atlas: new signals from a “hidden valley” matt strassler, u washington

ATLAS: New signals from a “Hidden Valley”

Matt Strassler, U Washington

Theoretical Motivation

Many top-down models, such as string theory or extended grand unified models, typically predict many sectors beyond the standard model.

Such sectors are also appearing regularly in solutions to the hierarchy problem (twin Higgs, folded supersymmetry…)

New sectors could be involved in SUSY-breaking, flavor, dark matter, …

Often these sectors continue to interact with our own down to low scales

Constraints on such sectors from LEP, cosmology, Tevatron are rather limited

Learning about these sectors, which may contain many particles, could open up an entirely new view of nature..

But as we will see… Cross-sections may be low, Signals are very unusual; novel phenomenology, special challenges Push the limit of (but do not exceed) the experiments’ capabilities

“Hidden valley”: signal that challenges the usual assumptions about how the LHC detectors are supposed to function –

do not get high-pT jets and isolated leptons

Typical signatures are Complex non-QCD-like multi-jet events Extreme event-to-event fluctuations Probably some missing energy (possibly a lot) Probably some heavy flavor (possibly a lot) Perhaps displaced jets Perhaps nonisolated moderate pT leptons

Could drastically affect “standard signals” such as standard Higgs supersymmetry little Higgs

Experimental Motivation

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 DiagramEnergy

Inaccessibility

Hidden Valley Models (w/ K. Zurek)

Basic minimal structure


Communicator


Communicators


New Z’ fromU(1)’


Communicators


Higgs BosonOr Bosons


Communicators


Lightest StandardModel Superpartner


Communicators


Heavy SterileNeutrinos


Communicators


Loops of ParticlesCharged Under

SM and HV


Note that the communicator for production need not be the communicator for the decays…




Higgs Bosons

Communicators

The Hidden Valley (“v”-)Sector


Communicator

Hidden ValleyQCD-like Theory



Communicator

Hidden ValleyGluons only



Communicator

Hidden ValleyQCD-like Theory

With N ColorsWith n1 Light QuarksAnd n2 Heavy Quarks



Communicator

Hidden ValleyGluons Plus

Adjoint Matter



Communicator

Hidden ValleyMultiple Gauge Groups

Simplest Class of Models

Clearly the number of possibilities is huge! Cannot address them one by one.

Key is to identify typical signatures of large classes of models.

Easiest model to understand … and simulate… is:



Hidden Valleyv-QCD with

two light v-quarks

Simplest Class of Models

This model is typical of a large class: QCD-like theory with a few light quarks and no heavy quarks

Other models can be quite different in their details; we’ll discuss a couple of them later.

For now, let’s explore this one in detail, since it’s the one in the current MC package.



Hidden Valleyv-QCD with

two light v-quarks

The Simple Model in the Program

Structure of the model Spectrum of the “v-hadrons” Decays of the v-hadrons Production of the v-hadrons Events

Along the way: Simulation techniques

Two-flavor (v)QCD

A model with N colors and two light v-quarks serves as a starting point.

The theory is asymptotically free and becomes strong at a scale v

All v-hadrons decay immediately to v-pions and v-nucleons.

All v-hadrons are electric and color neutral, since v-quarks are electric and color-neutral

If v-baryon number is conserved, v-baryons are stable (and invisible)

Two-flavor (v)QCD

All v-hadrons decay immediately to v-pions and the lightest v-baryons

Two of the three v-pions cannot decay via a Z’

But the third one can!

vQ1Q2 stable

vQ2Q1 stable

vQ1Q1 Q2Q2 (Z’)* f f

bb

bb

v Z’Z’

Pseudoscalars: their decays require a helicity flip; branching fractions proportional to fermion masses mf

2

Long lifetimes

The v-hadrons decay to standard model particles through a heavy Z’ boson.

Therefore – no surprise -- these particles may have long lifetimes

Notice the very strong dependence on what are essentially free parameters

LEP constraints are moderate; cosomological constraints weak

Thus displaced bottom-quark pairs and tau pairs are common in such models, but not required.

q q Q Q : v-quark production

qq

qq

QQ

QQ

Z’Z’

v-quarks

q q Q Q

qqQQ

qq QQ

Z’Z’

v-gluons

q q Q Q

qq

qq

QQ

QQ

Z’Z’

v, v

;v

q q Q Q

qq

qq

QQ

QQ

v, v

;v

v-pions

For now, take masses in range 20-350 GeV so that dominant v

decay is to b’s

Z’Z’

q q Q Q

qq

qq

QQ

QQ

v-pions

Z’Z’

q q Q Q

qq

qq

QQ

QQ

v-pionsThe v

, vare

invisible and stable

Z’Z’

q q Q Q

qq

qq

QQ

QQ

v-pions

Z’Z’

q q Q Q

qq

qq

QQ

QQ

v-pions But the vs

decay in the detector to bb pairs

Z’Z’

Production Rates for v-Hadrons

Other interesting processes

This is going to be [almost] the process addressed by the current simulation package

But first let’s step back… To keep perspective on what we will be able

to achieve with my current software, and what we cannot do directly but should have in the back of our minds, let’s consider other possible phenomena that would arise in other models, or even in this one…

What if Q_2 decays to Q_1 Z’*

Q2

Q1

qq

qq

v Z’Z’

v

Z’Z’FCNC; model dependent

Z’Z’

K+

K-

bb

bb

Kaons or other soft hadrons or leptons too soft to observe; essentially a decay to bottom quarks plus very soft stuff…

q q Q Q

qq

qq

QQ

QQ

v-pionsNow all or most v-pions decay in the detector

Z’Z’

Higgs decays to the v-sector

g

g

QQ

QQ

v-quarkshh hhvv

mixing

Higgs vpion vpion two displaced jet-pairs

A Discovery Channel at Tevatron! At LHC, trigger?!

Possibly in associated production or VBF? Needs study…

w/ K Zurek, May 06

SUSY decays to the v-sector

g

g

QQ

QQ

v-(s)quarks

If the standard model LSP is heavier than the v-sector LSP,then the former will decay to the latter(a v-squark or v-gluino in simplest models)

The traditional missing energy signal is replaced with multiple soft jets, reduced missing energy, and possibly multiple displaced vertices

July 06

q~

q*~

q

q

Q*Q*~

QQ~_

_

v-(s)hadrons

Many possibilities!!!

Other v-sector models

QCD with one flavor has a very different spectrum

The spectrum is not precisely known but the omega meson is stable against decay to hadrons

The v-omega can decay to any standard model fermion pair including muons and electrons

However its production will be accompanied by the production and decay of other v-hadrons, making it a challenge to detect the v-omega resonance in electron/muon pairs

Still this should be possible if the a sufficiently pure sample of events can be identified

Cascade decays of stable scalar and spin-two particles may be interesting and allow additional light-fermion production in three-body decays

Simulation package needed – w/ Skands

April 06

q q Q Q

qq

qq

QQ

QQ

v-pions But the vs

decay in the detector to bb pairs

Z’Z’

Back to our original model and our original process…

Returning to v-quark production

Our two-v-quark model is a very simple model to understand

But it is not simple to simulate; since it is different from QCD, it requires a new simulation package

For our current purposes, it is useful to consider a model which is almost exactly like QCD…

…“exactly” as far as the quarks and gluons are concerned, but with electroweak physics turned off, and with all mass scales scaled up by a constant factor…

… so that we can use existing Pythia software, suitably adjusted.

A third v-quark

Let’s add a third v-quark analogous to the s quark, but without allowing it to couple to the Z’ boson. (Charm and bottom quark physics will be a small effect; rarely

produced in Pythia showering, hadronization) Then

The Z’ does not decay directly to the third v-quark. Any v-hadrons containing the third v-quark cannot decay… except

through annihilation to v-pions. v-pion production is almost the same as in QCD v-Kaons are stable and invisible But the v-eta is different in this model than the eta is in Pythia.

In QCD the eta decays to two photons But there are no v-photons, so the v-eta decays to v-pions – which

Pythia does not currently simulate Similarly there are few differences among other v-hadrons

A third v-quark

Some of these differences between Pythia’s simulation of QCD and a perfect simulation of the v-sector could be adjusted for by changing hadron decay settings in Pythia

However these differences are rather small effects and I do not believe they will cause serious errors

Since the v-sector is not going to be exactly like QCD anyway, I see attempts to refine the simulation to this level as overkill

At worst there will be a few percent overestimate of the missing energy signal and a few percent underestimate of the vpion production in this particular model

Variations from model to model will be much larger than this!

A third v-quark

So the claim is that in this model with Two light quarks coupling to the Z’, and a third quark not

coupling to the Z’ Masses arranged to that all meson masses and decay

constants, relative to v, are the same as in QCD,

The use of a Pythia QCD simulation is a very good model of the showering and hadronization that would occur in this vQCD sector, except that v-pi-zeros decay not to v-photons [which don’t exist] but to

standard model fermion pairs, through the Z’. v-eta’s decay incorrectly – indeed all radiative decays are

incorrect [except v-pi-zero decays which are corrected for.] Some v-Kaon decays are not consistent within the model

Simulating this process To simulate the full process, we would need three steps

1. Simulate Z’ production and decay to v-quarks 2. Simulate v-showering and v-hadronization and the formation of a final state of v-

pions and v-baryons3. Simulate decays of the v-pi-zeros and the subsequent formation of standard model

b jets, tau final states; add in ISR and the UE.

The first is no problem. Z’ production is as always, though it depends on charges of SM particles under Z’ Decay of Z’ to v-quarks is like decay to quarks, but depends on charges of v-

quarks under Z’

The last is no problem. Since v-pions are spin-zero, decays are isotropic and are very similar to Higgs

boson decays The current program uses Higgs bosons as stand-ins for v-pions [but this may

change if ATLAS software requires it.] Pythia adds ISR, UE when event is generated

The second step is tricky, and compromises are necessary at present.

The procedure in step 2

qq Z’ QQ leaves us with a v-quark pair of invariant mass MQQ ~ mZ’ We scale down the mass MQQ by a factor / v :

m = MQQ / v

For example: if mZ’ ~ 3 TeV, v ~ 90 GeV, mv ~ 45 GeV, then m ~ 10 GeV We simulate (using Pythia) the showering and hadronization of an ordinary

quark-antiquark pair of invt mass m. Caution! If m lies too close to a bottomonium, charmonium, or light-quark resonance,

answers will be badly distorted. No current check to prevent problems!

Then scale all the particle energies by v / so that the invt mass of the hadronization products is again MQQ

Look in Pythia event record and grab all pions, throw away all other particles Store pi-zeros in event record as h0 bosons [these always decay] Store pi-plus/minus in event record as H0 bosons [these are usually stable, but

not in all variants of the model…] The resulting event record (in new LHA format) can be uploaded into Pythia

(with a simple Pythia card setting that turns off unwanted h0 and H0 decays and sets the lifetimes of these particles equal to v-pion lifetimes.)

This allows v-pion-decays/QCD-showers/QCD-hadronization to be simulated.

The interesting phenomena Case 1: particle lifetimes are short

Multiple moderate-to-low pT overlapping partons make an unusual looking event

Mapping of jets to partons very poor Light v-pions often make a single jet

Boosted b-jets overlap Not boosted only one jet is moderate pT, other soft

Many b quarks, but at moderate-to-low pT and overlapping, pose a tagging challenge

Case 2: particle lifetimes are long – displaced jets, displaced tau pairs and in many models, other displaced possibilities

Mixture of these is possible of course

Displaced jets

Questions I can’t answer but would like to:

Decays in beampipe – Tevatron expts would record these as b-tagged jets. Can one do better? What distinguishes them? How much background is there?

Decays in inner tracker – Any hints at trigger level? Muons that miss the beampipe? Is there a better

strategy? Any hints at reconstruction level? Vertices with hints of wide-angle tracks?

Decays in outer tracker, calorimeter – In a scatter plot of the number of reconstructed tracks versus the hadron/em

ratio in the calorimeter, late decays will be out on a tail (no tracks, normal had/em ratio). Can this be used? Study needed…

Extra hits in outer tracker near jet with no tracks? [what can TRT do at ATLAS?]

Since many events have multiple decays, it is important to combine these strategies be combined in a single analysis!

Trigger on ISR – how efficient? Can this be used to grab a few events even when majority cannot be triggered on?

atlas: new signals from a “hidden valley” matt strassler, u washington

Documents

vmatter slide

bosons hidden valley

hv hidden valley g v

hidden valley models

hidden valley matt strassler

theory slide

standard signals

adjoint matter slide