2. PHYSICS PERFORMANCE STUDY AND GENERAL SOFTWARE DEVELOPMENT

2.1 CMS DISCOVERY POTENTIAL

A number of studies were carried out taking into account detailed description of CMS detector response to investigate if CMS is adequate to the challenging tasks outlined in introduction. Main results of these studies are summarized below.

Standard Model (SM) Higgs boson

CMS will be able to discover SM Higgs boson in the entire mass range of interest (~ 80 GeV < mh < ~ 1 TeV). Excellent crystal ECAL performance increase a chance of discovery of the light Higgs (mh < ~ 140 GeV) in mode. The mass range up to 600 - 700 GeV will be explored with "gold - plated " mode thanks to the precise electron and muon momentum resolution. If contrary to the modern expectations Higgs would be very heavy (mh ~ 800 - 1000 GeV), nevertheless it can be discovered combining the observation in several complementary channels using the advantages of CMS calorimetry.

Supersymmetry

The variety of existing Supersymmetry models hampers the phenomenological analysis. At present more or less comprehensive study was done for minimal supersymmetric extension of SM model (MSSM). However even MSSM predicts an existence of 31 new particles! In particular, there are 4 Higgs bosons states (h0, H0, A,) and 28 superpartners of SM particles.

The essential and non trivial result of CMS physics group studies is that MSSM Higgs sector can be discovered in the most of model parameter space combining different observation channels (see Fig. 2.1). CMS will also discover strongly interacting sparticles (sqarks and gluino) with masses up to 2 - 2.5 TeV combining two general signatures:

Weakly interacting sparticles can be observed both in a clean leptons signature and in the cascade decays of sqarks and gluino. Chargino and neutralino reach in CMS found to be ~350 GeV. Sleptons can be detected up to ~400 GeV.

Search for physics beyond the SM and MSSM

There are a lot of possible new physics in TeV scale besides MSSM to be checked: Z' and W' vector bosons, SUSY with R-parity violation, compositeness, leptoquarks, scalar color octets, Kaluza-Klein states, Technicolor etc. The study is under way but of course not complete. First results indicate that CMS will essentially improve the existing bounds or will discover this new physics in TeV region.

B - physics

At , the cross section for production is about 500 µb. It means that ~1012 pairs will be produced at the initial phase of LHC operation at low luminosity. CMS intends to widely exploit this opportunity to study b - physics. The main goal is the search for CP - violation in B - meson decays. Detail simulations indicate that CMS sensitivity will be triangle angles. Other important CMS goals in B - physics is the investigation of oscillations. It was shown that CMS is adequate to investigate all interesting predicted range of the oscillation parameter xs due to the very precise decay vertex parameters determination provided by CMS central tracking system. LHC opens also unique opportunity to study rare decays of B - mesons due to the huge rate of the pairs production. One of the most interesting example is decay which width is extremely sensitive to the new physics beyond SM. In this channel the non standard physics which enhanced branching ratio by factors of 3-5 can be detected in CMS during one year of LHC running at low luminosity.

Heavy ion (HI) physics

The main goal of CMS HI program is the search for a new state of matter, namely, quark-gluon plasma (QGP). The QGP formation is predicted to be signaled by a strong suppression of upsilon prime and two prime resonance production relative to the upsilon production and by jet quenching phenomena. Detailed studies shows that the dimuons originated from upsilon can be detected in CMS despite enormous tracker occupancy with the efficiency 60% even in the worst case of the central Pb - Pb collision. Hard jets (Et > 100 GeV) can be recognized applying the dedicated jet reconstruction algorithm.


2.2 CONTRIBUTION OF RUSSIAN INSTITUTES IN CMS PHYSICS STUDIES

Russian physicists participated in CMS physics program from the very beginning of CMS. The results of their studies contribute to main CMS documents (including LOI and TP). The wide range of our interests included :

  • SM Higgs : associated production of light Higgs , heavy Higgs()

  • MSSM Higgs ()

  • Squarks and gluino search

  • B-physics ( oscillations, rare decay)

  • Heavy ion collisions (dimuons, jets)

In 1996 the efforts of CMS physics group were concentrated primarily on the SUSY study to meet the requirements of theorists to demonstrate the CMS discovery potential. The other goal of these studies was an optimization of CMS calorimetry system parameters which are crucial for SUSY search.

Our contribution to this study was basically the search of squarks and gluino in + jets + leptons final state, which is not so dependent on the instrumental . It was shown that mass range of squarks and gluino up to ~ 2-2.5 TeV can be explored in CMS (see contours in Fig. 2.2). The results of this study were presented at CERN SUSY Workshop in October 1996. We also continued our study of Higgs sector of MSSM. Two promising new channels:





enables one to observe the CP-odd MSSM Higgs boson (A0) in especially difficult range of the parameter space were studied in details. It was shown that these modes have a comparable reach in the parameter space, but newly suggested mode allows one a simultaneous search of both A0 and h0 Higgses even at the low luminosity stage of LHC. The results of these studies were published in Physical Review
() and presented at Snowmass-96 ().

In B-physics study the emphasis was done on the update of oscillations with new set of structure functions, lower muon trigger threshold and different options of the vertex detector. Main results were reported at PBARP-96 (Padova, Italy).

In HI sector a substantial progress in understanding of the CMS detector performance both for dimuons and jet physics was achieved. The new algorithm of dimuon reconstruction for the severe HI environment including muon track and dimuon vertex reconstruction was developed. It enables one to reconstruct dimuons with efficiency 66 % - 90 % depending on the 'centrality' of collision. The jet extraction algorithm specific for HI was developed and proved to be efficient to reconstruct jets with . The results of HI studies were reported at "Quark Matter 96" (Heidelberg, Germany), HI CMS workshops (Lion, France and Dubna), "XXXI rencontres de Moriond" (Moriond, France) and published in 6 papers.

In 1997-1998 we plan to continue and extend all the directions of activity briefly described above. Since our main milestones for 1997-1998 are subsystems TDR, the emphasis will be put on detailed detector simulations to optimize CMS performance for the physics program.

2.3 GENERAL SOFTWARE DEVELOPMENT

It is well understood in CMS collaboration that software is of the same importance as hardware. The software development for future experiment is a challenging task implying the joined efforts of a number of developers. At present time of TDR preparation, main efforts of CMS software group are concentrated on the development of the GEANT-based global CMS detector model (CMSIM). In 1996 our participation in CMSIM development was as follows:

  • quartz fiber HV geometry description, both detailed and fast (based on shower library) HV response simulation,

  • baseline calo trigger algorithms (including suggested specific tau -trigger)

  • semi-fast simulation package of CMS calorimetry (with the same data structure as detailed GEANT version, but much faster)

  • tracker reconstruction software including detailed simulation of Si and MSGC detectors response, track- and vertex-finding algorithms (see tracker chapter)

Besides CMSIM, a new versions of non-GEANT MC packages (CMSJET, TFAST) were developed for many of physics studies in CMS. Being not so precise and detailed as CMSIM, but are much faster, they are intensively used in a number of applications. In particular, CMSJET at present is the basic tool for physics studies.

The results of our software activity in 1996 were presented at ICCHEP-96 (Frascati, Italy), AIHENP-5 (Lausanne, Switzerland) and Vienna Pattern Recognition Workshop (Vienna, Austria).

In 1997 CMS software group goal in the light of TDRs is to provide fully debugged and tested simulation and reconstruction tools (CMSIM) for the benchmark physics processes chosen to demonstrate the CMS detector performance. Russian particular responsibility is TRACKER, ECAL and HV subdetectors software.

1998 is foreseen to be the beginning of transition period from the common fortran-based software to the Object-Oriented (OO) paradigm, which is decided to be a mainstream approach in CMS software. The russian participation in this activity at the prominent level implies self-teaching and further involving of new software specialists.


2.4 RADIATION STUDIES

New generation of collider experiments deals with an extremely hostile radiation environment. At project luminosity LHC will produce Dnear 1011 secondary particles per second in interaction point, additional portion of radiation are coming from beam halo particles losses near the experimental hall and beam-gas interactions inside vacuum chamber. Any experiment at LHC should be carefully shielded from the radiation excess to guarantee detectors performance and stability of their operation.

There are several groups of CMS experiment radiation problems under investigation:

  • radiation background calculations
  • radiation damage and radiation resistance studies
  • induced radioactivity calculations and access scenario planning

Work on these problems goes in strong connection with parallel intensive development of special software created for these purposes. To have correct description of radiation transport and dissipation one needs to take into account all main physical processes in a very wide energy interval, from TeV region down to thermal energies of neutrons. Radiation part of the CMS experiment design is based on the model calculation of the radiation transport and radiation effects inside detector. Radiation Physics Group at IHEP (Protvino) has created and developed during more than 20 years software package MARS for Monte-Carlo simulation of radiation physics problems. Except CMS MARS package was used for SDC experiment project at SSC, LHC-B experiment at LHC and for LHC accelerator design. Participation in the modern physics projects allows to survive and to support the MARS software project in Russia.


Radiation Background

Radiation background studies are very important for the optimization of the forward muon system and central tracker working conditions. Peak value of radiation background in FMS for the "trivial" version of shielding reaches up to 2000 charged particles/cm2/s. Special measures were designed to lower background particles production in the vicinity of sensitive elements. Optimization calculations of the FMS shielding should allow to suppress such values down to the safe level.

Proper design of the accelerator components located in the CMS hall is extremely important for the experiment radiation environment. Calculations show that interactions of the secondary particles with beam pipe material is one of the main sources of radiation background, so careful engineering design of the vacuum chamber and supporting system should be done to minimize the possibility of such interactions.

Calculations of the accelerator related background is the separate problem which is in strong connection with the accelerator systems design, such as beam cleaning system, vacuum system etc. MARS group participates in the LHC radiation impact studies. After intensive studies of LHC produced radiation in 1996 complex of shielding measures was proposed to suppress this source of background down to the negligible level.

Radiation Damage

Radiation damage studies and radiation doses distribution calculations will allow the proper choice of radiation hard materials and electronics where it would be necessary. The most damaged regions are the forward calorimeter, central tracker and end-cap calorimeters. Dose inside crystal (PbWO) end-cap EMC reaches 1.3 kGy/Y, in the plastic scintillator of end-cap HC - 8 kGy/Y, in the HF quartz fibers - 3 MGy/Y. Detailed maps of the dose and radiation defects distributions will used as a base for electronics positioning and detector construction in the "hot zones", where the detector life is limited by several months due to very high levels of radiation.

Experimental investigations of the materials radiation resistance are rather restricted due to simple fact that it is impossible to reproduce radiation fields that will be actual on the new accelerator. Results of the different radiation tests should be combined taking into account computational information to make the adequate analysis of the materials/electronics resistance in the mixed high-energy radiation fields at LHC.

Induced Radioactivity

Induced radioactivity would be the serious problem during the detector repair and upgrade between accelerator runs. It is obvious that the induced activity level will be rather high (up to 35 mSv/h near HF) and it will make difficult access to regions near the "hot zones". Excess access time will limit efficient time of detector operation. Proper design of the access scenarios needs preliminary calculation of the activity rates and study of the time dynamic of rates. Careful choice of materials would minimize influence of this factor on the detector repair time.


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