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Gustaaf Brooijmans CERN 2017
The Road to Discovery Searching for BSM Physics at Hadron Colliders
Gustaaf Brooijmans
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Gustaaf Brooijmans CERN 2017
Goals❖ Searches span very wide spectrum - Overview of many types of searches, with focus on experimental aspects
- Challenges posed at hadron colliders
- Ease of generating false positives
- Techniques to deal with limited knowledges
- But far from exhaustive!
❖ Thread in results - Hints?
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Gustaaf Brooijmans CERN 2017
Standard Model Today
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Triumph of Gauge Theories!
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Gustaaf Brooijmans CERN 2017
Standard Model Today❖ Higgs discovery completes the
Standard Model - Fully consistent, complete, precise
description of strong, electromagnetic and weak interactions
❖ Even generate fermion masses - But that is the only property of
fermions we “understand”
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Gustaaf Brooijmans CERN 2017
In Words❖ Matter is built of spin 1/2 particles that interact by exchanging 3 different
kinds of spin 1 particles corresponding to 3 different (gauge) interactions
❖ There appear to be 3 generations of matter particles
❖ The 4 different matter particles in each generation carry different combinations of quantized charges characterizing their couplings to the interaction bosons
❖ The matter fermions and the weak bosons have “mass”
❖ Gravitation is presumably mediated by spin 2 gravitons
❖ Gravitation is extremely weak for typical particle masses
❖ There appear to be 3 macroscopic space dimensions
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Gustaaf Brooijmans CERN 2017
About the Standard Model❖ It’s a theory of interactions: - Properties of fermions are inputs
- In gauge paradigm, fermion properties “generate” interactions
- Properties of interaction bosons in terms of couplings, propagations, masses are linked:
- Measuring a few allows us to predict the rest, then measure and compare with expectation
❖ It’s remarkably successful: - Predictions verified to be correct at sometimes incredible levels of precision
- After ~40 years, still no serious cracks
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Gustaaf Brooijmans CERN 2017
Precision Results
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muon g-2: 0.7 ppm!
B, K physicsLEP, SLD & Tevatron
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Gustaaf Brooijmans CERN 2017
Lacking in the Standard Model❖ Clear structure in fermionic sector
unexplained - No understanding of the “charges”
- Evidence of selective principle(s) - E.g. no neutral colored fermions
- q(down) = q(e)/Nc
- Interpreted as evidence for (grand) unification
- Grand or less grand? (One or more scales?)
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Gustaaf Brooijmans CERN 2017
Lacking in the Standard Model❖ Many cosmological issues - Dark matter and dark energy
- Not enough CP violation in the quark sector for baryogenesis
- Baryon number violation - Present in the SM through B-L (sphalerons)
- Baryogenesis through leptogenesis and B-L?
‣ Untestable?
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Gustaaf Brooijmans CERN 2017
Many Fundamental Questions❖ What exactly is spin? Or color? Or electric charge? Why are they
quantized? ❖ Are there only 3 generations? If so, why? ❖ Why are there e.g. no neutral, colored fermions? ❖ What is mass? Why are particles so light? ❖ Is there a link between particle and nucleon masses? ❖ How does all of this reconcile with gravitation? How many space-time
dimensions are there really? ❖ ...
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Gustaaf Brooijmans CERN 2017
Particles Solve Problems
(Problems Predict Particles)
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Gustaaf Brooijmans CERN 2017
Vector Boson Scattering
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• There was in fact one known problem with the Standard Model (+ a second, related, lesser one):
• If we collide W’s or Z’s (not so easy...), the scattering cross-section grows with the center of mass energy, and gets out of control (violates unitarity) at about 1.7 TeV: σ(WW → WW) ∼ s
• This is similar to “low” energy neutrino scattering:
• If q2 << (MW)2, looks like a “contact interaction”, and cross-section grows with center of mass energy: σ ∼ s
• But when q2 ≈ (MW)2, W-boson propagation becomes visible, and “cures” this problem
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Gustaaf Brooijmans CERN 2017
Vector Boson Scattering
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• There was in fact one known problem with the Standard Model (+ a second, related, lesser one):
• If we collide W’s or Z’s (not so easy...), the scattering cross-section grows with the center of mass energy, and gets out of control (violates unitarity) at about 1.7 TeV: σ(WW → WW) ∼ s
• This is similar to “low” energy neutrino scattering:
• If q2 << (MW)2, looks like a “contact interaction”, and cross-section grows with center of mass energy: σ ∼ s
• But when q2 ≈ (MW)2, W-boson propagation becomes visible, and “cures” this problem
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Gustaaf Brooijmans CERN 2017
The Higgs Boson❖ One way to solve WW, is to introduce a massive, spinless particle (of
mass < ~1 TeV) - Couplings to W and Z are fixed, quantum numbers are known...
- .... to be those of the vacuum
- Its mass is unknown, and its couplings to the fermions are unknown.... well, maybe
- Fermions can acquire mass by coupling to this Higgs boson, so their couplings could be proportional to their masses. This is called the “Standard Model Higgs”
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Gustaaf Brooijmans CERN 2017
Precision Measurements❖ In fact, we were able to say
something about the standard model Higgs mass
- If the fermions get their masses from the Higgs, we know all couplings and can infer the Higgs mass from precision measurements
- Result is very sensitive to measured top quark, W boson masses
- Really wants a “light” Higgs boson15
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Gustaaf Brooijmans CERN 2017
Precision Measurements❖ In fact, we were able to say
something about the standard model Higgs mass
- If the fermions get their masses from the Higgs, we know all couplings and can infer the Higgs mass from precision measurements
- Result is very sensitive to measured top quark, W boson masses
- Really wants a “light” Higgs boson16
✓
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Gustaaf Brooijmans CERN 2017
The Plot Thickens
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Gustaaf Brooijmans CERN 2017
New Physics?
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Could this be it?
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Gustaaf Brooijmans CERN 2017
Higgs Mass❖ Higgs, in fact, also acquires
mass from coupling to W’s, fermions, and itself!
- These “mass terms” are quadratically divergent
- Drive mass to limit of validity of the theory
❖ So we expect the Higgs mass to be close to the scale where new physics comes in....
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Gustaaf Brooijmans CERN 2017
New Physics?
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Could this be it?
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Gustaaf Brooijmans CERN 2017
New Physics?
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Could this be it?
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Gustaaf Brooijmans CERN 2017
Nevertheless❖ Clear structure in fermionic sector
unexplained - Evidence of some selective principle
(why are there no neutral colored fermions?)
- Proton stability, running of couplings suggestive of at least one other scale relevant to SM particles and interactions, ~1015 GeV
- Either fine-tuning, or a closer scale
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Gustaaf Brooijmans CERN 2017
The Tools
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Gustaaf Brooijmans CERN 2017
Energy Frontier❖ Currently, hadron colliders: - High energy implies probing of short
distances, and production of other, massive particles
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7 - 14 TeV center of mass energy
VLHCLHC
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Gustaaf Brooijmans CERN 2017
Hadron Colliders❖ Incoming longitudinal momentum not known: - “Hard interaction” is between one of the quarks and/or gluons from each
proton, other quarks/gluons are “spectators”
❖ Longitudinal boost “flattens” event to a pancake ➡ We usually work in the plane transverse to the beam
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Gustaaf Brooijmans CERN 2017
Detectors❖ Make best possible measurement of all particles coming out of collisions
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Gustaaf Brooijmans CERN 2017
CMS
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456%3.8T Solenoid
0>?@!76k scintillating PbWO4 crystals A>?@!Scintillator/brass
Interleaved ~7k ch
• Pixels (100x150 µm2) " ~ 1 m2 ~66M ch"• Si Strips (80-180 µm)" ~200 m2 ~9.6M ch!
g8T.#@%o%970>G.7!
5p+_%XQqqfZ!250 Drift Tubes (DT) and 480 Resistive Plate Chambers (RPC)
473 Cathode Strip Chambers (CSC) 432 Resistive Plate Chambers (RPC)
5p+_%f_i4Qg6!
!+&)1%B'#9"&%%%%%%%%%=4<<<%&%CD',)11%2#)('&',%%%=6%(%CD',)11%1'*9&"%%%%%%%;E:5%(%
eq+_%r+sf%
YBO YB1-2
Preshower Si Strips ~16 m2
~137k ch
Foward Cal Steel + quartz Fibers 2~k ch
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Gustaaf Brooijmans CERN 2017
Charged Particles❖ Combination of pixels, silicon strips (“SCT”) and straw tube transition
radiation tracker (TRT)
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Calorimetry❖ Liquid Argon & Pb accordion (EM & forward),
crystals ❖ Scintillator & steel/copper/tungsten (hadronic)
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Gustaaf Brooijmans CERN 2017
Muons❖ Air-core toroids/flux return; wire chambers
and RPCs
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Neutrinos*
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*(100% acceptance)
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Gustaaf Brooijmans CERN 2017 32
✓
✓✓
✓✓✓
✓
✓
✓ ✓ ✓
✓: Detect with high efficiency
✓: Detect by missing transverse energy
✓
✓
✓
✓
✓: Detect through decays: t→Wb, W/Z → leptons, ...
✓
H125500
0
✓
Detecting Particles
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Gustaaf Brooijmans CERN 2017
The Work
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Gustaaf Brooijmans CERN 2017
Steps in a Physics Analysis❖ Choose a topic (often theory-motivated) ❖ What is the final state? ⇒ “Preselection”
- For a search, sufficiently loose to be signal-poor - Prove you understand the detector response, physics processes contributing
- But sufficiently tight to have a manageable data volume - ATLAS/CMS write 1000 Hz × 1+ MB/event = 1+ GB/s
- “4-vectors” is not enough, need some amount of detector info
- In practice, often have preselected sample for frequent analysis, + looser sample for e.g. multijet background with rare passes
❖ Note that data volume ∝ running time, not ∫L34
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Gustaaf Brooijmans CERN 2017
Steps (II)❖ Determine preselected sample’s composition - MC and data to understand each contribution
- Multijet background to leptons often extracted from data: rejection factor ~10-4, difficult for simulation to be that accurate
- MC for most other processes, with corrections from data, since generators are (LO,) NLO, NNLO, (LL,) NLL, NNLL
- Also need to correct MC for real-life data conditions - Different alignment, dead channels etc.
- As statistics increase, more difficult, since mis-modelings not hidden by statistical uncertainties anymore
- Mis-modelings often show up in tails
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Anecdotes From the Field (I)❖ Everybody wants experimenters to produce results fast - Lots of pressure in the early days of LHC...
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Only jets, background easy
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Anecdotes From the Field (I)
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GEANT bug
❖ Everybody wants experimenters to produce results fast - Lots of pressure in the early days of LHC...
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Gustaaf Brooijmans CERN 2017
Anecdotes From the Field (I)
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GEANT bug
Cosmics
❖ Everybody wants experimenters to produce results fast - Lots of pressure in the early days of LHC…
- Sometimes it’s better to take the time needed to understand strange things…
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A Semi-Challenging Search: Higgs to τ µ
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❖ 20 fb-1 collected by end 2012 at 8 TeV
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400000 events in direct productioncan look for rare decays!
Producing Higgses
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Higgs Decay: 125 GeV is Golden
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Low MassH → bb, ττ, γγ
High MassH → WW, ZZ
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Gustaaf Brooijmans CERN 2017
µ+τ❖ Indirect constraints fairly weak (as opposed to e.g. e+µ)
- Indirect: BR(µτ) < ~10%; BR(eµ) < ~10-8
❖ Lepton Flavor remains a mystery - Observing LFV crucial in understanding origin
- Know it exists in the neutrino sector
❖ Experimentally: - With 400k Higgses produced,1% BR yields 4000 signal events (x efficiency)
- Two leptons ⇒ small to moderate background at hadron collider
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Gustaaf Brooijmans CERN 2017
Tau decays❖ Exploit two channels: - τ→eνν: BR = 18%
- τ→hν: BR = 49% (one charged particle) + 15% (three charged particles)
- Avoid Z → μμ background
❖ Final states are μτe and μτh - Irreducible background is Z → ττ
- Primary discriminating variable is μ-τ invariant mass - Unfortunately not directly reconstructible: neutrinos escape!
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Gustaaf Brooijmans CERN 2017
Collinear Mass❖ m(H) = 125 GeV, m(τ) = 1.8 GeV ➡ Tau is heavily boosted
➡ Tau decay products are collinear with tau
❖ Under that assumption, know neutrino direction - From direction and missing transverse momentum infer neutrino longitudinal momentum
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CMS: arXiv:1502.07400PreselectionPhys. Lett. B 749 (2015) 337
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Gustaaf Brooijmans CERN 2017
Categorize!❖ Different production mechanism (gluon
fusion vs. vector boson fusion) lead to different topologies
- In practice number of jets
❖ Different decay channels have different reducible backgrounds
- Hadronic tau decays are low multiplicity jets
❖ Categorize to exploit different S/B! - Assign corresponding weights (typically
ln(1+S/B)), to increase sensitivity
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Gustaaf Brooijmans CERN 2017
Backgrounds❖ Small signal ⇒ need very accurate
background estimate - Use data where possible
❖ In this case: - Z → ττ (irreducible): take Z → µµ events from data,
replace one muon with simulated tau
- Misidentified leptons: get control sample, and independently measure probability to fake e or τh, check in control region
- Rest: simulation46
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Gustaaf Brooijmans CERN 2017
Finally❖ Tighten cuts and look for signal ❖ Don’t forget systematic uncertainties - Difficult topic: estimators often have known
flaws, but “best we can do”
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Run 2
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Gustaaf Brooijmans CERN 2017
Higgs Drawbacks❖ So with the addition of a Higgs boson around 125 GeV particle
physics could be “complete” - Like Mendeleev’s table for chemistry, but not understood. By itself, the
Higgs is very unsatisfactory: - Why are the couplings to the fermions what they are?
‣ Dumb luck (aka landscape)?
- What is the link to gravity?
- What about Dark Matter?
- Why does the Higgs break the symmetry?
- Why are there 3....?
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