High p

T

physics at the LHC Lecture IV Searches

Miriam Watson, Juraj Bracinik (University of Birmingham) Warwick Week, April 2011

1. LHC machine 2. High PT experiments – Atlas and CMS 3. Standard Model physics 4. Searches 15/04/11 M. Watson, Warwick week 1

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

15/04/11 M. Watson, Warwick week 2

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 broken

– 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 massless 

Something has to give them mass as well

15/04/11 M. Watson, Warwick week 3

Electroweak Symmetry Breaking

• The gauge group for the GSW theory is SU(2) invariance of theory (SM) L ⊗ U(1) • This must be a broken symmetry, but do not want to destroy gauge • 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: 15/04/11 M. Watson, Warwick week 4

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 in the radial direction • As the universe fell into the ground state electroweak symmetry was “spontaneously” broken 15/04/11 M. Watson, Warwick week Vacuum expectation value (vev) = 246 GeV 5

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) (non-perturbative) • Theoretical bounds on the allowed Higgs mass  a chimney around 180 GeV extending to the Planck scale • Additional constraints from “fine tuning” limits  new physics O(TeV) Λ = cut-off scale at which new physics becomes important 15/04/11 M. Watson, Warwick week 6

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 m W , m t and m H 15/04/11 M. Watson, Warwick week Karl Jakobs, 2010 7

15/04/11

W and top mass measurements

D

M W /M W ~ 3.10

-4

Measurements up to July 2010 D

M t /M t ~ 6.10

-3 These measurements favour a light Higgs boson:

M H =89 +35 -26 GeV (68% CL) LEP2 direct search

M H > 114.4 GeV (95% CL)

M. Watson, Warwick week 8

Tevatron constraints on the Higgs Mass

• Recent CDF and D0 combination

excludes 158 < M H < 173 GeV

at 95% CL 15/04/11 M. Watson, Warwick week 9

Higgs processes at the LHC

• The Higgs will be produced through a variety of processes at the LHC • Some dominate (gg fusion) • Others are rare (ttH) • If a Higgs exists, it will be produced at the LHC • Finding it is another matter 15/04/11 M. Watson, Warwick week 10

SM Higgs production cross-sections

• Cross-sections O(100 pb)  significant no. of Higgs will be produced 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 processes 15/04/11 M. Watson, Warwick week 11

15/04/11

Standard Model Higgs decays

• For m H < 1 TeV, divide into

low, intermediate and high

mass regions • Decay modes change as a function of m H since the Higgs couples to mass and will decay to the heaviest particle(s) • Low mass  : dominant decay mode (bb) is essentially useless due to overwhelming QCD backgrounds concentrate on H  gg M. Watson, Warwick week 12

15/04/11

Low mass Higgs: H

 gg • Low branching ratio, but take advantage of the excellent photon resolution to see a narrow peak above continuum background • Need at least 10 fb -1 Simulation With good segmentation M. Watson, Warwick week 13

Low mass Higgs: vector boson fusion

• Tag two forward jets • Select Higgs bosons in the channel H→ tt (t 

l

or t  had ) • Decay products in central region, i.e. high p T • Make a collinear approximation (assume neutrinos in tau decays are in same direction as visible decay products) • Reconstruct Higgs mass  excess if sufficient luminosity Simulation 15/04/11 M. Watson, Warwick week 14

High mass Higgs: H



4 leptons

• Finding a high mass Higgs is much easier • Both H→WW→

l

n

l

n , and H→ZZ→4

l

are viable search modes (

l =

e, m ) • Multi-lepton signatures are relatively easy to discern above background • Both are easier if bosons are on shell (WW: m H > 160 GeV, ZZ: m H > 180 GeV) • H→ZZ→4

l

is considered to be the “golden mode” for Higgs searches • Low backgrounds (ZZ,Zbb,tt) 15/04/11 CMS simulation M. Watson, Warwick week 15

What has the LHC found so far?

H  WW  l n l n H  WW  l n l n 2010

Close to SM sensitivity in H



WW



l

n

l

n

(1.2 x SM) with 35 pb -1

H  ZZ  llqq/ll nn 15/04/11 Note different m H ranges on plots M. Watson, Warwick week H  gg 16

Prospects for SM Higgs in 2011-12

Indicates contributions from different channels Could exclude down to LEP limit with <4fb -1 ! (possibly) 15/04/11 M. Watson, Warwick week 17

Higgs boson properties

• If the Higgs boson is discovered, want to measure its properties: – mass, width – spin, CP (SM predicts 0++) – 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 – perform direct searches for extra Higgs bosons M H measurement dominated by ZZ  4

l

and H  gg modes Eventual precision ~0.1% over large mass range 15/04/11 M. Watson, Warwick week 18

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) • Unification of gauge coupling constants 15/04/11 SM appears to be a low energy approximation of a fundamental theory De Santo, 2007 M. Watson, Warwick week 19

Supersymmetry

• One favoured idea to solve the hierarchy problem is supersymmetry (SUSY) • Space-time symmetry between fermions and bosons • To make the SM lagrangian supersymmetric requires each bosonic particle to have a fermionic superpartner 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 15/04/11 M. Watson, Warwick week 20

Supersymmetric particles

• Superpartners have not been observed!

• Minimal Supersymmetric SM (MSSM): – Gauginos and higgsinos mix  2 charginos, 4 neutralinos – Two Higgs doublets  5 Higgs bosons (h,H; A, H ± ) 15/04/11 M. Watson, Warwick week Now have unification of gauge couplings: 21

R-parity

• SUSY allows for proton decay to occur via p → e + p 0 • But proton decay experiments have established that t p > 1.6 x 10 33 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: – Lightest SUSY particle (LSP) is stable – Sparticles can only be pair-produced 15/04/11 M. Watson, Warwick week 22

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,  1 0 • Whenever SUSY particles are produced they always cascade down to the massive but stable LSP  Missing energy is the canonical SUSY signature 15/04/11 M. Watson, Warwick week 23

SUSY Phenomenology

• There are a very large (>100) number of free parameters in the MSSM! – e.g. none of the masses are predicted • Impossible to make any phenomenological predictions without making further assumptions • Some possible constraints: 1.

2.

Impose boundary conditions at higher energy scale and evolve down to the weak scale via Renormalisation Group Equations (mSUGRA) 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 15/04/11 M. Watson, Warwick week 24

mSUGRA

• Only five parameters: – m 0 — universal scalar mass – m 1/2 – A 0 — universal gaugino mass — soft breaking parameter – tanβ — ratio of Higgs vevs – sgn(μ) — sign of SUSY m H term • Highly predictive – masses determined mainly by m 0 and m 1/2 • Useful framework to provide benchmark scenarios LHC experiments have agreed to examine 13 points in mSUGRA space • 9 at low mass (LM1->LM9) • 4 at high mass (HM1->HM4) 15/04/11 M. Watson, Warwick week 25

Searches for SUSY

• Signatures for SUSY: – Several high-p T jets; – High missing E T (R-conservation); – 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 • m_LSP(neutralino) > ~ 45 GeV • m_gluino > ~290 GeV • m_squark > ~375 GeV 15/04/11 M. Watson, Warwick week 26

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 • Should be found relatively quickly in one or more modes • Plot is for multi-jets + missing ET 15/04/11 M. Watson, Warwick week 27

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 • Will produce an experimental signature of multi-jets + leptons + missing E T • A useful variable is the effective mass • Typical selection: – n jets ≥ 4, E T > 100,50,50,50 GeV – 2 leptons E T > 20 GeV, – MET >100 GeV 15/04/11 M. Watson, Warwick week 28

Examples of results

Jets + MET+ b tagging 3 leptons + jets

15/04/11 • Some LHC SUSY limits are already similar to or better than TEVATRON M. Watson, Warwick week 29

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 possible – Other combinations within this chain: m(

l

q), m(

ll

q) – Different decay chains 15/04/11 M. Watson, Warwick week m(

ll

) / GeV 30

MSSM Higgs searches

• There are five Higgs bosons in the MSSM: h 0 , H 0 , H ± , A 0 • In nearly all models, the lightest neutral SUSY Higgs needs to be light (m h < ~130 GeV) • The phenomenology is sensitive to SUSY parameters, e.g. tan β • If tanβ is large, couplings to down-type fermions are enhanced and the role of b jets and t leptons become increasingly important – Production cross-sections are enhanced by ( tanβ) 2 – Event rates can be large 15/04/11 M. Watson, Warwick week M tt 31

An alternative to SUSY – Extra Dimensions

• The

hierarchy problem:

the weak force 10 -17 ) is much stronger than gravity (1/M Planck :1/M EW ~ • 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 • Two examples of models: – ADD (Arkani-Hamed, Dimopoulos, Dvali) – RS (Randall-Sundrum) 15/04/11 M. Watson, Warwick week 32

“Large” Extra-Dimensions (ADD)

• Electroweak interactions have been probed down to 1/M EW ~ 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) M Pl is a smaller number in ADD 15/04/11 M. Watson, Warwick week Hierarchy problem is solved 33

Detecting ADD extra dimensions

• Gravitons can escape into the extra dimensions and appear as missing energy at the LHC  Search for an overall excess of ETmiss Or an excess of monojet + ETmiss events Missing transverse energy plus single jet Dedicated experiments have also measured consistency with Newtonian gravity to scales < 10 100 μm

n

2 M D > [TeV] 2.37

3 1.98

4 1.77

15/04/11 M. Watson, Warwick week De Santo 34

“Warped” Extra Dimensions (RS Model)

• ONE small, highly curved (“warped”) extra dimension connects the SM brane at O(TeV) to the Planck scale brane • 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 15/04/11 M. Watson, Warwick week 35

Extra Dimensions in the

gg

channel

R = compactification radius, k = curvature, coupling defined by k/M PL 15/04/11 M. Watson, Warwick week 36

Micro Black Holes

• M Pl is the energy scale at which gravitational interactions become important • We normally assume this scale is 10 ignore the gravitational interaction of the colliding particles • But if, due to extra-dimensions, M Pl 19 ~ M GeV and we completely EW then gravitational interactions will be important • In fact, at length scales below 1/M Pl , gravity will dominate, and a micro-black hole will form 15/04/11 M. Watson, Warwick week 37

Micro Black Hole signature

• These micro black holes will rapidly evaporate via Hawking radiation and will radiate like a “black body” S T is the scalar sum of the E T of the N individual objects (jets, electrons, photons, and muons) • Democratic decays to all sorts of particle at the same time 15/04/11 Excludes the production of black holes with minimum mass of 3.5 -4.5 TeV M. Watson, Warwick week 38

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....

15/04/11 M. Watson, Warwick week 39

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 jets in 0.7<|η|<1.3

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

xcludes 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)

15/04/11 M. Watson, Warwick week 40

Inclusive searches: dileptons

• Study invariant mass spectrum to look for dilepton resonances (Z') • Also – String-theory-inspired E6 models – ADD extra dimensions 15/04/11 M. Watson, Warwick week 41

Inclusive searches: leptons+MET

• Example: W’ search • W • W ’ ’ has W-like fermionic couplings does not couple to other gauge bosons • Tevatron limits: m W ’ > 1.1TeV

q q

 W’ n

e e

 M W’ >1.56 TeV 15/04/11 M. Watson, Warwick week 42

Leptoquarks

• Leptoquarks possess both lepton and quark quantum numbers

q

LQ

e



q

LQ • Pair produced: search for qqll or qql ν daughters • Look at sum of transverse energy:

q e



q q q

LQ n 

q

LQ

q e

 15/04/11 M. Watson, Warwick week 43

Other models

• There are many other exotic possibilities...

– Stopped gluinos – Split SUSY models – 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...

15/04/11 M. Watson, Warwick week 44

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 detectors and the “background” to searches at higher mass and high E T • Numerous analyses are in place for searches • With 1-5 fb -1 in 2011-12 we

could

– A firm discovery of the Higgs – Indications of SUSY – New resonances – Other new physics have

• And we could find something completely unexpected!

15/04/11 M. Watson, Warwick week 45

Additional material (and acknowledgements)

• Last year’s lectures: – http://www2.warwick.ac.uk/fac/sci/physics/staff/academic/gershon/gradteach ing/warwickweek/material/lhcphysics • 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/AtlasPublic/WebHome • Moriond Electroweak and QCD: – http://indico.in2p3.fr/conferenceOtherViews.py?view=standard&confId=4403 – http://moriond.in2p3.fr/QCD/2011/MorQCD11Prog.html

15/04/11 M. Watson, Warwick week 46