First Physics at LHC: Exploring Non-perturbative QCD Phenomena

1. FIRST PHYSICS AT LHC Jean-Pierre Revol Guy Paic’s FestInstitute for Nuclear Sciences UNAM, Mexico September 21st, 2007 New Delhi November 22, 2001

2. Guy played a central role in shaping up the ALICE physics programme, from the very beginning: • Founding member • Physics Coordinator • November 7, 2000,contribution to First Physics with ALICE • Continuing to play an important role: • Deputy Physics Coordinator • We are all counting on Guy when LHC starts

3. Schematic layout of the LHC Update on the LHC machine already covered by Paolo Giubellino • The first proton-proton collisions are expected for the summer of 2008. • The first heavy ion collisions could in principle occur soon after.

4. LHC commissioning • Sure: LHC beam collision commissioning will be a most exciting time, a major event, perhaps even a historical event (general equipment commissioning has already started and is in final phase). • Not so sure: how in detail the LHC commissioning will happen, therefore, the subject of my talk required a few assumptions, in order to imagine a first physics scenario: • (1) The first (few?) collision data will be collected at a center of mass of 900 GeV; • (2) The bulk of the commissioning will be carried out at the highest possible center of mass energy (~ 14 TeV), for which I assume typically one month (106 seconds) of data, at a luminosity of 1031cm–2s–1. (extracted from LHC document on expected performance with 43 bunches)

5. CERN DG June 27, 2007

6. First Physics at LHC • The initial LHC physics will deal with large cross-section phenomena (strong interaction), most of it will be devoted to the study of the average non-elastic collision, which can be carried out during LHC commissioning. • “immorally successful theory of multiparticle production (Kaidalov et al.)” Alvaro de Rujula, Evian LHC Worshop, March 5-8, 1992 • However, even this needs to be verified at a new energy!!! • Regge theory gives a power law increase of the total cross-section which has to break down at some energy (at LHC?) • QCD is an excellent description of the strong interaction at large Q2, where asymptotic freedom is ideal for perturbative methods to work, but QCD needs to be also understood low Q2 (soft phenomena), in the non-perturbative regime (most of the mass of ordinary matter does not come from the Higgs mechanism!). • LHC should bring non-perturbative phenomena to relatively large Qs2 (~ 1 GeV2), large enough to be studied experimentally; • Experimental guidance is of interest for improving phenomenological application of QCD to the soft sector; • This new regime in proton-proton collisions is of course very relevant to the understanding of heavy ion collisions (Qs2 ~ A1/3); • Background to rare phenomena.

7. LHC Is a new parton regime accessible in pp collisions at LHC? • As low x (~Q2/s) values are reached, both the parton density and the parton transverse size increase, there must be a scale (at q2 < Qs2) where partons overlap. When this happens, the increase in the number of small x partons becomes limited by gluon fusion (gg  g). Beyond HERA,down to x ~ few 10–7 What is new at LHC is that this overlap should occur for relatively high pT partons ~ 1 GeV/c (Kharzeev Qs2 ~ 0.7 GeV2), where the effect could be observable

8. large x small x Why will partons overlap? • At high energies, the transverse size of a proton (or a heavy ion) increases as (T)1/2, and is limited by the Froissard Bound • As Qs2 ~ A1/3, the effect should be more spectacular with Pb-Pb collisions, for which Qs2 ~ 4 GeV2 . • Parton saturation may have been seen at RHIC (BRAHMS in Au-Au, forward region), if true can it be detected in pp collisions at LHC? D. Kharzeev, Academic training, CERN May 2007

9. Parton saturation and break down of Regge theory • From Regge theory (no parton saturation), in asymptotic regime, both the total cross-section and the particle density, at  = 0, grow as a power law of s: • On the other hand, the Froissard bound limits the growth of tot: • There will be an energy where the power law takes over the logarithm increase. However, with present value of p, at LHC top energy, both behaviours are the same: p(s) = intercept of the Pomeron trajectory at t = 0;  = p –1= 0.12 ±0.02 (Kaidalov,91)

10. Log and power law behaviours of total cross-section • LHC energy too low ! Unless something else is happening ... We need 100 TeV, to reach a 10% effect!  At LHC, we are still far from the asymptotic regime!

11. Observing parton saturation at LHC • Therefore, at 14 TeV, we expect: • The good news is: • both of these ratios, can be measured at LHC to a precision of a few % (ALICE multiplicity: 3-4 % absolute, 2.5 % ratio; TOTEM total cross section: 1%). Therefore, this can be precisely tested, provided we get some data at 0.9TeV. • definite predictions are available, such as with the “immorally successful” Quark Gluon String Model of Kaidalov, where the cross section ratio is 1.55 while the central multiplicity ratio is 1.67, a 8% difference! • In passing, we will verify that pp and ppbar do behave the same way at high energy: At =0, (dNch/dh)ap-p – (dNch/dh)p-p~ (s/s0)–½(from Regge theory) • at s ~ 103 GeV2 it’s measured to be 1%, therefore, at 900 GeV, we expect: (dNch/dh)ap-p – (dNch/dh)p-p=0.001 • The precision of anti-p–p measurement at SppbarS was a few %, therefore, it is sufficient to measure with similar precision, the 1/1000 level cannot be tested.

12. MB transverse momentum distribution • At LHC, parton saturation could perhaps be seen in proton-proton collisions, as a distortion of the pT distribution of charged particles, due to gg  g, in the range 0 to 2 GeV/c: hence the importance of a measurement of charged particle production down to the lowest possible PT. The region of interest is below 2 GeV/c, where the bulk of particle production occurs. How well can this be measured at LHC among the various experiments?

13. ATLAS inner tracker • Semiconductor pixel (3 layers) and strip detector (4 layers) (||≤2.6), followed by the Transition Radiation Tracker (TRT): straw-tubes interspersed with a radiator (e/ separation) providing on average 36 straw hits per track; Inside solenoid of 2T magnetic field. • MB trigger: scintillator hodoscope, and beam crossings. Courtesy of T. LeCompte, Maria Smizanska

14. CMS inner tracker • The CMS central tracker (||≤2.5) is based entirely on silicon technologies (pixels and micro-strips) (13 layers at  = 0); Inside solenoid of 4T magnetic field • MB trigger: on calorimeter information (some energy in the HF calorimeter), and beam crossings. • Tracklets from pixels (three layers) alone can be used to measure charged track multiplicity, and get sensitivity to lower pß. CMS central tracker Tracklets with CMS pixels Courtesy of P. Sphicas, P. De Barbaro

15. ALICE Pixels ALICE Tracker • The ALICE central tracker includes Si pixels, drift and strips detectors(6 planes at  = 0), a TPC and a TRD (6 layers) over a lever arm of 3.5 m. • MB Trigger: scintillator hodoscope (-1.7 to -3.7 and 2.8 to 5.1); Pixel FAST-OR (unique at LHC) and beam crossings.

16. Vertexing Tracking LHCb tracker • In LHCb the Vertex Locator (VELO) is the main tracking system before the dipole magnet (4 Tm), covering 2 ≤  ≤ 4.8 with a small gap at 4.3. It is followed by a Trigger Tracker made of Si strips (TT) and on the other side of the magnet by another tracker station (T1,T2&T3) (3 layers at 390 mrad to 15 layers at 60mrad and 4 layers at 15mrad) • MB trigger: on calorimeter information. Courtesy of O. Schneider; K. Harrison; P. Charpentier

17. 10.5 m ~14 m TOTEM tracker • The TOTEM tracker is made of two stations T1 (5 planes cathode strip chambers) and T2 (10 planes of GEM detectors) covering respectively 3.1≤  ≤ 4.7 and 5.3 ≤  ≤ 6.5 and same at negative  • MB Trigger: T1 and T2 (≥ one charged track), but losing diffractive can ask for p on one side and one single track in either T1 or T2 pointing to the vertex. Courtesy of V. Avati; M. Deile; K. Eggert

18. First physics measurement: charged particle density • Multiplicity measurements do no require full tracking, full alignment nor huge statistics (a few 10000 events is enough). It can be performed with the first few silicon detector layers, in the central region. • High precision measurements in the central region (ALICE, ATLAS & CMS) will be complemented at larger pseudorapidities by LHCb and TOTEM. Not clear for all experiments how low is the bias is in their Minimum Bias trigger; Use beam crossings? CDF: Phys. Rev. D65,72005(2002) Mean pT vs multiplicity

19. CMS outer/inner silicon ATLAS TRT/SCT ALICE TPC/ITS 1.2 X/X0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 2.0 h 3.0 Low momentum reach • Tracker capabilities at low pT depend on the amount of material traversed by particles, the number and precision of measuring points, the strength of the magnetic field, and the size of the detector. In general, there is a trade off between pseudorapidity coverage and thickness of material at  = 0 (sagitta goes as ~ L4). For instance, ALICE has the smallest  coverage but is the thinnest (each line is the sum of material after a given detector): Courtesy of K. Safarik For full tracking

20. Coverage in pseudorapidity for charged particles • The range here is arbitrarily defined such that pT/pT = 30%. There is complementarity between detectors.

21. Sector: 4 (outer) + 2 (inner) staves Half-Stave: 10 chips SPD: 10 sectors (1200 chips) Cut ALICE High Multiplicity Trigger • Motivation: study of the extreme energy densities, where parton overlap is maximum • Pixel Fast OR Trigger • Trigger on number of fired chips (per layer), or more sophisticated configurations using FPGAs

22. Simulated Statistics SPD Layer 1 (|h| < 2) • Very high statistics study possible! Trigger: 190 chips out of 400 in layer 1 10 times mean multiplicity 1031 cm-2 s-1 106 s PYTHIA distribution extrapolated above 250 evts in MB sample (~ 10) evts with trigger (~ 130K) Trigger rate 1Hz Purity 13%

23. Early Jet Physics? Within one month, 106 s at 1031cm–2s–1, ATLAS & CMS should reach 500 GeV jet, which covers already the Tevatron reach. As tests of compositeness will require calibrating the jet energy scale, the first physics will probably be the study of the underlying event. Courtesy of P. Sphicas

24. Courtesy of P. Sphicas

25. Baryon Transport to the Central Rapidity Region • Can gluons carry baryon number?(“yes”: Rossi, Veneziano String Junction model) • Baryon number can be transferred by specific configuration of the gluon field: (B. Z. Kopeliovitch and B. Zakharov, Z. Phys. C43 (1989) 241). • At LHC huge rapidity interval between incoming protons (yp = ± 9.6) and central rapidity. Veneziano’s model would result in substantial net baryon production in central rapidity region. – + p – + K+ + 2K0 HERA point plotted at rapidity 9.6 - 7.4 = 2.2

26. BRAHMS: Phys. Lett. B607 (2005) 42 Some hints from Heavy Ion collisions • Naively, in heavy ion collisions one would expect to find vanishing net baryon number at mid-rapidity: the valence quarks of the colliding nuclei are difficult to stop. String junction stopping might be easier to achieve!

27. Baryon Production in Central Rapidity Region • ALICE will make use of its sensitivity at low pT, and its unique Particle IDentification capability, to measure baryon-antibaryon asymmetries (p, , etc.) • As effects are expected to be small, the main effort will go into reducing the systematic uncertainties. Proton,+ Antiproton,– Courtesy of P. Kristakoglou & M. Oldenburg

28. g3-GHEISHA DATA FLUKA Challenge: reduce systematics to the 1% level • Sources of systematics: • uncertainties of the transport models (Comparison of various transport models, converging on FLUKA) • uncertainties of the detector material (needs to measure it with data to 5%) • beam-gas interactions (measure from data) • feed-down from hyperon decays (study them all) • effects of track and event cuts (will be a by product of basic mutiplicity and momentum measurements) experimental data taken from Bendiscioli and Kharzeev, Riv.Nuovo Cim.17N6, 1-142 (1994) Courtesy of P. Kristakoglou & M. Oldenburg

29. Systematic studies (preliminary) • Varying the density of the detector material: • From 90 to 110% of the initial (original) density with a step of 2.5% • We will try to measure it using data (gammagraphy of detector with gamma conversion) UNIQUE TO ALICE ! BOTH in pp and in HI Courtesy of P. Kristakoglou & M. Oldenburg

30. Heavy flavour production in pp collisions at 14 TeV • Important test of pQCD in a new energy domain • Remember the “15-years saga”* of b production from CERN SppbarS to the Tevatron! …and c production not yet fully understood • Huge rates at LHC, so clearly part of initial physics • NLO predictions for LHC: uncertainties of factor > 2–3 charm CDF, hep-ex/0307080 FONLL: Cacciari, Nason beauty Cacciari, Frixione, Mangano, Nason and Ridolfi, hep-ph/0312132 HERA-LHC Workshop Proceedings beauty MNR (NLO)

31. rec. track e Primary Vertex B X d0 Heavy flavour detection • Tracking & Vertexing to identify D (ct ~ 100–300 mm) and B mesons (ct ~ 500 mm) • Secondary vertex capabilities are crucial! Impact parameter resolution! HERA-LHC Workshop Proceedings ALICE wellequipped Courtesy of A. Dainese

32. ALICE (pp, s = 14 TeV, 1 LHC year) High sensitivity in comparison to pQCD, Importance of low pT : B  e + X D0 Kp 1 year at nominal luminosity (109 pp events) Courtesy of A. Dainese

33. 1 year at nominal luminosity ~1 month CMS & ATLAS • should have similar performance, • at least with muons • higher ptmin because more material • & larger field • should have similar performance • higher ptmin because lack of K id Significant pt coverage in the first month at 1031 ALICE D0 Kp B  e + X 1 to 10 GeV in one month! Charm Beauty Courtesy of A. Dainese

34. Courtesy of A. Dainese Quarkonia m+m- m+m- m+m- e+e- Quarkonia: • Complementarity between experiment coverages. • Low pT advantage for ALICE in +–.

35. Many measurements of Beauty cross section at LHC 1 year at nominal luminosity Courtesy of A. Dainese

36. Measurement of Charm cross section 1 year at nominal luminosity Courtesy of A. Dainese

37. LHCb: exclusive B Advantage of large rapidity Courtesy of A. Dainese

38. ATLAS: B+ J/yK+ and B0 J/yK0* • In the first month at LHC with 10–2fb–1, ATLAS reaches 25 GeV already! number of B’s with pT > pTmin in 30/fb (3 years at 1033cm-2s-1) pTmin [GeV] Courtesy of A. Dainese

39. Conclusion • Low pT properties of proton-proton collisions (not all discussed here) can be accessed during the LHC commissioning period, and promise to be exciting: • Exploration of a new energy domain! • The most efficient way for LHC to prepare for the Higgs search, goes through an optimized commissioning, between machine and experiments: • First collisions as early as possible (hence at 0.9TeV): • The by-product will be early detector preparation for the first and more challenging 14 TeV collisions as well as significant early physics in the domain of non-pertubative QCD. • ALICE, even though optimized for heavy ion studies, should play a major role in the First Physics at LHC, in addition to using proton-proton collisions as a reference for its main H.I. programme.

40. Happy 70th birthday Guy! Best wishes for a long Mexican career & your ALICE dream coming true!