The study of q q

production at LHC in the l l channel and sensitivity to other modelsMichihisa Takeuchi

~ ~L L

± ±

(hep-ph/0701190)

Kyoto Univ. (YITP), ＫＥＫ　D2

Collaboration with M. M. Nojiri

1. New Physics at TeV scaleFine tuning prob.

The Standard Model describes interactions among elementary particles well. But, radiative corrections for Higgs mass parameter is quadratic divergent.

We cannot doubt of the existence of the Dark matter in the universe. But, there is no candidate in the Standard Model.

New Physics at TeV

Assuming the DM is WIMP and the mass is

TeV mass DM is natural

Dark matter prob.

2. Models beyond the SM

For example, these models have this structure ＳＵＳＹ（ＭＳＳＭ） Littlest Higgs model with T-parity Universal Extra Dimension model 　 and so on…

・ Existence of stable DM

･ Fine tuning problem New symmetry

Partner particles for the SM particles

Constraint from ＬＥＰToo heavy

But, new particles are added in TeV scale.

(fine tuning problem is reintroduced)

Z odd particles are always pair produced

The lightest Z parity odd particlecannot decay into the SM particles

in a collider experiment

Similar typical partners’ mass spectra

　 All Z2 odd particles decay in cascade to the DM with emitting SM particles

1050 GeV

950 GeV

350 GeV

300 GeV

180 GeV

MSSM LHT

The litest Z2 odd particle

Is a good candidate of DM

Partners have the same features Models with

This is important feature to distinguish new physics from the SM processes at the LHC

At the LHC

p p

Strong interacting particles are produced in pair

Once new particles are produced, they decay in cascade.

Two lightest Z parity odd particles are finally produced

、 High jets 、 High leptons

Signal ：

Many hard jets

Large

At the LHC, the SM processes are also produced enormously. We must find out the signal beyond the SM.

Proton consists mainly of u, d and gluon.

To distinguish from SM events

Mass spectrum determination mass spectrum can be determined by the kinematics of the cascade decay.

After selecting events beyond the SM by proper cuts,

Partners

、 High jets 、 high leptons

Signal ：

Fine tuning prob.

Dark matter

Mass spectrum of new particles are extracted

But, what we can see is an evidence of new particles beyond the SM.

Distinguish modelsMass spectra are extracted by kinematics. (model independent way)

Measurement of production cross section separately helps this purpose

　　 We compare production cross section of 3 models with the same mass spectra

1050 GeV

950 GeV

350 GeV

300 GeV

180 GeV

Sample point

ＭＳＳＭ The model with an extended gluino sector ＬＨＴ

Difficult to distinguish models with partners and paity

To investigate the own features of the models are needed.

The MSSM

1050 GeV

950 GeV

350 GeV

300 GeV

180 GeV

Sample point

In the MSSM, once mass spectrum are determined,

we can calculate production cross sections of various processes.

We can verify the MSSM

to measure the production cross sections.

・ We focus on Left handed squark

From PDF of proton

<

The model with Extended gluino sector

1050 GeV

950 GeV

350 GeV

300 GeV

180 GeV

Sample point

Two gluinos

Left handed squark pair

productions are suppressed

MSSM + SU(3) adjoint fermion Inspired from N=2 model

Decay pattern is the same as MSSM・ We

assume

(JHEP 0208:035,2002. P. J. Fox, A. E. Nelson, N. Weiner)

Mass spectrum is completely the same

As the MSSM

Littlest Higgs model with T parity

No gluon partner

Decay pattern of T-odd quark is similar to squark

Quark partners production cross section is 4~5 times as large as that of the MSSM.

1050 GeV

950 GeV

350 GeV

300 GeV

180 GeV

Sample point

This model has T odd partners for SM particles

(JHEP 0410:067,2004, I. Low)Higgs is introduced as gold stone boson in this model

(T-parity would be weakly broken by anomaly, but collider signal doesn’t change)

ＭＳＳＭ The model with an extended gluino sector ＬＨＴ

Measurement of production cross section helps to distinguish these models

(Unit of pb)

Key point is 4 times of No prod.Few production

～４：１～２：１From u : d ～２：１ in PDF

Summary table of production cross sections

How to measure? Let’s consider the MSSM case at first.

SS2lepton channel easy to distinguish from SM events

Left handed squark decay into lepton

In the MSSM, How to Measureσ( q q ) LHC is hadron collider therefore we cannot tell easily what the initial

partons are. 　　　　 Identification of the produced particles is difficult.

We need a method to distinguish the production processes.

~ ~L L

BR=46% BR=44%

BR=20%

We can estimate production cross section from number of SS2 lepton events

Charge information is conserved

But, background SS2lepton events come from 　BR=4%

At 1TeV, 　　　　 is larger than 　 (5 times as) 　

Both contributions are same order

Separation of production processes

To measure ,we want to cut gluino production contribution• b-veto

60% of events with gluino are cut.Gluino decay into third generation squarks

2 jets1 jet

• Number of jets （We use Hemisphere cuts）

Originally due to large gluino contribution 　

After cutting gluino contribution, we expect

Hemisphere analysis

Using only kinematics

Model independent

hemisphere 2

hemisphere 1

2 jets or more1 jet

2 hemispheres

We define following variable in each hemisphere

We can likely identify the parent particles in each hemisphere.

Produced two particles decay independently and form two groups.It is useful to reconstruct these two groups.

We regard objects with similar momenta have the same parent.

to cut gluino contributions

vs distribution （ major 3 production processes of the MSSM)

2 jets1 jet generated by Herwig 6.5

： 3% left： 35% left

We impose

with AcerDET 1.0

Numerical results1. Basic cut ( : for cutting SM events)

2. b-veto

( : for cutting gluino contribution)

3. Hemisphere cuts

More than 97 % of events with gluino are cut after .

If we know the efficiency of cuts from MC and branching ratio, we can estimate the production cross sections using SS2l event rate.

Comparison of the models

Main source of

ＭＳＳＭ The model with an extended gluino sector ＬＨＴ

Absolute value 2573 ~1000 ~4000

after Hemisphere cuts

Ratio of after/before cuts

Gluino contribution is large 2 ：1Squark contribution is large 4 ： 1

If gluino contribution is large, number of events after the cut are reduced.

Unit is pb

Key point is 4 times of No prod.Few production

Summary The models which describe TeV physics have Ｚ 2 pa

rity, and collider signals are similar.　　　　　　　　　　ＭＳＳＭ、ＬＨＴ、ＵＥＤ、 etc.

It is important to measure each production cross section separately for distinguish these models, therefore also important to identify production process.

We investigate SS2l events and propose the method to distinguish between gluino and squark productions based on counting number of jets.