Preparing for the Early Years of the Large Hadron Collider Overview Lecture Given at the 4 th RTN Workshop Varna, Bulgaria Matthew J. Strassler Rutgers

Preparing for the Early Years of the

Large Hadron Collider Overview Lecture Given at the 4th RTN Workshop

Varna, Bulgaria

Matthew J. StrasslerRutgers University (New Jersey, USA)

The ContextThe LHC:

the best possible place to look for new physicsthe worst possible place to look for new physics What makes it so horrible?! How do we deal with the challenges

Jets are everywhere What are jets anyway?

The Standard Model is a source of large backgrounds to most signals What are the most important, and in what sense?

And now we want to find new physics But what does it look like? What kind of backgrounds must be understood? What should we expect? What should we be careful of?

What I won’t talk about

Heavy Ion Collisions and the connection with String Theory [see H. Liu’s talk]

Diffractive Higgs production and the connection with string theory [Pomeron] – not early LHC

All those different models of extra dimensions, deconstructed extra dimensions, theories dual to extra dimensions fermionic extra dimensions (well, a few words)

Black Holes

BBC Reporting

LHC Cross sections vary over many orders

of magnitude

Every aspect of the experiment is influenced

by this graph

Note the ratios of

• inelastic to b pairs

• b pairs to W

• W to top pairs

• top pairs to Higgs

• top pairs to TeV-scale SUSY

• Higgs to Higgs photons

If we kept all the events

we’d have 1015 / year

In fact 99.9999% must be instantly discarded

The trigger is necessary, crucial, and introduces unavoidable bias

Even after the trigger,

~109 events/year

~103 physicists

An automated system must quickly analyze the huge amount of data from each event.

Another necessary, crucial, and potentially biasing stage

Standard Model produces huge backgrounds: hard to calculate, or measure, or model

Theory is way behind

Experimentalists will determine many backgrounds by measuring

This is not always possible and is fraught with dangerous assumptions

Theory bias can creep in here as well.

ATLAS, CMS, LHCb, ALICE,... General purpose detectors ATLAS, CMS our main focus

What can these detectors do? What can’t they do?

Quick review: LHC Kinematics

Everything is oriented along (longitudinal) and perpendicular (transverse) to beam-pipe

The c.m. frame of proton-proton collision is the lab frame

But c.m. frame of scattering quarks/antiquarks/gluons is not the lab frame

Typical scattering is boosted along beampipe therefore total energy, z-momentum not known


7 TeV

7 TeV






x2 7 TeV

x1 7 TeV

The scattering “partons” carry fractions x1, x2 of their protons momentum






Can only apply conservation of transverse momentum Energy and z-momentum of scattering partons not known Not observable: destroyed proton debris lost down beampipe

Transverse momentum (2-d vector) : pT NOT longitudinal momentum NOT energy

Missing transverse momentum if pT not zero. Called “MET” or Missing (Transverse) Energy

x2 7 TeV

x1 7 TeV

The scattering “partons” carry fractions x1, x2 of their protons momentum

What’s easy, what’s hard?

Relatively easy: Detecting and measuring isolated electrons, muons, photons

Very hard Measuring jet energies, momenta for jets (pT> 50 GeV) Interpreting them as quark/gluon energies and momenta

Extremely hard Detecting/measuring low-pT jets Measuring missing transverse momentum

Virtually impossible Telling quark jets from gluon jets or antiquark jets Seeing electrons or photons inside of jets

What’s easy, what’s hard

Easy: Measuring cross-sections times branching ratios times efficienciesNumber of events proportional to Cross section for production , times Branching fraction into particular final state , times “Efficiency” for detecting the particular final state

Hard: Measuring cross-sections times branching ratios Measuring ratios of branching ratios

Need model for the shape of distributions to determine efficiency

Extremely hard: measuring cross-sections or branching ratios separately Must generally do accounting for 100 percent of the produced particle’s decays

But determining theory often requires cross-sections, branching ratios

What’s easy, what’s hard

Easy: Measuring masses of resonances in e, mu, gamma Measuring certain combinations of mass in dilepton decays Measuring charge quantum numbers of particles

Hard: Measuring masses of resonances in jets, taus Measuring non-resonant masses directly – but see new methods! Measuring spins of particles – but see new methods!

Extremely hard: Measuring small mass differences Measuring quark flavor quantum numbers (except t, b, maybe c) Measuring mixing angles [requires many measurements]

Unfortunately, extracting theory often requires masses and mixing angles

Example: Z’ resonance

A 1.5 TeV electron-positron resonance could be discovered by December 2008, or June 2009

Is it spin one? What are its couplings to Quarks vs Leptons? Special couplings to third generation? Tops? Bottoms? Taus? Couplings to Right-handed vs Left-handed Fermions? Does it decay to W+W-? Does it decay to Z Higgs? Does it decay to superpartners or other new particles? Does it decay invisibly, and if so, can we determine what?

So even if discovered right away, making a theory for Z’ will take years…

Lesson: expect to do model-building armed with fragmentary information

Everywhere at LHC: Jets, Jets, Jets

Not all LHC events make

*hard* (pT > 100 GeV) jets

Still the probability of a

pT ~ 20 GeV jet is very high

But what *are* jets?

Naïvely: quarks, antiquarks, gluons

produced in scattering turn into jets because of confinement, hadronization

D0 Dijet Event

Everywhere at LHC: Jets, Jets, Jets

Not all LHC events make

*hard* (pT > 100 GeV) jets

Still the probability of a

pT ~ 20 GeV jet is very high

But what *are* jets?

Naïvely: quarks, antiquarks, gluons

produced in scattering turn into jets because of confinement, hadronization

D0 Dijet Event

WRONG

qq

qq

ZZ

quarks

e+e- quark-antiquark

qq

qq

ZZ

gluons


Lack of particle states in an interacting QFT Once produced, a quark or gluon immediately begins to radiate

Nearly massless quarks Spin-one radiation patterns

The radiation is dominantly collinear (along direction of motion) Or soft (low energy, and subject to destructive interference)

Approximate conformal invariance The process is scale invariant and forms a fractal pattern

Weak (but nonzero!) ‘t Hooft coupling (sNc) The angular width of the fractal is small if the ‘t Hooft coupling is small

Jet, pre-confinement: a narrow fractal distribution of (mostly) gluons a fundamental object in an interacting gauge theory

Note this requires resummed perturbation theory

qq

qq

ZZ

gluons


qq

qq

ZZ


flux confined

Jets: role of confinement

Obviously, confinement has a role: turn the fractal pattern of gluons into a jet of mostly mesons, a

few baryons

But in fact confinement in QCD has very little effect and that this is critical for the phenomenon of jets

How are these statements consistent?

To understand this, consider string theory: Suppose I set an open string in motion in a particular state In what circumstances might you directly observe the state at infinity?

If open string coupling go = 0, closed string coupling gc << 1,

then typically string will oscillate, twist off closed strings –

“gravitational radiation”

Initial state scrambled

If open string coupling go << 1, closed string coupling gc = go

2,

then typically string will oscillate, snap into few open strings

Initial state scrambled

If open string coupling goND ~ 1, closed string coupling gc = go

2,

then typically string will Instantly breaks into many pieces

Initial state preserved

qq

qq

ZZ

gluons


qq

qq

ZZ


flux confined

hadrons

qq

qq

ZZ


Strings vs. QCD Flux Tubes Closed string coupling: gc vs. 1 / N2

Open string coupling: go vs. 1 / N Effective open string coupling: gcND vs. F / N

QCD has F = N = 3

If F = 0, no jets!

If 0 < F << N , no jets? Maybe not… Or quasi-jets, but jet momentum not ~ quark/gluon momentum

Light flavors!

Summary of Jets and Confinement N >> 1 and F << N good for non-perturbative aspects of QCD

But the failure of these same conditions allows parton-hadron dualitywhich allows us to precisely test Short-distance QCD scattering, decays of heavy particles (e.g. top), etc. The semi-perturbative process of jet formation

In short: We should not take the hadronic jets of QCD for granted!!

Too few flavors or many colors, confinement ruins jets Too many flavors, no confinement and no hadrons

This is one of the reasons why jets are so ill-defined theoretically [which means there’s more work to do!]

Standard Model Backgrounds Almost every new physics signal has

a large standard model background or a large detector background or both

Experimentalists spend much of their time Measuring backgrounds in data Predicting backgrounds in advance of an analysis Checking backgrounds in course of an analysis

A lot of theoretical calculation and simulation goes into this effort

Backgrounds are huge – though fortunately they are smaller at high energy

Quick Review: Why do backgrounds fall?Backgrounds fall with energy


Cross-section formulas - example:

All parton distribution functions fall like a power of x Parton-parton c.m. energy ~ (x1 x2)1/2 (14 TeV) Most parton-parton cross sections ~ 1/Energy^2


Cross-section formulas - example:

All parton distribution functions fall like a power of x Parton-parton c.m. energy ~ (x1 x2)1/2 (14 TeV) Most parton-parton cross sections ~ 1/Energy^2

What’s this?

The debris from the proton-proton collision! Unavoidably produced, always there.

The “Underlying Event”!

CMS experiment: Simulated g g Higgs Z Z e+e-+- + underlying event!



Can QCD theory of proton structure predict properties of underlying event?!?!?! A challenge to formal theorists!

Since we cannot currently model it, must measure it!

One of first measurements this year (at 10 TeV) and next year (at 14 TeV): the properties of the average underlying event: how many particles? What pT distribution?

Fluctuations in the underlying event are hard to measure – and can mask new physics

All LHC predictions are affected by the underlying event; if underlying events are more accurate than is guessed, it would cause some problems for the experiments

(Also every interesting proton-proton collision will be muddied by 4 – 20 simultaneous and boring ones )

What should theorists calculate?Tree-calculation solved – faster automation current goalBut trees are always ambiguous, so need first quantum correction

Dominant backgrounds are QCD multi-jet events, so most important calculation a theorist can do is pure QCD…? No!

For fixed # jets, many processes contribute # jets often does not equal # external legs Multijet events are poorly measured

Most measurements require at least one lepton or photon – they are “easy” So jets + lepton (i.e., W or Z) or jets + photon are most important

State of art: W + 3 jets [4 in reach?] But note: top-pairs = W + 4 jets already Lots of signals are lepton + 4 jets [SUSY!] Lots of signals are leptons + 6 jets [SUSY!]So there’s a long way to go – HELP!

What (not) to Compute Calculating total cross-sections is easier for theorists

But measuring total cross-sections is all but impossible

Therefore theorists must provide differential cross-sections to allow these effects to be properly modeled Harder for theorists; Analytic answers rare Need to produce a computer program which can compute value of

differential cross-section for a particular final state

Otherwise, experimentalists can only adjust the normalization of the tree-level calculation; shape still tree-level Hope d ~ dtree * (loop / tree)

This can fail badly when looking at tails of distributions…

Unfortunately theory still has a long way to go here

Aside from the fact that calculations are hard, there are deep conceptual problems (not new though, so not easy)

Fundamental problems of perturbation theory: an asymptotic series in a running coupling – essential ambiguities

Radiation of multiple gluons; breakdown of fixed-order perturbation theory resummations in branching processes in initial, final state

Inability to quantify theoretical errors on any given calculation

Example: g g Higgs boson (150 GeV) LO: 15 pb NLO: 25 pb NNLO: 30 pb

There are important, challenging, understudied formal problems in quantum field theory here; they deserve more attention!

Finally -- the Signals of New Physics!

???!!!???

Let’s talk a little about Supersymmetry (SUSY)

a possible solution to the hierarchy problem, yes… a favorite of string theorists popular even 25 years ago, so the detectors were optimized to find it

(along with the Higgs and “technicolor”)

Instructive LHC lessons even if SUSY isn’t found at the TeV scale

Now we have all heard SUSY Missing Energy i.e. Missing Transverse Momentum!and let’s recall why it is true […!...]

SUSY Missing Energy+Jets+LeptonsTrue in the simplest of the minimal SUSY models where SUSY particles are odd under a new Z2 symmetry (“R-parity”)

always produced in pairs decays of SUSY particles always have SUSY particles in final state the lightest one (“LSP”) can’t decay

The lightest one is neutral, colorless, and lives forever

LHC makes colored objects easily, colorless objects not Therefore LHC makes gluinos and squarks most often

if they are not too heavy.

Decaying colored objects must dump color into the final state But the LSP is neutral So the color must exit as quarks or gluons

JETS! Typically high pT

Often a partner of a Z or W is produced This often allows for a lepton or two to be produced as well

SUSY Missing Energy+Jets+LeptonsTrue in the simplest of the minimal SUSY models where SUSY particles are odd under a new Z2 symmetry (“R-parity”)

always produced in pairs decays of SUSY particles always have SUSY particles in final state the lightest one (“LSP”) can’t decay

The lightest one is neutral, colorless, and lives forever

LHC makes colored objects easily, colorless objects not Therefore LHC makes gluinos and squarks most often

if they are not too heavy.

Decaying colored objects must dump color into the final state But the LSP is neutral So the color must exit as quarks or gluons

JETS! Typically high pT

Often a partner of a Z or W is produced This often allows for a lepton or two to be produced as well

Many Models have similar signatures.

…or at least produce MET, high-pT jets and leptons in different combinations.

ATLAS detector: Supersymmetric event with jets, muons and MET – and U.E.

Missing Energy Especially Vague

Almost a useless discovery by itself… and not even clear cut…

If event has missing transverse momentum, only can conclude Something visible was mismeasured, or Something visible went into a crack or near the beam, or Something invisible was created and not observed

But maybe just neutrinos?

If it’s a new neutral particle, that’s great! But hardly SUSY.

We don’t yet know if The particle is a fermion or boson The particle is produced in pairs The particle is stable; lifetime > 10-8 sec, or decays to neutrinos?

Expect long time from first claim of SUSY to convincing evidence!!

Is the MSSM Well-Motivated… ?Minimal SUSY [“MSSM”]:

Superpartners for all known particles Two Higgs doublets, not one.

Supersymmetry is well motivated Stabilizes mW/mPl hierarchy against radiative corrections …As long as mu problem is solved…

Note – size of hierarchy NOT predicted

Minimality is not well motivated Solves nothing Makes theorists feel good – simplicity, beauty, elegance, Occham’s razor

But remember muon, 3rd generation, Z boson…

One should not give these two words equal weight!

Dangers of Minimalism

For a theorist, adding one or two new particles may Leave the main terms in the Lagrangian alone Leave the key mechanism unchanged Make the model look uglier Make the model less predictive (more parameters)So theorists always like minimal models (easier to publish!)

So do experimentalists (… why? …)

For an experimentalist, adding one or two new particles may Change the observable signatures 100 percent Contradict the “lore” as to how to discover Pose enormous challenges unrecognized in minimal model

Modifying the Higgs Sector

A light Higgs boson is a very sensitive creature

New particles in loops can dramatically alter cross-sections, photon branching fraction More scalars can generate mixing of eigenstates, new decay channels, new production

mechanisms.

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, <H>=v / √2 , <S>=0,

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

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> ≠ 0, a second ‘Higgs’

If <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

new mass terms and a mixing term, plus cubic, quartic couplings

Thus we have two eigenstates with masses m1 , m2

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

If <S> ≠ 0, a second ‘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

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

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

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

… it was just one little particle …Thus,

one additional particle can ruin your whole day

And it’s not even that unmotivated – in the first case it is a simple dark matter candidate.

At least we know about this one. It’s the particles we haven’t thought much about that could really

hurt us. We have to keep our eyes open.

We should always be very suspicious of potential Cultural Bias: The culture of theorists always prefers minimal models. Nature may not share this bias.

What does String Theory predict?Hard to find string models without extra matter!

“Millions of models without chiral exotics” [D.Luest’s talk] But these models typically have vector exotics, extra gauge sectors

what are their masses? how do they couple to SM fields?

Often some of vector matter is massless until a symmetry is broken This breaking scale, like weak scale, can naturally be set by SUSY breaking

Thus string theory suggests (to me, anyway) Non-minimal particle content

at or near the TeV scale, coupled to us with 1/TeV-scale interactions.

New heavy charged or colored particles (m > 100 GeV) New heavy or light neutral particles (m > 10 MeV?)

Hidden Valleys: a Subclass of Hidden

SectorsOf course, a hidden sector could be … well … hidden

Producing particles in such a sector could lead to only MET

To infer the structure of the hidden sector would require studying the distribution of MET and accompanying jets/leptons/photons

This would be exciting but very difficult and ambiguous

But it is also possible for a hidden sector NOT to be hidden at all !

Such is the case of a hidden valley

Hidden Valley Scenario (w/ K. Zurek)

A scenario: A Very Large Meta-Class of Models

Basic minimal structure

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

Communicator

Hidden ValleyGv with v-matter

hep-ph/0604261

A Conceptual DiagramEnergy

Inaccessibility

Hidden Valleys and high multiplicity Hidden sector particles may decay visibly, producing 2 or 3 SM particles each

Hidden sectors have their own interactions which can lead to decays or other processes that multiply the number of hidden particles

Let’s look at one example [simply for illustration – other examples can work very differently]

High-multiplicity final states have been considered Pairs of top-antitop pairs Various SUSY decays to 12 particles Black holes

But with the exception of the latter these are not the discovery channels

And black holes have large cross-sections – hidden valleys often don’t

qq

qq

QQ

QQ

Z’Z’

v-quarks

Analogous to e+e- hadrons

q q Q Q : v-quark production

q q Q Q

qqQQ

qq QQ

Z’Z’

v-gluons


q q Q Qv-hadrons


qq

qq

QQ

QQ

Z’Z’

q q Q Q


qq QQZ’Z’

qq QQ

v-hadrons

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

Some v-hadrons are stable and therefore invisible

A rare Z + many jets event?

Or an exotic decay of a heavy resonance?

And what if F is not ~ N ?

Exotic decays Above: a Z’ model with an exotic decay

Exotic decays could appear for other particles Higgs (up to 100% if light) Other neutral scalars (up to 100%) Lightest standard model superpartner (100%)

And other new dark matter candidates in other models Rare W, Z, top decays

These can be difficult to discover if they dominantly involve jets, have few leptons/photons

The phenomenology of Higgs, or SUSY, or Extra Dimensions, etc., can be altered 100%.

Or the effect could be subtle, but no less theoretically important

Hidden Valleys and long lifetimes

New neutral particles with mass m < TeV scale coupled to us by interactions at scale M ~ TeV scale have long lifetimes

~ m5/M4 [ dim 6 operator] inside detector for m > 1 GeV ~ m7/M6 [ dim 7 operator] inside detector for m > 10 GeV ~ m9/M8 [ dim 8 operator] inside detector for m > 100 GeV

Smaller masses most decay outside detector

Many hidden sectors will have several stable particles with different masses, couplings, approximate conservation laws… Consider for example QCD! Many different long lifetimes… Easy to extend lifetime by conservation law (e.g. helicity suppression)

Hidden sectors may appear along with new heavy metastable charged particles too (vectorlike exotics)

q q Q Q


qq QQZ’Z’

qq QQ

v-hadrons

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

Some v-hadrons are stable and therefore invisible

Clearly something new!

Or…

Discovering long-lived particlesATLAS and CMS were not built to find long-lived particles

Discovering long-lived particles – especially those decaying in flight to jets – is a complicated experimental analysis

The backgrounds don’t come from the Standard Model – they come from the detector

Warning: the ATLAS, CMS trackers are not as passive as would be ideal. Every jet of pions will contain One or more pion-tracker interaction One or more photon e+e- conversion

So there are lots of things that look like sprays of particles…

LHCb (somewhat accidentally) may have a better design:a larger matter-free region near the collision pointCould they be the first to discover new physics!? Even the Higgs?!

Clearly something new!

Or is it a rare event with many pion-tracker interactions?

Summary No matter how hard you think the LHC experiment is, it’s harder

Drowning in Data Incomplete Theory Looking for Gold in Golden Sands Many Years to Determine Particles and their Properties

There is still a lot of deep theoretical work to do for the LHC Jets – can the theory be advanced? Underlying Event – can it be treated properly? Standard Model Backgrounds – techniques, mathematics

Let’s keep our eyes (and those of our colleagues) wide open Minimality is not motivated and is a truly dangerous bias Non-minimal models can and do shake the assumptions that

underly the automated and human data analysts

A Theorist’s Worldview Heaven

The essential properties of the universe are simple and logical, and within our grasp.

All particles are well-motivated by basic principles All dynamical mechanisms are minimal and elegant With enough intelligent reasoning and a few more hints,

theorists can soon deduce the structure of the laws of nature Hell

The essential properties of the universe are complex and we have not yet even begun to understand their logic, if any.

Some particles are just there; they are not motivated by any theoretical requirement.

Most dynamical mechanisms are non-minimal and baroque Theorists are far from determining the principles, if any, that

govern the laws of nature, and therefore far from guessing what they are.

An Experimentalist’s Worldview Hell

The essential properties of the universe are simple and logical, and within our grasp.

All particles are well-motivated by basic principles All dynamical mechanisms are minimal and elegant With enough intelligent reasoning and a few more hints,

theorists can soon deduce the structure of the laws of nature Heaven

The essential properties of the universe are complex and we have not yet even begun to understand their logic, if any.

Some particles are just there; they are not motivated by any theoretical requirement.

Most dynamical mechanisms are non-minimal and baroque Theorists are far from determining the principles, if any, that

govern the laws of nature, and therefore far from guessing what they are.

Documents

Preparing for the Early Years of the Large Hadron Collider Overview Lecture Given at the 4 th RTN Workshop Varna, Bulgaria Matthew J. Strassler Rutgers