Finding The Higgs Boson A (hopefully) slightly better explained version of the events around July 4, 2012 Dr. B. Todd Huffman, Oxford University Dr. A. Weidberg, Oxford University

Explanation in two parts Particle Physicists  coffee Finding the Higgs Standard Model Higgs properties How to Find the Boson o Bump Hunting o Special Relativity July 4 th : the data Detector performance CMS and ATLAS results Stat. Confidence of Discovery

Standard Model Higgs (part 1) Standard Model does not predict the Higgs Mass though. Start with Higgs boson as a given. Standard Model is a quantitative theory. o Predicts Probability of a Higgs boson at the LHC Prediction is the cross- section (  h ) in “barns” What does this mean for us? B. Todd Huffman, Oxford University

Cross Section is an area. 10 pb = cm 2. Brightness = Lum.  # Particles per cm 2 per second = n/(cm 2 s)

Reason for Radiation Hard Electronics M h = 125 GeV/c 2 SM Higgs Production Rate = cm 2 x cm -2 s -1 = 0.1 Hz or one every 10 seconds.  pp ~ 100 mb that’s “millibarns” With L = cm -2 s -1 random interactions a billion times a second. (not Higgs) Beam bunches cross once every 50 ns. B. Todd Huffman, Oxford University

The LHC Once the Energy is fixed (ring size) Then the only thing we can tweak is Luminosity. This is a hard problem.

More Predictions: Higgs Decays 

Irreducible photon processes B. Todd Huffman, Oxford University quark Photon (  ) Standard Model shape: Number of photon pairs vs. energy time

Irreducible ZZ* processes B. Todd Huffman, Oxford University Standard Model shape: Number of 4-lepton pairs vs. energy quark ZoZo ZoZo l+ l- l+ Anti-quark

Why did we find it in the decays that are so rare? Higgs to  → 100 per year Higgs to ZZ * → 1000 per year (but Z to ee,  means ~5 per year)

One Step back: Special Relativity Two things happen! Explosion A At time t 0 and location x 0 Decay B At time t 1 and location x 1 But what if we were moving really fast to the left? B. Todd Huffman, Oxford University

V Two things happen! Explosion A At time t 0 ’ and location x 0 ’ Decay B At time t 1 ’ and location x 1 ’ The order and distance depends on the speed you travel! B. Todd Huffman, Oxford University

Special Relativity Time t 0 ; location x 0 t 1 ; x 1 But this quantity is the same in ALL frames of reference. Invariant Scalar

Special Relativity Momentum and Energy do this too! No momentum, P = 0, then you get E = MC 2 Throw in this fact of nature: o Energy and momentum are conserved. ALWAYS   M higgs

Invariant Mass M higgs (E 0 + E 1 ) 2 – (P 0 + P 1 ) 2 c 2 = M 2 higgs c 4 Works for any number of particles. Works no matter how fast or slow the Higgs is moving in the lab. e- e+   Does not work if they did not come from a Higgs B. Todd Huffman, Oxford University

Irreducible ZZ* processes B. Todd Huffman, Oxford University Standard Model shape: Number of 4-lepton pairs vs. energy quark ZoZo ZoZo l+ l- l+ Anti-quark time

Higgs Bump Hunting Many events have 4 lepton or two photon candidates. So just plug E and p of each one into the formula to find their scalar invariant mass. Mostly not Higgs. The scalar formula then puts a pip randomly on this histogram If there really is a parent  ALL combinations land at M higgs ; every time. 6 months

2 years B. Todd Huffman, Oxford University

Glad I did not book a flight to Stockholm. 15 years Last Paper for Theorist Prior to Managing a Hedge Fund B. Todd Huffman, Oxford University

ATLAS Features: o Standalone muon spectrometer (air-core toroid). o Conventional EM calorimeter (Pb/LAr).

B. Todd Huffman, Oxford UniversityCMS Some Powerful detectors (e.g. tracker). Less demanding on muon chamber technology.

Why The Pain is Worth It Backgrounds o H   Protons have quarks with electric charge. Two photons can result when q-qbar’s annihilate Neutral pions decay to photons     o Bad News; Quark jet could fake a photon o CMS and ATLAS detectors built to ID pions this way. o H  Z Z* then Z  e+ e- or  +  - Proton-Proton  Z Z* happens too No Higgs involved “Irreducible Background” We must deal with Backgrounds Careful Detector design. B. Todd Huffman, Oxford University

Geometric exploits     B. Todd Huffman, Oxford University Fine strip segmentation Very Useful!

B. Todd Huffman, Oxford University

The Data –   CMSATLAS B. Todd Huffman, Oxford University

The Data - ZZ* ATLASCMS B. Todd Huffman, Oxford University Next: Detector Resolution

Accurate Measurement Much Pain: to obtain track resolutions less than ten microns. To measure  and e energy as accurately as possible B. Todd Huffman, Oxford University

What would happen if tracking resolution was worse? LHC 2 years Meaning: What if the momentum we measure is further away from the true momentum?

B. Todd Huffman, Oxford University

Would have published earlier.

How do we know this is real? “The Data were inconclusive, so we applied Statistics” (A quote taken from Louis Lyons’ book) B. Todd Huffman, Oxford University

15 years Random events can, occasionally, fake a signal. B. Todd Huffman, Oxford University Basic Question: What is the probability, if it IS just random, that this “signal” is just a fake?

Discovery! And Limits ATLAS CMS

Nobel Choices B. Todd Huffman, Oxford University P. HiggsF. Englert T. Hagen G. Guralnik R. Brout (deceased) Who will win the prize? T. Kibble Any other questions?

References David Griffiths, "Introduction to Elementary Particles, 2nd ed.", Wiley-VCH, Weinheim, Germany, F. Halzen and A. D. Martin, “Quarks & Leptons: An Introductory Course in Modern Particle Physics” John Wiley & Sons. I. J. R. Aitchison and A. J. G. Hey, “Guage Theories in Particle Physics, 2 nd Ed.”, Adam Hilger, Bristol. B. Todd Huffman, Oxford University