the higgs boson and new physics at the large hadron...

April 8, 2020 Ian Lewis (University of Kansas) 1

The Higgs Boson and New Physics at the Large Hadron

Collider

Ian LewisUniversity of Kansas


July 4, 2012

● ATLAS and CMS announce discovery of a new particle.– Consistent with long sought-after Higgs boson.

"We have reached a milestone in our understanding of Nature". --- CERN Director General Rolf Heuer


Long Search

● 50+ years of work by theorists.● 25+ years of work by thousands of experimentalists.


Standard Model Complete

Quarks: charge +2/3 (up type) and -1/3 (down type)Leptons: charge -1 and 0


Role of the Higgs

● Higgs is the source of fundamental mass in the Standard Model.– Important for

understanding fundamental laws of nature.


Masses● Many massive

particles in Standard Model

● Massive Force Carriers.– W/Z.

● Photon and gluon are massless.

● What is the matter with mass?

● Natural units


What's the matter with mass?● Maxwell's equations:

● Invariant under the transformation (gauge invariance):

● Add mass, break gauge invariance:

● Why photon (gauge boson) is massless.


The Need to Explain Masses

● Have masses for gauge bosons.● What is source of gauge invariance breaking?● Explicit breaking:

– Equations of theory explicitly break an invariance.● Spontaneous breaking:

– Lowest lying energy state (vacuum) of theory breaks invariance.


Ferromagnetism● Before magnetization:.

● After magnetization:

● Spontaneous symmetry breaking.


Higgs Mechanism

● Introduce a Higgs.● Write fundamental equations (with Higgs) invariant

under all transformations.● Vacuum breaks gauge invariance.

– Higgs obtains a nonzero value throughout space.● Vacuum expectation value,

– Particles interact with Higgs vacuum expectation value, gaining mass.

● More massive particles have stronger interactions with the Higgs.


Standard Model Higgs Boson● Introduce complex Higgs with four degrees of

freedom.● Three degrees of freedom absorbed into weak

force carriers ( ) giving them masses.● One degree of freedom left, the physical Higgs

boson, h.


Large Hadron Collider (LHC) Overview

● 17 mile/27 km ring outside Geneva, Switzerland.

● Colliding protons at a center of mass energy of 7-14 TeV.– ~10 mph less than speed

of light.

– ~1.5 GJ of energy stored at 14 TeV (Aircraft carrier traveling ~20 mph/32 kmh)

● Purpose is to discover new physics at the TeV scale.

Hadron Collider● Hadrons (like a proton) are made of quarks and gluons.● LHC collides two protons at very high energy (7-14 TeV).● Constituents of proton annihilate at a typical energy of

~ 1 TeV:


LHC


Compact Muon Solenoid (CMS)


Detecting Final State


Amount of Data Produced● Over 1-2 billion collisions per second per

experiment.– Amount of data produced is 1 petabyte per second– Could fill ~200,0000 DVDs per second– Comparable to total amount of digital data produced

worldwide.● Experiments store and analyze less.

– Around 30 petabytes per year.– ~6,000,000 DVDs per year stored to be analyzed


Higgs Discovery!

● July 4, 2012 (mass of Cesium atom)● Created around 13,000,000 Higgs in through 2020.


Higgs production rate is small

Higgs rate is small, need to dig signal out of all the other Standard Model processes.


Higgs Production● Masses in Standard Model come from Higgs

mechanism.

– Completely predictive.– Vacuum expectation value

● Protons made mostly of light quarks and gluons.

●


Quantum Effects to the Rescue● Top quark can mediate coupling to gluon.● Dominant production mode at the LHC.

●


Other Production Modes


Higgs Production Rates

Other subdominant processes depend on W/Z and top quark couplings.

What about decay?


● WW and ZZ probes gauge boson mass generating mechanism.● Decays to di-photon at 0.2% of the time


Di-Photon

● Higgs discovered using quantum production and decay modes!


Reconstructed Higgs Mass

ATLAS-CONF-2018-018 ATLAS-CONF-2018-028

Higgs to 4 Lepton Higgs to Diphoton


Masses and Higgs Couplings

● Remarkably Standard Model like.

● Have measured Higgs rates to 10-20%.


Long Shut Down 2

● 2023 : 2 times current data ● 2030s: 20 times current data


Future Higgs Boson Measurements


What do Higgs measurements tell us?

● Consider very massive new physics.– Standard Model is leading order in a power

expansion of energies.– Precision measurements bound the next order in

the expansion:

– Where is the scale of new physics● Then measuring rates to ~5% is sensitive to

new physics scales in the TeV range.


Higgs is Central● Production and decay modes quantum effects.

– Sensitive to new physics● Expect new physics to be related to Higgs boson

properties.– Source of mass just starting to be probed.– Standard Model is simplest realization of this

mechanism.


How does Higgs obtain Vacuum Expectation Value?

● Important to test precisely the mechanics of generation of spontaneous breaking.– Higgs coupling measurements test if Higgs

generates mass in Standard Model.– How does the Higgs get vacuum expectation value?

● Consider harmonic oscillator example.


Harmonic Oscillator Example● Stored Potential energy:

– Invariant under

● Force equation:


Deformed Harmonic Oscillator● Stored Potential energy:

– Invariant under

● Shift to a minimum:

– Invariance not manifest at minimum:


Higgs Potential● Higgs potential:

● Have minimum:

● Expand about vacuum:


Higgs Self-Interactions● Higgs Potential:

● Potential has two parameters, everything determined:


● Need to measure potential to test Standard Model● Double Higgs production sensitive to trilinear coupling:

● Probing Higgs potential, source of mass. ● All couplings known in Standard Model.

Measuring Higgs Potential


Future Measurements of Potential

Di Micco et al., arXiv:1910.00012 [hep-ph]


Extended Scalar Sector● Standard Model Higgs source of all (fundamental)

mass.– Can have multiple sources of gauge invariance breaking.– Two Higgses are possible, sometimes required.

● New source of new gauge invariance breaking contribute to W/Z masses.– Alter Higgs couplings to W/Z

● Precision measurements of Higgs test if there are new sources of gauge invariance breaking.– Can even probe new scalars unrelated to gauge

invariance breaking.


New Scalar

● Consider a new scalar with no Standard Model charges

● After gauge invariance breaking, the Higgs boson has no charge.

● The two can mix quantum mechanically.

● Changes Higgs couplings


Higgs Couplings● Write equations in gauge invariant way.● Rotate into mass eigenstate basis● Original particles are superpositions of mass

eigenstates.● New scalar gains couplings to SM particles.● Higgs couplings are universally suppressed.


Higgs Precision Measurements Test This Scenario

● Suppressed coupling compared to Standard Model expectation.

● Simple interpretation of combined measurement of many production and decay channels:

Adhikari, I Lewis, Sullivan arXiv:2003.10449 [hep-ph]


New Scalar Potential● Any new scalar will alter potential

– S has no couplings to anything else in Standard Model● New potential

– Interaction terms:

– Expansion about vacuum – New Higgs Interactions.


Higgs and Scalar Interactions

● Comes from:


Double Higgs Resonance

● New Higgs process.● Resonant production can be

large.● There’s a complication: Higgs

potential has many minima now


Altering the Higgs potential● Implications of new scalars:

– Many more minima in potential● One Higgs: Minimize with respect to Higgs, calculate the

value of the Higgs field at the minimum (vacuum expectation value)

● Multiple scalars: Minimize with respect to all scalars.

● Global minimum must have correct gauge invariance breaking

– That is, gauge bosons have to have the mass that they have been measured to have.

– The scalar S cannot give W or Z masses.– Higgs vacuum expectation value has to be the same as in

the Standard Model.


Double Higgs Resonance● New Higgs process.● Potential has many minima

now

– Vacuum expectation value of S cannot give mass to W/Z.

– Require minimum with Standard Model Higgs vacuum expectation value is global minimum.

– Affects parameters in potential, and hence S-h-h coupling


Large Enhancements

Chen, Dawson, IL Phys.Rev. D91 (2015) 035015

● Ratio of double Higgs rate with S included to double Higgs rate in Standard Model

● Dashed lines have incorrect vacuum expectation value for Higgs.


● Ratio of double Higgs rate with S to Standard Model prediction

● Current Limit:

sin2q < 0.05

● Can still get upwards of 10 times Standard Model predictions

I Lewis, Sullivan Phys.Rev. D96 (2017) 035037


New InteractionsAdhikari, IL, Sullivan arXiv:2003.10449 [hep-ph]


Beyond the Simplest Model● Previous slides had the simplest possible model: Add a

new scalar that only couples to the Higgs.● That is the simplest self-consistent model, but there can

be additional new physics at high energies that cannot be produced at the LHC.

● This new physics introduces new “effective interactions” between the new scalar and Standard Model particles:– Scalar-gluons– Scalar-Weak force carriers– Scalar-fermions

● Also, on general principle, these new interactions cannot be avoided.


New Effective Interactions

● L is the mass scale of some heavy new physics.● Mix the scalar and the Higgs, and Higgs inherits these new interactions:


New Interactions● Higgs inherits new interactions:

● The production rates of Higgs are no longer simply suppressed by a mixing angle, there are new contributions altering the interpretation.


Higgs Measurements

● Blue dashed: no new interactions. Red and black: with the new interactions● Limits on sin q are drastically changed even with new physics at 3 TeV, an

order of magnitude above the Higgs scale.

Adhikari, IL, Sullivan arXiv:2003.10449 [hep-ph]


Scalar Searches● New production mechanisms for heavy scalar resonance

● LHC has many searches for the resonant production and decay of new scalars.


Combined Scalar Searches and Higgs Measurements

● Blue dashed: no new interactions. Black: with the new interactions● Limits on sin q are drastically changed even with new physics at 3 TeV, an

order of magnitude above the Higgs scale.

Adhikari, IL, Sullivan arXiv:2003.10449 [hep-ph]


Conclusions● LHC had two very successful runs.● Higgs boson discovery helps us to begin to understand the origin of

fundamental mass in the Standard Model.● LHC has been running at 13 TeV accumulating massive amounts of

data.● Higgs measurements sensitive to new physics.

– Test the origin of the fundamental masses of particles and understand symmetry breaking.

– Precision Higgs measurements and new physics searches still have much to tell us.

– Even in models we think we now well, “decoupled” new physics can make significant impact.

– We need precision measurements, and many of my fits depend on high precision calculations as well.

– There is still much to learn about the Standard Model and new physics, and we have not fully explored what new physics could appear at the LHC.

– In fact, from a theory point of view, I would argue that the effects I have shown are inevitably there and cannot be ignored.


Thank You


EXTRA SLIDES


Electroweak Baryogenesis


Baryon Asymmetry of the Universe

● We know there is more matter than anti-matter.● In 1967 Andrei Sakharov gave three conditions to generate a matter/anti-

matter asymmetry:– 1) Need Baryon number violating processes.

● If conserved, the amount of matter in = the amount of matter out.

– 2) Need charge conjugation and charge conjugation-parity violation.● There can be processes that generate more matter than anti-matter

– 3) need out of equilibrium interactions.● If baryon number violating processes in thermal equilibrium, they can be reversed and wash-out

any asymmetry.

● Standard Model has baryon number violating processes, and charge conjugation/charge conjugation-parity violation (but not enough)

● Need out of equilibrium interactions.– Can be decays of heavy particles that are not in thermal equilibrium.– Can appear in Higgs physics.


Strong First Order EW Phase Transition

● In early Universe, at high temperature, electroweak symmetry is restored.

● As temperature decreased, EW symmetry broke.

● If this breaking is first order, we have out of equilibrium interactions.– Tunnel from to

.– In second order phase

transition, smoothly transition from to

.– The SM is a second order

phase transition.– Need new physics.


Strong First Order EW Phase Transition

● EW symmetry breaking comes from the Higgs sector.

● To get strong first order EW phase transition, need to alter the Higgs potential.

● Measuring Higgs properties is vital to probing this scenario.

● Also, to change Higgs potential significantly, need new physics near the Higgs scale.– This scenario has a definite

scale attached to it that cannot be arbitrarily increased.

● Simplest to add a new scalar singlet, and it does the job.– Nice simple, benchmark model

to test the falsifiability of this scenario.


Zero Mixing Limit● Couplings between scalar and Higgs:

● Source of Higgs-scalar mixing is:

● In the limit of zero mixing a1→0 and only a2 survives

● If the scalar S does not mix with the Standard ModelHiggs, it only couples to the Higgs. Very difficult to produce and can be stable.

● a2 is the only term to drive the first order phase transition.

– Lower limit on how large it can be.– Gives h-h-S-S and h-S-S couplings, and so we have a lower bound on these.


Non-Zero Mixing

● Many di-boson production modes, all with different information about the potential.– Can have resonant di-Higgs if allowed.

● SS production depends on h-S-S coupling.– This is the coupling that must stay non-zero to have a strong first order electroweak phase

transition.


High Luminosity LHC

● 3 ab-1 at 14 TeV LHC.● Comparison of different methods of searching. ● Colored Dots: Compatible with strong first order electroweak phase transition.● Searches for h2h2 production: Yellow: Exclusion, Green: Discovery

● Red dashed curves: Higgs self-coupling limits at 30%.

Chen, Kozaczuk, Ian Lewis JHEP 1708 (2017) 096


100 TeV

● 30 ab-1 at 100 TeV, can probe much of the parameter space.● Colored Dots: Compatible with strong first order electroweak phase transition.● Searches for h2h2 production: Yellow: Exclusion, Green: Discovery

● Red dashed curves: Higgs self-coupling limits to 15%. Solid lines: Higgs-Z-boson coupling limits to 0.5%



Resonant Double Higgs Production

● Including interference effects important for determining viable parameter regions for strong first order electroweak phase transition.

Carena, Liu, Riembau PRD 97 (2018) 095032

● Much focus on relationship between resonant double Higgs production and a strong electroweak phase transition in the singlet model

Huang, et. Al PRD96 (2017) 035007; Profumo et al PRD91 (2015) 035018; Alves, Ghosh, Guo, Sinha 1808.08974; etc.


Additional Non-Resonant Modes

● New final states h1h2 and h2h2

● Different final state dominate in different parameter regimes.● Measurement of Higgs trilinear important.● Different production modes dominate in different regions.


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