Theories of Nature andThe Nature of Theories

Matthew Strassler, University of Washington

Based on:Lectures given at the CERN/Fermilab Hadron Collider Summer School,

August 2006Echoes of a Hidden Valley at Hadron Colliders

April 2006 (with K. Zurek)Discovering the Higgs through Highly Displaced Vertices

May 2006 (with K. Zurek)Possible Effects of a Hidden Valley on Supersymmetric Phenomenology

July 2006

Theories of Nature

Many theorists view the hierarchy between the weak scale and the gravitational scale as the greatest problem in particle physics

GN / GF = (MW / MPlanck )2 = 10-32

Favorite Theories of Theorists Supersymmetry

goal: stabilize hierarchy Technicolor, Randall-Sundrum | |

goal: stabilize hierarchy Flat extra dimensions, Randall-Sundrum |

goal: reconsider or solve hierarchy Little Higgs, Twin Higgs

goal: stabilize little hierarchy

The Nature of Theorists

For theorists (and many experimentalists) minimal = elegant = motivated = better

Non-minimal models are often ridiculed as “baroque” Non-minimal models are usually not published in good journals Model builders are rewarded with prestige for discovering elegant

solutions with minimal additional matter content Models always must have a “motivation”; they must solve a recognized

problem in particle physics. Almost never is anyone promoted for inventing and studying non-minimal

models Phenomenological studies (theoretical and experimental) usually ignore

non-minimal models

The design of the LHC detectors was based on the search for the standard model Higgs boson, and a few classic minimal hierarchy-solving models available in the 1980s.

Minimal “Supergravity” with R parity Technicolor

Higgs Physics

The standard model Higgs boson can be produced in abundance at LHC, and seen in a variety of decay modes gg h, qq qqh, qq Wh, Zh, ggttH h , h bb,, hWW, ZZ

Some other modes enhanced in 2 Higgs doublet models, such as gg bbh

A detector that has good efficiency, energy resolution for Photons Electrons Muons Taus Bottom jets

will do very well in searches for the Higgs boson

Minimal “Supergravity” with R parity Central high-momentum jets, leptons and large missing energy

Technicolor High-energy WW scattering – hard forward jets, central leptons/jets

Nature and Theories

But we don’t care what theorists (or even experimentalists) think. We care what nature does.

And what does nature do? Nature has 3 generations, not 1 Nature has neutral currents in addition to charged currents Nature has neutrino masses Nature has an apparent cosmological constant

All were viewed as non-minimal, inelegant, and unmotivated by many physicists before they were discovered

Minimalism, elegance and motivation are time-dependent human judgments They depend upon current experimental knowledge They depend upon current theoretical understanding

The “Minimal Standard Model” is not minimal from a theoretical standpoint; it is simply the minimal model that fits the data (and it did not originally include neutrino masses.)

So we should worry a bit that our cultural bias toward minimal models could mislead us about the physics that we are going to discover

Is this a serious concern?

This is a problem only if the experiments are poorly designed for the physics they will encounter

But of course CMS and ATLAS were designed as multi-purpose detectors that could find standard model Higgs supersymmetry technicolor, any other new physics which has high-momentum jets, leptons, missing energy,

photons, etc. Black holes decaying to many particles Extra top-quark-like states decaying to Zt, Wb.

Can an experiment that can see a minimal model miss a non-minimal model?

Naively, adding something non-minimal – one extra particle, for instance – can’t change a theory so drastically as to cause a problem

Let’s look at a simple example that shows this isn’t true…

Modifying the Higgs Sector

The Higgs boson is very sensitive to the presence of additional scalar particles More scalars can generate mixing of eigenstates, new decay channels, new

production mechanisms.

Let’s consider adding a single real scalar S to the standard model S carries no charges and couples to nothing except the Higgs, through the potential

If <S> = 0, an Invisible Decay

If, at the minimum of V(H,S), <H>=v / √2 , <S>=0,

then S2H2 (v+h)2S2 = v2 S2 + 2v hSS + hhSS

a shift in mass for S and a cubic coupling

This allows h SS (if mh > 2 mS) with a width ~ 2v2 / mh

This can easily exceed decays to bottom quarks, with width ~ yb2 mh !

So Br(h SS) could be substantial, even ~1 for a light Higgs boson, depending on But S is stable. There is an S -S symmetry. So this decay is invisible.

Therefore a light Higgs could be essentially invisible… (its existence might be inferred in VBF or diffractive Higgs production, with difficulty.)

If <S> nonzero, a 2nd ‘Higgs’

If, at the minimum of V(H,S), <H>=v / √2 , <S> = w / √2, S = (w+s) / √2 , then S2H2 (v+h)2(w+s)2 = v2 s2 + w2 h2 + 4vw hs + 2v hss + 2w hhs + hhss

we get new mass terms, new cubic couplings, new quartic couplings (Note I cheated slightly here; need to self-consistently find minimum)

The first two terms shift the masses; the third allows h and s to mix! Thus we have two eigenstates with masses m1 , m2

Both eigenstates couple to WW, ZZ, bb, gg, , through their h component; for instance,

If <S> nonzero, a 2nd ‘Higgs’

So there are two scalar particles that can be produced in gg collisions

And both decay to usual Higgs final states, via their h component --- thus

1has same branching fractions as an SM Higgs boson of mass m1

2 has same branching fractions as an SM Higgs boson of mass m2

So there are two Higgs-like states to find, each with a reduced production cross section, each standard-model-like in its branching fractions.

EXCEPTION: if m1 > 2 m2, then a new decay channel opens up:

122(bb)(bb), (bb)(), ()(), (bb)(), ()()

These exotic final states can occur in many models; recent heightened interest, since a light Higgs with these decay channels can escape LEP bounds.

Higgs decays to 4 fermions

g

g

hh SS

mixing

bb

bb

bb

bb

See Dermasek and Gunion 04-06 h aa bb bb, bb , , etc. and much follow up work by many authorsFox, Cheng, Weiner 05 have even found h 6 tau’s, h 8 b’s

SS

SShh

hh

Non-minimal and important

Clearly these types of multiparticle decays could have a huge impact on the ability to detect the Higgs boson Multi-b signatures are nearly impossible Multi-photon signatures are good, but rare Photons and taus?

It is possible for a light higgs to decay this way ~100%

Not yet clear what the best experimental search strategies are

Not yet clear whether ATLAS, CMS, or LHCb is best-positioned to find a Higgs that decays this way.

Non-minimal may be easier:

g

g

hh SS

mixing

w/ K Zurek, May 06

bb

bb

bb

bbSS

SShh

hh

Displaced vertex

Displaced vertex

An Overlooked Discovery Channel ! Appears in many models.

Other final states possible as well.

Opens possibility of Higgs discovery by LHCbMaybe powerful at CMS, ATLAS even with 1% branching ratio

Higgs decay to displaced vertices

Second decay occurs too far out for track reconstruction – jet without tracks.

Technical Problems

The ATLAS and CMS experiments are not optimized to look for long-lived particles decaying in flight

Some long-lived particles can be found with special purpose techniques that have been studied and validated [much work in Rome] If the particles are slow, timing can be used If the particles are charged and don’t decay, they may appear as strange tracks

with unusual energy deposition, shape, etc. If a particle decays inside the beampipe, vertexing can find them (but no searches

at Tevatron have been performed!)

But if the decay is between the beampipe and the outer edge of the muon detector, problems! The detector trigger, tracking and reconstruction software all assumes that particles

come from the primary vertex or very nearby Tracking is only available in the high-level trigger Tracking is too dilute to allow for high-efficiency identification of displaced vertices

outside the beampipe Decays in the calorimeters are difficult to recognize Backgrounds to displaced vertices at the Tevatron are known to be substantial

Efficiency Issues

These displaced vertices are precious They have no standard model background Gold-Plated They may be rare They may be recorded and found by accident, but should something so

potentially important be left to chance?

For these reasons, we would like to actively find them We would like to be able to find them at trigger level – or at least, find

hints that allow us to save candidate events with higher efficiency We would like to be able to flag events at trigger level that deserve

special-purpose initial reconstruction analysis

It has not yet been demonstrated that this is possible, but both CMS and ATLAS are engaged in preliminary studies Seattle – Rome working group within the ATLAS collaboration

Culture and Theory

Various non-MSUGRA versions of supersymmetry from the 1990s predict various possibilities for long-lived particles decaying in the detector Long-lived neutralino decaying to a gravitino plus

Photon – studied Z – not studied Higgs – not studied

Long-lived stau decaying to gravitino plus tau – studied Long-lived gluino decaying to gravitino plus jets – not studied Long-lived neutralino decaying to muon pairs plus neutrino – not studied Long-lived neutralino decaying to jets – not studied

Even MSUGRA with one extra particle can give these phenomenological signatures

These ideas came too late to affect the basic CMS, ATLAS design But it is surprising that so few Tevatron searches or LHC studies have yet

been done

Is this another example of the tyrannical hold that minimal models have on our culture?

Supersymmetry and its Signals

The classic supersymmetry signal: missing transverse energy Vast majority of SUSY searches focus on this variable Where does this really come from?

There are two neutral stable particles produced in every event

Why?

R-parity – a global symmetry (needed to forbid proton decay) under which The SUSY partners are charged The known SM particles are not

The chain of reasoning is this: Superpartners must be produced in pairs to conserve R-parity A superpartner must decay to a superpartner, to conserve R-parity

Therefore the lightest R-parity-odd particle --- the lightest superpartner (LSP) -- is stable New stable charged/colored particles are constrained, so the LSP is neutral Therefore the final state of superpartner production always has two stable neutral particles

Minimal supersymmetry with R parity implies a certain phenomenology Does this same phenomenology imply supersymmetry?

A New Exact Global SymmetrySuppose we have new particles X1 , X2 , X3 , X4 ,… Xn

(Let’s order them by mass, X1 the lightest, Xn the heaviest) All carry a new global X-charge

In any SM collision, initial state is global-neutral…so the final state is also global-neutral… therefore

Xi can’t be resonantly produced; cannot have q q X1

Instead they must be produced in pairs, e.g. q q X1 X1* q q X1 X2

(if X1 ,X2 have opposite global charge)

The LXP (the lightest particle carrying X-charge, X1) is stable Therefore the LXP must be electrically-neutral and color-neutral And thus it interacts very weakly with ordinary matter, at best

comparable to a neutrino, perhaps even more weakly. This makes it a potential dark matter candidate: a WIMP.

q

q

X1

X1*

q

q

X1

A Pair of Invisible Particles !

q

q X9*

X7

u

X4

X3

X1X6

u

h

bb

+

e+

e-

X9

X1*

u

d

Every XX* event has two invisible LXPs!

Transverse Momentum Imbalance (MET)

Collider consequences

MET in X-particle production Cascade decays of X-charged particles No resonant production or resonant decays of new X-charged particles Expect kinematic endpoints and edges from three-body decays, multiple two-

body decays, etc. with a missing particle

q q +- j j + MET

The invariant mass of the +- pair from the three body decay X2 +- X1 will have a

kinematic endpoint at m = m2 - m1

mZm

Example:

Signal plus background

Signal

Effect on quantum corrections

Suppression of higher-dimension operators

Suppression of loops

Thus the success of SM predictions for electroweak observables suggests that a global symmetry as well as weakly-interacting physics is present in TeV physics beyond the standard model

Low energy

Shifts W mass

The global symmetry is also partly responsible for the small corrections to electroweak precision measurements.

Suppressed by heavy mass

Signs of Supersymmetry? MET in X-particle production Cascade decays of X-charged particles No resonant production or resonant decays of new X-charged particles Kinematic endpoints and edges from 3-body decays, multiple 2-body decays, etc. with a missing particle Dark matter candidate Electroweak precision data is close to standard model prediction

All are characteristics often said to be those of SUSY. But they are not!

They are consequences of a conserved global symmetry in SUSY case, it’s R-parity

If you remove R-parity and keep supersymmetry, you lose these features If you keep R-parity and remove supersymmetry, you keep these features. If you take little Higgs or extra dimensions and add a corresponding “T-parity” or “KK-parity”,

you get the same features

Suppose LHC observes missing energy signals plus jets and leptons, and kinematic endpoints but no resonances, This will suggest new particles with a new global symmetry It does not tell us anything at all about whether supersymmetry is correct

Are we thinking too minimally?

Our reasons to expect SUSY are strong, but not as strong as is sometimes suggested.

Electroweak precision data gives us good reasons to expect a new global symmetry along with weakly-interacting TeV-scale physics – which may or may not involve SUSY.

Still, perhaps one can conclude that the phenomenology will be roughly similar to what one expects in SUSY studies, even if the global symmetry does not involve SUSY?

This is not the case! Electroweak precision data does not require

that the new global symmetry is exact (1-10% violations are ok) that it is carried only by particles that couple with electroweak strength or greater to the standard

model

The phenomenological consequences of a new global symmetry that we outlined above may be dramatically changed if either The symmetry is approximate (as in R-parity violation) [will not discuss here] The symmetry is exact (no R-parity violation) but is carried by particles in an sector ultra-weakly

coupled to the standard model.

An ultra-weakly coupled sector Suppose that the standard model and the particles X1 , X2 , X3 , X4 ,… Xn

carrying the new global charge all couple to each other with weak-interaction strength or stronger

But in addition there is another sector containing particles ,…m neutral under the new global charge particles ,…r charged under the new global charge

These particles may couple to each other rather strongly, but They couple to SM particles and the Xi ultra-weakly

(much more weakly than the weak interactions)

SM particles and

X1 , X2 , X3 , X4 ,… Xn

,…m

,…rVery weakcoupling

Simple case first: one Suppose the new sector consists only of one globally-charged particle

( or `’ for short) Suppose also that is lighter than X1, so that although X1 is the LXP in

the sector containing the SM (the `LsXP’), the true LXP is Remember the two sectors are ultra-weakly coupled to one another: Now reconsider:

q

q X9*

X7

u

X4

X3

X1X6

u

h

bb

+

e+

e-

X9

u

dThe couplings to the other sector are far too

small to affect any stage of this physical process

X1

… so nothing happens …

X1

… … … until … …

Late Decays

X1

u

ū

So X1 is long lived, and may decay anyplace inside or outside the detector.

An example: Gauge Mediated Supersymmetry Breaking

The global symmetry is R-parityand

is the gravitino, the true LSP

Phenomenological Consequences

The LsXP X1 need not be neutral, colorless (since it is unstable, there are no constraints)

The LsXP X1 (2 per event) may Be stable on detector timescales

Invisible if neutral Visible if charged Challenging if colored

Decay promptly Many possible final states for X1

A photon for each X1 A tau for each X1 A b-bbar pair for each X1 A Z boson for each X1 Etc.

Decay with a displaced vertex (inside or outside the beampipe) All of the above final states to consider Many different areas of detector to consider

So the addition of one extra particle dramatically widens the range of possible phenomena within any model with a new global symmetry Some of these are challenging for the LHC detectors Some have been studied in context of Gauge-Mediated SUSY or R-parity violation,

but many have not been.

Larger new sectors

This is what can happen if the ultra-weakly coupled sector has one particle

If it has more than one particle, so that decays within the new sector can occur (e.g. “hidden valley”), then the range of possible signals becomes much larger

Suppose there are three particles in the new sector, 1 , 2 that carry global charge, and that doesn’t carry it

And suppose the interactions among these particles are strong. (They are weakly coupled to the SM, remember, but they need not be

weakly coupled to each other.)

Late Decays

X1

u

ū

2

2

1

u

ū

b

b e+

e-

Yes, this happens in some models!

See recent work on Hidden Valleys

decay to SM particles is through very weak

interaction and may also produce (highly)

displaced vertices

X1 uuuubbe+e- 1

Multiparticle production

Cascade decays and/or strong interactions in a hidden sector may lead to multi-particle production

This can significantly reduce the MET signature of a theory with a new global symmetry

Complex, busy events can also pose analysis challenges Jet reconstruction may be unreliable and not well-matched with

underlying partons Lepton isolation cuts may be inefficient

If displaced vertices are present, then the challenge is to find them Similar issues to Higgs boson decays discussed above Number of displaced vertices may be larger, environment much busier

Lessons

What we learn from this is the following: Even if the most optimistic theorists are right and

Supersymmetry is truly the solution to the hierarchy problem R-parity is exact and stabilizes a dark matter candidate

a minimal amount of non-minimality may cause the phenomenology of supersymmetric models to differ wildly from the standard high-pT jets and leptons plus MET expectation –

rather few scenarios have been studied by theorists, fewer by experimentalists; some recently-discussed scenarios pose new experimental challenges and provide new opportunities

it will be a long road from the discovery of an R-parity-like global symmetry to an actual claim of having found supersymmetry.

General lesson

SU(3)xU(1)-neutral particles can serve as windows into new sectors of relatively light particles Higgs can decay often to unknown particles Neutralino LSP can decay always to unknown particles

Charged LSP can decay via virtual LSP to charged particle plus unknown particles

Z can only very rarely decay to unknown particles Sterile neutrino decays? Z’ decays?

Also, heavy charged particles can also decay into new unknown particles plus standard model ones

There are many places to go looking experimentally, not just Tevatron and LHC

Hidden Valley Models (w/ K. Zurek)

Basic minimal structure

Standard ModelSU(3)xSU(2)xU(1)

Communicator

Hidden ValleyGv with v-matter

April 06

A Conceptual Diagram

Energy

Inaccessibility

Instructive Class of Models

Easy subset of models to understand to find experimentally to simulate to allow exploration of a wide range

of phenomena This subset is part of a wide class of

QCD-like theories

Standard ModelSU(3)xSU(2)xU(1)

New Z’ fromU(1)’

Hidden Valleyv-QCD with

2 light v-quarks

q q Q Q : v-quark production

qq

qq

QQ

QQ

Z’Z’

v-quarks

qqQQ

qq QQ

Z’Z’

v-gluons

q q Q Q

qq

qq

QQ

QQ

Z’Z’

q q Q Q

qq

qq

QQ

QQ

v-hadrons

Z’Z’

q q Q Q

qq

qq

QQ

QQ

v-hadrons

Z’Z’

q q Q Q

qq

qq

QQ

QQ

v-hadrons

But some v-hadrons decay in the detector to visible particles, such as bb pairs, tau pairs, etc.

Z’Z’

Some v-hadrons are stable and therefore invisible

q q Q Q

qq

qq

QQ

QQ

v-hadrons In many models some v-hadrons have long lifetimes and produce highly displaced vertices

Z’Z’

q q Q Q

Unusual Phenomenology

Multi-particle production is typical Large numbers of quarks likely (e.g. 14 b quarks not unusual) Possibly some lepton pairs, often not isolated Number of jets and isolated leptons does not match well to number of quarks and

leptons produced! Underlying kinematics is almost completely scrambled; analysis challenge

Large event-to-event fluctuations are typical Number of v-hadrons produced Number of visible/invisible v-hadrons pT spectrum of visibly-decaying v-hadrons Usual jet fluctuations on top of this Challenge to collect events into a signal

Missing energy is likely Typically some v-hadrons carry a conserved charge (such as isospin-like or

baryon-like quantum number) and are stable Some v-hadrons may have lifetimes too long to see within the detectors

Multiple displaced vertices are possible Great if present – Gold-Plated Possibly challenging for trigger and tracking If absent, serious analysis challenge

Theory, Nature, and the Culture of Particle Physics

Theoretical and even Experimental Particle Physics often view “simplicity” as a mark of a good theory.

[It is well-known that every difficult problem has a solution which is simple, elegant, beautiful, and wrong.] Application of “Occham’s Razor” The Minimal XXXX Model The Next-to-Minimal XXXX Model

Yet history has not been kind to minimal models What seems non-minimal and complex today may seem minimal and elegant tomorrow

To the extent that our appreciation of minimal theories blinds us to experimental possibilities, it is important that we look more widely

I have argued that the physics of Higgs bosons, of supersymmetry, of Z’ bosons, and indeed of almost any well-studied phenomenon can be drastically altered by one or more non-minimal particles

Some phenomena that arise are known but have not been sufficiently considered; others are new and completely unstudied

The LHC experiments cannot prepare for everything, but it is important to expand the scope of their preparations to include the kinds of phenomena I have discussed here.