High p T physics at the LHC Lecture IV Searches Miriam Watson, Juraj Bracinik (University of Birmingham) Warwick Week, April /04/11M. Watson, Warwick week 1 1.LHC machine 2.High PT experiments – Atlas and CMS 3.Standard Model physics 4.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 15/04/11M. 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 15/04/11M. Watson, Warwick week 3

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: 15/04/11M. 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 15/04/11M. Watson, Warwick week 5 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) 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) 15/04/11M. Watson, Warwick week 6 Λ = cut-off scale at which new physics becomes important (non-perturbative)

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 15/04/11M. Watson, Warwick week 7 Karl Jakobs, 2010

W and top mass measurements 15/04/11M. Watson, Warwick week 8  M W /M W ~  M t /M t ~ These measurements favour a light Higgs boson: M H = GeV (68% CL) LEP2 direct search M H > GeV (95% CL) Measurements up to July 2010

Tevatron constraints on the Higgs Mass Recent CDF and D0 combination excludes 158 < M H < 173 GeV at 95% CL 15/04/11M. 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 15/04/11M. 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 15/04/11M. Watson, Warwick week 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   15/04/11M. Watson, Warwick week 12

Low mass Higgs: H   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 15/04/11M. Watson, Warwick week 13 With good segmentation Simulation

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 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 15/04/11M. Watson, Warwick week 14 Simulation

High mass Higgs: H  4 leptons Finding a high mass Higgs is much easier Both H→WW→ l l, and H→ZZ→4 l 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: 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) 15/04/11M. Watson, Warwick week 15 CMS simulation

What has the LHC found so far? 15/04/11M. Watson, Warwick week H   H  WW  l l H  ZZ  llqq/ll Close to SM sensitivity in H  WW  l l (1.2 x SM) with 35 pb -1 Note different m H ranges on plots H  WW  l l

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

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 15/04/11M. Watson, Warwick week 18 M H measurement dominated by ZZ  4 l 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) Unification of gauge coupling constants 15/04/11M. Watson, Warwick week 19 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) 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 These contribute with opposite sign to the loop corrections to the Higgs mass providing cancellation of the divergent terms! 15/04/11M. Watson, Warwick week 20 Spin differs by ½ Identical gauge numbers Identical couplings

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 ± ) 15/04/11M. Watson, Warwick week 21 Now have unification of gauge couplings:

R-parity SUSY allows for proton decay to occur via p → e +  0 But proton decay experiments have established that  p > 1.6 x 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 15/04/11M. 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, Whenever SUSY particles are produced they always cascade down to the massive but stable LSP  Missing energy is the canonical SUSY signature 15/04/11M. 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 15/04/11M. Watson, Warwick week 24 Some possible constraints: 1.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

mSUGRA Only five parameters: –m 0 — universal scalar mass –m 1/2 — universal gaugino mass –A 0 — 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 15/04/11M. Watson, Warwick week 25 LHC experiments have agreed to examine 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-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 15/04/11M. Watson, Warwick week 26

Searching for SUSY at the LHC 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 15/04/11M. Watson, Warwick week 27 Expected limits with 100 pb -1 – 1 fb -1

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) 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 15/04/11M. Watson, Warwick week 28 De Santo

Examples of results Some LHC SUSY limits are already similar to or better than TEVATRON 15/04/11M. Watson, Warwick week 29 Jets + MET+ b tagging 3 leptons + jets

Measuring SUSY masses 15/04/11M. Watson, Warwick week 30 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 m( ll ) / GeV

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. tanβ If tanβ is large, couplings to down-type fermions are enhanced and the role of b jets and  leptons become increasingly important –Production cross-sections are enhanced by (tanβ) 2 –Event rates can be large 15/04/11M. Watson, Warwick week 31 M 

An alternative to SUSY – Extra Dimensions The hierarchy problem: the weak force is much stronger than gravity (1/M Planck :1/M EW ~ )weak forcegravity 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) 15/04/11M. Watson, Warwick week 32

“Large” Extra-Dimensions (ADD) Electroweak interactions have been probed down to 1/M EW ~ O( 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) 15/04/11M. Watson, Warwick week 33 M Pl is a smaller 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  Search for an overall excess of ETmiss Or an excess of monojet + ETmiss events 15/04/11M. Watson, Warwick week 34 De Santo Missing transverse energy plus single jet n M D > [TeV] Dedicated experiments have also measured consistency with Newtonian gravity to scales < μm

“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 15/04/11M. Watson, Warwick week 35

Extra Dimensions in the  channel 15/04/11M. Watson, Warwick week 36 R = compactification radius, k = curvature, coupling defined by k/M PL

Micro Black Holes M Pl is the energy scale at which gravitational interactions become important We normally assume this scale is GeV and we completely ignore the gravitational interaction of the colliding particles But if, due to extra-dimensions, M Pl ~ M 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 15/04/11M. 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” Democratic decays to all sorts of particle at the same time 15/04/11M. Watson, Warwick week 38 S T is the scalar sum of the E T of the N individual objects (jets, electrons, photons, and muons) Excludes the production of black holes with minimum mass of 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 /04/11M. 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 Angular distribution sensitive to contact interactions 15/04/11M. Watson, Warwick week 40 Excludes quark compositeness for Λ<4.0TeV (95%CL) Lower limit on scale of contact interaction Λ=5.6 TeV (95% CL)

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

Inclusive searches: leptons+MET Example: W’ search W ’ has W-like fermionic couplings W ’ does not couple to other gauge bosons Tevatron limits: m W ’ > 1.1TeV 15/04/11M. Watson, Warwick week 42 W’  M W’ >1.56 TeV

Leptoquarks Leptoquarks possess both lepton and quark quantum numbers Pair produced: search for qqll or qqlν daughters Look at sum of transverse energy: 15/04/11M. Watson, Warwick week 43 LQ

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... 15/04/11M. 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 we could have –A firm discovery of the Higgs –Indications of SUSY –New resonances –Other new physics And we could find something completely unexpected! 15/04/11M. Watson, Warwick week 45

Additional material (and acknowledgements) Last year’s lectures: – ing/warwickweek/material/lhcphysicshttp://www2.warwick.ac.uk/fac/sci/physics/staff/academic/gershon/gradteach ing/warwickweek/material/lhcphysics CERN Academic Training lectures (Sphicas and Jakobs): – – London lectures (de Santo et al.): – ATLAS and CMS public results: – – Moriond Electroweak and QCD: – – 15/04/11M. Watson, Warwick week 46