A Quantum Diaries Survivor

Here is the introductory chapter (save a foreword which I will spare you from reading) of the proceedings for Corfu 2005.

As before, I accept corrections, criticism, modifications, flames, all with a good spirit.

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1 Introduction

1.1 The present
The Tevatron accelerator has been subjected at the turn of the millennium to a massive upgrade, with the construction of an entirely new ring, the Main Injector, and several improvements in the facility producing and storing antiprotons - the most challenging part of the whole project.

A sketch of the Run II setup of the Tevatron accelerating complex is shown in
Fig.1.

A modest increase in beam energy -from 900 to 980 GeV- was the by-product of an upgrade of the accelerating cavities. The Tevatron upgrade did not aim at the maximum possible center-of-mass energy, as much as at securing the best operating conditions that would guarantee the machine to consistently deliver the highest possible instantaneous luminosity, a crucial ingredient to keep hopes alive for a Higgs boson discovery, and for the measurement of mixing of Bs mesons, the other main goal for Run II of the CDF and DO experiments. The 9% increase in center-of-mass energy does grant a 30% increase in production rate of heavy particles such as the top quark or the
Higgs boson, but the impact of a larger number of protons and antiprotons and of a faster rate of data collection is more dramatic in the discovery reach of the Tevatron experiments in Run II. The Tevatron has recently surpassed the peak luminosity of 1.7 x 10^32 cm^-2 s^-1 (see Fig.2), and has been consistently delivering steady beam to the experiments. An integrated luminosity of about 1.5 fb^-1 per experiment has been collected at the time of writing.

At the time of writing a few crucial upgrades are being worked at to complete the plan of improvement of machine luminosity - to which the critical ingredient is the efficient production, storage, and transfer of antiprotons. The two principal upgrades are electron cooling of the antiproton beam and an increase of the stacktail bandwidth.

Electron cooling has been recently demonstrated in the recycler ring by merging a 4.8 MeV beam of electrons with the 8.9 GeV beam of antiprotons; because of the different masses, particles in the two beams move at the same speed. The electron current stabilizes the antiproton beam, removing some
of the longitudinal spread in antiproton velocities.

An increase in the capability of antiproton stacking, through an upgrade of the accumulator stacktail cooling system, is expected to significantly improve the speed of antiproton collection and the resulting amount of available particles
for Tevatron collisions.

If the described upgrades should not work as planned, the Tevatron will still be able to deliver a total of 4 fb^-1 to each collider experiment by 2009 -what has been called "base plan". But if the upgrades will produce the desired effect, the design luminosity of 8 fb^-1 can be collected by that date (see Fig.3).
This latter "design plan'" might change the perspective of Tevatron physics, with significant chances of a light Higgs boson discovery before or in coincidence with a LHC observation.

1.2 The future

The Large Hadron Collider (LHC), the accelerating complex in construction at the CERN laboratory in Geneva, is a truly daring enterprise, attempting to scale the two significant machine design parameters -beam energy and luminosity-
up by an order of magnitude from the previous records reached by its predecessor, the Tevatron collider of Fermilab.
 
From the 980 GeV beams of protons and antiprotons of the Tevatron to the 7000 GeV beams of protons of the LHC, the leap forwards is giant. It entailed the construction of 1232 superconducting magnets to provide the necessary bending force in the CERN $26.7~km$ tunnel, which until 2002 hosted the LEP II accelerator. The total stored beam energy at the design luminosity of 10^34 cm^-2 s^-1 equals the kinetic energy of a loaded Airbus A340-200 at landing speed.

Challenging is also the design and construction of a system capable of running with 2808 25 ns-spaced bunches of protons, achieving beam stability throughout the LHC tunnel, and squeezing the bunches transversely to a diameter of 34 um in the proximity of the CMS and Atlas experimental facilities.

Together with the technical challenges involved in the construction of the accelerating complex, the construction of the CMS and Atlas experimental pparata are also quite demanding.

CMS and ATLAS were born originally different in a few key design features, but gradually became more similar as the first lueprints evolved into the commissioning phase, in a process of dynamical ikening that was already observed between DO and CDF a decade ago.

The focus of the experiments, it appears, has gradually shifted from a marked accent to muon detection -with the aim of observing the ultra-clean signature of Higgs boson decay to a pair of Z bosons, p p -> H -> ZZ -> 4 muons- to a greater versatility. The motivation of that shift is the need to not only ensure detection of a Higgs boson whatever its mass, but also to do it with the smallest possible amount of data - i.e., in the shortest possible time. Moreover, if the original motivation for building the experiments was principally the detection of the Higgs boson, maintaining a rich physics program and constant financing for many years is a more important item in the agenda once running starts.

As we approach the starting date of the first collisions, most of the construction problems are on the way of being successfully solved -in some instances by inventing brilliant solutions, in others by throwing money at them. LHC must not fail: the high energy frontier will rely on that machine for the next ten years. Interestingly, a strong commitment to make it a success has been coming recently also from the United States, where the active participation in the experiments has been dubbed ''strategic'' for the plan of bringing back the leadership in frontier particle physics to american soil.

1.3 From the present to the future
 
For an experimentalist, the best way to exploit these few years that divide us from the time when the first data will be collected by the LHC is to prepare the ground for their successful exploitation, thinking in advance at what will be the dominant sources of uncertainty to the most intriguing measurements, and what solutions can be cooked up to reduce them.

A specific example of this preparatory action is the study of the effect of trigger selections to the calibration datasets which will be needed by many precision measurements. Take the top quark mass as a benchmark: in 2008 the relative uncertainty on this fundamental parameter of the Standard Model -after the combination of CDF and D0 results- will be likely smaller than 1.5 GeV, and quite possibly smaller than 1 GeV (see Fig.4).  If LHC experiments want to improve still further the precision of that number, they will need a very careful planning of the reduction of systematic uncertainties from the jet energy scale and from all other sources.

1.4 Plan of the writeup

The present writeup will attempt at offering some insight on several important challenges in the use of Tevatron results, data, and experience to prepare the ground for the LHC.

Section 2 will provide a short discussion of some details in the operation of those experimental apparata of relevance to the following topics.

In Section 3 I will discuss some of the most important methods currently
used to perform precise measurement of hadronic jets and efficient b-tagging, which are two critical ingredients to most of the high-Pt physics program of CDF and DO.

In Section 4 the status of Higgs boson searches at the Tevatron will be discussed, along with some speculation on the possible scenarios that the LHC experiments will be dealing with.

Section 5 will discuss top quark physics, and the challenges posed by precision measurements of important quantities in the top sector.

Section 6 will deal with electroweak physics measurements that may not only help improve the picture of Standard Model physics before the LHC, but also provide some opportunities to reduce key systematic uncertainties for precision measurements by LHC.

I will conclude in Section 7.