Recent Hera Results and Future Prospects

A few selected HERA results are presented and the prospects for future measurements with high luminosity are discussed, which will become available after the planned luminosity upgrade of the HERA storage ring planned for 2000. 1 Present HERA Performance HERA luminosity 1992-97 In te gr at ed L um in os ity ( pb -1 ) Mar Apr May Jun Jul Aug Sep Oct Nov 1992 1993 1994 1995 1996 1997 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 Figure 1: The integrated luminosity delivered by HERA to the ZEUS interaction region for the different years of operation versus the date in that year. The electron proton storage ring HERA at DESY, colliding 27.5GeV electrons or positrons on 820GeV protons, started operation in 1992. The instantaneous and integrated luminosities have been steadily increased as shown in Fig. 1. During the first two years of operation, when electrons were colliding on protons, the electron beam current was limited by breakdowns of the beam life time. These limitations were not present after the machine switched to positrons during the 1993/1994 winter shut-down. Operation in 1997 was very successful providing an increased instantaneous luminosity and a long duration of the running period. In total integrated luminosities of 2 pb−1 (e− p collisions, 1992 and 1993) and 71 pb−1 (e p collisions, since 1994) have been delivered to the colliding beam experiments H1 and ZEUS. Table 1 compares the machine parameters which were achieved in 1997 to the original design parameters. The maximum luminosity of 1.4×10 cm−2 s−1 is close to the original design goal (1.5× 10 cm−2 s−1). The accelerator is now operated in a two year cycle. Short winter shut-downs of one to two months in 1996/97 and 1998/99 alternate with long winter shut-downs in the following year. The major activity for HERA during the present winter shut-down is a modification of the vacuum system of the electron ring in order to allow high current electron running. The expected integrated luminosities per experiment (e−p collisions) for the next two years are about 15 pb−1 in 1998 (short running period) and >35 pb−1 in 1999 (long running period). The luminosity upgrade and installation of spin rotators in the south (ZEUS) and north (H1) straight sections are planned for the 1999/2000 winter shut-down. 2 The HERA Luminosity Upgrade The aim of the HERA luminosity upgrade is to increase the design luminosity from 1.5 × 10 cm−2 s−1 to about 7 × 10 cm−2 s−1 as shown in table 1. An integrated luminosity of about 1 fb−1 per experiment is expected to be delivered to the colliding beam experiments in the running period 2000 2005. The luminosity increase will be achieved by stronger focusing of both the electron and proton beam. The final focusing magnets have to be moved closer to the interaction point (IP). This necessitates an earlier separation of both beams at the interaction region, which requires new superconducting beam magnets to be placed inside the H1 and ZEUS detectors. The design of the new interaction region is challenging, because of a significant increase of the synchrotron radiation emitted near the IP. Details of the luminosity upgrade can be found in [1]. In table 1 the electron beam energy of 30GeV was chosen as HERA Parameters 1997 Design Upgrade p beam energy (GeV) 820 820 820 e beam energy (GeV) 27.5 30 30† Number of bunches (proton/electron) 180/189 210 180/189 Number of protons/bunch 7.7× 10 10× 10 10× 10 Number of electrons/bunch 2.9× 10 3.6× 10 4.2× 10 Proton current (mA) 105 160 140 Electron current (mA) 43 58 58 Hor. proton emittance (nm rad) 5.5 5.7 5.7 Hor. electron emittance (nm rad) 40 39 22 Proton beta function x/y (m) 7/0.5 10/1 2.45/0.18 Electron beta function x/y (m) 1/0.7 2/0.7 0.63/0.26 beam size σx × σy (μm) 200 × 54 247 × 78 118 × 32 Synchrotron radiation at IP (kW) 6.9 9.7 25 Specific luminosity (cm−2 s−1 mA−2) 7.6× 10 3.4× 10 1.6× 10 Luminosity (cm−2 s−1) 1.4× 10 1.5× 10 7.4× 10 Table 1: HERA parameters achieved in 1997, compared to the original design and the luminosity upgrade program. ⋆ Increase to 920GeV is being studied. † Maximum energy assumed for layout of IP. the maximum energy for background calculations and the design of the new interaction region. The actual electron energy will be slightly lower, because of constraints on the reliability and power of the RF system. There are plans for an increase of the proton beam energy from 820 to 920GeV. First tests were successfully performed during the machine development period in December 1997. If further tests scheduled during the re-commissioning in 1998 are successful too, the beam energy may already be increased for the 1998 running period. 3 Detector Modifications Both collaborations have made several modifications of their detectors during the last years. H1 installed a new backward tracking detector and a new rear calorimeter. Small electromagnetic calorimeters near the beam pipe (BPC) for tagging very low Q electrons were added to both detectors. In addition small Si trackers were placed in front of the BPCs in order to improve the measurement of the scattered electron. H1 installed a Si micro vertex detector with backward wheels. During the 1997/98 shut-down ZEUS implemented a forward plug calorimeter (FPC) for a better measurement of the proton remnant and very forward jets. These modifications, except the ZEUS FPC and the H1 barrel Si micro vertex detector, aim at improving the acceptance for electrons scattered at very small angle, and hence extending the kinematic region to very low x and Q physics. Several detector modifications have been proposed for the HERA luminosity upgrade: H1 is planning an upgrade of the central and forward tracking and an upgrade of the trigger; ZEUS will install a Si micro vertex detector with forward wheels. Both collaborations have to modify their luminosity monitors and leading proton spectrometers. The compensating solenoids will be removed. The acceptance at very small angles will be slightly reduced because of the final focusing magnet inside the detector volumes, i.e. detector components at very small angles (BPC and FPC) will have to be removed. 4 Selected Recent Results and Prospects 10 -1 1 10 10 2 10 3 10 4 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 1 x Q 2 (G eV 2 )