Introduction High pT physics at the LHC Lecture IV Searches · 2011-04-13 · – Unified electroweak theory involves massless gauge bosons only – Short range of the weak interaction

13/04/2011

1

High pT physics at the LHC Lecture IVSearches

Miriam Watson, Juraj Bracinik(University of Birmingham)

Warwick Week, April 2011

1. LHC machine

15/04/11 M. Watson, Warwick week 1

2. High PT experiments – Atlas and CMS3. Standard Model physics4. Searches

Introduction

• Topics I will cover today:– Higgs searches– SUSY– Extra Dimensions– Inclusive searches

• I will not cover– All the details of every search!

• I will concentrate on ATLAS and CMS


Why we think a Higgs field exists

• The SM is really two separate theories - QCD and GSW electroweak

• We know that the electroweak piece must be brokenSeparate EM and weak forces– Separate EM and weak forces

– Unified electroweak theory involves massless gauge bosons only– Short range of the weak interaction gauge bosons mediating

the weak force must be quite massive

• Something has to break the electroweak symmetry and something has to give the W,Z mass

• All the fermions that are masslessSomething has to give them mass as well


Electroweak Symmetry Breaking

• The gauge group for the GSW theory is SU(2)L⊗U(1)• This must be a broken symmetry, but do not want to destroy gauge

invariance of theory (SM)

• We want to add a new field to the SM that will initially have SU(2)L⊗U(1) symmetry. When this symmetry is broken, the massless bosons become the massive W,Z and a massless photon

• The addition of a single SU(2) doublet of complex scalar fields satisfies these requirements:


13/04/2011

2

Higgs Potential• Distance from the centre describes the

strength of the Higgs field• Height denotes the energy of a

particular field configuration.

• The zero-field configuration (centre) is unstable to small perturbations– system will fall into the lower energy

state in the moat– lowest energy state of space (the

vacuum) is not empty, but is permeated by the Higgs field

– in the ground state there is no symmetry Vac m e pectationin the ground state there is no symmetry in the radial direction

• As the universe fell into the ground state electroweak symmetry was “spontaneously” broken


Vacuum expectation value (vev) = 246 GeV

Theoretical constraints on the Higgs Mass• In order to confirm the existence of a

Higgs field and the Higgs mechanism, we need to find a quantum of this field (Higgs boson)( gg )

• Theoretical bounds on the allowed Higgs mass a chimney around 180 GeV extending to the Planck scale

(non-perturbative)

• Additional constraints from “fine tuning” limits new physics O(TeV)


Λ = cut-off scale at which new physics becomes important

Indirect limits from electroweak precision data

• W mass and top quark mass are fundamental parameters of the Standard Model

• There are well defined relationships between mW, mt and mH


Karl Jakobs, 2010

W and top mass measurementsMW/MW ~ 3.10-4

Mt/Mt ~ 6.10-3

Measurements up to July 2010


These measurements favour a light Higgs boson: MH=89 +35

-26 GeV (68% CL)

LEP2 direct search MH > 114.4 GeV (95% CL)

13/04/2011

3

Tevatron constraints on the Higgs Mass

• Recent CDF and D0 combination excludes 158 < MH < 173 GeV at 95% CL


Higgs processes at the LHC• The Higgs will be

produced through a variety of processes at the LHCLHC

• Some dominate (gg fusion)

• Others are rare (ttH)

• If a Higgs exists, it will be produced at pthe LHC

• Finding it is another matter


SM Higgs production cross-sections• Cross-sections

O(100 pb) significant no. of Higgs will be produced by theproduced by the LHC in a very short time (weeks/months)

• It will take longer than that to claim a discovery

• We have seen the relative cross-sections of Higgs and QCD/EW processes15/04/11 M. Watson, Warwick week 11

Standard Model Higgs decays

• For mH < 1 TeV, divide into low, intermediate and high mass regions

• Decay modes change as a function of mH since the Higgs couples to mass and will decay to the heaviest particle(s)

• Low mass: dominant decay mode (bb) is y ( )essentially useless due to overwhelming QCD backgrounds concentrate on H


13/04/2011

4

Low mass Higgs: H• Low branching ratio, but

take advantage of the excellent photon resolution to see a narrow peak above continuum backgroundg

• Need at least 10 fb-1

With good

Simulation


segmentation

Low mass Higgs: vector boson fusion• Tag two forward jets• Select Higgs bosons in the

channel H→ l or had

• Decay products in central region, i.e. high pT

• Make a collinear approximation (assume neutrinos in tau decays are in same direction as visible decay products)

Simulation

decay products)

• Reconstruct Higgs mass excess if sufficient luminosity


High mass Higgs: H 4 leptons

• Finding a high mass Higgs is much easier

• Both H→WW→ll, and H→ZZ→4l are viable search modes (l = e, )

• Multi-lepton signatures are relatively easy to discern above background

• Both are easier if bosons are on-shell (WW: mH > 160 GeV,

ZZ: m > 180 GeV)

CMS simulation

ZZ: mH > 180 GeV)

• H→ZZ→4l is considered to be the “golden mode” for Higgs searches

• Low backgrounds (ZZ,Zbb,tt)


What has the LHC found so far?

HWWll

Close to SM sensitivity in HWWllHWWll

2010

HWWll (1.2 x SM) with 35 pb-1

Note ff

HWWll


HHZZllqq/ll

different mH ranges on plots

13/04/2011

5

Prospects for SM Higgs in 2011-12

Indicates contributions from different channels


Could exclude down to LEP limit with <4fb-1 !

(possibly)

Higgs boson properties

• If the Higgs boson is discovered, want to measure its properties:– mass, width– spin, CP (SM predicts 0++)p ( p )– coupling to other bosons and to fermions– self-coupling

• … and check whether it is a SM Higgs, or if it is compatible with theories beyond the SM (e.g. SUSY)– in principle there could be more than one

Higgs boson

MH measurement dominated byHiggs boson

– perform direct searches for extra Higgs bosons


ZZ4l and H modes

Eventual precision ~0.1% over large mass range

Need for a theory beyond the Standard Model• Gravity is not included in the

Standard Model

• Hierarchy problem:– In order to avoid the significant fine-

tuning required to cancel quadratic divergences of the Higgs mass, some new physics is required (below ~10 TeV)

• Unificationof gauge

licoupling constants


SM appears to be a low-energy approximation of a fundamental theory

De Santo, 2007

Supersymmetry• One favoured idea to solve the

hierarchy problem is supersymmetry(SUSY)

S ti t b t• Space-time symmetry between fermions and bosons

• To make the SM lagrangiansupersymmetric requires each bosonicparticle to have a fermionicsuperpartner and vice-versa

Spin differs by ½

• These contribute with opposite sign to the loop corrections to the Higgs mass providing cancellation of the divergent terms!


Identical gauge numbers

Identical couplings

13/04/2011

6

Supersymmetric particles

• Superpartners have not been observed!• Minimal Supersymmetric SM (MSSM):

Now have unificationof gauge couplings:

p y ( )– Gauginos and higgsinos mix 2 charginos, 4 neutralinos

– Two Higgs doublets 5 Higgs bosons (h,H; A, H±)


R-parity• SUSY allows for proton decay to occur via

p → e+0

• But proton decay experiments have established that p > 1.6 x 1033 yrs

• This can be prevented by introducing a new symmetry in the theory, called R-parity:– All SM particles have even R-parity (R = 1)– All SUSY particles have odd R-parity (R= -1)

• R-parity conservation proton cannot decay

• Two consequences:• Two consequences:– Lightest SUSY particle (LSP) is stable– Sparticles can only be pair-produced


The LSP and Dark Matter

• The LSP would make a very good dark matter candidate:– Stable– Electrically neutral– Non-strongly interacting (weak and

gravitational interactions only)

• This is why many models are popular in which the LSP is the lightest neutralino,

• Whenever SUSY particles are produced they

01

~

• Whenever SUSY particles are produced they always cascade down to the massive but stable LSP Missing energy is the canonical SUSY

signature


SUSY Phenomenology• There are a very large (>100)

number of free parameters in the MSSM! – e.g. none of the masses are

predictedpredicted

• Impossible to make any phenomenological predictions without making further assumptions

• Some possible constraints:


p1. Impose boundary conditions at higher energy scale and evolve down

to the weak scale via Renormalisation Group Equations (mSUGRA)2. Constraints related to the way SUSY is broken (e.g. GMSB)

– we know it must be broken, because there are no sparticles with same mass as particles

13/04/2011

7

mSUGRA• Only five parameters:

– m0 — universal scalar mass– m1/2 — universal gaugino mass– A0 — soft breaking parameter

β i f Hi– tanβ — ratio of Higgs vevs– sgn(μ) — sign of SUSY mH term

• Highly predictive – masses determined mainly by m0 and m1/2

• Useful framework to provide benchmark scenarios


LHC experiments have agreed toexamine 13 points in mSUGRA space• 9 at low mass (LM1->LM9)• 4 at high mass (HM1->HM4)

Searches for SUSY

• Signatures for SUSY:– Several high-pT jets; – High missing ET (R-conservation);

P ibl l t d/ b j t– Possibly leptons and/or b-jets

• LEP and the Tevatron have set the most stringent limits to date on sparticle masses. Roughly speaking these are:

• m sleptons/charginos > ~ 95 GeV_ p g• m_LSP(neutralino) > ~ 45 GeV• m_gluino > ~290 GeV• m_squark > ~375 GeV


Searching for SUSY at the LHC

Expected limits with 100 pb-1 1 fb-1

• If any of the more common variants of SUSY do exist, the LHC will find it

100 pb 1 – 1 fb 1

• Should be found relatively quickly in one or more modes

• Plot is for multi-jets + missing ET


Example LHC Search Mode - Squark/Gluino Production

• These particles are strongly produced and thus have cross-sections comparable to QCD processes (at the same mass scale)

De Santo

scale)

• Will produce an experimental signature of multi-jets + leptons + missing ET

• A useful variable is the effective mass

• Typical selection:– njets ≥ 4, ET > 100,50,50,50 GeV– 2 leptons ET > 20 GeV, – MET >100 GeV


13/04/2011

8

Examples of results

Jets + MET+ b tagging

3 leptons + jets

• Some LHC SUSY limits are already similar to or better than TEVATRON


Measuring SUSY masses

• If SUSY is found, how can the underlying model be disentangled?

• Aim to map out the SUSY mass spectrum• One strategy is to measure the endpoint

of cascade decays• Make as many such measurements as

m(ll) / GeV


• Make as many such measurements as possible– Other combinations within this chain:

m(lq), m(llq)– Different decay chains

MSSM Higgs searches

• There are five Higgs bosons in the MSSM: h0, H0, H±, A0

• In nearly all models, the lightest neutral SUSY Higgs needs to be light (m < MSUSY Higgs needs to be light (mh < ~130 GeV)

• The phenomenology is sensitive to SUSY parameters, e.g. tanβ

• If tanβ is large, couplings to down-type fermions are enhanced and the role of b jets and leptons become i i l i t t

increasingly important– Production cross-sections are

enhanced by (tanβ)2

– Event rates can be large


An alternative to SUSY – Extra Dimensions• The hierarchy problem:

the weak force is much stronger than gravity (1/MPlanck:1/MEW ~ 10-17)

• Supersymmetry gives one solution to this problem

• Can also be addressed as a geometrical space-time phenomenon:

• Our 3D space could be a 3D “membrane” embedded in a much larger extra dimensional space

T l f d l• Two examples of models:– ADD (Arkani-Hamed, Dimopoulos, Dvali)– RS (Randall-Sundrum)


13/04/2011

9

“Large” Extra-Dimensions (ADD)• Electroweak interactions have been probed down to

1/MEW ~ O(10-15 m)• Gravitational interactions had only been studied to ~1 mm• Gravity may diverge from Newton’s Law at small distances

• For r << R, gravity behaves as if it were 4+n dimensonal (field lines spread out uniformly throughout the bulk) and is stronger

• For r ≥ R gravitational field lines are deformed since they are confined to the 4 dimensions (represented by a 3-D cylinder in the picture)

MPl is a smaller number in ADD


number in ADD

Hierarchy problem is solved

Detecting ADD extra dimensions

• Gravitons can escape into the extra dimensions and appear as missing energy at the LHC

De Santo

Search for an overall excess of ETmiss

Or an excess of monojet + ETmiss events Missing transverse

energy plus single jet

M >Dedicated experiments have


n MD>[TeV]

2 2.373 1.984 1.77

also measured consistency with Newtonian gravity to scales < 10-100 μm

“Warped” Extra Dimensions (RS Model)• ONE small, highly curved

(“warped”) extra dimension connects the SM brane at O(TeV) to the Planck scale branebrane

• Gravity is weak on the “weak brane” where SM fields are confined but increases in strength exponentially in the extra dimension (since space-time is accordingly “warped”)

• Signature: a series of narrow, high-mass resonances


Extra Dimensions in the channel

R = compactification radius,k = curvature,coupling defined by k/MPLp g y PL


13/04/2011

10

Micro Black Holes• MPl is the energy scale at which gravitational interactions become

important• We normally assume this scale is 1019 GeV and we completely

ignore the gravitational interaction of the colliding particles• But if due to extra dimensions M ~ M then gravitational• But if, due to extra-dimensions, MPl ~ MEW then gravitational

interactions will be important• In fact, at length scales below 1/MPl, gravity will dominate, and a

micro-black hole will form


Micro Black Hole signature• These micro black

holes will rapidly evaporate via Hawking radiation and will radiate like a

ST is the scalar sum of the ET of the N individual objects (jets, will radiate like a

“black body”

• Democratic decays to all sorts of particle at the same time

j (j ,electrons, photons, and muons)


Excludes the production of black holes with minimum mass of 3.5 -4.5 TeV

Inclusive searches: di-jets• Very early search for numerous

non-SM resonances: string resonance, excited quarks, axi-gluons, colorons, E6 diquarks, W’ & Z’, RS gravitons....Z , RS gravitons....


Di-jet centrality and angular distributions• Di-jet centrality ratio:

evts with two leading jets in |η|<0.7 compared to events with both leading jetswith both leading jets in 0.7<|η|<1.3

• Sensitive to deviations from the SM due to quark sub-structure, i.e. Compositeness

A l di t ib ti

Excludes quark compositeness for Λ<4.0TeV (95%CL)

• Angular distribution sensitive to contact interactions


Lower limit on scale of contact interaction Λ=5.6 TeV (95% CL)

13/04/2011

11

Inclusive searches: dileptons• Study invariant mass

spectrum to look for dileptonresonances (Z')

• Also– String-theory-inspired E6

models– ADD extra dimensions


Inclusive searches: leptons+MET

• Example: W’ search• W’ has W-like fermionic

couplings• W’ does not couple to other

W’

q e

pgauge bosons

• Tevatron limits: mW’ > 1.1TeV q e

MW’>1.56 TeV


Leptoquarks• Leptoquarks possess both

lepton and quark quantum numbers

P i d d h f llLQ

qe

LQq

q LQ

q

LQq

q• Pair produced: search for qqllor qqlν daughters

• Look at sum of transverse energy:

q e

q Qq e

q


Other models

• There are many other exotic possibilities...– Stopped gluinos– Split SUSY models– Hidden sectors– Hidden sectors– .....

• It would be impossible to cover all of these in one lecture (and too confusing!)

→ Please go and find out more!

→ Or, better still, find a particle...


13/04/2011

12

Summary• With ~40 pb-1 the LHC experiments have begun detailed

measurements of Standard Model physics

• The SM processes give a solid basis for understanding the d t t d th “b k d” t h t hi h ddetectors and the “background” to searches at higher mass and high ET

• Numerous analyses are in place for searches

• With 1-5 fb-1 in 2011-12 we could have – A firm discovery of the Higgs– Indications of SUSY– New resonances– Other new physics

• And we could find something completely unexpected!


Additional material (and acknowledgements)• Last year’s lectures:

– http://www2.warwick.ac.uk/fac/sci/physics/staff/academic/gershon/gradteaching/warwickweek/material/lhcphysics

• CERN Academic Training lectures (Sphicas and Jakobs):CERN Academic Training lectures (Sphicas and Jakobs):– http://indico.cern.ch/conferenceDisplay.py?confId=124047– http://indico.cern.ch/conferenceDisplay.py?confId=77835

• London lectures (de Santo et al.):– http://www.hep.ucl.ac.uk/~mw/Post_Grads/2007-8/Welcome.html

• ATLAS and CMS public results:https://twiki cern ch/twiki/bin/view/CMSPublic/PhysicsResults– https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResults

– https://twiki.cern.ch/twiki/bin/view/AtlasPublic/WebHome

• Moriond Electroweak and QCD:– http://indico.in2p3.fr/conferenceOtherViews.py?view=standard&confId=4403– http://moriond.in2p3.fr/QCD/2011/MorQCD11Prog.html


Documents

Introduction High pT physics at the LHC Lecture IV Searches · 2011-04-13 · – Unified electroweak theory involves massless gauge bosons only – Short range of the weak interaction