# Presentation: 948213

## Higgs Search Strategy at the LHC

**5.1 Cross Sections at the LHC**
**5.2 Higgs Loop Decays and Direct Decays**
**5.3 Higgs Production Rates**
**	****	****gg Fusion**
**	****	****WW Fusion and Tag Jets**
**	****	****Associated Production - HV, Htt**
**5.4 Higgs Branching Ratios and Search Strategy****	****ZZ --> 4l, ********, bb, ********, 2l +2********, 2l + 2J, WW**
**5.5 Lower limit on Higgs mass**
**5.6 Luminosity and Discovery Limits**
**5.7 Others- Composites, Extra Dimensions**

## Cross Sections at the LHC

**Resonances - narrow width approximation:**

**e.g.**

**There is a factor > 10****10 ****between the Higgs cross section and the total inelastic cross section. There is also the final state branching fraction to consider. This is why the LHC design luminosity is so high.**

**LHC Cross Sections:**

## Higgs Cross section vs Energy

**CDF and D0 successfully found the top quark, which has a cross section ~ 10****-10 ****the total cross section. A 500 GeV Higgs has a cross section 1000 times smaller at the Tevatron**
**A 500 GeV Higgs has a cross section ratio of  ~ 10****-11****, which requires great rejection power against backgrounds and a high luminosity. Multiple redundant measurements of the SM particles are needed. **

## LHC Rates at 1033/(cm2sec)

**“****low luminosity” at or near LHC startup is such that SUSY and low mass Higgs are still accessible.**

## Higgs - Loop Decays

**There are decay modes that are not accessible through diagrams with a single coupling constant - zero mass states.**
**The coupling is to the heaviest quark in the loop. For a light Higgs, ********(H --> ********)/M****H**** ~ ********2********W****/********2****(M****H****/M****W****)****2****(m****t****/M****H****)****8                          ****[sin****-1****(M****H****/2m****t****)]****4**
******(H --> gg)/M****H ****can be estimated by replacing ******** by ********s****.  This diagram is the major production mode at the LHC, g + g from p + p fusing into a Higgs boson. The loop integral |I| is ~ O(1). **
**For a light Higgs, ********(H --> gg)/M****H  ****~ [********s****2********W****/72********2****](M****H****/M****W****)****2****.**

**H**

****
****

**t**

**COMPHEP does not do loops**

## Higgs Decay Rates - Direct and Loop

**Direct:**
**	****Quarks and Leptons**

**Gauge Bosons**

**Loop Decays - Gauge Bosons:**

**Higgs couples to mass, with no direct H******** or Hgg coupling**

## Higgs - Production via gg Fusion

**The formation cross section is, **
**Using the expression for ********(H-->gg) and normalizing the gluon distribution with a = 6, **
**Note that the M****H****3 ****behavior of ******** cancels the 1/ M****H****3**** behavior of d********/dy , leaving a roughly constant cross section, **

- d/dy ~ **2(*H**gg*)/(*M**H*3)[*xg*(*x*)]*x*1[*xg*(*x*)]*x2*

*d*/*dy* ~ 49**2[(*H*  *gg*)/(4*M**H*3)][(1 - *M**H*/*s*)12]  ~  49**2(*H*  *gg*)/(4*M**H*3)

*d***/*dy* ~ 49|I|2s2W/[288*M**W*2].

## Higgs Event Rate Estimate

**Ignore |I| dependence on Higgs mass (n.b. peak at ~ twice the top mass). If M****H****= 500 GeV then the gluon source factor (1-M****H****/********s)****12**** is ~ 0.65. The falloff of the cross section with source function at the LHC is small for low mass Higgs.**

## Higgs Production Modes

**The Higgs cross section has as largest contribution g+g with an internal top loop. Note that qqH is quite large, followed by associated production modes**** including DY production of W, Z with H bremmstrahlung and H with top pairs**

## Tevatron Limits – Run II – gg Fusion

**Note that with 0.3 fb****-1**** integrated luminosity the limit is ~ 100 x above the SM expectation.**

## Higgs Production - Faked

**Higgs Production - Faked**

**Make a fake Hgg coupling and add to SM - Unitary gauge**

## VBF Mechanism – 2 Photon Physics

**In pp collisions there are valence quarks. The h-> Z is a good control sample. VBF is similar to 2 photon physics which is larger than 1 photon processes at sufficiently high energy. The 2 photon process is “tagged” by the observation of 2 small angle photons.**

**2 photon production means there is a quantum number “filter” –**
**J****PC ****= 0****++****, 2****++**

## Tag Jets in WW Fusion

**Soft and collinear radiation --> tag  jets in WW fusion. Distribution function of W calcuable in perturbation theory**

**Heavy Higgs formed by WW fusion is a large fraction of the total Higgs cross section**

## VBF at Tevatron and LHC

**Over the Higgs mass range (100,1000) GeV VBF is a substantial fraction of the Higgs production rate and it offers a good signature – 2 tag jets. The tevatron cross section is down by > 10 x. In addition the rapidity plateau is wider at the LHC which makes the tag jets, with <********>~ 3 more distinct there than at the Tevatron.**

## H Production from W+W

**Use the EW radiation of a W by a quark. The “effective W approximation” analogous to the WW approximation. Need good jet coverage to low P****T**** and small angles. Cross section depends only on the Higgs coupling to W, Z – isolate g****HWW****.**

- HF

- HE

- HB

## VBF is a ‘Discovery” Mode at the LHC

**The “continuum” WW cross section is small w.r.t. the Higgs contribution.**

**In a full study the main background is from top pairs and WWJJ radiative DY background.**

## Associated production - Tevatron

**Basically the D-Y process of W or Z production has an accompanying, radiated H (or Z as a control process)**

**There is a large background from W + b pairs. This is not a competitive strategy at the LHC.**

## Associated production Limits in Run II

** ****With ~ 0.4 fb****-1**** the limits set are ~ 100x above the rate expected for a SM Higgs.  If the logged luminosity rises to 4 fb****-1****, then **

## Higgs + Top Pairs at the LHC

**Top pair QCD production with a radiated accompanying H.**

**A top pair is ~ 0.35 TeV. This is too heavy for the Tevatron to make a useful rate. It is a useful mode at LHC for low mass Higgs with a good **
**ratio of  **

**Take Higgs cross section and divide by +- 2 sigma in the mass resolution ~ 20 GeV (recall Z) to get  **

## Z “Control” Sample

**In many cases the H can be replaced by a Z and the process will have a similar cross section and other dynamics. Then the dilepton decays of the Z can be used as a clean signature and the process can be validated prior to looking for the H itself.**

## Quartic gauge Couplings – D-Y, VBF 

**High WW or ZZ mass VBF processes will let us explore quartic couplings at the LHC.**

**Beyond the range of Run II at present. Seen at LEP -II**

## Higgs Pair Production at SLHC

**Higgs potential**
**vev at potential minimum**
**Expand about the minimum**

**There are 2 parameters in the potential. One is fixed by G. Once the Higgs mass is known the SM will be completely specified. Verify the SM by exploring the triple and quartic Higgs self-couplings?**

## Higgs – Self-couplings at the SLHC

**Cross section is ~ 20 fb @ 160 GeV. If SLHC is 1000 fb-1/yr, then 20,000 HH produced/yr. There are ~ 400 events with 2 leptons, missing energy and 4 jets**
**(2 W mass peaks).**

## Higgs Particle Production at the LHC

**The coupling of the Higgs field to the fermions is proportional to the fermion mass. Thus the Higgs field couples only weakly to ordinary matter, u,d quarks and gluons. Therefore, production cross sections are rather small, making discovery difficult.**
**The Higgs mass is unknown. For low masses bb and ********modes are favored. When energetically allowed , WW and ZZ modes dominate.**
**The large top mass makes the tt mode substantial, > 10%. The backgrounds are overwhelming, however.**
**The CMS detector is designed to discover the Higgs for all masses < 1 TeV in 1 year of full luminosity operation.**

## Higgs Branching ratios

**Note that q,l width ~ M while W,Z width ~ M****3****. Hence bb dominates below WW “threshold”. ******** is down by ~ 9 due to coupling to mass, and 1/3 color factor.**

**Similarly for WW***

## Higgs Width to q and l ~ M

## Higgs Width into V V ~ M3

**Below WW “threshold” there is a Wlv mode with an off shell W. Note that the Lorentzian width, ********W****~ 2.2 GeV takes the rate down by a factor ~[(********/2)/(M-Mo)]****2**** w.r.t. the WW rate. Going from WW at 160 GeV to 140 GeV WW*, expect width ~ 1.2 MeV ~ the bb width.**

## Higgs Strategy and BR

**H-->******** is a clean decay mode for low mass Higgs. The ZZ --> 4l mode is clean when it is above threshold at ~ 150 GeV. The dip in ZZ is due to WW rise above threshold at ~ 160 GeV. The WW decay mode does not have a mass peak and is unused save just at threshold, except in the VBF mode where it is a discovery mode.**

**LEP-II**

**VBF, H ->WW* -> 2l**

## bb QCD Backgrounds

**Assume calor reconstruction of bb at 100 GeV has ********M = 20 GeV. ******** /********M = 2pb/GeV. Thus Higgs is buried by ~ 1000x**

## Tau Pairs – Higgs and D-Y Background

**Use the BR given above. For M = 100 GeV, ******** B/********M = 0.2 pb/GeV**
**S/B is better due to reduced coupling and smaller source, g --> u **

**Although the S/********B is much more favorable. However, must identify hadronic tau decays in order not to be swamped by QC~ jets.**

## Tau ID and QCD J Rejection at LHC

**Leptonic decay – small BR**

**Hadronic decay – large BR with low multiplicity J and missing Et**

##  Background

****** ****mode is favorable w.r.t. bb as g --> u and strong --> EM coupling. ECAL mass resolution for ******** is ~ 10x better than bb. Still buried by ~ 50x in the continuum background**

**Recall Chpt. 4, u+u --> ******** +********  **

## Low Mass Higgs 

**H********: decay is rare (B~10****-3****)**
**But with good resolution, one gets a mass peak**
**Motivation for PbWO****4**** **
**     ****calorimeter**
**CMS: at 100 GeV, ******** 1GeV**
- S/B  1:20

## Higgs Search - Four Leptons

**ZZ* gives 1 resonant Z mass and 4 isolated leptons as a signature.**

**Z**

**Z***

## ZZ Background

**For Higgs mass = 350, 700 GeV the cross section is 10 and 1 pb. The ZZ branching fraction is 1/3. The mass window, ********M  is 70, 200 GeV. The natural width dominates at high mass.**

**ZZ continuum background is small w.r.t. Higgs signal – gold plated mode**

## Full Monte Carlo Study, H->ZZ->4l

## Higgs --> ZZ -->2l + 2 Jets

**For masses > 600 GeV, larger branching ratio decay modes are needed due to rate limitations..**

**e       e**

**J         J**

**Note HCAL granularity – Chpt.2**

## WW Mode -tt QCD Background

**tt has the same  Feynman diagrams and hence the same cross section as bb above threshold effects, WW background from t --> Wb**

## WW Backgrounds - 1/200 tt

**Can clean up top pairs using jet veto of the b quarks. However, there is no resonant mass ‘bump”. In VBF top pairs are a major background, but a mass bump can be formed using the tag jets.**

**D-Y Production of WW is an irreducible background**

**t pairs/100**

## Higgs Mass - Upper Limit

**The couplings are a function of the mass scale at which they are probed. We require that ********(Q) is well behaved from          = 174 GeV up to a scale ********, with 1/ ********(********) = 0 (strong coupling at ********), the running of ******** includes loops with H and t - with opposite sign.**

## Higgs Mass - Upper Limit

**If there is no new physics up to the GUT scale, then the Higgs mass must be < 160 GeV. Again a low mass Higgs is favored.**

**H           H,W,Z,t        H**

## Higgs Discovery Limits

**Limits from ATLAS for 30 fb****-1****, S/********B**

**LEP-II**

## Statistics and Significance

## Higgs Quantum Numbers

**If the Higgs is seen in the 2 photon decay mode , it cannot be a J=1 state (Yang’s theorem). Recall that the 2 photon state is a quantum number filter.**

**Suppose the Higgs is found in the WW decay mode. Look at the spin correlations expected for a 0****++**** state. The emission of the 2 leptons is then preferentially in the same direction.**

## H --> ZZ --> 4l, Spin and Parity

**Recall the classical pion parity in**
**for J = 0 into ZZ, CP requires that S = 0 for the ZZ, with a longitudinal and transverse Z polarization **

## Full Monte Carlo results

**0****-**** - decay planes are perpendicular**
**0****+**** - decay planes are parallel.**

## Higgs  Total Width Measurement

**Resonant mass of Higgs will be well measured**
**At low mass the natural width is dominated by detector resolutions. At high mass the natural width dominates and can be well measured at the 5% level.**

## Extraction of Partial Widths

**For a light Higgs several partial widths can be determined at the 10-20% level. **
**If the VBF method is successful with the WW and ZZ final state, then g****HWW**** and g****HZZ**** can be determined unambiguously.**

## Early Physics Reach – q*

**If the calorimetry is understood, resonances up to a few TeV in mass are accessible early in the LHC run. **

## Composites - DY

**Search for lepton composites in D-Y production of dilepton pairs. At masses above the Z there is no known resonant state. Reach is ~ 20 TeV. Early reach is ~ 5 TeV for 10 fb****-1****.**

## TeV Scale Extra Dimension 

**KK excitations of the ********, Z in D-Y**
** ****LHC at 600 fb****-1 ****has a reach to****      ****6  TeV. SLHC would push out 30% further.**

**Black hole production ******** Democratic Hawking evaporation ******** copious Higgs production. Study with full CMS simulation.**