FindingThe Higgs

BosonA (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 BosonoBump HuntingoSpecial Relativity

• July 4th: the data• Detector performance• CMS and ATLAS results• Stat. Confidence of Discovery

Standard Model Higgs(part 1)

Standard Model does not predictthe 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 (sh) in “barns”

• What does this mean for us?

B. Todd Huffman, Oxford University

Cross Section is an area. 10 pb = 10-35 cm2.

Brightness = Lum. # Particles per cm2 per second =

1034 n/(cm2s)

Reason for Radiation Hard

Electronics

Mh = 125 GeV/c2 SM Higgs Production Rate = 10-35 cm2 x 1034 cm-2s-1

= 0.1 Hz or one every 10 seconds.

But Hang on!spp ~ 100 mb

that’s “millibarns”With L = 1034 cm-2s-1 random interactions a billion times a second. (not Higgs)

Beam bunches cross once every 50 ns.

B. Todd Huffman, Oxford University

50 INTERACTIONS/CROSSING

The LHCOnce the Energy is fixed(ring size)

Then the only thing we can tweak is Luminosity.

This is a hard problem.

More Predictions:Higgs Decays

g g

Irreducible photon processes

B. Todd Huffman, Oxford University

quark

Photon (g)

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

Zo

Zo

l+l- l- l+

Anti-quark

Why did we find it in the decays that are so

rare?

Higgs to gg → 100 per year

Higgs to ZZ* → 1000 per year (but Z to ee, mm means ~5 per year)

One Step back: Special Relativity

Two things happen!

Explosion A

At time t0 and location x0

Decay B

At time t1 and location x1

But what if we were moving really fast to the left?

B. Todd Huffman, Oxford University

V

Two things happen!

Explosion A

At time t0’ and location x0’

Decay B

At time t1’ and location x1’

The order and distance depends on the speed you travel!

B. Todd Huffman, Oxford University

Special Relativity

Time t0 ; location x0

t1 ; x1

But this quantity is the same in ALL frames of reference.

 

Invariant Scalar

Special Relativity• Momentum and Energy do this too!

E2 - P2C2 = M2C4• No momentum, P = 0, then you get E = MC2

• Throw in this fact of nature:o Energy and momentum are conserved. ALWAYS

g0g1Mhiggs

Invariant Mass

E2-P2C2=M2C4

Mhiggs

(E0 + E1)2 – (P0 + P1)2c2 = M2higgsc4

Works for any number of particles.Works no matter how fast or slow the Higgs is moving in the lab.

e-

e+

-m

+m

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

Zo

Zo

l+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 eachone into the formula to find their scalar invariant mass.

Mostly not Higgs. The scalarformula then puts a pip randomly on this histogram

If there really is a parent ALL combinations land at Mhiggs; 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 TheoristPrior to Managing a Hedge Fund

B. Todd Huffman, Oxford University

B. Todd Huffman, Oxford University

ATLAS

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

B. Todd Huffman, Oxford University

CMS

• Some Powerful detectors (e.g. tracker).• Less demanding on muon chamber technology.

Why The Pain is Worth It

• Backgroundso H g g

• Protons have quarks with electric charge. Two photons can result when q-qbar’s annihilate

• Neutral pions decay to photons po g go Bad News; Quark jet could fake a photono CMS and ATLAS detectors built to ID pions this way.

o H Z Z* then Z e+ e- or m+ m-• 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 exploitspo g g

B. Todd Huffman, Oxford University

Fine strip segmentation

Very Useful!

B. Todd Huffman, Oxford University

The Data – g gCMS ATLAS

B. Todd Huffman, Oxford University

The Data - ZZ*ATLAS CMS

B. Todd Huffman, Oxford University

Next: Detector Resolution

Accurate Measurement

Much Pain: to obtain track resolutions less than ten microns.

To measure m and eenergy as accuratelyas possible

B. Todd Huffman, Oxford University

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

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)

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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. Higgs F. Englert T. HagenG. 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, 2008.

• 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, 2nd Ed.”, Adam Hilger, Bristol.

B. Todd Huffman, Oxford University