.Lett. 106 (2011) 121803